Redox Signal-mediated Enhancement of the Temperature Sensitivity of Transient Receptor Potential Melastatin 2 (TRPM2) Elevates Glucose-induced Insulin Secretion from Pancreatic Islets*

Background: Transient receptor potential melastatin 2 (TRPM2) is a temperature-sensitive Ca2+-permeable ion channel involved in glucose-induced insulin secretion. Results: Hydrogen peroxide treatment caused a TRPM2-dependent increase in intracellular Ca2+ concentration. Glucose-induced insulin secretion from pancreatic islets was temperature-, antioxidant-, and TRPM2-dependent. Conclusion: Redox signal-mediated TRPM2 sensitization elevates glucose-induced insulin secretion. Significance: These results provide new insights into the involvement of TRPM2 sensitization in insulin secretion. Transient receptor potential melastatin 2 (TRPM2) is a thermosensitive Ca2+-permeable cation channel expressed by pancreatic β cells where channel function is constantly affected by body temperature. We focused on the physiological functions of redox signal-mediated TRPM2 activity at body temperature. H2O2, an important molecule in redox signaling, reduced the temperature threshold for TRPM2 activation in pancreatic β cells of WT mice but not in TRPM2KO cells. TRPM2-mediated [Ca2+]i increases were likely caused by Ca2+ influx through the plasma membrane because the responses were abolished in the absence of extracellular Ca2+. In addition, TRPM2 activation downstream from the redox signal plus glucose stimulation enhanced glucose-induced insulin secretion. H2O2 application at 37 °C induced [Ca2+]i increases not only in WT but also in TRPM2KO β cells. This was likely due to the effect of H2O2 on KATP channel activity. However, the N-acetylcysteine-sensitive fraction of insulin secretion by WT islets was increased by temperature elevation, and this temperature-dependent enhancement was diminished significantly in TRPM2KO islets. These data suggest that endogenous redox signals in pancreatic β cells elevate insulin secretion via TRPM2 sensitization and activity at body temperature. The results in this study could provide new therapeutic approaches for the regulation of diabetic conditions by focusing on the physiological function of TRPM2 and redox signals.


Transient receptor potential melastatin 2 (TRPM2) is a thermosensitive Ca 2؉ -permeable cation channel expressed by pancreatic ␤ cells where channel function is constantly affected by body temperature. We focused on the physiological functions of redox signal-mediated TRPM2 activity at body temperature. H 2 O 2 , an important molecule in redox signaling, reduced the temperature threshold for TRPM2 activation in pancreatic ␤ cells of WT mice but not in TRPM2KO cells. TRPM2-mediated [Ca
] i increases were likely caused by Ca 2؉ influx through the plasma membrane because the responses were abolished in the absence of extracellular Ca 2؉ . In addition, TRPM2 activation downstream from the redox signal plus glucose stimulation enhanced glucose-induced insulin secretion. H 2 O 2 application at 37°C induced [Ca 2؉ ] i increases not only in WT but also in TRPM2KO ␤ cells. This was likely due to the effect of H 2 O 2 on K ATP channel activity. However, the N-acetylcysteine-sensitive fraction of insulin secretion by WT islets was increased by temperature elevation, and this temperature-dependent enhancement was diminished significantly in TRPM2KO islets. These data suggest that endogenous redox signals in pancreatic ␤ cells elevate insulin secretion via TRPM2 sensitization and activity at body temperature. The results in this study could provide new therapeutic approaches for the regulation of diabetic conditions by focusing on the physiological function of TRPM2 and redox signals.
The transient receptor potential (TRP) 2 ion channel superfamily consists of 28 channels in six subfamilies in mammals.
Some channels detect a wide range of environmental factors, including thermal, mechanical, and chemical stimuli, in various species (1). Among them, nine members (TRPA1, TRPV1,  TRPV2, TRPV3, TRPV4, TRPM2, TRPM4, TRPM5, and  TRPM8) have been reported to have temperature sensitivity and are called thermoTRPs (2). TRPM3 has recently been reported to be activated by elevated temperature (3). The ambient temperature around peripheral sensory nerve endings in the skin can change dynamically and is detected by several ther-moTRPs, such as TRPA1, TRPV1, TRPV2, and TRPM8, covering a wide range of temperatures from noxious cold to noxious heat (4). In addition to sensory neurons, skin keratinocytes reportedly detect ambient temperature with TRPV3, one of the thermoTRPs, and transmit the temperature information to sensory neurons (5,6). Furthermore, many deep organs that are not normally exposed to dynamic temperature changes also express thermoTRPs whose activities are continuously affected by body temperature (1). Among them, the activity of TRPM2 at body temperature is regulated by intracellular endogenous ligands, including ADP-ribose and cyclic ADP-ribose (7,8), and by environmental reactive oxygen species (ROS), so-called redox signals (9). Therefore, TRPM2 activity could be modulated by these regulatory molecules at body temperature and could be involved in physiological functions of cells or tissues in which TRPM2 is expressed.
