Pax7 reporter mouse models: a pocket guide for satellite cell research

Since their discovery, satellite cells have showcased their need as primary contributors to skeletal muscle maintenance and repair. Satellite cells lay dormant, but when needed, activate, differentiate, fuse to fibres and self-renew, that has bestowed satellite cells with the title of muscle stem cells. The satellite cell specific transcription factor Pax7 has enabled researchers to develop animal models against the Pax7 locus in order to isolate and characterise satellite cell-mediated events. This review focuses specifically on describing Pax7 reporter mouse models. Here we describe how each model was generated and the key findings obtained. The strengths and limitations of each model are also discussed. The aim is to provide new and current satellite cell enthusiasts with a basic understanding of the available Pax7 reporter mice and hopefully guide selection of the most appropriate Pax7 model to answer a specific research question.

A Novel Method for Thyroarytenoid Myofiber Culture

OBJECTIVES/HYPOTHESIS

Myofiber culture has been employed to investigate muscle physiology in vitro and is well-established in the rodent hind limb. Thyroarytenoid (TA) myofiber culture has not been described, providing an opportunity to employ this method to investigate distinct TA myofiber functions. The purpose of this study was to assess the feasibility of a TA myofiber culture model.

STUDY DESIGN

In vitro.

METHODS

TA muscles from five Sprague Dawley rats were independently isolated and digested for 90 min. A smooth-tip, wide-bored pipette dissociated TA myofibers from cartilage, and the fibers were distributed on collagen-coated dishes and incubated at 37°C, 5% CO2 for 2 h. Myofiber specificity was determined via immunolabeling for desmin and myosin heavy chain (MHC). Myofibers viability was assessed over 7 days via esterase assay. Additional myofibers were immunolabeled for satellite cell marker Pax-7. Glucocorticoid (GC) receptor (GR) was immunolabeled following GC treatment.

RESULTS

The harvest technique yielded ~120 myofibers per larynx. By day 7, ~60% of the fibers remained attached and were calcein AM-positive/ethidium homodimer-negative, indicating viability. Myofibers were positive for desmin and MHC, indicating muscle specificity. Cells surrounding myofibers were positive for Pax-7, indicating the presence of myogenic satellite cells. Myofibers also responded to GC treatment as determined by GR nuclear translocation.

CONCLUSION

TA myofibers remained viable in culture for at least 7 days with a predictable response to exogenous stimuli. This technique provides novel investigative opportunities regarding TA structure and function.

LEVEL OF EVIDENCE

N/A Laryngoscope, 133:3109-3115, 2023.

c-Abl Kinase Is Required for Satellite Cell Function Through Pax7 Regulation

Satellite cells (SCs) are tissue-specific stem cells responsible for adult skeletal muscle regeneration and maintenance. SCs function is critically dependent on two families of transcription factors: the paired box (Pax) involved in specification and maintenance and the Muscle Regulatory Factors (MRFs), which orchestrate myogenic commitment and differentiation. In turn, signaling events triggered by extrinsic and intrinsic stimuli control their function via post-translational modifications, including ubiquitination and phosphorylation. In this context, the Abelson non-receptor tyrosine kinase (c-Abl) mediates the activation of the p38 α/β MAPK pathway, promoting myogenesis. c-Abl also regulates the activity of the transcription factor MyoD during DNA-damage stress response, pausing differentiation. However, it is not clear if c-Abl modulates other key transcription factors controlling SC function. This work aims to determine the role of c-Abl in SCs myogenic capacity via loss of function approaches in vitro and in vivo. Here we show that c-Abl inhibition or deletion results in a down-regulation of Pax7 mRNA and protein levels, accompanied by decreased Pax7 transcriptional activity, without a significant effect on MRF expression. Additionally, we provide data indicating that Pax7 is directly phosphorylated by c-Abl. Finally, SC-specific c-Abl ablation impairs muscle regeneration upon acute injury. Our results indicate that c-Abl regulates myogenic progression in activated SCs by controlling Pax7 function and expression.

