Umeå University's logo

Purpose: We have previously described a novel type of multiterminal en plaque motor endplates in the human extraocular muscles (EOMs). This study aimed to investigate whether multiterminal en plaque motor endplates are conserved in EOMs among vertebrates.

Methods: The motor endplates were identified with α-bungarotoxin (α-BTx) and antibodies against synaptic proteins and neurofilament in the EOMs of zebrafish, rabbits and mice. Transcriptomic data were re-analyzed to identify acetylcholine receptor (AChR) subunits in EOMs and trunk muscles of wild-type zebrafish at five and 20 months of age.

Results: In addition to the two typical types of single en plaque and multiple en grappe motor endplates, the third type of multiterminal en plaque motor endplates were observed in the EOMs of zebrafish, rabbits, and mice. The EOMs of zebrafish showed a significantly higher proportion of myofibers containing multiterminal en plaque motor endplates compared to EOMs of rabbits and mice. RNA sequencing data revealed significantly higher AChR subunits in the zebrafish EOMs compared to trunk muscles.

Conclusions: Multiterminal en plaque motor endplates are not exclusive to human EOMs but are also present in the EOMs of other vertebrate species, suggesting a conserved feature of the EOMs.

In muscular dystrophies, muscle fibers loose integrity and die, causing significant suffering and premature death. Strikingly, the extraocular muscles (EOMs) are spared, functioning well despite the disease progression. Although EOMs have been shown to differ from body musculature, the mechanisms underlying this inherent resistance to muscle dystrophies remain unknown. Here, we demonstrate important differences in gene expression as a response to muscle dystrophies between the EOMs and trunk muscles in zebrafish via transcriptomic profiling. We show that the LIM-protein Fhl2 is increased in response to the knockout of desmin, plectin and obscurin, cytoskeletal proteins whose knockout causes different muscle dystrophies, and contributes to disease protection of the EOMs. Moreover, we show that ectopic expression of fhl2b can partially rescue the muscle phenotype in the zebrafish Duchenne muscular dystrophy model sapje, significantly improving their survival. Therefore, Fhl2 is a protective agent and a candidate target gene for therapy of muscular dystrophies.

Purpose: The cytoskeleton of the extraocular muscles (EOMs) is significantly different from that of other muscles. We aimed to investigate the role of obscurin, a fundamental cytoskeletal protein, in the EOMs.

Methods: The distribution of obscurin in human and zebrafish EOMs was compared using immunohistochemistry. The two obscurin genes in zebrafish, obscna and obscnb, were knocked out using CRISPR/Cas9, and the EOMs were investigated using immunohistochemistry, qPCR, and in situ hybridization. The optokinetic reflex (OKR) in five-day-old larvae and adult obscna−/−;obscnb−/− and sibling control zebrafish was analyzed. Swimming distance was recorded at the same age.

Results: The obscurin distribution pattern was similar in human and zebrafish EOMs. The proportion of slow and fast myofibers was reduced in obscna−/−;obscnb−/− zebrafish EOMs but not in trunk muscle, whereas the number of myofibers containing cardiac myosin myh7 was significantly increased in EOMs of obscurin double mutants. Loss of obscurin resulted in less OKRs in zebrafish larvae but not in adult zebrafish.

Conclusions: Obscurin expression is conserved in normal human and zebrafish EOMs. Loss of obscurin induces a myofiber type shift in the EOMs, with upregulation of cardiac myosin heavy chain, myh7, showing an adaptation strategy in EOMs. Our model will facilitate further studies in conditions related to obscurin.

Muscular dystrophies (MDs) are caused by dysregulation of over 40 proteins but commonly share features of muscle weakness, myofiber death and regeneration, loss of ambulation and premature death. A MD involves a broken link anywhere in the connection from extracellular matrix through the sarcolemma to the sarcomere. Thus, any protein which is a part of this link causes MD if misfolded, dysregulated or absent. In MD, the most common causes of death are cardiac or respiratory failure, when the muscles involved in these processes fail. Although MDs affect 1:3500-5000 births worldwide there are currently no cures available. Extraocular muscles (EOMs) are strikingly not affected by MDs, however, the mechanisms behind this native resistance remain elusive. We have recently shown that the EOMs cytoskeleton differs significantly from that of other muscles and hypothesized that investigation of their cytoskeleton in MD models would provide important clues. Furthermore, we hypothesized that application of the EOMs strategies to trunk muscle tissue would decrease the detrimental impact of MD overall.

The zebrafish model system has recently increased vastly in popularity, and has quickly become a MD model. Due to its compatibility with the CRISPR/Cas9 method, genetic knockout studies can be utilized to generate novel mutant lines tailored to fit various aspects in studies of the zebrafish skeletal muscle. In this thesis I present nine new zebrafish lines which I used to study muscle biology processes, including muscle regeneration and the EOM cytoskeleton. 

