The Homeobox Transcription Factor Barx2 Regulates Plasticity of Young Primary Myofibers

Adult mammalian muscle retains incredible plasticity. Muscle growth and repair involves the activation of undifferentiated myogenic precursors called satellite cells. In some circumstances, it has been proposed that existing myofibers may also cleave and produce a pool of proliferative cells that can re-differentiate into new fibers. Such myofiber dedifferentiation has been observed in the salamander blastema where it may occur in parallel with satellite cell activation. Moreover, ectopic expression of the homeodomain transcription factor Msx1 in differentiated C2C12 myotubes has been shown to induce their dedifferentiation. While it remains unclear whether dedifferentiation and redifferentiaton occurs endogenously in mammalian muscle, there is considerable interest in induced dedifferentiation as a possible regenerative tool.We previously showed that the homeobox protein Barx2 promotes myoblast differentiation. Here we report that ectopic expression of Barx2 in young immature myotubes derived from cell lines and primary mouse myoblasts, caused cleavage of the syncytium and downregulation of differentiation markers. Microinjection of Barx2 cDNA into immature myotubes derived from primary cells led to cleavage and formation of mononucleated cells that were able to proliferate. However, injection of Barx2 cDNA into mature myotubes did not cause cleavage. Barx2 expression in C2C12 myotubes increased the expression of cyclin D1, which may promote cell cycle re-entry. We also observed differential muscle gene regulation by Barx2 at early and late stages of muscle differentiation which may be due to differential recruitment of transcriptional activator or repressor complexes to muscle specific genes by Barx2.We show that Barx2 regulates plasticity of immature myofibers and might act as a molecular switch controlling cell differentiation and proliferation

Identification of Pax6-Dependent Gene Regulatory Networks in the Mouse Lens

Lineage-specific DNA-binding transcription factors regulate development by activating and repressing particular set of genes required for the acquisition of a specific cell type. Pax6 is a paired domain and homeodomain-containing transcription factor essential for development of central nervous, olfactory and visual systems, as well as endocrine pancreas. Haploinsufficiency of Pax6 results in perturbed lens development and homeostasis. Loss-of-function of Pax6 is incompatible with lens lineage formation and results in abnormal telencephalic development. Using DNA microarrays, we have identified 559 genes expressed differentially between 1-day old mouse Pax6 heterozygous and wild type lenses. Of these, 178 (31.8%) were similarly increased and decreased in Pax6 homozygous embryonic telencephalon [Holm PC, Mader MT, Haubst N, Wizenmann A, Sigvardsson M, Götz M (2007) Loss- and gain-of-function analyses reveals targets of Pax6 in the developing mouse telencephalon. Mol Cell Neurosci 34: 99–119]. In contrast, 381 (68.2%) genes were differently regulated between the lens and embryonic telencephalon. Differential expression of nine genes implicated in lens development and homeostasis: Cspg2, Igfbp5, Mab21l2, Nrf2f, Olfm3, Spag5, Spock1, Spon1 and Tgfb2, was confirmed by quantitative RT-PCR, with five of these genes: Cspg2, Mab21l2, Olfm3, Spag5 and Tgfb2, identified as candidate direct Pax6 target genes by quantitative chromatin immunoprecipitation (qChIP). In Mab21l2 and Tgfb2 promoter regions, twelve putative individual Pax6-binding sites were tested by electrophoretic mobility shift assays (EMSAs) with recombinant Pax6 proteins. This led to the identification of two and three sites in the respective Mab21l2 and Tgfb2 promoter regions identified by qChIPs. Collectively, the present studies represent an integrative genome-wide approach to identify downstream networks controlled by Pax6 that control mouse lens and forebrain development

The two faces of pannexins: new roles in inflammation and repair

Helen P Makarenkova,1 Sameer B Shah,2,3 Valery I Shestopalov4–6 1Department of Molecular Medicine, Scripps Research Institute, 2Departments of Orthopaedic Surgery and Bioengineering, University of California, 3Research Division, Veterans Affairs San Diego Healthcare System, San Diego, CA, 4Bascom Eye Institute, Department of Ophthalmology, University of Miami, Miami, FL, USA; 5Vavilov Institute for General Genetics, Russian Academy of Sciences, 6Kharkevich Institute for Information Transmission Problems, Russian Academy of Sciences, Moscow, Russia Abstract: Pannexins belong to a family of ATP-release channels expressed in almost all cell types. An increasing body of literature on pannexins suggests that these channels play dual and sometimes contradictory roles, contributing to normal cell function, as well as to the pathological progression of disease. In this review, we summarize our understanding of pannexin “protective” and “harmful” functions in inflammation, regeneration and mechanical signaling. We also suggest a possible basis for pannexin’s dual roles, related to extracellular ATP and K+ levels and the activation of various types of P2 receptors that are associated with pannexin. Finally, we speculate upon therapeutic strategies related to pannexin using eyes, lacrimal glands, and peripheral nerves as examples of interesting therapeutic targets. Keywords: pannexin, Panx1, ATP, purinergic signaling, inflammation, regeneratio

GDNF/Ret signaling and renal branching morphogenesis: From mesenchymal signals to epithelial cell behaviors

Signaling by GDNF through the Ret receptor tyrosine kinase is required for the normal growth and morphogenesis of the ureteric bud (UB) during kidney development. Recent studies have sought to understand the precise role of Ret signaling in this process, and the specific responses of UB cells to GDNF. Surprisingly, the requirement for Gdnf and Ret was largely relieved by removing the negative regulator Spry1, revealing unexpected functional overlap between GDNF and FGF10. However, the kidneys that developed without Gdnf/Ret and Spry1 displayed significant branching abnormalities, suggesting a unique role for GDNF in fine-tuning UB branching. GDNF/Ret signaling alters patterns of gene expression in UB tip cells, and one critical event is upregulation of the ETS transcription factors Etv4 and Etv5. Mice lacking Etv4 and Etv5 fail to develop kidneys. Thus, these genes represent key components of a regulatory network downstream of Ret. Studies of chimeric embryos in which a subset of cells lack either Ret, Etv4/5 or Spry1 have revealed an important role for this pathway in cell movement. Ret signaling, via Etv4 and Etv5, promotes competitive cell rearrangements in the nephric duct, in which the cells with the highest level of Ret signaling preferentially migrate to form the first ureteric bud tip