Cell Communication- Introduction

A cell always needs a command from its surrounding inorder to maintain its structure and function. So, either cells or proteins present in the extracellular region that can bind specifically to a receptor are called ligand. A ligand can either be a small peptide or large globular protein molecule.  Hormones which are produced from endocrine gland act as ligand and it is also termed to be first messengers, because they basically trigger the signaling cascade inside the cell through a receiver protein molecule called receptor.

 Receptor is basically found attached to the cell membrane and it gets activated only when a matched ligand binds to it. First messengers cannot enter the cell membrane; hence they have to bind to the receptor molecule Inorder to activate the signaling cascade. Certain ligands like steroid hormones need not be requiring receptor for mediating the signaling cascade; instead they directly pass through the cell membrane and trigger the target molecules such as transcription factors, or nuclear receptor.

So, the ligand binds to the receptor of the cell and then what happens?

The activated receptor will activate the downstream molecules like protein kinase enzyme and then thereby leads to the release of second messengers inside the cell. The second messenger will further activate other enzyme, which modulates the ion channels and transcriptional activities. 

Cell signaling versus signal transduction

Cell signalling is regarding the generation and transmission of a signal, the reception of the signal and the propagation of that signal within the receiving cell.

Signal transduction is the process whereby an extracellular signal is converted into a different form of intracellular signal.

Secretions by glands

Glands can be divided into two types: exocrine gland and endocrine gland. Exocrine glands secretes either in the gastrointestinal tract or into the surface of the body. Its secretion is not involved in the signalling, but only play general role in digestive or innate immune system.

Endocrine glands secretes informational molecules into the blood stream and not all first messenger travel through blood, instead some can act locally or even self- stimulate via autocrine secretion.

The four crines

Autocrine

Cell produces a hormone for which it has a specific receptor on itself. Cell becomes self stimulating. For example breast cancer cells produce TGFα which causes uncontrolled growth via interaction with the EGF receptor.

Paracrine

Hormones are secreted locally and the producing cells have no receptors. They will diffuse through the interstitial fluid to nearby target cells. For example in breast cancer cells the PDGF produced will stimulate the nearby fibroblasts which possess the PDGF receptors.

Juxtacrine

Membrane bound hormone precursor interacts with receptor on the adjacent cell.

Intracrine

Hormone receptor complex forms inside cell from newly synthesised proteins.

Endocrine

The signaling molecule act on target cells distant from their site of synthesis by cells of the various endocrine organs. Blood carries the hormone from the glands to the target cell.

“Cell signalling means cell to cell or within cell communication”

In this communication, the following steps are involved à

1. Synthesis
2. Release of the signalling molecule by the signalling cell
3. Transport of the signal to the target cell
4. Binding of the signal by a specific receptor protein leading to its activation
5. Initiation of one or more intracellular signal-transduction pathways by the activated receptor
6. Specific changes in cellular function, metabolism, or development
7. Removal of the signal – termination of the cellular response