A novel mechanism of imeglimin-mediated insulin secretion via the cADPR-TRP channel pathway

Imeglimin modulates mitochondria biology and facilitates mitokine secretion in 3T3-L1 adipocytes

AIMS

Imeglimin, a novel antidiabetic drug, has recently been reported to affect pancreatic β-cells and hepatocytes. Adipose tissue plays a crucial role in systemic metabolism. However, its effect on adipocytes remains unexplored. Herein, we investigated the effects of imeglimin on adipocytes, particularly in the mitochondria.

MAIN METHODS

The 3T3-L1 adipocytes were treated with imeglimin. Mitochondrial respiratory complex I activity and NAD+, NADH, and AMP levels were measured. Protein expression levels were determined by western blotting, mitochondrial DNA and mRNA expression levels were determined using quantitative polymerase chain reaction, and secreted adipocytokine and mitokine levels were determined using adipokine array and enzyme-linked immunosorbent assay.

KEY FINDINGS

Imeglimin inhibited complex I activity, decreased the NAD+/NADH ratio, and increased AMP levels, which were associated with the enhanced phosphorylation of AMP-activated protein kinase. In addition, imeglimin increased the mitochondrial DNA content and levels of mitochondrial transcription factor A and peroxisome proliferator-activated receptor-γ coactivator 1-α mRNA, which were abolished by Ly294002, a phosphoinositide 3-kinase inhibitor. Furthermore, imeglimin facilitated the expression levels of markers of the mitochondrial unfolded protein response, and the gene expression and secretion of two mitokines, fibroblast growth factor 21 and growth differentiation factor 15. The production of both mitokines was transcriptionally regulated and abolished by phosphoinositide 3-kinase and Akt inhibitors.

SIGNIFICANCE

Imeglimin modulates mitochondrial biology in adipocytes and may exert a mitohormetic effect through mitokine secretion.

Imeglimin enhances glucagon secretion through an indirect mechanism and improves fatty liver in high-fat, high-sucrose diet-fed mice

AIMS/INTRODUCTION

Imeglimin is a recently approved oral antidiabetic agent that improves insulin resistance, and promotes insulin secretion from pancreatic β-cells. Here, we investigated the effects of imeglimin on glucagon secretion from pancreatic α-cells.

MATERIALS AND METHODS

Experiments were carried out in high-fat, high-sucrose diet-fed mice. The effects of imeglimin were examined using insulin and glucose tolerance tests, glucose clamp studies, and measurements of glucagon secretion from isolated islets. Glucagon was measured using both the standard and the sequential protocol of Mercodia sandwich enzyme-linked immunosorbent assay; the latter eliminates cross-reactivities with other proglucagon-derived peptides.

RESULTS

Plasma glucagon, insulin and glucagon-like peptide-1 levels were increased by imeglimin administration in high-fat, high-sucrose diet-fed mice. Glucose clamp experiments showed that the glucagon increase was not caused by reduced blood glucose levels. After both single and long-term administration of imeglimin, glucagon secretions were significantly enhanced during glucose tolerance tests. Milder enhancement was observed when using the sequential protocol. Long-term administration of imeglimin did not alter α-cell mass. Intraperitoneal imeglimin administration did not affect glucagon secretion, despite significantly decreased blood glucose levels. Imeglimin did not enhance glucagon secretion from isolated islets. Imeglimin administration improved fatty liver by suppressing de novo lipogenesis through decreasing sterol regulatory element binding protein-1c and carbohydrate response element binding protein and their target genes, while enhancing fatty acid oxidation through increasing carnitine palmitoyltransferase I.

CONCLUSIONS

Overall, the present results showed that imeglimin enhances glucagon secretion through an indirect mechanism. Our findings also showed that glucagon secretion promoted by imeglimin could contribute to improvement of fatty liver through suppressing de novo lipogenesis and enhancing fatty acid oxidation.

