The correct polarity of a neuron underlies its ability to integrate precise circuitries in the nervous system. The goal of my thesis was to investigate the pathways that establish and maintain neuron polarity/orientation in vivo. To accomplish this, I used bipolar VC4/5 motor neurons, which innervate the C. elegans egg-laying musculature, as a model system. Vulval proximal VC4/5 neurons extend axons in the left-right (LR) orientation, around the vulva; whereas vulval distal VC1-3,6 neurons extend axons along the anterior-posterior (AP) axis. A previous study showed that vang-1, a core planar cell polarity (PCP) gene, suppresses AP axon growth in VC4/5 neurons. In order to identify new components of this pathway we performed genetic screens for mutants with abnormal VC4/5 polarity/morphology. We isolated and mapped alleles of farnesyl transferase b (fntb-1) and of core PCP genes, prickle- 1 (prkl-1) and dishevelled-1 (dsh-1); all of which display tripolar VC4/5 neurons, similar to vang-1 lof. In prkl-1 and dsh-1 mutants, primary LR and ectopic AP VC4/5 axons are born simultaneously, suggesting an early role in establishing polarity. In addition, prkl-1 and dsh-1 act persistently to maintain neuron morphology/orientation. Genetic analysis of double mutants suggests that prkl-1 interacts with vang-1 in a common PCP pathway to prevent AP axon growth, while dsh-1 also acts in a parallel pathway. Furthermore, prkl-1 functions cell autonomously in neurons, whereas dsh-1 acts both cell autonomously and cell nonautonomously in epithelial cells. Notably, prkl-1 overexpression results in unipolar VC4/5 neurons, in a dose-dependent manner. In contrast, dsh-1 overexpression in VC4/5 neurons results in a lof phenotype, similar to vang-1 lof and overexpression phenotype. Remarkably, prkl-1 overexpression restores normal VC4/5 polarity in dsh-1 and vang-1 mutants, which is suggestive of a downstream role for prkl-1. Both PRKL-1 and DSH-1 are expressed in iii uniformly distributed puncta at the plasma membrane of VC4/5, similar to VANG-1; suggesting that their asymmetric distribution is not critical for neuron polarity. Furthermore, we found that the vulva epithelium induces prkl-1 expression in VC4/5; indicating a functional relationship between the egg-laying organ and neuron morphology. Moreover, a structure-function analysis of PRKL-1 revealed that the conserved PET domain and the Cterminal region are crucial to prevent AP axon growth, whereas the three LIM domains are dispensable for this role. In addition, we showed that dsh-1 also regulates the morphology of AP-oriented PDE neurons. dsh-1 promotes the formation of PDE posterior axons, contrary to its function in VC5 neurons; which indicates a context-dependent role for dsh-1 in neuronal polarity. Altogether, this thesis implicates the PCP signalling pathway in a previously unknown role, in establishing and maintaining neuronal polarity, by controlling AP axon growth in response to organ-derived polarizing cues.

The correct polarity of a neuron underlies its ability to integrate precise circuitries in the nervous system. The goal of my thesis was to investigate the pathways that establish and maintain neuron polarity/orientation in vivo. To accomplish this, I used bipolar VC4/5 motor neurons, which innervate the C. elegans egg-laying musculature, as a model system. Vulval proximal VC4/5 neurons extend axons in the left-right (LR) orientation, around the vulva; whereas vulval distal VC1-3,6 neurons extend axons along the anterior-posterior (AP) axis. A previous study showed that vang-1, a core planar cell polarity (PCP) gene, suppresses AP axon growth in VC4/5 neurons. In order to identify new components of this pathway we performed genetic screens for mutants with abnormal VC4/5 polarity/morphology. We isolated and mapped alleles of farnesyl transferase b (fntb-1) and of core PCP genes, prickle- 1 (prkl-1) and dishevelled-1 (dsh-1); all of which display tripolar VC4/5 neurons, similar to vang-1 lof. In prkl-1 and dsh-1 mutants, primary LR and ectopic AP VC4/5 axons are born simultaneously, suggesting an early role in establishing polarity. In addition, prkl-1 and dsh-1 act persistently to maintain neuron morphology/orientation. Genetic analysis of double mutants suggests that prkl-1 interacts with vang-1 in a common PCP pathway to prevent AP axon growth, while dsh-1 also acts in a parallel pathway. Furthermore, prkl-1 functions cell autonomously in neurons, whereas dsh-1 acts both cell