SUMMARY La Crosse virus (LACV), a zoonotic Bunyavirus, is a major cause of pediatric viral encephalitis in the United States. A hallmark of neurological diseases caused by LACV and other encephalitic viruses is the induction of neuronal cell death. Innate immune responses have been implicated in neuronal damage, but no mechanism has been elucidated. By using in vitro studies in primary neurons and in vivo studies in mice, we have shown that LACV infection induced the RNA helicase, RIG-I, and mitochondrial antiviral signaling protein (MAVS) signaling pathway, resulting in upregulation of the sterile alpha and TIR-containing motif 1 (SARM1), an adaptor molecule that we found to be directly involved in neuronal damage. SARM1-mediated cell death was associated with induced oxidative stress response and mitochondrial damage. These studies provide an innate-immune signaling mechanism for virus-induced neuronal death and reveal potential targets for development of therapeutics to treat encephalitic viral infections.

Aging is a complex biological process and environmental risk factors like pesticide exposure have been implicated in the increased incidence of age-related neurodegenerative diseases like Parkinson’s disease (PD) but the etiology remains unknown. There is also lack of a proper animal model system to study the progressive effect of these environmental toxins on age-associated neurodegeneration. In this study, we established a drosophila model of aging to study the age-dependent vulnerability to the environmental toxin rotenone that has been implicated in sporadic cases of PD. We demonstrate that age plays a determining role in the increased susceptibility to chronic rotenone exposure that is accompanied by severe locomotor deficits, decreased lifespan and loss of dopaminergic (DA) neurons. Chronic low dose exposure to rotenone results in the rapid induction of the neurodegenerative molecule SARM1/dSarm. Further, the age-dependent dSarm induction is accompanied by a heightened inflammatory response (increased expression of Eiger and Relish) that is independent of reactive oxygen species (ROS) generation in the observed rotenone-induced neurotoxicity. dSarm induction and subsequent locomotor deficits is reversed in the presence of the anti-inflammatory molecule resveratrol. Thus, dSarm and heightened inflammatory responses may play a crucial role in age-dependent vulnerability to the pesticide rotenone thus making it an attractive target to help develop cost-effective therapeutic strategies to prevent ongoing dopaminergic neuronal loss as seen in PD.

Neuronal apoptosis is a key aspect of many different neurological diseases, but the mechanisms remain unresolved. Recent studies have suggested a mechanism of innate immune-induced neuronal apoptosis that may act through the stimulation of toll-like receptors (TLR) in neurons. TLRs are stimulated both by pathogen associated molecular patterns (PAMPs) as well as by damage-associated molecular patterns (DAMPs), including micro-RNAs released by damaged neurons. In the current study, we identified the mechanism responsible for TLR7/TLR9-mediated neuronal apoptosis. TLR-induced apoptosis required endosomal localization of TLRs but was independent of MyD88 signaling. Instead, apoptosis required the TLR adaptor molecule, sterile alpha armadillo motif (SARM1), which localized to the mitochondria following TLR activation and was associated with mitochondrial accumulation in neurites. Deficiency in SARM1 inhibited both mitochondrial accumulation in neurites and TLR-induced apoptosis. These studies identify a non-MyD88 pathway of TLR7/TLR9 signaling in neurons and provide a mechanism for how innate immune responses in the CNS directly induce neuronal damage.