Hyperglycemia (HG) is a well-established risk factor for secondary osteoporosis, primarily due to suppressed osteoblast activity. While gut microbiota (GM) dysbiosis has been implicated in various diseases, its role in HG-induced osteoporosis remains poorly understood. Here, we demonstrate that HG mice develop low-turnover osteoporosis accompanied by reduced GM diversity. Fecal microbiota transplantation (FMT) from HG mice (GMHG-FMT) induced osteoporosis in recipient mice, independent of blood glucose levels. A depletion of Bifidobacterium pseudolongum was associated with bone loss, whereas supplementation with either microbiota of normoglycemic mice or B. pseudolongum alleviated osteoporosis in HG mice. Both HG and GMHG-FMT recipient mice exhibited elevated serum interleukin-17A (IL-17A) levels, and anti-IL-17A antibody treatment mitigated osteoporosis in the GMHG-FMT model. Furthermore, decanoic acid levels were elevated in the feces of HG mice and the serum of GMHG-FMT recipients. Decanoic acid promoted the differentiation of naive CD4+ T cells into T helper17 cells, leading to increased IL-17A production. These findings reveal a microbiome dysbiosis-driven decanoic acid/IL-17A axis in HG-induced osteoporosis and highlight the therapeutic potential of microbiome-associated targets.

Maternal obesity is a known risk factor for metabolic dysfunction in offspring; however, its effect on metabolism during pregnancy in female offspring remains unclear. This study investigated how maternal obesity, induced by high-fat (HF) feeding in C57BL/6J mice, affects the metabolic adaptation to pregnancy in female offspring. Dams were fed an HF diet (60% fat) or chow for 3 months before and during pregnancy. Offspring of HF diet-fed dams (OF-HFD) exhibited reduced fetal growth, followed by rapid postnatal catch-up and increased adult adiposity, compared with offspring of chow-fed dams (OF-CD), despite having similar baseline glucose and insulin levels. During pregnancy, OF-HFD exhibited diminished increases in maternal body fat, blood triglycerides, and insulin concentrations, accompanied by glucose intolerance. In cultured islets, glucose-stimulated insulin secretion was markedly reduced in pregnant OF-HFD, despite unchanged β-cell mass or proliferation. Hepatic triglyceride secretion was decreased, whereas liver insulin signaling was enhanced, suggesting alterations in lipid and glucose metabolism. Feeding OF-HFD an HF diet before and during pregnancy further impaired fetal growth. These findings indicate that maternal obesity impairs the metabolic adaptation to pregnancy in female offspring, characterized by insulin insufficiency and disrupted lipid homeostasis. This may initiate a transgenerational cycle of metabolic dysfunction, potentially increasing the risk of gestational diabetes in subsequent generations. Our findings underscore the need for more research to explore these mechanisms in humans and develop strategies to reduce the long-term effects of maternal obesity.

Type 1 diabetes (T1D) is characterized by autoimmune destruction of insulin-producing β-cells. Recent evidence has implicated hybrid insulin peptides (HIPs) as targets of autoreactive CD4 T cells in human T1D patients and as critical autoantigens recognized by diabetogenic T cells in nonobese diabetic (NOD) mice. HIPs form within pancreatic islets through cross-linking reactions between proinsulin fragments and various β-cell peptides. In the NOD mouse model, highly pathogenic CD4 T cells specifically target HIPs generated through transpeptidation mediated by cathepsin D (CatD). These disease-relevant HIPs consistently incorporate a C-peptide fragment terminating in a critical leucine residue that binds to other β-cell peptides. In vitro experiments demonstrated that substituting isoleucine for this leucine residue in human C-peptide inhibited CatD-mediated HIP formation. To investigate the in vivo significance of this finding, we engineered NOD mice carrying a leucine-to-isoleucine mutation in the insulin 2 gene (NOD INS2I/I). Mass spectrometric analysis revealed significantly reduced HIP formation in islets from NOD INS2I/I mice. Significantly decreased activation of HIP-reactive T cells to islets from these mice was also observed. Furthermore, the NOD INS2I/I mice showed significantly delayed diabetes onset, with 43% remaining disease-free at 1 year compared with only 10% of wild-type NOD controls. These findings implicate HIPs as key mediators in T1D pathogenesis and demonstrate that targeted disruption of HIP formation significantly alters disease progression. Inhibiting CatD-mediated transpeptidation represents a promising therapeutic approach for preventing or delaying T1D onset in genetically susceptible individuals.

