SLC52A2 [p.P141T] and SLC52A3 [p.N21S] causing Brown-Vialetto-Van Laere Syndrome in an Indian patient: First genetically proven case with mutations in two riboflavin transporters

BACKGROUND

Brown-Vialetto-Van Laere Syndrome (BVVLS), a rare neurological disorder characterized by bulbar palsies and sensorineural deafness, is mainly associated with defective riboflavin transporters encoded by the SLC52A2 and SLC52A3 genes.

METHODS

Here we present a 16-year-old BVVLS patient belonging to a five generation consanguineous family from Indian ethnicity with two homozygous missense mutations viz., c.421C>A [p.P141T] in SLC52A2 and c.62A>G [p.N21S] in SLC52A3.

RESULTS

Functional characterization based on ³H-riboflavin uptake assay and live-cell confocal imaging revealed that the effect of mutation c.421C>A [p.P141T] identified in SLC52A2 had a slight reduction in riboflavin uptake; on the other hand, the c.62A>G [p.N21S] identified in SLC52A3 showed a drastic reduction in riboflavin uptake, which appeared to be due to impaired trafficking and membrane targeting of the hRFVT-3 protein.

CONCLUSIONS

This is the first report presenting mutations in both riboflavin transporters hRFVT-2 and hRFVT-3 in the same BVVLS patient. Also, c.62A>G [p.N21S] in SLC52A3 appears to contribute more to the disease phenotype in this patient than c.421C>A [p.P141T] in SLC52A2.

Vitamin C Enhances Antiviral Functions of Lung Epithelial Cells

Vitamin C is well documented to have antiviral functions; however, there is limited information about its effect on airway epithelial cells-the first cells to encounter infections. Here, we examined the effect of vitamin C on human bronchial epithelium transformed with Ad12-SV40 2B (BEAS-2B) cells, and observed that sodium-dependent vitamin C transporter 2 (SVCT2) was the primary vitamin C transporter. Transcriptomic analysis revealed that treating BEAS-2B cells with vitamin C led to a significant upregulation of several metabolic pathways and interferon-stimulated genes (ISGs) along with a downregulation of pathways involved in lung injury and inflammation. Remarkably, vitamin C also enhanced the expression of the viral-sensing receptors retinoic acid-inducible gene 1 (RIG-1) and melanoma differentiation-associated protein 5 (MDA-5), which was confirmed at the protein and functional levels. In addition, the lungs of l-gulono-γ-lactone oxidase knockout (GULO-KO) mice also displayed a marked decrease in these genes compared to wild-type controls. Collectively, our findings indicate that vitamin C acts at multiple levels to exert its antiviral and protective functions in the lungs.

Structure/functional aspects of the human riboflavin transporter-3 (SLC52A3): role of the predicted glycosylation and substrate-interacting sites

The human riboflavin (RF) transporter-3 (hRFVT-3; product of the SLC52A3 gene) plays an essential role in the intestinal RF absorption process and is expressed exclusively at the apical membrane domain of polarized enterocytes. Previous studies have characterized different physiological/biological aspects of this transporter, but nothing is known about the glycosylation status of the hRFVT-3 protein and role of this modification in its physiology/biology. Additionally, little is known about the residues in the hRFVT-3 protein that interact with the ligand, RF. We addressed these issues using appropriate biochemical/molecular approaches, a protein-docking model, and established intestinal/renal epithelial cells. Our results showed that the hRFVT-3 protein is glycosylated and that glycosylation is important for its function. Mutating the predicted N-glycosylation sites at Asn94 and Asn168 led to a significant decrease in RF uptake; it also led to a marked intracellular (in the endoplasmic reticulum, ER) retention of the mutated proteins as shown by live-cell confocal imaging studies. The protein-docking model used in this study has identified a number of putative substrate-interacting sites: Ser16, Ile20, Trp24, Phe142, Thr314, and Asn315 Mutating these potential interacting sites was indeed found to lead to a significant inhibition in RF uptake and to intracellular (ER) retention of the mutated proteins (except for the Phe142 mutant). These results demonstrate that the hRFVT-3 protein is glycosylated and this glycosylation is important for its function and cell surface expression. This study also identified a number of residues in the hRFVT-3 polypeptide that are important for its function/cell surface expression.

