A robust and well-organized rhythm is a key feature of many neuronal networks, including those that regulate essential behaviors such as circadian rhythmogenesis, breathing, and locomotion. Here we show that excitatory V3-derived neurons are necessary for a robust and organized locomotor rhythm during walking. When V3-mediated neurotransmission is selectively blocked by the expression of the tetanus toxin light chain subunit (TeNT), the regularity and robustness of the locomotor rhythm is severely perturbed. A similar degeneration in the locomotor rhythm occurs when the excitability of V3-derived neurons is reduced acutely by ligand-induced activation of the allatostatin receptor. The V3-derived neurons additionally function to balance the locomotor output between both halves of the spinal cord, thereby ensuring a symmetrical pattern of locomotor activity during walking. We propose that the V3 neurons establish a regular and balanced motor rhythm by distributing excitatory drive between both halves of the spinal cord.

Primary cilia mediate Hh signalling and mutations in their protein components affect Hh activity. We show that in mice mutant for a cilia intraflagellar transport (IFT) protein, IFT88/polaris, Shh activity is increased in the toothless diastema mesenchyme of the embryonic jaw primordia. This results in the formation of ectopic teeth in the diastema, mesial to the first molars. This phenotype is specific to loss of polaris activity in the mesenchyme since loss of Polaris in the epithelium has no detrimental affect on tooth development. To further confirm that upregulation of Shh activity is responsible for the ectopic tooth formation, we analysed mice mutant for Gas1, a Shh protein antagonist in diastema mesenchyme. Gas1 mutants also had ectopic diastema teeth and accompanying increased Shh activity. In this context, therefore, primary cilia exert a specific negative regulatory effect on Shh activity that functions to repress tooth formation and thus determine tooth number. Strikingly, the ectopic teeth adopt a size and shape characteristic of premolars, a tooth type that was lost in mice around 50-100 million years ago.

Holoprosencephaly (HPE) is a common birth defect predominantly affecting the forebrain and face and has been linked to mutations in the sonic hedgehog (SHH) gene. HPE is genetically heterogeneous, and clinical presentation represents a spectrum of phenotypes. We have previously shown that Gas1 encodes a cell-autonomous Hedgehog signaling enhancer. Combining cell surface binding, in vitro activity, and explant culture assays, we provide evidence that SHH contains a previously unknown unique binding surface for its interaction with GAS1 and that this surface is also important for maximal signaling activity. Within this surface, the Asn-115 residue of human SHH has been documented to associate with HPE when mutated to lysine (N115K). We provide evidence that HPE associated with this mutation can be mechanistically explained by a severely reduced binding of SHH to GAS1, and we predict a similar result if a mutation were to occur at Tyr-80. Our data should encourage future searches for mutations in GAS1 as possible modifiers contributing to the wide spectrum of HPE.

Myogenic potential, survival and expansion of mammalian muscle progenitors depend on the myogenic determinants Pax3 and Pax7 embryonically(1), and Pax7 alone perinatally(2-5). Several in vitro studies support the critical role of Pax7 in these functions of adult muscle stem cells(5-8) (satellite cells), but a formal demonstration has been lacking in vivo. Here we show, through the application of inducible Cre/loxP lineage tracing(9) and conditional gene inactivation to the tibialis anterior muscle regeneration paradigm, that, unexpectedly, when Pax7 is inactivated in adult mice, mutant satellite cells are not compromised in muscle regeneration, they can proliferate and reoccupy the sublaminal satellite niche, and they are able to support further regenerative processes. Dual adult inactivation of Pax3 and Pax7 also results in normal muscle regeneration. Multiple time points of gene inactivation reveal that Pax7 is only required up to the juvenile period when progenitor cells make the transition into quiescence. Furthermore, we demonstrate a cell-intrinsic difference between neonatal progenitor and adult satellite cells in their Pax7-dependency. Our finding of an age-dependent change in the genetic requirement for muscle stem cells cautions against inferring adult stem-cell biology from embryonic studies, and has direct implications for the use of stem cells from hosts of different ages in transplantation-based therapy.