Exploring new mechanisms of Imeglimin in diabetes treatment: Amelioration of mitochondrial dysfunction

With the increasing prevalence of type 2 diabetes mellitus (T2DM), it has become critical to identify effective treatment strategies. In recent years, the novel oral hypoglycaemic drug Imeglimin has attracted much attention in the field of diabetes treatment. The mechanisms of its therapeutic action are complex and are not yet fully understood by current research. Current evidence suggests that pancreatic β-cells, liver, and skeletal muscle are the main organs in which Imeglimin lowers blood glucose levels and that it acts mainly by targeting mitochondrial function, thereby inhibiting hepatic gluconeogenesis, enhancing insulin sensitivity, promoting pancreatic β-cell function, and regulating energy metabolism. There is growing evidence that the drug also has a potentially volatile role in the treatment of diabetic complications, including metabolic cardiomyopathy, diabetic vasculopathy, and diabetic neuroinflammation. According to available clinical studies, its efficacy and safety profile are more evident than other hypoglycaemic agents, and it has synergistic effects when combined with other antidiabetic drugs, and also has potential in the treatment of T2DM-related complications. This review aims to shed light on the latest research progress in the treatment of T2DM with Imeglimin, thereby providing clinicians and researchers with the latest insights into Imeglimin as a viable option for the treatment of T2DM.

Characteristics and Biological Properties of Imeglimin Hydrochlo ride, A Novel Antidiabetic Agent: A Systematic Review

BACKGROUND

WHO indicates that diabetes will become the 7th leading reason for death by 2030. The physiopathology of dysfunctioning is associated with obesity, weight gain and predominantly insulin resistance in insulin-sensitive cells and continuous deterioration of pancreatic beta cell function..Imeglimin is an investigational novel oral anti-diabetic drug.

OBJECTIVES

The motive of the review is to comprehensively explore the chemistry, biological and analytical analysis of the Imeglimin hydrochloride.

METHODS

To enhance the understanding, a systematic review was conducted by forming a database of relevant existing studies from electronic resources like Web of Science, ScienceDirect and PubMed. The methodology is reflected in the PRISMA design.

RESULT

The drug was approved in the year 2021 for therapeutic purposes in Japan. It is the novel and first approved drug for this type of Anti-diabetic treatment. It is a small molecular drug whose molecular weight is 191.6 grams per mole utilized for oral administration. Imeglimin is thought to have both activities, as the amount of glucose is dependent on insulin secretory impact and insulin sensitivity is increased.

CONCLUSION

Therapeutic, pharmacological, and analytical considerations for the novel drug Imeglimin hydrochloride are discussed in this review.

Imeglimin-mediated glycemic control in maternally inherited deafness and diabetes

Mitochondrial dysfunction causes maternally inherited deafness and diabetes (MIDD). Herein, we report improved glycemic control in a 47-year-old Japanese woman with MIDD using imeglimin without major adverse effects. Biochemical tests and metabolome analysis were performed before and after imeglimin administration. Blood glucose level fluctuations were determined. Sulfonylureas, dipeptidyl peptidase-4 inhibitors (DPP4is), and sodium glucose transporter-2 inhibitors (SGLT2i) were administered to evaluate the efficacy of their combination with imeglimin. Imeglimin decreased the HbA1c and ammonia levels and increased the time-in-range, C-peptide reactivity, and glucagon level. Elevated citrulline and histamine levels were decreased by imeglimin. The hypoglycemic effect was not enhanced by imeglimin when combined with sulfonylurea or DPP4i, but the blood glucose level was improved when combined with SGLT2i. Imeglimin improved glucose concentration-dependent insulin secretion and maximized the insulin secretory capacity by improving mitochondrial function and glutamine metabolism and urea circuit abnormalities by promoting glucagon secretion. Imeglimin could improve glycemic control in MIDD.

Imeglimin-mediated glycemic control in maternally inherited deafness and diabetes

Mitochondrial dysfunction causes maternally inherited deafness and diabetes (MIDD). Herein, we report improved glycemic control in a 47-year-old Japanese woman with MIDD using imeglimin without major adverse effects. Biochemical tests and metabolome analysis were performed before and after imeglimin administration. Blood glucose level fluctuations were determined. Sulfonylureas, dipeptidyl peptidase-4 inhibitors (DPP4is), and sodium glucose transporter-2 inhibitors (SGLT2i) were administered to evaluate the efficacy of their combination with imeglimin. Imeglimin decreased the HbA1c and ammonia levels and increased the time-in-range, C-peptide reactivity, and glucagon level. Elevated citrulline and histamine levels were decreased by imeglimin. The hypoglycemic effect was not enhanced by imeglimin when combined with sulfonylurea or DPP4i, but the blood glucose level was improved when combined with SGLT2i. Imeglimin improved glucose concentration-dependent insulin secretion and maximized the insulin secretory capacity by improving mitochondrial function and glutamine metabolism and urea circuit abnormalities by promoting glucagon secretion. Imeglimin could improve glycemic control in MIDD.

