Calcitonin Receptor (CALCR) is a Class B (secretin) G protein-coupled receptor (GPCR) that binds calcitonin, calcitonin gene-related peptide (CGRP), amylin, and related peptides. While primarily studied in bone metabolism and migraine, CALCR is increasingly recognized for its roles in neuroprotection, pain modulation, and neuroinflammatory processes relevant to Alzheimer's disease, Parkinson's disease, and other neurodegenerative conditions 1. The receptor is encoded by the CALCR gene and signals through multiple G protein pathways to regulate diverse physiological processes in the central and peripheral nervous systems.
| Protein Name | Calcitonin Receptor (CALCR) |
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| Gene | [CALCR](/genes/calcr) |
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| UniProt ID | [P30988](https://www.uniprot.org/uniprot/P30988) |
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| PDB Structures | 7XBW, 7XBY, 6X10 |
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| Molecular Weight | ~64 kDa (n glycosylated) |
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| Subcellular Localization | Plasma membrane |
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| Protein Family | Class B GPCR (Secretin family) |
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| Expression | Neurons, glia, pituitary, bone, kidney |
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The CALCR protein possesses the characteristic architecture of Class B GPCRs, consisting of distinct domains optimized for peptide hormone binding and signal transduction 10.
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N-terminal Extracellular Domain (ECD): The large ECD (approximately 150 amino acids) contains multiple disulfide bonds and a conserved fold that serves as the primary ligand-binding site. This domain recognizes the C-terminal region of peptide ligands including calcitonin, CGRP, and amylin. The ECD adopts a unique fold characterized by a short α-helix and a β-hairpin, creating a binding pocket that achieves high-affinity ligand recognition through a network of hydrogen bonds and hydrophobic interactions.
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Seven Transmembrane Domain (TMD): Like other GPCRs, CALCR contains seven transmembrane helices (TM1-TM7) connected by three extracellular loops (ECL1-3) and three intracellular loops (ICL1-3). The transmembrane domain adopts the classic seven-helix bundle conformation that undergoes conformational changes upon ligand binding, enabling coupling to intracellular G proteins. The Class B signature includes a conserved proline in TM6 and specific residues in the DRY motif at the boundary of TM3 and ICL2.
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C-terminal Intracellular Domain: The C-terminal tail (approximately 80 amino acids) contains serine/threonine residues that serve as phosphorylation sites for GPCR kinases (GRKs). This domain also contains motifs important for receptor internalization and desensitization through β-arrestin recruitment.
¶ Ligand-Binding Pocket Architecture
The ligand-binding mechanism involves two distinct binding sites:
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Primary Binding Site (ECD): The extracellular domain provides the primary high-affinity binding interface for peptide ligands. Structural studies have revealed that the C-terminal "message" region of CGRP engages the ECD, while the N-terminal region interacts with the transmembrane domain to trigger signal transduction.
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Secondary Binding Site (TMD): The transmembrane domain contributes to ligand recognition, particularly for small molecule antagonists (gepants). The interface between the ECD and TMD creates a complex binding pocket that can be targeted by different drug classes.
CALCR undergoes several important post-translational modifications:
- N-linked Glycosylation: The extracellular domain contains multiple N-glycosylation sites that affect receptor folding, trafficking, and ligand-binding affinity
- Disulfide Bonds: Conserved disulfide bonds in the ECD maintain structural stability
- Palmitoylation: C-terminal cysteine residues undergo S-acylation, anchoring the receptor in the plasma membrane
CALCR activates multiple intracellular signaling cascades through coupling to different G protein subtypes 8:
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Gs-coupled cAMP/PKA Pathway
- Primary signaling pathway upon ligand binding
- Activates adenylyl cyclase, increasing intracellular cAMP
- Promotes PKA activation and downstream phosphorylation events
- Mediates bone resorption inhibition (calcitonin) and neuromodulation (CGRP)
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Gq/11-coupled PLC/IP3/DAG Pathway
- Activates phospholipase C (PLC)
- Generates inositol trisphosphate (IP3) and diacylglycerol (DAG)
- Increases intracellular calcium release
- Involved in pain signaling and vasodilation
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Gβγ-mediated Pathways
- Activates PI3K/Akt signaling
- Modulates ion channel function
- Contributes to neuroprotective effects
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β-arrestin Signaling
- Independent of G protein signaling
- Mediates receptor internalization
- Activates MAPK pathways (ERK1/2, p38)
¶ Tissue Distribution and Function
In the central nervous system, CALCR (particularly as the CGRP receptor complex) is expressed in regions involved in pain processing, mood, and autonomic function:
- Trigeminal Nucleus Caudalis: Primary site for processing craniofacial pain
- Hypothalamus: Regulates stress responses and autonomic function
- Locus Coeruleus: Modulates arousal and pain perception
- Periaqueductal Gray: Involved in descending pain inhibition
- Hippocampus: Expressed in CA1/CA3 regions, implicated in memory
- Dorsal Root Ganglion (DRG): Primary sensory neurons expressing CALCR
- CGRP-positive afferents: Involved in neurogenic inflammation
- Enteric nervous system: Modulates gastrointestinal function
- Osteoclasts: Calcitonin binding inhibits bone resorption
- Kidney: Regulates calcium reabsorption
- Parathyroid: Feedback regulation of calcitonin secretion
The CGRP receptor (CALCR + RAMP1) is central to migraine pathophysiology 2:
- CGRP is released during migraine attacks from trigeminal afferents
- CGRP causes meningeal vasodilation and neurogenic inflammation
- Receptor activation on trigeminal nociceptors propagates pain signals
- Temporal correlation between CGRP levels and migraine attacks
CGRP Monoclonal Antibodies (Preventive therapy):
- Erenumab: Anti-CALCR antibody (CGRP receptor antagonist)
- Fremanezumab: Anti-CGRP antibody (ligand antagonist)
- Galcanezumab: Anti-CGRP antibody
- Eptinezumab: Anti-CGRP antibody (IV formulation)
Gepants (Small molecule antagonists) 14:
- Rimegepant: Oral CGRP receptor antagonist
- Ubrogepant: Oral CGRP receptor antagonist
- Atogepant: Oral CGRP receptor antagonist
- Zavegepant: Nasal spray formulation
These therapeutics block the CALCR-RAMP1 complex, preventing CGRP binding and subsequent signal transduction, thereby terminating migraine attacks or reducing their frequency.
