Regulation of motility of myogenic cells in filling limb muscle anlagen by Pitx2

A single-cell census of mouse limb development identifies complex spatiotemporal dynamics of skeleton formation

Limb development has long served as a model system for coordinated spatial patterning of progenitor cells. Here, we identify a population of naive limb progenitors and show that they differentiate progressively to form the skeleton in a complex, non-consecutive, three-dimensional pattern. Single-cell RNA sequencing of the developing mouse forelimb identified three progenitor states: naive, proximal, and autopodial, as well as Msx1 as a marker for the naive progenitors. In vivo lineage tracing confirmed this role and localized the naive progenitors to the outer margin of the limb, along the anterior-posterior axis. Sequential pulse-chase experiments showed that the progressive transition of Msx1⁺ naive progenitors into proximal and autopodial progenitors coincides with their differentiation to Sox9⁺ chondroprogenitors, which occurs along all the forming skeletal segments. Indeed, tracking the spatiotemporal sequence of differentiation showed that the skeleton forms progressively in a complex pattern. These findings suggest an alternative model for limb skeleton development.

Pitx2 Differentially Regulates the Distinct Phases of Myogenic Program and Delineates Satellite Cell Lineages During Muscle Development

The knowledge of the molecular mechanisms that regulate embryonic myogenesis from early myogenic progenitors to myoblasts, as well as the emergence of adult satellite stem cells (SCs) during development, are key concepts to understanding the genesis and regenerative abilities of the skeletal muscle. Several previous pieces of evidence have revealed that the transcription factor Pitx2 might be a player within the molecular pathways controlling somite-derived muscle progenitors’ fate and SC behavior. However, the role exerted by Pitx2 in the progression from myogenic progenitors to myoblasts including SC precursors remains unsolved. Here, we show that Pitx2 inactivation in uncommitted early myogenic precursors diminished cell proliferation and migration leading to muscle hypotrophy and a low number of SCs with decreased myogenic differentiation potential. However, the loss of Pitx2 in committed myogenic precursors gave rise to normal muscles with standard amounts of SCs exhibiting high levels of Pax7 expression. This SC population includes few MYF5+ SC-primed but increased amount of less proliferative miR-106b+cells, and display myogenic differentiation defects failing to undergo proper muscle regeneration. Overall our results demonstrate that Pitx2 is required in uncommitted myogenic progenitors but it is dispensable in committed precursors for proper myogenesis and reveal a role for this transcription factor in the generation of diverse SC subpopulations.

Pitx genes in development and disease

Homeobox genes encode sequence-specific transcription factors (SSTFs) that recognize specific DNA sequences and regulate organogenesis in all eukaryotes. They are essential in specifying spatial and temporal cell identity and as a result, their mutations often cause severe developmental defects. Pitx genes belong to the PRD class of the highly evolutionary conserved homeobox genes in all animals. Vertebrates possess three Pitx paralogs, Pitx1, Pitx2, and Pitx3 while non-vertebrates have only one Pitx gene. The ancient role of regulating left-right (LR) asymmetry is conserved while new functions emerge to afford more complex body plan and functionalities. In mouse, Pitx1 regulates hindlimb tissue patterning and pituitary development. Pitx2 is essential for the development of the oral cavity and abdominal wall while regulates the formation and symmetry of other organs including pituitary, heart, gut, lung among others by controlling growth control genes upon activation of the Wnt/ß-catenin signaling pathway. Pitx3 is essential for lens development and migration and survival of the dopaminergic neurons of the substantia nigra. Pitx gene mutations are linked to various congenital defects and cancers in humans. Pitx gene family has the potential to offer a new approach in regenerative medicine and aid in identifying new drug targets.

