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IMS > Membranes and lipids- membrane proteins and carbohydrates, membrane transport, extracellular and intracellular signalling, cholesterol and lipoproteins > Flashcards

Membranes and lipids- membrane proteins and carbohydrates, membrane transport, extracellular and intracellular signalling, cholesterol and lipoproteins Flashcards

1
Q

Lipid bilayer

A
  • Hydrophilic molecules dissolve in water- contain charged groups or uncharged groups that form electrostatic interactions or HB with water molecules
  • Hydrophobic molecules are insoluble in water- most of their atoms are uncharged and nonpolar- can’t form energetically favourable interactions with water
  • In water, hydrophobic molecules force adjacent water molecules to reorganise
  • If hydrophobic molecules (or hydrophobic part of molecule) cluster together a small number of water molecules are affected-lower free energy cost
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2
Q

Fatty acids

A

Fatty acids vary in chain length, DB number, DB position and hydroxylation. Two fatty acid chains in a lipid can be different in length.

Nomenclature:

XX:Y n-y

  • XX denotes the number of carbons in the chain.
  • Y indicates the level of chain saturation (the number of double bonds).
  • n-y is the position of the first double bonded carbon counting from the methyl terminus (first DB is from the first C)
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3
Q

Diversity of membrane lipids

A

atoms. This lipids tail are relatively straight.
Unsaturated lipid tails: Fatty acids in lipid tails that contain one or more double bonds between adjacent carbon atoms. Unsaturated lipid tails can have a cis double bond - makes a 30o kink or a trans double bond that does not affect their structure.

Differences in length and saturation of the fatty acid tails influence how phospholipids pack against one another (membrane rigidity).

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4
Q

Membranes contain 3 lipids

A
  • Glycerophospholipids (phospholipids)
  • Sphingolipids
  • Sterols
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5
Q

Glycerophospholipids

A
  • Chemical diversity arises from the combination of two fatty acids, linkage at SN1 position and the head group
  • SN1 fatty acid is usually saturated or monounsaturated
  • SN2 fatty acid is more often monounsaturated or polyunsaturated (multiple double bonds)
  • Fatty acid linkage by an ester bond
  • Glycerol is derivatised to glycerol-3-phosphate
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6
Q

Sphingolipids

A
  • Sphingolipids are built on a sphingoid base, N-acyl chain and head group.
  • Most common sphingolipid is sphingomyelin (SM) that has a PC headgroup
  • The amide group has the ability to form HB- allows interactions of sphingolipds with cholesterol or polar parts of proteins
  • The N-acyl chains of sphingolipids tend to be more saturated and can be longer than the acyl chains of glycerophospholipids
  • An acyl chain is attached via amide linkage
  • Sphingolipids in mammals usually have saturated acyl chains (up to 24 C that enable them to pack tightly)
  • Don’t have ester bonds
  • Both tails are saturated
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7
Q

Glycosphingolipids (Glycolipids)

A
  • Complex glycosphingolipids have different oligosaccharides as head groups. Their structures are composed of various building blocks (mainly sugars)
  • Glycosphingolipids are found exclusive in the outer leaflet of the membrane
  • Small percentage of the outer leaflet- 5%
  • In the plasma membrane, sugar groups are exposed at the cell surface
  • Important role in interactions of the cell with its surroundings (cell-cell adhesion)
  • Allow membranes to act as recognition sites for certain chemicals
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8
Q

Sterols

A
  • Have a hydroxyl group and a hydrocarbon tail
  • Cholesterol most common sterol in animal. Ergosterol is found in yeast and fungi membrane. Sitosterol and stigmasterol are found in plants.
  • Its size and shape allows cholesterol to interact with pockets in membrane proteins
  • Its presence increase thickness, packing and compressibility of membranes while it decreases mobility of lipids and proteins
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9
Q

Cholesterol

A
  • Due to shape it can align better with saturated side chains e.g. with sphingomyelin
  • Interactions with POPC- its OH group is not buried in the complex
  • Interactions with sphingomyelin- a HB is formed between the OH group of cholesterol and the NH group of the sphingolipid
  • The OH group of cholesterol is masked by the polar head of sphingomyelin. Make the membrane thicker due to close packing
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10
Q

Membrane curvature

A
  • The relative size of the head group and hydrophobic tails of lipids affect the shape of the lipid and the spontaneous curvature of the membrane
  • The negative spontaneous curvature of PE leads to bilayer-disrupting properties, which might promote processes that involve the generation of non-bilayer membrane intermediates, such as fusion
  • PIP has an inverted-conical structure
  • Negative and positive charge important- when trying to separate and fuse together
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11
Q

Types of membrane curvature and phospholipid

A

Cylindrical- PC, PS
Conical-PE, PA
Inverted-conical- Lyso-GPLs and phosphoinositides

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12
Q

Lipid diversity- two types

A
  1. Chemical/structural- defines specific properties of lipids
  2. Compositional diversity between tissues, organelles and leaflets within the same membrane- affects the collective behaviour of lipids
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13
Q

Asymmetry

A
  • Erythrocyte membrane has a complex lipid composition
  • 50% cholesterol
  • High conc. of PC and SM lipids in the outer leaflet
  • High conc. of PS and PE lipids in the inner leaflet
  • Lipid asymmetry is functionally important.
  • Phosphatidylserine in animal cells translocate to the extracellular monolayer when such cells undergo cell death, or apoptosis. This acts as a signal to neighbouring cells, e.g. macrophages, to phagocytose the dead cell and digest it
  • The movement of the PS lipids occurs via scramblases
  • Glycolipids are orientated towards the exterior of the cell
  • Second messengers in signalling pathways are orientated towards the interior of the cell e.g. hydrolysis of PI(4,5)P2 , allows to recruit membrane protein for the formation of secondary messengers diacylglycerol (DAG) and inositol-1,4,5-triphosphate (IP3)
  • Lipid interdigitation occurs because of the lipid length asymmetry in membranes. Interdigitation can couple the two leaflets together and decreases the lipids thickness
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14
Q

Lipid movement

A
  1. Rotational- the spinning of a lipid around its axis. Doesn’t alter its position but affects its interaction with neighbouring molecules.
  2. Lateral (sideways)-Neighbouring lipids exchange places. It allows lipids to change position within a bilayer leaflet.
  3. Transverse- Exchange of molecular between leaflets. Lipids can move across a bilayer of membrane spontaneously by transverse diffusion or their translocation can be mediated by proteins (unidirectional or bidirectional). Protein-mediated lipids translocation may require the input energy e.g. ATP hydrolysis. Some proteins are relatively nonspecific, whereas others only translocate specific lipid species. P-type flippases carry out inward movement of lipids; ABC proteins mediate outward movement of lipids. Scramblase perform non-specific Ca2+ -dependent randomisation of lipids across the bilayer.
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15
Q

