Understanding age-related pathologic changes in TDP-43 functions and the consequence on RNA splicing and signalling in health and disease

TAR DNA binding protein-43 (TDP-43) is a key component in RNA splicing which plays a crucial role in the aging process. In neurodegenerative diseases such as amyotrophic lateral sclerosis, frontotemporal dementia and limbic-predominant age-related TDP-43 encephalopathy, TDP-43 can be mutated, mislocalised out of the nucleus of neurons and glial cells and form cytoplasmic inclusions. These TDP-43 alterations can lead to its RNA splicing dysregulation and contribute to mis-splicing of various types of RNA, such as mRNA, microRNA, and circular RNA. These changes can result in the generation of an altered transcriptome and proteome within cells, ultimately changing the diversity and quantity of gene products. In this review, we summarise the findings of novel atypical RNAs resulting from TDP-43 dysfunction and their potential as biomarkers or targets for therapeutic development.

Cyclin F, Neurodegeneration, and the Pathogenesis of ALS/FTD

Amyotrophic lateral sclerosis (ALS) is the most common form of motor neuron disease and is characterized by the degeneration of upper and lower motor neurons of the brain and spinal cord. ALS is also linked clinically, genetically, and pathologically to a form of dementia known as frontotemporal dementia (FTD). Identifying gene mutations that cause ALS/FTD has provided valuable insight into the disease process. Several ALS/FTD-causing mutations occur within proteins with roles in protein clearance systems. This includes ALS/FTD mutations in CCNF, which encodes the protein cyclin F: a component of a multiprotein E3 ubiquitin ligase that mediates the ubiquitylation of substrates for their timely degradation. In this review, we provide an update on the link between ALS/FTD CCNF mutations and neurodegeneration.

Cyclin F can alter the turnover of TDP-43

Previously, we demonstrated that the SCFcyclin F complex directly mediates the poly-ubiquitylation of TDP-43, raising the question of whether cyclin F can be used to enhance the turnover of TDP-43. A hurdle to the use of cyclin F, however, is that the overexpression of cyclin F can lead to the initiation of cell death pathways. Accordingly, the aim of this study was to identify and evaluate a less toxic variant of cyclin F. To do so, we first confirmed and validated our previous findings that cyclin F binds to TDP-43 in an atypical manner. Additionally, we demonstrated that mutating the canonical substrate region in cyclin F (to generate cyclin FMRL/AAA) led to reduced binding affinity to known canonical substrates without impacting the interaction between cyclin F and TDP-43. Notably, both wild-type and cyclin FMRL/AAA effectively reduced the abundance of TDP-43 in cultured cells whilst cyclin FMRL/AAA also demonstrated reduced cell death compared to the wild-type control. The decrease in toxicity also led to a reduction in morphological defects in zebrafish embryos. These results suggest that cyclin F can be modified to enhance its targeting of TDP-43, which in turn reduces the toxicity associated with the overexpression of cyclin F. This study provides greater insights into the interaction that occurs between cyclin F and TDP-43 in cells and in vivo.

RNA-binding properties orchestrate TDP-43 homeostasis through condensate formation in vivo

Insoluble cytoplasmic aggregate formation of the RNA-binding protein TDP-43 is a major hallmark of neurodegenerative diseases including Amyotrophic Lateral Sclerosis. TDP-43 localizes predominantly in the nucleus, arranging itself into dynamic condensates through liquid-liquid phase separation (LLPS). Mutations and post-translational modifications can alter the condensation properties of TDP-43, contributing to the transition of liquid-like biomolecular condensates into solid-like aggregates. However, to date it has been a challenge to study the dynamics of this process in vivo. We demonstrate through live imaging that human TDP-43 undergoes nuclear condensation in spinal motor neurons in a living animal. RNA-binding deficiencies as well as post-translational modifications can lead to aberrant condensation and altered TDP-43 compartmentalization. Single-molecule tracking revealed an altered mobility profile for RNA-binding deficient TDP-43. Overall, these results provide a critically needed in vivo characterization of TDP-43 condensation, demonstrate phase separation as an important regulatory mechanism of TDP-43 accessibility, and identify a molecular mechanism of how functional TDP-43 can be regulated.

A transient protein folding response targets aggregation in the early phase of TDP-43-mediated neurodegeneration

Understanding the mechanisms that drive TDP-43 pathology is integral to combating amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration (FTLD) and other neurodegenerative diseases. Here we generated a longitudinal quantitative proteomic map of the cortex from the cytoplasmic TDP-43 rNLS8 mouse model of ALS and FTLD, and developed a complementary open-access webtool, TDP-map ( https://shiny.rcc.uq.edu.au/TDP-map/ ). We identified distinct protein subsets enriched for diverse biological pathways with temporal alterations in protein abundance, including increases in protein folding factors prior to disease onset. This included increased levels of DnaJ homolog subfamily B member 5, DNAJB5, which also co-localized with TDP-43 pathology in diseased human motor cortex. DNAJB5 over-expression decreased TDP-43 aggregation in cell and cortical neuron cultures, and knockout of Dnajb5 exacerbated motor impairments caused by AAV-mediated cytoplasmic TDP-43 expression in mice. Together, these findings reveal molecular mechanisms at distinct stages of ALS and FTLD progression and suggest that protein folding factors could be protective in neurodegenerative diseases.

The E3 Ubiquitin Ligase SCF Cyclin F Promotes Sequestosome-1/p62 Insolubility and Foci Formation and is Dysregulated in ALS and FTD Pathogenesis

Amyotrophic lateral sclerosis (ALS)- and frontotemporal dementia (FTD)-linked mutations in CCNF have been shown to cause dysregulation to protein homeostasis. CCNF encodes for cyclin F, which is part of the cyclin F-E3 ligase complex SCFcyclinF known to ubiquitylate substrates for proteasomal degradation. In this study, we identified a function of cyclin F to regulate substrate solubility and show how cyclin F mechanistically underlies ALS and FTD disease pathogenesis. We demonstrated that ALS and FTD-associated protein sequestosome-1/p62 (p62) was a canonical substrate of cyclin F which was ubiquitylated by the SCFcyclinF complex. We found that SCFcyclin F ubiquitylated p62 at lysine(K)281, and that K281 regulated the propensity of p62 to aggregate. Further, cyclin F expression promoted the aggregation of p62 into the insoluble fraction, which corresponded to an increased number of p62 foci. Notably, ALS and FTD-linked mutant cyclin F p.S621G aberrantly ubiquitylated p62, dysregulated p62 solubility in neuronal-like cells, patient-derived fibroblasts and induced pluripotent stem cells and dysregulated p62 foci formation. Consistently, motor neurons from patient spinal cord tissue exhibited increased p62 ubiquitylation. We suggest that the p.S621G mutation impairs the functions of cyclin F to promote p62 foci formation and shift p62 into the insoluble fraction, which may be associated to aberrant mutant cyclin F-mediated ubiquitylation of p62. Given that p62 dysregulation is common across the ALS and FTD spectrum, our study provides insights into p62 regulation and demonstrates that ALS and FTD-linked cyclin F mutant p.S621G can drive p62 pathogenesis associated with ALS and FTD.

