Single-cell transcriptional profiles in human skeletal muscle.

Skeletal muscle is a heterogeneous tissue comprised of muscle fiber and mononuclear cell types that, in addition to movement, influences immunity, metabolism and cognition. We investigated the gene expression patterns of skeletal muscle cells using RNA-seq of subtype-pooled single human muscle fibers and single cell RNA-seq of mononuclear cells from human vastus lateralis, mouse quadriceps, and mouse diaphragm. We identified 11 human skeletal muscle mononuclear cell types, including two fibro-adipogenic progenitor (FAP) cell subtypes. The human FBN1+ FAP cell subtype is novel and a corresponding FBN1+ FAP cell type was also found in single cell RNA-seq analysis in mouse. Transcriptome exercise studies using bulk tissue analysis do not resolve changes in individual cell-type proportion or gene expression. The cell-type gene signatures provide the means to use computational methods to identify cell-type level changes in bulk studies. As an example, we analyzed public transcriptome data from an exercise training study and revealed significant changes in specific mononuclear cell-type proportions related to age, sex, acute exercise and training. Our single-cell expression map of skeletal muscle cell types will further the understanding of the diverse effects of exercise and the pathophysiology of muscle disease.

Noninvasive ultrasound imaging for assessment of intact microstructure of extracellular matrix in tissue engineering.

Development of artificial tissues or organs is one of the actual tasks in regenerative medicine that requires observation and evaluation of intact volume microstructure of tissue engineering products at all stages of their formation, from native donor tissues and decellularized scaffolds to recipient cell migration in the matrix. Unfortunately in practice, methods of vital noninvasive imaging of volume microstructure in matrixes are absent. In this work, we propose a new approach based on high-frequency acoustic microscopy for noninvasive evaluation and visualization of volume microstructure in tissue engineering products. The results present the ultrasound characterization of native rat diaphragms and lungs and their decellularized scaffolds. Verification of the method for visualization of tissue formation in the matrix volume was described in the model samples of diaphragm scaffolds with stepwise collagenization. Results demonstrate acoustic microscopic sensitivity to cell content concentration, variation in local density, and orientation of protein fibers in the volume, micron air inclusions, and other inhomogeneities of matrixes.

Comparative Transcriptomic Analysis Identifies a Range of Immunologically Related Functional Elaborations of Lymph Node Associated Lymphatic and Blood Endothelial Cells.

Lymphatic and blood vessels are formed by specialized lymphatic endothelial cells (LEC) and blood endothelial cells (BEC), respectively. These endothelial populations not only form peripheral tissue vessels, but also critical supporting structures in secondary lymphoid organs, particularly the lymph node (LN). Lymph node LEC (LN-LEC) also have been shown to have important immunological functions that are not observed in LEC from tissue lymphatics. LN-LEC can maintain peripheral tolerance through direct presentation of self-antigen via MHC-I, leading to CD8 T cell deletion; and through transfer of self-antigen to dendritic cells for presentation via MHC-II, resulting in CD4 T cell anergy. LN-LEC also can capture and archive foreign antigens, transferring them to dendritic cells for maintenance of memory CD8 T cells. The molecular basis for these functional elaborations in LN-LEC remain largely unexplored, and it is also unclear whether blood endothelial cells in LN (LN-BEC) might express similar enhanced immunologic functionality. Here, we used RNA-Seq to compare the transcriptomic profiles of freshly isolated murine LEC and BEC from LN with one another and with freshly isolated LEC from the periphery (diaphragm). We show that LN-LEC, LN-BEC, and diaphragm LEC (D-LEC) are transcriptionally distinct from one another, demonstrating both lineage and tissue-specific functional specializations. Surprisingly, tissue microenvironment differences in gene expression profiles were more numerous than those determined by endothelial cell lineage specification. In this regard, both LN-localized endothelial cell populations show a variety of functional elaborations that suggest how they may function as antigen presenting cells, and also point to as yet unexplored roles in both positive and negative regulation of innate and adaptive immune responses. The present work has defined in depth gene expression differences that point to functional specializations of endothelial cell populations in different anatomical locations, but especially the LN. Beyond the analyses provided here, these data are a resource for future work to uncover mechanisms of endothelial cell functionality.

