Pancreatic islets undergo coordinated cellular remodeling during obesity-induced insulin resistance. However, longitudinal changes across endocrine and non-endocrine compartments remain largely unexplored. We present a comprehensive high-resolution atlas using longitudinal single-cell RNA sequencing (scRNA-seq) and single-cell ATAC sequencing (scATAC-seq) on islets from C57BL/6 mice subjected to high-fat diet (HFD) feeding for 8, 16, and 24 weeks, along with age-matched controls on regular chow (RC). We mapped dynamic changes in islet cell composition and transcriptional states. Trajectory inference indicated diversification of beta-cell programs into adaptive and inflammatory states under HFD. Progression of insulin resistance induced shrinkage and transcriptional remodeling of glucagon-secreting alpha-cells, marked by upregulation of genes related to intracellular transport and oxidative stress, accompanied by the emergence of a polyhormonal alpha-cell subpopulation. Similarly, we identified delta-cell subpopulations exhibiting beta-like transcriptional signatures and polyhormonal identity under nutritional stress, suggesting adaptive delta-cell plasticity that may partially compensate for beta-cell loss during insulin resistance.

The islet microenvironment exhibited robust expansion of proinflammatory M1 macrophages, reaching a plateau by 16 weeks of HFD, indicating niche saturation. Cell-cell communication analyses revealed disruption of key signaling pathways within endocrine and between endocrine and non-endocrine cells under HFD conditions. Notably, CCL27a–chemokine receptor signaling between beta-cells and M1 macrophages was significantly reduced in HFD islets, likely driven by reduced Ccl27a expression and chromatin accessibility in a distinct beta cell subpopulation, which we further validated using INS-1 cells exposed to HFD-like conditions. Comparative analysis with scRNA seq of human islets confirmed conserved stress signatures. Furthermore, genetic variants at the CCL27 locus were associated with increased T2D risk and HOMA-IR in human populations, establishing a novel link between beta-cell stress and systemic inflammation. This resource provides a hierarchical framework for understanding islet failure and identifies potential therapeutic nodes for type 2 diabetes.