Glycogen Synthase Kinase-3 (GSK-3) is a highly conserved serine/threonine kinase comprised of two mammalian homologues, GSK-3α and β, encoded by independent genes. This thesis reports the characterization of GSK-3-null primary mouse embryonic fibroblasts (MEFs) generated by gene targeting to gain insight into the physiological functions of this protein kinase. Combined inactivation of both alleles of GSK-3α and GSK-β led to elevated sensitivity to TNFα-induced apoptosis, altered organization of focal adhesion complexes, defects in cell spreading on fibronectin, decreased cell growth associated with altered cell cycle progression through the G2/M phase and increased spontaneous apoptosis. Future work will focus on unraveling the molecular mechanisms responsible for these effects and identifying the common and distinct cellular roles for GSK-3α and β, and specific variants of these isoforms.

Glycogen Synthase Kinase-3 (GSK-3) is a highly conserved serine/threonine kinase comprised of two mammalian homologues, GSK-3α and β, encoded by independent genes. This thesis reports the characterization of GSK-3-null primary mouse embryonic fibroblasts (MEFs) generated by gene targeting to gain insight into the physiological functions of this protein kinase. Combined inactivation of both alleles of GSK-3α and GSK-β led to elevated sensitivity to TNFα-induced apoptosis, altered organization of focal adhesion complexes, defects in cell spreading on fibronectin, decreased cell growth associated with altered cell cycle progression through the G2/M phase and increased spontaneous apoptosis. Future work will focus on unraveling the molecular mechanisms responsible for these effects and identifying the common and distinct cellular roles for GSK-3α and β, and specific variants of these isoforms.

Bcl-2 associated factor 1 (Bclaf1; previously known as Btf) is a nuclear protein that was originally identified as an interacting partner for the adenoviral anti-apoptotic Bcl-2 family member E1B-19K. Surprisingly, Bclaf1 does not share structural homology with the Bcl-2 family of proteins, but rather exhibits protein structure and subcellular distribution patterns reminiscent of proteins that regulate mRNA processing. In addition, Bclaf1 appears to be expressed at high levels in skeletal muscle and was recently shown to associate with emerin, a protein linked to muscular dystrophy. Despite these observations, roles for Bclaf1 in RNA processing and/or skeletal muscle differentiation remain to be elucidated. In an effort to identify new roles for Bclaf1 I conducted protein-protein interaction screens to identify candidate interacting proteins and pathways. I identified p32 and 9G8 as novel interacting partners for Bclaf1. Additional subsequent experiments demonstrated an interaction of Bclaf1 with tip associated protein (Tap) and association of Bclaf1 with ribonucleoprotein complexes. Given that all of these proteins have been linked to mRNA processing, a role for Bclaf1 in this pathway was investigated. Using several approaches, I demonstrated that Bclaf1 is able to associate with splicing complexes and mRNA species at various stages of processing. The function of Bclaf1 in the context of skeletal muscle differentiation was also explored using skeletal muscle cell lines and primary mouse myoblasts. Skeletal muscle differentiation led to a dramatic decrease in nuclear Bclaf1 steady-state protein, with the unexpected appearance of smaller Bclaf1 protein species that accumulated in the cytoplasm during differentiation due to cleavage by caspases. Furthermore, Bclaf1 depletion in a myoblast cell line led to increased myoblast fusion and myofiber dimensions during differentiation. Overall our findings indicate roles for Bclaf1 in the skeletal muscle differentiation program and in molecular events that regulate pre-mRNA splicing and related events.

Bcl-2 associated factor 1 (Bclaf1; previously known as Btf) is a nuclear protein that was originally identified as an interacting partner for the adenoviral anti-apoptotic Bcl-2 family member E1B-19K. Surprisingly, Bclaf1 does not share structural homology with the Bcl-2 family of proteins, but rather exhibits protein structure and subcellular distribution patterns reminiscent of proteins that regulate mRNA processing. In addition, Bclaf1 appears to be expressed at high levels in skeletal muscle and was recently shown to associate with emerin, a protein linked to muscular dystrophy. Despite these observations, roles for Bclaf1 in RNA processing and/or skeletal muscle differentiation remain to be elucidated. In an effort to identify new roles for Bclaf1 I conducted protein-protein interaction screens to identify candidate interacting proteins and pathways. I identified p32 and 9G8 as novel interacting partners for Bclaf1. Additional subsequent experiments demonstrated an interaction of Bclaf1 with tip associated protein (Tap) and association of Bclaf1 with ribonucleoprotein complexes. Given that all of these proteins have been linked to mRNA processing, a role for Bclaf1 in this pathway was investigated. Using several approaches, I demonstrated that Bclaf1 is able to associate with splicing complexes and mRNA species at various stages of processing. The function of Bclaf1 in the context of skeletal muscle differentiation was also explored using skeletal muscle cell lines and primary mouse myoblasts. Skeletal muscle differentiation led to a dramatic decrease in nuclear Bclaf1 steady-state protein, with the unexpected appearance of smaller Bclaf1 protein species that accumulated in the cytoplasm during differentiation due to cleavage by caspases. Furthermore, Bclaf1 depletion in a myoblast cell line led to increased myoblast fusion and myofiber dimensions during differentiation. Overall our findings indicate roles for Bclaf1 in the skeletal muscle differentiation program and in molecular events that regulate pre-mRNA splicing and related events.

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