Vertebrates undergo an amazing and rapid process of growth during early development to form the trunk and tail of the body.  Zebrafish, like all vertebrates, grow from the neck down by progressively adding cells from a progenitor population, which contributes cells to both the mesodermal (muscle and bone) and neural (spinal cord) tissues, located at the most posterior end of the embryo.  A major interest in the lab is to understand the mechanisms that regulate this process.  To date, a principal focus has been on the signaling factors that pattern and regulate the mesodermal progenitors, including Wnts, Fgfs, Bmps, Nodals and retinoic acid, and their interaction with the transcription factors that regulate the progenitors, particularly members of the T-box family.  We recently showed (Martin and Kimelman, 2012) that the posterior of the body forms from a bipotential progenitor population, which produces both the neurons and mesoderm, with Wnt signaling determining which fate the progenitors will adopt. Wnt expression is controlled by the T-box transcription factor Brachyury (called No tail in zebrafish) in an autoregulatory loop (Martin and Kimelman, 2008), which sustains the progenitors in a multipotent state.

One major current focus is on the proliferation of the progenitors, which is needed to provide enough cells to complete the embryonic body.  Studying proliferation beyond the early cleavage stages in vertebrates has been very difficult because of the large number of cells and the complex cell movements that occur during early development. However, with newly developed photoconvertible fluorescent proteins and improved confocal microscopy, it has become feasible to study this process. Surprisingly, we find that progenitor proliferation is highly regulated, with rapid proliferation during the gastrula stages and then a complete absence of progenitor proliferation as the body extends. Using a new fluorescent transgenic zebrafish line, we observed that proliferation of the bipotent progenitors is regulated by tight control of the cell cycle. When we altered the cell cycle through timed misexpression of one of the principle regulators, we observed major defects in the formation of the body, which we are currently studying. These results demonstrate that the cell cycle needs to be precisely regulated in order for the vertebrate body to form normally.

Another major focus is on the regulation of morphogenesis of the mesodermal cells. As a multipotent progenitor cell begins to differentiate as mesoderm, it undergoes an epithelial to mesenchymal (EMT)-like transition. We showed that the T-box gene Spadetail/Tbx16 plays a key role in this process (Row et al., 2011). In spadetail mutants, cells begin the transition but get stuck in an intermediate highly blebbing state, preventing them from completing the morphogenetic transformation, resulting in partially differentiated cells trapped at the most posterior end of the body and a failure to form normal muscle. These results reveal a novel intermediate step in the EMT process.

 

The Kimelman Lab

University of Washington

Department of Biochemistry