Optogenetics in Pancreatic Islets: Actuators and Effects

Abstract

The islets of Langerhans reside within the endocrine pancreas as highly vascularized microorgans that are responsible for the secretion of key hormones, such as insulin and glucagon. Islet function relies on a range of dynamic molecular processes that include Ca2+ waves, hormone pulses, and complex interactions between islet cell types. Dysfunction of these processes results in poor maintenance of blood glucose homeostasis and is a hallmark of diabetes. Recently, the development of optogenetic methods that rely on light-sensitive molecular actuators has allowed perturbation of islet function with near physiological spatiotemporal acuity. These actuators harness natural photoreceptor proteins and their engineered variants to manipulate mouse and human cells that are not normally light-responsive. Until recently, optogenetics in islet biology has primarily focused on controlling hormone production and secretion; however, studies on further aspects of islet function, including paracrine regulation between islet cell types and dynamics within intracellular signaling pathways, are emerging. Here, we discuss the applicability of optogenetics to islets cells and comprehensively review seminal as well as recent work on optogenetic actuators and their effects in islet function and diabetes mellitus.

Figure 1

Optogenetic actuators to control the transcription of GLP-1 and insulin. A: LightOn employs the synthetic transcription factor GAVPO that contains Gal4, VVD, and p65. VVD dimerizes in the presence of blue light, allowing for Gal4 to bind galactose-responsive upstream activation sequence (UASG) elements to drive transcription. B: GBOI is a two-component system whereby the GI-Gal4 fusion protein is under the control of the glucose-sensitive promoter P_GIP. The constitutively expressed LOV-VP16 forms a complex with GI-Gal4 only in the presence of high glucose and blue light to drive insulin expression. C: The FRL-activated BphS system converts GTP to c-di-GMP, which binds to FRTA (which contains the BldD transcription factor, VP64, and p65). Activated FRTA subsequently drives transcription from BldD-specific DNA operator sites (P_FRL). Coexpression of the c-di-GMP phosphodiesterase YhjH can be used to reduce basal intracellular c-di-GMP. D: REDMAP uses a fused ΔphyA-Gal4 construct along with FHY1 fused to a VP64 transactivator. The ΔphyA-FHY1 interaction is promoted by red light to mediate transgene expression. The system can be reversed with FRL. E: OPN4 can drive endogenous PLC signaling, Ca²⁺ influx, and NFAT cell–mediated transcription. F: The Glow Control system uses a membrane-anchored TtCBD and a second TtCBD domain fused to VP64, p65, and Rta (VPR) and TetR. In the dark, TtCBD domains are fused. Green light promotes dissociation, leading to transgene expression. hGLP1, human glucagon-like peptide 1; TetO, tetracycline operator.

Figure 2

Optogenetic actuators deployed into islet cells to control intracellular signaling, Ca²⁺ influx, and insulin secretion. A: Native G_q-coupled GPCRs promote PLC signaling to raise cytoplasmic Ca²⁺ concentrations and potentiate insulin secretion. OPN4 can be used to achieve light-dependent control of this process. B: The light-sensitive ion channel ChR2 can be used to promote membrane depolarization (not shown) and influx of cations such as Ca²⁺. C: Ca²⁺-specific influx can be achieved through activation of native CRACs using light-sensitive monSTIM1. High intracellular Ca²⁺ stimulates insulin production and exocytosis of insulin from secretory granules. D: AC activity can enhance insulin secretion through the cAMP/PKA pathway, and this can be optically controlled with bPAC. The native GLP-1 pathway is also shown here. E: Secretion can also be light-regulated at the post-transcriptional level from the ER using optoPASS. F: Finally, the anion pump, NpHR inhibits insulin secretion in a light-dependent manner by promoting the influx of Cl⁻ into the cell. Dotted lines indicate indirect insulin release. DAG, diacylglycerol; EPAC, exchange protein activated by cAMP; IP₃, inositol triphosphate; PIP₂, phosphatidylinositol 4,5-bisphosphate; VGCC, voltage-gated Ca²⁺ channel.

Figure 3

Illumination strategies applied in vivo (rodent models). A: Animals can be stimulated using LED lamps, matrixes, or arrays installed above, around, or within the animal cage; these illuminate multiple animals (left). Direct local stimulation to the pancreas or a cell implant can be achieved using tethered optical fibers (right). B: Wireless LED implants allow for tether-free light delivery in freely behaving animals. They can be coencapsulated with designer cells, as represented here, or be used to directly stimulate a tissue of interest. Wireless LEDs are powered through a field generator wire surrounding the cage. C: UCNPs achieve minimally invasive tether-free light delivery deep into tissues by converting NIR light into higher-energy visible light. This is particularly useful for actuators that require short wavelengths or for tissues that are difficult to target with external illumination strategies. Animals can be stimulated externally with NIR - typically using LED lamps, arrays, or matrixes as illustrated in A. UCNPs can be injected into the pancreas for direct tissue stimulation or coencapsulated with designer cells for transplantation.