The MuSK-BMP pathway regulates synaptic Nav1.4 localization and muscle excitability.

The neuromuscular junction (NMJ) is the linchpin of nerve-evoked muscle contraction. Broadly considered, the function of the NMJ is to transduce a nerve action potential into a muscle fiber action potential (MFAP). Efficient information transfer requires both cholinergic signaling, responsible for the generation of endplate potentials (EPPs), and excitation, the activation of postsynaptic voltage-gated sodium channels (Nav1.4) to trigger MFAPs. In contrast to the cholinergic apparatus, the signaling pathways that organize Nav1.4 and muscle fiber excitability are poorly characterized. Muscle-specific kinase (MuSK), in addition to its Ig1 domain-dependent role as an agrin-LRP4 receptor, is also a BMP co-receptor that binds BMPs via its Ig3 domain and shapes BMP-induced signaling and transcriptional output. Here we probed the function of the MuSK-BMP pathway at the NMJ using mice lacking the MuSK Ig3 domain ('ΔIg3-MuSK'). Synapses formed normally in ΔIg3-MuSK animals, but the postsynaptic apparatus was fragmented from the first weeks of life. Anatomical denervation was not observed at any age examined. Moreover, spontaneous and nerve-evoked acetylcholine release, AChR density, and endplate currents were comparable to WT. However, trains of nerve-evoked MFAPs in ΔIg3-MuSK muscle were abnormal as revealed by increased jitter and blocking in single fiber electromyography. Further, nerve-evoked compound muscle action potentials (CMAPs), as well as twitch and tetanic muscle torque force production, were also diminished. Finally, Nav1.4 levels were reduced at ΔIg3-MuSK synapses but not at the extrajunctional sarcolemma, indicating that the observed excitability defects are the result of impaired localization of this voltage-gated ion channel at the NMJ. We propose that MuSK plays two distinct roles at the NMJ: as an agrin-LRP4 receptor necessary for establishing and maintaining cholinergic signaling, and as a BMP co-receptor required for maintaining proper Nav1.4 density, nerve-evoked muscle excitability and force production. The MuSK-BMP pathway thus emerges as a target for modulating excitability and functional innervation, which are defective in conditions such as congenital myasthenic syndromes and aging.

Calcium-dependent maintenance of agrin-induced postsynaptic specializations.

Although much progress has been made in understanding synapse formation, little is known about the mechanisms underlying synaptic maintenance and loss. The formation of agrin-induced AChR clusters on cultured myotubes requires both activation of the receptor tyrosine kinase MuSK and intracellular calcium fluxes. Here, we provide evidence that such AChR clusters are maintained by agrin/MuSK-induced intracellular calcium fluxes. Clamping intracellular calcium fluxes after AChR clusters have formed leads to rapid MuSK and AChR tyrosine dephosphorylation and cluster dispersal, even in the continued presence of agrin. Both the dephosphorylation and the dispersal are inhibited by the tyrosine phosphatase inhibitor pervanadate. In contrast, clamping intracellular calcium at the time of initial agrin stimulation has no effect on agrin-induced MuSK or AChR phosphorylation, but blocks AChR cluster formation. These findings suggest an avenue by which postsynaptic stability can be regulated by modification of intracellular signaling pathways that are distinct from those used during synapse formation.

A role for the cytoplasmic polyadenylation element in NMDA receptor-regulated mRNA translation in neurons.

