Stiff person syndrome-associated autoantibodies to amphiphysin mediate reduced GABAergic inhibition

Pathogenesis and immunopathology of paraneoplastic disorders

Paraneoplastic neurologic syndromes (PNS) represent a rare group of immune-mediated complications associated with an underlying tumor. Ectopic protein expression in neoplastic cells or an aberrant immune regulation in the course of hematooncologic diseases or thymomas trigger an autoimmune response that may affect any part of the central and/or peripheral nervous system. Recent advances in drug therapies as well as novel animal models and neuropathologic studies have led to further insights on the immune pathomechanisms of PNS. Although the syndromes share common paths in pathogenesis, they may differ in the disease course, prognosis, and therapy targets, depending on the localization and type of antibody epitope. Neuropathologic hallmarks of PNS associated with antibodies directed against intracellular epitopes are characterized by T cell-dominated inflammation, reactive gliosis including microglial nodules, and neuronal degeneration. By contrast, the neuropathology of cell surface antibody-mediated PNS strongly depends on the targeted antigen and varies from B cell/plasma cell-dominated inflammation and well-preserved neurons together with a reduced expression of the target antigen in anti-NMDAR encephalitis to irreversible Purkinje cell loss in anti-P/Q-type VGCC antibody-associated paraneoplastic cerebellar degeneration. The understanding of different pathomechanisms in PNS is important because they strongly correspond with therapy response and prognosis, and should guide treatment decisions.

General features, pathogenesis, and laboratory diagnostics of autoimmune encephalitis

Autoimmune encephalitis (AE) is a group of inflammatory conditions that can associate with the presence of antibodies directed to neuronal intracellular, or cell surface antigens. These disorders are increasingly recognized as an important differential diagnosis of infectious encephalitis and of other common neuropsychiatric conditions. Autoantibody diagnostics plays a pivotal role for accurate diagnosis of AE, which is of utmost importance for the prompt recognition and early treatment. Several AE subgroups can be identified, either according to the prominent clinical phenotype, presence of a concomitant tumor, or type of neuronal autoantibody, and recent diagnostic criteria have provided important insights into AE classification. Antibodies to neuronal intracellular antigens typically associate with paraneoplastic neurological syndromes and poor prognosis, whereas antibodies to synaptic/neuronal cell surface antigens characterize many AE subtypes that associate with tumors less frequently, and that are often immunotherapy-responsive. In addition to the general features of AE, we review current knowledge on the pathogenic mechanisms underlying these disorders, focusing mainly on the potential role of neuronal antibodies in the most frequent conditions, and highlight current theories and controversies. Then, we dissect the crucial aspects of the laboratory diagnostics of neuronal antibodies, which represents an actual challenge for both pathologists and neurologists. Indeed, this diagnostics entails technical difficulties, along with particularly interesting novel features and pitfalls. The novelties especially apply to the wide range of assays used, including specific tissue-based and cell-based assays. These assays can be developed in-house, usually in specialized laboratories, or are commercially available. They are widely used in clinical immunology and in clinical chemistry laboratories, with relevant differences in analytic performance. Indeed, several data indicate that in-house assays could perform better than commercial kits, notwithstanding that the former are based on non-standardized protocols. Moreover, they need expertise and laboratory facilities usually unavailable in clinical chemistry laboratories. Together with the data of the literature, we critically evaluate the analytical performance of the in-house vs commercial kit-based approach. Finally, we propose an algorithm aimed at integrating the present strategies of the laboratory diagnostics in AE for the best clinical management of patients with these disorders.

Pathophysiological Effects of Autoantibodies in Autoimmune Encephalitides

The heterogeneity of autoantibody targets in autoimmune encephalitides presents a challenge for understanding cellular and humoral pathophysiology, and the development of new treatment strategies. Thus, current treatment aims at autoantibody removal and immunosuppression, and is primarily based on data generated from other autoimmune neurological diseases and expert consensus. There are many subtypes of autoimmune encephalitides, which now entails both diseases with autoantibodies targeting extracellular antigens and classical paraneoplastic syndromes with autoantibodies targeting intracellular antigens. Here, we review the current knowledge of molecular and cellular effects of autoantibodies associated with autoimmune encephalitis, and evaluate the evidence behind the proposed pathophysiological mechanisms of autoantibodies in autoimmune encephalitis.

