Effects on DHEA levels by estrogen in rat astrocytes and CNS co-cultures via the regulation of CYP7B1-mediated metabolism

Neuroactive steroids fluctuate with regional specificity in the central and peripheral nervous system across the rat estrous cycle

Neuroactive steroids (i.e., sex steroid hormones and neurosteroids) are important physiological regulators of nervous function and potential neuroprotective agents for neurodegenerative and psychiatric disorders. Sex is an important component of such effects. However, even if fluctuations in sex steroid hormone level during the menstrual cycle are associated with neuropathological events in some women, the neuroactive steroid pattern in the brain across the ovarian cycle has been poorly explored. Therefore, we assessed the levels of pregnenolone, progesterone, and its metabolites (i.e., dihydroprogesterone, allopregnanolone and isoallopregnanolone), dehydroepiandrosterone, testosterone and its metabolites (i.e., dihydrotestosterone, 3α-diol and 17β-estradiol) across the rat ovarian cycle to determine whether their plasma fluctuations are similar to those occurring in the central (i.e., hippocampus and cerebral cortex) and peripheral (i.e., sciatic nerve) nervous system. Data obtained indicate that the plasma pattern of these molecules generally does not fully reflect the events occurring in the nervous system. In addition, for some neuroactive steroid levels, the pattern is not identical between the two brain regions and between the brain and peripheral nerves. Indeed, with the exception of progesterone, all other neuroactive steroids assessed here showed peculiar regional differences in their pattern of fluctuation in the nervous system during the estrous cycle. These observations may have important diagnostic and therapeutic consequences for neuropathological events influenced by the menstrual cycle.

Syncytin-1, an endogenous retroviral protein, triggers the activation of CRP via TLR3 signal cascade in glial cells

Schizophrenia is a devastating psychiatric disorder that impacts on social functioning and quality of life, and there is accumulating evidence that inflammation is a potential pathogenic mechanism of schizophrenia. However, the mechanism of inflammation possibly occurred in schizophrenia has not been well understood. The endogenous retroviral protein syncytin-1 and inflammatory marker CRP are both abnormally expressed in schizophrenia patients. CRP is one of the markers of bacterial infection generally. Less clear is whether virus or viral protein can trigger the activation of CRP. Here, we detected a robust increase of the levels of syncytin-1 and CRP in schizophrenia patients, and displayed a positive correlation and marked consistency between expressions of syncytin-1 and CRP in schizophrenia patients. Furthermore, overexpression of syncytin-1 significantly elevated the levels of CRP, TLR3, and IL-6 in both human microglia and astrocytes. TLR3 deficiency impaired the expressions of CRP and IL-6 induced by syncytin-1. Importantly, we observed a cellular co-localization and a direct interaction between syncytin-1 and TLR3. Additionally, knockdown of IL-6 inhibited the syncytin-1-induced CRP expression. Thus, the totality of these results showed that viral protein syncytin-1 could trigger the activation of CRP, which might explain the elevated CRP in sterile inflammation and exhibit a novel mechanism for regulation of inflammation by syncytin-1 in schizophrenia.

Expression and regulation of CYP17A1 and 3β-hydroxysteroid dehydrogenase in cells of the nervous system: Potential effects of vitamin D on brain steroidogenesis

Steroids are reported to have diverse functions in the nervous system. Enzymatic production of steroid hormones has been reported in different cell types, including astrocytes and neurons. However, the information on some of the steroidogenic enzymes involved is insufficient in many respects. Contradictory results have been reported concerning the relative importance of different cell types in the nervous system for expression of CYP17A1 and 3β-hydroxysteroid dehydrogenase (3β-HSD). 3β-HSD is important in all basic steroidogenic pathways and CYP17A1 is required to form sex hormones. In the current investigation we studied the expression of these enzymes in cultured primary rat astrocytes, in neuron-enriched cells from rat cerebral cortex and in human neuroblastoma SH-SY5Y cells, a cell line often used as an in vitro model of neuronal function and differentiation. As part of this study we also examined potential effects on CYP17A1 and 3β-HSD by vitamin D, a compound previously shown to have regulatory effects in steroid hormone-producing cells outside the brain. The results of our study indicate that astrocytes are a major site for expression of 3β-HSD whereas expression of CYP17A1 is found in both astrocytes and neurons. The current data suggest that neurons, contrary to some previous reports, are not involved in 3β-HSD reactions. Previous studies have shown that vitamin D can influence gene expression and hormone production by steroidogenic enzymes in some cells. We found that vitamin D suppressed CYP17A1-mediated activity by 20% in SH-SY5Ycells and astrocytes. Suppression of CYP17A1 mRNA levels was considerably stronger, about 50% in SH-SY5Y cells and 75% in astrocytes. In astrocytes 3β-HSD was also suppressed by vitamin D, about 20% at the enzyme activity level and 60% at the mRNA level. These data suggest that vitamin D-mediated regulation of CYP17A1 and 3β-HSD, particularly on the transcriptional level, may play a role in the nervous system.

Re-adopting classical nuclear receptors by cholesterol metabolites

Since the first cloning of the human estrogen receptor (ER) α in 1986 and the subsequent cloning of human ERβ, there has been extensive investigation of the role of estrogen/ER. Estrogens/ER play important roles not only in sexual development and reproduction but also in a variety of other functions in multiple tissues. Selective Estrogen Receptor Modulators (SERMs) are ER lignds that act as agonists or antagonists depending on the target genes and tissues, and until recently, only synthetic SERMs have been recognized. However, the discovery of the first endogenous SERM, 27-hydroxycholesterol (27HC), opened a new dimension of ER action in health and disease. In addition to the identification of 27HC as a SERM, oxysterols have been recently demonstrated as indirect modulators of ER through interaction with the nuclear receptor Liver X Receptor (LXR) β. In this review, the recent progress on these novel roles of oxysterols in ER modulation is summarized.

