Rab4A Control of Liver Dysfunction in Mouse Models of Systemic Lupus Erythematosus.

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Summary Systemic Lupus Erythematosus (SLE) is a devastating autoimmune disease with a poorly understood etiology which affects 20-150 per 100,000 people worldwide. Dysfunction of the immune system leads to T cell autoreactivity, overproduction of autoantibodies, dysregulated cytokine production, immune complex deposition, and increased oxidative stress. Understanding both the immunological and biochemical pathways that lead to SLE pathogenesis is critical to not only treating this disease, but also to prevent, and potentially cure, SLE. We study HRES-1/Rab4 (Rab4A), a GTPase involved in the endosomal trafficking of cell surface proteins such as CD4, which is involved in the formation and maintenance of the immune synapse between T cells and antigen presenting cells (APCs). Rab4A is also involved in the maintenance of mitochondrial membrane potential, as its disruption is linked to mitochondrial hyperpolarization that is seen in the T cells of SLE patients. Injury to the liver, the largest metabolic organ of the body, is present in about 20% of patients, however, a gap exists in our knowledge in understanding its role in disease pathogenesis and its relationship to T cell dysfunction in SLE. Preliminary studies unveiled that constitutive activation of Rab4A in lupus-prone C57Bl/6 lpr (B6.lprQ72L) mice promoted mitochondrial reactive oxygen species (ROS) generation and inflammation in the liver. Deletion of Rab4A in T cells further promoted dysfunction in the livers of B6.lprCD4KO mice. Notably, we observed increased isocitrate and decreased NADPH in the isolated mitochondria of the livers of these mice. Preliminary studies also show a significant increase in the expression of the regulatory T cell (Treg) effector protein CTLA-4 (CD152) on CD4+ T cells with constitutive activation of Rab4A, which is then reduced to baseline with T cell-specific deletion of Rab4A. The combination of increased oxidative stress with constitutive activation of Rab4A and the potentially decreased activities of Tregs with T cell-specific deletion of Rab4A suggest that Rab4A may regulate hepatocellular mitophagy and regulatory T cell function through the modulation of dynamin related protein 1 (Drp1) and the recycling of CTLA-4 (CD152). In Aim 1, we will test the hypothesis that the constitutive activation of Rab4A increases oxidative stress in hepatocytes by inhibiting mitophagy in a Drp1-dependent manner, thus preventing the turnover of IDH2 which promotes the buildup of ROS. In Aim 2, we will test the hypothesis that T cell-specific deletion of Rab4A inhibits Treg activity by blocking the trafficking of CTLA-4. We will use a recycling assay to assess the ability of Rab4A to traffic CTLA-4 to the surface of the cell, use a Treg suppression assay to test the inhibitory functions of isolated and cultured Treg cells, and use high throughput imaging flow cytometry to assess Treg function in inhibiting the immune synapse. These studies will take place at SUNY Upstate Medical University and will help provide intensive training to lay the groundwork for a career in academic medicine. These studies will elucidate a novel and concise mechanism linking Rab4A with hepatocellular mitophagy and Treg function that explains the pathogenesis of Rab4A-mediated liver dysfunction in mouse models of SLE.