Muscle-specific kinase (MuSK) autoantibodies suppress the MuSK pathway and ACh receptor retention at the mouse neuromuscular junction.

Muscle-specific kinase (MuSK) autoantibodies from myasthenia gravis patients can block the activation of MuSK in vitro and/or reduce the postsynaptic localization of MuSK. Here we use a mouse model to examine the effects of MuSK autoantibodies upon some key components of the postsynaptic MuSK pathway and upon the regulation of junctional ACh receptor (AChR) numbers. Mice became weak after 14 daily injections of anti-MuSK-positive patient IgG. The intensity and area of AChR staining at the motor endplate was markedly reduced. Pulse-labelling of AChRs revealed an accelerated loss of pre-existing AChRs from postsynaptic AChR clusters without a compensatory increase in incorporation of (newly synthesized) replacement AChRs. Large, postsynaptic AChR clusters were replaced by a constellation of tiny AChR microaggregates. Puncta of AChR staining also appeared in the cytoplasm beneath the endplate. Endplate staining for MuSK, activated Src, rapsyn and AChR were all reduced in intensity. In the tibialis anterior muscle there was also evidence that phosphorylation of the AChR ß-subunit-Y390 was reduced at endplates. In contrast, endplate staining for ß-dystroglycan (through which rapsyn couples AChR to the synaptic basement membrane) remained intense. The results suggest that anti-MuSK IgG suppresses the endplate density of MuSK, thereby down-regulating MuSK signalling activity and the retention of junctional AChRs locally within the postsynaptic membrane scaffold.

Drug-induced thrombocytopenia: development of a novel NOD/SCID mouse model to evaluate clearance of circulating platelets by drug-dependent antibodies and the efficacy of IVIG.

Drug-induced immune thrombocytopenia (DITP) is an adverse drug effect mediated by drug-dependent antibodies. Intravenous immunoglobulin (IVIG) is frequently used to treat DITP and primary immune thrombocytopenia (ITP). Despite IVIG's proven beneficial effects in ITP, its efficacy in DITP is unclear. We have established a nonobese diabetic/severe combined immunodeficient (NOD/SCID) mouse model of DITP in which human platelets survive for more than 24 hours, allowing platelet clearance by DITP/ITP antibodies to be studied. Rapid human platelet clearance was uniformly observed with all quinine-induced thrombocytopenia (QITP) patient sera studied (mean platelet lifespans: QITP 1.5 ± 0.3 hours vs controls 16.5 ± 4.3 hours), consistent with the clinical presentation of DITP. In contrast, clearance rates with ITP antibodies were more variable. IVIG treatment partially prevented platelet clearance by DITP and ITP antibodies. Our results suggest that the NOD/SCID mouse model is useful for investigating the efficacy of current and future DITP therapies, an area in which there is little experimental evidence to guide treatment.

Polysaccharides from the root of Angelica sinensis promotes hematopoiesis and thrombopoiesis through the PI3K/AKT pathway.

