Central Motivation of Depression; An Expanded Kynurenine Theory

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Project Summary Encephalitis attributed to viral infection is often refractory to established drugs. Enteroviruses (Picornaviridae) are a common cause of encephalitis with neurological sequelae; as are Arborviruses (West Nile and Zika that are emerging infectious pathogens spreading across the United States). Viral encephalitis impairs neurological function that may lead to acute seizures (ictogenesis), neurodegeneration and persistent comorbid conditions including cognitive impairment, depression and anxiety. Therefore, finding new and novel therapeutics is important for public health. Only recently was a suitable animal model developed to investigate how viral encephalitis causes this neuropathogenic sequelae. When infected with Theiler's murine encephalomyelitis virus (TMEV, a Picornavirus), C57BL/6 mice survive the infection, but develop acute seizures followed by persistent comorbid conditions including cognitive impairment and anxiety. Activation of the innate immune system is strongly implicated in development of these symptoms. Although modifications are frequently investigated as a means to limit damage to the brain, this immune response is designed and indeed critical to fight the infection. Effective therapies are needed to prevent neurological sequelae without compromising immune function. We have uncovered two proteins that appear to meet these criteria. The indoleamine 2,3-dixogenases (Ido's: Ido1 and Ido2) encode inflammation-dependent rate-limiting enzymes that initiate metabolism of tryptophan to kynurenine. Kynurenine is then metabolized in a cell-specific manner to generate either neurotoxic/seizure- inducing or neuroprotective/anti-seizure metabolites. By genetic modification of the Ido's, we have generated mice with diminished or enhanced neurological sequelae following viral encephalitis. We propose to identify the source of Ido responsible for these sequelae. To achieve this goal, we will conduct a series of rigorous multidisciplinary (ex vivo, genetic and pharmaceutical) experiments with the TMEV model of encephalitis. This mouse model recapitulates clinical observations including neurodegeneration, formation of a glial scar and the development of spontaneous seizures. Specific Aim 1 will characterize in detail the ability of viral infection to modulate Ido activity within cells implicated in the progression of encephalitis. Specific Aim 2 will define which cell's Ido activity is mediating ictogenesis and neurodegeneration. Specific Aim will characterize Ido-dependent persistent behaviors. And Specific Aim 4 will initiate translational research to examine the ability of Ido-inhibitors to attenuate neurological sequelae associated with encephalitis. Our data support a paradigm shifting hypothesis where changes in Ido activity has the potential to prevent viral-induced neurological sequelae without affecting the immune system's ability to fight infection. Data generated from the proposed experiments should have broad implications across all types of encephalitis. Understanding the molecular pathway and cellular specificity by which this process occurs will create an advanced understanding of the mechanisms operating during viral and inflammatory diseases and identify cellular and molecular targets for the development of novel therapies.