The control of muscle glucose uptake (MGU) is distributed across delivery, transport, and phosphorylation of glucose. These steps have been defined as control points of MGU in vivo due to the application of isotopic tracer techniques to transgenic mouse models. Using these techniques in a classic study published in Diabetes, Fueger et al. demonstrated that overexpression in skeletal muscle of hexokinase II (HKII), the enzyme responsible for intracellular glucose phosphorylation, enhanced MGU in insulin-sensitive but not in insulin-resistant mice. Conversely, HKII overexpression enhanced MGU in insulin-resistant mice in response to exercise. Since exercise reduces barriers of glucose delivery and transport, this suggested that these two processes contribute to the dysregulation of MGU in insulin-resistant states. These fundamental findings have spurred subsequent studies highlighting the contribution of glucose delivery and transport to the regulation of MGU in health and disease.

Low birth weight (LBW) is a risk factor for type 2 diabetes (T2D). We hypothesized that 4 weeks of carbohydrate overfeeding (COF) with +25% energy would unmask key T2D perturbations among 22 nonobese LBW men, including five with screen-detected metabolic dysfunction-associated steatotic liver disease (MASLD), compared with 21 healthy control participants with normal birth weight (NBW). Body weight, lean and fat mass, and hepatic fat content increased to the same extent in both groups during COF, whereas fasting glucose and insulin resistance increased significantly more in LBW compared with NBW participants. The differential COF responses were most pronounced in LBW participants without MASLD, including increased resting energy expenditure. Plasma adiponectin was lower, whereas fibroblast growth factor 21 levels increased more during COF in LBW participants. Subcutaneous adipose tissue (SAT) density was lower in LBW participants and decreased during COF in both groups. Serum alanine, phosphatidylcholines, and triglycerides increased significantly more in LBW participants during COF. Multiomics analysis of SAT RNA sequencing, serum lipidomics, and metabolomics uncovered impaired peroxisome proliferator-activated receptor signaling as well as aberrant collagen and extracellular matrix regulation in LBW participants. The results document differential and MASLD-independent metabolic perturbations in LBW participants during COF.

Adipocyte size is linked to insulin resistance and the risk of developing type 2 diabetes. We aimed to generate a surrogate method to estimate adipocyte size by measuring adipose tissue gene expression using quantitative real-time PCR (qRT-PCR), which could be used alongside systemic measures of insulin sensitivity to predict type 2 diabetes risk. We examined the relationship of 40,591 genes with abdominal subcutaneous adipocyte size in 132 adults and validated the findings in additional cohorts with available transcriptomic and adipocyte size data. qRT-PCR analysis of gene expression in abdominal adipose tissue biopsies was used to develop a standardized adipocyte size estimate. This estimate was compared alongside systemic and adipose insulin sensitivity measures, including adipocyte lipogenesis, hyperinsulinemic-euglycemic clamp, adipose insulin resistance, and HOMA. Transcriptome-wide analyses found that UCHL1 gene expression strongly correlated with adipocyte size, independent of other genes and additional cofactors, such as insulin resistance (β-coefficient 0.32; P = 0.002). Using qRT-PCR, UCHL1 expression accurately estimated adipocyte size across a wide range of adipocyte volumes with high precision (receiver operating characteristic area under the curve 0.94) and showed strong correlations with all insulin sensitivity measures (adjusted r2 = 0.2-0.6; P < 0.0001). We scaled the measurement of UCHL1 expression to 25-mg adipose biopsies and provided a standard operating procedure for routinely estimating adipocyte size. In summary, we provide a simple, accurate, and accessible surrogate measure to estimate an individual's adipocyte size, which may be useful in clinical insulin resistance studies.

Diabetes currently affects ∼37 million adults in the U.S. and 537 million people worldwide, with type 2 diabetes (T2D) accounting for 90%-95% of the diabetes burden. The transition from normal glucose regulation (NGR) to T2D is via an intermediate stage of prediabetes, characterized by impaired fasting glucose (IFG) and impaired glucose tolerance (IGT). Prediabetes affects ∼98 million adults in the U.S.; worldwide, more than 541 million adults have IGT and 319 million adults have IFG. Prediabetes is associated with increased risks of developing vascular and neuropathic complications, besides the risk of progression to T2D. Discussed herein are the demographic, anthropometric, biobehavioral, biochemical, and molecular factors associated with the transition from NGR to prediabetes. The natural history of prediabetes predicts time-dependent progression to T2D, as sustained recovery from prediabetes is uncommon without intervention. Lifestyle modification and certain medications interrupt the progression to T2D and may restore NGR. The landmark intervention trials are discussed, with an interpretive focus on their limitations and the need for novel approaches for durable reversal of prediabetes.

