A coordinated paracrine islet cell network regulating β-cell Ca2+ dynamics.

Abstract

Pulsatile insulin secretion originates from oscillations in cytosolic free Ca²⁺ in pancreatic islet β-cells. Decades of work have identified multiple determinants of these oscillations, including glucose metabolism, ion channels, ER Ca²⁺ handling, cellular heterogeneity, and paracrine and neurohormonal signaling. Yet their relative contributions under physiological conditions remain unresolved because each experimental preparation preserves only a subset of these regulatory layers. Here, we review evidence that intercellular paracrine signaling, in addition to cell-autonomous β-cell machinery, is an important determinant of oscillation frequency and coordination. Simultaneous Ca²⁺ imaging of α-cells and β-cells reveals tightly coordinated activity, with α-cells following β-cells after a conserved ∼20-s delay. Perturbation of glucagon signaling further indicates that α-cell signaling can modulate β-cell oscillation frequency. Complementing these findings, in vivo imaging of islets engrafted in the anterior chamber of the eye shows that α-cell expansion enhances β-cell Ca²⁺ coordination in a glucagon-receptor-dependent manner. Extending this circuit to δ-cells, somatostatin limits oscillation frequency primarily by suppressing α-cell glucagon release, thereby enabling the δ-cell-α-cell-β-cell paracrine circuit to shift β-cell oscillations between fast, mixed, and slow regimes. Importantly, a glycemia-dependent slow-to-fast transition is lost in diabetic mice and rescued by GLP-1 receptor agonism. Under anesthesia-free conditions, however, islets in awake animals occupy a slow, highly synchronized oscillatory regime in which glucose modulates amplitude, period, and plateau fraction without altering network coordination. Together, these findings suggest that the δ-cell-α-cell-β-cell circuit represents an important regulatory layer shaping β-cell Ca²⁺ oscillations alongside intrinsic β-cell mechanisms and extrinsic physiological inputs. β-cell dysfunction may therefore involve not only altered oscillation frequency and coordination, but also disrupted paracrine regulation by glucagon and somatostatin.