Liver X Receptor as a Possible Drug Target for Blood-Brain Barrier Integrity

Cholesterol metabolism: physiological versus pathological aspects in intracerebral hemorrhage

Cholesterol is an important component of plasma membranes and participates in many basic life functions, such as the maintenance of cell membrane stability, the synthesis of steroid hormones, and myelination. Cholesterol plays a key role in the establishment and maintenance of the central nervous system. The brain contains 20% of the whole body's cholesterol, 80% of which is located within myelin. A huge number of processes (e.g., the sterol regulatory element-binding protein pathway and liver X receptor pathway) participate in the regulation of cholesterol metabolism in the brain via mechanisms that include cholesterol biosynthesis, intracellular transport, and efflux. Certain brain injuries or diseases involving crosstalk among the processes above can affect normal cholesterol metabolism to induce detrimental consequences. Therefore, we hypothesized that cholesterol-related molecules and pathways can serve as therapeutic targets for central nervous system diseases. Intracerebral hemorrhage is the most severe hemorrhagic stroke subtype, with high mortality and morbidity. Historical cholesterol levels are associated with the risk of intracerebral hemorrhage. Moreover, secondary pathological changes after intracerebral hemorrhage are associated with cholesterol metabolism dysregulation, such as neuroinflammation, demyelination, and multiple types of programmed cell death. Intracellular cholesterol accumulation in the brain has been found after intracerebral hemorrhage. In this paper, we review normal cholesterol metabolism in the central nervous system, the mechanisms known to participate in the disturbance of cholesterol metabolism after intracerebral hemorrhage, and the links between cholesterol metabolism and cell death. We also review several possible and constructive therapeutic targets identified based on cholesterol metabolism to provide cholesterol-based perspectives and a reference for those interested in the treatment of intracerebral hemorrhage.

LXR agonism for CNS diseases: promises and challenges

The unfavorable prognosis of many neurological conditions could be attributed to limited tissue regeneration in central nervous system (CNS) and overwhelming inflammation, while liver X receptor (LXR) may regulate both processes due to its pivotal role in cholesterol metabolism and inflammatory response, and thus receives increasing attentions from neuroscientists and clinicians. Here, we summarize the signal transduction of LXR pathway, discuss the therapeutic potentials of LXR agonists based on preclinical data using different disease models, and analyze the dilemma and possible resolutions for clinical translation to encourage further investigations of LXR related therapies in CNS disorders.

PSD-95 inhibitor Tat-NR2B9c (NA-1) protects the integrity of the blood-brain barrier after transient middle artery occlusion in rats by downregulating matrix metalloprotease-9 and upregulating endothelial nitric oxide synthase

BACKGROUND

Protection against ischemic stroke may be most effective when multiple components of the neurovascular unit are protected, yet current treatments target mainly neurons. Here we explored whether the PSD-95 inhibitor Tat-NR2B9c (NA-1) can protect not only neurons but also the blood-brain barrier.

METHODS

Adult male Sprague-Dawley rats were randomly divided into three groups, which were subjected to either sham surgery or transient cerebral ischemia-reperfusion, after which some animals were treated with Tat-NR2B9c. The therapeutic efficacy of Tat-NR2B9c was assessed in terms of the degree of neurological deficit and cerebral infarction, integrity of the blood-brain barrier, cerebral water content, as well as expression of PSD-95, nitric oxide synthase, and matrix metalloprotease-9.

RESULTS

Tat-NR2B9c (NA-1) ameliorated neurofunctional deficit, reduced cerebral infarction, mitigated blood-brain barrier injury and improved its integrity following ischemia-reperfusion, leading to less cerebral edema. These improvements were associated with upregulation of tight junction proteins in the blood-brain barrier. At the same time, Tat-NR2B9c (NA-1) downregulated neuronal nitric oxide synthase and matrix metalloprotease-9, while reversing the ischemia-induced downregulation of endothelial nitric oxide synthase in brain. We report here the first evidence that PSD-95 is expressed in vascular endothelial cells in the brain.

CONCLUSION

Our experiments in a rat model of transient occlusion of the middle cerebral artery suggest that Tat-NR2B9c (NA-1) can mitigate ischemic injury to the blood-brain barrier, and that it may do so by downregulating matrix metalloprotease-9 and upregulating endothelial nitric oxide synthase.

24-Hydroxycholesterol Moderates the Effects of Amyloid-β on Expression of HMG-CoA Reductase and ABCA1 Proteins in Mouse Astrocytes

Background

Elevated brain cholesterol increases the risk of Alzheimer's disease. Production of 24-hydroxycholesterol (24s-OHC) by neurons prevents cholesterol accumulation in the brain. In this study, we investigated the effect of 24s-OHC on the HMG-COA reductase and ABCA1 which are involved in the brain cholesterol homeostasis with or without β-amyloid in astrocytes.

Methods and Materials

Astrocytes were treated with 24s-OHC with or without Aβ. Western blot and real-time polymerase chain reaction were done to detect protein and gene expression of β-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGCR) and ABCA1, respectively. Cholesterol release was determined using a quantitation kit.

Results

Protein levels of HMGCR and ABCA1 were significantly increased by Aβ; however, the 24s-OHC was able to restore their levels and diminish the effect of amyloid-β. Aβ did not have a significant effect on HMGCR expression, while 24s-OHC reduced it by 68%. Aβ-induced ABCA1 expression did not increase cholesterol efflux as the lower levels of cholesterol in conditioned medium of Aβ-treated cells were found.

Conclusion

Our novel findings show that Aβ affects two key elements in the brain cholesterol homeostasis, HMGCR and ABCA1, which are crucial in cholesterol synthesis and efflux. Since 24s-OHC could suppress the Aβ effects on enhancement of HMGCR and ABCA1, therefore the cytochrome P450 46A1 (Cyp46A1), which is exclusively expressed in the central nervous system and responsible for producing of 24s-OHC, could consider as a therapeutic target in the cholesterol-related neurodegenerative diseases such as Alzheimer's disease.