Recombinant Mouse Chloride channel CLIC-like protein 1 (Clcc1)

What is the subcellular localization of Clcc1 and how can this be verified experimentally?

Clcc1 primarily localizes to the endoplasmic reticulum (ER) and has been identified at ER-mitochondria contact sites known as mitochondrial-associated ER membranes (MAMs). Experimental verification approaches include:

• Immunofluorescence microscopy: Use specific antibodies against Clcc1 alongside organelle markers such as mCherry-KDEL for the ER and MitoTracker for mitochondria .

• Co-localization studies: Examine co-localization with established ER proteins such as Calreticulin and Calnexin, or MAM proteins like SigmaR1 .

• Subcellular fractionation: Perform Western blotting on ER-enriched fractions to detect Clcc1.

• Fluorescently-tagged constructs: Express tagged Clcc1 constructs (e.g., FLAG-tagged) followed by immunostaining .

Recent findings show that Clcc1 co-localizes with SigmaR1 at both the ER and MAMs, confirming its presence at these critical membrane contact sites .

What are the known functions of Clcc1 in cellular homeostasis?

Clcc1 serves multiple critical functions in maintaining cellular homeostasis:

• Ion homeostasis: Functions as an ER anion channel, primarily conducting chloride ions. Maintains steady-state [Cl-]ER and [K+]ER levels crucial for proper ER function .

• ER stress regulation: Prevents ER stress and accumulation of misfolded proteins. Loss of Clcc1 leads to increased expression of UPR target genes like GRP78 (BiP) and accumulation of ubiquitin-positive inclusions in neurons before their degeneration .

• Counter-ion mechanism: Likely provides a counter-ion current to balance charge during Ca2+ release from the ER. Depletion of Clcc1 reduces ER Ca2+ release .

• Nuclear pore complex assembly: Contributes to the fusion of inner and outer nuclear envelopes during nuclear pore complex formation .

• Lipid metabolism: Regulates hepatic neutral lipid flux, with its loss leading to increased TAG biosynthesis, decreased TAG breakdown, and hepatic steatosis .

• Viral cycle regulation: Facilitates membrane fusion during herpesvirus nuclear egress .

These diverse functions highlight Clcc1's importance in maintaining cellular homeostasis across multiple systems.

What experimental models are available for studying Clcc1 function?

Several experimental models have been developed to study Clcc1 function:

Cell Culture Models:

• CRISPR-Cas9-mediated Clcc1 knockout in cell lines (HEK293, HeLa)

• siRNA/shRNA knockdown systems

• Overexpression of wild-type or mutant Clcc1 using plasmid transfection

Mouse Models:

• Complete Clcc1 knockout (embryonically lethal)

• Conditional tissue-specific knockout (e.g., liver-specific Clcc1 HepKO)

• C3H/HeSnJ inbred strain (carries a naturally occurring Clcc1 mutation - IAP retrotransposon insertion)

• BAC transgenic mice for rescue experiments

• CLCC1 point mutation knock-in mice (e.g., D25E mutation)

Zebrafish Models:

• Morpholino-mediated clcc1 knockdown, resulting in reduced eye size and thinner retinal layers

• Expression analysis showing high clcc1 expression in hindbrain, swim bladder, and eye

Biochemical Systems:

• Purified Clcc1 incorporated into lipid bilayers for electrophysiological studies

• Microsomal preparations from Clcc1-expressing cells

Each model system offers unique advantages for studying different aspects of Clcc1 function.

How can mutations in Clcc1 affect channel conductance and cellular function?

Mutations in Clcc1 have diverse effects on both channel conductance and cellular function:

D25E mutation (associated with retinitis pigmentosa):

• Alters the cytoplasmic N-terminus of Clcc1

• Changes protein interaction profile, with increased interactions with cytoplasmic proteins

• Affects retinal development, particularly cone and rod photoreceptors

• May alter Ca2+ binding and regulation of channel activity, as D25 is a conserved residue responsible for Ca2+ binding

ALS-associated mutations:

• Impair channel conductance

• Lead to increased ER stress and accumulation of misfolded proteins

• Cause motor neuron loss and ALS-like pathologies

• K298A mutation (PIP2-sensing residue): Exhibits dominant-negative effects, with 10% of heterozygous mice developing ALS-like symptoms

IAP retrotransposon insertion (in C3H/HeSnJ mice):

• Results in aberrant splicing and >10-fold decrease in normal Clcc1 transcripts

• Dramatically reduces protein levels

• Causes late-onset cerebellar degeneration, ataxia and peripheral neuropathy

These mutations highlight the relationship between altered channel function and disease pathology, providing important insights into Clcc1's physiological roles.