Generation of a Functioning and Self-Renewing Diaphragmatic Muscle Construct.

Surgical repair of large muscular defects requires the use of autologous graft transfer or prosthetic material. Naturally derived matrices are biocompatible materials obtained by tissue decellularization and are commonly used in clinical practice. Despite promising applications described in the literature, the use of acellular matrices to repair large defects has been only partially successful, highlighting the need for more efficient constructs. Scaffold recellularization by means of tissue engineering may improve not only the structure of the matrix, but also its ability to functionally interact with the host. The development of such a complex construct is challenging, due to the complexity of the native organ architecture and the difficulties in recreating the cellular niche with both proliferative and differentiating potential during growth or after damage. In this study, we tested a mouse decellularized diaphragmatic extracellular matrix (ECM) previously described by our group, for the generation of a cellular skeletal muscle construct with functional features. The decellularized matrix was stored using different conditions to mimic the off-the-shelf clinical need. Pediatric human muscle precursors were seeded into the decellularized scaffold, demonstrating proliferation and differentiation capability, giving rise to a functioning three-dimensional skeletal muscle structure. Furthermore, we exposed the engineered construct to cardiotoxin injury and demonstrated its ability to activate a regenerative response in vitro promoting cell self-renewal and a positive ECM remodeling. Functional reconstruction of an engineered skeletal muscle with maintenance of a stem cell pool makes this a promising tool toward future clinical applications in diaphragmatic regeneration. Stem Cells Translational Medicine 2019;8:858&869.

Morphological Evaluation of the Tissue Reaction to Subcutaneous Implantation of Decellularized Matrices.

Based on the data of morphological analysis, we performed histological evaluation of rat tissue reaction to subcutaneous implantation of decellularized matrices of intrathoracic organs and tissues. Cell composition of the inflammatory infiltrate was analyzed, and the dynamics of macrophage and T and B lymphocyte content was assessed on days 7 and 14 of the experiment. It was found that the reaction to implantation depended not only on the quality of decellularization and efficiency of removal of antigen molecules, but also on the original histological structure and quality of preimplantation processing of the transplant.

Differential effects of spinal motor neuron-derived and skeletal muscle-derived Rspo2 on acetylcholine receptor clustering at the neuromuscular junction.

We recently reported that R-spondin 2 (Rspo2), a secreted activator of Wnt/ß-catenin signaling, promotes acetylcholine receptor (AChR) clustering and neuromuscular junction (NMJ) formation via its receptor, Lgr5. Rspo2 is expressed highly in spinal motor neurons (SMNs) and marginally in the skeletal muscle, but the origin of Rspo2 at the NMJ remains elusive. We rescued Rspo2-deficient (Rspo2-/-) mice by specifically expressing Rspo2 in the skeletal muscle and SMNs. SMN-specific Rspo2 mitigated or over-corrected abnormal features of the NMJs and AChR clusters observed in Rspo2-/- mice including (i) abnormal broadening of enlarged AChR clusters, (ii) three of six abnormal ultrastructural features, and (iii) abnormal expression of nine genes in SMNs and the diaphragm. In contrast, muscle-specific Rspo2 normalized all six abnormal ultrastructural features, but it had no effect on AChR clustering and NMJ formation at the light microscopy level or on abnormal gene expression in SMNs and the diaphragm. These results suggest that SMN-derived Rspo2 plays a major role in AChR clustering and NMJ formation in the postsynaptic region, and muscle-derived Rspo2 also plays a substantial role in juxtaposition of the active zones and synaptic folds.

Decellularized Diaphragmatic Muscle Drives a Constructive Angiogenic Response In Vivo.