Efficacy, safety and tolerability of imeglimin in patients with type 2 diabetes mellitus: A meta-analysis of randomized controlled trials

AIMS/INTRODUCTION

This meta-analysis aimed to evaluate the efficacy and safety/tolerability of imeglimin, a novel oral antihyperglycemic agent, administered as monotherapy and adjunctive therapy in patients with type 2 diabetes mellitus.

MATERIALS AND METHODS

Parallel-group randomized controlled trials comparing imeglimin with placebo in adults with type 2 diabetes mellitus were included. Risk ratios or weighted mean differences (WMD) and 95% confidence intervals (CIs) were calculated using random effects models. The primary outcome for efficacy was the change in glycated hemoglobin (HbA1c). Secondary outcomes included other efficacy-related outcomes, specific adverse events, and changes in body weight and lipid parameters.

RESULTS

Nine randomized controlled trials (n = 1,655) were included. When analyzed by dose, there was a significant difference in glycated hemoglobin (%) between imeglimin monotherapy and placebo at doses >1,000 mg twice daily (1,000 mg: studies N = 3, patients n = 517, WMD = -0.714, P < 0.001; 1,500 mg: N = 5, n = 448, WMD = -0.531, P = 0.020; 2,000 mg: N = 1, n = 149, WMD = -0.450, P = 0.005). Imeglimin adjunctive therapy significantly improved glycated hemoglobin over placebo at doses of 1,000 mg (N = 1, n = 214, WMD = -0.600, P < 0.001) and 1,500 mg (N = 2, n = 324, WMD = -0.576, P < 0.001). Subgroup analysis of the primary outcome showed that imeglimin was effective regardless of chronic kidney disease category, with studies carried out in Japan and in patients with lower body mass index showing a trend toward improved imeglimin efficacy. There were no significant differences between imeglimin and placebo in the risk of all-cause discontinuation and the proportion of patients who presented with at least one adverse event.

CONCLUSIONS

Imeglimin is efficacious, safe, and well tolerated as monotherapy and adjunctive therapy.

A large-scale functional analysis of genes expressed differentially in insulin secreting MIN6 sublines with high versus mildly reduced glucose-responsiveness

Molecular mechanisms of glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells are not fully understood. GSIS deteriorations are believed to underlie the pathogenesis of type 2 diabetes mellitus. By comparing transcript levels of 3 insulin secreting MIN6 cell sublines with strong glucose-responsiveness and 3 with mildly reduced responsiveness, we identified 630 differentially expressed genes. Using our recently developed system based on recombinase-mediated cassette exchange, we conducted large-scale generation of stable clones overexpressing such genes in the doxycycline-regulated manner. We found that overexpressions of 18, out of 83, genes altered GSIS. Sox11 ((sex determining region Y)-box 11) was selected to confirm its roles in regulating insulin secretion, and the gene was subjected to shRNA-mediated suppression. While Sox11 overexpression decreased GSIS, its suppression increased GSIS, confirming the role of Sox11 as a negative regulator of insulin secretion. Furthermore, metabolic experiments using radiolabelled glucose showed Sox11 to participate in regulating glucose metabolism. Our data suggested that overexpression screening is a feasible option for systemic functional testing to identify important genes in GSIS.

Improving the Effects of Imeglimin on Endothelial Function: A Prospective, Single-Center, Observational Study

INTRODUCTION

Endothelial dysfunction is a risk factor for cardiovascular disease in patients with diabetes. We hypothesized that imeglimin, a novel oral hypoglycemic agent, would improve endothelial function.

METHODS

In this study, imeglimin was administered to patients with type 2 diabetes and HbA1c ≥ 6.5% who were not receiving insulin therapy. A meal tolerance test (592 kcal, glucose 75.0 g, fat 28.5 g) was performed before and 3 months after administration, and endothelial function, blood glucose, insulin, glucagon, and triglycerides were evaluated. Endothelial function was assessed by flow-mediated dilation (FMD).

