An altered phenotype in a conditional knockout of Pitx2 in extraocular muscle

Extreme Tolerance of Extraocular Muscles to Diseases and Aging: Why and How?

The extraocular muscles (EOMs) possess unique characteristics that set them apart from other skeletal muscles. These muscles, responsible for eye movements, exhibit remarkable resistance to various muscular dystrophies and aging, presenting a significant contrast to the vulnerability of skeletal muscles to these conditions. In this review, we delve into the cellular and molecular underpinnings of the distinct properties of EOMs. We explore their structural complexity, highlighting differences in fiber types, innervation patterns, and developmental origins. Notably, EOM fibers express a diverse array of myosin heavy-chain isoforms, retaining embryonic forms into adulthood. Moreover, their motor innervation is characterized by a high ratio of nerve fibers to muscle fibers and the presence of unique neuromuscular junctions. These features contribute to the specialized functions of EOMs, including rapid and precise eye movements. Understanding the mechanisms behind the resilience of EOMs to disease and aging may offer insights into potential therapeutic strategies for treating muscular dystrophies and myopathies affecting other skeletal muscles.

Developmental, Physiological and Phylogenetic Perspectives on the Expression and Regulation of Myosin Heavy Chains in Craniofacial Muscles

This review deals with the developmental origins of extraocular, jaw and laryngeal muscles, the expression, regulation and functional significance of sarcomeric myosin heavy chains (MyHCs) that they express and changes in MyHC expression during phylogeny. Myogenic progenitors from the mesoderm in the prechordal plate and branchial arches specify craniofacial muscle allotypes with different repertoires for MyHC expression. To cope with very complex eye movements, extraocular muscles (EOMs) express 11 MyHCs, ranging from the superfast extraocular MyHC to the slowest, non-muscle MyHC IIB (nmMyH IIB). They have distinct global and orbital layers, singly- and multiply-innervated fibres, longitudinal MyHC variations, and palisade endings that mediate axon reflexes. Jaw-closing muscles express the high-force masticatory MyHC and cardiac or limb MyHCs depending on the appropriateness for the acquisition and mastication of food. Laryngeal muscles express extraocular and limb muscle MyHCs but shift toward expressing slower MyHCs in large animals. During postnatal development, MyHC expression of craniofacial muscles is subject to neural and hormonal modulation. The primary and secondary myotubes of developing EOMs are postulated to induce, via different retrogradely transported neurotrophins, the rich diversity of neural impulse patterns that regulate the specific MyHCs that they express. Thyroid hormone shifts MyHC 2A toward 2B in jaw muscles, laryngeal muscles and possibly extraocular muscles. This review highlights the fact that the pattern of myosin expression in mammalian craniofacial muscles is principally influenced by the complex interplay of cell lineages, neural impulse patterns, thyroid and other hormones, functional demands and body mass. In these respects, craniofacial muscles are similar to limb muscles, but they differ radically in the types of cell lineage and the nature of their functional demands.

TWIST1, a gene associated with Saethre-Chotzen syndrome, regulates extraocular muscle organization in mouse

Heterozygous loss of function mutations in TWIST1 cause Saethre-Chotzen syndrome, which is characterized by craniosynostosis, facial asymmetry, ptosis, strabismus, and distinctive ear appearance. Individuals with syndromic craniosynostosis have high rates of strabismus and ptosis, but the underlying pathology is unknown. Some individuals with syndromic craniosynostosis have been noted to have absence of individual extraocular muscles or abnormal insertions of the extraocular muscles on the globe. Using conditional knock-out alleles for Twist1 in cranial mesenchyme, we test the hypothesis that Twist1 is required for extraocular muscle organization and position, attachment to the globe, and/or innervation by the cranial nerves. We examined the extraocular muscles in conditional Twist1 knock-out animals using Twist2-cre and Pdgfrb-cre drivers. Both are expressed in cranial mesoderm and neural crest. Conditional inactivation of Twist1 using these drivers leads to disorganized extraocular muscles that cannot be reliably identified as specific muscles. Tendons do not form normally at the insertion and origin of these dysplastic muscles. Knock-out of Twist1 expression in tendon precursors, using scleraxis-cre, however, does not alter EOM organization. Furthermore, developing motor neurons, which do not express Twist1, display abnormal axonal trajectories in the orbit in the presence of dysplastic extraocular muscles. Strabismus in individuals with TWIST1 mutations may therefore be caused by abnormalities in extraocular muscle development and secondary abnormalities in innervation and tendon formation.

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.

