In the developing brain, cell polarization guides mitosis, migration, axodendritic outgrowth, synapse assembly, and circuit formation. Polarity is established early in development as precursor cells divide. This initial polarization is verified and adjusted at later stages of development, and so provides a correctable framework for subsequent brain circuitry. The broad objective of the research proposed here is to understand these two phases of polarity – initiation and maintenance – in molecular terms. In our studies, we focus on cerebellar granule cell precursors (GCPs), a relatively homogeneous cell population that migrates in a synchronous way from a proliferative zone in the developing cerebellum to distal locations to become part of mature brain circuitry. Previous studies under auspices of this grant have established brain-derived growth factor (BDNF) as a key extracellular cue for orientation of GCPs and subsequent migration.
Our preliminary work gives rise to a testable model. We hypothesize that 1) early in development GCPs divide symmetrically and give rise to two proliferative cells, 2) at later times GCPs divide asymmetrically to produce one proliferative cell and one cell that migrates towards the high concentration of BDNF in the IGL, 3) the BDNF gradient corrects and/or reinforces pre-existing polarity of post-mitotic daughter cells using a conserved molecular cascades important for polarity that involves the endocytic protein numb, and 4) numb promotes endocytosis and localization of TrkB receptors in leading processes, and thereby stimulates chemotactic migration. This model generates testable predictions; the two specific aims of our study plan will test these predictions:
Aim 1: To test the prediction that BDNF regulates numb and numblike in migrating GCPs and these components promote localized accumulation of TrkB receptors in signaling endosomes.
Aim 2: To test the prediction that molecules of the numb family are needed for radial migration of GCPs from external to internal granule cell layer (EGL to IGL).
Our studies on polarity and migration will lead to approaches for restricting the extensive migration of malignant brain tumor cells, and for correcting errors in migration that predispose to defects in cerebellar circuitry in neuropsychiatric disorders such as dyslexia and autism.