Research Interests

How is a complex brain structure formed from a simple embryonic primordium? We have been tackling this question in mouse utilizing a broad range of genetic approaches. The brain is divided into three basic regions (forebrain, midbrain, and hindbrain) and then further divided into structures that carry out particular functions. The cerebellum (Cb) develops from the anterior hindbrain and is the primary center for coordinating motor function. While the Cb consists of a simple layered cytoarchitecture with only a few cell types, it has an intricate set of folds along the anterior-posterior (AP) axis that are thought to serve as a scaffold to organize circuits that control related motor functions. In addition to the folds, the Cb is compartmentalized along the medial-lateral (ML) axis into sagittal bands based on molecular markers and afferent projections. During evolution of mammals, although the number of folia and sagittal bands has increased, the basic patterns have been preserved suggesting that an underlying conserved genetic mechanism determines the position of each fold and sagittal band. We are studying how the folds and bands are patterned and whether they direct circuit formation.  A second new area of research is the role of the morphogen Sonic Hedgehog (Shh) is regulating the biology of adult stem cells.

How is the Cb primordium established?

Fundamental to understanding organ development is the question of how spatial cues are established and interpreted. The midbrain/Cb region has become a paradigm for studying this question since a centrally located organizing center (isthmus) patterns the AP axis of each structure. The primary organizer molecule is the secreted factor Fgf8, expressed in the isthmus. We study how Fgf8 expression is regulated and how this single gene differentially specifies the midbrain and Cb. Our genetic studies have shown that the two transcription factors Otx2 and Gbx2, expressed in the midbrain and Cb respectively, are required to position the Fgf8 expression domain. We also demonstrated that four Fgf proteins expressed near the isthmus have distinct functions: Fgf8a, Fgf17b or Fgf18 induce midbrain development, whereas Fgf8b induces a Cb. We are now using a conditional mutagenesis approach to determine when Fgf signaling is necessary for midbrain and Cb development, and how Fgf8 specifies different cell types.

What cell movements underlie Cb development?

We recently developed a technique for marking embryonic cells and following them into the adult (Genetic Inducible Fate Mapping, GIFM). Using this approach, we discovered that the Otx2/Gbx2 border is a cell lineage restriction that inhibits midbrain and Cb cells from mixing. Thus, the organizer forms at a compartment border and then influences cells on either side to form different tissues. We then marked different domains of the Cb primordium and demonstrated that the axes of the Cb shift during development. First, the anterior hindbrain undergoes a 90º rotation, converting the AP axis to the ML axis. Then all but the granule cells maintain their ML position into the adult; granule cells initially respect their position but later migrate lateral to medial as they begin to differentiate. We are now modifying GIFM to determine how the circuitry of the Cb is established and whether it is dependent on cues from the folds and sagittal stripes.

How is the pattern of folds and stripes determined?

The folds form concomitant with the major proliferation of granule cells after birth and Shh is expressed by Purkinje cells and regulates proliferation of granule cells. We found that the length of exposure to Shh signaling determines the complexity Cb foliation. Thus, Shh Shh acts permissively to allow an underlying genetic code regulating the pattern of foliation to unveil. By analyzing an array of mouse mutants we constructed in the mouse Engrailed (En) homeobox genes we found these genes play a fundamental role in patterning the position of folia and saggital stripes. We are exploring the temporal and cell type specific requirement for the En genes in each patterning the folds and stripes, as well as the circuitry.

How is proliferation of adult stem cells regulated?

Using GIFM we found that Shh signals to both quiescent stem cells in the adult brain and dormant stem cells in epithelial tissues. Furthermore, Shh regulates the expansion of stem cells in response to tissue depletion.  We are combining GIFM and conditional mutagenesis to determine how Shh regulates adult stem cell maintenance and response to injury.