Cellular Biology Flashcards
What are the 4 main types of cell signalling?
- Endocrine - hormones secreted into the blood to act on a receptor at a different site (eg. insulin)
- Paracrine - hormones secreted to act on adjacent target cell (eg. vWF)
- Autocrine - target site on same cell (eg. immune)
- Juxtacrine - signalling by direct cell-cell linkage, often through membrane associated substances (eg. gap junctions in cardiac myocytes)
What is signal transduction?
- Extracellular signalling molecule (ligand) binding to and activating a cell surface receptor, to form a receptor-ligand complex
- This elicits a physiological response in the cell, by way of GTPase switch proteins, kinases, phosphates and secondary messengers
THE FUNCTION OF THIS PATHWAY IS TO AMPLIFY THE RESPONSE
In terms of therapeutic targets of signal transduction, where is the best point in the signal to ‘interrupt’?
Higher up - it is best to target the receptor itself, in order to have a predictable response and avoid unwanted side effects
Give some examples of secondary messengers
Ions (ca++) Lipid derivatives Carbohydrate derivatives Nucelotides (cAMP, cGMP) Proteins (Ras, JAK, Raf)
What are the 4 main types of cell-surface receptor?
- Direct ligand gated receptor channel (ION)
- G protein coupled receptor
- Tyrosine kinase linked receptor (ENZYME)
- Intracellular
How does a direct ligand gated receptor work?
- Ligand binds to receptor
- Conformational change
- Ion influx through a channel in the receptor
Give some examples of ligand gated receptors?
Nicotinic Ach (dysfunctional in myasthenia gravis)
GABA receptors
Neurotransmitters (serotonin)
How does a G protein coupled receptor work?
- Ligand binds to receptor
- Conformational change
- Causes activation of an associated G protein (GDP-GTP)
- GTP binds to alpha subunit and activates it
- Activated alpha dissociates from beta/gamma
- Trigger secondary messengers such as cAMP
How many domains and subunits is a G protein coupled receptor composed of
7 membrane spanning domains 3 subunits (alpha, beta, gamma)
Describe 3 specific G protein pathways
- Gas
- activates adenylyl cyclase
- generates cAMP
- activates protein kinase
- signal activation
- causes positive inotropy - Gai
- inhibits adenylyl cyclase
- causes negative inotropy - Gaq
- activates phospholipase C
- produces IP3 and DAG
- stimulates release of ca from endoplasmic reticulum
- smooth muscle contraction
Give some examples of processes that involve G protein coupled receptors?
Vision Taste Smell Behavioural and mood regulation Immune regulation (TLR responses) Inflammation (histamine) Autonomic nervous system (sympathetic and parasympathetic)
What G protein coupled receptor is found in the collecting ducts of the renal tubule, which is bound by ADH?
V2 Vasopressin receptor (V2R)
What condition results from an inactivating mutation of V2R?
Congenital diabetes insipidus
How does a tyrosine kinase linked receptor work?
- Ligand binds to receptor
- Receptor dimerizes, allowing tyrosine to be transphosphorylated by its partner receptor
- This allows other proteins to ‘dock’ and become phosphorylated
- This amplifies a signal through the membrane
- The process is aided by relay proteins
Give some examples of tyrosine kinase linked receptors?
Epidermal growth factor receptor
Fibroblast growth factor receptor
Vascular endothelial growth factor
What is the link between EGFR and cancer?
EGFR is a member of the HER growth receptor family.
It is involved in growth, differentiation and survival.
Many tumours express EGFR.
EGFR involves STAT, AKT etc
Which tumours express EGFR?
Colorectal, head and neck, pancreatic, lung, breast, renal, glioblastoma
What therapies inhibit EGFR as a form of chemotherapy?
Monoclonal antibodies (trastuzamab aka herceptin in breast cancer)
Tyrosine kinase inhibitors (eg. imatinib in CML)
Antisense (prevents transcription)
How does an intracellular receptor work? What are the 3 domains involved?
- Small/hydrophobic chemical messengers cross the membrane
- Ligand activate the receptors (found in nucleus/cytosol)
- Activated complex can act as a transcription factor, turning on specific genes
3 domains:
- Ligand binding domain
- DNA binding domain
- Activation domain
What hormones often bind to intracellular receptors?
Steroid hormones (eg. ENaC) Thyroid hormones
They are both hydrophobic
What type of receptor do hydrocodone and lisinopril target?
G protein coupled receptor
What are the functions of endothelial cells?
- Autoregulation of tissue perfusion
- Facilitate exchange of blood gases, nutrients and waste
- Tissue repair and angiogenesis
- Homeostasis of blood coagulation
- Direct immune cells to their destination
What junctions can endothelial cells be joined by?
Tight junctions
Gap junctions
Adherens junctions
What is the role of tight junctions?