autonomously and cell nonautonomously in epithelial cells. Notably, prkl-1 overexpression results in unipolar VC4/5 neurons, in a dose-dependent manner. In contrast, dsh-1 overexpression in VC4/5 neurons results in a lof phenotype, similar to vang-1 lof and overexpression phenotype. Remarkably, prkl-1 overexpression restores normal VC4/5 polarity in dsh-1 and vang-1 mutants, which is suggestive of a downstream role for prkl-1. Both PRKL-1 and DSH-1 are expressed in iii uniformly distributed puncta at the plasma membrane of VC4/5, similar to VANG-1; suggesting that their asymmetric distribution is not critical for neuron polarity. Furthermore, we found that the vulva epithelium induces prkl-1 expression in VC4/5; indicating a functional relationship between the egg-laying organ and neuron morphology. Moreover, a structure-function analysis of PRKL-1 revealed that the conserved PET domain and the Cterminal region are crucial to prevent AP axon growth, whereas the three LIM domains are dispensable for this role. In addition, we showed that dsh-1 also regulates the morphology of AP-oriented PDE neurons. dsh-1 promotes the formation of PDE posterior axons, contrary to its function in VC5 neurons; which indicates a context-dependent role for dsh-1 in neuronal polarity. Altogether, this thesis implicates the PCP signalling pathway in a previously unknown role, in establishing and maintaining neuronal polarity, by controlling AP axon growth in response to organ-derived polarizing cues.

The correct polarity of a neuron underlies its ability to integrate precise circuitries in the nervous system. The goal of my thesis was to investigate the pathways that establish and maintain neuron polarity/orientation in vivo. To accomplish this, I used bipolar VC4/5 motor neurons, which innervate the C. elegans egg-laying musculature, as a model system. Vulval proximal VC4/5 neurons extend axons in the left-right (LR) orientation, around the vulva; whereas vulval distal VC1-3,6 neurons extend axons along the anterior-posterior (AP) axis. A previous study showed that vang-1, a core planar cell polarity (PCP) gene, suppresses AP axon growth in VC4/5 neurons. In order to identify new components of this pathway we performed genetic screens for mutants with abnormal VC4/5 polarity/morphology. We isolated and mapped alleles of farnesyl transferase b (fntb-1) and of core PCP genes, prickle- 1 (prkl-1) and dishevelled-1 (dsh-1); all of which display tripolar VC4/5 neurons, similar to vang-1 lof. In prkl-1 and dsh-1 mutants, primary LR and ectopic AP VC4/5 axons are born simultaneously, suggesting an early role in establishing polarity. In addition, prkl-1 and dsh-1 act persistently to maintain neuron morphology/orientation. Genetic analysis of double mutants suggests that prkl-1 interacts with vang-1 in a common PCP pathway to prevent AP axon growth, while dsh-1 also acts in a parallel pathway. Furthermore, prkl-1 functions cell autonomously in neurons, whereas dsh-1 acts both cell autonomously and cell nonautonomously in epithelial cells. Notably, prkl-1 overexpression results in unipolar VC4/5 neurons, in a dose-dependent manner. In contrast, dsh-1 overexpression in VC4/5 neurons results in a lof phenotype, similar to vang-1 lof and overexpression phenotype. Remarkably, prkl-1 overexpression restores normal VC4/5 polarity in dsh-1 and vang-1 mutants, which is suggestive of a downstream role for prkl-1. Both PRKL-1 and DSH-1 are expressed in iii uniformly distributed puncta at the plasma membrane of VC4/5, similar to VANG-1; suggesting that their asymmetric distribution is not critical for neuron polarity. Furthermore, we found that the vulva epithelium induces prkl-1 expression in VC4/5; indicating a functional relationship between the egg-laying organ and neuron morphology. Moreover, a structure-function analysis of PRKL-1 revealed that the conserved PET domain and the Cterminal region are crucial to prevent AP axon growth, whereas the three LIM domains are dispensable for this role. In addition, we showed that dsh-1 also regulates the morphology of AP-oriented PDE neurons. dsh-1 promotes the formation of PDE posterior axons, contrary to its function in VC5 neurons; which indicates a context-dependent role for dsh-1 in neuronal polarity. Altogether, this thesis implicates the PCP signalling pathway in a previously unknown role, in establishing and maintaining neuronal polarity, by controlling AP axon growth in response to organ-derived polarizing cues.