The Hedgehog (Hh) pathway is essential for normal embryonic development and tissue repair. The role of Hh signaling in hematopoiesis has been studied primarily by modulating the activity of Patched and Smoothened, but results have been conflicting. Some studies demonstrate a requirement for pathway activity in hematopoiesis, whereas others report that it is dispensable. Hh activity converges on the Gli transcription factors, but the specific role of these downstream effectors in hematopoiesis has not been reported. We have analyzed hematopoietic stem cell (HSC) and progenitor function in mice with a homozygous deletion of Gli1 (Gli1 null ). Gli1 null mice have more longterm HSCs that are more quiescent and show increased engraftment after transplantation. In contrast, myeloid development is adversely affected with decreased in vitro colony formation, decreased in vivo response to granulocyte colonystimulating factor (G-CSF), and impaired leukocyte recovery after chemotherapy. Levels of the proto-oncogene Cyclin D1 are reduced in Gli1 null mice and may explain the loss of proliferation seen in HSCs and progenitor cells. These data demonstrate that Gli1 regulates normal and stress hematopoiesis. Moreover, they suggest that Gli1 and Smoothened may not be functionally redundant, and direct GLI1 inhibitors may be needed to effectively block HH/GLI1 activity in human disease. (Blood. 2010;115:2391-2396) IntroductionThe proliferation of hematopoietic stem cells (HSCs) and progenitors is tightly regulated during normal homeostasis. HSCs are normally quiescent in the adult mouse but they can be induced to proliferate in response to stress or cytokine stimulation. In contrast, progenitors are highly proliferative to maintain a constant supply of infection-fighting white blood cells. Precisely how HSC and progenitor proliferation are regulated is not completely understood, but recent data have implicated a role for developmental signaling pathways such as Wnt and Notch in the regulation of stem cell proliferation, self-renewal, and differentiation. [1][2][3][4] The Hedgehog (Hh) signaling pathway in mammals consists of 3 closely related ligands, Sonic Hh (Shh), Indian Hh (Ihh), and Desert Hh (Dhh), that can each bind to the transmembrane protein Patched (Ptch). Upon ligand binding, Ptch inhibition of the positive effector Smoothened (Smo) is released and signaling is transduced. Three zinc finger transcription factors, Gli1, Gli2, and Gli3, lie downstream of Smo and mediate Hh's effects. Gli1 is a positive effector of signaling, Gli3 is predominantly a transcriptional inhibitor, and Gli2 can function in both roles. 5 The precise role of Hh signaling in normal hematopoiesis, however, is not known and the literature is contradictory. One group has reported that loss of Smo activity leads to a severe defect in HSC function, 6 whereas others have reported a more modest phenotype, 7 or none at all. 8,9 All of these studies have focused primarily on the upstream modulators of pathway activity, Ptch and Smo. To better understand ...

There is a lack of pharmacological interventions available for sarcopenia, a progressive age-associated loss of muscle mass, leading to a decline in mobility and quality of life. We found mTORC1 (mammalian target of rapamycin complex 1), a well-established positive modulator of muscle mass, to be surprisingly hyperactivated in sarcopenic muscle. Furthermore, partial inhibition of the mTORC1 pathway counteracted sarcopenia, as determined by observing an increase in muscle mass and fiber type cross-sectional area in select muscle groups, again surprising because mTORC1 signaling has been shown to be required for skeletal muscle mass gains in some models of hypertrophy. Additionally, several genes related to senescence were downregulated and gene expression indicators of neuromuscular junction denervation were diminished using a low dose of a “rapalog” (a pharmacological agent related to rapamycin). Therefore, partial mTORC1 inhibition may delay the progression of sarcopenia by directly and indirectly modulating multiple age-associated pathways, implicating mTORC1 as a therapeutic target to treat sarcopenia.

Skeletal myogenesis is regulated by signal transduction, but the factors and mechanisms involved are not well understood. The group I Paks Pak1 and Pak2 are related protein kinases and direct effectors of Cdc42 and Rac1. Group I Paks are ubiquitously expressed and specifically required for myoblast fusion in Drosophila. We report that both Pak1 and Pak2 are activated during mammalian myoblast differentiation. One pathway of activation is initiated by N-cadherin ligation and involves the cadherin coreceptor Cdo with its downstream effector, Cdc42. Individual genetic deletion of Pak1 and Pak2 in mice has no overt effect on skeletal muscle development or regeneration. However, combined muscle-specific deletion of Pak1 and Pak2 results in reduced muscle mass and a higher proportion of myofibers with a smaller cross-sectional area. This phenotype is exacerbated after repair to acute injury. Furthermore, primary myoblasts lacking Pak1 and Pak2 display delayed expression of myogenic differentiation markers and myotube formation. These results identify Pak1 and Pak2 as redundant regulators of myoblast differentiation in vitro and in vivo and as components of the promyogenic Ncad/Cdo/Cdc42 signaling pathway.KEYWORDS Pak, cell adhesion, cell differentiation, myogenesis, regeneration, signal transduction C ell differentiation is a complex process whereby precursor cells take on tissuespecific structure and function. Lineage-restricted transcription factors lie at the heart of cell differentiation, but the process is often initiated and fortified by ubiquitous signaling pathways that function in many biological contexts. Skeletal myogenesis serves as a paradigm for cell differentiation. Differentiation of skeletal myoblasts is a coordinated process involving adoption of a cell-type-specific transcriptional program and morphological changes, including fusion into multinucleated myofibers (1, 2). MyoD family proteins (MyoD, Myf5, myogenin, and MRF4) are muscle-specific transcription factors that act in concert with other, more broadly expressed transcription factors to establish the muscle phenotype (1, 3). The activities of these factors are regulated posttranslationally by non-muscle-specific signal transduction pathways. One such pathway is the p38␣/␤ mitogen-activated protein kinase (MAPK; here simply p38) pathway (4). p38 is activated during myogenic differentiation in vitro, and its inhibition results in impaired differentiation (5-7). Furthermore, mice lacking p38␣ exhibit delayed myofiber growth and maturation (8).The signals that initiate p38 activity during myoblast differentiation are poorly understood. One mechanism is via activation of a signaling complex nucleated at sites of cadherin-based cell-cell adhesion (9, 10). The transmembrane IgSF coreceptor Cdo (also called Cdon) is bound in cis to N-cadherin (Ncad) in myoblasts. During myoblast differentiation, or acutely upon Ncad ligation, the Cdo intracellular region associates directly with (i) Bnip-2, a scaffold protein for Cdc42, and (ii) JLP, a scaffo...

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