Recombinant Rat Transmembrane protein 110 (Tmem110)

What is Tmem110 and what alternative designations exist in the literature?

Tmem110, also designated as STIMATE (STIM activating enhancer), is an endoplasmic reticulum (ER)-resident multi-transmembrane protein identified through proteomic studies focusing on ER-plasma membrane (ER-PM) junctions. These specialized junctional sites, also known as membrane contact sites, connect the endoplasmic reticulum and plasma membrane and are closely implicated in controlling lipid and calcium homeostasis in mammalian cells . The protein plays a critical role in calcium signaling regulation at cellular interfaces, particularly at the ER-PM junctional regions where calcium transport is tightly regulated .

What cellular compartments contain Tmem110?

Tmem110 is primarily localized to the cortical endoplasmic reticulum, the endoplasmic reticulum membrane, and specifically at contact sites between the endoplasmic reticulum and plasma membrane . It is a trans-ER protein with C-terminal and N-terminal regions projecting into the cytosol, making it strategically positioned to participate in membrane organization and calcium signaling regulation . This specific localization pattern enables Tmem110 to facilitate communication between different cellular compartments.

What is the primary functional role of Tmem110 in cellular signaling?

Tmem110 functions as a positive modulator of calcium flux mediated by the STIM-ORAI signaling pathway in vertebrates. The protein physically associates with STIM1 to promote a conformational switch from inactive toward an activated state, thereby coupling to and gating the ORAI calcium channels on the plasma membrane . This molecular interaction is fundamental to store-operated calcium entry, which is essential for numerous cellular processes including immune cell activation, muscle contraction, and gene expression .

What genetic manipulation approaches are effective for studying Tmem110 function?

Several genetic manipulation techniques have proven effective for investigating Tmem110 function:

• RNAi knockdown: Small interfering RNA (siRNA) targeting TMEM110 has been successfully employed to deplete the protein and study resulting phenotypes .

• CRISPR-Cas9 gene editing: Complete knockout of TMEM110 using CRISPR-Cas9 technology has been achieved in HEK293 cells, providing valuable tools for loss-of-function studies .

• Expression of RNAi-resistant TMEM110: This approach allows for rescue experiments to confirm specificity of RNAi-mediated phenotypes .

When designing siRNAs targeting TMEM110, researchers should carefully evaluate target sequences to ensure specificity and efficacy, as demonstrated in previous studies that successfully reduced TMEM110 expression and altered downstream calcium signaling and autophagy pathways .

What imaging techniques are most suitable for analyzing Tmem110 localization and function at ER-PM junctions?

Multiple complementary imaging approaches have been employed to study Tmem110 localization and function:

• Total Internal Reflection Fluorescence (TIRF) microscopy: This technique is particularly useful for visualizing ER structures near the plasma membrane. Studies have used DsRed-ER or RFP-ER markers combined with TIRF microscopy to monitor ER-PM junctions in living cells .

• Electron microscopy: This approach provides higher resolution for examining ER-PM junctions specifically. Researchers have used this method to identify junctions with spacing less than 20 nm, revealing that TMEM110-depleted cells have fewer ER-PM junctions per μm of plasma membrane compared to control cells .

• Fluorescence confocal microscopy: This technique has been used to analyze protein distributions and interactions, particularly in studies examining autophagy markers in relation to TMEM110 function .

The choice of imaging method should be guided by the specific research question, with TIRF microscopy offering advantages for monitoring dynamic changes across the entire cell footprint, while electron microscopy provides more definitive structural information about junction formation.

How can researchers effectively measure the impact of Tmem110 on calcium signaling?

To assess Tmem110's impact on calcium signaling, researchers can employ several methodological approaches:

• Calcium imaging with fluorescent indicators to directly measure calcium flux in response to store depletion.

• Monitoring STIM1 puncta formation at ER-PM junctions following store depletion in cells with normal or depleted TMEM110 levels .

• Assessing downstream calcium-dependent signaling pathways, such as the calcium/calcineurin/NFAT signaling axis, using reporter assays or by measuring activation of pathway components .

• Quantifying TIRF-layer ER fluorescence as a percentage of total ER fluorescence to indirectly assess the ER structures capable of supporting calcium signaling .

These approaches provide complementary information about Tmem110's functional role in calcium homeostasis and signaling.

How does Tmem110 regulate STIM1-ORAI1 interaction during calcium signaling?

