The Avian Retroviral Receptor Tva Mediates the Uptake of Transcobalamin Bound Vitamin B ₁₂ (Cobalamin)

Abstract

The Avian sarcoma and leukosis viruses (ASLVs) are important chicken pathogens. Some of the virus subgroups, including ASLV-A and K, utilize the Tva receptor for cell entrance. Though Tva was identified three decades ago, its physiological function remains unknown. Previously, we have noted an intriguing resemblance and orthology between the chicken gene coding for Tva and the human gene coding for CD320, a receptor involved in cellular uptake of transcobalamin (TC) in complex with vitamin B12/cobalamin (Cbl). Here we show that both the transmembrane and the glycosylphosphatidylinositol (GPI)-anchored form of Tva in the chicken cell line DF-1 promotes the uptake of Cbl with help of expressed and purified chicken TC. The uptake of TC-Cbl complex was monitored using an isotope- or fluorophore-labeled Cbl. We show that (i) TC-Cbl is internalized in chicken cells; and (ii) the uptake is lower in the Tva-knockout cells and higher in Tva-overexpressing cells when compared with wild type chicken cells. The relation between physiological function of Tva and its role in infection was elaborated by showing that infection with ASLV subgroups (targeting Tva) impairs the uptake of TC-Cbl, while this is not the case for cells infected with ASLV-B (not recognized by Tva). In addition, exposure of the cells to a high concentration of TC-Cbl alleviates the infection with Tva-dependent ASLV. IMPORTANCE: We demonstrate that the ASLV receptor Tva participates in the physiological uptake of TC-Cbl, because the viral infection suppresses the uptake of Cbl and vice versa. Our results pave the road for future studies addressing the issues: (i) whether a virus infection can be inhibited by TC-Cbl complexes in vivo; and (ii) whether any human virus employs the human TC-Cbl receptor CD320. In broader terms, our study sheds light on the intricate interplay between physiological roles of cellular receptors and their involvement in virus infection.

Knock-Out of Retrovirus Receptor Gene Tva in the Chicken Confers Resistance to Avian Leukosis Virus Subgroups A and K and Affects Cobalamin (Vitamin B12)-Dependent Level of Methylmalonic Acid

“…The absence of distinct phenotype is consistent with the mouse model, where CD320 knock-out does not completely abrogate the cbl import, and clinically asymptomatic human cases with CD320 defect [49,50], implying a parallel transport mechanism [23,24]. An alternative cbl uptake pathway is also visible in the tva chicken cells, where the uptake of cbl-transcobalamin remains significantly above the background level [21]. MMA is the most sensitive marker of altered cbl metabolism, whereas other metabolites will be analyzed in the inbred Tva KO line.…”

“…In addition to its function as ALV receptor, Tva was recently identified as a specific receptor for cbl-saturated transcobalamin necessary for efficient cbl internalization into cells [21]. Thus, Tva ensures the bioavailability of cbl for internal conversions of MMA-CoA to succinyl-CoA and remethylation of homocysteine.…”

“…Its orthology with the mammalian CD320 (originally denoted 8D6A) gene [9] provided a clue when CD320 was identified as a cellular receptor for the uptake of cobalamin (cbl, vitamin B 12 )/transcobalamin II complex in humans [20]. This function of Tva has recently been confirmed by showing internalization of labeled cbl in chicken cells: the tva knock-out decreases cbl uptake ten times, whereas overexpression of tva displays a tenfold increase in cbl uptake [21]. Furthermore, ALV-A infection interferes with cbl uptake.…”

Residues E53, L55, H59, and G70 of the cellular receptor protein Tva mediate cell binding and entry of the novel subgroup K avian leukosis virus

“…7 E ). Studies have indicated that Tva is related to the uptake of transcobalamin-bound vitamin B 12 /Cbl ( 29 ). The ability to internalize vitamin B 12 /Cbl of DF-1-rTva was detected using the confocal assay.…”

“…Therefore, precise editing of key sites of cell receptors can not only resist virus infection but will also not affect the physiological function of the receptor protein, which will be an important direction of antiviral research in the future. Tva is involved in TC-mediated Cbl uptake ( 29 ). Although Tva-deficient gene-edited chickens have been generated and could completely resist ALV-K infection, they exhibit specific Cbl metabolic disturbances due to the lack of Tva ( 42 ).…”