Cloning and characterization of the human PAX7 promoter

'Enhancing' skeletal muscle and stem cells in three-dimensions: genome regulation of skeletal muscle in development and disease

The noncoding genome imparts important regulatory control over gene expression. In particular, gene enhancers represent a critical layer of control that integrates developmental and differentiation signals outside the cell into transcriptional outputs inside the cell. Recently, there has been an explosion in genomic techniques to probe enhancer control, function, and regulation. How enhancers are regulated and integrate signals in stem cell development and differentiation is largely an open question. In this review, we focus on the role gene enhancers play in muscle stem cell specification, differentiation, and progression. We pay specific attention toward the identification of muscle-specific enhancers, the binding of transcription factors to these enhancers, and how enhancers communicate to their target genes via three-dimensional looping.

The mesodermal and myogenic specification of hESCs depend on ZEB1 and are inhibited by ZEB2

Human embryonic stem cells (hESCs) can differentiate into any cell lineage. Here, we report that ZEB1 and ZEB2 promote and inhibit mesodermal-to-myogenic specification of hESCs, respectively. Knockdown and/or overexpression experiments of ZEB1, ZEB2, or PAX7 in hESCs indicate that ZEB1 is required for hESC Nodal/Activin-mediated mesodermal specification and PAX7+ human myogenic progenitor (hMuP) generation, while ZEB2 inhibits these processes. ZEB1 downregulation induces neural markers, while ZEB2 downregulation induces mesodermal/myogenic markers. Mechanistically, ZEB1 binds to and transcriptionally activates the PAX7 promoter, while ZEB2 binds to and activates the promoter of the neural OTX2 marker. Transplanting ZEB1 or ZEB2 knocked down hMuPs into the muscles of a muscular dystrophy mouse model, showing that hMuP engraftment and generation of dystrophin-positive myofibers depend on ZEB1 and are inhibited by ZEB2. The mouse model results suggest that ZEB1 expression and/or downregulating ZEB2 in hESCs may also enhance hESC regenerative capacity for human muscular dystrophy therapy.

Lauric acid impairs insulin-induced Akt phosphorylation by upregulating SELENOP expression via HNF4α induction

Selenoprotein P (SeP; encoded by SELENOP in humans, Selenop in rodents) is a hepatokine that is upregulated in the liver of humans with type 2 diabetes. Excess SeP contributes to the onset of insulin resistance and various type 2 diabetes-related complications. We have previously reported that the long-chain saturated fatty acid, palmitic acid, upregulates Selenop expression, whereas the polyunsaturated fatty acids (PUFAs) downregulate it in hepatocytes. However, the effect of medium-chain fatty acids (MCFAs) on Selenop is unknown. Here we report novel mechanisms that underlie the lauric acid-mediated Selenop gene regulation in hepatocytes. Lauric acid upregulated Selenop expression in Hepa1-6 hepatocytes and mice liver. A luciferase promoter assay and computational analysis of transcription factor-binding sites identified the hepatic nuclear factor 4α (HNF4α) binding site in the SELENOP promoter. A chromatin immunoprecipitation (ChIP) assay showed that lauric acid increased the binding of HNF4α to the SELENOP promoter. The knockdown of Hnf4α using siRNA canceled the upregulation of lauric acid-induced Selenop. Thus, the lauric acid-induced impairment of Akt phosphorylation brought about by insulin was rescued by the knockdown of either Hnf4α or Selenop. These results provide new insights into the regulation of SeP by fatty acids and suggest that SeP may mediate MCFA-induced hepatic insulin signal reduction.

Generation of PAX7 Reporter Cells to Investigate Skeletal Myogenesis from Human Pluripotent Stem Cells

This protocol describes the use of CRISPR/Cas9-mediated homology-directed recombination to construct a PAX7-GFP reporter in human pluripotent stem cells (hPSCs). PAX7 is a key transcription factor and regulator of skeletal muscle stem/progenitor cells. We obtained heterozygous knockin reporter cells and validated their PAX7 expression using both artificial activation by the CRISPR/dCas9-VPR system and physiological activation during hPSC myogenic differentiation. These cells can serve as tools for better understanding of in vitro hPSC myogenesis and enriching myogenic cells for downstream analysis. For complete details on the use and execution of this protocol, please refer to Xi et al. (2017) and Xi et al. (2020).

