INTRODUCTION:

Cellular Communication:

The word “cellular” is used in biology, biophysics and biochemistry to classify different types of communication methods among the living cells.This process allows millions of cells to communicate and work together and to perform important process that are necessary to survival. Both multicellular and unicellular organisms greatly rely on cell to cell communication. [1]

Three Stages of Cell Communication:

The communications between cells are various three stages as follows:

1. Reception

2. Transduction

3. Response

1. Reception:

Reception is the process when the target cell receives a signal generally from the molecule called ligand for binding to receptor proteins. The ligand receptor complex then capable to transmit the received signals and the conformational change takes place. [2]

2. Transduction:

The specific cellular responses are the results of the transformed signal generally transduction requires a change in a sequence of similar molecules is called a signal transduction pathway but at a time can occur in a particular step.The molecules that are compose these pathway are called as relay molecules. These are multi step process of the transduction stage often composed of the activation of proteins by phosphorylation or removal of phosphate groups to release of ligand molecules that can act as messengers.

3. Response:

This result of transduced signal in the ultimate stage of cell signaling.The response can be any cellular activity i.e. present in the body. It can stimulate the move of the cytoskeleton or even as catalysis via an enzyme. These are three steps involved in cell signaling and ensure that the right cell behaving told the right time and in organization with other cells. Finally, the signal pathway leads to the regulation of a cellular activity. A majority of signal pathway control protein synthesis via certain gene in the nucleus. [3]

Figure 1: Diagrammatic Representation of Cell Communication.

Stimulus.

A stimulus is a demonstrable change in the in the internal and external environment. The skill of an organism or organ to react to external stimuli is called sensitivity. When a stimulus id applied to a sensory receptors. [4]

Ligands:

A ligand is a chemical substance that a complex with a bio-molecule to apply biological purpose. which produces a signal via binding to a site on a target protein and results in a change of conformation of the target protein. [5] They are binds to the receptors altered the conformation in acting the three dimensional direction. The conformation of a receptor protein composes the functional conditions. Ligands are includes substrate, inhibitor, activator, neurotransmitter, the rate of binding is called affinity.

The majority of signal transduction pathways are involved in binding of signal molecule is known as ligands to receptors that trigger occur inside the cell.

(i) Endogenous- Hormones, NTS (Neurotransmitters)

(ii) Exogenous- Drug outside the body. [6]

Neurotransmitters:

An extracellular signaling molecule that is released by the pre-synaptic neuron at a chemical synapse and relays the signal to the postsynaptic cell. The response elicited by a neurotransmitter, either excitatory or inhibitory is determined by its receptor on the postsynaptic cell.Neurotransmitter is also known as Chemical messenger. These are endogenous chemical that allow to neurotransmission. They convey signal across a synapse such as neuromuscular junction from one neuron to another target neuron, neuron cell.[7] These neurotransmitters released from synaptic vesicle in synapse into the synaptic cleft. Where they are received via receptor on the target cell. [8]

History of receptors:

In 1901, John Langley challenged that drug act at nerve endings via demonstrating at sympathetic ganglia so far after the degeneration of the severed pre-ganglionic nerve endings. [9] In 1905 he introduced the concept of a accessible substance on the surface of skeletal muscle that mediated the action of a drug. It also postulated that these accessible substances were different in different species. Ehrlich was trying to understand the basis of selectivity of Receptors. [10]

Theories of drug-receptor interaction:

1. Occupation Theory

2. Rate Theory

3. Induced Fit Theory

4. Macromolecular perturbation Theory

5. Activation-aggregation Theory

1. Occupation Theory:

The receptor occupation theory was introduced by A. J. Clark (1926) and had been elaborated upon by E. J. Ariens and his co-workers. This theory is based on law of mass action for the reaction of receptor sites with drug molecules. [11] The theory states that, the intensity of the pharmacological effect is directly proportional to the number of receptors occupied by the drug. The theory was then modified to account for partial agonist that the drug-receptor interactions involve two stages; firstly there is a complexation of the drug with the receptor which they both termed as the affinity and secondly; there is an initiation of a biological effect which termed as the intrinsic activity or efficacy. Here, the term affinity refers to the measure of a capacity of a drug to bind to the receptor, and is dependent on the molecular complementary of the drug and receptor, where as the efficacy refers to the property of a compound that produces the maximum biological response. [12]

2. Rate Theory:

The intensity of the response of agonist or stimulant activity is proportional to the Rate of drug-receptor combination rather than the number of occupied receptors. Agonist activity is the result of a series of rapid association and dissociation of the drug and the receptor An antagonist has a high association rate but a low rate of dissociation. [13]

