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Many transcription factors regulate specific temporal-spatial events during cardiac differentiation; however, the mechanisms that regulate such events are largely unknown. Using a modified subtractive hybridization method to identify specific genes that influence early cardiac development, we found that Bop is expressed specifically in

The contribution of changes in cis-regulatory elements or trans-acting factors to interspecies differences in gene expression is not well understood. The mammalian β-globin loci have served as a paradigm for gene regulation during development. Transgenic mice harboring the human β-globin locus, consisting of the linked embryonic (ε), fetal (γ) and adult (β) genes, have been used as a model system to study the temporal switch from fetal to adult hemoglobin, as occurs in humans. We show that the human γ-globin genes in these mice behave as murine embryonic globin genes, revealing a limitation of the model and demonstrating that critical differences in the trans-acting milieu have arisen during mammalian evolution. We show that the expression of BCL11A, a repressor of human γ-globin expression identified through genome-wide association studies, differs between mouse and human. Developmental silencing of the mouse embryonic globin and human γ-globin genes fails to occur in mice in the absence of BCL11A. Thus, BCL11A is a critical mediator of species-divergent globin switching. By comparing the ontogeny of β-globin gene regulation in mice and humans, we have shown that alterations in expression of a trans-acting factor constitute a critical driver of gene expression changes during evolution.

Forkhead transcription factors are key participants in development and immune regulation. Here we demonstrate that absence of the gene encoding the forkhead transcription factor Foxp1 resulted in a profound defect in early B cell development. Foxp1 deficiency was associated with decreased expression of all B lineage genes in B220+ fetal liver cells as well as with a block in the transition from pro-B cell to pre-B cell involving diminished expression of recombination-activating genes 1 and 2. Foxp1 bound to the Erag enhancer and was involved in controlling variable-(diversity)-joining recombination of the gene encoding immunoglobulin heavy chain in a B cell lineage-specific way. Our results identify Foxp1 as an essential participant in the transcriptional regulatory network of B lymphopoiesis.

We have recently described a new subfamily of Fox genes, Foxp1/2/4, which are transcriptional repressors and are thought to regulate important aspects of development in several tissues, including the lung, brain, thymus and heart. Here, we show that Foxp1 is expressed in the myocardium as well as the endocardium of the developing heart. To further explore the role of Foxp1 in cardiac development, we inactivated Foxp1 through gene targeting in embryonic stem cells. Foxp1 mutant embryos have severe defects in cardiac morphogenesis, including outflow tract septation and cushion defects, a thin ventricular myocardial compact zone caused by defects in myocyte maturation and proliferation, and lack of proper ventricular septation. These defects lead to embryonic death at E14.5 and are similar to those observed in other mouse models of congenital heart disease, including Sox4 and Nfatc1 null embryos. Interestingly, expression of Sox4 in the outflow tract and cushions of Foxp1 null embryos is significantly reduced, while remodeling of the cushions is disrupted, as demonstrated by reduced apoptosis and persistent Nfatc1 expression in the cushion mesenchyme. Our results reveal a crucial role for Foxp1in three aspects of cardiac development: (1) outflow tract development and septation, (2) tissue remodeling events required for cardiac cushion development, and (3) myocardial maturation and proliferation.

Recently, transplantation of mouse donor spermatogonial stem cells from a fertile testis to an infertile recipient mouse testis was described. The donor cells established spermatogenesis in the seminiferous tubules of the host, and normal spermatozoa were produced. In the most successful transplants, the recipient mice were fertile and sired up to 80 per cent of progeny from donor cells. Here we examine the feasibility of transplanting spermatogonial stem cells from other species to the mouse seminiferous tubule to generate spermatogenesis. Marked testis cells from transgenic rats were transplanted to the testes of immunodeficient mice, and in all of 10 recipient mice (in 19 of 20 testes), rat spermatogenesis occurred. Epididymides of eight mice were examined, and the three from mice with the longest transplants (> or = 110 days) contained rat spermatozoa with normal morphology. The generation of rat spermatogenesis in mouse testes suggests that spermatogonial stem cells of many species could be transplanted, and opens the possibility of xenogeneic spermatogenesis for other species.

To enable vital observation of glia at the neuromuscular junction, transgenic mice were generated that express proteins of the green fluorescent protein family under control of transcriptional regulatory sequences of the human S100B gene. Terminal Schwann cells were imaged repetitively in living animals of one of the transgenic lines to show that, except for extension and retraction of short processes, the glial coverings of the adult neuromuscular synapse are stable. In other lines, subsets of Schwann cells were labeled. The distribution of label suggests that Schwann cells at individual synapses are clonally related, a finding with implications for how these cells might be sorted during postnatal development. Other labeling patterns, some present in unique lines, included astrocytes, microglia, and subsets of cerebellar Bergmann glia, spinal motor neurons, macrophages, and dendritic cells. We show that lines with labeled macrophages can be used to follow the accumulation of these cells at sites of injury.

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