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LET'S TALK LIFE-SCIENCE CELL BIOLOGY

Suraj Prakash Sharma | Ekta Chotia

CELL SIGNALLING
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4.5.         Intracellular Hormone Receptors

Steroid and thyroid hormone family of receptors works as transcription factors as after hormones binding they activates the gene expression. The steroid-thyroid hormone receptor superfamily [e.g. glucocorticoid (GR), vitamin D (VDR), retinoic acid (RAR) and thyroid hormone TR) receptors] Their receptor is located in cytoplasm and bind their lipophilic hormone ligands in this compartment as these hormones are capable of freely penetrating the hydrophobic plasma membrane. Upon binding ligand the hormone-receptor complex translocates to the nucleus and binds to specific DNA sequences termed hormone response elements (HREs). The binding of the complex to an HRE results in altered transcription rates of the associated gene. Analysis of the human genome has revealed 48 nuclear receptor genes.

Many of these genes are capable of yielding more than one receptor isoform. The nuclear receptors all contain a ligand-binding domain (LBD) and a DNA-binding domain (DBD). Steroid receptor III bind to DNA as homodimers  eg estrogen receptor (ER), mineralocorticoid receptor (MR), progesterone receptor (PR), androgen receptor (AR), and the glucocorticoid receptor (GR). Steroid receptor I binds to DNA as heterodimers. The retinoid X receptors (RXRs), the liver X receptors (LXRs), the farnesoid X receptors (FXRs) and the peroxisome proliferator-activated receptors (PPARs) are the example of the receptor that bind with lipophilic ligands  just like steroid hormone receptor and thyroid hormone receptors.

The steroid hormones are all derived from cholesterol. Moreover, with the exception of vitamin D, they all contain the same cyclopentanophenanthrene ring and atomic numbering system as cholesterol. Steroids with 21 carbon atoms are known as pregnanes, whereas those containing 19 and 18 carbon atoms are known as androstanes and estranes, respectively. Retinoic acid and vitamin D are not derived from pregnenolone, but from vitamin A and cholesterol respectively remaining all are steroid hormones are derived from pregneolone.

All the steroid hormones exert their action by passing through the plasma membrane and binding to intracellular receptors. The hormone – receptor complex work as transcription factor. The complex moves to nucleus binds to its DNA sequences known as hormone response elements and activates the genes.

4.6.         Two component system :

In bacteria and plant, signal transduction mediates by two component system (TCS), involve in cell-cell communication and to respond extracellular signal. In bacteria two component systems is ubiquitous. TCS is not present in human and other mammals thus become target for drug.

Two component system contain a sensor, which is a homodimeric transmembrane protein called Histidine kinase placed, which is having autophosphorylating activity along with a conserved histidine residue and a response regulator located after histidine kinase, which contains a conserve aspartate residue. Histidine kinase (HK) has two domain, one histidine phospho transfer domain, which possesses specific histidine and second ATP binding domain. Response regulator (RR) also had two domain, one conserved receiver domain, which comprises conserved aspartate and second effector domain.

When a ligand comes and binds to the N terminal of histidine kinase, in turn causes the activation of histidine kinase autophosphorylating activity. As a result, it causes transfer of a phosphate residue from ATP to the conserved histidine present in kinase domain present at C terminal. This leads to the transfer of this phosphate from the histidine to conserve aspartate present in present in conserved receiver domain of response regulator. Phosphorylation of aspartate result in conformational change in RR, in turn causes the activation of effector domain of RR, as a result signal get generated to mediate cellular response specifically off or on gene expression.

Histidine kinase also present in hybrid form called hybrid histidine kinase, which histidine kinase also contain one internal receiver domain, as ligand bind to the hybrid histidine kinase, it autophosphorylates itself of histidine by same mechanism. Then transfer this phosphate to internal receiver domain’s aspartate residue ,after that this phosphate transfer to histidine phosphotransfer protein or histidine phosphotransferase, which transfer this phosphate to terminal response regulater containing conserved aspartate residue. This system is called as phosphorelay system.

4.7.         Quorum sensing

Quorum sensing defines as a mechanism through which regulation of physiological process (motility, competence, conjugation, symbiosis, virulence, sporulation and antibiotic production) and cooperative activity takes place in bacteria because it control gene expression. Through this mechanism, communication between bacterial cells occurs by sensing and responding a secreted small low molecular weight signal molecule, which is diffusible in nature and known as autoinducer, which concentration define the bacterial cells density, because both had directly proportional correlation. This mechanism is help bacteria to carry out various function like, allow bacterial cells to identify their population density, in formation of biofilms, in colonization of bacteria, during protection against competitors and provide ability to adapt changing environment. Vibrio fischeri, a marine bioluminescent, is the first one into which quorum sensing gets described.

Quorum sensing responsible for initiation of coordinated activity governing gene’s expression, which is done when those gene expression governing transcriptional activator or sensor interact with its respective autoinducer, due to this signalling autoinducer also induce its own gene expression. Quorum sensing carried out in response to the bacterial population density and change according to the fluctuation takes place in bacterial population, in turn change the coordinated activity governing gene’s expression also takes place because in this situation interaction of gene expression governing transcriptional activator or sensor with its autoinducer also change with respect to situation. Alteration in gene expression takes place when autoinducer concentration is detected as minimal threshold stimulatory concentration level. Quorum sensing mechanism is used by both gram negative and gram positive bacteria.

