Osteopontin/Secreted Phosphoprotein-1 Behaves as a Molecular Brake Regulating the Brain Translocator Protein-Dependent Neuroinflammatory Response to Chronic Viral Infection

Abstract Background Osteopontin (OPN) as a secreted signaling protein, is dramatically induced in response to cellular injury and neurodegeneration. Microglial inflammatory responses in the brain are tightly associated with the neuropathologic hallmarks of neurodegenerative disease, but understanding of the molecular mechanisms remains in several contexts, poorly understood. Methods Positron emission tomography (PET) neuroimaging using radioligands to detect increased expression of the translocator protein (TSPO) receptor in the brain, is a non-invasive tool used to track neuroinflammation in living mammals. Results In humanized, chronically HIV-infected mice in which OPN expression was knocked down with functional aptamers, uptake of TSPO radioligand, DPA-713 was markedly upregulated in the hippocampus, cortex, olfactory bulb, cerebellum and significantly increased in other key brain regions analyzed compared to controls. TSPO+ microglia were detected by immunolabeling of post-mortem brain tissue, thus validating the neuroimaging findings. Unexpectedly, two types of neurons also selectively stained positively for TSPO. The reactive cells were the specialized neurons of the cerebellum, Purkinje cells, and a subset of tyrosine hydroxylase positive neurons of the substantia nigra. Two-way ANOVA of immunoreactivity revealed that a well-validated marker of microglial activation in brain tissue, ionized calcium-binding adaptor molecule-1 (Iba-1) was significantly increased, in an interaction that depended on HIV replication. Interestingly, similar analyses of TSPO immunoreactivity showed a significant interaction with OPN expression. Conclusions Collectively, these findings using a model of chronic HIV-infection revealed for the first time two key findings: 1) two different pathways of neuroinflammation are activated, and 2) osteopontin acts as a molecular brake regulating in the brain, the inflammatory response to HIV infection.