Lecture 4 - Cell Adhesion and Communication Flashcards

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1
Q

There are 5 types of lateral connections. Name them.

A
  1. Tight junctions.
  2. Adherens juntions.
  3. Desmosomes.
  4. Gap junctions.
  5. ‘Non-junctional’ adhesions (proteins such as cadherins, selectins and integrins).
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2
Q

Connections with the basement membrane?

A

Non-junctional (integrins) and attachment junctions (hemidesmosomes and focal adhesions).

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3
Q

Define the 3 distinct classes of cell adhesion junctions.

A
  1. Occluding (tight) junctions - form a barrier between epithelial layers that prevents the movement of molecules from passing through the intracellular space between cells.
  2. Attachment junctions - links two cells using their cytoskeleton, or links a cell to the basement membrane.
  3. Communication junctions - links two adjacent cells using their cytoplasm.
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4
Q

Where are occluding (tight) junctions found?

A

Just below the apical surface.

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5
Q

What do tight junctions consist of?

A

Made of bands of interconnected strands of integral membrane proteins that wrap around the cell. Transmembrane proteins in these include claudins, junctional adhesion molecules (JAMs) and occludins.

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6
Q

How many claudins are there?

A

24

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7
Q

The claudins are the core proteins of tight junctions. What structure do they form?

A

Paracellular pores.

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8
Q

Define paracellular transport.

A

Transfer of substances through the intracellular space inbetween the cells.

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9
Q

Why is transcellular transport beneficial?

A

Allows the transport of substances against their concentration gradient (active transport).

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10
Q

Briefly explain the active transport mechanism that is used to absorb glucose.

A

Sodium-potassium ATPase pumps Na+ out of the cell, exchanged for K+. The concentration of Na+ is now low in the cell = diffusion = shuttles glucose as it does. Na+ glucose symporter used. The concentration of glucose is now high in the intestinal epithelial cell, enters the blood via GLUT2.

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11
Q

Two types of attachment junctions?

A

Adherens junctions and desmosomes (hemidesmoses)

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12
Q

Main protein that makes up an adheren junction?

A

Cadherin (dimer, one from each cell).

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13
Q

Adheren junctions link what type of filament from each cell? Which protein (besides cadherin) is involved?

A

Actin filaments, and catenin.

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14
Q

Desmosomes link what type of filament from each cell?

A

Intermediate filaments.

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15
Q

Name the 4 types of proteins involved in desmosomes.

A

Cadherins (desmocollins, desmogleins) and the adaptor proteins plagoglobulins and desmoplakin (form the cytoplasmic plaque).

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16
Q

Hemidesmosomes are attachment junctions that link the extracellular matrix to the intermediate filaments. What transmembrane proteins are involved in this structure?

A

Integrins.

17
Q

Hemidesmosomes also form cytoplasmic plaques. Which adaptor protein do they use?

A

Plectin.

18
Q

Describe the structure of integrin.

A

Heterodimer, one alpha and one beta subunit.

19
Q

Why can integrins bind to different components of the ECM?

A

There are 18 different alpha subunits and 8 different beta subunits.

20
Q

How are integrins regulated?

A

By signalling cascades that indicate whether a cell needs to adhere more tightly to the ECM.

21
Q

‘Inside-out signalling’

A

When signalling inside the cell affects the role of integrins on the outside of the cell.

22
Q

‘Outside-in signalling’

A

Cell binding to ECM can cluster integrins, forming docking sites for signalling molecules.

23
Q

How are integrins recycled for focal adhesions?

A

Integrin is endocytosed, breaking the attachment to the substrate. The internalised integrin is then recycled to establish a new focal adhesion in the direction of movement.

24
Q

What do gap junctions form?

A

Channels between cells that allow the transport of small molecules.

25
Q

Describe the structure of a gap junction.

A

They consist of two halves (connexons). Each connexon consists of 6 subunits, called connexins. Each connexin consists of 4 TM alpha helices.

26
Q

How are gap junctions regulated?

A

They can alternate between open and closed states, regulated by the phosphorylation of connexins. This is in response to changes in intracellular pH and/or calcium concentration. Can also be protective, e.g. if adjacent cells undergo apoptosis.