Schnyder corneal dystrophy-associated UBIAD1 mutations cause corneal cholesterol accumulation by stabilizing HMG-CoA reductase

Structure and mechanism of vitamin-K-dependent γ-glutamyl carboxylase

γ-Glutamyl carboxylase (GGCX) is the sole identified enzyme that uses vitamin K (VK) as a cofactor in humans. This protein catalyses the oxidation of VK hydroquinone to convert specific glutamate residues to γ-carboxyglutamate residues in VK-dependent proteins (VDPs), which are involved in various essential biological processes and diseases1-3. However, the working mechanism of GGCX remains unclear. Here we report three cryogenic electron microscopy structures of human GGCX: in the apo state, bound to osteocalcin (a VDP) and bound to VK. The propeptide of the VDP binds to the lumenal domain of GGCX, which stabilizes transmembrane helices 6 and 7 of GGCX to create the VK-binding pocket. After binding of VK, residue Lys218 in GGCX mediates the oxidation of VK hydroxyquinone, which leads to the deprotonation of glutamate residues and the construction of γ-carboxyglutamate residues. Our structural observations and results from binding and cell biological assays and molecular dynamics simulations show that a cholesterol molecule interacts with the transmembrane helices of GGCX to regulate its protein levels in cells. Together, these results establish a link between cholesterol metabolism and VK-dependent pathways.

Hepatic lipid metabolism is altered in Ubiad1+/- mice of both sexes

UbiA prenyltransferase domain containing 1 (Ubiad1) has the potential to affect cholesterol and phospholipid levels in different cell types. We previously identified Ubiad1 as a candidate gene for regulating subcutaneous fat pad weight in a mouse genome-wide association study. Here we evaluated the relationship between Ubiad1 and obesity-related traits in cohorts of humans and mice, and in Ubiad1+/- mice fed a high-fat diet. In both humans and mice, adipose tissue Ubiad1 mRNA expression correlated negatively with adiposity and positively with mitochondria-related genes. To determine the role of Ubiad1 in high-fat diet-induced obesity, we disrupted the Ubiad1 gene in mice. Deletion of Ubiad1 was embryonically lethal in C57BL/6 N mice, preventing analysis of adult Ubiad1-/- mice. Thus, male and female Ubiad1+/+ and Ubiad1+/- mice were fed high-fat diet for 10 weeks, with no difference in weight gain and adipose tissue organ weights observed between the genotypes. Analysis of liver mRNA expression revealed that Ubiad1 heterozygosis (Ubiad1+/-) altered several pathways involved in lipid metabolism. Detailed lipid quantification with HPLC-qTOF/MS showed increased levels of hepatic ceramides in female Ubiad1+/- mice, whereas phosphatidylglycerols, phosohatidylinositol and lysophosphatidylethanolamines were reduced in male Ubiad1+/- mice. Our findings reveal sex-specific effects of Ubiad1 expression on hepatic lipid metabolism.

Hepatic Dyrk1b impairs systemic glucose homeostasis by modulating Wbp2 expression in a kinase activity-dependent manner

Patients with gain-of-function mutations of Dyrk1b have higher fasting blood glucose (FBG) levels. However, the role of Dyrk1b in glucose metabolism is not fully elucidated. Herein, we found that hepatic Dyrk1b overexpression in mice impaired systemic glucose tolerance and hepatic insulin signaling. Dyrk1b overexpression in vitro attenuated insulin signaling in a kinase activity-dependent manner, and its kinase activity was required for its effect on systemic glucose homeostasis and hepatic insulin signaling in vivo. Dyrk1b ablation improved systemic glucose tolerance and hepatic insulin signaling in mice. Quantitative proteomic analyses showed that Dyrk1b downregulated WW domain-binding protein 2 (Wbp2) protein abundance. Mechanistically, Dyrk1b enhanced Wbp2 ubiquitylation and proteasomal degradation. Restoration of hepatic Wbp2 partially rescued the impaired glucose homeostasis in Dyrk1b overexpression mice. In addition, Dyrk1b inhibition with AZ191 moderately improved systemic glucose homeostasis. Our study uncovers that hepatic Dyrk1b impairs systemic glucose homeostasis via its modulation of Wbp2 expression in a kinase activity-dependent manner.

