Transplantation-based screen identifies inducers of muscle progenitor cell engraftment across vertebrate species.

Stem cell transplantation presents a potentially curative strategy for genetic disorders of skeletal muscle, but this approach is limited by the deleterious effects of cell expansion in vitro and consequent poor engraftment efficiency. In an effort to overcome this limitation, we sought to identify molecular signals that enhance the myogenic activity of cultured muscle progenitors. Here, we report the development and application of a cross-species small-molecule screening platform employing zebrafish and mice, which enables rapid, direct evaluation of the effects of chemical compounds on the engraftment of transplanted muscle precursor cells.

Evidence for RNA or protein transport from somatic tissues to the male reproductive tract in mouse.

The development of tools to manipulate the mouse genome, including knockout and transgenic technology, has revolutionized our ability to explore gene function in mammals. Moreover, for genes that are expressed in multiple tissues or at multiple stages of development, the use of tissue-specific expression of the Cre recombinase allows gene function to be perturbed in specific cell types and/or at specific times. However, it is well known that putative tissue-specific promoters often drive unanticipated 'off-target' expression.

Building and Maintaining the Skin.

The skin forms a crucial, dynamic barrier between an animal and the external world. In mammals, three stem cell populations possess robust regenerative potential to maintain and repair the body's protective surface: epidermal stem cells, which maintain the stratified epidermis; hair follicle stem cells, which power the cyclic growth of the hair follicle; and melanocyte stem cells, which regenerate pigment-producing melanocytes to color the skin and hair. These stem cells reside in complex microenvironments ("niches") comprising diverse cellular repertoires that enable stem cells to rejuvenate tissues during homeostasis and regenerate them upon injury.

Stress-associated ectopic differentiation of melanocyte stem cells and ORS amelanotic melanocytes in an ex vivo human hair follicle model.

Hair greying depends on the altered presence and functionality of hair follicle melanocytes. Melanocyte stem cells (MelSCs) reside in the bulge of hair follicles and give rise to migrating and differentiating progeny during the anagen phase. Ageing, genotoxic stress, redox stress and multiple behaviour-associated acute stressors have been seen to induce hair greying by depleting the MelSC pool, a phenomenon which is accompanied by ectopic pigmentation of these cells, followed by their depletion from the stem cell niche.

FOS licenses early events in stem cell activation driving skeletal muscle regeneration.

Muscle satellite cells (SCs) are a quiescent (non-proliferative) stem cell population in uninjured skeletal muscle. Although SCs have been investigated for nearly 60 years, the molecular drivers that transform quiescent SCs into the rapidly dividing (activated) stem/progenitor cells that mediate muscle repair after injury remain largely unknown. Here we identify a prominent FBJ osteosarcoma oncogene (Fos) mRNA and protein signature in recently activated SCs that is rapidly, heterogeneously, and transiently induced by muscle damage.

Melanocortin 1 receptor is dispensable for acute stress induced hair graying in mice.

Stress is a risk factor for many skin conditions, but the cellular and molecular mechanisms of its impacts have only begun to be revealed. In mice, acute stress induces loss of melanocyte stem cells (MeSCs) and premature hair greying. Our previous work demonstrated that the loss of MeSCs upon acute stress is caused by the hyperactivation of the sympathetic nervous system.

Growth inhibition associated with disruption of the actin cytoskeleton by Latrunculin A in rhabdomyosarcoma cells.

Functional genomic screening of KRAS-driven mouse sarcomas was previously employed to identify proliferation-relevant genes. Genes identified included Ubiquitin-conjugating enzyme E2 (Ube2c), Centromere Protein E (Cenpe), Hyaluronan Synthase 2 (Has2), and CAMP Responsive Element Binding Protein 3 Like 2 (Creb3l2). This study examines the expression and chemical inhibition of these candidate genes, identifying variable levels of protein expression and significant contributions to rhabdomyosarcoma (RMS) cell proliferation.

Hyperactivation of sympathetic nerves drives depletion of melanocyte stem cells.

Empirical and anecdotal evidence has associated stress with accelerated hair greying (formation of unpigmented hairs), but so far there has been little scientific validation of this link. Here we report that, in mice, acute stress leads to hair greying through the fast depletion of melanocyte stem cells. Using a combination of adrenalectomy, denervation, chemogenetics, cell ablation and knockout of the adrenergic receptor specifically in melanocyte stem cells, we find that the stress-induced loss of melanocyte stem cells is independent of immune attack or adrenal stress hormones.

