Cellular Junctions: Types and Functions

Cells in eukaryotic organisms require constant interaction for proper functioning. Different cellular junctions assist in this interactions. Usually, animal cells release materials, like proteins abundant in collagen, into their extracellular space. The collagen fibers wove with the proteoglycans (carbohydrate with protein) to form the extracellular matrix.

These matrices bind the cells together and also allow cell-cell interaction. The interaction occurs via the binding of a molecule (present in the matrix) with a protein receptor present in the cell’s plasma membrane. This interaction brings a conformational change in the microfilaments. The change signals turn on and off the transcription of a specific section of DNA, affecting the production of associated proteins and changing the activities within the cell. 

Besides, the extracellular matrix cells can also interact with each other by direct contact with the help of intercellular junctions. These junctions differ in plant and animal cells. There are a few types of cellular junctions in animal cells: tight, gap, and anchoring junctions. In contrast, there is only one type of junction in plant cells: plasmodesmata.

Types of Cellular Junctions

The types of cellular junctions in the animal and plant cells are; tight junctions (occluding junctions), gap junctions, anchoring junctions, and plasmodesmata (only junctions present in plant cells).

Tight Junctions (Zonula Occludens)

1. Tight or occluding junctions are the impermeable or selectively permeable seals that surround cells and bind them together. The cell portion exposed to the lumen is the apical surface/border beneath where the tight junctions are situated. 
2. In the thin section of cell, the tight junction of two adjacent plasma membranes appears fused at various points. In contrast, the three-dimensional structure appears as a network of ridges in the cytoplasmic part of the membrane with complementary grooves in the outer half. 
3. These ridges comprise two rows of protein that are like zippers, each from the two adjacent cells. These provide the leakproof effect so also termed sealing strands. These strands are anastomosis, a row of stitches in a quilted surface, and form a series of interconnected lines.
4. These occur in vertebrate and invertebrate animals and are most common in epithelial tissues, especially those that provide the interface between the masses of cells and a space or cavity (lumen). These are termed septate junctions in invertebrates.
5. The septate junctions of invertebrates differ in the proteins found in the gaps, the parallel row or septae occurrence, and the adjacent plasma membrane surfaces that are not in direct contact with each other.

Functions 

1. They are the permeability barrier that limits the passage of molecules and ions through the space between cells. For example, it is the indirect diffusion of nutrients from the intestinal lumen to the blood.  
2. These serve as a support mechanism for keeping the epithelium together.
3. These block the movement of integral membrane proteins between the adjacent cells to preserve the special functions of cells. For example, exocytosis occurs at the basolateral surface, and receptor-mediated endocytosis on the apical surface of the cells.   

Anchoring Junctions

The anchoring cellular junctions are present in the tissues where the epithelial and endothelial cells come in contact. These are present in the apical end of the cells near the tight junctions. The anchoring junctions comprise a mixture of primary transmembrane adherence proteins called cadherins and catenins, plaque proteins. 

Functions of anchoring junctions

1. Maintain plasticity of the cells.
2. Regulate the passage of solutes, water, and lymphoid cells across the connected cells.

The anchoring cells are of four types; desmosomes, adherens junctions, hemidesmosomes, and focal adherens. 

Desmosomes

Desmosomes are present in tissues that withstand severe mechanical stress like skin epithelia, bladder, cardiac muscles, and neck of the uterus and vagina. The desmosomes are of three types; spot desmosomes, hemidesmosomes, and belt desmosomes. 

The spot desmosomes are also called macula adherens. These act like rivets or spot welds for holding the epithelial cells together and are circular areas with a 0.5 µm diameter at the point of contact. The plasma membranes of the adjacent cells are at a distance of 30-50 nm. The intercellular core or the central stratum between the two plasma membranes has specific desmosomal material rich in proteins and mucopolysaccharides. The spot desmosome facing the plasma membrane consists of discoidal intracellular plaque non-glycosylated protein like desmoplakins I, II, and III. 10 nm thick keratin proteins called tonofilaments connect at the plaque proteins. Other thinner filaments of glycosylate proteins called desmogleins I and II also arise from the dense plaque and into the plasma membrane forming trans-membrane linkers. These linkers provide mechanical support.  

Adherens junctions

These are also known as belt desmosomes or zonula adherens. These are present in the interface between columnar cells just below tight junctions. Around the inner surface of the plasma membrane, these junction proteins form a girdle after forming a band. The bands have 6-7 nm actin microfilaments and other intermediate microfilaments of 10 nm. The actin microfilaments are contractile and intermediate filaments have a structural role. The adjacent cells’ plasma membranes are parallel, thicker than usual, and 15-20 nm apart. The intercellular space between the cells fills with amorphous material.    

Hemidesosomes

These are half desmosomes that are similar to spot desmosomes, but these join at the basal surface of the epithelial cell to the basal lamina. These help anchor extracellular proteins like collagen and other proteins to the cell.

Focal adherens 

The focal adherens connect to the cytoskeleton by the actin filaments. These contain proteins integrins in the intercellular membrane and the plaque proteins vinculin are present at the side facing the plasma membrane.   

Gap Junctions

1. The tissues of higher animals couple together with the help of interconnecting gap junctions. These are also called nexuses, macula communicans, or communicating junctions. 
2. Structurally, the gap junctions have hollow round channels called pores. The channel comprises of two connexons, called hemichannel, with six protein subunits, each with four transmembrane domains. The number of gap junctions varies from cell to cell. The diameter of the pores ranges from 1.5-2 nm.  
3. The subunits’ movement leads to the junctions’ opening and closing. Its permeability is indirectly proportional to the level of Caᐩᐩ ions. The small ions and molecules (up to 10,000 daltons) are easily permeable through the junctions.
4. The permeability of ions creates ionic or electronic connections between adjacent cells. These help in changing the membrane potential between connected cells.  
5. The gap junctions are present in the embryonic cells. In adult tissues, these are present in epithelia, cardiac, and liver cells.

Functions

1. Permeability of inorganic ions, sugars, amino acids, nucleotides, AMP, ADP, ATP, cAMP, and vitamins between adjacent cells.
2. Transfer of action potential in between different cells likes; cardiac muscle cells to generate rhythmic contraction of a heartbeat, electrical synapses in the brain, etc.

Plasmodesmata

The fine cytoplasmic channels that connect the neighboring cells of higher plants are called plasmodesmata (singular, plasmodesma). A plasmodesma is an almost cylindrical, membrane-lined channel with a 20-40 nm diameter. The center of plasmodesma has a narrow cylindrical structure called desmotubule, which is continuous with the molecules of membranes of SER (smooth endoplasmic reticulum) of each connected cell. The innermost part of plasmodesma is the plasma membrane and in between the desmotubule and plasma membrane is the annulus of cytosol. The annulus of the cytosol appears constricted, which regulates the movement of molecules through the annulus that joins the two cytosols.      

Functions

1. It helps pass molecules, like mRNA, large proteins, etc., from cell to cell by passive diffusion.
2. The plasmodesmata play a crucial role in the movement of nutrients in vascular tissues. 

References

• Adil, M., Narayanan, S., & Somanath, P. (2020). Cell-cell junctions: structure and regulation in physiology and pathology. Tissue Barriers, 9(1), 1848212. https://doi.org/10.1080/21688370.2020.1848212
• Verma, P., & Agarwal, V. (2005). Cell biology, genetics, molecular biology, evolution and ecology (23rd ed., pp. 112-153). S Chand and Company Ltd.