Diabetic kidney disease (DKD) progression involves NIMA-related kinase 7 (NEK7)-driven podocyte pyroptosis, with hyperglycemia-induced O-GlcNAcylation as a key posttranslational regulator. This study elucidates how O-GlcNAc modification governs NEK7 stability and its pathological role. We used clinical DKD specimens, high-glucose-stimulated podocytes, and streptozotocin-induced diabetic mice to first examine NEK7, O-GlcNAc, O-GlcNAc transferase (OGT), and glutamine fructose-6-phosphate amidotransferase 1 (GFPT1) expression, confirming the pyroptosis role of NEK7 via siRNA knockdown. Bioinformatic analysis predicted O-GlcNAcylation motifs, validated by T302A mutagenesis and coimmunoprecipitation. Protein stability was assessed using cycloheximide chase and ubiquitination assays. Therapeutic efficacy of the GFPT1 inhibitor (6-diazo-5-oxo-l-norleucine) DON was evaluated in vitro and in vivo through biochemical parameters, histopathology, and pyroptosis markers. Chronic hyperglycemia activated the hexosamine biosynthetic pathway (HBP), elevating pathology-associated O-GlcNAc modifications that promoted NEK7 accumulation via posttranslational stabilization. This was accompanied by upregulated O-GlcNAc, OGT, and GFPT1 in DKD glomeruli and high-glucose podocytes. Crucially, threonine 302 was identified as the primary O-GlcNAcylation site of NEK7. This modification reduced proteasomal degradation, extended NEK7 half-life, and enhanced NLRP3 inflammasome activation and interleukin release. Pharmacological HBP inhibition using DON normalized O-GlcNAcylation, suppressed pyroptosis, and mitigated renal injury. We report the discovery of the glucose/O-GlcNAc/NEK7/NLRP3 axis driving podocyte pyroptosis in DKD, proposing threonine 302 as a potential therapeutic target. These findings establish a novel posttranslational modification mechanism and suggest a dual-target therapeutic strategy for DKD management.

Somatostatin is a powerful inhibitor of insulin secretion and β-cell electrical activity, but the effects are weak in intact islets, possibly because of high intraislet somatostatin levels. We used optogenetics in conjunction with hormone secretion measurements, electrophysiology, and cytoplasmic free Ca2+ concentration ([Ca2+]i) imaging to interrogate the relative roles of paracrine and electrical control of β-cells by δ-cells. We confirm that optoactivation and optoinhibition of δ-cells stimulated and inhibited their electrical activity and somatostatin secretion, respectively. Unexpectedly, neither optoactivation nor optoinhibition of δ-cells had any effect on insulin secretion at 1 or 20 mmol/L glucose. Paradoxically, optoactivation of δ-cells at 6 mmol/L glucose increased insulin secretion by 113%, an effect that correlated with β-cell action potential firing. In [Ca2+]i imaging experiments, optoactivation of δ-cells induced islet-wide β-cell [Ca2+]i transients and synchronized the oscillatory pattern induced by 7 mmol/L glucose. Conversely, optoinhibition of δ-cells and somatostatin secretion reduced rather than increased β-cell electrical activity and [Ca2+]i in the <10% of β-cells situated <20 µm from δ-cells. We propose that δ-cells, in addition to subserving an inhibitory paracrine effect, play a role in the rapid propagation of electrical signals across the islet, possibly contributing to the coordination of β-cell activity.

Accurate prediction of diabetic kidney disease progression is challenging, but mandatory. Urinary Dickkopf-3 (uDKK3), a tubular, epithelial-derived glycoprotein and marker of tubular injury, is a promising biomarker for kidney function decline. We explored the clinical utility of uDKK3 to predict kidney function decline and adverse cardiovascular events in patients with type 2 diabetes mellitus (T2DM) in a primary health care setting. In this cohort study, 3,232 patients with T2DM were analyzed. The primary end point was a composite of a sustained estimated glomerular filtration rate (eGFR) decline ≥40%; a sustained increase in albuminuria of at least 30%, including a transition in albuminuria class; progression to end-stage kidney disease; and death from kidney failure. After adjustment for confounding variables, uDKK3 values >200 pg/mg creatinine were associated with a higher risk of the composite kidney end point during a median follow-up of 4.26 years. Furthermore, uDKK3 improved the prediction of the 1-year eGFR decline on top of albuminuria. Individuals with high uDKK3 levels also had an increased risk for adverse cardiovascular events and all-cause mortality. uDKK3 identifies patients with T2DM at high risk for kidney function decline on top of established biomarkers (albuminuria and eGFR). In primary care, uDKK3 may help to identify high-risk patients who might benefit from intensified treatment and/or referrals to specialists.

