Cellular Adhesion and Extracellular Matrix
Cadherins and Adherens Junctions
Cadherins are transmembrane proteins that act as adapters between the cell's internal fibers and the external environment.
They are crucial for adherens junctions. The term 'adherens' is related to 'adherin'.
Classical cadherins exist in various forms, but memorizing specific names is not crucial.
Cadherins possess inherent domains that facilitate their function.
They function as homophilic adhesion proteins, meaning they bind to the same type of cadherin on adjacent cells (like binds to like).
For example, Fat cadherin binds to Fat cadherin, and Flamingo binds to Flamingo.
Homophilic binding involves the same adapter protein binding to its identical counterpart.
Heterophilic binding (different cadherins binding) does not occur.
Cadherin Conformations and Calcium Dependence
Cadherins have two main conformations influenced by calcium.
Cadherin domains stiffen when calcium ions bind to hinge regions between them.
This stiffening allows the terminal domains to interact with cadherins on adjacent cells.
In the presence of calcium, cadherins maintain a rigid structure, enabling intercellular adhesion.
The absence of calcium causes cadherins to become floppy and lose their adhesive properties.
EDTA, a common reagent in cell culture, is used to remove calcium, causing cells to detach from surfaces. Trypsin and EDTA are used together.
Cadherins as Velcro
Cadherins function like Velcro, with multiple interactions providing strong adhesion.
Shear forces (sideways pulling) are less effective at disrupting cadherin bonds compared to direct pulling.
Multiple cadherin interactions make it difficult to separate cells through shear force.
Cadherins in Embryogenesis
Cadherins play a vital role in embryogenesis by sorting cells based on their type.
Different cadherin types expressed during development cause cells to segregate into distinct regions.
For instance, during ectoderm development into the neural tube and neural crest, different cadherins facilitate cell separation.
Cells expressing E-cadherin and N-cadherin will separate and bind to like cells.
E-cadherin is typical of epithelial cells, while N-cadherin is found in mesenchymal cells.
Cell sorting also occurs based on cadherin expression levels, with high and low expression levels leading to further segregation.
Specific cadherins guide cells to their correct locations during development.
Adapter Proteins and Plaque Proteins
Adapter (plaque) proteins, located beneath the cell membrane, connect cadherins to the actin cytoskeleton.
Vinculin binds to alpha-catenin, which directly binds to actin, linking the transmembrane protein to the actin cytoskeleton.
Alpha-catenin adapts directly to actin, and vinculin can bind to alpha-catenin to initiate another actin filament, resulting in two filaments connected to one alpha-catenin molecule.
Beta-catenin and other catenins stabilize the region, connecting the cell's exterior to its interior.
Alpha-Catenin and Tension-Dependent Binding
Alpha-catenin's interaction with vinculin is tension-dependent.
When folded, alpha-catenin allows only one actin filament to bind.
Upon tension, alpha-catenin unfolds, exposing a binding site for vinculin, which then binds to a secondary actin filament.
Increased tension leads to more actin fibers being created and linked, reinforcing the connection.
Rho family proteins (G proteins) organize the cytoskeleton and bring actin filaments to cadherins.
Cadherin binding triggers signal transduction pathways that reinforce and strengthen the connection.
Connections become stronger over time unless calcium is removed.
Adhesion Belt in Intestines
In the intestines, an adhesion belt exists just below the tight junctions.
It is a band of actin connecting the cell's sides.
Each connection is an adherens junction.
Actin extends into microvilli, giving them structure and tying them into adherens junctions.
Desmosomes
Desmosomes connect intermediate filaments to transmembrane proteins, providing mechanical strength to epithelial layers.
Desmosomal transmembrane proteins are not classical cadherins; they don't rely on calcium for stiffening or direct grabbing.
They still contain cadherin domains and interact with each other.
A plaque of adapter proteins, including desmoplakin, plagoglobulin, and plagophilin, pulls the structure together.
Nonclassical cadherins in desmosomes include desmoglein and desmocollin, which alternate in the junction.
These nonclassical cadherins bind to each other in a like-to-like manner.
Cytokeratins, a type of epithelial intermediate filament, provide mechanical strength to epithelial layers in the intestines, skin, etc.
Desmosomes connect cells not just horizontally but also vertically, reinforcing the cytoskeleton's structure.
