It has been well-established that in the evolution of obesity and diabetes, hypothalamic weight- and glucose-regulatory neurons become insensitive to signals (including hormones such as leptin or insulin) that work to inhibit incremental weight gain and increased circulating glucose. However, the specific mechanisms of these insensitivities are not well-understood. We postulate that hypothalamic accumulation of the sphingolipid ceramide can drive attenuation of hypothalamic leptin and insulin signaling. Ceramide accumulation has been demonstrated to drive insulin resistance in a number of peripheral tissues, and some evidence suggests it may antagonize central leptin/insulin receptor signaling also. One set of neurons within the hypothalamus known to regulate both body weight and circulating glucose and that become insensitive to leptin/insulin in the obese state are characterized by expression of proopiomelanocortin (POMC). We hypothesize that POMC neurons accumulate ceramides in the obese state, and these ceramides drive resistance of these cells to leptin/insulin signal transduction. To test this, we will inducibly overexpress an enzyme which degrades ceramides, acid ceramidase, in POMC neurons of adult mice. The construct used will be a tetracycline response element- driven acid ceramidase, which when mated to a mouse containing both flox-stop-flox-Rosa26-rtTA and POMC- Cre cassettes, will produce acid ceramidase in the POMC neuron upon administration of doxycycline in the chow. This will be complemented with similar mice which force ceramide glucosylation or knockout mice which prevent ceramide glucosylation, as glucosylceramide derivatives may have independent effects on glucose and energy homeostasis. We will use these mice to interrogate whether ceramides have a causative role in the pathogenesis of obesity, whether reducing ceramides in POMC neurons can improve glucose homeostasis independent of weight change, and the molecular pathways which ceramides modulate within the POMC neuron. This work will thus demonstrate a possible novel drug target in the brain to decrease both body weight gain and blood glucose in obese or diabetic individuals, respectively.
The rates of obesity in the United States have exploded in the last few decades, and it remains unclear how the brain becomes less responsive to nutritional and hormonal signals that regulate blood sugar and body weight. We hypothesize that a saturated fat derived metabolite, a sphingolipid termed ceramide, drives impairs hormonal signaling in the hypothalamus, and that ceramide lowering agents might be a powerful new tool in the fight against obesity and diabetes.