Anti-amphiphysin antibody-associated paraneoplastic brainstem encephalitis with pruritus and dysphagia as the first symptoms: A case report

RATIONALE

Anti-amphiphysin antibodies are uncommonly detected in paraneoplastic neurologic syndromes (PNS), especially in patients with small cell lung cancer. Here, we report the first case of anti-amphiphysin antibody-associated PNS with pruritus and dysphagia as the first complaints.

PATIENT CONCERNS

The patient was a 58-year-old man who sought medical advice with a chief complaint of dysphagia and the lung occupancy. We found that he had developed progressive pruritus several months ago.

DIAGNOSES

In the outer basal segment of the right lung lower lobe, PET-CT revealed small occupancies with hypermetabolism. Later, the pathology showed small cell lung cancer. And anti-amphiphysin antibodies were detected in serum. Above all, the patient's symptoms improved significantly after antitumor treatment. Even neither of the 2 cranial enhancement MRIs showed any meaningful imaging signs, the above evidence could confirm the diagnosis of PNS.

INTERVENTIONS

The chemotherapy regimen was etoposide 0.1g d1-3+cisplatin 40 mg d1-3 (q3w). Paroxetine 20 mg/day was given to relieve the itching.

OUTCOMES

After the treatment, the Watian water swallowing test dropped from grade 5 to grade 1, the intense itching also became tolerable.

LESSONS

Clinicians should consider diagnoses other than anxiety states or esophageal cancer in a patient with pruritus and dysphagia, such as PNS.

Case Report: Brain tumor's pitfalls: two cases of high-grade brain tumors mimicking autoimmune encephalitis with positive onconeuronal antibodies

Background

Glioblastoma (GBM) is the most common primary brain tumor in adulthood. Initial diagnosis is generally based on clinical and MRI findings, which may be misinterpreted as other neurological pictures, including autoimmune encephalitis (AE). AE is a heterogeneous group of neuroinflammatory diseases due to the presence of auto-antibodies targeting antigens on neuronal synaptic or cell surface. In the present report, we describe two peculiar cases of GBM initially misdiagnosed as AE, focusing on the diagnostic pitfalls and the treatment strategies.

Methods

We report the case of two patients with high-grade brain tumors, initially misdiagnosed and treated for AE. Clinical, laboratory, and neuroradiological data are discussed in terms of differential diagnosis between AE and GBM.

Results

The presence of atypical brain MRI findings and the unresponsiveness to immunosuppressive treatment are major red flags in the differential diagnosis between AE and GBM. In these cases, a brain biopsy is necessary to confirm the diagnosis.

Conclusions

Atypical brain tumor presentation causes a diagnostic and therapeutic delay. A positive onconeural autoantibodies result should always be interpreted cautiously, considering the possibility of a false-positive test. A brain biopsy is mandatory for a definite diagnosis.

Case report: Reversible brain atrophy with low titer anti-amphiphysin antibodies related to gastric adenocarcinoma

Amphiphysin (AMPH) autoimmunity is associated with a variety of neurological complications, including encephalitis, peripheral neuropathy, myelopathy, and cerebellar syndrome. Its diagnosis is based on clinical neurological deficits and the presence of serum anti-AMPH antibodies. Active immunotherapy, such as intravenous immunoglobulins, steroids, and other immunosuppressive therapies, has been reported to be effective in most patients. However, the extent of recovery varies depending on the case. Herein, we report the case of a 75-year-old woman with semi-rapidly progressive systemic tremors, visual hallucinations, and irritability. Upon hospitalization, she developed a mild fever and cognitive impairment. Brain magnetic resonance imaging (MRI) showed semi-rapidly progressive diffuse cerebral atrophy (DCA) over 3 months, while no clear abnormal intensities were observed. The nerve conduction study revealed sensory and motor neuropathy in the limbs. The fixed tissue-based assay (TBA) failed to detect antineuronal antibodies; however, based on commercial immunoblots, the presence of anti-AMPH antibodies was suspected. Therefore, serum immunoprecipitation was performed, which confirmed the presence of anti-AMPH antibodies. The patient also had gastric adenocarcinoma. High-dose methylprednisolone, and intravenous immunoglobulin were administered and tumor resection was performed, resulting in resolution of the cognitive impairment and improvement in the DCA on the post-treatment MRI. After immunotherapy and tumor resection, the patient's serum was analyzed using immunoprecipitation, which showed a decrease in the level of anti-AMPH antibodies. This case is noteworthy because the DCA showed improvement after immunotherapy and tumor resection. Additionally, this case demonstrates that negative TBA with positive commercial immunoblots do not necessarily indicate false positive results.

