The forebrain is the primary centre of higher cognitive and motor functions in vertebrates. It originates from the anterior part of the neural plate and becomes progressively subdivided during embryonic development, first into the telencephalon anteriorly and the diencephalon posteriorly and later into increasingly specialised brain areas that subserve distinct functions. This regionalisation is regulated by secreted signalling factors.
We are investigating the role of Hedgehog and Wnt signals in forebrain development. We use electroporation, in situ hybridisation, immunohistochemistry, fate mapping and reporter gene technology in chick and mouse embryos to assess how these signals regulate cell fate, proliferation, neurogenesis and morphogenesis in different forebrain subdivisions.
We are particularly interested in (a) the thalamus (a structure affected in the stroke-induced thalamic syndrome and in fatal familial insomnia) as well as in (b) patterning of the telencephalon part of which gives rise to the neocortex in mammals. Congenital malformations of the human neocortex have increasingly been implicated in the etiology of psychiatric disorders such as autism, depression, intellectual disability and schizophrenia. A thorough understanding of the genetic program underlying the development of these brain regions is required to fully grasp the mechanisms of such neuropathological diseases. Eventually, our research should provide us with more targeted strategies to produce specific types of neurons in a Petri dish for stem cell replacement therapies.
Hedgehog signalling in thalamic development
Sonic hedgehog (Shh) is secreted from two sources flanking the developing thalamus: the diencephalic basal plate and the zona limitans intrathalamica (ZLI), a narrow transverse stripe of cells between the thalamus and prethalamus. We showed previously that ZLI-derived Shh regulates regional gene expression in those areas and that the differential response of thalamus and prethalamus to Shh is mediated by the Iroquois-related transcription factor Irx3. Shh is able to specify cell fates dose-dependently (high levels= GABAergic rostral thalamus, low levels=glutamatergic caudal thalamus); however, it has also been suggested that Shh from the ZLI and the basal plate have qualitatively different inductive effects. We are currently performing in ovo electroporation experiments using mutant forms of Shh receptors to further characterise the effects of Shh on thalamic cells, to identify additional “competence factors” that regulate the cellular response to Shh and to analyse how the Shh signal spreads in thalamic neuroepithelium.
Wnts in thalamic development
A surprisingly large number of Wnt genes, Wnt receptors and downstream Wnt signal transducers are expressed in highly characteristic spatiotemporal patterns in the developing diencephalon. Previously, we found that Wnt4 expression in the emerging thalamus is negatively regulated by Shh from the ZLI. We are now testing the effects of ectopically expressing Wnt4, or a dominant-negative form of Wnt4, in the chick forebrain. Wnt4 has been described to activate either the canonical, b-catenin-dependent, Wnt pathway or the noncanonical planar cell polarity pathway depending on the model organism and on the cellular context. We use different reporter constructs to test which pathway is activated by Wnt4 in the diencephalon.
Hedgehog and Wnt signalling in dorsoventral (DV) patterning of the telencephalon
The embryonic telencephalon becomes subdivided (from dorsal to ventral) into the hippocampus, the dorsal, lateral and ventral pallium (that give rise to different parts of the cortex and the amygdala in mammals) and the subpallium (that gives rise to the basal ganglia). This regionalisation is controlled by a ventral signalling centre, the lamina terminalis that secretes Shh, and a dorsal signalling centre, the pallial hem (cortical hem in mammals) that secretes Wnts and bone morphogenetic proteins. Halfway between these two signalling centres, the pallial-subpallial boundary secretes inhibitors of Wnts (secreted Frizzled-related proteins, Sfrps) and has hence been termed "antihem", as it could act as a sink for the Wnts that are released dorsally. Although it is generally accepted that Shh and Wnt signalling regulate telencephalic DV patterning, our understanding of the genetic network underlying this process remains sketchy. We are using in ovo electroporations to distinguish between direct and indirect effects of signalling pathways on DV marker gene expression and to test how Shh and Wnt signalling are integrated at the cellular level.