Cell signalling

Cell signaling refers to communication between cells.

Regulates:


 * the development of cells and their organization into tissues
 * cell growth and cell division
 * coordination of cellular functions

Endocrine (hormonal)
Cells secrete signalling molecules (primarily hormones), that enter the blood and the circulatory system to the target cell, to which the receptor binds.


 * Action – remote
 * Speed – in minutes

Paracrine
The cells release chemical substances (e.g. growth factors) into the ECF, which act as local mediators and affect the cells in the immediate vicinity. For example, signalling molecules that regulate inflammation at the site of infection or cell proliferation during wound healing work in this way.

Autocrine
The secreted signaling molecule binds to the receptor of the cell that produced it.

Synaptic
This signaling is specific to the nervous system of animals. A nerve cell produces a chemical signal (neurotransmitter), that is transmitted to another nerve cell through synaptic cleft.


 * Speed − up to 100 m/s.

By direct contact

 * 1) Junctional complexes ensure the continuity of adjacent cells. In animal cells using nexus (gap junctions), in plant cells using plasmodesmata.
 * 2) Interaction of cell surface molecules.

Through signalling molecules
Signal molecules are substances that are capable of transmitting a signal. They can be divided according to their chemical nature into several groups: e.g. cytokine interactions between cells of the immune system




 * 1) Lipophilic signalling molecules:
 * 2) *steroid hormones,
 * 3) *thyroid hormones,
 * 4) *fatty acid derivatives (eicosanoids),
 * 5) *retinoids (retinal).
 * 6) Peptide and protein signalling molecules:
 * 7) *peptide hormones (e.g., liberins, statins, insulin, glucagon, vasopressin),
 * 8) *growth factors and cytokines.
 * 9) Amino acid derivatives:
 * 10) *hormones (e.g., adrenaline, noradrenaline),
 * 11) *neurotransmitters (e.g., GABA, glutamate, glycine),
 * 12) *mediators (e.g., histamine).
 * 13) Malé anorganické molekuly a ionty:
 * 14) *NO,
 * 15) *CO,
 * 16) *Ca2+.
 * 17) Nukleotides:
 * 18) *cAMP,
 * 19) *cGMP.

Mechanism of action of signaling molecules

 * It depends on whether the signaling molecule is water soluble (hydrophilic) or fat soluble (hydrophobic).
 * Cytoplasmic membrane of cells is permeable to hydrophobic (= lipophilic) signal molecules and to small inorganic molecules such as NO. These molecules bind to cytoplasmic or nuclear receptors, which mostly act as ligand-directed transcription factors and, after binding the signaling molecule, affect gene transcription.
 * For hydrophilic signalling molecules (peptides, proteins) and amino acid derivatives, the cytoplasmic membrane is impermeable and therefore their signalling must take place via receptors located in the cytoplasmic membrane of the target cells (so-called membrane receptors). After binding of a signal molecule (ligand) to a membrane receptor, signal transduction occurs; signal transmission from the receptor to the interior of the cell. Intracellular signalling follows, often involving second messengers or specific protein kinases. These subsequently regulate the activity of effector proteins and the behavior of the cell will change. Effector proteins can be enzymes affecting metabolism, transcription factors, components of the cytoskeleton or ion channels.

Signalling stages

 * 1) Production of a signalling molecule by the cell (based on superior stimulation – e.g. hormones controlled by the Hypothalamic-pituitary-gonadal axis, or when the concentration of certain molecules changes – e.g., glucose or ions).
 * 2) Reception of the signal by the target cell → the signaling molecule binds to the receptor.
 * 3) Signal transmission (= signal transduction) – can be one-step or involve a cascade of changes in molecules (the so-called signaling pathway).
 * 4) A signal triggers a specific response.
 * 5) Degradation of the signaling molecule.

Types of membrane receptors
They differ in the signal that is created inside the cell after the binding of the extracellular signal molecule to the receptor.

'''Only hydrophilic substances bind to membrane receptors. Hydrophobic substances pass through the membrane without specific carriers and bind to their receptors only in the cytoplasm or in the nucleus (the best-known hydrophobic substances are steroids and thyroid hormones).'''

