Radiomanganese PET Detects Changes in Functional β-Cell Mass in Mouse Models of Diabetes

Abstract

The noninvasive measurement of functional β-cell mass would be clinically valuable for monitoring the progression of type 1 and type 2 diabetes as well as the viability of transplanted insulin-producing cells. Although previous work using MRI has shown promise for functional β-cell mass determination through voltage-dependent Ca²⁺ channel (VDCC)-mediated internalization of Mn²⁺, the clinical utility of this technique is limited by the cytotoxic levels of the Mn²⁺ contrast agent. Here, we show that positron emission tomography (PET) is advantageous for determining functional β-cell mass using ⁵²Mn²⁺ (t_1/2: 5.6 days). We investigated the whole-body distribution of ⁵²Mn²⁺ in healthy adult mice by dynamic and static PET imaging. Pancreatic VDCC uptake of ⁵²Mn²⁺ was successfully manipulated pharmacologically in vitro and in vivo using glucose, nifedipine (VDCC blocker), the sulfonylureas tolbutamide and glibenclamide (K_ATP channel blockers), and diazoxide (K_ATP channel opener). In a mouse model of streptozotocin-induced type 1 diabetes, ⁵²Mn²⁺ uptake in the pancreas was distinguished from healthy controls in parallel with classic histological quantification of β-cell mass from pancreatic sections. ⁵²Mn²⁺-PET also reported the expected increase in functional β-cell mass in the ob/ob model of pretype 2 diabetes, a result corroborated by histological β-cell mass measurements and live-cell imaging of β-cell Ca²⁺ oscillations. These results indicate that ⁵²Mn²⁺-PET is a sensitive new tool for the noninvasive assessment of functional β-cell mass.

Figure 1

Tissue distribution and pharmacokinetics of ⁵²Mn²⁺. A: Serial PET images of ICR mice injected i.v. with ⁵²Mn²⁺ (no anesthesia except during the PET scans). Coronal PET image slices were selected to best show pancreatic uptake. Arrows point to P, pancreas; H, heart; L, liver; I, intestines; and SG, salivary gland. B: ROI-based quantification of ⁵²Mn²⁺ uptake in the heart, liver, kidneys, muscle, pancreas, and submandibular salivary gland. C: Ex vivo ⁵²Mn²⁺ biodistribution of euthanized mice after the last PET scans, determined by gamma counting (n = 4).

Figure 2

Rapid kinetics of tissue ⁵²Mn²⁺ uptake revealed by single i.v. bolus injection or continuous i.v. infusion. Dynamic PET TACs derived from hand-drawn ROIs for the pancreas, heart/blood, liver, kidneys, salivary gland, and muscle. The blue curves indicate TACs in mice injected with a rapid i.v. bolus of ⁵²Mn²⁺, and the red curves indicate an i.v. infusion of ⁵²Mn²⁺ over the first 30 min of the scans. The inset shows the blood/heart distribution of ⁵²Mn²⁺ within the first 4 min after i.v. bolus injection.

Figure 3

Pharmacological manipulation of VDCC in isolated islets. A: Cartoon of the β-cell–triggering pathway. Molecular structures in blue indicate compounds that activate Ca²⁺ influx through VDCC, whereas compounds in red are inhibitory. B: Uptake of ⁵²Mn²⁺ by isolated ob/ob mouse islets. Groups of 50 islets from three preparations were incubated with ⁵²Mn²⁺ (370 kBq) in the presence of glucose and VDCC modulators as indicated. Data are presented as mean ± SD. *P < 0.05; **P < 0.01; ***P < 0.001.

Figure 4

In vivo assessment of functional β-cell mass by ⁵²Mn²⁺-PET. A: Coronal PET images at 1 h postinjection showing the pancreas of ICR mice given i.p. injections of diazoxide (20 mg/kg), nifedipine (20 mg/kg), or glibenclamide (5 mg/kg) before the administration of a ⁵²Mn²⁺ rapid bolus. The pancreas (P) is demarcated by white dashed contours. B: Manual ROI-based quantification of ⁵²Mn²⁺ uptake in various tissues from static PET images acquired at 1 h postinjection. C: Ex vivo biodistribution analysis after PET imaging at 1 h postinjection. Significantly reduced pancreatic uptake of ⁵²Mn²⁺ is observed in mice that received nifedipine and diazoxide before radiotracer administration. Mice that received glibenclamide (5 mg/kg) before radiotracer administration had significantly higher pancreatic uptake of ⁵²Mn²⁺ than the control mice, based on both PET imaging (P = 0.02) and biodistribution (P = 0.047) studies. Data are presented as mean ± SD (n = 3–4 mice per group). ***P < 0.001.

Figure 5

⁵²Mn²⁺-PET imaging in STZ-induced type 1 diabetes. A: After the administration of an acute dose of STZ (180 mg/kg), ICR mice started to show symptoms of diabetes: reduced body weight and high blood glucose level (BGL; >250 mg/dL). B: At 1 h postinjection, coronal PET images of healthy (left panel) or diabetic (center/right panels) ICR mice show clearly reduced PET signal in the pancreas of the STZ-diabetic mice. The pancreas (P) is demarcated by white dashed contours. The significant decline in ⁵²Mn²⁺ uptake in the pancreas of STZ-diabetic mice was confirmed quantitatively by ROI analysis of the PET images (C) and ex vivo biodistribution (D) (n = 3 mice/group). E: Quantification of β-cell mass for control and STZ-treated ICR mice. Data are shown as the mean ± SD (n = 4 mice/group). Scale bars = 400 μm. *P < 0.05, ***P < 0.001. MIP, maximum intensity projection.

Figure 6

⁵²Mn²⁺-PET imaging in the ob/ob model of pretype 2 diabetes. A: Coronal PET images acquired at 1 h after ⁵²Mn²⁺ administration in ob/ob mice and C57BL/6J controls. The pancreas (P) is demarcated by white dashed contours. B: Image-derived quantification expressed as SUV indicated a significant difference in ⁵²Mn²⁺ pancreatic uptake between groups (mean ± SD; n = 3). C: Quantification of β-cell mass for wild-type and ob/ob mice (n = 3 mice/group). Scale bars = 400 μm. D: Recordings of islet Ca²⁺ in response to glucose (10 mmol/L and 7 mmol/L) from wild-type and ob/ob C57BL/6J mice. E: Contingency plot shows the range of islet behaviors at each glucose level. F: The oscillatory plateau fraction, reflecting the plasma membrane glucose sensitivity, was calculated as the fraction of time spent in the active state during each oscillation at 10 mmol/L glucose. C57BL/6J, n = 141 islets from 4 mice; ob/ob, n = 126 islets from 4 mice. Results reflect mean ± SD. *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001.