Enhanced ER-associated degradation of HMG CoA reductase causes embryonic lethality associated with Ubiad1 deficiency

UbiA prenyltransferase domain-containing protein-1 (UBIAD1) synthesizes the vitamin K subtype menaquinone-4 (MK-4). Previous studies in cultured cells (Schumacher et al., 2015) revealed that UBIAD1 also inhibits endoplasmic reticulum (ER)-associated degradation (ERAD) of ubiquitinated HMG CoA reductase (HMGCR), the rate-limiting enzyme of the mevalonate pathway that produces cholesterol and essential nonsterol isoprenoids. Gene knockout studies were previously attempted to explore the function of UBIAD1 in mice; however, homozygous germ-line elimination of the Ubiad1 gene caused embryonic lethality. We now report that homozygous deletion of Ubiad1 is produced in knockin mice expressing ubiquitination/ERAD-resistant HMGCR. Thus, embryonic lethality of Ubiad1 deficiency results from depletion of mevalonate-derived products owing to enhanced ERAD of HMGCR rather than from reduced synthesis of MK-4. These findings provide genetic evidence for the significance of UBIAD1 in regulation of cholesterol synthesis and offer the opportunity in future studies for the discovery of new physiological roles of MK-4.

Schnyder corneal dystrophy-associated UBIAD1 inhibits ER-associated degradation of HMG CoA reductase in mice

Autosomal-dominant Schnyder corneal dystrophy (SCD) is characterized by corneal opacification owing to overaccumulation of cholesterol. SCD is caused by mutations in UBIAD1, which utilizes geranylgeranyl pyrophosphate (GGpp) to synthesize vitamin K₂. Using cultured cells, we previously showed that sterols trigger binding of UBIAD1 to the cholesterol biosynthetic enzyme HMG CoA reductase (HMGCR), thereby inhibiting its endoplasmic reticulum (ER)-associated degradation (ERAD) (Schumacher et al. 2015). GGpp triggers release of UBIAD1 from HMGCR, allowing maximal ERAD and ER-to-Golgi transport of UBIAD1. SCD-associated UBIAD1 resists GGpp-induced release and is sequestered in ER to inhibit ERAD. We now report knockin mice expressing SCD-associated UBIAD1 accumulate HMGCR in several tissues resulting from ER sequestration of mutant UBIAD1 and inhibition of HMGCR ERAD. Corneas from aged knockin mice exhibit signs of opacification and sterol overaccumulation. These results establish the physiological significance of UBIAD1 in cholesterol homeostasis and indicate inhibition of HMGCR ERAD contributes to SCD pathogenesis.

Active surveillance for antenatally detected ureteroceles: Predictors of success

INTRODUCTION

Historically, ureteroceles were surgically treated, as patients were diagnosed after developing symptoms. However, with the advance of fetal medicine, antenatal detection has provided an opportunity to look at the natural history of ureteroceles.

OBJECTIVES

With data derived from a retrospective chart review of patients with ureteroceles that were detected antenatally, the current study aimed to determine which group of children would be at risk for failure on active surveillance. It was hypothesized that single system ureteroceles (SSU) and male patients with duplex system ureteroceles (DSU) would be ideal for observation.

METHODS

Outcomes were assessed by descriptive statistics. Kaplan-Meier curves were utilized to estimate median duration on active surveillance in both single and duplex cohorts. Breakthrough febrile urinary tract infection (fUTI) and surgery were determined by Cox regression in the duplex system cohort. Surgery was considered surveillance failure.

RESULTS

A total of 102 patients (64 females/38 males) met the criteria: 78 (76.5%) had DSU and 24 (23.5%) SSU. The overall median observation was 1.2 years (range 0.7-3.1). Follow-up ranged from 0.3 to 11.7 years for SSU, and from 0.02 to 17.3 years for DSU. The predictors of failure of active surveillance (AS) in DSU (surgical intervention) were male gender (HR 1.8, 1.0-3.3, P = 0.037), or fUTI (HR 3.1, 1.7-5.8, P = 0.002). Predictors of fUTI were contralateral hydroureter or ipsilateral hydronephrosis ± hydroureter (OR 9.5, 1.2-71.7, P = 0.028). Interestingly, vesicoureteral reflux (VUR) was not a predictor of fUTI. The SSU patients were ideal for AS, while in DSU, surveillance was successful in 30% of patients who were primarily females without contralateral hydroureter or ipsilateral hydronephrosis ± hydroureter. However, in contradiction to the hypothesis, males were at higher risk for surgical intervention in the DSU cohort.

CONCLUSION

Active surveillance is an option for patients with antenatally detected ureteroceles, but careful long term follow up is mandatory. Parents should be advised that surgical intervention may still be necessary, particularly in males with DSU.

Abstract 457: Feedback Regulation of 3-Hydroxy-3-methylglutaryl-coenzyme a Reductase in Livers of Mice

Introduction: HMG-CoA reductase is a membrane protein of the endoplasmic reticulum that catalyzes reduction of HMG-CoA to mevalonate, a rate-limiting step in the synthesis of cholesterol and nonsterol isoprenoids, which exert feedback control on HMGCR through multiple mechanisms. These mechanisms ensure constant synthesis of essential nonsterol isoprenoids, while avoiding toxic cholesterol accumulation. One mechanism involves sterol-induced ubiquitination of HMGCR, marking the enzyme for degradation from ER membranes, a process augmented by nonsterol isoprenoids. We examine the contribution of sterol-accelerated ubiquitination/degradation to overall regulation of HMGCR in livers of mice.

