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BIOC212 > Dr Reinhardt > Flashcards

Dr Reinhardt Flashcards

1
Q

Occluding junctions (examples)

A

Seal cells together in an epithelium, with no leaking from one side to the other
Examples:
Tight junctions - vertebrates
Separate junctions - invertebrates mainly

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2
Q

Anchoring Junctions

A

Mechanically attach cells to the neighbors or the ECM

Examples:

  • cell-cell junctions
  • cell-matrix junctions
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3
Q

Communicating Junctions (examples)

A

mediate the passage of signals from one cell to the next

Examples:

  • gap junctions
  • chemical synapses
  • plasmodesmata (plants only)
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4
Q

Anchoring junctions with actin filament attachment sites

A

cell-cell junctions (adherens junction)
cell-matrix junction (focal adhesion)

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5
Q

Anchoring junctions with intermediate filament attachment sites

A

cell-cell junctions (desmosomes)
cell-matrix junctions (hemidesmosomes)

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6
Q

The role of tight junctions in transcellular transport

A

Transport proteins are confined to different regions of the plasma membrane in epithelial cells of the small intestine. This segregation permits a vectorial transfer of nutrients across the epithelium from the gut lumen to the blood.
Tight junctions are thought to confine the transport proteins to their appropriate membrane domains by acting as diffusion barriers within the lipid bilayer of the plasma membrane; these junctions also block the backflow of glucose from the basal side of the epithelium into the gut lumen.

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7
Q

Glucose transport in epithelial cells
basolateral membrane.

A

glucose is actively transported into the cell by Na+-driven glucose symports at the apical surface (higher Na+ concentrations in the lumen), and it diffuses out of the cell by facilitated diffusion mediated by glucose carriers in the basolateral membrane

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8
Q

visible light region

A

400-700 nm

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9
Q

Light versus electron microscopy: particles and sizes

A

Source of electrons in electron microscopy: cathode on top

Acceleration in electron microscopy: anode on top

Dark areas, dense materials

Electron smaller wavelengths 0.5nm to 100um

Light larger wavelength 500 nm to 1mm+

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10
Q

Freeze fracture electron microhraph

A

start with rapid freexing of cell

then the frozen cells are cleaved along a fracture plane

the e face is the inner face of the outer lipid monolayer

the complemented surface is the p face (the inner surface of the inner leaflet of the bilayer)

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11
Q

Tight Junction proteins

A

claundin, occludin

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12
Q

Claudin

A

Claudin (The claudin superfamily consists of at least 24 homologous proteins (24 genes) in humans ranging from 20 to 27 kDa)

Specificity: claudin in kidney epithelia (claudin-16 is required for Mg2+ to be reabsorbed from the urine into the blood. A mutation in the gene encoding this claudin results in excessive loss of Mg2+ in the urine)

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13
Q

Occludin

A

unknown function

  • There are two isoforms of occludin that result from alternative mRNA splicing, but have similar tissue distributions. Localization of occludin to tight junctions is regulated by phosphorylation in both epithelial and endothelial cells.
  • Occludin is a 65-kDa protein
  • Occludins not as essential as claudins
  • Occludin deficient mice show complex phenotypes: chronic inflammation, gastric epithelium calcification etc. -> not easily explained by barrier function. Occludins might be involved in epithelial differentiation.
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14
Q

ZO proteins

A

intracellular connection to cytoskeleton for both proteins

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15
Q

Claudins and occludins structure

A

4 alpha-helical trans-membrane segments with two extracellular loops

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16
Q

sealing strands

A
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17
Q

Three major types of cytoskeletal fibers

A

Actin filaments: actin

Intemediate fiolaments: keratin, desmin, vitemin

Microtubles: tubulin

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18
Q

Tight Junction associated hereditary diseases Cldn-1, Cldn-14, Cldn-16

A

Cldn-1: Neonatal ichthyosis and sclerosing cholangitis

Cldn-14: Non-syndromic deafness (DFNB29)

Cldn-16: Familial hypomagnesemia with hypercalciuria and nephrocalcinosis

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19
Q

Why is it important to have anchoring junctions?

A

cells

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20
Q

Where are anchoring junctions most abundant?

A
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21
Q

2 principle proteins in anchor junctions

A

Transmembrane adhesion proteins - green (cytosplasmatic tail, TM, extracellular domain)

Intracellular anchor proteins - blue (distinct plaque => connect to actin, intermediate filaments)

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22
Q

Anchor junction parts

Cell-cell

cell-matrix

Actin filaments

IF

In Adherens junction, Desmosome, Focal adhesion, Hemidesmosome

A

Often: intracellular signalling proteins

Cell-cell: Adherens junctions, desmosomes (Cadherins)

Cell-ECM: focal adhesions, hemidesmosomes (Integrins)

Actin filaments: Adherens junctions, focal adhesions

IF: Desmosomes and hemidesmosomes

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23
Q

belt-like junction

A

Encircles each of the interacting cells. Its most obvious feature is a contractile bundle of actin filaments running along the cytoplasmic surface of the junctional plasma membrane.

