Giovanna Lalli

Signalling pathways in neural progenitor migration and differentiation
The discovery of neurogenic niches in the adult mammalian brain has opened the exciting possibility of alternative therapeutic strategies exploiting neurogenesis in trauma and neurodegeneration. For this purpose, some key questions have to be addressed:

1. How can we direct the stem cell-derived neural progenitors towards the site of injury?
2. How can we manipulate the neural progenitors to functionally integrate into pre-existing circuits to replace degenerating neurons?

We seek to dissect the signalling mechanisms controlling the migration and differentiation of stem cell-derived neural progenitors in the postnatal brain using a combination of biochemical and cell biology approaches together with high-resolution time-lapse imaging applied both in vitro and in vivo.

So far, two neurogenic niches have been identified in the normal adult mammalian brain: the subgranular zone (SGZ) of the dentate gyrus in the hippocampus and the subventricular (SVZ) zone of the lateral ventricles. Quiescent stem cells in the SVZ give rise to proliferating transient amplifying progenitors, which in turn generate neuroblasts. In the normal brain, these neuroblasts migrate tangentially in chains through a stereotyped path (known as the rostral migratory stream or RMS) towards the olfactory bulb (OB), where they disperse radially and functionally integrate into pre-existing circuitry as inhibitory interneurons, ultimately modulating neuronal activity.

Importantly, several studies have highlighted the ability of SVZ-derived neuroblasts to migrate to areas affected by trauma, stroke or neurodegeneration. However, the molecular mechanisms underlying the control of their motility and their potential to differentiate into functional neurons have yet to be determined. The possibility to culture and manipulate SVZ-derived neuroblasts both in vitro and in vivo makes them an ideal model system to investigate the signalling pathways involved in migration and differentiation of stem cell-derived progenitors.

Small GTPase signalling in neuronal polarisation and migration Neuronal migration and differentiation rely on the control of cell shape and polarity, events involving dynamic cytoskeletal reorganisation and polarized membrane delivery. These fundamental processes can be tightly coordinated by the activation of molecular switches such as the Ras family of small GTPases. The two isoforms of the Ras-like GTPase Ral (RalA and RalB) can promote neurite branching in distinct neuronal cell types. We have recently identified an additional role for RalA and one if its effectors, the exocyst complex, in neuronal polarisation by promoting the correct localisation of PAR-3, a crucial polarity determinant. We are currently investigating the function of Ral in SVZ-derived neural progenitors. Moreover, we are interested in the role other small GTPases and their regulators, including GEFs and GAPs and downstream targets may have in neuroblast motility.

Molecular regulation of neuroblast migration We have used a microarray-based approach to identify novel regulators of neuroblast migration and have started to characterise their function using a variety of techniques including RNA interference, live cell confocal microscopy and biochemistry as well as in vitro migration assays with RMS explants. We are testing the role of the most interesting candidate molecules in vivo by postnatal electroporation and slice cultures, techniques that enable us to perform DNA overexpression or shRNA studies related to the control of neuroblast motility and differentiation in a physiologically relevant context. Understanding the mechanisms that trigger and facilitate the recruitment of neuroblasts in an adult brain environment will be instrumental for the development of novel brain repair strategies.

Cytoskeletal and membrane traffic mechanisms controlling neurogenesis and neuronal morphology; neuronal cell biology; signalling.