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Gap junction belong to the category of intercellular connections that are formed between a multitude of animal cell types. They are clumps of intercellular channels that allow ions and tiny molecules to flow freely between cells.
- Gap junction are low-resistance ion channels that connect excitable cells (nerve and muscle) in solid tissues.
- The junctions were first photographed in 1952 but officially appeared in 1969.
- They were 2-4 nm gaps that act as a bridge between cell membranes, which was more characterised by 1967.
Gap junction intercellular communication has been adapted to a range of roles and has many regulation mechanisms thanks to its lengthy evolutionary history.
- Innexins in precordates make up gap-junctional channels.
- It allows the exchange of ions and small metabolites between adjacent cells.
Gap junction functions have been investigated using genetic mutations in flies, worms, and humans and targeted gene disruption in mice.
Key Terms: Gap Junction, Cytoplasm, Vertebrates, Animal Cell, Cells, Plant Cells, Tissues, Connexon, Pannexins, RNA, DNA, Cell Membrane, Functions of Gap Junction, Organs, Retina, Connexin
Gap Junction
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Gap junction are intercellular connectors that connect a variety of animal cell types. They connect the cytoplasm of two cells directly, allowing numerous chemicals, ions, and electrical impulses to move directly between cells through a regulated gate.
- Two protein hexamers (or hemichannels) called connexons in vertebrates and innexons in invertebrates make up a gap junctions channel.
- The hemichannel pair bridges the gap between two cells by connecting across the intercellular space.
- They are similar to plasmodesmata, which connect plant cells.
Gap junction can be found in almost all body tissues, with the exception of fully mature adult skeletal muscle and mobile cell types like sperm or erythrocytes.
- Simpler species, such as sponges and slime moulds, lack gap junctions.
- There are 26 different types of connexins that are responsible for the formation of gap junction.
- Its components include ZO-1, which holds the membranes close together, sodium channels, and aquaporin.
Gap Junction
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Gap Junction Structure
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Gap junction hemichannels in vertebrates are largely homo- or hetero-hexamers of connexin proteins. They are invertebrates that are made up of proteins from the innexin family.
- Connexins and innexins have no significant sequence homology.
- Innexins are similar enough to connexins that they form gap junctions in vivo in the same way connexins do, despite their sequence differences.
The pannexin family, which was once considered to form intercellular channels (due to an amino acid sequence similar to innexins), is now known to function as a single-membrane channel. It communicates with the extracellular environment and can pass calcium and ATP. The intercellular space at gap junctions is between 2 and 4 nm, and the hemichannels in each cell's membrane are aligned with one another.
- Homotypic gap junction channels have two identical hemichannels.
- They have a consistent protein composition.
- Heterotypic gap junction channels have two different hemichannels.
- These hemichannels have different proteins.
The gap junction channel function is influenced by channel composition. Prior to the discovery of innexins and connexins, the genes that code for connexin gap junction channels were divided into three groups based on gene mapping and sequence similarity: A, B, and C. (for example, GJA1, GJC1).
Structure of Gap Junction
Levels of Organization
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There are various levels of organization in gap junction, which are as follows:
- The hemichannel genes (DNA) are transcribed into RNA and subsequently translated into hemichannel proteins.
- There are four transmembrane domains which make one hemichannel protein.
- One hemichannel is made up of six hemichannel proteins.
- A heteromeric hemichannel is formed when different hemichannel proteins combine to produce one hemichannel.
A gap junction channel is made up of two hemichannels that are connected across a cell membrane. A homotypic GJ channel is formed when two identical hemichannel proteins combine to create these channels.
- A heterotypic gap junction channel is formed when one homomeric hemichannel and one heteromeric hemichannel combine.
- A heterotypic gap junction channel is formed when two heteromeric hemichannels combine.
- A gap junction plaque is a macromolecular structure that contains hundreds of gap junction channels.
Occurence of Gap Junction
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Gap junction appear to be present in all animal organs and tissues, and it will be interesting to see if there are any exceptions, aside from cells that are not generally in communication with one another.