In Vivo Gene Editing of Muscle Stem Cells with Adeno-Associated Viral Vectors in a Mouse Model of Duchenne Muscular Dystrophy

Delivery of therapeutic transgenes with adeno-associated viral (AAV) vectors for treatment of myopathies has yielded encouraging results in animal models and early clinical studies. Although certain AAV serotypes efficiently target muscle fibers, transduction of the muscle stem cells, also known as satellite cells, is less studied. Here, we used a Pax7nGFP;Ai9 dual reporter mouse to quantify AAV transduction events in satellite cells. We assessed a panel of AAV serotypes for satellite cell tropism in the mdx mouse model of Duchenne muscular dystrophy and observed the highest satellite cell labeling with AAV9 following local or systemic administration. Subsequently, we used AAV9 to interrogate CRISPR/Cas9-mediated gene editing of satellite cells in the Pax7nGFP;mdx mouse. We quantified the level of gene editing using a Tn5 transposon-based method for unbiased sequencing of editing outcomes at the Dmd locus. We also found that muscle-specific promoters can drive transgene expression and gene editing in satellite cells. Lastly, to demonstrate the functionality of satellite cells edited at the Dmd locus by CRISPR in vivo, we performed a transplantation experiment and observed increased dystrophin-positive fibers in the recipient mouse. Collectively, our results confirm that satellite cells are transduced by AAV and can undergo gene editing to restore the dystrophin reading frame in the mdx mouse.

Altered satellite cell dynamics accompany skeletal muscle atrophy during chronic illness, disuse, and aging

PURPOSE OF REVIEW

This review explores recent research investigating the contribution of satellite cells (skeletal muscle stem cells) during muscle fiber atrophy as seen in periods of disuse, illness, and aging.

RECENT FINDINGS

Studies indicate reduced satellite cell activity and density in a variety of acute and chronic conditions characterized by robust muscle wasting. The direct contribution of satellite cells to unloading/denervation and chronic illness-induced atrophy remains controversial. Inflammation that accompanies acute trauma and illness likely impedes proper satellite cell differentiation and myogenesis, promoting the rapid onset of muscle wasting in these conditions. Transgenic mouse studies provide surprising evidence that age-related declines in satellite cell function and abundance are not causally related to the onset of sarcopenia in sedentary animals.

SUMMARY

Recent clinical and preclinical studies indicate reduced abundance and dysregulated satellite cell activity that accompany muscle atrophy during periods of disuse, illness, and aging, providing evidence for their therapeutic potential.

Mechanisms of silver_nanoparticles induced hypopigmentation in embryonic zebrafish

Silver_nanoparticles (AgNPs) have been reported to inhibit specification of erythroid cells and to induce spinal cord deformities and cardiac arrhythmia in vertebrates, but have not been implicated in development of neural crest (NC) and pigment cells in an in vivo model yet. In current study, down-regulated expressions of NC genes pax7 and foxd3, melanophore genes mitfa and dct, and xanthophore gene gch2 in AgNPs-exposed embryos were revealed by microarray, qRT-PCR and whole-mount in situ hybridization (WISH). Then, the down-regulated expressions of melanophore genes mitfa and dct but not xanthophore gene gch2 in AgNPs-exposed embryos were found to be recovered by melanogenesis agonists palmitic acid and dibutyryl cyclic AMP (dbcAMP). Finally, Ag⁺ chelating and AgNPs coating compound l-cysteine was found to neutralize AgNPs-induced hypopigmentation in AgNPs-exposed embryos, and to recover the down-regulated expressions of both dct and gch2 to nearly normal level in embryos, suggesting that AgNPs-releasing Ag⁺ might mediate their biological effects on zebrafish pigmentation mostly. This study was firstly to unveil that AgNPs might specifically act up-stream of mitfa and pax7 genes to suppress specification and differentiation of melanophore and xanthophore lineages respectively by their releasing Ag⁺ during vertebrate embryogenesis.