Hemidesmosomes and Tight Junctions
Hemidesmosomes are found at the base of cells, connecting intermediate filaments to the extracellular matrix via integrins.
Tight junctions link cell membranes together without connecting to internal filaments.
They prevent substances from passing between intestinal cells.
Tight junctions resemble quilting, with proteins tightly stitching cell membranes together.
Claudins and occludins are key components of tight junctions.
Tight Junctions and Signaling
Tight junctions facilitate signaling, transmitting information about cellular connections to the cell's interior.
Various domains (PDZ, SH3, GK, P) bind to different intracellular components.
Claudins bind on one side, and occludins on the other, linking them together.
Signaling mechanisms allow cells to sense whether they are properly connected to adjacent cells.
Gap Junctions
Gap junctions allow the passage of small molecules (less than 1 kDa) between cells.
Ions can pass through with their solvent shells, unlike membrane transport proteins.
Large proteins cannot pass through gap junction channels.
Gap Junction Structure
Gap junctions are made of connexons.
Each connexon is formed by six connexin subunits.
Connexins are transmembrane proteins that create a pore.
Gap junctions connect the pores of two adjacent cells, preventing leakage into the interstitial fluid.
Heartbeats are transmitted through gap junctions via calcium signaling.
Selectins
Selectins mediate transient adhesions, typically in the bloodstream.
They possess a lectin domain that binds to sugars on glycoproteins and proteoglycans.
Lectins, also present in the ER, recognize glycosylated proteins, but selectins use lectins for adhesion.
White blood cells are drawn to chemokines and roll along the endothelium's surface due to selectin interactions.
Selectins are tied to actin and thus the whole cell.
White blood cells roll until they encounter high concentrations of chemokines, then use integrin-dependent binding for tight adhesion.
They then slip through the cellular junctions into an infection.
Extracellular Matrix
Glycoproteins are mostly protein with some sugar, whereas proteoglycans are mostly sugar attached to a protein.
Glycosaminoglycans form gel-like structures within the matrix.
Glycoproteins here are mostly small in size.
Glycosaminoglycans and Their Function
Glycosaminoglycans have numerous negative charges due to the presence of sulfate and uronic acid groups.
The negative charges repel each other, attracting water and forming a gel-like substance.
This gel resists impacts and prevents cells from being crushed.
Hyaluronan is a large glycosaminoglycan (8,000,000 Daltons) that acts as a space filler during tissue repair.
Aggrecan
Aggrecan is another type of proteoglycan with sugars branching out from the protein core.
Aggrecan also binds to water and forms a gel.
These substances are often included in skincare products.
Collagens
Collagens consist of three amino acid chains with glycine, wrapped around each other to give strength.
Collagen fibers in the extracellular matrix resist pulling forces in one direction.
There are various types of collagen, including type II (long) and type IX (small, branching).
Type IX collagens connect different collagen fibers, making it harder to pull them apart.
Collagen gives cushion to the extracellular matrix.
Elastin
Elastin is an extracellular matrix protein that provides elasticity, and it decreases with age.
Elastin molecules have domains that curl up; disulfide bonds crosslink them.
When stretched, elastin maintains its crosslinks and acts as a spring.
Loss of elastin causes sagging and wrinkling of the skin.
Fibronectin
Fibronectin binds collagen, integrins, heparin, and fibrin.
Integrins, remember, are in those actin mediated cell junctions with the matrix.
Fibronectin has cryptic binding sites that are exposed when the molecule is pulled apart (beta barrel).
*The RGD sequence on fibronectin is used to coat cell culture plates to promote integrin-mediated cell adhesion.
is required for cells that require extensive extracellular matrix attachments in order to survive.
Basal Lamina
Connective tissue makes up what we call the basal lamina.
The basal lamina (a type of extracellular matrix) is composed of proteins that bind to each other.
It is not a membrane in the same sense, because they're not made of fatty acids.
Integrins connect cells to the basal lamina, linking the actin cytoskeleton to the extracellular matrix.
Muscle, epithelial, and kidney cells are all connected to the basal lamina.
Laminins
Laminins are multi-adapter proteins that tie different extracellular proteins together.
Laminins bind to integrins, self-assemble, and have coiled-coil domains.
Collagen, laminins, perlecan, and nidogen form the basal lamina.
Integrins on the cell surface connect the cell to this complex extracellular matrix structure.
Junctions in the basal lamina are similar but slightly different from adherens junctions.