- A probable exception is adult skeletal muscle.
- Gap junctions, if present in skeletal muscle, might potentially propagate contractions in an arbitrary manner among muscle cells.
- This may not be the case in all circumstances, as evidenced by other muscle types with these junctions.
- Analysis of malignancies or the ageing process may provide insight into what happens when the junctions are reduced or absent.
Functions of Gap Junction
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Gap junctions can be thought of as a direct cell-to-cell conduit for electrical currents, tiny molecules, and ions at their most basic level. Controlling this communication has a variety of functions, which are discussed below.
- Gap junction provide metabolic and electrical coupling between cells.
- They help in exchange through hemichannels.
- It consists of tumour suppressor genes like Cx43, Cx32 and Cx36.
- The adhesive function is independent of the conductive junction channel, which acts as neural migration in the neocortex.
- Gap junctions play an important role in carboxyl-terminal in signalling cytoplasmic pathways (Cx43).
Other important functions of Gap Junctions are as follows:
Embryonic, Organ and Tissue Development
Gap junction communication's more subtle but no less crucial responsibilities were studied in the 1980s. The communication was discovered to be inhibited by injecting anti-connexin antibodies into embryonic cells.
Areas of Electrical Coupling
Gap junctions connect cells electrically and chemically throughout most animals' bodies. The electrical connection has the potential to act quickly.
- The functions of the tissues in this section have been observed to be coordinated via gap junctions, with inter-cellular signalling occurring in microseconds or less.
Retina
Neurons in the retina exhibit considerable connection, both within and between populations of the same cell type.
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Things to Remember
- Gap junctions are low-resistance ion channels that connect excitable cells (nerve and muscle) in solid tissues.
- It is a very important topic included in NCERT Class 11 Biology.
- They are first identified as low-resistance ion channels interconnecting excitable cells (nerve and muscle).
- The heart, eye lens, neurons and retina are areas of electrical coupling.
- It is used to distribute metabolic demands among cell groups.
- The junction acts as a buffer against spatial gradients of nutrients or signalling chemicals.
Sample Questions
Ques. What is a Cytoplasm? (3 marks)
Ans. Except for the cell nucleus, cytoplasm refers to any material within a eukaryotic cell that is surrounded by the cell membrane. The nucleoplasm is the substance found inside the nucleus and within the nuclear membrane. Cytosol (a gel-like fluid), organelles (the cell's internal sub-structures), and numerous cytoplasmic inclusions are the basic components of the cytoplasm. The cytoplasm is typically whitish and made up of roughly 80% water.
Ques.What is ATP? (2 marks)
Ans. Adenosine triphosphate (ATP) is a hydrotrope and an organic substance that supplies energy for various functions in living cells, including muscular contraction, nerve impulse propagation, condensate dissolving, and chemical synthesis. ATP is the "molecular unit of currency" of intracellular energy transfer and can be found in all known forms of life.
Ques. What is Plasmodesamata? (2 marks)
Ans. Plasmodesmata (singular: plasmodesma) are microscopic channels that run through the cell walls of plant and algae cells, allowing them to communicate and carry information. Plasmodesmata originated separately in various lineages, and members of the Charophyceae, Charales, Coleochaetales, and Phaeophyceae (algae) as well as all embryophytes (land plants) have these structures.
Ques. What is Homology? (2 marks)
Ans. Homology refers to the resemblance of two structures or genes from different taxa due to their shared ancestry. The forelimbs of vertebrates are a good example of homologous structures, as bats and birds' wings, primates' arms, whales' front flippers, and dogs' and crocodiles' forelegs are all derived from the same original tetrapod structure.
Ques. What is an Ephapse? (3 marks)
Ans. Ephaptic coupling is a type of nervous system communication that differs from direct communication methods like electrical and chemical synapses. It could refer to the coupling of nearby (touching) nerve fibres induced by ion exchange between cells, or it could refer to nerve fibre coupling caused by local electric fields.
- Ephaptic coupling can affect the synchronisation and timing of action potential firing in neurons in either circumstance.
