Cells can detect as well as respond in real time what is going around them. All types of cells may it be in unicellular or multicellular organisms send and receive a wide array of messages in chemical form. Examining the core principles of cell communication will start by understanding how cell to cell signalling operates, followed by the different types of short and long range communication that occur in different cells. Typically, cells communicate through chemical signals. These signals, mostly protein in nature, are often secreted by a sender cell are introduced to the extracellular space after which they flow to the neighbouring cells. To be precise, not all cells have the capability to receive a particular message. To recieve a chemical message, the target cell must contain the appropriate receptor for that signal. The movement and shape of the receptor immediately changes when a signalling molecule attaches to it, therefore, causing changes inside the cell. This action results in the intercellular signal converting to an intracellular signal that generates a response. The signalling molecules referred to as ligands are molecules whose primary purpose is to attach themselves to other molecules, in this case, the receptors.
Unicellular organisms also communicate through chemical signals. Quorum sensing is the method used by single cell organisms such as bacteria and yeast. Here, the unicellular organism monitors the population density based on the signals. When the signal approaches a certain level, all the cells in their entirety alter their performance and gene expression thus indicating communication. The purpose of this essay is to look into the categories of chemical signalling in multicellular organisms, signalling through G protein coupled receptors while also explaining inter-kingdom signalling between bacteria and host, cell to cell signalling of Pseudomonas aeruginosa, cell signalling by receptor Tyrosine Kinases, utilising primary literature from scholarly journals.
Communication between cells in multicellular organisms
Paracrine signalling is a form of signalling where cells achieve communication over relatively short distances. Regularly, cells that are close to one another communicate by the use of ligands that penetrate through the intercellular spaces. This type of signalling permits local coordination of activities between neighbouring cells (Welle, 1999, p. 124). Paracrine signals are particularly important during cell differentiation, where they facilitate communication in cellular identity although also used in many various contexts. Synapse signalling is a unique illustration of paracrine signalling whereby signals pass through the nerve cells. This type of communication occurs in the synapse which is the area joining two nerve cells.
Another form of signalling is known as autocrine signalling. The cell signals to itself causing it to release a ligand that attracts its receptors. Although a strange phenomenon, autocrine signalling is central to many roles and processes. Case in point, this form of signalling is imperative during cell development, assisting cells to specialise (Welle, 1999, p. 133). From a medical point of view, autocrine signalling is essential in cancer and plays a major role in metastasis. In most circumstances, a signal may contain both paracrine and autocrine properties causing binding to the sender cells as well as the surrounding similar cells in the organism.
Endocrine signalling is the third form of multicellular signalling. Here, the circulatory system offers a network for the distribution of signals over long distances. In endocrine signalling, specialised cells produce the signals which are then taken to the bloodstream and further on to the receptors in distant cells (Welle, 1999, p. 142). Hormones, for instance, are signals produced in specific parts of a body and are transmitted through the circulatory system to far away destinations. Endocrine glands in multicellular organisms such as humans include the thyroid, the pituitary, the hypothalamus, the pancreas as well as the gonads. Each gland is responsible for the production of one or more hormones whose aim is physiology and development. For instance, the pituitary produces the growth hormone (GH), which encourages the cellular development of the cartilage and skeleton. Similar to most hormones, the growth hormone affects various cells throughout the body. Nonetheless, cartilage cells offer one example of how the hormone works: it clings to receptors on target cells and facilitates division.
Signaling through cell-cell contact is the final form of signalling these essays investigates. Gap junctions and plasmodesmata in animals and plants respectively refer to minuscule channels that connect cells directly (Welle, 1999, p. 151). They allow small signalling molecules, intracellular mediators, to pass through mostly due to the presence of water. Molecules such as calcium ions Ca2+ can diffuse through them whereas large molecules such as protein and DNA cannot diffuse through without external interference.
Signaling through G protein coupled receptors
Ga, Gb/Gg subunits are Heterogenic G proteins that form one of the essential portions of cell signalling cascade. G Protein Coupled Receptors (GPCRs) convert a lot of extra cellular signals to heterotrimeric G proteins and further converts the signals intracellular to effectors, therefore, playing a significant role in numerous signalling paths (Tuteja, 2009, p. 943). These receptors belong to a superfamily of membrane receptors that are made up of seven transmembrane sections that bind to various ligands. Once activated by a ligand, the GPCR transforms and activates the G proteins by facilitating the exchange of GDP/GTP linked with the subunit known as Ga. Separation of Gb/Gg from Ga is the outcome of this exchange. The separated components then become free to act upon their effectors and thus triggering unique signalling responses. After signal transmission, Ga becomes inactive (Ga-GDP) because the GTP of Ga-GTP is hydrolyzed to GDP, which results in its re-combination with Gb/Gg to create the inactive heterotrimeric molecule. The GPCR is also able to transform the signal using a G protein independent path. GPCRs also control cell cycle advancement (Tuteja, 2009, p. 945). Many years of biological research have shown that GPCRs are common in the animal kingdom while only single GPCR are present in a plant system, which proves to be cell cycle regulated and also takes part in ABA and stress signalling.
