A KRAB/KAP1-miRNA Cascade Regulates Erythropoiesis Through Stage-Specific Control of Mitophagy

Abstract

During hematopoiesis, lineage-and stage-specific transcription factors work in concert with chromatin modifiers to direct the differentiation of all blood cells. Here, we explored the role of KRAB-containing zinc finger proteins (KRAB-ZFPs) and their cofactor KAP1 in this process. Hematopoietic-restricted deletion of Kap1 in the mouse resulted in severe hypoproliferative anemia. Kap1-deleted erythroblasts failed to induce mitophagy-associated genes and retained mitochondria. This was due to persistent expression of microRNAs targeting mitophagy transcripts, itself secondary to a lack of repression by stage-specific KRAB-ZFPs. The KRAB/ KAP1-miRNA regulatory cascade is evolutionary conserved, as it also controls mitophagy during human erythropoiesis. Thus, a multilayered transcription regulatory system is present, where protein-and RNA-based repressors are super-imposed in combinatorial fashion to govern the timely triggering of an important differentiation event.Through the process of erythropoiesis, about one hundred billion new red cells are generated every day in the human adult bone marrow. This process is initiated by the differentiation of hematopoietic stem cells (HSC) into the earliest erythroid progenitor, which was identified ex vivo as a slowly growing burst-forming unit-erythroid (BFU-E). This erythroid progenitor morphs into the rapidly dividing CFU-E (colony-forming unit-erythroid), the proliferation of which is stimulated by the hypoxia-induced hormone erythropoietin. Further differentiation occurs through a highly sophisticated program orchestrated by lineage-and stage-specific combinations of protein-and RNA-based transcription regulators (1-3). It culminates in the elimination of intracellular organelles including mitochondria and the nucleus to yield the fully mature erythrocyte, containing on the order of 250 million molecules of hemoglobin as almost sole cargo. Much is still to be learned about the molecular mechanisms of these events, not only to understand the cause of red cell disorders, but also to aid the in vitro manufacturing of the large supplies of oxygen-carrying cells for transfusion.Higher vertebrate genomes encode hundreds of KRAB-ZFPs that can bind DNA in a sequence-specific fashion through a C-terminal array of C2H2 zinc fingers and recruit the ‡ Corresponding author. didier.trono@epfl.ch.

The WTX Tumor Suppressor Interacts with the Transcriptional Corepressor TRIM28

“…These proteins were enriched for components of chromatin- modifying enzyme complexes (TRIM28, DBC1, RBBP7, RUVBL2, and RUVBL1) and RNA-binding proteins (SFR14, HNRPF, DDX5, and SAM68), suggesting a potential role of WTX in chromatin regulation and RNA binding. Among these proteins, the transcriptional corepressor TRIM28 (also known as KAP1/TIF1␤/KRIP1) was the most highly enriched nuclear WTX-interacting protein, and it was noteworthy in that it has been implicated in cellular differentiation as well as chromatin silencing (17)(18)(19)(20). The co-immunoprecipitation with WTX of RBBP7, a component of the TRIM28-associated NurD complex (21), also raised the possibility of functional interaction between WTX and TRIM28.…”

“…The effect of WTX on ␤-catenin stability is most evident in its contribution to early stage lineage specification (6). Similarly, constitutive knock-out of TRIM28 in mice leads to an early embryonic lethal phenotype at the postimplantation stage, but tissue-specific inactivation is associated with differentiation defects in various hematopoietic lineages and in germ cells (17,19,20). Our results indicate that the nuclear interaction of WTX and TRIM28 modulates a later stage in mesenchymal cell differentiation.…”

“…Its impact on ␤-catenin stability appears to contribute to this lineage specification step. Similarly, constitutive knock-out of TRIM28 leads to an early embryonic lethal phenotype at the postimplantation stage, but tissue-specific inactivation of TRIM28 has uncovered a role in differentiation of various hematopoietic lineages and germ cells (17,19,20). Thus, both WTX and TRIM28 mediate complex developmental phenotypes that may affect lineage specificity in different cell types.…”

TRIM-mediated precision autophagy targets cytoplasmic regulators of innate immunity

“…The concept of platforms for assembly of autophagic machinery in mammalian cells also extends to generic, starvation-induced autophagy, which utilizes exocyst components specifically endowed with Exo84 (Bodemann et al, 2011). However, TRIM engagement with autophagy may entail other mechanisms, as for example TRIM28 has multiple (both positive and negative) proposed mechanisms of action (Barde et al, 2013;Yang et al, 2013;Pineda et al, 2015), whereas the mechanism of autophagy induction for TRIM13 in response to the ER stress has not been fully delineated (Tomar et al, 2012), although it shows a relationship with p62 and DFCP, an ER-derived autophagy precursor compartment termed omegasome (Axe et al, 2008).…”

“…IFN-γ induces autophagy (Inbal et al, 2002;Gutierrez et al, 2004;Fabri et al, 2011) and influences cytokine networks and polarization of immune systems (Ghezzi and Dinarello, 1988;Schroder and Tschopp, 2010;Mishra et al, 2013), whereas TRIMs are involved in immune responses (Kawai and Akira, 2011) and, through an assortment of proposed mechanisms affect autophagy (Niida et al, 2010;Tomar et al, 2012;Barde et al, 2013;Pizon et al, 2013;Yang et al, 2013;Khan et al, 2014;Mandell et al, 2014;Pineda et al, 2015). IFN-γ can induce expression of a subset of TRIMs (Carthagena et al, 2009).…”