Recombinant Human Transmembrane protein 79 (TMEM79)

What is TMEM79 and what cellular pathways does it regulate?

TMEM79 (MATTRIN) is an orphan five-pass transmembrane protein that acts as a specific antagonist of Wnt signaling pathways. It was identified through CRISPR/Cas9 genome-wide loss-of-function screening as a negative regulator of Wnt signaling activity. TMEM79 functions by down-regulating Frizzled (FZD) protein levels through promoting FZD ubiquitination and subsequent degradation during protein biogenesis. This regulatory pathway is independent of the well-established ZNRF3/RNF43 ubiquitin ligase pathway that also targets FZD receptors .

The specificity of TMEM79 for the Wnt pathway has been confirmed through multiple experimental approaches. Knockdown of TMEM79 using siRNAs increases the activity of WNT-responsive luciferase reporter (TOP-Flash) with or without Wnt3a stimulation, while overexpression of TMEM79 decreases Wnt3a-induced activation of TOP-Flash. Importantly, TMEM79 does not affect other stimuli-responsive reporters such as those activated by epidermal growth factor (EGF), demonstrating its specificity for Wnt signaling .

What methodologies are effective for studying TMEM79 function in cell culture?

For studying TMEM79 function in cell culture, researchers can employ several complementary methodologies:

• CRISPR/Cas9 genome editing: To generate TMEM79 knockout cell lines, researchers can transiently transfect cells with sgRNA plasmids containing Cas9, followed by puromycin selection for 2-3 days. Monoclonal populations can be isolated by limiting dilution in 96-well plates. Target regions should be PCR amplified and characterized by Sanger sequencing, with PCR products showing multiple peaks around the target site in chromatogram being cloned and sequenced to ensure frame shift indels .

• Reporter assays: WNT-responsive luciferase reporters (TOP-Flash) can be used to measure Wnt signaling activity in cells with TMEM79 knockdown or overexpression. The specificity of TMEM79 for Wnt signaling can be verified by testing other pathway reporters (e.g., FOP-Flash or EGF-responsive reporters) .

• Biochemical interaction studies: Co-immunoprecipitation (co-IP) experiments can be performed to assess TMEM79 interactions with FZD receptors and other proteins. For example, HEK293T cells stably expressing TMEM79-Myc-Flag can be lysed, and proteins can be immunoprecipitated using Anti-Flag M2 affinity gel followed by Anti-c-Myc Agarose beads. Bound proteins can be eluted with specific peptides and analyzed by mass spectrometry .

• Subcellular localization: Immunostaining via confocal microscopy can be used to determine the subcellular localization of TMEM79, which has been shown to predominantly localize to the endoplasmic reticulum (ER) and to some extent the lysosome .

How does TMEM79 depletion affect embryonic development?

TMEM79 depletion significantly impacts embryonic development, particularly in Xenopus embryos. When Tmem79 is depleted from the animal region/naive ectoderm using antisense morpholino oligonucleotides (MO), several developmental defects occur:

• Anterior development deficiency: Tmem79 depletion results in deficiencies in anterior development and anterior gene expression. This phenotype can be rescued by co-injection of mouse Tmem79 mRNA, demonstrating the specificity of the MO. Importantly, the anterior deficiency can also be rescued or even over-rescued by co-depletion of either β-catenin or Usp8, indicating that Tmem79 functions as a Wnt antagonist in naive ectoderm for anterior patterning through inhibition of Usp8 and β-catenin signaling .

• Neural plate formation reduction: Depletion of Tmem79 in naive ectoderm leads to a reduction in neural plate formation. In classical neural induction assays, animal pole explants from Tmem79-depleted embryos fail to be neuralized by the BMP antagonist Noggin and instead adopt an epidermal fate. This neural plate deficiency can be rescued by mouse Tmem79 mRNA and by co-depletion of β-catenin or Usp8 .

• Pigmentation defects: Tmem79 depletion drastically reduces pigmentation in embryos, suggesting a defect in melanocytes derived from neural crest precursors. Indeed, Tmem79 depletion diminishes expression of neural crest markers Foxd3 and Snail1. The pigmentation deficiency can be rescued by co-depletion of Usp8 or β-catenin, further supporting the role of Tmem79 in inhibiting Wnt/β-catenin signaling for proper neural crest formation .