Our results clearly demonstrate the need for understanding compensatory mechanisms in biology. Interestingly, pax3 and pax7 were shown to functionally compensate for each other both in appendicular muscle formation and in muscle regeneration, respectively, two processes where these individual genes have great impact in other organisms. This finding would also prove to be important in aiding our understanding of the EOM biology in adaptive strategies towards MDs. Furthermore, our results show that zebrafish EOMs are a good model to study cytoskeletal composition, as they share important features with human EOMs. Utilizing the CRISPR/Cas9 genome editing technique, I developed several knockout models of cytoskeletal proteins (desmin, obscurin, plectin) and studied their importance for the function of the EOMs.  In studies of zebrafish EOMs lacking obscurin, we found that EOMs functionally adapt their myosin composition over time via upregulation of myh7, a cardiac specific myosin. Furthermore, an RNA-sequencing screen on a CRISPR/Cas9 induced desminopathy model (desma; desmb double mutant) identified several protective genes of interest. We show that a four and a half LIM-domain protein (Fhl2) is upregulated in EOMs in several muscular dystrophy models and that fhl2b protects EOMs from excessive myonuclei turnover and hypertrophy. Furthermore, its ectopic expression in trunk muscle can also protect an additional muscle dystrophy model (dmd) from acute early death, improve myofiber function and stabilize neuromuscular junctions. Importantly, this protein was also detected in both human and mouse EOMs, indicating a potentially conserved role in the EOMs across species.

In summary, we identified several novel strategies of adaptation to disease progression in the EOMs. Together, these findings have contributed significantly to a better understanding of the EOMs and suggest new treatment strategies for MD that may have important future clinical applications.

Background: Migrating muscle progenitors delaminate from the somite and subsequently form muscle tissue in distant anatomical regions such as the paired appendages, or limbs. In amniotes, this process requires a signaling cascade including the transcription factor paired box 3 (Pax3).

Results: In this study, we found that, unlike in mammals, pax3a/3b double mutant zebrafish develop near to normal appendicular muscle. By analyzing numerous mutant combinations of pax3a, pax3b and pax7a, and pax7b, we determined that there is a feedback system and a compensatory mechanism between Pax3 and Pax7 in this developmental process, even though Pax7 alone is not required for appendicular myogenesis. pax3a/3b/7a/7b quadruple mutant developed muscle-less pectoral fins.

Conclusions: We found that Pax3 and Pax7 are redundantly required during appendicular myogenesis in zebrafish, where Pax7 is able to activate the same developmental programs as Pax3 in the premigratory progenitor cells.

Purpose: To study the medial rectus (MR) muscle of zebrafish (Danio rerio) with respect to the pattern of distribution of desmin and its correlation to distinct types of myofibers and motor endplates.

Methods: The MRs of zebrafish were examined using confocal microscopy in whole-mount longitudinal specimens and in cross sections processed for immunohistochemistry with antibodies against desmin, myosin heavy chain isoforms, and innervation markers. Desmin patterns were correlated to major myofiber type and type of innervation. A total of 1382 myofibers in nine MR muscles were analyzed.

Results: Four distinct desmin immunolabeling patterns were found in the zebrafish MRs. Approximately a third of all slow myofibers lacked desmin, representing 8.5% of the total myofiber population. The adult zebrafish MR muscle displayed en grappe, en plaque, and multiterminal en plaque neuromuscular junctions (NMJs) with intricate patterns of desmin immunolabeling.

Conclusions: The MRs of zebrafish showed important similarities with the human extraocular muscles with regard to the pattern of desmin distribution and presence of the major types of NMJs and can be regarded as an adequate model to further study the role of desmin and the implications of heterogeneity in cytoskeletal protein composition.

Translational Relevance: The establishment of a zebrafish model to study the cytoskeleton in muscles that are particularly resistant to muscle disease opens new avenues to understand human myopathies and muscle dystrophies and may provide clues to new therapies.

We constantly direct our eyes to the object of interest with the help of the extraocular muscles, andthereby use foveal fixation to attain the best possible visual acuity. The muscles around the eye arerather different from other skeletal muscles, being, for example, simultaneously the fastest musclesin the body and impossible to exhaust. The most exciting property of the extraocular muscles is theirunique response to disease, as they often remain unaffected in muscle conditions which lead tosevere handicap and premature death. Understanding the coping strategies that allow the extraocularmuscles to remain unaffected may provide clues for the future treatment of severe diseases such asmuscle dystrophies.

The hepatocyte growth factor receptor C-met plays an important role in cellular migration, which is crucial for many developmental processes as well as for cancer cell metastasis. Cmet has been linked to the development of mammalian appendicular muscle, which are derived from migrating muscle progenitor cells (MMPs) from within the somite. Mammalian limbs are homologous to the teleost pectoral and pelvic fins. In this study we used Crispr/Cas9 to mutate the zebrafish met gene and found that the MMP derived musculature of the paired appendages was severely affected. The mutation resulted in a reduced muscle fibre number, in particular in the pectoral abductor, and in a disturbed pectoral fin function. Other MMP derived muscles, such as the sternohyoid muscle and posterior hypaxial muscle were also affected in met mutants. This indicates that the role of met in MMP function and appendicular myogenesis is conserved within vertebrates.

The pigment pattern of many animal species is a result of the arrangement of different types of pigment-producing chromatophores. The zebrafish has three different types of chromatophores: black melanophores, yellow xanthophores, and shimmering iridophores arranged in a characteristic pattern of golden and blue horizontal stripes. In the zebrafish embryo, chromatophores derive from the neural crest cells. Using pax7a and pax7b zebrafish mutants, we identified a previously unknown requirement for Pax7 in xanthophore lineage formation. The absence of Pax7 results in a severe reduction of xanthophore precursor cells and a complete depletion of differentiated xanthophores in embryos as well as in adult zebrafish. In contrast, the melanophore lineage is increased in pax7a/pax7b double-mutant embryos and larvae, whereas juvenile and adult pax7a/pax7b double-mutant zebrafish display a severe decrease in melanophores and a pigment pattern disorganization indicative of a xanthophore-deficient phenotype. In summary, we propose a novel role for Pax7 in the early specification of chromatophore precursor cells.