Cellular dynamics of myogenic cell migration: molecular mechanisms and implications for skeletal muscle cell therapies

Directional cell migration is a critical process underlying morphogenesis and post-natal tissue regeneration. During embryonic myogenesis, migration of skeletal myogenic progenitors is essential to generate the anlagen of limbs, diaphragm and tongue, whereas in post-natal skeletal muscles, migration of muscle satellite (stem) cells towards regions of injury is necessary for repair and regeneration of muscle fibres. Additionally, safe and efficient migration of transplanted cells is critical in cell therapies, both allogeneic and autologous. Although various myogenic cell types have been administered intramuscularly or intravascularly, functional restoration has not been achieved yet in patients with degenerative diseases affecting multiple large muscles. One of the key reasons for this negative outcome is the limited migration of donor cells, which hinders the overall cell engraftment potential. Here, we review mechanisms of myogenic stem/progenitor cell migration during skeletal muscle development and post-natal regeneration. Furthermore, strategies utilised to improve migratory capacity of myogenic cells are examined in order to identify potential treatments that may be applied to future transplantation protocols.

Requirement of Pitx2 for skeletal muscle homeostasis

Skeletal muscle is generated by the successive incorporation of primary (embryonic), secondary (fetal), and tertiary (adult) fibers into muscle. Conditional excision of Pitx2 function by an MCKCre driver resulted in animals with histological and ultrastructural defects in P30 muscles and fibers, respectively. Mutant muscle showed severe reduction in mitochondria and FoxO3-mediated mitophagy. Both oxidative and glycolytic energy metabolism were reduced. Conditional excision was limited to fetal muscle fibers after the G1-G0 transition and resulted in altered MHC, Rac1, MEF2a, and alpha-tubulin expression within these fibers. The onset of excision, monitored by a nuclear reporter gene, was observed as early as E16. Muscle at this stage was already severely malformed, but appeared to recover by P30 by the expansion of adjoining larger fibers. Our studies demonstrate that the homeodomain transcription factor Pitx2 has a postmitotic role in maintaining skeletal muscle integrity and energy homeostasis in fetal muscle fibers.

Hyaluronic acid, CD44 and RHAMM regulate myoblast behavior during embryogenesis

Hyaluronic acid (HA) is an extracellular matrix (ECM) component that has been shown to play a significant role in regulating muscle cell behavior during repair and regeneration. For instance, ECM remodeling after muscle injury involves an upregulation in HA expression that is coupled with skeletal muscle precursor cell recruitment. However, little is known about the role of HA during skeletal muscle development. To gain insight into the way in which HA mediates embryonic myogenesis, we first determined the spatial distribution and gene expression of CD44, RHAMM and other HA related proteins in embryonic day (E)10.5 to E12.5 murine forelimbs. While HA and CD44 expression remained high, RHAMM decreased at both the protein (via immunohistochemistry) and RNA (via qPCR) levels. Next, we determined that 4-methylumbelliferone-mediated knockdown of HA synthesis inhibited the migration and proliferation of E11.5/E12.5 forelimb-derived cells. Then, the influence of CD44 and RHAMM on myoblast and connective tissue cell behavior was investigated using antibodies against these receptors. Anti-RHAMM, but not anti-CD44, significantly decreased the total distance myogenic progenitors migrated over 24 h, whereas both inhibited connective tissue cell migration. In contrast, anti-CD44 inhibited the proliferation of connective tissue cells and muscle progenitors, but anti-RHAMM had no effect. However, when myoblasts and connective tissue cells were depleted of CD44 and RHAMM by shRNA, motility and proliferation were significantly inhibited in both cells indicating that blocking cell surface-localized CD44 and RHAMM does not have as pronounced effect as global shRNA-mediated depletion of these receptors. These results show, for the first time, the distribution and activity of RHAMM in the context of skeletal muscle. Furthermore, our data indicate that HA, through interactions with CD44 and RHAMM, promotes myogenic progenitor migration and proliferation. Confirmation of the role of HA and its receptors in directing myogenesis will be useful for the design of regenerative therapies that aim to promote the restoration of damaged or diseased muscle.