What critical structural domains and residues are important for Clcc1 function?

Clcc1 contains several key structural domains and functional residues:

• N-terminal cytoplasmic domain: Contains D25, a residue mutated in retinitis pigmentosa (p.D25E) that affects Ca2+ binding and regulation .

• Transmembrane domains: Form the ion-conducting pore of the channel. Clcc1 is predicted to have multiple transmembrane segments that create the chloride-conducting pathway .

• D181 residue: Along with D25, responsible for Ca2+ binding and mediates luminal Ca2+-dependent inhibition of channel open probability .

• K298 residue: Located in the intraluminal loop, this is a critical PIP2-sensing residue that facilitates channel activity when bound to phosphatidylinositol 4,5-bisphosphate .

• Multimerization domains: Clcc1 forms homomultimers as demonstrated by:

  • Cross-linking with disuccinimidyl suberate (DSS) revealing high molecular weight complexes

  • Co-immunoprecipitation of differentially tagged Clcc1 co-expressed in cells

• Alternative splicing regions: Clcc1 produces different isoforms through alternative splicing, which may have slightly different functional properties .

Understanding these structure-function relationships is essential for interpreting the effects of mutations and designing therapeutic strategies.

What phenotypes are observed in Clcc1 knockout/knockdown models?

Clcc1 knockout/knockdown models exhibit diverse phenotypes:

Complete knockout:

• Embryonic lethality in mice and zebrafish

• Severe defects in neural development

Conditional/tissue-specific knockout:

• Liver-specific knockout:

  • Hepatic steatosis and 2-fold increase in liver weight/body weight ratio

  • Enlarged whitened livers with dramatic increases in TAG and CE

  • Reduced plasma TAG and HDL levels

  • Liver damage indicated by elevated AST levels

  • Severe MASH (metabolic dysfunction-associated steatohepatitis) pathologies in as little as 4 weeks without dietary challenge

• Neuron-specific knockout:

  • Motor neuron loss, ER stress, misfolded protein accumulation

  • ALS-like pathologies in the spinal cord

Knockdown models:

• Zebrafish morphants:

  • Reduced eye size (0.13 vs. control 0.178 mm²)

  • Decreased lens size (0.019 vs. control 0.026 mm²)

  • Significantly thinner inner plexiform layer and outer nuclear layer

• Cell culture:

  • Increased sensitivity to ER stress

  • Upregulation of UPR genes

  • Accumulation of ubiquitinated proteins

C3H/HeSnJ inbred mouse strain (natural Clcc1 mutation):

• Late-onset cerebellar degeneration

• Ataxia and peripheral neuropathy

These phenotypes demonstrate Clcc1's critical roles across different tissues and developmental stages.

How can I measure Clcc1 channel activity in experimental settings?

Measuring Clcc1 channel activity requires specialized electrophysiological techniques:

Planar lipid bilayer recordings:

• Purify recombinant Clcc1 protein

• Incorporate the purified protein into a synthetic lipid bilayer

• Measure ion currents across the bilayer using voltage-clamp techniques

• Test channel conductance with different ion concentrations

• Assess effects of regulators like luminal Ca2+ (inhibitory) and PIP2 (facilitative)

• Evaluate how mutations affect channel properties, as demonstrated with ALS-associated mutations

Microsomal patch-clamp:

• Isolate microsomes (ER-derived vesicles) from cells expressing Clcc1

• Perform patch-clamp recordings on these microsomes

• Characterize channel conductance, ion selectivity, and regulation

Indirect measurements in intact cells:

• Use chloride-sensitive fluorescent dyes to monitor [Cl-] changes

• Employ ER-targeted chloride indicators to specifically measure [Cl-]ER

• Compare chloride dynamics in wild-type vs. Clcc1-deficient cells

• Assess how perturbations of Clcc1 affect ER calcium release, as Clcc1 depletion reduces ER Ca2+ release

When performing these measurements, it's critical to account for the effects of the identified regulatory mechanisms: inhibition by luminal Ca2+ through residues D25 and D181, and facilitation by PIP2 through residue K298 .

