TMEM27 Human

What is TMEM27 and where is it primarily expressed in human tissues?

TMEM27 was first described in the kidney as an angiotensin-converting enzyme 2 (ACE2) homologue involved in amino acid transport. In humans, TMEM27 is primarily expressed in pancreatic islets, specifically in beta cells, and in kidney proximal tubular cells . The protein is cleaved and shed by pancreatic beta cells, which led to its proposal as a potential beta cell mass biomarker.

Research methods to study expression include:

• Immunohistochemistry with specific antibodies

• RT-PCR analysis of tissue samples

• RNA sequencing of isolated cell populations

How does TMEM27 expression differ between healthy and diabetic human islets?

TMEM27 mRNA levels are significantly decreased in islets from type 2 diabetic donors compared to non-diabetic controls. Specifically, studies show that TMEM27 mRNA levels are reduced by approximately 65% in diabetic islets (healthy islets 1.00 ± 0.26, diabetic islets 0.35 ± 0.15 arbitrary units) . Interestingly, while expression is reduced, the localization pattern remains unchanged in diabetic donors - TMEM27 is still primarily produced in insulin-positive cells.

This finding aligns with whole-genome profiling data identifying TMEM27 as one of 370 genes differentially expressed between diabetic and control human islets . Additionally, TMEM27 expression is reported to be tenfold lower in islets from recently diagnosed type 1 diabetic pancreatic donors compared with healthy controls .

What are the suggested functions of TMEM27 in human pancreatic beta cells?

The functional role of TMEM27 in beta cells has been the subject of scientific debate. Studies have suggested two potential primary functions:

• Insulin secretion: Overexpression studies demonstrate that TMEM27 significantly enhances glucose-induced insulin secretion in both beta cell lines and isolated islets .

• Cell proliferation: Some studies have linked TMEM27 to beta cell proliferation, though evidence for this function is more controversial .

The evidence more strongly supports a role in insulin secretion, with overexpression studies showing significant enhancement of glucose-stimulated insulin secretion but only modest or no effects on proliferation .

What molecular mechanisms explain TMEM27's role in glucose-stimulated insulin secretion?

TMEM27's gene expression correlates with insulin and SNAPIN expression in human islets . This correlation with SNAPIN is particularly notable as SNAPIN is involved in insulin exocytosis, suggesting that TMEM27 may function in the insulin secretory pathway through interaction with the exocytosis machinery.

Experimental evidence supporting TMEM27's role in insulin secretion includes:

• Overexpression of TMEM27 (from rat, mouse, or human) in INS-1 cells significantly enhances glucose-stimulated insulin secretion

• Similar enhancement of glucose-stimulated insulin secretion is observed when rat Tmem27 is overexpressed in isolated rat pancreatic islets

• The effect on insulin secretion is consistent across species despite potential differences in protein processing

Researchers investigating the molecular mechanisms should consider:

• Examining interactions between TMEM27 and components of the insulin exocytosis machinery

• Studying calcium signaling in TMEM27-overexpressing cells

• Investigating potential changes in glucose sensing or metabolism

How is TMEM27 processed in human cells, and is this processing tissue-specific?

TMEM27 is a membrane protein that undergoes cleavage, releasing its extracellular portion (approximately 25 kDa) into the extracellular space. This extracellular fragment is glycosylated . Initially, this cleavage was thought to be specific to beta cells, which led to the hypothesis that shed TMEM27 could serve as a beta cell mass biomarker.

• TMEM27 is cleaved and shed by both pancreatic islets and renal proximal tubular cells

• The same size fragment (~25 kDa) is detected in the culture media of both cell types

• The fragment can be deglycosylated, confirming it corresponds to the released fragment of TMEM27

These findings challenge the potential use of shed TMEM27 as a beta cell-specific biomarker .

Table 1: TMEM27 Cleavage in Different Cell Types

How do species differences affect TMEM27 function and processing?

