Metabolic stress levels influence the ability of the myelin transcription factors to regulate ß-cell identity and survival.

A hallmark of type 2 diabetes (T2D) is endocrine islet ß-cell failure, which can occur via cell dysfunction, loss-of-identity, and/or death. How each is induced remains largely unknown. Here, we use mouse ß-cells that are deficient for Myelin transcription factors (Myt TFs, including Myt1, 2, and 3) to address this question. We have reported that inactivating all three Myt genes in pancreatic progenitor cells (MytPancΔ) causes ß-cell failure and late onset diabetes in mice. Their lower expression in human ß-cells is correlated with ß-cell dysfunction and SNPs in MYT2 and MYT3 are associated in higher risk of T2D. We now show that these Myt TF-deficient postnatal ß-cells also de-differentiate by reactivating several progenitor markers. Intriguingly, mosaic Myt TF inactivation in only a portion of islet ß-cells does not results in overt diabetes, but this creates a condition where Myt TF-deficient ß-cells stay alive while activating several markers of Ppy-expressing islet cells. By transplanting MytPancΔ islets into the anterior eye chambers of immune-compromised mice, we directly show that glycemic and obesity-related conditions influence cell fate, with euglycemia inducing several Ppy+ cell markers while hyperglycemia and insulin resistance inducing additional cell death. These findings suggest that the observed ß-cell defects in T2D depend on not only their inherent genetic/epigenetic defects, but also the metabolic load.

Atf4 protects islet ß-cell identity and function under acute glucose-induced stress but promotes ß-cell failure in the presence of free fatty acid.

Glucolipotoxicity, caused by combined hyperglycemia and hyperlipidemia, results in ß-cell failure and type 2 diabetes (T2D) via cellular stress-related mechanisms. Activating transcription factor 4 (Atf4) is an essential effector of stress response. We show here that Atf4 expression in ß-cells is dispensable for glucose homeostasis in young mice, but it is required for ß-cell function during aging and under obesity-related metabolic stress. Henceforth, aged Atf4- deficient ß-cells display compromised secretory function under acute hyperglycemia. In contrast, they are resistant to acute free fatty acid-induced loss-of identity and dysfunction. At molecular level, Atf4 -deficient ß-cells down-regulate genes involved in protein translation, reducing ß-cell identity gene products under high glucose. They also upregulate several genes involved in lipid metabolism or signaling, likely contributing to their resistance to free fatty acid-induced dysfunction. These results suggest that Atf4 activation is required for ß-cell identity and function under high glucose, but this paradoxically induces ß-cell failure in the presence of high levels of free fatty acids. Different branches of Atf4 activity could be manipulated for protecting ß-cells from metabolic stress-induced failure. Highlights: Atf4 is dispensable in ß-cells in young miceAtf4 protects ß-cells under high glucoseAtf4 exacerbate fatty acid-induced ß-cell defectsAtf4 activates translation but depresses lipid-metabolism.

Endocrine islet ß-cell subtypes with differential function are derived from biochemically distinct embryonic endocrine islet progenitors that are regulated by maternal nutrients.

Endocrine islet b cells comprise heterogenous cell subsets. Yet when/how these subsets are produced and how stable they are remain unknown. Addressing these questions is important for preventing/curing diabetes, because lower numbers of b cells with better secretory function is a high risk of this disease. Using combinatorial cell lineage tracing, scRNA-seq, and DNA methylation analysis, we show here that embryonic islet progenitors with distinct gene expression and DNA methylation produce b-cell subtypes of different function and viability in adult mice. The subtype with better function is enriched for genes involved in vesicular production/trafficking, stress response, and Ca2+-secretion coupling, which further correspond to differential DNA methylation in putative enhancers of these genes. Maternal overnutrition, a major diabetes risk factor, reduces the proportion of endocrine progenitors of the b-cell subtype with better-function via deregulating DNA methyl transferase 3a. Intriguingly, the gene signature that defines mouse b-cell subtypes can reliably divide human cells into two sub-populations while the proportion of b cells with better-function is reduced in diabetic donors. The implication of these results is that modulating DNA methylation in islet progenitors using maternal food supplements can be explored to improve b-cell function in the prevention and therapy of diabetes.

CAMSAP2 localizes to the Golgi in islet ß-cells and facilitates Golgi-ER trafficking.

Glucose stimulation induces the remodeling of microtubules, which potentiates insulin secretion in pancreatic ß-cells. CAMSAP2 binds to microtubule minus ends to stabilize microtubules in several cultured clonal cells. Here, we report that the knockdown of CAMSAP2 in primary ß-cells reduces total insulin content and attenuates GSIS without affecting the releasability of insulin vesicles. Surprisingly, CAMSAP2 knockdown does not change microtubule stability. Unlike in cultured insulinoma cells, CAMSAP2 in primary ß-cells predominantly localizes to the Golgi apparatus instead of microtubule minus ends. This novel localization is specific to primary ß- but not α-cells and is independent of microtubule binding. Consistent with its specific localization at the Golgi, CAMSAP2 promotes efficient Golgi-ER trafficking in primary ß-cells. Moreover, primary ß-cells and insulinoma cells likely express different CAMSAP2 isoforms. We propose that a novel CAMSAP2 isoform in primary ß-cells has a non-canonical function, which promotes Golgi-ER trafficking to support efficient production of insulin and secretion.

Oxytocin receptors in the midbrain dorsal raphe are essential for postpartum maternal social and affective behaviors.

Oxytocin receptors (OTRs) in the midbrain dorsal raphe (DR; the source of most forebrain serotonin) have recently been identified as a potential pharmacological target for treating numerous psychiatric disorders. However, almost all research on this topic has been conducted on males and the role of DR OTRs in female social and affective behaviors is mostly unknown. This may be particularly relevant during early motherhood, which is a time of high endogenous oxytocin signaling, but also a time of elevated risk for psychiatric dysfunction. To investigate whether OTRs in the DR are necessary for postpartum female social and affective behaviors, we constructed and then injected into the DR an adeno-associated virus permanently expressing an shRNA targeting OTR mRNA. We then observed a suite of social and affective behaviors postpartum. OTR knockdown in the maternal DR led to pup loss after parturition, decreased nursing, increased aggression, and increased behavioral despair. These effects of OTR knockdown in the DR may be due to disrupted neuroplasticity in the primary somatosensory cortex (S1), which mediates maternal sensitivity to the tactile cues from young, as we found significantly more plasticity-restricting perineuronal nets (PNNs) in the S1 rostral barrel field and fewer PNNs in the caudal barrel field of OTR-knockdown mothers. These results demonstrate that OTRs in the midbrain DR are essential for postpartum maternal social and affective behaviors, are involved in postpartum cortical plasticity, and suggest that pharmacotherapies targeting OTRs in the DR could be effective treatments for some peripartum affective disorders.