Need proteins to tell lipids to change leaflets- examples

A

P-type flippase- form outside to inner side of cell. Require energy from ATP.
ABC flippase- flip from inner to outside. Require energy from ATP.
Scramblase- needs calcium to change- Can move from outer to inner and inner to outer.
Spontaneous- no energy needed

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16
Q

Lipid phases

A

When a membrane is in a lamellar phase it can be in the following phases:

1) lamellar liquid crystalline (liquid disorder)
2) solid gel- move slow as very tight
3) liquid-ordered- well packed- lipids can still move fast

1) lamellar liquid crystalline (liquid disorder): In this phase lipid e.g. glycerophospholipids that have unsaturated tails are not well packed and thus the membrane is more fluid and lipids can move fast in the bilayer.
2) solid gel: Sphingolipids have long saturated and pack more tightly, adopting the solid gel phases. Lipids can’t move fast in the membrane.
3) liquid-ordered: the presence of cholesterol with lipids with saturated tails e.g. SM results in a lipid ordered domain (raft). Well packed but lipids can still move fast in the membrane.

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17
Q

How is the membrane organised?

A
  • The Fluid Mosaic Model (old model): The bulk of the lipids forms the bilayer providing the solvent for embedded proteins.
  • Most proteins are inserted in the membrane (integral proteins), peripheral proteins also exist.
  • The bilayer is fluid – lateral mobility of lipids and of some proteins. It is mosaic in the sense that proteins are scattered across it.
  • The Fluid Mosaic Model emphasises the fluidity of the bulk lipids allowing random diffusion.

New model:

  • Possibility of lateral (more) organisation in membranes, e.g. signaling receptors.
  • The concept of membrane domains or rafts. Certain proteins can come together to form a domain or raft within the membrane which interact with certain lipids. This model suggests that in an eukaryotic plasma membrane there will be a large number of such domains.
  • These domains are enriched in cholesterol and sphingomyelin. Lipid ordered phase.
  • Proteins are either excluded or included in the raft regions.
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18
Q

Lipid droplets what are they?

A
  • Lipid droplets are storage organelles that help to maintain the lipid and energy homeostasis.
  • Hydrophobic core of neutral lipids enclosed by a phospholipid monolayer that also has specific proteins.
  • They originate from the ER and are initiated when neutral lipids are produced.
  • Neutral lipids result from the esterification of a fatty acid to triacylglycerol or a sterol (such as cholesterol) to sterol ester.
  • At low concentrations, neutral lipids are dispersed between the leaflets of the ER bilayer. As their con¬centration increases, neutral lipids accumulate –cluster together (de-mixing).
  • If fatty acid storage in lipid droplets is impaired it can result in diseases, such as type 2 diabetes and non-alcoholic fatty liver disease.
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19
Q

Type of membrane proteins

A
  1. Integral (intrinsic) membrane protein
  2. Lipid-linked membrane protein
  3. Peripheral (extrinsic) membrane protein
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20
Q

Integral (intrinsic) membrane protein- what are they?

A
  • Span the membrane with single or multiple transmembrane (TM) segments
  • Interact with fatty acid chains in hydrophobic interior of bilayer
  • TM regions made up predominantly of amino acids with hydrophobic side chains
  • Can only be extracted from the membrane by disrupting the membrane with organic solvents or detergents due to interactions
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21
Q

Examples of integral membrane proteins-Glycophorin A

A

Single TM domain

  • Asymmetrically orientated amino terminus at top, carboxyl terminus at bottom- has to be this way up
  • Extracellular domain glycosylated (sugar groups added)
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22
Q

Examples of integral membrane proteins- Bacteriorhodopsin

A

Multiple TMs

  • 20-30 amino acids in an a-helix required to cross the 45A thick membrane
  • Multiple TM domains packed in bundle
  • Short loops on either side of membrane
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23
Q

Lipid-linked membrane proteins

A

Several proteins are stably attached to the membrane through direct covalent interaction with lipids, called acylated or lipid modified proteins. These include proteins with a glycosyl-phoshatidylinositol (GPI) anchor attaching to a protein, such as a prion protein, various viral and cellular proteins that contain myristic acid (myristoylated).

Lipid-linked proteins

  • Proteins are covalently linked to a lipid
  • This lipid is inserted in the membrane
  • Different proteins use different lipids for attachment
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24
Q

Lipid-linked protein examples

A

Prion proteins, viral proteins, signalling proteins and insulin receptor.

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25
Q

Peripheral (extrinsic) membrane protein

A

Peripheral membrane proteins don’t interact with the hydrophobic core of the bilayer. They interact with the lipid headgroups or with other membrane proteins through ionic interactions. These ionic interactions can be disrupted with a high salt (Na+Cl-) solution, which will wash the peripheral membrane proteins off the bilayer in a soluble form.

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26
Q

Peripheral (extrinsic) membrane protein- examples- cytoskeletal proteins

A

Spectrins- form long filaments
Ankyrin- bridges spectrin and band 3 protein
Actin- joins spectrin filaments
Band 4.1- stabilises spectrin-actin interaction

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27
Q

Peripheral Membrane Proteins – RBC Cytoskeleton

A

If RBC peripheral proteins are removed:

  • RBCs ‘ghost’-lose rigid shape
  • Membrane proteins become laterally mobile
  • Cytoskeleton is important in maintaining shape and rigidity of cell and in restricting the lateral motion (moving sideways) of integral membrane proteins.
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28
Q

Hereditary diseases affecting the cytoskeleton

A

Hereditary spherocytosis and elliptocytosis:
• Mutations in genes encoding spectrin or ankyrin
• Result in abnormally shaped erythrocytes
• Degraded more rapidly by spleen
• Results in anaemia

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29
Q

Alzheimer’s and membrane proteins

A

Dementia- serious deterioration in mental functions, such as memory, language, orientation and judgement

Alzheimer’s- most common cause of dementia

Symptoms caused by nerve cells in brain dying and connections between nerves cells degenerating.
The loss of the part of the brain dealing with memory (hippocampus) usually caused the first symptoms.
Progressive disease, getting worse over time.

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30
Q

Clinical features of Alzheimer’s disease

A

Amnesia- recent memories initially affected
Aphasia- language problems
Agnosia- difficulty recognising and naming objects e.g. auto prosopagnosia (failure to recognise own face)
Apraxia- difficulties in complex tasks
Visuospatial difficulties
Functional impairment- can’t do finances/can’t dress/ do personal things
Mood disorders
Psychosis- delusions and hallucinations
Personality change- ‘living bereavement’

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31
Q

Alzheimer’s disease- neurone malfunction

A

Neurons malfunction and eventually die. As they malfunction both the chemical signalling between the nerve cells and the electrical signalling within them goes wrong.