Myosin heavy chains in extraocular muscle fibres: Distribution, regulation and function

This review examines kinetic properties and distribution of the 11 isoforms of myosin heavy chain (MyHC) expressed in extraocular muscle (EOM) fibre types and the regulation and function of these MyHCs. Although recruitment and discharge characteristics of ocular motoneurons during fixation and eye movements are well documented, work directly linking these properties with motor unit contractile speed and MyHC composition is lacking. Recruitment of motor units according to Henneman's size principle has some support in EOMs but needs consolidation. Both neurogenic and myogenic mechanisms regulate MyHC expression as in other muscle allotypes. Developmentally, multiply-innervated (MIFs) and singly-innervated fibres (SIFs) are derived presumably from distinct myoblast lineages, ending up expressing MyHCs in the slow and fast ends of the kinetic spectrum respectively. They modulate the synaptic inputs of their motoneurons through different retrogradely transported neurotrophins, thereby specifying their tonic and phasic impulse patterns. Immunohistochemical analyses of EOMs regenerating in situ and in limb muscle beds suggest that the very impulse patterns driving various ocular movements equip effectors with appropriate MyHC compositions and speeds to accomplish their tasks. These experiments also suggest that satellite cells of SIFs and MIFs are distinct lineages expressing different MyHCs during regeneration. MyHC compositions and functional characteristics of orbital fibres show longitudinal variations that facilitate linear ocular rotation during saccades. Palisade endings on global MIFs are postulated to respond to active and passive tensions by triggering axon reflexes that play important roles during fixation, saccades and vergence. How EOMs implement Listings law during ocular rotation is discussed.

Mouse Extraocular Muscles and the Musculotopic Organization of Their Innervation

The organization of extraocular muscles (EOMs) and their motor nuclei was investigated in the mouse due to the increased importance of this model for oculomotor research. Mice showed a standard EOM organization pattern, although their eyes are set at the side of the head. They do have more prominent oblique muscles, whose insertion points differ from those of frontal-eyed species. Retrograde tracers revealed that the motoneuron layout aligns with the general vertebrate plan with respect to nuclei and laterality. The mouse departed in some significant respects from previously studied species. First, more overlap between the distributions of muscle-specific motoneuronal pools was present in the oculomotor nucleus (III). Furthermore, motoneuron dendrites for each pool filled the entire III and extended beyond the edge of the abducens nucleus (VI). This suggests mouse extraocular motoneuron afferents must target specific pools based on features other than dendritic distribution and nuclear borders. Second, abducens internuclear neurons are located outside the VI. We concluded this because no unlabeled abducens internuclear neurons were observed following lateral rectus muscle injections and because retrograde tracer injections into the III labeled cells immediately ventral and ventrolateral to the VI, not within it. This may provide an anatomical substrate for differential input to motoneurons and internuclear neurons that allows rodents to move their eyes more independently. Finally, while soma size measurements suggested motoneuron subpopulations supplying multiply and singly innervated muscle fibers are present, markers for neurofilaments and perineuronal nets indicated overlap in the size distributions of the two populations. Anat Rec, 302:1865-1885, 2019. © 2019 American Association for Anatomy.

The ancient sarcomeric myosins found in specialized muscles

Striated muscles express an array of sarcomeric myosin motors that are tuned to accomplish specific tasks. Each myosin isoform found in muscle fibers confers unique contractile properties to the fiber in order to meet the demands of the muscle. The sarcomeric myosin heavy chain (MYH) genes expressed in the major cardiac and skeletal muscles have been studied for decades. However, three ancient myosins, MYH7b, MYH15, and MYH16, remained uncharacterized due to their unique expression patterns in common mammalian model organisms and due to their relatively recent discovery in these genomes. This article reviews the literature surrounding these three ancient sarcomeric myosins and the specialized muscles in which they are expressed. Further study of these ancient myosins and how they contribute to the functions of the specialized muscles may provide novel insight into the history of striated muscle evolution.

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.

Effects of retinoic acid signaling on extraocular muscle myogenic precursor cells in vitro

One major difference between limb and extraocular muscles (EOM) is the presence of an enriched population of Pitx2-positive myogenic precursor cells in EOM compared to limb muscle. We hypothesize that retinoic acid regulates Pitx2 expression in EOM myogenic precursor cells and that its effects would differ in leg muscle. The two muscle groups expressed differential retinoic acid receptor (RAR) and retinoid X receptor (RXR) levels. RXR co-localized with the Pitx2-positive cells but not with those expressing Pax7. EOM-derived and LEG-derived EECD34 cells were treated with vehicle, retinoic acid, the RXR agonist bexarotene, the RAR inverse agonist BMS493, or the RXR antagonist UVI 3003. In vitro, fewer EOM-derived EECD34 cells expressed desmin and fused, while more LEG-derived cells expressed desmin and fused when treated with retinoic acid compared to vehicle. Both EOM and LEG-derived EECD34 cells exposed to retinoic acid showed a higher percentage of cells expressing Pitx2 compared to vehicle, supporting the hypothesis that retinoic acid plays a role in maintaining Pitx2 expression. We hypothesize that retinoic acid signaling aids in the maintenance of large numbers of undifferentiated myogenic precursor cells in the EOM, which would be required to maintain EOM normalcy throughout a lifetime of myonuclear turnover.

Extraocular Muscle Repair and Regeneration

PURPOSE OF REVIEW

The goal of this review is to summarize the unique regenerative milieu within mature mammalian extraocular muscles (EOMs). This will aid in understanding disease propensity for and sparing of EOMs in skeletal muscle diseases as well as the recalcitrance of the EOM to injury.

