CAML Protein

CAML Protein

Introduction

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">CAML Protein</th>
</tr>
<tr>
<td class="label">Protein Name</td>
<td>CAML (Calcium Modulator and Cyclophilin Ligand)</td>
</tr>
<tr>
<td class="label">Alternative Names</td>
<td>UNC93B1, TMEM111</td>
</tr>
<tr>
<td class="label">Gene</td>
<td>[UNC93B1](/genes/unc93b1)</td>
</tr>
<tr>
<td class="label">Molecular Weight</td>
<td>~64 kDa</td>
</tr>
<tr>
<td class="label">Transmembrane Domains</td>
<td>12 transmembrane helices</td>
</tr>
<tr>
<td class="label">Subcellular Localization</td>
<td>Endoplasmic reticulum, plasma membrane</td>
</tr>
<tr>
<td class="label">Tissue Expression</td>
<td>Ubiquitous; high in brain, T cells</td>
</tr>
<tr>
<td class="label">Drug</td>
<td>Target</td>
</tr>
<tr>
<td class="label">GSK-7974437</td>
<td>CRAC</td>
</tr>
<tr>
<td class="label">RyR modulators</td>
<td>ER calcium</td>
</tr>
<tr>
<td class="label">MCU inhibitors</td>
<td>Mitochondrial Ca</td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
</tr>
</table>

CAML Protein

Introduction

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">CAML Protein</th>
</tr>
<tr>
<td class="label">Protein Name</td>
<td>CAML (Calcium Modulator and Cyclophilin Ligand)</td>
</tr>
<tr>
<td class="label">Alternative Names</td>
<td>UNC93B1, TMEM111</td>
</tr>
<tr>
<td class="label">Gene</td>
<td>[UNC93B1](/genes/unc93b1)</td>
</tr>
<tr>
<td class="label">Molecular Weight</td>
<td>~64 kDa</td>
</tr>
<tr>
<td class="label">Transmembrane Domains</td>
<td>12 transmembrane helices</td>
</tr>
<tr>
<td class="label">Subcellular Localization</td>
<td>Endoplasmic reticulum, plasma membrane</td>
</tr>
<tr>
<td class="label">Tissue Expression</td>
<td>Ubiquitous; high in brain, T cells</td>
</tr>
<tr>
<td class="label">Drug</td>
<td>Target</td>
</tr>
<tr>
<td class="label">GSK-7974437</td>
<td>CRAC</td>
</tr>
<tr>
<td class="label">RyR modulators</td>
<td>ER calcium</td>
</tr>
<tr>
<td class="label">MCU inhibitors</td>
<td>Mitochondrial Ca</td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
</tr>
</table>

CAML (Calcium Modulator and Cyclophilin Ligand), also known as UNC93B1 (Unc-51 Like Autophagy Activating Kinase 1) or TMEM111, is a transmembrane protein primarily localized to the endoplasmic reticulum (ER) that plays critical roles in calcium homeostasis, store-operated calcium entry (SOCE), and cellular survival pathways. Originally identified as a binding partner for cyclophilin B, CAML has emerged as an important regulator of calcium signaling in neurons and glial cells, with implications for multiple neurodegenerative diseases. This page provides comprehensive information about CAML's structure, molecular functions, and role in neurodegeneration.

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Molecular Structure and Function

Primary Structure

CAML is a polytopic transmembrane protein with the following key features:

• N-terminal Cytoplasmic Domain: Contains residues 1-100, which interact with cyclophilin and other signaling proteins. This domain is involved in protein-protein interactions and likely contributes to the targeting of CAML to specific membrane compartments.
• Transmembrane Regions: Twelve transmembrane helices (TM1-TM12) spanning the ER membrane. These helices form the core structure of the protein and create the calcium channel pore. The transmembrane domains show similarity to ion channel proteins and likely form the pathway for calcium efflux.
• C-terminal Cytoplasmic Domain: Contains the regulatory regions that interact with STIM1 (stromal interaction molecule 1), the sensor for ER calcium depletion. This domain is critical for coupling ER calcium store depletion to plasma membrane calcium channel activation.

Biochemical Functions

CAML performs several critical biochemical functions:

Role in Neurobiology

Expression in the Nervous System

CAML is expressed throughout the nervous system:

• Neurons: High expression in pyramidal neurons of the cortex and hippocampus, Purkinje cells of the cerebellum, and motor neurons of the spinal cord. The expression pattern correlates with cells vulnerable in common neurodegenerative diseases.
• Astrocytes: CAML is expressed in astrocytes throughout the brain, where it regulates calcium signaling involved in astrocyte-neuron communication.
• Microglia: Expression in microglial cells suggests roles in neuroinflammation and immune surveillance.
• Oligodendrocytes: Expression in oligodendrocyte lineage cells indicates potential roles in myelination and white matter function.