The ability of neurons to modify synaptic connections based on activity is essential for information processing and storage in the brain. The induction of long-lasting changes in synaptic strength requires new protein synthesis and is often mediated by NMDA-type glutamate receptors (NMDARs). We used a dark-rearing paradigm to examine mRNA translational regulation in the visual cortex after visual experience-induced synaptic plasticity. In this model system, we demonstrate that visual experience induces the translation of mRNA encoding the alpha-subunit of calcium/calmodulin-dependent kinase II in the visual cortex. Furthermore, this increase in translation is NMDAR dependent. One potential source for newly synthesized proteins is the translational activation of dormant cytoplasmic mRNAs. To examine this possibility, we developed a culture-based assay system to study translational regulation in neurons. Cultured hippocampal neurons were transfected with constructs encoding green fluorescent protein (GFP). At 6 hr after transfection, approximately 35% of the transfected neurons (as determined by in situ hybridization) expressed detectable GFP protein. Glutamate stimulation of the cultures at this time induced an increase in the number of neurons expressing GFP protein that was NMDAR dependent. Importantly, the glutamate-induced increase was only detected when the 3'-untranslated region of the GFP constructs contained intact cytoplasmic polyadenylation elements (CPEs). Together, these findings define a molecular mechanism for activity-dependent synaptic plasticity that is mediated by the NMDA receptor and requires the CPE-dependent translation of an identified mRNA.

Internalization of ionotropic glutamate receptors in response to mGluR activation.

Activation of group 1 metabotropic glutamate receptors (mGluRs) stimulates dendritic protein synthesis and long-term synaptic depression (LTD), but it remains unclear how these effects are related. Here we provide evidence that a consequence of mGluR activation in the hippocampus is the rapid loss of both AMPA and NMDA receptors from synapses. Like mGluR-LTD, the stable expression of this change requires protein synthesis. These data suggest that expression of mGluR-LTD is at least partly postsynaptic, and that a functional consequence of dendritic protein synthesis is the regulation of glutamate receptor trafficking.

PTEN mutation, EGFR amplification, and outcome in patients with anaplastic astrocytoma and glioblastoma multiforme.

BACKGROUND: Survival of patients with anaplastic astrocytoma is highly variable. Prognostic markers would thus be useful to identify clinical subsets of such patients. Because specific genetic alterations have been associated with glioblastoma, we investigated whether similar genetic alterations could be detected in patients with anaplastic astrocytoma and used to identify those with particularly aggressive disease. METHODS: Tissue specimens were collected from 174 patients enrolled in Mayo Clinic Cancer Center and North Central Cancer Treatment Group clinical trials for newly diagnosed gliomas, including 63 with anaplastic astrocytoma and 111 with glioblastoma multiforme. Alterations of the EGFR, PTEN, and p53 genes and of chromosomes 7 and 10 were examined by fluorescence in situ hybridization, semiquantitative polymerase chain reaction, and DNA sequencing. All statistical tests were two-sided. RESULTS: Mutation of PTEN, amplification of EGFR, and loss of the q arm of chromosome 10 were statistically significantly less common in anaplastic astrocytoma than in glioblastoma multiforme (P =.033, P =.001, and P<.001, respectively), and mutation of p53 was statistically significantly more common (P<.001). Univariate survival analyses of patients with anaplastic astrocytoma identified PTEN (P =.002) and p53 (P =.012) mutations as statistically significantly associated with reduced and prolonged survival, respectively. Multivariate Cox analysis of patients with anaplastic astrocytoma showed that PTEN mutation remained a powerful prognostic factor after adjusting for patient age, on-study performance score, and extent of tumor resection (hazard ratio = 4.34; 95% confidence interval = 1.82 to 10.34). Multivariate classification and regression-tree analysis of all 174 patients identified EGFR amplification as an independent predictor of prolonged survival in patients with glioblastoma multiforme who were older than 60 years of age. CONCLUSION: PTEN mutation and EGFR amplification are important prognostic factors in patients with anaplastic astrocytoma and in older patients with glioblastoma multiforme, respectively.

A phase III study of radiation therapy plus carmustine with or without recombinant interferon-alpha in the treatment of patients with newly diagnosed high-grade glioma.