The TRPM2 channel is expressed in the brain, liver, spleen, and pancreas, where its functions are continuously affected by body temperature (10). We demonstrated previously that one of the molecular mechanisms for reducing the temperature threshold for TRPM2 activation utilized hydrogen peroxide (H 2 O 2 ). H 2 O 2 is a highly versatile and functional molecule in redox signaling systems (9), and it likely regulates a number of physiological functions as a second messenger in vivo. In the absence of H 2 O 2 , the temperature threshold for TRPM2 activation is ϳ47°C. H 2 O 2 can lower the temperature threshold for TRPM2 activation toward physiological body temperature, depending on H 2 O 2 concentration and treatment duration (9). We also explored its involvement in cytokine release and the phagocytic activity of mouse peritoneal macrophages, in which the binding of pathogen-associated molecular patterns to Tolllike receptors activates NADPH oxidase and elicits the production of ROS for microbicidal activity. These findings illustrate that TRPM2 activity at body temperature could be regulated by endogenous redox signals.
Pancreatic ␤ cells play a crucial role in blood glucose regulation because they secrete hypoglycemic insulin when blood glucose levels are elevated. The primary pathway for glucose-induced insulin secretion is well known to be mediated by Ca 2ϩ influx through voltage-gated Ca 2ϩ channels upon membrane depolarization following ATP-sensitive K ϩ (K ATP ) channel closure. However, recent studies have revealed the significant contribution of many other ion channels to the increases in intracellular Ca 2ϩ concentrations upon glucose stimulation (11). In particular, several thermoTRPs (TRPA1, TRPV2, TRPV4, TRPM2, TRPM3, TRPM4, and TRPM5) have been reported to be expressed in pancreatic ␤ cells, although their functions are not yet fully elucidated (12). We have reported previously that TRPM2 has high Ca 2ϩ permeability and is expressed in pancreatic ␤ cells (7). It regulates insulin secretion evoked by glucose or glucose together with incretin hormones at body temperature (13,14).
Pancreatic ␤ cells produce ROS in response to many kinds of extracellular signals, including insulin, cytokines, hormones, and blood glucose elevation (15,16). Interestingly, the expression levels of the ROS-eliminating enzymes catalase and glutathione reductase are extremely low in the pancreas compared with other tissues (17). Pharmacological and genetic inhibition of NADPH oxidase reportedly suppresses glucose-induced insulin secretion (18), and H 2 O 2 application with low concentrations of glucose apparently enhances insulin secretion (16). Therefore, physiological concentrations of ROS could function as favorable signaling molecules in pancreatic ␤ cells (15,16). On the other hand, low levels of antioxidant enzymes in the pancreas might explain its high vulnerability to supranormal levels of ROS that lead to cellular malfunction or death of ␤ cells and progression of type II diabetes (19). In this study, we focused on the physiological function of TRPM2 sensitization in glucose-induced insulin secretion from pancreatic islets and the effect of temperature on this process.

EXPERIMENTAL PROCEDURES
Animals-Male C57BL/6NCr mice (Japan SLC) and mutant TRPM2-deficient (TRPM2KO) mice were provided by Yasuo Mori (Kyoto University, Kyoto, Japan) (20) and were used at 10 -16 weeks of age. WT and TRPM2KO mice were housed in a controlled environment (12-h light/12-h dark cycle; room temperature, 22 -24°C; relative humidity, 50 -60%) with free access to food and water. All procedures involving the care and use of animals were approved by the National Institute for Physiological Science and carried out in accordance with the National Institutes of Health guidelines for the care and use of laboratory animals.
