Using retroviruses that cause leukemia or neurological diseases in rodents, our laboratory conducts studies to obtain basic information on how these viruses induce molecular changes in normal cells that result in pathological consequences. Our goal is to design rational strategies to counteract the molecular changes observed and to eventually apply this to human diseases. The major focus of our research is to understand the molecular basis for leukemia development by utilizing retrovirus-based mouse model systems. We are currently studying the Friend spleen focus-forming virus (SFFV), which is an excellent model for studying how deregulation of signal transduction pathways can lead to leukemia. When injected into susceptible strains of mice, SFFV induces a rapid erythroleukemia consisting of two stages. In the first stage, virus-infected erythroid cells proliferate, differentiate and survive in the absence of their normal regulator erythropoietin (Epo), leading to erythroid hyperplasia and polycythemia. Our studies have shown that this is due to expression of the unique SFFV envelope glycoprotein, gp55, which interacts with the Epo receptor complex and a short form of the receptor tyrosine kinase Stk, sf-Stk, at the cell surface. This results in activation of sf-Stk and the constitutive activation of various components of the Epo signal transduction pathway, which mediate proliferation, survival and differentiation of the virus-infected cells in the absence of Epo. Studies are in progress to determine how sf-Stk becomes activated in SFFV-infected cells and to identify sf-Stk substrates. Also, studies have been initiated to test pharmacological inhibitors of signal transducing molecules activated by SFFV for their ability to block the development of SFFV-induced erythroleukemia. The second stage of erythroleukemia induced by SFFV is characterized by the outgrowth of a rare, truly malignant erythroid cell in which SFFV has activated the myeloid transcription factor PU.1. These cells are blocked in differentiation, allowing the outgrowth of transformed erythroid cells. Our studies indicate that these cells are specifically blocked in the activation of Stats 1 and 3 DNA binding activity. There is a direct correlation between cells expressing high levels of PU.1 and those blocked in Stat DNA-binding activity, and studies are in progress to determine how PU.1 expression in these cells results in a block in Stat DNA-binding activity. We have also determined that transformed erythroid cells from SFFV-infected mice require c-Jun N-terminal kinase (JNK) for their proliferation and survival and have initiated studies to test a specific JNK inhibitor for its ability to block the growth of these cells in mice. As a second retroviral model system, we have been studying PVC-211 murine leukemia virus (MuLV), which causes a rapid neurodegenerative disease in rats. PVC-211 MuLV provides an important model for understanding how retroviruses can undergo genetic changes that alter their interaction with cells in the host to cause novel biological effects. Our previous studies indicated that PVC-211 MuLV, unlike other MuLVs, can efficiently infect brain capillary endothelial cells (BCEC). Further analysis indicated that the envelope protein of PVC-211 MuLV contains two amino acid changes in its receptor binding domain that result in a unique heparin-binding site that may facilitate its interaction with negatively charged BCECs. Since BCECs are the only cells within the central nervous system infected with PVC-211 MuLV, we have been examining these cells for changes that may indirectly lead to the death of neurons. Our studies have shown that BCEC from PVC-211 MuLV-infected rats express high levels of inducible nitric oxide synthase (iNOS) and the chemokine LIX, and studies are in progress to determine how virus infection results in their activation and how their expression in BCECs may indirectly cause the death of neurons. We have also initiated studies to determine if we can block virus-induced neurodegeneration in rats using pharmacological inhibitors of iNOS or by giving rats erythropoietin, which was recently shown to be neuroprotective.
