Cholesterol synthesis is a tightly regulated process, both transcriptionally and post-translationally. Transcriptional control of cholesterol synthesis is relatively well-understood. However, of the ∼20 enzymes in cholesterol biosynthesis, post-translational regulation has only been examined for a small number. Three of the four sterol reductases in cholesterol production, 7-dehydrocholesterol reductase (DHCR7), 14-dehydrocholesterol reductase (DHCR14), and lamin-B receptor (LBR), share evolutionary ties with a high level of sequence homology and predicted structural homology. DHCR14 and LBR uniquely share the same Δ-14 reductase activity in cholesterol biosynthesis, yet little is known about their post-translational regulation. We have previously identified specific modes of post-translational control of DHCR7, but it is unknown whether these regulatory mechanisms are shared by DHCR14 and LBR. Using CHO-7 cells stably expressing epitope-tagged DHCR14 or LBR, we investigated the post-translational regulation of these enzymes. We found that DHCR14 and LBR undergo differential post-translational regulation, with DHCR14 being rapidly turned over, triggered by cholesterol and other sterol intermediates, whereas LBR remained stable. DHCR14 is degraded via the ubiquitin-proteasome system, and we identified several DHCR14 and DHCR7 putative interaction partners, including a number of E3 ligases that modulate DHCR14 levels. Interestingly, we found that gene expression across an array of human tissues showed a negative relationship between the C14-sterol reductases; one enzyme or the other tends to be predominantly expressed in each tissue. Overall, our findings indicate that whereas LBR tends to be the constitutively active C14-sterol reductase, DHCR14 levels are tunable, responding to the local cellular demands for cholesterol.

The cholesterol is a vital component of cell membranes and myelin sheaths, and a precursor for essential molecules such as steroid hormones. In humans, cholesterol is partially obtained through the diet, while the majority is synthesized in the body, primarily in the liver. However, the limited exchange between the central nervous system and peripheral circulation, due to the presence of the blood-brain barrier, necessitates cholesterol in the brain to be exclusively acquired from local de novo synthesis. This cholesterol is reutilized efficiently, rendering a much slower overall turnover of the compound in the brain as compared with the periphery. Furthermore, brain cholesterol is regulated independently from peripheral cholesterol. Numerous enzymes, proteins, and other factors are involved in cholesterol synthesis and metabolism in the brain. Understanding the unique mechanisms and pathways involved in the maintenance of cholesterol homeostasis in the brain is critical, considering perturbations to these processes are implicated in numerous neurodegenerative diseases. This review focuses on the developing understanding of cholesterol metabolism in the brain, discussing the sites and processes involved in its synthesis and regulation, as well as the mechanisms involved in its distribution throughout, and elimination from, the brain.

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Cholesterol synthesis is a tightly regulated process, both transcriptionally and post‐translationally. Transcriptional control of cholesterol synthesis is relatively well understood. However, post‐translational regulation has only been examined for a few of the ~20 enzymes in cholesterol biosynthesis. Two of the four sterol reductases, DHCR14 and LBR, share evolutionary ties with a high level of sequence homology, and they uniquely share the same delta‐14 reductase activity in cholesterol biosynthesis. However, the post‐translational regulation of these twin enzymes is poorly understood. Using CHO‐7 cells stably expressing epitope tagged DHCR14 or LBR, we investigated the post‐translational regulation of these enzymes. We found that DHCR14 and LBR undergo differential post translational regulation, with DHCR14 being rapidly turned over, triggered by cholesterol and other sterol intermediates while LBR remained stable. DHCR14 is degraded via the ubiquitin‐proteasome system and via mass‐spectrometry we identified several E3 ligases that modulate basal DHCR14 levels. Significantly, we found that gene expression across an array of human tissues showed an inverse relationship between the C14‐sterol reductases; one enzyme or the other tends to be predominantly expressed in each tissue. Overall, our findings indicate that while LBR tends to be the constitutively active delta‐14 reductase, DHCR14 levels are tuneable, responding to the local cellular demands for cholesterol. Support or Funding Information This work was supported by Australian Research Council Grant DP170101178. PhD Candidates were funded by the Research Training Program from the Australian Government Department of Education.

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