Cell Culture-Islets of Langerhans were isolated from mouse pancreas by collagenase digestion as described previously (21) with minor modifications. Animals were anesthetized with sodium pentobarbital (80 mg/kg intraperitoneally, Dainippon Sumitomo Pharma Co., Ltd.) followed by injection of 1.14 mg/ml collagenase (Sigma-Aldrich, St. Louis, MO) into the common bile duct ligated upper and lower sites. The collagenase was dissolved in Ca 2ϩ -free HEPES-supplemented Krebs-Ringer bicarbonate buffer solution (HKRB(Ϫ) (129 mM NaCl, 5 mM NaHCO 3 , 4.7 mM KCl, 1.2 mM KH 2 PO 4 , 1.2 mM MgSO 4 , 10 mM HEPES, 3.3 mM glucose, and 0.1% BSA (pH 7.4)). The pancreas was removed by dissection and incubated in collagenase solution at 37°C for 23 min. The islets were washed twice with HKRB(Ϫ) to remove collagenase and were then used for experiments. For [Ca 2ϩ ] i imaging experiments, islets were incubated with 1 mM EGTA in HKRB(Ϫ) and dispersed into single cells. The dissociated single pancreatic ␤ cells were suspended in RPMI 1640 medium (WAKO Pure Chemical Industries, Ltd.) containing 10% FBS, 100 units/ml penicillin, 100 g/ml streptomycin, and 5.6 mM glucose unless indicated otherwise. Dispersed cells were seeded onto poly-L-lysine (100 M)-coated glass coverslips and used for fluorescence measurements within 12-24 h of seeding. The concentration of glucose (5.6 mM) in culture medium matched the fasting blood glucose level (13).
Fluorescence Measurements-Fura-2 fluorescence of mouse pancreatic ␤ cells was measured in 2 mM Ca 2ϩ -containing HKRB(ϩ) (129 mM NaCl, 5 mM NaHCO 3 , 4.7 mM KCl, 1.2 mM KH 2 PO 4 , 1.2 mM MgSO 4 , 2.0 mM CaCl 2 , 10 mM HEPES, and 2.8 mM glucose (pH 7.4)). Ca 2ϩ -free HKRB(Ϫ) used in the Ca 2ϩfree experiments was made by adding 5 mM EGTA instead of 2 mM CaCl 2 . Thermal stimulation was applied by increasing the bath temperature with preheated solution through an inline heater (SH-27B, Warner Instruments). The proximal temperature of the recording area was monitored with a thermocouple (TA-29, Warner Instruments). Fura-2 loaded in the cells was excited with 340-and 380-nm wavelengths, and emission was monitored at 510 nm with a (complementary metal-oxidesemiconductor) camera (Zyla 5.5, Andor Technology). Data were acquired using iQ2.8 software (Andor Technology) and analyzed by ImageJ (http://rsbweb.nih.gov/ij/). The cells that reacted to tolbutamide (300 M) with a ratio increase over 0.3 from the basal ratio were identified as pancreatic ␤-cells. Ionomycin (5 M) was applied to confirm cell viability, and ratio increases from basal level were normalized to those evoked by ionomycin for each experiment. In some experiments, [Ca 2ϩ ] i was calculated according to an in vitro calibration using a K d value of fura-2 (224 nM) at 37°C.
Measurement of insulin release from mouse pancreatic islets of Langerhans-Islets were collected in RPMI of the same composition as that in cell culture and incubated for 2 h and then preincubated in Krebs-Ringer buffer, KRB(ϩ) (129 mM NaCl, 5 mM NaHCO 3 , 5.2 mM KCl, 1.3 mM KH 2 PO 4 , 2.7 mM CaCl 2 , 1.3 mM MgSO 4 , 0.2% BSA, pH 7.4) containing 3.3 mM glucose for 30 min at 37°C, and then 10 islets/10 l were sorted into 1.5 ml tubes and used for the in vitro insulin secretion assay. All of the

TRPM2 Sensitization and Insulin Secretion
in vitro insulin secretion assays were conducted in triplicate and their average values were used. Insulin secretion was elicited by adding 400 l of 16.7 mM glucose-containing KRB(ϩ) and incubated for 60 min at temperatures of 33, 37 and 40°C in the presence or absence of NAC (300 M). KRB(ϩ) with 3.3 mM glucose was used as the negative control. After 60 min incubation, the supernatants were collected and used for the measurement of insulin content by ELISA assay (Morinaga) following the manufacturer's instructions.