These developmental effects demonstrate that TMEM79 plays a crucial role in embryonic development, particularly in anterior patterning, neural induction, and neural crest formation through its function as a Wnt antagonist.

What is the molecular mechanism by which TMEM79 regulates Frizzled receptor levels?

TMEM79 regulates Frizzled (FZD) receptor levels through a complex molecular mechanism involving protein-protein interactions and regulation of deubiquitination:

• TMEM79-FZD interaction: TMEM79 exhibits remarkable specificity in binding to FZD receptors. Upon overexpression, TMEM79 co-immunoprecipitates with endogenous FZD5 but not with LRP6 (another Wnt co-receptor). Importantly, TMEM79 interacts with all ten human FZD proteins (FZD1-10) but not with Smoothened (SMO), a distant member of the FZD subfamily required for Shh signaling. This binding specificity mirrors TMEM79's functional specificity in regulating Wnt signaling .

• Selective interaction with immature FZD forms: TMEM79 preferentially co-immunoprecipitates with the immature/unglycosylated form of FZD5 that is predominantly located in the endoplasmic reticulum (ER) during protein biogenesis. This interaction is distinct from the action of ZNRF3, which co-immunoprecipitates with mature FZD5 at the plasma membrane .

• Inhibition of USP8-mediated deubiquitination: TMEM79 forms a complex with FZD and USP8 (a deubiquitinating enzyme) and inhibits USP8's ability to remove ubiquitin from FZD during biogenesis. This inhibition promotes trafficking of ubiquitinated FZD from the ER to the lysosome for destruction. The capacity of TMEM79 to function as a Wnt/FZD signaling antagonist relies fully on its inhibition of USP8 and is abolished in USP8 KO cells .

• Lysosomal degradation pathway: Treatment with the lysosomal inhibitor Bafilomycin A1 (Baf A1) results in elevated levels of both immature and mature forms of FZD5, suggesting lysosomal degradation of both forms. Baf A1 blocks immature FZD5 degradation without increasing the level of mature FZD5 at the plasma membrane, consistent with the notion that TMEM79 directs FZD from the ER to the lysosome for degradation .

This mechanism highlights a synthesis-ubiquitination-destruction cycle for FZD from the ER to the lysosome, with USP8 acting to prevent FZD destruction and to allow FZD to mature to the plasma membrane. TMEM79's role in this process represents a previously unrecognized FZD degradation pathway that is independent of and complementary to the ZNRF3/RNF43-mediated endocytic-lysosomal pathway .

How does TMEM79 function differ from ZNRF3/RNF43 in the regulation of FZD receptors?

TMEM79 and ZNRF3/RNF43 regulate FZD receptor levels through distinct but complementary mechanisms:

• Subcellular localization and site of action:

  • TMEM79 is predominantly localized at the endoplasmic reticulum (ER) and to some extent in lysosomes, but is not found at the Golgi or plasma membrane. It acts on immature FZD during protein biogenesis .

  • ZNRF3/RNF43 functions at the plasma membrane as Rspo receptors and acts on mature FZD at the cell surface .

• FZD interaction specificity:

  • TMEM79 preferentially co-immunoprecipitates with immature/unglycosylated FZD5 that is predominantly located in the ER during biogenesis .

  • ZNRF3 co-immunoprecipitates with mature FZD5 at the plasma membrane .

• Pathway independence:

  • TMEM79 antagonizes Wnt signaling in ZNRF3/RNF43 double knockout (ZRKO) cells, and ZNRF3 antagonizes Wnt signaling in TMEM79 knockout (T79KO) cells, indicating that they function independently of each other .

  • siRNA depletion of ZNRF3 further elevates TOP-Flash activation in T79KO cells, and siRNA depletion of TMEM79 elevates TOP-Flash in ZRKO cells, further supporting their independent actions .

• Mechanism of action:

  • TMEM79 forms a complex with FZD and USP8 and inhibits USP8 deubiquitination of FZD during biogenesis, promoting trafficking of ubiquitinated FZD from ER to lysosome for destruction .

  • ZNRF3/RNF43 are E3 ubiquitin ligases that ubiquitinate mature FZD at the plasma membrane, leading to endocytosis and lysosomal degradation .

