Neuroinflammation in Parkinson's disease: Its role in neuronal death and implications for therapeutic intervention

Parkinson's disease (PD) is the second most common neurodegenerative disease, after Alzheimer's disease. The potential causes of PD remain uncertain, but recent studies suggest neuroinflammation and microglia activation play important roles in PD pathogenesis. Major unanswered questions include whether protein aggregates cause the selective loss of dopaminergic neurons in the substantia nigra that underlies the clinical symptoms and whether neuroinflammation is a consequence or a cause of nigral cell loss. Within the microenvironment of the brain, glial cells play a critical role in homeostatic mechanisms that promote neuronal survival. Microglia have a specialized immune surveillance role and mediate innate immune responses to invading pathogens by secreting a myriad of factors that include, cytokines, chemokines, prostaglandins, reactive oxygen and nitrogen species, and growth factors. Some of these factors have neuroprotective and trophic activities and aid in brain repair processes; while others enhance oxidative stress and trigger apoptotic cascades in neurons. Therefore, pro- and anti-inflammatory responses must be in balance to prevent the potential detrimental effects of prolonged or unregulated inflammation-induced oxidative stress on vulnerable neuronal populations. In this review, we discuss potential triggers of neuroinflammation and review the strongest direct evidence that chronic neuroinflammation may have a more important role to play in PD versus other neurodegenerative diseases. Alternatively, we propose that genetic deficiency is not the only way to reduce protective factors in the brain which may function to keep microglial responses in check or regulate the sensitivity of DA neurons. If chronic inflammation can be shown to decrease the levels of neuroprotective factors in the midbrain, in essence genetic haploinsufficiency of protective factors such as Parkin or RGS10 may result from purely environmental triggers (aging, chronic systemic disease, etc.), increasing the vulnerability to inflammation-induced nigral DA neuron death and predisposing an individual to development of PD. Lastly, we review the latest epidemiological and experimental evidence supporting the potential use of anti-inflammatory and immunomodulatory drugs as neuroprotective agents to delay the progressive nigrostriatal degeneration that leads to motor dysfunction in PD.