Haploinsufficiency of Insm1 Impairs Postnatal Baseline β-Cell Mass

Abstract

Baseline b-cell mass is established during the early postnatal period when b-cells expand. In this study, we show that heterozygous ablation of Insm1 decreases baseline b-cell mass and subsequently impairs glucose tolerance. When exposed to a high-fat diet or on an ob/ob background, glucose intolerance was more severe in Insm1 +/lacZ mice compared with Insm1 +/+ mice, although no further decrease in the b-cell mass was detected. In islets of early postnatal Insm1 +/lacZ mice, the cell cycle was prolonged in b-cells due to downregulation of the cell cycle gene Ccnd1. Although Insm1 had a low affinity for the Ccnd1 promoter compared with other binding sites, binding affinity was strongly dependent on Insm1 levels. We observed dramatically decreased binding of Insm1 to the Ccnd1 promoter after downregulation of Insm1 expression. Furthermore, downregulation of Ccnd1 resulted in a prolonged cell cycle, and overexpression of Ccnd1 rescued cell cycle abnormalities observed in Insm1-deficient b-cells. We conclude that decreases in Insm1 interfere with b-cell specification during the early postnatal period and impair glucose homeostasis during metabolic stress in adults. Insm1 levels are therefore a factor that can influence the development of diabetes.Pancreatic b-cell mass is regulated both during development and in the adult. Lineage tracing in animals indicates that embryonic b-cells mainly differentiate from pancreatic progenitor cells (1), whereas postnatal increases in b-cell mass arise through self-renewal (2,3). b-Cell replication slows considerably in adults, although variations in insulin demand can lead to adaptive changes in b-cell mass (4). Sufficient b-cell mass is essential for normal regulation of blood glucose levels. Loss of b-cell mass by an immune attack or metabolic stressors results in type 1 and type 2 diabetes, respectively. In addition, although b-cell mass varies in individuals, low b-cell mass is a risk factor for prediabetes and diabetes (5). At least two factors contribute to total b-cell mass: replication capacity and baseline b-cell mass. The replication rate of adult b-cells is low, and massive efforts have been made to restore diabetic b-cell loss by enhancing b-cell replication. In contrast, the establishment of the baseline b-cell mass is not well investigated, and it is not yet fully understood how postnatal b-cell expansion varies in different individuals.The baseline b-cell mass is established in the early postnatal period in both mice and humans (2,6-8). Recent identified factors that modulate the postnatal b-cell expansion regulate the metabolic pathways or the cell cycle (9-11). The cyclin genes Ccnd1 and Ccnd2 are essential for postnatal b-cell growth and regulate the progression through G 1 through interaction with . Heterozygous mutations in Ccnd1 combined with complete knockout of Ccnd2 have dose-dependent effects on b-cell mass (14). Cell cycle inhibitors are another group of essential regulators in the postnatal b-cell mass expansion. p16 INK4a is a b-cell replication inhibi...

Insm1, Neurod1, and Pax6 promote murine pancreatic endocrine cell development through overlapping yet distinct RNA transcription and splicing programs

“…Insm1 is specifically important for expression of many genes involved in inducible secretory function of endocrine cells, including protein secretion and Ca 2+ regulated vesicle exocytosis. This is consistent with the role of Insm1 in the proliferative and secretory functions of mature endocrine cells ( Tao et al , 2018 ). Our new data, together with re-analysis of previously reported ChIP-Seq data, suggests that 89% of genes dysregulated in endocrine progenitor cells of Insm1 KO mice contain binding sites for Insm1 with 5 kb of the transcriptional start site.…”

“…Our findings strongly point to a negative epistasis between Insm1 and Pax6 , as already noted, due to the fewer than expected numbers of Insm1 GFPCre/+ ; Pax6 fl/fl animals at E18.5. Consistent with this, a recent report showed that mice with only a single functional Insm1 allele ( Insm1 +/ − ) have impaired β-cell proliferation that results in impaired glucose tolerance in adult animals ( Tao et al 2018 ). However, the authors observed no impairments in β-cell mass at birth and argued that Insm1 haploinsufficiency has little to no effect on embryonic endocrine development ( Tao et al 2018 ).…”

“…Consistent with this, a recent report showed that mice with only a single functional Insm1 allele ( Insm1 +/ − ) have impaired β-cell proliferation that results in impaired glucose tolerance in adult animals ( Tao et al 2018 ). However, the authors observed no impairments in β-cell mass at birth and argued that Insm1 haploinsufficiency has little to no effect on embryonic endocrine development ( Tao et al 2018 ). Previous Pax6 KO studies have also not reported any unexpected difficulties in obtaining Pax6 null embryos, and animals died just after birth ( Sander et al 1997 ; St-Onge et al 1997 ; Ashery-Padan et al 2004 ; Heller et al 2004 ).…”

Patterns of senescence and apoptosis during development of branchial arches, epibranchial placodes, and pharyngeal pouches

“…For example, Upadhya et al 71 identified the inhibition of G2/M phase progression in fetal lens cells from mitogen‐activated protein kinase 1 knockout mice by comparing the fraction of proliferating lens cells specifically detected with the TEC‐3 antibody (G1‐M phases) with those labeled after BrdU application (S phase) or anti‐phosphorylated histone‐H3 immunostaining (G2‐M phases). Similarly, Tao et al 72 successfully used the TEC‐3 antibody in BrdU pulse‐chase experiments to detect a cell cycle delay in developing pancreatic β‐cells of Insm1 haploinsufficient mice. Thus, after application of a single dose of BrdU followed by a 23‐h BrdU‐free (chase) period, the cell cycle length could be determined from the ratio of BrdU + /Ki67 + and BrdU + /Ki67 − cells in wild‐type and transgenic mice.…”

Insulinoma-Associated-1: From Neuroendocrine Tumor Marker to Cancer Therapeutics

“…Additional evidence supports that Insm1 is a transcription factor that acts in a pan-endocrine manner, which is essential for the differentiation of endocrine cells into pancreas, intestine, adrenal gland, and anterior pituitary gland. A recent study suggested that haplo-insufficiency of insm1 impaired postnatal baseline b-cell mass (38). The SNAG domain at the N-terminus of INSM1 recruits histone-modifying factors, lysine (K)-specific demethylase 1A (Kdm1a), histone deacetylase 1/2 (Hdac1/2), and REST corepressor 1-3 (Rcor1-3) and other proteins implicated in transcriptional regulation high mobility group 20a/b (Hmg20a/b) and genetic suppressor element 1 (Gse1) in the anterior pituitary gland (15).…”