| dc.description.abstract | Migration of neurons is essential for proper nervous system development. Defects in neural development can lead to several neurological disorders. Hence it is important to understand the mechanism of neuronal migration along with the signaling pathways required. Caenorhabditis elegans is a useful system to study neuronal migration due to its well-characterized nervous system and fully sequenced genome. We use Q neuroblasts to study neuronal cell migration. The QR and QL neuroblasts, born in the posterior lateral region of the worm, undergo initial polarizations in the anterior and posterior directions respectively. They then migrate in the direction of protrusion and divide to produce three neurons of which AQR (from QR) migrates anteriorly to near the anterior deirid, and PQR (from QL) posteriorly to near the phasmid ganglia. Secreted Wnt ligands control the direction of Q descendent migrations, but the initial protrusions and migrations are independent of the EGL-20/Wnt signal. Previous studies have shown that the transmembrane proteins UNC-40/DCC, PTP-3/LAR and MIG-21 are required to direct the early Q cell migrations (Honigberg and Kenyon, 2000) (Middelkoop et al., 2012). To elucidate the genetic interaction between unc-40, ptp-3 and mig-21, we built double mutants and used statistics to compare the defects observed to the single mutants. Our mutant analyses showed that MIG-21 and PTP-3 function in the same genetic pathway in both QR and QL. In QL, UNC-40 acts redundant to MIG-21/PTP-3 in directing posterior migration. In QR, UNC-40 and MIG-21/PTP-3 inhibit each other's role in posterior migration to direct anterior migration. Cell specific rescue experiments showed that unc-40, ptp-3 and mig-21 act cell autonomously in directing Q neuroblast migration. We wanted to identify other genes that function with unc-40, ptp-3 and mig-21 in QR and QL. We isolated cdh-4, a fat like cadherin from a forward genetic screen. Previous work (Schmitz et al., 2008) has shown that CDH-4 is required for Q descendent migration. cdh-4 mutants show defects in early QL and QR(~75% migrate posterior) migrations. To understand how cdh-4 interacts with unc-40, ptp-3 and mig-21, we built double mutants of cdh-4 with the above genes. In QR, both unc-40RNAi and ptp-3RNAi significantly suppressed the posterior migration seen in cdh-4 mutants suggesting that, the posterior migration seen in cdh-4 mutants required functional UNC-40 and PTP-3. The above result shows that CDH-4 has a role in both UNC-40 and PTP-3 pathways, which might explain why cdh-4 mutants show a high percentage of QR migrating posterior. In QL however, unc-40RNAi; cdh-4 mutants show an increase in percentage of QL migrating anterior, suggesting that UNC-40 and CDH-4 are required in redundant pathways for posterior QL migration. In contrast, ptp-3RNAi; cdh-4 mutants resembled cdh-4 mutants alone, suggesting that PTP-3 and CDH-4 might function in the same genetic pathway. CDH-4 driven by its endogenous promoter is expressed in Q cells during its migrations. We wanted to check further if cdh-4 functions in the Q cell to direct its migration. Mosaic analysis experiments showed that cdh-4 functions non-cell autonomously to regulate Q cell migrations despite being expressed in the Q cell during its migration. Together, we have identified the roles of transmembrane proteins UNC-40, PTP-3, MIG-21 and CDH-4 in directing Q cell migrations. We have also characterized a signaling network through which these proteins function and thereby provide some insight into mechanism of neuronal cell migration. As mentioned earlier, we have identified and characterized the role of genes that are required to direct both early and subsequent descendant Q neuroblast migration. However, genes like egl-20/Wnt and mab-5/Hox are exclusively required to direct Q descendant migration. We wanted to identify other genes that function to direct QR and QL descendant migrations. We identified sdn-1/Syndecan, a heparan sulfate proteoglycan, and characterized its role in directing anterior QR descendant migration. To understand how sdn-1 interacts with mig-13, a previously characterized gene in anterior QR guidance, we built sdn-1; mig-13 double mutants. In AQR, sdn-1; mig-13 double mutants significantly enhanced the defects observed in the single mutant backgrounds suggesting that they function in parallel pathways to direct AQR migrations. Also, sdn-1 mutants suppressed the anterior migration of PQR seen in a mab-5 loss of function background, suggesting that SDN-1 functioned genetically downstream of MAB-5 and was possibly regulated by MAB-5. In sum, we are starting to classify the genetic pathways required to direct both early QR and QL and their descendant neuron migration. These signaling pathways characterize the inherent asymmetry between these left-right cells. Understanding these interactions will help us uncover the more complex signaling systems in mammalian nervous system development. | |