A Human Skeletal Muscle Atlas Identifies the Trajectories of Stem and Progenitor Cells across Development and from Human Pluripotent Stem Cells

The developmental trajectory of human skeletal myogenesis and the transition between progenitor and stem cell states are unclear. We used single-cell RNA sequencing to profile human skeletal muscle tissues from embryonic, fetal, and postnatal stages. In silico, we identified myogenic as well as other cell types and constructed a "roadmap" of human skeletal muscle ontogeny across development. In a similar fashion, we also profiled the heterogeneous cell cultures generated from multiple human pluripotent stem cell (hPSC) myogenic differentiation protocols and mapped hPSC-derived myogenic progenitors to an embryonic-to-fetal transition period. We found differentially enriched biological processes and discovered co-regulated gene networks and transcription factors present at distinct myogenic stages. This work serves as a resource for advancing our knowledge of human myogenesis. It also provides a tool for a better understanding of hPSC-derived myogenic progenitors for translational applications in skeletal muscle-based regenerative medicine.

RAGE signaling deficiency in rhabdomyosarcoma cells causes upregulation of PAX7 and uncontrolled proliferation

Embryonal rhabdomyosarcomas (ERMSs) show elevated levels of PAX7, a transcription factor that marks quiescent adult muscle stem (satellite) cells and is important for proliferation and survival of activated satellite cells and whose timely repression is required for myogenic differentiation. However, the mechanism of PAX7 accumulation in ERMSs and whether high PAX7 causes uncontrolled proliferation in ERMS remains to be elucidated. The receptor for advanced glycation end-products (RAGE, encoded by AGER) transduces a myogenic and anti-proliferative signal in myoblasts, and stable transfection of the ERMS cell line TE671, which does not express RAGE, with AGER results in reduced proliferation and formation of tumor masses in vivo, and enhanced apoptosis and myogenic differentiation. Herein, we show that RAGE expression is low or absent in human ERMSs. We also show that in ERMS cells (1) PAX7 accumulates owing to absent or low RAGE signaling; (2) elevated PAX7 levels reduce RAGE expression and levels of MyoD and myogenin, muscle-specific transcription factors required for myoblast proliferation arrest and differentiation, respectively; (3) PAX7 supports myoblast proliferation by reducing the levels of MyoD, primarily by promoting its degradation; and (4), when ectopically expressed in ERMS cells, that RAGE upregulates myogenin which upregulates MyoD and downregulates PAX7, with consequent inhibition of proliferation and stimulation of differentiation. Thus, failure to express RAGE and, hence, MyoD and myogenin above a critical level in ERMS cells might result in deregulated PAX7 expression leading to uncontrolled proliferation and, potentially, to rhabdomyosarcomagenesis.

Pax7 is regulated by cMyb during early neural crest development through a novel enhancer

The neural crest (NC) is a migratory population of cells unique to vertebrates that generates many diverse derivatives. NC cells arise during gastrulation at the neural plate border (NPB), which is later elevated as the neural folds (NFs) form and fuse in the dorsal region of the closed neural tube, from where NC cells emigrate. In chick embryos, Pax7 is an early marker, and necessary component of NC development. Unlike other early NPB markers, which are co-expressed in lateral ectoderm, medial neural plate or posterior-lateral mesoderm, Pax7 early expression seems more restricted to the NPB. However, the molecular mechanisms controlling early Pax7 expression remain poorly understood. Here, we identify a novel enhancer of Pax7 in avian embryos that replicates the expression of Pax7 associated with early NC development. Expression from this enhancer is found in early NPB, NFs and early emigrating NC, but unlike Pax7, which is also expressed in mesodermal derivatives, this enhancer is not active in somites. Further analysis demonstrates that cMyb is able to interact with this enhancer and modulates reporter and endogenous early Pax7 expression; thus, cMyb is identified as a novel regulator of Pax7 in early NC development.

