Which Dopamine Neurons Live or Die in Parkinson's Disease: A Balancing Act Between Protective Astrocytes and Deleterious Microglia
Parkinson's disease (PD) is a chronic, progressive neurodegenerative disorder affecting as many as one million Americans and arises from a loss of dopamine (DA) signaling within the striatum due to the progressive degeneration of DA neurons of the substantia nigra (SN), whereas the neighboring ventral tegmental area (VTA) DA neurons are relatively spared from death, which suggests intrinsic differences between these two closely related regions. The studies presented in this thesis sought to deepen our understanding of these regional differences, and examine potential underlying mechanisms which result in differential vulnerability to disease. Studies were conducted using a new hTH-GFP reporter rat model, which aided in sub-dissection of the SN and VTA to examine not only the DA neurons, but also astrocytes and microglia. In vitro modeling of DA neurodegeneration demonstrated a novel vulnerability of VTA DA neurons, and exacerbation of SN DA neuron susceptibility, to the PD mimetic toxin MPP+ when astrocytes are mostly absent, suggesting a role for VTA astrocytes in neuroprotection. Conditioned media experiments and subsequent RNAseq of astrocytes of these sub-regions demonstrated that VTA astrocytes secrete factor(s) which are neuroprotective of both SN and VTA DA neurons and that sub-regional astrocytes exhibit vastly different transcriptional profiles. We demonstrated that GDF15, which is expressed 230-fold higher in VTA than SN astrocytes, recapitulates neuroprotection of SN, VTA and iPSC DA neurons, presumably through the receptor GFRAL expressed by these cells. The role of microglia in selective vulnerability was similarly examined in vitro. We demonstrated that microglia exhibit basal and MPP + induced sub-region specific cytokine profiles, though they are not deleterious to DA neurons at baseline. However, when coupled with MPP + treatment, microglia result in exacerbation of toxicity of SN, but not VTA DA neurons. Finally, we demonstrate that VTA astrocytes are able to mediate protection of both SN and VTA DA neurons from MPP+ toxicity, despite the presence of exacerbatory microglia, though greater numbers of microglia overburden these protective effects. Together, these studies deepen our understanding of the intrinsic differences of the SN and VTA and lay the foundations for future studies into therapeutic mechanisms of GDF15.
Kostuk, Eric Wildon, "Which Dopamine Neurons Live or Die in Parkinson's Disease: A Balancing Act Between Protective Astrocytes and Deleterious Microglia" (2018). ETD Collection for Thomas Jefferson University. AAI10749809.