Date of Graduation
Doctor of Philosophy (PhD)
Sharon Y. R. Dent, Ph.D
Rick A. Finch, Ph.D.
Mark T. Bedford, Ph.D.
Xiaobing Shi, Ph.D.
Taiping Chen, Ph.D.
Aberrant levels of histone ubiquitination are involved in various human diseases including neurodegenerative disorders and cancers. Particularly, Histone H2B monoubiquitination (H2Bub1) is highly associated with gene regulation in both normal cells and diseases. Many deubiquitinases (mainly USPs) are defined to regulate global H2Bub1 levels. However, how these USPs are regulated and how they contribute to diseases are not well understood.
USP22, part of the deubiquitination module (DUBm) in the SAGA complex, is a well-defined regulator of H2Bub1 levels. ATXN7, another crucial subunit of the SAGA DUBm, is involved in a neurodegenerative disease, spinocerebellar ataxia type 7 (SCA7), due to a polyglutamine (polyQ) expansion in its N-terminal region. Given the allosteric regulation of USP22 DUB activity within the DUBm, whether and how the polyQ expansion in ATXN7 affects SAGA DUB activity was not known. To address this question, we reconstituted the SAGA DUBm in vitro with wild type ATXN7 or the ATXN7polyQ mutant. We found that the DUBm has minimal activity in the absence of ATXN7. ATXN7-92Q, which corresponds to a pathogenic form of the protein, is largely insoluble unless it is incorporated into the DUBm. Importantly, we demonstrated that ATXN7-92Q enhances DUB activity to a similar extent as WT ATXN7. Consistent with these in vitro results, co-overexpression of DUBm components in human astrocytes promotes the solubility of ATXN7-92Q, and greatly inhibits its aggregation into nuclear inclusions that sequester other DUBm components including ATXN7L3, leading to global increases in H2Bub levels. Co-overexpression of ATXN7L3 and ENY2 with ATXN7-92Q reverses this effect. Finally, we found that global H2Bub levels are increased in cerebellums taken from a SCA7 mouse model, consistent with reduced DUB activity in the presence of ATXN7 polyQ mutant. Taken together, our study suggests that ATXN7-polyQ expansions do not change the biochemical activity of the DUBm, but they likely contribute to SCA7 by initiating aggregate formation that sequesters DUBm components away from H2B and other substrates. To further study the contribution of the DUB activity of USP22 to SCA7 disease, we also planned to delete USP22 in Purkinje cells and Bergman glia, both of which are involved in the pathogenesis of SCA7 disease, through establishing a FloxP-Usp22 mouse model. We generated Usp22FloxFRT mice by in vitro fertilization using sperm obtained from the Knock Out Mouse Project (KOMP). After crossing with FLpase mice, we obtained the Usp22Flox mouse model. Currently, we are crossing Usp22Flox mice with Pcp2-Cre and GFAP-Cre mice, which will specifically delete Usp22 in Purkinje cells and Bergman glia, respectively. We will analyze the potential “Ataxia” phenotypes of the mice with Usp22 deletion in Purkinje cells or Bergman glia.
Similar to USP22, USP44 is also involved in H2Bub1 regulation. The USP44 deubiquitinase is implicated in various biological processes including ES cell differentiation, DNA damage repair, and cancer development. However, USP44 alone is not active, and whether USP44 directly deubiquitinates histone H2B and what other partners are required for its DUB activity are not known. Using tandem affinity purification and Multidimensional Protein Identification Technology (MudPIT), we identified USP44 as an integral subunit of the Nuclear receptor co-repressor complex (N-CoR complex). As part of N-CoR, USP44 deubiquitinates H2B in vitro and in vivo. Depletion of USP44 impairs the repressive activity of N-CoR complex targeted to a chromatinized luciferase reporter. ChIP-qPCR experiments indicate that USP44 is recruited to the promoter of luciferase reporter, leading to decreased H2Bub levels. These data indicate that USP44 and HDAC3 work together within N-CoR, to repress transcription of target genes. Furthermore, USP44 expression is higher, and H2Bub1 levels are lower, in MDA-MB-231 triple negative breast cancer cells than in MCF-10A cells. Depletion of USP44 impairs invasiveness of MDA-MB-231 cells in vitro and leads to an increase of global H2Bub1 levels, as does depletion of another essential subunit of NCoR, TBL1XR1. Together, our findings indicate that USP44 and N-CoR may contribute to metastatic behaviors of triple negative breast cancer cells.
In summary, both USP22 and USP44 participate in multiple biological processes through modulating H2Bub1 levels and defective activity of these DUBs likely contributes to various human diseases. Our data highlight the importance of appropriate DUB activity and H2Bub1 levels for normal cell growth and behavior.
SAGA; USP22; SCA7; USP44; N-CoR; H2Bub1
Available for download on Tuesday, January 31, 2017