Identification of phosphorylated tau protein interactors in progressive supranuclear palsy (PSP) reveals networks involved in protein degradation, stress response, cytoskeletal dynamics, metabolic processes, and neurotransmission

Progressive supranuclear palsy (PSP) is a late-onset neurodegenerative disease defined pathologically by the presence of insoluble phosphorylated-Tau (p-Tau) in neurons and glia. Identifying co-aggregating proteins within p-Tau inclusions may reveal important insights into processes affected by the aggregation of Tau. We used a proteomic approach, which combines antibody-mediated biotinylation and mass spectrometry (MS) to identify proteins proximal to p-Tau in PSP. Using this proof-of-concept workflow for identifying interacting proteins of interest, we characterized proteins proximal to p-Tau in PSP cases, identifying >84% of previously identified interaction partners of Tau and known modifiers of Tau aggregation, while 19 novel proteins not previously found associated with Tau were identified. Furthermore, our data also identified confidently assigned phosphorylation sites that have been previously reported on p-Tau. Additionally, using ingenuity pathway analysis (IPA) and human RNA-seq datasets, we identified proteins previously associated with neurological disorders and pathways involved in protein degradation, stress responses, cytoskeletal dynamics, metabolism, and neurotransmission. Together, our study demonstrates the utility of biotinylation by antibody recognition (BAR) approach to answer a fundamental question to rapidly identify proteins in proximity to p-Tau from post-mortem tissue. The application of this workflow opens up the opportunity to identify novel protein targets to give us insight into the biological process at the onset and progression of tauopathies.

Microglia morphophysiological diversity and its implications for the CNS

Microglia are mononuclear phagocytes of mesodermal origin that migrate to the central nervous system (CNS) during the early stages of embryonic development. After colonizing the CNS, they proliferate and remain able to self-renew throughout life, maintaining the number of microglia around 5-12% of the cells in the CNS parenchyma. They are considered to play key roles in development, homeostasis and innate immunity of the CNS. Microglia are exceptionally diverse in their morphological characteristics, actively modifying the shape of their processes and soma in response to different stimuli. This broad morphological spectrum of microglia responses is considered to be closely correlated to their diverse range of functions in health and disease. However, the morphophysiological attributes of microglia, and the structural and functional features of microglia-neuron interactions, remain largely unknown. Here, we assess the current knowledge of the diverse microglial morphologies, with a focus on the correlation between microglial shape and function. We also outline some of the current challenges, opportunities, and future directions that will help us to tackle unanswered questions about microglia, and to continue unravelling the mysteries of microglia, in all its shapes.

Brain-Targeted Codelivery of Bcl-2/Bcl-xl and Mcl-1 Inhibitors by Biomimetic Nanoparticles for Orthotopic Glioblastoma Therapy

Glioblastoma (GBM) is among the most treatment-resistant solid tumors and often recurrs after resection. One of the mechanisms through which GBM escapes various treatment modalities is the overexpression of anti-apoptotic Bcl-2 family proteins (e.g., Bcl-2, Bcl-xl, and Mcl-1) in tumor cells. Small-molecule inhibitors such as ABT-263 (ABT), which can promote mitochondrial-mediated cell apoptosis by selectively inhibiting the function of Bcl-2 and Bcl-xl, have been proven to be promising anticancer agents in clinical trials. However, the therapeutic prospects of ABT for GBM treatment are hampered by its limited blood-brain barrier (BBB) penetration, dose-dependent thrombocytopenia, and the drug resistance driven by Mcl-1, which is overexpressed in GBM cells and further upregulated upon treatment with ABT. Herein, we reported that the Mcl-1-specific inhibitor A-1210477 (A12) can act synergistically with ABT to induce potent cell apoptosis in U87 MG cells, drug-resistant U251 cells, and patient-derived GBM cancer stem cells. We further designed a biomimetic nanomedicine, based on the apolipoprotein E (ApoE) peptide-decorated red blood cell membrane and pH-sensitive dextran nanoparticles, for the brain-targeted delivery of ABT and A12. The synergistic anti-GBM effect was retained after encapsulation in the nanomedicine. Additionally, the obtained nanomedicine possessed good biocompatibility, exhibited efficient BBB penetration, and could effectively suppress tumor growth and prolong the survival time of mice bearing orthotopic GBM xenografts without inducing detectable adverse effects.

Enhanced Antioxidant Effects of the Anti-Inflammatory Compound Probucol when Released from Mesoporous Silica Particles

Brain endothelial cells mediate the function and integrity of the blood brain barrier (BBB) by restricting its permeability and exposure to potential toxins. However, these cells are highly susceptible to cellular damage caused by oxidative stress and inflammation. Consequent disruption to the integrity of the BBB can lead to the pathogenesis of neurodegenerative diseases. Drug compounds with antioxidant and/or anti-inflammatory properties therefore have the potential to preserve the structure and function of the BBB. In this work, we demonstrate the enhanced antioxidative effects of the compound probucol when loaded within mesoporous silica particles (MSP) in vitro and in vivo zebrafish models. The dissolution kinetics were significantly enhanced when released from MSPs. An increased reduction in lipopolysaccharide (LPS)-induced reactive oxygen species (ROS), cyclooxygenase (COX) enzyme activity and prostaglandin E₂ production was measured in human brain endothelial cells treated with probucol-loaded MSPs. Furthermore, the LPS-induced permeability across an endothelial cell monolayer by paracellular and transcytotic mechanisms was also reduced at lower concentrations compared to the antioxidant ascorbic acid. Zebrafish pre-treated with probucol-loaded MSPs reduced hydrogen peroxide-induced ROS to control levels after 24-h incubation, at significantly lower concentrations than ascorbic acid. We provide compelling evidence that the encapsulation of antioxidant and anti-inflammatory compounds within MSPs can enhance their release, enhance their antioxidant effects properties, and open new avenues for the accelerated suppression of neuroinflammation.

