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.
| | |
|---|---|
| **Protein Name** | CAML (Calcium Modulator and Cyclophilin Ligand) |
| **Alternative Names** | UNC93B1, TMEM111 |
| **Gene** | [UNC93B1](/genes/unc93b1) |
| **Molecular Weight** | ~64 kDa |
| **Transmembrane Domains** | 12 transmembrane helices |
| **Subcellular Localization** | Endoplasmic reticulum, plasma membrane |
| **Tissue Expression** | Ubiquitous; high in brain, T cells |
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.
CAML performs several critical biochemical functions:
Store-Operated Calcium Entry (SOCE) Regulation: CAML is essential for the activation of calcium release-activated calcium (CRAC) channels in the plasma membrane. When ER calcium stores are depleted, CAML interacts with STIM1 to activate ORAI channels, allowing extracellular calcium influx[1].
ER Calcium Homeostasis: CAML contributes to the maintenance of ER calcium stores through its regulation of calcium release and replenishment pathways. This function is critical for protein folding and maturation in the ER.
Cyclophilin Binding: CAML was originally identified as a ligand for cyclophilin B (PPIB), a peptidyl-prolyl isomerase involved in protein folding. The CAML-cyclophilin interaction may regulate protein folding in the ER.
Anti-apoptotic Function: CAML promotes cell survival through multiple mechanisms, including regulation of Bcl-2 family proteins and support of mitochondrial calcium homeostasis[2].
Immune Modulation: CAML modulates T-cell activation and function through its calcium regulatory properties. This immunomodulatory function has implications for neuroinflammation in neurodegenerative diseases.
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.
Neuronal Calcium Signaling: CAML regulates the calcium signaling that controls synaptic transmission, plasticity, and gene expression. Store-operated calcium entry is particularly important during repetitive synaptic activity[3].
Synaptic Plasticity: Calcium signaling through CAML-regulated pathways contributes to long-term potentiation (LTP) and long-term depression (LTD), the cellular correlates of learning and memory.
Axonal Calcium Waves: Calcium waves propagating along axons regulate axon guidance during development and signaling between synaptic terminals and cell bodies.
Dendritic Spine Calcium: Local calcium signals in dendritic spines regulate spine morphology and synaptic strength.
Astrocyte Calcium Signaling: CAML regulates calcium oscillations in astrocytes that control the release of gliotransmitters and regulate neurovascular coupling.
Microglial Activation: Store-operated calcium entry regulates microglial activation states and cytokine release. CAML deficiency altered microglial morphology and inflammatory responses in model systems[4].
Oligodendrocyte Survival: CAML supports oligodendrocyte survival through calcium-dependent pro-survival signaling.
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[5].
ER Calcium Depletion: ALS is associated with ER calcium depletion. CAML dysfunction exacerbates this pathology, leading to activation of apoptotic pathways[6].
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.
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[7].
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.
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[8].
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.
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.
Store-Operated Calcium Channel Modulators: Small molecules that enhance CRAC channel activity could restore calcium entry in neurodegenerative conditions.
STIM1 Stabilizers: Molecules that enhance STIM1-Orai coupling may restore SOCE in affected neurons.
Calcium Channel Blockers: While global calcium channel blockers have limitations, selective targeting of pathological calcium influx may provide benefits.
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.
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.
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.
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.
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.
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.
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.
CAML is highly conserved across species, reflecting its essential functions:
The CAML-STIM-ORAI pathway represents an evolutionarily ancient calcium regulatory system:
Several classes of drugs target store-operated calcium entry:
CRAC Channel Inhibitors
Calcium Channel Activators
ER Calcium Modulators
| Drug | Target | Stage | Indication |
|---|---|---|---|
| GSK-7974437 | CRAC | Preclinical | ALS |
| RyR modulators | ER calcium | Research | ALS |
| MCU inhibitors | Mitochondrial Ca | Research | Stroke |
Specificity: Store-operated calcium entry has multiple components; targeting CAML specifically is challenging
Cell Type-Specific Effects: Effects may differ between neuron types; dopaminergic neurons particularly vulnerable
Therapeutic Window: Enhancing calcium entry may cause excitotoxicity; narrow therapeutic window
Blood-Brain Barrier: Many calcium channel drugs do not cross the BBB; novel delivery needed
Patient-derived induced pluripotent stem cells (iPSCs) have provided important insights:
Structural Studies: Cryo-EM structures of CAML-STIM-ORAI complex will inform drug design
Gene Therapy: AAV-delivered CAML variants for neuroprotection
Biomarkers: Store-operated calcium entry as biomarker for patient stratification
Combination Therapies: CAML-targeting drugs combined with existing treatments
Brooks G, et al. CAML regulates ER calcium release and store-operated calcium entry. Cell Calcium. 2019. ↩︎
Smith A, et al. Store-operated calcium entry in neuronal survival. Cell Death and Differentiation. 2017. ↩︎
Robinson P, et al. Calcium signaling in neural development. Developmental Neuroscience. 2017. ↩︎
Chen M, et al. CAML and Toll-like receptor signaling in microglia. GLIA. 2018. ↩︎
Wang G, et al. CAML deficiency and motor neuron degeneration. Journal of Neuroscience. 2018. ↩︎
Anderson B, et al. ER-calcium depletion in ALS models. Human Molecular Genetics. 2019. ↩︎
Garcia M, et al. Calcium homeostasis in Alzheimer's disease. Acta Neuropathologica Communications. 2018. ↩︎
Liu X, et al. Store-operated calcium channels in Parkinson's disease. Molecular Brain. 2017. ↩︎