Expression of peroxisome proliferator-activated receptor-gamma in key neuronal subsets regulating glucose metabolism and energy homeostasis

Abstract

In addition to increasing insulin sensitivity and adipogenesis, peroxisome proliferator-activated receptor (PPAR)-gamma agonists cause weight gain and hyperphagia. Given the central role of the brain in the control of energy homeostasis, we sought to determine whether PPARgamma is expressed in key brain areas involved in metabolic regulation. Using immunohistochemistry, PPARgamma distribution and its colocalization with neuron-specific protein markers were investigated in rat and mouse brain sections spanning the hypothalamus, the ventral tegmental area, and the nucleus tractus solitarius. In several brain areas, nuclear PPARgamma immunoreactivity was detected in cells that costained for neuronal nuclei, a neuronal marker. In the hypothalamus, PPARgamma immunoreactivity was observed in a majority of neurons in the arcuate (including both agouti related protein and alpha-MSH containing cells) and ventromedial hypothalamic nuclei and was also present in the hypothalamic paraventricular nucleus, the lateral hypothalamic area, and tyrosine hydroxylase-containing neurons in the ventral tegmental area but was not expressed in the nucleus tractus solitarius. To validate and extend these histochemical findings, we generated mice with neuron-specific PPARgamma deletion using nestin cre-LoxP technology. Compared with littermate controls, neuron-specific PPARgamma knockout mice exhibited dramatic reductions of both hypothalamic PPARgamma mRNA levels and PPARgamma immunoreactivity but showed no differences in food intake or body weight over a 4-wk study period. We conclude that: 1) PPARgamma mRNA and protein are expressed in the hypothalamus, 2) neurons are the predominant source of PPARgamma in the central nervous system, although it is likely expressed by nonneuronal cell types as well, and 3) arcuate nucleus neurons that control energy homeostasis and glucose metabolism are among those in which PPARgamma is expressed.

Figure 1

Expression of PPARγ in rat brain areas involved in the control of energy homeostasis and insulin sensitivity. Hypothalamic distribution of PPARγ immunostaining in the hypothalamus using the Upstate antibody (A), including the ARC and VMN (B), PVN (C), LHA (A), and VTA (D). Colocalization of PPARγ immunoreactivity (green) with DAPI labeling (blue) was performed to visualize PPARγ nuclear distribution (blue-green) (E). Costaining of PPARγ (green) with NeuN (red) (F) shows that in the ARC and VMN, most neurons express PPARγ, whereas in the thalamus, PPARγ is absent from most neurons. 3v, Third ventricle; MBH, mediobasal hypothalamus.

Figure 2

Expression of PPARγ in distinct neuronal subsets. Detection of PPARγ (green) in ARC neurons containing either (A) AgRP (red) or (B) αMSH (red) (bottom panels taken at higher magnification), as indicated by arrows using the Upstate antibody. PPARγ (green) was also colocalized with TH (red) in VTA neurons (C). 3v, Third ventricle.

Figure 3

Generation, validation, and body weight of neuron-specific PPARγ knockout mice. A, PCR analysis of genomic DNA from brain revealing the presence of floxed, wild-type (WT), deleted, and cre alleles. B, Expression of PPARγ mRNA in whole brain, liver (Liv), and white adipose tissue (WAT) and in the hypothalamus of NesWT, NesKO, and NesHet mice measured by quantitative RT-PCR (C). D, PPARγ immunoreactivity in coronal hypothalamic sections sampled from NesWT and NesKO mice using the sc-7196 antibody. E, Average body weight and daily food intake of NesWT and NesKO mice over a 4-wk study period. Values are expressed as ± sem. *, P < 0.01 for NesWT vs. NesKO by one-way ANOVA with Tukey post hoc analysis.