TMEM27 protein is highly homologous among rats, mice, and humans . Despite this homology, immunoblot analysis of islet protein extracts from these species reveals different band patterns, suggesting potential inter-species differences in processing and/or post-translational modifications .

To investigate whether these differences affect function, researchers have adenovirally expressed rat, mouse, and human TMEM27/Tmem27 in the same cellular context (INS-1 cells). Results showed that:

• All three orthologs elicited an increase in glucose-stimulated insulin secretion

• All showed a slight increase in proliferation, though this was not accompanied by an increase in cell number

• The different orthologs appeared to exert similar effects when expressed in the same cellular context

These findings suggest that despite potential differences in processing, the fundamental function of TMEM27 in enhancing insulin secretion is conserved across species.

What experimental approaches are recommended for studying TMEM27 function in islets?

Based on published research, several experimental approaches have proven effective for studying TMEM27:

• Adenoviral expression systems: Researchers have successfully used recombinant adenoviruses to express TMEM27/Tmem27 in cell lines and isolated islets . This approach allows for efficient transduction and controlled expression levels.

Method overview:

• cDNA for TMEM27/Tmem27 is amplified from islet total RNA

• cDNA is myc-tagged at the 3′ end for detection

• The construct is subcloned into an adenoviral vector

• Recombinant adenoviruses are generated and titrated

• Transduction efficiency can be monitored using control adenoviruses expressing markers like β-galactosidase

• Functional assays:

  • Glucose-stimulated insulin secretion assays

  • Proliferation measurements (including BrdU incorporation)

  • Cell number quantification

  • Western blot analysis of protein processing

• Correlation analyses: Examining relationships between TMEM27 expression and other genes (e.g., insulin, SNAPIN) in human islet samples

How can researchers reconcile contradictory findings regarding TMEM27's role in beta cell proliferation?

The scientific literature contains contradictory findings regarding TMEM27's role in beta cell proliferation:

• One study linked TMEM27 production to beta cell proliferation and suggested its potential use as a beta cell mass biomarker

• Other studies, including the overexpression experiments in INS-1 cells and islets, showed only modest or no effects on proliferation

• Tmem27-knockout mice exhibited normal beta cell mass and replication

To reconcile these contradictions, researchers should consider:

• Experimental context: Different cell lines, primary cells, or in vivo models may respond differently

• Expression levels: The level of TMEM27 overexpression might influence proliferative effects

• Timing: Acute versus chronic expression might produce different outcomes

• Interaction with other factors: The proliferative response might depend on other cellular factors that vary between experimental systems

A comprehensive approach would include:

• Dose-response studies with varying TMEM27 expression levels

• Temporal analysis of TMEM27 effects

• Studies in multiple model systems

• Investigation of potential interacting partners

What is the potential relevance of TMEM27 in diabetes research and therapeutics?

The reduced expression of TMEM27 in diabetic islets, combined with its role in enhancing glucose-stimulated insulin secretion, suggests that TMEM27 dysfunction might contribute to the pathophysiology of diabetes .

Key considerations for diabetes researchers include:

• Diagnostic relevance: While shed TMEM27 is not a specific beta cell biomarker due to its expression and cleavage in kidney cells, changes in TMEM27 expression may still provide insights into beta cell function in diabetes .

• Therapeutic potential: Understanding the mechanisms through which TMEM27 enhances insulin secretion could lead to novel therapeutic approaches to improve beta cell function in diabetes.

• Pathophysiological insights: The correlation between reduced TMEM27 expression and diabetes provides a potential mechanism contributing to impaired insulin secretion in the disease.

Table 2: Experimental Models for Studying TMEM27

What techniques are recommended for detecting and quantifying TMEM27 expression?