Tmem110 (STIMATE) physically associates with STIM1 to promote its conformational switch from an inactive toward an activated state . This conformational change is critical for STIM1's ability to couple to and gate ORAI calcium channels on the plasma membrane. When TMEM110 is depleted through RNAi knockdown or Cas9-mediated gene disruption, the puncta formation of STIM1 at ER-PM junctions is substantially reduced . This reduction in STIM1 puncta directly impacts the calcium/calcineurin/NFAT signaling axis, demonstrating that Tmem110 plays a crucial role in facilitating STIM1-ORAI1 interactions necessary for store-operated calcium entry .

What is the impact of Tmem110 depletion on calcium signaling pathways?

Depletion of Tmem110 has significant consequences for calcium signaling:

• Reduced STIM1 puncta formation at ER-PM junctions

• Diminished calcium flux mediated by STIM-ORAI signaling

• Inhibition of the calcium/calcineurin/NFAT signaling axis

• Decreased availability of ER-PM junctions competent for calcium signaling

These effects highlight Tmem110's importance as a positive regulator of store-operated calcium entry, which is essential for numerous cellular processes including immune cell activation, muscle function, and gene expression.

How does Tmem110 contribute to the maintenance of ER-PM junctions?

Tmem110 plays a critical role in maintaining ER-PM junctions, as evidenced by several experimental findings:

• Cells depleted of endogenous TMEM110 show significantly less TIRF-layer ER fluorescence as a percentage of total ER fluorescence compared to control cells, both at rest and after store depletion .

• Expression of RNAi-resistant TMEM110 restores the level of TIRF-layer ER fluorescence in depleted cells .

• TMEM110 knockout (TMEM110 -/-) cells created using CRISPR-Cas9 technology have considerably less TIRF-layer ER fluorescence than parental cells .

• Electron microscopy confirms that TMEM110-depleted cells have fewer ER-PM junctions per μm plasma membrane than control cells in both unstimulated and store-depleted conditions .

These findings collectively demonstrate that Tmem110 is required to maintain a normal complement of ER-PM junctions, which are essential for calcium signaling and other cellular processes.

What structural features enable Tmem110 to function at ER-PM junctions?

Tmem110 is a trans-ER protein with C-terminal and N-terminal regions projecting into the cytosol . This structural arrangement is critical for its function at ER-PM junctions. The majority of Tmem110 protein interactors are cytoskeletal components, ER- or PM-resident proteins, and proteins involved in intracellular membrane trafficking and posttranslational modification . These interactions suggest that Tmem110 serves as a scaffold protein that helps organize and maintain the specialized architecture of ER-PM junctions through multiple protein-protein interactions across the junction interface .

How does Tmem110 influence autophagy in cellular systems?

Recent research has uncovered Tmem110's involvement in autophagy regulation. When TMEM110 is blocked with siRNA, significant changes occur in autophagy markers:

• Altered LC3II/I ratio: a key indicator of autophagosome formation and maturation

• Changes in Beclin1 expression: a central regulator of autophagy

• Altered p62 expression: a marker of autophagy flux

Transmission electron microscopy (TEM) analysis and fluorescence confocal microscopy further confirmed that blocking TMEM110 significantly affects autophagy dynamics . Considering Tmem110's localization at the ER membrane and ER-PM contact sites, researchers have speculated that it may be related to the unfolded protein response-ER stress-related autophagy pathway, though further studies are needed to confirm this hypothesis .

What is the relationship between Tmem110, melatonin signaling, and osteogenesis?

Tmem110 plays a critical role in melatonin-induced osteogenesis through an autophagy-dependent mechanism. Research has shown that:

• Blocking TMEM110 expression with siRNA decreases melatonin-induced osteogenesis, as determined by alkaline phosphatase (ALP) staining and activity analysis, Alizarin Red S (ARS) staining, and mineralized nodes analysis .

• The expression of osteogenesis-related genes at both mRNA and protein levels is significantly decreased when TMEM110 is inhibited .

• TMEM110 appears to function as an enhancer of autophagy, which in turn facilitates osteogenic differentiation in response to melatonin .

These findings suggest that Tmem110 represents a potential target for enhancing periodontal tissue regeneration through modulation of autophagy and osteogenic differentiation.

How do different experimental models affect the study of Tmem110 function?

Different experimental models provide complementary insights into Tmem110 function:

When designing experiments, researchers should carefully consider which model system best addresses their specific research questions, particularly when investigating tissue-specific functions of Tmem110.

What are the most significant research gaps in understanding Tmem110 function?

Despite recent advances, several critical knowledge gaps remain in Tmem110 research:

• The precise molecular mechanisms by which Tmem110 induces autophagy remain unclear .