RESULTS

Twelve patients (50% male) with a median age of 55.5 years old (interquartile range [IQR] 51.3-66.0) were enrolled. Fasting FMD did not differ before or 3 months after imeglimin administration (from 6.1 [3.9-8.5] to 6.6 [3.9-9.0], p = 0.092), but 2 h postprandial FMD was significantly improved 3 months after imeglimin administration (from 2.3 [1.9-3.4] to 2.9 [2.4-4.7], p = 0.013). In terms of the glucose profile, imeglimin administration significantly improved HbA1c (from 7.2 ± 0.6% to 6.9 ± 0.6%, p = 0.007), fasting glucose (from 138 ± 19 mg/dL to 128 ± 20 mg/dL, p = 0.020), and 2 h postprandial glucose (from 251 ± 47 mg/dL to 215 ± 68 mg/dL, p = 0.035). The change in 2 h postprandial FMD between before and 3 months after imeglimin administration (Δ2 h postprandial FMD) was negatively correlated with Δ2 h postprandial glucose (r = - 0.653, p = 0.021) in a univariate correlation coefficient analysis. Both patients with and without decreased postprandial glucose 3 months after imeglimin administration had improved postprandial FMD.

CONCLUSION

In this small study, imeglimin administration improved 2 h postprandial FMD. Both glycemic control-dependent and -independent mechanisms might contribute to improved endothelial function.

TRIAL REGISTRATION

This research was registered in the University Hospital Medical Information Network (UMIN, UMIN000046311).

Efficacy and safety of imeglimin in type 2 diabetes: A systematic review and meta-analysis of randomized placebo-controlled trials

BACKGROUND & AIMS

Imeglimin is a novel new oral compound recently approved for treating type 2 diabetes (T2D) in India. We conducted a systematic review and meta-analysis to evaluate the efficacy of imeglimin in people with T2D in the approved dose of 1000 mg twice daily (BID).

METHODS

We systematically searched the database of PubMed until December 20, 2022, and retrieved all published double-blind, randomized, placebo-controlled trials (RCTs) conducted with imeglimin 1000 mg BID, using appropriate keywords and MeSH terms. A meta-analysis was conducted to study the HbA1c lowering effect of imeglimin 1000 mg BID in people with T2D using the Comprehensive meta-analysis (CMA) software Version 3, Biostat Inc. Englewood, NJ, USA.

RESULTS

Of the seven Phase 2 studies and three Phase 3 studies conducted so far, only three published double-blind RCTs have reported the efficacy and safety of imeglimin 1000 mg BID against the placebo. Our meta-analysis using the random-effects model from two monotherapy studies (n = 360) showed imeglimin 1000 mg BID reduce HbA1c significantly (Δ -0.9%, 95% Confidence Interval [CI], -1.1 to -0.74%; P < 0.0001) against the placebo, without any heterogeneity (I² = 0%). The pooled meta-analysis from all three RCTs (n = 574) found a significant reduction in HbA1c with imeglimin 1000 mg BID (Δ -0.79%; 95% CI, -1.00 to -0.59%; P < 0.0001) compared to placebo with high heterogeneity.

CONCLUSIONS

This meta-analysis found a significant HbA1c lowering effect of imeglimin in people with T2D with an acceptable tolerability profile. Still, larger and longer studies are needed.

[Pharmacological profile and clinical efficacy of imeglimin hydrochloride (TWYMEEG®Tablets), the orally drug for type 2 diabetes mellitus with the first dual mode of action in the world]

Imeglimin hydrochloride (imeglimin) is an orally drug for type 2 diabetes mellitus, which was approved in Japan for the first in the world, with dual mode of actions: pancreatic action means amplifying glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells, and extrapancreatic action means improving insulin sensitivity by which gluconeogenesis suppresses in hepatocytes and glucose uptake increases in skeletal muscles. Although the molecular target of imeglimin is still unknown, imeglimin exerts some of its actions through modulation of the mitochondrial function. In pancreatic islets, imeglimin enhanced adenosine triphosphate and Ca²⁺ under high-glucose conditions. Furthermore, imeglimin induced the synthesis of oxidized form nicotinamide adenine dinucleotide (NAD⁺) via the 'salvage pathway', and NAD⁺ metabolites may contribute to the increase in intracellular Ca²⁺. The in vivo studies indicated that imeglimin enhanced the sensitivity to insulin and modulated the mitochondrial function (restoring the deficient Complex III activity, decreasing Complex I activity and reactive oxygen species production), which contribute to the improvement of glucose metabolism in hepatocytes and skeletal muscles. In clinical trials, imeglimin's dual effects were demonstrated in foreign type 2 diabetic patients who received 1500 mg bid, which is different from the domestic approved dose. Imeglimin has been shown to evidence of statistically significant glucose lowering, a generally favorable safety and tolerability profile in patients with type 2 diabetes by monotherapy and combination therapy with 1,000 mg bid in four Japanese trials. Since imeglimin has dual effects, it may have shown a newly effective option, regardless of the pathophysiology of type 2 diabetic patients.

