Cells in their social context: Cell junctions and the extracellular matrix Flashcards
(140 cards)
1
Q
What is the primary function of cell cohesion in multicellular organisms?
A
To hold cells together, enabling the formation of organized multicellular structures.
2
Q
What is the extracellular matrix?
A
A complex network of proteins and polysaccharide chains secreted by cells.
3
Q
How do mechanisms of cell cohesion affect tissues and organs?
A
They govern the architecture, shape, strength, and arrangement of different cell types.
4
Q
What are cell–matrix junctions?
A
Connections that link connective tissue cells to the extracellular matrix.
5
Q
What characterizes connective tissues?
A
They are formed from an extracellular matrix produced by sparsely distributed cells.
6
Q
How do epithelial tissues differ from connective tissues?
A
Epithelial tissues consist of tightly bound cells forming sheets, with a less pronounced extracellular matrix.
7
Q
What is the basal lamina?
A
A thin mat of extracellular matrix underlying epithelial tissue.
8
Q
What are adherens junctions?
A
Cell–cell junctions connected to actin filaments.
9
Q
What are desmosomes?
A
Cell–cell junctions linked to intermediate filaments.
10
Q
What is the function of tight junctions?
A
To seal the gap between epithelial cells and prevent leakage.
11
Q
What do gap junctions allow?
A
The passage of small water-soluble molecules from cell to cell.
12
Q
What two superfamilies do the cytoskeleton-linked transmembrane adhesion proteins belong to?
A
Cadherin superfamily and integrin superfamily.
13
Q
What is the main role of cadherins?
A
To mediate cell–cell adhesion.
14
Q
What is the significance of Ca2+ ions for cadherins?
A
They are essential for maintaining the adhesive properties of cadherins.
15
Q
What is homophilic adhesion?
A
The binding of cadherin molecules of the same or closely related subtype on adjacent cells.
16
Q
What happens to cadherin structures when Ca2+ is removed?
A
The hinges flex, making the structure floppy and weakening binding affinity.
17
Q
How do cadherins achieve strong attachments?
A
Through the formation of many weak bonds in parallel.
18
Q
What are the four major types of anchoring junctions?
A
- Adherens junctions * Desmosomes * Actin-linked cell–matrix junctions * Hemidesmosomes
19
Q
What are the main cellular components linked by adherens junctions?
A
Actin filaments.
20
Q
What are hemidesmosomes responsible for?
A
Anchoring intermediate filaments to the extracellular matrix.
21
Q
What type of junctions are primarily responsible for transmitting mechanical stresses in epithelial tissues?
A
Anchoring junctions.
22
Q
What is a junctional complex?
A
A structure formed by tight junctions, adherens junctions, and desmosomes.
23
Q
What type of binding do cadherins typically exhibit?
A
Cadherins typically bind to their partners with relatively low affinity
This allows for strong attachments through the formation of many weak bonds in parallel.
24
Q
What is the ‘Velcro principle’ in the context of cell adhesion?
A
The ‘Velcro principle’ refers to how strong cell–cell junctions can be easily disassembled, similar to how Velcro can be pulled apart
This principle applies to cell–cell and cell–matrix adhesions formed by transmembrane adhesion proteins.
25
What are some members of the cadherin superfamily?
* Classical cadherin (E-cadherin)
* Fat cadherin
* Flamingo
* Ret
* Desmocollin
* Cadherin 23
* Protocadherins (Pcdh γ)
* T-cadherin
26
What structural feature is common among classical cadherins?
Classical cadherins contain multiple copies of the extracellular cadherin domain
## Footnote
Vertebrate classical cadherins have 5 domains, while some nonclassical cadherins have more than 30.
27
What role does calcium play in cadherin function?
Calcium ions bind to hinge regions of cadherins, preventing flexibility and maintaining adhesion at higher concentrations
## Footnote
A decrease in calcium concentration can lead to failure of adhesion.
28
What is homophilic binding in cadherins?
Homophilic binding refers to cadherins binding to identical cadherins on adjacent cells
## Footnote
This is in contrast to heterophilic binding, where different types of cell adhesion molecules bind.
29
How do cadherins contribute to tissue organization during development?
Cadherins mediate highly selective recognition that enables cells of similar types to adhere and segregate from different types
## Footnote
This was demonstrated in experiments with amphibian embryos.
30
What is the correlation between cadherin expression and embryonic development?
Changes in cadherin expression correlate with developmental stages where cells regroup and change contacts to form new tissue structures.
31
What happens to cadherin expression in neural tube formation?
Neural tube cells lose E-cadherin and acquire N-cadherin as they form and detach from the ectoderm.
32
What is the significance of cadherin-dependent cell sorting?
Cadherin-dependent cell sorting allows cells expressing different cadherins to segregate and aggregate separately
## Footnote
This reflects similar behavior seen in tissues expressing different cadherins.
