Neuronal migration is essential for the morphogenesis of the developing brain, and defective migration or germinal zone (GZ) exit contributes to profound developmental and cognitive disorders, such as mental retardation, epilepsy and pediatric cancer [1-4]. Despite recent advances implicating the cytoskeleton as a critical regulator of neuronal motility [5, 6], a key remaining challenge is to understand how disparate elements (substrate adhesions, cytoskeletal components and signaling molecules) are coordinated to cooperatively execute complex neuronal motility programs, such as nucleokinesis or GZ exit. Studies in my laboratory using the cerebellar granule neuron (CGN) model illustrate that signaling through the partitioning defective (PAR) polarity complex regulates multiple aspects of neuronal motility, including radial migration initiation, neuronal adhesion to migration substrates, the cadence of nucleokinesis (i.e., centrosomal and somal motility) and a potential functional interaction between the PAR complex and Myosin II, a molecular motor that is essential for nucleokinesis [7-9];however, the upstream regulators and downstream effectors of the PAR during these processes are currently unclear. The long-term goal of this proposal is to characterize upstream regulators and downstream effector(s) of the PAR complex critical for migration to elucidate how cytoskeletal organization, adhesion dynamics and migration initiation are globally coordinated during brain development. We will use gain- and loss-of-function approaches in combination with advanced live cell imaging in ex vivo cerebellar slice preparations to test three hypotheses related to our long term goal: I. Myosin II, an actin-based motor, powers polarized organelle motility and leading-process adhesion dynamics during nucleokinesis. II. The PAR complex regulates myosin II motors to orchestrate cytoskeletal and adhesion dynamics required for nucleokinesis. III. A competitive balance between Shh signaling and Par6 regulates CGN GZ exit and radial migration initiation through JAM-C adhesion. We propose three Aims to address each of these hypotheses:
Aim 1 : Determine whether leading process Myosin II motors are necessary for centrosomal and somal motility and leading process adhesion dynamics during radial migration.
Aim 2 : Demonstrate that Par6 regulates Myosin II activity and JAM-C adhesions by scaffolding actomyosin components via an IQ motif.
Aim3 : Determine whether excess Shh activity in Patched heterozygous CGNs regulates Par6 dependant GZ exit and JAM-C adhesion. At the end of this study, we will create a new conceptual framework for an integrated model of neuronal motility and provide novel insight into the pathological mechanisms of neuronal positioning disorders and pediatric cancer.

Public Health Relevance

Proper regulation of neuronal positioning directs the formation of the neuronal laminae that are the foundation of neuronal circuitry. Errors in migration lead to developmental abnormalities that are the basis of diseases like mental retardation, epilepsy and pediatric cancer. The goal of this proposal is to understand the function of key signaling proteins and molecular motors, which are promising targets to understand the forces that power the migration of neurons in the developing brain, information that will be essential to eventually prevent or treat neuronal positioning disorders.

National Institute of Health (NIH)
National Institute of Neurological Disorders and Stroke (NINDS)
Research Project (R01)
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Neurodifferentiation, Plasticity, and Regeneration Study Section (NDPR)
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Riddle, Robert D
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St. Jude Children's Research Hospital
United States
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Ong, Taren; Solecki, David J (2017) Seven in Absentia E3 Ubiquitin Ligases: Central Regulators of Neural Cell Fate and Neuronal Polarity. Front Cell Neurosci 11:322
Trivedi, Niraj; Stabley, Daniel R; Cain, Blake et al. (2017) Drebrin-mediated microtubule-actomyosin coupling steers cerebellar granule neuron nucleokinesis and migration pathway selection. Nat Commun 8:14484
Singh, Shalini; Howell, Danielle; Trivedi, Niraj et al. (2016) Zeb1 controls neuron differentiation and germinal zone exit by a mesenchymal-epithelial-like transition. Elife 5:
Singh, Shalini; Solecki, David J (2015) Polarity transitions during neurogenesis and germinal zone exit in the developing central nervous system. Front Cell Neurosci 9:62
Ramahi, Joseph S; Solecki, David J (2014) The PAR polarity complex and cerebellar granule neuron migration. Adv Exp Med Biol 800:113-31
Trivedi, Niraj; Ramahi, Joseph S; Karakaya, Mahmut et al. (2014) Leading-process actomyosin coordinates organelle positioning and adhesion receptor dynamics in radially migrating cerebellar granule neurons. Neural Dev 9:26
Famulski, Jakub K; Solecki, David J (2013) New spin on an old transition: epithelial parallels in neuronal adhesion control. Trends Neurosci 36:163-73
Solecki, David J (2012) Sticky situations: recent advances in control of cell adhesion during neuronal migration. Curr Opin Neurobiol 22:791-8