RhoA as a Signaling Hub Controlling Glucagon Secretion From Pancreatic α-Cells

Regulation of Type 1 Diabetes via Brown Adipocyte-Secreted Proteins and the Novel Glucagon Regulator Nidogen-2

Current treatments for type 1 diabetes (T1D) focus on insulin replacement. We demonstrated the therapeutic potential of a secreted protein fraction from embryonic brown adipose tissue (BAT) that mediates insulin receptor-dependent recovery of euglycemia in a T1D, nonobese diabetic (NOD) mouse model, by suppressing glucagon secretion. This fraction promoted white adipocyte differentiation and browning, maintained healthy BAT, and enhanced glucose uptake in adipose tissue, skeletal muscle, and liver. We identified nidogen-2 as a critical BAT-secreted protein that reverses hyperglycemia in NOD mice, inhibits glucagon secretion from pancreatic α-cells, and mimics other actions of the entire secreted fraction. Secretions from a BAT cell line with siRNA knockdown of nidogen-2 failed to inhibit glucagon secretion and restore euglycemia. These findings demonstrate that BAT-secreted peptides represent a novel therapeutic approach to diabetes management. Furthermore, our research reveals a novel signaling role for nidogen-2 beyond its traditional classification as an extracellular matrix protein.

ARTICLE HIGHLIGHTS

Large embryonic brown adipose tissue-secreted proteins (CB-100) suppress glucagon and normalize glycemia in mice with type 1 diabetes (T1D) without changing insulin. CB-100 prevented T1D-induced whitening, promoted browning of adipose tissue, and enhanced glucose uptake via an insulin receptor-dependent pathway. Within CB-100, nidogen-2 regulated glucagon secretion and restored euglycemia in T1D. Nidogen-2 and CB-100 unveil a therapeutic strategy for diabetes management beyond insulin-centric paradigms by modulating glucagon secretion and enhancing glucose metabolism.

Beta cells intrinsically sense and limit their secretory activity via mTORC1-RhoA signaling

Precise regulation of insulin secretion by pancreatic β cells is essential to prevent excessive insulin release. Here, we show that the nutrient sensor mechanistic Target of Rapamycin Complex 1 (mTORC1) is rapidly activated by glucose in β cells via the insulin secretion machinery, positioning mTORC1 as a sensor of β cell activity. Acute pharmacological inhibition of mTORC1 during glucose stimulation enhances insulin release, suggesting that mTORC1 acts as an intrinsic feedback regulator that restrains insulin secretion. Phosphoproteomic profiling reveals that mTORC1 modulates the phosphorylation of proteins involved in actin remodeling and vesicle trafficking, with a prominent role in the RhoA-GTPase pathway. Mechanistically, mTORC1 promotes RhoA activation and F-actin polymerization, limiting vesicle movement and dampening the second phase of insulin secretion. These findings identify a glucose-mTORC1-RhoA signaling axis that forms an autonomous feedback loop to constrain insulin exocytosis, providing insight into how β cells prevent excessive insulin release and maintain metabolic balance.

TMEM55A-mediated PI5P signalling regulates alpha cell actin depolymerisation and glucagon secretion

AIMS/HYPOTHESIS

Diabetes is associated with the dysfunction of glucagon-producing pancreatic islet alpha cells, although the underlying mechanisms regulating glucagon secretion and alpha cell dysfunction remain unclear. While insulin secretion from pancreatic beta cells has long been known to be controlled partly by intracellular phospholipid signalling, very little is known about the role of phospholipids in glucagon secretion. Using patch-clamp electrophysiology and single-cell RNA sequencing, we previously found that expression of PIP4P2 (encoding TMEM55A, a lipid phosphatase that dephosphorylates phosphatidylinositol-4,5-bisphosphate [PIP2] to phosphatidylinositol-5-phosphate [PI5P]) correlates with alpha cell function. We hypothesise that TMEM55A is involved in glucagon secretion and aim to validate the role of TMEM55A and its potential signalling molecules in alpha cell function and glucagon secretion.

METHODS

Correlation analysis was generated from the data in www.humanislets.com . Human islets were isolated at the Alberta Diabetes Institute IsletCore. Electrical recordings were performed on dispersed human or mouse islets with scrambled siRNA or si-PIP4P2 (si-Pip4p2 for mouse) transfection. Glucagon secretion was measured using an islet perfusion system with intact mouse islets. TMEM55A activity was measured using an in vitro on-beads phosphatase assay and live-cell imaging. GTPase activity was measured using an active GTPase pull-down assay. Confocal microscopy was used to quantify F-actin intensity using primary alpha cells and alphaTC1-9 cell lines after chemical treatment.

