Recombinant Xenopus laevis Transmembrane protein 135 (tmem135)

What is the molecular structure of TMEM135 and how conserved is it across species?

TMEM135 is a transmembrane protein with five predicted transmembrane helices in wild-type organisms. Structural analysis using the TMHMM program (v. 1.0) reveals that these transmembrane domains are critical for proper protein orientation and function . When mutations occur that affect these domains, as seen in the FUN025 mouse model, the protein structure is significantly altered. The FUN025 mutation results in the loss of the 4th and 5th transmembrane helices and reverses the orientation of the remaining three helices within the membrane .

The protein contains both N-terminal and C-terminal regions that extend into cellular compartments, with the C-terminal region being particularly important for function. Mutations leading to a shortened C-terminal region, as observed in the FUN025 mice, likely impair normal protein functions .

Regarding conservation, TMEM135 appears to be functionally conserved between mammals and nematodes (C. elegans), suggesting it likely maintains similar functions in amphibians like Xenopus laevis . This conservation across evolutionarily distant species underscores its fundamental importance in cellular physiology.

What are the primary cellular functions of TMEM135?

TMEM135 functions in three primary cellular processes:

The interplay between these functions appears critical for cellular homeostasis, as demonstrated by phenotypes arising from both TMEM135 deficiency and overexpression in model organisms.

What approaches are recommended for generating recombinant Xenopus laevis TMEM135?

When generating recombinant Xenopus laevis TMEM135, researchers should consider these methodological approaches:

• Gene identification and cloning: Begin by identifying the Xenopus laevis TMEM135 ortholog through comparative genomic analysis. Use PCR-based cloning with primers designed from conserved regions of TMEM135 genes across species.

• Expression system selection: For membrane proteins like TMEM135, two main approaches have proven effective:

  • Bacterial expression systems with specific membrane protein optimization protocols

  • Eukaryotic expression systems (particularly insect or mammalian cells) that better preserve native conformation

• Fusion tag optimization: From mammalian studies, N-terminal tagging appears less disruptive to TMEM135 function than C-terminal tagging, as antibodies recognizing the N-terminus of TMEM135 successfully detected both wild-type and mutant proteins in western blot analysis .

• Validation strategy: Confirm recombinant protein functionality through:

  • Subcellular localization studies (mitochondrial and peroxisomal markers)

  • Complementation studies in TMEM135-deficient cells

  • Assessment of mitochondrial membrane potential using Mitotracker Red staining, as successfully employed in C. elegans studies

• Protein purification considerations: Due to its multiple transmembrane domains, standard detergent-based approaches must be optimized to maintain structural integrity.

How can TMEM135 mutations be effectively generated and validated?

Based on successful approaches in mouse models, researchers should consider:

• CRISPR/Cas9 system: This has proven effective for introducing specific point mutations in the TMEM135 gene, as demonstrated in the creation of T>C mice used in complementation testing with FUN025 mice . For Xenopus studies, this would involve:

  • Designing sgRNAs targeting conserved regions of Xenopus TMEM135

  • Using repair templates to introduce specific mutations

  • Microinjection into fertilized Xenopus eggs

• Targeting strategy: Focus on:

  • Splice site mutations (as in the FUN025 mouse model)

  • Transmembrane domain-encoding regions

  • C-terminal region modifications

• Validation approaches:

  • Genomic sequencing to confirm mutation

  • RT-PCR to assess splicing effects

  • Western blotting to confirm protein expression and size

  • Structural prediction analysis using programs like TMHMM to assess potential changes in protein topology

• Complementation testing: Cross different mutant lines or perform rescue experiments with wild-type protein to confirm phenotype causality, similar to the approach used in confirming the FUN025 phenotype in mice .

What functional assays best evaluate TMEM135 activity in cellular systems?

To comprehensively assess TMEM135 function, researchers should employ multiple assays targeting its diverse cellular functions:

• Mitochondrial function assays:

  • Mitochondrial morphology assessment via fluorescent microscopy

  • Mitochondrial membrane potential using Mitotracker Red staining (40.7% reduction observed in C. elegans tmem135 mutants)

  • Mitochondrial fragmentation analysis using time-lapse imaging

• Peroxisomal function assays:

  • Peroxisome proliferation assessment

  • Measurement of peroxisomal β-oxidation enzymes activity

  • Analysis of peroxisomal biogenesis factors (Pex3, Pex5) expression levels

• Lipid metabolism assays:

  • Oil Red O staining for neutral lipid quantification

  • Docosahexaenoic acid (DHA) measurement, which is significantly affected in TMEM135 mutants

  • Lipid profiling using mass spectrometry

• Stress response assays:

  • Cold stress survival tests (particularly relevant based on C. elegans findings)

  • Assessment of cellular survival under metabolic stress conditions

  • FoxO/DAF-16 expression analysis (60.7% reduction in tmem135 mutant worms)

• Age-related phenotype assessment:

  • Long-term culture studies monitoring cellular aging markers

  • Analysis of aging-related damage in specialized cells like RPE

How does TMEM135 mediate the interplay between mitochondria and peroxisomes?

