During spermiogenesis (the maturation of spermatids into spermatozoa) in many vertebrate species, protamines replace histones to become the primary DNA-packaging protein.It has long been thought that this process is facilitated by the hyperacetylation of histone H4. However, the responsible histone acetyltransferase enzymes are yet to be identified. CDY is a human Y-chromosomal gene family expressed exclusively in the testis and implicated in male infertility. Its mouse homolog Cdyl, which is autosomal, is expressed abundantly in the testis. Proteins encoded by CDY and its homologs bear the ''chromodomain,'' a motif implicated in chromatin binding. Here, we show that (i) human CDY and mouse CDYL proteins exhibit histone acetyltransferase activity in vitro, with a strong preference for histone H4; (ii) expression of human CDY and mouse Cdyl genes during spermatogenesis correlates with the occurrence of H4 hyperacetylation; and (iii) CDY and CDYL proteins are localized to the nuclei of maturing spermatids where H4 hyperacetylation takes place. Taken together, these data link human CDY and mouse CDYL to the histone-to-protamine transition in mammalian spermiogenesis. This link offers a plausible mechanism to account for spermatogenic failure in patients bearing deletions of the CDY genes.histone acetylation ͉ spermiogenesis ͉ chromodomain ͉ infertility T he acetylation of N-terminal lysine residues in core histones has been implicated in three distinct cellular processes. The first is the deposition of free histones onto newly synthesized DNA (1). The second is the regulation of gene expression (reviewed in ref.2). The third is the displacement of histones by transition proteins and protamines during vertebrate spermatogenesis (3-6). Various histone acetyltransferase (HAT) enzymes involved in the first two processes have been characterized. HATs involved in the third process have yet to be identified.In mammals and many other vertebrates, dramatic chromatin remodeling occurs during spermiogenesis, whereby histones are displaced from chromatin, first by transition proteins and later by protamines (7). With protamines, DNA in mature spermatozoa is packaged into an extremely condensed, functionally inert configuration (8). Several lines of evidence led to the view that chromatin remodeling during spermiogenesis is facilitated by hyperacetylation of histone H4. First, studies of numerous vertebrate species have shown that H4 hyperacetylation correlates with the occurrence of the histone-to-protamine transition in spermatogenesis. Extensive H4 hyperacetylation occurs in the testes of species where histones are replaced by protamines during spermiogenesis (3-6), but not in species that completely retain somatic-type histones in mature spermatozoa (4, 9). Second, the timing of H4 hyperacetylation is consistent with its role in promoting histone displacement. Studies in the rat, for instance, demonstrated that H4 hyperacetylation during spermiogenesis immediately precedes the histone-to-protamine transition (5, 6). Finally...

Lymphocyte development and differentiation are regulated by the basic helix-loop-helix (bHLH) transcription factors encoded by the E2A and HEB genes. These bHLH proteins bind to E-box enhancers in the form of homodimers or heterodimers and, consequently, activate transcription of the target genes. E2A homodimers are the predominant bHLH proteins present in B-lineage cells and are shown genetically to play critical roles in B-cell development. E2A-HEB heterodimers, the major bHLH dimers found in thymocyte extracts, are thought to play a similar role in T-cell development. However, disruption of either the E2A or HEB gene led to only partial blocks in T-cell development. The exact role of E2A-HEB heterodimers and possibly the E2A and HEB homodimers in T-cell development cannot be distinguished in simple disruption analysis due to a functional compensation from the residual bHLH homodimers. To further define the function of E2A-HEB heterodimers, we generated and analyzed a dominant negative allele of HEB, which produces a physiological amount of HEB proteins capable of forming nonfunctional heterodimers with E2A proteins. Mice carrying this mutation show a stronger and earlier block in T-cell development than HEB complete knockout mice. The developmental block is specific to the ␣/␤ T-cell lineage at a stage before the completion of V(D)J recombination at the TCR␤ gene locus. This defect is intrinsic to the T-cell lineage and cannot be rescued by expression of a functional T-cell receptor transgene. These results