BACKGROUND: The current study was conducted to determine whether the addition of interferon-alpha (IFN-alpha) to treatment with radiation therapy and carmustine (BCNU) improves time to disease progression or overall survival in patients with high-grade glioma. METHODS: Patients with anaplastic astrocytoma, anaplastic oligoastrocytoma, glioblastoma multiforme, or gliosarcoma received radiation therapy plus BCNU as initial therapy. Subsequently, patients without tumor progression at the completion of radiation therapy were stratified by age, extent of surgery, tumor grade and histology, Eastern Cooperative Oncology Group performance status, and treating institution, and then were randomly assigned to receive either BCNU alone (200 mg/m(2) on Day 1) or BCNU (150 mg/m(2) on Day 3) plus IFN--alpha (12 million U/m(2) on Days 1-3, Weeks 1, 3, and 5) every 7 weeks for a maximum of 6 cycles. RESULTS: Of the 383 patients enrolled in the study, 275 eligible patients were randomized. There was no significant difference with regard to time to disease progression or overall survival between the two groups. Patients receiving IFN-alpha experienced more fever, chills, myalgias, and neurocortical symptoms including somnolence, confusion, and exacerbation of neurologic deficits. Cox multivariate regression models confirmed known favorable prognostic variables including younger age, Grade 3 tumor (according to World Health Organization criteria), and greater extent of surgery. Cox and classification and regression tree analysis models also demonstrated that a normal baseline Folstein mini-mental status examination (MMSE) score was associated with better prognosis. CONCLUSIONS: IFN-alpha does not appear to improve time to disease progression or overall survival in patients with high-grade glioma and appears to add significantly to toxicity. The baseline MMSE score may serve as an independent prognostic factor and warrants further investigation.

Efficacy of neuroradiological imaging, neurological examination, and symptom status in follow-up assessment of patients with high-grade gliomas.

OBJECT: It is standard practice for the oncological follow-up of patients with brain tumors (especially in the setting of clinical trials) to include neurological examination and neuroradiological studies such as computerized tomography (CT) or magnetic resonance (MR) imaging in addition to evaluation of the patients' symptomatology and performance score. The validity of this practice and its impact on the welfare of patients with high-grade gliomas has not been adequately assessed. The purpose of this study is to provide such an assessment. METHODS: The authors studied 231 similarly treated patients who were participating in three prospective North Central Cancer Treatment Group or Mayo Clinic trials who developed progressive disease during follow up. According to the protocol, the symptom status, performance score, results of neurological examination, and CT or MR status were recorded prospectively in each patient at each evaluation (every 6-8 weeks). At progression, 177 (77%) of 231 patients experienced worsening of their baseline symptoms or they developed new ones. In the remaining 54 asymptomatic patients (23%), neuroradiological imaging revealed the progression. Asymptomatic progression was more likely to be detected on MR imaging compared with CT studies (p < 0.01). In no asymptomatic patient was progression detected on neurological examination alone. The median survival time after tumor recurrence was 13.3 weeks in symptomatic patients compared with 41.7 weeks in the asymptomatic group (p < 0.0001). Asymptomatic patients were more aggressively treated, with surgery (p < 0.0001) and second-line chemotherapy (p < 0.0002). Multivariate analysis of survival time following first progression by using both classification and regression trees and Cox models showed that treatment at recurrence was the most important prognostic variable. CONCLUSIONS: Symptoms are the most frequent indicators of progression in patients with high-grade gliomas (77%). All asymptomatic progressions were detected on neuroradiological studies; MR imaging was more likely than CT scanning to reveal asymptomatic recurrences. Survival after disease progression was significantly longer in asymptomatic patients and could be related both to treatment following progression and to other favorable prognostic factors such as performance score.

Agrin and microvascular damage in Alzheimer's disease.