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32
Q

Alzheimer’s disease- neurone malfunction

A

The disease is characterised by loss of short term memory and progressive dementia. Post-mortem, the brain of an Alzheimer’s individual has senile plaques which consist mainly of short amyloid-beta (Abeta) peptide. The peptide is derived from the larger membrane bound amyloid precursor protein (APP).

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33
Q

Alzheimer’s disease (AD)- impact on cholesterol

A
  • Increased incidence of amyloid plaques in those dying of heart disease
  • Apolipoprotein E4 involved in cholesterol transport more prevalent in AD
  • AD prevalence 70% lower in people taking satins (cholesterol-lowering drugs)
  • Statins lower Aβ production in cells
  • Statins alter cholesterol content and hence fluidity of membrane rafts
  • Statins don’t slow down progression in AD patients
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34
Q

Cholesterol rich domains

A

Location of lipids into lipid domains
• Plasma membrane is not homogenous
• Consists of different domains

Lipid rafts/domains rich in cholesterol, glycosphingolipids and sphingomyelin
• Certain proteins cluster in rafts
• Important in many biological processes like AD

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35
Q

AD- APP processing in rafts

A

The proteolytic processing of APP in the cholesterol-rich lipid rafts produces the toxic amyloid-beta peptide, whereas cleavage of APP in other regions of the membrane prevent the formation of amyloid-beta.

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36
Q

The importance of carbohydrates- where are they found in the membrane?

A
  • Attached to lipids-glycolipids
  • Attached to proteins-glycoproteins
  • Both located on extracellular face of membrane
  • Carbohydrates (sugars) can be attached either to membrane lipids or proteins.
  • Sugars located in extracellular face of membrane
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37
Q

The importance of carbohydrates – Glycoproteins

A

O-linked glycoproteins contain a carbohydrate attached to a Serine or Threonine amino acid.

N-linked glycoproteins contain a carbohydrate attached to an asparagine amino acid Asn-X-Ser/Thr (as long as X is not Pro). Large branched structures with as many as 30-40 sugar residues.

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38
Q

The importance of carbohydrates – Function

A
  • Carbohydrates on the lipids and proteins are involved in stabilising
  • Intercellular recognition e.g. blood group antigens (ABO)- on lipids
  • No extra sugar in O antigen
  • N-acetyl-galactosamine in A antigen
  • Galactose in B antigen
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39
Q

Treatment for cholera

A

Symptoms- severe diarrhoea, vomiting
Cause- Vibrio cholerae bacteria in contaminated water and food. Virulence factor is the cholera toxin.
Treatment- Oral rehydration therapy (water, salts, glucose)

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40
Q

Permeability of phospholipid bilayers

A

Pure lipid bilayer permeable to gases, hydrophobic molecules and small polar molecules.

Pure lipid bilayer impermeable to large polar molecules, such as glucose and other sugars, ions, even small ones such as K+, and charged polar molecules, such as amino acids and phosphorylated sugars.

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41
Q

Types of membrane transport (small molecules)

A
  1. Passive transport (no input of metabolic energy is required to move the molecule across the membrane):
    • Simple diffusion (e.g. gases, hydrophobic molecules)
    • Facilitated diffusion (e.g. ionophores, ion channels, glucose uptake, aquaporins)
  2. Active transport (where energy is required):
    • ATP-driven (e.g. K+/Na+ ATPase)
    • Ion-driven (e.g. Na+/glucose transporter, Na+/Ca2+ exchanger)
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42
Q

Simple diffusion

A

Simple diffusion
• No metabolic energy required
• Small molecules (e.g. O2, CO2, NO, urea)
• No specificity
• Rate of diffusion proportional to concentration gradient
• Net movement down conc. gradient

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43
Q

Facilitated diffusion (FD)

A
  • Occurs down concentration gradient- high to low conc.
  • No energy required
  • Depends on integral membrane proteins (carriers, permeases, channels, transporters)
  • Proteins are specific
  • Similar kinetics to enzymes - dependent on temp, pH, saturable, inhibitable etc.
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44
Q

Examples of facilitated diffusion- ionophores

A

Two main types -
(1) Carrier ionophores transport ions across membrane. These are hydrophobic molecules that carry the ion in their core and shield it from the hydrophobic membrane environment.

(2) Channel-forming ionophores. Membrane-spanning hydrophobic proteins (outside) that form a hydrophilic channel (in the middle) that allows ions (sodium and potassium) to pass freely through the cell membrane.

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45
Q

Examples of facilitated diffusion- ion channels

A
  • Allow rapid and gated (channels closed normally and open by a specific stimulus) passage of anions (-) and cations (+)
  • Highly selective
  • Allow ions to flow across membrane down conc. gradient
  • Essential for: maintaining osmotic balance, signal transduction and nerve impulses.
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46
Q

Examples of facilitated diffusion- glucose transport

A

Transport of glucose into erythrocytes
• Integral membrane protein - glucose transporter (GLUT1) facilitates movement of glucose across plasma membrane. The direction of movement is dependent on relative concentrations of the glucose either side of the membrane.
• 12 transmembrane alpha helices

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47
Q

Glucose uptake by erythrocytes

A

Glucose transporter move glucose either way across membrane- direction dependent on relative concentrations of glucose on either side of membrane – glucose is always taken up into the erythrocyte and as soon as enters cytosol acted on by hexokinase and converted into glucose-6-phosphate (by adding a phosphate group) which is no longer a substrate for the glucose transporter. This maintains glucose conc. gradient where glucose always wants to move into the cell. The glucose transporter is very specific for the D-isomer of glucose relative to the L-isomer or to other related sugars.

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48
Q

What glucose transporter is the most specific?

A

L-glucose

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49
Q

Examples of facilitated diffusion- aquaporins

A
  • Water channel proteins required for the bulk flow of H2O across cell membranes
  • Protein, 6 trans-membrane a-helices
  • Tetramer with 4 pores through which H2O can pass
  • Certain circumstances more water needs to be moved across the membrane than can be accommodated by simple diffusion (e.g. in erythrocytes and kidney). In these cases the aquaporin water channel protein is involved
  • Needed when want to move large amount of water rapidly in in erythrocytes, kidney cells
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50
Q

Active transport

A
  • ATP-driven - Na+, K+, Ca2+ and H+ transport is directly coupled to ATP hydrolysis e.g. Na+/K+ ATPase
  • Ion-driven -Coupled to movement of ion (Na+ or H+) down its concentration gradient e.g. Na+/glucose transporter, Na+/Ca+ exchanger
  • AT requires energy can come from the direct hydrolysis of ATP (“ATP-driven”) or from the movement of an ion down its concentration gradient (“ion-driven”)
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51
Q

ATP- driven AT

A

High [K+}, low [Na+] in cell (high concentration of Na+ outside and a low concentration inside cell)= Na+/K+ gradient:
• controls cell volume
• makes nerve and muscle cells electrically excitable
• facilitates ion-driven active transport of amino acids and sugars
Maintained by Na+/K+ ATPase

Na+/K+ ATPase:
• Tetramer: a2b2
• Pumps 3 Na+ ions out and 2 K+ into the cell
• This polarises the cell membrane (+ on outside)
• ATP hydrolysis induces conformational changes, pumping Na+ and K+ against their concentration gradients- thus need input of energy
• Coupled system: ATP is not hydrolysed unless Na+ and K+ are transported and the ions are not moved unless ATP is hydrolysed

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52
Q

Ion-driven AT

A

Movement of molecule is coupled to ion movement (Na+ or H+).