What experimental approaches can be used to study Clcc1's role in ER stress and unfolded protein response?

To investigate Clcc1's role in ER stress and the unfolded protein response (UPR), researchers should consider these experimental approaches:

UPR marker analysis:

• Measure expression levels of canonical UPR markers (BiP/GRP78, CHOP, XBP1 splicing) by qRT-PCR and Western blotting

• GRP78, the major HSP70 family chaperone in the ER, is upregulated in Clcc1-deficient granule cells in vivo

• Use reporter constructs containing UPR-responsive elements to monitor UPR activation

ER stress induction:

• Apply ER stress-inducing agents (tunicamycin, thapsigargin) to Clcc1-deficient and control cells

• Acute knockdown of Clcc1 expression in cultured cells increases sensitivity to ER stress

• Monitor cell viability, UPR activation, and recovery kinetics

Misfolded protein detection:

• Immunostaining for ubiquitinated proteins in Clcc1-deficient cells/tissues

• Ubiquitinated proteins accumulate in Clcc1-deficient neurons before their degeneration

• Pulse-chase experiments to evaluate protein folding and secretion efficiency

ER homeostasis assessment:

• Monitor ER calcium levels using targeted calcium indicators

• Measure ER redox state using redox-sensitive fluorescent proteins

• Evaluate ER morphology using high-resolution microscopy

Rescue experiments:

• Test whether wild-type Clcc1 expression can reverse UPR activation in knockout models

• Determine if mutant forms of Clcc1 can rescue UPR defects

• BAC transgenic mice carrying wild-type Clcc1 can be used to rescue phenotypes

These approaches can reveal the mechanisms by which Clcc1 contributes to ER homeostasis and prevents pathological activation of the UPR.

How can I investigate Clcc1 interactions with other proteins at ER-mitochondria contact sites?

Investigating Clcc1 interactions at ER-mitochondria contact sites (MAMs) requires specialized techniques:

Proximity labeling approaches:

• Express Clcc1 fused to promiscuous biotin ligases (BioID, TurboID) to identify nearby proteins

• Use APEX2-Clcc1 fusions for electron microscopy visualization of interaction sites

Advanced microscopy techniques:

• Super-resolution microscopy to visualize co-localization at MAMs

• Co-localization analysis with known MAM proteins like SigmaR1

• Treatment with MitoTracker™ Red shows that Clcc1 and SigmaR1 associate at regions between the ER and mitochondria

Biochemical fractionation:

• Isolate MAM fractions using differential centrifugation protocols

• Perform immunoprecipitation from these fractions to identify Clcc1 interactors

• Use chemical crosslinking with agents like disuccinimidyl suberate (DSS) to capture protein complexes

Mass spectrometry analysis:

• Liquid chromatography-mass spectrometry (LC-MS) to identify Clcc1-interacting proteins

• Compare wild-type Clcc1 interactors with those of mutant variants (e.g., D25E)

• Analysis of the Clcc1D25E mutant reveals altered protein interactions, with increased cytoplasmic protein interactions compared to wild-type

Validation studies:

• Co-immunoprecipitation to confirm interactions identified by mass spectrometry

• Immunocytochemistry to visualize co-localization patterns

• Functional studies to determine the significance of these interactions

Recent research has validated Calnexin and SigmaR1 as novel Clcc1 interactors, confirming Clcc1's important role at MAMs .

What techniques are recommended for studying Clcc1's role in neurodegeneration and retinal development?