BACKGROUND: Dozens of Traditional Chinese Medicine (TCM) formulas have been used for promotion of "blood production" for centuries, and we are interested in developing novel thrombopoietic medicines from these TCMs. Our previous studies have demonstrated the hematopoietic effects of DangGui BuXue Tong (DBT), a formula composed of Radix Angelicae Sinensis and Radix Astragali in animal and cellular models. As a step further to identify and characterize the active chemical components of DBT, we tested the hematopoietic and particularly, thrombopoietic effects of polysaccharide-enriched fractions from the root of Radix Angelicae Sinensis (APS) in this study. METHODS: A myelosuppression mouse model was treated with APS (10 mg/kg/day). Peripheral blood cells from APS, thrombopoietin and vehicle-treated samples were then counted at different time-points. Using the colony-forming unit (CFU) assays, we determined the effects of APS on the proliferation and differentiation of hematopoietic stem/progenitor cells and megakaryocytic lineages. Using a megakaryocytic cell line M-07e as model, we analyzed the cellular apoptosis progression with and without APS treatment by Annexin V, Mitochondrial Membrane Potential and Caspase 3 assays. Last, the anti-apoptotic effect of APS on cells treated with Ly294002, a Phosphatidylinositol 3-Kinse inhibitor (PI3K) was also tested. RESULTS: In animal models, APS significantly enhanced not only the recovery of platelets, other blood cells and their progenitor cells, but also the formation of Colony Forming Unit (CFU). In M-07e cells, we observed the anti-apoptotic effect of APS. Treatment by Ly294002 alone increased the percentage of cells undergoing apoptosis. However, addition of APS to Ly294002-treated cells significantly reduced the percentage of cells undergoing apoptosis. CONCLUSIONS: APS promotes hematopoiesis and thrombopoiesis in the mouse model. This effect likely resulted from the anti-apoptosis activity of APS and is likely to involve the PI3K/AKT pathway.

Differentiation of CD45­/CD31+ lung side population cells into endothelial and smooth muscle cells in vitro.

Side population (SP) cells are a small subpopulation of cells found in many mammalian tissues and organs, identified by their capacity to efflux Hoechst 33342 dye. They are enriched for stem/progenitor cell activity. SP cells isolated from the adult mouse lung can be separated into a CD45+ subset (bone marrow­derived) and a CD45­ subset that can be subdivided into CD31­ and CD31+ subpopulations. CD45­/CD31­ lung SP (LSP) cells are known to be mesenchymal stem cells. However, CD45­/CD31+ LSP cells are not fully characterized. In the present study, it was found that CD45­/CD31+ LSP cells were able to form colonies. Based on the expression of vascular endothelial growth factor receptor 2 (VEGFR2), these cells were separated into VEGFR2­ and VEGFR2+ cells. The CD45­/CD31+/VEGFR2­ LSP cells expressed genes characteristic of smooth muscle and endothelial progenitors, and were able to differentiate into smooth muscle and endothelial cells in vitro. The CD45­/CD31+/VEGFR2+ LSP cells expressed genes characteristic of endothelial progenitors and gave rise to endothelial cells, although not smooth muscle, in vitro. The data demonstrate that CD45­/CD31+/VEGFR2­ LSP cells differentiated into CD45­/CD31+/VEGFR2+ LSP cells and then endothelial cells, indicating that CD45­/CD31+/VEGFR2+ LSP cells are likely to be derived from CD45­/CD31+/VEGFR2­ LSP cells. Taken together, the results suggest that CD45­/CD31+ LSP cells can be separated into CD45­/CD31+/VEGFR2­ LSP cells, which may be progenitors of endothelial and smooth muscle, whereas CD45­/CD31+/VEGFR2+ LSP cells may serve as late commitment endothelial progenitors in the adult mouse lung.

The mouse passive-transfer model of MuSK myasthenia gravis: disrupted MuSK signaling causes synapse failure.

While the majority of myasthenia gravis patients express antibodies targeting the acetylcholine receptor, the second most common cohort instead displays autoantibodies against muscle-specific kinase (MuSK). MuSK is a transmembrane tyrosine kinase found in the postsynaptic membrane of the neuromuscular junction. During development, MuSK serves as a signaling hub, coordinating the alignment of the pre- and postsynaptic components of the synapse. Adult mice that received repeated daily injections of IgG from anti-MuSK+ myasthenia gravis patients developed muscle weakness, associated with neuromuscular transmission failure. MuSK autoantibodies are predominantly of the IgG4 type. They suppress the kinase activity of MuSK and the phosphorylation of target proteins in the postsynaptic membrane. Loss of postsynaptic acetylcholine receptors is the primary cause of neuromuscular transmission failure. MuSK autoantibodies also disrupt the capacity of the motor nerve terminal to adaptively increase acetylcholine release in response to the reduced postsynaptic responsiveness to acetylcholine. The passive IgG transfer model of MuSK myasthenia gravis has been used to test candidate treatments. Pyridostigmine, a first-line cholinesterase inhibitor drug, exacerbated the disease process, while 3,4-diaminopyridine and albuterol were found to be beneficial in this mouse model.

