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