TMEM135 represents a critical nexus in organelle crosstalk, particularly between mitochondria and peroxisomes. Current evidence suggests several mechanisms:

• Translocation capability: TMEM135 can translocate from peroxisomes to mitochondria during fission events, suggesting it may serve as a mobile regulator between these organelles .

• Coordinated regulation: Research indicates that TMEM135-mediated mitochondrial fragmentation may originate from altered peroxisomal function. Studies in mouse embryonic fibroblasts show that inhibition of peroxisome biogenesis (through Pex3 or Pex5 elimination) promotes mitochondrial fragmentation, mirroring phenotypes observed in TMEM135 transgenic mice .

• Peroxisomal enzyme regulation: TMEM135 transgenic mice show decreased expression of multifunctional protein 2 (Mfp2/Hsd17b4), a critical enzyme for peroxisomal β-oxidation. This decreased expression correlates with observed mitochondrial abnormalities .

• Shared metabolic pathways: The interdependence between peroxisomal and mitochondrial function regulated by TMEM135 appears particularly crucial for fatty acid metabolism. In C. elegans, TMEM135 deletion results in reduced mitochondrial potential (40.7% reduction measured by Mitotracker Red), which correlates with impaired fat metabolism and cold sensitivity .

For researchers working with Xenopus systems, this suggests experimental designs should simultaneously monitor both organelle systems when manipulating TMEM135 expression.

What role does TMEM135 play in tissue-specific aging processes?

TMEM135 demonstrates remarkable tissue-specific effects on aging processes:

• Retinal aging: The most well-characterized aging phenotype involves retinal degeneration. TMEM135 mutation (FUN025) leads to accelerated age-dependent photoreceptor cell degeneration, coinciding with:

  • Visual function loss

  • Ectopic synapse development

  • Neuroinflammation (Müller glia activation and immune cell infiltration)

  • RPE abnormalities including autofluorescence and lipid accumulation

• Cardiac aging: TMEM135 overexpression impacts cardiac health, with transgenic mice developing:

  • Cardiac hypertrophy

  • Increased fibrosis

  • Ultrastructural abnormalities including large vacuoles between myofibrils

• Metabolic aging: TMEM135 significantly influences longevity through metabolic regulation. In C. elegans:

  • TMEM135 deletion shortens lifespan at both normal (20°C) and low (15°C) temperatures

  • TMEM135 overexpression extends lifespan under cold conditions (15°C)

• Molecular mechanism of aging modulation: TMEM135 appears to regulate aging partly through FoxO/DAF-16 signaling pathways, which are well-established longevity factors:

  • TMEM135 deletion reduces FoxO/DAF-16 expression by 60.7%

  • TMEM135 overexpression increases FoxO/DAF-16 levels 2-fold

These findings suggest that in Xenopus studies, researchers should expect tissue-specific effects when manipulating TMEM135, with particular attention to eye, heart, and metabolic tissues.

How does TMEM135 modulation affect cellular responses to environmental stressors?

TMEM135 plays a critical role in cellular stress responses, particularly under metabolic and temperature stress conditions:

• Cold stress resistance:

  • In C. elegans, TMEM135 overexpression increases cold tolerance and extends lifespan at 15°C

  • TMEM135 deletion renders worms more cold-sensitive with reduced survival

  • This cold-stress response appears linked to both fat storage regulation and mitochondrial function

• Metabolic stress adaptation:

  • TMEM135 expression is induced in VLCAD-deficient mice (a model of fatty acid oxidation disorders)

  • It appears to be part of a feedback regulatory circuit that enhances survival during metabolic stress

  • This suggests TMEM135 as a critical adaptive response element to metabolic challenges

• Mechanism of stress response regulation:

  • TMEM135 modulates mitochondrial activity (measured by membrane potential)

  • It influences fat storage and utilization pathways

  • It regulates expression of stress-responsive transcription factors like FoxO/DAF-16

• Tissue-specific stress responses:

  • Different tissues show variable sensitivity to TMEM135-mediated stress responses

  • The retina appears particularly sensitive, with accelerated aging phenotypes under TMEM135 dysfunction

  • Cardiac tissue shows distinct stress responses with vacuolization and fibrosis

For Xenopus studies, researchers should design experiments that challenge organisms with various stressors (particularly temperature variations and metabolic challenges) to fully characterize TMEM135 function.

How should researchers reconcile contradictory phenotypes between TMEM135 deficiency and overexpression models?

