The mechanisms behind the development and progression of Parkinson’s disease (PD) are still not fully understood. Several pathophysiological mechanisms have been implicated in the progression of the disease including oxidative stress, mitochondrial dysfunction, excitotoxicity, protein aggregation, and neuroinflammation. Since current therapeutic options for PD treat only the symptoms without influencing disease progression, recent translational studies have focused on identifying factors that protect against putative pathophysiological mechanisms of PD. We show herein, that the chemokine-like signaling protein, Prokineticin-2 (PK2), is expressed and secreted at higher levels in dopaminergic neurons during the acute phase of inflammatory or neurotoxic stress. Furthermore, we are the first to demonstrate that this protein can protect dopaminergic neurons from MPP+-induced cell death through the activation of the MAPK and Akt pathways. PK2 treatment also protected dopaminergic neurons from mitochondrial fragmentation, while increasing the mitochondrial biogenesis mediators Pgc1α and TFAM. Importantly, overexpression of PK2 by injecting PK2 AAV 2/5 into the mouse striatum significantly protected against MPTP-induced behavioral deficits and dopaminergic neuronal loss, whereas blocking PK2 signaling with the receptor antagonist PKRA7 exacerbates both of these MPTP effects.

Activation of microglia and astrocytes are both hallmarks of PD in the brain, and blocking microglia-produced inflammation has been found to protect dopaminergic neurons in animal models of PD. However, the mechanisms behind the activation of these cells are not fully understood, especially in astrocytes. Both microglia and astrocytes constantly survey their environment and respond to specific stimuli. Microglia can shift their phenotype by expressing either protective signaling molecules or inflammation-related factors, depending on the type of stimuli. Recent studies indicate that astrocytes undergo similar phenotypic changes that were previously known only for microglia.

During our investigations of PK2, we made novel discoveries about this secreted chemokine-like protein’s role in microglial and astrocytic activation, and its contribution to neuroinflammation. PK2 treatment in microglia lead to a shift in the microglial activation state towards a more anti-inflammatory phenotype by reducing basal levels of damaging reactive oxygen species (ROS) and inflammatory cytokines and increasing protective factors such as Arginase1 and Nucelear factor E2-related factor 2 (Nrf2). Similarly, recombinant PK2 (rPK2) attenuated the LPS-induced production of ROS, nitric oxide, pro-inflammatory cytokines and mitochondrial bioenergetic dysfunction. Overexpression of PK2 in vivo by injecting PK2 AAV into the striatum led to morphological changes in microglia, such as increased number and length of branch processes. Interestingly, PK2 AAV 2/5 protected microglia from MPTP-induced morphological changes, whereas the PK2 receptor antagonist PKRA7 exacerbated these effects. Our studies in astrocytes produced similar results, wherein PK2 treatment could induce the expression of protective factors such as Arginase1 and Nrf2 while decreasing inflammatory factors such as ROS and reactive nitrogen species (RNS). PK2 treatment in both mouse and human astrocytes increased the production and secretion of the neuroprotective glial cell-line derived neurotrophic factor (GDNF). Importantly, the GDNF produced and secreted by PK2 overexpression significantly protected a human dopaminergic neuronal culture from MPP+-induced cell death. Overall, we found that PK2 signaling has a direct neuroprotective effect, and that it can promote microglia and astrocytes to protect neurons through the reduction of inflammation and the production of neuroprotective factors.