Brain-Targeted Aggregation-Induced-Emission Nanoparticles with Near-Infrared Imaging at 1550 nm Boosts Orthotopic Glioblastoma Theranostics

A remaining challenge in the treatment of glioblastoma multiforme (GBM) is surmounting the blood-brain barrier (BBB). Such a challenge prevents the development of efficient theranostic approaches that combine reliable diagnosis with targeted therapy. In this study, brain-targeted near-infrared IIb (NIR-IIb) aggregation-induced-emission (AIE) nanoparticles are developed via rational design, which involves twisting the planar molecular backbone with steric hindrance. The resulting nanoparticles can balance competing responsiveness demands for radiation-mediated NIR fluorescence imaging at 1550 nm and non-radiation NIR photothermal therapy (NIR-PTT). The brain-targeting peptide apolipoprotein E peptide (ApoE) is grafted onto these nanoparticles (termed as ApoE-Ph NPs) to target glioma and promote efficient BBB traversal. A long imaging wavelength 1550 nm band-pass filter is utilized to monitor the in vivo biodistribution and accumulation of the nanoparticles in a model of orthotopic glioma, which overcomes previous limitations in wavelength range and equipment. The results demonstrate that the ApoE-Ph NPs have a higher PTT efficiency and significantly enhanced survival of mice bearing orthotopic GBM with moderate irradiation (0.5 W cm^-2 ). Collectively, the work highlights the smart design of a brain-targeted NIR-II AIE theranostic approach that opens new diagnosis and treatment options in the photonic therapy of GBM.

Cytokine Signalling at the Microglial Penta-Partite Synapse

Microglial cell processes form part of a subset of synaptic contacts that have been dubbed microglial tetra-partite or quad-partite synapses. Since tetrapartite may also refer to the presence of extracellular matrix components, we propose the more precise term microglial penta-partite synapse for synapses that show a microglial cell process in close physical proximity to neuronal and astrocytic synaptic constituents. Microglial cells are now recognised as key players in central nervous system (CNS) synaptic changes. When synaptic plasticity involving microglial penta-partite synapses occurs, microglia may utilise their cytokine arsenal to facilitate the generation of new synapses, eliminate those that are not needed anymore, or modify the molecular and structural properties of the remaining synaptic contacts. In addition, microglia–synapse contacts may develop de novo under pathological conditions. Microglial penta-partite synapses have received comparatively little attention as unique sites in the CNS where microglial cells, cytokines and other factors they release have a direct influence on the connections between neurons and their function. It concerns our understanding of the penta-partite synapse where the confusion created by the term “neuroinflammation” is most counterproductive. The mere presence of activated microglia or the release of their cytokines may occur independent of inflammation, and penta-partite synapses are not usually active in a neuroimmunological sense. Clarification of these details is the main purpose of this review, specifically highlighting the relationship between microglia, synapses, and the cytokines that can be released by microglial cells in health and disease.

Riluzole does not ameliorate disease caused by cytoplasmic TDP-43 in a mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease commonly treated with riluzole, a small molecule that may act via modulation of glutamatergic neurotransmission. However, riluzole only modestly extends lifespan for people living with ALS, and its precise mechanisms of action remain unclear. Most ALS cases are characterised by accumulation of cytoplasmic TAR DNA binding protein of 43 kDa (TDP-43), and understanding the effects of riluzole in models that closely recapitulate TDP-43 pathology may provide insights for development of improved therapeutics. We therefore investigated the effects of riluzole in female transgenic mice that inducibly express nuclear localisation sequence (NLS)-deficient human TDP-43 in neurons (NEFH-tTA/tetO-hTDP-43ΔNLS, 'rNLS8', mice). Riluzole treatment from the first day of hTDP-43ΔNLS expression did not alter disease onset, weight loss or performance on multiple motor behavioural tasks. Riluzole treatment also did not alter TDP-43 protein levels, solubility or phosphorylation. Although we identified a significant decrease in GluA2 and GluA3 proteins in the cortex of rNLS8 mice, riluzole did not ameliorate this disease-associated molecular phenotype. Likewise, riluzole did not alter the disease-associated atrophy of hindlimb muscle in rNLS8 mice. Finally, riluzole treatment beginning after disease onset in rNLS8 mice similarly had no effect on progression of late-stage disease or animal survival. Together, we demonstrate specific glutamatergic receptor alterations and muscle fibre-type changes reminiscent of ALS in female rNLS8 mice, but riluzole had no effect on these or any other disease phenotypes. Future targeting of pathways related to accumulation of TDP-43 pathology may be needed to develop better treatments for ALS.

Polymeric nanoparticle mediated inhibition of miR-21 with enhanced miR-124 expression for combinatorial glioblastoma therapy

Glioblastoma (GBM) is the most common and fatal form of malignant brain tumor. Despite intensive effort, there is still no effective GBM treatment. Therefore, novel and more effective GBM therapeutic approaches are highly desired. In this study, we combined polymeric nanotechnology with microRNA (miRNA) regulation technology to develop a targeted polymeric nanoparticle to co-deliver anti-miR-21 and miR-124 into the brain to effectively treat GBM. The polymeric nanoparticle decorated with Angiopep-2 peptide not only can encapsulate miRNA via triple-interaction (electrostatic, hydrogen bond and hydrophobic bonding) to protect miRNA against enzyme degradation in the blood, but also is capable of crossing blood brain barrier (BBB) and allowing targeted delivery of miRNAs to GBM tissue due to the dual-targeting function of Angiopep-2. Moreover, the co-delivered anti-miR-21 and miR-124 simultaneously regulated the mutant RAS/PI3K/PTEN/AKT signaling pathway in tumor cells, consequently achieving combinatorial GBM therapy. This combinatorial effect was confirmed by our results showing that these miRNA nanomedicines can effectively reduce tumor cell proliferation, migration and invasion as well as reducing tumor angiogenesis. Consequently, effective suppression of tumor growth and significantly improved medium survival time are observed when these miRNA nanomedicines were assessed in an orthotopic GBM xenograft model. This work indicated that our new polymeric nanoparticles successfully mediate inhibition of miR-21 and miR-124 supplementation to significantly reduce tumorigenesis, and may have strong potential in GBM therapy.