Discovery of Natural Potent HMG-CoA Reductase Degraders for Lowering Cholesterol

Exploitation of key protected wild plant resources makes great sense, but their limited populations become the major barrier. A particular strategy for breaking this barrier was inspired by the exploration of a resource-saving fungal endophyte Penicillium sp. DG23, which inhabits the key protected wild plant Schisandra macrocarpa. Chemical studies on the cultures of this strain afforded eight novel indole diterpenoids, schipenindolenes A-H (1-8), belonging to six diverse skeleton types. Importantly, semisyntheses suggested some key nonenzymatic reactions constructing these molecules and provided targeted compounds, in particular schipenindolene A (Spid A, 1) with low natural abundance. Remarkably, Spid A was the most potent HMG-CoA reductase (HMGCR) degrader among the indole diterpenoid family. It degraded statin-induced accumulation of HMGCR protein, decreased cholesterol levels and acted synergistically with statin to further lower cholesterol. Mechanistically, transcriptomic and proteomic profiling suggested that Spid A potentially activated the endoplasmic reticulum-associated degradation (ERAD) pathway to enhance the degradation of HMGCR, while simultaneously inhibiting the statin-activated expression of many key enzymes in the cholesterol and fatty acid synthesis pathways, thereby strengthening the efficacy of statins and potentially reducing the side effects of statins. Collectively, this study suggests the potential of Spid A for treating cardiovascular disease.

Cholesterol metabolism: physiological regulation and diseases

Cholesterol homeostasis is crucial for cellular and systemic function. The disorder of cholesterol metabolism not only accelerates the onset of cardiovascular disease (CVD) but is also the fundamental cause of other ailments. The regulation of cholesterol metabolism in the human is an extremely complex process. Due to the dynamic balance between cholesterol synthesis, intake, efflux and storage, cholesterol metabolism generally remains secure. Disruption of any of these links is likely to have adverse effects on the body. At present, increasing evidence suggests that abnormal cholesterol metabolism is closely related to various systemic diseases. However, the exact mechanism by which cholesterol metabolism contributes to disease pathogenesis remains unclear, and there are still unknown factors. In this review, we outline the metabolic process of cholesterol in the human body, especially reverse cholesterol transport (RCT). Then, we discuss separately the impact of abnormal cholesterol metabolism on common diseases and potential therapeutic targets for each disease, including CVD, tumors, neurological diseases, and immune system diseases. At the end of this review, we focus on the effect of cholesterol metabolism on eye diseases. In short, we hope to provide more new ideas for the pathogenesis and treatment of diseases from the perspective of cholesterol.

Mechanisms of 3-Hydroxyl 3-Methylglutaryl CoA Reductase in Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disease worldwide and has a high incidence in the elderly. Unfortunately, there is no effective therapy for AD owing to its complicated pathogenesis. However, the development of lipid-lowering anti-inflammatory drugs has heralded a new era in the treatment of Alzheimer's disease. Several studies in recent years have shown that lipid metabolic dysregulation and neuroinflammation are associated with the pathogenesis of AD. 3-Hydroxyl 3-methylglutaryl CoA reductase (HMGCR) is a rate-limiting enzyme in cholesterol synthesis that plays a key role in cholesterol metabolism. HMGCR inhibitors, known as statins, have changed from being solely lipid-lowering agents to neuroprotective compounds because of their effects on lipid levels and inflammation. In this review, we first summarize the main regulatory mechanism of HMGCR affecting cholesterol biosynthesis. We also discuss the pathogenesis of AD induced by HMGCR, including disordered lipid metabolism, oxidative stress, inflammation, microglial proliferation, and amyloid-β (Aβ) deposition. Subsequently, we explain the possibility of HMGCR as a potential target for AD treatment. Statins-based AD treatment is an ascent field and currently quite controversial; therefore, we also elaborate on the current application prospects and limitations of statins in AD treatment.

