Showing papers by "Lin Mei published in 2012"

Autoantibodies to Lipoprotein-Related Protein 4 in Patients With Double-Seronegative Myasthenia Gravis

Abstract: Objectives To determine whether patients with myasthenia gravis (MG) have serum antibodies to lipoprotein-related protein 4 (LRP4), a newly identified receptor for agrin that is essential for neuromuscular junction formation, and to establish whether such antibodies contribute to MG pathogenesis. Design Serum samples from patients with MG with known status of serum antibodies to the acetylcholine receptor (AChR) and muscle-specific kinase (MuSK) and serum samples from control subjects (healthy individuals and individuals with other diseases) were tested for antibodies to LRP4. Serum samples with such antibodies were tested to determine whether they had the ability to inhibit 2 different functions of LRP4 at the neuromuscular junction. Setting Serum samples were collected at the Hellenic Pasteur Institute and Wayne State University. Samples were tested for LRP4 autoantibodies at Georgia Health Sciences University. Other immunoreactivities of the samples were tested at the Hellenic Pasteur Institute, Athens, Greece, or processed through University Laboratories of the Detroit Medical Center, Michigan. Patients The study included 217 patients with MG, 76 patients with other neurologic or psychiatric diseases, and 45 healthy control subjects. Results Anti-LRP4 antibodies were detected in 11 of 120 patients with MG without detectable anti-AChR or anti-MuSK antibodies (double seronegative) and in 1 of 36 patients without anti-AChR antibodies but with anti-MuSK antibodies, but they were not detected in any of the 61 patients with anti-AChR antibodies. No healthy control subjects and only 2 of the 76 control patients with neurologic disease had anti-LRP4 antibodies. Serum samples from patients with MG with anti-LRP4 antibodies were able to inhibit the LRP4-agrin interaction and/or alter AChR clustering in muscle cells. Conclusions Anti-LRP4 antibodies were detected in the serum of approximately 9.2% of patients with double-seronegative MG. This frequency is intermediate compared with 2 recent studies showing anti-LRP4 antibodies in 2% and 50% of patients with double-seronegative MG from different geographic locations. Together, these observations indicate that LRP4 is another autoantigen in patients with MG, and anti-LRP4 autoantibodies may be pathogenic through different immunopathogenic processes.

Structural basis of agrin–LRP4–MuSK signaling

Abstract: Synapses are the fundamental units of neural circuits that enable complex behaviors. The neuromuscular junction (NMJ), a synapse formed between a motoneuron and a muscle fiber, has contributed greatly to understanding of the general principles of synaptogenesis as well as of neuromuscular disorders. NMJ formation requires neural agrin, a motoneuron-derived protein, which interacts with LRP4 (low-density lipoprotein receptor-related protein 4) to activate the receptor tyrosine kinase MuSK (muscle-specific kinase). However, little is known of how signals are transduced from agrin to MuSK. Here, we present the first crystal structure of an agrin–LRP4 complex, consisting of two agrin–LRP4 heterodimers. Formation of the initial binary complex requires the z8 loop that is specifically present in neuronal, but not muscle, agrin and that promotes the synergistic formation of the tetramer through two additional interfaces. We show that the tetrameric complex is essential for neuronal agrin-induced acetylcholine receptor (AChR) clustering. Collectively, these results provide new insight into the agrin–LRP4–MuSK signaling cascade and NMJ formation and represent a novel mechanism for activation of receptor tyrosine kinases.

Neuregulin 1 represses limbic epileptogenesis through ErbB4 in parvalbumin-expressing interneurons

