A conditional allele of Rspo3 reveals redundant function of R-spondins during mouse limb development.

R-spondins are secreted ligands that bind cell surface receptors and activate Wnt/ß-catenin signaling. Human mutations and gene inactivation studies in mice have revealed a role for these four proteins (RSPO1-4) in diverse developmental processes ranging from sex determination to limb development. Among the genes coding for R-spondins, only inactivation of Rspo3 shows early embryonic lethality (E10.5 in mice). Therefore, a conditional allele of this gene is necessary to understand the function of R-spondins throughout murine development. To address this need, we have produced an allele in which loxP sites flank exons 2-4 of Rspo3, allowing tissue-specific deletion of these exons in the presence of Cre recombinase. We used these mice to investigate the role of Rspo3 during limb development and found that limbs ultimately developed normally in the absence of Rspo3 function. However, severe hindlimb truncations resulted when Rspo3 and Rspo2 mutations were combined, demonstrating redundant function of these genes.

Shox2 function couples neural, muscular and skeletal development in the proximal forelimb.

The mouse Shox2 gene codes for a homeodomain transcription factor that is required to form the proximal bones of the limbs, the humerus and femur. Shox2 is the only gene known to be essential for the specific development of these skeletal elements. Shox2 is also of special interest because it is closely related to the human SHOX gene, deficiencies of which cause the short stature in Turner, Langer and Léri-Weill syndromes. In order to understand in more detail the development of the proximal limb, we searched for Shox2-dependent gene expression patterns using Affymetrix microarrays. We compared the mRNA of Shox2-mutant and wild-type forelimb buds at 10.5 and 11.5 days of embryonic development (E10.5 and E11.5) and successfully identified a set of genes whose wild-type expression pattern requires Shox2 function, as confirmed by in situ hybridization for eleven of the candidates. Strikingly, several of the identified genes were predicted to have functions in tissues other than the skeleton, including nerves and muscle precursors, prompting us to analyze neural and muscular patterning in Shox2 mutants. We report here an axonal migration defect in Shox2 mutants resulting in a profound innervation deficiency of the dorsal forelimb, including the complete absence of the radial and axillary nerves. Muscular development was also altered as early as E11.5. Specifically, the triceps muscles that develop along the posterior face of the humerus had severe abnormalities. These data demonstrate that Shox2 is required for normal skeletal, neural and muscular development in the forelimb at a similar early developmental stage in each tissue.

Arabidopsis At5g39790 encodes a chloroplast-localized, carbohydrate-binding, coiled-coil domain-containing putative scaffold protein.

BACKGROUND: Starch accumulation and degradation in chloroplasts is accomplished by a suite of over 30 enzymes. Recent work has emphasized the importance of multi-protein complexes amongst the metabolic enzymes, and the action of associated non-enzymatic regulatory proteins. Arabidopsis At5g39790 encodes a protein of unknown function whose sequence was previously demonstrated to contain a putative carbohydrate-binding domain. RESULTS: We here show that At5g39790 is chloroplast-localized, and binds starch, with a preference for amylose. The protein persists in starch binding under conditions of pH, redox and Mg(+2) concentrations characteristic of both the day and night chloroplast cycles. Bioinformatic analysis demonstrates a diurnal pattern of gene expression, with an accumulation of transcript during the light cycle and decline during the dark cycle. A corresponding diurnal pattern of change in protein levels in leaves is also observed. Sequence analysis shows that At5g39790 has a strongly-predicted coiled-coil domain. Similar analysis of the set of starch metabolic enzymes shows that several have strong to moderate coiled-coil potential. Gene expression analysis shows strongly correlated patterns of co-expression between At5g39790 and several starch metabolic enzymes. CONCLUSION: We propose that At5g39790 is a regulatory scaffold protein, persistently binding the starch granule, where it is positioned to interact by its coiled-coil domain with several potential starch metabolic enzyme binding-partners.