Enzymotropic receptors (catalytic receptors, enzyme-linked receptors or receptors with intrinsic enzymatic activity)

 * Some proteins pass through the phospholipid bilayer of the membrane only once. They consist of an extracellular part of a protein with a binding site for a signaling molecule, a transmembrane α-helix, a cytoplasmic part that either contains its own enzymatic activity or is associated with an enzyme.
 * A receptor is either a ligand-directed enzyme, or a protein that binds to the enzyme. Many receptors contain a cytoplasmic portion that functions as a tyrosine protein kinase.
 * After binding the signal molecule, 2 receptor proteins join and a dimer is formed. This activates the tyrosine kinase parts of the receptor, which phosphorylate tyrosine (using phosphate groups from ATP) of the receptor itself.
 * Phosphorylated tyrosines serve as binding sites for various proteins, which themselves become active after binding.
 * For example, the signalling molecule Ras (GTP-binding protein) is activated, which subsequently activates other protein kinases. As a consequence, a change in gene expression occurs. Signal termination is catalyzed by protein-tyrosine-phosphatase, or activated receptors can undergo Endocytosis and be degraded by Lysosomes.
 * E.g., growth factor or insulin, bind to receptors with tyrosine kinase activity (more detail below).
 * The group of enzymotropic receptors also includes receptors with serine/threonine kinase, guanylate cyclase or tyrosine phosphatase activity.

Receptors with tyrosine kinase activity
They are predominantly receptors for most growth and differentiation factors such as EGF (epidermal growth factor), PDGF (platelet-derived growth factor), IGF-1 (insulin-like growth factor) and the insulin receptor. After the binding of the ligand to the receptor, it is activated and the phosphate group is transferred from ATP to specific tyrosines. Either the tyrosines of the receptor proteins themselves (autophosphorylation) or the tyrosines of specific cellular proteins (intracellular protein kinases) are phosphorylated. This initiates a cascade of intracellular signal transmission.

Ras proteins are among the important intracellular signal proteins that are primarily involved in signal transmission from a receptor with tyrosine kinase activity to the interior of the cell, where they activate the serine/threonine phosphorylation cascade. Ras proteins are anchored in the cytoplasmic part of the plasma membrane. It belongs to the family of monomeric GTPases (as opposed to G proteins – trimeric GTPases). However, the activation and function of monomeric and trimeric GTPases is similar. They are in a constant transition between an active state when GTP is bound to them, and an inactive state when GDP is bound. Ras proteins are phosphorylated (activated) by receptor tyrosine kinases, inactivated by phosphatases and GTP hydrolysis, which they themselves carry out.

The tyrosine phosphorylation of Ras proteins, which is carried out by receptor tyrosine kinases on the cytoplasmic side of the plasma membrane, is soon terminated by dephosphorylation by specific tyrosine phosphatases. Activated Ras proteins also inactivate themselves by hydrolysis of bound GTP to GDP. Stimulating cells to proliferate and differentiate, however, requires long-term signaling. Further signal transmission is ensured by phosphorylation of serines and threonines by MAP-protein kinases (mitogen-activated protein kinases) . Phosphorylation of serines and threonines has a longer duration than tyrosine phosphorylation of Ras proteins.

The active Ras/GTP complex binds to Raf-kinase (MAP-kinase 1) and activates it by phosphorylating serines and threonines.

Other protein kinases are also involved in the regulation of Raf-kinase activity:


 * 1) activation of Raf-protein kinase increases Src-protein kinase by tyrosine phosphorylation
 * 2) protein kinase C by serine phosphorylation
 * 3) serine phosphorylation by protein kinase A has an inhibitory effect

Active Raf-kinase activates MAP-kinase 2 by phosphorylation, which activates MAP-kinase 3, which enters the nucleus. Here, a regulatory protein is activated, which stimulates the activity of genes involved mainly in the regulation of cell proliferation.

Activated Ras protein phosphorylate and thereby activate a cascade of three types of MAP-kinases. Binding of the first MAP-kinase (referred to as Raf) to the activated Ras protein results in its phosphorylation and thus activation. This then catalyzes serine/threonine phosphorylation of another MAP-kinase, and this enzyme activates another (third) MAP-kinase. Activation of the last MAP-kinase in the cascade by MAP-kinase phosphorylation requires phosphorylation of both threonine and tyrosine. After entering the nucleus, the third MAP-kinase activated in this way first phosphorylates the regulatory protein, which is bound to a short DNA sequence in the regulatory region of the early response genes - the myc, jun a fos genes. This results in their transcription.

Late response gene products are involved in the regulation of cell proliferation. These include, for example, the main components of the cell cycle control system – cyclins and cyclin-dependent protein kinases.

