Recombinant Rat Transmembrane protein 97 (Tmem97)

What is TMEM97 and what is its molecular identity?

TMEM97 (Transmembrane Protein 97) has been identified as the molecular entity corresponding to the Sigma-2 receptor, a pharmacologically described receptor that remained molecularly uncharacterized for decades despite extensive pharmacological investigation. TMEM97 consists of an "EXPERA" domain, which in humans is shared among several proteins including TM6SF1, TM6SF2, EBP, and EBPL. The protein is predicted to have a four-pass transmembrane topology with cytosolic N and C termini, with the C-terminus containing a predicted ER-retention sequence "KRKKK" . Evolutionary coupling analysis and structural prediction suggest TMEM97 likely possesses a four-helix bundle fold with two critical aspartate residues (D29 and D56) that are essential for ligand binding and are positioned in close proximity to each other within the predicted structural model .

How is the structure of TMEM97 related to its function?

TMEM97's structure features two conserved aspartate residues (D29 and D56) that are critical for ligand binding, as demonstrated by site-directed mutagenesis experiments showing that D29N and D56N mutations abolish all binding to 3H DTG . This requirement for two acidic groups resembles the Sigma-1 receptor, where both Asp126 and Glu172 interact directly with each other as part of a hydrogen bond network that includes the basic amine of the ligand . The four-transmembrane domain structure appears to create a binding pocket that accommodates the basic amines found in all Sigma-2 ligands. This structural arrangement explains TMEM97's ability to interact with various ligands including RHM-4, SW43, and AG-205, which exhibit nanomolar binding affinities .

What protein complexes does TMEM97 form in neuronal cells?

TMEM97 forms a trimeric complex with Progesterone Receptor Membrane Component 1 (PGRMC1) and Low Density Lipoprotein Receptor (LDLR). This trimeric complex has been confirmed in primary rat cortical neurons through proximity ligation assays demonstrating pairwise interactions between LDLR and PGRMC1, LDLR and TMEM97, and TMEM97 and PGRMC1 . The intact complex is necessary for efficient uptake of lipoproteins such as LDL and apolipoprotein E (apoE). Additionally, this complex plays a crucial role in the internalization of Amyloid Beta 1-42 (Aβ42) monomers and oligomers, with implications for Alzheimer's Disease pathology . The complex is present not only in neuronal cell models but also in human brain tissue from both normal individuals and Alzheimer's Disease patients .

What are the recommended methods for recombinant expression of TMEM97?

For recombinant expression of TMEM97, researchers have successfully used multiple expression systems. For mammalian expression, the TMEM97 gene can be cloned into a pTARGET vector followed by a porcine teschovirus-1 2A skip peptide and a fluorescent protein (such as mCardinal) to assess transfection efficiency . This construct can be transfected into Expi293 cells with expression confirmed after 36 hours via flow cytometry analysis of fluorescence levels.

For insect cell expression, human TMEM97 can be cloned into the vector pVL1392, with baculovirus prepared using a system such as BestBac . For large-scale production, Sf9 insect cells can be infected at a density of 4 × 10^6 cells/mL and shaken at 27°C for 60 hours before harvest . This insect cell system is particularly valuable for ligand binding studies as Sf9 cells lack an endogenous TMEM97 homolog and show no appreciable 3H DTG binding, making them an excellent background-free system for characterizing the pharmacological properties of the recombinant protein .

How can researchers perform binding assays to characterize TMEM97-ligand interactions?

Competition binding assays can be performed using 3H DTG (ditolylguanidine) as a radioligand. The general protocol involves:

• Prepare membranes from cells expressing TMEM97 (either Sf9 insect membranes overexpressing TMEM97 or MCF-7 membranes)

• Incubate membranes (2.5-30 μg of total protein per reaction) in a 100-μL reaction buffered with 50 mM Tris (pH 8.0)

• Add 30 nM 3H DTG and a range of concentrations (10-100 μM) of the competing cold ligand

• Include 1.8 μM (+)SKF-10,047 to block Sigma-1 receptor sites when testing in MCF-7 membranes or when testing TMEM97 ligands in Sf9 membranes

• Incubate reactions for 90 minutes at 37°C

• Terminate reactions by filtration through a glass fiber filter (pre-soaked in 0.3% polyethylenimine) using a cell harvester

• Perform all reactions in triplicate using a 96-well block format

This methodology allows for the determination of binding affinities for various ligands and comparison between different experimental conditions or mutant proteins.