Proliferation, differentiation and migration of SCA1-/CD31- cardiac side population cells in vitro and in vivo.

BACKGROUND: Side-population (SP) cells, identified by their capacity to efflux Hoechst dye, are highly enriched for stem/progenitor cell activity. They are found in many mammalian tissues, including mouse heart. Studies suggest that cardiac SP (CSP) cells can be divided into SCA1+/CD31-, SCA1+/CD31+ and SCA1-/CD31- CSP subpopulations. SCA1+/CD31- were shown to be cardiac and endothelial stem/progenitors while SCA1+/CD31+ CSP cells are endothelial progenitors. SCA1-/CD31- CSP cells remain to be fully characterized. In this study, we characterized SCA1-/CD31- CSP cells in the adult mouse heart, and investigated their abilities to proliferate, differentiate and migrate in vitro and in vivo. METHODS AND RESULTS: Using fluorescence-activated cell sorting, reverse transcriptase/polymerase chain reaction, assays of cell proliferation, differentiation and migration, and a murine model of myocardial infarction we show that SCA1-/CD31- CSP cells are located in the heart mesenchyme and express genes characteristic of stem cells and endothelial progenitors. These cells were capable of proliferation, differentiation, migration and vascularization in vitro and in vivo. Following experimental myocardial infarction, the SCA1-/CD31- CSP cells migrated from non-infarcted areas to the infarcted region within the myocardium where they differentiated into endothelial cells forming vascular (tube-like) structures. We further demonstrated that the SDF-1α/CXCR4 pathway may play an important role in migration of these cells after myocardial infarction. CONCLUSIONS: Based on their gene expression profile, localization and ability to proliferate, differentiate, migrate and vascularize in vitro and in vivo, we conclude that SCA1-/CD31- CSP cells may serve as endothelial progenitor cells in the adult mouse heart.

Gene expression profiling and localization of Hoechst-effluxing CD45- and CD45+ cells in the embryonic mouse lung.

Hoechst-effluxing cells (side population cells) are a rare subset of cells found in adult tissues that are highly enriched for stem and progenitor cell activity. To identify potential stem and progenitor cells during lung development, we generated gene expression profiles for CD45- and CD45+ side population cells in the embryonic day 17.5 lung. We found that side population cells comprise 1% of total embryonic day 17.5 lung cells (55% CD45+, 45% CD45-). Gene profiling data demonstrated an overrepresentation of endothelial genes within the CD45- side population. We used expression of several distinct genes to identify two types of CD45- side population cells: 1) von Willebrand factor+/smooth muscle actin+ cells that reside in the muscular layer of select large vessels and 2) von Willebrand factor+/intercellular adhesion molecule+ cells that reside within the endothelial layer of select small vessels. Gene profiling of the CD45+ side population indicated an overrepresentation of genes associated with myeloid cell differentiation. Consistent with this, culturing CD45+ side population cells was associated with induction of mature dendritic markers (CD86). The microarray results suggested that expression of myeloperoxidase and proteinase-3 might be used to identify CD45+ side population cells. By immunohistochemistry, we found that myeloperoxidase+/proteinase-3+ cells represent a small subset of total CD45+ cells in the embryonic day 17.5 lung and that they reside in the mesenchyme and perivascular regions. This is the first detailed information regarding the phenotype and localization of side population cells in a developing organ.

Forced expression of muscle specific kinase slows postsynaptic acetylcholine receptor loss in a mouse model of MuSK myasthenia gravis.

