A novel optical tracer for VMAT2 applied to live cell measurements of vesicle maturation in cultured human β-cells

Imaging in Type 1 Diabetes, Current Perspectives and Directions

Type 1 diabetes (T1D) is characterized by the autoimmune-mediated attack of insulin-producing beta cells in the pancreas, leading to reliance on exogenous insulin to control a patient's blood glucose levels. As progress is being made in understanding the pathophysiology of the disease and how to better develop therapies to treat it, there is an increasing need for monitoring technologies to quantify beta cell mass and function throughout T1D progression and beta cell replacement therapy. Molecular imaging techniques offer a possible solution through both radiologic and non-radiologic means including positron emission tomography, magnetic resonance imaging, electron paramagnetic resonance imaging, and spatial omics. This commentary piece outlines the role of molecular imaging in T1D research and highlights the need for further applications of such methodologies in T1D.

Utilization of commercial collagens for preparing well-differentiated human beta cells for confocal microscopy

Introduction

With technical advances, confocal and super-resolution microscopy have become powerful tools to dissect cellular pathophysiology. Cell attachment to glass surfaces compatible with advanced imaging is critical prerequisite but remains a considerable challenge for human beta cells. Recently, Phelps et al. reported that human beta cells plated on type IV collagen (Col IV) and cultured in neuronal medium preserve beta cell characteristics.

Methods

We examined human islet cells plated on two commercial sources of Col IV (C6745 and C5533) and type V collagen (Col V) for differences in cell morphology by confocal microscopy and secretory function by glucose-stimulated insulin secretion (GSIS). Collagens were authenticated by mass spectrometry and fluorescent collagen-binding adhesion protein CNA35.

Results

All three preparations allowed attachment of beta cells with high nuclear localization of NKX6.1, indicating a well-differentiated status. All collagen preparations supported robust GSIS. However, the morphology of islet cells differed between the 3 preparations. C5533 showed preferable features as an imaging platform with the greatest cell spread and limited stacking of cells followed by Col V and C6745. A significant difference in attachment behavior of C6745 was attributed to the low collagen contents of this preparation indicating importance of authentication of coating material. Human islet cells plated on C5533 showed dynamic changes in mitochondria and lipid droplets (LDs) in response to an uncoupling agent 2-[2-[4-(trifluoromethoxy)phenyl]hydrazinylidene]-propanedinitrile (FCCP) or high glucose + oleic acid.

Discussion

An authenticated preparation of Col IV provides a simple platform to apply advanced imaging for studies of human islet cell function and morphology.

Design, synthesis, and characterization of a novel fluoroprobe for live human islet cell imaging of serotonin 5-HT1A receptor

Mounting evidence suggests that the serotonin system serves in signal transmission to regulate insulin secretion in pancreatic islets of Langerhans. Among the 5-HT receptor subtype found in pancreatic islets, serotonin receptor 1A (5-HT 1A ) demonstrates a unique ability to inhibit beta-cell insulin secretion. We report on the design, synthesis, and characterization of two novel fluorescent probes for the 5-HT 1A receptor. The compounds were prepared by conjugating the scaffold of the 5-HT 1A receptor agonist 8-OH-DPAT with two fluorophores suitable for live-cells imaging. Compound 5a showed a high affinity for the 5-HT 1A receptor ( K i = 1.8 nM). Fluoroprobe 5a was able to label the 5-HT 1A receptor in pancreatic islet cell cultures in a selective manner, as the fluorescence emission was significantly attenuated by co-administration of the 5-HT 1A receptor antagonist WAY-100635. Thus, fluoroprobe 5a showed useful properties to further characterize this unique receptor's role.

Elucidating the Relationship Between Diabetes Mellitus and Parkinson's Disease Using 18F-FP-(+)-DTBZ, a Positron-Emission Tomography Probe for Vesicular Monoamine Transporter 2

Diabetes mellitus (DM) and Parkinson’s disease (PD) have been and will continue to be two common chronic diseases globally that are difficult to diagnose during the prodromal phase. Current molecular genetics, cell biological, and epidemiological evidences have shown the correlation between PD and DM. PD shares the same pathogenesis pathways and pathological factors with DM. In addition, β-cell reduction, which can cause hyperglycemia, is a striking feature of DM. Recent studies indicated that hyperglycemia is highly relevant to the pathologic changes in PD. However, further correlation between DM and PD remains to be investigated. Intriguingly, polycystic monoamine transporter 2 (VMAT2), which is co-expressed in dopaminergic neurons and β cells, is responsible for taking up dopamine into the presynaptic vesicles and can specifically bind to the β cells. Furthermore, we have summarized the specific molecular and diagnostic functions of VMAT2 for the two diseases reported in this review. Therefore, VMAT2 can be applied as a target probe for positron emission tomography (PET) imaging to detect β-cell and dopamine level changes, which can contribute to the diagnosis of DM and PD during the prodromal phase. Targeting VMAT2 with the molecular probe ¹⁸F-FP-(+)-DTBZ can be an entry point for the β cell mass (BCM) changes in DM at the molecular level, to clarify the potential relationship between DM and PD. VMAT2 has promising clinical significance in investigating the pathogenesis, early diagnosis, and treatment evaluation of the two diseases.