What techniques can be used to verify TMEM97 expression and complex formation in neuronal cells?

Several complementary techniques can be used to verify TMEM97 expression and complex formation:

• Proximity Ligation Assay (PLA): This technique can demonstrate pairwise protein interactions, as was used to confirm the trimeric complex of TMEM97/PGRMC1/LDLR in primary rat cortical neurons. The assay can detect interactions between LDLR and PGRMC1, LDLR and TMEM97, and TMEM97 and PGRMC1 .

• Radioligand Binding: Radiolabeled ligands such as RHM-4 can bind to TMEM97 in primary neuron homogenates, allowing determination of binding parameters. In primary rat neurons, RHM-4 showed a Kd of 1.37 nM and a Bmax of 1448 fmol/mg, indicating high expression of TMEM97 .

• Immunofluorescence Staining: This can be used to visualize the presence and localization of TMEM97 and its complex partners in neurons, as well as to verify neuronal identity using markers such as MAP2 .

• siRNA Knockdown: Reducing TMEM97 expression by approximately 60% via siRNA in PC-12 cells resulted in a nearly identical reduction in Sigma-2 receptor expression as measured by 3H DTG binding, confirming the identity of TMEM97 as the Sigma-2 receptor .

How does TMEM97 contribute to Aβ42 uptake in neuronal cells?

TMEM97, as part of the trimeric complex with PGRMC1 and LDLR, plays a crucial role in the internalization of Aβ42 in both monomeric (mAβ42) and oligomeric (oAβ42) forms. Experimental evidence from both HeLa cell models and primary neuronal cells demonstrates that the intact complex is necessary for efficient uptake of these Aβ42 species .

The mechanism involves:

• Direct interaction of the TMEM97/PGRMC1/LDLR complex with Aβ42

• Enhanced uptake when Aβ42 is complexed with apolipoprotein E (apoE)

• Differential uptake depending on apoE isoform, with a rank order of apoE2 < apoE3 < apoE4 for facilitating Aβ42 internalization

This process contributes to the accumulation and aggregation of Aβ42 within neurons, which can eventually lead to plaque formation and neuronal death. The apoE4 allele, being the greatest risk factor associated with the development of Alzheimer's Disease, makes this complex particularly relevant for understanding disease pathogenesis .

What pharmacological approaches can inhibit TMEM97-mediated Aβ42 uptake?

Several pharmacological inhibitors targeting TMEM97 and PGRMC1 have been demonstrated to reduce Aβ42 uptake:

• RHM-4: A TMEM97 ligand with high affinity (Kd ~ 0.2 nM) that significantly reduces the internalization of mAβ42 and oAβ42, both alone and when complexed with apoE3 .

• SW43: Another TMEM97 ligand (Kd ~ 12 nM) that demonstrates similar inhibitory effects on Aβ42 uptake .

• AG-205: A PGRMC1 ligand (Kd ~ 1 μM against Sigma2R) that also reduces cellular uptake of Aβ42 species .

All three compounds, when used at a concentration of 500 nM, significantly reduce the capacity for internalizing mAβ42 and oAβ42 alone or when complexed with apoE3 in cell models. They also result in significantly less uptake of apoE3 alone or when in a complex with any of the Aβ42 aggregated states . These findings suggest that TMEM97 and its complex partners represent promising therapeutic targets for reducing neuronal Aβ42 accumulation, potentially slowing Alzheimer's Disease progression.

How does the interaction between TMEM97 and different apoE isoforms affect Alzheimer's disease risk?

The interaction between TMEM97 and different apoE isoforms has significant implications for Alzheimer's disease risk, particularly given that the apoE4 allele is the greatest genetic risk factor for late-onset Alzheimer's disease. Research shows that:

• The TMEM97/PGRMC1/LDLR complex mediates internalization of Aβ42 in complex with all apoE isoforms (apoE2, apoE3, and apoE4)

• There is differential uptake of Aβ42 when complexed with the various apoE isoforms in a rank order of apoE2 < apoE3 < apoE4

• The enhanced uptake associated with apoE4 leads to greater accumulation of Aβ42 within neurons, potentially explaining the increased risk of Alzheimer's disease in apoE4 carriers

• Pharmacological inhibition of TMEM97 and PGRMC1 results in decreased internalization of all apoE isoforms and their complexes with Aβ42, offering a potential therapeutic approach that might be particularly beneficial for apoE4 carriers

This differential effect based on apoE isoform provides insight into why apoE4 carriers have an increased risk of Alzheimer's disease and suggests that TMEM97-targeted therapies might be especially valuable for this genetic subgroup.