Symport (both molecules travel in same direction)- Movement of Na+ ions into the cell down the concentration gradient can be coupled to movement of glucose into the cell via the Na+/glucose transporter.

Antiport (molecules moving in opposite directions)- Na+ movement into the cell can also be coupled to movement of molecules out of the cell (antiport) - for example the Na+/Ca2+ exchanger which helps to maintain a low concentration of Ca2+ inside the cell.

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53
Q

Cardiac Glycosides (cardiotonic steroids)

A
  • Plant steroids, e.g. digitalin (digitalis), ouabain, digoxin, digitoxin
  • Inhibit Na+/K+ ATPase
  • Increase conc. Na+ inside cell
  • Decreases Na+ gradient across membrane
  • Decreases Ca2+-Na+ exchange across membrane
  • As this Na+ gradient is required for the Na+/Ca2+ exchanger to move Ca2+ out of the cell
  • Increases conc. Ca2+ inside cell
  • Enhances strength of contraction of heart muscle
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54
Q

Intestinal Epithelial Cells

A
  • Line the lumen of small intestine
  • Large surface area for absorption
  • The apical membrane has finger-like (brush-like) projections of the membrane that increase the surface area for absorption of nutrients from digested food (sugars, amino acids, lipids, etc.)
  • Transfer nutrients into the blood
  • Similar epithelial cells are found lining the kidney tubules where they are involved in the reabsorption of molecules from the urine.
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55
Q

Intestinal Epithelial Cells- movement of glucose

A
  • Movement of glucose from gut into the epithelial cell driven by movement of Na+ ions down concentration gradient through the Na+/glucose symport transporter in the apical membrane
  • Glucose diffuses across basolateral membrane through glucose transporter into blood stream, an example of facilitated diffusion
  • Concentration of Na+ ions maintained at low level by Na+/K+ ATPase in the basolateral membrane
  • As glucose moves through the epithelial cell, water follows by osmosis
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56
Q

Oral rehydration therapy

A
  • Uptake of glucose dependent on Na+, therefore give oral solution of glucose and Na+
  • This increases the osmotic pressure in epithelial cells, therefore H2O follows
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57
Q

Membrane transport of macromolecules: Exocytosis

A 
Constitutive
     - All cells
     - Secreted proteins &
       plasma membrane     
       proteins

Regulated

 - Specialised cells
   e. g. neuronal (NTM), pancreatic (insulin release)       
 - Ca2+-dependent

Regulated exocytosis in the nerve terminal-Example of regulated exocytosis - the nerve terminal.

  • Nerve stimulation leads to depolarisation of nerve cell membrane resulting in action potential that travels along the axon to the nerve terminal
  • Activates voltage-gated ion channels allow Ca to enter nerve terminal
  • Synaptic vesicles containing neurotransmitters fuse with synaptic membrane, releasing their contents into synaptic cleft
  • Stimulate receptors on the post-synaptic cell
58
Q

Membrane transport of macromolecules: Endocytosis

A

1.Phagocytosis - “cell eating” - ingestion of large particles (bacteria/debris) by specialised cells

Phagocytosis-“Professional phagocytes” are found in the immune system:
• macrophages, neutrophils and dendritic cells
• Can ingest bacteria then degraded by enzymes in the lysosomes and the remnants are displayed on the cell surface of the phagocyte to alert other immune cells to fight the infection.

  1. Receptor-mediated endocytosis:
    • Selective (receptor recognition)
    • Involves clathrin-coated pits & vesicles
    • The ligand being taken up must first bind to a specific cell surface receptor. The receptor-molecule complexes accumulate in a clathrin coated pit and then endocytosed in a clathrin-coated vesicle
    • Good for concentrating low levels of macromolecules

3.Pinocytosis - “cell drinking” - uptake of fluid - all cells

59
Q

How do cells communicate with one another?

A

Cells can communicate with each other in 3 main ways:
• Remote signalling- they secrete chemicals that signal to cells some distance away
• Contact signalling- they display plasma membrane-bound signalling molecules that influence other cells by direct physical contact (juxtracrine signalling)
• Contact signalling- they form gap junctions that directly join the cytoplasm’s of interacting cells, thereby allowing the passage of chemical and electrical signals from one interacting cell to its partner

60
Q

Gap junction signalling example – Cardiomyocyte contraction

A
  • Connexin proteins form gap junctions between adjacent cells – movement of cytoplasmic contents + fast electrical coupling
  • Connexin-43 gap junctions couple adjacent cardiomyocytes = important electrical contraction
61
Q

Extracellular signalling molecule examples

A

• Extracellular signalling molecules (1st messengers)- released from one cell and have an effect on another cell
– growth factors
– neurotransmitters
– hormones
– cytokines
• Synthesised and secreted by signalling cells
• Produce a specific response in target cells that have specific receptors for the signalling molecule

62
Q

Types of intercellular signalling

A
  • Paracrine - the signalling molecule acts on nearby cells
  • Autocrine - cells respond to a signalling molecule secreted by itself
  • Endocrine - signalling molecule is released into the blood where it can circulate the whole body to act on target cells distant from its site of synthesis
  • Neuronal - in response to a nerve impulse, neurotransmitters are released locally from a nerve terminal to act on target cells. This signal can act either on the releasing nerve cell (autocrine), or on the nearby target (paracrine), which may be an effector, such as a muscle or gland, or another nerve. Stimulus to electrical signal to chemical signal will then affect another cell type
63
Q

Hormone

A

A hormone is a chemical messenger released by a cell, a gland, or an organ in one part of the body.

64
Q

Endocrine hormones

A

Secreted directly into blood from endocrine glands

65
Q

Paracrine hormones

A

Act locally diffuse through interstitial tissues to target cells

66
Q

What do hormones regulate?

