Genetic regulation of macrophage activation: understanding the function of Nramp1 (=Ity/Lsh/Bcg).

The Nramp1 gene was originally described as Ity/Lsh/Bcg, a single gene controlling resistance and susceptibility of inbred mice to a range of intramacrophage pathogens. Functional studies demonstrated that Ity/Lsh/Bcg had multiple pleiotropic effects on macrophage activation pathways, broadening interest in the gene to include its candidacy as an autoimmune disease susceptibility gene. In 1993 the gene was positionally cloned and found to encode a polytopic integral membrane protein of unknown function. Subsequent studies have localized the protein to late endosomal and lysosomal compartments, and demonstrated that it functions as an iron transporter. Precisely how this function influences macrophage activation pathways is still under investigation, but is likely to include direct effects on pathogen survival in the endosomal/lysosomal compartment as well as influences on intracellular signalling pathways and in regulating mRNA stability. Several studies now provide evidence for a role for NRAMP1 in determining human susceptibility to autoimmune (rheumatoid arthritis. juvenile rheumatoid arthritis, diabetes, Crohn's disease) and infectious (tuberculosis, leprosy) diseases. Amongst these. data are accumulating to support the hypothesis that a functional Z-DNA forming repeat polymorphism in the promoter region of human NRAMP1 contributes directly to disease susceptibility. Four alleles have been observed, alleles 1 and 4 are rare (gene frequencies approximately equal to 0.001), alleles 2 and 3 occur at gene frequencies approximately 0.25 and approximately 0.75, respectively. In the absence of exogenous stimuli, alleles 1, 2 and 4 are poor promoters of gene expression in a luciferase reporter gene system; allele 3 drives high expression. Allele 3 shows allelic association with autoimmune disease susceptibility, allele 2 with infectious disease susceptibility. Hence, balancing selection is likely to be maintaining these two alleles in human populations. Although the association of NRAMP1 with autoimmune disease susceptibility may be related to any one of the multiple pleiotropic effects associated with macrophage activation, the function of NRAMP1 as an iron transporter now prompts more interesting speculation that regulation of iron transport may contribute directly to the disease phenotype in arthritic disease. Patients suffering from rheumatoid arthritis show increased deposition of iron in the synovial membrane, which may contribute to free radical generation and local inflammation. Further analysis of NRAMP1 function will continue to be of importance in understanding the molecular basis to autoimmune and infectious disease susceptibility.

Genetic regulation of leishmanial and mycobacterial infections: the Lsh/Ity/Bcg gene story continues.

A common basis to genetic regulation of leishmanial and mycobacterial infections is provided by the action of the murine Lsh/Ity/Bcg gene in controlling the priming/activation of macrophages for antimicrobial activity. This relies on the TNF-alpha-dependent sustained expression of the inducible nitric oxide synthase (iNOS) gene responsible for the generation of large amounts of toxic nitric oxide (NO). The Lsh/Ity/Bcg gene has many pleiotropic effects, including differential expression of the early response gene KC following stimulation of macrophages with bacterial lipopolysaccharide (LPS) and mycobacterial lipoarabinomannan (LAM). The major signal transduction pathway involved in KC induction requires the generation of low levels of NO via constitutive nitric oxide synthase (cNOS) activity, leading to activation of guanylate cyclase and the cGMP-dependent kinase pathway. NO therefore appears to provide a common link between the early influence of Lsh in regulating the expression of genes which mediate many pleiotropic effects, and the later production of NO as the final effector mechanism for kill. The recently cloned candidate for Lsh/Ity/Bcg, designated Nramp for Natural resistance associated macrophage protein, encodes a polytopic integral membrane protein that has structural features common to prokaryotic and eukaryotic transporters and includes a conserved binding-protein-dependent transport motif which may be involved in interaction with peripheral ATP-binding subunits. The N-terminal sequence also carries a proline/serine rich putative SH3 binding domain, consistent with a role for tyrosine kinases in regulating Nramp function. (ABSTRACT TRUNCATED AT 250 WORDS)