One of the most intriguing aspects of TMEM135 research is the seemingly contradictory phenotypes observed between deficiency and overexpression models:

• Balanced expression requirement:

  • Both deficiency and overexpression of TMEM135 can lead to pathology, suggesting that "proper balance of TMEM135 function is vital for the health of the retina and other tissues"

  • This indicates TMEM135 may function optimally within a specific expression range

• Retinal pathology interpretation:

  • TMEM135 FUN025 mutation causes photoreceptor degeneration

  • TMEM135 overexpression leads to RPE degeneration

  • These differential effects may reflect tissue-specific dependencies on TMEM135-regulated processes

• Mitochondrial dynamics contradictions:

  • TMEM135 is required for proper mitochondrial function (deficiency reduces mitochondrial potential)

  • Yet excessive TMEM135 causes mitochondrial fragmentation

  • Researchers should interpret this as indication that TMEM135 regulates a dynamic process requiring precise balance

• Experimental approach to reconcile contradictions:

  • Employ dose-response studies with graduated expression levels

  • Conduct tissue-specific conditional expression studies

  • Perform detailed temporal analysis, as effects may vary with developmental stage or age

A recommended experimental design would include generating multiple Xenopus models with varying TMEM135 expression levels to establish the optimal functional range in different tissues.

What technical challenges should be anticipated when working with recombinant TMEM135?

Researchers working with recombinant TMEM135 should anticipate several technical challenges:

• Protein solubility and stability issues:

  • As a multi-pass transmembrane protein, TMEM135 is inherently challenging to maintain in stable, functional conformations

  • Extraction protocols must be carefully optimized to preserve native structure

• Subcellular localization complexity:

  • TMEM135 localizes to both mitochondria and peroxisomes

  • It can translocate between these organelles

  • Experimental designs must account for this dynamic localization to properly interpret results

• Functional assay selection challenges:

  • Due to TMEM135's multiple functions, no single assay adequately captures its activity

  • Researchers should implement parallel assays tracking:

    • Mitochondrial morphology and function

    • Peroxisomal proliferation

    • Lipid metabolism parameters

• Mutation effect interpretation:

  • The FUN025 mutation impacts protein topology, causing loss of transmembrane helices and reversed orientation

  • When designing mutations in Xenopus TMEM135, researchers should use prediction tools like TMHMM to assess potential structural changes

• Species-specific differences:

  • While functionally conserved, there may be species-specific variations in TMEM135 regulation

  • Cross-species experimental designs may help identify conserved versus divergent aspects

How can findings from model organisms inform understanding of TMEM135 function in Xenopus laevis?

Integrating findings from multiple model organisms can enhance Xenopus laevis TMEM135 research through comparative biology approaches:

• Evolutionary conservation insights:

  • TMEM135 function is conserved from nematodes to mammals, suggesting fundamental cellular roles

  • Comparing sequence conservation between mouse, C. elegans, and Xenopus TMEM135 can identify critical functional domains

• Phenotype prediction framework:

  • Mouse models show retinal and cardiac phenotypes

  • C. elegans shows longevity and metabolic effects

  • Researchers can design targeted screens for these phenotypes in Xenopus TMEM135 models

• Methodological adaptation strategy:

  • CRISPR/Cas9 approaches successful in mouse models can be adapted for Xenopus

  • Mitochondrial assays validated in C. elegans (Mitotracker Red) can be implemented in Xenopus cells

• Developmental context consideration:

  • Xenopus provides excellent opportunities to study TMEM135 in embryonic development

  • Comparative development studies between models can reveal stage-specific requirements for TMEM135

• Stress response evaluation:

  • Cold sensitivity findings in C. elegans suggest testing temperature-dependent phenotypes in poikilothermic Xenopus

  • This unique aspect of amphibian biology makes Xenopus particularly valuable for studying TMEM135's role in temperature adaptation

What are the implications of TMEM135 research for understanding human diseases?

TMEM135 research has significant implications for various human diseases:

• Age-related macular degeneration (AMD) connections:

  • Intriguing similarities exist between retinal abnormalities in TMEM135 mutant mice and AMD pathologies

  • Gene expression profiles of TMEM135 mutant eyecups parallel RPE/choroid samples from multiple AMD stages

• Metabolic disorders relevance:

  • TMEM135 is part of a genetic network regulating fat metabolism

  • It may have implications for VLCAD deficiency and other fatty acid oxidation disorders

  • TMEM135 has been associated with non-alcoholic fatty liver disease

• Cancer associations:

  • Published associations exist between TMEM135 and multiple cancers:

    • Breast cancer

    • Prostate cancer

    • Melanoma

    • Non-small lung cancer

    • Glioblastoma multiforme

• Other potential disease connections:

  • Osteoporosis

  • Cognitive disorders

  • Metabolic diseases

• Therapeutic potential direction:

  • Given TMEM135's role in organelle homeostasis, researchers should explore:

    • Small molecule modulators of TMEM135 activity

    • Gene therapy approaches for TMEM135-related diseases

    • Whether TMEM135 could serve as a biomarker for disease progression or treatment response

For Xenopus researchers, these disease connections suggest potential value in developing amphibian models of these conditions through TMEM135 manipulation, particularly for high-throughput therapeutic screening applications.