How can site-directed mutagenesis be used to investigate TMEM97 structure-function relationships?

Site-directed mutagenesis is a powerful approach for investigating TMEM97 structure-function relationships, as demonstrated by research identifying critical residues for ligand binding. A methodological approach includes:

• Rational selection of mutation targets: Based on the hypothesis that acidic residues might serve as counter-ions to the basic amine present in all Sigma-2 ligands, researchers systematically mutated all Glu and Asp residues in TMEM97 .

• Generation of point mutants: Using techniques such as KAPA polymerase-based site-directed mutagenesis to create specific amino acid substitutions (e.g., D29N and D56N) .

• Expression of mutants: Expressing the mutant constructs in systems like Expi293 cells to produce sufficient protein for functional analysis .

• Functional characterization: Testing the effect of mutations on ligand binding using radioligand binding assays (e.g., 3H DTG binding) .

This approach successfully identified two conserved aspartate residues (D29 and D56) that are essential for ligand binding, as their mutation to asparagine (D29N and D56N) abolished all binding to 3H DTG . Combined with evolutionary coupling analysis and structural prediction, this mutagenesis data provided critical insights into the structure of the ligand-binding site, suggesting a mechanism similar to that of the Sigma-1 receptor where multiple acidic residues form part of a hydrogen bond network interacting with the basic amine of ligands .

What are the implications of TMEM97's role in cholesterol homeostasis for neurological disease research?

TMEM97's involvement in cholesterol homeostasis has significant implications for neurological disease research:

• Connection to lipid metabolism disorders: TMEM97 belongs to the "EXPERA" domain family, which in humans includes proteins implicated in cholesterol biology (TM6SF1, TM6SF2, EBP, and EBPL). One member, TM6SF2, has been linked to nonalcoholic fatty liver disease .

• Role in lipoprotein receptor function: TMEM97 forms a complex with LDLR, a key receptor in cholesterol metabolism, and this complex is necessary for efficient uptake of LDL .

• Link to Alzheimer's disease pathology: Cholesterol metabolism is increasingly recognized as an important factor in Alzheimer's disease pathogenesis. The TMEM97/PGRMC1/LDLR complex mediates uptake of apoE, a major cholesterol transport protein in the brain, along with Aβ42 .

• Therapeutic potential: TMEM97 ligands that were originally developed as Sigma-2 receptor binders may now be applied to study pathologies associated with aberrant cholesterol trafficking. This creates opportunities for repurposing existing compounds to target cholesterol-related aspects of neurological diseases .

• Potential relevance to other neurodegenerative diseases: The mechanism involving TMEM97 may have implications in other CNS disorders involving protein aggregates, such as alpha-synuclein in Parkinson's Disease or tau aggregation in tauopathies .

The identification of TMEM97 as the Sigma-2 receptor thus bridges two previously separate research fields, enabling investigators to leverage both the rich pharmacology of Sigma-2 ligands and the growing understanding of cholesterol homeostasis in neurological disease.

How might TMEM97 be involved in the spread of protein aggregates in neurodegenerative diseases?

TMEM97 may play a critical role in the spread of protein aggregates in neurodegenerative diseases through several mechanisms:

• Facilitation of aggregate uptake: The TMEM97/PGRMC1/LDLR complex mediates uptake of Aβ42 oligomers, suggesting it may facilitate the internalization of protein aggregates by neurons .

• Exosome-mediated spread: Recent evidence suggests that protein aggregates like Aβ42, tau, and α-synuclein can be packaged into exosomes and transferred between cells. Given that LDLR or LRP1 may play a role in exosome uptake, the TMEM97-PGRMC1-LDLR complex could potentially mediate the spread of these toxic aggregates between neurons .

• Differential effects based on protein type: While the research has focused primarily on Aβ42, there is speculation that TMEM97 might similarly affect the transfer of other aggregated proteins such as alpha-synuclein and tau .

• Therapeutic implications: Pharmacological targeting of TMEM97 could potentially reduce not only the accumulation of Aβ42 within individual neurons but also limit the cell-to-cell spread of various protein aggregates, thus slowing disease progression across multiple neurodegenerative conditions .

Research delineating the role of TMEM97 in neuronal transfer of various aggregated proteins represents an important frontier in neurodegenerative disease research, with potential implications extending beyond Alzheimer's disease to conditions like Parkinson's disease and other tauopathies .