Sox9 is required in regeneration of pancreatic β cells following injury

The reduction of insulin secretion due to pancreatic β cell injury caused by autoimmune reaction is the pathological basis of Type 1 diabetes mellitus (T1DM). Therefore, seeking new molecular targets for alleviating pancreatic β cell injury will provide experimental basis for the prevention and treatment of T1DM. SRY-box 9 (Sox9) is not only an important molecule regulating the development of various organs, but also its high expression can aggravate the pathological process of various diseases. In addition, Sox9⁺ cells are also pancreatic progenitor cells, participating in pancreatic repair reaction induced by injury. In our study, elevated blood glucose and lack of pancreatic β cells almost returned to normal over time after streptozotocin (STZ)-induced pancreatic β cell damage, implying that pancreatic β cells were regenerated after STZ-induced injury. In particular, the expression of Sox9 was significantly elevated during pancreatic β cell regeneration. On this basis, we conducted in vitro experiments to verify whether overexpression of Sox9 could inhibit the damage of pancreatic β cells by inflammatory factors. Our results showed that overexpression of Sox9 alleviated the damage of pancreatic β cells by inflammatory factors and improved the inhibitory effect of inflammatory factors on insulin secretion of pancreatic β cells. Unsurprising, blood glucose levels, insulin content and pancreatic β cell number failed to return to near-normal levels timely after pancreatic β cells specific knockout Sox9 mice were treated with STZ, further confirming the importance of Sox9 in facilitating pancreatic β cell repair or regeneration. Our study indicate that enhanced Sox9 activity might protect pancreatic β cells from autoimmune induced damage and thus improve the pathological process of T1DM.

Current understanding of imeglimin action on pancreatic β-cells: Involvement of mitochondria and endoplasmic reticulum homeostasis

Recent preclinical studies have provided insight on imeglimin's action on pancreatic β-cells and the mechanisms underlying its clinical benefits. Imeglimin may enhance glucose-induced insulin secretion (GIIS) and inhibit apoptosis of pancreatic ß-cells leading to preserved β-cell mass by maintaining or restoring the functional and structural integrity of the mitochondria and the endoplasmic reticulum homeostasis in pancreatic β-cells.

Phase 2 trial with imeglimin in patients with Type 2 diabetes indicates effects on insulin secretion and sensitivity

INTRODUCTION

The aim of the present study was to evaluate the effect of 18-week monotherapy with imeglimin on glucose tolerance and on insulin secretion/sensitivity in type 2 diabetic (T2D) patients.

METHODS

The study was an 18-week, double-blind clinical trial in T2D subjects previously treated with stable metformin therapy and washed out for 4 weeks. Subjects were randomized 1:1 to receive a 1500 mg bid of imeglimin or placebo. The primary endpoint was the effect of imeglimin vs placebo on changes from baseline to week 18 in glucose tolerance (glucose area under the curve [AUC]) during a 3 h-glucose tolerance test [OGTT]). Secondary endpoints included glycaemic control and calculated indices of insulin secretion and sensitivity.

RESULTS

A total of 59 subjects were randomized, 30 receiving imeglimin and 29 receiving placebo. The study met its primary endpoint. Least squares (LS) mean difference between treatment groups (imeglimin - placebo) for AUC glucose from baseline to week 18 was -429.6 mmol/L·min (p = .001). Two-hour post-dose fasting plasma glucose was significantly decreased with LS mean differences of -1.22 mmol/L (p = .022) and HbA1c was improved with LS mean differences of -0.62% (p = .013). The AUC_0-180min ratio C-peptide/glucose [LS mean differences of 0.041 nmol/mmol (p < .001)] and insulinogenic index were significantly increased by imeglimin treatment. The increase in insulin secretion was associated with an increase in beta-cell glucose sensitivity. Additionally, the insulin sensitivity indices derived from the OGTT Stumvoll (p = .001) and Matsuda (not significant) were improved in the imeglimin group vs placebo. Imeglimin was well tolerated with 26.7% of subjects presenting at least one treatment-emergent adverse event versus 58.6% of subjects in the placebo group.

CONCLUSIONS

Results are consistent with a mode of action involving insulin secretion as well as improved insulin sensitivity and further support the potential for imeglimin to improve healthcare in T2D patients.

