Recombinant Xenopus tropicalis Transmembrane protein 97 (tmem97)

What is TMEM97 and why is Xenopus tropicalis used as a model for studying it?

TMEM97 (Transmembrane Protein 97), also known as Sigma-2 receptor (S2R), is a five-pass transmembrane protein that plays important roles in various biological processes. Xenopus tropicalis is emerging as a powerful model organism for studying TMEM97 and other human disease genes for several compelling reasons. Unlike Xenopus laevis, X. tropicalis possesses a true diploid genome with high conservation of gene synteny with the human genome, making it particularly valuable for biomedical research . Additionally, X. tropicalis has a relatively short life cycle compared to other amphibian models, facilitating faster experimental timelines . Modern genome editing techniques such as CRISPR/Cas9 and TALEN can be applied with high efficiency in this organism, allowing for rapid and cost-effective generation of genetic models for human diseases .

What is the structure and key features of recombinant Xenopus tropicalis TMEM97?

Recombinant full-length Xenopus tropicalis TMEM97 protein consists of 171 amino acids and is typically produced with an N-terminal His tag when expressed in E. coli expression systems . The full amino acid sequence is:
MAVCARLLEWIFFFYFFSHIPITLLVDLQAVLPPSLYPQELLDLMKWYTVAFKDHLMANPPPWFKSFVYCEAILQLPFFPVAAYAFFKGGCKWIRIPAIVYSAHVATTVIAIIGHILFGEFPKSDVIAPLTQKDRLTLVSIYAPYLLVPVLLLLTMLFSPRYRQEEKRKRK

In database entries, Xenopus tropicalis TMEM97 is identified by the UniProt ID Q6DFQ5 and has several synonyms including Sigma intracellular receptor 2, Sigma-2 receptor, and Sigma2 receptor .

How should recombinant Xenopus tropicalis TMEM97 be handled and stored for optimal stability?

For optimal stability and functionality, recombinant Xenopus tropicalis TMEM97 protein should be handled according to the following protocol:

• Upon receipt, briefly centrifuge the vial to bring contents to the bottom

• Reconstitute the lyophilized protein in deionized sterile water to a concentration of 0.1-1.0 mg/mL

• Add glycerol to a final concentration of 5-50% (typically 50% is recommended)

• Aliquot for long-term storage at -20°C or -80°C

• Avoid repeated freeze-thaw cycles as they significantly degrade protein quality

• For short-term use, working aliquots can be stored at 4°C for up to one week

The protein is typically supplied in a Tris/PBS-based buffer containing 6% Trehalose at pH 8.0 .

What are the recommended experimental applications for recombinant Xenopus tropicalis TMEM97?

• Protein-protein interaction studies: To investigate binding partners of TMEM97, particularly within the Wnt signaling pathway where it interacts with components like USP8 and FZD

• Immunization for antibody production: The purified protein (>90% purity) can serve as an antigen for generating specific antibodies

• Functional assays: For studying TMEM97's role in Wnt signaling inhibition through use in cell culture systems

• Structure-function analysis: To identify key domains responsible for its regulation of FZD proteins

When designing experiments, researchers should consider controls that account for the His-tag presence, which may influence certain interactions or functional assays.

How can TMEM97 be used to study Wnt signaling pathways in Xenopus embryogenesis?

TMEM97 has been identified as a specific antagonist of Wnt signaling in Xenopus development. To effectively study its role in Wnt pathway regulation:

• Loss-of-function studies: Use antisense morpholino oligonucleotides (MOs) targeted against Tmem79 in naïve ectoderm to observe effects on anterior development, neural plate formation, and neural crest development

• Rescue experiments: Co-depletion of Usp8 or β-catenin with Tmem79 can confirm pathway specificity by demonstrating rescue of developmental defects

• Explant assays: Animal pole explants can be used to test TMEM97's effects on multiple signaling pathways (Wnt, FGF, Nodal/TGF-β, BMP, Shh) to confirm specificity

• Epistasis analysis: Test TMEM97 inhibition at different levels of the Wnt pathway (upstream/downstream of β-catenin) to identify its precise point of action

Research has demonstrated that TMEM97 blocks Wnt8 signaling but not FGF, Nodal/TGF-β, BMP, or Shh signaling in Xenopus embryos, and acts upstream of β-catenin .

What genome editing approaches can be used to study TMEM97 function in Xenopus tropicalis models?

For investigating TMEM97 function in Xenopus tropicalis, several genome editing approaches have proven effective:

• CRISPR/Cas9 system: The most widely used method due to its efficiency and simplicity

  • Design guide RNAs targeting the early exons of tmem97

  • Inject Cas9 protein and guide RNA into fertilized eggs at the one-cell stage

  • Verify editing efficiency by sequencing and T7 endonuclease assays

  • Generate F0 mosaic embryos for rapid phenotypic analysis

• TALEN (Transcription Activator-Like Effector Nuclease):

  • Design TALEN pairs targeting the coding region of tmem97

  • Inject TALEN mRNAs into fertilized eggs

  • Screen for mutations and establish stable lines

• Antisense morpholino oligonucleotides (MOs):

  • Design MOs targeting the translation start site or splice junctions

  • Inject into specific blastomeres at early cleavage stages

  • Include control MOs and rescue experiments with wild-type mRNA

Mosaic disruption of tmem97 allows for the generation of highly penetrant and low latency developmental phenotypes, facilitating the study of its role in embryogenesis and signaling pathways .