Studying Clcc1's role in neurodegeneration and retinal development requires multidisciplinary approaches:

Retinal development analysis:

• Histological examination of retinal layers in Clcc1-deficient models

• Clcc1 morpholino-treated zebrafish show significantly thinner inner plexiform layer (7.82 vs. control 12.30μm) and outer nuclear layer (9.1 vs. control 12.9μm)

• In situ hybridization reveals Clcc1 expression is strongest in the ganglion cell layer, outer nuclear layer, and retinal pigmented epithelium

• Electroretinography (ERG) to assess functional development of the retina

• Optical coherence tomography (OCT) for in vivo imaging of retinal structure

Neurodegeneration models:

• Temporal analysis of neuron loss in Clcc1-deficient mice

• C3H/HeSnJ inbred strain with Clcc1 mutation shows late-onset cerebellar degeneration

• Conditional knockout mice show cell-autonomous motor neuron loss and ALS-like pathologies

• Immunohistochemical detection of protein aggregates and neuroinflammation

Cell death and ER stress evaluation:

• TUNEL staining to detect apoptotic cells

• Assessment of UPR marker expression in specific neuronal populations

• GRP78 is upregulated in Clcc1-deficient granule cells in vivo

• Analysis of ubiquitinated protein inclusions accumulating in neurons before degeneration

Genetic rescue approaches:

• BAC transgenic mice carrying wild-type Clcc1 for rescue experiments

• Temporal control of Clcc1 expression using inducible systems

• Introduction of mutations affecting specific Clcc1 functions

Human disease modeling:

• Analysis of patient samples with CLCC1 mutations (c.75C>A, p.D25E) associated with retinitis pigmentosa

• Generation of mutation knock-in mouse models to recapitulate human disease

These techniques can reveal how Clcc1 dysfunction leads to specific neurodegenerative phenotypes and retinal abnormalities.

How does Clcc1 contribute to lipid metabolism in hepatocytes and what methods can be used to study this?

Clcc1 plays a critical role in hepatic lipid metabolism that can be investigated using the following methods:

Lipid analysis techniques:

• Thin-layer chromatography (TLC) shows increased TAG (1.5-2 fold) in Clcc1 knockout cells

• Liver-specific knockout mice (Clcc1 HepKO) show dramatic increases in TAG and CE

• Oil Red O staining reveals neutral lipid accumulation in Clcc1 HepKO livers

• Electron microscopy demonstrates accumulation of enlarged lipid droplets

Lipid flux measurements:

• Pulse-chase assays using fluorescently labeled fatty acids show that loss of Clcc1 increases TAG biosynthesis and decreases TAG breakdown

• VLDL secretion assays to assess hepatic lipid export

• Analysis of plasma indicates reduced TAG and HDL in Clcc1 HepKO mice, indicating defects in hepatic lipid secretion

Phenotypic assessments:

• Liver-specific Clcc1 knockout results in:

  • 2-fold increase in liver weight/body weight ratio

  • Enlarged, whitened livers indicating lipid accumulation

  • Liver damage indicated by elevated AST levels

  • Severe MASH pathologies including hepatocyte ballooning, fibrosis, and immune cell infiltration

  • Livers that float in water due to high lipid content, unlike control livers which sink

Molecular mechanism investigation:

• Analysis of lipid droplet proteins shows reduced PLIN2 levels in Clcc1 KO cells despite high amounts of lipid droplets

• Proteasome inhibitor MG132 rescues PLIN2 levels, indicating post-translational degradation

• Immunofluorescence reveals large lipid droplets devoid of PLIN2 staining

Rescue experiments:

• Clcc1 overexpression rescues PLIN2-GFP localization to lipid droplets and PLIN2-GFP levels in Clcc1 KO cells

• Re-introduction of Clcc1 to knockout mice reverses pathologic defects including whitening appearance, hepatocyte ballooning, fibrosis, and immune cell infiltration

These findings demonstrate that Clcc1 is critical for normal hepatic lipid metabolism and protection against lipotoxicity.

What are the challenges in interpreting data from Clcc1 overexpression experiments?