Histone deacetylase inhibitors regulate vitamin C transporter functional expression in intestinal epithelial cells

Intestinal absorption of vitamin C in humans is mediated via the sodium-dependent vitamin C transporters (hSVCT1 and hSVCT2). hSVCT1 and hSVCT2 are localized at the apical and basolateral membranes, respectively, of polarized intestinal epithelia. Studies have identified low plasma levels of vitamin C and decreased expression of hSVCT1 in patients with several inflammatory conditions including inflammatory bowel disease (IBD). Investigating the underlying mechanisms responsible for regulating hSVCT1 expression are critical for understanding vitamin C homeostasis, particularly in conditions where suboptimal vitamin C levels detrimentally affect human health. Previous research has shown that hSVCT1 expression is regulated at the transcriptional level, however, little is known about epigenetic regulatory pathways that modulate hSVCT1 expression in the intestine. In this study, we found that hSVCT1 expression and function were significantly decreased in intestinal epithelial cells by the histone deacetylase inhibitors (HDACi), valproic acid (VPA), and sodium butyrate (NaB). Further, expression of transcription factor HNF1α, which is critical for SLC23A1 promoter activity, was significantly down regulated in VPA-treated cells. Chromatin immunoprecipitation (ChIP) assays showed significantly increased enrichment of tetra-acetylated histone H3 and H4 within the SLC23A1 promoter following VPA treatment. In addition, knockdown of HDAC isoforms two, and three significantly decreased hSVCT1 functional expression. Following VPA administration to mice, functional expression of SVCT1 in the jejunum was significantly decreased. Collectively, these in vitro and in vivo studies demonstrate epigenetic regulation of SVCT1 expression in intestinal epithelia partly mediated through HDAC isoforms two and three.

Upregulation of Vitamin C Transporter Functional Expression in 5xFAD Mouse Intestine

The process of obtaining ascorbic acid (AA) via intestinal absorption and blood circulation is carrier-mediated utilizing the AA transporters SVCT1 and SVCT2, which are expressed in the intestine and brain (SVCT2 in abundance). AA concentration is decreased in Alzheimer’s disease (AD), but information regarding the status of intestinal AA uptake in the AD is still lacking. We aimed here to understand how AA homeostasis is modulated in a transgenic mouse model (5xFAD) of AD. AA levels in serum from 5xFAD mice were markedly lower than controls. Expression of oxidative stress response genes (glutathione peroxidase 1 (GPX1) and superoxide dismutase 1 (SOD1)) were significantly increased in AD mice jejunum, and this increase was mitigated by AA supplementation. Uptake of AA in the jejunum was upregulated. This increased AA transport was caused by a marked increase in SVCT1 and SVCT2 protein, mRNA, and heterogeneous nuclear RNA (hnRNA) expression. A significant increase in the expression of HNF1α and specific protein 1 (Sp1), which drive SLC23A1 and SLC23A2 promoter activity, respectively, was observed. Expression of hSVCT interacting proteins GRHPR and CLSTN3 were also increased. SVCT2 protein and mRNA expression in the hippocampus of 5xFAD mice was not altered. Together, these investigations reveal adaptive up-regulation of intestinal AA uptake in the 5xFAD mouse model.

Calsyntenin-3 interacts with the sodium-dependent vitamin C transporter-2 to regulate vitamin C uptake

