Longer T(2) relaxation time is a marker of hypothalamic gliosis in mice with diet-induced obesity

Abstract

A hallmark of brain injury from infection, vascular, neurodegenerative, and other disorders is the development of gliosis, which can be detected by magnetic resonance imaging (MRI). In rodent models of diet-induced obesity (DIO), high-fat diet (HFD) consumption rapidly induces inflammation and gliosis in energy-regulating regions of the mediobasal hypothalamus (MBH), and recently we reported MRI findings suggestive of MBH gliosis in obese humans. Thus, noninvasive imaging may obviate the need to assess MBH gliosis using histopathological end points, an obvious limitation to human studies. To investigate whether quantitative MRI is a valid tool with which to measure MBH gliosis, we performed analyses, including measurement of T(2) relaxation time from high-field MR brain imaging of mice fed HFD and chow-fed controls. Mean bilateral T(2) relaxation time was prolonged significantly in the MBH, but not in the thalamus or cortex, of HFD-fed mice compared with chow-fed controls. Histological analysis confirmed evidence of increased astrocytosis and microglial accumulation in the MBH of HFD-fed mice compared with controls, and T(2) relaxation times in the right MBH correlated positively with mean intensity of glial fibrillary acidic protein staining (a marker of astrocytes) in HFD-fed animals. Our findings indicate that T(2) relaxation time obtained from high-field MRI is a useful noninvasive measurement of HFD-induced gliosis in the mouse hypothalamus with potential for translation to human studies.

Keywords: gliosis; high-fat diet; hypothalamus; magnetic resonance imaging; obesity.

Fig. 1.

Regions of interest and representative images of scan parameters. A: high-resolution 2-dimensional rapid acquisition with refocused echoes image for a slice selected to include the midregion of the arcuate nucleus. B: T₂ map generated from multiecho sequence. C and D: diffusion tensor imaging for fractional anisotropy (C) and tensor trace measurement (D). Regions indicate areas of analysis. R, right; L, left; MBH, mediobasal hypothalamus. Scale bar, 1 mm.

Fig. 2.

Results of multiparametric imaging in high-fat diet (HFD)-fed mice and chow-fed controls. A: T₂ relaxation time is higher in HFD-fed animals in the MBH (t₁₄ = 2.93; *P = 0.01) but not in the control regions of the thalamus and cortex (diet × region interaction: F_2,42 = 3.84, P = 0.03). B: there were no diet effects for fractional anisotropy, but there was an effect of region (F_2,39 = 3.28, P < 0.05). C: there were no group differences in tensor trace measurements; n = 8 for each group except in fractional anisotropy and tensor trace analyses, where n = 7 for HFD due to an image artifact.

Fig. 3.

Histopathological analyses. A–D: representative images of immunofluorescence analysis of MBH sections obtained from mice fed chow (A and C) or HFD for 21 wk (B and D). A and B: glial fibrillary acidic protein (GFAP) immunoreactivity (red) marks astrocyte cell bodies and processes. C and D: ionized calcium binding adaptor molecule 1 immunoreactivity (green) identifies microglial cells and their processes. E: mean arcuate nucleus GFAP staining intensity (determined by densitometry) was higher in HFD- compared with chow-fed mice (t₁₄ = 2.68). F: T₂ relaxation time in the right MBH was positively correlated with mean GFAP staining intensity from the right ARC in HFD-fed mice. G: bilateral ARC microglial number was higher in HFD- compared with chow-fed mice (t₁₄ = 2.58). Representative regions of interest used for quantification of astrocyte and microglial cells within the ARC of the MBH are indicated by solid lines in A–D. Scale bar in A represents 50 μM. 3V, 3rd ventricle. *P = 0.02.