An allelic series at pax7a is associated with colour polymorphism diversity in Lake Malawi cichlid fish

Despite long-standing interest in the evolution and maintenance of discrete phenotypic polymorphisms, the molecular genetic basis of such polymorphism in the wild is largely unknown. Female sex-associated blotched colour polymorphisms found in cichlids of Lake Malawi, East Africa, represent a highly successful polymorphic phenotype, found and maintained in four genera across the geographic expanse of the lake. Previously, we identified an association with an allelic variant of the paired-box transcription factor gene pax7a and blotched colour morphs in Lake Malawi cichlid fishes. Although a diverse range of blotched phenotypes are present in Lake Malawi cichlid species, they all appeared to result from an allele of pax7a that produces increased levels of transcript. Here, we examine the developmental and genetic basis of variation among blotched morphs. First, we confirm that pax7a-associated blotch morphs result primarily from modulation of melanophore development and survival. From laboratory crosses and natural population studies, we identify at least three alleles of pax7a associated with discrete subtypes of blotched morphs, in addition to the ancestral pax7a allele. Genotypes at pax7a support initial evolution of a novel pax7a allele to produce the blotched class of morphs, followed by subsequent evolution of that pax7a blotched allele to produce additional alleles associated with discrete colour morphs. Variant alleles of pax7a produce different levels of pax7a transcript, correlating with pigmentation phenotype at the cellular level. This naturally selected allelic series should serve as a case study for understanding the molecular genetic control of pax7a expression and the evolution of sex-associated alleles.

Generation of skeletal muscle cells from pluripotent stem cells: advances and challenges

Human pluripotent stem cells (hPSCs) possess unlimited proliferative potential while maintaining the ability to differentiate into any cell type including skeletal muscle cells (SMCs). hPSCs are amenable to genetic editing and can be derived from patient somatic cells, and thus represent a promising option for cell therapies for the treatment of degenerative diseases such as muscular dystrophies. There are unresolved challenges however associated with the derivation and scale-up of hPSCs and generation of differentiated cells in large quantity and high purity. Reported myogenic differentiation protocols are long, require cell sorting and/or rely on ectopic expression of myogenic master regulators. More recent advances have been made with the application of small molecules to enhance the myogenic differentiation efficiency and the identification of more selective markers for the enrichment of myogenic progenitors with enhanced regenerative potential. Here we review the field of myogenic differentiation and highlight areas requiring further research.

Influence of exercise contraction mode and protein supplementation on human skeletal muscle satellite cell content and muscle fiber growth

Skeletal muscle satellite cells (SCs) are involved in remodeling and hypertrophy processes of skeletal muscle. However, little knowledge exists on extrinsic factors that influence the content of SCs in skeletal muscle. In a comparative human study, we investigated the muscle fiber type-specific association between emergence of satellite cells (SCs), muscle growth, and remodeling in response to 12 wk unilateral resistance training performed as eccentric (Ecc) or concentric (Conc) resistance training ± whey protein (Whey, 19.5 g protein + 19.5 g glucose) or placebo (Placebo, 39 g glucose) supplementation. Muscle biopsies (vastus lateralis) were analyzed for fiber type-specific SCs, myonuclei, and fiber cross-sectional area (CSA). Following training, SCs increased with Conc in both type I and type II fibers (P < 0.01) and exhibited a group difference from Ecc (P < 0.05), which did not increase. Myonuclei content in type I fibers increased in all groups (P < 0.01), while a specific accretion of myonuclei in type II fibers was observed in the Whey-Conc (P < 0.01) and Placebo-Ecc (P < 0.01) groups. Similarly, whereas type I fiber CSA increased independently of intervention (P < 0.001), type II fiber CSA increased exclusively with Whey-Conc (P < 0.01) and type II fiber hypertrophy correlated with whole muscle hypertrophy exclusively following Conc training (P < 0.01). In conclusion, isolated concentric knee extensor resistance training appears to constitute a stronger driver of SC content than eccentric resistance training while type II fiber hypertrophy was accentuated when combining concentric resistance training with whey protein supplementation.