Vitamin K and the Visual System-A Narrative Review

Vitamin K occupies a unique and often obscured place among its fellow fat-soluble vitamins. Evidence is mounting, however, that vitamin K (VK) may play an important role in the visual system apart from the hepatic carboxylation of hemostatic-related proteins. However, to our knowledge, no review covering the topic has appeared in the medical literature. Recent studies have confirmed that matrix Gla protein (MGP), a vitamin K-dependent protein (VKDP), is essential for the regulation of intraocular pressure in mice. The PREDIMED (Prevención con Dieta Mediterránea) study, a randomized trial involving 5860 adults at risk for cardiovascular disease, demonstrated a 29% reduction in the risk of cataract surgery in participants with the highest tertile of dietary vitamin K1 (PK) intake compared with those with the lowest tertile. However, the specific requirements of the eye and visual system (EVS) for VK, and what might constitute an optimized VK status, is currently unknown and largely unexplored. It is, therefore, the intention of this narrative review to provide an introduction concerning VK and the visual system, review ocular VK biology, and provide some historical context for recent discoveries. Potential opportunities and gaps in current research efforts will be touched upon in the hope of raising awareness and encouraging continued VK-related investigations in this important and highly specialized sensory system.

UbiA prenyltransferase domain-containing protein 1 (UBIAD1) variant c.695 A > G identified in a multigenerational Japanese family with Schnyder corneal dystrophy

PURPOSE

We aimed to identify pathogenic variations in the UbiA prenyltransferase domain-containing protein 1 (UBIAD1) gene in a Japanese family with Schnyder corneal dystrophy (SCD).

STUDY DESIGN

Clinical study METHODS: Three clinically diagnosed SCD patients from a single pedigree participated. Patients 1 and 2 were 69- and 65-year-old sisters, and patient 3 was the 42-year-old daughter of patient 1. Blood samples from the patients were obtained for genetic analysis. Mutation screening of the two UBIAD1 exons was performed using polymerase chain reaction (PCR)-based DNA sequencing.

RESULTS

All participants were found to be heterozygous for the pathogenic missense variation c.695 A > G (p.Asn232Ser) in exon 2 of UBIAD1.

CONCLUSION

This is the first report on the pathogenic UBIAD1 variation c.695 A > G (p.Asn232Ser) in a Japanese population. SCD is a rare corneal dystrophy, and further research on additional cases will aid in the elucidation of disease mechanisms and development of therapeutic strategies.

Post-Translational Regulation of HMG CoA Reductase

3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) is an endoplasmic reticulum (ER)-localized integral membrane protein that catalyzes the rate-limiting step in the synthesis of cholesterol and many nonsterol isoprenoids including geranylgeranyl pyrophosphate (GGpp). HMGCR is subjected to strict feedback control through multiple mechanisms to ensure cells constantly produce essential nonsterol isoprenoids, but do not overaccumulate cholesterol. Here, we focus on the mechanism of feedback control of HMGCR that involves its sterol-induced ubiquitination and ER-associated degradation (ERAD) that is augmented by GGpp. We will also discuss the how GGpp-regulated intracellular trafficking of the vitamin K2 synthetic enzyme UbiA prenyltransferase domain-containing protein-1 (UBIAD1) inhibits HMGCR ERAD to balance the synthesis of sterol and nonsterol isoprenoids. Finally, we will summarize various mouse models, the characterization of which establish that sterol-accelerated, UBIAD1-modulated ERAD plays a major role in regulation of HMGCR and cholesterol metabolism in vivo.

