Recombinant Mouse V-type proton ATPase 16 kDa proteolipid subunit (Atp6v0c)

What is the basic structure and function of mouse Atp6v0c in the V-ATPase complex?

Atp6v0c is the c-subunit of the V0 domain of vacuolar ATPase (V-ATPase), a 155-amino acid H+ transport protein that forms the proteolipid c-ring of the V-ATPase complex. The V-ATPase is composed of two domains: a peripheral V1 domain (consisting of A, B, C, D, E, F, G, and H subunits) that hydrolyzes ATP, and an integral V0 domain (consisting of a, b, c, d, and e subunits) that translocates protons. The Atp6v0c subunit plays a pivotal role in forming the proteolipid pore that enables proton flow across membranes, which is critical for establishing and maintaining pH gradients in cellular compartments .

The protein is highly conserved across species, underscoring its essential functions. Structural studies have revealed that multiple c-subunits form a ring-like structure within the V0 domain that rotates during the proton translocation process, coupling the energy of ATP hydrolysis to proton movement .

How does Atp6v0c expression vary across different tissues in mice?

Atp6v0c is ubiquitously expressed across multiple tissues in mice, but shows particularly high expression in the central nervous system, predominantly in the brain cortex, cerebellum, hypothalamus, and hippocampus. The expression pattern remains relatively consistent throughout the lifespan, suggesting a continuous requirement for V-ATPase function in these regions .

Expression analysis using in situ hybridization techniques in zebrafish has demonstrated that the ortholog atp6v0c2 is expressed in a subset of CNS neurons, beginning several hours after the emergence of post-mitotic neurons. This neuron-specific expression pattern suggests specialized roles in neural function beyond general cellular pH regulation .

What are the recommended methods for knocking out Atp6v0c in mouse models?

Creating Atp6v0c knockout mouse models requires careful consideration due to the embryonic lethality observed in homozygous knockouts. The recommended approach involves:

• Generating conditional knockout models using the Cre-loxP system, as demonstrated in published research where loxP sites were placed at intron 1-2 and the 3'-untranslated region of the Atp6v0c gene .

• For germline deletion, crossing the floxed Atp6v0c mice with mice expressing EIIa-Cre recombinase, which results in complete deletion of exons 2 and 3 (encoding 129 of the 155 amino acids) .

• For tissue-specific studies, crossing with tissue-specific Cre lines to circumvent embryonic lethality.

It's important to note that homozygous Atp6v0c knockout mice exhibit neonatal lethality, with embryos unable to survive beyond E5.0-E5.5. This early lethality highlights the critical requirement for Atp6v0c function during early embryogenesis .

What techniques are most effective for studying Atp6v0c localization and function in cellular models?

Several complementary techniques have proven effective for investigating Atp6v0c localization and function:

• Immunofluorescence microscopy: Using antibodies against Atp6v0c along with markers for cellular compartments (e.g., lysosomes, endosomes) to determine subcellular localization.

• LysoSensor fluorescence assays: To measure lysosomal acidification, which directly reflects V-ATPase function. Studies have shown reduced LysoSensor fluorescence in cells with ATP6V0C variants, indicating impaired acidification .

• pH-sensitive fluorescent proteins: Fusion proteins with pH-sensitive GFP variants targeted to different cellular compartments can provide real-time monitoring of pH changes.

• Co-localization studies: ATP6V0C2 protein has been shown to co-localize with the presynaptic vesicle marker SV2, indicating its involvement in neurotransmitter storage and/or secretion in neurons .

• CRISPR-Cas9 genome editing: For creating precise mutations to study specific protein domains and their functions.

• Heterologous expression systems: Studies using yeast models have demonstrated the utility of expressing ATP6V0C variants in VMA3 (yeast homolog) deletion strains to assess functional consequences of specific mutations .

How does Atp6v0c function affect embryonic development in mouse models?

Atp6v0c plays critical roles in early embryonic development:

• Embryonic lethality: Homozygous Atp6v0c knockout mice exhibit embryonic lethality around E5.0-E5.5, immediately after implantation. This indicates an essential role for V-ATPase function during early embryogenesis .

