The ‘endocrine synapse’: A novel mechanism for the control of insulin secretion

Kylie Deng, The University of Sydney, Australia

 

Aims

Diabetes mellitus (DM) is a major global health problem that currently affects over 450 million people worldwide, placing enormous burdens on both individuals and healthcare systems. Crucially, defective insulin secretion from pancreatic β-cells is as a causal factor in DM^[1]. Understanding the fundamental mechanisms that control β-cell secretion is therefore a critical part of tackling the growing DM problem.

Our lab has exciting new data indicating the presynaptic scaffold protein, liprin, plays a key role in regulating β-cell secretion, in mechanisms analogous to synaptic transmission. We are the first to show that liprin is enriched, as required for synaptic-like control, at the β-cell-capillary interface where secretion is targeted^[2]. Building on this work, I have been able to demonstrate a functional role of liprin in regulating glucose-stimulated insulin secretion, and in spatially constraining β-cell granule fusion. While these observations are consistent with presynaptic-like control of secretory function, mechanisms around this remain unclear.

In neurons, liprin plays a central role in synapse formation and the tight spatial regulation of neurotransmitter release^[3]. If we were to demonstrate an equivalent role in β-cells, this would not only identify a new functional protein, but provide the foundation for a novel conceptual framework for stimulus-secretion coupling.

 

Evaluation

This proposal sought to investigate presynaptic-like mechanisms for the control of insulin secretion in pancreatic β-cells, specifically, the role of liprin (a presynaptic scaffold protein) in the spatial regulation of insulin granule fusion. All experiments outlined in this proposal have now been completed. I have been able to show that:

1. When observing β-cells in situ in their intact islet environment using pancreatic tissue slices, liprin is locally enriched at the β-cell-capillary interface where insulin exocytosis is targeted. As predicted, a truncated N-terminal liprin mutant (missing SAM domains which are thought to bind transmembrane proteins such as LAR) did not localise to the capillary interface, while a truncated C-terminal mutant (including SAM domains) did.
2. Liprin aggregates into microdomain clusters at the β-cell-capillary interface in a glucose dependent manner. Insulin granules preferentially fuse near (<0.2μm) these liprin aggregates, suggesting that liprin might be tethering granules to specific membrane sites for fusion.
3. Knockdown of liprin in mouse β-cells reduces glucose-stimulated insulin secretion (GSIS). While our truncated N-terminal liprin mutant showed diffuse localisation across the β-cell (no enrichment at the capillary-interface), functional experiments showed that N-liprin is necessary and sufficient for bulk GSIS. Surprisingly, overexpression of C-liprin significantly reduced GSIS.

Together, these data add further weight to a presynaptic-like model for the control of insulin secretion in pancreatic β-cells, with the first functional evidence for a role in liprin in spatially regulating insulin granule fusion. This model has now been described in a review article 10.1016/j.ceca.2022.102585 and these results are currently being prepared for a manuscript. In addition, I have also been able to present these data at numerous conferences both local and international, including the Australasian Diabetes Congress 2022/2023 as well as the EASD (European Association for the Study of Diabetes) Annual Meeting 2022.

 

Grant awarded: £4,884