Splicing factor proline and glutamine rich intron retention, reduced expression and aggregate formation are pathological features of amyotrophic lateral sclerosis

AIM

Splicing factor proline and glutamine rich (SFPQ) is an RNA-DNA binding protein that is dysregulated in Alzheimer's disease and frontotemporal dementia. Dysregulation of SFPQ, specifically increased intron retention and nuclear depletion, has been linked to several genetic subtypes of amyotrophic lateral sclerosis (ALS), suggesting that SFPQ pathology may be a common feature of this heterogeneous disease. Our study aimed to investigate this hypothesis by providing the first comprehensive assessment of SFPQ pathology in large ALS case-control cohorts.

METHODS

We examined SFPQ at the RNA, protein and DNA levels. SFPQ RNA expression and intron retention were examined using RNA-sequencing and quantitative PCR. SFPQ protein expression was assessed by immunoblotting and immunofluorescent staining. At the DNA level, SFPQ was examined for genetic variation novel to ALS patients.

RESULTS

At the RNA level, retention of SFPQ intron nine was significantly increased in ALS patients' motor cortex. In addition, SFPQ RNA expression was significantly reduced in the central nervous system, but not blood, of patients. At the protein level, neither nuclear depletion nor reduced expression of SFPQ was found to be a consistent feature of spinal motor neurons. However, SFPQ-positive ubiquitinated protein aggregates were observed in patients' spinal motor neurons. At the DNA level, our genetic screen identified two novel and two rare SFPQ sequence variants not previously reported in the literature.

CONCLUSIONS

Our findings confirm dysregulation of SFPQ as a pathological feature of the central nervous system of ALS patients and indicate that investigation of the functional consequences of this pathology will provide insight into ALS biology.

In vivo Validation of Bimolecular Fluorescence Complementation (BiFC) to Investigate Aggregate Formation in Amyotrophic Lateral Sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is a form of motor neuron disease (MND) that is characterized by the progressive loss of motor neurons within the spinal cord, brainstem, and motor cortex. Although ALS clinically manifests as a heterogeneous disease, with varying disease onset and survival, a unifying feature is the presence of ubiquitinated cytoplasmic protein inclusion aggregates containing TDP-43. However, the precise mechanisms linking protein inclusions and aggregation to neuronal loss are currently poorly understood. Bimolecular fluorescence complementation (BiFC) takes advantage of the association of fluorophore fragments (non-fluorescent on their own) that are attached to an aggregation-prone protein of interest. Interaction of the proteins of interest allows for the fluorescent reporter protein to fold into its native state and emit a fluorescent signal. Here, we combined the power of BiFC with the advantages of the zebrafish system to validate, optimize, and visualize the formation of ALS-linked aggregates in real time in a vertebrate model. We further provide in vivo validation of the selectivity of this technique and demonstrate reduced spontaneous self-assembly of the non-fluorescent fragments in vivo by introducing a fluorophore mutation. Additionally, we report preliminary findings on the dynamic aggregation of the ALS-linked hallmark proteins Fus and TDP-43 in their corresponding nuclear and cytoplasmic compartments using BiFC. Overall, our data demonstrates the suitability of this BiFC approach to study and characterize ALS-linked aggregate formation in vivo. Importantly, the same principle can be applied in the context of other neurodegenerative diseases and has therefore critical implications to advance our understanding of pathologies that underlie aberrant protein aggregation.

Unbiased Label-Free Quantitative Proteomics of Cells Expressing Amyotrophic Lateral Sclerosis (ALS) Mutations in CCNF Reveals Activation of the Apoptosis Pathway: A Workflow to Screen Pathogenic Gene Mutations

The past decade has seen a rapid acceleration in the discovery of new genetic causes of ALS, with more than 20 putative ALS-causing genes now cited. These genes encode proteins that cover a diverse range of molecular functions, including free radical scavenging (e.g., SOD1), regulation of RNA homeostasis (e.g., TDP-43 and FUS), and protein degradation through the ubiquitin-proteasome system (e.g., ubiquilin-2 and cyclin F) and autophagy (TBK1 and sequestosome-1/p62). It is likely that the various initial triggers of disease (either genetic, environmental and/or gene-environment interaction) must converge upon a common set of molecular pathways that underlie ALS pathogenesis. Given the complexity, it is not surprising that a catalog of molecular pathways and proteostasis dysfunctions have been linked to ALS. One of the challenges in ALS research is determining, at the early stage of discovery, whether a new gene mutation is indeed disease-specific, and if it is linked to signaling pathways that trigger neuronal cell death. We have established a proof-of-concept proteogenomic workflow to assess new gene mutations, using CCNF (cyclin F) as an example, in cell culture models to screen whether potential gene candidates fit the criteria of activating apoptosis. This can provide an informative and time-efficient output that can be extended further for validation in a variety of in vitro and in vivo models and/or for mechanistic studies. As a proof-of-concept, we expressed cyclin F mutations (K97R, S195R, S509P, R574Q, S621G) in HEK293 cells for label-free quantitative proteomics that bioinformatically predicted activation of the neuronal cell death pathways, which was validated by immunoblot analysis. Proteomic analysis of induced pluripotent stem cells (iPSCs) derived from patient fibroblasts bearing the S621G mutation showed the same activation of these pathways providing compelling evidence for these candidate gene mutations to be strong candidates for further validation and mechanistic studies (such as E3 enzymatic activity assays, protein-protein and protein-substrate studies, and neuronal apoptosis and aberrant branching measurements in zebrafish). Our proteogenomics approach has great utility and provides a relatively high-throughput screening platform to explore candidate gene mutations for their propensity to cause neuronal cell death, which will guide a researcher for further experimental studies.