Interpreting data from Clcc1 overexpression experiments presents several challenges that researchers should consider:

Expression level considerations:

• Strong promoters may yield non-physiological protein levels

• In immunofluorescence studies, transfected cells express approximately 2-3 times the amount of Clcc1 compared to endogenous levels

• Transient or stable overexpression of Clcc1-CR under control of a strong promoter reduces HSV-1 nuclear egress in control cells and poorly rescues the nuclear egress defect in Clcc1-KO cells

• Stable expression under a weak promoter provides better rescue of phenotypes, suggesting optimal expression levels are critical

Localization artifacts:

• Excessive Clcc1 expression can saturate ER retention mechanisms

• Both wild-type and p.D25E mutant Clcc1 proteins show a reticular pattern consistent with ER localization when overexpressed

• Verification of proper localization using co-localization with ER markers is essential

Functional effects of tagging:

• Tags may interfere with channel function or protein interactions

• The use of CRISPR-resistant (CR) gene variants with silent mutations may be necessary for rescue experiments

Multimerization considerations:

• Clcc1 forms homomultimers, and overexpression may alter the stoichiometry of these complexes

• Dominant-negative effects can occur with certain mutations (e.g., K298A)

Rescue experiment design:

• Single-cell Clcc1 rescue clones show varying degrees of phenotype rescue (30-80% in some clones, >90% in others)

• Multiple clones should be tested to account for clonal variation

• Both bulk rescue pools and single-cell clones should be evaluated

To address these challenges, researchers should use complementary approaches such as rescue of knockout phenotypes with near-physiological expression levels rather than relying solely on overexpression data.

What are the experimental considerations when studying the interplay between Clcc1 and calcium signaling?

Studying the interplay between Clcc1 and calcium signaling requires consideration of several factors:

Regulatory mechanisms:

• Clcc1 channel activity is inhibited by luminal Ca2+ through conserved residues D25 and D181

• This creates a complex regulatory relationship where changes in ER calcium can affect Clcc1 function

Counter-ion hypothesis:

• Depletion of Clcc1 reduces ER Ca2+ release, suggesting a counter-ion mechanism

• Clcc1 may provide an anion current that balances charge during calcium release

• Experimental designs should test this relationship by manipulating both calcium and chloride levels

Calcium-binding domains:

• D25 (mutated in retinitis pigmentosa) and D181 are key residues for Ca2+ binding

• Mutation of these residues can be used to study calcium-dependent regulation

• The D25E mutation associated with retinitis pigmentosa may affect calcium binding and regulation

MAM localization implications:

• Clcc1 localizes to MAMs where it can influence calcium transfer between ER and mitochondria

• Co-localization with SigmaR1, a MAM-associated protein involved in calcium signaling, suggests functional interaction

• Experiments should examine whether Clcc1 affects calcium signaling at these contact sites

Disease-relevant conditions:

• Conditional knockout of Clcc1 causes motor neuron loss and characteristic ALS pathologies

• Investigation of calcium dysregulation in disease models can provide insights into pathological mechanisms

• Comparison of calcium handling in cells expressing wild-type versus disease-associated Clcc1 mutants

When designing experiments, researchers should remember that the relationship between Clcc1 and calcium involves multiple feedback mechanisms and interactions with other ion channels and transporters.

How can researchers determine the significance of Clcc1 genetic variants in disease contexts?

Determining the significance of Clcc1 genetic variants requires a comprehensive approach:

Genetic and population studies:

• Identify variants in patient cohorts (e.g., c.75C>A, p.D25E in retinitis pigmentosa)

• Determine allele frequencies in general and disease populations

• Analyze haplotypes to identify potential founder effects, as seen with the D25E mutation in Pakistani families

Functional characterization:

• Electrophysiological studies to measure changes in channel conductance, as performed with ALS-associated mutations

• Cell biological approaches to assess protein localization and ER stress responses

• Protein interaction studies using methods like LC-MS to identify altered interactomes, as shown with the D25E variant

Animal modeling:

• Generate knock-in mice carrying specific mutations

• 10% of K298A heterozygous mice develop ALS-like symptoms, demonstrating dominant-negative effects

• Compare phenotypes to human disease manifestations

• Assess tissue-specific effects of mutations (e.g., retina for D25E, motor neurons for ALS variants)

Rescue experiments:

• Test whether wild-type Clcc1 can rescue phenotypes in knockout or mutant models

• Expression of wild-type Clcc1 in trans can rescue defects in Clcc1-KO cells

• Determine if the rescue is complete or partial, providing insights into mutation mechanisms

Structure-function analysis:

• Map mutations to specific domains (e.g., D25 in N-terminal cytoplasmic domain, K298 in intraluminal loop)

• Correlate mutations with specific functional defects (e.g., calcium binding, PIP2 sensing)

• Use computational approaches to predict structural impacts of mutations

Domain-specific experiments:

• The D25E mutation affects the cytoplasmic N-terminus, potentially altering its structure and interactions

• Analysis of the Clcc1D25E interactome reveals changes in protein interactions compared to wild-type

These approaches can help determine whether variants are pathogenic and elucidate the mechanisms by which they cause disease.