These two FZD degradation pathways may act in parallel or in concert in different developmental or homeostatic contexts to ensure a low and rate-limiting FZD protein level at the plasma membrane. This dual regulation allows cellular responsiveness to Rspo proteins, which stabilize/increase FZD protein levels through inhibition of ZNRF3/RNF43 .

What are the critical experimental considerations when studying TMEM79 in animal models?

When studying TMEM79 in animal models, particularly Xenopus embryos, several critical experimental considerations should be addressed:

• Morpholino design and validation:

  • Design specific antisense morpholino oligonucleotides (MOs) targeting Tmem79 (e.g., Tmem79.S MO: 5'-TCTGGAGCAACCATTGGACTTCTGT-3' and Tmem79.L MO: 5'-TGTTTCAGGAGACACCATTGGACTT-3') .

  • Validate MO specificity by demonstrating that it blocks synthesis of Xenopus Tmem79 protein specifically .

  • Include rescue experiments with mouse Tmem79 mRNA to confirm phenotype specificity .

• Injection parameters for embryonic studies:

  • For animal cap assays, carefully titrate mRNA doses (e.g., Tmem79: 100-200 pg; Xwnt8: 10 pg; β-catenin: 50 pg; Xnr1: 250 pg; BMP4: 100 pg) .

  • Inject into the animal pole at the 4-cell stage for studying effects on naive ectoderm .

  • For MO experiments, use approximately 20 ng of MO injected into the animal pole at the 4-cell stage .

• Dissection and culture timing:

  • For animal caps treated with Xwnt8, Xnr1, or BMP4 mRNA, dissect at stage 9 .

  • For animal caps treated with recombinant proteins (bFGF: 100 ng/mL; hShh: 1.5 ng/mL), dissect at stage 8.5 and culture until stage 10 before RT-PCR analysis .

  • For neural induction assays, dissect animal caps at stage 9 and treat with recombinant Noggin protein (500 ng/mL) until stage 18 before RT-PCR analysis .

• Co-depletion experiments:

  • Include co-depletion of β-catenin or Usp8 to verify pathway specificity .

  • Use Usp8 MO (5'-GCTGCAAATTCTCCTCTCTTATCAA-3') at comparable doses to Tmem79 MO .

• Phenotypic and molecular analysis:

  • Assess multiple developmental processes affected by Tmem79, including anterior patterning, neural plate formation, and neural crest development .

  • Examine expression of appropriate marker genes through in situ hybridization or RT-PCR .

  • Document phenotypes thoroughly, including anterior deficiency, neural plate reduction, and pigmentation defects .

These experimental considerations ensure rigorous investigation of TMEM79 function in vivo, allowing for accurate assessment of its role in embryonic development and signaling pathway regulation.

How does TMEM79 contribute to disease pathogenesis?

TMEM79 has been identified as a predisposition gene for atopic dermatitis (AD), suggesting a potential role in disease pathogenesis:

• Association with atopic dermatitis:

  • TMEM79 is a predisposition gene for atopic dermatitis (AD), the most common chronic and relapsing inflammatory skin disease that affects 20% of children and 5-10% of adults .

  • The molecular etiology of AD remains poorly defined, and the identification of TMEM79 as a predisposition gene provides new insights into potential pathogenic mechanisms .

• Potential mechanisms linking TMEM79 to AD:

  • Given TMEM79's role as a Wnt signaling antagonist, the research findings imply that deregulation of Wnt/FZD signaling may be a possible cause of AD pathogenesis .

  • Wnt signaling is known to regulate skin development, homeostasis, and immune responses, all of which are relevant to AD pathology.

  • TMEM79's function in regulating protein degradation and trafficking could potentially affect skin barrier function, which is compromised in AD.

• Pathway connections:

  • The functional connection between TMEM79 and USP8 may be particularly relevant, as dysregulation of deubiquitination processes has been implicated in various skin disorders.

  • The independent but complementary functions of TMEM79 and ZNRF3/RNF43 in FZD regulation suggest that perturbations in either pathway could contribute to disease through altered Wnt signaling.

While the research primarily establishes TMEM79's molecular function and developmental roles, the connection to atopic dermatitis points to potential therapeutic avenues targeting the TMEM79-USP8-FZD regulatory axis for treating this common skin condition .

What are effective methods for producing and purifying recombinant TMEM79 protein?