33
What is the role of actin cytoskeleton changes in cell adhesion?
Changes in the actin cytoskeleton are necessary for the assembly of strong cell–cell adhesions, including a decrease in cortical tension.
34
What GTPases are involved in the regulation of cortical tension during cell adhesion?
Rac and Rho are the small GTPases involved in regulating cortical tension during the formation of adherens junctions.
35
How does cortical tension affect cell adhesion?
High cortical tension prevents the spreading of adhesion surfaces, requiring local reduction of tension for effective adhesion to occur.
36
What is the long-term effect of cadherin clustering at adherens junctions?
In the long term, large adherens junctions inhibit Rac and stimulate Rho, promoting the formation of actin–myosin fibers that interact with cadherins.
37
What is the role of Rac activation in adherens junctions?
Rac activation stimulates local actin protrusions, contributing to junction expansion.
38
What happens to Rac and Rho during the formation of adherens junctions?
Rac is inhibited and Rho is activated, promoting assembly of linear actin filament bundles.
39
What are catenins?
Catenins are proteins that link classical cadherins to the actin cytoskeleton.
40
Which proteins interact with the cytoplasmic tail of cadherins?
* β-catenin
* p120-catenin
* α-catenin
41
What do adherens junctions respond to?
They respond to mechanical stresses and generate biochemical signals.
42
What is mechanotransduction?
It is the process by which junctions sense mechanical stresses and generate biochemical responses.
43
What happens when myosin activity is inhibited?
Inhibition of myosin activity results in the disassembly of many adherens junctions.
44
How do adherens junctions maintain balance of forces?
They sense forces and modify local actin and myosin behavior to balance forces on both sides.
45
What structural change occurs in α-catenin during increased contractile activity?
α-catenin unfolds from a folded to an extended conformation.
46
What is the function of vinculin in adherens junctions?
Vinculin promotes the recruitment of more actin to the junction.
47
What role do adherens junctions play in tissue remodeling?
They coordinate actin-mediated contraction with cell–cell adhesion.
48
What is the form of adherens junctions in epithelial tissues?
They often form a continuous adhesion belt (zonula adherens).
49
What is the main function of desmosomes?
To provide mechanical strength to tissues.
50
What type of filaments do desmosomes link to?
Intermediate filaments.
51
What do tight junctions do?
They form a seal between cells and a fence between plasma membrane domains.
52
What is the role of epithelial sheets in animals?
They enclose and partition the body, creating internal compartments.
53
What happens to adherens junctions during cell intercalation?
Specific adherens junctions are lost to allow cells to insert between others.
54
What type of contractile forces are involved in cell–cell adhesion remodeling?
Actin-based contractile forces.
55
What is the main structural function of intermediate filaments in desmosomes?
To form a network of tensile strength throughout the tissue.
56
What proteins are primarily involved in the assembly of desmosomes?
* Desmogleins
* Desmocollins
57
What occurs at the vertical cell boundaries during contraction of bundles?
Removal of E-cadherin leads to shrinkage and loss of adhesion along the vertical axis
## Footnote
This process allows for new cadherin-based adhesions to form along horizontal boundaries.
58
What are the two sides of an epithelium called?
Basal side and apical side
## Footnote
The basal side is anchored to other tissue, while the apical side is free.
59
What mediates the attachment of epithelia to underlying tissue?
Basal lamina
## Footnote
This structure lies at the interface with the underlying tissue.
60
What common function do all epithelia perform?
Serve as selective permeability barriers
## Footnote
They separate fluids with different chemical compositions on their basal and apical sides.
61
What type of junctions seal adjacent epithelial cells together?
Tight junctions
## Footnote
This sealing prevents molecules from leaking across the cell sheet.
62
What type of epithelium is characterized by a simple columnar structure?
Epithelium of the small intestine
## Footnote
It consists of a single layer of tall (columnar) cells, primarily absorptive cells.
63
What is the function of absorptive cells in the small intestine?
Transport selected nutrients across the epithelium
## Footnote
This transport occurs from the lumen into the extracellular fluid.
64
What are the two sets of transport proteins involved in nutrient transport?
* Apical surface transporters
* Basolateral surface transporters
## Footnote
The apical transporters actively transport molecules into the cell, while basolateral transporters allow molecules to leave the cell.
65
What role do tight junctions play in the distribution of transport proteins?
They function as 'fences' to prevent apical or basolateral proteins from diffusing into the wrong regions
## Footnote
This ensures that transport proteins remain in their designated membrane domains.
66
What is paracellular transport?
Movement of ions and other molecules between epithelial cells
## Footnote
This transport differs based on the proteins forming tight junctions.
67
What are the main transmembrane proteins in tight junctions?
