BIOL2021: Chapter 19 - Cell Junctions and the Extracellular Matrix

What are the 2 basic strategies/mechanisms for holding cells together?

• Cell Junctions (point where cells interact) and the Cytoskeleton

• Extracellular Matrix (ECM)

What are the 2 major categories of animal tissues that use junctions and ECM?

• Epithelial tissue

• Connective tissue

What does epithelial tissue cover?

surfaces (body, cavities)

What does connective tissue do?

• Supports, connects, or separates tissues

• Mostly ECM with a few cells; handles stress, tension, compression

What are the 3 broad categories of cell junctions?

• Cell-Cell contact

• Cell-ECM contact

• Some connect to actin/intermediate filaments

What do Tight junctions do?

• Hold cells close together near the apical side

• Seals gap between cells

• Prevents diffusion of proteins and lipids from apical to basolateral surface

What are Gap junctions?

Channels that allow passage of small water soluble molecules

Where do anchoring junctions anchor to?

The cytoskeleton

What type of junction are Cell matrix junctions and desmosomes, and what do they connect to?

Anchoring junctions; Cell-Cell contact

• Adherens junctions bind actin

• Desmosomes bind intermediate filaments

What type of junction are Cell-matrix and hemidesmosomes, and what do they connect to?

Anchoring Junctions; Cell-ECM

• Cell matrix binds actin

• Hemidesmosomes bind intermediate filaments

What 3 components do anchoring junctions involve

• Transmembrane adhesion proteins (cadherins for cell-cell, integrins for cell-ECM

• Intracellular adaptor proteins

• Cytoskeleton

What are Cadherins?

• The transmembrane link in cell-cell contacts

• Diverse family

• Found in almost all cell types

• Resist external forces to prevent cells from being pulled apart

What do cadherins bind to?

Actin (adherens) or Intermediate filaments (desmosomes)

What do cadherins do?

Mediate Homophilic adhesion between cells

True/False; All cells have the same type of cadherin

False; Different cell types have different cadherins

What does the extracellular region of a cadherin have?

Extracellular cadherin (EC) domain

What impact does Ca²⁺ binding to the EC domain of cadherin have?

Provides rigidity for homophilic binding

In adherens junctions, what happens to adaptor proteins linked to actin under tension?

They extend

What does an α-catenin attach to?

Binds to actin and cadherin, as well as vinculin (which is binded to another actin filament)

What are desmosomes structurally similar to?

Adherens junctions

What is the function of Desmosomes in Epithelia cells?

To provide mechanical strength

What is the purpose of tight junctions?

• Form a seal between cells

• Restrict diffusion of membrane proteins and lipids

• Restrict diffusion of extracellular molecules

What are claudins?

Transmembrane adhesion proteins found in tight junctions that form a seal

What do gap junctions do?

• Couple cells electrically and metabolically

• Allow synchronization in excitable tissue

• Allow exchange of inorganic ion and small water soluble molecules

What are Connexins and Innexins and what organisms are they found in?

Gap junctions

• Connexins; Vertebrates

• Innexins; Invertebrates

What are Plasmodesmata?

• Found in plant cells

• Perform many of the same functions as gap junctions

• Fused plasma membrane with smooth ER in between

• Passes ions, small molecules

What is pectin and what does it do?

• Polysaccharide

• Attaches cell walls of adjacent cells

What do selectins do?

Mediate Transient Cell-Cell adhesions in the bloodstream

What are the 3 main types of cell-cell adhesion proteins

• selectins

• Integrins

• Immunoglobulin (Ig) superfamily

What are selectins?

Cell-surface carbohydrate-binding proteins

What is the general structure of a selectin?

• Extracellular lectin domain (carbohydrate-binding)

• EGF (epidermal growth factor)-like domain

• Single transmembrane domain

• intracellular domain; binds to anchoring proteins which bind to actin filaments

What are the 3 types of selectins found in the bloodstream?

• L-selectins (lymphocytes)

• P-selectins (platelets and endothelial cells)

• E-selectins (endothelial cells)

What do L selectins do?

Interact weakly with oligosaccharides on endothelial cells

How do selectins mediate transient cell-cell adhesions in the bloodstream?

• P- and E- selectins are expressed, which bind to oligosaccharides on lymphocytes

• Integrins expressed on lymphocytes bind to endothelial Ig superfamily proteins, which strengthen lymphocyte binding

• Selectins and integrins allow lymphocytes to migrate out of the blood

• Cell-cell adhesion by selectins and integrins are heterophilic

What mediates Ca²⁺- independent cell-cell adhesion

Members of the Immunoglobulin (Ig) Superfamily

• ICAM (intracellular cell adhesion molecule)

• VCAM (vascular cell adhesion molecule)

• NCAM (neural cell adhesion molecule)

What is the general structure of endothelial Ig superfamily proteins?

• Extracellular Ig-like domains

• Transmembrane domain

• Cytoplasmic C-terminal

What type of binding do ICAM and VCAM mediate?

Heterophilic binding to integrins on lymphocytes

What is the extracellular matrix made and oriented by?

• The cells within it; Proteins, polysaccharides, sometimes minerals

• Hard or gel

What are the components of the ECM produced by?

Fibroblast

• Osteoblasts; bone

• Chondroblasts; Cartilage

What are the 3 main classes of molecules that the ECM s made from?

