Recombinant Human Transmembrane 7 superfamily member 3 (TM7SF3)

What is the structural organization of TM7SF3?

TM7SF3 is a seven-transmembrane protein with a unique structure. In silico analysis using AlphaFold2 reveals an extended N-terminal domain resembling Class B GPCRs (such as GLP-1R), but with distinct characteristics. The N-terminal region possesses prominent barrel-like β-pleated sheets that potentially serve as a ligand-binding domain. The protein's structural analysis suggests both transmembrane domains and nuclear localization capabilities, which explains its ability to function both at membrane surfaces and within nuclear speckles . For recombinant protein studies, researchers have successfully used N-terminus fragments (aa 24-290) and C-terminus fragments (aa 496-564) with 6xHis and MBP tags .

How does TM7SF3 regulate cellular homeostasis?

TM7SF3 functions as a homeostatic factor primarily by attenuating endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). In multiple cell types, TM7SF3 maintains protein homeostasis by:

• Inhibiting caspase 3/7 activation (decreasing activity by approximately 65% in human islets)

• Reducing cellular content of pro-apoptotic proteins (FAS, FADD, caspase-8)

• Suppressing iNOS expression and subsequent NO production (silencing TM7SF3 increases iNOS mRNA levels 1.7-fold and NO production 2-fold)

• Preventing inhibitory phosphorylation of eIF2α during stress conditions

• Inhibiting stress-induced expression of ATF3, ATF4, and C/EBP homologous protein (CHOP)

These protective mechanisms operate both under basal conditions and under stress induced by thapsigargin, tunicamycin, or pro-inflammatory cytokines .

What are the optimal techniques for expressing and purifying recombinant TM7SF3?

For recombinant TM7SF3 expression, researchers have successfully employed bacterial expression systems using plasmids with N-terminus fragments (aa 24-290) and C-terminus fragments (aa 496-564), both tagged with 6xHis and MBP-TEVH . The optimal cloning approach involves Transfer-PCR (TPCR), which combines PCR amplification from mouse TM7SF3-Myc vectors followed by integration into recipient vectors (e.g., pETMBPH-TEVH) . Primers should be designed with target-gene specific sequences at the 3′ end and sequences corresponding to the integration site in the recipient vector at the 5′ end. After PCR, treatment with Dpn1 is necessary to remove parental methylated DNA before transformation into competent E. coli XL-1Blue .

What methodologies are effective for studying TM7SF3's role in alternative splicing?

To investigate TM7SF3's role in alternative splicing, researchers should employ:

• RNAseq analysis following TM7SF3 silencing in cell models (e.g., U2OS or MIN6 cells)

• Analysis of local splicing variations (LSVs) using the MAJIQ/VOILA algorithm

• Validation of specific splicing events using RT-PCR for selected target genes

• Motif enrichment analysis using tools such as ATtRACT and HOMER to identify significantly enriched motifs in regions affected by TM7SF3 knockdown

• Co-immunoprecipitation studies to identify TM7SF3's interactions with splicing factors

• Concurrent silencing of TM7SF3 and its partner proteins (e.g., HNRNPK) to determine functional interactions

This comprehensive approach has revealed that TM7SF3 regulates alternative splicing of >330 genes, with particularly significant effects at the 3′end of introns .

How does TM7SF3 interact with the p53 pathway?

TM7SF3 and p53 form a regulatory feedback loop:

• p53 directly regulates TM7SF3 transcription by binding to the TM7SF3 gene at a site approximately 1000 bp downstream of the transcription start site (within the first intron)

• Chromatin immunoprecipitation analysis reveals approximately 5-fold enrichment in p53 binding to this site

• Activation of p53 by Nutlin increases TM7SF3 expression in a time-dependent manner, while silencing of p53 abrogates this effect

• Conversely, TM7SF3 inhibits p53 activity - silencing TM7SF3 significantly potentiates stress-induced p53 activity as measured by increased p21 mRNA levels

This negative feedback loop represents a novel mechanism whereby p53 promotes expression of TM7SF3, which then acts to inhibit p53 activity, potentially serving as a homeostatic regulatory mechanism .

What is the relationship between TM7SF3 and the unfolded protein response (UPR)?

TM7SF3 functions as a negative regulator of the UPR pathway:

• Silencing TM7SF3 accelerates ER stress and UPR activation through:

  • Increased inhibitory phosphorylation of eIF2α

  • Enhanced expression of ATF3, ATF4, and CHOP (approximately 2-fold increase in protein levels)

  • Induction of apoptosis via caspase 3/7 activation

• TM7SF3 levels are themselves regulated by ER stress:

  • Treatment with thapsigargin (an ER stress inducer) reduces TM7SF3 mRNA levels

  • This suggests a complex regulatory relationship between TM7SF3 and ER stress pathways

• The protective effects of TM7SF3 against UPR activation are observed across multiple cell types, including:

  • Pancreatic β-cells (MIN6 cell line and human islets)

  • Liver cells (HepG2)

  • Bone cells (U2-OS)

  • Kidney cells (HEK293)

How does TM7SF3 influence pancreatic β-cell function and diabetes pathophysiology?