- The interactions are considered to be inhibited by myelination.
Ques. What is Transmembrane Protein? (3 marks)
Ans. The integral membrane protein transmembrane protein (TP) crosses the entire cell membrane. Many transmembrane proteins act as portals, allowing some molecules to pass through the membrane.
- To transfer a material through the membrane, they often go through major conformational changes.
- They are often hydrophobic, aggregating and precipitating in water.
- They must be removed using detergents or nonpolar solvents, while some (beta-barrels) can be extracted with denaturing agents.
Ques. What are Ion Channels? (3 marks)
Ans. Ion channels are membrane proteins that produce pores that allow ions to pass through. Establishing a resting membrane potential, forming action potentials and other electrical signals by gating the flow of ions across the cell membrane, managing the flow of ions across secretory and epithelial cells, and regulating cell volume are all tasks of these cells. All cells have ion channels in their membranes. Ion channels are one of two types of ionophoric proteins; ion transporters are the other.
Ques. What are Connexins? (3 marks)
Ans. Gap junction proteins, or connexins (Cx) (TC# 1.A.24), are structurally similar transmembrane proteins that assemble to create vertebrate gap junctions. Invertebrates use a distinct class of proteins called innexins to generate gap junctions.
- Each gap junction is made up of two hemichannels, or connexons, that are made up of homo- or heterohexameric arrangements of connexins.
- The connexon in one plasma membrane docks end-to-end with a connexon in the membrane of a cell that is close to it.
Ques. What is a Protein Domain? (2 marks)
Ans. A protein domain is a self-stabilizing and fold-independent portion of the polypeptide chain of a protein. Each domain is a three-dimensional folded structure that is compact. Several domains make up many proteins. A single domain can be found in a wide range of proteins. Domains are used as building blocks in molecular evolution, and they can be recombined in many ways to produce proteins with various functions.
Ques. What are Skeletal Muscles? (2 marks)
Ans. Skeletal muscles (also known as muscles) are organs of the vertebrate muscular system that are primarily linked to the skeleton by tendons. Skeletal muscle cells are substantially longer than those found in other types of muscle tissue, and are referred to as muscle fibres. A skeletal muscle's muscle tissue is striated, giving it a striped look due to the arrangement of sarcomeres.
Ques. What are Pannexins? (2 marks)
Ans. Pannexins are a group of vertebrate proteins that share a lot of similarities with invertebrate innexins. While invertebrates have innexins that form gap junctions, pannexins have been discovered to primarily function as vast transmembrane channels that connect the intracellular and extracellular spaces, allowing ions and tiny molecules to move between them (such as ATP and sulforhodamine B).
Ques. What are the three types of cell junctions? (3 marks)
Ans. The three types of cell junctions are as follows:
- Tight Junctions: It is the outermost layer of cell junctions where adjacent cells fuse together and act as a barrier between epithelial cells.
- Gap Junctions: Gap junctions are different types of intercellular channels that allow direct communication among cells. They are basically found in nerve cells, hepatocytes, etc.
- Anchoring Junctions: Anchoring junctions are junctions that are connected to one another and form a connection with the extracellular matrix.
Ques. How gap junction help in human mutations? (4 marks)
Ans. Given the extensive evolutionary history of gap junctions in metazoans, it is not unexpected that numerous cell types have adapted them to carry out a variety of physiological functions. Numerous cell- and tissue-specific functions of GJIC have been identified by human mutations and targeted connexin gene deletion in mice.
- Mutations in Cx32 result in X-linked neuropathy, and mutations in Cx47 cause Pelizaeus-Merzbacher-Like-illness, a central demyelinating illness in humans.
- An often occurring peripheral demyelination neuropathy is Charcot-Marie-Tooth syndrome.
- Over half of all cases of profound hereditary deafness are caused by mutations in Cx26; these diseases are often syndromic and involve skin problems.
- Familial cataracts are often associated with mutations in the ocular lens-specific genes Cx46 or Cx50, whose expression is largely localized to the lens of the eye.
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