Location, Location, Location: Signals in Muscle Specification

Muscles control body movement and locomotion, posture and body position and soft tissue support. Mesoderm derived cells gives rise to 700 unique muscles in humans as a result of well-orchestrated signaling and transcriptional networks in specific time and space. Although the anatomical structure of skeletal muscles is similar, their functions and locations are specialized. This is the result of specific signaling as the embryo grows and cells migrate to form different structures and organs. As cells progress to their next state, they suppress current sequence specific transcription factors (SSTF) and construct new networks to establish new myogenic features. In this review, we provide an overview of signaling pathways and gene regulatory networks during formation of the craniofacial, cardiac, vascular, trunk, and limb skeletal muscles.

Mapping the Chromatin State Dynamics in Myoblasts

Background

Genome-wide mapping reveals chromatin landscapes unique to cell states. Histone marks of regulatory genes involved in cell specification and organ development provide a powerful tool to map regulatory sequences. H3K4me3 marks promoter regions; H3K27me3 marks repressed regions, and Pol II presence indicates active transcription. The presence of both H3K4me3 and H3K27me3 characterize poised sequences, a common characteristic of genes involved in pattern formation during organogenesis.

Results

We used genome-wide profiling for H3K27me3, H3K4me3, and Pol II to map chromatin states in mouse embryonic day 12 forelimbs in wild type (control) and Pitx2-null mutant mice. We compared these data with previous gene expression studies from forelimb Lbx1⁺ migratory myoblasts and correlated Pitx2-dependent expression profiles and chromatin states. During forelimb development, several lineages including myoblast, osteoblast, neurons, angioblasts etc., require synchronized growth to form a functional limb. We identified 125 genes in the developing forelimb that are Pitx2-dependent. Genes involved in muscle specification and cytoskeleton architecture were positively regulated, while genes involved in axonal path finding were poised.

Conclusion

Our results have established histone modification profiles as a useful tool for identifying gene regulatory states in muscle development, and identified the role of Pitx2 in extending the time of myoblast progression, promoting formation of sarcomeric structures, and suppressing attachment of neuronal axons.

PITX2 Loss-of-Function Mutation Contributes to Congenital Endocardial Cushion Defect and Axenfeld-Rieger Syndrome

Congenital heart disease (CHD), the most common type of birth defect, is still the leading non-infectious cause of infant morbidity and mortality in humans. Aggregating evidence demonstrates that genetic defects are involved in the pathogenesis of CHD. However, CHD is genetically heterogeneous and the genetic components underpinning CHD in an overwhelming majority of patients remain unclear. In the present study, the coding exons and flanking introns of the PITX2 gene, which encodes a paired-like homeodomain transcription factor 2essential for cardiovascular morphogenesis as well as maxillary facial development, was sequenced in 196 unrelated patients with CHD and subsequently in the mutation carrier's family members available. As a result, a novel heterozygous PITX2 mutation, p.Q102X for PITX2a, or p.Q148X for PITX2b, or p.Q155X for PITX2c, was identified in a family with endocardial cushion defect (ECD) and Axenfeld-Rieger syndrome (ARS). Genetic analysis of the pedigree showed that the nonsense mutation co-segregated with ECD and ARS transmitted in an autosomal dominant pattern with complete penetrance. The mutation was absent in 800 control chromosomes from an ethnically matched population. Functional analysis by using a dual-luciferase reporter assay system revealed that the mutant PITX2 had no transcriptional activity and that the mutation eliminated synergistic transcriptional activation between PITX2 and NKX2.5, another transcription factor pivotal for cardiogenesis. To our knowledge, this is the first report on the association of PITX2 loss-of-function mutation with increased susceptibility to ECD and ARS. The findings provide novel insight into the molecular mechanisms underpinning ECD and ARS, suggesting the potential implications for the antenatal prophylaxis and personalized treatment of CHD and ARS.

