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4.14:_Secondary_Messengers
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<p class="lt-bio-4657">Second messengers are molecules that relay signals received at receptors on the cell surface — such as the arrival of protein hormones, growth factors, etc. — to target molecules in the cytosol and/or nucleus. But in addition to their job as relay molecules, second messengers serve to greatly <strong>amplify the strength</strong> of the signal. Binding of a ligand to a single receptor at the cell surface may end up causing massive changes in the biochemical activities within the cell.</p> <p class="lt-bio-4657">There are 3 major classes of second messengers:</p> <ol> <li class="lt-bio-4657">cyclic nucleotides (e.g., <strong>cAMP</strong> and <strong>cGMP</strong>)</li> <li class="lt-bio-4657">inositol trisphosphate (<strong>IP<sub><font size="2">3</font></sub></strong>) and diacylglycerol (<strong>DAG</strong>)</li> <li class="lt-bio-4657">calcium ions (Ca<sup><small><font size="1">2+</font></small></sup>)</li> </ol> <span id="Cyclic_Nucleotides"></span><span id="Cyclic_Nucleotides"></span><h2 class="lt-bio-4657">Cyclic Nucleotides</h2> <figure><img class="internal" alt="cAMP+cGMP.png" loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6381/cAMP%252BcGMP.png?revision=1" /><figcaption>Figure <mjx-container class="MathJax CtxtMenu_Attached_0" jax="SVG" overflow="linebreak" tabindex="0" ctxtmenu_counter="96" 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data-semantic-level-number="1" data-speech-node="true">.</mo></mrow><mn data-latex="14" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:14" data-semantic-level-number="1" data-speech-node="true">14</mn><mrow data-mjx-texclass="ORD" data-latex="{.}"><mo data-latex="." data-semantic-type="punctuation" data-semantic-role="fullstop" data-semantic-annotation="nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:{.};texclass:ORD" data-semantic-operator="punctuated" data-semantic-level-number="1" data-speech-node="true">.</mo></mrow><mn data-latex="1" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:1" data-semantic-level-number="1" data-speech-node="true">1</mn></mrow></math></mjx-assistive-mml></mjx-container>: Cyclic Nucleotides</figcaption></figure> <span id="Cyclic_AMP_(cAMP)"></span><span id="Cyclic_AMP_(cAMP)"></span><h3 class="lt-bio-4657">Cyclic AMP (cAMP)</h3> <p class="lt-bio-4657">Some of the hormones that achieve their effects through cAMP as a second messenger:</p> <ul> <li class="lt-bio-4657">adrenaline</li> <li class="lt-bio-4657">glucagon</li> <li class="lt-bio-4657">luteinizing hormone (LH)</li> </ul> <p class="lt-bio-4657">Cyclic AMP is synthesized from ATP by the action of the enzyme <strong>adenylyl cyclase</strong>.</p> <ul> <li class="lt-bio-4657">Binding of the hormone to its receptor activates</li> <li class="lt-bio-4657">a <strong>G protein</strong> which, in turn, activates</li> <li class="lt-bio-4657">adenylyl cyclase.</li> <li class="lt-bio-4657">The resulting rise in cAMP turns on the appropriate response in the cell by either (or both): <ul> <li class="lt-bio-4657">changing the molecular activities in the cytosol, often using <strong>P</strong>rotein <strong>K</strong>inase <strong>A</strong> (<strong>PKA</strong>) — a c<strong>A</strong>MP-dependent protein kinase that phosphorylates target proteins</li> <li class="lt-bio-4657">turning on a new pattern of gene transcription</li> </ul> </li> </ul> <span id="Cyclic_GMP_(cGMP)"></span><span id="Cyclic_GMP_(cGMP)"></span><h2 class="lt-bio-4657">Cyclic GMP (cGMP)</h2> <p class="lt-bio-4657">Cyclic GMP is synthesized from the nucleotide GTP using the enzyme <strong>guanylyl cyclase</strong>. Cyclic GMP serves as the second messenger for</p> <ul> <li class="lt-bio-4657">atrial natriuretic peptide (ANP)</li> <li class="lt-bio-4657">nitric oxide (NO)</li> <li class="lt-bio-4657">the response of the rods of the retina to light</li> </ul> <p class="lt-bio-4657">Some of the effects of cGMP are mediated through <strong>P</strong>rotein <strong>K</strong>inase <strong>G</strong> (<strong>PKG</strong>) — a c<strong>G</strong>MP-dependent protein kinase that phosphorylates target proteins in the cell.