Lipoprotein lipase in the brain and nervous system

Deficiency of Lipoprotein Lipase in Neurons Decreases AMPA Receptor Phosphorylation and Leads to Neurobehavioral Abnormalities in Mice

Alterations in lipid metabolism have been found in several neurodegenerative disorders, including Alzheimer’s disease. Lipoprotein lipase (LPL) hydrolyzes triacylglycerides in lipoproteins and regulates lipid metabolism in multiple organs and tissues, including the central nervous system (CNS). Though many brain regions express LPL, the functions of this lipase in the CNS remain largely unknown. We developed mice with neuron-specific LPL deficiency that became obese on chow by 16 wks in homozygous mutant mice (NEXLPL-/-) and 10 mo in heterozygous mice (NEXLPL+/-). In the present study, we show that 21 mo NEXLPL+/- mice display substantial cognitive function decline including poorer learning and memory, and increased anxiety with no difference in general motor activities and exploratory behavior. These neurobehavioral abnormalities are associated with a reduction in the 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl) propanoic acid (AMPA) receptor subunit GluA1 and its phosphorylation, without any alterations in amyloid β accumulation. Importantly, a marked deficit in omega-3 and omega-6 polyunsaturated fatty acids (PUFA) in the hippocampus precedes the development of the neurobehavioral phenotype of NEXLPL+/- mice. And, a diet supplemented with n-3 PUFA can improve the learning and memory of NEXLPL+/- mice at both 10 mo and 21 mo of age. We interpret these findings to indicate that LPL regulates the availability of PUFA in the CNS and, this in turn, impacts the strength of synaptic plasticity in the brain of aging mice through the modification of AMPA receptor and its phosphorylation.

Lipoprotein lipase is an important modulator of lipid uptake and storage in hypothalamic neurons

LPL is the rate-limiting enzyme for uptake of TG-derived FFA in peripheral tissues, and the enzyme is expressed in the brain and CNS. We previously created a mouse which lacks neuronal LPL. This animal becomes obese on a standard chow, and we observed reduced lipid uptake in the hypothalamus at 3 months preceding obesity. In our present study, we replicated the animal phenotype in an immortalized mouse hypothalamic cell line (N41) to examine how LPL affects expression of AgRP as well as entry and storage of lipids into neurons. We show that LPL is able to modulate levels of the orexigenic peptide AgRP. LPL also exerts effects on lipid uptake into culture neurons, and that uptake of neutral lipid can be enhanced even by mutant LPL lacking catalytic activity. N41 cells also accumulate neutral lipid in droplets, and this is at least in part regulated by LPL. These data in addition to those published in mice with neuron-specific deletion of LPL suggest that neuronal LPL is an important regulator of lipid homeostasis in neurons and that alterations in LPL levels may have important effects on systemic metabolism and neuronal lipid biology.

Differential effects of angiopoietin-like 4 in brain and muscle on regulation of lipoprotein lipase activity

OBJECTIVE

Lipoprotein lipase (LPL) is a key regulator of circulating triglyceride rich lipoprotein hydrolysis. In brain LPL regulates appetite and energy expenditure. Angiopoietin-like 4 (Angptl4) is a secreted protein that inhibits LPL activity and, thereby, triglyceride metabolism, but the impact of Angptl4 on central lipid metabolism is unknown.

METHODS

We induced type 1 diabetes by streptozotocin (STZ) in whole-body Angptl4 knockout mice (Angptl4(-/-) ) and their wildtype littermates to study the role of Angptl4 in central lipid metabolism.

RESULTS

In type 1 (streptozotocin, STZ) and type 2 (ob/ob) diabetic mice, there is a ~2-fold increase of Angptl4 in the hypothalamus and skeletal muscle. Intracerebroventricular insulin injection into STZ mice at levels which have no effect on plasma glucose restores Angptl4 expression in hypothalamus. Isolation of cells from the brain reveals that Angptl4 is produced in glia, whereas LPL is present in both glia and neurons. Consistent with the in vivo experiment, in vitro insulin treatment of glial cells causes a 50% reduction of Angptl4 and significantly increases LPL activity with no change in LPL expression. In Angptl4(-/-) mice, LPL activity in skeletal muscle is increased 3-fold, and this is further increased by STZ-induced diabetes. By contrast, Angptl4(-/-) mice show no significant difference in LPL activity in hypothalamus or brain independent of diabetic and nutritional status.

CONCLUSION

Thus, Angptl4 in brain is produced in glia and regulated by insulin. However, in contrast to the periphery, central Angptl4 does not regulate LPL activity, but appears to participate in the metabolic crosstalk between glia and neurons.

