Nicole Avitahl

Aiolos, a lymphoid restricted transcription factor that interacts with Ikaros to regulate lymphocyte differentiation

Development of the lymphoid system is dependent on the activity of zinc finger transcription factors encoded by the Ikaros gene. Differences between the phenotypes resulting from a dominant‐negative and a null mutation in this gene suggest that Ikaros proteins act in concert with another factor with which they form heterodimers. Here we report the cloning of Aiolos, a gene which encodes an Ikaros homologue that heterodimerizes with Ikaros proteins. In contrast to Ikaros_which is expressed from the pluripotent stem cell to the mature lymphocyte_Aiolos is first detected in more committed progenitors with a lymphoid potential and is strongly up‐regulated as these differentiate into pre‐T and pre‐B cell precursors. The expression patterns of Aiolos and Ikaros, the relative transcriptional activity of their homo‐ and heteromeric complexes, and the dominant interfering effect of mutant Ikaros isoforms on Aiolos activity all strongly suggest that Aiolos acts in concert with Ikaros during lymphocyte development. We therefore propose that increasing levels of Ikaros and Aiolos homo‐ and heteromeric complexes in differentiating lymphocytes are essential for normal progression to a mature and immunocompetent state.

Aiolos Regulates B Cell Activation and Maturation to Effector State

Aiolos encodes a zinc finger DNA-binding protein that is highly expressed in mature B cells and is homologous to Ikaros. In the periphery of mice homozygous for an Aiolos-null mutation, B cells exhibit an activated cell surface phenotype and undergo augmented antigen receptor (BCR)-mediated in vitro proliferative responses, even at limiting amounts of stimulant. In vivo, T cell-dependent B cell responses, including the formation of germinal centers and elevated serum IgG and IgE, are detected in Aiolos-deficient mice in the absence of immunization. Auto-antibodies and development of B cell lymphomas are frequently seen among aging Aiolos mutants. In sharp contrast to conventional B cells, B cells of the peritoneum, of the marginal zone, and the recirculating bone marrow population are greatly reduced.

Helios, a novel dimerization partner of Ikaros expressed in the earliest hematopoietic progenitors

BACKGROUNDnNormal hematopoietic development depends on the activity of the Ikaros transcription factor, which contains distinct zinc-finger domains that mediate DNA binding and protein dimerization. Mice homozygous for a transgene encoding a dominant-negative version of Ikaros that lacks the DNA-binding domain but not the dimerization domain have a more severe phenotype than Ikaros null mice. This observation suggests the presence of factor(s) that can dimerize with Ikaros and partially complement its function. One previously identified factor, Aiolos, probably serves this role in the lymphoid system; a related factor involved in hematopoietic progenitors remains unknown, however.nnnRESULTSnHere, we describe the cloning of an Ikaros-related gene, Helios. Analysis of the primary sequences of Helios, Ikaros and Aiolos revealed that the DNA-binding, transcriptional activation and dimerization domains are functionally conserved. Helios activated transcription from Ikaros DNA-binding sites and could dimerize with itself, Ikaros or Aiolos. Expression of Helios was detected in the earliest hematopoietic sites of the embryo, in hematopoietic stem cells in the adult and was subsequently restricted to a subset of cells in the T cell lineage. Helios co-localized with Ikaros and Aiolos proteins in macromolecular nuclear structures and formed stable complexes in vivo with the dominant-negative version of Ikaros.nnnCONCLUSIONSnDistinct but overlapping expression patterns of members of the Ikaros gene family during hematopoiesis might result in the formation of different multimeric complexes that have specific roles in lineage progression. The preferential expression of Helios in the earliest stages of hematopoiesis suggests that this gene functions predominantly in early progenitors.

Ikaros Sets Thresholds for T Cell Activation and Regulates Chromosome Propagation

T cell activation involves the sustained accumulation of T cell receptor (TCR) and IL-2 receptor (IL-2R) mediated signaling events that promote cell cycle entry and progression. The Ikaros family of nuclear factors regulate this process by providing thresholds overcome by receptor signaling. T cells with reduced levels of Ikaros activity require fewer TCR engagement events for activation, exhibit a greater proliferative response to IL-2, and are less sensitive to inhibitors of TCR and IL-2R signaling. Upon T cell activation, Ikaros proteins localize in a higher-order chromatin structure where they colocalize with components of the DNA replication machinery. Proliferating T cells with reduced Ikaros activity display chromosome abnormalities. We propose that participation of Ikaros in higher-order chromatin structures controls cell cycle transitions and restricts DNA replication.

The Ikaros Gene Family in Hemopoietic Differentiation

The molecular events that enable the progeny of a hemopoietic stem cell (HSC) to become committed to the erythroid, myeloid, or lymphoid lineages are still to be defined. Differentiated hemopoietic lineages originate from common multipotent progenitor cells that undergo a series of divisions and commitment steps, giving rise to precursor cells with increasingly restricted differentiation potentials. The functional and phenotypic changes that occur as a cell differentiates are determined by changes in gene expression. These changes are directed by cell signaling events, which induce a cascade of regulatory factors that ultimately affect the transcriptional program of the cell. Therefore, an important key to understanding hemopoiesis is identifying the transcription factors that regulate the transition through stages of differentiation. The Ikaros family of transcription factors has been shown to play an integral role in the determination and differentiation of the lymphoid lineage. This chapter will discuss the founder of this family of transcription factors, Ikaros, as well as its more lymphoid-restricted homolog, Aiolos, and what is known to date about the roles of these proteins in hemopoiesis and lymphocyte differentiation.