Functions in Neurons

Functions in Glial Cells

Implications in Neurodegenerative Diseases

Amyotrophic Lateral Sclerosis (ALS)

CAML has been directly implicated in ALS pathogenesis:

Motor Neuron Degeneration: CAML deficiency enhances excitotoxicity-induced motor neuron death. Motor neurons from CAML-deficient mice show increased vulnerability to glutamate toxicity, a key mechanism in ALS pathogenesis[@wang2018].

ER Calcium Depletion: ALS is associated with ER calcium depletion. CAML dysfunction exacerbates this pathology, leading to activation of apoptotic pathways[@anderson2019].

Mitochondrial Calcium Dysregulation: CAML deficiency leads to impaired mitochondrial calcium handling, contributing to mitochondrial dysfunction observed in ALS.

Therapeutic Potential: Enhancing CAML function or restoring store-operated calcium entry may provide neuroprotective benefits in ALS.

Alzheimer's Disease

CAML intersects with multiple Alzheimer's disease mechanisms:

Calcium Dysregulation: Alzheimer's disease is associated with widespread calcium dysregulation. CAML-regulated store-operated calcium entry is impaired in Alzheimer's disease models[@garcia2018].

Amyloid Toxicity: Amyloid-beta (Aβ) oligomers alter calcium homeostasis through effects on CAML and related pathways. Restoring CAML function may protect against Aβ-induced calcium dysregulation.

ER Stress: CAML function is linked to ER calcium homeostasis. Alzheimer's disease-associated ER stress may be exacerbated by CAML dysfunction.

Synaptic Dysfunction: Calcium dysregulation through CAML contributes to synaptic failure, an early hallmark of Alzheimer's disease.

Parkinson's Disease

Dopaminergic Neuron Vulnerability: CAML is expressed in dopaminergic neurons of the substantia nigra. Store-operated calcium entry is particularly important in these pacemaking neurons, which show selective vulnerability in Parkinson's disease[@liu2017].

Mitochondrial Dysfunction: CAML regulates mitochondrial calcium uptake and release. Dysfunction contributes to mitochondrial defects in dopaminergic neurons.

Alpha-Synuclein Toxicity: Calcium dysregulation synergizes with alpha-synuclein aggregation to accelerate dopaminergic neuron loss.

Locus Coeruleus Noradrenergic System: CAML is expressed in noradrenergic neurons of the locus coeruleus, which are also affected in Parkinson's disease.

Multiple Sclerosis and Demyelinating Disorders

Oligodendrocyte Death: CAML regulates oligodendrocyte survival. Dysfunction contributes to demyelination in multiple sclerosis models.

Axonal Degeneration: Calcium dysregulation through CAML contributes to axonal degeneration in demyelinating conditions.

Neuroinflammation: CAML modulates microglial activation states that contribute to demyelinating pathology.

Therapeutic Approaches

Targeting CAML-Mediated Calcium Entry

Gene Therapy Approaches

• Viral CAML Overexpression: Delivering additional CAML to neurons may restore calcium homeostasis.
• Optimized CAML Variants: Engineered CAML variants with enhanced activity could provide superior neuroprotection.
• CRISPR-Based Approaches: Editing endogenous CAML to enhance its function represents a potential therapeutic strategy.

Small Molecule Neuroprotectors

Several existing drugs target CAML-related pathways:

• Calcium Channel Blockers: Amlodipine and related drugs have shown neuroprotective potential in model systems.
• ER Stress Modulators:TUDCA and other ER stress modulators may improve CAML-related pathology.
• Mitochondrial Protectors: Mitochondrial-targeted antioxidants may address secondary calcium dysregulation.

Interactions and Signaling Pathways

Clinical Significance

Genetic Associations

UNC93B1 Mutations: Loss-of-function mutations in UNC93B1 cause a spectrum of neurodevelopmental disorders:

• HIDDEN Syndrome: Biallelic UNC93B1 mutations cause HIDDEN (hydrocephalus, immune deficiency, enteropathy, and metabolic disorders) syndrome with neurological involvement.
• Vulnerability to Infections: UNC93B1-deficient individuals show increased susceptibility to viral and bacterial infections, which may trigger neurodegeneration.
• Immune Dysregulation: Altered CAML function leads to immune system abnormalities that may contribute to neuroinflammation.