A 
Regulate:
–	body energy needs
–	protein and nucleic acid metabolism
–	mineral and electrolyte metabolism
–	synthesis and release of hormones
•	Activity is regulated through negative or positive feedback mechanisms
67
Q

Four main classes of receptor (“Superfamilies”)- 1. Ligand-gated ion channels

A

• Ionotropic receptors
• Binding and channel opening is very fast (milliseconds)
• Involved in fast synaptic transmission
• Binding causes a conformational change in the channel protein such that specific ions (Na+, K+, Ca2+) flow through the channel (in or out of the cell) to alter the cells electric potential (membrane potential)
• Ligand binding site on the extracellular side
• Comprise 4 or 5 heterometric subunits surrounding a central pore
• Example: Nicotinic acetylcholine receptor (nAChR) - ACh binds
• Increases Na+ and K+ permeability
- Na+ moves in, K+ moves out of cell
- Causes membrane depolarisation

68
Q

Four main classes of receptor (“Superfamilies”)- 2. G-protein-coupled receptors (GPCRs)

A 
•	Metabotropic or heptahelical receptors
•	Timescale-seconds 
•	Couple to an intracellular effector system via a G-protein
•	G protein carries signal inside cell
•	Targets include:
–	peptide hormones
–	neurotransmitters
•	Examples:
–	muscarinic ACh receptor
–	adrenoceptors
–	angiotensin II receptors

• Integral membrane protein receptors and consist of a single polypeptide, comprising 7 membrane-spanning alpha-helical regions connected by alternating extracellular and intracellular loops

69
Q

GPCR signalling- 2nd messenger

A
  • Binding of hormone (ligand) to specific GPCR results in conformational change of receptor
  • Facilitating interaction with the signal-transducing heterotrimeric G-protein
  • Modulates activities (activation or inhibition) of downstream effector proteins
  • Effector proteins modulate levels of 2nd messenger molecules (e.g. cAMP, cGMP, IP3, DAG and Ca2+) or regulate ion channel opening and determine cells membrane potential.
70
Q

Renin-Angiotensin system (RAS)

A
  • Feedback system controlling BP, blood volume and electrolyte homeostasis
  • Stimulated by decrease in blood volume, blood Na+ or blood pressure.
  • RAS consists of two main enzymes working in series (i) renin and (ii) angiotensin-converting enzyme (ACE).
  • Renin cleaves a decapeptide from angiotensinogen (plasma globulin made in the liver)
  • ACE cleaves C-terminal dipeptides from angiotensin and other peptides

When stop producing renin, system stops when blood pressure falls down.

Angiotensin II (Ang II):

  • Peptide hormone
  • Octapeptide- 8 amino acids long
  • Angiotensin II receptors are GPCRs
71
Q

Therapeutic intervention of the renin-angiotensin system

A

• RAS plays an important role in the pathogenesis of heart failure
• Strategies devised to control RAS activity include:
– Inhibit renin release and/or activity
– ACE inhibitors (e.g. Ramipril)
– AT1 receptor antagonists (e.g. Losartan)- activation has effects such as vasoconstriction, increased noradrenaline release, aldosterone secretion from kidney cortex, vascular growth
– Aldosterone receptor antagonists

72
Q

Four main classes of receptor (“Superfamilies”)- 3.Kinase-linked receptors

A 
•	Timescale-hours
•	Single transmembrane helix
–	large extracellular binding domain
–	intracellular catalytic domain
•	Catalytic receptors
–	receptor is itself an enzyme
–	e.g. insulin
•	Non-catalytic receptors
–	act through cytoplasmic tyrosine kinases
–	e.g. cytokines

• Large heterogeneous group (>100 receptors)
• Receptor dimerisation follows ligand binding
• Act by indirectly regulating gene transcription
• Roles in controlling:
– cell division
– tissue repair
– apoptosis

73
Q

Four main classes of receptor (“Superfamilies”)- 4. Nuclear hormone receptors and the mechanism

A 
•	Timescale-hours
•	Intracellular receptors
–	cytosol or nucleus
–	“ligand-activated transcription factors”
–	Monomeric structure
–	separate ligand- and DNA-binding domains
•	Regulate gene transcription
•	Examples:
–	steroid hormones
–	thyroid hormones

Mechanism:
• Hormones diffuse across the plasma membrane
• Interact with cytosolic or nuclear receptors
• Form hormone-receptor complexes in nucleus
• Bind to regions of the DNA (“hormone-responsive elements”) and affect gene transcription

74
Q

Nerve function- structure

A
  • Nerve cells (neurones)-electrically excitable
  • Dendrites-receive information (nerve impulses)
  • Cell body- assimilates the information
  • Axon-ends at the nerve terminal
  • Axon passes this information on to the target cell (e.g. post-synaptic cell). The axon ends at the nerve terminal (terminal boutons) where neurotransmitters are stored and released.
75
Q

Neurotransmission

A

When AP reaches nerve terminal, causes synaptic vesicles to fuse with plasma membrane and release NTM by exocytosis (this process is Ca2+-dependent). The neurotransmitter diffuses across the synaptic cleft, binds to specific post-synaptic receptors and initiates a cellular response.

76
Q

Neurotransmitters: chemical messengers of the nervous system- examples

A

• Acetylcholine (ACh)
• Monoamines (biogenic (produced by living organism) amines)
– noradrenaline, adrenaline, dopamine, histamine, serotonin
• Amino acids
– glutamate, aspartate, glycine, gamma-Aminobutyric acid (GABA)
• Peptides
– endorphins, substance P, neurokinins, neurotensin
• Lipids
– Anandamide (the “natural” cannabis)

77
Q

Life cycle of a neurotransmitter

A

1) Synthesis: in the nerve terminal
2) Storage: in synaptic vesicles within nerve terminals
3) Release: into the synaptic cleft from pre-synaptic vesicles by exocytosis (this is a Ca2+-dependent process) in response to an action potential
4) Receptor activation: diffuse across the synaptic cleft and act on specific receptors located on the post-synaptic cell (n.b –ve feedback by pre-synaptic autoreceptor activation)
5) Neurotransmitter inactivation: action is short lived due to enzyme metabolism and/or re-uptake into pre-synaptic nerve terminal

78
Q

Depression

A

• Affective disorder
-i.e. disorders of mood rather than disturbances of thought/cognition
• Two distinct types of depressive syndrome
-Unipolar depression- feeling of low mood
-Bipolar affective disorder- cycles between low mood and high mood
• Functional deficit of monoaminergic transmission
-Noradrenaline, dopamine, serotonin

79
Q

Treatments for depression

A
  1. Monoamine reuptake inhibitors
    • TCAs - tricyclic antidepressants
    • SSRIs - selective serotonin reuptake inhibitors
    • SNRIs - serotonin/noradrenaline reuptake inhibitors
  2. Monoamine oxidase inhibitors (MAOIs)
  3. Miscellaneous “atypical” antidepressants
  4. Electroconvulsive therapy (ECT)
  5. Mood-stabilising drugs (e.g. Lithium)
80
Q

Sites of antidepressant drug action

A

Reuptake of monoamine NTs from the synaptic cleft by monoamine transporters allows termination of monoaminergic signalling and the maintenance of presynaptic monoamine storage levels.