Heparan sulfate proteoglycans (HSPGs) are ubiquitously present within the perivascular basement membrane, and have been shown to be altered in patients with Alzheimer's Disease (AD). Although the HSPG agrin clearly orchestrates the differentiation of the neuromuscular junction, its role in the brain remains unclear. Growing evidence suggests that agrin may be an important vascular basement membrane (VBM)-associated HSPG. In previous studies, we demonstrated that agrin is present throughout the brain microvasculature, as well as in neuronal cell bodies. AD brains exhibited fragmentation of VBM-associated agrin. Agrin immunoreactivity was also seen within senile plaques and neurofibrillary tangles. These changes were accompanied by the appearance of an additional pool of insoluble agrin. In the present study, we provide further evidence for microvascular damage in AD, by examining the distribution of agrin and laminin within the VBM, and by measuring the agrin concentration within hippocampus and prefrontal cortex. Furthermore, we assessed blood-brain-barrier (BBB) leakage by examining the perivascular distribution of prothrombin immunoreactivity. Soluble agrin levels were increased approximately 30% in Braak stage III-VI AD patients relative to age-matched controls. Furthermore, agrin and laminin exhibited identical patterns of VBM fragmentation in AD and colocalized with beta-amyloid in senile plaques. Microvascular changes were associated with the appearance of perivascular prothrombin immunoreactivity. Our data suggest that agrin is an important VBM-associated HSPG in the brain and that agrin levels are altered in association with microvascular damage in AD.

Molecular mechanisms for activity-regulated protein synthesis in the synapto-dendritic compartment.

The creation of enduring modifications in synaptic efficacy requires new protein synthesis. Neurons face the formidable challenge of directing these newly made proteins to the appropriate subset of synapses. One attractive solution to this problem is the local translation of mRNAs that are targeted to dendrites and perhaps to synapses themselves. The molecular mechanisms mediating such local protein synthesis, notably CPEB-mediated cytoplasmic polyadenylation, are now being elucidated.

The small leucine-rich repeat proteoglycan biglycan binds to alpha-dystroglycan and is upregulated in dystrophic muscle.

The dystrophin-associated protein complex (DAPC) is necessary for maintaining the integrity of the muscle cell plasma membrane and may also play a role in coordinating signaling events at the cell surface. The alpha-/beta-dystroglycan subcomplex of the DAPC forms a critical link between the cytoskeleton and the extracellular matrix. A ligand blot overlay assay was used to search for novel dystroglycan binding partners in postsynaptic membranes from Torpedo electric organ. An approximately 125-kD dystroglycan-binding polypeptide was purified and shown by peptide microsequencing to be the Torpedo ortholog of the small leucine-rich repeat chondroitin sulfate proteoglycan biglycan. Biglycan binding to alpha-dystroglycan was confirmed by coimmunoprecipitation with both native and recombinant alpha-dystroglycan. The biglycan binding site was mapped to the COOH-terminal third of alpha-dystroglycan. Glycosylation of alpha-dystroglycan is not necessary for this interaction, but binding is dependent upon the chondroitin sulfate side chains of biglycan. In muscle, biglycan is detected at both synaptic and nonsynaptic regions. Finally, biglycan expression is elevated in muscle from the dystrophic mdx mouse. These findings reveal a novel binding partner for alpha-dystroglycan and demonstrate a novel avenue for interaction of the DAPC and the extracellular matrix. These results also raise the possibility of a role for biglycan in the pathogenesis, and perhaps the treatment, of muscular dystrophy.

Building inhibitory synapses: exchange factors getting into the act? [ comment].

Gephyrin is a synaptic protein that is required for clustering of glycine and GABAA receptors. Gephyrin itself may be regulated by a newly identified protein named collybistin.

Detection of p16, RB, CDK4, and p53 gene deletion and amplification by fluorescence in situ hybridization in 96 gliomas.

Inactivation of the p53 gene is a common early event of astrocytoma tumorigenesis. Alternatively, since the p16, retinoblastoma (RB), and CDK4 genes have been implicated in malignant progression, detection of losses or amplifications of these genes in gliomas could be diagnostically, prognostically, and therapeutically important. We obtained smear preparations from 96 diffuse gliomas and 10 nonneoplastic specimens. Dual-color fluorescence in situ hybridizations using paired probes for CEN9/p16, CEN8/RB, CEN17/p53, and CEN12/CDK4 were performed and revealed expected frequencies of abnormalities, except for p53 losses, which were low (7%). The latter supports the concept that p53 inactivation usually occurs by mitotic recombination. Detected abnormalities of the p16/RB/CDK4 pathway were highly associated with astrocytic differentiation and were univariately associated with decreased patient survival. However, only patient age and histologic classification retained statistical significance on multivariate analysis. We conclude that in diffuse gliomas, p16/RB/CDK4 abnormalities are markers of astrocytic phenotype. Thus, their detection by fluorescence in situ hybridization may have diagnostic usefulness in cases with equivocal morphologic features. Although our numbers are small, we find no additional prognostic significance to these genetic abnormalities one age, grade, and oligodendroglial histology are taken into account.