Cerebrospinal fluid proteomics indicates immune dysregulation and neuronal dysfunction in antibody associated autoimmune encephalitis

Autoimmune Encephalitis (AE) spans a group of non-infectious inflammatory conditions of the central nervous system due to an imbalanced immune response. Aiming to elucidate the pathophysiological mechanisms of AE, we applied an unsupervised proteomic approach to analyze the cerebrospinal fluid (CSF) protein profile of AE patients with autoantibodies against N-methyl-d-aspartate receptor (NMDAR) (n = 9), leucine-rich glioma-inactivated protein 1 (LGI1) (n = 9), or glutamate decarboxylase 65 (GAD65) (n = 8) compared to 9 patients with relapsing-remitting multiple sclerosis as inflammatory controls, and 10 patients with somatic symptom disorder as non-inflammatory controls. We found a dysregulation of the complement system, a disbalance between pro-inflammatory and anti-inflammatory proteins on the one hand, and dysregulation of proteins involved in synaptic transmission, synaptogenesis, brain connectivity, and neurodegeneration on the other hand to a different extent in all AE subtypes compared to non-inflammatory controls. Furthermore, elevated levels of several proteases and reduction in protease inhibitors could be detected in all AE subtypes compared to non-inflammatory controls. Moreover, the different AE subtypes showed distinct protein profiles compared to each other and inflammatory controls which may facilitate future identification of disease-specific biomarkers. Overall, CSF proteomics provides insights into the complex pathophysiological mechanisms of AE, including immune dysregulation, neuronal dysfunction, neurodegeneration, and altered protease function.

Brain blood vessel autoantibodies in patients with NMDA and GABAA receptor encephalitis: identification of unconventional Myosin-X as target antigen

Introduction: The antibody repertoire from CSF-derived antibody-secreting cells and memory B-cells in patients with encephalitis contains a considerable number of antibodies that do not target the disease-defining autoantigen such as the GABA or NMDA receptors. This study focuses on the functional relevance of autoantibodies to brain blood vessels in patients with GABA_A and NMDA receptor encephalitis. Methods: We tested 149 human monoclonal IgG antibodies from the cerebrospinal fluid of six patients with different forms of autoimmune encephalitis on murine brain sections for reactivity to blood vessels using immunohistochemistry. Positive candidates were tested for reactivity with purified brain blood vessels, effects on transendothelial electrical resistance (TEER), and expression of tight junction proteins as well as gene regulation using human brain microvascular endothelial hCMEC/D3 cells as in vitro blood-brain barrier model. One blood-vessel reactive antibody was infused intrathecally by pump injection in mice to study in vivo binding and effects on tight junction proteins such as Occludin. Target protein identification was addressed using transfected HEK293 cells. Results: Six antibodies reacted with brain blood vessels, three were from the same patient with GABA_AR encephalitis, and the other three were from different patients with NMDAR encephalitis. One antibody from an NMDAR encephalitis patient, mAb 011-138, also reacted with cerebellar Purkinje cells. In this case, treatment of hCMEC/D3 cells resulted in decreased TEER, reduced Occludin expression, and mRNA levels. Functional relevance in vivo was confirmed as Occludin downregulation was observed in mAb 011-138-infused animals. Unconventional Myosin-X was identified as a novel autoimmune target for this antibody. Discussion: We conclude that autoantibodies to blood vessels occur in autoimmune encephalitis patients and might contribute to a disruption of the blood-brain barrier thereby suggesting a potential pathophysiological relevance of these antibodies.