Receptory s tyrosinfosfatasovou aktivitou
Specifická aktivita těchto enzymů zajišťuje, že fosforylace tyrosinů trvá velmi krátkou dobu, a že v klidových buňkách je tyrosinů fosforylováno jen malé množství. Příkladem receptoru s tyrosinfosfatasovou aktivitou je membránový glykoprotein CD45, který se nachází na povrchu bílých krvinek. Účastní se aktivace T a B lymfocytů po setkání s cizími antigeny.

Receptory s guanylátcyklasovou aktivitou
Jde například o receptor vázající atriální natriuretické peptidy (ANPs), což je skupina peptidových hormonů. Nacházejí se v buňkách ledvin a v buňkách hladké svaloviny krevních cév. Atriální natriuretické peptidy jsou secernovány svalovými buňkami srdeční předsíně při vzestupu krevního tlaku. Stimulují ledviny k exkreci Na+ a vody a navozují relaxaci svalových buněk ve stěnách krevních cév. Oba tyto účinky vedou ke snížení krevního tlaku.

Receptory mají extracelulární oblast pro vazbu ANPs a intracelulární guanylátcyklasovou katalytickou doménu. Vazba ligandu s receptorem aktivuje cyklasu k produkci cyklického 3‘,5‘-GMP (cGMP). cGMP se váže k cGMP-dependentní proteinkinase a tím ji aktivuje k fosforylaci serinů a threoninů specifických proteinů, které se podílejí na dalším přenosu signálu a realizaci konečného projevu.

Receptory s připojenou tyrosinkinasovou aktivitou
Liší se od receptorů s tyrosinkinasovou aktivitou tím, že tyrosinkinasa je v tomto případě kódována dalším samostatným genem (např. protoonkogenem src) a je nekovalentně připojena k cytoplasmatické části receptorového polypeptidického řetězce. Tyto receptory tvoří velkou heterogenní skupinu. Jsou to například receptory pro většinu cytokinů, které regulují proliferaci a diferenciaci buněk hemopoetického systému; antigen-specifické receptory na T a B lymfocytech; receptory hormonů (např. růstový hormon, prolaktin) a další.

Antigen je prezentován molekulami MHC a rozeznán receptory T lymfocytů (TCR); TCR je aktivován a předává signál prostřednictvím signálních molekul do jádra. Následně dochází k expresi cytokinů.

Sekretovaný cytokin se váže a aktivuje membránový receptor B lymfocytu s připojenou tyrosinkinasovou aktivitou. Tyrosinkinasa je kódována protoonkogenem src.

Receptory spojené s iontovými kanály (ionotropní receptory)
right| 250px Některé receptorové proteiny regulují navázáním signální molekuly činnost iontových kanálů. Jejich otevírání a zavírání je vlastní signalizační odpovědí. Po navázání nervového mediátoru se změní konformace receptoru a iontový kanál se uzavře nebo naopak otevře pro specifické ionty, které se pohybují po svém elektrochemickém gradientu a dochází ke změně membránového potenciálu. Tento typ buněčné signalizace se vyskytuje v tzv. vzrušivých tkáních - nervové soustavě a svalech.

Receptory spojené s G-proteinem (GPCR = G-protein–coupled receptor)
right |250px Receptor je polypeptidový řetězec, který sedmkrát prochází membránou. V klidovém stavu se G-protein s receptorem pravděpodobně ani nedotýká. Je tvořen třemi podjednotkami α, β, γ. Na α podjednotce je v klidu navázán GDP. Po navázání ligandu se receptor spojí s G-proteinem a GDP je nahrazeno GTP. Ukončení signálu je doprovázeno hydrolýzou GTP zpět na GDP (α podjednotka má GTPázovou aktivitu). Cílem působení aktivovaného G-proteinu (jeho disociované α podjednotky nebo βγ komplexu) mohou být iontové kanály nebo enzymy v membráně. Nejčastěji je aktivována adenylátcykláza (tvorba cAMP) a fosfolipáza C (tvorba IP3 a DAG).

Obecné schéma signální dráhy
Hormon → membránový receptor → G-protein → adenylátcykláza → cAMP → proteinkináza A → 


 * 1) fosforylace enzymů ovlivňujících metabolismus (rychlé účinky);
 * 2) fosforylace genových regulačních proteinů → ovlivnění transkripce genů (pomalé účinky).

Související články

 * Hormony
 * G-protein
 * Inzulin
 * Cytokiny
 * Růstové faktory
 * Druzí poslové

Použitá literatura