• Cell polarity maintenance: In Atp6v0c mutant embryos, the apical-basal polarity of the visceral endoderm (VE) epithelium is not properly maintained, resulting in abnormal tissue morphology. This suggests that V-ATPase function is imperative for establishing and/or maintaining epithelial cell polarity during early embryogenesis .

• Tissue organization: While mutant embryos can differentiate extraembryonic ectoderm (ExE), epiblast, and VE around E5.2, the ExE and epiblast are reduced in size, and the patterning of epiblast and VE is impaired, with some regions showing abnormal expression of both OCT3/4 and GATA6 .

• Protein localization defects: In Atp6v0c-deficient embryos, polarity markers such as atypical protein kinase C (aPKC) show broader distribution, indicating expansion and/or randomization of apical characteristics. Additionally, ezrin, which normally connects the apical cell surface to the cortical actin cytoskeleton, becomes mislocalized to the entire plasma membrane .

What is the relationship between ATP6V0C variants and epilepsy in humans?

Research has established significant connections between ATP6V0C variants and epilepsy:

• Clinical phenotypes: Heterozygous ATP6V0C variants have been identified in patients with febrile seizures (FS) and epilepsy with febrile seizures plus (EFS+). These patients typically experience their first febrile seizure at 7-8 months of age .

• Variant types: Both missense variants (e.g., c.64G>A/p.Ala22Thr) and frameshift variants (e.g., c.361_373del/p.Thr121Profs*7) have been identified in affected families, with co-segregation of the variants with the disease phenotype .

• Functional impact: The variants are predicted to alter the structure of the proteolipid c-ring or produce truncated transcripts, leading to impaired V-ATPase function. This disruption affects synaptic vesicle acidification and neurotransmitter packaging, potentially contributing to seizure susceptibility .

• Differential diagnosis: ATP6V0C mutation screening has clinical significance in differentiating patients with ATP6V0C-related epilepsy from those with Dravet syndrome caused by SCN1A mutations, as the conditions present with similar clinical manifestations but respond differently to antiepileptic treatments .

The prevalence of ATP6V0C variants in epilepsy cohorts was found to be significantly higher than in control populations, strengthening the evidence for its role as a causative gene in these conditions .

How can model organisms be used to assess the functional impact of ATP6V0C variants identified in human patients?

Multiple model organisms provide valuable insights into ATP6V0C variant pathogenicity:

• Yeast models: Expression of patient-derived ATP6V0C variants in yeast VMA3 (ATP6V0C homolog) deletion strains allows assessment of growth in different media conditions. Studies have revealed that cells expressing patient variants show reduced growth in media containing varying concentrations of CaCl₂, indicating impaired V-ATPase function .

• Drosophila models: Knockdown of ATP6V0C in Drosophila has resulted in increased duration of seizure-like behavior, directly modeling the neurological phenotypes observed in patients .

• C. elegans models: Expression of selected patient variants in C. elegans has demonstrated reduced growth, motor dysfunction, and reduced lifespan, providing evidence for pathogenicity and insights into disease mechanisms .

• Zebrafish models: Studies of zebrafish atp6v0c2 have shown its involvement in neurotransmitter storage/secretion and control of neuronal excitability, offering a platform to study neurological phenotypes associated with variant function .

The combined use of these models allows for comprehensive assessment of variant effects across evolutionary conserved functions and provides complementary evidence for pathogenicity determination.

What methods are most effective for assessing V-ATPase activity in relation to ATP6V0C variants?

Several sophisticated methods can be employed to evaluate V-ATPase activity:

• LysoSensor fluorescence assays: These directly measure lysosomal acidification capacity. Patient-derived cells or model systems expressing ATP6V0C variants have shown reduced LysoSensor fluorescence, indicating impaired acidification .

• Proton transport assays: Using pH-sensitive dyes or electrodes to measure proton transport across membranes in isolated vesicles or organelles.