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24
Q

Intracellular anchor proteins:

A

catenins, vinculin, alpha-actinin

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25
Q

Requirement for formation of tight junctions

A

Contraction of actin network by myosin motor proteins (fundamental

processes in morphogenesis)

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26
Q

Contractile actin filament in Anchor junctions

A

parallel to plasma membrane, attached to adhesion belt

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27
Q

What do cadherins form in anchoring junctions

A

homo dimers

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28
Q

The folding of an epithelial sheet forms

A

an epithelial tube

the oriented contraction of the bundles of actin filaments running along adhesion belts causes the epithelial cells to narrow at their apex and helps the epithelial sheet to roll up into a tube. An example is the formation of the neural tube in early vertebrate development.

Rearrangements of the cells within the epithelial sheet are also thought to have an important role in the process.

29
Q

Anchoring junctions in Desmosomes

A

•Button-like points, rivets cells together

  • Anchor for IF
  • IF linked to a network
  • Epithelial cells: keratin filaments

•Heart muscle: desmin filaments

attachment of intermediate filaments.

30
Q

Cadherin family proteins in desmosomes

A

desmoglein and desmocollin

31
Q

Dense plaque in anchoring junction of desmosomes

A

plakoglobin, desmoplakin

32
Q

Transmembrane adhesion proteins in anchor desmosomes junction

A

desmoglein [pronounce: desmoglin], desmocollin (cadherin family)

33
Q

Pemphigous vulgaris

A

autoimmune blistering diseases that affect the skin and mucous membranes. In pemphigus, autoantibodies form against desmoglein. When autoantibodies attack desmogleins, the cells become separated from each other and the epidermis becomes “unglued”, a phenomenon called acantholysis.

34
Q

BINDING IN DESMOSOMES

A

Some of the molecular components of a desmosome. Desmoglein and desmocollin are members of the cadherin family of adhesion proteins. Their cytoplasmic tails bind plakoglobin (g-catenin), which in turn binds to desmoplakin. Desmoplakin also binds to the sides of intermediate filaments, thereby tying the desmosome to these filaments. In the mid-region of desmoplakin, a coiled-coil rod domain is responsible for homodimerization.

35
Q

importance of anchoring proteins

A

regulation through affinities, mutations could cause cardiomyopathy and skin diseases

36
Q

Where do anchoring junctions in focal adhesion bind to

A

Anchoring junctions cell to extracellular matrix: Focal adhesions, hemidesmosomes

• Anchoring Cell-Matrix: Integrins (18 alpha subunits, 8 beta subunits, 24 integrin heterodimers)

37
Q

Focal adhesions

A

cell-ECM contact
• attach to actin via vinculin, alpha-actinin, filamin
• myotendinous junction, fibroblast on plastic surface

38
Q

Anchoring junctions hemidesmosomes

A

hemidesmosomes resemble desmosomes:

connecting IF to ECM

act as rivets

distribute forces

laminins, collagen in basement membrane

intracellular: plectin (binds to keratin IF)

Difference: IF ends in hemidesmosomes while making lateral attachments in desmosomes

Pathology

39
Q

Epidermoylsis Bullosa

A

The level of tissue separation in the simplex, hemidesmosomal, junctional, and dystrophic forms of epidermolysis bullosa is indicated on the right

Hemidesmosomal EB: Plectin mutations

40
Q

Communication junctions: Gap junctions

A

Most cells have gap junctions

Communication

Functional pore size 1.5 nm (fluorescent dyes)

Characteristic gap between cells (2-4 nm)

small molecules (<1000 Dalton) like water sugars, amino acids, vitamins, second messengers

metabolic and electric coupling, pH

Macromolecules cannot pass

41
Q

Gap junction proteins

A

Connexins

4 pass transmembrane proteins

humans: 14 different connexins (different genes, different tissue distribution)

6 form a connexon

continuous aqueous channel

different properties (permeability) in different tissues

most cells: more than 1 connexin

homomeric and heteromeric connexons

42
Q

What forms a connexon?

A

6 connexins

43
Q

he intercellular channels can be

A

homotypic or heterotypic.

44
Q

Are gap junctions dynamic or static?

