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Sphingosine-1-phosphate (S1P) activates a widely expressed family of G protein-coupled receptors, serves as a muscle trophic factor and activates muscle stem cells called satellite cells (SCs) through unknown mechanisms. Here we show that muscle injury induces dynamic changes in S1P signaling and metabolism in vivo. These changes include early and profound induction of the gene encoding the S1P biosynthetic enzyme SphK1, followed by induction of the catabolic enzyme sphingosine phosphate lyase (SPL) 3 days later. These changes correlate with a transient increase in circulating S1P levels after muscle injury. We show a specific requirement for SphK1 to support efficient muscle regeneration and SC proliferation and differentiation. Mdx mice, which serve as a model for muscular dystrophy (MD), were found to be S1P-deficient and exhibited muscle SPL upregulation, suggesting that S1P catabolism is enhanced in dystrophic muscle. Pharmacological SPL inhibition increased muscle S1P levels, improved mdx muscle regeneration and enhanced SC proliferation via S1P receptor 2 (S1PR2)-dependent inhibition of Rac1, thereby activating Signal Transducer and Activator of Transcription 3 (STAT3), a central player in inflammatory signaling. STAT3 activation resulted in p21 and p27 downregulation in a S1PR2-dependent fashion in myoblasts. Our findings suggest that S1P promotes SC progression through the cell cycle by repression of cell cycle inhibitors via S1PR2/STAT3-dependent signaling and that SPL inhibition may provide a therapeutic strategy for MD.

In the visual system, differential gene expression underlies development of the anterior-posterior and dorsal-ventral axes. Here we present the results of a microarray screen to identify genes differentially expressed in the developing retina. We assayed gene expression in nasal (anterior), temporal (posterior), dorsal, and ventral embryonic mouse retina. We used a statistical method to estimate gene expression between different retina regions. Genes were clustered according to their expression pattern and were ranked within each cluster. We identified groups of genes expressed in gradients or with restricted patterns of expression as verified by in situ hybridization. A common theme for the identified genes is the differential expression in the dorsal-ventral axis. By analyzing gene expression patterns, we provide insight into the molecular organization of the developing retina.A fundamental aspect of nervous system development is the spatial and temporal regulation of gene expression that underlies cell fate specification and connectivity. In the visual system, generation of the anterior-posterior (A-P) and dorsalventral (D-V) axes underlies positional specificity within the retina. The best example is the formation of a topographic map on the retinal axon output target, the superior colliculus (SC). Axons from D and V retina project to V and D SC, respectively, while A (also defined as nasal, N) and P (also defined as temporal, T) retinal axons project to P and A SC, respectively (1-4). Map formation depends upon complementary gradients of positional labels in the retina and the SC (5), specifically retinal EphA receptors and collicular ephrin-A ligands. In chick, EphA3 is expressed in a high-T to low-N gradient in the retina (6), whereas ephrin-A2 and ephrin-A5 are expressed in corresponding P-A gradients across the target (6, 7). Current models suggest that retinocollicular mapping is independent of the absolute level of EphA receptor signaling in retinal ganglion cells (RGCs), but, rather, is dependent on relative differences between neighboring RGCs (8).Other molecules are differentially expressed in the visual system. Two EphB receptors and A and B type ephrins are expressed in gradients in the retina (9-12). The early eye can also be subdivided by expression of retinoic acid-generating enzymes along the D-V axis (13-15). Transcription factors and signaling components are expressed in gradients in the retina during development (16)(17)(18)(19). To gain a broader view of gene expression patterns in the retina, we applied a microarray-based approach to identify new genes with restricted patterns of expression during development. Our results provide insight into the organization of the retina structure as well as identify candidate genes with potential roles in positional identity. Materials and MethodsMicroarray Hybridizations. Embryonic day 14.5 (E14.5) CD-1 mouse retinas were divided into N, T, D, and V portions for RNA isolation with TRIzol (Invitrogen) and RNeasy columns (Qiagen, Valencia, CA). RNA amp...

As an approach toward understanding the molecular mechanisms of neuronal differentiation, we utilized DNA microarrays to elucidate global patterns of gene expression during pontocerebellar development. Through this analysis, we identified groups of genes specific to neuronal precursor cells, associated with axon outgrowth, and regulated in response to contact with synaptic target cells. In the cerebellum, we identified a phase of granule cell differentiation that is independent of interactions with other cerebellar cell types. Analysis of pontine gene expression revealed that distinct programs of gene expression, correlated with axon outgrowth and synapse formation, can be decoupled and are likely influenced by different cells in the cerebellar target environment. Our approach provides insight into the genetic programs underlying the differentiation of specific cell types in the pontocerebellar projection system.

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