Steady-state and regenerative hematopoiesis occurs normally in mice in the absence of GDF11.

Tightly regulated production of mature blood cells is essential for health and survival in vertebrates and dependent on discrete populations of blood-forming (hematopoietic) stem and progenitor cells. Prior studies suggested that inhibition of growth differentiation factor 11 (GDF11) through soluble activin receptor type II (ActRII) ligand traps or neutralizing antibodies promotes erythroid precursor cell maturation and red blood cell formation in contexts of homeostasis and anemia. As Gdf11 is expressed by mature hematopoietic cells, and erythroid precursor cell expression of Gdf11 has been implicated in regulating erythropoiesis, we hypothesized that genetic disruption of Gdf11 in blood cells might perturb normal hematopoiesis or recovery from hematopoietic insult.

Anagen hair follicle repair: Timely regenerative attempts from plastic extra-bulge epithelial cells.

Anagen hair follicle repair (AHFR) is the regenerative scheme activated to restore the structure and hair growth following injuries to anagen hair follicles. Compared with telogen-to-anagen regeneration and hair follicle neogenesis, AHFR is a clinically important, yet relatively unexplored regenerative feature of hair follicles. Due to their highly proliferative character, germinative cells and matrix cells within hair bulbs are highly susceptible to injuries, such as chemotherapy and radiotherapy.

Emerging roles of transit-amplifying cells in tissue regeneration and cancer.

Most regenerative tissues employ transit-amplifying cells (TACs) that are positioned in between stem cells and differentiated progeny. In a classical hierarchical model, stem cells undergo limited divisions to produce TACs, which then proliferate rapidly to expand the system and produce diverse differentiated cell types. Although TACs are indispensable for generating tissues, they have been largely viewed as a transit point between stem cells and downstream lineages.

Two tissue-resident progenitor lineages drive distinct phenotypes of heterotopic ossification.

Fibrodysplasia ossificans progressiva (FOP), a congenital heterotopic ossification (HO) syndrome caused by gain-of-function mutations of bone morphogenetic protein (BMP) type I receptor ACVR1, manifests with progressive ossification of skeletal muscles, tendons, ligaments, and joints. In this disease, HO can occur in discrete flares, often triggered by injury or inflammation, or may progress incrementally without identified triggers. Mice harboring an Acvr1 knock-in allele recapitulate the phenotypic spectrum of FOP, including injury-responsive intramuscular HO and spontaneous articular, tendon, and ligament ossification.

Established cell surface markers efficiently isolate highly overlapping populations of skeletal muscle satellite cells by fluorescence-activated cell sorting.

Background: Fluorescent-activated cell sorting (FACS) has enabled the direct isolation of highly enriched skeletal muscle stem cell, or satellite cell, populations from postnatal tissue. Several distinct surface marker panels containing different positively selecting surface antigens have been used to distinguish muscle satellite cells from other non-myogenic cell types. Because functional and transcriptional heterogeneity is known to exist within the satellite cell population, a direct comparison of results obtained in different laboratories has been complicated by a lack of clarity as to whether commonly utilized surface marker combinations select for distinct or overlapping subsets of the satellite cell pool.

The Vitamin D Receptor Regulates Tissue Resident Macrophage Response to Injury.

Ligand-dependent actions of the vitamin D receptor (VDR) play a pleiotropic role in the regulation of innate and adaptive immunity. The liganded VDR is required for recruitment of macrophages during the inflammatory phase of cutaneous wound healing. Although the number of macrophages in the granulation tissue 2 days after wounding is markedly reduced in VDR knockout (KO) compared with wild-type mice, VDR ablation does not alter macrophage polarization.

Polycomb-Mediated Repression and Sonic Hedgehog Signaling Interact to Regulate Merkel Cell Specification during Skin Development.

An increasing amount of evidence indicates that developmental programs are tightly regulated by the complex interplay between signaling pathways, as well as transcriptional and epigenetic processes. Here, we have uncovered coordination between transcriptional and morphogen cues to specify Merkel cells, poorly understood skin cells that mediate light touch sensations. In murine dorsal skin, Merkel cells are part of touch domes, which are skin structures consisting of specialized keratinocytes, Merkel cells, and afferent neurons, and are located exclusively around primary hair follicles.