The insulin receptor tyrosine kinase substrate p58/53 and the insulin receptor are components of CNS synapses.

The synapse is the primary locus of cell-cell communication in the nervous system. It is now clear that the synapse incorporates diverse cell signaling modalities in addition to classical neurotransmission. Here we show that two components of the insulin pathway are localized at CNS synapses, where they are components of the postsynaptic density (PSD). An immunochemical screen revealed that polypeptides of 58 and 53 kDa (p58/53) were highly enriched in PSD fractions from rat cerebral cortex, hippocampus, and cerebellum. These polypeptides were purified and microsequenced, revealing that p58/53 is identical to the insulin receptor tyrosine kinase substrate p58/53 (IRSp53). Our analysis of IRSp58/53 mRNA suggests that within rat brain there is one coding region for IRSp58 and IRSp53; we find no evidence of alternative splicing. We demonstrate that IRSp58/53 is expressed in the synapse-rich molecular layer of the cerebellum and is highly concentrated at the synapses of cultured hippocampal neurons, where it co-localizes with the insulin receptor. Together, these data suggest that insulin signaling may play a role at CNS synapses.

Clinicopathologic study of 85 similarly treated patients with anaplastic astrocytic tumors. An analysis of DNA content (ploidy), cellular proliferation, and p53 expression.

BACKGROUND: The biologic behavior of anaplastic (World Health Organization Grade III) astrocytomas and oligoastrocytomas is highly variable, ranging from rapid progression to prolonged survival. It is difficult to predict the outcome of an individual patient based on morphology alone. METHODS: To determine the prognostic value of commonly used clinicopathologic markers, we reviewed our experience with 85 similarly treated patients enrolled in 3 North Central Cancer Treatment Group high grade glioma protocols. The pathology was comprised exclusively of primary anaplastic astrocytic tumors (66 astrocytomas and 19 oligoastrocytomas). Variables examined included patient age, morphologic type, preoperative performance score, extent of surgery, solitary versus multiple mitoses, DNA flow cytometric and image morphometric parameters, and expression of proliferating cell nuclear antigen, MIB-1, and p53 expression. RESULTS: The study was comprised of 48 men and 37 women ranging in age from 14-79 years (median age, 47 years). Overall survival ranged from <1 month to >12 years (median, 21.6 months). Statistical analyses revealed that age accounted for the majority of this extensive variability in survival. The median survival times were 65. 5 months, 22.1 months, and 4.4 months, respectively, for the groups <40 years, 40-59 years, and >/=60 years, respectively (P < 0.0001). On univariate analyses, aneuploidy by flow cytometry and a low performance score also predicted a better survival (P values of 0.04 and 0.009, respectively). Statistical trends predicting a better survival were observed for patients with a solitary mitosis and p53 immunopositivity. However, only patient age remained significant in multivariate models. CONCLUSIONS: In a small but relatively uniformly treated cohort of patients with anaplastic astrocytomas and oligoastrocytomas, patient age was associated strongly and inversely with overall survival. Once patient age was taken into account, the clinical and pathologic markers tested appeared to be of limited prognostic value.

Tumor suppressor gene alterations in malignant gliomas: histopathological associations and prognostic evaluation.