Statistical analysis-Data are presented as means Ϯ S.E. or means Ϯ S.D. Statistical analysis was performed using the Student t test, paired t test or two-way analysis of variance followed by the Bonferroni-type post-hoc multiple t tests. p values less than 0.05 were considered significant.

Temperature Sensitivity in Pancreatic ␤ Cells Was Enhanced by H 2 O 2 Treatment-
We have reported previously that the temperature threshold for TRPM2 activation was reduced from a supraphysiological to a physiological temperature range by H 2 O 2 , a kind of ROS, termed "sensitization," involved in macrophage functions (9). To examine whether TRPM2 sensitization was also observed in pancreatic ␤ cells, we first compared heatevoked changes in intracellular Ca 2ϩ concentrations between WT and TRPM2KO ␤ cells using a Ca 2ϩ imaging method. ␤ cells were identified by their reactivity to tolbutamide (300 M), a K ATP channel inhibitor. Heat-evoked responses in WT ␤ cells were enhanced by H 2 O 2 treatment in a dose-dependent manner (Fig. 1, A and C), similar to heterologously expressed TRPM2 channels and WT mouse peritoneal macrophages (9). On the other hand, TRPM2KO cells did not show any statistically significant enhancement of the heat-evoked responses even after treatment with high concentrations of H 2 O 2 (Fig. 1, B  and C), whereas the responses to a high concentration of K ϩ (40 mM) were comparable with those in WT cells (Fig. 1D). These data indicated that redox signal-mediated changes in the temperature threshold for TRPM2 activation could also occur in pancreatic ␤ cells, suggesting that TRPM2 sensitization is a broad phenomenon in many kinds of tissues in which TRPM2 is expressed.
The Heat-evoked Response Enhanced by H 2 O 2 Was Dependent on Extracellular Ca 2ϩ -The H 2 O 2 effects on heat-evoked responses were not observed under Ca 2ϩ -free extracellular conditions in WT cells ( Fig. 2A), indicating that Ca 2ϩ influx from the extracellular space mediated the heat-evoked [Ca 2ϩ ] i increases. In the experiment shown in Fig. 2A, we used a different concentration of glucose from the previous analyses. Therefore, we investigated the responses in the presence of 10 mM glucose both in the culture medium and extracellular bath solu-  MAY 8, 2015 • VOLUME 290 • NUMBER 19

JOURNAL OF BIOLOGICAL CHEMISTRY 12437
tion, similar to a previous report (22). However, we again failed to observe [Ca 2ϩ ] i increases under conditions in which the extracellular medium was Ca 2ϩ -free (Fig. 2B). As shown in Fig.  2B, the responses to tolbutamide were small in the presence of 10 mM glucose because [Ca 2ϩ ] i levels remained high, probably because of the closure of the K ATP channel in the presence of the high concentration of glucose. One group reported that TRPM2 acted as a lysosomal Ca 2ϩ release channel in pancreatic ␤ cells (22). Therefore, we compared the lysosomal Ca 2ϩ levels between WT and TRPM2KO ␤ cells under extracellular Ca 2ϩfree conditions in the presence of glycyl-L-phenylalanine-2nephthylamide (GPN), a reagent destructive for lysosomes. 10 mM glucose evoked Ca 2ϩ oscillations in ␤ cells of both genotypes, and removal of extracellular Ca 2ϩ suppressed the oscillatory [Ca 2ϩ ] i responses (Fig. 2, C and D). After the removal of extracellular Ca 2ϩ , 200 M GPN was applied to elicit Ca 2ϩ

TRPM2 Sensitization and Insulin Secretion
release from lysosomes. [Ca 2ϩ ] i increases upon GPN application were similar in WT and TRPM2KO ␤ cells (Fig. 2, C-E (Fig. 3, A-C), suggesting that H 2 O 2 also affected proteins other than TRPM2, such as the K ATP channel, to regulate [Ca 2ϩ ] i in pancreatic ␤ cells.
