DLX Genes in the Development and Maintenance of the Vertebrate Skeleton: Implications for Human Pathologies.

Skeletal shape and mechanical properties define, to a large extent, vertebrate morphology and physical capacities. During development, skeletal morphogenesis results from dynamic communications between chondrocytes, osteoblasts, osteoclasts, and other cellular components of the skeleton. Later in life, skeletal integrity depends on the regulatory cascades that assure the equilibrium between bone formation and resorption. Finally, during aging, skeletal catabolism prevails over anabolism resulting in progressive skeletal degradation. These cellular processes depend on the transcriptional cascades that control cell division and differentiation in each cell type. Most Distal-less (Dlx) homeobox transcription factors are directly involved in determining the proliferation and differentiation of chondrocytes and osteoblasts and, indirectly, of osteoclasts. While the involvement of Dlx genes in the regulation of skeletal formation has been well-analyzed thanks to several mutant mouse models, the role of these genes in the maintenance of bone integrity has been only partially studied. The importance of Dlx genes for adult bone tissues is evidenced by their central role in the regulatory pathways involving Osx/Sp7 and Runx2, the two major master genes of osteogenesis. Dlx genes appear to be involved in several bone pathologies including, for example, osteoporosis. Indeed, at least five large-scale GWAS studies which aimed to detect loci associated with human bone mineral density (BMD) have identified a known DLX5/6 regulatory region within chromosome 7q21.3 in proximity of SEM1/FLJ42280/DSS1 coding sequences, suggesting that DLX5/6 expression is critical in determining healthy BMD. This review aims to summarize the major findings concerning the involvement of Dlx genes in skeletal development and homeostasis and their involvement in skeletal aging and pathology.

Posterior axis formation requires Dlx5/Dlx6 expression at the neural plate border.

Neural tube defects (NTDs), one of the most common birth defects in human, present a multifactorial etiology with a poorly defined genetic component. The Dlx5 and Dlx6 bigenic cluster encodes two evolutionary conserved homeodomain transcription factors, which are necessary for proper vertebrate development. It has been shown that Dlx5/6 genes are essential for anterior neural tube closure, however their role in the formation of the posterior structures has never been described. Here, we show that Dlx5/6 expression is required during vertebrate posterior axis formation. Dlx5 presents a similar expression pattern in neural plate border cells during posterior neurulation of zebrafish and mouse. Dlx5/6-inactivation in the mouse results in a phenotype reminiscent of NTDs characterized by open thoracic and lumbar vertebral arches and failure of epaxial muscle formation at the dorsal midline. The dlx5a/6a zebrafish morphants present posterior NTDs associated with abnormal delamination of neural crest cells showing altered expression of cell adhesion molecules and defects of motoneuronal development. Our findings provide new molecular leads to decipher the mechanisms of vertebrate posterior neurulation and might help to gather a better understanding of human congenital NTDs etiology.

Dlx5 and Dlx6 control uterine adenogenesis during post-natal maturation: possible consequences for endometriosis.

Dlx5 and Dlx6 are two closely associated homeobox genes which code for transcription factors involved in the control of steroidogenesis and reproduction. Inactivation of Dlx5/6 in the mouse results in a Leydig cell defect in the male and in ovarian insufficiency in the female. DLX5/6 are also strongly expressed by the human endometrium but their function in the uterus is unknown. The involvement of DLX5/6 in human uterine pathology is suggested by their strong downregulation in endometriotic lesions and upregulation in endometrioïd adenocarcinomas. We first show that Dlx5/6 expression begins in Müllerian ducts epithelia and persists then in the uterine luminal and glandular epithelia throughout post-natal maturation and in the adult. We then use a new mouse model in which Dlx5 and Dlx6 can be simultaneously inactivated in the endometrium using a Pgr(cre/+) allele. Post-natal inactivation of Dlx5/6 in the uterus results in sterility without any obvious ovarian involvement. The uteri of Pgr(cre/+); Dlx5/6(flox/flox) mice present very few uterine glands and numerous abnormally large and branched invaginations of the uterine lumen. In Dlx5/6 mutant uteri, the expression of genes involved in gland formation (Foxa2) and in epithelial remodelling during implantation (Msx1) is significantly reduced. Furthermore, we show that DLX5 is highly expressed in human endometrial glandular epithelium and that its expression is affected in endometriosis. We conclude that Dlx5 and Dlx6 expression determines uterine architecture and adenogenesis and is needed for implantation. Given their importance for female reproduction, DLX5 and DLX6 must be regarded as interesting targets for future clinical research.

cAMP oscillations and retinal activity are permissive for ephrin signaling during the establishment of the retinotopic map.

Spontaneous activity generated in the retina is necessary to establish a precise retinotopic map, but the underlying mechanisms are poorly understood. We demonstrate here that neural activity controls ephrin-A-mediated responses. In the mouse retinotectal system, we show that spontaneous activity of the retinal ganglion cells (RGCs) is needed, independently of synaptic transmission, for the ordering of the retinotopic map and the elimination of exuberant retinal axons. Activity blockade suppressed the repellent action of ephrin-A on RGC growth cones by cyclic AMP (cAMP)-dependent pathways. Unexpectedly, the ephrin-A5-induced retraction required cAMP oscillations rather than sustained increases in intracellular cAMP concentrations. Periodic photo-induced release of caged cAMP in growth cones rescued the response to ephrin-A5 when activity was blocked. These results provide a direct molecular link between spontaneous neural activity and axon guidance mechanisms during the refinement of neural maps.