Cell Junctions: Hanging Together in Tension - Notes | BIOL 605 | Study notes Biology

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Cell Junctions: Hanging Together in Tension

There are two types of cell junctions: those that stick cells to cells, and those that stick cells to the

extracellular matrix.

Cell junctions can alter/affect cell function – for example, when liver cells are taken and grown in

culture, they cease to act like liver cells and begin to act like amoebae. The way cells stick together is

almost as important in determining their nature as their genes.

Plant cells stick together differently than animal cells – they are embedded in a common, rigid ECM and

do not move (are permanently fixed). Because of this, there is no equivalent in plants to cancer in

animals. While tumors form, they are not able to metastasize (move to different places) as they are in

animal cells.

In animal cells, cells are connected through the ECM as well as with a group of specific cell junction

proteins, which are for the most part not present in plant cells.

Junctions in animal cells can either be temporary, semi-permanent and more stable, or virtually

permanent. As an example, white blood cells tumble along in vessels due to temporary connections

made with the surrounding vessel tissue, and can squeeze between vascular cells using more stable,

longer connections with the surrounding cells (extravasation).

In skin cell junctions, there are both cell to cell junctions and cell to ECM junctions. These help to

distribute stress throughout the skin using cytoskeleton fibers that are attached throughout the ECM as

well as cell to cell connections.

Cell Adhesion Molecules (CAMs) in Animals

Some CAMs are calcium dependent (cadherins), while others are calcium independent.

There are both cell to cell, or hemophilic, junctions (molecules stick together at their midpoint using

calcium, antibody/antigen specific connections) and cell to ECM, or heterophilic, junctions (integrins

bind cells to ECM components, selectins bind to certain proteins with sugars attached).

Desmosomes

Cadherins that link cells to other cells.

Called ‘spot desmosomes’ because of their structure

All spot desmosomes within one cell are linked together through cytoskeleton fibers in the cell;

one spot desmosome from one cell connects to one from an adjacent cell – this means that the

internal structures of both cells are all connected to each other through the spot desmosome

junction

These connections help to distribute stress and relieve tension

The spot desmosomes are plaques of protein – mostly plakoglobin and desmoplakin

Integral proteins embedded in the plaque – desmocollin and desmoglein – are woven through

the cell membrane and ECM and link to other desmocollin and desmoglein proteins in the

adjacent cell’s desmosome using calcium

Inside the cell, intermediate filaments of the cytoskeleton are also embedded in the plaque

(tonofilaments) – it is these filaments that link all the spot desmosomes in one cell together,

and these filaments are mostly made from keratin

Integrins

Cadherins that link cells to the ECM

Consist of alpha and beta subunits

Can be linked to the cytoskeleton of the cell

Involved in signaling and linking spot desmosomes

Hemidesmosomes

Use integrin proteins to attach cells to the ECM

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Cell Junctions: Hanging Together in Tension There are two types of cell junctions: those that stick cells to cells, and those that stick cells to the extracellular matrix. Cell junctions can alter/affect cell function – for example, when liver cells are taken and grown in culture, they cease to act like liver cells and begin to act like amoebae. The way cells stick together is almost as important in determining their nature as their genes. Plant cells stick together differently than animal cells – they are embedded in a common, rigid ECM and do not move (are permanently fixed). Because of this, there is no equivalent in plants to cancer in animals. While tumors form, they are not able to metastasize (move to different places) as they are in animal cells. In animal cells, cells are connected through the ECM as well as with a group of specific cell junction proteins, which are for the most part not present in plant cells. Junctions in animal cells can either be temporary, semi-permanent and more stable, or virtually permanent. As an example, white blood cells tumble along in vessels due to temporary connections made with the surrounding vessel tissue, and can squeeze between vascular cells using more stable, longer connections with the surrounding cells (extravasation). In skin cell junctions, there are both cell to cell junctions and cell to ECM junctions. These help to distribute stress throughout the skin using cytoskeleton fibers that are attached throughout the ECM as well as cell to cell connections. Cell Adhesion Molecules (CAMs) in Animals Some CAMs are calcium dependent (cadherins), while others are calcium independent. There are both cell to cell, or hemophilic , junctions (molecules stick together at their midpoint using calcium, antibody/antigen specific connections) and cell to ECM, or heterophilic , junctions (integrins bind cells to ECM components, selectins bind to certain proteins with sugars attached). Desmosomes  Cadherins that link cells to other cells.  Called ‘spot desmosomes’ because of their structure  All spot desmosomes within one cell are linked together through cytoskeleton fibers in the cell; one spot desmosome from one cell connects to one from an adjacent cell – this means that the internal structures of both cells are all connected to each other through the spot desmosome junction  These connections help to distribute stress and relieve tension  The spot desmosomes are plaques of protein – mostly plakoglobin and desmoplakin  Integral proteins embedded in the plaque – desmocollin and desmoglein – are woven through the cell membrane and ECM and link to other desmocollin and desmoglein proteins in the adjacent cell’s desmosome using calcium  Inside the cell, intermediate filaments of the cytoskeleton are also embedded in the plaque ( tonofilaments ) – it is these filaments that link all the spot desmosomes in one cell together, and these filaments are mostly made from keratin Integrins  Cadherins that link cells to the ECM  Consist of alpha and beta subunits  Can be linked to the cytoskeleton of the cell  Involved in signaling and linking spot desmosomes Hemidesmosomes  Use integrin proteins to attach cells to the ECM

 Also a protein plaque made mostly of plectin  Links cells to the basal lamella; stabilizes the cell Belt Desmosome  Cadherin that links cells to other cells  Two dense regions, one on each cell, link to each other  Linked to actin molecules – gives cells the possibility of movement and contraction  Involved in closing wounds and tissue movement  Not as strong as spot desmosomes Tight Junction  Cadherin that connect cells to other cells  Structured as a belt near the tops of cells  Threads of protein crisscross throughout the junction and match to patterns in the adjacent cell; attached using calcium  The three main proteins involved are occludin, claudin, and JAM. Occludin links to occluding, claudin links to claudin, and JAM links to JAM using calcium bridges  Tight junctions stop apoplastic movement (through the ECM) and block things in the ECM from moving between cells  Also block the movement of integrin proteins so cells can put proteins in membranes that will not diffuse through (divides the cell into the half above the tight junction and the half below it) Gap Junctions  Cadherins used for communication  Weak junctions that can form and re-form  Made of proteins called connexin that group together into groups of 6, which are then called connexons  Connexons form in such a way that there is a narrow water channel in the middle; match up with connexons on other cells and form a water channel between the two cells  Can open and close; about 1.5nm in diameter – allow small, individual molecules through (ions, calcium, ATP, cAMP)  Important in coordinating activity (heartbeat for example, which is regulated by ions that move between muscle cells) Junctions in Plants Plasmodesmata  Communication junction in plants similar to the gap junction in animals  Holes in the cell wall 30-50nm in diameter connect cells  These holes are cytoplasmic filled channels that the plasma membrane, endoplasmic reticulum, and large molecules can move through  Provides a direct, continuous connection of the cell membrane – one cell or many?  Plasmodesmata can be closed and re-formed or newly formed where there were none before  Allow symplastic movement – diffusion from one cell to another Casparian Strip  Similar to tight junction in animal cells  Prevent apoplastic movement (regulate it)  Important in root cells – the strip is a waterproof layer in the cell wall of the endoplasm that prevents molecules from entering the vascular system unless they are able to move into the cell through the cell membrane and move through the plasmodesmata.

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