How does TMEM97 mechanistically regulate FZD protein levels in Xenopus, and what experimental approaches can verify this mechanism?

TMEM97 regulates FZD (Frizzled) protein levels through a specific mechanistic pathway that can be experimentally verified:

• Mechanism: TMEM97 down-regulates FZD protein levels by promoting FZD ubiquitination and degradation during biogenesis. It functions by:

  • Associating with USP8 (a deubiquitinase)

  • Inhibiting USP8's deubiquitination of FZD specifically

  • Acting independently of the ZNRF3/RNF43 ubiquitin ligase pathway

• Experimental verification approaches:

  • Co-immunoprecipitation: To demonstrate physical interaction between TMEM97 and USP8

  • Ubiquitination assays: To measure changes in FZD ubiquitination levels in the presence/absence of TMEM97

  • Protein stability assays: To track FZD half-life using cycloheximide chase experiments

  • Rescue experiments: Test if FZD overexpression can rescue Tmem79 depletion phenotypes

  • Compound inhibitor studies: Use IWP-2 (a PORCUPINE inhibitor) to demonstrate that autocrine Wnt production is required for enhanced Wnt signaling upon TMEM79 depletion

The pathway has been validated by demonstrating that co-depletion of Usp8 rescues the developmental defects caused by Tmem79 depletion in Xenopus embryos .

What are the primary phenotypic consequences of TMEM97 depletion in Xenopus embryos and how can these be quantitatively assessed?

TMEM97 depletion in Xenopus embryos results in several distinctive developmental phenotypes that can be quantitatively assessed:

• Anterior development deficiency:

  • Quantification: Measure head size and anterior marker gene expression (e.g., Otx2, Pax6)

  • Assessment methods: Whole-mount in situ hybridization, qRT-PCR, morphometric analysis

• Neural plate formation defects:

  • Quantification: Analyze neural marker expression patterns and neural plate dimensions

  • Assessment methods: Immunostaining for neural markers, analysis of Sox2/Sox3 expression

• Neuralization failure:

  • Quantification: Measure the response to neural inducers like Noggin in explant assays

  • Assessment methods: Animal cap assays with neural markers, qRT-PCR for neural genes

• Neural crest formation defects:

  • Quantification: Assess neural crest marker expression and migration patterns

  • Assessment methods: In situ hybridization for neural crest markers (Slug, FoxD3, Twist)

These phenotypes can be confirmed as TMEM97-specific by rescue experiments through co-depletion of Usp8 or β-catenin, which validates the mechanistic link to Wnt signaling inhibition .

How does the function of TMEM97 in Xenopus compare to its role in mammalian models?

The function of TMEM97 exhibits both similarities and critical differences between Xenopus and mammalian models:

The differences may be explained by either: (1) stronger maternal Wnt influence in Xenopus requiring more antagonist activity, or (2) genetic compensation that occurs in mouse knockout models but not in acute MO knockdowns in Xenopus .

What is the role of TMEM97 in modulating behavior and neuropathic pain, and how might this inform therapeutic applications?

TMEM97 (Sigma-2 receptor/TMEM97) plays a significant role in modulating behavior and neuropathic pain, with important implications for therapeutic applications:

• Behavioral modulation:

  • Female Tmem97 knockout mice show reduced anxiety-like and depressive-like behaviors in specific behavioral tests (light/dark preference and tail suspension tests)

  • These effects are not observed in all behavioral paradigms (open field, elevated plus maze, and forced swim tests), suggesting context-specific effects

• Neuropathic pain implications:

  • Wild-type mice develop prolonged neuropathic pain-induced depressive-like phenotypes after nerve injury

  • Tmem97 knockout mice are protected from developing these pain-induced depressive-like behaviors

  • This suggests TMEM97 mediates the affective comorbidities of chronic pain

• Therapeutic potential:

  • TMEM97 could be a valuable target for treating both neuropathic pain and its associated affective disorders

  • Sex differences in TMEM97 function suggest potential for sex-specific therapeutic approaches

  • Sigma-2 receptor/TMEM97 ligands have shown anxiolytic/antidepressant-like properties in rodent models

These findings highlight the translational potential of research on Xenopus tropicalis TMEM97 for understanding and treating neuropsychiatric aspects of pain disorders.

What are the key considerations when designing experiments using recombinant Xenopus tropicalis TMEM97 protein?

