Background
Short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD) is a ubiquitously expressed mitochondrial enzyme with a role in the degradation of fatty acids. Because the protein also is a negative regulator of insulin secretion in pancreatic β-cells, inactivating mutations in the SCHAD gene (HADH) cause congenital hyperinsulinism of infancy (CHI) and severe hypoglycemia. Here we sought to identify novel interaction partners of SCHAD that might be particularly relevant for the endocrine pancreas.

Results
Employing the SCHAD protein as bait, we performed yeast 2-hybrid screening of a cDNA library made from human islets of Langerhans. Surprisingly, the screening revealed the intermediate filament protein keratin 8 (K8) as a putative interaction partner of SCHAD with very high confidence. Previous reports have linked K8 to glucose homeostasis, and we confirmed the SCHAD interaction by co-immunoprecipitation in HEK293 cells. SCHAD and K8 expression were then characterized in the human β-cell model EndoC-βH1. By using proximity ligation assay, we demonstrated that stimulating the cells with a high level of glucose triggered a transient increase in the interaction. However, when studying knockout mice, we found that the loss of either K8 or SCHAD did not change the expression level of the other interaction partner. Still, when K8 knockout mice were challenged with a ketogenic diet, upregulation of SCHAD expression was blunted compared to the upregulation observed in wildtype littermates.

Conclusions
We propose that the SCHAD protein interacts with K8 in a way that might be relevant for proper functioning of the pancreatic β-cell. Whether the SCHAD-K8 interaction influences the phenotype of CHI remains to be demonstrated.

The role of N6-methyladenosine (m6A) RNA methylation in liver regeneration is unclear. This study aimed to determine the role of m6A methylation in liver regeneration after a 70% hepatectomy (HEPA) using liver-specific methyltransferase-like 14 (Mettl14) knockout (KO) male mice. Analysis was conducted on postoperative days 1, 3, or 7 (HEPA1, 3, or 7) in control (Flox) mice. In Flox mice, cyclin D1 protein expression was highest on postoperative day 3 (HEPA3) consistent with a dynamic increase in hepatocyte replication. The abundance of Mettl14 protein presented a similar pattern on HEPA3. Then, we performed hepatectomy in Mettl14 KOs (M14KO) and Flox controls and observed significantly higher postsurgical mortality in mutants. In Flox mice, cyclin D1 protein levels and Ki-67 were markedly increased on HEPA3 compared to sham operation, while being downregulated in M14KO. Characterizing the m6A epitranscriptomic changes in Flox mice after hepatectomy and contrasting them to hepatectomy in M14KO in HEPA3 revealed enrichment for gene ontology terms associated with endoplasmic reticulum, inflammation, and apoptosis. Differentially methylated genes in M14KO compared to Flox on HEPA3 were also enriched for peroxisome proliferator-activated receptor (PPAR) and AMPK signaling. Finally, we identified hypomethylated transcripts involved in fibrinogen synthesis, such as Fga, Fgb, and Fgg, by comparing differentially m6A-decorated genes in M14KO vs. Flox on HEPA3. Knockdown of fibrinogen leads to suppression of proliferation via activation of p21 protein in AML12 cells. Together, these data point to m6A RNA methylation being significant in decorating genes involved in fibrinogen synthesis in liver regeneration.

Glucagon, secreted by pancreatic alpha cells, is essential for maintaining normal blood glucose levels. In type 1 and advanced type 2 diabetes, alpha cells often fail to respond to low glucose, yet the mechanisms underlying  their stress resistance remain unclear. To investigate this, we performed a genome-wide CRISPR screen and identify Sec31A, a gene involved in transporting proteins from the endoplasmic reticulum (ER), as a key regulator of alpha cell survival under stress. We show that loss of Sec31A enhances survival in stressed mouse alpha cells and in C. elegans. In human islets, SEC31A expression increases in alpha cells under inflammatory stress, and this upregulation is reversed by reducing ER stress. Functional studies in lab-grown human islet clusters reveal distinct responses in alpha versus beta cells following Sec31A suppression. We also find that Sec31A interacts with the insulin receptor, suggesting a link between stress adaptation and insulin signaling in alpha cells.

