Hippocampal lipoprotein lipase regulates energy balance in rodents

Abstract

Brain lipid sensing is necessary to regulate energy balance. Lipoprotein lipase (LPL) may play a role in this process. We tested if hippocampal LPL regulated energy homeostasis in rodents by specifically attenuating LPL activity in the hippocampus of rats and mice, either by infusing a pharmacological inhibitor (tyloxapol), or using a genetic approach (adeno-associated virus expressing Cre-GFP injected into Lpl (lox/lox) mice). Decreased LPL activity by either method led to increased body weight gain due to decreased locomotor activity and energy expenditure, concomitant with increased parasympathetic tone (unchanged food intake). Decreased LPL activity in both models was associated with increased de novo ceramide synthesis and neurogenesis in the hippocampus, while intrahippocampal infusion of de novo ceramide synthesis inhibitor myriocin completely prevented body weight gain. We conclude that hippocampal lipid sensing might represent a core mechanism for energy homeostasis regulation through de novo ceramide synthesis.

Keywords: AAV, adeno-associated virus; ANS, autonomic nervous system; CERS, ceramide synthase; CNS, central nervous system; Ceramides; Energy expenditure; GFP, green fluorescent protein; LPL, lipoprotein lipase; Lipid sensing; Obesity; Parasympathetic nervous system; RQ, respiratory quotient; SMPD1, acid sphingomyelin phosphodiesterase 1; SPHK1, sphingosine kinase 1; SPT, serine palmitoyltransferase; TG, triglycerides.

Graphical abstract

Figure 1

Decreased LPL activity in the hippocampus increases body weight gain in mice and rats without affecting food intake. (A) LPL activity in the hippocampus, hypothalamus and cortex of rats infused with vehicle or tyloxapol for 28 days, through an osmotic minipump connected to a cannula implanted in hippocampus; n≥6. FFA: free fatty acids. (B) LPL activity in the hippocampus and cortex of mice with a specific deletion of the Lpl gene in the hippocampus after AAV Cre-GFP injection (LPL Hip−/− mice) and their control littermates (LPL Hip+/+); n≥3. Inset in (B): Cre-GFP fusion protein fluorescence in right hippocampus (coordinates X: +1 mm; Y: −2.06 mm; Z: −1.55 mm, 20× magnification) 4 weeks after injection shows specific expression of the protein within hippocampus with high fluorescence in CA1 pyramidal layer. (C and D) Body weight gain after the beginning of intrahippocampal tyloxapol infusion in rats (C) or AAV Cre-GFP injection in Lpl^lox/lox mice (D); n≥6. (E and F) Time course of food intake reported to body weight in rats (E) and mice (F) during the 28 days of treatment; n≥6 for both rats and mice.

Figure 2

Decreased LPL activity is associated with increased hippocampal ceramide content through the activation of the de novo synthesis pathway. (A) Ceramide synthesis pathways: the de novo synthesis pathway that is inhibited by myriocin (left) and the sphingomyelin hydrolysis pathway (right). (B) Changes in gene expression of enzymes involved in ceramide synthesis pathways in tyloxapol-treated rats vs. controls; n≥5. (C and D) Hippocampal ceramide content in tyloxapol-treated and control rats co-infused with myriocin or vehicle (C), and hippocampal ceramide content in LPL Hip+/+ and LPL Hip−/− mice with or without myriocin treatment (D); n≥6 in rats and n≥3 in mice. (E and F) LPL activity in rats and mice in the four groups above; n≥6 and n≥3, respectively. *: p<0.05, **: p<0.01 vs. controls.

Figure 3

Myriocin infusion into the hippocampus completely prevents increased body weight gain in both models. (A and B) Body weight gain over 28 days in tyloxapol-treated and control rats with or without co-infusion with myriocin (A) and LPL Hip+/+ and LPL Hip−/− mice with or without myriocin treatment (B); n≥6 each. (A and D) Time course of food intake reported to body weight in rats (E) and mice (F) during the 28 days of treatment; n≥6 for both rats and mice. (E and F) Energy expenditure in tyloxapol-treated and control rats with or without co-infusion with myriocin (C) and LPL Hip+/+ and LPL Hip−/− mice with or without myriocin treatment (D); n≥6 each. (G and H) Locomotor activity in the 4 groups of rats (E) and mice (F); n≥6 for both. *: p<0.05, ***: p<0.001; tyloxapol or LPL Hip+/+ vs. controls. ##: p<0.01; LPL Hip−/− myriocin vs. LPL Hip+/+.

Figure 4

Increased body weight gain is related to increased parasympathetic nervous system activity and is prevented by myriocin treatment. (A and B) Parasympathetic nervous system activity recorded in tyloxapol-treated and control rats with or without co-infusion with myriocin (A) and LPL Hip+/+ and LPL Hip−/− mice with or without myriocin treatment (B); n≥4 each. (C and D) Quantification of parasympathetic nervous system activity in the 4 groups of rats (C) and mice (D); n≥4 each. *: p<0.05, ***: p<0.001 vs. controls.