ABSTRACT
Thrombopoietin (Thpo) signaling through the c-Mpl receptor promotes either quiescence or proliferation of hematopoietic stem cells (HSCs) in a concentration-dependent manner; however, in vivo Thpo serum levels are responsive to platelet mass rather than HSC demands, suggesting additional regulation exists. Ott1 (Rbm15), a spliceosomal component originally identified as a fusion partner in t(1;22)-associated acute megakaryocytic leukemia, is also essential for maintaining HSC quiescence under stress. Ott1 controls the alternative splicing of a dominant negative isoform, Mpl-TR, capable of inhibiting HSC engraftment and attenuating Thpo signaling. Ott1, which associates with Hdac3 and the histone methyltransferase, Setd1b, binds to both c-Mpl RNA and chromatin and regulates H4 acetylation and H3K4me3 marks. Histone deacetylase or histone methyltransferase inhibition also increases Mpl-TR levels, suggesting that Ott1 uses an underlying epigenetic mechanism to control alternative splicing of c-Mpl. Manipulation of Ott1-dependent alternative splicing may therefore provide a novel pharmacologic avenue for regulating HSC quiescence and proliferation in response to Thpo.
Subject(s)
Alternative Splicing , Hematopoietic Stem Cells/metabolism , RNA-Binding Proteins/metabolism , Receptors, Thrombopoietin/genetics , Thrombopoietin/metabolism , Animals , Cells, Cultured , Chromatin/genetics , Chromatin/metabolism , Hematopoietic Stem Cell Transplantation , Hematopoietic Stem Cells/cytology , Humans , Mice , Mice, Knockout , NIH 3T3 Cells , Protein Isoforms/chemistry , Protein Isoforms/genetics , RNA, Messenger/genetics , RNA, Messenger/metabolism , RNA-Binding Proteins/genetics , Receptors, Thrombopoietin/chemistry , Signal TransductionABSTRACT
Innate immune recognition of pathogens is critical to the prompt control of infections, permitting the host to survive to develop long-term immunity via an adaptive immune response. Poxviruses encode a family of proteins that inhibit signaling by Toll-like receptors to their downstream signaling components, severely limiting nuclear translocation of transcription factors such as IRF3 and NF-κB and thereby decreasing production of host interferons and cytokines. We describe bioinformatics techniques for identifying candidate poxviral inhibitors of the innate immune response based on similarity to the family of proteins that includes A52, A46, and N1. Robust luciferase assays can determine whether a given poxviral gene affects innate immune signaling, and in combination with other approaches can identify the cellular targets of poxviral innate immune evasion genes. Because apoptosis is an innate immune response of the cell to viral infection, assays for identifying poxviral genes that inhibit apoptosis can also be employed. Novel poxviral innate immune inhibitors are being identified via several approaches and these techniques promise to identify further complexities in the way that poxviruses interact with the host innate immune system.