Insulin-like growth factor-2 receptor (IGF2R), also known as cation-independent mannose-6-phosphate receptor, is localized in cytosolic vesicles and is unique in its ability to transport enzymes to the lysosome and to clear IGF2 from the cell surface by acting as a scavenger receptor. To evaluate the direct role of IGF2R in β-cell biology, we undertook complementary in vitro knockdown and in vivo knockout approaches. A β-cell line with a stable knockdown of IGF2R (IGF2RKD) exhibited decreased glucose-induced insulin secretion and enhanced cell proliferation. Tamoxifen-inducible β-cell–specific IGF2R knockout mice exhibited impaired glucose tolerance and blunted insulin secretion after high-fat-diet loading that was likely secondary to reduced β-cell mass due to attenuated proliferation. β-cells with IGF2RKD had fewer autophagosomes after starvation and reduced expression of p62, LC3B, and ULK1. Aged mice also had impaired autophagy in βIGF2R-deficient β-cells. Reduced IGF2R function and N6-methyladenosine (m6A) mRNA methylation were observed in islets from both mouse and human type 2 diabetes. Taken together, these data point to IGF2R as an important regulator of insulin secretion, cell proliferation, and autophagy in mammalian β-cells.

Lipodystrophy and obesity are associated with insulin resistance and metabolic syndrome accompanied by fat tissue dysregulation. Here, we show that serine protease inhibitor A1 (SerpinA1) expression in the liver is increased during recovery from lipodystrophy caused by the adipocyte-specific loss of insulin signaling in mice. SerpinA1 induces the proliferation of white and brown preadipocytes and increases the expression of uncoupling protein 1 (UCP1) to promote mitochondrial activation in mature white and brown adipocytes. Liver-specific SerpinA1 transgenic mice exhibit increased browning of adipose tissues, leading to increased energy expenditure, reduced adiposity and improved glucose tolerance. Conversely, SerpinA1 knockout mice exhibit decreased adipocyte mitochondrial function, impaired thermogenesis, obesity, and systemic insulin resistance. SerpinA1 forms a complex with the Eph receptor B2 and regulates its downstream signaling in adipocytes. These results demonstrate that SerpinA1 is an important hepatokine that improves obesity, energy expenditure and glucose metabolism by promoting preadipocyte proliferation and activating mitochondrial UCP1 expression in adipocytes.

N⁶-methyladenosine (m⁶A) is the most abundant chemical modification in mRNA and plays important roles in human and mouse embryonic stem cell pluripotency, maintenance, and differentiation. We have recently reported that m⁶A is involved in the postnatal control of β-cell function in physiological states and in type 1 and 2 diabetes. However, the precise mechanisms by which m⁶A acts to regulate the development of human and mouse pancreas are unexplored. Here, we show that the m⁶A landscape is dynamic during human pancreas development, and that METTL14, one of the m⁶A writer complex proteins, is essential for the early differentiation of both human and mouse pancreatic cells.

Brown adipose tissue (BAT) regulates systemic metabolism by releasing signaling lipids. N6-methyladenosine (m6A) is the most prevalent and abundant post-transcriptional mRNA modification and has been reported to regulate BAT adipogenesis and energy expenditure. Here, we demonstrate that the absence of m6A methyltransferase-like 14 (METTL14) modifies the BAT secretome to improve systemic insulin sensitivity independent of UCP1. Using lipidomics, we identify prostaglandin E2 (PGE2) and prostaglandin F2a (PGF2a) as BAT-secreted insulin sensitizers. PGE2 and PGF2a inversely correlate with insulin sensitivity in humans and protect mice from high-fat-diet-induced insulin resistance by suppressing specific AKT phosphatases. Mechanistically, METTL14-mediated m6A promotes the decay of PTGES2 and CBR1, the genes encoding PGE2 and PGF2a biosynthesis enzymes, in brown adipocytes via YTHDF2/3. Consistently, BAT-specific knockdown of Ptges2 or Cbr1 reverses the insulin-sensitizing effects in M14KO mice. Overall, these findings reveal a novel biological mechanism through which m6A-dependent regulation of the BAT secretome regulates systemic insulin sensitivity.

Pancreatic β cell dysfunction is a key feature of type 2 diabetes, and novel regulators of insulin secretion are desirable. Here, we report that succinate receptor 1 (SUCNR1) is expressed in β cells and is upregulated in hyperglycemic states in mice and humans. We found that succinate acted as a hormone-like metabolite and stimulated insulin secretion via a SUCNR1-Gq-PKC-dependent mechanism in human β cells. Mice with β cell-specific Sucnr1 deficiency exhibited impaired glucose tolerance and insulin secretion on a high-fat diet, indicating that SUCNR1 is essential for preserving insulin secretion in diet-induced insulin resistance. Patients with impaired glucose tolerance showed an enhanced nutrition-related succinate response, which correlates with the potentiation of insulin secretion during intravenous glucose administration. These data demonstrate that the succinate/SUCNR1 axis is activated by high glucose and identify a GPCR-mediated amplifying pathway for insulin secretion relevant to the hyperinsulinemia of prediabetic states.