RESULTS

TMEM55A regulated alpha cell exocytosis and glucagon secretion. TMEM55A knockdown in both human and mouse alpha cells reduced exocytosis at low glucose levels and this was rescued by the direct reintroduction of PI5P. PI5P, instead of PIP2 increased the glucagon secretion using intact mouse islets. This did not occur through an effect on Ca2+ channel activity but through a remodelling of cortical F-actin dependent on TMEM55A lipid phosphatase activity, which occurred in response to oxidative stress. TMEM55A- and PI5P-induced F-actin remodelling depends on the inactivation of GTPase and RhoA, instead of Ras-related C3 botulinum toxin substrate 1 or CDC42.

CONCLUSIONS/INTERPRETATION

We reveal a novel pathway by which TMEM55A regulates alpha cell exocytosis by controlling intracellular PI5P and the F-actin network.

TMEM55A-mediated PI5P signaling regulates α-cell actin depolymerization and glucagon secretion

Diabetes is associated with the dysfunction of glucagon-producing pancreatic islet α-cells, although the underlying mechanisms regulating glucagon secretion and α-cell dysfunction remain unclear. While insulin secretion from pancreatic β-cells has long been known to be partly controlled by intracellular phospholipid signaling, very little is known about the role of phospholipids in glucagon secretion. Here we show that TMEM55A, a lipid phosphatase that dephosphorylates phosphatidylinositol-4,5-bisphosphate (PIP2) to phosphatidylinositol-5-phosphate (PI5P), regulates α-cell exocytosis and glucagon secretion. TMEM55A knockdown in both human and mouse α-cells reduces exocytosis at low glucose, and this is rescued by the direct reintroduction of PI5P. This does not occur through an effect on Ca2+ channel activity, but through a re-modelling of cortical F-actin dependent upon TMEM55A lipid phosphatase activity which occurs in response to oxidative stress. In summary, we reveal a novel pathway by which TMEM55A regulates α-cell exocytosis by manipulating intracellular PI5P level and the F-actin network.

Role of Complexin 2 in the regulation of hormone secretion from the islet of Langerhans

Regulated secretion of insulin from β-cells, glucagon from α-cells, and somatostatin from δ-cells is necessary for the maintenance of glucose homeostasis. The release of these hormones from pancreatic islet cells requires the assembly and disassembly of the SNARE protein complex to control vesicle fusion and exocytosis. Complexin 2 (Cplx 2) is a small soluble synaptic protein that participates in the priming and release steps of vesicle fusion. It plays a dual role as a molecular switch that first clamps and prevents fusion pore opening, and subsequently undergoes a conformational change upon Ca 2+ binding to synaptotagmin to facilitate exocytosis. Using a Cplx 2 knockout (KO) mouse model, we show a direct inhibitory role of Cplx 2 for glucagon and somatostatin secretion, along with an indirect role in the paracrine inhibition of insulin secretion by somatostatin. Deletion of Cplx 2 increases glucagon and somatostatin secretion from intact mouse islets, while there is no difference in insulin secretion between WT and Cplx 2 KO islets. The normal paracrine inhibition of insulin secretion by somatostatin is disrupted in Cplx 2 KO islets. On the contrary, deletion of Cplx 2 did not affect the known role of somatostatin in the paracrine inhibition of glucagon at elevated glucose levels, since the paracrine inhibition of glucagon secretion by somatostatin is similar for both WT and Cplx 2 KO islets. In both β- and α-cells, the secretion profiles are parallel to Ca 2+ activity changes following somatostatin treatment of WT and Cplx 2 KO islets. The loss of paracrine inhibition of insulin secretion is substantiated by direct measurements of insulin vesicle fusion events in Cplx 2 KO islets. Together, these data show a differential role for Cplx 2 in regulating hormone secretion from pancreatic islets.

Insulin-Independent Regulation of Type 1 Diabetes via Brown Adipocyte-Secreted Proteins and the Novel Glucagon Regulator Nidogen-2

Current treatments for type 1 diabetes (T1D) focus on insulin replacement. We demonstrate the therapeutic potential of a secreted protein fraction from embryonic brown adipose tissue (BAT), independent of insulin. The large molecular weight secreted fraction mediates insulin receptor-dependent recovery of euglycemia in a T1D animal model, nonobese diabetic (NOD) mice, by suppressing glucagon secretion. This fraction also promotes white adipocyte differentiation and browning, maintains healthy BAT, and enhances glucose uptake in adipose tissue, skeletal muscle, and liver. From this fraction, we identify nidogen-2 as a critical BAT-secreted protein that reverses hyperglycemia in NOD mice, inhibits glucagon secretion from pancreatic α-cells, and mimics other actions of the entire secreted fraction. These findings confirm that BAT transplants affect physiology and demonstrate that BAT-secreted peptides represent a novel therapeutic approach to diabetes management. Furthermore, our research reveals a novel signaling role for nidogen-2, beyond its traditional classification as an extracellular matrix protein.