Abstract: Epilepsy is a common and refractory neurological disorder, but the neuronal regulatory mechanisms of epileptogenesis remain largely unclear. Activity-dependent transcription of genes for neurotrophins such as brain-derived neurotrophic factor (BDNF) has been shown to promote epileptogenesis; however, little is known about factors that may act as intrinsic, homeostatic or counterbalancing mechanisms. Using rodent models, here we show that limbic seizure activity upregulated NRG1-ErbB4 signaling and that epileptogenesis was inhibited by infusing NRG1 intracerebrally but exacerbated by neutralizing endogenous NRG1 with soluble ErbB4 extracellular domain, by inhibiting ErbB4 activation or by deleting the Erbb4 gene. Furthermore, specific depletion of ErbB4 in parvalbumin-expressing interneurons abolished NRG1-mediated inhibition of epileptogenesis and promoted kindling progression, resulting in increased spontaneous seizures and exuberant mossy fiber sprouting. In contrast, depleting ErbB4 in CaMKII alpha-positive pyramidal neurons had no effect. Thus, NRG1-induced activation of ErbB4 in parvalbumin-expressing inhibitory interneurons may serve as a critical endogenous negative-feedback mechanism to suppress limbic epileptogenesis.

Wnt proteins regulate acetylcholine receptor clustering in muscle cells

Abstract: Background: The neuromuscular junction (NMJ) is a cholinergic synapse that rapidly conveys signals from motoneurons to muscle cells and exhibits a high degree of subcellular specialization characteristic of chemical synapses. NMJ formation requires agrin and its coreceptors LRP4 and MuSK. Increasing evidence indicates that Wnt signaling regulates NMJ formation in Drosophila, C. elegans and zebrafish. Results: In the study we systematically studied the effect of all 19 different Wnts in mammals on acetylcholine receptor (AChR) cluster formation. We identified five Wnts (Wnt9a, Wnt9b, Wnt10b, Wnt11, and Wnt16) that are able to stimulate AChR clustering, of which Wnt9a and Wnt11 are expressed abundantly in developing muscles. Using Wnt9a and Wnt11 as example, we demonstrated that Wnt induction of AChR clusters was dose-dependent and non-additive to that of agrin, suggesting that Wnts may act via similar pathways to induce AChR clusters. We provide evidence that Wnt9a and Wnt11 bind directly to the extracellular domain of MuSK, to induce MuSK dimerization and subsequent tyrosine phosphorylation of the kinase. In addition, Wnt-induced AChR clustering requires LRP4. Conclusions: These results identify Wnts as new players in AChR cluster formation, which act in a manner that requires both MuSK and LRP4, revealing a novel function of LRP4.

Synaptic dysfunction in schizophrenia.

Abstract: Schizophrenia alters basic brain processes of perception, emotion, and judgment to cause hallucinations, delusions, thought disorder, and cognitive deficits Unlike neurodegeneration diseases that have irreversible neuronal degeneration and death, schizophrenia lacks agreeable pathological hallmarks, which makes it one of the least understood psychiatric disorders With identification of schizophrenia susceptibility genes, recent studies have begun to shed light on underlying pathological mechanisms Schizophrenia is believed to result from problems during neural development that lead to improper function of synaptic transmission and plasticity, and in agreement, many of the susceptibility genes encode proteins critical for neural development Some, however, are also expressed at high levels in adult brain Here, we will review evidence for altered neurotransmission at glutamatergic, GABAergic, dopaminergic, and cholinergic synapses in schizophrenia and discuss roles of susceptibility genes in neural development as well as in synaptic plasticity and how their malfunction may contribute to pathogenic mechanisms of schizophrenia We propose that mouse models with precise temporal and spatial control of mutation or overexpression would be useful to delineate schizophrenia pathogenic mechanisms

VPS35 regulates developing mouse hippocampal neuronal morphogenesis by promoting retrograde trafficking of BACE1

Abstract: VPS35, a major component of the retromer, plays an important role in the selective endosome-to-Golgi retrieval of membrane proteins. Dysfunction of retromer is a risk factor for neurodegenerative disorders, but its function in developing mouse brain remains poorly understood. Here we provide evidence for VPS35 promoting dendritic growth and maturation, and axonal protein transport in developing mouse hippocampal neurons. Embryonic hippocampal CA1 neurons suppressing Vps35 expression by in utero electroporation of its micro RNAs displayed shortened apical dendrites, reduced dendritic spines, and swollen commissural axons in the neonatal stage, those deficits reflecting a defective protein transport/trafficking in developing mouse neurons. Further mechanistic studies showed that Vps35 depletion in neurons resulted in an impaired retrograde trafficking of BACE1 (β1-secretase) and altered BACE1 distribution. Suppression of BACE1 expression in CA1 neurons partially rescued both dendritic and axonal deficits induced by Vps35-deficiency. These results thus demonstrate that BACE1 acts as a critical cargo of retromer in vitro and in vivo, and suggest that VPS35 plays an essential role in regulating apical dendritic maturation and in preventing axonal spheroid formation in developing hippocampal neurons.

