Dr. Joni Seeling, Ph.D.
Professor - Department of Biological Sciences
Office: LDB 134
Lab: LDB 133
Regulation of Wnt signaling by protein phosphatase 2A.
Dr. Seeling studies the role of phosphorylation in the Wnt cell-cell signaling pathway, which is critical in development and disease.
Although signaling cascades have been intensely studied, relatively little is known about the role that phosphatases play in them. While there are over 400 serine/threonine (S/T) kinase genes in the human genome, there are roughly 40 S/T phosphatase catalytic subunits to counter these kinases. This was initially interpreted to mean that S/T phosphatases have broad, constitutive activities, however, it was later found that S/T phosphatases achieve diversity by forming multimeric complexes. In the case of protein phosphatase 2A (PP2A),
a heterotrimer is formed containing a structural A, catalytic C, and regulatory B subunit. The B subunits confer substrate specificity and subcellular localization on the holoenzyme. PP2A can form as many as 200 distinct heterotrimers due to the combinatorial association of its subunits, each of which (A, C, and B55, B56, and B72 regulatory subunits) are encoded by multigene families.
PP2A influences cell growth and tumorigenesis. The phosphatase inhibitor okadaic acid induces skin and gastrointestinal tract cancers, and small and middle t DNA tumor virus antigens enhance cell growth through PP2A inhibition. Moreover, mutations in the A subunit have been found in colon,
breast, and lung cancers. Subsets of these mutations are able to bind all B subunits except B56, suggesting that the loss of B56-specific PP2A activity is important for tumorigenesis. PP2A provides the majority of S/T phosphatase activity in the cell, and is therefore likely to play an important role, if not multiple roles, in most signaling pathways. PP2A's B56 subunits are encoded by five genes in humans (B56alpha, B56beta, B56gamma, B56delta, and B56epsilon) that share a highly conserved core region surrounded by variable N- and C-termini. The Wnt signaling pathway is a key regulator of development and tumorigenesis. In canonical Wnt signaling, Wnt blocks beta-catenin degradation via a phosphorylation-regulated signal transduction cascade. This results in the activation of beta-catenin-dependent transcription of cell cycle regulators. Acting through noncanonical/convergent extension signaling, Wnt induces actin cytoskeletal changes involved in cell movements required for body axis elongation.
We found that B56alpha
reduces beta-catenin abundance and beta-catenin-dependent gene expression in multiple experimental systems.
In addition, ectopic B56alpha rescues Wnt-induced secondary axes in Xenopus laevis. Epistasis and coimmunoprecipitation data suggest both that PP2A:B56alpha is a component of the beta-catenin degradation complex, and that casein kinase Idelta/epsilon (CKIdelta/epsilon) dissociates PP2A from that complex. We also found that B56 functional diversity is achieved in part through the synthesis of a novel mixed-isoform B56delta/gamma transcript, which is expected to have a unique set of substrates due to its B56delta N-terminal domain and its B56gamma core and C-terminal domains. The demonstration that B56epsilon is required for Wnt signaling reveals that B56 isoforms can either inhibit or activate Wnt signaling. Recently, B56 isoforms have been found to have roles in noncanonical/convergent extension Wnt signaling. To aid in our understanding of the multiple roles of PP2A in Wnt signaling, we are studying the mechanisms by which B56 subunits differentially influence Wnt signaling.
We have also characterized the role of phosphorylation on another component of the Wnt signaling pathway, Dapper (Dpr/Frodo/Dact).
We found that Dpr inhibits canonical Wnt signaling when unphosphorylated, but promotes Wnt signaling when phosphorylated by CKIdelta/epsilon, and we continue to study the role of Dpr phosphorylation in Wnt signaling.