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Patterning the Vertebrate Body Axis
Summary: Olivier Pourquié wants to understand the genetic and developmental mechanisms that control segmentation in vertebrates, a striking feature of the early growth of embryos that has long been thought to be a key aspect of the basic design of many animals.
The goal of my research is to gain a better understanding of the segmentation process in vertebrates. Segmentation is the embryonic process whereby the body axis forms as a series of repeated anatomical modules. In humans, the segmented aspect is particularly conspicuous at the level of the vertebral column. The segmental pattern is established during embryogenesis when somites, the precursors of muscles and vertebrae, are segregated in a periodic fashion from precursors located in the presomitic mesoderm (PSM). My laboratory identified a molecular oscillator, termed the segmentation clock, that ticks in somitic precursors with a rhythm paralleling that of somite formation. Our ongoing research focuses on the elucidation of the molecular mechanism underlying the clock oscillator, as well as on the precise role of the clock in the vertebrate segmentation process.
Segmentation Is a Major Embryonic Patterning Process
The segmented or metameric aspect of the body axis is a basic characteristic of many animal species, ranging from invertebrates to humans, and segmentation has long been thought to be a key aspect of the basic design of animals. Conservation of the segmented body pattern among very distantly related species provided a strong argument in favor of the "unity of animal body plan" idea developed at the beginning of the 19th century. Because body segmentation is one of the most salient features of the embryo, it was used as a morphological criterion in the pioneering genetic screens performed in the fruit fly by Christiane Nüsslein-Volhard (Max Planck Institute for Developmental Biology, Tübingen) and Eric Wieschaus (HHMI, Princeton University) during the late 1970s. These screens led to the identification of the genetic cascade involved in establishing the metameric pattern of the fly embryo. Many of the genes identified through these screens (e.g., wingless or hedgehog) proved to be part of major signaling systems that are deregulated in diseases such as cancer.
Somite Formation
The vertebrate body is built on a metameric organization that consists of a repetition along the anteroposterior axis of functionally equivalent units, each comprising a vertebra, its associated muscles, peripheral nerves, and blood vessels. At the functional level, segmentation is critical to ensure the movements of a rod-like structure such as the vertebral column. The segmented distribution of the vertebrae derives from the earlier metameric pattern of the embryonic somites that are epithelial spheres generated in a rhythmic fashion from the mesenchymal PSM (Figure 1).
In contrast to the fly embryo, in which segments are determined simultaneously, vertebrate segmentation is a sequential process that proceeds synchronously with the posterior extension of the embryo. After the completion of gastrulation, during which the superficial tissues are internalized to form the mesoderm and the endoderm, the embryo begins to elongate at its posterior end. This elongation process leads to the sequential formation of embryonic tissues in an anterior-to-posterior sequence. This progressive mode of body formation establishes a gradient of maturation along the anteroposterior axis. Somite formation follows this differentiation gradient and proceeds rhythmically from head to tail in all vertebrate embryos, including humans. In the mouse embryo, a new pair of somites is added immediately posterior to the last formed somite pair every 120 minutes until 65 somite pairs are formed.
The Vertebrate Segmentation Clock
Theoretical models of vertebrate segmentation proposed the existence of an oscillator in PSM cells, acting to generate a temporal periodicity then translated into the spatial periodicity of somite boundaries. Work from my laboratory reporting the existence of rhythmic waves of expression of the mRNA coding for the transcription factor c-hairy1, exhibiting a period similar to that of somitogenesis, provided the first evidence for the existence of an oscillator associated with the segmentation process (Figure 2). We and others subsequently showed that this oscillator, the segmentation clock, controls the rhythmic transcription of a group of genes now referred to as cyclic genes. The segmentation clock has now been identified in fish, chick, and mouse, indicating that it represents a conserved feature among vertebrates. One output of the oscillator is the rhythmic activation of Notch in the PSM, which could act as a periodic trigger, initiating the process of somite boundary specification. Our current understanding is that the clockwork of the oscillator involves a series of negative-feedback loops involving Notch and Wnt signaling. Recently, we have been developing a microarray approach to identify all the cyclic genes in the mouse transcriptome. Unraveling the molecular processes underlying the segmentation clock is a major focus of my laboratory.
FGF Signaling Plays a Major Role in Segmentation
Although the segmentation clock is believed to set the rhythm of somitogenesis, it does not specify the positioning of somite boundaries along the anteroposterior axis. We recently demonstrated that the mechanism controlling the spacing of the future somite boundaries in the forming PSM relies on a traveling threshold of FGF signaling. We showed that segments become genetically defined in the PSM at a permissive level of FGF signaling—called the determination front—where cells become competent to respond to a periodic signal from the segmentation clock (see the movie). This system results in the segment-wide expression of genes (e.g., the transcription factors of the Mesp family) that control subsequent steps of somite formation.
We observed that fgf8 mRNA is constantly transcribed in the precursors of the PSM in the tail bud during axis extension and that its transcription stops in descendants of these cells when they enter the posterior PSM. The posterior growth of the vertebrate axis coupled to the progressive decay of the fgf8 mRNA in the PSM results in the formation of an mRNA gradient along the PSM. Due to the axis elongation process, which results in the constant addition of new cells expressing high levels of fgf8 mRNA selectively in the posterior PSM, the gradient is dynamic and is constantly displaced posteriorly (Figure 2). This mechanism ensures a tight coupling of segmentation to the axis formation.
Control of Somite Left-Right Symmetry and Regional Identity of Somitic Derivatives
A striking feature of somites is their perfect symmetry along the left-right axis, which can be contrasted with the general asymmetry of the internal organs such as heart or liver. This symmetrical pattern results from the coordinated production of pairs of somites at the anterior extremity of the PSM. We recently showed that retinoic acid plays a critical role in this coordination process by preventing the response of future somitic cells to signals from the left-right patterning machinery. We are interested in understanding the mechanisms that control the left-right symmetry of somite production and of the embryo in general.
We are also interested in the mechanisms involved in the control of the regionalization of somite derivatives. Hox genes play a major role in specifying the regional identity of vertebrae, and we are studying how Hox gene expression is coordinated to the segmentation process.
Human Segmentation Syndromes
Congenital vertebral malformations in humans are a major therapeutic challenge due to the intricate neural and musculoskeletal anatomy of the spine. Our research is expected to have a strong impact in the field of congenital spine anomalies, currently an understudied biomedical problem, and will be of use in the elucidation of the etiology and eventual prevention of these disorders. This work is also expected to further our understanding of the major signaling pathways underlying segmentation and establishment of the vertebrate body plan, which include Notch, Wnt, FGF, and retinoic acid pathways—all known to play important roles in a wide array of human diseases.
A grant from the National Institutes of Health provided support for this work. Additional support was provided by the the Stowers Institute for Medical Research, the Centre National de la Recherche Scientifique (CNRS), the Association Francaise contre les Myopathies (AFM), and the Human Frontier Science Program (HFSP).
Last updated April 06, 2009
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