List the 4 types of cellular junctions
1) Tight or Occluding Junctions (Zonula Occludens)
2) Anchoring Junctions
3) Gap Junctions
4) Signal-Relaying Junctions (Synapse)
Tight or Occluding Junctions (Zonula Occuldens)
They involve claudin proteins. They are involved in sealing gaps between epithelial cells. They have 3main functions:
1) seal cells together to create a permeability barrier:
2) regulate paracellular transport: they can be regulated to promote leakage between cells or paracellular transport
3) Apical vs. Basolateral protein/lipid sorting: The fence function of tight junctions separates apical from basolateral plasma membrane domains
Plaque or Anchoring Junctions
Link cell to cell (typically via transmembrane cadherin proteins) or cell to matrix (typically via transmembrane integrin proteins). They mechanically attach cells and their cytoskeleton to their neighbors or the extracellular matrix to stabilize against mechanical stress.
-2 Main Categories (cell-to-cell or cell-to-matrix)
- 4 Junction types:
1) Adherens Junction: Use classical cadherins as their transmembrane adhesion protein and are cell-to-cell. They utilize actin filaments as their intracellular cytoskeletal attachment.
2) Desmosomes: These are cell-to-cell and they use a non-classical cadherin as their transmembrane protein. They also use intermediate filaments as their cytoskeletal attachment.
3) Focal Contacts/Focal Adhesions: Actin-linked cell-matrix junction: These use Integrin as the transmembrane adhesion protein and actin as the cytoskeletal filament.
4) Hemidesmosomes: They use integrin as well and involve intermediate filaments. They are also cell-matrix junctions.
Gap Junctions
They allow small molecules and electrical signals to pass between interacting cells.
-Connexins are the proteins that constitute gap junctions and they are 4 pass transmembrane proteins. 6 connections form a functional pore, called a connexon. Also, the gap junction permeability varies with connexin composition (21 genes in humans)
Gap Junction Functions:
1) Electrically connect cells because ions can flow through: Heart muscle cells for example
2) Metabolically couples cells by average small molecules throughout a tissue: Liver.
3) More specialized cells uncouple from cells with different cell fates: Embryogenesis
Signal-Relaying Junctions: Synapses
Scaffold proteins organize adhesive proteins, ion channels, receptors, etc. Know that these scaffolding proteins are crucial for the organization of these junctions.
- These tend to be very complicated
- Have some features of adhesion junctions because they have actin and some adhesion junctions between them but they are not very robust like a desmosome
- They are a little weaker adhesion but there needs to be a little bit of adhesion for the two membranes to be located near each other
- There is a scaffolding protein that will anchor signaling molecules like ion channels and signal receptors.
- Signal relaying junctions can be very different
Explain the function of tight junctions in blood vessels and in the epithelial cells which line the intestinal lumen
The tight junctions in the epithelial cells perform a multitude of functions, most notably their prevention of the leakage of gastric enzymes into the body would could have detrimental effects. Also, they help establish an apical and basal side of the cell such that certain receptors in the cell are on one side and others on another. They also put specific channels on one side or the other for nutrient diffusion out of the intestines so we get nutrient uptake. The tight junctions in the blood vessels prevent blood loss and keep the blood in circulation as well as all for the intake of nutrients from the intestines.
Explain the importance of transcellular transport of nutrients across sheets of epithelial cells that are linked by tight junctions
- Transcellular Transport: Active transport of glucose from gut lumen into cells via Na-driven glucose symporter (active transport). Glucose has to go through the cells because of the tight junctions. Has to go through the cell!
- However, if the junctions are disrupted, we get paracellular transport –> Can now go around the cells
It keeps the permeability barriers preserved!
The transcellular transport is crucial to ensure that nutrients can still move across the cell barrier since paracellular transport is not permissible with tight junctions.
Explain the process of paracellular transport
Paracellular transport is simply the ability of molecules to move in between cells since they are not as tightly linked to one another. There is a gap between the cells for molecules to pass through. It is the “leakage” between cells.
List the 4 different types of anchoring junctions
1) Adheren Junctions
2) Desmosomes
3) Focal Contacts/Focal Adhesions: Actin-linked cell-matrix junction
4) Hemidesmosomes
For each of the 4 types of anchoring junctions, identify the class of transmembrane adhesive proteins and the cytoskeletal proteins to which each adhesive proteins attach
1) Adheren Junctions:
- Class of transmembrane adhesive protein: Classical Cadherins
- Cytoskeletal proteins to which each adhesive proteins attach: Actin filaments
- The adapter proteins are catenin
2) Desmosomes:
- Class of transmembrane adhesive protein: Non-classical Cadherins
- Cytoskeletal proteins to which each adhesive proteins attach: Intermediate Filaments
- The adapter proteins are a dense plaque of adapter proteins.
3) Focal Contacts/Focal Adhesions: Actin-linked cell-matrix junction:
- Class of transmembrane adhesive protein: Integrin
- Cytoskeletal proteins to which each adhesive proteins attach: Actin Filaments
4) Hemidesmosomes:
- Class of transmembrane adhesive protein: alpha6-beta4 Integrin, type XVII Collagen
- Cytoskeletal proteins to which each adhesive proteins attach: Intermediate Filaments
Illustrate the importance of desmosomes using a clinical example
Pemphigus is an autoimmune disease where auto-antibodies against desmosomal cadherins cause the loss of cell-cell adhesion and get severe blistering
Explain the role of selectins with the interactions of white blood cells and endothelial cells
Selectins are Lectins: Carbohydrate binding proteins
Transient, calcium-dependent adhesion
Heterophilic. Cooperate with integrins
E Selectin: Activated endothelial cells
Selecting are critical for allowing the carbohydrate markers of white blood cells to bind to the selectins via the lectin domain. The selectins are crucial for allowing the WBCs to “roll” in the vein. Then, when integrin begins to have a larger role, the WBC can bind tightly and go through the epithelial cells.