RECENT FINDINGS

The EOMs continually remodel throughout life and contain an extremely enriched number of myogenic precursor cells that differ in number and functional characteristics from those in limb skeletal muscle. The EOMs also contain a large population of Pitx2-positive myogenic precursor cells that provide the EOMs with many of their unusual biological characteristics, such as myofiber remodeling and skeletal muscle disease sparing. This environment provides for rapid and efficient remodeling and regeneration after various types of injury. In addition, the EOMs show a remarkable ability to respond to perturbations of single muscles with coordinated changes in the other EOMs that move in the same plane.

SUMMARY

These data will inform Ophthalmologists as they work toward developing new treatments for eye movement disorders, new approaches for repair after nerve or direct EOMs injury, as well as suggest potential explanations for the unusual disease propensity and disease sparing characteristics of human EOM.

Sparing of the extraocular muscles in mdx mice with absent or reduced utrophin expression: A life span analysis

Sparing of the extraocular muscles in muscular dystrophy is controversial. To address the potential role of utrophin in this sparing, mdx:utrophin(+/-) and mdx:utrophin(-/-) mice were examined for changes in myofiber size, central nucleation, and Pax7-positive and MyoD-positive cell density at intervals over their life span. Known to be spared in the mdx mouse, and contrary to previous reports, the extraocular muscles from both the mdx:utrophin(+/-) and mdx:utrophin(-/-) mice were also morphologically spared. In the mdx:utrophin(+/)(-) mice, which have a normal life span compared to the mdx:utrophin(-/-) mice, the myofibers were larger at 3 and 12 months than the wild type age-matched eye muscles. While there was a significant increase in central nucleation in the extraocular muscles from all mdx:utrophin(+/)(-) mice, the levels were still very low compared to age-matched limb skeletal muscles. Pax7- and MyoD-positive myogenic precursor cell populations were retained and were similar to age-matched wild type controls. These results support the hypothesis that utrophin is not involved in extraocular muscle sparing in these genotypes. In addition, it appears that these muscles retain the myogenic precursors that would allow them to maintain their regenerative capacity and normal morphology over a lifetime even in these more severe models of muscular dystrophy.

Muscle stem cells contribute to myofibres in sedentary adult mice

Skeletal muscle is essential for mobility, stability, and whole body metabolism, and muscle loss, for instance during sarcopenia, has profound consequences. Satellite cells (muscle stem cells) have been hypothesized, but not yet demonstrated, to contribute to muscle homeostasis and a decline in their contribution to myofiber homeostasis to play a part in sarcopenia. To test their role in muscle maintenance, we genetically labeled and ablated satellite cells in adult sedentary mice. We demonstrate via genetic lineage experiments that even in the absence of injury, satellite cells contribute to myofibers in all adult muscles, although the extent and timing differs. However, genetic ablation experiments showed that satellite cells are not globally required to maintain myofiber cross-sectional area of uninjured adult muscle.

Susceptibility of murine induced pluripotent stem cell-derived cardiomyocytes to hypoxia and nutrient deprivation

Introduction

Induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs) may be suitable for myocardial repair. While their functional and structural properties have been extensively investigated, their response to ischemia-like conditions has not yet been clearly defined.

Methods

iPS-CMs were differentiated and enriched from murine induced pluripotent stem cells expressing enhanced green fluorescent protein (eGFP) and puromycin resistance genes under the control of an α-myosin heavy chain (α-MHC) promoter. iPS-CMs maturity and function were characterized by microscopy, real-time PCR, calcium transient recordings, electrophysiology, and mitochondrial function assays, and compared to those from neonatal murine cardiomyocytes. iPS-CMs as well as neonatal murine cardiomyocytes were exposed for 3 hours to hypoxia (1% O₂) and glucose/serum deprivation, and viability, apoptosis markers, reactive oxygen species, mitochondrial membrane potential and intracellular stress signaling cascades were investigated. Then, the iPS-CMs response to mesenchymal stromal cell-conditioned medium was determined.

Results

iPS-CMs displayed key morphological and functional properties that were comparable to those of neonatal cardiomyocytes, but several parameters indicated an earlier iPS-CMs maturation stage. During hypoxia and glucose/serum deprivation, iPS-CMs exhibited a significantly higher proportion of poly-caspase-active, 7-aminoactinomycin D-positive and TUNEL-positive cells than neonatal cardiomyocytes. The average mitochondrial membrane potential was reduced in “ischemic” iPS-CMs but remained unchanged in neonatal cardiomyocytes; reactive oxygen species production was only increased in “ischemic” iPS-CMs, and oxidoreductase activity in iPS-CMs dropped more rapidly than in neonatal cardiomyocytes. In iPS-CMs, hypoxia and glucose/serum deprivation led to upregulation of Hsp70 transcripts and decreased STAT3 phosphorylation and total PKCε protein expression. Treatment with mesenchymal stromal cell-conditioned medium preserved oxidoreductase activity and restored pSTAT3 and PKCε levels.

