Maturation-promoting activity of SATB1 in MGE-derived cortical interneurons

The generation of cortical interneuron subtypes is controlled by genetic programs that are activated in the ventral forebrain and unfold during the prolonged period of inhibitory neuron development. The LIM-homeodomain protein LHX6 is critical for the development of all cortical interneurons originating in the medial ganglionic eminence, but the molecular mechanisms that operate downstream of LHX6 to control the terminal differentiation of somatostatin- and parvalbumin-expressing interneurons within the cortex remain unknown. Here, we provide evidence that the nuclear matrix and genome organizer protein SATB1 is induced by neuronal activity and functions downstream of Lhx6 to control the transition of tangentially migrating immature interneurons into the terminally differentiated Somatostatin (SST)-expressing subtype. Our experiments provide a molecular framework for understanding the genetic and epigenetic mechanisms by which specified but immature cortical interneurons acquire the subtype-defining molecular and morphophysiological characteristics that allow them to integrate and function within cortical circuits.

Differential RET signaling pathways drive development of the enteric lymphoid and nervous systems

During the early development of the gastrointestinal tract, signaling through the receptor tyrosine kinase RET is required for initiation of lymphoid organ (Peyer's patch) formation and for intestinal innervation by enteric neurons. RET signaling occurs through glial cell line-derived neurotrophic factor (GDNF) family receptor α co-receptors present in the same cell (signaling in cis). It is unclear whether RET signaling in trans, which occurs in vitro through co-receptors from other cells, has a biological role. We showed that the initial aggregation of hematopoietic cells to form lymphoid clusters occurred in a RET-dependent, chemokine-independent manner through adhesion-mediated arrest of lymphoid tissue initiator (LTin) cells. Lymphoid tissue inducer cells were not necessary for this initiation phase. LTin cells responded to all RET ligands in trans, requiring factors from other cells, whereas RET was activated in enteric neurons exclusively by GDNF in cis. Furthermore, genetic and molecular approaches revealed that the versatile RET responses in LTin cells were determined by distinct patterns of expression of the genes encoding RET and its co-receptors. Our study shows that a trans RET response in LTin cells determines the initial phase of enteric lymphoid organ morphogenesis, and suggests that differential co-expression of Ret and Gfra can control the specificity of RET signaling.

Pre-TCR signaling and CD8 gene bivalent chromatin resolution during thymocyte development

The CD8 gene is silent in CD4(-)CD8(-) double-negative thymocytes, expressed in CD4(+)CD8(+) double-positive cells, and silenced in cells committing to the CD4(+) single-positive (SP) lineage, remaining active in the CD8(+) SP lineage. In this study, we show that the chromatin of the CD8 locus is remodeled in C57BL/6 and B6/J Rag1(-/-) MOM double-negative thymocytes as indicated by DNaseI hypersensitivity and widespread bivalent chromatin marks. Pre-TCR signaling coincides with chromatin bivalency resolution into monovalent activating modifications in double-positive and CD8 SP cells. Shortly after commitment to CD4 SP cell lineage, monovalent repressive characteristics and chromatin inaccessibility are established. Differential binding of Ikaros, NuRD, and heterochromatin protein 1α on the locus during these processes may participate in the complex regulation of CD8.

CD8 locus nuclear dynamics during thymocyte development

Nuclear architecture and chromatin reorganization have recently been shown to orchestrate gene expression and act as key players in developmental pathways. To investigate how regulatory elements in the mouse CD8 gene locus are arranged in space and in relation to each other, three-dimensional fluorescence in situ hybridization and chromosome conformation capture techniques were employed to monitor the repositioning of the locus in relation to its subchromosomal territory and to identify long-range interactions between the different elements during development. Our data demonstrate that CD8 gene expression in murine lymphocytes is accompanied by the relocation of the locus outside its subchromosomal territory. Similar observations in the CD4 locus point to a rather general phenomenon during T cell development. Furthermore, we show that this relocation of the CD8 gene locus is associated with a clustering of regulatory elements forming a tight active chromatin hub in CD8-expressing cells. In contrast, in nonexpressing cells, the gene remains close to the main body of its chromosomal domain and the regulatory elements appear not to interact with each other.

Variegated expression of CD8 alpha resulting from in situ deletion of regulatory sequences

The developmental and subset-specific expression of the CD8 genes is under the control of a complex array of regulatory elements distributed along the locus and characterized by DNaseI hypersensitivity. Here we describe the phenotype of mice in which hypersensitive sites 1 and 2 (HSS1 and 2) of DNaseI hypersensitive Cluster II (CII), which are located upstream of the CD8 alpha gene, were deleted by targeted homologous recombination of the endogenous locus. Knockout mice exhibit a variegated expression of the CD8 alpha gene, particularly among the immature CD4(+)8(+) TCR(lo) thymocyte population. We propose that HSS Cluster II regulatory elements are essential in ensuring initiation of chromatin remodeling and establishment of an open configuration in all developing thymocytes that undergo the double-negative to double-positive transition. Furthermore, these sequences contribute to the levels of expression of the CD8 alpha gene.

Hierarchical interactions of control elements determine CD8alpha gene expression in subsets of thymocytes and peripheral T cells

CD4 and CD8 are crucial for the development and function of T cells. An intergenic deoxyribonuclease I hypersensitive site region (cluster CIII) directs expression in mature CD8 T cells only. Here, we show that two further independent regions from the CD8 gene locus in conjunction with cluster CIII restore transgene expression in appropriate immature thymocytes. Deletion of two of the intergenic cluster CIII DNaseI-HSS in homozygous mutant mice affects expression of CD8alphaalpha homodimers on intraepithelial T cells (IEL), particularly on the gammadeltaTCR+ subset. Surprisingly, none of the thymocyte or peripheral alphabetaTCR T cell subsets are affected by this mutation, indicating hierarchical activation of these elements within the different T cell subsets.