Hypothalamic lipid signaling in metabolism regulation

Our recent studies have demonstrated that mitochondrial fission is tightly connected to lipid metabolism and more specifically, that mitochondrial fission is an inherent element in oxidation of long chain fatty acids by the orexigenic AgRP neurons (Jin et al., 2021). Intriguingly, while mitochondrial fission is associated with lipid catabolic processes, mitochondrial fusion is associated with lipid anabolism. More specifically, mitofusin 2, critical mitochondrial fusion protein, plays a critical role in the formation of the endoplasmic reticulum (ER)-mitochondria contact sites, relevant sites of lipid metabolism where intact fatty acids are used as precursors for the generation, for example, of sphingolipids. Besides mitofusin 2, other proteins have been shown to regulate the ER- mitochondria interaction. Among those, the Neurite OutGrowth inhibitor (Nogo), a member of the reticulon family of proteins (Reticulon 4 gene; Rtn4) located on the ER, plays also a critical role in regulating sphingolipids production (Cantalupo et al., 2015). Among sphingolipids, Sphingosine-1-phosphate (S1P) has been shown to play a crucial role in a large number of physiological processes including most recently feeding behavior via its action in the hypothalamus. However, the specific site of synthesis of S1P and its target within the hypothalamus have not been identified. Our preliminary data have shown that AgRP neurons are enriched of enzymes involved in the S1P de novo biosynthesis and their mRNA levels are regulated by the metabolic state, with fasting upregulating Nogo mRNA levels while downregulating all the enzymes involved in the synthesis of S1P. In line with this, we observed that S1P levels in the arcuate nucleus are downregulated during food deprivation. As the multitude of different S1P-mediated actions is linked to its capacity to be secreted, we found that S1P receptors are expressed in the neighboring anorexigenic POMC neurons where S1P significantly induced their activation by in vivo calcium imaging. Interestingly, we also observed that the expression of Nogo and several of the enzymes involved in the S1P de novo synthesis, together with S1P levels, are altered in diet-induced obesity (DIO). Altogether our data gave impetus to the central hypothesis that Nogo is a critical regulator of AgRP neuronal function and feeding behavior by regulating fatty acid metabolic pathways (catabolism versus anabolism) and that dysregulation of fatty acid metabolism during high fat diet (HFD) plays a role in DIO. Specifically, we hypothesize that when activated during fasting in AgRP neurons, Nogo by inhibiting S1P de novo biosynthesis will direct fatty acids to oxidation by the mitochondria (catabolic pathway) thus, activating AgRP neurons and inducing feeding behavior (Aim 1). On the other hand, Nogo downregulation in AgRP neurons during fed state will disinhibit S1P de novo biosynthesis (thus promoting the anabolic pathway), and by acting via its receptors, S1P will affect AgRP target neurons resulting in decreased food intake (Aim 2). Finally, dysregulation of this pathway and the resulting imbalance in sphingolipid metabolism (increased ceramides production but decreased S1P generation) during HFD plays a role in DIO (Aim 3).