Conclusion

iPS-CMs appear to be particularly sensitive to hypoxia and nutrient deprivation. Counteracting the ischemic susceptibility of iPS-CMs with mesenchymal stromal cell-conditioned medium may help enhance their survival and efficacy in cell-based approaches for myocardial repair.

Human cardiac extracellular matrix supports myocardial lineage commitment of pluripotent stem cells

OBJECTIVES

Cross-talk between organ-specific extracellular matrix (ECM) and stem cells is often assumed but has not been directly demonstrated. We developed a protocol for the preparation of human cardiac ECM (cECM) and studied whether cECM has effects on pluripotent stem cell differentiation that may be useful for future cardiac regeneration strategies in patients with end-stage heart failure.

METHODS

Of note, 0.3 mm-thick cECM slices were prepared from samples of myocardium from patients with end-stage non-ischaemic dilated cardiomyopathy, using a three-step protocol involving hypotonic lysis buffer, sodium dodecyl sulphate (SDS) and foetal bovine serum (FBS). Murine embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs) and mesenchymal stromal cells (MSCs) were seeded and grown in standard culture, on cECM or on non-specific ECM preparations (Matrigel® or Geltrex®). Cell attachment, apoptosis induction (Caspase 3/7 activity) and metabolic activity (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium conversion) were followed. Transcriptional activation of genes involved in pluripotency; early and late myocardial development; and endothelial, ectodermal or endodermal commitment were monitored by quantitative real-time polymerase chain reaction (rtPCR). Protein expression of selected markers was confirmed by immunohistology.

RESULTS

cECM supported the proliferation of ESCs and iPSCs, and Caspase 3/7 activity was significantly lower compared with standard culture. Cardiac lineage commitment was favoured when ESCs or iPSCs were grown on cECM, as evidenced by the significantly increased mRNA expression of cardiac alpha myosin heavy polypeptide 6 (Myh6), cardiac troponin T2 (Tnnt2) and NK2 homeobox 5 (Nkx2.5) as well as positive immunohistology for cardiac troponin T and heavy-chain cardiac myosin protein. In contrast, Matrigel or Geltrex did not induce cardiac-specific markers. MSCs showed no evidence of cardiomyocyte differentiation.

CONCLUSIONS

Human cardiac ECM seems to direct differentiation of pluripotent stem cells towards a cardiomyocyte phenotype. This phenomenon supports the use of cardiac ECM preparations for guided stem cell differentiation and myocardial repair, and may ultimately increase the therapeutic efficacy of cell therapy in heart failure patients.

Pitx2, an atrial fibrillation predisposition gene, directly regulates ion transport and intercalated disc genes

BACKGROUND

Pitx2 is the homeobox gene located in proximity to the human 4q25 familial atrial fibrillation (AF) locus. When deleted in the mouse germline, Pitx2 haploinsufficiency predisposes to pacing-induced AF, indicating that reduced Pitx2 promotes an arrhythmogenic substrate. Previous work focused on Pitx2 developmental functions that predispose to AF. Although Pitx2 is expressed in postnatal left atrium, it is unknown whether Pitx2 has distinct postnatal and developmental functions.

METHODS AND RESULTS

To investigate Pitx2 postnatal function, we conditionally inactivated Pitx2 in the postnatal atrium while leaving its developmental function intact. Unstressed adult Pitx2 homozygous mutant mice display variable R-R interval with diminished P-wave amplitude characteristic of sinus node dysfunction, an AF risk factor in human patients. An integrated genomics approach in the adult heart revealed Pitx2 target genes encoding cell junction proteins, ion channels, and critical transcriptional regulators. Importantly, many Pitx2 target genes have been implicated in human AF by genome-wide association studies. Immunofluorescence and transmission electron microscopy studies in adult Pitx2 mutant mice revealed structural remodeling of the intercalated disc characteristic of human patients with AF.

CONCLUSIONS

Our findings, revealing that Pitx2 has genetically separable postnatal and developmental functions, unveil direct Pitx2 target genes that include channel and calcium handling genes, as well as genes that stabilize the intercalated disc in postnatal atrium.

Dystrophic changes in extraocular muscles after gamma irradiation in mdx:utrophin(+/-) mice

Extraocular muscles (EOM) have a strikingly different disease profile than limb skeletal muscles. It has long been known that they are spared in Duchenne (DMD) and other forms of muscular dystrophy. Despite many studies, the cause for this sparing is not understood. We have proposed that differences in myogenic precursor cell properties in EOM maintain normal morphology over the lifetime of individuals with DMD due to either greater proliferative potential or greater resistance to injury. This hypothesis was tested by exposing wild type and mdx:utrophin(+/-) (het) mouse EOM and limb skeletal muscles to 18 Gy gamma irradiation, a dose known to inhibit satellite cell proliferation in limb muscles. As expected, over time het limb skeletal muscles displayed reduced central nucleation mirrored by a reduction in Pax7-positive cells, demonstrating a significant loss in regenerative potential. In contrast, in the first month post-irradiation in the het EOM, myofiber cross-sectional areas first decreased, then increased, but ultimately returned to normal compared to non-irradiated het EOM. Central nucleation significantly increased in the first post-irradiation month, resembling the dystrophic limb phenotype. This correlated with decreased EECD34 stem cells and a concomitant increase and subsequent return to normalcy of both Pax7 and Pitx2-positive cell density. By two months, normal het EOM morphology returned. It appears that irradiation disrupts the normal method of EOM remodeling, which react paradoxically to produce increased numbers of myogenic precursor cells. This suggests that the EOM contain myogenic precursor cell types resistant to 18 Gy gamma irradiation, allowing return to normal morphology 2 months post-irradiation. This supports our hypothesis that ongoing proliferation of specialized regenerative populations in the het EOM actively maintains normal EOM morphology in DMD. Ongoing studies are working to define the differences in the myogenic precursor cells in EOM as well as the cellular milieu in which they reside.