Skeletal muscle tissue engineering (TE) aims to efficiently repair large congenital and acquired defects. Biological acellular scaffolds are considered a good tool for TE, as decellularization allows structural preservation of tissue extracellular matrix (ECM) and conservation of its unique cytokine reservoir and the ability to support angiogenesis, cell viability, and proliferation. This represents a major advantage compared to synthetic scaffolds, which can acquire these features only after modification and show limited biocompatibility. In this work, we describe the ability of a skeletal muscle acellular scaffold to promote vascularization both ex vivo and in vivo. Specifically, chicken chorioallantoic membrane assay and protein array confirmed the presence of pro-angiogenic molecules in the decellularized tissue such as HGF, VEGF, and SDF-1α. The acellular muscle was implanted in BL6/J mice both subcutaneously and ortotopically. In the first condition, the ECM-derived scaffold appeared vascularized 7 days post-implantation. When the decellularized diaphragm was ortotopically applied, newly formed blood vessels containing CD31⁺, αSMA⁺, and vWF⁺ cells were visible inside the scaffold. Systemic injection of Evans Blue proved function and perfusion of the new vessels, underlying a tissue-regenerative activation. On the contrary, the implantation of a synthetic matrix made of polytetrafluoroethylene used as control was only surrounded by vWF⁺ cells, with no cell migration inside the scaffold and clear foreign body reaction (giant cells were visible). The molecular profile and the analysis of macrophages confirmed the tendency of the synthetic scaffold to enhance inflammation instead of regeneration. In conclusion, we identified the angiogenic potential of a skeletal muscle-derived acellular scaffold and the pro-regenerative environment activated in vivo, showing clear evidence that the decellularized diaphragm is a suitable candidate for skeletal muscle tissue engineering and regeneration.

In vitro effects of Crotalus atrox snake venom on chick and mouse neuromuscular preparations.

The neuromuscular effect of venoms is not a major clinical manifestation shared between rattlesnakes native to the Americas, which showed two different venom phenotypes. Taking into account this dichotomy, nerve muscle preparations from mice and chicks were used to investigate the ability of Crotalus atrox venom to induce in vitro neurotoxicity and myotoxicity. Unlike crotalic venoms of South America, low concentrations of C. atrox venom did not result in significant effects on mouse neuromuscular preparations. The venom was more active on avian nerve-muscle, showing reduction of twitch heights after 120 min of incubation with 10, 30 and 100 µg/mL of venom with diminished responses to agonists and KCl. Histological analysis highlighted that C. atrox was myotoxic in both species of experimental animals; as evidenced by degenerative events, including edematous cells, delta lesions, hypercontracted fibers and muscle necrosis, which can lead to neurotoxic action. These results provide key insights into the myotoxicity and low neurotoxicity of C. atrox in two animal models, corroborating with previous genomic and proteomic findings and would be useful for a deeper understanding of venom evolution in snakes belonging to the genus Crotalus.

Regeneration of diaphragm with bio-3D cellular patch.

Neonates with congenital diaphragmatic hernia often require surgical defect closure with a patch. Alternatives to native diaphragmatic tissue are critically needed for this paediatric surgery. The clinical efficacy of mesh patches is limited by complications associated with residual foreign material and by hernia recurrence. In this study, we used a novel bio-3D printer method to generate large scaffold-free tissue patches composed of human cells. The resulting large tissue constructs had high elasticity and strength. Cellular patches were transplanted into rats with surgically created diaphragmatic defects. Rats survived for over 710 days after implantation of tissue constructs. CT confirmed complete tissue integration of the grafts during rat growth. Histology revealed regeneration of muscle structure, neovascularization, and neuronal networks within the reconstructed diaphragms. Our results demonstrate that created cellular patches are a highly safe and effective therapeutic strategy for repairing diaphragmatic defects, and thus pave the way for a clinical trial.

Glucocorticoids Improve Myogenic Differentiation In Vitro by Suppressing the Synthesis of Versican, a Transitional Matrix Protein Overexpressed in Dystrophic Skeletal Muscles.