Chronic myopathy due to immunoglobulin light chain amyloidosis

Amyloid myopathy associated with a plasma cell dyscrasia is a rare cause of muscle hypertrophy. It can be a challenging diagnosis, since pathological findings are often elusive. In addition, the mechanism by which immunoglobulin light-chain deposition stimulates muscle overgrowth remains poorly understood. We present a 53-year old female with a 10-year history of progressive generalized muscle overgrowth. Congo-red staining and immunohistochemistry revealed perivascular lambda light chain amyloid deposits, apparent only in a second muscle biopsy. The numbers of central nuclei and satellite cells were increased, suggesting enhanced muscle progenitor cell formation. Despite the chronicity of the light chain disease, the patient showed complete resolution of hematologic findings and significant improvement of her muscle symptoms following autologous bone marrow transplantation. This case highlights the importance of early diagnosis and therapy for this treatable cause of a chronic myopathy with muscle hypertrophy.

Generating tamoxifen-inducible Cre alleles to investigate myogenesis in mice

Gene inactivation has become the gold standard for determining gene function in the mouse. Many genes inactivated in the germ line cause early lethality that precludes phenotypic assessment at a later time point. Conditional gene inactivation using Cre recombinase expressed via a tissue specific promoter/enhancer allows phenotypic analyses of selected tissues, but lacks temporal control. Recent development of the tamoxifen-inducible Cre-ER (T2) offers both cell type-specific and temporal control of conditional gene inactivation. As an example, we describe the design and step-wise construction of a Cre-ER (T2) knock-in allele at the Pax7 locus using the recombineering method - Pax7 is selectively expressed in embryonic muscle progenitors and adult muscle stem cells. The resulting Pax7-Cre- ER (T2) (Pax7 (CE)) allele has been successfully applied to embryos and adults for tamoxifen-regulated myogenic lineage tracing and gene inactivation (Nature 460:627-631, 2009; Genesis 48:424-436, 2010).

Skeletal muscle satellite cells: background and methods for isolation and analysis in a primary culture system

Repair of adult skeletal muscle depends on satellite cells, myogenic stem cells located between the basal lamina and the plasmalemma of the myofiber. Standardized protocols for the isolation and culture of satellite cells are key tools for understanding cell autonomous and extrinsic factors that regulate their performance. Knowledge gained from such studies can contribute important insights to developing strategies for the improvement of muscle repair following trauma and in muscle wasting disorders. This chapter provides an introduction to satellite cell biology and further describes the basic protocol used in our laboratory to isolate and culture satellite cells from adult skeletal muscle. The cell culture conditions detailed herein support proliferation and differentiation of satellite cell progeny and the development of reserve cells, which are thought to reflect the in vivo self-renewal ability of satellite cells. Additionally, this chapter describes our standard immunostaining protocol that allows the characterization of satellite cell progeny by the temporal expression of characteristic transcription factors and structural proteins associated with different stages of myogenic progression. Although emphasis is given here to the isolation and characterization of satellite cells from mouse hindlimb muscles, the protocols are suitable for other muscle types (such as diaphragm and extraocular muscles) and for muscles from other species, including chicken and rat. Altogether, the basic protocols described are straightforward and facilitate the study of diverse aspects of skeletal muscle stem cells.

Origin of vertebrate limb muscle: the role of progenitor and myoblast populations

Muscle development, growth, and regeneration take place throughout vertebrate life. In amniotes, myogenesis takes place in four successive, temporally distinct, although overlapping phases. Understanding how embryonic, fetal, neonatal, and adult muscle are formed from muscle progenitors and committed myoblasts is an area of active research. In this review we examine recent expression, genetic loss-of-function, and genetic lineage studies that have been conducted in the mouse, with a particular focus on limb myogenesis. We synthesize these studies to present a current model of how embryonic, fetal, neonatal, and adult muscle are formed in the limb.

Muscle stem cells in developmental and regenerative myogenesis

PURPOSE OF REVIEW

Skeletal muscle development serves as a paradigm for cell lineage specification and cell differentiation. Adult skeletal muscle has high regenerative capacity, with satellite cells the primary source of this capability. The present review describes recent findings on developmental and adult myogenesis with emphasis on emerging distinctions between various muscle groups and stages of myogenesis.