In art and literature, originals are revered and copies frowned upon. But do men use germline stem cell copies to pass traits to their children? In Science, Nakagawa et al. (2010) show that new stem cell copies regularly arise in mouse testes by fragmentation of downstream germ cell clusters.

Stem cell differentiation is accompanied by a gradual cellular morphogenesis and transcriptional changes. Identification of morphological regulators that control cell behavior during differentiation could shed light on how cell morphogenesis is coupled to transcriptional changes during development. By analyzing cellular behavior during differentiation of mouse embryonic stem cells (ESCs), we uncover a role of Borg5 (binder of Rho guanosine 5'-triphosphatase 5) in regulating trophectoderm (TE) cell morphogenesis. We report that differentiation of ESCs toward TE is accompanied by enhanced actin protrusion and cell motility that require upregulation of Borg5. Borg5 interacts with both Cdc42 and atypical protein kinase C (aPKC) and functions downstream of Cdc42 to enhance TE cell motility. Borg5 is required for the sorting of differentiating TE to the outside of ESCs in vitro. In developing embryos, Borg5 protein localizes to cell cell contacts and the cytoplasm after compaction. It exhibits higher levels of expression in outer cells than in inner cells in morula and blastocysts. Reduction of Borg5 disrupts aPKC localization and inhibits blastocyst formation. Since Cdx2 and Borg5 facilitate each other's expression as ESCs differentiate toward TE, we propose that cell morphogenesis is coupled with transcriptional changes to regulate TE differentiation. Our studies also demonstrate the utility of ESCs in identifying morphological regulators important for development. STEM CELLS 2010;28:1030-1038

We have generated a mouse strain carrying a Cre-ERT2 knock-in allele at the Pax7 locus, the Pax7(CE) allele (Lepper etal., 2009, Nature 460:627-631). Combining Pax7(CE) and the R26(LacZ) Cre reporter allele, here we describe temporal-specific tamoxifen (tmx)-inducible lineage tracing of embryonic Pax7-expressing cells. In particular, we focus on the somitic lineage. Tmx-inducible Cre activity directed by the Pax7(CE) allele is similar to the endogenous Pax7 expression pattern. The somitic Pax7-expressing cells selectively marked at embryonic day 9.5 (E9.5) give rise to dorsal dermis and brown adipose tissue, in addition to dorsal aspects of trunk muscles and the diaphragm muscle. However, they do not contribute to ventral body wall and limb muscles. After E12.5, marked Pax7-expressing cells become lineage restricted to muscles. Descendants of these early marked Pax7-expressing cells begin to occupy sublaminal positions associated with the myofibers around E16.5, characteristic of embryonic satellite cells. Furthermore, they contribute to adult myofibers and regeneration competent satellite cells in the tibialis anterior muscle, providing evidence that some adult satellite cells are of embryonic origin. genesis 48:424-436, 2010. (C) 2010 Wiley-Liss, Inc.

Skeletal muscle tissue provides mechanical force for locomotion of all vertebrate animals. It is prone to damage from acute physical trauma and physiological stress. To cope with this, it possesses a tremendous capacity for rapid and effective repair that is widely held to be accomplished by the satellite cells lying between the muscle fiber plasmalemma and the basement membrane. Cell transplantation and lineage-tracing studies have demonstrated that Pax7-expressing (Pax7(+)) satellite cells can repair damaged muscle tissue repeatedly after several bouts of acute injury. These findings provided evidence that Pax7(+) cells are muscle stem cells. However, stem cells from a variety of other origins are also reported to contribute to myofibers upon engraftment into muscles, questioning whether satellite cells are the only stem cell source for muscle regeneration. Here, we have engineered genetic ablation of Pax7(+) cells to test whether there is any significant contribution to muscle regeneration after acute injury from cells other than this source. We find that such elimination of Pax7+ cells completely blocks regenerative myogenesis either following injury to the tibialis anterior (TA) muscle or after transplantation of extensor digitorum longus (EDL) muscles into nude mice. As Pax7 is specifically expressed in satellite cells, we conclude that they are essential for acute injury-induced muscle regeneration. It remains to be established whether there is any significant role for stem cells of other origins. The implications of our results for muscle stem cell-based therapy are discussed.