We have identified a feline cDNA that confers susceptibility to FeLV-A infection. We showed by several criteria that a 3.5-kb feline cDNA that was isolated by a gene transfer strategy encodes the FeLV-A cellular receptor. Expression of this feline cDNA in normally nonpermissive MDTF cells resulted in specific binding of FeLV-A envelope protein and rendered the cells susceptible to FeLV-A infection. Analysis of the predicted polyprotein sequence encoded by this feline cDNA revealed a high amino acid identity (~93%) to the human thiamine transport protein huTHTR1. Indeed, the human thiamine transport protein rendered cells permissive to FeLV-A, further indicating that the feline receptor is the orthologue of huTHTR1. Thus, the feline receptor is most likely a thiamine transport protein in feline cells, and we have therefore called this receptor feTHTR1.
In general, the receptors for many of the gammaretroviruses are multiple transmembrane proteins that transport small molecules. Some examples include mCAT, the cationic amino acid transporter utilized by ecotropic murine leukemia viruses (29
); Pit1 and Pit2, the sodium-dependent phosphate symporters utilized by murine leukemia viruses, gibbon ape leukemia virus, and some FeLVs (28
); and FLVCR, a heme export protein used by FeLV-C (48
). Multiple membrane transport receptors are also used by other retroviruses, including beta- and deltaretroviruses. For example, certain human endogenous retroviruses and baboon endogenous virus use as receptors the neutral amino acid transporters ASCT1 and ASCT2 (50
). Human T-cell leukemia virus type 1 also appears to require a transport protein for infection, the glucose transporter Glut1 (36
). Thus, the finding that the thiamine transport protein THTR1 is a receptor for FeLV-A is in keeping with the general preference for multiple membrane transport molecules as receptors for retroviruses.
The human THTR1 protein rendered MDTF cells permissive to FeLV-A, but MDTF/huTHTR1 cells were less readily infected than MDTF/feTHTR1 cells. These findings are consistent with previous findings showing that human cells were less permissive to FeLV-A than feline cells (9
). Indeed, we found that FeLV-A infection was ~50- to 100-fold lower in human cells (HEK293T) and ~10- to 30-fold lower in MDTF-huTHTR1 cells compared to feline cells (AH927) and MDTF-feTHTR1 cells. This difference in susceptibility between human 293T cells and MDTF-huTHTR1 cells could be due to lower endogenous expression of the huTHTR1 in 293 cells. In addition, binding by FeLV-A was less readily detected with the human receptor versus the feline receptor. This may suggest that human THTR has a lower binding affinity for FeLV-A, which may explain the lower infectivity observed with this receptor orthologue.
The THTR1 transporter belongs to the reduced folate family of transporters, SLC19, of which there are three members. There are two thiamine transporters, THTR1 and THTR2 and a reduced folate carrier, RFC1. The THTR1 and thiamine transporter 2 (THTR-2), which share ~65% sequence homology, both transport thiamine but not folate (14
); the third member, RFC1, transports folate but not thiamine (40
). Human THTR1 is expressed in absorptive tissue such as the small intestine, liver, and kidney, and is also expressed in skeletal muscle and peripheral blood leukocytes (12
). This pattern of THTR1 expression appears similar in feline tissue, where high levels of feTHTR1 are found in small intestine, liver, and kidney. FeTHTR1 was also expressed at relatively high levels in cells of the lymphoid system; the expression pattern of huTHTR1 has not been examined for the lymphoid system in humans.
This broad pattern of feTHTR1 expression is consistent with the observation that FeLV-A infects many tissues in the infected cat (21
). It will be of interest to examine the expression of different feTHTR1 cell populations in the oral mucosa, as this is a presumed site of FeLV-A shedding as well as a possible portal of entry for the virus (19
). The observed expression of feTHTR1 in lymphoid cells is consistent with reports that in the early phase of FeLV-A infection, the virus can be found replicating in mononuclear leukocytes in the local lymph nodes of the head and neck (55
It is perhaps noteworthy that the autosomal recessive disorder Rogers syndrome, or thiamine responsive megaloblastic anemia, has been linked to mutations in the THTR1 gene (12
). Rogers syndrome is characterized by the occurrence of megaloblastic anemia, diabetes mellitus, sensorineural deafness, and abnormalities of the retina and heart anomalies. The decrease in intracellular thiamine in these patients leads to decreased activity of enzymatic reactions that are dependent on thiamine pyrophosphate (46
). These include enzymes in the pentose phosphate shunt pathway, namely the enzyme transketolase, which is involved in the production of ribose synthesis required for nucleic acids that are important for replication and cell function (46
). Because megaloblastic anemia has been described in some cats with FeLV-infection (11
), it is tempting to speculate that FeLV-A infection may disrupt thiamine transport function.
Down-regulation of this particular transporter in persistent retroviral infection could contribute in some manner to FeLV-induced disease. FeLV-A defines a unique receptor interference group among retroviruses; no other retroviruses are thought to share the same receptor (45
). This receptor was one of the few remaining elusive retroviral receptors to be defined and may be of particular importance given the role of the A subgroup in FeLV transmission. Moreover, recent studies have indicated a role for the FeLV-C receptor, which functions in heme export, in the specific pathology associated with this FeLV subgroup (1
). Thus, the identification of feTHR1 as the receptor for FeLV-A will allow further investigations into the mechanism of FeLV transmission and pathogenesis, including whether the function of the receptor in thiamine transport has any specific role in the disease process.