RECENT FINDINGS

Muscle progenitors of the body are derived from multipotent cells of the dermomyotome and express the transcription factors Pax3 and Pax7. These cells self-renew or induce expression of myogenic regulatory factors (MRFs) and differentiate. The roles of Pax3, Pax7 and specific myogenic regulatory factor progenitor populations in trunk and limb myogenesis have been identified through cell ablation in the mouse. Various head muscles and associated satellite cells have differing developmental origins, and rely on distinct combinations of transcriptional regulators, than trunk and limb muscles. Several genetic and sorting protocols demonstrate that satellite cells are heterogeneous with some possessing stem cell properties; the relative roles of lineage and niche in these properties are being explored. Although cellular mechanisms of developmental, postnatal and adult regenerative myogenesis are thought to be similar, recent studies reveal distinct genetic requirements for embryonic, fetal, postnatal and adult regenerative myogenesis.

SUMMARY

Genetic determinants of formation or repair of various muscles during different stages of myogenesis are unexpectedly diverse. Future studies should illuminate these differences, as well as mechanisms that underlie stem cell properties of satellite cells.

Graft outcomes influenced by co-expression of Pax7 in graft and host tissue

Cell replacement therapies offer promise in the treatment of neurotrauma and neurodegenerative disorders and have concentrated on the use of primary fetal brain tissue. However, there is a growing promise of using neural stem cells, in which case other factors may be important in their successful engraftment. We therefore investigated whether the co-expression of the major developmental transcription factor (Pax7 in this study) of donor tissue to graft site influences transplant survival and differentiation in the rat midbrain. Neural progenitor cells were prepared from either the Pax7-expressing dorsal (DM) or non-Pax7-expressing ventral mesencephalon (VM) of embryonic EGFP(+/+) rats. Cells were dissociated and grafted into the adult rat superior colliculus (SC) lesioned with quinolinic acid 3 days previously, a time shown to be associated with the up-regulation of Pax7. Grafts were then examined 4 weeks later. Our results suggest the origin of the graft tissue did not alter graft survival in the SC; however, dorsal grafts appear to have a higher incidence of neuronal survival, whereas ventral grafts have a higher incidence of astrocytic survivors.

Lineage-specific responses to reduced embryonic Pax3 expression levels

Pax3 is an essential paired- and homeodomain-containing transcription factor that is necessary for closure of the neural tube, and morphogenesis of the migratory neural crest and myoblast lineages. Homozygous loss-of-function mutation results in mid-gestational lethality with defects in myogenesis, neural tube closure and neural crest-derived lineages including melanocytes, Schwann cells and insufficient mesenchymal cells to septate the cardiac outflow tract. To address the function of Pax3 in later fetal stages and in specific adult tissues, we generated a floxed Pax3 allele (Pax3(flox)). An intermediate allele (Pax3(neo)) was produced via creation of the floxed allele, in which the TK-neo(R) cassette is present between exons 5 and 6. It was deduced to be a hypomorph, as Pax3 protein expression is reduced by 80% and homozygote hypomorphs die postnatally. To assess the consequences of reduced Pax3 levels on the various Pax3-expressing lineages and to determine the underlying cause of lethality, we examined Pax3 spatiotemporal expression and the resultant defects. Defective limb and tongue musculature were observed and lethality was due to an inability to suckle. However, the heart, diaphragm, trunk musculature, as well as the various neural crest-derived lineages and neural tube were all unaffected by reduced Pax3 levels. Significantly, elevated levels of the related Pax7 protein were present in unaffected neural tube and epaxial somatic component. The limb and tongue myogenic defects were found to be due to a significant increase in apoptosis within the somites that leads to a paucity of migratory hypaxial myoblasts. These effects were attributed to the hypomorphic effect of the Pax3(neo) allele, as removal of the TK-neo(R) cassette completely relieves the hypomorphic effect, as 100% of the Pax3(flox/flox) mice were normal. These data demonstrate a lineage-specific response to approximately 80% loss of Pax3 protein expression, with myogenesis of limb and tongue being most sensitive to reduced Pax3 levels. Thus, we have established that there are different minimum threshold requirements for Pax3 within different Pax3-expressing lineages.