Discovery of an insulin-induced gene binding compound that ameliorates nonalcoholic steatohepatitis by inhibiting sterol regulatory element-binding protein-mediated lipogenesis

BACKGROUND AND AIMS

NASH is associated with high levels of cholesterol and triglyceride (TG) in the liver; however, there is still no approved pharmacological therapy. Synthesis of cholesterol and TG is controlled by sterol regulatory element-binding protein (SREBP), which is found to be abnormally activated in NASH patients. We aim to discover small molecules for treating NASH by inhibiting the SREBP pathway.

APPROACH AND RESULTS

Here, we identify a potent SREBP inhibitor, 25-hydroxylanosterol (25-HL). 25-HL binds to insulin-induced gene (INSIG) proteins, stimulates the interaction between INSIG and SCAP, and retains them in the endoplasmic reticulum, thereby suppressing SREBP activation and inhibiting lipogenesis. In NASH mouse models, 25-HL lowers levels of cholesterol and TG in serum and the liver, enhances energy expenditure to prevent obesity, and improves insulin sensitivity. 25-HL dramatically ameliorates hepatic steatosis, inflammation, ballooning, and fibrosis through down-regulating the expression of lipogenic genes. Furthermore, 25-HL exhibits both prophylactic and therapeutic efficacies of alleviating NASH and atherosclerosis in amylin liver NASH model diet-treated Ldlr^-/- mice, and reduces the formation of cholesterol crystals and associated crown-like structures of Kupffer cells. Notably, 25-HL lowers lipid contents in serum and the liver to a greater extent than lovastatin or obeticholic acid. 25-HL shows a good safety and pharmacokinetics profile.

CONCLUSIONS

This study provides the proof of concept that inhibiting SREBP activation by targeting INSIG to lower lipids could be a promising strategy for treating NASH. It suggests the translational potential of 25-HL in human NASH and demonstrates the critical role of SREBP-controlled lipogenesis in the progression of NASH by pharmacological inhibition.

Synthesis, function, and regulation of sterol and nonsterol isoprenoids

Cholesterol, the bulk end-product of the mevalonate pathway, is a key component of cellular membranes and lipoproteins that transport lipids throughout the body. It is also a precursor of steroid hormones, vitamin D, and bile acids. In addition to cholesterol, the mevalonate pathway yields a variety of nonsterol isoprenoids that are essential to cell survival. Flux through the mevalonate pathway is tightly controlled to ensure cells continuously synthesize nonsterol isoprenoids but avoid overproducing cholesterol and other sterols. Endoplasmic reticulum (ER)-localized 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase (HMGCR), the rate limiting enzyme in the mevalonate pathway, is the focus of a complex feedback regulatory system governed by sterol and nonsterol isoprenoids. This review highlights transcriptional and post-translational regulation of HMGCR. Transcriptional regulation of HMGCR is mediated by the Scap-SREBP pathway. Post-translational control is initiated by the intracellular accumulation of sterols, which causes HMGCR to become ubiquitinated and subjected to proteasome-mediated ER-associated degradation (ERAD). Sterols also cause a subfraction of HMGCR molecules to bind the vitamin K₂ synthetic enzyme, UbiA prenyltransferase domain-containing protein-1 (UBIAD1). This binding inhibits ERAD of HMGCR, which allows cells to continuously synthesize nonsterol isoprenoids such as geranylgeranyl pyrophosphate (GGPP), even when sterols are abundant. Recent studies reveal that UBIAD1 is a GGPP sensor, dissociating from HMGCR when GGPP thresholds are met to allow maximal ERAD. Animal studies using genetically manipulated mice disclose the physiological significance of the HMGCR regulatory system and we describe how dysregulation of these pathways contributes to disease.

