RILP Restricts Insulin Secretion Through Mediating Lysosomal Degradation of Proinsulin

2020

Front. Endocrinol.

Inhibition of glucagon hypersecretion from pancreatic α-cells is an appealing strategy for the treatment of diabetes. Our hypothesis is that proteins that associate with glucagon within alpha cell secretory granules will regulate glucagon secretion, and may provide druggable targets for controlling abnormal glucagon secretion in diabetes. Recently, we identified a dynamic glucagon interactome within the secretory granules of the α cell line, αTC1-6, and showed that select proteins within the interactome could modulate glucagon secretion. In the present study, we show that one of these interactome proteins, the neuronal protein stathmin-2, is expressed in αTC1-6 cells and in mouse pancreatic alpha cells, and is a novel regulator of glucagon secretion. The secretion of both glucagon and Stmn2 was significantly enhanced in response to 55 mM K + , and immunofluorescence confocal microscopy showed co-localization of stathmin-2 with glucagon and the secretory granule markers chromogranin A and VAMP-2 in αTC1-6 cells. In mouse pancreatic islets, Stathmin-2 co-localized with glucagon, but not with insulin, and co-localized with secretory pathway markers. To show a function for stathmin-2 in regulating glucagon secretion, we showed that siRNA-mediated depletion of stathmin-2 in αTC1-6 cells caused glucagon secretion to become constitutive without any effect on proglucagon mRNA levels, while overexpression of stathmin-2 completely abolished both basal and K + -stimulated glucagon secretion. Overexpression of stathmin-2 increased the localization of glucagon into the endosomal-lysosomal compartment, while depletion of stathmin-2 reduced the endosomal localization of glucagon. Therefore, we describe stathmin-2 as having a novel role as an alpha cell secretory granule protein that modulates glucagon secretion via trafficking through the endosomal-lysosomal system. These findings describe a potential new pathway for the regulation of glucagon secretion, and may have implications for controlling glucagon hypersecretion in diabetes.

Section: Discussionmentioning

confidence: 98%

Stathmin-2 Mediates Glucagon Secretion From Pancreatic α-Cells

2020

Front. Endocrinol.

“…These results suggest a mechanism whereby an increase in Stmn2 in the alpha cell inhibits glucagon secretion by targeting glucagon secretory granules for degradation in the endosome-lysosome pathway, perhaps in a manner similar to that of insulin secretory granules in Type 2 diabetes (39). Very recently, it has been shown that insulin secretion can be inhibited by the targeting of proinsulin for lysosomal degradation by Rab7-interacting lysosomal protein (RILP) (40). The proposed mechanism of action was through interactions between RILP and an insulin secretory granule membrane protein, Rab26.…”

Section: Discussionmentioning

confidence: 99%

Stathmin-2 Mediates Glucagon Secretion from Pancreatic α-Cells

2019

Preprint

21Inhibition of glucagon hypersecretion from pancreatic α-cells is an appealing strategy for the treatment of 22 diabetes. Our hypothesis is that proteins that associate with glucagon within alpha cell secretory granules 23 will regulate glucagon secretion, and may provide druggable targets for controlling abnormal glucagon 24 secretion in diabetes. Recently, we identified a dynamic glucagon interactome within the secretory granules 25 of the α cell line, αTC1-6, and showed that select proteins within the interactome could modulate glucagon 26 secretion. In the present study, we show that one of these interactome proteins, the neuronal protein 27 stathmin-2, is expressed in aTC1-6 cells and in mouse pancreatic alpha cells, and is a novel regulator of 28 glucagon secretion. Stathmin-2 was co-secreted with glucagon in response to 55 mM K+, and 29 immunofluorescence confocal microscopy showed co-localization of stathmin-2 with glucagon and the 30 secretory granule markers chromogranin A and VAMP-2 in αTC1-6 cells. In mouse pancreatic islets, 31Stathmin-2 co-localized with glucagon, but not with insulin, indicating that it is an alpha cell protein.To 32 show a function for stathmin-2 in regulating glucagon secretion, we showed that siRNA -mediated 33 depletion of stathmin-2 in aTC1-6 cells caused glucagon secretion to become constitutive without any effect 34 on proglucagon mRNA levels, while overexpression of stathmin-2 completely abolished both basal and 35 K+-stimulated glucagon secretion. Overexpression of stathmin-2 increased the localization of glucagon into 36 the endosomal-lysosomal compartment, while depletion of stathmin-2 reduced the endosomal localization 37 of glucagon. Therefore, we describe stathmin-2 as having a novel role as an alpha cell secretory granule 38 protein that modulates glucagon secretion via trafficking through the endosomal-lysosomal system. These 39 findings describe a potential new pathway for the regulation of glucagon secretion, and may have 40 implications for controlling glucagon hypersecretion in diabetes. 41 42 43 44 Glucagon secretion and Stathmin-2 3 Introduction 45 Hyperglucagonemia is a characteristic sign of diabetes, causing fasting hyperglycemia and 46 glycemic volatility. Clinically, glycemic variability contributes to the development of diabetes 47 complications (1). Persistent hyperglucagonemia may exacerbate abnormal glucose metabolism 48 in patients with type 2 diabetes and lead to metabolic disturbances in obese and prediabetic 49 individuals (2). Accordingly, controlling this excess glucagon secretion may be a potential 50 therapeutic strategy for diabetes (3) so that glycemia and glucose metabolism may be better 51 regulated. Such an approach has been suggested as a priority for the treatment of diabetes (1). 52Combating hyperglucagonemia could be theoretically achieved by i) inhibition of glucagon action 53 at target organs by blocking the glucagon receptor, or ii) inhibition of glucagon secretion from the 54 pancreatic α cells. While in the short-term the former...

“…Identifying new pathways or networks that control glucagon granule biogenesis and trafficking may identify novel targets for the control of hyperglucagonemia in addition to yielding a greater understanding of alpha cell biology in both health and disease. There is an emerging hypothesis that glucagon secretion can be controlled by trafficking through the endosomal-lysosomal pathway, similar to insulin ( 136 ), and below, we highlight some recent studies that suggest glucagon may regulated through such an alternate trafficking pathway.…”

Section: Novel Proteins In the Regulation Of Glucagon Trafficking And Secretionmentioning

confidence: 95%

Pathways of Glucagon Secretion and Trafficking in the Pancreatic Alpha Cell: Novel Pathways, Proteins, and Targets for Hyperglucagonemia

2021

Front. Endocrinol.

Patients with diabetes mellitus exhibit hyperglucagonemia, or excess glucagon secretion, which may be the underlying cause of the hyperglycemia of diabetes. Defective alpha cell secretory responses to glucose and paracrine effectors in both Type 1 and Type 2 diabetes may drive the development of hyperglucagonemia. Therefore, uncovering the mechanisms that regulate glucagon secretion from the pancreatic alpha cell is critical for developing improved treatments for diabetes. In this review, we focus on aspects of alpha cell biology for possible mechanisms for alpha cell dysfunction in diabetes: proglucagon processing, intrinsic and paracrine control of glucagon secretion, secretory granule dynamics, and alterations in intracellular trafficking. We explore possible clues gleaned from these studies in how inhibition of glucagon secretion can be targeted as a treatment for diabetes mellitus.