The mechanism for signal transduction through G proteins and GPCR starts in an inactive state where Ga is attached to GDP and Gbg dimer. The signalling commences by binding a certain agonist molecule that results in GPCR activation. The receptor is a guanine nucleotide factor that encourages GDP/GTP exchange (Tuteja, 2009, p. 947). The now active GPCR speeds up exchange process on the Ga subunit, conformational changes occur as a result leading to separation of Gbg from Ga, therefore, activating various G protein molecules. The activated G proteins form a representation of GPCR. In turn, active Ga and Gbg attach to effectors thus switching then either on or off, in doing so the effectors continue to propagate the signal to other secondary messengers.
Hormonal signals are the way in which microorganisms and their hosts communicate. This form of signalling involves hormones and hormone-like chemicals present in bacteria. Recent studies show that quorum sensing is not restricted to bacteria only but also permits communication between host and microorganisms (Nash and Boosma, 2008, p. 60). Bacterial signals referred to as quorum sensing signals can modulate and cross-signal with mammalian host signals that result in a modification of bacterial gene expression.
There exist three main categories of mammalian hormones: proteins (peptides), amino-acids (amines) and steroids (lipidic hormones). The location of the receptor depends on the hormones structure. Peptides, as well as amine hormones, are not able to diffuse through cell membranes therefore binding to surface receptors in the case of GPCRs and receptor kinase. However, steroid hormones cross through membranes and freely bind to the receptors (Schlessinger, 2000, p. 212). Signaling through receptor kinase is another mechanism this essay looks at since signalling through GPCR is similar in bacteria as well as in inter-kingdom signalling.
Receptor kinase is a receptor located on the surface of cells that contain intrinsic tyrosine that gets activated once a hormone binds to the extracellular section of the receptor. The outcome of activation is the phosphorylation and recruitment of downstream proteins that initiate the signalling process. The EGF receptor is critical in communication between host and microorganism. The aforementioned receptor is a cell-surface receptor that becomes active by the binding of small proteins, EGFs that act as signals. The EGFRs undergo a structural change to an active homodimer from the inactive form caused by the EGF activation (Schlessinger, 2000, p. 219). This change stimulates the tyrosine-kinase action of EGFR. Signaling as well as downstream activation by other proteins is as a result of autophosphorylation that initiates the signal conversion. Signaling through EGFR is essential for growth, cell-fate specification and survival throughout stages of cell development.
Calcium ions are vital in cell signalling. Once they penetrate the cytoplasm, they induce various regulatory effects on proteins as well as enzymes. Calcium forms an integral part in signal transduction which refers to the process of transmitting physical or chemical signals through a series of events. Calcium also acts as a second messenger resulting from utilisation of signal transduction paths such as receptor tyrosine kinase (Putney and Tomita, 2017, p. 158). Calcium signalling follows the release of calcium ions through cell stimulation or when the ions diffuse through the plasma membrane channel. Phospholipase C pathway happens to be the most common path that aids to increase calcium ion concentration.
The phospholipase C enzyme (PLC) undergoes activation by the assistance of various cell surfaces receptors such as GPCR and RTK. PLC then separates a membrane phospholipid (PIP2) into two second messengers diacylglycerol (DAG) and IP3. Protein kinase C (PKC) binds to the plasma membrane with the help of diacylglycerol. The IP3 then penetrates the endoplasmic reticulum and attaches itself to the IP3 receptor. A calcium ion channel is now made available through this receptor where the release of calcium ions from the reticulum occurs. Through this type of movement, calcium signalling is very much evident.
Calcium ions (Ca2+) also act as good secondary messengers exhibiting multiple physiological roles. Roles such as neuronal transmission, muscle contraction, cellular motility, cell growth, neurogenesis and fertilisation in addition to secretion of saliva are all functions of Ca2+.
Cell to cell signaling in Pseudomonas aeruginosa
Quorum sensing systems, especially in Gram-negative bacteria, operate through an antoinducer, a cell to cell signal molecule made up of a homoserine lactone plus a fatty acid chain. The human pathogen Pseudomonas aeruginosa is an example of such a bacteria (Pesci, n.d., p. 25). This bacteria is made up of two quorum sensing systems known as las and rhl that function through the help of the antoinducers namely N-(3-oxododecanoyl)-L-homoserine lactone and N-butyryl-L-homoserine lactone. Observation of these molecules reveals they attach to as well as trigger transcriptional proteins that induce a large number of Pseudomonas aeruginosa genes. P. aeruginosa, in turn, produces an...
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