Estrogen-mediated regulation of steroid metabolism in rat glial cells; effects on neurosteroid levels via regulation of CYP7B1-mediated catalysis

Many neuroactive steroids, including dehydroepiandrosterone (DHEA), pregnenolone, 27-hydroxycholesterol and 17β-estradiol, are known to affect development and function of the brain and nervous system. These and other steroids can undergo tissue and/or cell-specific enzymatic conversions into steroid metabolites. Carefully regulated production of steroids with various physiological effects is important for cells of the nervous system. Astrocytes express many steroidogenic enzymes and are considered important producers of brain steroids. The quantitative roles of different pathways for steroid metabolism in rat astrocytes are not clear. In the current study we examined effects of estrogens on steroid metabolism catalyzed by CYP7B1 and other enzymes in primary cultures of rat astrocytes. The CYP7B1 enzyme, which has been linked to neurodegenerative disease, is involved in the metabolism of several important neurosteroids. In the present study, we found that 7α-hydroxylation, performed by CYP7B1, is the quantitatively most important pathway for DHEA metabolism in rat astrocytes. In addition, our present experiments on catalytic steroid conversions revealed that estrogens significantly suppress the CYP7B1-catalyzed metabolism of not only DHEA but also of pregnenolone and 27-hydroxycholesterol in rat astrocytes. These novel findings point to a regulatory mechanism for control of the cellular levels of these neurosteroids via CYP7B1. Our hypothesis that estrogens can regulate neurosteroid levels via this enzymatic reaction was supported by experiments using ELISA to assay levels of DHEA and pregnenolone in the presence or absence of estrogen. Furthermore, the present results show that estrogen suppresses CYP7B1-catalyzed 7α-hydroxylation also in primary cultures of rat Schwann cells, indicating that regulation by estrogen via this enzyme may be of relevance in both the CNS and the PNS.

Age-related changes in brain support cells: Implications for stroke severity

Stroke is one of the leading causes of adult disability and the fourth leading cause of mortality in the US. Stroke disproportionately occurs among the elderly, where the disease is more likely to be fatal or lead to long-term supportive care. Animal models, where the ischemic insult can be controlled more precisely, also confirm that aged animals sustain more severe strokes as compared to young animals. Furthermore, the neuroprotection usually seen in younger females when compared to young males is not observed in older females. The preclinical literature thus provides a valuable resource for understanding why the aging brain is more susceptible to severe infarction. In this review, we discuss the hypothesis that stroke severity in the aging brain may be associated with reduced functional capacity of critical support cells. Specifically, we focus on astrocytes, that are critical for detoxification of the brain microenvironment and endothelial cells, which play a crucial role in maintaining the blood brain barrier. In view of the sex difference in stroke severity, this review also discusses studies of middle-aged acyclic females as well as the effects of the estrogen on astrocytes and endothelial cells.

Mutations in phospholipase DDHD2 cause autosomal recessive hereditary spastic paraplegia (SPG54)

Hereditary spastic paraplegias (HSP) are a genetically heterogeneous group of disorders characterized by a distal axonopathy of the corticospinal tract motor neurons leading to progressive lower limb spasticity and weakness. Intracellular membrane trafficking, mitochondrial dysfunction and myelin formation are key functions involved in HSP pathogenesis. Only recently defects in metabolism of complex lipids have been implicated in a number of HSP subtypes. Mutations in the 23 known autosomal recessive HSP genes explain less than half of autosomal recessive HSP cases. To identify novel autosomal recessive HSP disease genes, exome sequencing was performed in 79 index cases with autosomal recessive forms of HSP. Resulting variants were filtered and intersected between families to allow identification of new disease genes. We identified two deleterious mutations in the phospholipase DDHD2 gene in two families with complicated HSP. The phenotype is characterized by early onset of spastic paraplegia, mental retardation, short stature and dysgenesis of the corpus callosum. Phospholipase DDHD2 is involved in intracellular membrane trafficking at the golgi/ endoplasmic reticulum interface and has been shown to possess phospholipase A1 activity in vitro. Discovery of DDHD2 mutations in HSP might therefore provide a link between two key pathogenic themes in HSP: membrane trafficking and lipid metabolism.

Human cytochromes P450 in health and disease

There are 18 mammalian cytochrome P450 (CYP) families, which encode 57 genes in the human genome. CYP2, CYP3 and CYP4 families contain far more genes than the other 15 families; these three families are also the ones that are dramatically larger in rodent genomes. Most (if not all) genes in the CYP1, CYP2, CYP3 and CYP4 families encode enzymes involved in eicosanoid metabolism and are inducible by various environmental stimuli (i.e. diet, chemical inducers, drugs, pheromones, etc.), whereas the other 14 gene families often have only a single member, and are rarely if ever inducible or redundant. Although the CYP2 and CYP3 families can be regarded as largely redundant and promiscuous, mutations or other defects in one or more genes of the remaining 16 gene families are primarily the ones responsible for P450-specific diseases-confirming these genes are not superfluous or promiscuous but rather are more directly involved in critical life functions. P450-mediated diseases comprise those caused by: aberrant steroidogenesis; defects in fatty acid, cholesterol and bile acid pathways; vitamin D dysregulation and retinoid (as well as putative eicosanoid) dysregulation during fertilization, implantation, embryogenesis, foetogenesis and neonatal development.