Heterochronic Developmental Shifts Underlying Squamate Cerebellar Diversity Unveil the Key Features of Amniote Cerebellogenesis

Despite a remarkable conservation of architecture and function, the cerebellum of vertebrates shows extensive variation in morphology, size, and foliation pattern. These features make this brain subdivision a powerful model to investigate the evolutionary developmental mechanisms underlying neuroanatomical complexity both within and between anamniote and amniote species. Here, we fill a major evolutionary gap by characterizing the developing cerebellum in two non-avian reptile species—bearded dragon lizard and African house snake—representative of extreme cerebellar morphologies and neuronal arrangement patterns found in squamates. Our data suggest that developmental strategies regarded as exclusive hallmark of birds and mammals, including transit amplification in an external granule layer (EGL) and Sonic hedgehog expression by underlying Purkinje cells (PCs), contribute to squamate cerebellogenesis independently from foliation pattern. Furthermore, direct comparison of our models suggests the key importance of spatiotemporal patterning and dynamic interaction between granule cells and PCs in defining cortical organization. Especially, the observed heterochronic shifts in early cerebellogenesis events, including upper rhombic lip progenitor activity and EGL maintenance, are strongly expected to affect the dynamics of molecular interaction between neuronal cell types in snakes. Altogether, these findings help clarifying some of the morphogenetic and molecular underpinnings of amniote cerebellar corticogenesis, but also suggest new potential molecular mechanisms underlying cerebellar complexity in squamates. Furthermore, squamate models analyzed here are revealed as key animal models to further understand mechanisms of brain organization.Peer reviewe

Functions of peroxisome proliferator-activated receptors (PPAR) in skin homeostasis

The peroxisome proliferator-activated receptors (PPAR) are ligand-activated transcription factors that belong to the nuclear hormone receptor family. Three isotypes (PPARα, PPARβ or δ, and PPARγ) with distinct tissue distributions and cellular functions have been found in vertebrates. All three PPAR isotypes are expressed in rodent and human skin. They were initially investigated for a possible function in the establishment of the permeability barrier in skin because of their known function in lipid metabolism in other cell types. In vitro studies using specific PPAR agonists and in vivo gene disruption approaches in mice indeed suggest an important contribution of PPARα in the formation of the epidermal barrier and in sebocyte differentiation. The PPARγ isotype plays a role in stimulating sebocyte development and lipogenesis, but does not appear to contribute to epidermal tissue differentiation. The third isotype, PPARβ, regulates the late stages of sebaceous cell differentiation, and is the most effective isotype in stimulating lipid production in these cells, both in rodents and in humans. In addition, PPARβ activation has pro-differentiating effects in kera-tinocytes under normal and inflammatory conditions. Finally, preliminary studies also point to a potential role of PPAR in hair follicle growth and in melanocyte differentiation. By their diverse biological effects on cell proliferation and differentiation in the skin, PPAR agonists or antagonists may offer interesting oppotunities for the treatment of various skin disorders characterized by inflammation, cell hyperproliferation, and aberrant differentiatio

Additive and global functions of HoxA cluster genes in mesoderm derivatives

Hox genes encode transcription factors that play a central role in the specification of regional identities along the anterior to posterior body axis. In the developing mouse embryo, Hox genes from all four genomic clusters are involved in range of developmental processes, including the patterning of skeletal structures and the formation of several organs. However, the functional redundancy observed either between paralogous genes, or among neighboring genes from the same cluster, has hampered functional analyses, in particular when synergistic, cluster-specific functions are considered. Here, we report that mutant mice lacking the entire HoxA cluster in mesodermal lineages display the expected spectrum of postnatal respiratory, cardiac and urogenital defects, previously reported for single gene mutations. Likewise, mild phenotypes are observed in both appendicular and axial skeleton. However, a striking effect was uncovered in the hematopoietic system, much stronger than that seen for Hoxa9 inactivation alone, which involves stem cells (HSCs) as well as the erythroid lineage, indicating that several Hoxa genes are necessary for normal hematopoiesis to occur. Finally, the combined deletions of Hoxa and Hoxd genes reveal abnormalities in axial elongation as well as skin morphogenesis that are likely the results of defects in epithelial-mesenchymal interactions