81
Q

Monoamine reuptake inhibitors and their function

A

E.g. TCAs, SSRIs and SNRIs enhance transmission by binding to pre-synaptic nerve terminal monoamine transporters, thereby inhibiting reuptake and raising NT levels in the synaptic cleft.

82
Q

Monoamine oxidases (MAOs)

A

Enzymes that catalyse the oxidative deamination (breakdown) of monoamines in the presynaptic nerve terminal

83
Q

MAOIs (monoamine oxidase inhibitors)

A

Prevent the breakdown of monoamines within the nerve terminal; and thus ensures more monoamine NTs are available for release.

84
Q

Gasotransmitters

A

Gaseous molecules synthesised in the body

85
Q

Carbon monoxide

A

Silent killer:

  • Highly toxic gas
  • Exposure is common
  • > 50% of fatal poisonings
  • Chronic exposure leads to long-term neurological and cardiovascular disorders
Unlikely hero:
•	Important endogenous signalling molecule
•	Cardioprotective
•	Neuroprotective
•	In clinical trials
86
Q

Features of signal transduction- hierarchy

A

Components of signal transduction pathway arranged in specific order to transmit a signal from the outside of the cell to inside of the cell (e.g. nucleus) to affect a change in cellular function.

Example:
Cell: liver or skeletal muscle
First messenger: Adrenaline (epinephrine)-yellow
Receptor: Beta-adrenergic receptor (β-AR)
Cell response: Breakdown of energy reserves (glycogen) to form glucose

The first messenger binds to receptor on cell surface, resulting in activation of the G-protein which stimulates an effector enzyme (e.g. adenylyl cyclase).

The effector enzyme catalyses the production of second messenger molecules (e.g. cyclic AMP) that stimulate a protein kinase (e.g. protein kinase A) to phosphorylate specific target proteins, thus altering their function and eliciting a cellular response.

87
Q

Features of signal transduction- amplification

A

Signal transduction pathways also serve to AMPLIFY the signal.
• G-protein activation - the G-protein is activated by the receptor
• Effector enzyme -catalyse the production of many molecules of second messenger
• Protein kinase -phosphorylation of many molecules of protein substrate

88
Q

Features of signal transduction- specificity

A

Cell A- 1st messenger may bind to single receptor to elicit a single response.

Cell B- Same 1st messenger acting on same receptor can stimulate a different response in another cell type due to differential expression of signalling components (e.g. G-proteins, effector enzymes, kinases etc).

Cell C- Other 1st messengers bind to different receptors on the same cell may modulate the cellular response by activating or inhibiting the original signal transduction pathway. This interaction between different signalling pathways is called “cross-talk”- two different signalling pathways but merge.

Cell D - same 1st messenger may act on different type of receptor and stimulate a different signal transduction pathway to produce a very different cellular response. Thus, cellular response to a particular stimulus depends on many variables and is not always the same for different cell types.

89
Q

Features of signal transduction- complexity

A

Signal transduction pathways are highly complex
• 1000+ GPCRs
• 500+ protein kinases
• Cross-talk- signalling pathways interact with one another- despite different pathways and receptors
• Cell-type specificity

90
Q

G-proteins

A

Two major groups:

G-proteins (receptor-associated):
• Heterotrimeric (a, b, g subunits)
• e.g. Gas, Gai , and Gaq

Small GTPases
• monomeric
• e.g. Ras, Rho

Lipid tails are how they are anchored to the membrane of a cell.

G-proteins are molecular switches:
• Switched “ON” by ligand binding to receptor
• Switched “OFF” by intrinsic GTPase activity

G-proteins act as “molecular switches”, being active when bound to GTP, and inactive when bound to GDP.

  • Binding of the ligand 1st messenger to the receptor conformational change in the receptor allowing the G-protein to bind to receptor
  • Stimulates G-protein to exchange GDP for GTP, thus switching it “on”
  • Can then activate the effector enzyme
  • G-protein subsequently hydrolyses GTP back to GDP, thereby returning it to the “off” position.
91
Q

Receptor associated G-proteins

A

Different alpha subunits of G-proteins that are coupled to different effector enzymes, resulting in changes in cellular function. The main groups are Gi (“inhibitory”), Gs (“stimulatory”), Gq

92
Q

Receptor associated G-proteins- Gs- mechanism

A

Receptor should have 7 TM domains.
Alpha subunit of Gs is bound to GDP and is therefore in the “OFF” state (inactive). Adenylyl cyclase is also inactive as has not been activated by G-protein.

  1. Binding of ligand to receptor causes G-protein to release GDP and swap it for GTP, thus switching the G-protein to the “ON” state. The GTP bound alpha subunit dissociates from the beta and gamma subunits.
  2. The GTP-bound Gs-alpha subunit binds to and activates adenylyl cyclase which catalyses conversion of ATP to the second messenger cyclic AMP.
  3. The GTPase activity of the Gs-alpha subunit hydrolyses GTP to GDP (with release of inorganic phosphate, Pi), thus reverting the G-protein back to the “OFF” state.
  4. The GDP-bound alpha subunit then re-associates with the beta and gamma subunits.
  5. Cyclic AMP is broken down to AMP by phosphodiesterases. If the receptor is still active, the process will be repeated so the signal continues to be amplified.
93
Q

Gs and Gi have opposing effects on cyclic AMP levels

A

A drug coupled to activation of Gs stimulates adenylate cylase and elevates cAMP.

A drug that is coupled to activation of Gi inhibits adenylate cyclase and reduces cAMP levels. cAMP is broken down by specific phosphodiesterases (PDEs) to form AMP.

94
Q

G-protein diseases

A
  • Gs - cholera toxin (Cholera)
  • Gi - pertussis toxin (Whooping cough)

Gs and Gi G-proteins are targets of cholera toxin and pertussis toxin which are produced by the bacteria that cause cholera and whooping cough respectively.

95
Q

Cholera toxin mechanism (Gs)

A

Cholera- effects of cholera are due to cholera toxin activating the Gs G-protein. Cholera toxin is a “virulence factor” - a molecule that contributes to the pathogenicity of the V. cholerae bacteria.

Gs switched off as bound to GDP, switched on when ligand binds to receptor when GDP exchanges to GTP. This is the same with Gi.

Cholera toxin prevents GTPase activity of Gs, therefore GTP remains bound to Gs and it stays in the “ON” state. This leads to over-stimulation of adenylate cyclase and accumulation of cyclic AMP.

In intestinal epithelial cells, elevated cAMP increases loss of Cl- ions through chloride channels. The resultant osmotic gradient leads to water being excreted into the intestinal lumen and hence diarrhoea and dehydration.

96
Q

G-protein diseases- whooping cough

A

Effects of pertussis toxin due to inhibition of Gi G-protein.