The role of Pitx2 in maintaining the phenotype of myogenic precursor cells in the extraocular muscles

Many differences exist between extraocular muscles (EOM) and non-cranial skeletal muscles. One striking difference is the sparing of EOM in various muscular dystrophies compared to non-cranial skeletal muscles. EOM undergo continuous myonuclear remodeling in normal, uninjured adults, and distinct transcription factors are required for the early determination, development, and maintenance of EOM compared to limb skeletal muscle. Pitx2, a bicoid-like homeobox transcription factor, is required for the development of EOM and the maintenance of characteristic properties of the adult EOM phenotype, but is not required for the development of limb muscle. We hypothesize that these unique properties of EOM contribute to the constitutive differences between EOM and non-craniofacial skeletal muscles. Using flow cytometry, CD34(+)/Sca1(-/)CD45(-/)CD31(-) cells (EECD34 cells) were isolated from extraocular and limb skeletal muscle and in vitro, EOM EECD34 cells proliferated faster than limb muscle EECD34 cells. To further define these myogenic precursor cells from EOM and limb skeletal muscle, they were analyzed for their expression of Pitx2. Western blotting and immunohistochemical data demonstrated that EOM express higher levels of Pitx2 than limb muscle, and 80% of the EECD34 cells expressed Pitx2. siRNA knockdown of Pitx2 expression in EECD34 cells in vitro decreased proliferation rates and impaired the ability of EECD34 cells to fuse into multinucleated myotubes. High levels of Pitx2 were retained in dystrophic and aging mouse EOM and the EOM EECD34 cells compared to limb muscle. The differential expression of Pitx2 between EOM and limb skeletal muscle along with the functional changes in response to lower levels of Pitx2 expression in the myogenic precursor cells suggest a role for Pitx2 in the maintenance of constitutive differences between EOM and limb skeletal muscle that may contribute to the sparing of EOM in muscular dystrophies.

Pitx genes are redeployed in adult myogenesis where they can act to promote myogenic differentiation in muscle satellite cells

Skeletal muscle retains a resident stem cell population called satellite cells. Although mitotically quiescent in mature muscle, satellite cells can be activated to produce myoblast progeny to generate myonuclei for skeletal muscle homoeostasis, hypertrophy and repair. Regulation of satellite cell function in adult requires redeployment of many of the regulatory networks fundamental to developmental myogenesis. Involved in such control of muscle stem cell fate in embryos are members of the Pitx gene family of bicoid-class homeodomain proteins. Here, we investigated the expression and function of all three Pitx genes in muscle satellite cells of adult mice. Endogenous Pitx1 was undetectable, whilst Pitx2a, Pitx2b and Pitx2c were at low levels in proliferating satellite cells, but increased during the early stages of myogenic differentiation. By contrast, proliferating satellite cells expressed robust amounts of Pitx3, with levels then decreasing as cells differentiated, although Pitx3 remained expressed in unfused myoblasts. To examine the role of Pitx genes in satellite cell function, retroviral-mediated expression of Pitx1, all Pitx2 isoforms or Pitx3, was used. Constitutive expression of any Pitx isoform suppressed satellite cell proliferation, with the cells undergoing enhanced myogenic differentiation. Conversely, myogenic differentiation into multinucleated myotubes was decreased when Pitx2 or Pitx3 levels were reduced using siRNA. Together, our results show that Pitx genes play a role in regulating satellite cell function during myogenesis in adult.

Dynamics of abducens nucleus neurons in the awake mouse

The mechanics of the eyeball and orbital tissues (the "ocular motor plant") are a fundamental determinant of ocular motor signal processing. The mouse is used increasingly in ocular motor physiology, but little is known about its plant mechanics. One way to characterize the mechanics is to determine relationships between extraocular motoneuron firing and eye movement. We recorded abducens nucleus neurons in mice executing compensatory eye movements during 0.1- to 1.6-Hz oscillation in the light. We analyzed firing rates to extract eye position and eye velocity sensitivities, from which we determined time constants of a viscoelastic model of the plant. The majority of abducens neurons were already active with the eye in its central rest position, with only 6% recruited at more abducted positions. Firing rates exhibited largely linear relationships to eye movement, although there was a nonlinearity consisting of increasing modulation in proportion to eye movement as eye amplitudes became small (due to reduced stimulus amplitude or reduced alertness). Eye position and velocity sensitivities changed with stimulus frequency as expected for an ocular motor plant dominated by cascaded viscoelasticities. Transfer function poles lay at approximately 0.1 and 0.9 s. Compared with previously studied animal species, the mouse plant is stiffer than the rabbit but laxer than cat and rhesus. Differences between mouse and rabbit can be explained by scaling for eye size (allometry). Differences between the mouse and cat or rhesus can be explained by differing ocular motor repertoires of animals with and without a fovea or area centralis.