ALS/FTD-causing mutation in cyclin F causes the dysregulation of SFPQ

Previously, we identified missense mutations in CCNF that are causative of familial and sporadic amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Hallmark features of these diseases include the build-up of insoluble protein aggregates as well as the mislocalization of proteins such as transactive response DNA binding protein 43 kDa (TDP-43). In recent years, the dysregulation of SFPQ (splicing factor proline and glutamine rich) has also emerged as a pathological hallmark of ALS/FTD. CCNF encodes for the protein cyclin F, a substrate recognition component of an E3 ubiquitin ligase. We have previously shown that ALS/FTD-linked mutations in CCNF cause disruptions to overall protein homeostasis that leads to a build-up of K48-linked ubiquitylated proteins as well as defects in autophagic machinery. To investigate further processes that may be affected by cyclin F, we used a protein-proximity ligation method, known as Biotin Identification (BioID), standard immunoprecipitations and mass spectrometry to identify novel interaction partners of cyclin F and infer further process that may be affected by the ALS/FTD-causing mutation. Results demonstrate that cyclin F closely associates with proteins involved with RNA metabolism as well as a number of RNA-binding proteins previously linked to ALS/FTD, including SFPQ. Notably, the overexpression of cyclin F(S621G) led to the aggregation and altered subcellular distribution of SFPQ in human embryonic kidney (HEK293) cells, while leading to altered degradation in primary neurons. Overall, our data links ALS/FTD-causing mutations in CCNF to converging pathological features of ALS/FTD and provides a link between defective protein degradation systems and the pathological accumulation of a protein involved in RNA processing and metabolism.

A Robust Intrinsically Green Fluorescent Poly(Amidoamine) Dendrimer for Imaging and Traceable Central Nervous System Delivery in Zebrafish

Intrinsically fluorescent poly(amidoamine) dendrimers (IF-PAMAM) are an emerging class of versatile nanoplatforms for in vitro tracking and bio-imaging. However, limited tissue penetration of their fluorescence and interference due to auto-fluorescence arising from biological tissues limit its application in vivo. Herein, a green IF-PAMAM (FGP) dendrimer is reported and its biocompatibility, circulation, biodistribution and potential role for traceable central nervous system (CNS)-targeted delivery in zebrafish is evaluated, exploring various routes of administration. Key features of FGP include visible light excitation (488 nm), high fluorescence signal intensity, superior photostability and low interference from tissue auto-fluorescence. After intravenous injection, FGP shows excellent imaging and tracking performance in zebrafish. Further conjugating FGP with transferrin (FGP-Tf) significantly increases its penetration through the blood-brain barrier (BBB) and prolongs its circulation in the blood stream. When administering through local intratissue microinjection, including intracranial and intrathecal injection in zebrafish, both FGP and FGP-Tf exhibit excellent tissue diffusion and effective cellular uptake in the brain and spinal cord, respectively. This makes FGP/FGP-Tf attractive for in vivo tracing when transporting to the CNS is desired. The work addresses some of the major shortcomings in IF-PAMAM and provides a promising application of these probes in the development of drug delivery in the CNS.

Observation and characterisation of macrophages in zebrafish liver

Kupffer cells are liver-resident macrophages that play an important role in mediating immune-related functions in mammals and humans. They are well-known for their capacity to phagocytose large amounts of waste complexes, cell debris, microbial particles and even malignant cells. Location, appearance and functional aspects are important features used to identify these characteristic cells of the liver sinusoid. To-date, there is limited information on the occurrence of macrophages in zebrafish liver. Therefore, we aimed to characterise the ultrastructural and functional aspects of liver-associated macrophages in the zebrafish model by taking advantage of the latest advances in zebrafish genetics and multimodal correlative imaging. Herein, we report on the occurrence of macrophages within the zebrafish liver exhibiting conventional ultrastructural features (e.g. presence of pseudopodia, extensive lysosomal apparatus, a phagolysosome and making up ∼3% of the liver volume). Intriguingly, these cells were not located within the sinusoidal vascular bed of hepatic tissue but instead resided between hepatocytes and lacked phagocytic function. While our results demonstrated the presence and structural similarities with liver macrophages from other experimental models, their functional characteristics were distinctly different from Kupffer cells that have been described in rodents and humans. These findings illustrate that the innate immune system of the zebrafish liver has some distinctly different characteristics compared to other animal experimental models. This conclusion underpins our call for future studies in order to have a better understanding of the physiological role of macrophages residing between the parenchymal cells of the zebrafish liver.

Muscle specific kinase protects dystrophic mdx mouse muscles from eccentric contraction-induced loss of force-producing capacity

KEY POINTS

Adeno-associated viral vector was used to elevate the expression of muscle specific kinase (MuSK) and rapsyn (a cytoplasmic MuSK effector protein) in the tibialis anterior muscle of wild-type and dystrophic (mdx) mice. In mdx mice, enhanced expression of either MuSK or rapsyn ameliorated the acute loss of muscle force associated with strain injury. Increases in sarcolemmal immunolabelling for utrophin and β-dystroglycan suggest a mechanism for the protective effect of MuSK in mdx muscles. MuSK also caused subtle changes to the structure and function of the neuromuscular junction, suggesting novel roles for MuSK in muscle physiology and pathophysiology.

ABSTRACT

Muscle specific kinase (MuSK) has a well-defined role in stabilizing the developing mammalian neuromuscular junction, but MuSK might also be protective in some neuromuscular diseases. In the dystrophin-deficient mdx mouse model of Duchenne muscular dystrophy, limb muscles are especially fragile. We injected the tibialis anterior muscle of 8-week-old mdx and wild-type (C57BL10) mice with adeno-associated viral vectors encoding either MuSK or rapsyn (a cytoplasmic MuSK effector protein) fused to green fluorescent protein (MuSK-GFP and rapsyn-GFP, respectively). Contralateral muscles injected with empty vector served as controls. One month later mice were anaesthetized with isoflurane and isometric force-producing capacity was recorded from the distal tendon. MuSK-GFP caused an unexpected decay in nerve-evoked tetanic force, both in wild-type and mdx muscles, without affecting contraction elicited by direct electrical stimulation of the muscle. Muscle fragility was probed by challenging muscles with a strain injury protocol consisting of a series of four strain-producing eccentric contractions in vivo. When applied to muscles of mdx mice, eccentric contraction produced an acute 27% reduction in directly evoked muscle force output, affirming the susceptibility of mdx muscles to strain injury. mdx muscles overexpressing MuSK-GFP or rapsyn-GFP exhibited significantly milder force deficits after the eccentric contraction challenge (15% and 14%, respectively). The protective effect of MuSK-GFP in muscles of mdx mice was associated with increased immunolabelling for utrophin and β-dystroglycan in the sarcolemma. Elevating the expression of MuSK or rapsyn revealed several distinct synaptic and extrasynaptic effects, suggesting novel roles for MuSK signalling in muscle physiology and pathophysiology.