Biglycan is an extracellular MuSK binding protein important for synapse stability

Abstract: The receptor tyrosine kinase MuSK is indispensable for nerve-muscle synapse formation and maintenance. MuSK is necessary for prepatterning of the endplate zone anlage and as a signaling receptor for agrin-mediated postsynaptic differentiation. MuSK-associated proteins such as Dok7, LRP4, and Wnt11r are involved in these early events in neuromuscular junction formation. However, the mechanisms regulating synapse stability are poorly understood. Here we examine a novel role for the extracellular matrix protein biglycan in synapse stability. Synaptic development in fetal and early postnatal biglycan null (bgn −/o ) muscle is indistinguishable from wild-type controls. However, by 5 weeks after birth, nerve-muscle synapses in bgn −/o mice are abnormal as judged by the presence of perijunctional folds, increased segmentation, and focal misalignment of acetylcholinesterase and AChRs. These observations indicate that previously occupied presynaptic and postsynaptic territory has been vacated. Biglycan binds MuSK and the levels of this receptor tyrosine kinase are selectively reduced at bgn −/o synapses. In bgn −/o myotubes, the initial stages of agrin-induced MuSK phosphorylation and AChR clustering are normal, but the AChR clusters are unstable. This stability defect can be substantially rescued by the addition of purified biglycan. Together, these results indicate that biglycan is an extracellular ligand for MuSK that is important for synapse stability.

Erbin is required for myelination in regenerated axons after injury

Abstract: Neuregulin 1 (NRG1) is an axon-derived factor that is critical for Schwann cell (SC) development and myelinogenesis in a manner dependent on transmembrane tyrosine kinases ErbB2 and ErbB3. Recent studies suggest that NRG1 signaling plays a role in remyelination of regenerated nerves after injury. In this study, we investigated the role of Erbin, a protein that interacts with ErbB2 in remyelination of injured nerves. We show that Erbin expression increased dramatically in injured nerves. Myelinated axons were fewer, and g-ratios of those that were myelinated were increased in erbin−/− mice, which were impaired in functional recovery from nerve injury. These results indicate a necessary role of Erbin in remyelination of regenerating axons. Erbin ablation had little effect on numbers of BrdU-labeled and TUNEL-labeled SCs, suggesting mechanisms independent of altered proliferation or apoptosis. We demonstrated that Erbin mutant mice were impaired in raising or maintaining the levels of ErbB2 and in producing NRG1 in axons. Together, these observations demonstrate that Erbin is required for remyelination of regenerated axons after injury, probably by regulating ErbB2 and NRG1 levels, identifying a novel player in regulating remyelination.

β-Catenin gain of function in muscles impairs neuromuscular junction formation

Abstract: Neuromuscular junction (NMJ) formation requires proper interaction between motoneurons and muscle cells. β-Catenin is required in muscle cells for NMJ formation. To understand underlying mechanisms, we investigated the effect of β-catenin gain of function (GOF) on NMJ development. In HSA-β-catflox(ex3)/+ mice, which express stable β-catenin specifically in muscles, motor nerve terminals became extensively defasciculated and arborized. Ectopic muscles were observed in the diaphragm and were innervated by ectopic phrenic nerve branches. Moreover, extensive outgrowth and branching of spinal axons were evident in the GOF mice. These results indicate that increased β-catenin in muscles alters presynaptic differentiation. Postsynaptically, AChR clusters in HSA-β-catflox(ex3)/+ diaphragms were distributed in a wider region, suggesting that muscle β-catenin GOF disrupted the signal that restricts AChR clustering to the middle region of muscle fibers. Expression of stable β-catenin in motoneurons, however, had no effect on NMJ formation. These observations provide additional genetic evidence that pre- and postsynaptic development of the NMJ requires an intricate balance of β-catenin activity in muscles.