Yeast two-hybrid analysis of a human trabecular meshwork cDNA library identified EFEMP2 as a novel PITX2 interacting protein

PURPOSE

Mutations in the homeobox transcription factor paired-like homeodomain transcription factor 2 (PITX2) cause Axenfeld-Reiger syndrome (ARS), which is associated with anterior segment dysgenesis (ASD) and glaucoma. To understand ARS pathogenesis, it is essential to know the normal functions of PITX2 and the proteins with which PITX2 interacts in the eye. Therefore, we used a unique cDNA library that we created from human trabecular meshwork (TM) primary cells to discover PITX2-interacting proteins (PIPs).

METHODS

A human TM cDNA library was created from primary cells in the ProQuest Two-Hybrid prey vector: pEXP-AD502. Human PITX2A and PITX2C isoforms were used independently as "bait" to identify novel PIPs. A total of 1.25×10⁶ clones were screened by yeast two-hybrid (Y2H) analyses. PIPs obtained from each Y2H experiment were confirmed by yeast retransformation and mammalian co-immunoprecipitation assays.

RESULTS

EGF-containing fibulin-like extracellular matrix protein 2 (EFEMP2) was identified by both PITX2A and PITX2C isoforms as a novel PIP from Y2H analyses. EFEMP2 is 443 amino acids long with six epidermal growth factor (EGF)-like modules and one fibulin-like module. The PITX2-interaction domain in EFEMP2 lies between the second EGF-like module and the COOH-terminal fibulin-like module. Co-immunoprecipitation assays in COS-7 cells confirmed the interaction between PITX2 and EFEMP2.

CONCLUSIONS

We discovered EFEMP2 as a novel PITX2-interacting protein. Further, our cDNA library made from human TM primary cells is a unique and effective resource to identify novel interacting proteins for glaucoma and ASD candidates. This resource could be used both for discovery and validation of interactomes identified from in silico analysis.

Genomic profiling reveals Pitx2 controls expression of mature extraocular muscle contraction-related genes

PURPOSE

To assess the influence of the Pitx2 transcription factor on the global gene expression profile of extraocular muscle (EOM) of mice.

METHODS

Mice with a conditional knockout of Pitx2, designated Pitx2(Δflox/Δflox) and their control littermates Pitx2(flox/flox), were used. RNA was isolated from EOM obtained at 3, 6, and 12 weeks of age and processed for microarray-based profiling. Pairwise comparisons were performed between mice of the same age and differentially expressed gene lists were generated. Select genes from the profile were validated using real-time quantitative polymerase chain reaction and protein immunoblot. Ultrastructural analysis was performed to evaluate EOM sarcomeric structure.

RESULTS

The number of differentially expressed genes was relatively small. Eleven upregulated and 23 downregulated transcripts were identified common to all three age groups in the Pitx2-deficient extraocular muscle compared with littermate controls. These fell into a range of categories including muscle-specific structural genes, transcription factors, and ion channels. The differentially expressed genes were primarily related to muscle contraction. We verified by protein and ultrastructural analysis that myomesin 2 was expressed in the Pitx2-deficient mice, and this was associated with development of M lines evident in their orbital region.

CONCLUSIONS

The global transcript expression analysis uncovered that Pitx2 primarily regulates a relatively select number of genes associated with muscle contraction. Pitx2 loss led to the development of M line structures, a feature more typical of other skeletal muscle.

The anatomical identification of saccadic omnipause neurons in the rat brainstem

Omnipause neurons (OPNs) represent a crucial component for the generation of saccadic eye movements. They inhibit saccadic premotor neurons in the paramedian pontine reticular formation (PPRF) as well as in the rostral interstitial nucleus of the medial longitudinal fascicle (RIMLF) during the intersaccadic interval. In turn, inhibition of OPNs is a prerequisite in order to generate saccadic eye movements. Although the anatomy of the saccadic system including the OPNs has been extensively studied in primates and cats, no detailed anatomical description of these neurons in rats has been performed so far. The aim of the present study was the identification of putative OPNs in the rat brainstem based on their projection target, localization, and histochemical characteristics. Stereotactic tract-tracer injections into the rostral mesencephalon including the RIMLF in rat resulted in back-labeling of a neuron group adjacent to the midline at the level of traversing fibers of the abducens nerve, which are considered as OPNs lying in the nucleus raphe interpositus. Combined immunohistochemical staining for various markers revealed in these neurons the expression of parvalbumin, chondroitin sulfate proteoglycan, and glycine, but a lack of serotonin. The results of our study demonstrate the striking similarity between individual elements of the premotor saccadic network in rats and primates. The exact knowledge of their location in rats provides a basis for in vitro studies of the OPNs in rat brainstem slices.

