Running rings around chromosomes to trim axons and target dendrites

CyclinD1 Down-Regulation and Increased Apoptosis Are Common Features of Cohesinopathies

Genetic variants within components of the cohesin complex (NIPBL, SMC1A, SMC3, RAD21, PDS5, ESCO2, HDAC8) are believed to be responsible for a spectrum of human syndromes known as "cohesinopathies" that includes Cornelia de Lange Syndrome (CdLS). CdLS is a multiple malformation syndrome affecting almost any organ and causing severe developmental delay. Cohesinopathies seem to be caused by dysregulation of specific developmental pathways downstream of mutations in cohesin components. However, it is still unclear how mutations in different components of the cohesin complex affect the output of gene regulation. In this study, zebrafish embryos and SMC1A-mutated patient-derived fibroblasts were used to analyze abnormalities induced by SMC1A loss of function. We show that the knockdown of smc1a in zebrafish impairs neural development, increases apoptosis, and specifically down-regulates Ccnd1 levels. The same down-regulation of cohesin targets is observed in SMC1A-mutated patient fibroblasts. Previously, we have demonstrated that haploinsufficiency of NIPBL produces similar effects in zebrafish and in patients fibroblasts indicating a possible common feature for neurological defects and mental retardation in cohesinopathies. Interestingly, expression analysis of Smc1a and Nipbl in developing mouse embryos reveals a specific pattern in the hindbrain, suggesting a role for cohesins in neural development in vertebrates.

Cornelia de Lange Syndrome: NIPBL haploinsufficiency downregulates canonical Wnt pathway in zebrafish embryos and patients fibroblasts

Cornelia de Lange Syndrome is a severe genetic disorder characterized by malformations affecting multiple systems, with a common feature of severe mental retardation. Genetic variants within four genes (NIPBL (Nipped-B-like), SMC1A, SMC3, and HDAC8) are believed to be responsible for the majority of cases; all these genes encode proteins that are part of the 'cohesin complex'. Cohesins exhibit two temporally separated major roles in cells: one controlling the cell cycle and the other involved in regulating the gene expression. The present study focuses on the role of the zebrafish nipblb paralog during neural development, examining its expression in the central nervous system, and analyzing the consequences of nipblb loss of function. Neural development was impaired by the knockdown of nipblb in zebrafish. nipblb-loss-of-function embryos presented with increased apoptosis in the developing neural tissues, downregulation of canonical Wnt pathway genes, and subsequent decreased Cyclin D1 (Ccnd1) levels. Importantly, the same pattern of canonical WNT pathway and CCND1 downregulation was observed in NIPBL-mutated patient-specific fibroblasts. Finally, chemical activation of the pathway in nipblb-loss-of-function embryos rescued the adverse phenotype and restored the physiological levels of cell death.

Expression of cohesin and condensin genes during zebrafish development supports a non-proliferative role for cohesin

Cohesin and condensin are similar, but distinct multi-subunit protein complexes that have well-described roles in sister chromatid cohesion and chromosome condensation, respectively. Recently it has emerged that cohesin, and proteins that regulate cohesin function have additional developmental roles. To further understand the role of cohesin in development, we analyzed the expression of genes encoding cohesin and condensin subunits in developing zebrafish embryos and juvenile brain. We found that cohesin subunits are expressed in a pattern that is similar (but not quite identical) to the expression of condensin subunits. Cohesin genes smc1a, rad21, pds5b and smc3 were expressed in the forebrain ventricular zone, the tectum, the mid-hindbrain boundary, the fourth ventricle, branchial arches, the otic vesicle, the eye and faintly in the developing pectoral fins. Condensin genes smc2 and smc4 were expressed in the forebrain ventricular zone, the tectum, the mid-hindbrain boundary, the fourth ventricle, branchial arches, eye and pectoral fins. Condensin genes were additionally expressed in the hindbrain proliferative zone, an area in which cohesin genes were not detected. A comparison with pcna expression and BrdU incorporation revealed that the expression of cohesins and condensins closely overlap with zones of proliferation. Interestingly, cohesin genes were expressed in non-proliferating cells flanking rhombomere boundaries in the developing brain. In mature brain and eye, cohesin was expressed in both proliferating cells and in broad zones of post-mitotic cells. The distribution of cohesin and condensin mRNAs supports existing evidence for a non-cell cycle role for cohesin in the developing brain.

Cohesin, gene expression and development: lessons from Drosophila

The cohesin complex, discovered through its role in sister chromatid cohesion, also plays roles in gene expression and development in organisms from yeast to human. This review highlights what has been learned about the gene control and developmental functions of cohesin and the Nipped-B (NIPBL/Scc2) cohesin loading factor in Drosophila. The Drosophila studies have provided unique insights into the aetiology of Cornelia de Lange syndrome (CdLS), which is caused by mutations affecting sister chromatid cohesion proteins in humans. In vivo experiments with Drosophila show that cohesin and Nipped-B have dosage-sensitive effects on the functions of many evolutionarily conserved genes and developmental pathways. Genome-wide studies with Drosophila cultured cells show that Nipped-B and cohesin co-localize on chromosomes, and bind preferentially, but not exclusively, to many actively transcribed genes and their regulatory sequences, including many of the proposed in vivo target genes. In contrast, the cohesion factors are largely excluded from genes silenced by Polycomb group (PcG) proteins. Combined, the in vivo genetic data and the binding patterns of cohesin and Nipped-B in cultured cells are consistent with the hypothesis that they control the action of gene regulatory sequences, including transcriptional enhancers and insulators, and suggest that they might also help define active chromatin domains and influence transcriptional elongation.