In Duchenne muscular dystrophy (DMD), a dysregulated extracellular matrix (ECM) directly exacerbates pathology. Glucocorticoids are beneficial therapeutics in DMD, and have pleiotropic effects on the composition and processing of ECM proteins in other biological contexts. The synthesis and remodelling of a transitional versican-rich matrix is necessary for myogenesis; whether glucocorticoids modulate this transitional matrix is not known. Here, versican expression and processing were examined in hindlimb and diaphragm muscles from mdx dystrophin-deficient mice and C57BL/10 wild type mice. V0/V1 versican (Vcan) mRNA transcripts and protein levels were upregulated in dystrophic compared to wild type muscles, especially in the more severely affected mdx diaphragm. Processed versican (versikine) was detected in wild type and dystrophic muscles, and immunoreactivity was highly associated with newly regenerated myofibres. Glucocorticoids enhanced C2C12 myoblast fusion by modulating the expression of genes regulating transitional matrix synthesis and processing. Specifically, Tgfß1, Vcan and hyaluronan synthase-2 (Has2) mRNA transcripts were decreased by 50% and Adamts1 mRNA transcripts were increased three-fold by glucocorticoid treatment. The addition of exogenous versican impaired myoblast fusion, whilst glucocorticoids alleviated this inhibition in fusion. In dystrophic mdx muscles, versican upregulation correlated with pathology. We propose that versican is a novel and relevant target gene in DMD, given its suppression by glucocorticoids and that in excess it impairs myoblast fusion, a process key for muscle regeneration.

Depletion of Pax7+ satellite cells does not affect diaphragm adaptations to running in young or aged mice.

KEY POINTS: Satellite cell depletion does not affect diaphragm adaptations to voluntary wheel running in young or aged mice. Satellite cell depletion early in life (4 months of age) has minimal effect on diaphragm phenotype by old age (24 months). Prolonged satellite cell depletion in the diaphragm does not result in excessive extracellular matrix accumulation, in contrast to what has been reported in hind limb muscles. Up-regulation of Pax3 mRNA+ cells after satellite cell depletion in young and aged mice suggests that Pax3+ cells may compensate for a loss of Pax7+ satellite cells in the diaphragm. Future investigations should focus on the role of Pax3+ cells in the diaphragm during adaptation to exercise and ageing. ABSTRACT: Satellite cell contribution to unstressed diaphragm is higher compared to hind limb muscles, which is probably attributable to constant activation of this muscle to drive ventilation. Whether satellite cell depletion negatively impacts diaphragm quantitative and qualitative characteristics under stressed conditions in young and aged mice is unknown. We therefore challenged the diaphragm with prolonged running activity in the presence and absence of Pax7+ satellite cells in young and aged mice using an inducible Pax7CreER -R26RDTA model. Mice were vehicle (Veh, satellite cell-replete) or tamoxifen (Tam, satellite cell-depleted) treated at 4 months of age and were then allowed to run voluntarily at 6 months (young) and 22 months (aged). Age-matched, cage-dwelling, Veh- and Tam-treated mice without wheel access served as activity controls. Diaphragm muscles were analysed from young (8 months) and aged (24 months) mice. Satellite cell depletion did not alter diaphragm mean fibre cross-sectional area, fibre type distribution or extracellular matrix content in young or aged mice, regardless of running activity. Resting in vivo diaphragm function was also unaffected by satellite cell depletion. Myonuclear density was maintained in young satellite cell-depleted mice regardless of running, although it was modestly reduced in aged sedentary (-7%) and running (-19%) mice without satellite cells (P < 0.05). Using fluorescence in situ hybridization, we detected higher Pax3 mRNA+ cell density in both young and aged satellite cell-depleted diaphragm muscle (P < 0.05), which may compensate for the loss of Pax7+ satellite cells.

Size and Proportions of Slow-Twitch and Fast-Twitch Muscle Fibers in Human Costal Diaphragm.