Nucleo-cytoplasmic transport of TDP-43 studied in real time: impaired microglia function leads to axonal spreading of TDP-43 in degenerating motor neurons

Transactivating DNA-binding protein-43 (TDP-43) deposits represent a typical finding in almost all ALS patients, more than half of FTLD patients and patients with several other neurodegenerative disorders. It appears that perturbation of nucleo-cytoplasmic transport is an important event in these conditions but the mechanistic role and the fate of TDP-43 during neuronal degeneration remain elusive. We have developed an experimental system for visualising the perturbed nucleocytoplasmic transport of neuronal TDP-43 at the single-cell level in vivo using zebrafish spinal cord. This approach enabled us to image TDP-43-expressing motor neurons before and after experimental initiation of cell death. We report the formation of mobile TDP-43 deposits within degenerating motor neurons, which are normally phagocytosed by microglia. However, when microglial cells were depleted, injury-induced motor neuron degeneration follows a characteristic process that includes TDP-43 redistribution into the cytoplasm, axon and extracellular space. This is the first demonstration of perturbed TDP-43 nucleocytoplasmic transport in vivo, and suggests that impairment in microglial phagocytosis of dying neurons may contribute towards the formation of pathological TDP-43 presentations in ALS and FTLD.

Real-time visualization of oxidative stress-mediated neurodegeneration of individual spinal motor neurons in vivo

Generation of reactive oxygen species (ROS) has been shown to be important for many physiological processes, ranging from cell differentiation to apoptosis. With the development of the genetically encoded photosensitiser KillerRed (KR) it is now possible to efficiently produce ROS dose-dependently in a specific cell type upon green light illumination. Zebrafish are the ideal vertebrate animal model for these optogenetic methods because of their transparency and efficient transgenesis. Here we describe a zebrafish model that expresses membrane-targeted KR selectively in motor neurons. We show that KR-activated neurons in the spinal cord undergo stress and cell death after induction of ROS. Using single-cell resolution and time-lapse confocal imaging, we selectively induced neurodegeneration in KR-expressing neurons leading to characteristic signs of apoptosis and cell death. We furthermore illustrate a targeted microglia response to the induction site as part of a physiological response within the zebrafish spinal cord. Our data demonstrate the successful implementation of KR mediated ROS toxicity in motor neurons in vivo and has important implications for studying the effects of ROS in a variety of conditions within the central nervous system, including aging and age-related neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis.

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Motor neurons can be targeted for oxidative stress using optogenetics in zebrafish.

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KillerRed expressing neurons undergo characteristic sequence of neurodegeneration.

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Targeted neurons show microglial activation as part of the physiological response.

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ROS toxicity has important implications for mechanisms driving neurodegeneration.

Casein kinase II phosphorylation of cyclin F at serine 621 regulates the Lys48-ubiquitylation E3 ligase activity of the SCF(cyclin F) complex

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder that is characterized by progressive weakness, paralysis and muscle loss often resulting in patient death within 3–5 years of diagnosis. Recently, we identified disease-linked mutations in the CCNF gene, which encodes the cyclin F protein, in cohorts of patients with familial and sporadic ALS and frontotemporal dementia (FTD) (Williams KL et al. 2016 Nat. Commun.7, 11253. (doi:10.1038/ncomms11253)). Cyclin F is a part of a Skp1-Cul-F-box (SCF) E3 ubiquitin-protein ligase complex and is responsible for ubiquitylating proteins for degradation by the proteasome. In this study, we investigated the phosphorylation status of cyclin F and the effect of the serine to glycine substitution at site 621 (S621G) on E3 ligase activity. This specific mutation (S621G) was found in a multi-generational Australian family with ALS/FTD. We identified seven phosphorylation sites on cyclin F, of which five are newly reported including Ser621. These phosphorylation sites were mostly identified within the PEST (proline, glutamic acid, serine and threonine) sequence located at the C-terminus of cyclin F. Additionally, we determined that casein kinase II (CK2) can phosphorylate Ser621 and thereby regulate the E3 ligase activity of the SCF^{(cyclin F)} complex. Furthermore, the S621G mutation in cyclin F prevents phosphorylation by CK2 and confers elevated Lys48-ubiquitylation activity, a hallmark of ALS/FTD pathology. These findings highlight the importance of phosphorylation in regulating the activity of the SCF^{(cyclin F)} E3 ligase complex that can affect downstream processes and may lead to defective motor neuron development, neuron degeneration and ultimately ALS and FTD.

Cannabinoid-induced increase of quantal size and enhanced neuromuscular transmission

Cannabinoids exert dynamic control over many physiological processes including memory formation, cognition and pain perception. In the central nervous system endocannabinoids mediate negative feedback of quantal transmitter release following postsynaptic depolarization. The influence of cannabinoids in the peripheral nervous system is less clear and might have broad implications for the therapeutic application of cannabinoids. We report a novel cannabinoid effect upon the mouse neuromuscular synapse: acutely increasing synaptic vesicle volume and raising the quantal amplitudes. In a mouse model of myasthenia gravis the cannabinoid receptor agonist WIN 55,212 reversed fatiguing failure of neuromuscular transmission, suggesting future therapeutic potential. Our data suggest an endogenous pathway by which cannabinoids might help to regulate transmitter release at the neuromuscular junction.

Aurora kinase B regulates axonal outgrowth and regeneration in the spinal motor neurons of developing zebrafish

Aurora kinase B (AurkB) is a serine/threonine protein kinase with a well-characterised role in orchestrating cell division and cytokinesis, and is prominently expressed in healthy proliferating and cancerous cells. However, the role of AurkB in differentiated and non-dividing cells has not been extensively explored. Previously, we have described a significant upregulation of AurkB expression in cultured cortical neurons following an experimental axonal transection. This is somewhat surprising, as AurkB expression is generally associated only with dividing cells Frangini et al. (Mol Cell 51:647-661, 2013); Hegarat et al. (J Cell Biol 195:1103-1113, 2011); Lu et al. (J Biol Chem 283:31785-31790, 2008); Trakala et al. (Cell Cycle 12:1030-1041, 2014). Herein, we present the first description of a role for AurkB in terminally differentiated neurons. AurkB was prominently expressed within post-mitotic neurons of the zebrafish brain and spinal cord. The expression of AurkB varied during the development of the zebrafish spinal motor neurons. Utilising pharmacological and genetic manipulation to impair AurkB activity resulted in truncation and aberrant motor axon morphology, while overexpression of AurkB resulted in extended axonal outgrowth. Further pharmacological inhibition of AurkB activity in regenerating axons delayed their recovery following UV laser-mediated injury. Collectively, these results suggest a hitherto unreported role of AurkB in regulating neuronal development and axonal outgrowth.