- Weak adhesion and rolling is selection-dependent
- Strong adhesion and emigration is integrin-dependent
Cadherins
Ca2+-dependent transmembrane adhesion proteins. Involved in Adheren Junctions and Desmosomes for cell-cell junctions.
- Cadherins are extremely important adhesion molecules
- They are calcium dependent
- They have a variety of cadherin repeats in them and the presence of calcium allows the cadherins to become very stiff and then reach out to encounter one another at the amino terminus.
- The two amino terminuses interact.
- To detach cells, you have to wash them with calcium to stop them from binding to each other
- Need calcium in order to function
- It is homophilic binding: Different types of cadherins so it will only bind to another cell with the SAME cadherin on it because it is homophilic.
- These cadherins being homotypic allowed for a self sorting adhesion so that cells with a similar fate will adhere to one another to get appropriate development
- Cadherin forms adhesion junctions that link adjacent cells
- When cells are not touching each other, the cadherin is not very stable and the turnover number is much greater than when they are touching and interacting with cadherin on other cells
- Homophilic binding is important during embryogenesis
Integrins
Transmembrane ECM binding proteins. They are a dimer and they bind indirectly to the actin filaments via adapter proteins.
Identify the transmembrane protein that forms gap junctions
Connexins
- 4 pass transmembrane proteins
- 6 connexins form a functional pore: Connexon
- Permeability varies with connexin composition
Connexins are the transmembrane proteins which form the connexon or the gap junction (pore). 6 connexins make up a connexon and they are 4 pass transmembrane proteins.
Identify specific types of molecules that are able to pass through gap junctions and those that are unable to pass through gap junctions
In terms of molecular weight, those with a molecular weight below 1000 tend to be able to pass through gap junctions. More specifically, Ions, Sugars (mono/disaccharides) and Nucleotides can pass through the gap junctions. What can pass through a specific gap junction or pore depends on the composition of the connexins that make up the connexons.
Identify the cells that secrete most of the matrix molecules of connective tissue
1) fibroblasts
2) chondroblasts
3) osteoblasts
4) epithelium
The Extracellular Matrix
A substantial volume of tissues consists of extracellular space, which is filled by an intricate network of proteins and carbohydrate-containing molecules of the extracellular matrix. The components of the extracellular matrix include proteoglycans and glycosaminoglycans (GAGs), structural proteins, and adhesive proteins.
- The ECM is a Hydrated network of protein and carbohydrate containing molecules and it is very abundant in connective tissues.
- Its components are secreted by fibroblasts, chondroblasts, osteoblasts, epithelium
- The ECM bears most of mechanical stress
-It is a hydrated network of protein and carbohydrate containing molecules
The ECM is a whole group of different types of molecules like proteins and sugars and makes up the most of the connective tissue
Most of the components of the ECM will absorb the force that was put on the epithelium above them
There are some specialized cells
It is a contribution of multiple cell types that make it up.
Give examples of the major types of components that comprise the extracellular matrix
1) Glycosaminoglycans
2) Hyaluronan (Hyaluronic Acid, Hyaluronate)
3) Proteoglycans
4) Collagen
5) Elastin
6) Fibronectin
Identify the characteristics of Glycosaminoglycans (GAGs)
- Unbranched polysaccharide chains composed of repeating disaccharide units (N-acetylglucoamine and glucuronic acid)
- One amino sugar is usually sulfated and the other is a uronic acid
- Absorb large amounts of water: Occupy space
- The high negative charge from the sulfates attracts Na+ ions and water.
- Uronic acid is where the OH is a COO- to get the acid group which is seen on C6
Hyaluronan (Hyaluronic Acid, Hyaluronate)
- This is the simplest GAG and none of the sugars are sulfanated in this which is unusual. It is very long, containing up to 25,000 disulfide units.
- Synthesized from the basal side of an epithelial sheet
- Creates a cell-free space into which cells can migrate: Embryogenesis
- Diffusion of nutrients, metabolites, etc.
- Resists compressive forces
- Degraded by Hyaluronidase – Wound Healing
- Think of it like a sponge to create a gradient
Proteoglycans
- These are glycans so they are a sugar based molecule with a protein associated with it
- Glycosaminoglycans covalently attached to a core protein
- These contain at least 1 GAG, so any protein with at least one GAG is referred to as a proteoglycan
- Very high carbohydrate content
- Long, unbranched side chains
- Regulate movement of molecules and cells
- Chemical signaling between cells
- Proteoglycans have a core protein and then GAGs attached to that core protein.
-Glycoproteins have few, short, branched carbohydrate chains and a low carbohydrate content, this is a protein with a sugar group attached to it. They tend to me much smaller than proteoglycans. They are proteins at their root, meaning they are proteins with associated sugars.
Explain the relationship of Glycosaminoglycans (GAGs) to proteoglycans
GAGs are completely composed of sugars. They are not associated with a protein when they are referred to as GAGs. However, GAGs can associated with proteins at which point they are referred to as proteoglycans.
Collagen
Collagens are the most abundant proteins of the ECM and are primarily responsible for the strength of the matrix. Three alpha chains arising from combinations of over 40 collagen genes assemble to give rise to distinct structures and functions. Individual collagen polypeptide chains are synthesized as larger precursor molecules, which undergo several modifications before secretion and assembly into the triple helix.
- The most abundant proteins of the extracellular matrix
- 25% of protein mass in mammal
- Responsible for the strength of the ECM