Identification of the GTPase-activating protein DEP domain containing 1B (DEPDC1B) as a transcriptional target of Pitx2

Pitx2 is a bicoid-related homeobox transcription factor implicated in regulating left-right patterning and organogenesis. However, only a limited number of Pitx2 downstream target genes have been identified and characterized. Here we demonstrate that Pitx2 is a transcriptional repressor of DEP domain containing 1B (DEPDC1B). The first intron of the human and mouse DEP domain containing 1B genes contains multiple consensus DNA-binding sites for Pitx2. Chromatin immunoprecipitation assays revealed that Pitx2, along with histone deacetylase 1, was recruited to the first intron of Depdc1b. In contrast, RNAi-mediated depletion of Pitx2 not only enhanced the acetylation of histone H4 in the first intron of Depdc1b, but also increased the protein level of Depdc1b. Luciferase reporter assays also showed that Pitx2 could repress the transcriptional activity mediated by the first intron of human DEPDC1B. The GAP domain of DEPDC1B interacted with nucleotide-bound forms of RAC1 in vitro. In addition, exogenous expression of DEPDC1B suppressed RAC1 activation and interfered with actin polymerization induced by the guanine nucleotide exchange factor TRIO. Moreover, DEPDC1B interacted with various signaling molecules such as U2af2, Erh, and Salm. We propose that Pitx2-mediated repression of Depdc1b expression contributes to the regulation of multiple molecular pathways, such as Rho GTPase signaling.

Pitx2c is reactivated in the failing myocardium and stimulates myf5 expression in cultured cardiomyocytes

BACKGROUND

Pitx2 (paired-like homeodomain 2 transcription factor) is crucial for heart development, but its role in heart failure (HF) remains uncertain. The present study lays the groundwork implicating Pitx2 signalling in different modalities of HF.

METHODOLOGY/PRINCIPAL FINDINGS

A variety of molecular, cell-based, biochemical, and immunochemical assays were used to evaluate: (1) Pitx2c expression in the porcine model of diastolic HF (DHF) and in patients with systolic HF (SHF) due to dilated and ischemic cardiomyopathy, and (2) molecular consequences of Pitx2c expression manipulation in cardiomyocytes in vitro. In pigs, the expression of Pitx2c, physiologically downregulated in the postnatal heart, is significantly re-activated in left ventricular (LV) failing myocardium which, in turn, is associated with increased expression of a restrictive set of Pitx2 target genes. Among these, Myf5 was identified as the top upregulated gene. In vitro, forced expression of Pitx2c in cardiomyocytes, but not in skeletal myoblasts, activates Myf5 in dose-dependent manner. In addition, we demonstrate that the level of Pitx2c is upregulated in the LV-myocardium of SHF patients.

CONCLUSIONS/SIGNIFICANCE

The results provide previously unrecognized evidence that Pitx2c is similarly reactivated in postnatal/adult heart at distinct HF phenotypes and suggest that Pitx2c is involved, directly or indirectly, in the regulation of Myf5 expression in cardiomyocytes.

Gene Networks during Skeletal Myogenesis

Mammalian skeletal muscles are derived from mesoderm segments flanking the embryonic midline. Upon receiving inductive cues from the adjacent neural tube, lateral plate mesoderm, and surface ectoderm, muscle precursors start to delaminate, migrate to their final destinations and proliferate. Muscle precursor cells become committed to the myogenic fate, become differentiated muscle cells, and fuse to form myofibers. Myofibers then fuse together to form the muscle groups. Muscle precursor cells have the ability to proliferate, and differentiate during development, while a subset remains capable of regeneration and repair of local injuries in adulthood. When the process of muscle development is perturbed such as in muscular dystrophies and injuries, ways to intervene and allow for proper muscle development or repair are the focus of regenerative medicine. Thus, understanding the developmental program of muscle at the genetic, cellular, and molecular levels has become a major focus of skeletal muscle regeneration research in the last few years.

