Leptin-receptor-expressing neurons in the dorsomedial hypothalamus and median preoptic area regulate sympathetic brown adipose tissue circuits.

Brown adipose tissue (BAT) thermogenesis is critical to maintain homoeothermia and is centrally controlled via sympathetic outputs. Body temperature and BAT activity also impact energy expenditure, and obesity is commonly associated with decreased BAT capacity and sympathetic tone. Severely obese mice that lack leptin or its receptor (LepRb) show decreased BAT capacity, sympathetic tone, and body temperature and thus are unable to adapt to acute cold exposure (Trayhurn et al., 1976). LepRb-expressing neurons are found in several hypothalamic sites, including the dorsomedial hypothalamus (DMH) and median preoptic area (mPOA), both critical sites to regulate sympathetic, thermoregulatory BAT circuits. Specifically, a subpopulation in the DMH/dorsal hypothalamic area (DHA) is stimulated by fever-inducing endotoxins or cold exposure (Dimicco and Zaretsky, 2007; Morrison et al., 2008). Using the retrograde, transsynaptic tracer pseudorabies virus (PRV) injected into the BAT of mice, we identified PRV-labeled LepRb neurons in the DMH/DHA and mPOA (and other sites), thus indicating their involvement in the regulation of sympathetic BAT circuits. Indeed, acute cold exposure induced c-Fos (as a surrogate for neuronal activity) in DMH/DHA LepRb neurons, and a large number of mPOA LepRb neurons project to the DMH/DHA. Furthermore, DMH/DHA LepRb neurons (and a subpopulation of LepRb mPOA neurons) project and synaptically couple to rostral raphe pallidus neurons, consistent with the current understanding of BAT thermoregulatory circuits from the DMH/DHA and mPOA (Dimicco and Zaretsky, 2007; Morrison et al., 2008). Thus, these data present strong evidence that LepRb neurons in the DMH/DHA and mPOA mediate thermoregulatory leptin action.

Critical roles for the TSC-mTOR pathway in ß-cell function.

TSC1 is a tumor suppressor that associates with TSC2 to inactivate Rheb, thereby inhibiting signaling by the mammalian target of rapamycin (mTOR) complex 1 (mTORC1). mTORC1 stimulates cell growth by promoting anabolic cellular processes, such as translation, in response to growth factors and nutrient signals. To test roles for TSC1 and mTORC1 in ß-cell function, we utilized Rip2/Cre to generate mice lacking Tsc1 in pancreatic ß-cells (Rip-Tsc1cKO mice). Although obesity developed due to hypothalamic Tsc1 excision in older Rip-Tsc1cKO animals, young animals displayed a prominent gain-of-function ß-cell phenotype prior to the onset of obesity. The young Rip-Tsc1cKO animals displayed improved glycemic control due to mTOR-mediated enhancement of ß-cell size, mass, and insulin production but not determinants of ß-cell number (proliferation and apoptosis), consistent with an important anabolic role for mTOR in ß-cell function. Furthermore, mTOR mediated these effects in the face of impaired Akt signaling in ß-cells. Thus, mTOR promulgates a dominant signal to promote ß-cell/islet size and insulin production, and this pathway is crucial for ß-cell function and glycemic control.

Critical role for hypothalamic mTOR activity in energy balance.

The mammalian target of rapamycin (mTOR) promotes anabolic cellular processes in response to growth factors and metabolic cues. The TSC1 and TSC2 tumor suppressors are major upstream inhibitory regulators of mTOR signaling. Mice with Rip2/Cre-mediated deletion of Tsc1 (Rip-Tsc1cKO mice) developed hyperphagia and obesity, suggesting that hypothalamic disruption (for which Rip2/Cre is well known) of Tsc1 may dysregulate feeding circuits via mTOR activation. Indeed, Rip-Tsc1cKO mice displayed increased mTOR signaling and enlarged neuron cell size in a number of hypothalamic populations, including Pomc neurons. Furthermore, Tsc1 deletion with Pomc/Cre (Pomc-Tsc1cKO mice) resulted in dysregulation of Pomc neurons and hyperphagic obesity. Treatment with the mTOR inhibitor, rapamycin, ameliorated the hyperphagia, obesity, and the altered Pomc neuronal morphology in developing or adult Pomc-Tsc1cKO mice, and cessation of treatment reinstated these phenotypes. Thus, ongoing mTOR activation in Pomc neurons blocks the catabolic function of these neurons to promote nutrient intake and increased adiposity.