indicate that E2A-HEB heterodimers play obligatory roles both before and after TCR␤ gene rearrangement during the ␣/␤ lineage T-cell development.T lymphocytes are derived in the thymus following a stepwise developmental pathway. Each T cell acquires a unique T-cell receptor (TCR), composed of either an ␣/␤ or ␥/␦ heterodimer, on the cell surface after the V(D)J recombination at the corresponding TCR gene loci. The ␣/␤ cell lineage development in the thymus has been conveniently divided into several stages based on the expression of TCR and its coreceptor CD4 and CD8 surface molecules. The most immature population is negative for TCR, CD4, and CD8 expression (double negative, or DN). These cells progress and expand to the TCR low CD4 ϩ CD8 ϩ (double positive, or DP) stage, which makes up 70 to 80% of total cell mass of the thymus (11). DP cells are then subject to major histocompatibility complexmediated positive and negative selection before maturing into either TCR ϩ CD4 Ϫ CD8 ϩ cytotoxic T cells or TCR ϩ CD4 ϩ CD8 Ϫ helper T cells (single positive, or SP). These cytotoxic and helper T cells exit the thymus to peripheral lymph organs, where they provide and mediate antigen-specific immune responses, respectively.Lineage commitment and initiation of V(D)J recombination occur in the DN population, which is composed of less than 2% of total thymocytes in young adult mice. With additional markers such as CD44 and CD25, the DN cells can be further divided into precommitment (CD44 ϩ CD25 Ϫ , or DN1) and postcom...

The mammalian E2A, HEB, and E2-2 genes encode a unique class of basic helix-loop-helix (bHLH) transcription factors that are evolutionarily conserved and essential for embryonic and postnatal development. While the structural and functional similarities among the gene products are well demonstrated, it is not clear why deletion of E2A, but not HEB or E2-2, leads to a complete arrest in B-lymphocyte development. To understand the molecular basis of the functional specificity between E2A and HEB/E2-2 in mammalian development, we generated and tested a panel of E2A knockin mutations including subtle mutations in the E12 and E47 exons and substitution of both E12 and E47 exons with a human HEB cDNA. We find that the alternatively spliced E12 and E47 bHLH proteins of the E2A gene play similar and additive roles in supporting B lymphopoiesis. Further, we find that HEB driven by the endogenous E2A promoter can functionally replace E2A in supporting B-cell commitment and differentiation toward completion. Finally, the postnatal lethality associated with E2A disruption is fully rescued by the addition of HEB. This study suggests that the functional divergence among E12, E47, and HEB in different cell types is partially defined by the context of gene expression.B lymphocytes in mammals are derived from hematopoietic stem cells (HSC) present in the liver during fetal development and bone marrow in adult life. The same HSC also generate T lymphocytes, erythrocytes, macrophages, and other cell types in blood and the lymphoid organs. Once committed to the B-cell lineage, the HSC follow a stepwise differentiation pathway to become B lymphocytes, which subsequently participate in diverse humoral immune responses. It is not entirely known how and when B-lineage cells are first specified from HSC and how B lymphopoiesis is maintained throughout life.B-lineage development can be divided roughly into three discrete stages: progenitor (pro-B), precursor (pre-B), and mature B-cell stages. Rearrangements of immunoglobulin (Ig) genes initiate at the pro-B stage and proceed to completion at the pre-B stage. Pre-B cells expressing functional but nonselfreactive B-cell receptors are selected for survival and expansion to become mature B cells (18). In addition, B cells at various stages of development express lineage-specific markers, such as B220, CD43, and CD19 surface antigens (9, 19). These lineage markers, combined with the status of Ig rearrangements and expression, establish road signs of normal and abnormal progression of B lymphopoiesis (9).Regulation at the transcriptional level is crucial in each step of B-cell development. Indeed, recent studies have shown that E2A, EBF, Pax5, Ikaros, and several other transcription factors play key roles in the pro-and pre-B stages of development (2,14,29,31,35). Disruption of genes encoding each one of these transcription factors arrests B-cell development at either the pro-or pre-B-cell stage. Since deletion of E2A blocks B-cell development prior to the initiation of Ig gene rearrangement, ...