Form an impermeable barrier to hold cells together
What is the role of gap junctions?
Facilitate communication
What is the role of an adherens junction?
Provide anchorage and structure
Which chemicals cause endothelial vasodilatation?
Nitric oxide
Prostaglandins
Bradykinin
Which chemicals cause endothelial vasoconstriction?
Endothelin 1
Angiotensin II
What are the four methods of transport across the endothelium?
Diffusion - concentration gradient
Filtration - pressure gradient
Vesicular - transcytosis
Active transport - against concentration gradient
What are the 3 types of capillary endothelium?
Continuous - facilitates blood flow along the vessel, many tight junctions
Fenestrated -passage of small molecules through transcellular pores
Discontinuous - passage of large molecules for exchange, poorly structured basement membrane
Where do you get continuous endothelium?
Skin
Muscle
Lung
CNS
Where do you get fenestrated endothelium?
Exocrine
Glomeruli
Intestinal mucosa
Where do you get discontinuous endothelium?
Liver
Spleen
Bone marrow
Describe the endothelium of the BBB
Highly specialised, only allows small, lipid soluble molecules to pass via active transport
Describe the endothelium of the liver
No basement membrane, very low pressure and leaky to allow waste exchange
Describe the endothelium of the kidney
Fenestrated endothelium lines the glomeruli, acting as a second filter after the podocytes. Allows urea, sodium and rate to go through but not protein and cells
Describe the process of angiogenesis
- CLONAL EXPANSION - division of mature endothelial cells
- Migration towards damaged tissue in response to growth receptors (VEGF - tyrosine kinase linked)
- Growth of new blood vessels
- Supplementation of oxygen and nutrients
Describe the process of leukocyte extravasion (leakage). What molecules are involved in this process?
ADHERINS AND SELECTINS
- They bind to oligosaccharides on the neutrophil, which slows the movement of it
- Neutrophil expresses P-selectin and integrin, which binds ICAM-1
- This causes the neutrophils to roll along the wall (margination)
- Bonds become stronger and rolling slows
- Neutrophils move through the endothelium to sites of infection (diapedesis)
- Weakened bonds between endothelial cells allow other molecules and fluid to cross
The main site of this is in post capillary venues
NB this process is also involved in tumours
What is margination?
When neutrophils roll along the surface of the endothelium in response to adherins and selection in inflammation
What is diapedesis?
When neutrophils move through the endothelium to reach the site of infection, in response to adherins and selectin
Which pro-inflammatory cytokines activate the endothelium, and how do these lead to sepsis?
IL1, IL6, TNFa
- Trigger localised apoptosis and necrosis
- Weaken junctions causing oedema
- Increased vasodilation
- Increased production of pro-thrombotic factors such as VWF
What pro-atherosclerotic agents activate the endothelium, and what is the pathophysiology?
Lipids, glucose and smoking
- Increase cellular permeability to lipoprotein
- Increase production of extracellular matrix resulting in plaque formation
- Altered expression of cell surface receptors regulating anticoagulation and diapedesis
How do endothelial cells counteract coagulation?
Production of anti platelets, fibrinolytic agents (antithrombin) and anticoagulants
How does an endothelium become stimulated/activated in disease?
- Stimulation - e.g. from vWF release or P-selectin expression
- Activation - prolonged exposure to stimules, alters gene transcription
What is the role of TNF?
Pro-inflammatory cytokine:
- Activates macrophages and neutrophils
- Stimulates T cell activation
- Key mediator of response to gram negative septicaemia
- Anti-tumour effect
Name some TNF blockers. What are the side effects?
Infliximab, adalimumab - used in severe rheumatoid.
SE - reactivation of latent TB and demyelination
What is VEGF-A?
Growth factor - acts on microvascular endothelial cells to initiate ‘vascular sprouting’
Secreted by macrophages in response to hypoxia due to disruption of blood supply.
Promotes chemotaxis in angiogenesis and tissue repair and is overexpressed in cancer.
Where do foam cells usually accumulate in atherosclerosis?
Tunica intima
Which structure is responsible for storage of vWF and P-selectin?
Weibel Palade body
What is the pathophysiology of myasthenia gravis?
Autoimmune destruction of the nicotinic ACH receptor (ligand gated), so there is less depolarisation
What are the symptoms of MG?
Fatiguability
Eyelid drooping
Slurring of speech
How is MG investigated?
- Ice test - leads to improvement of eyelid dropping
- Anti-AchR autoantibodies
- Edrophonium tests - blocks ACHase and improves symptoms
How is MG treated?
Neostigmine - acetylcholinesterase inhibitor
What is the AKT pathway?
G protein coupled pathway
IGF binds to G protein, activates PIP3 and the AKT pathway.
AKT up regulates many other pathways (eg. mTOR) and this can BLOCK APOPTOSIS (implication in cancer)
