Charlotte A. Collins

Stem Cell Function, Self-Renewal, and Behavioral Heterogeneity of Cells from the Adult Muscle Satellite Cell Niche

Satellite cells are situated beneath the basal lamina that surrounds each myofiber and function as myogenic precursors for muscle growth and repair. The source of satellite cell renewal is controversial and has been suggested to be a separate circulating or interstitial stem cell population. Here, we transplant single intact myofibers into radiation-ablated muscles and demonstrate that satellite cells are self-sufficient as a source of regeneration. As few as seven satellite cells associated with one transplanted myofiber can generate over 100 new myofibers containing thousands of myonuclei. Moreover, the transplanted satellite cells vigorously self-renew, expanding in number and repopulating the host muscle with new satellite cells. Following experimental injury, these cells proliferate extensively and regenerate large compact clusters of myofibers. Thus, within a normally stable tissue, the satellite cell exhibits archetypal stem cell properties and is competent to form the basal origin of adult muscle regeneration.

Pax7 and myogenic progression in skeletal muscle satellite cells

Skeletal muscle growth and regeneration are attributed to satellite cells - muscle stem cells resident beneath the basal lamina that surrounds each myofibre. Quiescent satellite cells express the transcription factor Pax7 and when activated, coexpress Pax7 with MyoD. Most then proliferate, downregulate Pax7 and differentiate. By contrast, others maintain Pax7 but lose MyoD and return to a state resembling quiescence. Here we show that Pax7 is able to drive transcription in quiescent and activated satellite cells, and continues to do so in those cells that subsequently cease proliferation and withdraw from immediate differentiation. We found that constitutive expression of Pax7 in satellite-cell-derived myoblasts did not affect MyoD expression or proliferation. Although maintained expression of Pax7 delayed the onset of myogenin expression it did not prevent, and was compatible with, myogenic differentiation. Constitutive Pax7 expression in a Pax7-null C2C12 subclone increased the proportion of cells expressing MyoD, showing that Pax7 can act genetically upstream of MyoD. However these Pax7-null cells were unable to differentiate into normal myotubes in the presence of Pax7. Therefore Pax7 may be involved in maintaining proliferation and preventing precocious differentiation, but does not promote quiescence.

Lrig1 Expression Defines a Distinct Multipotent Stem Cell Population in Mammalian Epidermis

Summary Lrig1 is a marker of human interfollicular epidermal stem cells and helps maintain stem cell quiescence. We show that, in mouse epidermis, Lrig1 defines the hair follicle junctional zone adjacent to the sebaceous glands and infundibulum. Lrig1 is a Myc target gene; loss of Lrig1 increases the proliferative capacity of stem cells in culture and results in epidermal hyperproliferation in vivo. Lrig1-expressing cells can give rise to all of the adult epidermal lineages in skin reconstitution assays. However, during homeostasis and on retinoic acid stimulation, they are bipotent, contributing to the sebaceous gland and interfollicular epidermis. β-catenin activation increases the size of the junctional zone compartment, and loss of Lrig1 causes a selective increase in β-catenin-induced ectopic hair follicle formation in the interfollicular epidermis. Our results suggest that Lrig1-positive cells constitute a previously unidentified reservoir of adult mouse interfollicular epidermal stem cells.

