Recombinant Mouse GRAM domain-containing protein 1C (Gramd1c)

What is GRAMD1C and what is its primary cellular function?

GRAMD1C is a cholesterol transport protein localized primarily to the endoplasmic reticulum (ER). Its primary function involves regulating cholesterol transport between cellular organelles, particularly between the mitochondria and ER. GRAMD1C acts as a negative regulator of starvation-induced autophagy and influences mitochondrial oxidative phosphorylation .

To investigate GRAMD1C's function experimentally, researchers typically use genetic approaches to deplete or overexpress the protein, followed by functional assays measuring autophagy flux, cholesterol distribution, and mitochondrial respiration. Knockout or knockdown models using CRISPR-Cas9 or siRNA techniques allow for detailed examination of its regulatory effects on autophagy and mitochondrial function .

What domains are present in GRAMD1C and what are their specific functions?

GRAMD1C contains two primary functional domains:

• GRAM domain: This domain is responsible for targeting the protein to specific membranes and mediates interaction with mitochondria. Experimental evidence shows that the GRAM domain alone (EGFP-GRAM) can be detected in isolated mitochondria and interacts with outer mitochondrial membrane proteins like TOMM70/TOMM70A .

• VASt domain: This domain is essential for cholesterol transport activity. Experiments with GRAMD1C mutants lacking the VASt domain (ΔVASt) demonstrate that this domain is critical for GRAMD1C's function in regulating autophagy and mitochondrial bioenergetics .

To study domain functionality, rescue experiments in GRAMD1C knockout cells are particularly informative. While wild-type GRAMD1C can rescue the phenotypes of knockout cells, mutants lacking either the GRAM domain (ΔGRAM) or the VASt domain (ΔVASt) fail to do so, indicating the essential nature of both domains for proper protein function .

How does GRAMD1C relate to other GRAM domain-containing proteins?

GRAMD1C belongs to a family of GRAM domain-containing proteins that includes GRAMD1A, GRAMD1B, GRAMD2, and GRAMD3. Phylogenetic analysis reveals that these proteins are orthologs of a family anchored by the presence of the GRAM domain .

While these proteins share structural similarities, they exhibit functional specialization:

• GRAMD1A and GRAMD1B: Associated with distinct ER-PM contact sites compared to GRAMD1C. They show an opposite pattern of expression in cancer progression compared to GRAMD1C, with high expression of GRAMD1A and GRAMD1B being unfavorable for survival in clear cell renal carcinoma (ccRCC) .

• GRAMD2a: Defines distinct ER-PM contacts from GRAMD1A and functions as a tether that pre-marks sites for STIM1 recruitment and store-operated calcium entry (SOCE) .

To experimentally differentiate between these family members, researchers can use specific antibodies for immunoprecipitation and immunofluorescence, as well as gene-specific siRNAs to selectively deplete individual family members .

How does GRAMD1C regulate autophagosome biogenesis and autophagy progression?

GRAMD1C negatively regulates starvation-induced autophagy at the initiation stage. Mechanistically, this regulation appears to involve:

• Regulation of membrane curvature: GRAMD1C may suppress membrane curvature at ER-associated autophagy initiation sites .

• Control of early autophagic marker recruitment: GRAMD1C-depleted cells show increased numbers of ATG13, ATG16L1, and WIPI2B puncta (early autophagic markers) under starvation conditions .

• Cholesterol modulation at autophagosome formation sites: Since the autophagosome membrane is enriched in unsaturated fatty acids and has low cholesterol content, GRAMD1C may regulate autophagosome biogenesis by controlling local cholesterol levels .

To measure autophagy flux in GRAMD1C-depleted cells, researchers utilize several methodologies:

• mRFP-GFP-LC3 reporter assays to quantify autophagosomes (yellow puncta) and autolysosomes (red-only puncta)

• Turnover of radioactively labeled long-lived proteins

• Western blotting for autophagy markers like LC3-II

• Treatment with Bafilomycin A1 (BafA1) to block autolysosome formation and assess flux

Experimental data shows a significant increase in autolysosomes (red-only puncta) in GRAMD1C-depleted cells compared to controls, indicating enhanced autophagic flux .

What is the role of GRAMD1C in mitochondrial cholesterol transport and how does it affect mitochondrial function?

GRAMD1C facilitates cholesterol transport from mitochondria to the ER. This role is evidenced by:

• Increased mitochondrial cholesterol in GRAMD1C knockout cells: Using mCherry-D4 (a cholesterol-binding domain) and cholesterol oxidase-based quantification, researchers have demonstrated higher mitochondrial cholesterol levels in GRAMD1C knockout cells compared to control cells .

• Reduced ER cholesterol in GRAMD1C knockout cells: This is indicated by increased expression of SREBP target genes, suggesting compensatory responses to lower ER cholesterol levels .