Ascorbic acid (AA) uptake in neurons occurs via a Na⁺-dependent carrier-mediated process mediated by the sodium-dependent vitamin C transporter-2 (SVCT2). Relatively little information is available concerning the network of interacting proteins that support human (h)SVCT2 trafficking and cell surface expression in neuronal cells. Here we identified the synaptogenic adhesion protein, calsyntenin-3 (CLSTN3) as an hSVCT2 interacting protein from yeast two-hybrid (Y2H) screening of a human adult brain cDNA library. This interaction was confirmed by co-immunoprecipitation, mammalian two-hybrid (M2H), and co-localization in human cell lines. Co-expression of hCLSTN3 with hSVCT2 in SH-SY5Y cells led to a marked increase in AA uptake. Reciprocally, siRNA targeting hCLSTN3 inhibited AA uptake. In the J20 mouse model of Alzheimer's disease (AD), mouse (m)SVCT2 and mCLSTN3 expression levels in hippocampus were decreased. Similarly, expression levels of hSVCT2 and hCLSTN3 were markedly decreased in hippocampal samples from AD patients. These findings establish CLSTN3 as a novel hSVCT2 interactor in neuronal cells with potential pathophysiological significance.

Nedd4-1 regulates human sodium-dependent vitamin C transporter-2 functional expression in neuronal and epithelial cells

The ubiquitin-proteasomal pathway regulates the functional expression of many membrane transporters in a variety of cellular systems. Nothing is currently known about the role of ubiquitin E3 ligase, neural precursor cell-expressed developmentally down-regulated gene 4 (Nedd4-1) and the proteasomal degradation pathway in regulating human vitamin C transporter-2 (hSVCT2) in neuronal cells. hSVCT2 mediates the uptake of ascorbic acid (AA) and is the predominantly expressed vitamin C transporter isoform in neuronal systems. Therefore, we addressed this knowledge gap in our study. Analysis of mRNA revealed markedly higher expression of Nedd4-1 in neuronal samples than that of Nedd4-2. Interestingly, Nedd4-1 expression in the hippocampus was higher in patients with Alzheimer's disease (AD) and age-dependently increased in the J20 mouse model of AD. The interaction of Nedd4-1 and hSVCT2 was confirmed by coimmunoprecipitation and colocalization. While the coexpression of Nedd4-1 with hSVCT2 displayed a significant decrease in AA uptake, siRNA-mediated knockdown of Nedd4-1 expression up-regulated the AA uptake. Further, we mutated a classical Nedd4 protein interacting motif ("PPXY") within the hSVCT2 polypeptide and observed markedly decreased AA uptake due to the intracellular localization of the mutated hSVCT2. Also, we determined the role of the proteasomal degradation pathway in hSVCT2 functional expression in SH-SY5Y cells and the results indicated that the proteasomal inhibitor (MG132) significantly up-regulated the AA uptake and hSVCT2 protein expression level. Taken together, our findings show that the regulation of hSVCT2 functional expression is at least partly mediated by the Nedd4-1 dependent ubiquitination and proteasomal pathways.

Valproic acid upregulates sodium-dependent vitamin C transporter-2 functional expression in neuronal cells

Neuronal uptake of ascorbic acid (AA) in humans occurs via the human sodium-dependent vitamin C transporter-2 (hSVCT2). Recent studies show that a significantly lower level of vitamin C is present in the blood of epileptic patients. Consequently, focused studies investigating the involved molecular mechanisms for hSVCT2 regulation are vital to enhance vitamin C body homeostasis. Currently, little is known about the role of valproic acid (VPA), a drug utilized to treat epilepsy and a class I histone deacetylase inhibitor (HDACi), on AA uptake in neuronal systems. Thus, this study aims to examine the effect of VPA on hSVCT2 functional expression in neuronal cells. VPA treatment upregulated the AA uptake and this increased AA uptake was associated with a significant increase in hSVCT2 expression and SLC23A2 promoter activity in SH-SY5Y cells. Knockdown of HDAC2, a predominant isoform in neuronal systems, significantly increased hSVCT2 functional expression. VPA treatment in mice displayed increased mouse (m)SVCT2 protein, mRNA and heterogenous nuclear RNA (hnRNA) expression in the brain. In addition, Yin Yang-1 (YY1), a transcription factor that drives the SLC23A2 promoter activity, protein and mRNA expression levels were markedly upregulated in VPA-treated SH-SY5Y cells and mice brain. Together, our findings suggest that VPA upregulates the functional expression of SVCT2 via HDAC2 and transcriptional mechanism(s).