  • Pertussis toxin prevents GDP/GTP exchange by Gi.
  • Gi protein is locked in the “off” position.
  • Unable to inhibit adenylate cyclase, resulting in accumulation of cyclic AMP.
  • Many physiological effects including increased insulin secretion and increased sensitivity to histamine that contribute to some of the symptoms of the disease.

Inhibitory protein so would normally lower adenylate cyclase.

97
Q

Second messengers

A
  • Short-acting intracellular molecules that are rapidly formed or released as a result of receptor activation
  • Response is short-lived (minutes)
98
Q

How are cAMP produced?

A

Production of cyclic AMP from ATP; a reaction catalysed by adenylate cyclase (also called adenylyl cyclase)

99
Q

How are cGMP produced?

A

Production of cyclic GMP from GTP; a reaction catalysed by guanylate cyclase (also called guanylyl cyclase). Either by activation of soluble guanylate cyclase by nitric oxide (NO), or by activation of membrane-bound guanylate cyclase in response to e.g. neuropeptides such as atrial natriuretic peptide (ANP).

100
Q

How is cyclic GMP broken down?

A

Cyclic GMP is broken down by specific phosphodiesterases (PDEs) to form GMP.

Cyclases and PDEs:

  • Some phosphodiesterases (PDEs) break down cyclic AMP to AMP
  • Others break down cyclic GMP to GMP
  • A third group can break down both cyclic AMP and cyclic GMP.
  • PDEs reduce the levels of cAMP and cGMP and therefore reduce the response (important in switching off the signalling pathway). Two well-known PDE inhibitors are caffeine and Viagra (sildenafil).
101
Q

Gq activation - DAG and IP3 production

A

DAG, IP3 and Ca- all produced as a result of activation of a different G-protein Gq.

  1. Ligand binding to receptor causes receptor to associate with G protein (Gq). This stimulates displacement of GDP by GTP (switching G-protein “on”) and alpha-subunit dissociates from beta/gamma subunits.
  2. GTP-bound Gq stimulates membrane-localised phospholipase C (PLC) which catalyses production of two different second messengers DAG and IP3 from the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP2).

IP3 released into cytosol, where it triggers calcium release.

DAG remains in membrane, where it activates protein kinase C.

102
Q

IP3 and intracellular calcium

A

Released into cytosol from intracellular stores (e.g. endoplasmic reticulum) or enters the cell from extracellular sources (via calcium channels).

  • Calcium is sequestered in the endoplasmic reticulum (ER) and mitochondria under normal conditions so the cytosolic concentration is low
  • Receptor activation, IP3 binds to receptors in ER membrane resulting in efflux of Ca from the ER
  • Ca acts as second messenger to activate various molecules (e.g calcium-dependent kinases) to modulate cellular function
  • In some cells, increase in intracellular Ca can also trigger opening of calcium channels in the plasma membrane so that even more Ca floods into the cell
  • Ca is then taken back up into the ER through a calcium ATPase in the ER membrane, or is pumped or exchanged out of the cell
103
Q

Protein kinases- what are they?

A

Enzymes that facilitate transfer of a phosphate group from ATP to a specific amino acid residue (Ser, Thr or Tyr) on a specific protein.

G-proteins (always switched on by GTP), phosphorylation may activate or inhibit protein function.

Kinases add a phosphate group from ATP onto SPECIFIC serine, threonine or tyrosine residues on SPECIFIC proteins. All three of these amino acids have side groups containing a hydroxyl group (OH) - this is where the phosphate group from ATP is added.

104
Q

Phosphoproteins- on what amino acids does phosphorylation take place?

A

Phosphorylation occurs on Ser, Thr and Tyr residues- only on intracellular domains of the protein - as this is where the kinases are located

105
Q

3 main groups for protein kinases

A
  • Serine/Threonine kinases- Phosphorylate Ser and/or Thr residues, e.g. PKA, PKC, PKG
  • Tyrosine kinases-Phosphorylate only Tyr residues-Receptor TKs e.g. Insulin Receptor- Non-receptor TKs e.g. Src
  • Dual-specificity kinases- Phosphorylate Ser/Thr and Tyr residues- e.g. MAP kinase kinases (MKKs)
106
Q

Protein kinases and phosphatases

A
  • Phosphatases remove phosphate groups from amino acids residues to oppose the effects of kinases and switch the switch in the opposite direction.
  • Two groups of phosphatases are Ser/Thr-directed phosphoprotein phosphatases (PPPs) and the Tyr-directed phosphotyrosine phosphatases (PTPs).
  • The broad specificity of phosphatases means that there are not as many different phosphatases as there are kinases.
107
Q

Protein kinase cascades

A

Modulate protein function by:

  • Phosphorylation to induce change in protein conformation / function. Alters function of that particular protein. This may have a knock on effect on other proteins e.g. in a kinase cascade.
  • Phosphorylation of a transcription factor that alters gene transcription and hence protein expression levels
108
Q

Protein kinase inhibitors as therapeutic agents

A 
Protein kinase inhibitors are in use or being developed as therapeutic agents for many diseases including:
•	  Cancer
•	  Cardiovascular Disease
•	  HIV/AIDS
•	  Rheumatoid Arthritis
•	  Alzheimer’s Disease
109
Q

Lipoproteins definition

A

The carriers for lipids which are otherwise insoluble in the blood.

110
Q

Classifications of lipoproteins- chylomicrons

A

Chylomicrons- transport of dietary fats (triglycerides, cholesterol) from intestine to tissues. Largest and least dense.

111
Q

Classifications of lipoproteins-VLDL

A

Transport lipids made in liver to peripheral tissues.

112
Q

Classifications of lipoproteins- LDL

A

Provide cholesterol for peripheral tissues. The main cholesterol carrier.

113
Q

Classifications of lipoproteins- HDL

A

Transport cholesterol to liver from peripheral tissues. Made by blood and take cholesterol away from tissue to liver.

114
Q

Structure of lipoproteins

A
  • Each lipoprotein has a particular function, mediated by specific protein strands embedded in surface of lipoproteins- apolipoproteins
  • Lipoprotein distinguishing functions are mediated by specific protein strands on the surface called apolipoproteins which determine start and end points for cholesterol transport
  • Lipid+ apolipoprotein= lipoprotein
115
Q

Synthesis and function of apolipoproteins

A
  • Synthesis in intestine regulated by dietary fat intake
  • Synthesis in liver also controlled by hormones and drugs
  • Regulate key enzymes in lipoprotein metabolism
  • Are ligands for interaction with lipoprotein receptors, targeting lipoproteins to the correct tissues
116
Q

Classes of apolipoproteins- ApoA, ApoB, ApoC, ApoE

A

ApoA – present in HDL, cholesterol out of peripheral cells and into liver
ApoB – recognizes apoB/apoE receptors, LDL uptake
ApoC – activator of lipoprotein lipase, transferred between lipoproteins
ApoE – stablises VLDL, ligand for apoB/E LDL receptor

117
Q

Structure of LDL

A
  • LDL-Apo B- encircles the lipoprotein
  • Surface monolayer of phospholipids and free cholesterol
  • Hydrophobic core of triglyceride and cholesterol esters
  • LDL not proinflammatory, but particle can become modified. It is the modified LDL particle that is proinflammatory and proatherogenic.
  • LDL is oxidised and in oxidised state so proatherogenic
118
Q

Atherogenic

A

Tending to promote the formation of fatty deposits in the arteries

119
Q

Structure of HDL

A
  • HDL particles smaller and contain different apolipoproteins, mainly apo A-I and apo A-II
  • Both apolipoproteins have properties that protect the lipids against oxidative modification
  • HDL resistant to being oxidised, and why anti-inflammatory properties of HDL
120
Q

Where are chylomicrons made and what do they transport?