Glucose-induced Insulin Secretion from pancreatic Islets Was Temperature-, Antioxidant-, and TRPM2-dependent-We have reported previously that glucose-induced insulin secretion was enhanced by the functional expression of TRPM2 in pancreatic ␤ cells (13), although the detailed mechanisms have not been fully elucidated. In this study, we focused on the involvement of TRPM2 sensitization in glucose-induced insulin secretion because pancreatic ␤ cells produce ROS in response to blood glucose elevation (16). We compared insulin secretion between WT and TRPM2KO pancreatic islets in the presence or absence of an antioxidant, NAC, to investigate the contribution of ROS and TRPM2 sensitization to insulin secretion. In addition, we conducted insulin secretion assays at three different temperatures (33, 37, and 40°C) to clarify the involvement of TRPM2 sensitization. Consistent with our previous work (13), insulin secretion by TRPM2KO islets exposed to 16.7 mM glucose (16.7G) was significantly lower than that from WT islets at 37 and 40°C but not at 33°C, and the statistical significance of the decrease became larger in a temperature-dependent manner (Fig. 4, C, F, and I). In addition, NAC treatment significantly attenuated the 16.7 mM glucose-induced insulin secretion by WT islets but not by TRPM2KO islets (Fig.  4, A-I). Interestingly, the difference in insulin secretion between WT and TRPM2KO islets completely disappeared with NAC treatment, and no statistical significance among the WT (16.7G ϩ NAC), TRPM2KO (16.7G), and TRPM2KO (16.7G ϩ NAC) groups was observed at any of the temperatures. Specifically, we observed 17.4 Ϯ 2.4 ng/10 islets, 19.8 Ϯ 1.3 ng/10 islets, and 19.3 Ϯ 1.8 ng/10 islets at 33°C; 44.9 Ϯ 5.3 ng/10 islets, 41.2 Ϯ 3.9 ng/10 islets, and 39.8 Ϯ 3.6 ng/10 islets at 37°C; and 50.9 Ϯ 3.5 ng/10 islets, 55.4 Ϯ 2.9 ng/10 islets, and 51.3 Ϯ 2.5 ng/10 islets at 40°C in these three groups, respectively (two-way analysis of variance followed by the Bonferronitype post hoc multiple t tests).
To more clearly evaluate the effects of TRPM2 sensitization and temperature on insulin secretion, we extracted the NACsensitive fraction from each experimental batch (Fig. 4J). WT islets showed clear temperature-dependent increases in NACsensitive insulin secretion, whereas such temperature-dependent NAC-sensitive insulin secretion was almost gone in TRPM2KO islets. In addition, a statistically significant correlation between temperature and genotype (p ϭ 0.022, two-way analysis of variance) was also observed, suggesting that temperature has an important role in the physiological function of TRPM2 under conditions where a redox signal is present.
These results are consistent with our previous study using mouse peritoneal macrophages (9).

DISCUSSION
We found that the heat-evoked responses in pancreatic ␤ cells could be regulated by redox signaling that could occur in ␤ cells in response to in vivo physiological stimuli like cytokines, insulin, and blood glucose elevation. The heat-evoked responses in ␤ cells disappeared when extracellular Ca 2ϩ was lacking or when TRPM2KO cells were examined (Figs. 1B and   's t test). Glucose-induced insulin secretion was attenuated in TRPM2KO islets compared with the WT, and the statistical significance of the difference between genotypes increased with temperature elevation (C, F, and I). J, NAC-sensitive fractions of glucose-induced insulin secretion at each temperature and genotype. Not only statistical significance between the WT and TRPM2KO (p Ͻ 0.001) but also a significant correlation between genotype and temperature (p Ͻ 0.05) was observed (two-way analysis of variance). NS, p Ͼ 0.05; **, p Ͻ 0.01 between the WT and TRPM2KO under each temperature condition (Student's t test). A and B). Therefore, the heat-evoked responses observed in WT ␤ cells were likely mediated by Ca 2ϩ influx through TRPM2 channels.

