Michael Rudnicki

Degrees:

Ph.D. University of Ottawa 1995
Post-doctoral Research Massachusetts Institute of Technology

Research Interest:

We are interested in understanding the molecular mechanisms that regulate the determination, proliferation, and differentiation of stem cells during embryonic development and during tissue regeneration. We have conducted extensive studies into both embryonic myogenesis and the function of pluripotent stem cells in adult skeletal muscle. Towards this end, we are employing molecular biological approaches together with transgenesis and gene targeting to determine the function and roles played by regulatory genes. Outlined below are examples of the different avenues of research being pursued in our laboratory.

1) Developmental Regulation of Myogenesis

Gene targeting experiments have established unequivocally that the bHLH transcription factors Myf5 and MyoD play key regulatory roles during embryonic skeletal-muscle development. Myf5 and MyoD are required for myogenic determination, whereas myogenin and MRF4 are required during myocyte differentiation. Analysis of Myf5- and MyoD-mutant embryos revealed a striking difference in the temporal-spatial patterns of myogenesis. Myf5 is required for the appropriate development of epaxial musculature (muscles of the deep back), whereas MyoD is required for the appropriate development of hypaxial musculature (muscles of the jaw, abdominal wall and limbs). This work provided compelling molecular evidence that Myf5 and MyoD define the origins of distinct myogenic lineages. Therefore, we are conducting an experimental program to investigate the molecular mechanisms by which Myf5 and MyoD differentialy regulate the development of these myogenic lineages.

2) Establishing and Maintaining Muscle Progenitor Cells during Embryonic Development

How does the fate of a multipotential progenitor cell become progressively restricted to yield unipotential progeny cells? This four-way collaboration (S. Tajbakhsh, G. Cossu, B. Wold, and M.A. Rudnicki) has been forged to directly address this question in skeletal muscle development. The transcription factors Myf5 and MyoD, regulate downstream target genes and so determine muscle progenitor fate, phenotype, and survival or proliferation. Upstream signaling molecules that include Wnts, BMP4, Sonic Hedgehog and noggin, play causal roles in either inducing or inhibiting muscle cell fate choice in the embryo. Various tissues near the prospective muscle progenitors produce these extracellular signaling molecules. Activation of these signaling cascades leads to expression of Myf5 and/or MyoD.

A three-pronged approach is underway to study the specification of multipotential progenitor cells that stand between inductive signals and muscle identity. We have identified high priority candidate genes for important functions in progenitor/progeny cell fate specification and these will be examined in single cells for coexpression (RNA and protein) with the first Myf5+ and MyoD+ cells. New or previously unappreciated genes specific for muscle progenitor cells or their immediate progeny are being identified by selective micro-cDNA cloning and RDA from marked cells in wildtype and Myf5/MyoD mutant embryos. Microsurgery and explant cultures of mouse embryos provide a unique means to assay inductive interactions that specify muscle progenitor cells. These assays will be used to ask how altered signaling affects the expression and action of candidate genes identified.

3) Molecular Regulation of Myogenic Stem Cell Function

Satellite cells, the stem cell of adult skeletal muscle, reside beneath the basal lamina closely juxtaposed to muscle fibers and make up 2-7% of the nuclei associated with a particular fiber. Satellite cells are normally mitotically quiescent but are activated in response to stress or injury, and give rise to proliferative myogenic precursor cells that effect the regeneration of the damaged tissue. Our experiments have led to the insight that MyoD expression is required for satellite cells to efficiently form proliferative myogenic precursor cells. Activated satellite cells first express either Myf5 alone or MyoD alone, prior to co-expressing Myf5 and MyoD, and subsequently progressing through the myogenic program. Together, these data suggest the hypothesis that expression of Myf5 alone may define an intermediate developmental stage that provides a mechanism for satellite cell self-renewal. We are therefore conducting an experimental program to investigate this hypothesis and to elucidate the molecular mechanisms that regulate myogenic stem cell activity.