Lowering low-density lipoprotein cholesterol: from mechanisms to therapies

Low-density lipoprotein (LDL) is the main carrier of cholesterol and cholesteryl ester in circulation. High plasma levels of LDL cholesterol (LDL-C) are a major risk factor of atherosclerotic cardiovascular disease (ASCVD). LDL-C lowering is recommended by many guidelines for the prevention and treatment of ASCVD. Statins, ezetimibe, and proprotein convertase subtilisin/kexin type 9 inhibitors are the mainstay of LDL-C-lowering therapy. Novel therapies are also emerging for patients who are intolerant to statins or respond poorly to standard treatments. Here, we review the most recent advances on LDL-C-lowering drugs, focusing on the mechanisms by which they act to reduce LDL-C levels. The article starts with the cornerstone therapies applicable to most patients at risk for ASCVD. Special treatments for those with little or no LDL receptor function then follow. The inhibitors of ATP-citrate lyase and cholesteryl ester transfer protein, which are recently approved and still under investigation for LDL-C lowering, respectively, are also included. Strategies targeting the stability of 3-hydroxy-3-methylglutaryl-coenzyme A reductase and cholesterol catabolism can be novel regimens to reduce LDL-C levels and cardiovascular risk.

Regulated degradation of HMG CoA reductase requires conformational changes in sterol-sensing domain

3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) is the rate-limiting enzyme in cholesterol synthesis and target of cholesterol-lowering statin drugs. Accumulation of sterols in endoplasmic reticulum (ER) membranes accelerates degradation of HMGCR, slowing the synthesis of cholesterol. Degradation of HMGCR is inhibited by its binding to UBIAD1 (UbiA prenyltransferase domain-containing protein-1). This inhibition contributes to statin-induced accumulation of HMGCR, which limits their cholesterol-lowering effects. Here, we report cryo-electron microscopy structures of the HMGCR-UBIAD1 complex, which is maintained by interactions between transmembrane helix (TM) 7 of HMGCR and TMs 2-4 of UBIAD1. Disrupting this interface by mutagenesis prevents complex formation, enhancing HMGCR degradation. TMs 2-6 of HMGCR contain a 170-amino acid sterol sensing domain (SSD), which exists in two conformations-one of which is essential for degradation. Thus, our data supports a model that rearrangement of the TMs in the SSD permits recruitment of proteins that initate HMGCR degradation, a key reaction in the regulatory system that governs cholesterol synthesis.

Naturally occurring UBIAD1 mutations differentially affect menaquinone biosynthesis and vitamin K-dependent carboxylation

UbiA prenyltransferase domain-containing protein-1 (UBIAD1) is responsible for the biosynthesis of menaquinone-4 (MK-4), a cofactor for extrahepatic carboxylation of vitamin K-dependent (VKD) proteins. Genetic variations of UBIAD1 are mainly associated with Schnyder corneal dystrophy (SCD), a disease characterized by abnormal accumulation of cholesterol in the cornea. Results from in vitro studies demonstrate that SCD-associated UBIAD1 mutations are defective in MK-4 biosynthesis. However, SCD patients do not exhibit typical phenotypes associated with defects of MK-4 or VKD carboxylation. Here, we coupled UBIAD1's biosynthetic activity of MK-4 with VKD carboxylation in HEK293 cells that stably express a chimeric VKD reporter protein. The endogenous Ubiad1 gene in these cells was knocked out by CRISPR-Cas9-mediated genome editing. The effect of UBIAD1 mutations on MK-4 biosynthesis and VKD carboxylation was evaluated in Ubiad1-deficient reporter cells by determining the production of MK-4 or by measuring the efficiency of reporter-protein carboxylation. Our results show that the hot-spot mutation N102S has a moderate impact on MK-4 biosynthesis (retained ˜ 82% activity) but does not affect VKD carboxylation. However, the G186R mutation significantly affected both MK-4 biosynthesis and VKD carboxylation. Other mutations exhibit varying degrees of effects on MK-4 biosynthesis and VKD carboxylation. These results are consistent with in vivo results obtained from gene knock-in mice and SCD patients. Our findings suggest that UBIAD1's MK-4 biosynthetic activity does not directly correlate with the phenotypes of SCD patients. The established cell-based assays in this study provide a powerful tool for the functional studies of UBIAD1 in a cellular milieu.