Prediction of gene network models in limb muscle precursors

The ventrolateral dermomyotome gives rise to all muscles of the limbs through the delamination and migration of cells into the limb buds. These cells proliferate and form myoblasts, withdraw from the cell cycle and become terminally differentiated. The myogenic lineage colonizes pre-patterned regions to form muscle anlagen as muscle fibers are assembled. The regulatory mechanisms that control the later steps of this myogenic program are not well understood. The homeodomain transcription factor Pitx2 is expressed in the muscle lineage from the migration of precursors to adult muscle. Ablation of Pitx2 results in distortion, rather than loss, of limb muscle anlagen, suggesting that its function becomes critical during the colonization of, and/or fiber assembly in, the anlagen. Gene expression arrays were used to identify changes in gene expression in flow-sorted migratory muscle precursors, labeled by Lbx1(EGFP), which resulted from the loss of Pitx2. Target genes of Pitx2 were clustered using the "David Bioinformatics Functional Annotation Tool" to bin genes according to enrichment of gene ontology keywords. This provided a way to both narrow the target genes and identify potential gene families regulated by Pitx2. Representative target genes in the most enriched bins were analyzed for the presence and evolutionary conservation of Pitx2 consensus binding sequence, TAATCY, on the -20kb, intronic, and coding regions of the genes. Fifteen Pitx2 target genes were selected based on the above analysis and were identified as having functions involving cytoskeleton organization, tissue specification, and transcription factors. Data from these studies suggest that Pitx2 acts to regulate cell motility and expression of muscle specific genes in the muscle precursors during forelimb muscle development. This work provides a framework to develop the gene network leading to skeletal muscle development, growth and regeneration.

In vitro cell migration and invasion assays

Determining the migratory and invasive capacity of tumor and stromal cells and clarifying the underlying mechanisms is most relevant for novel strategies in cancer diagnosis, prognosis, drug development and treatment. Here we shortly summarize the different modes of cell travelling and review in vitro methods, which can be used to evaluate migration and invasion. We provide a concise summary of established migration/invasion assays described in the literature, list advantages, limitations and drawbacks, give a tabular overview for convenience and depict the basic principles of the assays graphically. In many cases particular research problems and specific cell types do not leave a choice for a broad variety of usable assays. However, for most standard applications using adherent cells, based on our experience we suggest to use exclusion zone assays to evaluate migration/invasion. We substantiate our choice by demonstrating that the advantages outbalance the drawbacks e.g. the simple setup, the easy readout, the kinetic analysis, the evaluation of cell morphology and the feasibility to perform the assay with standard laboratory equipment. Finally, innovative 3D migration and invasion models including heterotypic cell interactions are discussed. These methods recapitulate the in vivo situation most closely. Results obtained with these assays have already shed new light on cancer cell spreading and potentially will uncover unknown mechanisms.

Loss of abdominal muscle in Pitx2 mutants associated with altered axial specification of lateral plate mesoderm

Sequence specific transcription factors (SSTFs) combinatorially define cell types during development by forming recursively linked network kernels. Pitx2 expression begins during gastrulation, together with Hox genes, and becomes localized to the abdominal lateral plate mesoderm (LPM) before the onset of myogenesis in somites. The somatopleure of Pitx2 null embryos begins to grow abnormally outward before muscle regulatory factors (MRFs) or Pitx2 begin expression in the dermomyotome/myotome. Abdominal somites become deformed and stunted as they elongate into the mutant body wall, but maintain normal MRF expression domains. Subsequent loss of abdominal muscles is therefore not due to defects in specification, determination, or commitment of the myogenic lineage. Microarray analysis was used to identify SSTF families whose expression levels change in E10.5 interlimb body wall biopsies. All Hox9-11 paralogs had lower RNA levels in mutants, whereas genes expressed selectively in the hypaxial dermomyotome/myotome and sclerotome had higher RNA levels in mutants. In situ hybridization analyses indicate that Hox gene expression was reduced in parts of the LPM and intermediate mesoderm of mutants. Chromatin occupancy studies conducted on E10.5 interlimb body wall biopsies showed that Pitx2 protein occupied chromatin sites containing conserved bicoid core motifs in the vicinity of Hox 9-11 and MRF genes. Taken together, the data indicate that Pitx2 protein in LPM cells acts, presumably in combination with other SSTFs, to repress gene expression, that are normally expressed in physically adjoining cell types. Pitx2 thereby prevents cells in the interlimb LPM from adopting the stable network kernels that define sclerotomal, dermomyotomal, or myotomal mesenchymal cell types. This mechanism may be viewed either as lineage restriction or specification.