A population of myogenic stem cells that survives skeletal muscle aging

Age‐related decline in integrity and function of differentiated adult tissues is widely attributed to reduction in number or regenerative potential of resident stem cells. The satellite cell, resident beneath the basal lamina of skeletal muscle myofibers, is the principal myogenic stem cell. Here we have explored the capacity of satellite cells within aged mouse muscle to regenerate skeletal muscle and to self‐renew using isolated myofibers in tissue culture and in vivo. Satellite cells expressing Pax7 were depleted from aged muscles, and when aged myofibers were placed in culture, satellite cell myogenic progression resulted in apoptosis and fewer total differentiated progeny. However, a minority of cultured aged satellite cells generated large clusters of progeny containing both differentiated cells and new cells of a quiescent satellite‐cell‐like phenotype characteristic of self‐renewal. Parallel in vivo engraftment assays showed that, despite the reduction in Pax7+ cells, the satellite cell population associated with individual aged myofibers could regenerate muscle and self‐renew as effectively as the larger population of satellite cells associated with young myofibers. We conclude that a minority of satellite cells is responsible for adult muscle regeneration, and that these stem cells survive the effects of aging to retain their intrinsic potential throughout life. Thus, the effectiveness of stem‐cell‐mediated muscle regeneration is determined by both extrinsic environmental influences and diversity in intrinsic potential of the stem cells themselves.

Self-renewal of the adult skeletal muscle satellite cell.

The concept of the stem cell has evolved in dynamic systems such as those involved inembryonic development and, in the adult, in tissues such as blood and skin which arecontinuously renewed. It has proved difficult to establish whether stem cell mechanismsunderlie the maintenance of the more stable tissues that form the majority of the adultbody. We have investigated skeletal muscle, a low-turnover and largely postmitotictissue which nevertheless maintains a remarkable capacity to regenerate itself followinginjury. The contractile units of muscle are myofibers, elongated syncytial cells eachcontaining many hundreds of postmitotic myonuclei. Satellite cells are resident beneaththe basal lamina of myofibers and function as myogenic precursors during muscleregeneration. We have recently demonstrated that as few as seven Pax7+ satellite cellsassociated with one myofiber can regenerate a hundred or more new myofiberscontaining thousands of myonuclei. Satellite cells also undergo self-renewal, givingthem the ability to participate in multiple rounds of injury-induced regeneration. Thesatellite cell may thus serve as a prototype for stem cell function in stable adult tissues: atissue-specific progenitor which is normally quiescent but which has self-renewalproperties similar to those of better known stem cells.

Integrated Functions of Pax3 and Pax7 in the Regulation of Proliferation, Cell Size and Myogenic Differentiation

Pax3 and Pax7 are paired-box transcription factors with roles in developmental and adult regenerative myogenesis. Pax3 and Pax7 are expressed by postnatal satellite cells or their progeny but are down regulated during myogenic differentiation. We now show that constitutive expression of Pax3 or Pax7 in either satellite cells or C2C12 myoblasts results in an increased proliferative rate and decreased cell size. Conversely, expression of dominant-negative constructs leads to slowing of cell division, a dramatic increase in cell size and altered morphology. Similarly to the effects of Pax7, retroviral expression of Pax3 increases levels of Myf5 mRNA and MyoD protein, but does not result in sustained inhibition of myogenic differentiation. However, expression of Pax3 or Pax7 dominant-negative constructs inhibits expression of Myf5, MyoD and myogenin, and prevents differentiation from proceeding. In fibroblasts, expression of Pax3 or Pax7, or dominant-negative inhibition of these factors, reproduce the effects on cell size, morphology and proliferation seen in myoblasts. Our results show that in muscle progenitor cells, Pax3 and Pax7 function to maintain expression of myogenic regulatory factors, and promote population expansion, but are also required for myogenic differentiation to proceed.

Dynamic regulation of retinoic acid-binding proteins in developing, adult and neoplastic skin reveals roles for β-catenin and Notch signalling

Retinoic acid (RA) signalling is essential for epidermal differentiation; however, the mechanisms by which it acts are largely unexplored. Partitioning of RA between different nuclear receptors is regulated by RA-binding proteins. We show that cellular RA-binding proteins CRABP1 and CRABP2 and the fatty acid-binding protein FABP5 are dynamically expressed during skin development and in adult tissue. CRABP1 is expressed in embryonic dermis and in the stroma of skin tumours, but confined to the hair follicle dermal papilla in normal postnatal skin. CRABP2 and FABP5 are expressed in the differentiating cells of sebaceous gland, interfollicular epidermis and hair follicles, with FABP5 being a prominent marker of sebaceous glands and anagen follicle bulbs. All three proteins are upregulated in response to RA treatment or Notch activation and are negatively regulated by Wnt/beta-catenin signalling. Ectopic follicles induced by beta-catenin arise from areas of the sebaceous gland that have lost CRABP2 and FABP5; conversely, inhibition of hair follicle formation by N-terminally truncated Lef1 results in upregulation of CRABP2 and FABP5. Our findings demonstrate that there is dynamic regulation of RA signalling in different regions of the skin and provide evidence for interactions between the RA, beta-catenin and Notch pathways.