Producing and purifying recombinant TMEM79 protein presents unique challenges due to its transmembrane nature. Based on methodologies described for similar transmembrane proteins, the following approach can be effective:

• Expression system selection:

  • For functional studies, mammalian expression systems such as HEK293T cells are preferred to ensure proper folding and post-translational modifications of TMEM79 .

  • For structural studies or large-scale production, insect cell systems (Sf9 or Hi5) may provide higher yields while maintaining eukaryotic processing capabilities.

• Construct design considerations:

  • Include epitope tags (such as Myc-Flag tandem tags) to facilitate detection and purification .

  • Consider truncation constructs that remove portions of the transmembrane domains if studying specific protein-protein interactions.

  • For membrane protein expression, fusion partners such as GFP or MBP can improve stability and folding.

• Purification strategy:

  • Use a two-step affinity purification approach, such as the tandem affinity purification (TAP) method described in the research:

    • First affinity step: Incubate lysates with Anti-Flag M2 affinity gel at 4°C for 4 hours

    • Wash beads five times with lysis buffer

    • Elute bound proteins with Flag peptide (N-Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys-C)

    • Second affinity step: Incubate protein elution with Anti-c-Myc Agarose beads overnight at 4°C

    • Elute proteins with c-Myc peptide (N-Glu-Gln-Lys-Leu-Ile-Ser-Glu-Glu-Asp-Leu-C)

• Detergent considerations:

  • For membrane protein extraction, mild non-ionic detergents (e.g., DDM, LMNG, or digitonin) can be used to solubilize TMEM79 while preserving protein-protein interactions.

  • NP-40 lysis buffer has been successfully used for TMEM79 extraction in co-IP experiments .

• Quality control and validation:

  • Assess protein purity using SilverQuest Silver Staining Kit after resolution on 4-12% NuPage gels .

  • Confirm protein identity and integrity using mass spectrometry analysis.

  • Verify functional activity through binding assays with known interaction partners such as USP8 and FZD.

These methodological approaches provide a framework for producing recombinant TMEM79 protein suitable for biochemical and structural studies while maintaining its functional properties.

What techniques are most effective for studying TMEM79-FZD interactions?

Several complementary techniques have proven effective for studying TMEM79-FZD interactions:

• Co-immunoprecipitation (co-IP):

  • This approach has successfully demonstrated TMEM79 interaction with all ten FZD proteins (FZD1-10) but not with Smoothened (SMO) .

  • For optimal results, perform experiments in HEK293T cells with tagged constructs (e.g., TMEM79-Myc-Flag and HA-FZD) .

  • Use NP40 lysis buffer for cell lysis and protein extraction .

  • To distinguish between mature and immature forms of FZD, analyze the co-IP results by Western blotting, noting that the upper/slow-migrating band reflects mature/fully glycosylated FZD while the lower/fast-migrating band represents immature/unglycosylated FZD .

• Cell surface biotinylation:

  • This technique helps differentiate between cell surface and intracellular pools of proteins.

  • It has revealed that TMEM79 co-IPs with the immature FZD5 only, while causing reduction of mature FZD5 at the plasma membrane .

  • Importantly, while ZNRF3 is surface-biotinylated, TMEM79 is not, suggesting that TMEM79 is not localized at the plasma membrane .

• Subcellular fractionation and localization studies:

  • Immunostaining via confocal microscopy has shown that endogenous TMEM79 localizes predominantly at the ER and to some extent in lysosomes, but not or little at the Golgi or plasma membrane .

  • This approach complements biochemical analyses and provides spatial context for the TMEM79-FZD interaction.

• Mutational analysis:

  • The use of FZD5-K0, a mutant FZD5 with cytoplasmic lysine (K) residues replaced by arginine (R) residues, has demonstrated that FZD5 ubiquitination is involved in the action of TMEM79 .

  • This mutant is resistant to reduction of its cell surface level by both ZNRF3 and TMEM79 .

• Inhibitor studies:

  • Treatment with lysosomal inhibitor Bafilomycin A1 (Baf A1) blocks immature FZD5 degradation without increasing the level of mature FZD5 at the plasma membrane, supporting the notion that TMEM79 directs FZD from the ER to the lysosome .

  • Baf A1 also prevents lysosomal degradation of TMEM79 itself, which is found in the lysosome and likely chaperones FZD to degradation there .