Smaller diaphragmatic motor unit potentials (MUPs) compared to MUPs of limb muscles lead to the hypothesis that diaphragmatic muscle fibers, being the generators of MUPs, might be also smaller. We compared autopsy samples of costal diaphragm and vastus lateralis of healthy men with respect to fibers' size and expression of slow myosin heavy chain isoform (MyHC-1) and fast 2A isoform (MyHC-2A). Diaphragmatic fibers were smaller than fibers in vastus lateralis with regard to the mean minimal fiber diameter of slow-twitch (46.8 versus 72.2 µm, p < 0.001), fast-twitch (45.1 versus 62.4 µm, p < 0.001), and hybrid fibers (47.3 versus 65.0 µm, p < 0.01) as well as to the mean fiber cross-sectional areas of slow-twitch (2376.0 versus 5455.9 µm2, p < 0.001), fast-twitch (2258.7 versus 4189.7 µm2, p < 0.001), and hybrid fibers (2404.4 versus 4776.3 µm2, p < 0.01). The numerical proportion of slow-twitch fibers was higher (50.2 versus 36.3%, p < 0.01) in costal diaphragm and the numerical proportion of fast-twitch fibers (47.2 versus 58.7%, p < 0.01) was lower. The numerical proportion of hybrid fibers did not differ. Muscle fibers of costal diaphragm have specific characteristics which support increased resistance of diaphragm to fatigue.

A novel approach for targeted delivery to motoneurons using cholera toxin-B modified protocells.

BACKGROUND: Trophic interactions between muscle fibers and motoneurons at the neuromuscular junction (NMJ) play a critical role in determining motor function throughout development, ageing, injury, or disease. Treatment of neuromuscular disorders is hindered by the inability to selectively target motoneurons with pharmacological and genetic interventions. NEW METHOD: We describe a novel delivery system to motoneurons using mesoporous silica nanoparticles encapsulated within a lipid bilayer (protocells) and modified with the atoxic subunit B of the cholera toxin (CTB) that binds to gangliosides present on neuronal membranes. RESULTS: CTB modified protocells showed significantly greater motoneuron uptake compared to unmodified protocells after 24h of treatment (60% vs. 15%, respectively). CTB-protocells showed specific uptake by motoneurons compared to muscle cells and demonstrated cargo release of a surrogate drug. Protocells showed a lack of cytotoxicity and unimpaired cellular proliferation. In isolated diaphragm muscle-phrenic nerve preparations, preferential axon terminal uptake of CTB-modified protocells was observed compared to uptake in surrounding muscle tissue. A larger proportion of axon terminals displayed uptake following treatment with CTB-protocells compared to unmodified protocells (40% vs. 6%, respectively). COMPARISON WITH EXISTING METHOD(S): Current motoneuron targeting strategies lack the functionality to load and deliver multiple cargos. CTB-protocells capitalizes on the advantages of liposomes and mesoporous silica nanoparticles allowing a large loading capacity and cargo release. The ability of CTB-protocells to target motoneurons at the NMJ confers a great advantage over existing methods. CONCLUSIONS: CTB-protocells constitute a viable targeted motoneuron delivery system for drugs and genes facilitating various therapies for neuromuscular diseases.

Regulation of lymphangiogenesis in the diaphragm by macrophages and VEGFR-3 signaling.

Lymphatic vessels play important roles in fluid drainage and in immune responses, as well as in pathological processes including cancer progression and inflammation. While the molecular regulation of the earliest lymphatic vessel differentiation and development has been investigated in much detail, less is known about the control and timing of lymphatic vessel maturation in different organs, which often occurs postnatally. We investigated the time course of lymphatic vessel development on the pleural side of the diaphragmatic muscle in mice, the so-called submesothelial initial diaphragmatic lymphatic plexus. We found that this lymphatic network develops largely after birth and that it can serve as a reliable and easily quantifiable model to study physiological lymphangiogenesis in vivo. Lymphangiogenic growth in this tissue was highly dependent on vascular endothelial growth factor receptor (VEGFR)-3 signaling, whereas VEGFR-1 and -2 signaling was dispensable. During diaphragm development, macrophages appeared first in a linearly arranged pattern, followed by ingrowth of lymphatic vessels along these patterned lines. Surprisingly, ablation of macrophages in colony-stimulating factor-1 receptor (Csf1r)-deficient mice and by treatment with a CSF-1R-blocking antibody did not inhibit the general lymphatic vessel development in the diaphragm but specifically promoted branch formation of lymphatic sprouts. In agreement with these findings, incubation of cultured lymphatic endothelial cells with conditioned medium from P7 diaphragmatic macrophages significantly reduced LEC sprouting. These results indicate that the postnatal diaphragm provides a suitable model for studies of physiological lymphangiogenic growth and maturation, and for the identification of modulators of lymphatic vessel growth.