Isolation and Grafting of Single Muscle Fibres

Satellite cells are mononucleate muscle precursor cells resident beneath the basal lamina, which surrounds each skeletal muscle fibre. Normally quiescent in adult muscle, in response to muscle damage satellite cells are activated and proliferate to generate a pool of muscle precursor cells, which subsequently differentiate and fuse together to repair and replace terminally differentiated muscle fibre syncytia. Cells prepared by enzymatic digestion of whole muscle tissue are likely to contain myogenic cells derived both from the satellite cell niche and from other populations in the muscle interstitium and vasculature. Single muscle fibre preparations, in which satellite cells retain their normal anatomical position beneath the basal lamina, are free of interstitial and vascular tissue and can therefore be used to investigate satellite cell behaviour in the absence of other myogenic cell types. Here, we describe methods for the isolation of viable muscle fibres and for grafting of muscle fibres and their associated satellite cells into mouse muscles to assess the contribution of satellite cells to muscle regeneration.

β-Catenin Stabilization in Skin Fibroblasts Causes Fibrotic Lesions by Preventing Adipocyte Differentiation of the Reticular Dermis

The Wnt/β-catenin pathway plays a central role in epidermal homeostasis and regeneration, but how it affects fibroblast fate decisions is unknown. We investigated the effect of targeted β-catenin stabilization in dermal fibroblasts. Comparative gene expression profiling of stem cell antigen 1- (Sca1-) and Sca1+ neonatal fibroblasts from upper and lower dermis, respectively, confirmed that Sca1+ cells had a preadipocyte signature and showed differential expression of Wnt/β-catenin–associated genes. By targeting all fibroblasts or selectively targeting Dlk1+ lower dermal fibroblasts, we found that β-catenin stabilization between developmental stages E16.5 and P2 resulted in a reduction in the dermal adipocyte layer with a corresponding increase in dermal fibrosis and an altered hair cycle. The fibrotic phenotype correlated with a reduction in the potential of Sca1+ fibroblasts to undergo adipogenic differentiation ex vivo. Our findings indicate that Wnt/β-catenin signaling controls adipogenic cell fate within the lower dermis, which potentially contributes to the pathogenesis of fibrotic skin diseases.

Polyclonal origin and hair induction ability of dermal papillae in neonatal and adult mouse back skin

Hair follicle development and growth are regulated by Wnt signalling and depend on interactions between epidermal cells and a population of fibroblasts at the base of the follicle, known as the dermal papilla (DP). DP cells have a distinct gene expression signature from non-DP dermal fibroblasts. However, their origins are largely unknown. By generating chimeric mice and performing skin reconstitution assays we show that, irrespective of whether DP form during development, are induced by epidermal Wnt activation in adult skin or assemble from disaggregated cells, they are polyclonal in origin. While fibroblast proliferation is necessary for hair follicle formation in skin reconstitution assays, mitotically inhibited cells readily contribute to DP. Although new hair follicles do not usually develop in adult skin, adult dermal fibroblasts are competent to contribute to DP during hair follicle neogenesis, irrespective of whether they originate from skin in the resting or growth phase of the hair cycle or skin with β-catenin-induced ectopic follicles. We propose that during skin reconstitution fibroblasts may be induced to become DP cells by interactions with hair follicle epidermal cells, rather than being derived from a distinct subpopulation of cells.