• The specific targeted molecules or interacting partners of Tmem110 in the autophagy pathway have not been clearly identified .

• The relationship between Tmem110's role in calcium signaling and its function in autophagy regulation requires further clarification .

• The physiological significance of Tmem110 in different tissue contexts, particularly in vivo, remains to be fully elucidated.

• The potential role of Tmem110 in pathological conditions such as calcium signaling disorders or autophagy-related diseases deserves further investigation.

Addressing these knowledge gaps will require innovative experimental approaches combining genetic manipulation, high-resolution imaging, and physiological validation in relevant model systems.

What are common challenges when working with recombinant Tmem110 proteins?

When working with recombinant Tmem110 proteins, researchers often encounter several technical challenges:

• Protein solubility issues: As a multi-transmembrane protein, Tmem110 can be difficult to express and purify in soluble form. Using appropriate detergents or lipid environments is crucial for maintaining protein stability and function.

• Proper folding: Ensuring correct folding of recombinant Tmem110 is essential for functional studies. Researchers should consider using mammalian expression systems rather than bacterial systems for complex multi-transmembrane proteins to achieve proper folding and post-translational modifications.

• Storage stability: Repeated freeze-thaw cycles can significantly reduce protein activity. Storage recommendations include aliquoting the protein upon receipt and avoiding repeated freeze-thaw cycles . For recombinant transmembrane proteins, storage in buffer containing 6% Trehalose at pH 8.0 has been effective, with reconstitution in deionized sterile water to a concentration of 0.1-1.0 mg/mL and addition of 5-50% glycerol for long-term storage at -20°C/-80°C .

What controls are essential when studying Tmem110 function in calcium signaling and ER-PM junctions?

To ensure robust and reproducible results when studying Tmem110 function, several essential controls should be included:

• RNAi-resistant TMEM110 rescue: When depleting Tmem110 using RNAi, expression of an RNAi-resistant version of TMEM110 should restore the phenotype, confirming specificity of the knockdown effect .

• Multiple siRNA sequences: Using multiple independent siRNA sequences targeting different regions of TMEM110 helps rule out off-target effects .

• Appropriate imaging controls: For TIRF microscopy studies of ER-PM junctions, researchers should include controls for total ER fluorescence to accurately calculate the TIRF-layer ER fluorescence as a percentage of total .

• Positive controls for calcium signaling: Include treatments known to activate store-operated calcium entry, such as thapsigargin, to confirm the experimental system is responsive .

• Quantitative assessment: Use multiple quantitative measures (e.g., puncta formation, calcium flux, downstream signaling activation) to comprehensively assess Tmem110 function .

What emerging techniques might advance our understanding of Tmem110 function?

Several emerging techniques hold promise for deepening our understanding of Tmem110 function:

• Cryo-electron microscopy: This technique could reveal the detailed molecular structure of Tmem110 and its interactions with STIM1 and other proteins at ER-PM junctions.

• Proximity labeling approaches: Methods such as BioID or APEX2 could identify the complete interactome of Tmem110 in different cellular contexts, revealing new functional partners.

• Optogenetic tools: Light-inducible control of Tmem110 function or localization could provide temporal precision in studying its dynamic roles in calcium signaling and autophagy.

• Tissue-specific conditional knockout models: These models would allow investigation of Tmem110 function in specific tissues in vivo without developmental complications of constitutive knockout.

• Single-molecule imaging: These approaches could reveal the dynamic behavior of individual Tmem110 molecules during calcium signaling events at ER-PM junctions.

How might Tmem110 research inform therapeutic approaches for calcium signaling disorders or autophagy-related conditions?

Understanding Tmem110 function has potential therapeutic implications:

• Modulation of calcium signaling: As Tmem110 positively regulates STIM-ORAI calcium signaling, targeting this interaction could provide novel approaches for disorders characterized by aberrant calcium signaling, such as immune deficiencies, muscle disorders, or certain neurological conditions .

• Enhancement of autophagy: Given Tmem110's role in promoting autophagy, particularly in the context of melatonin signaling, it represents a potential target for enhancing autophagy in conditions where this process is impaired, such as neurodegenerative diseases or aging-related disorders .

• Tissue regeneration: The involvement of Tmem110 in melatonin-induced osteogenesis suggests potential applications in periodontal tissue regeneration and possibly other regenerative medicine contexts .

Further research into these therapeutic applications would require detailed understanding of Tmem110's tissue-specific functions and the development of targeted approaches to modulate its activity in relevant pathological contexts.