AbstractNeuronal nuclei are a recurrent organisational motif in the vertebrate CNS, yet the mechanisms governing their assembly (nucleogenesis) remain poorly understood. Previous work has demonstrated that several classes of cell adhesion molecules, including type II cadherins and immunoglobulins, are required for the correct organisation of motor neurons during development. However, the functional importance of establishing nuclear topography for circuit formation is still unresolved. The zebrafish ocular motor system, which comprises three nuclei which innervate six extraocular muscles that control eye movements such as the optokinetic response (OKR), represents an excellent model in which to examine this question in vivo.
Using live imaging, I have described the wild-type development of ocular motor nuclei and identified critical time windows for nucleogenesis and circuit formation. Furthermore, I find that ocular motor neurons differentially express several classical cadherins during this time, indicating a role for cadherin-mediated adhesion in their development.
Perturbing this adhesion using a cell-specific dominant negative approach results in subpopulation-specific defects to oculomotor neuronal positioning, including scattering and defective contralateral migration, while the trochlear nucleus becomes elongated along the dorsoventral axis. Furthermore, I discover that adhesions between different oculomotor subpopulations also contribute to their normal positioning. Interestingly, I find that perturbations to the clustering of dorsal oculomotor neurons, which control the larval OKR, impairs this behaviour significantly, suggesting that the correct spatial organisation of ocular motor neurons also plays an important role in the function of the developing motor circuit.
Knocking out γ-catenin, a cytoplasmic binding partner of type II cadherins, leads to disaggregation of the trochlear nucleus along the dorsoventral axis, but leaves oculomotor nucleogenesis unperturbed, suggesting that oculomotor neurons may utilise an additional, or entirely different, cytoplasmic binding partner during cadherin-mediated adhesion.
During the timeframe of nucleogenesis, ocular motor neurons also express the immunoglobulin cell adhesion molecule MDGA2A. Downregulation of this protein causes a number of defects to ocular motor system development, including ectopic migration of ocular motor neurons out of the neuroepithelium onto the oculomotor nerve, suggesting that MDGA2A is required to restrict cell bodies to their appropriate positions within the CNS. Knockdown of MDGA2A also has significant consequences for the larval OKR.
Taken together, my results show that different classes of cell adhesion molecules regulate distinct aspects of ocular motor neuron positioning. Crucially, they indicate that the correct spatial organisation of ocular motor neurons may also play an important part in the function of the motor circuit. This finding offers valuable insight into the importance of establishing nuclear topography during development.
|Date of Award||1 Nov 2019|
|Supervisor||Jon Clarke (Supervisor)|