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Specification of left-right symmetry and asymmetry in the nervous system

Final Report Summary - NEUROSYMMASYMM (Specification of left-right symmetry and asymmetry in the nervous system)

The Poole lab uses the power of C. elegans as an in vivo genetic model system with single-cell resolution to reveal fundamental conserved principles of neural development, which should subsequently provide the basis for novel therapies and brain repair strategies. Despite the overt L/R bilateral symmetry of our nervous system, it is both functionally and neuroanatomically asymmetric. The aim of this project is to elucidate the molecular and cellular mechanisms that regulate left-right (L/R) asymmetric neurogenesis. Given that L/R asymmetries have been linked to human pathologies, such as the lateralised onset on Parkinson’s disease, such knowledge will allow insight into the formation and nature of neuronal lateralisation with respect to human pathology.

We focus on the production of two neurons that are generated only on the left side of the C. elegans embryo. Their specification depends on the asymmetric expression of the proneural bHLH transcription factor hlh-14/acheate-scute. How neuronal potential is segregated in this lineage to generate the asymmetric expression of hlh-14/acheate-scute is not understood. To address this question the specific objectives of this project were too: (1) identify upstream regulators of proneural gene expression through cis-regulatory analysis; (2) Identify novel trans-acting factors required for asymmetric neurogenesis through forward genetic screening and a 4D-lineage based screen; (3) characterise the roles of these molecular factors and other cellular mechanisms required for asymmetric neurogenesis.

We have conducted two genetic screens and isolated a collection of asymmetric neurogenesis defective (and) mutants which fail to produce these two neurons. Using a “mapping-by-sequencing” approach we developed, we find that and-6 is an allele of hlh-2/daughterless, a bHLH transcription factor that heterodimerizes with other bHLH transcription factors, including hlh-14 and we show this is also asymmetrically expressed in this lineage. We find that and-4 is an allele of let-19/mdt-13, a member of the Mediator complex, which we show acts to regulate an unequal (in size) cell division three divisions before the birth of the neurons. In let-19 mutants this division is symmetrised leading to a loss of L/R asymmetric expression of hlh-2 and hlh-14, with a concomitant loss of neuronal cell fate markers and ectopic expression of hypodermal cell fate markers. These results demonstrate that through the regulation of an unequal cell division earlier in the lineage, let-19 acts to regulate asymmetric neurogenesis upstream of the L/R asymmetric expression of hlh-2 and the proneural gene hlh-14. This suggests that asymmetric segregation of an unidentified neuronal determinant has a critical role in this lineage and that let-19 can regulate cell size and cell fate concomitantly.

In addition, the lab has expanded research themes in a new, exciting direction, to address the mechanisms of plasticity and transdifferentiation during glial-derived neurogenesis. We have identified two previously undescribed glia-to-neuron cell fate switches, both of which occur during male sexual maturation. In one case a glial cell pair divides asymmetrically to self-renew and produce an interneuron cell pair crucial for male-specific associative learning. In the other case we find a that a pair of glial cells directly transdifferentiate into a sensory neuron pair required for specific steps of mating. We are currently undertaking genetic screens to identify the molecular players that regulate these in vivo glia-to-neuron cell fate switches supported by a recently acquired Wellcome Trust Senior Research Fellowship.