Radiologic evidence that hypothalamic gliosis is associated with obesity and insulin resistance in humans

Abstract

Objective: To use quantitative magnetic resonance imaging (MRI) to test whether mediobasal hypothalamic (MBH) gliosis is associated with obesity and insulin resistance in humans.

Methods: Sixty-seven participants underwent a fasting blood draw and MRI. Cases with radiologic evidence of MBH gliosis (N = 22) were identified as the upper tertile of left MBH T2 relaxation time and were compared to controls (N = 23) from the lowest tertile. In a separate postmortem study, brain slices (N = 10) through the MBH were imaged by MRI and stained for glial fibrillary acidic protein (GFAP).

Results: In all participants, longer T2 relaxation time in the left MBH was associated with higher BMI (P = 0.01). Compared with controls, cases had longer T2 relaxation times in the right MBH (P < 0.05), as well as higher BMI (P < 0.05), fasting insulin concentrations (P < 0.01), and HOMA-IR values (P < 0.01), adjusted for sex and age. Elevations in insulin and HOMA-IR were also independent of BMI. In the postmortem study, GFAP staining intensity was positively associated with MBH T2 relaxation time (P < 0.05), validating an MRI-based method for the detection of MBH gliosis in humans.

Conclusions: These findings link hypothalamic gliosis to insulin resistance in humans and suggest that the link is independent of the level of adiposity.

Figure 1. Measurement of T2 relaxation time

Regions of interest (ROIs) were placed on high resolution coronal sections (A: TE 20 & B: TE 170 ms) then transferred to a parametric map of T2 relaxation times (C) derived from the multi-echo sequence to measure the mean (SD) T2 relaxation time within each ROI. The MBH ROI was placed adjacent to the 3^rd ventricle to encompass the location of the arcuate nucleus. Reference ROIs were placed in the amygdala and putamen. The ROI areas were: MBH ~3.9 mm² (with corresponding tissue volume 9.75 mm³), putamen ~3.56 mm² (8.90 mm³), and amygdala ~2.97 mm² (7.43mm³). Scale bar in C represents 1 cm. MBH: mediobasal hypothalamus.

Figure 2. Hypothalamic T2 relaxation time, adiposity and insulin resistance

Longer T2 relaxation times in the left mediobasal hypothalamus are associated with higher BMI (A) and higher HOMA-IR scores (B). N=67.

Figure 3. Nested case-control study of radiologic evidence of gliosis

Cases were identified as the tertile with the highest mean T2 relaxation time in the left mediobasal hypothalamus (MBH). Cases with radiologic evidence of gliosis have higher BMIs (A), fasting insulin concentrations (C) and HOMA-IR (D) but not fasting glucose concentrations (B). Data are least square means ± SEM; P-values determined from linear regression. Data are adjusted for sex and age. N=23 (Controls), 22 (Cases). *P<0.05, **P<0.01.

Figure 4. Representative images of mediobasal hypothalamus (MBH) ROI and GFAP staining in autopsy samples

Coronal brain slices that included the hypothalamus were imaged, sectioned, stained for glial fibrillary acidic protein (GFAP) via immunohistochemistry and digitally scanned. ROIs were placed on digital images (A, captured at 0.4X, scale bar represents 5 mm) and on a parametric map of T2 relaxation times derived from the multi-echo sequence for radiologic analysis (B, scale bar represents 5 mm). In both cases, the MBH ROI was placed adjacent to the 3^rd ventricle to encompass the location of the arcuate nucleus. Panels C-D show representative sections through the MBH from a brain slice with a high MBH T2 relaxation time and panels E-F are derived from a brain slice with a low MBH T2 relaxation time. Images in panels C and E depict immunostaining for GFAP (brown) and hematoxylin (blue) whereas panels D and F depict the identical region processed with digital color deconvolution software to measure staining intensity. Pseudocolors in D and F represent staining intensity (blue or white=no stain, yellow=low intensity and red=moderate or high intensity). Images in C-F captured at 40X; scale bar = 50 μm.