Dietary triglycerides act on mesolimbic structures to regulate the rewarding and motivational aspects of feeding

Circulating triglycerides (TGs) normally increase after a meal but are altered in pathophysiological conditions, such as obesity. Although TG metabolism in the brain remains poorly understood, several brain structures express enzymes that process TG-enriched particles, including mesolimbic structures. For this reason, and because consumption of high-fat diet alters dopamine signaling, we tested the hypothesis that TG might directly target mesolimbic reward circuits to control reward-seeking behaviors. We found that the delivery of small amounts of TG to the brain through the carotid artery rapidly reduced both spontaneous and amphetamine-induced locomotion, abolished preference for palatable food and reduced the motivation to engage in food-seeking behavior. Conversely, targeted disruption of the TG-hydrolyzing enzyme lipoprotein lipase specifically in the nucleus accumbens increased palatable food preference and food-seeking behavior. Finally, prolonged TG perfusion resulted in a return to normal palatable food preference despite continued locomotor suppression, suggesting that adaptive mechanisms occur. These findings reveal new mechanisms by which dietary fat may alter mesolimbic circuit function and reward seeking.

Physiological and pathophysiological implications of lipid sensing in the brain

Fatty acid (FA)-sensitive neurons are present in the brain, especially the hypothalamus, and play a key role in the neural control of energy homeostasis. Through neuronal output, FA may modulate feeding behaviour as well as insulin secretion and action. Subpopulations of neurons in the ventromedial and arcuate hypothalamic nuclei are selectively either inhibited or activated by FA. Molecular effectors of these FA effects probably include chloride or potassium ion channels. While intracellular metabolism and activation of the ATP-sensitive K⁺ channel appear to be necessary for some of the signalling effects of FA, at least half of the FA responses in ventromedial hypothalamic neurons are mediated by interaction with FAT/CD36, an FA transporter/receptor that does not require intracellular metabolism to activate downstream signalling. Thus, FA or their metabolites can modulate neuronal activity as a means of directly monitoring ongoing fuel availability by brain nutrient-sensing neurons involved in the regulation of energy and glucose homeostasis. Recently, the role of lipoprotein lipase in FA sensing has also been shown in animal models not only in hypothalamus, but also in hippocampus and striatum. Finally, FA overload might impair neural control of energy homeostasis through enhanced ceramide synthesis and may contribute to obesity and/or type 2 diabetes pathogenesis in predisposed subjects.

Mice with altered brain lipoprotein metabolism display maladaptive responses to environmental challenges that may predispose to weight gain

BACKGROUND

Three-month-old neuron-specific lipoprotein lipase (LPL)-depleted mice (NEXLP(-/-)) mice are preobese and have normal body weight before developing obesity by 4.5 months. This series of experiments investigated responses to novel environment stimuli and acute sleep deprivation in preobese NEXLPL(-/-)) mice to test the hypothesis that neuron-specific LPL deletion alters normal adaptive metabolic responses to environmental challenges.

METHODS

Three-month-old, age- and weight-matched, male NEXLPL(-/-)) (n=10) and wild-type (WT) (n=10) mice were housed in individual metabolic chambers with a 12-hr dark cycle. Food and water intake, locomotor activity, and calorimetry data were recorded in 12-min intervals. Novel environmental responses were elicited by first-time introduction to chambers at dark onset, followed by acclimation, baseline recording, and 6-hr of sleep deprivation on subsequent experimental days.

RESULTS

NEXLPL(-/-)) mice displayed a 1.5-fold greater increase in activity in response to a novel environment than seen in WT controls (P=0.0308), and a two-fold greater increase in food intake following acute sleep deprivation (P=0.0117). NEXLPL(-/-)) mice averaged a 27% higher metabolic rate than WT mice throughout the experiments (P<0.0001). Body weight, composition, and temperature did not differ between murine groups throughout the experiments. Levels of free fatty acid, insulin, glucose, and triglycerides were similar between groups at the terminus.

CONCLUSIONS

A deficiency in neuronal LPL signaling disrupts normal responses to novel environmental exposure and acute sleep deprivation, a maladaptive response that may contribute to weight gain in genetically predisposed mice, and perhaps humans.

What are lipoproteins doing in the brain?

Lipoproteins in plasma transport lipids between tissues, however, only high-density lipoproteins (HDL) appear to traverse the blood-brain barrier (BBB); thus, lipoproteins found in the brain must be produced within the central nervous system. Apolipoproteins E (ApoE) and ApoJ are the most abundant apolipoproteins in the brain, are mostly synthesized by astrocytes, and are found on HDL. In the hippocampus and other brain regions, lipoproteins help to regulate neurobehavioral functions by processes that are lipoprotein receptor-mediated. Moreover, lipoproteins and their receptors also have roles in the regulation of body weight and energy balance, acting through lipoprotein lipase (LPL) and the low-density lipoprotein (LDL) receptor-related protein (LRP). Thus, understanding lipoproteins and their metabolism in the brain provides a new opportunity with potential therapeutic relevance.

Hippocampal lipoprotein lipase regulates energy balance in rodents

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.