A Review on the Role of TRP Channels and Their Potential as Drug Targets_An Insight Into the TRP Channel Drug Discovery Methodologies

Transient receptor potential (TRP) proteins are a large group of ion channels that control many physiological functions in our body. These channels are considered potential therapeutic drug targets for various diseases such as neurological disorders, cancers, cardiovascular disease, and many more. The Nobel Prize in Physiology/Medicine in the year 2021 was awarded to two scientists for the discovery of TRP and PIEZO ion channels. Improving our knowledge of technologies for their study is essential. In the present study, we reviewed the role of TRP channel types in the control of normal physiological functions as well as disease conditions. Also, we discussed the current and novel technologies that can be used to study these channels successfully. As such, Flux assays for detecting ionic flux through ion channels are among the core and widely used tools for screening drug compounds. Technologies based on these assays are available in fully automated high throughput set-ups and help detect changes in radiolabeled or non-radiolabeled ionic flux. Aurora’s Ion Channel Reader (ICR), which works based on label-free technology of flux assay, offers sensitive, accurate, and reproducible measurements to perform drug ranking matching with patch-clamp (gold standard) data. The non-radiolabeled trace-based flux assay coupled with the ICR detects changes in various ion types, including potassium, calcium, sodium, and chloride channels, by using appropriate tracer ions. This technology is now considered one of the very successful approaches for analyzing ion channel activity in modern drug discovery. It could be a successful approach for studying various ion channels and transporters, including the different members of the TRP family of ion channels.

Pharmacokinetics of Imeglimin in Caucasian and Japanese Healthy Subjects

BACKGROUND

Imeglimin is a first-in-class novel oral antidiabetic marketed in Japan as TWYMEEG^® to treat type 2 diabetes mellitus. Its mode of action is distinct from all other anti-hyperglycemic classes.

OBJECTIVE

To assess the pharmacokinetic and safety profile of imeglimin in Caucasian and Japanese healthy individuals.

METHODS

Two randomized placebo-controlled phase 1 clinical studies were conducted in Caucasian subjects after single (250-8000 mg) and multiple (250-2000 mg twice daily) ascending doses and in Japanese subjects after single (500-6000 mg) and multiple (500-2000 mg twice daily) ascending doses. Imeglimin plasma and urine concentrations were measured.

RESULTS

All imeglimin doses achieved maximal concentration between 1 and 3.5 h in Caucasians, and 1.5 and 3 h in Japanese subjects. The elimination half-lives (t_1/2) were dose-independent and means ranged between 9.03 and 20.2 h for Caucasians, and 4.45 and 12 h for Japanese subjects. Dose-normalized area under the plasma concentration-time curve decreased with dose in the 250-8000 mg and in the 500-6000 mg dose range in Caucasians and Japanese, respectively, suggesting a dose-dependent but less than dose-proportional effect in imeglimin exposure. Plasma accumulation was minimal following repeated dosing, and food did not affect the pharmacokinetics in either population. Exposures were generally similar between Caucasian and Japanese subjects with less than 20% difference, although there was a tendency for exposures in Japanese to be slightly higher. Imeglimin had an acceptable safety and tolerability profile, with dose-dependent mild gastrointestinal adverse events.

CONCLUSION

Imeglimin was safe and well tolerated in these two phases 1 studies, with pharmacokinetics comparable between the two populations.

CLINICAL TRIAL REGISTRATIONS

EudraCT 2005-001946-18 and 2014-004679-21.

The calcium signaling enzyme CD38 - a paradigm for membrane topology defining distinct protein functions

CD38 is a single-pass transmembrane enzyme catalyzing the synthesis of two nucleotide second messengers, cyclic ADP-ribose (cADPR) from NAD and nicotinic acid adenine dinucleotide phosphate (NAADP) from NADP. The former mediates the mobilization of the endoplasmic Ca²⁺-stores in response to a wide range of stimuli, while NAADP targets the endo-lysosomal stores. CD38 not only possesses multiple enzymatic activities, it also exists in two opposite membrane orientations. Type III CD38 has the catalytic domain facing the cytosol and is responsible for producing cellular cADPR. The type II CD38 has an opposite orientation and is serving as a surface receptor mediating extracellular functions such as cell adhesion and lymphocyte activation. Its ecto-NADase activity also contributes to the recycling of external NAD released by apoptosis. Endocytosis can deliver surface type II CD38 to endo-lysosomes, which acidic environment favors the production of NAADP. This article reviews the rationale and evidence that have led to CD38 as a paradigm for membrane topology defining distinct functions of proteins. Also described is the recent discovery of a hitherto unknown cADPR-synthesizing enzyme, SARM1, ushering in a new frontier in cADPR-mediated Ca²⁺-signaling.