A zebrafish model of axenfeld-rieger syndrome reveals that pitx2 regulation by retinoic acid is essential for ocular and craniofacial development

PURPOSE

The homeobox transcription factor PITX2 is a known regulator of mammalian ocular development, and human PITX2 mutations are associated with Axenfeld-Rieger syndrome (ARS). However, the treatment of patients with ARS remains mostly supportive and palliative.

METHODS

The authors used molecular genetic, pharmacologic, and embryologic techniques to study the biology of ARS in a zebrafish model that uses transgenes to mark neural crest and muscle cells in the head.

RESULTS

The authors demonstrated in vivo that pitx2 is a key downstream target of retinoic acid (RA) in craniofacial development, and this pathway is required for coordinating neural crest, mesoderm, and ocular development. pitx2a knockdown using morpholino oligonucleotides disrupts jaw and pharyngeal arch formation and recapitulates ocular characteristics of ARS, including corneal and iris stroma maldevelopment. These phenotypes could be rescued with human PITX2A mRNA, demonstrating the specificity of the knockdown and evolutionary conservation of pitx2a function. Expression of the ARS dominant negative human PITX2A K50E allele also caused ARS-like phenotypes. Similarly, inhibition of RA synthesis in the developing eye (genetic or pharmacologic) disrupted craniofacial and ocular development, and human PITX2A mRNA partially rescued these defects.

CONCLUSIONS

RA regulation of pitx2 is essential for coordinating interactions among neural crest, mesoderm, and developing eye. The marked evolutionary conservation of Pitx2 function in eye and craniofacial development makes zebrafish a potentially powerful model of ARS, amenable to in vivo experimentation and development of potential therapies.

Barx homeobox family in muscle development and regeneration

Homeobox transcription factors are key intrinsic regulators of myogenesis. In studies spanning several years, we have characterized the homeobox factor Barx2 as a novel marker for muscle progenitor cells and an important regulator of muscle growth and repair. In this review, we place the expression and function of Barx2 and its paralogue Barx1 in context with other muscle-expressed homeobox factors in both embryonic and adult myogenesis. We also describe the structure and regulation of Barx genes and possible gene/disease associations. The functional domains of Barx proteins, their molecular interactions, and cellular functions are presented with particular emphasis on control of genes and processes involved in myogenic differentiation. Finally, we describe the patterns of Barx gene expression in vivo and the phenotypes of various Barx gene perturbation models including null mice. We focus on the Barx2 null mouse model, which has demonstrated the critical roles of Barx2 in postnatal myogenesis including muscle maintenance during aging, and regeneration of acute and chronic muscle injury.

PITX2 is involved in stress response in cultured human trabecular meshwork cells through regulation of SLC13A3

PURPOSE

Mutations of the PITX2 gene cause Axenfeld-Rieger syndrome (ARS) and glaucoma. In this study, the authors investigated genes directly regulated by the PITX2 transcription factor to gain insight into the mechanisms underlying these disorders.

METHODS

RNA from nonpigmented ciliary epithelium cells transfected with hormone-inducible PITX2 and activated by mifepristone was subjected to microarray analyses. Data were analyzed using dCHIP algorithms to detect significant differences in expression. Genes with significantly altered expression in multiple microarray experiments in the presence of activated PITX2 were subjected to in silico and biochemical analyses to validate them as direct regulatory targets. One target gene was further characterized by studying the effect of its knockdown in a cell model of oxidative stress, and its expression in zebrafish embryos was analyzed by in situ hybridization.

RESULTS

Solute carrier family 13 sodium-dependent dicarboxylate transporter member 3 (SLC13A3) was identified as 1 of 47 potential PITX2 target genes in ocular cells. PITX2 directly regulates SLC13A3 expression, as demonstrated by luciferase reporter and chromatin immunoprecipitation assays. Reduction of PITX2 or SLC13A3 levels by small interfering RNA (siRNA)-mediated knockdown augmented the death of transformed human trabecular meshwork cells exposed to hydrogen peroxide. Zebrafish slc13a3 is expressed in anterior ocular regions in a pattern similar to that of pitx2.

CONCLUSIONS

The results indicate that SLC13A3 is a direct downstream target of PITX2 transcriptional regulation and that levels of PITX2 and SLC13A3 modulate responses to oxidative stress in ocular cells.