* Claudins
* Occludin
* Tricellulin
## Footnote
Claudins are essential for tight-junction formation, occludin limits permeability, and tricellulin prevents leakage at junctions where three cells meet.
68
What is the role of scaffold proteins in tight junctions?
They organize and position the sealing strands of tight junctions
## Footnote
Scaffold proteins can bind to multiple junctional proteins and the actin cytoskeleton.
69
What is a junctional complex?
An assembly of tight junctions, adherens junctions, and desmosomes
## Footnote
These components depend on each other for their formation.
70
What type of junctions allow cells to couple electrically and metabolically?
Gap junctions
## Footnote
They facilitate communication between adjacent cells.
71
What is the junctional complex?
A junctional complex is an assembly of tight junctions, adherens junctions, and desmosomes that bond cells together mechanically.
72
What is the primary function of tight junctions?
Tight junctions block the passageways between epithelial cells, preventing leakage of extracellular molecules.
73
What are gap junctions?
Gap junctions are structures that bridge gaps between adjacent cells, creating direct channels from the cytoplasm of one cell to that of another.
74
In which types of tissues are gap junctions found?
Gap junctions are present in most animal tissues, including connective tissues, epithelia, and heart muscle.
75
What is the pore size of gap junctions, and what does it allow?
Gap junctions have a pore size of about 1.4 nm, allowing the exchange of inorganic ions and small water-soluble molecules.
76
What are the two families of channel-forming proteins in gap junctions?
The two families are connexins and innexins.
77
What role do gap junctions play in electrically excitable tissues?
They allow the rapid spread of action potentials from cell to cell.
78
What is the function of plasmodesmata in plants?
Plasmodesmata connect the cytoplasms of adjacent plant cells, allowing for direct cell–cell communication.
79
What structure runs through the center of most plasmodesmata?
The desmotubule, which is continuous with elements of the smooth endoplasmic reticulum.
80
What triggers changes in the open and closed states of gap-junction channels?
Voltage differences, membrane potential, and various chemical properties of the cytoplasm.
81
What is a dynamic feature of gap-junctional plaques?
They can assemble, disassemble, or be remodeled, containing a variable number of connexons.
82
What are plasmodesmata?
Molecules between certain cells or groups of cells that are connected by apparently normal plasmodesmata
83
What is the function of plasmodesmata?
They connect adjacent plant cells and allow communication
84
What is the structure that lines each plasmodesma?
Plasma membrane
85
What is a desmotubule?
A fine tubular structure derived from smooth endoplasmic reticulum found within plasmodesmata
86
What is the role of selectins in the bloodstream?
They mediate transient cell–cell adhesion interactions
87
Which types of cells express selectins?
White blood cells, blood platelets, and endothelial cells
88
What are the three types of selectins?
* L-selectin on white blood cells
* P-selectin on blood platelets and activated endothelial cells
* E-selectin on endothelial cells
89
What role do selectins play in white blood cell migration?
They control the binding of white blood cells to endothelial cells, enabling migration into tissues
90
What is the relationship between selectins and integrins?
Selectins mediate weak adhesion; integrins strengthen the binding of blood cells to the endothelium
91
What are ICAMs and VCAMs?
Intercellular cell adhesion molecules and vascular cell adhesion molecules expressed on endothelial cells
92
What type of binding do ICAMs and VCAMs mediate?
Heterophilic binding to integrins
93
What is NCAM?
Neural cell adhesion molecule that mediates homophilic binding
94
What is a characteristic feature of cadherins?
They bind to one another homophilically
95
What do tight junctions create between epithelial cells?
A barrier to the diffusion of molecules
96
What are gap junctions composed of?
Plaques of clustered connexons
97
What size of molecules can pass through gap junctions?
Molecules smaller than about 1000 daltons
98
What are the three additional classes of transmembrane adhesion proteins?
* Selectins
* Ig superfamily members
* Integrins
99
How do integrins primarily function?
They attach cells to the extracellular matrix
100
What is the basal lamina?
A specialized type of extracellular matrix that underlies epithelial cells and surrounds certain cells like muscle and Schwann cells.
101
What role does the basal lamina play in multicellular animals?
It is a defining feature that appeared early in their evolution, providing structural support and various functions.
102
What is the primary organizer of the sheet structure of the basal lamina?
Laminin
103
What is the structure of laminin?
Composed of three long polypeptide chains (α, β, and γ) held together by disulfide bonds, forming an asymmetric bouquet shape.
104
How do laminins self-assemble?
They can self-assemble in vitro into a network through interactions between their heads but require interactions with cells to organize into a sheet.
105
What type of collagen is essential for the basal lamina?
Type IV collagen
106
How do type IV collagen molecules differ from fibrillar collagens?
Type IV collagen has interruptions in its triple-stranded helical structure, allowing for bends, and forms a flexible network.
