Numerous studies over the last decades have established the VMH as an important satiety “center” 6, 36, 37
. The signals controlling VMH neuron activity are increasingly well defined, partly through the use of novel techniques including those of targeted transgenesis and cell-type specific knockout mice 1, 4, 6, 8
. Thus, abrogation of leptin signaling in SF-1-positive VMH neurons causes obesity and VMH-restricted alterations of BDNF affect energy homeostasis, supporting the notion that activation of glutamatergic SF-1 VMH neurons is critical for the suppression of feeding 5–7, 37
. Finally, insulin and also nutrients such as glucose have been previously demonstrated to control VMH neuronal activity, however, the physiological importance of this regulation remained unclear as of now. The present study establishes that deletion of the insulin receptor gene and subsequent inactivation of insulin-stimulated signaling provides partial protection from high-fat diet-induced hyperphagia, weight gain and obesity.
From a mechanistic point of view, our study identifies insulin as a crucial regulator of neuronal excitability, via PI3k-dependent activation of KATP
channels, as insulin’s ability to inhibit firing of SF-1-positive VMH neurons is abrogated by tolbutamide. The majority of insulin-responsive neurons are clustered in an area in the mediobasal VMH. Interestingly, the same area provides glutamatergic innervations to anorexigenic POMC neurons in the ARC as revealed by photoactivated, glutamate-uncaging mapping 35
. Thus, we propose, that overactivated insulin signaling in the VMH as present under high-fat diet conditions inhibits glutamatergic projections to POMC neurons. This notion is consistent with the observed increase of POMC cell firing in SF-1ΔIR
-mice, when the mice are exposed to HFD.
Interestingly, under normal chow diet, insulin-dependent silencing of these cells appears not to occur because SF-1ΔIR
-mice are phenotypically indistinguishable from controls under these conditions. Only when insulin levels rise, as upon high-fat feeding, insulin action appears to reach a threshold for profound PI3k activation, subsequent KATP
channel activation and ultimately neuronal silencing. This notion is directly supported by two findings of the present study. First, high-fat feeding results in increased PIP3
formation, and second this increase can be significantly attenuated by abrogation of IR signaling in VMH neurons. These experiments reveal that there appears to exist a relatively high threshold for PI3k-dependent KATP
channel activation. This is consistent with our previous findings in POMC neurons where profound PI3k activation - either following insulin stimulation or via genetic ablation of the PIP3
phosphatase PTEN - activate KATP
channels to cause neuronal silencing. On the other hand, leptin stimulates PI3k signaling to much lesser extent and thus fails to activate KATP
channels but rather promotes POMC cell firing via activation of unspecific cation channels19, 38, 39
. The differential threshold for insulin’s and leptin’s ability to activate apparently also holds true for VMH neurons, as insulin silences these cells, while leptin promotes firing in the majority of SF-1 VMH neurons6
; it has also been demonstrated that PI3k signaling in SF-1 cells is critical for leptin’s ability to increase firing 40
Another important finding of the present study is that while high-fat feeding has been demonstrated to cause insulin resistance in the arcuate nucleus of the hypothalamus41
, increased insulin action in the VMH upon high-fat feeding indicates that regional differences appear within the hypothalamus ranging from resistance to enhanced basal signaling under conditions of diet-induced obesity. This novel finding of differentially regulated insulin sensitivity within the hypothalamus may stem from differences in insulin accessibility of the neurons in these different locations. While the blood brain barrier in the ARC is highly permeable42
, POMC and AgRP neurons may sense elevated circulating insulin concentrations, which lead to signal desensitization. Indeed, it could be demonstrated in cultured POMC neurons, that chronic insulin stimulation results in signal attenuation via activation of proteasome-mediated IRS degradation42, 43
. On the other hand, VMH neurons may be exposed to elevated, but relatively lower insulin concentrations than ARC neurons leading to increased tonic insulin signaling. Alternatively, we and others have recently demonstrated that fatty acids can cause leptin and insulin resistance in ARC neurons via TLR-dependent activation of inflammatory signaling26
. Differential accessibility of VMH neurons to circulating fatty acids may prevent that insulin resistance in response to high-fat feeding occurs in this anatomical localization. Whatever the molecular basis for this phenomenon is, our experiments reveal an intriguing concept, that differential, nucleus-specific dysregulation of insulin action upon high-fat feeding, i.e. insulin resistance in POMC neurons and overactivation of insulin action in the VMH can cooperate to cause obesity.
Insulin stimulation enhances production of PIP3
, which will subsequently bind to and open KATP
channels, resulting in neuronal silencing. However, in POMC neurons leptin- and insulin-stimulated PI3k activation is also required for POMC transcription and neuropeptide synthesis in a FOXO1-dependent manner 19, 21, 44
. In contrast to this, mice lacking the principle PIP3
-activated downstream kinase, PDK1, in SF-1 cells, were neither protected nor more sensitive to diet-induced obesity and hyperglycemia (BFB and JCB, unpublished observations), indicating that in SF-1 neurons insulin primarily regulates neuronal activity and thus neurotransmitter/neuropeptide release from these cells.
Taken together, using multiple complementary mouse models, we demonstrate that high-fat feeding-induced hyperinsulinemia leads to cell autonomous hyperpolarization of SF-1 neurons, leading to functional changes in the synaptic input onto POMC neurons, causing obesity and glucose intolerance.