For researchers studying TMEM27, multiple complementary techniques should be considered:

• mRNA expression:

  • Real-time PCR for quantification of TMEM27 transcript levels

  • RNA sequencing for genome-wide expression analysis

• Protein detection:

  • Western blotting for detection of full-length and cleaved TMEM27

  • Immunoprecipitation for enrichment of low-abundance forms

  • ELISA for quantification in culture media or biological fluids

• Localization:

  • Immunofluorescence to determine cellular and subcellular localization

  • Co-staining with markers like insulin to confirm cell-type specificity

When studying TMEM27 processing, analysis of both cell lysates and culture media is essential to detect the cleaved and shed extracellular portion .

How can researchers effectively overexpress or knockdown TMEM27 in beta cells?

Based on published methodologies, effective approaches include:

• Overexpression:

  • Adenoviral vectors have been successfully used to express myc-tagged TMEM27 in both beta cell lines and primary islets

  • The construct should include appropriate tags (e.g., myc tag) to facilitate detection

  • Efficiency can be monitored using reporter genes like β-galactosidase

• Knockdown/Knockout:

  • While not explicitly described in the search results, standard approaches would include:

  • siRNA or shRNA for transient or stable knockdown

  • CRISPR/Cas9 for genomic editing and knockout

  • Comparison with Tmem27 knockout mouse models which have been generated

For any genetic manipulation, appropriate controls should be included:

• Empty vector controls for overexpression

• Scrambled siRNA for knockdown studies

• Isogenic wild-type controls for knockout models

What are the recommended assays for studying TMEM27's functional effects?

To investigate TMEM27's proposed functions in insulin secretion and potentially proliferation, researchers should consider these assays:

• Insulin secretion:

  • Static glucose-stimulated insulin secretion assays, measuring insulin release at basal and stimulatory glucose concentrations

  • Dynamic perifusion studies to assess the kinetics of insulin release

  • Measurement of insulin content to normalize secretory data

• Proliferation:

  • BrdU incorporation to measure DNA synthesis

  • Cell counting to assess changes in cell number over time

  • Ki67 immunostaining to identify proliferating cells

  • EdU incorporation as an alternative to BrdU

• Protein processing:

  • Western blot analysis of cell lysates to detect full-length TMEM27

  • Analysis of culture media to detect the cleaved extracellular domain

  • Deglycosylation experiments to confirm the identity of cleaved fragments

What recent advances have been made in understanding TMEM27's role in human beta cells?

The identification of TMEM27 as one of the genes differentially expressed between diabetic and control human islets represents an important advance . Additionally, the clarification that TMEM27 primarily influences insulin secretion rather than proliferation has helped resolve some of the contradictions in the literature .

Key recent findings include:

• The demonstration that TMEM27 cleavage is not beta cell-specific, occurring also in kidney cells, which challenges its potential use as a beta cell mass biomarker

• The observation that TMEM27 mRNA levels are significantly reduced in both type 1 and type 2 diabetic islets compared to controls

• The correlation between TMEM27 expression and SNAPIN, suggesting a potential mechanistic link to the insulin secretory machinery

What are promising directions for future TMEM27 research?

Based on current knowledge and gaps, several promising research directions emerge:

• Mechanistic studies: Investigating the precise molecular mechanisms through which TMEM27 enhances glucose-stimulated insulin secretion, particularly potential interactions with SNAPIN and other components of the exocytosis machinery.

• Regulatory mechanisms: Exploring how TMEM27 expression is regulated in beta cells and why it is decreased in diabetic islets.

• Therapeutic potential: Determining whether strategies to restore TMEM27 expression or function could improve beta cell function in diabetes.

• Processing enzymes: Identifying the specific proteases responsible for TMEM27 cleavage in beta cells and kidney cells.

• Signaling functions: Investigating whether the cleaved extracellular domain has signaling functions beyond serving as a potential biomarker.

• Clinical correlations: Examining relationships between TMEM27 levels and clinical parameters in diabetic patients.

TMEM27 has also been identified in human genetics studies alongside other targets like CUBN, CLDN2, and TM4SF4 in the context of HNF1A function, suggesting potential roles in broader genetic networks related to metabolic diseases .