107
What are the roles of the basal lamina in tissue regeneration?
It provides a scaffold for regenerating cells to migrate along, helping to reconstruct original tissue architecture.
108
What is the role of integrins in the basal lamina?
They are cell-surface receptors that help organize the assembly of the basal lamina.
109
What happens if a cell lacks enzymes to degrade the surrounding matrix?
It is inhibited from dividing and hindered from migrating.
110
What is a key function of the basal lamina concerning cell migration?
It serves as a highway for cell migration.
111
What is the role of the basal lamina in nerve regeneration?
It directs the regenerating nerve to the original synaptic site.
112
What happens when only the muscle is allowed to regenerate?
The junctional basal lamina causes newly made acetylcholine receptors to accumulate at the original synaptic site.
113
Why is matrix degradation important in cancer?
It is important for the spread of cancer cells and their ability to proliferate in invaded tissues.
114
What are the two general classes of proteolytic enzymes that degrade matrix components?
* Matrix metalloproteases
* Serine proteases
115
What is a characteristic of matrix metalloproteases?
They depend on bound Ca2+ or Zn2+ for activity.
116
What must be tightly controlled to prevent the collapse of the body fabric?
The activities of the proteases that degrade the matrix.
117
What can the proteolytic cleavage of matrix proteins generate?
Protein fragments with specific biological activities.
118
What happens when specific proteoglycans are inactivated by mutation?
Severe developmental defects can occur.
119
What are the fundamental roles of the extracellular matrix?
* Scaffolding for tissue structure
* Substrate for cell anchorage and migration
* Influencing cell signaling
120
What are glycosaminoglycans (GAGs)?
Negatively charged polysaccharide chains linked to protein to form proteoglycans.
121
What provides tensile strength to the extracellular matrix?
Fibrillar collagens.
122
What is the main component that organizes the basal lamina?
Laminin molecules.
123
What are the functions of basal laminae?
* Provide mechanical support for epithelia
* Form interface and attachment between epithelia and connective tissue
* Serve as filters in the kidney
* Act as barriers to keep cells in proper compartments
* Influence cell polarity and differentiation
* Guide cell migration during development and tissue regeneration
124
What are integrins?
Integrins are the principal receptors on animal cells for binding most extracellular matrix proteins
## Footnote
They are part of the fundamental architectural toolkit of multicellular animals.
125
How do integrins transmit signals?
Integrins transmit signals in both directions across the plasma membrane
## Footnote
They can send messages into the cell and receive signals from the cell interior.
126
What happens to integrins when tension is applied?
Tension causes integrins to tighten their grip on structures
## Footnote
Loss of tension can loosen their hold, affecting molecular signaling complexes.
127
What is the structure of integrins?
Integrins are composed of two noncovalently associated glycoprotein subunits, α and β
## Footnote
Both subunits span the cell membrane.
128
What specific sequence do integrins commonly bind to?
Integrins commonly bind to the RGD sequence found in fibronectin
## Footnote
Some also bind to the LDV sequence.
129
What are hemidesmosomes?
Hemidesmosomes are attachment sites in epithelia where integrins anchor cells to laminin
## Footnote
They connect to keratin intermediate filaments via adaptor proteins.
130
What is the function of β2 integrins?
β2 integrins are crucial for white blood cells to fight infection
## Footnote
They mediate cell–cell interactions rather than cell–matrix interactions.
131
What are the two major activity states of integrins?
Integrins exist in inactive (folded) and active (extended) states
## Footnote
The transition between these states involves conformational changes.
132
What is an 'outside-in' mechanism in integrin activation?
An 'outside-in' mechanism refers to the activation of integrins by binding external matrix proteins
## Footnote
This drives integrins to switch from low-affinity to high-affinity states.
133
What are the two major activity states of integrins?
Inactive (folded) and active (extended) structures.
134
How do integrins differ from cell-surface receptors for hormones?
Integrins usually bind their ligand with lower affinity and are present at a 10- to 100-fold higher concentration on the cell surface.
135
What is anchorage dependence?
The dependence of cell growth, proliferation, and survival on attachment to a substratum, mainly mediated by integrins.
136
What happens to cells that lose contact with the extracellular matrix?
They undergo apoptosis.
137
What does mechanotransduction at cell–matrix junctions involve?
It involves the ability of junctions to sense and respond to mechanical forces acting on them.
138
What is the significance of the conformational switch in integrins?
It allows for allosteric coupling between binding to the matrix and binding to the cytoskeleton.
139
What cellular behaviors can integrin signaling influence?
* Proliferation
* Survival
* Cell polarity
* Guidance of migration
140
How do integrins contribute to cell–matrix adhesion strength?
By recruiting additional integrins and proteins in response to mechanical tension.