Methods: Forty mice, including 20 wild-type (WT) and 20 knock-in (Ki) mice expressing ubiquitination-resistant HMGCR, were fed diets containing only chow, or chow supplemented with 0.1, 0.3, or 1% cholesterol. After five days of feeding, livers were harvested for measurements of cholesterol and triglycerides, immunoblot analysis, and qRT-PCR of genes related to cholesterol, nonsterol isoprenoid, and fatty acid synthesis.

Results: Normalization of mRNA to protein levels indicates that HMGCR Ki livers contain a more HMGCR protein despite mRNA downregulation. Protein and gene expression of SREBP2 and its target genes, which contribute to cholesterol synthesis, decreased as expected with increased dietary cholesterol. Conversely, protein and gene expression of SREBP1 and its target genes increased, likely due to SREBP1c predominance toward fatty acid synthesis, which prevents cholesterol accumulation.

Conclusion: The increase in HMGCR protein relative to mRNA suggests that significant post-transcriptional regulation exists in the form of impaired degradation. Furthermore, these normalized values indicate that accumulation of protein is primarily due to impaired degradation at lower cholesterol levels (chow, 0.1%); however, at high cholesterol levels (0.3, 1%), a greater degree of transcriptional control from sterol-mediated inhibition of SREBP2 regulates HMGCR due to negative feedback. This study demonstrates the role of degradative control on inhibition of HMGCR and may assist in reducing HMGCR accumulation during statin therapy.

Contribution of Accelerated Degradation to Feedback Regulation of 3-Hydroxy-3-methylglutaryl Coenzyme A Reductase and Cholesterol Metabolism in the Liver

Accumulation of sterols in endoplasmic reticulum membranes stimulates the ubiquitination of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR), which catalyzes a rate-limiting step in synthesis of cholesterol. This ubiquitination marks HMGCR for proteasome-mediated degradation and constitutes one of several mechanisms for feedback control of cholesterol synthesis. Mechanisms for sterol-accelerated ubiquitination and degradation of HMGCR have been elucidated through the study of cultured mammalian cells. However, the extent to which these reactions modulate HMGCR and contribute to control of cholesterol metabolism in whole animals is unknown. Here, we examine transgenic mice expressing in the liver the membrane domain of HMGCR (HMGCR (TM1-8)), a region necessary and sufficient for sterol-accelerated degradation, and knock-in mice in which endogenous HMGCR harbors mutations that prevent sterol-induced ubiquitination. Characterization of transgenic mice revealed that HMGCR (TM1-8) is appropriately regulated in the liver of mice fed a high cholesterol diet or chow diet supplemented with the HMGCR inhibitor lovastatin. Ubiquitination-resistant HMGCR protein accumulates in the liver and other tissues disproportionately to its mRNA, indicating that sterol-accelerated degradation significantly contributes to feedback regulation of HMGCR in vivo Results of these studies demonstrate that HMGCR is subjected to sterol-accelerated degradation in the liver through mechanisms similar to those established in cultured cells. Moreover, these studies designate sterol-accelerated degradation of HMGCR as a potential therapeutic target for prevention of atherosclerosis and associated cardiovascular disease.

Transcription Factor Hepatocyte Nuclear Factor-1β Regulates Renal Cholesterol Metabolism

HNF-1β is a tissue-specific transcription factor that is expressed in the kidney and other epithelial organs. Humans with mutations in HNF-1β develop kidney cysts, and HNF-1β regulates the transcription of several cystic disease genes. However, the complete spectrum of HNF-1β-regulated genes and pathways is not known. Here, using chromatin immunoprecipitation/next generation sequencing and gene expression profiling, we identified 1545 protein-coding genes that are directly regulated by HNF-1β in murine kidney epithelial cells. Pathway analysis predicted that HNF-1β regulates cholesterol metabolism. Expression of dominant negative mutant HNF-1β or kidney-specific inactivation of HNF-1β decreased the expression of genes that are essential for cholesterol synthesis, including sterol regulatory element binding factor 2 (Srebf2) and 3-hydroxy-3-methylglutaryl-CoA reductase (Hmgcr). HNF-1β mutant cells also expressed lower levels of cholesterol biosynthetic intermediates and had a lower rate of cholesterol synthesis than control cells. Additionally, depletion of cholesterol in the culture medium mitigated the inhibitory effects of mutant HNF-1β on the proteins encoded by Srebf2 and Hmgcr, and HNF-1β directly controlled the renal epithelial expression of proprotein convertase subtilisin-like kexin type 9, a key regulator of cholesterol uptake. These findings reveal a novel role of HNF-1β in a transcriptional network that regulates intrarenal cholesterol metabolism.

[Effect of the ototoxicity of gentamycin on the vestibulo-ocular reflexes in the cat]

In this paper we show the effect of gentamicin upon the VOR and ViVOR gain and phase in pendular and impulse rotatory tests. We show the early effect of gentamicin upon the gain in low frequency tests, and the small effect upon the phase of the response.