An intrinsic hallmark of type 1 diabetes is the correlation between appearance of autoantibodies directed against islet cell autoantigens with subsequent development of the disease. We recently studied effects of human monoclonal autoantibodies (mAbs) derived from a patient with prediabetes and demonstrated that a GAD65mAb penetrated and accumulated in β-cells and significantly reduced the insulin secretion rate (ISR). Accordingly, in the current study, we performed more detailed analyses of the effects of this GAD65mAb on rat and human islets. ISR was suppressed by ∼40% after 3 days of exposure. Mechanisms mediating the effects were found to involve inhibition of mitochondrial generation of ATP, which decreased in parallel with that of ISR. As expected, the GAD65mAb inhibited γ-aminobutyric acid secretion. The effects of GAD65mAb were observed in rat and human islets but not in mouse islets, which do not express GAD65. GAD65mAb also reduced insulin secretion in vivo, where decreased insulin levels after intraperitoneal (i.p.) injection of glucose were observed in rats after i.p. injection of GAD65mAb. Thus, it appears that an islet cell autoantibody against GAD65 can directly impact and impair secretory function in islets in vitro and in vivo through a mechanism that involves inhibition of mitochondrial energetics.

Postbariatric hypoglycemia (PBH) is a serious complication of Roux-en-Y gastric bypass (RYGB), characterized by severe hypoglycemia that may lead to loss of consciousness and seizures. The exact mechanism of PBH is poorly understood. One potential mechanism is β-cell expansion. To this end, we investigated β-cell mass in individuals with and without PBH after RYGB using [68Ga]Ga-NODAGA-exendin-4 positron emission tomography/computed tomography imaging (PET/CT). Individuals with PBH (n = 10) and without PBH (n = 9) after RYGB were included. PET/CT imaging was performed after infusion with 102.2 ± 6.9 MBq of the [68Ga]Ga-NODAGA-exendin-4 tracer to quantify pancreatic β-cell mass. The two groups did not differ with respect to sex, age, BMI, and total body weight loss after RYGB. Time between RYGB and inclusion was longer for individuals with PBH compared with those without. β-Cell mass did not differ between the groups. Individuals with PBH had a smaller pancreas than those without. β-Cell mass correlated neither with body weight parameters nor with metabolic parameters. Our data indicating that β-cell mass does not differ between individuals with and without PBH after RYGB argue against expansion of β-cell mass to explain PBH.

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.

The presence of macrophages surrounding lipotoxic tubular epithelial cells (TECs) is a hallmark of diabetic nephropathy (DN). Nevertheless, the mechanisms of communication between these cell types are not well understood. Previous studies have revealed a unique subset of macrophages that express triggering receptor expressed on myeloid cells 2 (Trem2) in kidneys of human patients and mice with DN. Here, we explored the characteristics and the function of Trem2+ macrophages in the progress of DN. RNA-sequencing of macrophages in kidneys of Trem2 knockout (KO) mice fed a high-fat diet plus streptozotocin (HFD/STZ) revealed functional enrichment of metabolic processes, cytokine production, positive regulation of extracellular signal-regulated kinase (ERK) cascades, and the regulation of phagocytosis. In vivo studies demonstrated that Trem2+ macrophages reduced lipid accumulation and mitigated ferroptosis of TECs in diabetic mice. Mechanistically, Trem2-deficient macrophages amplified the production of interleukin-1β (IL-1β) through activating the ERK signaling pathway. Furthermore, IL-1β triggered CD36 expression via the transcription factor NF-κB. Bioinformatics and functional assays showed NF-κB binds the CD36 promoter, which directly bound to the promoters of CD36 to facilitate its transcription. Inhibition of NF-κB blocked IL-1β-induced CD36 production. This mechanism is exacerbated in Trem2-deficient macrophages, which release excess IL-1β to activate NF-κB in tubular cells, promoting CD36-dependent lipid uptake and ferroptosis. Additionally, we found Trem2 plays a role in enhancing the phagocytosis and clearance of ferroptotic cells by bone marrow-derived macrophages. Altogether, our results suggest Trem2+ macrophages maintain homeostasis of the renal microenvironment and exert a protective function in DN.