• ATP hydrolysis assays: Measuring the rate of ATP hydrolysis by the V1 domain, which is coupled to proton transport by the V0 domain containing ATP6V0C.

• Electron microscopy and structural studies: To assess whether variants affect the assembly of the c-ring structure, which is critical for V-ATPase function.

• Growth assays in selective media: Yeast expressing ATP6V0C variants show reduced growth in media containing varying concentrations of CaCl₂, providing a functional readout of V-ATPase activity .

• Subcellular localization studies: Co-localization analysis with compartment-specific markers can reveal whether variants affect proper targeting of the V-ATPase complex.

These complementary approaches provide multifaceted evidence of functional impacts and help elucidate the mechanisms by which specific variants impair V-ATPase activity.

How does ATP6V0C interact with microRNAs and what are the implications for gene regulation?

Research has revealed important interactions between ATP6V0C and microRNAs:

• Target site identification: ATP6V0C contains a 7-mer target site for the microRNA US25-1 within its open reading frame (ORF), specifically starting at nucleotide 186 with the sequence GAGCGGT .

• Functional validation: Luciferase assays using constructs containing the ATP6V0C target site have confirmed that US25-1 induces significant reduction in expression. Furthermore, mutation of the target seed region restores luciferase activity, indicating that the identified target site is both sufficient and necessary for US25-1-specific inhibition .

• Regulatory significance: As one of the most highly enriched targets identified in systematic microRNA analysis, ATP6V0C regulation by microRNAs may represent an important mechanism for modulating V-ATPase activity in different cellular contexts .

• Methodological approaches: The study of these interactions has employed several techniques:

  • Luciferase reporter assays with wild-type and mutated target sites

  • Co-transfection with microRNA mimics

  • Western blot analysis to confirm protein-level changes

  • Bioinformatic prediction of target sites

These findings suggest that post-transcriptional regulation of ATP6V0C may be an important mechanism controlling V-ATPase function and cellular pH homeostasis in various physiological and pathological contexts.

What is known about the role of ATP6V0C in cancer and how might it serve as a potential therapeutic target?

ATP6V0C has emerging implications in cancer biology:

• Expression in cancer: ATP6V0C has been identified as differentially expressed in acute myeloid leukemia (AML) samples. Specifically, it is among the overexpressed genes unique to the AML-M2 subtype, suggesting a potential role in leukemia pathogenesis .

• Acidification and tumor microenvironment: As a component of V-ATPase, ATP6V0C contributes to the acidification of the tumor microenvironment, which can promote cancer cell invasion, metastasis, and drug resistance.

• V-ATPase inhibition in cancer: Pharmacological inhibitors of V-ATPase, such as bafilomycin and concanamycin, have shown anti-cancer effects in preclinical models, suggesting that targeting ATP6V0C function might have therapeutic potential.

• Research methodologies: Studies investigating ATP6V0C in cancer contexts have employed:

  • Gene expression profiling using microarrays

  • RT-PCR validation of differential expression

  • Functional studies in cancer cell lines

  • Analysis of clinical samples from cancer patients

The involvement of ATP6V0C in fundamental cellular processes such as vesicular acidification, autophagy, and membrane trafficking makes it a potentially important contributor to cancer cell biology, warranting further investigation as a therapeutic target.

How do ATP6V0C mutations affect neurodevelopment beyond seizure susceptibility?

Recent research has uncovered broader neurodevelopmental impacts of ATP6V0C variants:

• Neurodevelopmental syndrome: Heterozygous ATP6V0C variants have been identified in 27 patients with a novel syndrome characterized by developmental delay, epilepsy, and other neurological features .

• Variant spectrum: The identified variants include 22 missense substitutions (18 unique), four frameshifting variants, and one stop-loss variant. Some variants (p.A138P, p.A149T, and p.L150F) are recurrent, being found in multiple unrelated individuals .