• Altered abundance of cholesterol-associated proteins: Proteomic analysis of GRAMD1C-depleted cells shows changes in various cholesterol-associated proteins including STARD9, ERLIN, SQLE, NPC2, and APOB .

The impact of GRAMD1C on mitochondrial function includes:

• Enhanced mitochondrial respiration: GRAMD1C depletion increases ATP-production linked respiration and maximal respiratory capacity as measured by Seahorse XF Analyzer .

• Preserved OXPHOS protein levels: Western blot analysis shows no significant changes to oxidative phosphorylation (OXPHOS) proteins in GRAMD1C knockdown cells, suggesting that the enhanced respiration is not due to changes in the mitochondrial proteome .

• Unaltered mitochondrial membrane potential and ROS: Despite changes in respiration, mitochondrial membrane potential and total cellular reactive oxygen species remain unchanged in GRAMD1C knockdown cells .

To experimentally assess these functions, researchers use:

• Seahorse XF Analyzer for measuring oxygen consumption rates

• Cholesterol quantification in isolated mitochondria

• Western blotting for OXPHOS proteins

• qPCR for SREBP target gene expression

How does GRAMD1C interact with mitochondria?

GRAMD1C interacts with mitochondria through its GRAM domain. This interaction has been demonstrated through several experimental approaches:

• Live microscopy: Studies using EGFP-GRAM domain constructs show dynamic interaction with mitochondrial structures .

• Mitochondrial isolation: The GRAM domain of GRAMD1C (EGFP-GRAM) is detected in isolated mitochondria .

• Immunoprecipitation: Assays confirm the interaction of the GRAM domain with the outer mitochondrial membrane protein TOMM70/TOMM70A .

• Proteomic analysis: GRAMD1C primarily interacts with proteins of mitochondrial origin. GO-term enrichment analysis of GRAMD1C interactome reveals enrichment for mitochondrial proteins .

• Specific mitochondrial interactors: Several mitochondrial proteins such as NDUFAF2, SHDB, and ATAD3A, as well as ER-mitochondria contact site proteins like VDAC1 and ACSL4, are enriched in the interactome of full-length GRAMD1C but absent in that of the ΔGRAM mutant .

These findings suggest that GRAMD1C localizes to ER-mitochondria contact sites and facilitates cholesterol transport between these organelles through its GRAM domain-mediated interactions with the mitochondrial outer membrane.

What is the clinical significance of GRAMD1C expression in cancer, particularly in clear cell renal carcinoma (ccRCC)?

GRAMD1C expression has significant clinical implications in cancer, especially in clear cell renal carcinoma (ccRCC):

• Correlation with survival: Higher GRAMD1C expression is associated with improved patient survival in ccRCC. By contrast, low expression of GRAMD1A and GRAMD1B is favorable for survival .

• Correlation with tumor stage: GRAMD1C expression decreases in advanced stage tumors, while GRAMD1A and GRAMD1B show increased expression in late-stage tumor samples compared to early-stage tumor samples .

• Association with immune infiltration: GRAMD1C transcript levels have been shown to positively correlate with the level of immune cell infiltration in ccRCC patients .

• Co-expression with mitochondrial genes: GRAMD1C is co-expressed with several mitochondrial genes in ccRCC samples, including AUH, AK3, MICU2, and SIRT5, with Pearson's correlation values above 0.45 .

Experimental validation of these clinical associations includes:

• Colony formation assays showing that depletion of GRAMD1A and GRAMD1B significantly decreases the ability of 786-O ccRCC cells to form colonies

• Migration assays (wound healing) showing slightly decreased migration in siGRAMD1B-treated cells

• Seahorse analysis demonstrating that GRAMD1C depletion promotes ATP-production linked respiration in ccRCC 786-O cells

What experimental approaches can be used to study GRAMD1C function and localization?

Multiple experimental approaches can be employed to study GRAMD1C:

For functional studies:

• Genetic manipulation:

  • siRNA-mediated knockdown using different siRNA oligos targeting GRAMD1C

  • CRISPR-Cas9-mediated knockout to generate GRAMD1C KO cell lines

  • Rescue experiments with wild-type and mutant constructs (ΔGRAM, ΔVASt)

• Autophagy assays:

  • mRFP-GFP-LC3 reporter assays to quantify autophagosomes and autolysosomes

  • Turnover of radioactively labeled long-lived proteins

  • Treatment with autophagy inhibitors like Bafilomycin A1 (BafA1)

  • Quantification of early autophagic markers (ATG13, ATG16L1, WIPI2B)

• Mitochondrial function assays:

  • Seahorse XF Analyzer for measuring oxygen consumption rates and ATP production

  • Western blotting for OXPHOS proteins

  • Mitochondrial membrane potential measurements

  • Reactive oxygen species (ROS) quantification

• Cholesterol transport and quantification:

  • mCherry-D4 (cholesterol-binding domain) for visualizing cholesterol

  • Cholesterol oxidase-based quantification in isolated mitochondria

  • qPCR for SREBP target gene expression

For localization studies:

• Fluorescence microscopy:

  • Expression of GRAMD1C-EGFP, ΔGRAM-EGFP, and EGFP-GRAM domain

  • Co-localization studies with organelle markers

  • Live-cell imaging to observe dynamic interactions

• Biochemical fractionation:

  • Isolation of mitochondria and detection of GRAMD1C or its domains

  • Density gradient fractionation to separate different membrane compartments

• Protein-protein interaction studies:

  • Immunoprecipitation followed by mass spectrometry

  • Co-immunoprecipitation with specific binding partners

  • Proximity labeling approaches to identify proteins in the vicinity of GRAMD1C

These methodologies provide complementary approaches to comprehensively characterize GRAMD1C function, regulation, and interactions in cellular physiology and pathophysiology.

How can researchers effectively validate GRAMD1C knockout or knockdown models?

• Validation of genetic manipulation:

  • Western blotting with specific antibodies against GRAMD1C to confirm protein reduction

  • qRT-PCR to assess mRNA levels

  • Using multiple siRNA oligos to rule out off-target effects

  • Sequencing confirmation of genomic edits in CRISPR-Cas9 generated knockout lines

• Rescue experiments:

  • Re-expression of wild-type GRAMD1C in knockout or knockdown cells to restore normal phenotypes

  • Use of domain-specific mutants (ΔGRAM, ΔVASt) as negative controls that should not rescue phenotypes

  • Dose-dependent rescue to establish specificity

• Phenotypic validation:

  • Assessment of expected consequences of GRAMD1C depletion:

    • Enhanced starvation-induced autophagy

    • Increased mitochondrial cholesterol levels

    • Enhanced mitochondrial respiration

• Controls for experimental procedures:

  • Include appropriate pharmacological controls (e.g., Bafilomycin A1 for autophagy assays)

  • Use non-targeting siRNAs as controls for knockdown experiments

  • Include wild-type cells as controls for knockout studies

What are the recommended experimental conditions for studying GRAMD1C-regulated autophagy?

To effectively study GRAMD1C's role in autophagy regulation, researchers should consider these experimental conditions:

• Autophagy induction methods:

  • Nutrient starvation: Incubation in EBSS (Earle's Balanced Salt Solution) is the preferred method to study GRAMD1C's role in starvation-induced autophagy

  • Avoid chemical inducers like rapamycin initially, as GRAMD1C's effects appear specific to starvation-induced autophagy

• Autophagy flux assessment:

  • Include Bafilomycin A1 (BafA1) treatment to block autophagosome-lysosome fusion

  • Compare both basal and starvation-induced conditions

  • Include appropriate time points (e.g., 2-4 hours for starvation)

• Reporter systems:

  • mRFP-GFP-LC3 is particularly useful as it distinguishes autophagosomes (yellow puncta) from autolysosomes (red-only puncta)

  • Quantify puncta per cell across multiple cells (at least 30-50 cells per condition)

  • Include controls for reporter expression levels

• Additional markers:

  • Examine early autophagic markers (ATG13, ATG16L1, WIPI2B) to assess GRAMD1C's role in autophagy initiation

  • Use co-localization analysis to study GRAMD1C's relationship with autophagy initiation sites

• Complementary methods:

  • Combine imaging with biochemical assays like LC3 turnover and long-lived protein degradation

  • Consider measuring related processes like mitochondrial function simultaneously

What are the challenges in studying GRAMD1C at ER-mitochondria contact sites?

Studying GRAMD1C at ER-mitochondria contact sites presents several technical challenges:

• Resolution limitations:

  • ER-mitochondria contact sites are typically 10-30 nm apart, below the resolution of conventional light microscopy

  • Super-resolution microscopy techniques (STED, STORM, PALM) are recommended for detailed visualization of these contacts

• Dynamic nature of contacts:

  • ER-mitochondria contacts are highly dynamic, requiring live-cell imaging with high temporal resolution

  • Careful consideration of imaging frequency and duration to avoid photobleaching and phototoxicity

• Protein tagging considerations:

  • Fluorescent tags may interfere with GRAMD1C function or localization, especially when placed near functional domains

  • C-terminal tagging is generally preferable since the C-terminus faces the cytosol

  • Validation of tagged constructs against endogenous protein behavior is essential

• Biochemical isolation challenges:

  • Purification of intact ER-mitochondria contact sites is technically difficult

  • Mitochondrial isolation protocols may disrupt transient interactions

  • Consider using proximity labeling approaches (BioID, APEX) to identify proteins in the vicinity of GRAMD1C at contact sites

• Functional assessment:

  • Measuring cholesterol transfer at specific contact sites requires specialized probes

  • Consider using targeted cholesterol sensors to specifically measure cholesterol levels at ER-mitochondria contacts

  • Correlate structural observations with functional measurements of cholesterol transport and mitochondrial function