What strategies can address low expression yields of recombinant TMEM97?

When facing low expression yields of recombinant TMEM97, researchers can implement several strategies:

• Optimize expression systems: While both mammalian (Expi293) and insect cell (Sf9) systems have been successfully used for TMEM97 expression , yields can vary. For membrane proteins like TMEM97, insect cells often provide higher expression levels due to their ability to accommodate high levels of membrane protein production.

• Codon optimization: Adapting the TMEM97 coding sequence to the codon usage bias of the expression host can significantly improve translation efficiency and protein yield.

• Expression tags and fusion partners: Utilizing solubility-enhancing tags or fusion partners may improve expression. The use of fluorescent protein tags like mCardinal not only allows for monitoring expression levels but can also enhance protein stability .

• Temperature modulation: Lowering the expression temperature (e.g., from 37°C to 30°C for mammalian cells or from 27°C to 24°C for insect cells) can slow protein production and improve folding, potentially increasing the yield of functional protein.

• Media supplements: Addition of specific chemical chaperones or lipids that promote membrane protein folding and stability can enhance expression yields.

• Induction optimization: For inducible systems, optimizing the concentration of inducer and the timing of induction relative to cell density can significantly impact yields.

• Cell density optimization: For insect cell expression, infection at the optimal cell density (as demonstrated with 4 × 10^6 cells/mL for Sf9 cells ) can be critical for maximizing protein production.

How can researchers validate the functionality of recombinant TMEM97?

Validating the functionality of recombinant TMEM97 requires multiple complementary approaches:

• Ligand binding assays: Radioligand binding using 3H DTG is the gold standard for confirming that recombinant TMEM97 retains its binding properties. Saturation binding experiments should demonstrate a Kd value consistent with published values (approximately 11.3 nM for human TMEM97 expressed in Sf9 cells) .

• Competition binding profiles: Testing a panel of known Sigma-2/TMEM97 ligands in competition binding assays should yield affinity values consistent with published data. This includes both classical Sigma-2 ligands (haloperidol, DTG, PB-28, SAS-1121) and TMEM97 ligands (Elacridar, Ro 48-8071) .

• Functional assays: For cellular studies, assessing the ability of recombinant TMEM97 to form a complex with PGRMC1 and LDLR using techniques like proximity ligation assays can confirm proper protein-protein interactions .

• Mutational analysis: Creating known function-disrupting mutations like D29N and D56N should abolish ligand binding if the protein is properly folded and functioning .

• Cell-based uptake assays: In cellular contexts, the ability of recombinant TMEM97 to mediate uptake of labeled lipoproteins or Aβ42 species can demonstrate functional activity .

• Pharmacological response: Confirming that known inhibitors like RHM-4, SW43, and AG-205 can modulate TMEM97 function in the expected concentration ranges provides additional validation of proper folding and functionality .

What are potential pitfalls in interpreting TMEM97 knockout or inhibition studies?

When interpreting TMEM97 knockout or inhibition studies, researchers should be aware of several potential pitfalls:

• Compensatory mechanisms: Long-term knockout of TMEM97 may lead to upregulation of compensatory pathways that mask the true phenotype. Acute inhibition using pharmacological tools or inducible knockout systems may provide more accurate insights into TMEM97 function.

• Complex disruption effects: Since TMEM97 forms a trimeric complex with PGRMC1 and LDLR, knockout or inhibition may disrupt this entire complex. Effects observed might be due to the loss of the complex rather than specific TMEM97 functions, requiring careful experimental design to distinguish these possibilities .

• Ligand specificity concerns: Many Sigma-2/TMEM97 ligands also interact with other targets. For example, some compounds might affect both TMEM97 and PGRMC1. Careful consideration of ligand specificity is essential when interpreting inhibition studies .

• Cell type-specific effects: TMEM97 function may vary across cell types. Effects observed in HeLa or MCF-7 cells may not directly translate to neuronal systems, requiring validation in the most relevant cell types for the research question .

• Developmental compensation: In knockout models, developmental adaptations may occur that are not relevant to acute inhibition in adult systems. Conditional knockout models may help address this issue.

• Differential effects on various substrates: Inhibition of TMEM97 may affect uptake of different substrates (e.g., various forms of Aβ42, different apoE isoforms) to different degrees, requiring comprehensive analysis of all relevant substrates when interpreting results .

• Context-dependent effects: The effect of TMEM97 inhibition may vary depending on the disease state or presence of specific genetic risk factors like apoE4, necessitating studies across multiple relevant models .