These methodological approaches provide a comprehensive toolkit for studying the specificity, subcellular localization, and functional consequences of TMEM79-FZD interactions, enabling detailed characterization of this regulatory pathway.

What are the key unresolved questions about TMEM79 function?

Despite significant advances in understanding TMEM79 function, several key questions remain unresolved:

• Structural determinants of specificity:

  • What structural features of TMEM79 confer its specificity for FZD receptors?

  • Which domains of TMEM79 are responsible for its interaction with USP8 and inhibition of USP8's deubiquitinating activity?

  • How does TMEM79 distinguish between different FZD family members, if at all?

• Regulatory mechanisms:

  • How is TMEM79 expression and activity regulated in different developmental contexts and disease states?

  • Are there post-translational modifications that modulate TMEM79 function?

  • What triggers TMEM79-mediated degradation versus maturation of FZD receptors?

• Connection to atopic dermatitis:

  • What is the precise mechanism by which TMEM79 dysfunction contributes to atopic dermatitis pathogenesis?

  • Are there specific TMEM79 variants associated with different subtypes or severities of atopic dermatitis?

  • Could targeting the TMEM79-USP8-FZD axis provide therapeutic benefits in atopic dermatitis?

• Ubiquitination machinery:

  • Which ubiquitin ligases ubiquitinate immature FZD during biogenesis and work in concert with TMEM79?

  • The research notes that "Ubiquitin ligases that ubiquitinate immature FZD and LRP6 remain to be identified" .

• Developmental regulation:

  • How is the balance between TMEM79 and ZNRF3/RNF43 pathways regulated during development?

  • Are there tissue-specific or developmental stage-specific differences in TMEM79 function?

  • What are the downstream effects of TMEM79-mediated Wnt antagonism on gene expression and cellular differentiation?

Addressing these questions will require integrated approaches combining structural biology, biochemistry, genetics, and developmental biology. The answers will not only enhance our understanding of TMEM79 function but may also reveal new therapeutic targets for conditions like atopic dermatitis.

How might TMEM79 be targeted for therapeutic purposes?

Given TMEM79's role in Wnt signaling regulation and its association with atopic dermatitis, several therapeutic targeting strategies could be explored:

• Modulation of TMEM79-USP8 interaction:

  • Small molecules that enhance TMEM79 inhibition of USP8 could potentially reduce Wnt signaling in contexts where hyperactive Wnt contributes to disease.

  • Conversely, compounds that disrupt the TMEM79-USP8 interaction might boost Wnt signaling in conditions where it is deficient.

  • The specificity of this interaction offers a potentially selective target that might avoid the broad effects of targeting Wnt signaling directly.

• TMEM79 expression regulation:

  • For atopic dermatitis, where TMEM79 dysfunction may contribute to pathogenesis, gene therapy approaches to restore proper TMEM79 expression could be explored.

  • RNA-based therapeutics (e.g., antisense oligonucleotides or siRNAs) could be designed to modulate TMEM79 expression in specific tissues.

• Targeting the FZD degradation pathway:

  • Understanding the complete FZD degradation pathway that TMEM79 participates in could reveal additional therapeutic targets.

  • The research has shown that "Ubiquitin ligases that ubiquitinate immature FZD and LRP6 remain to be identified" - identifying these enzymes could provide new drug targets.

• Skin barrier function in atopic dermatitis:

  • If TMEM79's role in atopic dermatitis involves skin barrier function, topical treatments that compensate for TMEM79 dysfunction could be developed.

  • Since TMEM79 is a predisposition gene for atopic dermatitis, early intervention strategies in genetically susceptible individuals might prevent disease development.

• Combination therapies:

  • Given the parallel but independent functions of TMEM79 and ZNRF3/RNF43 in FZD regulation, combination approaches targeting both pathways might provide synergistic therapeutic effects.

  • Combining TMEM79-targeted therapies with existing treatments for atopic dermatitis might enhance efficacy or address treatment-resistant cases.

While these therapeutic approaches are speculative and require extensive validation, the emerging understanding of TMEM79's molecular function provides a foundation for rational drug discovery efforts. Future research clarifying TMEM79's precise role in disease pathogenesis will be crucial for translating these basic scientific insights into clinical applications.