β-Catenin gain of function in muscles impairs neuromuscular junction formation

Abstract: Neuromuscular junction (NMJ) formation requires proper interaction between motoneurons and muscle cells. β-Catenin is required in muscle cells for NMJ formation. To understand underlying mechanisms, we investigated the effect of β-catenin gain of function (GOF) on NMJ development. In HSA-β-catflox(ex3)/+ mice, which express stable β-catenin specifically in muscles, motor nerve terminals became extensively defasciculated and arborized. Ectopic muscles were observed in the diaphragm and were innervated by ectopic phrenic nerve branches. Moreover, extensive outgrowth and branching of spinal axons were evident in the GOF mice. These results indicate that increased β-catenin in muscles alters presynaptic differentiation. Postsynaptically, AChR clusters in HSA-β-catflox(ex3)/+ diaphragms were distributed in a wider region, suggesting that muscle β-catenin GOF disrupted the signal that restricts AChR clustering to the middle region of muscle fibers. Expression of stable β-catenin in motoneurons, however, had no effect on NMJ formation. These observations provide additional genetic evidence that pre- and postsynaptic development of the NMJ requires an intricate balance of β-catenin activity in muscles.

Differential regulation of myosin X movements by its cargos, DCC and neogenin

Abstract: Myosin X (Myo X), also known as MYO10, is an unconventional actin-based motor protein that plays an important role in filopodium formation. Its intra-filopodia movement, an event tightly associated with the function of Myo X, has been extensively studied. However, how the motor activity of Myo X and the direction of its movements are regulated remains largely unknown. In our previous study, we demonstrated that DCC (for 'deleted in colorectal carcinoma') and neogenin (neogenin 1, NEO1 or NGN), a family of immunoglobin-domain-containing transmembrane receptors for netrins, interact with Myo X and that DCC is a cargo of Myo X to be delivered to the neurites of cultured neurons. Here, we provide evidence for DCC and neogenin as regulators of Myo X. DCC promotes movement of Myo X along basal actin filaments and enhances Myo-X-mediated basal filopodium elongation. By contrast, neogenin appears to suppress Myo X movement on the basal side, but increases its movement towards the apical and dorsal side of a cell, promoting dorsal filopodium formation and growth. Further studies have demonstrated that DCC, but not neogenin, enhances integrin-mediated tyrosine phosphorylation of focal adhesion kinase and basal F-actin reorganization, providing a cellular mechanism underlying their distinct effects on Myo X. These results thus demonstrate differential regulatory roles on Myo X activity by its cargo proteins, DCC and neogenin, revealing different cellular functions of DCC and neogenin.

Essential role of PIKE GTPases in neuronal protection against excitotoxic insults.

Abstract: Phosphoinositide 3-kinase enhancer (PIKE) is a group of novel GTPases that regulate the PI3K/Akt activity through direct interactions. Previous in vitro studies suggest that PIKE are involved in numerous mechanisms to prevent apoptosis in neurons but their roles in intact animal remain unexplored. Here, we show that neurons with PIKE ablation (PIKE −/−) are hypersensitive to neuroexcitotoxic stimulation as higher Ca2+ permeability and cell death are detected in kainic acid (KA)-treated PIKE −/− neurons. We further examined the neuroprotective role of PIKE in vivo by administrating KA into the knockout animals via intraperitoneal injection. Immunohistological and biochemical analysis reveal that KA injection causes severer neuronal damage in PIKE> −/− brains. The discharge frequency of electroencephalogram is also higher in PIKE −/− mice after KA administration, results in a stronger epileptogenic behavior in the mutant line. Therefore, our data provide compelling evidence on the neuroprotective role of PIKE GTPases in animals that PIKE proteins are essential for protecting against neurotoxic insults.