Neural progenitor cells, asymmetric cell division, and embryonic cortical development
Research in our lab focuses on understanding the mechanisms by which neural stem/progenitor cells know when and where to self-renew or differentiate during brain development, and on the potential of this knowledge being translated to develop treatment for brain cancers. We study these problems in the embryonic mouse cerebral cortex, using combined biochemical, molecular, cellular and genetic approaches. Current projects attempt to address the following three issues: (1) molecular mechanisms that regulate symmetric vs. asymmetric cell division; (2) epigenetic and transcriptional mechanisms that control cortical neurogenesis and gliogenesis; and (3) asymmetric cell division and cancers.
1. The mechanisms of symmetric versus asymmetric cell division
The ability to divide asymmetrically is a defining property of stem/progenitor cells that allows them to simultaneously maintain the stem/progenitor cell pool and generate cellular diversity. The mechanism of asymmetric cell division (ACD) has been studied extensively in the central nervous system of Drosophila and C. elegans, however, the process as it occurs in the mammalian brains remains poorly understood.
In the past a few years, we have discovered that a regulator of G protein signaling (RGS) mediated ephrin-B signaling pathway and the Galpha signaling pathway are together crucial for controlling neuronal differentiation in radial glial cells (RGC) of the developing mouse cerebral cortex. Our data have shown that the ephrin-B/RGS and Galpha pathways are two opposing forces in RGCs and the balance between these two forces regulates the balance between self-renewal (symmetric cell division) and differentiation (asymmetric cell division). The ephrin-B/RGS pathway promotes RGC state, the Galpha signaling pathway encourages neurogenesis, and the two are linked by the RGS.
We postulated that the ephrin-B/RGS and Galpha signaling pathways regulate the progression of neurogenesis by directly controlling symmetric versus asymmetric cell division in RGCs. Consistent with this idea, we have identified an RGS-binding mitotic kinesin (RBMK) that interacts with and recruits the ephrin-B/RGS proteins into the midbody of dividing cells. RBMK can therefore be a potential bridge that links the ephrin-B/RGS pathway to the cell division machinery. We are currently investigating how the interaction network of ephrin-B/RGS/RBMK/Galpha contributes to balance symmetric versus asymmetric cell division during cortical development.
2. Epigenetic and transcriptional control of neural progenitor cell state
The decision of a stem/progenitor cell to choose between the state of self-renewal and the state of differentiation is determined by the gene expression potential of its genomic DNA. This is thought to be achieved, in large part, through specific epigenetic modifications, in particular via DNA methylation and histone modification. We are interested to understand the epigenetic and transcriptional mechanisms that guide neural progenitor cells to choose between different states, particularly with a focus on the transition to neuronal and glial differentiation.
The epigenetic marks that are crucial for controlling progenitor cell states are expected to display dynamic changes during neuronal differentiation in accordance with expression changes of the marked genes. Therefore, to identify key epigenetic marks that regulate the transition of neural progenitor cells from self-renewal to neuronal differentiation, our approach is to isolate neural progenitor cells and their neuronal progeny from the same animals and then compare the global landscape of epigenetic marks and gene expression profiles between the two cell populations.
To facilitate isolation of endogenous brain cells, we have developed a genetic dual reporter strategy for purification of neural progenitor cells from the developing mouse cerebral cortex. In this strategy, progenitor cells are labeled with GFP from a progenitor-specific promoter (e.g., Nestin promoter) and differentiated neurons are labeled with RFP expressed from a young neuron-specific promoter (e.g., DCX promoter). The use of a differentiation reporter in conjunction with a progenitor cell reporter helps prevent the misidentification of progeny cells as progenitors (which can occur due to “carryover” of GFP from a primitive cell to progeny), thus enabling selective purification of neural progenitor cells from the brain. Combining the Nestin-GFP and DCX-RFP reporter mice, we were able to simultaneously purify cortical neural progenitor cells and their daughter neurons. We are currently conducting global analyses of transcriptome and epigenetic marks of these two distinct cell populations and will use this information to further investigate the mechanisms of epigenetic and transcriptional regulation during cortical neurogenesis.
3. Asymmetric cell division and cancers
Defect in asymmetric cell division can lead to un-regulated progenitor cell proliferation which may be one cause of tumorigenesis. We are interested to explore this idea using the asymmetric cell division regulators identified in our studies. For example, we are interested to generate engineered mice which can be induced to over-express ephrin-B/RGS/RBMK proteins in the adult brains and test if these mice will develop spontaneous brain tumors. In addition, molecules that can disrupt the ephrin-B/RGS/RBMK network may be tested for their potential for inhibiting brain tumor initiation, progression or growth.
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