HMGB1-RAGE regulates muscle satellite cell homeostasis through p38-MAPK- and myogenin-dependent repression of Pax7 transcription

Expression of the paired-box 7 (PAX7) transcription factor is regulated during both myoblast proliferation and differentiation: high levels of PAX7 compromise myogenic differentiation because of excess and prolonged proliferation, whereas low levels of PAX7 result in precocious differentiation. We showed that myogenin repressed Pax7 transcription in differentiating myoblasts by binding to specific recognition sites in the Pax7 promoter, and that high-mobility group box 1 (HMGB1)-receptor for advanced glycation end-products (RAGE) signaling was required for myogenin induction and myogenin-dependent repression of Pax7 transcription. In addition, PAX7 negatively and myogenin positively regulated RAGE expression. RAGE, a multiligand receptor of the immunoglobulin superfamily, was not expressed in adult skeletal muscles, and was transiently expressed in activated, proliferating and differentiating satellite cells (SCs) in injured muscles. Compared with wild-type muscles, Rage(-/-) muscles exhibited increased numbers of basal SCs that were further increased in injured Rage(-/-) muscles following elevated myoblast asymmetric division; complete regeneration of injured Rage(-/-) muscles was found to be delayed by ~1 week. Thus, RAGE signaling physiologically repressed Pax7 transcription in SCs by upregulating myogenin, thereby accelerating muscle regeneration and limiting SC self-renewal.

Distinct enhancers regulate neural expression of Pax7

The murine Pax7 gene has emerged as an important regulator of neural and somite development. It is expressed in discrete domains of the central nervous system, including cranial neural crest, dorsal neural tube, and mesencephalic tectum, pretectum, and base, and at the midbrain-hindbrain boundary. It is also expressed by nasal epithelia and neural crest-derived facial structures. Here, we define the 5' end of the cDNA for murine Pax7 and identify the transcriptional start site. We clarify gene structure and the murine coding sequence, and we define regions of noncoding sequence that are conserved between mice and humans. Using transgenic approaches, we identify upstream and intronic regulatory elements that confer distinct domains of neural expression in the cranial neural crest, facial mesenchyme, mesencephalon, and pontine reticular nucleus. These enhancer regions will be useful for gene expression studies and for the identification of upstream regulators of Pax7 expression.

Structural and functional characterization of the human PAX7 5'-flanking regulatory region

The human PAX7 gene is a member of the paired box containing gene family of transcription factors implicated in development of the skeletal muscle of the trunk and limbs as well as elements of the central nervous system. To understand the molecular mechanisms involved in its expression, we have localized the transcription start sites in adult skeletal muscle and functionally characterized the 5'-flanking regulatory region responsible for PAX7 expression in this tissue. The major transcription start was identified 664 bp upstream from the ATG codon using primer extension and 5'-rapid amplification of cDNA ends (5'-RACE). Analysis of the 5'-flanking sequence revealed the absence of a TATA-box and the presence of an inverted CCAAT-box. Several consensus sites for common transcriptional regulators including Oct-1, NF1, AP2, AP4, CREB, Sp1, Nkx2.5, and MyoD are present in the promoter region. To determine the sites critical for the function of the PAX7 promoter, a series of deletion fragments of the 5'-flanking region were cloned adjacent to luciferase reporter gene and expressed in RD, Cos-7 and JAR cell lines. The maximal promoter activity was achieved by a fragment extending from the position -403 to +373. No strong positive or negative regulatory elements were discovered by adding of further sequences (up to 2.97 kb). A polymorphic (CCT)(n) repeat sequence was found 107 bp upstream of the transcription initiation site. PCR-based systematic screening for length variations in 227 unrelated individuals of a Caucasian population showed a bimodal distribution of three alleles containing 8, 10 or 11 repeat units. When different variants of this PAX7 gene-linked polymorphic region (PAX7-LPR) were fused to a luciferase reporter gene and transfected into RD cells, the variant with 11 repeat units revealed higher transcriptional efficiency compared to the 8 or 10 repeat alleles.