Pitx2 regulates myosin heavy chain isoform expression and multi-innervation in extraocular muscle

Extraocular muscle is fundamentally distinct from other skeletal muscle and demonstrates specific anatomical divisions, unique innervation, diverse myosin isoform expression patterns, a distinct genomic profile and differential involvement in neuromuscular disorders. The paired-like homeodomain transcription factor 2 (Pitx2) is known to regulate the formation of extraocular muscle development and in this report we show that its expression in adulthood also defines certain extraocular muscle traits. We found that expression of slow-MyHC and slow-tonic MyHC, along with contractile regulatory proteins troponin I and troponin T, is reduced during the first 3 weeks after birth in mice with conditional knockout of Pitx2, designated Pitx2(Δflox/Δflox). En grappe endplates, which are normally only found on slow-MyHC expressing fibres, were not identified in the Pitx2(Δflox/Δflox) extraocular muscle, suggesting that altered innervation was responsible for the loss in slow-MyHC expression. Extraocular muscle (EOM)-specific MyHC expressing fibres were dramatically reduced at P14 and rarely detected at 3 months in the Pitx2(Δflox/Δflox) mice. 2A-MyHC fibres, which are excluded from mid-belly region in wild-type mice, dominated the orbital layer with no apparent longitudinal variation in the Pitx2(Δflox/Δflox) mice. Pure 2X-MyHC fibres, only present at distal ends in the wild-type mice, populated the outer global layer in the mid-belly region of the Pitx2(Δflox/Δflox) mice. Pitx2 influences slow-MyHC, slow-tonic MyHC and EOM-MyHC expression in extraocular muscle and its absence leads to increased expression of 2X-MyHC and 2A-MyHC. Precise definition of the regulation of MyHC isoforms in extraocular muscle may allow their rational manipulation, in order to alter muscle contractility for therapeutic purposes.

Identification of four SNPs and association analysis with meat quality traits in the porcine Pitx2c gene

The association of the porcine Pitx2c gene with meat quality traits was investigated in the present study. A total of eight single nucleotide polymorphisms (SNPs) were found. Allele frequencies of four SNPs were further detected in four commercial breeds and eight Chinese indigenous breeds. Single SNP and meat quality associations were analyzed in a Yorkshire×Meishan F(2) population. The SNPs c.474C>T (P<0.01) and c.636C>T (P<0.05) showed a significant association with meat color (MCV1). The SNPs c.*37G>A and c.*47G>A were significantly associated with drip loss rate (DLR), water holding capacity (WHC) and meat color value (MCV1) consistently (P<0.05). Linkage disequilibrium (LD) analysis revealed that the adjacent SNPs were in LD. Two major haplotypes were identified, and association analysis between haplotype combinations and meat quality indicated that the presence of two copies of haplotype 1 -CCGG- may improve meat quality.

Pitx2 is an upstream activator of extraocular myogenesis and survival

The transcription factors required to initiate myogenesis in branchial arch- and somite-derived muscles are known, but the comparable upstream factors required during extraocular muscle development have not been identified. We show Pax7 is dispensable for extraocular muscle formation, whereas Pitx2 is cell-autonomously required to prevent apoptosis of the extraocular muscle primordia. The survival requirement for Pitx2 is stage-dependent and ends following stable activation of genes for the muscle regulatory factors (e.g. Myf5, MyoD), which is reduced in the absence of Pitx2. Further, PITX2 binds and activates transcription of the Myf5 and MyoD promoters, indicating these genes are direct targets. Collectively, these data demonstrate that PITX2 is required at several steps in the development of extraocular muscles, acting first as an anti-apoptotic factor in pre-myogenic mesoderm, and subsequently to activate the myogenic program in these cells. Thus, Pitx2 is the first demonstrated upstream activator of myogenesis in the extraocular muscles.

Myosin heavy chain expression in mouse extraocular muscle: more complex than expected

PURPOSE

To characterize the expression patterns of myosin heavy chain (MyHC) isoforms in mouse extraocular muscles (EOMs) during postnatal development.

METHODS

MyHC isoform expression in mouse EOMs from postnatal day (P)0 to 3 months was evaluated by quantitative polymerase chair reaction (qPCR) and immunohistochemistry. The longitudinal and cross-sectional distribution of each MyHC isoform and coexpression of certain isoforms in single muscle fibers was determined by single, double, and triple immunohistochemistry.

RESULTS

MyHC isoform expression in postnatal EOMs followed the developmental rules observed in other skeletal muscles; however, important exceptions were found. First, developmental isoforms were retained in the orbital layer of the adult EOMs. Second, expression of emb-MyHC, neo-MyHC, and 2A-MyHC was restricted to the orbital layer and that of 2B-MyHC to the global layer. Third, although slow-MyHC and 2B-MyHC did not exhibit obvious longitudinal variations, emb-MyHC, neo-MyHC, and 2A-MyHC were more abundant distally and were excluded from the innervational zone, whereas eom-MyHC complemented their expression and was more abundant in the mid-belly region in both the orbital and global layers. Fourth, coexpression of MyHC isoforms in single global layer fibers was rare, but it was common among the orbital layer fibers.

CONCLUSIONS

MyHC isoforms have complex expression patterns, exhibiting not only longitudinal and cross-sectional variation of each isoform, but also of coexpression in single fibers. The highly heterogeneous MyHC expression reflects the complex contractile profiles of EOMs, which in turn are a function of the requirements of eye movements, which range from extremely fast saccades to sustained position, each with a need for precise coordination of each eye.