CALCA encodes calcitonin gene-related peptide (CGRP), one of the most potent neuropeptides in the human body and a central player in migraine pathophysiology, neuroinflammation, and pain modulation. The CALCA gene produces two alternatively spliced peptides: calcitonin and α-CGRP (also known as CGRP), through tissue-specific processing of the CALCA mRNA transcript. While calcitonin is primarily involved in calcium homeostasis, CGRP has emerged as a critical signaling molecule in the nervous system with profound implications for neurodegenerative diseases[@russell2014]. The peptide is expressed in sensory neurons of the trigeminal ganglion, where it plays a pivotal role in the pathogenesis of migraine, and in various brain regions where it modulates neuroinflammation, pain perception, and vascular tone.
The discovery of CGRP's central role in migraine has led to the development of revolutionary therapeutic approaches, including CGRP receptor antagonists (gepants) and monoclonal antibodies against CGRP or its receptor. These therapies have transformed migraine treatment and validated CGRP as a druggable target. Beyond migraine, emerging research has revealed connections between CGRP and neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease, where the peptide may play roles in neuroinflammation, blood-brain barrier dysfunction, and neuronal survival[@poo2019].
CALCA encodes calcitonin gene-related peptide (CGRP), one of the most potent neuropeptides in the human body and a central player in migraine pathophysiology, neuroinflammation, and pain modulation. The CALCA gene produces two alternatively spliced peptides: calcitonin and α-CGRP (also known as CGRP), through tissue-specific processing of the CALCA mRNA transcript. While calcitonin is primarily involved in calcium homeostasis, CGRP has emerged as a critical signaling molecule in the nervous system with profound implications for neurodegenerative diseases[@russell2014]. The peptide is expressed in sensory neurons of the trigeminal ganglion, where it plays a pivotal role in the pathogenesis of migraine, and in various brain regions where it modulates neuroinflammation, pain perception, and vascular tone.
The discovery of CGRP's central role in migraine has led to the development of revolutionary therapeutic approaches, including CGRP receptor antagonists (gepants) and monoclonal antibodies against CGRP or its receptor. These therapies have transformed migraine treatment and validated CGRP as a druggable target. Beyond migraine, emerging research has revealed connections between CGRP and neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease, where the peptide may play roles in neuroinflammation, blood-brain barrier dysfunction, and neuronal survival[@poo2019].
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<tr><th colspan="2" style="background:#e8f4f8; text-align:center; font-size:1.1em;">Calcitonin Gene-Related Peptide (α-CGRP)</th></tr>
<tr><td><strong>Protein Name</strong></td><td>Calcitonin gene-related peptide 1 (α-CGRP)</td></tr>
<tr><td><strong>Gene Symbol</strong></td><td>CALCA</td></tr>
<tr><td><strong>Alternative Names</strong></td><td>CGRP, α-CGRP, CGRP-I</td></tr>
<tr><td><strong>Mature Peptide</strong></td><td>37 amino acids</td></tr>
<tr><td><strong>Molecular Weight</strong></td><td>3,800 Da</td></tr>
<tr><td><strong>UniProt ID</strong></td><td>[P01258](https://www.uniprot.org/uniprot/P01258)</td></tr>
<tr><td><strong>Cellular Location</strong></td><td>Secreted peptide</td></tr>
<tr><td><strong>Protein Family</strong></td><td>Calcitonin family</td></tr>
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<td class="label">KG Connections</td>
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The CALCA gene is located on chromosome 11 and consists of six exons. Alternative splicing of the primary transcript gives rise to two different mRNAs: one encoding calcitonin and the other encoding α-CGRP. The choice between these two splicing pathways is tissue-specific, with calcitonin being the predominant product in thyroid C-cells and CGRP being the predominant product in neurons of the peripheral and central nervous system. This alternative splicing is regulated by tissue-specific splicing factors that recognize distinct regulatory sequences in the CALCA pre-mRNA[@russell2014].
The CGRP precursor (prepro-CGRP) is a 128-amino acid precursor protein that undergoes proteolytic processing to generate the mature 37-amino acid peptide. This processing occurs in the secretory pathway through the action of prohormone convertases. The mature peptide contains a disulfide bond between cysteine residues at positions 2 and 7, which is essential for receptor binding and biological activity.
CGRP exerts its effects through a specific receptor system consisting of the calcitonin receptor-like receptor (CLR) and receptor activity-modifying proteins (RAMPs). The RAMP proteins (RAMP1, RAMP2, and RAMP3) are required for proper receptor trafficking and ligand specificity. When CLR is co-expressed with RAMP1, it forms the CGRP receptor, which has high affinity for CGRP. Co-expression with RAMP2 forms the adrenomedullin 1 receptor (AM1), while RAMP3 forms the AM2 receptor[@hay2018].