3. Induced Fit Theory:

The induced fit theory was proposed in terms of precise orientation of catalytic groups is required for enzymatic action with the substrate and causes an significant change in the three-dimensional relationship of the amino acids at the active site, and the changes in the protein structure caused by the substrate will bring the catalytic groups into the proper configuration whereas a nonsubstrate will not causes changes in protein structure.[14] Substrate or drug binding to the receptor induces 3 dimensional conformational changes in the macromolecule positioning catalytic groups in the correct position to conduct productive chemistry or altering membrane behavior. [15]

4. Macromolecular perturbation Theory:

This theory recommended that interaction of a drug with a receptor,leads to two common types of macromolecular perturbations.It can be specific conformational perturbation, makes possible the binding of certain molecules that produce biological response (an agonist) and Nonspecific conformational perturbation accommodates other types of molecules that do not elicit a response (an antagonist). If the drug contributes to both macromolecular perturbation a mixture of two complexes will result (partial agonist) This theory does not deal with the concept of inverse agonist.

5. Activation-aggregation Theory:

A receptor is in state of dynamic equilibrium between an activated form (R₀), which is responsible for the biological response and an inactive form (T₀) which is not responsible for the biological response. Agonist binds to an activated form and shifts the equilibrium state.Antagonists bind to the inactive form (T₀), and partial agonist bind to both conformations. In this model the agonist binding site in the activated form conformation can be different from the antagonist binding site in the inactive form conformation. This theory however does not deal with inverse agonists.The Two-State (Multistate) Model of Receptor Activation. This model proposes that the absence of a natural ligand or agonist, the receptor exist in equilibrium between active state and resting state. Full agonist binds to the active state and alters the equilibrium to the active state causing maximum response; Partial agonists preferentially bind to the active state, but not to the extent that as full agonist does, so maximum response is not attained. Full inverse agonists bind to the resting state and alter the equilibrium fully to the resting state, causing a negative efficacy an antagonist have equal affinities for both states.[16]

Types of Receptors:

1. Ligand-Gated Ion Channel:

Ligand-gated ion channels (LICs), are also referred as an ionotropic receptors. They are a group of transmembrane ion channel proteins which open to allow ions such as Na⁺, K⁺, Ca²⁺, and Cl⁻ to go through the membrane in response to the binding of a chemical messenger such as a neurotransmitter.[17]These proteins are consist of atleast two different domains a transmembrane domain which includes the ion pore and an extracellular domain which includes the ligand binding location (an allosteric binding site). This modularly has enabled a part and overcome advance to finding the structure of the proteins.The function of such a receptors located at synapses is to altered the chemical signal of presynaptic released neurotransmitter directly and very rapidly into a presynaptic electrical signals. Many ligands gated Ions Channels are also modulated by allosteric ligands via Channel blocker. Ligand gated Ions Channals are classified into three Channels which lack evolutionary correlation: cys-loop receptors, Ionotropic glutamate receptor and ATP gated channel. The receptors are subdivided with respect to the type of ion that they behavior (anionic or cationic) and further into defined by the endogenous ligand. They are generally pentameric with 2α, β, γ, δ subunit containing four transmembrane helices constituting the transmembrane domain.[18]

Figure 2:- Ligand gated ion channel.

2. G protein–coupled receptors (GPCRs):

In 2012 Nobel prize in chemistry was awarded to Brian Kobilka and Robert Lefkowitz for their work that was “crucial for understanding how G-protein coupled receptor function”[19] G protein coupled receptors (GPCRs) are also known as seven-transmembrane domain receptors (7TM), heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR). It is compose a proteineous family of receptors to identify molecule outside the cell and active internal signal transduction pathway and produce a cellular response and coupling with G protein. It is called as 7 transmembrane receptors, because they pass through the cell membrane seven times. [20] The ligands that bind and activate these receptors including light sensitive compounds, hormones, and neurotransmitter. GPCR are involved in many diseases and are also the target of approximately 40% of all modern medicinal drugs. [21]

There are two principal signal transduction pathways involving the G protein–coupled receptors:

· The cAMP signal pathway and

· The phosphatidylinositol signal pathway. [22]

When a ligand binds to the GPCR thereby causes a conformational change in the GPCR, which allows acting as a guanine nucleotide exchange factor (GEF). The GPCR can be activates an associated G protein by exchanging the GDP bound to GTP. The G protein's α subunit, together with the bound GTP, can then dissociate from the β and γ subunits to further affect intracellular signaling proteins or target functional proteins directly depending on the α subunit.[23]

cAMP signal pathway:

Cyclic adenosine 3′, 5′-monophosphate (cAMP) is the second messenger to be identified and acting fundamental roles in cellular responses to many hormones and neurotransmitters.The cAMP signal transduction their is contains major characters:

· stimulative hormone receptor (Rs) or inhibitory hormones receptors (Ri)

· stimulative regulative G protein (Gs) or inhibitory regulative G protein (Gi)

· Adenylyl cyclase (AC)

· Protein kinase A (PKA)

Stimulative hormone receptor (Rs) is a receptor which can binds with stimulative signal molecules and while inhibitory hormone receptor (Ri) is a receptor which can binds with inhibitory signal molecules. Stimulative regulative G-protein is a G-protein linked to stimulative hormone receptor (Rs), and it’s α subunit further on activation could stimulate the activity of an enzyme not at all inhibitory regulative G-protein is linked to an inhibitory hormone receptor and its α subunit further on activation could inhibit the activity of an enzyme. Adenylyl cyclase is a 12-transmembrane glycoprotein that catalyzes ATP to form cAMP with the help of cofactor Mg²⁺ or Mn²⁺.

Figure 3: Cyclic adenosine 3′, 5′-monophosphate (cAMP) Signaling Pathway.

The cAMP produced is a second messenger in cellular metabolism and is an allosteric activator of protein kinase A. The Protein kinase A is an important enzyme in cell metabolism due to its ability to regulate cell metabolism by phosphorylation with specific enzymes in the metabolic pathway. It can also regulate specific gene expression, cellular secretion, and membrane permeability. When cAMP binds to the regulatory subunits, their conformation is changed their causing the dissociation of the regulatory subunits, which activates protein kinase A and allows further biological effects or activities. These signals then can be terminated by cAMP phosphodiesterase, which is an enzyme that degrades cAMP to 5'-AMP and inactivates protein kinase A. [24]

Phosphatidyl inositol signal pathway:

In the Phosphatidyl inositol signal pathway, the extracellular signal molecule binds with the Gprotein receptors (Gq) on the cell surface and activate phospholipase C. which is located on the plasma membrane. The lipase hydrolyzes Phosphatidyl inositol 4, 5-bisphosphate(PIP₂) into two second messengers:

· Inositol 1,4,5-triphosphate(IP₃) and

· Diacylglycerol (DAG).

· Inositol 1,4,5 triphosphate and Diacylglycerol

Inositol 1,4,5 triphosphate are also known as IP₃together with DAG, is a secondary messenger molecule used in signal transduction and lipid signaling in biological cell. It is made by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂) a phospholipid that is located in the plasma membrane, by phospholipase C (PLC).[25] IP3 binds with the IP3 receptor in the membrane of the smooth endoplasmic reticulum and mitochondria due to open Ca²⁺ channels then DAG through activate the protein kinase C (PKC), which phosphorylates or many other proteins, changed their catalytic activities and leading to cellular responses. They things of Ca²⁺ are also significant with DAG in activating PKC and they can activate the CAM (calcium-modulated protein calmodulin) kinase pathway. In which CaM binds Ca²⁺undergoes change in conformation and activates the CaM kinase II. Which has single ability to increases its binding affinity to CAM by auto phosphorylation process? The kinase then phosphorylates target enzymes and regulating their activities and contraction. [24]

Figure 4:- Phospholipase-c(PLC) signaling pathway.

3. Tyrosin Kinase:

Tyrosine phosphorylation is a extremely regulated that essential for inter or intracellular communication in cell. The enzymes that catalyze phosphoryl transfer to tyrosine residues in protein substrates by using ATP as a phosphate donor they are the protein tyrosine kinases receptor(RTKs).[26] The RTKs family includes among others epidermal growth factor receptor (EGFR), fibroblast growth factor receptors (FGFRs), vascular endothelial growth factor receptors, and the insulin receptor. RTKs are essential components of cellular signaling pathways that are active during embryonic development and adult homeostasis. Because of their roles as growth factor receptors many RTKs have been implicated in the onset or progression of various cancers, either through receptor gain-of-function mutations or through receptor/ligand over expression.[27] RTKs are single pass type I receptors resident in the plasma membrane. Generally RTKs are activated through ligand-induced oligomerization typically dimerization, which juxtaposes the cytoplasmic tyrosine kinase domains. [28]

These are 2 major pathways of the Receptor Tyrosine Kinases (RTKs)

· RAS/RAF/MEK/ERK pathway

· JAK/STAT signaling pathway

RAS/RAF/MEK/ERK pathway:

