In vivo and in vitro approaches to characterising axon growth and metabolic alterations in Parkinson's disease

A recent pathological hypothesis of Parkinson's disease suggests that at the time of disease onset, midbrain dopaminergic cells lose their synaptic connectivity before dying. The preposition that dopaminergic axons degenerate initially and predominantly in PD offers new critical opportunities for the development of neuroprotective and restorative therapies. However, we know little about how dopaminergic neurons innervate their target areas. Because of their functional importance in PD, understanding the mechanisms underlying the development and function of midbrain dopaminergic circuits is of considerable interest. Therefore, the initial focus of my research was on exploring novel pathways governing the development of dopaminergic circuitry. Using a mouse model, my work defined a novel role for Nolz1, a developmentally expressed transcription factor, in regulating aspects of dopaminergic axon growth and target innervation. I show that Nolz1 regulates critical aspects of striatal development in the mouse embryo and that aberrant striatal patterning alters the outgrowth of nigrostriatal projection neurons (Chapter 2). The premature degeneration of axonal projections in PD has been linked to changes in mitochondrial function. Impairment of mitochondrial dynamics, transport, and loss of plasticity of axon terminals precedes the onset of neuronal degeneration in this neurodegenerative disease. PINK1 is a mitochondrial kinase that ensures mitochondrial health and mutations in PINK1 cause PD. My work explored how mutations in PINK1 alter cellular metabolism, using a Drosophila model of PD. Multi-omics analysis of pink1 mutant flies uncovered metabolic and transcriptional modifications in cysteine metabolism which coincided with defects in mitochondrial respiration. These findings confirmed PINK1's canonical role in mitochondrial function, while highlighting the relevance of cysteine metabolism to compensate for early and selective defects in mitochondrial respiration and ATP production (Chapter 3). To answer if we can recapitulate these changes in a human model, I then used human induced pluripotent stem cells (iPSCs). I differentiated human induced pluripotent stem cells (iPSCs) carrying a recently discovered I368N mutation in PINK1 into neural precursor cells (NPCs) and examined their metabolic profile. I found that the PINK1 I368N mutation causes global metabolic changes that broadly validate the comprehensive number of hits recovered in pink1 mutant flies, suggesting a significant overlap between my integrated networks. This convergence of disease phenotypes points to a common disease mechanism that may offer a unifying perspective on early-stage PD pathology (Chapter 4). Together, these results shed light on novel genes and pathways that could be exploited for therapeutic intervention.