We have examined a series of 135 gliomas for alterations of the p53, CDKN2A (p16) and PTEN tumor suppressor genes (TSGs) in order to evaluate the incidence of their inactivation as a function of tumor malignancy and cellular differentiation, and to examine potential associations with patient outcome. The composition of this series, classified using WHO criteria, is as follows: 27 grade 2 tumors (11 astrocytomas, 12 oligoastrocytomas, 4 oligodendrogliomas), 42 grade 3 tumors (22 astrocytomas, 16 oligoastrocytomas, 4 oligodendrogliomas), and 66 grade 4 tumors (63 astrocytomas and 3 oligoastrocytomas). Similar frequencies of p53 mutation were observed among grade 2 (37.0%), and grade 3 tumors (38.1%), as well as between astrocytomas and mixed tumors. CDKN2A and PTEN mutations were clearly associated with increasing tumor malignancy (occurring in 0% of grade 2 tumors, 14.3% and 4.8% respectively of grade 3 tumors, and 27.3% and 30.3% respectively of grade 4 tumors) and were observed at substantially higher rates among astrocytomas. For the tumor suppressor genes examined, there was no relationship between the occurrence of any two TSG inactivation events. With regard to outcome, the p53 genetic status showed no significant relationship with patient survival. The CDKN2 and PTEN alterations were negative prognostic indicators of survival when evaluated in all 135 gliomas, but failed to predict outcome when evaluated in either of the high grade (3 or 4) tumor groups.

Agrin in Alzheimer's disease: altered solubility and abnormal distribution within microvasculature and brain parenchyma.

Agrin is a heparan sulfate proteoglycan that is widely expressed in neurons and microvascular basal lamina in the rodent and avian central nervous system. Agrin induces the differentiation of nerve-muscle synapses, but its function in either normal or diseased brains is not known. Alzheimer's disease (AD) is characterized by loss of synapses, changes in microvascular architecture, and formation of neurofibrillary tangles and senile plaques. Here we have asked whether AD causes changes in the distribution and biochemical properties of agrin. Immunostaining of normal, aged human central nervous system revealed that agrin is expressed in neurons in multiple brain areas. Robust agrin immunoreactivity was observed uniformly in the microvascular basal lamina. In AD brains, agrin is highly concentrated in both diffuse and neuritic plaques as well as neurofibrillary tangles; neuronal expression of agrin also was observed. Furthermore, patients with AD had microvascular alterations characterized by thinning and fragmentation of the basal lamina. Detergent extraction and Western blotting showed that virtually all the agrin in normal brain is soluble in 1% SDS. In contrast, a large fraction of the agrin in AD brains is insoluble under these conditions, suggesting that it is tightly associated with beta-amyloid. Together, these data indicate that the agrin abnormalities observed in AD are closely linked to beta-amyloid deposition. These observations suggest that altered agrin expression in the microvasculature and the brain parenchyma contribute to the pathogenesis of AD.

Phase II North Central Cancer Treatment Group study of 2-cholorodeoxyadenosine in patients with recurrent glioma.

There is no standard treatment for patients with recurrent gliomas, and their prognosis remains poor. 2-Chlorodeoxyadenosine is a purine analogue that has significant activity in many low-grade lymphoproliferative disorders. The authors conducted a phase II study to determine the efficacy of 2-chlorodeoxyadenosine in patients with recurrent gliomas. Patients with a histologically confirmed primary brain tumor with evidence of progression after radiation therapy were eligible. Protocol treatment consisted of 2-chlorodeoxyadenosine 7.0 mg/m2 intravenously on days 1 through 5 every 28 days. For those with a history of prior nitrosourea therapy, the dose of 2-chlorodeoxyadenosine was reduced to 5.6 mg/m2 on days 1 through 5. Treatment was continued until progression or a maximum of 12 cycles. Fifteen patients with recurrent astrocytomas or oligoastrocytomas of all grades were entered in the study. Treatment was well tolerated. Major toxicities were myelosuppression and neurotoxicity. No responses were seen. The authors conclude that although 2-chlorodeoxyadenosine is well tolerated, no demonstrable activity in patients with recurrent gliomas was established.