The CGRP receptor is a G protein-coupled receptor (GPCR) that signals primarily through the Gs protein pathway, leading to activation of adenylate cyclase and increased cAMP production. This signaling cascade mediates most of the biological effects of CGRP, including vasodilation, modulation of neurotransmitter release, and regulation of inflammatory responses. Receptor desensitization and internalization are important mechanisms for regulating CGRP signaling.
CGRP's role in migraine is mediated primarily through its actions in the trigeminovascular system, which is the primary pain pathway for migraine headache. CGRP is released from trigeminal nerve endings that innervate the meninges and cerebral blood vessels. This release can be triggered by various migraine triggers, including stress, hormonal changes, and certain foods. Once released, CGRP produces meningeal vasodilation, plasma protein extravasation, and activation of trigeminal nociceptors, leading to the generation of migraine pain[@iyengar2017].
The trigeminal ganglion contains cell bodies of sensory neurons that innervate the meninges and facial structures. These neurons synthesize and release CGRP upon activation. In migraine patients, there is evidence of both increased CGRP release during attacks and possibly increased baseline CGRP levels between attacks. This has led to the hypothesis that CGRP plays a causal role in migraine pathogenesis.
CGRP has been implicated in cortical spreading depression (CSD), the neurological phenomenon believed to underlie migraine aura. CSD is a wave of neuronal depolarization that propagates across the cerebral cortex, followed by a prolonged suppression of neuronal activity. This wave is associated with changes in blood flow, blood-brain barrier permeability, and the release of inflammatory mediators, including CGRP[@eikermann-Haerter2019].
Studies have shown that CGRP can facilitate the induction and propagation of CSD, while CGRP receptor antagonists can suppress CSD. This suggests that CGRP may act as a modulator of cortical excitability and that its inhibition could prevent both migraine aura and the subsequent headache phase.
The role of CGRP in migraine is strongly supported by clinical evidence. Intravenous infusion of CGRP can trigger migraine-like attacks in susceptible individuals, while CGRP receptor antagonists can relieve migraine pain. Furthermore, monoclonal antibodies against CGRP or its receptor have proven effective in migraine prevention, providing long-lasting protection with monthly or quarterly dosing[@edvinsson2019].
The successful development of CGRP-targeted therapies has validated the CGRP pathway as a migraine target and has provided new hope for patients with chronic migraine who were previously refractory to conventional treatments. These therapies include the small molecule receptor antagonists (gepants) such as rimegepant and ubrogepant, as well as the monoclonal antibodies erenumab (targeting the CGRP receptor) and eptinezumab, fremanezumab, and galcanezumab (targeting CGRP itself).
Emerging evidence links CGRP to Alzheimer's disease pathophysiology through multiple mechanisms. CGRP is expressed in brain regions affected by AD pathology, including the hippocampus and cerebral cortex. The peptide can modulate neuroinflammation, which is a key contributor to AD pathogenesis. Studies have shown that CGRP can both promote and suppress neuroinflammatory responses depending on the context and receptor subtype involved[@messenger2019].
CGRP may also interact with amyloid-β (Aβ) pathology, a hallmark of AD. Some studies suggest that CGRP can protect neurons against Aβ-induced toxicity, while others indicate that CGRP may exacerbate neuroinflammation in AD. The relationship between CGRP and AD is complex and may depend on the specific disease stage and brain region examined.
The blood-brain barrier (BBB) is disrupted in AD, and CGRP may play a role in regulating BBB permeability. CGRP can affect endothelial cell function and tight junction integrity, potentially contributing to the BBB dysfunction observed in AD. This relationship has made CGRP a protein of interest for understanding vascular contributions to AD.
In Parkinson's disease, CGRP is implicated in both motor and non-motor aspects of the disorder. The peptide is expressed in dopaminergic neurons of the substantia nigra and can modulate dopaminergic signaling. Studies have shown that CGRP levels are altered in the substantia nigra and cerebrospinal fluid of PD patients, suggesting a role in disease pathophysiology[@walker2019].
CGRP may also contribute to non-motor symptoms of PD, including autonomic dysfunction and gastrointestinal problems. The peptide is widely expressed in the autonomic nervous system and can regulate smooth muscle function, gut motility, and cardiovascular control. These effects may be relevant to the autonomic dysfunction observed in PD.
The neuroinflammatory response in PD involves activation of microglia and release of pro-inflammatory cytokines. CGRP can modulate this response through its receptors on immune cells, potentially influencing the progression of neuroinflammation in PD.