</p> <span id="Inositol_trisphosphate_(IP3)_and_diacylglycerol_(DAG)"></span><span id="Inositol_trisphosphate_(IP3)_and_diacylglycerol_(DAG)"></span><h2 class="lt-bio-4657">Inositol trisphosphate (IP<sub><font size="5">3</font></sub>) and diacylglycerol (DAG)</h2> <p class="lt-bio-4657">Peptide and protein hormones like vasopressin, thyroid-stimulating hormone (<strong>TSH</strong>), and angiotensin and neurotransmitters like GABA bind to G protein-coupled receptors (GPCRs) that activate the intracellular enzyme <strong>phospholipase C</strong> (<strong>PLC</strong>).</p> <figure><img class="internal" alt="Diagram showing phospholipase C activation: PIP₂ conversion to DAG and IP₃, leading to Ca²⁺ release from the endoplasmic reticulum, represented with arrows and labeled components." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6382/DAG_IP3.gif?revision=1" /><figcaption>Figure 4.14.3 IP3 and DAG</figcaption></figure> <p class="lt-bio-4657">As its name suggests, it hydrolyzes phospholipids — specifically phosphatidylinositol-4,5-bisphosphate (PIP<sub><small><font size="1">2</font></small></sub>) which is found in the inner layer of the plasma membrane. Hydrolysis of PIP<sub><small><font size="1">2</font></small></sub> yields two products:</p> <ul> <li class="lt-bio-4657"><strong>diacylglycerol</strong> (<strong>DAG</strong>): DAG remains in the inner layer of the plasma membrane. It recruits <strong>P</strong>rotein <strong>K</strong>inase <strong>C</strong> (<strong>PKC</strong>) — a <strong>c</strong>alcium-dependent kinase that phosphorylates many other proteins that bring about the changes in the cell. As its name suggests, activation of PKC requires calcium ions. These are made available by the action of the other second messenger — IP<sub><font size="2">3</font></sub>.</li> <li class="lt-bio-4657"><strong>inositol-1,4,5-trisphosphate</strong> (<strong>IP<sub><font size="2">3</font></sub></strong>): This soluble molecule diffuses through the cytosol and binds to receptors on the endoplasmic reticulum causing the release of calcium ions (Ca<sup><small><font size="1">2+</font></small></sup>) into the cytosol. The rise in intracellular calcium triggers the response.</li> </ul> <p class="lt-bio-4657"><strong>Example</strong>:</p> <p class="lt-bio-4657">The calcium rise is needed for NF-AT (the "nuclear factor of activated T cells") to turn on the appropriate genes in the nucleus.</p> <p class="lt-bio-4657">The remarkable ability of <strong>tacrolimus</strong> and <strong>cyclosporine</strong> to prevent <strong>graft rejection</strong> is due to their blocking this pathway.</p> <p class="lt-bio-4657">The binding of an antigen to its receptor on a B cell (the BCR) also generates the second messengers DAG and IP<sub><font size="2">3</font></sub>.</p> <span id="Calcium_ions_(Ca2.2B)"></span><span id="Calcium_ions_(Ca2.2B)"></span><h2 class="lt-bio-4657">Calcium ions (Ca<sup><small><font size="3">2+</font></small></sup>)</h2> <p class="lt-bio-4657">As the functions of IP<sub><font size="2">3</font></sub> and DAG indicate, calcium ions are also important intracellular messengers. In fact, calcium ions are probably the most widely used intracellular messengers.</p> <p class="lt-bio-4657">In response to many different signals, a rise in the concentration of Ca<sup><small><font size="1">2+</font></small></sup> in the cytosol triggers many types of events such as</p> <ul> <li class="lt-bio-4657">muscle contraction</li> <li class="lt-bio-4657">exocytosis, e.g. <ul> <li class="lt-bio-4657">release of neurotransmitters at synapses (and essential for the long-term synaptic changes that produce Long-Term Potentiation (LTP) and Long-Term Depression (LTD);</li> <li class="lt-bio-4657">secretion of hormones like insulin</li> </ul> </li> <li class="lt-bio-4657">activation of T cells and B cells when they bind antigen with their antigen receptors (TCRs and BCRs respectively)</li> <li class="lt-bio-4657">adhesion of cells to the extracellular matrix (ECM)</li> <li class="lt-bio-4657">apoptosis</li> <li class="lt-bio-4657">a variety of biochemical changes mediated by <strong>P</strong>rotein <strong>K</strong>inase <strong>C</strong> (<strong>PKC</strong>).