Emerging evidence links CALCR/CGRP signaling to AD pathophysiology 6:
- CGRP Expression: Altered CGRP levels in AD patient CSF and brain tissue
- Neuroprotective Effects: CGRP promotes neuronal survival through cAMP/PKA and PI3K/Akt pathways 9
- Amyloid Interaction: CGRP may modulate amyloid-beta production and aggregation
- Synaptic Function: CGRP signaling affects synaptic plasticity and memory
- Calcium Homeostasis: CALCR-mediated calcium signaling is dysregulated in AD
- Neuroinflammation: CGRP modulates microglial activation and cytokine release 13
CALCR has been implicated in PD through several mechanisms 15:
- Dopaminergic Neurons: CALCR is expressed in substantia nigra pars compacta neurons
- Neuroprotection: CGRP can protect dopaminergic neurons from oxidative stress
- Neuroinflammation: Modulates microglial activation in the substantia nigra
- Motor Function: CALCR signaling affects basal ganglia circuits
- Therapeutic Potential: CALCR agonists may slow dopaminergic degeneration
- Amyotrophic Lateral Sclerosis (ALS): CGRP levels altered in patient CSF; CALCR expressed in motor neurons
- Multiple Sclerosis: CGRP modulates demyelination and neuroinflammation
- Frontotemporal Dementia: CALCR involvement in tau pathology
- Huntington's Disease: CGRP signaling affects medium spiny neuron survival
- RAMP1 (Receptor Activity Modifying Protein 1): Essential for forming functional CGRP receptor; changes receptor pharmacology and trafficking
- RAMP2: Forms AM (adrenomedullin) receptor
- RAMP3: Forms AM2/AMY2 receptor
- Gs protein: Couples to cAMP production
- Gq/11 protein: Couples to PLC signaling
- β-arrestin 1/2: Mediates internalization and G protein-independent signaling
- CGRP (CALCA gene product): Primary ligand
- Amylin (IAPP gene product): Alternate ligand
- Receptorchesterin (RAMP1): Accessory protein
| Model |
Description |
Phenotype |
Relevance |
| CALCR knockout mice |
Germline CALCR deletion |
Bone turnover, pain behavior |
Receptor function |
| RAMP1 knockout mice |
Lacks CGRP receptor |
Migraine resistance |
CGRP biology |
| CALCR floxed mice |
Conditional deletion |
Tissue-specific knockout |
CNS function |
| Human CALCR knock-in |
Human CALCR expression |
Rescue of knockout |
Drug testing |
- Migraine models: Cortical spreading depression, trigeminal activation
- AD models: APP/PS1 mice, 5xFAD mice
- PD models: MPTP-treated mice, α-synuclein transgenic mice
- Erenumab (Aimovig): Anti-CALCR monoclonal antibody; preventive migraine treatment
- Rimegepant (Nurtec ODT): Gepant; acute and preventive migraine
- Ubregepant (Ubrelvy): Gepant; acute migraine treatment
- Atogepant (Qulipta): Gepant; preventive migraine treatment
- Zavegepant: Nasal spray gepant (FDA approved 2024)
- Rimegepant: Preventive migraine indication
- Atogepant: Expansion to other indications
¶ Adverse Effects and Limitations
- Constipation: Most common side effect
- Hypersensitivity reactions: Antibody formulations
- Liver enzyme elevations: Gepants
- Cardiovascular effects: RAMP1 expression in vasculature
- Medication overuse headache: Challenge in clinical practice
- Does CGRP/CALCR signaling contribute to AD progression?
- Can CALCR agonists provide neuroprotection in PD?
- What determines patient response to different drug classes?
- Is there a role for CALCR in ALS?
- Single-cell sequencing: CALCR expression in specific neuronal populations
- Cryo-EM structures: High-resolution receptor-ligand complexes 15
- Biomarkers: CGRP as migraine biomarker
- Gene therapy: AAV-mediated CALCR modulation