• Functional consequences: These variants impair V-ATPase function, affecting processes such as:

  • Synaptic vesicle acidification

  • Neurotransmitter packaging and release

  • Neuronal excitability

  • Endosomal-lysosomal function, which is critical for neuronal development and synaptic plasticity

• Research approaches: Investigation of these effects has employed:

  • Patient cohort studies

  • Variant identification through next-generation sequencing

  • Functional characterization in model organisms

  • Computational modeling of variant effects on protein structure

Understanding the broader neurodevelopmental impacts of ATP6V0C variants is an emerging area of research that may provide insights into the role of V-ATPase in brain development and function beyond seizure susceptibility.

What are the latest findings regarding ATP6V0C interactions with viral proteins and implications for infectious disease research?

Emerging research has identified important interactions between ATP6V0C and viral processes:

• Host-virus interactome: ATP6V0C has been identified as a significant host factor in genome-wide CRISPR-Cas screens for influenza A virus. It is among the top ranked proviral factors, indicating its importance for viral replication .

• Vacuolar acidification pathway: ATP6V0C's role in the vacuolar acidification pathway is particularly relevant for viruses that require endosomal acidification for entry and uncoating, including influenza A virus .

• Therapeutic targeting potential: The consistent identification of ATP6V0C across multiple screens suggests it could be a promising target for broad-spectrum antiviral strategies.

• Research methodologies: Studies investigating these interactions have employed:

  • Pooled genome-wide CRISPR-Cas screens

  • Meta-analysis approaches such as the algorithm of meta-analysis by information content (MAIC)

  • Hierarchical clustering of screen results

  • Pathway enrichment analysis

These findings highlight the importance of ATP6V0C in viral infection processes and suggest potential applications in the development of novel antiviral strategies targeting host factors rather than viral proteins.

What are the challenges in generating recombinant mouse ATP6V0C for in vitro studies?

Production of functional recombinant mouse ATP6V0C presents several technical challenges:

• Hydrophobic nature: As a membrane protein with multiple transmembrane domains, ATP6V0C is highly hydrophobic, which complicates expression, purification, and solubilization.

• Structural integrity: Maintaining the native structure of ATP6V0C during recombinant expression is challenging, as it normally functions as part of a multi-subunit complex.

• Expression systems: Traditional bacterial expression systems often result in inclusion body formation, necessitating careful optimization of expression conditions or alternative expression systems such as:

  • Insect cell expression systems

  • Mammalian cell expression systems

  • Cell-free protein synthesis systems with added lipids or detergents

• Purification considerations: Purification typically requires specialized detergents for solubilization and chromatography techniques optimized for membrane proteins.

• Functional validation: Assessing the functionality of recombinant ATP6V0C requires reconstitution into liposomes or nanodiscs to enable proton transport assays.

These technical challenges have led researchers to pursue alternative approaches such as studying ATP6V0C in native membrane preparations or employing tagged versions that can be co-purified with interaction partners.

How can researchers accurately quantify ATP6V0C gene expression in different experimental contexts?

Precise quantification of ATP6V0C expression requires appropriate methodological considerations:

• RT-qPCR optimization: For accurate RT-qPCR analysis:

  • Selection of appropriate reference genes is crucial, as common housekeeping genes may vary across tissues or experimental conditions

  • Primer design should account for potential splice variants

  • cDNA synthesis protocols should be optimized for consistent reverse transcription efficiency

• Digital PCR approaches: Droplet digital PCR (ddPCR) has been used successfully for ATP6V0C quantification, offering absolute quantification without the need for standard curves .

• Protein-level quantification: Western blotting for ATP6V0C requires:

  • Careful membrane protein extraction

  • Appropriate antibody validation

  • Quantification relative to stable membrane protein controls

• Expression profiling approaches: Microarray and RNA-seq analyses have been employed to measure ATP6V0C expression in various tissues and disease states, requiring:

  • Rigorous data normalization

  • Statistical analysis accounting for multiple testing

  • Validation of findings using independent methods

• Single-cell analysis: Recent advances in single-cell RNA-seq can provide insights into cell-type-specific expression patterns of ATP6V0C.