</li> </ul> <p class="lt-bio-4657">Normally, the level of calcium in the cell is very low (~100 nM). There are two main depots of Ca<sup><small><font size="1">2+</font></small></sup> for the cell:</p> <ul> <li class="lt-bio-4657">The extracellular fluid (ECF — made from blood), where the concentration is ~ 2 mM or 20,000 times higher than in the cytosol;</li> <li class="lt-bio-4657">the endoplasmic reticulum ("sarcoplasmic" reticulum in skeletal muscle).</li> </ul> <p class="lt-bio-4657">However, its level in the cell can rise dramatically when channels in the plasma membrane open to allow it in from the extracellular fluid or from depots within the cell such as the endoplasmic reticulum and mitochondria.</p> <span id="Getting_Ca2.2B_into_(and_out_of)_the_cytosol"></span><span id="Getting_Ca2.2B_into_(and_out_of)_the_cytosol"></span><h3 class="lt-bio-4657">Getting Ca<sup><small><font size="2">2+</font></small></sup> into (and out of) the cytosol</h3> <ul> <li class="lt-bio-4657">Voltage-gated channels <ul> <li class="lt-bio-4657">open in response to a change in membrane potential, e.g. the depolarization of an action potential</li> <li class="lt-bio-4657">are found in excitable cells: <ul> <li class="lt-bio-4657">skeletal muscle</li> <li class="lt-bio-4657">smooth muscle (These are the channels blocked by drugs, such as felodipine [Plendil®], used to treat high blood pressure. The influx of Ca<sup><small><font size="1">2+</font></small></sup> contracts the smooth muscle walls of the arterioles, raising blood pressure. The drugs block this.)</li> <li class="lt-bio-4657">neurons. When the action potential reaches the presynaptic terminal, the influx of Ca<sup><small><font size="1">2+</font></small></sup> triggers the release of the neurotransmitter.</li> <li class="lt-bio-4657">the taste cells that respond to salt.</li> </ul> </li> <li class="lt-bio-4657">allow some 10<sup><small><font size="1">6</font></small></sup> ions to flow in each second following the steep concentration gradient.</li> </ul> </li> <li class="lt-bio-4657">Receptor-operated channels<br /> These are found in the post-synaptic membrane and open when they bind the neurotransmitter. Example: NMDA receptors.</li> <li class="lt-bio-4657">G-protein-coupled receptors (GPCRs). These are not channels but they trigger a release of Ca<sup><small><font size="1">2+</font></small></sup> from the endoplasmic reticulum as described above. They are activated by various hormones and neurotransmitters (as well as bitter substances on taste cells in the tongue).</li> </ul> <p class="lt-bio-4657">Ca<sup><small><font size="1">2+</font></small></sup> ions are returned</p> <ul> <li class="lt-bio-4657">to the ECF by active transport using <ul> <li class="lt-bio-4657">an ATP-driven pump called a Ca<sup><small><font size="1">2+</font></small></sup> ATPase;</li> <li class="lt-bio-4657">two Na<sup><font size="2">+</font></sup>/Ca<sup><small><font size="1">2+</font></small></sup> exchangers. These antiport pumps harness the energy of <ul> <li class="lt-bio-4657">3 Na<sup><font size="2">+</font></sup> ions flowing DOWN their concentration gradient to pump one Ca<sup><small><font size="1">2+</font></small></sup> against its gradient and</li> <li class="lt-bio-4657">4 Na<sup><font size="2">+</font></sup> ions flowing down to pump 1 Ca<sup><small><font size="1">2+</font></small></sup> and 1 K<sup><small><font size="1">+</font></small></sup> ion up their concentration gradients.</li> </ul> </li> </ul> </li> <li class="lt-bio-4657">to the endoplasmic (and sarcoplasmic) reticulum using another Ca<sup><small><font size="1">2+</font></small></sup> ATPase.</li> </ul> <p class="lt-bio-4657">How can such a simple ion like Ca<sup><small><font size="1">2+</font></small></sup> regulate so many different processes? Some factors at work:</p> <ul> <li class="lt-bio-4657">localization within the cell (e.g., released at one spot — the T-system is an example — or spread throughout the cell)</li> <li class="lt-bio-4657">by the amount released (amplitude modulation, "AM")</li> <li class="lt-bio-4657">by releasing it in pulses of different frequencies (frequency modulation, "FM")</li> </ul> <footer class="mt-content-footer"> <style>/*<![CDATA[*/#mt-toc-container {display: none !important;}/*]]>*/</style><script type="text/javascript">/*<![CDATA[*/ $(function() { if(!window['autoDefinitionList']){ window['autoDefinitionList'] = true; $('dl').find('dt').on('click', function() { $(this).next().toggle('350'); }); } });/*]]>*/</script> <script defer="true" src="https://static.cloudflareinsights.com/beacon.min.js" data-cf-beacon="{"token": "483ec2414e274209a7e93c253192df0b"}"></script><script src="https://cdn.libretexts.net/github/LibreTextsMain/Miscellaneous/h5p-resizer.js"></script><script src="https://cdnjs.cloudflare.com/ajax/libs/iframe-resizer/4.2.11/iframeResizer.contentWindow.min.js" integrity="sha512-FOf4suFgz7OrWmBiyyWW48u/+6GaaAFSDHagh2EBu/GH/1+OQSYc0NFGeGeZK0gZ3vuU1ovmzVzD6bxmT4vayg==" crossorigin="anonymous"></script><script src="https://cdnjs.cloudflare.com/ajax/libs/iframe-resizer/4.2.11/iframeResizer.min.js" integrity="sha512-HY1lApSG7xxx8mYzs/lxRs+c5AaDThRaa3pvQB6puiswvf2lWqMJVf+8qSGiL4ZXfHQoPIqbd1TlpqfycPo3cQ==" crossorigin="anonymous"></script><script>/*<![CDATA[*/window.addEventListener('load', function(){$('iframe').iFrameResize({warningTimeout:0, scrolling: 'omit'});})/*]]>*/</script><script>/*<![CDATA[*/ window.PageNum = "auto"; window.InitialOffset = "false"; window.PageName = "4.14: Secondary Messengers"; /*]]>*/</script> <script type="text/javascript">/*<![CDATA[*/ // var front = window.PageNum.trim(); if(front=="auto"){ front = window.PageName.replace('\"', '\\\"').trim(); //front = "'..string.matchreplace(PageName,'\"','\\\"')..'".trim(); if(front.includes(":")){ front = front.split(":")[0].trim(); if(front.includes(".")){ front = front.split("."); front = front.map((int)=>int.includes("0")?parseInt(int,10):int).join("."); } front+="."; } else { front = ""; } } front = front.trim(); function loadMathJaxScript() { try { const script = document.createElement('script'); script.id = "mathjax-script"; script.src = "https://cdn.jsdelivr.net/npm/mathjax@4/tex-mml-svg.js"; script.type = "text/javascript"; script.defer = true; document.head.appendChild(script); } catch (err) { console.error(err); } } document.addEventListener('DOMContentLoaded', (e) => { loadMathJaxScript(); }); if (window.PageName !== 'Realtime MathJax'){ MathJax = { options: { ignoreHtmlClass: "tex2jax_ignore", processHtmlClass: "tex2jax_process", menuOptions: { settings: { zscale: "150%", zoom: "Double-Click", assistiveMml: true, // true to enable assitive MathML collapsible: false, // true to enable collapsible math }, }, }, output: { scale: 0.85, mtextInheritFont: false, displayOverflow: "linebreak", linebreaks: { width: "100%", }, }, startup: { pageReady: () => { if (window.activateBeeLine) { window.activateBeeLine(); } return MathJax.startup.defaultPageReady(); }, }, chtml: { matchFontHeight: true, }, tex: { tags: "all", tagformat: { number: (n) => { if (window.InitialOffset) { const offset = Number(window.InitialOffset); if(!offset) { return front + n; // If offset is falsy (nan, undefined, etc.) } const added = Number(n) + offset; return front + added; } else { return front + n; } }, }, macros: { eatSpaces: ['#1', 2, ['', ' ', '\\endSpaces']], PageIndex: ['{' + front.replace(/\./g, '{.}') + '\\eatSpaces#1 \\endSpaces}', 1], test: ["{" + front + "#1}", 1], mhchemrightleftharpoons: "{\\unicode{x21CC}\\,}", xrightleftharpoons: ['\\mhchemxrightleftharpoons[#1]{#2}', 2, ''] }, packages: { "[+]": [ "mhchem", "color", "cancel", "ams", "tagformat" ], }, }, loader: { '[tex]/mhchem': { ready() { const {MapHandler} = MathJax._.input.tex.MapHandler; const mhchem = MapHandler.getMap('mhchem-chars'); mhchem.lookup('mhchemrightarrow')._char = '\uE42D'; mhchem.lookup('mhchemleftarrow')._char = '\uE42C'; } }, load: [ "[tex]/mhchem", "[tex]/color", "[tex]/cancel", "[tex]/tagformat", ], }, }; }; ///*]]>*/</script> <hr class="autoattribution-divider" /><div class="autoattribution"><p>This page titled <a target="_blank" class="internal mt-self-link" href="/Sandboxes/johnnyphung/biology/04:_Cell_Metabolism/4.14:_Secondary_Messengers">4.14: Secondary Messengers</a> is shared under a <a rel="nofollow" href="https://creativecommons.org/licenses/by/3.0" target="_blank">CC BY 3.0</a> license and was authored, remixed, and/or curated by <a rel="nofollow" target="_blank" href="http://www.biology-pages.info/">John W. 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