CGRP is expressed in motor neurons and may play a role in ALS pathogenesis. Studies have shown altered CGRP expression in ALS models and patients. The peptide can affect motor neuron survival through modulation of excitotoxicity and neuroinflammation. The relationship between CGRP and ALS is an active area of investigation.
CGRP plays a complex role in pain modulation, acting as both a pro-nociceptive and anti-nociceptive mediator depending on the context and receptor subtype. In the peripheral nervous system, CGRP promotes nociception by sensitizing nociceptors to noxious stimuli and enhancing neurogenic inflammation. This pro-nociceptive effect is mediated primarily through the CGRP receptor (CLR/RAMP1) on sensory nerve endings[@vanderah2018].
In the central nervous system, CGRP can produce analgesia through activation of descending pain modulatory pathways. This paradoxical effect may involve different receptor subtypes or distinct neuronal populations. The complexity of CGRP's pain-modulating effects has made it a challenging but interesting target for pain therapy.
CGRP contributes to inflammatory pain by promoting neurogenic inflammation and sensitizing nociceptors. The peptide is released from peripheral nerve endings upon activation by inflammatory mediators, where it produces vasodilation, plasma protein extravasation, and recruitment of immune cells. This cascade amplifies the inflammatory response and contributes to pain hypersensitivity.
CGRP receptor antagonists have shown efficacy in treating inflammatory pain conditions, including arthritis and musculoskeletal pain. The development of CGRP-targeted drugs for migraine has provided tools for testing these approaches in other pain conditions.
CGRP can modulate microglial activation, the key cellular players in neuroinflammation. Microglia express CGRP receptors, and CGRP signaling can influence the release of pro-inflammatory cytokines and chemokines. The effect of CGRP on microglia is context-dependent, with both pro-inflammatory and anti-inflammatory effects reported[@poo2019].
The modulation of microglial activation by CGRP has implications for neurodegenerative diseases where neuroinflammation plays a key role. Understanding how CGRP influences microglial phenotype may lead to new therapeutic strategies for conditions like AD, PD, and ALS.
CGRP modulates the peripheral immune system through its effects on immune cells, including T cells, B cells, and macrophages. The peptide can inhibit the production of pro-inflammatory cytokines and promote the activation of anti-inflammatory immune responses. This immunomodulatory function positions CGRP at the interface of the nervous and immune systems.
The immunomodulatory effects of CGRP have relevance for autoimmune conditions and inflammatory diseases. CGRP receptor antagonists, originally developed for migraine, are being investigated for potential use in inflammatory and autoimmune conditions.
CGRP is one of the most potent vasodilators known, producing vasodilation through activation of the CGRP receptor on vascular endothelial cells. This effect is mediated primarily by increased cAMP production and subsequent activation of endothelial nitric oxide synthase (eNOS). The resulting nitric oxide release produces vasodilation of cerebral and peripheral blood vessels[@russell2014].
The vasodilatory effects of CGRP are particularly relevant to migraine, where they contribute to the vascular changes during attacks. They also have implications for cardiovascular homeostasis and may be relevant to conditions involving impaired vasodilation.
CGRP plays a role in blood pressure regulation through its vasodilatory effects and actions on the heart. The peptide can lower blood pressure in experimental models, suggesting potential for cardiovascular therapy. However, the complex interactions between CGRP and other vasodilatory systems have made it challenging to exploit this property therapeutically.
Gepants are small molecule CGRP receptor antagonists that have been developed for migraine treatment. These drugs block the CGRP receptor, preventing CGRP from binding and activating its effects. Rimegepant, ubrogepant, and atogepant have been approved for migraine treatment, with others in development. These drugs offer advantages over traditional migraine medications, including efficacy in patients who do not respond to triptans[@edvinsson2019].
Four monoclonal antibodies targeting the CGRP pathway have been approved for migraine prevention: erenumab (targets CGRP receptor), eptinezumab, fremanezumab, and galcanezumab (all target CGRP). These antibodies offer long-lasting protection with monthly or quarterly dosing and have proven effective in reducing migraine frequency and severity. They are particularly valuable for patients with chronic migraine or those who cannot tolerate other preventive therapies.
The safety profile of CGRP-targeted therapies has been generally favorable, with the most common adverse effects being mild and self-limiting. However, the long-term effects of CGRP pathway inhibition are still being studied. Concerns have been raised about potential cardiovascular effects, as CGRP is involved in vasodilation and blood pressure regulation. Clinical trials have not shown significant cardiovascular risks to date, but post-marketing surveillance continues.
The study of Calca Protein has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.
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