Cellular and Molecular Biology of Forebrain Development

The brain is so formidably complex that, despite a century or more of study, we are still far from understanding its detailed structure, let alone its higher functions. However, a powerful approach lies in studying the embryonic brain, deciphering the genetic instructions that propel the emergence of structural and functional complexity. The genes involved are remarkably similar in all vertebrates, and many are shared even with the fruit fly, Drosophila. Indeed, it is the powerful genetics of fruit fly development that first identified most of the key genes and developmental mechanisms involved in building the vertebrate brain. We are working on some of the earliest steps in development, during which the embryo sets aside a group of cells and instructs them to make the brain; different regions of this primordial brain -- at this stage a simple-looking tube of cells -- are given different growth capacities and molecular character. Our main concern is how this process of regionalization endows specific subregions of what will become the forebrain with the potential to develop distinct cellular architecture and function. Throughout the process, different genes are activated in different places so as to precisely control the availability of their specific protein product, which may regulate the activity of other genes in the same cell or may be a signal to neighbouring cells. In development generally, an important regionalizing mechanism involves the formation of a small group of specialized cells that inform neighbouring cells about their position and fate. We have found such an ?organizer? in the middle of the embryonic forebrain which, we have shown, has a crucial influence on the growth and development of a major part of the forebrain. Our work in this new programme is to investigate many of the processes involved in forebrain development in this new light. We will look for new genes involved in the control of these processes and test the function of these, and genes we already know of, using zebrafish and chick embryos in which we can readily suppress gene activity or activate genes in the wrong cells --- both approaches give significant information on what a particular gene does. This work will enhance our understanding of normal brain development, and therefore give insight into developmental malformation; it will also contribute to understanding the developmental history of region-specific neuronal cell types, essential for the future of stem cell therapy for brain degeneration or injury.

Developmental mechanisms responsible for regionalizing the neural plate and for conferring different regions of the emerging brain with distinct fates in respect of architecture, cell types and functions, are poorly understood. We have been studying how the forebrain is regionalized and have discovered, during the course of our previous MRC Programme, that a well-known intrazonal boundary in the diencephalon, the zona limitans intrathalamica (ZLI), acts as a local organizer of this part of the forebrain through the actions of a long-range morphogen, Sonic hedgehog (Shh). Our previous cell lineage and fate mapping studies have shown that the ZLI is a lineage-restricted developmental compartment that initial incorporates as much as one third of the prosencephalon, but subsequently condenses into a narrow stripe of cells. We found that Shh, which is expressed in the ZLI at high level, is responsible for informing cells in the presumptive thalamus and prethalamus about their position and fate. We now propose a detailed study of patterning processes in the forebrain, including the following: 1) Studying morphogenetic events surrounding formation of the ZLI from a major part of the prosencephalic vesicle, possibly involving convergent-extension mechanisms; 2) Fate mapping the Wnt8b+ presumptive (pr)ZLI in zebrafish; 3) Investigating preliminary findings that Wnt8b signalling from the prZLI has a patterning role that embraces the rostral brain; 4) Performing an expression screen to find new molecules involved in lineage-restricting the prZLI; 5) Exploring molecular controls that establish how the prZLI is positioned along the anteroposterior axis; 6) Determining how Shh spreads through tissues and how the range of Shh signalling from the ZLI is controlled; 7) Examining how the various ZLI signalling pathways are integrated; 8) Exploring the relative importance of signal identity and competence factors in the response of diencephalic neuroepithelia to ZLI signals; 9) Discovering new developmental control genes for forebrain development by transcriptome profiling of emerging subregions; 10) Spatiotemporal dissection of transcription factor networks underlying the emergence of patterned forebrain areas. 10)
All of the above studies will involve either chick or zebrafish or both. Each of these species offers distinct advantages for experimental design.