Accumulation of sterols in membranes of the endoplasmic reticulum (ER) leads to the accelerated ubiquitination and proteasomal degradation of 3-hydroxy-3-methylglutaryl coenzyme A reductase, a rate-limiting enzyme in synthesis of cholesterol and nonsterol isoprenoids. This degradation results from sterol-induced binding of reductase to the Insig-1 or Insig-2 proteins of ER membranes. We previously reported that in immortalized human fibroblasts (SV-589 cells) Insig-1, but not Insig-2, recruits gp78, a membranebound RING-finger ubiquitin ligase. We now report that both Insig-1 and Insig-2 bind another membrane-bound RING-finger ubiquitin ligase called Trc8. Knockdown of either gp78 or Trc8 in SV-589 cells through RNA interference (RNAi) inhibited sterolinduced ubiquitination of reductase and inhibited sterol-induced degradation by 50-60%. The combined knockdown of gp78 and Trc8 produced a more complete inhibition of degradation (>90%). Knockdown of gp78 led to a three to fourfold increase in levels of Trc8 and Insig-1 proteins, which opposed the inhibitory action of gp78. In contrast, knockdown of Trc8 had no effect on gp78 or Insig-1. The current results suggest that sterol-induced ubiquitination and proteasomal degradation of reductase is dictated by the complex interplay of at least four proteins: Insig-1, Insig-2, gp78, and Trc8. Variations in the concentrations of any one of these proteins may account for differences in cell-and/or tissue-specific regulation of reductase degradation.cholesterol metabolism | ER-associated degradation
The endoplasmic reticulum (ER)-associated degradation (ERAD) pathway in the yeast Saccharomyces cerevisiae is mediated by two membrane-bound ubiquitin ligases, Doa10 and Hrd1. These enzymes are found in distinct multiprotein complexes that allow them to recognize and target a variety of substrates for proteasomal degradation. Although multiprotein complexes containing mammalian ERAD ubiquitin ligases likely exist, they have yet to be identified and characterized in detail. Here, we identify two ER membrane proteins, SPFH2 and TMUB1, as associated proteins of mammalian gp78, a membrane-bound ubiquitin ligase that bears significant sequence homology with mammalian Hrd1 and mediates sterol-accelerated ERAD of the cholesterol biosynthetic enzyme HMG-CoA reductase. Co-immunoprecipitation studies indicate that TMUB1 bridges SPFH2 to gp78 in ER membranes. The functional significance of these interactions is revealed by the observation that RNA interference (RNAi)-mediated knockdown of SPFH2 and TMUB1 blunts both the sterol-induced ubiquitination and degradation of endogenous reductase in HEK-293 cells. These studies mark the initial steps in the characterization of the mammalian gp78 ubiquitin ligase complex, the further elucidation of which may yield important insights into mechanisms underlying gp78-mediated ERAD.
IMPORTANCE A neurophysiologic signature of the melanopsin-mediated persistent constriction phase of the pupillary light reflex may represent a surrogate biomarker for the integrity of the retinohypothalamic tract, with potential utility for investigating alterations in homeostatic mechanisms associated with brain disorders and implications for identifying new treatments.OBJECTIVE To characterize abnormalities of retinal architecture in patients with multiple sclerosis (MS) and corresponding alterations in the melanopsin-mediated sustained pupillary constriction response. DESIGN, SETTING, AND PARTICIPANTSThe case-control study was an experimental assessment of various stimulus-induced pupillary response characteristics and was conducted at a university clinical center for MS from September 6, 2012, to February 2015. Twenty-four patients with MS (48 eyes) and 15 individuals serving as controls (30 eyes) participated. The melanopsin-mediated, sustained pupillary constriction phase response following cessation of a blue light stimulus was compared with the photoreceptor-mediated pupillary constriction phase response following cessation of a red light stimulus. Optical coherence tomography was used to characterize the association between pupillary response characteristics and alterations in retinal architecture, specifically, the thickness of the retinal ganglion cell layer and inner plexiform layer (GCL + IPL). MAIN OUTCOMES AND MEASURESAssociation of pupillary response characteristics with alterations in retinal architecture. RESULTSOf 24 patients with MS included in the analysis, 17 were women (71%); mean (SD) age was 47 (11) years. Compared with eyes from individuals with MS who had normal optical coherence tomography-derived measures of retinal GCL + IPL thickness, eyes of patients who had GCL + IPL thickness reductions to less than the first percentile exhibited a correspondingly significant attenuation of the melanopsin-mediated sustained pupillary response (mean [SD] pupillary diameter ratios at a point in time, 0.18 [0.1] vs 0.33 [0.09]; P < .001, generalized estimating equation models accounting for age and within-patient intereye correlations). CONCLUSIONS AND RELEVANCEIn this case-control study, attenuation of the melanopsin-mediated sustained pupillary constriction response was significantly associated with thinning of the GCL + IPL sector of the retina in the eyes of patients with MS, particularly those with a history of acute optic neuritis. Melanopsin-containing ganglion cells in the retina represent, at least in part, the composition of the retinohypothalamic tract. As such, our findings may signify the ability to elucidate a putative surrogate neurophysiologic signature that correlates with a constellation of homeostatic mechanisms in both health and illness.
Patients with progressive forms of multiple sclerosis have various symptoms which affect their quality of life significantly including depression, cognitive decline, sleep changes, bladder dysfunction, sexual dysfunction, and spasticity. Despite recent promising results on the effects of ocrelizumab on neurological disability in patients with PPMS, currently none of the immunomodulatory therapies are approved for progressive forms of multiple sclerosis. Therefore, clinicians currently mostly focus on management of well-recognized comorbidities of this disease phenotype in order to improve patients' quality of life. There are very few studies evaluating strategies of symptomatic management on progressive forms of multiple sclerosis and most of the data is derived from studies on relapsing forms of multiple sclerosis. Understanding of the risks, benefits, and limitations of these therapies can significantly affect patient care. In this article, we review common comorbidities associated with progressive forms of multiple sclerosis and outline important strategies for their symptomatic management.
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