Comprendre organogenèse précoce aide d'un simplifié<em> In Situ</em> Protocole hybridation in<em> Xenopus</em

The expression pattern of a specific gene is an important piece of information in determining the potential role for that gene in the development of a specific organ or cell type. Simply put, if it is not expressed at the right time and place it is unlikely to play a key role. In Xenopus, as in most early embryos, the most commonly used assay for detecting the expression of a gene is whole mount in situ hybridization using labeled antisense RNA probes. The use of antibody staining to assess expression of a gene in Xenopus is becoming more common as researchers discover antibodies, usually raised against mammalian proteins, that cross react to the Xenopus homologue or generate their own ^1-3. However, the vast majority of studies on Xenopus organogenesis still utilize antisense RNA probes. When antibodies are used, each individual antibody often requires optimization for the primary antibody concentration or fixation protocols. In contrast, the protocol for in situ hybridizations is essentially invariant for different probes. The basic concept is relatively simple and an excellent standard protocol has been well established ⁴. Our protocol is a streamlined version of the original protocol ⁴ that still provides excellent detection of gene expression patterns in the early embryo. The embryos are fixed and then prepared for hybridization by changing solutions and temperatures such that it allows for high stringency binding of the labeled antisense RNA probe to its target mRNA. The unbound probe is washed away and the embryos are then prepared for binding of an antibody against the label on the RNA probes. Excess antibody is then washed away and an enzymatic color reaction is used to localize where the RNA probe is bound in the embryo. There are now a number of Xenopus transgenic lines that drive expression of fluorescent proteins in specific tissues and these are available at the Xenopus stock centers such as the National Xenopus Resource in Woods Hole. While very useful for many experiments that require examining organogenesis in living embryos, this option requires separate housing for the transgenic lines.

In situ hybridization can clearly delineate where specific organs or cell types will form in the early embryo (Figure 1). The technique is remarkably sensitive given that one can detect gene expression in a small number of cells in a single embryo ⁵. However, in situ hybridization using the intensity of colorimetric staining is not considered quantifiable because the color reaction is not a linear one. Despite difficulty in quantifying staining intensity, changes in expression are often quite noticeable; particularly when the in situ hybridization shows quantifiable increases or decreases in the size of expression domains ^6,7.

The clear advantages of whole mount in situ hybridization make it a critical assay in the study of early development. However, it is a time consuming one that requires many steps over several days. This protocol is a simplified version of the standard protocol that eliminates several steps without reducing the quality of the in situ result. The simplification also eliminates sources of variability, making trouble shooting easier if an in situ hybridization is not optimal. Specifically, we have eliminated the use of proteinase K and RNAse treatments of the embryo, two steps that can depend on reagent quality and can also reduce signal intensity if overdone. The protocol also provides some degree of cost saving due to eliminating the use of several reagents. Finally, this protocol also provides some simple guidelines for improved capturing of images of in situ hybridization results. Although this protocol is optimized for work in Xenopus embryos, it is likely that at least some of the simplifications will be applicable to in situ hybridization work in other embryo systems.