In bacteria three quorum sensing classes present which are mentioned below:

First class is governed by LuxI/LuxR system which possesses acyl-homoserine lactone (AHL) as their signal molecule and this type of quorum sensing present in Gram-negative bacteria. LuxI like protein called ALH synthase responsible for the synthesis of acyl-homoserine lactone (AHL), AHL is formed by the coupling of homocystein moiety of S-adenosylmetionine (SAM) to a specific acyl-acyl carrier protein (acyl-ACP), in this coupling homocystein moiety joins with acyl side chain of acyl-ACP and lactonization of this intermediate result in the formation of acyl-HSL along with release of methylthioadenosine. Unique AHL is produced by each bacterial species as a result of a particular bacterial species member respond and recognise a specific signal molecule. After synthesis it get diffused and get recognised and binded by a cognate LuxR protein, in turn activation of LuxR occur then the complex of AHL-LuxR binds to the promoter of the target gene and transcription of that gene get starts.

This is the diagram of quorum sensing in Gram-negative bacteria, define transcriptional activation require the particular threshold concentration to activate the transcription of gene, below that concentration not any kind of transcription takes place.

Second class governs oligopeptide mediated two component system which possesses small peptide as their signal molecule and this type of quorum sensing present in Gram-positive bacteria. In Gram-positive bacteria autoinducer is not able to cross the plasma membrane and the sensor or receptor of this inducer called autoinducing peptide (AIP- 5to 25 amino acid) are transmembrane protein, here two-component signal transduction system are present which contain  receptor of AIP is called histidine kinase protein along with a cytoplasmic response regulator which proceed the signal transduction by mediate the regulation of gene expression via peptide signalling. AIP get secreted into external environment form interior of the cell by ABC transpoter.

Third class governs by luxS encoded autoinducer 2 and this type of quorum sensing present in Gram-negative as well as Gram-positive bacteria.

Now LET'S TALK about the example of Vibrio fischeri, a marine bioluminescent. Vibrio fischeri reside in symbiotic relationship with a number of marine animal host. Vibrio fischeri produces light by the production of luciferase enzyme. Thus called bioluminescent and bacteria produce luminescence which is blue-green light, when bacteria is present in large concentration in response to AHLs quorum sensing. Light production takes place in specialized organ present in marine organism called light organ when bacteria get colonized in high concentration in this light organ but Vibrio fischeri does not produce luminescence when present in free state and this luminesence appears in dark.

Chemotaxis in bacteria

Chemotexis is a phenomenon which explains the movement of bacteria in response to chemical stimulus, in the specific direction. Chemotaxis plays an important role in bacteria’s flagella movement, searching of food and in case of protection like feel for poisons. If movement takes place towards higher concentration of chemical, it called as positive chemotaxis, as reverse, If movement takes place in opposite direction from the higher concentration of chemical, it called as negative chemotaxis. Chemotaxis inducer in motile cell called chemoattractant (chemokines and formyl peptides) and chemorepellent (amino acid, inorganic salts and some chemokines), if chemoattractant is presents cell moves in forward direction and if chemorepellent present then cell moves in opposite direction or away from the chemical. Both chemical perform their signalling by interact with its receptor, which is a transmembrane protein. Chemotaxis performs by two component system, which contains histidine kinase protein as transmembrane receptor along with a cytoplasmic response regulator which proceed the signal transduction by mediate the regulation of gene expression in response to particular chemical.

Flagellar rotation in E.coli governed by chemotaxis and movement of flagella correlated with the swimming behaviour of bacteria, during counter-clockwise flagellar rotation, bacteria move forward direction which is also called run along with this bacteria swim in straight line, this type of movement get achieved because counter-clockwise rotation causes alignment of flagella into a single rotating bundle. During clockwise flagellar rotation, bacteria movement in forward direction get cease along with this bacterium get tumble in place. This type of movement takes place because clockwise rotation breaks the flagella bundle separately, here each flagellum points in separate direction. If chemical gradient is not present, the movement of bacteria is random, in this case bacteria moves forward /run. Thus swims and after some time gets stop, thus gets tumble. If chemical gradient is present, in case of presence of chemoattractant tumble is less frequent and longer run occur or in case of presence of chemorepellent, longer run occur in opposite direction along with less tumble.

Flagellar movement is occured by two component system as mentioned above, here the receptor is known as Methyl-accepting Chemotaxis protein (MCP) and methylation of receptor done by a methyltransferase name CheR, CheW a adaptor protein binds to receptor in one side and bind to CheA to other side , thus linking the CheA with a sensor protein. CheA a sensor histidine kinase possess a conserve histidine residue. When a chemorepellent comes and binds to the MCP in turn activate the MCP, which activate the CheW and which activate the CheA in cascade manner, activated CheA cause autophosphorylation of its own conserve histidine residue and after that CheA transfer it phosphate to CheY, which is a response regulator and possess a conserve aspartate residue, as a result diffusion of ChsY takes place and it interacts with flagellar switch protein FliM or flagellar motor protein, this leads to the change of flagellum rotation from counter-clockwise to clockwise manner.

CheY is responsible for the control of flageller motor. As the change in rotation of single flagellum occurs, it causes disruption of entire flagella bundle, which result in tumble. Phosphorylation state of CheY persists for few sec, and CheY dephosphorylate by CheZ, which is responsible for signal termination and known as Asp specific phosphatise. Inactivation of CheY done by CheZ. Binding of attractant exert opposite effect, it cause inactivation of receptor, in turn phosphorylation of CheA and CheY get decreased, as a result counter clock wise rotation of flagella occurs thus bacteria runs and swims in forward direction. Bacteria get desensitized if higher concentration of ligand is present and which is more than the usual higher concentration.

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