Association of TM4SF4 with the human thiamine transporter-2 in intestinal epithelial cells

Vitamin C transport in neurons and epithelia is regulated by secretory carrier-associated membrane protein-2 (SCAMP2)

The human sodium-dependent vitamin C transporter-1 (hSVCT1) is localized at the apical membrane domain of polarized intestinal and renal epithelial cells to mediate ascorbic acid (AA) uptake. Currently, little is known about the array of interacting proteins that aid hSVCT1 trafficking and functional expression at the cell surface. Here we used an affinity tagging ('One-STrEP') and proteomic approach to identify hSVCT1 interacting proteins, which resolved secretory carrier-associated membrane protein-2 (SCAMP2) as a novel accessary protein partner. SCAMP2 was validated as an accessory protein by co-immunoprecipitation with hSVCT1. Co-expression of hSVCT1 and SCAMP2 in HEK-293 cells revealed both proteins co-localized in intracellular structures and at the plasma membrane. Functionally, over-expression of SCAMP2 potentiated ¹⁴C-AA uptake, and reciprocally silencing endogenous SCAMP2 decreased ¹⁴C-AA uptake. Finally, knockdown of endogenous hSVCT1 or SCAMP2 impaired differentiation of human-induced pluripotent stem cells (hiPSCs) toward a neuronal fate. These results establish SCAMP2 as a novel hSVCT1 accessary protein partner that regulates AA uptake in absorptive epithelia and during neurogenesis.

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.

Identification of transmembrane protein 237 as a novel interactor with the intestinal riboflavin transporter-3 (RFVT-3): role in functionality and cell biology

The apically localized riboflavin (RF) transporter-3 (RFVT-3) is involved in intestinal absorption of vitamin B2. Previous studies have characterized different physiological/biological aspects of the RFVT-3, but there is a lack of knowledge regarding possible existence of interacting partner(s) and consequence of interaction(s) on its function/cell biology. To address the latter, we performed yeast two-hybrid (Y2H) screening of a human colonic cDNA library and have identified transmembrane protein 237 (TMEM237) as a putative interactor with the human (h)RFVT-3; the interaction was further confirmed via "1-by-1" Y2H assay that involved appropriate positive and negative controls. TMEM237 was found to be highly expressed in human native intestine and in human intestinal epithelial cell lines; further, confocal images showed colocalization of the protein with hRFVT-3. The interaction between TMEM237 with hRFVT-3 in human intestinal epithelial HuTu-80 cells was established by coimmunoprecipitation. Expressing TMEM237 in HuTu-80 cells led to a significant induction in RF uptake, while its knockdown (with the use of gene-specific siRNA) led to a significant reduction in uptake. Transfecting TMEM237 into HuTu-80 cells also led to a marked enhancement in hRFVT-3 protein stability (reflected by an increase in the protein half-life). Interestingly, the level of expression of TMEM237 was found to be markedly reduced following treatment with TNF-α (a proinflammatory cytokine that inhibits intestinal RF uptake), while its expression was significantly upregulated following treatment with butyrate (an inducer of intestinal RF uptake). These findings identify TMEM237 as an interactor with the intestinal hRFVT-3 and show that the interaction has physiological/biological significance.

Gastrointestinal Handling of Water-Soluble Vitamins

Nine compounds are classified as water-soluble vitamins, eight B vitamins and one vitamin C. The vitamins are mandatory for the function of numerous enzymes and lack of one or more of the vitamins may lead to severe medical conditions. All the vitamins are supplied by food in microgram to milligram quantities and in addition some of the vitamins are synthesized by the intestinal microbiota. In the gastrointestinal tract, the vitamins are liberated from binding proteins and for some of the vitamins modified prior to absorption. Due to their solubility in water, they all require specific carriers to be absorbed. Our current knowledge concerning each of the vitamins differs in depth and focus and is influenced by the prevalence of conditions and diseases related to lack of the individual vitamin. Because of that we have chosen to cover slightly different aspects for the individual vitamins. For each of the vitamins, we summarize the physiological role, the steps involved in the absorption, and the factors influencing the absorption. In addition, for some of the vitamins, the molecular base for absorption is described in details, while for others new aspects of relevance for human deficiency are included. © 2018 American Physiological Society. Compr Physiol 8:1291-1311, 2018.

Distinct metabolic network states manifest in the gene expression profiles of pediatric inflammatory bowel disease patients and controls

Information on biological networks can greatly facilitate the function-orientated interpretation of high-throughput molecular data. Genome-wide metabolic network models of human cells, in particular, can be employed to contextualize gene expression profiles of patients with the goal of both, a better understanding of individual etiologies and an educated reclassification of (clinically defined) phenotypes. We analyzed publicly available expression profiles of intestinal tissues from treatment-naive pediatric inflammatory bowel disease (IBD) patients and age-matched control individuals, using a reaction-centric metabolic network derived from the Recon2 model. By way of defining a measure of 'coherence', we quantified how well individual patterns of expression changes matched the metabolic network. We observed a bimodal distribution of metabolic network coherence in both patients and controls, albeit at notably different mixture probabilities. Multidimensional scaling analysis revealed a bisectional pattern as well that overlapped widely with the metabolic network-based results. Expression differences driving the observed bimodality were related to cellular transport of thiamine and bile acid metabolism, thereby highlighting the crosstalk between metabolism and other vital pathways. We demonstrated how classical data mining and network analysis can jointly identify biologically meaningful patterns in gene expression data.