HIGHLIGHTS

The large molecular weight brown adipocyte-secreted protein fraction suppresses glucagon secretion and normalizes glycemia in mouse models of type 1 diabetes (T1D), independent of insulin, offering a novel therapeutic strategy for disease management.Nidogen-2, a critical component of this fraction, is identified as an inhibitor of glucagon secretion in pancreatic α-cells by regulating intracellular messenger activities.The large-secreted protein fraction prevents T1D-related whitening of brown adipose tissue, promotes adipocyte differentiation, and enhances browning of inguinal white adipose tissue.This fraction enhances glucose uptake in adipose tissue, skeletal muscle, and liver through an insulin receptor-dependent pathway.

An orally available compound suppresses glucagon hypersecretion and normalizes hyperglycemia in type 1 diabetes

Suppression of glucagon hypersecretion can normalize hyperglycemia during type 1 diabetes (T1D). Activating erythropoietin-producing human hepatocellular receptor type-A4 (EphA4) on α cells reduced glucagon hypersecretion from dispersed α cells and T1D islets from both human donor and mouse models. We synthesized a high-affinity small molecule agonist for the EphA4 receptor, WCDD301, which showed robust plasma and liver microsome metabolic stability in both mouse and human preparations. In islets and dispersed islet cells from nondiabetic and T1D human donors, WCDD301 reduced glucagon secretion comparable to the natural EphA4 ligand, Ephrin-A5. In diabetic NOD and streptozotocin-treated mice, once-daily oral administration of WCDD301 formulated with a time-release excipient reduced plasma glucagon and normalized blood glucose for more than 3 months. These results suggest that targeting the α cell EphA4 receptor by sustained release of WCDD301 is a promising pharmacologic pathway for normalizing hyperglycemia in patients with T1D.

Small GTPase control of pituitary hormone secretion: Evidence from studies in the goldfish (Carassius auratus) neuroendocrine model

The secretion of vertebrate pituitary hormones is regulated by multiple hypothalamic factors, which, while generally activating unique receptor systems, ultimately propagate signals through interacting intracellular regulatory elements to modulate hormone exocytosis. One important family of intracellular regulators is the monomeric small GTPases, a subset of which (Arf1/6, Rac, RhoA, and Ras) is highly conserved across vertebrates and regulates secretory vesicle exocytosis in many cell types. In this study, we investigated the roles of these small GTPases in basal and agonist-dependent hormone release from dispersed goldfish (Carassius auratus) pituitary cells in perifusion experiments. Inhibition of these small GTPases elevated basal LH and GH secretion, except for Ras inhibition which only increased basal LH release. However, variable responses were observed with regard to LH and GH responses to the two goldfish native gonadotropin-releasing hormones (GnRH2 and GnRH3). GnRH-dependent LH release, but not GH secretion, was mediated by Arf1/6 GTPases. In contrast, inhibition of Rac and RhoA GTPases selectively enhanced GnRH3- and GnRH2-dependent GH release, respectively, while Ras inhibition only enhanced GnRH3-evoked LH secretion. Together, our results reveal novel divergent cell-type- and ligand-specific roles for small GTPases in the control of goldfish pituitary hormone exocytosis in unstimulated and GnRH-evoked release.

GC-Globulin/Vitamin D-Binding Protein Is Required for Pancreatic α-Cell Adaptation to Metabolic Stress

GC-globulin (GC), or vitamin D-binding protein, is a multifunctional protein involved in the transport of circulating vitamin 25(OH)D and fatty acids, as well as actin scavenging. In the pancreatic islets, the gene encoding GC, GC/Gc, is highly localized to glucagon-secreting α-cells. Despite this, the role of GC in α-cell function is poorly understood. We previously showed that GC is essential for α-cell morphology, electrical activity, and glucagon secretion. We now show that loss of GC exacerbates α-cell failure during metabolic stress. High-fat diet-fed GC-/- mice have basal hyperglucagonemia, which is associated with decreased α-cell size, impaired glucagon secretion and Ca2+ fluxes, and changes in glucose-dependent F-actin remodelling. Impairments in glucagon secretion can be rescued using exogenous GC to replenish α-cell GC levels, increase glucagon granule area, and restore the F-actin cytoskeleton. Lastly, GC levels decrease in α-cells of donors with type 2 diabetes, which is associated with changes in α-cell mass, morphology, and glucagon expression. Together, these data demonstrate an important role for GC in α-cell adaptation to metabolic stress.