• Glycosaminoglycan; large charged polysaccharides

• Fibrous proteins; primality collagen family

• Glycoproteins; non-collagen proteins with carbohydrate

What are Glycosaminoglycan (GAG) chains?

• Long unbranched polysaccharide chains of repeating disaccharide units

• Strongly hydrophilic; attract cations which draw in water

• Can link to protein to form proteoglycans

• Occupy large amounts if space

• Form hydrated Gels

What are the major proteins of the ECM?

Collagens

What are Collagens?

• Major protein in connective tissue

• Long, stiff, triple-stranded helical structure

• Multiple strands = filament

• 42 collagen genes in humans; 25%of protein mass

What do secreted Fibril-Associated Collagens do?

• Help organize collagen fibrils fibrils

What do Elastin fibers do?

• Give tissues resilience to recoil after stretching

• Collagen fibers are interwoven with elastin fibers to prevent the tissue from tearing

What are Glycoproteins and what do they do?

• Large scaffold proteins with some carbohydrates

• Often have binding sites for other ECM proteins

• Help organize the matrix and allow cells to attach to it

What is the Basal Lamina?

• Specialized form of ECM

• Thin, tough, and flexible sheet of ECM

• Contains laminin, collagen, fibronectin etc

• Has several functions

What are the functions of the basal lamina?

• Structural role; connection between cell and connective tissue, surrounds some cells, separates cell types

• Selective filter between extracellular compartments

• Determine cell polarity

• Organize plasma membrane proteins of adjacent cell

What is Laminin?

• Glycoprotein

• Composed of 3 large and long polypeptide chains (alpha, beta, and gamma) held together by disulfide bonds

What is Epidermolysis Bullosa Caused by?

Genetic mutation of the LAMB3 (laminin) gene, preventing normal epidermal resistance to anchoring, making the skin fragile

What do matrix receptors do?

Bridge the cytoskeleton to ECM

What are integrins?

• Transmembrane heterodimers that link the ECM to the cytoskeleton

• Comprised of alpha and beta subunits (not covalently attached)

• Bind actin or intermediate filaments on inside and laminin or fibronectin on outside

• mediate signaling for cell behavior, including survival

• Major matrix receptor

What do Type IV collagen, laminin, perlecan (glycoprotein) and nidogen (proteoglycan) form?

Cross-link network

What is laminin anchored to the cell by?

Binding to integrins

What does Laminin bind to?

Other laminins, collagen, nidogen, and perlecan

What happens when an integrin is in an inactive state?

• Extracellular domain of integrin dimer is folded and cannot bind to matrix proteins

• Cytoplasmic tails are hooked together and cannot bind adaptor proteins

What form is an integrin in when it is active?

• Extracellular domain is unfolded and extended

• Cytoplasmic tails unhook to expose adaptor protein binding sites

What is an “Outside-In” activation of integrins?

• ECM proteins switch integrins from inactive to active state

• Leads to exposure of internal talin binding sites and subsequent actin filament binding

What is an “Inside-Out” activation of integrins?

• Activation of GPCR leads to activation of small monomeric GTPase (Rap1)

• Activated Rap1 small GTPase interacts with RIAM (RAP1-GTP interacting adaptor molecule)

• RIAM recruits talin to the plasma membrane to allow it to bind to integrin beta subunit

• Talin recruits vinculin, which allows integrin to associate with actin filaments

• Talin binding also unfolds external domain of integrins to allow it to bind to matrix proteins

True/False; Plant cell wall is Thicker, stronger, and more rigid than the extracellular matrix.

True

What is the difference between the primary and secondary cell wall in plant cells?

• Primary is thin and extensible, follows cell division until growth stops

• Secondary is ridged and thicker, with addition of ligin (polymer)

What is the plant cell wall mostly made of?

Almost entirely of polymers; cellulose and ligin (no nitrogen), very little protein

What is the primary cell wall built from?

Cellulose microfibrils interwoven with a network of pectin polysaccharides

What is cellulose? What is it formed by?

• Polysaccharide

• Formed by cross-linked glycans

• Glucose molecule is inverted with respect to neighboring glucose

What is a cellulose microfibril?

• Assembly of multiple cellulose molecules

• Most abundant organic macromolecule on Earth

What allows cellulose to give cell wall tensile strength?

• Hydrogen bonds between glucose molecules

• Crystalline aggregates of cellulose chains (microfibrils)

What is the plant cell wall comprised of?

• Orthogonal array of cellulose microfibrils

• Cross-linking glycans

• Pectin; polysaccharide

How are orthogonal arrays of cellulose microfibrils arranged?

In layers or lamellae

What are cross-linking glycans?

Heterogenous branched polysaccharides that bind tightly to cellulose microfibril

What charge does pectin have?

negative

Which lamella is pectin abundant in and why?

Middle lamella, helps to cement walls of adjacent cells

What part of the cell wall governs direction of cell expansion (growth)

The direction of cellulose microfibrils; aligned parallel to each other, and perpendicular to axis of cell elongation

Where does cellulose microfibril synthesis occur?

At the plasma membrane

Where are ECM proteins synthesized?

In the ER and Golgi apparatus.

What is the cellulose synthase complex?

• A hexagonal rosette subunit made of of 3 isoforms of cellulose synthase (CesA)

• Assemble at the plasma membrane

Where are the distal and proximal end of cellulose microfibrils located?

• Distil; Integrated into cell wall

• Proximal; Pushes cellulose synthase complex along plasma membrane