Infectious diseases have re-emerged as a major health problem in North America, in part die to the widespread emergence of antibiotic resistance. Tuberculosis is a dramatic example of this threat, with the appearance of highly virulent and multidrug resistant M. tuberculosis, and the prevalence of TB in AIDS patients. The mechanisms of defense against intracellular parasites, and the bacterial strategies underlying survival and replication in host phagocytes remain poorly understood. A better understanding of host defenses against such infections may suggest new strategies for intervention in these diseases. Using a genetic approach, the PI has identified a new component (Nramp1) of anti-microbial defenses of phagocytes. Mutations at Nramp1 in mice cause susceptibility to several intracellular infections, and polymorphic variants at human NRAMP1 are associated with susceptibility to TB and leprosy in endemic areas of disease. Nramp1 is part of a large family of membrane transporters that has been highly conserved from bacteria to man. Nramp1 is expressed in the lysosomal compartment of macrophages and is targeted to the membrane of bacterial phagosomes soon after phagocytosis. By homology with the known substrates of other Nramp family members, it is proposed that Nramp1 functions as a divalent cation efflux pump at the phagosomal membrane to suppress bacterial replication. The current proposal has five major goals. The first is to study in the mouse the role of Nramp1 and other genes in regulating Mtb replication in the lungs. The second is to understand how Nramp1 delivery affects the physiological properties of the phagosome including maturation, acidification and bactericidal activity. The role of Nramp1 in neutrophil function will also be studied. The third goal is to identify the substrate and mechanism of transport of Nramp1 at the phagosomal membrane. The fourth is to identify protein determinants responsible for Nramp1 targeting to the lysosome and residues essential for substrate binding and transport. The fifth is to study the role of the close Nramp2 homologue in divalent cation transport in normal tissues, including at the phagosomal membrane. Together, these studies should clarify the role and mechanism of action of Nramp1 in phagocyte anti-microbial defenses, which may in turn suggest new avenues for intervention in TB and other infectious diseases.