We investigated the influence of postsynaptic tyrosine kinase signaling in a mouse model of muscle-specific kinase (MuSK) myasthenia gravis (MG). Mice administered repeated daily injections of IgG from MuSK MG patients developed impaired neuromuscular transmission due to progressive loss of acetylcholine receptor (AChR) from the postsynaptic membrane of the neuromuscular junction. In this model, anti-MuSK-positive IgG caused a reduction in motor endplate immunolabeling for phosphorylated Src-Y418 and AChR ß-subunit-Y390 before any detectable loss of MuSK or AChR from the endplate. Adeno-associated viral vector (rAAV) encoding MuSK fused to enhanced green fluorescent protein (MuSK-EGFP) was injected into the tibialis anterior muscle to increase MuSK synthesis. When mice were subsequently challenged with 11 daily injections of IgG from MuSK MG patients, endplates expressing MuSK-EGFP retained more MuSK and AChR than endplates of contralateral muscles administered empty vector. Recordings of compound muscle action potentials from myasthenic mice revealed less impairment of neuromuscular transmission in muscles that had been injected with rAAV-MuSK-EGFP than contralateral muscles (empty rAAV controls). In contrast to the effects of MuSK-EGFP, forced expression of rapsyn-EGFP provided no such protection to endplate AChR when mice were subsequently challenged with MuSK MG IgG. In summary, the immediate in vivo effect of MuSK autoantibodies was to suppress MuSK-dependent tyrosine phosphorylation of proteins in the postsynaptic membrane, while increased MuSK synthesis protected endplates against AChR loss. These results support the hypothesis that reduced MuSK kinase signaling initiates the progressive disassembly of the postsynaptic membrane scaffold in this mouse model of MuSK MG.

Migration of resident cardiac stem cells in myocardial infarction.

Ischemic heart disease is a major cause of morbidity and mortality worldwide. Stem cell-based therapy, which aims to restore cardiac structure and function by regeneration of functional myocardium, has recently been proposed as a novel alternative treatment modality. Resident cardiac stem cells (CSCs) in adult hearts are a key cell type under investigation. CSCs have been shown to be able to repair damaged myocardium and improve myocardial function in both human and animal studies. This approach relies not only on the proliferation of the CSCs, but also upon their migration to the site of injury within the heart. Here, we briefly review reported CSC populations and discuss signaling factors and pathways required for the migration of CSCs.

Differentiation and migration of Sca1+/CD31- cardiac side population cells in a murine myocardial ischemic model.

BACKGROUND: Side population cells are a rare subset of cells found in the adult heart that are highly enriched for stem and progenitor cell activity. Recent studies have suggested that Sca1+/CD31- cardiac side population cells are capable of differentiation into cardiomyocytes in vitro. However, the response of these cells to myocardial injury remains unknown in vivo. METHODS: Sca1+/CD31- cardiac side population cells were isolated from mouse (C57BL6/J) hearts by FACS. These cells were labeled and delivered via an intramyocardial injection into an infracted mouse heart. The differentiation potential of these cells was determined by immunohistochemistry two weeks later. We further tested the migration potential and the relationship of SDF-1alpha/CXCR4 to these cells. RESULTS: The transplanted cells were found to express cardiomyocyte or endothelial cell specific markers. Furthermore, when these cells were transplanted into non-infarct myocardium after myocardial infarction, they were found in the damaged myocardium. Consistent with their homing property, we found that SDF-1alpha and CXCR4 were up-regulated in the damaged myocardium and on Sca1+/CD31- cardiac side population cells respectively following myocardial infarction. We also show that SDF-1alpha induced migration of Sca1+/CD31- cardiac side population cells in vitro. CONCLUSIONS: Our results have suggested that Sca1+/CD31- cardiac side population cells are able to migrate into damaged myocardium from non-ischemic area of the heart and differentiate into both cardiomyocyte- and endothelial-like cells following acute ischemic injury. The SDF-1alpha/CXCR4 system might play an important role in the migration of these cells.