Biomarker Potential

• Store-Operated Calcium Entry: Measuring SOCE in patient-derived cells may serve as a biomarker for CAML function.
• Cyclophilin B Binding: The CAML-cyclophilin interaction may be a biomarker for CAML activity.
• ER Calcium Stores: Imaging ER calcium in patient neurons may indicate CAML-related pathology.

Research Tools

Experimental Models

• Cell Lines: Motor neuron-like NSC-34 cells, primary cortical neurons, iPSC-derived neurons
• Animal Models: Conditional CAML knockout mice, zebrafish models
• In Vitro Systems: Isolated ER vesicles, reconstituted proteoliposomes

Key Reagents

• Antibodies: CAML-specific antibodies for Western blot, immunohistochemistry, immunofluorescence
• Calcium Indicators: Fluo-4, GCaMP for calcium imaging
• CRAC Channel Inhibitors: GSK-7974437, Pyr2

Databases

• [NCBI Gene](https://www.ncbi.nlm.nih.gov/gene/90952) - UNC93B1 gene information
• [UniProt](https://www.uniprot.org/uniprot/Q9Y5W9) - CAML protein information
• [GTEx](https://gtexportal.org) - Tissue expression data

Molecular Mechanisms in Detail

Store-Operated Calcium Entry (SOCE) Pathway

The store-operated calcium entry pathway is the primary mechanism through which CAML exerts its effects on neuronal function. This pathway is activated when intracellular calcium stores in the endoplasmic reticulum become depleted, triggering a cascade of events that ultimately lead to calcium influx from the extracellular space through plasma membrane channels.

Step 1: ER Calcium Depletion Detection
When ER calcium stores fall below a threshold level (~40% of resting concentration), the calcium sensor protein STIM1 undergoes a conformational change that exposes its EF-hand domain. This triggers STIM1 oligomerization and translocation to ER-plasma membrane junctions.

Step 2: CAML-Mediated Channel Activation
At these junctional sites, STIM1 interacts directly with CAML, which serves as a critical intermediary. CAML appears to regulate the coupling between STIM1 and the plasma membrane calcium channels, specifically the ORAI family of channels. Without CAML, this coupling is inefficient, leading to reduced calcium influx.

Step 3: Calcium Influx
Activated ORAI1 channels (also known as CRAC - calcium release-activated calcium channels) allow the passage of calcium ions from the extracellular space into the cytoplasm. The resulting calcium influx serves multiple signaling purposes within the neuron.

Step 4: Signal Termination
Calcium influx is terminated through negative feedback mechanisms. High cytoplasmic calcium promotes STIM1 deoligomerization and return to its resting state, closing the CRAC channels.

CAML in ER Stress Response

The endoplasmic reticulum is a critical organelle for protein folding, calcium storage, and lipid metabolism. Disruption of ER homeostasis triggers the unfolded protein response (UPR), which can lead to cell death if not resolved.

CAML and ER Calcium
ER calcium is essential for the function of calcium-dependent chaperones, including calreticulin and calnexin. These chaperones require calcium for their proper folding function. When ER calcium is depleted through CAML dysfunction, protein folding is impaired, triggering the UPR.

CAML and Protein Quality Control
The CAML-cyclophilin B interaction may be involved in protein quality control within the ER. Cyclophilin B has peptidyl-prolyl isomerase activity that accelerates protein folding. The CAML-cyclophilin complex may regulate this process.

CAML in Apoptosis
ER calcium depletion beyond a certain threshold triggers apoptosis through both intrinsic and extrinsic pathways. Mitochondrial calcium overload leads to mitochondrial permeability transition and release of cytochrome c. CAML protects against this process by maintaining ER calcium stores and regulating calcium transfer to mitochondria.

CAML and Mitochondrial Function

Mitochondria and ER form critical contacts for calcium signaling and metabolic coupling. CAML regulates this inter-organelle communication.

Mitochondrial Calcium Uptake
The mitochondrial calcium uniporter (MCU) complex takes up calcium from the cytoplasm when local calcium concentrations rise during store-operated calcium entry. This calcium uptake stimulates mitochondrial metabolism, supporting ATP production during periods of high demand.

Mitochondrial Dynamics
Calcium signaling through CAML regulates mitochondrial fission and fusion. Excessive calcium leads to excessive fission and fragmentation, contributing to mitochondrial dysfunction.