Functional study of SCCD pathogenic gene UBIAD1 (Review)

Schnyder's crystalline corneal dystrophy (SCCD) is a rare autosomal dominant genetic disorder that is characterized by progressive corneal opacity, owing to aberrant accumulation of cholesterol and phospholipids in the cornea. A number of SCCD affected families have been reported in the world since 1924, when it was first described. In 2007, the molecular basis of SCCD was demonstrated to be associated with a tumor suppressor, UbiA prenyltransferase domain-containing 1 (UBIAD1), which was isolated from the bladder mucosa and demonstrated to be involved in vitamin K₂ and CoQ10 biosynthesis. This sterol triggers the binding of UBIAD1 to 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCR) at endoplasmic reticulum (ER) membranes, which is regulated by an intracellular geranylgeranyl diphosphate (GGpp) molecule. The inability of SCCD-associated UBIAD1 to bind GGpp results in the consistent binding of UBIAD1 to HMGCR at ER membranes. This binding leads to HMGCRs being redundant. Therefore, they cannot be degraded through ER-associated degradation to synthesize abundant cholesterol in tissue cells. Excess corneal cholesterol accumulation thus leads to SCCD disease. After decades, the efforts of numerous ophthalmologists and scientists have helped clarify the molecular basis and pathogenesis of SCCD, which has guided the effective diagnosis and treatment of this genetic disorder. However, more studies need to be conducted to understand the pathogenesis of SCCD disease from a genetic basis by studying the defective gene, UBIAD1. Results would guide effective diagnosis and treatment of the inherited eye disease.

Enzymes in the Cholesterol Synthesis Pathway: Interactomics in the Cancer Context

A global protein interactome ensures the maintenance of regulatory, signaling and structural processes in cells, but at the same time, aberrations in the repertoire of protein-protein interactions usually cause a disease onset. Many metabolic enzymes catalyze multistage transformation of cholesterol precursors in the cholesterol biosynthesis pathway. Cancer-associated deregulation of these enzymes through various molecular mechanisms results in pathological cholesterol accumulation (its precursors) which can be disease risk factors. This work is aimed at systematization and bioinformatic analysis of the available interactomics data on seventeen enzymes in the cholesterol pathway, encoded by HMGCR, MVK, PMVK, MVD, FDPS, FDFT1, SQLE, LSS, DHCR24, CYP51A1, TM7SF2, MSMO1, NSDHL, HSD17B7, EBP, SC5D, DHCR7 genes. The spectrum of 165 unique and 21 common protein partners that physically interact with target enzymes was selected from several interatomic resources. Among them there were 47 modifying proteins from different protein kinases/phosphatases and ubiquitin-protein ligases/deubiquitinases families. A literature search, enrichment and gene co-expression analysis showed that about a quarter of the identified protein partners was associated with cancer hallmarks and over-represented in cancer pathways. Our results allow to update the current fundamental view on protein-protein interactions and regulatory aspects of the cholesterol synthesis enzymes and annotate of their sub-interactomes in term of possible involvement in cancers that will contribute to prioritization of protein targets for future drug development.

Regulation of glucose and lipid metabolism in health and disease

Glucose and fatty acids are the major sources of energy for human body. Cholesterol, the most abundant sterol in mammals, is a key component of cell membranes although it does not generate ATP. The metabolisms of glucose, fatty acids and cholesterol are often intertwined and regulated. For example, glucose can be converted to fatty acids and cholesterol through de novo lipid biosynthesis pathways. Excessive lipids are secreted in lipoproteins or stored in lipid droplets. The metabolites of glucose and lipids are dynamically transported intercellularly and intracellularly, and then converted to other molecules in specific compartments. The disorders of glucose and lipid metabolism result in severe diseases including cardiovascular disease, diabetes and fatty liver. This review summarizes the major metabolic aspects of glucose and lipid, and their regulations in the context of physiology and diseases.