Neuronal Lipoprotein Lipase Deficiency alters Neuronal Function and Hepatic Metabolism

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.