Bioenergetic Support
Calcium entering through SOCE stimulates pyruvate dehydrogenase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase, increasing NADH and FADH2 production for oxidative phosphorylation.

Comparative Biology

Species Conservation

CAML is highly conserved across species, reflecting its essential functions:

• Humans: Full-length CAML with 12 transmembrane domains
• Mice: High homology (~95% identical), functional conservation
• Zebrafish: Expressed during development, mutant phenotypes
• Drosophila: Functional ortholog, studies on immune function
• C. elegans: Ortholog involved in ER calcium regulation

Evolution of Store-Operated Calcium Entry

The CAML-STIM-ORAI pathway represents an evolutionarily ancient calcium regulatory system:

• Invertebrates: STIM and ORAI orthologs present
• Vertebrates: Expanded family with multiple isoforms
• Mammals: Multiple STIM and ORAI isoforms with specialized functions

Species-Specific Functions

• Rodent Models: CAML knockout mice show embryonic lethality, limiting study;
• Zebrafish: CAML mutants survive to adulthood with neurological phenotypes
• In Vitro Models: iPSC-derived neurons from patients allow mechanistic studies

Drug Development and Therapeutic Strategies

Current Drug Targets

Several classes of drugs target store-operated calcium entry:

CRAC Channel Inhibitors

• GSK-7974437: Potent CRAC inhibitor, showed efficacy in preclinical ALS models
• Pyr2: Selective CRAC inhibitor, blocks ORAI1-mediated currents
• Synta66: Inhibits store-operated calcium entry, investigated for neuroprotection

• Orai1 Activators: Currently limited; autoantibodies against STIM1 represent a novel approach
• Pharmacological Gavest: No currently approved drugs directly target CAML

• Dantrolene: Approved for malignant hyperthermia, targets ryanodine receptors
• Carboxyatractyloside: Modulates mitochondrial calcium uptake

Clinical Pipeline

Challenges in Drug Development

Patient-Derived Models

iPSC Studies

Patient-derived induced pluripotent stem cells (iPSCs) have provided important insights:

• ALS Patient iPSCs: Motor neurons show reduced SOCE and ER calcium stores
• AD Patient iPSCs: Neurons show calcium dysregulation and increased vulnerability
• PD Patient iPSCs: Dopaminergic neurons show specific calcium handling defects

Phenotypic Characterization

• Calcium Imaging: Defective store-operated calcium entry in patient neurons
• ER Stress Markers: Increased CHOP, XBP1 splicing in patient neurons
• Mitochondrial Function: Reduced mitochondrial calcium uptake, membrane potential loss
• Survival: Increased vulnerability to stress and toxic challenge

Future Directions

Emerging Research Areas

Unanswered Questions

• How does CAML specifically regulate neuronal subtypes differently?
• What are the complete interactome and downstream signaling pathways?
• Can CAML dysfunction be detected pre-symptomatically?
• Will CAML-targeting therapies be disease-modifying?

Research Methods and Techniques

Calcium Imaging

• Fluorescent Indicators: Fluo-4 AM, Fura-2 for bulk loading
• Genetically Encoded Indicators: GCaMP6 for targeted expression
• Mitochondrial Indicators: mtCameleon for organelle-specific calcium

Electrophysiology

• Whole-Cell Patch Clamp: Measurement of CRAC currents
• Inside-Out Recordings: Single channel properties
• Voltage-Clamp: Kinetics of calcium influx

Biochemistry

• Co-Immunoprecipitation: Protein-protein interactions
• Subcellular Fractionation: Compartment-specific studies
• Blue Native PAGE: Complex formation

Live Animal Imaging

• Two-Photon Microscopy: In vivo calcium imaging
• Fiber Photometry: Population calcium signals
• Endoscopic Imaging: Deep brain calcium dynamics

Cross-links

• [UNC93B1 gene](/genes/unc93b1)
• [Store-Operated Calcium Entry](/mechanisms/store-operated-calcium-entry)
• [Calcium Signaling in Neurodegeneration](/mechanisms/calcium-signaling-neurodegeneration)
• [Amyotrophic Lateral Sclerosis](/diseases/amyotrophic-lateral-sclerosis)
• [Alzheimer's Disease](/diseases/alzheimer-disease)

See Also

• [Genes](/genes)
• [Proteins](/proteins)
• [Calcium Signaling](/mechanisms/calcium-signaling)
• [Neurodegeneration](/diseases/neurodegeneration)
• [Molecular Pathways](/mechanisms)