DNA nanoclew templated spherical nucleic acids for siRNA delivery

A superior biocompatible spherical nucleic acid (SNA) conjugate was fabricated by grafting siRNA onto the surface of a core composed of a spherical DNA nanostructure that we have termed DNA nanoclew (DC).

A versatile upconversion surface evaluation platform for bio-nano surface selection for the nervous system

There is considerable interest in developing diagnostic nanotools for early detection and delivery of various therapeutic agents for treatment of neurodegenerative diseases. However, a key challenge remains in the selection of suitable surfaces to overcome the nano-bio interface issue, namely that many nanoparticle surfaces demonstrate instability when administered into biological environments and show substantial cytotoxicity to the central nervous system. In this study, we fabricated an evaluation platform for bio-nano surface selection based on the combination of upconversion nanoparticles (UCNPs), cultured neural cells and zebra fish, and systemically demonstrated how it can evaluate the suitability of nanoparticle surfaces for applications in the central nervous system. Firstly, we fabricated highly lanthanide-doped UCNPs, which generate the strongest tissue penetrable emission at 800 nm. We then functionalized these UCNPs with four popular surfaces for evaluation. Next, we systematically evaluated the spectral emission properties, biophysical stability, cytotoxicity and cell uptake capability of these surface-functionalized UCNPs in biological solutions or with cultured NSC-34 cells. Through these studies, PEG-COOH proved to be the superior surface modification. Accordingly, we further confirmed the bioavailability of unmodified and surface modified UCNPs in the spinal cord of living zebrafish. As predicted, PEG-UCNPs displayed excellent dispersal and uptake into spinal motor neurons in living zebrafish. Collectively, this study developed a versatile upconversion platform for systematic evaluation of nanoparticle surfaces, which can provide valuable information via systemic surface evaluation in vitro and in vivo for future construction of multifunctional nanosystems for theranostic applications in neurodegenerative diseases.

Improving the Delivery of SOD1 Antisense Oligonucleotides to Motor Neurons Using Calcium Phosphate-Lipid Nanoparticles

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease affecting the upper and lower motor neurons in the motor cortex and spinal cord. Abnormal accumulation of mutant superoxide dismutase I (SOD1) in motor neurons is a pathological hallmark of some forms of the disease. We have shown that the orderly progression of the disease may be explained by misfolded SOD1 cell-to-cell propagation, which is reliant upon its active endogenous synthesis. Reducing the levels of SOD1 is therefore a promising therapeutic approach. Antisense oligonucleotides (ASOs) can efficiently silence proteins with gain-of-function mutations. However, naked ASOs have a short circulation half-life and are unable to cross the blood brain barrier (BBB) warranting the use of a drug carrier for effective delivery. In this study, calcium phosphate lipid coated nanoparticles (CaP-lipid NPs) were developed for delivery of SOD1 ASO to motor neurons. The most promising nanoparticle formulation (Ca/P ratio of 100:1), had a uniform spherical core-shell morphology with an average size of 30 nm, and surface charge (ζ-potential) of -4.86 mV. The encapsulation efficiency of ASO was 48% and stability studies found the particle to be stable over a period of 20 days. In vitro experiments demonstrated that the negatively charged ASO-loaded CaP-lipid NPs could effectively deliver SOD1-targeted ASO into a mouse motor neuron-like cell line (NSC-34) through endocytosis and significantly down-regulated SOD1 expression in HEK293 cells. The CaP-lipid NPs exhibited a pH-dependant dissociation, suggesting that that the acidification of lysosomes is the likely mechanism responsible for facilitating intracellular ASO release. To demonstrate tissue specific delivery and localization of these NPs we performed in vivo microinjections into zebrafish. Successful delivery of these NPs was confirmed for the zebrafish brain, the blood stream, and the spinal cord. These results suggest that CaP-lipid NPs could be an effective and safe delivery system for the improved delivery of SOD1 ASOs to motor neurons. Further in vivo evaluation in transgenic mouse models of SOD1 ALS are therefore warranted.

Utility and reliability of non-invasive muscle function tests in high-fat-fed mice

NEW FINDINGS

What is the central question of this study? Non-invasive muscle function tests have not been validated for use in the study of muscle performance in high-fat-fed mice. What is the main finding and its importance? This study shows that grip strength, hang wire and four-limb hanging tests are able to discriminate the muscle performance between chow-fed and high-fat-fed mice at different time points, with grip strength being reliable after 5, 10 and 20 weeks of dietary intervention. Non-invasive tests are commonly used for assessing muscle function in animal models. The value of these tests in obesity, a condition where muscle strength is reduced, is unclear. We investigated the utility of three non-invasive muscle function tests, namely grip strength (GS), hang wire (HW) and four-limb hanging (FLH), in C57BL/6 mice fed chow (chow group, n = 48) or a high-fat diet (HFD group, n = 48) for 20 weeks. Muscle function tests were performed at 5, 10 and 20 weeks. After 10 and 20 weeks, HFD mice had significantly reduced GS (in newtons; mean ± SD: 10 weeks chow, 1.89 ± 0.1 and HFD, 1.79 ± 0.1; 20 weeks chow, 1.99 ± 0.1 and HFD, 1.75 ± 0.1), FLH [in seconds per gram body weight; median (interquartile range): 10 weeks chow, 2552 (1337-4964) and HFD, 1230 (749-1994); 20 weeks chow, 2048 (765-3864) and HFD, 1036 (717-1855)] and HW reaches [n; median (interquartile range): 10 weeks chow, 4 (2-5) and HFD, 2 (1-3); 20 weeks chow, 3 (1-5) and HFD, 1 (0-2)] and higher falls [n; median (interquartile range): 10 weeks chow, 0 (0-2) and HFD, 3 (1-7); 20 weeks chow, 1 (0-4) and HFD, 8 (5-10)]. Grip strength was reliable in both dietary groups [intraclass correlation coefficient (ICC) = 0.5-0.8; P < 0.05], whereas FLH showed good reliability in chow (ICC = 0.7; P < 0.05) but not in HFD mice after 10 weeks (ICC < 0.5). Our data demonstrate that non-invasive muscle function tests are valuable and reliable tools for assessment of muscle strength and function in high-fat-fed mice.

