Gyorgy Nagy

Regulation of CD4 Expression via Recycling by HRES-1/RAB4 Controls Susceptibility to HIV Infection

A novel 2986-base transcript encoded by the antisense strand of the HRES-1 human endogenous retrovirus was isolated from peripheral blood lymphocytes. This transcript codes for a 218-amino acid protein, termed HRES-1/Rab4, based on homology to the Rab4 family of small GTPases. Antibody 13407 raised against recombinant HRES-1/Rab4 detected a native protein of identical molecular weight in human T cells. HRES-1 nucleotides 2151-1606, located upstream of HRES-1/Rab4 exon 1, have promoter activity when oriented in the direction of HRES-1/Rab4 transcription. The human immunodeficiency virus, type 1 (HIV-1), tat gene stimulates transcriptional activity of the HRES-1/Rab4 promoter via trans-activation of the HRES-1 long terminal repeat. Transfection of HIV-1 tat into HeLa cells or infection of H9 and Jurkat cells by HIV-1 increased HRES-1/Rab4 protein levels. Overexpression of HRES-1/Rab4 in Jurkat cells abrogated HIV infection, gag p24 production, and apoptosis, whereas dominant-negative HRES-1/Rab4S27N had the opposite effects. HRES-1/Rab4 inhibited surface expression of CD4 and targeted it for lysosomal degradation. HRES-1/Rab4S27N enhanced surface expression, recycling, and total cellular CD4 content. Infection by HIV elicited a coordinate down-regulation of CD4 and up-regulation of HRES-1/Rab4 in PBL. Moreover, overexpression of HRES-1/Rab4 reduced CD4 expression on peripheral blood CD4+ T cells. Stimulation by HIV-1 of HRES-1/Rab4 expression and its regulation of CD4 recycling reveal novel coordinate interactions between an infectious retrovirus and the human genome.

Transaldolase deficiency influences the pentose phosphate pathway, mitochondrial homoeostasis and apoptosis signal processing.

TAL (transaldolase) was originally described in the yeast as an enzyme of the PPP (pentose phosphate pathway). However, certain organisms and mammalian tissues lack TAL, and the overall reason for its existence is unclear. Recently, deletion of Ser(171) (TALDeltaS171) was found in five patients causing inactivation, proteasome-mediated degradation and complete deficiency of TAL. In the present study, microarray and follow-up Western-blot, enzyme-activity and metabolic studies of TALDeltaS171 TD (TAL-deficient) lymphoblasts revealed co-ordinated changes in the expression of genes involved in the PPP, mitochondrial biogenesis, oxidative stress, and Ca(2+) fluxing. Sedoheptulose 7-phosphate was accumulated, whereas G6P (glucose 6-phosphate) was depleted, indicating a failure to recycle G6P for the oxidative branch of the PPP. Nucleotide analysis showed depletion of NADPH and NAD(+) and accumulation of ADP-ribose. TD cells have diminished Deltapsi(m) (mitochondrial transmembrane potential) and increased mitochondrial mass associated with increased production of nitric oxide and ATP. TAL deficiency resulted in enhanced spontaneous and H(2)O(2)-induced apoptosis. TD lymphoblasts showed increased expression of CD38, which hydrolyses NAD(+) into ADP-ribose, a trigger of Ca(2+) release from the endoplasmic reticulum that, in turn, facilitated CD20-induced apoptosis. By contrast, TD cells were resistant to CD95/Fas-induced apoptosis, owing to a dependence of caspase activity on redox-sensitive cysteine residues. Normalization of TAL activity by adeno-associated-virus-mediated gene transfer reversed the elevated CD38 expression, ATP and Ca(2+) levels, suppressed H(2)O(2)- and CD20-induced apoptosis and enhanced Fas-induced cell death. The present study identified the TAL deficiency as a modulator of mitochondrial homoeostasis, Ca(2+) fluxing and apoptosis.

Molecular mimicry and immunomodulation by the HRES-1 endogenous retrovirus in SLE

Genetic and environmental factors are believed to influence development of systemic lupus erythematosus (SLE). Endogenous retroviruses (ERV) correspond to the integrated proviral form of infectious retroviruses, which are trapped within the genome due to mutations. ERV represent a key molecular link between the host genome and infectious viral particles. ERV-encoded proteins are recognized by antiviral immune responses and become targets of autoreactivity. Alternatively, ERV protein may influence cellular processes and the life cycle of infectious viruses. As examples, the HRES-1 human ERV encodes a 28-kDa nuclear autoantigen and a 24-kDa small GTP-ase, termed HRES-1/Rab4. HRES-1/p28 is a nuclear autoantigen recognized by cross-reactive antiviral antibodies, while HRES-1/Rab4 regulates surface expression of CD4 and the transferrin receptor (TFR) through endosome recycling. Expression of HRES-1/Rab4 is induced by the tat gene of HIV-1, which in turn down-regulates expression of CD4 and susceptibility to re-infection by HIV-1. CD4 and the TFR play essential roles in formation of the immunological synapse (IS) during normal T-cell activation by a cognate MHC class II peptide complex. The key intracellular transducer of T-cell activation, Lck, is brought to the IS via binding to CD4. T-cell receptorζ (TCRζ) chain binds to the TFR. Abnormal T-cell responses in SLE have been associated with reduced lck and TCRζ chain levels. HRES-1 is centrally located on chromosome 1 at q42 relative to lupus-linked microsatellite markers and polymorphic HRES-1 alleles have been linked to the development of SLE. 1q42 is one of the three most common fragile sites in the human genome, and is inducible by DNA demethylation, a known mechanism of retroviral gene activation. Molecular mimicry and immunomodulation by a ERV, such as HRES-1, may contribute to self-reactivity and abnormal T and B-cell functions in SLE.