Diaphragmatic lymphatic vessel behavior during local skeletal muscle contraction.

The mechanism through which the stresses developed in the diaphragmatic tissue during skeletal muscle contraction sustain local lymphatic function was studied in 10 deeply anesthetized, tracheotomized adult Wistar rats whose diaphragm was exposed after thoracotomy. To evaluate the direct effect of skeletal muscle contraction on the hydraulic intraluminal lymphatic pressures (Plymph) and lymphatic vessel geometry, the maximal contraction of diaphragmatic fibers adjacent to a lymphatic vessel was elicited by injection of 9.2 nl of 1 M KCl solution among diaphragmatic fibers while Plymph was recorded through micropuncture and vessel geometry via stereomicroscopy video recording. In lymphatics oriented perpendicularly to the longitudinal axis of muscle fibers and located at <300 µm from KCl injection, vessel diameter at maximal skeletal muscle contraction (Dmc) decreased to 61.3 ± 1.4% of the precontraction value [resting diameter (Drest)]; however, if injection was at >900 µm from the vessel, Dmc enlarged to 131.1 ± 2.3% of Drest. In vessels parallel to muscle fibers, Dmc increased to 122.8 ± 2.9% of Drest. During contraction, Plymph decreased as much as 22.5 ± 2.6 cmH2O in all submesothelial superficial vessels, whereas it increased by 10.7 ± 5.1 cmH2O in deeper vessels running perpendicular to contracting muscle fibers. Hence, the three-dimensional arrangement of the diaphragmatic lymphatic network seems to be finalized to efficiently exploit the stresses exerted by muscle fibers during the contracting inspiratory phase to promote lymph formation in superficial submesothelial lymphatics and its further propulsion in deeper intramuscular vessels.

Presynaptic localization of Smn and hnRNP R in axon terminals of embryonic and postnatal mouse motoneurons.

Spinal muscular atrophy (SMA) is caused by deficiency of the ubiquitously expressed survival motoneuron (SMN) protein. SMN is crucial component of a complex for the assembly of spliceosomal small nuclear ribonucleoprotein (snRNP) particles. Other cellular functions of SMN are less characterized so far. SMA predominantly affects lower motoneurons, but the cellular basis for this relative specificity is still unknown. In contrast to nonneuronal cells where the protein is mainly localized in perinuclear regions and the nucleus, Smn is also present in dendrites, axons and axonal growth cones of isolated motoneurons in vitro. However, this distribution has not been shown in vivo and it is not clear whether Smn and hnRNP R are also present in presynaptic axon terminals of motoneurons in postnatal mice. Smn also associates with components not included in the classical SMN complex like RNA-binding proteins FUS, TDP43, HuD and hnRNP R which are involved in RNA processing, subcellular localization and translation. We show here that Smn and hnRNP R are present in presynaptic compartments at neuromuscular endplates of embryonic and postnatal mice. Smn and hnRNP R are localized in close proximity to each other in axons and axon terminals both in vitro and in vivo. We also provide new evidence for a direct interaction of Smn and hnRNP R in vitro and in vivo, particularly in the cytosol of motoneurons. These data point to functions of SMN beyond snRNP assembly which could be crucial for recruitment and transport of RNA particles into axons and axon terminals, a mechanism which may contribute to SMA pathogenesis.

Lubricating recovery of damaged pleural mesothelium: effect of time and of phosphatidylcholines.