Multifunctional Hybrid Nanoparticles for Traceable Drug Delivery and Intracellular Microenvironment-Controlled Multistage Drug-Release in Neurons

Innovative nanoparticles hold promising potential for disease therapy as drug delivery systems. For brain-disease therapy, a drug delivery system that can sustainably control drug-release and monitor fluorescence of the drug cargos is highly desirable. In this study, a light-traceable and intracellular microenvironment-responsive drug delivery system was developed based on the combination of glutathione-responsive autoflurescent nanogel, dendrimer-like mesoporous silica nanoparticles, and gold nanoparticles. The resulting hybrid nanoparticles represent a new class of delivery system that can efficiently load, transport, and control multistage-release of sulfydryl-containing drugs into neurons, with light-traceable monitoring for future brain-disease therapy.

Triggering Cell Stress and Death Using Conventional UV Laser Confocal Microscopy

Using a standard confocal setup, a UV ablation method can be utilized to selectively induce cellular injury and to visualize single-cell responses and cell-cell interactions in the CNS in real-time. Previously, studying these cell-specific responses after injury often required complicated setups or the transfer of cells or animals into different, non-physiological environments, confounding immediate and short-term analysis. For example, drug-mediated ablation approaches often lack the specificity that is required to study single-cell responses and immediate cell-cell interactions. Similarly, while high-power pulsed laser ablation approaches provide very good control and tissue penetration, they require specialized equipment that can complicate real-time visualization of cellular responses. The refined UV laser ablation approach described here allows researchers to stress or kill an individual cell in a dose- and time-dependent manner using a conventional confocal microscope equipped with a 405-nm laser. The method was applied to selectively ablate a single neuron within a dense network of surrounding cells in the zebrafish spinal cord. This approach revealed a dose-dependent response of the ablated neurons, causing the fragmentation of cellular bodies and anterograde degeneration along the axon within minutes to hours. This method allows researchers to study the fate of an individual dying cell and, importantly, the instant response of cells-such as microglia and astrocytes-surrounding the ablation site.

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 β‐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.

The established and emerging roles of astrocytes and microglia in amyotrophic lateral sclerosis and frontotemporal dementia

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are two progressive, fatal neurodegenerative syndromes with considerable clinical, genetic and pathological overlap. Clinical symptoms of FTD can be seen in ALS patients and vice versa. Recent genetic discoveries conclusively link the two diseases, and several common molecular players have been identified (TDP-43, FUS, C9ORF72). The definitive etiologies of ALS and FTD are currently unknown and both disorders lack a cure. Glia, specifically astrocytes and microglia are heavily implicated in the onset and progression of neurodegeneration witnessed in ALS and FTD. In this review, we summarize the current understanding of the role of microglia and astrocytes involved in ALS and FTD, highlighting their recent implications in neuroinflammation, alterations in waste clearance involving phagocytosis and the newly described glymphatic system, and vascular abnormalities. Elucidating the precise mechanisms of how astrocytes and microglia are involved in ALS and FTD will be crucial in characterizing these two disorders and may represent more effective interventions for disease progression and treatment options in the future.

In vivo characterization of microglial engulfment of dying neurons in the zebrafish spinal cord

Microglia are specialized phagocytes in the vertebrate central nervous system (CNS). As the resident immune cells of the CNS they play an important role in the removal of dying neurons during both development and in several neuronal pathologies. Microglia have been shown to prevent the diffusion of damaging degradation products of dying neurons by engulfment and ingestion. Here we describe a live imaging approach that uses UV laser ablation to selectively stress and kill spinal neurons and visualize the clearance of neuronal remnants by microglia in the zebrafish spinal cord. In vivo imaging confirmed the motile nature of microglia within the uninjured spinal cord. However, selective neuronal ablation triggered rapid activation of microglia, leading to phagocytic uptake of neuronal debris by microglia within 20-30 min. This process of microglial engulfment is highly dynamic, involving the extension of processes toward the lesion site and consequently the ingestion of the dying neuron. 3D rendering analysis of time-lapse recordings revealed the formation of phagosome-like structures in the activated microglia located at the site of neuronal ablation. This real-time representation of microglial phagocytosis in the living zebrafish spinal cord provides novel opportunities to study the mechanisms of microglia-mediated neuronal clearance.

Electrophysiological analysis of neuromuscular synaptic function in myasthenia gravis patients and animal models

Study of the electrophysiological function of the neuromuscular junction (NMJ) is instrumental in the understanding of the symptoms and pathophysiology of myasthenia gravis (MG), an autoimmune disorder characterized by fluctuating and fatigable muscle weakness. Most patients have autoantibodies to the acetylcholine receptor at the NMJ. However, in recent years autoantibodies to other crucial postsynaptic membrane proteins have been found in previously 'seronegative' MG patients. Electromyographical recording of compound and single-fibre muscle action potentials provides a crucial in vivo method to determine neuromuscular transmission failure while ex vivo (miniature) endplate potential recordings can reveal the precise synaptic impairment. Here we will review these electrophysiological methods used to assess NMJ function and discuss their application and typical results found in the diagnostic and experimental study of patients and animal models of the several forms of MG.

The neuromuscular junction: measuring synapse size, fragmentation and changes in synaptic protein density using confocal fluorescence microscopy

The neuromuscular junction (NMJ) is the large, cholinergic relay synapse through which mammalian motor neurons control voluntary muscle contraction. Structural changes at the NMJ can result in neurotransmission failure, resulting in weakness, atrophy and even death of the muscle fiber. Many studies have investigated how genetic modifications or disease can alter the structure of the mouse NMJ. Unfortunately, it can be difficult to directly compare findings from these studies because they often employed different parameters and analytical methods. Three protocols are described here. The first uses maximum intensity projection confocal images to measure the area of acetylcholine receptor (AChR)-rich postsynaptic membrane domains at the endplate and the area of synaptic vesicle staining in the overlying presynaptic nerve terminal. The second protocol compares the relative intensities of immunostaining for synaptic proteins in the postsynaptic membrane. The third protocol uses Fluorescence Resonance Energy Transfer (FRET) to detect changes in the packing of postsynaptic AChRs at the endplate. The protocols have been developed and refined over a series of studies. Factors that influence the quality and consistency of results are discussed and normative data are provided for NMJs in healthy young adult mice.

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 β-subunit-Y390 was reduced at endplates. In contrast, endplate staining for β-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.