Centre for Neuroscience - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemThe Role of Wnts in dopamine axon growth and guidance( 2011)Cell transplantation, to replace lost dopamine (DA) neurons in Parkinson’s disease (PD), offers hope for many patients. While proof of principle has been demonstrated in the clinic, a number of limitations have prevented its widespread application. One such limitation has been the lack of sufficient innervation by the grafted tissue, as well as the failure to restore normal brain circuitry. Improving the connectivity of these grafted cells will be dependent on understanding how these neurons are normally wired during development. Several growth and guidance cues have been identified in the maturation of the DA pathways, however, they fail to account for all developmental events. This thesis explores the role of Wnt proteins in this context; focusing on understanding the roles of Wnts in DA axon growth and guidance during development as well as their potential for in vivo application to promote graft integration in PD. Wnts are a family of secreted proteins that have previously been shown to play important roles in the proliferation and differentiation of midbrain DA neurons during development. Elsewhere within the central nervous system, Wnts have been shown to also have roles in axon growth, guidance, and synapse formation. This research examines whether Wnts play a later role in the maturation of the DA pathways, regulating axon morphogenesis and chemotaxis. Our results demonstrate that while several Wnts are expressed within the developing midbrain, Wnt5a and Wnt7a show highly controlled spatial and temporal patterns suggestive of roles in DA axon growth and guidance. We further pursued the role of Wnt5a. We show that early in development Wnt5a, expressed within the ventral midbrain (VM), is capable of repelling DA axons out of the VM and promoting axonal elongation. Later in development of the pathway, Wnt5a expression within the VM ensures DA axons maintain their rostral trajectory, but interestingly, preventing neurite growth. We show that this negative effect on neurite growth corresponds to axonal stalling; a development event that occurs prior to entry into the target tissue (the striatum). These findings were validated in Wnt5a knockout mice. Further investigations demonstrate that the non canonical Wnt planar cell polarity (PCP) pathway is implicated in these effects on axon outgrowth and chemorepulsion, with signaling mediated, at least in part through both the Frizzled-3 and Ryk receptors A more extensive examination of the Ryk receptors shows that it not only plays a newly identified role in connectivity of DA neurons, but additionally in DA neurogenesis. We show that Ryk is temporally expressed within the VM and is specifically localized on DA neurons. Using a functional blocking antibody and examination of Ryk knockout mice, we show that Ryk influences the proliferation of VM progenitors, and thereby DA neurons. Loss of signaling through the Ryk receptor results in a significant decrease in VM progenitors, DA precursors and DA neurons; a phenotype that, unlike many Wnt-related knockouts, is maintained into later development. Together these results demonstrate that Wnt5a is the guidance cue present within the VM which is responsible for some of the earliest growth and guidance events and that the Ryk receptor is mediating some of these novel effects, in conjunction with the Frizzled family of receptors. In light of the role of Wnt5a in DA growth and guidance, we examined the effect of Wnt5a on grafted DA neurons in an animal model of PD. Our findings showed that Wnt5a maintained effects on DA axons, causing repulsion away from the Wnt5a signal. However, exposure of VM grafts to Wnt5a had no effect on axonal length. These results suggest that although Wnt5a may be capable of driving axons of grafted neurons out of the midbrain (following homotypic transplantation into the substantia nigra) additional cues are required to promote axon extension. In summary, the work presented identifies several novel roles for Wnts in the development of the midbrain DA pathways, additionally identifying receptors and signaling cascades involved. While extending our knowledge of DA development, these findings also provide new insights into regulating the connectivity of transplanted DA neurons for the treatment of PD.