What techniques are available for studying Clcc1's role in viral replication cycles?

Recent discoveries have highlighted Clcc1's importance in viral replication, particularly for herpesviruses:

Nuclear egress quantification:

• Flow-cytometry-based assay to measure capsid nuclear egress in herpes simplex virus 1 (HSV-1)

• Loss of Clcc1 results in a defect in HSV-1 nuclear egress, with efficiencies reduced to ~20%, comparable to control UL34-null HSV-1 mutant

Viral titer measurements:

• Multiple-step growth curves to assess viral replication

• Loss of Clcc1 results in ~1000-fold drop in HSV-1 titer

• Similar defects observed in closely related herpesviruses (HSV-2 and pseudorabies virus)

Ultrastructural analysis:

• Electron microscopy to visualize viral capsid localization

• Loss of Clcc1 causes accumulation of capsid-containing primary enveloped virions (PEVs)

Rescue experiments:

• Generation of CRISPR-resistant (CR) Clcc1 variants

• Introduction of silent mutations to destroy sgRNA target sites

• Expression of Clcc1-CR under a weak promoter in knockout cells rescues nuclear egress defects and viral replication

Evolutionary analysis:

• Identification of Clcc1 homologs in diverse herpesvirus genomes

• Homologs found in Malacoherpesviridae and Alloherpesviridae (infecting mollusks and fish)

• Analysis suggests ancient evolutionary relationship between Clcc1 and herpesviruses

Mechanistic studies:

• Investigation of membrane fusion defects in Clcc1-deficient cells

• Relationship between nuclear pore complex insertion defects and viral nuclear egress

• Analysis of how Clcc1 facilitates membrane fusion during viral replication

These techniques reveal how Clcc1 plays a crucial role in membrane fusion during both normal nuclear envelope morphogenesis and herpesvirus nuclear egress.

How can microscale thermophoresis be used to study the interaction between Clcc1 and its regulatory molecules?

Microscale thermophoresis (MST) offers several advantages for studying Clcc1 interactions with regulatory molecules:

Principle and advantages:

• MST measures changes in fluorescence signal as molecules move along temperature gradients

• Requires minimal sample volumes and can detect interactions in near-native conditions

• Suitable for membrane proteins like Clcc1 when properly solubilized

Application to Clcc1-Ca2+ interaction:

• Label purified Clcc1 with a fluorescent dye or use GFP-tagged Clcc1

• Titrate increasing concentrations of Ca2+ and measure binding isotherms

• Determine dissociation constants (Kd) for interaction with the conserved residues D25 and D181

• Compare wild-type binding with D25E mutant to understand how this disease-associated mutation affects Ca2+ binding

PIP2 binding studies:

• Use fluorescently labeled Clcc1 and titrate PIP2-containing liposomes

• Alternatively, use fluorescently labeled PIP2 analogs and titrate Clcc1

• Investigate the role of the K298 residue in PIP2 binding

• Compare binding affinities of different Clcc1 mutants

Protein-protein interaction analysis:

• Study Clcc1 interactions with validated partners like SigmaR1 and Calnexin

• Compare binding profiles of wild-type vs. D25E mutant Clcc1

• Correlate interaction changes with alterations in cellular function

Experimental considerations:

• Ensure proper protein folding and stability throughout the experiment

• Optimize detergent conditions to maintain native-like membrane protein structure

• Include appropriate controls to account for non-specific binding

• Consider using nanodiscs or amphipols to provide a more native-like environment for Clcc1

MST can provide quantitative binding parameters that help explain the regulatory mechanisms governing Clcc1 function and how these are affected by disease-associated mutations.