Fate by Chance, not by Choice: Epidermal Stem Cells Go Live.

The skin epidermis is constantly renewed by epidermal stem cells. In a recent Science paper, Rompolas et al. utilize live imaging to track epidermal stem cells over their lifetimes.

Dual SMAD Signaling Inhibition Enables Long-Term Expansion of Diverse Epithelial Basal Cells.

Functional modeling of many adult epithelia is limited by the difficulty in maintaining relevant stem cell populations in culture. Here, we show that dual inhibition of SMAD signaling pathways enables robust expansion of primary epithelial basal cell populations. We find that TGFβ/BMP/SMAD pathway signaling is strongly activated in luminal and suprabasal cells of several epithelia, but suppressed in p63+ basal cells.

In vivo gene editing in dystrophic mouse muscle and muscle stem cells.

Frame-disrupting mutations in the DMD gene, encoding dystrophin, compromise myofiber integrity and drive muscle deterioration in Duchenne muscular dystrophy (DMD). Removing one or more exons from the mutated transcript can produce an in-frame mRNA and a truncated, but still functional, protein. In this study, we developed and tested a direct gene-editing approach to induce exon deletion and recover dystrophin expression in the mdx mouse model of DMD.

Young systemic factors as a medicine for age-related neurodegenerative diseases.

It is widely known that neurogenesis, brain function and cognition decline with aging. Increasing evidence suggests that cerebrovascular dysfunction is a major cause of cognitive impairment in the elderly but is also involved in age-related neurodegenerative diseases. Finding ways and molecules that reverse this process and ameliorate age- and disease-related cognitive impairment by targeting vascular and neurogenic deterioration would be of great therapeutic value.

How stem cells get "turned on".

Recent research began to link autophagic processes to the functional integrity of certain stem cells. A novel study published in this issue of reports on autophagic flux as crucial checkpoint to meet the energy demands during muscle stem cell activation.

Low incidence of off-target mutations in individual CRISPR-Cas9 and TALEN targeted human stem cell clones detected by whole-genome sequencing.

Genome editing has attracted wide interest for the generation of cellular models of disease using human pluripotent stem cells and other cell types. CRISPR-Cas systems and TALENs can target desired genomic sites with high efficiency in human cells, but recent publications have led to concern about the extent to which these tools may cause off-target mutagenic effects that could potentially confound disease-modeling studies. Using CRISPR-Cas9 and TALEN targeted human pluripotent stem cell clones, we performed whole-genome sequencing at high coverage in order to assess the degree of mutagenesis across the entire genome.

Permanent alteration of PCSK9 with in vivo CRISPR-Cas9 genome editing.

Rationale: Individuals with naturally occurring loss-of-function proprotein convertase subtilisin/kexin type 9 (PCSK9) mutations experience reduced low-density lipoprotein cholesterol levels and protection against cardiovascular disease.

Objective: The goal of this study was to assess whether genome editing using a clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated system can efficiently introduce loss-of-function mutations into the endogenous PCSK9 gene in vivo.

Methods And Results: We used adenovirus to express CRISPR-associated 9 and a CRISPR guide RNA targeting Pcsk9 in mouse liver, where the gene is specifically expressed.

Skeletal muscle degenerative diseases and strategies for therapeutic muscle repair.

Skeletal muscle is a highly specialized, postmitotic tissue that must withstand chronic mechanical and physiological stress throughout life to maintain proper contractile function. Muscle damage or disease leads to progressive weakness and disability, and manifests in more than 100 different human disorders. Current therapies to treat muscle degenerative diseases are limited mostly to the amelioration of symptoms, although promising new therapeutic directions are emerging.

Transcription factor-mediated epigenetic reprogramming.

Input from various signaling pathways in conjunction with specific transcription factors (TFs), noncoding RNAs, and epigenetic modifiers governs the maintenance of cellular identity. Endogenous or exogenous TFs operate within certain boundaries, which are set, in part, by the cell type-specific epigenetic landscape. Ectopic expression of selected TFs can override the cellular identity and induce reprogramming to alternative fates.