4) Genetic Regulation of Skeletal Muscle Repair

Our laboratory has established that mice carrying a targeted null mutation MyoD exhibit a profound deficiency in skeletal muscle regeneration due to a failure of myogenic stem cells to efficiently progress through their developmental program. This result led to the important insight that MyoD is required for satellite cells to efficiently give rise to myogenic precursor cells (mpc) whose proliferation and differentiation are necessary for muscle regeneration. We are therefore undertaking a research program designed to further our understanding of the molecular mechanisms that regulate muscle regeneration.

5) Cell-Cycle Control of the Switch from Proliferation to Differentiation

The Rb-family of proteins is believed to a play a central role in regulating the switch from cellular proliferation to withdrawal from the cell cycle and terminal differentiation. The loss of Rb function in many types of cancer attests to the importance of this developmental role played by the Rb-family. We have undertaken a research program to investigate how the distinct activities of the different members of the Rb-family contribute to this regulation. Our primary focus will be to investigate how the switch from proliferation to terminal differentiation is regulated by the Rb-family during the development of the embryonic cell lineage that gives rise to skeletal muscle. Myogenesis provides a powerful model system to investigate these issues as the molecular mechanisms that regulate determination and differentiation are well understood. In addition, these experiments will elucidate the discrete roles played by Rb, p107 and p130 in development, and furthermore, should reveal to what extent Rb-family members substitute for one another in development and in cancer. Targeted p107- and p130-mutant mice display no apparent phenotype in a C57BL/6J genetic background, whereas profound phenotypes are observed in a Balb/cJ genetic background. Preliminary genetic analysis suggests that multiple recessive mutations in C57BL/J suppress the mutant phenotypes. Therefore, we are continuing to molecularly characterize the phenotype of p130 and p107-mutant deficient animals in a Balb/cJ genetic background. These experiments will provide important new information concerning the role of Rb-family members in regulating the switch from proliferation to differentiation during development.

Selected Publications:

Kuang S, Charge SB, Seale P, Huh M, Rudnicki MA. Distinct roles for Pax7 and Pax3 in adult regenerative myogenesis. Journal of Cell Biology. J Cell Biol. 2006 Jan 2;172(1):103-13. Abstract

Grenier G, Rudnicki MA. The potential use of myogenic stem cells in regenerative medicine. Handb Exp Pharmacol. 2006;(174):299-317. Abstract

Ishibashi J, Perry RL, Asakura A, Rudnicki MA. MyoD induces myogenic differentiation through cooperation of its NH2- and COOH-terminal regions. J Cell Biol. 2005 Nov 7;171(3):471-82. Abstract

Huh MS, Smid JK, Rudnicki MA. Muscle function and dysfunction in health and disease. Birth Defects Res C Embryo Today. 2005 Sep;75(3):180-92. Abstract

Holterman CE, Rudnicki MA. Molecular regulation of satellite cell function. Semin Cell Dev Biol. 2005 Aug-Oct;16(4-5):575-84. Abstract

Perez-Iratxeta C, Palidwor G, Porter CJ, Sanche NA, Huska MR, Suomela BP, Muro EM, Krzyzanowski PM, Hughes E, Campbell PA, Rudnicki MA, Andrade MA. Study of stem cell function using microarray experiments. FEBS Lett. 2005 Mar 21;579(8):1795-801. Abstract

McKinnell IW, Rudnicki MA. Molecular mechanisms of muscle atrophy. Cell. 2004 Dec 29;119(7):907-10. Abstract

Vanderluit JL, Ferguson KL, Nikoletopoulou V, Parker M, Ruzhynsky V, Alexson T, McNamara SM, Park DS, Rudnicki M, Slack RS. p107 regulates neural precursor cells in the mammalian brain. J Cell Biol. 2004 Sep 13;166(6):853-63. Abstract

Huh MS, Parker MH, Scime A, Parks R, Rudnicki MA. Rb is required for progression through myogenic differentiation but not maintenance of terminal differentiation. J Cell Biol. 2004 Sep 13;166(6):865-76. Abstract

Charge SB, Rudnicki MA. Cellular and molecular regulation of muscle regeneration. Physiol Rev. 2004 Jan;84(1):209-38. Abstract