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Faculty List
Barbara Wold
woldb@caltech.edu
Ph.D., 1978, California Institute of Technology
Regulation of Muscle Differentiation
A substantial biological challenge is to understand the regulation and execution of developmental decisions that lead from multipotential, undifferentiated precursor cells to their specialized differential products. In our group we are interested in several interrelated aspects of this problem, and we also work to develop new methods for studying it. The particular cell lineage problem we study begins with the specification of mesoderm in early development and continues to the final differentiation of skeletal muscle or cardiac muscle in the fully developed animal. The experimental system we use is the mouse.
Regulatory proteins of the MyoD family are involved in regulating the developmental cell fate decisions in the skeletal muscle lineages of diverse organisms, and are therefore a key experimental focus. In vertebrate mesoderm, some multipotential precursor cells will become restricted to the myogenic lineage. The resulting unipotential cells are now myoblasts, a proliferating population of precursor cells that will later become skeletal muscle, but that do not yet express the repertoire of specialized gene products that are characteristic of muscle (muscle type actins, myosins, acetylcholine receptors, etc.). Upon differentiation, myoblasts cease proliferating and become myocytes. These myocytes express the full range of muscle specific proteins, and eventually fuse into multinucleate myotubes. The evidence that members of the MyoD family are important in this pathway rests on several observations, the pivotal one being that when MyoD expression is experimentally forced in a diverse range of otherwise non-myogenic cells, these are now redirected to differentiate along the skeletal muscle pathway. This is true for MyoD or any of three closely related transcription factors: myogenin, myf-5, or MRF4/herculin. Important questions are: How are the myogenic regulators themselves regulated during development? To what extent are the effects of these regulators governed by the cellular environment? How are expression and activity of MyoD family regulators affected by the cell cycle? What would be the effect on mouse development of mutating into inactivity one or more of the MyoD family genes? Projects that address these questions use cell culture model systems and transgenic mice. Molecular-level analyses include studies of in vitro and in vivo protein-DNA interactions by MyoD family members, and by ancillary factors that amplify or suppress MyoD activity.
An important aspect of regulation by MyoD family regulators is that they are not active as monomers, but instead must form dimers or higher order oligomers to function. Different potential pairing partners for MyoD family regulators are present in cells, and the choice of different partners can lead to very different activities from e resulting dimer. These range from inactivation to the very efficient, sequence-specific binding to target DNA sites. Several ongoing projects examine the general problem of how pair-choice regulates the network of myogenic regulators and their more distantly related pairing partners.
Finally, an active collaboration with the Dervan group in the chemistry division is focused on forming, detecting, and manipulating triple-stranded nucleic acid complexes in living cells and in systems that model physiological conditions (in vitro transcription systems, for example). We wish to understand the range of interactions that can occur under physiological conditions between specific single-stranded RNA or DNA oligonucleotides and a target double-helical DNA sequence. If such interactions are sufficiently sequence-specific, they might be used either in nature or by the experimenter to recognize specific genes and alter their pattern of expression.