Fatty acid transporter CD36 mediates hypothalamic effect of fatty acids on food intake in rats

Variations in plasma fatty acid (FA) concentrations are detected by FA sensing neurons in specific brain areas such as the hypothalamus. These neurons play a physiological role in the control of food intake and the regulation of hepatic glucose production. Le Foll et al. previously showed in vitro that at least 50% of the FA sensing in ventromedial hypothalamic (VMH) neurons is attributable to the interaction of long chain FA with FA translocase/CD36 (CD36). The present work assessed whether in vivo effects of hypothalamic FA sensing might be partly mediated by CD36 or intracellular events such as acylCoA synthesis or β-oxidation. To that end, a catheter was implanted in the carotid artery toward the brain in male Wistar rats. After 1 wk recovery, animals were food-deprived for 5 h, then 10 min infusions of triglyceride emulsion, Intralipid +/− heparin (IL, IL_H, respectively) or saline/heparin (S_H) were carried out and food intake was assessed over the next 5 h. Experimental groups included: 1) Rats previously injected in ventromedian nucleus (VMN) with shRNA against CD36 or scrambled RNA; 2) Etomoxir (CPT1 inhibitor) or saline co-infused with IL_H/S_H; and 3) Triacsin C (acylCoA synthase inhibitor) or saline co-infused with IL_H/S_H. IL_H significantly lowered food intake during refeeding compared to S_H (p<0.001). Five hours after refeeding, etomoxir did not affect this inhibitory effect of IL_H on food intake while VMN CD36 depletion totally prevented it. Triacsin C also prevented IL_H effects on food intake. In conclusion, the effect of FA to inhibit food intake is dependent on VMN CD36 and acylCoA synthesis but does not required FA oxidation.

Lipoprotein lipase (LPL) is associated with neurite pathology and its levels are markedly reduced in the dentate gyrus of Alzheimer's disease brains

Lipoprotein lipase (LPL) is involved in regulation of fatty acid metabolism, and facilitates cellular uptake of lipoproteins, lipids and lipid-soluble vitamins. We evaluated LPL distribution in healthy and Alzheimer’s disease (AD) brain tissue and its relative levels in cerebrospinal fluid. LPL immunostaining is widely present in different neuronal subgroups, microglia, astrocytes and oligodendroglia throughout cerebrum, cerebellum and spinal cord. LPL immunoreactivity is also present in leptomeninges, small blood vessels, choroid plexus and ependymal cells, Schwann cells associated with cranial nerves, and in anterior and posterior pituitary. In vitro studies have shown presence of secreted LPL in conditioned media of human cortical neuronal cell line (HCN2) and neuroblastoma cells (SK-N-SH), but not in media of cultured primary human astrocytes. LPL was present in cytoplasmic and nuclear fractions of neuronal cells and astrocytes in vitro. LPL immunoreactivity strongly associates with AD-related pathology, staining diffuse plaques, dystrophic and swollen neurites, possible Hirano bodies and activated glial cells. We observed no staining associated with neurofibrillary tangles or granulovacuolar degeneration. Granule cells of the dentate gyrus and the associated synaptic network showed significantly reduced staining in AD compared to control tissue. LPL was also reduced in AD CSF samples relative to those in controls.

RNA-seq characterization of spinal cord injury transcriptome in acute/subacute phases: a resource for understanding the pathology at the systems level

Spinal cord injury (SCI) is a devastating neurological disease without effective treatment. To generate a comprehensive view of the mechanisms involved in SCI pathology, we applied RNA-Sequencing (RNA-Seq) technology to characterize the temporal changes in global gene expression after contusive SCI in mice. We sequenced tissue samples from acute and subacute phases (2 days and 7 days after injury) and systematically characterized the transcriptomes with the goal of identifying pathways and genes critical in SCI pathology. The top enriched functional categories include “inflammation response,” “neurological disease,” “cell death and survival” and “nervous system development.” The top enriched pathways include LXR/RXR Activation and Atherosclerosis Signaling, etc. Furthermore, we developed a systems-based analysis framework in order to identify key determinants in the global gene networks of the acute and sub-acute phases. Some candidate genes that we identified have been shown to play important roles in SCI, which demonstrates the validity of our approach. There are also many genes whose functions in SCI have not been well studied and can be further investigated by future experiments. We have also incorporated pharmacogenomic information into our analyses. Among the genes identified, the ones with existing drug information can be readily tested in SCI animal models. Therefore, in this study we have described an example of how global gene profiling can be translated to identifying genes of interest for functional tests in the future and generating new hypotheses. Additionally, the RNA-Seq enables splicing isoform identification and the estimation of expression levels, thus providing useful information for increasing the specificity of drug design and reducing potential side effect. In summary, these results provide a valuable reference data resource for a better understanding of the SCI process in the acute and sub-acute phases.