Changes in Hox genes' structure and function during the evolution of the squamate body plan

Hox genes are central to the specification of structures along the anterior-posterior body axis, and modifications in their expression have paralleled the emergence of diversity in vertebrate body plans. Here we describe the genomic organization of Hox clusters in different reptiles and show that squamates have accumulated unusually large numbers of transposable elements at these loci, reflecting extensive genomic rearrangements of coding and non-coding regulatory regions. Comparative expression analyses between two species showing different axial skeletons, the corn snake and the whiptail lizard, revealed major alterations in Hox13 and Hox10 expression features during snake somitogenesis, in line with the expansion of both caudal and thoracic regions. Variations in both protein sequences and regulatory modalities of posterior Hox genes suggest how this genetic system has dealt with its intrinsic collinear constraint to accompany the substantial morphological radiation observed in this group

Shared and differential features of Robo3 expression pattern in amniotes

In Bilaterians, commissural neurons project their axons across the midline of the nervous system to target neurons on the opposite side. In mammals, midline crossing at the level of the hindbrain and spinal cord requires the Robo3 receptor which is transiently expressed by all commissural neurons. Unlike other Robo receptors, mammalian Robo3 receptors do not bind Slit ligands and promote midline crossing. Surprisingly, not much is known about Robo3 distribution and mechanism of action in other vertebrate species. Here, we have use whole-mount immunostaining, tissue clearing and light-sheet fluorescent microscopy to study Robo3 expression pattern in multiple embryonic tissue from diverse representatives of amniotes at distinct stages, including squamate (African house snake), birds (chicken, duck, pigeon, ostrich, emu and zebra finch), early postnatal marsupial mammals (fat-tailed dunnart), and eutherian mammals (mouse and human). The analysis of this rich and unique repertoire of amniote specimen reveals conserved features of Robo3 expression in midbrain, hindbrain and spinal cord commissural circuits, which together with subtle but meaningful modifications could account for species-specific evolution of sensory-motor and cognitive capacities. Our results also highlight important differences of precerebellar nuclei development across amniotes. This article is protected by copyright. All rights reserved

Distinct roles and regulations for hoxd genes in metanephric kidney development

Hox genes encode homeodomain-containing proteins that control embryonic development in multiple contexts. Up to 30 Hox genes, distributed among all four clusters, are expressed during mammalian kidney morphogenesis, but functional redundancy between them has made a detailed functional account difficult to achieve. We have investigated the role of the HoxD cluster through comparative molecular embryological analysis of a set of mouse strains carrying targeted genomic rearrangements such as deletions, duplications, and inversions. This analysis allowed us to uncover and genetically dissect the complex role of the HoxD cluster. Regulation of metanephric mesenchyme-ureteric bud interactions and maintenance of structural integrity of tubular epithelia are differentially controlled by some Hoxd genes during renal development, consistent with their specific expression profiles. We also provide evidence for a kidney-specific form of colinearity that underlies the differential expression of two distinct sets of genes located on both sides and overlapping at the Hoxd9 locus. These insights further our knowledge of the genetic control of kidney morphogenesis and may contribute to understanding certain congenital kidney malformations, including polycystic kidney disease and renal hypoplasia. Citation: Di-Poï N, Zákány J, Duboule D (2007) Distinct roles and regulations for Hoxd genes in metanephric kidney development. PLoS Genet 3(12): e232. doi:10.1371/journal. pgen.003023

Pogona vitticeps tooth transcriptomics

Paired-end sequencing reads (Illumina technology) of successional dental lamina tissues of acrodont teeth from Pogona vitticeps lizard, biological replicate

Pogona vitticeps tooth transcriptomics

Paired-end sequencing reads (Illumina technology) of dental mesenchyme tissues of pleurodont teeth from Pogona vitticeps lizard, biological replicate