Regulation of Calvarial Osteogenesis by Concomitant De-repression of GLI3 and Activation of IHH Targets

Loss-of-function mutations in GLI3 and IHH cause craniosynostois and reduced osteogeneiss, respectively. In this study, we show that ihh ligand, the receptor Ptch1 and Gli transcription factors are differentialyy expressed in embryonic mouse calvaria osteogenic condenstions. We show that in both ihh(-/-) and Gli3(Xt-J/Xt-J) embroyonic mice, the normal gene expression architecture is lost and this results in disorganized calvarial bone developement. RUNX2 is a master regulatory transciption factor controlling osteogenesis. In the absence of Gli3, RUNX2 isoform II and IHH are upregulated, and RUNX2 isoform I downregulated. This is consistent with the expandeed and aberant osteogenesis observed in Gli3Xt-J/Xt-J mice, and consistent RunX2-t expression by relatively immature osteoprogenitors. ihh-/- mice exhibited small calvarial bones HH target genes, Ptch1 and Gli1, were absent. This indicates that IHH is the functional HH ligand, and that it is not compensated by another HH ligand. To decipher the roles and potential interaction of Gli3 and ihh. we generated ihh-/-; gli3Xt-J/Xt-J compound mutant mice. Even in the absence of ihh, Gli3 deletion was sufficient to induce aberrant precocious ossification across the developing suture, indicating that the carniosyostosis pehnotype of Gli3Xt-J/Xt-J mice is not dependent on IHH ligand. Also we found that ihh was not required for Runx2 expression as the expression of RUNX2 target genes was unaffected by deletion of Ihh. To test whether RUNX2 has a role upstream of IHH we performed RUNX2 siRNA knock down experiements in WT calvarial osteoblasts and explants and found that Ihh expression is suppressed. Our results show that IHH is the functional HH ligand in the embroynic mouse calvaria osteogenic condensations, where it regulates the progression of osteoblastic differentation. As GLI3 represses the expression of Runx2-II abd Ihh, and also elevats the Runx2-I expression, and as IHH may be regulated by RUNX2 these results raise the possibility of a regualtory feedback circuit to control calvarial osteogenesis and suture patency. Taken together RUNX2-controlled osteoblastic cell fate is regulated by IHH through concomitant inhibition of GLI3-repressor formation and activation of downstreams targets.Peer reviewe

Neuromuscular imaging in inherited muscle diseases

Driven by increasing numbers of newly identified genetic defects and new insights into the field of inherited muscle diseases, neuromuscular imaging in general and magnetic resonance imaging (MRI) in particular are increasingly being used to characterise the severity and pattern of muscle involvement. Although muscle biopsy is still the gold standard for the establishment of the definitive diagnosis, muscular imaging is an important diagnostic tool for the detection and quantification of dystrophic changes during the clinical workup of patients with hereditary muscle diseases. MRI is frequently used to describe muscle involvement patterns, which aids in narrowing of the differential diagnosis and distinguishing between dystrophic and non-dystrophic diseases. Recent work has demonstrated the usefulness of muscle imaging for the detection of specific congenital myopathies, mainly for the identification of the underlying genetic defect in core and centronuclear myopathies. Muscle imaging demonstrates characteristic patterns, which can be helpful for the differentiation of individual limb girdle muscular dystrophies. The aim of this review is to give a comprehensive overview of current methods and applications as well as future perspectives in the field of neuromuscular imaging in inherited muscle diseases. We also provide diagnostic algorithms that might guide us through the differential diagnosis in hereditary myopathies