Effect of time and phosphatidylcholines (PCs) on lubrication of damaged mesothelium has been investigated. Marked increase in coefficient of kinetic friction (µ) of pleural specimens after mesothelial blotting and rewetting decreased by 23.4±3.5%, 41.8±3.8%, and 40.5±2.7% after 30min, 1h, and 2h. Hence, damaged mesothelium is able to partially reset lubricating molecules on its surface. Increase in µ of post-blotting Ringer 2h after addition of unsaturated PCs (3mg/ml) decreased a little more than after 2h Ringer. Effects of unsaturated and saturated PCs were similar, contrary to expectation raised by their different percentage in pleural and alveolar lavage. Effect of PCs did not increase at 6mg/ml, and was nil at 0.4mg/ml. Increase of µ after short phospholipase treatment decreased by 45.9±2.0% after 2h Ringer, and a little more after addition of unsaturated or saturated PCs. Hence, PCs, as other phospholipids, have a small effect, likely because of difficulty in resetting their relationships with main lubricating molecules.

The FgfrL1 receptor is required for development of slow muscle fibers.

FgfrL1, which interacts with Fgf ligands and heparin, is a member of the fibroblast growth factor receptor (Fgfr) family. FgfrL1-deficient mice show two significant alterations when compared to wildtype mice: They die at birth due to a malformed diaphragm and they lack metanephric kidneys. Utilizing gene arrays, qPCR and in situ hybridization we show here that the diaphragm of FgfrL1 knockout animals lacks any slow muscle fibers at E18.5 as indicated by the absence of slow fiber markers Myh7, Myl2 and Myl3. Similar lesions are also found in other skeletal muscles that contain a high proportion of slow fibers at birth, such as the extraocular muscles. In contrast to the slow fibers, fast fibers do not appear to be affected as shown by expression of fast fiber markers Myh3, Myh8, Myl1 and MylPF. At early developmental stages (E10.5, E15.5), FgfrL1-deficient animals express slow fiber genes at normal levels. The loss of slow fibers cannot be attributed to the lack of kidneys, since Wnt4 knockout mice, which also lack metanephric kidneys, show normal expression of Myh7, Myl2 and Myl3. Thus, FgfrL1 is specifically required for embryonic development of slow muscle fibers.

Differential role of APP and APLPs for neuromuscular synaptic morphology and function.

The analysis of mouse models indicated that APP and the related APLPs are important for synapse formation and function. The synaptic role of APP is, however, complex due to partially overlapping functions within the gene family. APP/APLPs are proteolytically cleaved and have both adhesive and signaling properties. Mice lacking individual APP family members are viable, whereas APP/APLP2 and APLP1/APLP2 double knockout (DKO) mice die shortly after birth. Here, we analyzed the morphology of the neuromuscular junction (NMJ) of lethal APLP1/APLP2-DKO mice in comparison to lethal APP/APLP2-DKO mutants and viable single KO mice. We report that, surprisingly, the NMJ phenotype of APLP1/APLP2-DKO mice shows striking differences as compared to APP/APLP2-DKO mice. Unexpectedly, APLP1/APLP2-DKO mice exhibit normal endplate patterning and lack presynaptic nerve terminal sprouting. However, at the level of individual synapses we show that APLP1/APLP2-DKO mice exhibit reduced size of pre- and postsynaptic compartments and reduced colocalization. As APP/APLP2-DKO and APLP1/APLP2-DKO mice show similar penetrance of early postnatal lethality, this suggests that deficits at the level of individual synapses due to impaired synaptic apposition and/or deficits in transmitter release may cause lethality. Using an in vitro cell-adhesion assay, we observed that APP trans-dimerization is considerably less efficient than APLP2 trans-interaction. Thus, differences between APP/APLP2 and APP/APLP1 NMJ formation may be in part explained by differences in APP/APLP2 trans-dimerization properties. Collectively, our study further highlights the distinct and essential role of APLP2 at NMJ synapses that cannot be compensated by APP.