Summary Neurons expressing agouti-related protein (AgRP) are essential for feeding. The majority of these neurons are located outside the blood-brain barrier (BBB), allowing them to directly sense circulating metabolic factors. Here we show that in adult mice, AgRP neurons outside the BBB (AgRPOBBB) were rapidly ablated by peripheral administration of monosodium glutamate (MSG), whereas AgRP neurons inside the BBB and most proopiomelanocortin (POMC) neurons were spared. MSG treatment induced proliferation of tanycytes, the putative hypothalamic neural progenitor cells, but the newly proliferated tanycytes did not become neurons. Intriguingly, AgRPOBBB neuronal number increased within a week after MSG treatment, and newly emerging AgRP neurons were derived from post-mitotic cells including some from the Pomc-expressing cell lineage. Our study reveals that the lack of protection by the BBB renders AgRPOBBB vulnerable to lesioning by circulating toxins, but that the rapid re-emergence of AgRPOBBB is part of a reparative process to maintain energy balance.

BackgroundThe proper establishment of hypothalamic feeding circuits during early development has a profound influence on energy homeostasis, and perturbing this process could predispose individuals to obesity and its associated consequences later in life. The maturation of hypothalamic neuronal circuitry in rodents takes place during the initial postnatal weeks, and this coincides with a dramatic surge in the circulating level of leptin, which is known to regulate the outgrowth of key neuronal projections in the maturing hypothalamus. Coincidently, this early postnatal period also marks the rapid proliferation and expansion of astrocytes in the brain.MethodsHere we examined the effects of leptin on the proliferative capacity of astrocytes in the developing hypothalamus by treating postnatal mice with leptin. Mutant mice were also generated to conditionally remove leptin receptors from glial fibrillary acidic protein (GFAP)-expressing cells in the postnatal period.Results and conclusionsWe show that GFAP-expressing cells in the periventricular zone of the 3rd ventricle were responsive to leptin during the initial postnatal week. Leptin enhanced the proliferation of astrocytes in the postnatal hypothalamus and conditional removal of leptin receptors from GFAP-expressing cells during early postnatal period limited astrocyte proliferation. While increasing evidence demonstrates a direct role of leptin in regulating astrocytes in the adult brain, and given the essential function of astrocytes in modulating neuronal function and connectivity, our study indicates that leptin may exert its metabolic effects, in part, by promoting hypothalamic astrogenesis during early postnatal development.

Background: The autonomic regulation of hepatic metabolism offers a novel target for the treatment of non-alcoholic fatty liver disease (NAFLD). However, the molecular characteristics of neurons that regulate the brain-liver axis remain unclear. Since mice lacking neuronal lipoprotein lipase (LPL) develop perturbations in neuronal lipid-sensing and systemic energy balance, we reasoned that LPL may be a component of pre-autonomic neurons involved in the regulation of hepatic metabolism. Methods: Measures of glucose homeostasis in mice homozygous (NEXLPL-/-) and heterozygous (NEXLPL+/-) for neuronal LPL deficiency were compared to that of WT mice. A detailed analysis of hepatic glucose and lipid metabolism was also determined in NEXLPL+/- at 6-18 mo. To determine the effect of neuronal LPL deficiency on neuronal physiology, liver-related neurons were identified in the paraventricular nucleus (PVN) of the hypothalamus using the transsynaptic retrograde tracer PRV-152. In addition, we used Fluorescence Lifetime Imaging Microscopy (FLIM) as a novel method to visualize changes in neuronal metabolism following LPL-depletion directly in the PVN. Results: Here we show that despite obesity, mice with reduced neuronal LPL also show improved glucose tolerance and reduced hepatic lipid accumulation with aging, concomitant with reduced hepatic lipogenic gene expression (e.g. SCD1 and FADS2). Retroviral tracing and patch clamp studies revealed reduced inhibitory post-synaptic currents in liver-related neurons lacking LPL. Quantification of the free versus bound Nicotinamide Adenine Dinucleotide (NADH) and Flavin Adenine Dinucleotide (FAD), revealed that LPL loss resulted in altered substrate utilization characterized by increased glucose utilization and TCA cycle flux. These findings were recapitulated by analysis of global metabolites from hypothalamic cell lines either deficient in, or over-expressing, LPL. Conclusions: Our data suggest that LPL is a novel feature of liver–related preautonomic neurons in the PVN. Moreover, LPL loss is sufficient to alter neuronal metabolism and function, leading to changes in systemic glucose metabolism including improved hepatic function with age.

The autonomic regulation of hepatic metabolism offers a novel target for the treatment of non-alcoholic fatty liver disease (NAFLD). However, the molecular characteristics of neurons that regulate the brain-liver axis remain unclear. Since mice lacking neuronal lipoprotein lipase (LPL) develop perturbations in neuronal lipid-sensing and systemic energy balance, we reasoned that LPL might be a component of pre-autonomic neurons involved in the regulation of hepatic metabolism. Here, we show that, despite obesity, mice with reduced neuronal LPL (NEXCreLPLflox (LPL KD)) show improved glucose tolerance and reduced hepatic lipid accumulation with aging compared to wilt type (WT) controls (LPLflox). To determine the effect of LPL deficiency on neuronal physiology, liver-related neurons were identified in the paraventricular nucleus (PVN) of the hypothalamus using the transsynaptic retrograde tracer PRV-152. Patch-clamp studies revealed reduced inhibitory post-synaptic currents in liver-related neurons of LPL KD mice. Fluorescence lifetime imaging microscopy (FLIM) was used to visualize metabolic changes in LPL-depleted neurons. Quantification of free vs. bound nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) revealed increased glucose utilization and TCA cycle flux in LPL-depleted neurons compared to controls. Global metabolomics from hypothalamic cell lines either deficient in or over-expressing LPL recapitulated these findings. Our data suggest that LPL is a novel feature of liver-related preautonomic neurons in the PVN. Moreover, LPL loss is sufficient to cause changes in neuronal substrate utilization and function, which may precede changes in hepatic metabolism.