Table of Contents
Ways of Communication in Cell Signalling
Intercellular communication through gap junctions
- Cytoplasmic communication between two cells, without involvement of an extracellular fluid. The connexin proteins form a cylindrical structure between the lateral surfaces of two adjacent cells. This cylindrical structure with pore is called connexon. Connexon can open or close in response to a certain signal.
Receptor-ligand complex formation leads to an intracellular response.
Cell membrane has specific proteins present on its surface. These proteins have a three dimensional structure which gives them a unique binding site. They are known as cell surface receptors and are only complementary in shape to certain molecules called ligand. When ligand binds to its cell surface protein, a response is produced.
Modes of Communication Between Cells
Cells adopt one of the following modes of communication with each other:
- Endocrine signals are released by the endocrine glands, which release the chemical substances called hormones. Hormones travel through the blood to distant target organs with specific receptors for that particular hormone. Pancreas, an endocrine gland, releases the hormone insulin, which travels through blood and finds its receptors on specific cells such as skeletal muscle cells or liver cells.
- Paracrine signals are given off by one cell to its surrounding cells. For example, the neurons release neurotransmitters, which act on the neighbouring neurons. Signal is therefore transmitted from one neuron to another.
Autocrine signals are produced by the target cells and act on the cell itself through cell surface receptors. An example is the B lymphocyte, which sends signals to itself for cellular proliferation.
Intracrine signalling refers to the production of signals which act intracellularly. For example, fibroblast growth factor acts on the same cell, without being released as in the case of autocrine signal.
Juxtacrine signals, also called contact dependent signals, are the ones transmitted from one cell to its adjacent cell. Immune cells are known to observe juxtacrine mechanism of signal transmission.
Types of Receptors
There are different types of receptors, which produce a specific type of response. They are broadly classified into the following types:
G-protein-coupled receptors activate G protein when the ligand binds to the receptor. G protein in return activates an effector protein to generate a second messenger inside the cell as shown in the figure beside. Examples include epinephrine, glucagon, and insulin.
Receptors with an intrinsic enzymatic activity are receptors which are activated when a specific ligand binds to it. For example, tyrosine-kinase receptors have catalytic ability to phosphorylate other proteins. They gain their catalytic activity when a ligand binds to them. Janus kinase (JAK) is a family of tyrosine-kinase.
Ion channel linked protein receptors are linked to channel proteins on the surface of cell membrane. When a ligand binds to the receptor, it causes a conformational change in the structure of the channel protein. This opens the gate of the channel protein and allows ions to diffuse in or out of the cell.
Cytosolic receptors are present inside the cell. Lipid soluble molecules diffuse into the cell and bind with receptors in the cytoplasm. They then move inside the nucleus and initiate transcription of proteins. The new proteins formed alter the cell function.
Modification of Receptors
Receptors may undergo following modifications:
- Up-regulation is referred to an increase in the number of specific receptors on the surface of the surface membrane. Down-regulation is when the number of receptors decreases.
- Internalization is the process in which the receptors undergo endocytosis while externalization is when they are expressed on cell surface membrane.
Desensitization of receptors occurs when they undergo a conformational change in their structure and therefore are unable to produce a response. Sensitization occurs when receptors produce a response after the formation of a receptor-ligand complex.
Steps of Cell Signalling
Cell signalling comprises of three stages:
Reception: when the target cell receives the signalling molecule.
Transduction: consists of a cascade of interactions between the receptor and target molecule.
Response: the required outcome for example transcription of proteins.