Characterizing gene expression during lens formation in Xenopus laevis: Evaluating the model for embryonic lens induction

Abstract

Few directed searches have been undertaken to identify the genes involved in vertebrate lens formation. In the frog Xenopus, the larval cornea can undergo a process of transdifferentiation to form a new lens once the original lens is removed. Based on preliminary evidence, we have shown that this process shares many elements of a common molecular/genetic pathway to that involved in embryonic lens development. A subtracted cDNA library, enriched for genes expressed during cornea-lens transdifferentiation, was prepared. The similarities/identities of specific clones isolated from the subtracted cDNA library define an expression profile of cells undergoing cornea-lens transdifferentiation ("lens regeneration") and corneal wound healing (the latter representing a consequence of the surgery required to trigger transdifferentiation). Screens were undertaken to search for genes expressed during both transdifferentiation and embryonic lens development. Significantly, new genes were recovered that are also expressed during embryonic lens development. The expression of these genes, as well as others known to be expressed during embryonic development in Xenopus, can be correlated with different periods of embryonic lens induction and development, in an attempt to define these events in a molecular context. This information is considered in light of our current working model of embryonic lens induction, in which specific tissue properties and phases of induction have been previously defined in an experimental context. Expression data reveal the existence of further levels of complexity in this process and suggests that individual phases of lens induction and specific tissue properties are not strictly characterized or defined by expression of individual genes.

Three matrix metalloproteinases are required in vivo for macrophage migration during embryonic development

“…The restricted and specific expression of MMP-9 and -18 to the macrophage cell lineage is consistent with their known functions later in embryogenesis where they have been associated with wound healing, metamorphosis and regeneration (Carinato et al, 2000;Henry et al, 2002;Tashiro et al, 2006). Very little is currently known about the substrates of the Xenopus MMPs.…”

Beyond early development: Xenopus as an emerging model for the study of regenerative mechanisms

“…Several genes encoding transcription factors involved in initial development of the lens during embryonic stages (pax6, prox1, otx2, and sox3) are re-expressed in transdifferentiating corneal cells, suggesting that regeneration is at least in part a recapitulation of ontogeny (Schaefer et al, 1999;Henry et al, 2002). A recent study showed that the lens regenerating competence correlates specifically to localized pax6 expression in the lentogenic area, and that misexpression of pax6 in noncompetent flank epidermis enables transdifferentiation into lens in response to the vitreous factor .…”

“…Using a subtractive library approach, Henry et al isolated and sequenced 250 clones preferentially expressed in transdifferentiating corneas 6 in the first 4 days after lentectomy (Henry et al, 2002). After selection to remove duplicated genes, a further 157 clones were sequenced, resulting in identification of a total of 233 unique 5Ј EST sequences.…”

Knockdown of SPARC leads to decreased cell–cell adhesion and lens cataracts during post-gastrula development in Xenopus laevis

“…Consistent with the critical requirement of SPARC for proper lens development, lens cataracts were observed during late tailbud development in embryos injected with a concentration of XSMOs that had no visible effect on other tissues, including the optic cup. In contrast to Sparc-null mice, Xenopus lenses are eventually regenerated, not unexpected as levels of XSMOs are diluted by this stage and that Xenopus has the capacity to regenerate lenses until at least metamorphosis (Henry et al 2002).…”