A

Chylomicrons are made in the intestine, transport triglycerides and cholesterol in the blood.

Chylomicrons shrink, remnants transported back to liver.

121
Q

What are triglycerides hydrolysed by?

A

Triglycerides are hydrolysed by lipoprotein lipase to fatty acids that are taken up by target tissues and used for energy production (eg muscle) or stored (adipose tissue).

122
Q

Where are VLDL made and where are they transported to?

A

VLDL are made by the liver and transport lipids to target tissues, acted on by lipoprotein lipase to release fatty acids & taken up by target tissues . VLDL remnants remain in the blood, become LDL that are then taken up by target cells by the LDL receptor, digested in the lysosome to release the cholesterol.

123
Q

Lipoprotein receptors

A
  • Membrane-bound receptors to enable cholesterol entry to hepatic (liver) and peripheral cells
  • LDL receptor gene expression is regulated by intracellular cholesterol concentration
  • Binding of LDL to receptor stimulates endocytosis
1-LDL taken in by clatharin protein
2-ApoB binds to receptor
3-Forms a coated vesicle
4- vesicle uncoat- LDL separated from receptor
5- receptors move to another vesicle
6- recycled back to the membrane
124
Q

How is the LDL receptor activity controlled?

A
  • High intracellular levels of cholesterol suppress LDL receptor synthesis
  • The decreased synthesis of LDL receptor prevents excessive cholesterol uptake by cells
  • As a result, excess dietary cholesterol remains in the blood as LDL.
  • Potentially deleterious consequences
125
Q

What is HDL? What does it do?

A
  • HDL – “Good cholesterol”- as can remove cholesterol and return to liver
  • High blood levels of HDL correlate with low incidence of atherosclerosis
  • HDL is resistant to oxidation through apoA-I and AII
  • Scavenges cholesterol from cells and other lipoproteins and returns to the liver, excreted in bile
  • HDL can protect against atherosclerosis through Reverse Cholesterol Transport
126
Q

Atherosclerosis

A

Atherosclerosis is a disease in which plaque builds up inside your arteries.

127
Q

What is reverse cholesterol transport?

A

Cholesterol in macrophage, nascent HDL then mature HDL then bind to scavenger receptors on liver. HDL carried back to liver for secretion into bile and excretion via the intestine.
FC = free cholesterol
CE = cholesterol ester

128
Q

What is dsylipidaemias?

A
  • Mutations affecting LDL receptor associated with inherited form- Familial hypercholesterolaemia
  • Cells lacking functional LDL receptors cannot take up LDL
  • Thus amount of circulating LDL increases, excess cholesterol is deposited in the arteries leading to enhanced risk of developing atherosclerosis
129
Q

How does cholesterol and atherosclerosis link?

A
  • Cholesterol-structural component of cell membranes, synthesis of bile acids, steroid hormones, fat-soluble vitamins (vitamins A, D, E & K)
  • High serum levels indicative of risk for cardiovascular diseases (atherosclerosis)
  • A major constituent of atherosclerotic plaques is cholesterol-enriched LDL
130
Q

Clinical manifestations of atherosclerosis

A
  • Cardiac-Chest pain, palpitations, heart attack
  • Cerebral-Stroke, cerebral haemorrhage
  • Peripheral-Pain (walking), ischaemia, ulceration & gangrene
131
Q

What is the cholesterol-synthetic pathway (in liver)?

A

HMG-CoA reductase catalyses the rate limiting step forming mevalonate.

132
Q

Hypercholesterolaemia

A

LDL and HDL levels are elevated

133
Q

How can hypercholesterolaemia be treated?

A

Two organs primarily control blood cholesterol levels:
• Liver - produces cholesterol and bile acids
• Intestine - absorbs cholesterol from food and bile

134
Q

Pharmacological Control of Hypercholesterolaemia

A

Statins- prevent cholesterol synthesis in the liver by inhibiting HMG-CoA reductase. This leads to lowered intracellular cholesterol levels and increased expression of LDL receptor, more uptake of cholesterol from the blood.

Cholesterol absorption inhibitors- prevent uptake from the intestine.

Fibrates- reduce triglycerides and increase HDL, less effective LDL.

135
Q

Statins- pleiotropic effect- what is it?

A
  • Actions other than those for which the agent was specifically developed
  • Usually unanticipated
  • Undesirable (side effects/toxicity)
  • Beneficial in the case of statins
136
Q

Statins’ pleiotrophic effects:

A
  • Improve endothelial dysfunction
  • Antioxidant properties
  • Inhibition of inflammatory responses
  • Stabilise atherosclerotic plaques-less likely to rupture
  • Reducing prenylation of the small G proteins Ras and Rho (and hence preventing cellular signalling of “harmful” pathways)
137
Q

What are isoprenoids?

A
  • Other intermediates of mevalonate called isoprenoids
  • These are hydrophobic molecules that are required for prenylation (sticking a lipid tail onto intracellular signalling molecules) called small G-proteins (or GTPases).

Important for small G proteins to be prenylated- activate G proteins- lipid tail that sticks the G protein on the cell membrane.

138
Q

Prenylation

A

Sticking a lipid tail onto intracellular signalling molecules

139
Q

Examples of isoprenoids and what they become after prenylation

A
  • Farnesyl pyrophosphate (FPP)
  • Geranylgeranyl pyrophosphate (GGPP)

Prenlylation:
• Ras – Farnesylation (of FPP)- farnesylated
• Rho – Geranylgeranylation (of GGPP)- geranylgeranylated

140
Q

Prenylation of G proteins

A

The key G-proteins are those of the Ras and Rho families.

Ras and Rho need a isoprenoid to be able to signal.
In turn, they become cell membrane associated which can then initiate a signalling cascade in the cell.

Prenylation of G proteins – activates signalling pathways impacting cell functions
• Changes in nuclear transcription factors + gene expression
• Impact significant on cellular functions for cardiovascular disease

IMS (13 decks)

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