The MAPK/ERK pathway are also known as the Ras-Raf-MEK-ERK pathway this is a chain of proteins in the cell that the communicates a signal to a receptor on the surface of the cell membrane to the DNA in the nucleus of the cell.The signal starts when a signaling molecule binds to the receptor on the cell surface and ends when the DNA in the nucleus expresses a protein and produced some change in the cell, such as cell division. The pathway includes many proteins, including “mitogen activated protein kinase (MAPK)also calledextracellular signal regulating kinase(ERK)” which communicate by accumulation of phosphate groups to a neighboring protein, which acts such as an "on" or "off" switch.[29]

Figure 5: The Organization and Function of the RAS- RAF- MEPK Pathway

The Ras/Raf/MEK/ERK cascade couples signals beginning cell surface receptors to transcription factors, which is regulate gene expression.[30]A increase in the complication of this pathway as there are multiple members of the kinase, transcription factor, apoptotic regulator families, which can be activated by protein phosphorylation. At also this pathway can induce the transcription of certain genes. RAF, either from downstream MEK and ERK it can induce the phosphorylation of proteins which control apoptosis. In additional signal transduction pathways interact with the Raf/MEK/ERK pathway to positively or negatively regulate its activity. [31]

JAK/STAT signaling pathway:

The Janus kinase or signal transducers and activators of transcription (JAK/STAT) pathway is one of the handful of pleiotropic cascades used to transduce a multitude of signals for development and homeostasis. The JAK/STAT pathway is the principal signaling mechanism for a wide display of cytokines and growth factors the approaches the receptor tyrosine kinase. Upon associate of ligand with receptor develop a conformational change that is dimerization. The activation stimulates cell proliferation, differentiation, [32]Intracellular activation occurs when ligand binding induces the process of multi dimerization of receptor subunits.Signal propagation through either homodimers or heterodimers now become activated by phosphorylation. These phosphorylation tyrosine kinase attract JAK to get attach at catalytic site and JAK now phosphorylation. The phosphorylation JAK complex and STAT to associated to it phosphorylated attached with JAK and become JAK-STAT complex and their again phosphorylation. STATs are latent transcription factors that reside in the cytoplasm until activated in the nucleus, dimerized STATs bind specific regulatory sequences to activate and transcription of target genes.[33]

Figure 6: JAK STAT Signaling Pathway.

4. Nuclear Receptor:

The molecular mechanisms of action and the role of nuclear transcription factors in their functional environment, the cell nucleus have been intensively studied and partially release. This holds true in particular for steroid and thyroid hormone receptors a major class of the super family of nuclear receptors, representing ligand activated transcription factors, involved in the regulation of metabolic, growth, developmental and immune processes.[34]

In the ligand unbound, non-activated state, these receptors are components of a macromolecular complex with heat shock and other proteins.[35].

Activated by extracellular or intracellular regulatory molecules involved in cell growth, differentiation survival and apoptosis. Until recently, research on how the nuclear receptors and the other transcription factors exert their physiological functions was centered mainly on their action within the nuclear environment and their interaction with nuclear genes. [36]

Figure7:Signal transduction mechanism of Nuclear Receptor.

CONCLUSION:

Most of the drugs produce their effects by binding to protein molecules. Drug binding often leads to a conformational change in the protein. Four primary drug targets are ion channels, enzymes, carrier molecules and receptors. The drug binds to such molecules altering their transport ability there by permeation of ions between either sides of the membrane. Agonist is a drug that has affinity as well as intrinsic activity which combines with receptor and elicits a response. Here, affinity refers to a measure of how tightly a drug binds to the receptor. Antagonist is one that combines with a receptor without producing responses that, it blocks the action of the agonist (has affinity but no or zero intrinsic activity). The Partial agonist combines with its receptor and evokes a response as a full agonist but produces sub-maximal effect regardless of concentration (affinity and partial efficacy). Ion channels are protein molecules that span the cell membrane, and can switch between open and closes states, allowing the controlled entry or exit of specific ions across the cell membrane in their open state. Ion channels are highly selective for the ions that they control. Similarly other types of receptors are eliciting their pharmacological response after binding with their specific ligands. These receptors function as an enzyme that transfers phosphate groups from ATP to tyrosine residues on intracellular target proteins which is act as an off-on switch there by catalytic response if evoked.

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Received on 28.10.2017 Modified on 11.01.2018

Res. J. Pharmacology and Pharmacodynamics.2018; 10(1): 17-23.

DOI: 10.5958/2321-5836.2018.00004.6