When designing experiments with recombinant Xenopus tropicalis TMEM97 protein, researchers should consider several critical factors:

• Protein solubility and stability:

  • TMEM97 is a transmembrane protein that may have limited solubility in aqueous buffers

  • Avoid repeated freeze-thaw cycles which significantly diminish protein activity

  • Store working aliquots at 4°C for short-term use (up to one week)

  • Consider detergent selection carefully for solubilization while maintaining protein functionality

• Tag interference:

  • The N-terminal His tag may affect protein folding or function in certain assays

  • Include appropriate controls to account for potential tag effects

  • Consider tag cleavage for applications where native structure is critical

• Experimental controls:

  • Include proper negative controls (buffer-only, irrelevant protein)

  • Use wild-type protein alongside mutant versions for comparative studies

  • When testing interactions, consider using a non-related transmembrane protein as control

• Species compatibility:

  • While Xenopus tropicalis TMEM97 shares high homology with human TMEM97, cross-species interactions may vary

  • Validate key findings using species-matched experimental systems when possible

• Quantification methods:

  • Ensure protein concentration determination is accurate (Bradford/BCA assay)

  • Use carefully validated antibodies for detection and quantification

  • Account for batch-to-batch variations in protein preparation

How can researchers address challenges in studying TMEM97 function across different developmental stages in Xenopus?

Studying TMEM97 function across different developmental stages in Xenopus presents specific challenges that require tailored approaches:

• Dynamic expression pattern management:

  • TMEM97 exhibits broad and dynamic expression throughout embryogenesis

  • Expression is detected in animal blastomeres at four-cell stage, broadly in animal region in blastula, and shifts during neurulation

  • Use stage-specific targeted interventions (temporal control of gene editing or protein activity)

• Maternal vs. zygotic contribution:

  • Address maternal protein/mRNA contribution using strategies like maternal protein depletion

  • Design antisense oligonucleotides that specifically target maternal or zygotic transcripts

  • Use tissue-specific CRISPR approaches for later developmental stages

• Tissue-specific function analysis:

  • Employ blastomere injection techniques to target specific lineages

  • Use tissue-specific promoters for transgenic approaches

  • Perform explant cultures from specific embryonic regions

• Phenotype interpretation:

  • Document phenotypes across multiple developmental timepoints

  • Use marker gene analysis to distinguish primary from secondary effects

  • Perform rescue experiments with temporally controlled expression

  • Consider redundancy with other Wnt antagonists expressed at different stages

• Technical solutions:

  • Combine knockdown/knockout approaches with gain-of-function studies

  • Use inducible systems for temporal control

  • Employ lineage tracing to follow cell fates after TMEM97 manipulation

What are the primary unanswered questions about TMEM97 function in Xenopus development?

Despite significant progress, several critical questions about TMEM97 function in Xenopus development remain unanswered:

• Molecular specificity: How does TMEM97 achieve specificity for FZD protein regulation among various transmembrane proteins during biogenesis?

• Developmental timing: What mechanisms regulate the dynamic expression pattern of TMEM97 during embryogenesis, particularly its initial expression in the anterior neural plate and subsequent exclusion from CNS primordia?

• Pathway integration: How does TMEM97-mediated Wnt antagonism integrate with other signaling pathways during neural induction and patterning?

• Species differences: Why does TMEM97 depletion cause severe developmental defects in Xenopus but not in mouse models? Is this due to differential maternal Wnt influence or compensatory mechanisms?

• Regulatory network: What transcription factors and epigenetic regulators control TMEM97 expression during development?

• Functional domains: Which specific domains of TMEM97 are responsible for its interaction with USP8 and inhibition of FZD deubiquitination?

• Evolutionary conservation: How conserved is the TMEM97-USP8-FZD regulatory axis across vertebrate species, and what evolutionary pressures shaped this pathway?

These questions represent promising avenues for future research that could significantly advance our understanding of TMEM97 biology and developmental regulation.

How might integrating findings from Xenopus TMEM97 research contribute to understanding human disease mechanisms?

Integrating findings from Xenopus TMEM97 research offers substantial potential for understanding human disease mechanisms:

• Neurodevelopmental disorders: Given TMEM97's role in anterior neural development and Wnt signaling regulation, findings may inform understanding of neurodevelopmental disorders linked to Wnt pathway dysregulation

• Atopic dermatitis: TMEM97 is a predisposition gene for atopic dermatitis (AD), and Xenopus research suggests that deregulation of Wnt/FZD signaling may contribute to AD pathogenesis

• Psychiatric conditions: The finding that TMEM97 modulates anxiety-like and depressive-like behaviors in mouse models suggests potential relevance to psychiatric disorders

• Chronic pain: TMEM97's role in neuropathic pain-induced depressive phenotypes highlights its potential as a therapeutic target for treating both pain and its affective comorbidities

• Cancer biology: As a regulator of Wnt signaling, which is frequently dysregulated in cancer, TMEM97 findings may inform understanding of malignancy development, particularly hematologic malignancies that Xenopus models are being developed to study

• Drug development: Structure-function studies of Xenopus TMEM97 could guide development of targeted therapeutics for conditions where TMEM97 dysfunction contributes to pathology

By leveraging the experimental advantages of the Xenopus system (external development, transparency, efficient genome editing), researchers can rapidly generate and test hypotheses about TMEM97 function that may translate to human health applications.