TM7SF3 serves as a protective factor for pancreatic β-cells through multiple mechanisms:

• Inhibition of cytokine-induced apoptosis:

  • Overexpression of TM7SF3 in human islets decreases cytokine-induced caspase 3/7 activity by approximately 45%

  • Reduces cellular content of pro-apoptotic proteins (FAS, FADD, caspase-8)

• Promotion of insulin secretion:

  • Silencing of TM7SF3 inhibits insulin secretion from pancreatic β-cells

  • Maintains cellular reducing power within physiological levels

• Protection against ER stress:

  • Attenuates cytokine-induced ER stress and UPR activation

  • Suppresses iNOS expression and subsequent NO production

These findings suggest that TM7SF3 might serve as a potential therapeutic target for diabetes, particularly for preserving β-cell mass and function in inflammatory environments typical of both type 1 and type 2 diabetes .

What is the role of TM7SF3 in liver fibrosis and metabolic-dysfunction-associated steatohepatitis (MASH)?

Recent research (2024) has revealed TM7SF3's critical role in liver fibrosis:

• TM7SF3 regulates hepatic stellate cell (HSC) activation:

  • Deletion of TM7SF3 accelerates HSC activation in liver organoids, primary human HSCs, and in vivo MASH mouse models

  • TM7SF3 knockdown leads to activation of fibrogenic programs and HSC proliferation

• Molecular mechanism involves TEAD1 alternative splicing:

  • Under normal conditions, TM7SF3 keeps hnRNPU inactivated, leading to production of a standard, relatively inactive TEAD1 protein

  • Under fibrosis-promoting conditions or when TM7SF3 is absent, hnRNPU becomes active and triggers alternative splicing of TEAD1

  • This alternative splicing results in exclusion of the inhibitory exon 5, generating a shorter, more active form of TEAD1 that drives HSC activation

• Therapeutic potential:

  • Inhibiting TEAD1 alternative splicing with a specific antisense oligomer (ASO) deactivates HSCs in vitro and reduces MASH diet-induced liver fibrosis

  • This suggests targeted modulation of TM7SF3 or its downstream pathways could serve as a potential treatment for MASH-related fibrosis

How does a transmembrane protein like TM7SF3 localize to nuclear speckles, and what are the implications?

TM7SF3 challenges conventional understanding by localizing to nuclear speckles despite being a seven-transmembrane protein. This unexpected localization has significant implications:

• Subcellular localization studies:

  • TM7SF3 predominantly resides in nuclear speckles, eukaryotic nuclear bodies enriched in splicing factors

  • This suggests either a novel translocation mechanism for transmembrane proteins or potential dual functionality

• Formation of stable complexes with splicing machinery:

  • TM7SF3 forms stable complexes with pre-mRNA splicing factors including DHX15, LARP7, HNRNPU, RBM14, and HNRNPK

  • These complexes remain associated even following extraction with strong detergents (deoxycholate and SDS), indicating tight association

• Functional consequences of nuclear localization:

  • Allows direct regulation of alternative splicing of >330 genes

  • Preferentially affects splicing at the 3′end of introns, particularly the last 100 nt before splice acceptors

  • This represents the first identified seven-transmembrane protein with such unique nuclear localization and function as a modulator of the splicing machinery

Understanding this unusual localization may reveal new principles about protein trafficking and multifunctionality across cellular compartments.

What are the cell-specific effects of TM7SF3 on alternative splicing and how might these contribute to tissue-specific functions?

TM7SF3 demonstrates remarkable cell-type specificity in its regulation of alternative splicing:

This cell-type specificity and context-dependent regulation suggests TM7SF3 may contribute to tissue-specific alternative splicing programs relevant to both normal physiology and disease states.

How might recombinant TM7SF3 be optimized for therapeutic applications targeting cellular stress pathways?

For therapeutic development targeting TM7SF3, researchers should consider:

• Structural optimization approaches:

  • Focus on the N-terminal domain (aa 24-290) which contains prominent barrel-like β-pleated sheets that potentially serve as ligand-binding domains

  • Consider development of mimetic peptides based on functional domains of TM7SF3 that retain stress-protective activities

• Delivery strategies for recombinant TM7SF3:

  • Cell-penetrating peptide fusions to enhance cellular uptake

  • Nuclear localization signal incorporation to ensure appropriate subcellular targeting

  • Liposomal or nanoparticle-based delivery systems to protect protein integrity

• Cell-specific targeting considerations:

  • For pancreatic β-cell protection: Conjugation with β-cell-specific targeting moieties

  • For liver fibrosis applications: Hepatic stellate cell-targeted delivery systems

  • Stimulus-responsive release systems activated by markers of cellular stress

• Alternative approach using antisense oligonucleotides (ASOs):

  • Recent research shows success using ASO 56 to specifically target TEAD1 alternative splicing downstream of TM7SF3

  • This ASO effectively reduced liver fibrosis in MASH mouse models, suggesting similar approaches might be effective for modulating TM7SF3-dependent pathways

• Consideration of potential side effects:

  • Given TM7SF3's widespread effects on alternative splicing (>330 genes)

  • Tissue-specific and context-dependent effects must be carefully evaluated

  • Potential interactions with the p53 pathway require thorough safety assessment