A

Gap junctions are dynamic structures that flip between open/closed (closed: ↑ Ca2+ or ↓ pH)

45
Q

Functions of Gap Junctions

A

signals spread rapidly (nerve, muscle)

Electric coupling in vertebrates synchronizes the contractions of heart muscle cells and smooth muscle (intestine)

Non excitable cells: Sharing of the small metabolites and ions

Example in liver: noradrenaline/Glucose/Glycogen

46
Q

Location of various cell junctions

A

Tight junction in the most apical position

Adherens junctions (adhesion belt)

Desmosomes

All three above together: junctional complex

Gap junctions and additional desmosomes are less regularly organized.

47
Q

Cell junction versus cell adhesion

A

Cell junctions (EM)

  • permanent anchorage
  • stability
  • communication

Cell adhesion (functional assay)

  • transient anchorage
  • cell-cell recongnition
  • typically before cell junctions are established
  • partially overlapping molecules (cadherins, integrins)
48
Q

CAM

A

Cell Adhesion Molecules

develops neural tube (epithelial cells)

49
Q

EDTA

A

Embryonic tissue

binds calcium stronger than proteins, extracts calcium from cadherins

Also, use trypsin (protease) to digest proteins that keep cells together

Mixing cells from different embryonic organs: cells from the same organ find each other

50
Q

Three major mechanisms of cell-cell adhesion

A

Homophilic binding

Heterophilic binding

Binding through an extracellular linker molecule

51
Q

Homophilic binding

A

two of the same proteins bind to each other

52
Q

Heterophilic binding

A

two different proteins binding

53
Q

Which bindings are most preferred and most frequently

A

Homophilic and heterophilic binding are most preferred, homophilic binding occurs most frequently

54
Q

Cadherins in cell adhesion molecules

A

homophilic interactions, use calcium

55
Q

Selectins in cell adhesion molecules

A

heterophilic interactions, use calcium

56
Q

Integrins in cell adhesion molecules

A

heterophilic interactions, use calcium

57
Q

Ig-superfamily CAMs in cell adhesion

A

homophilic interactions, don’t use calcium

58
Q

Structure of classical cadherins:

A

Cadherin repeat extends into extracellular matrix

Cadherin repeat has lots of beta pleated sheets, with small alpha helices

Calcium binding domains in between cadherin repeats, stabilizes cadherin structure

59
Q

Non-classical cadherins include:

A

Fat-like cadherins: long cadherin domains

Seven-pass transmembrane cadherins

Protein kinase cadherins: intracellular signaling activities (Ret)

Desmosomal cadherins, similar to non-classical

T-cadherins: tethered to PM by other means, with a GPI anchor

60
Q

When are Gap junctions open

when are they closed?

pH and ion concentration

A

Closed: High Ca2+ Low pH

Open: Low Ca2+ High pH

61
Q

Function of calcium in cadherins

A

to stabilize the structure

62
Q

Cadherin homophilic interaction mediated

A

by the N-terminal cadherin repeat

N-terminal cadherin domains fit into each other, similar to key-and-lock

Generates the individual forces for cell adhesion

Generates a typical gap of 38.5 nm, relatively low binding affinity

Cadherins often come in patches, increasing the interaction increases the strength of the binding

63
Q

Binding affinity vs binding avidity

A

avidity is the apparent affinity of the entire patch of cadherins

64
Q

Cadherin diversity in the CNS

A

Classical cadherins are expressed in distinct regions in the brain and spinal cord

Protocadherins are also expressed in the CNS, provide heterogeneity

There are a number of protocadherin gene clusters in the CNS, exons are variable and code for the extracellular region and the transmembrane domain

Constant region exons code for intracellular portion of the cadherin

65
Q

Linkage and signaling of classical cadherins

A

Cadherin dimers are linked by adaptor proteins to actin filaments in the cell

Normal: Rac1 (GTP binding signaling molecule) is bound to GDI (GDP dissociation inhibitor Rho) and GDP

When a cadherin dimer interacts with another dimer on another cell, GDI dissociates from Rac1, localizing Rac1 to the plasma membrane

Cadherin signals to PI3K which activates a GEF

GEF exchanges GDP to GTP on Rac1, now Rac1 is activated

Activated Rac1 activates actin binding protein Arp2/3

Arp2/3 helps actin filaments to form, generating a force that pushes the cell membrane against the other membrane

66
Q

Structure of selectins

A

Lectin domain: bind to oligosaccharides on the other cell

Number of extracellular domains to space the lectin domain away from plasma membrane

Anchor proteins that bind to actin filament in the cell

67
Q

L-selectin vs E-selectin vs P-selectin

A

L-selectin: lymphocytes

E-selectin: activated endothelial cells

P-selectin: platelets, endothelial cells

68
Q

Selectins are important in the

A
  • migration of white blood cells along endothelial tissue
    • Weak adhesion and rolling is selectin dependent
    • Endothelial cells express selectins, oligosaccharides are located on lymphocytes
    • Strong adhesion and diapedesis into the tissue is integrin dependent

BIOC212 (7 decks)

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