2,
We did not observe heat-mediated increases in [Ca 2ϩ ] i upon H 2 O 2 treatment under extracellular Ca 2ϩ -free conditions (Fig.  2, A and B) regardless of glucose concentrations (5.6 mM or 10 mM) in culture medium or extracellular bath solution. These results suggest little involvement of lysosomes in the H 2 O 2induced [Ca 2ϩ ] i increases. Another group reported that TRPM2 functioned as a Ca 2ϩ release channel in the lysosomes of pancreatic ␤ cells (22), results that apparently contradict our results. However, we note that the experimental conditions were different, especially regarding glucose concentrations. However, similar GPN-evoked [Ca 2ϩ ] i increases between WT and TRPM2KO ␤ cells suggest that lysosomal Ca 2ϩ levels are not different between the two genotypes. In addition, the fact that lysosomal lumina are maintained at an acidic pH (Ͻ 5.0) for enzyme reactions (23) and that TRPM2 activity is almost completely inhibited under acidic conditions (24) would make the involvement of lysosome TRPM2 in [Ca 2ϩ ] i mobilization in ␤ cells unlikely. Other possible factors might contribute to the difference between our results and the work mentioned above. In addition to TRPM2, other thermoTRPs (TRPV2, TRPV4, TRPM3, TRPM4, and TRPM5) are reportedly expressed in pancreatic ␤ cells (12), and, among them, TRPV4 and TRPM3 are sensitive to physiological temperature and have Ca 2ϩ permeability. However, these channels are not likely to contribute to the heat-evoked responses we observed in this study because the responses were completely lost in TRPM2KO ␤ cells. In addition, activation of TRPM4 and TRPM5, monovalent-selective channels, could cause [Ca 2ϩ ] i increases upon heat stimulation because their activation leads to membrane depolarization followed by the activation of voltage-gated Ca 2ϩ channels. However, the fact that the phenomenon was lost in TRPM2KO ␤ cells argues against that possibility as well. Accordingly, participation of TRPM2 channels is the most likely explanation for the H 2 O 2 -evoked events.
We asked whether H 2 O 2 actually acted as a signaling molecule and regulated intracellular Ca 2ϩ and insulin secretion from pancreatic ␤ cells because H 2 O 2 evoked [Ca 2ϩ ] i increases both in WT and TRPM2KO ␤ cells (Fig. 3) and because ␤ cells highly express K ATP channels whose activity is largely affected by ROS. ATP depletion following oxidative stress enhances the opening of K ATP channels (25), whereas oxidation of sulfhydryl groups on K ATP channels suppresses channel activity (26). Therefore, [Ca 2ϩ ] i increases observed in TRPM2KO cells could be due to K ATP channel closure in response to ROS and/or ROS actions on other proteins. Considering the extremely low expression level of ROS-quenching enzymes in pancreatic ␤ cells (17), ROS concentrations could be dynamically changed in response to physiological stimuli, including blood glucose elevation initiated by food consumption. Chronic oxidative stress is well known to be involved in diabetic pathogenesis. However, ROS also have a role as signaling molecules regulating glucoseinduced insulin secretion from pancreatic islets (27). Therefore, we investigated the involvement of TRPM2 sensitization in insulin secretion from pancreatic islets and found that the sensitization of TRPM2 mediated by redox signals that accom-panied glucose stimulation enhanced glucose-induced insulin secretion. In addition, NAC-sensitive (redox signal-dependent) insulin secretion was elevated in a temperature-dependent manner in WT but not in TRPM2KO pancreatic islets (Fig. 4J). ROS modulate diverse protein functions as signaling molecules (28). Regarding the proteins involved in [Ca 2ϩ ] i regulation in pancreatic islets, redox signals have dual effects on K ATP channels (25,26). Increases in insulin secretion from TRPM2KO islets exhibited a slight temperature dependence (Fig. 4, B, E, andH)thatcouldbeexplainedbythegeneraltemperaturedependence of physicochemical reactions because of the change in molecular movement (29) as well as activation of other thermosensitive TRP channels, such as TRPM5 (30). However, we found that the NAC-sensitive fraction of insulin secretion was drastically diminished in TRPM2KO islets and that no statistically meaningful difference was observed between WT (16.7 mM glucose ϩ NAC), TRPM2 (16.7 mM glucose), and TRPM2KO (16.7 mM glucose ϩ NAC). Therefore, TRPM2 can be viewed as one of the main targets of redox signaling upon glucose stimulation, causing changes in the temperature sensitivity of insulin secretion from ␤ cells, although the effects of TRPM2-mediated depolarization and [Ca 2ϩ ] i increases on the function of other molecules cannot be excluded.
Along with the ROS-mediated regulation of TRPM2 function, the activity of TRPM2 is also regulated by incretin hormone through G s -coupled receptor activation, which changes insulin secretion from pancreatic islets (13,14). Therefore, TRPM2 function is thought to be regulated by various mechanisms working in concert to correctly modulate insulin secretion from pancreatic islets. Elucidating the detailed mechanisms controlling the role of TRPM2 in islet function could lead to a better understanding of the pathogenesis of diabetes and also provide new therapeutic approaches to regulate diabetic conditions.