Autoimmune RNA dysregulation and seizures: therapeutic prospects in neuropsychiatric lupus

Lupus autoimmunity frequently presents with neuropsychiatric manifestations, but underlying etiology remains poorly understood. Human brain cytoplasmic 200 RNA (BC200 RNA) is a translational regulator in neuronal synapto-dendritic domains. Here, we show that a BC200 guanosine-adenosine dendritic transport motif is recognized by autoantibodies from a subset of neuropsychiatric lupus patients. These autoantibodies impact BC200 functionality by quasi irreversibly displacing two RNA transport factors from the guanosine-adenosine transport motif. Such anti-BC autoantibodies, which can gain access to brains of neuropsychiatric lupus patients, give rise to clinical manifestations including seizures. To establish causality, naive mice with a permeabilized blood-brain barrier were injected with anti-BC autoantibodies from lupus patients with seizures. Animals so injected developed seizure susceptibility with high mortality. Seizure activity was entirely precluded when animals were injected with lupus anti-BC autoantibodies together with BC200 decoy autoantigen. Seizures are a common clinical manifestation in neuropsychiatric lupus, and our work identifies anti-BC autoantibody activity as a mechanistic cause. The results demonstrate potential utility of BC200 decoys for autoantibody-specific therapeutic interventions in neuropsychiatric lupus.

Stiff person syndrome spectrum disorders; more than meets the eye

Stiff person syndrome spectrum disorders (SPSD) are a group of rare neuroimmunological disorders that often include painful spasms and rigidity. However, patients have highly heterogeneous signs and symptoms which may reflect different mechanistic disease processes. Understanding subsets of patients based on clinical phenotype may be important for prognosis and guiding treatment. The goal of this review is to provide updates on SPSD and its expanding clinical spectrum, prognostic markers, and treatment considerations. Further, we describe the current understanding in immunopathogenesis and highlight gaps in our knowledge appropriate for future research directions. Examples of revised diagnostic criteria for SPSD based on phenotype are also presented.

Inositol 1,4,5-trisphosphate receptor type 1 autoantibody (ITPR1-IgG/anti-Sj)-associated autoimmune cerebellar ataxia, encephalitis and peripheral neuropathy: review of the literature

Background

In 2014, we first described novel autoantibodies to the inositol 1,4,5-trisphosphate receptor type 1 (ITPR1-IgG/anti-Sj) in patients with autoimmune cerebellar ataxia (ACA) in this journal. Here, we provide a review of the available literature on ITPR1-IgG/anti-Sj, covering clinical and paraclinical presentation, tumour association, serological findings, and immunopathogenesis.

Methods

Review of the peer-reviewed and PubMed-listed English language literature on ITPR1-IgG/anti-Sj. In addition, we provide an illustrative report on a new patient with ITPR1-IgG-associated encephalitis with cognitive decline and psychosis.

Results

So far, at least 31 patients with serum ITPR1-IgG/anti-Sj have been identified (clinical information available for 21). The most common manifestations were ACA, encephalopathy with seizures, myelopathy, and (radiculo)neuropathy, including autonomic neuropathy. In 45% of cases, an underlying tumour was present, making the condition a facultative paraneoplastic neurological disorder. The neurological syndrome preceded tumour diagnosis in all but one case. In most cases, immunotherapy had only moderate or no effect. The association of ITPR1-IgG/anti-Sj with manifestations other than ACA is corroborated by the case of a 48-year-old woman with high-titre ITPR1-IgG/anti-Sj antibodies and rapid cognitive decline, affecting memory, attention and executive function, and psychotic manifestations, including hallucinations, investigated here in detail. FDG-PET revealed right-temporal glucose hypermetabolism compatible with limbic encephalitis. Interestingly, ITPR1-IgG/anti-Sj mainly belonged to the IgG2 subclass in both serum and cerebrospinal fluid (CSF) in this and further patients, while it was predominantly IgG1 in other patients, including those with more severe outcome, and remained detectable over the entire course of disease. Immunotherapy with intravenous methylprednisolone, plasma exchange, and intravenous immunoglobulins, was repeatedly followed by partial or complete recovery. Long-term treatment with cyclophosphamide was paralleled by relative stabilization, although the patient noted clinical worsening at the end of each treatment cycle.

Conclusions

The spectrum of neurological manifestations associated with ITPR1 autoimmunity is broader than initially thought. Immunotherapy may be effective in some cases. Studies evaluating the frequency of ITPR1-IgG/anti-Sj in patients with cognitive decline and/or psychosis of unknown aetiology are warranted. Tumour screening is essential in patients presenting with ITPR1-IgG/anti-Sj.

Anti-IgLON5 antibodies cause progressive behavioral and neuropathological changes in mice

BACKGROUND

Anti-IgLON5 disease is a rare neurological disorder associated with autoantibodies against the neuronal cell adhesion protein, IgLON5. Cellular investigations with human IgLON5 antibodies have suggested an antibody-mediated pathogenesis, but whether human IgLON5 autoantibodies can induce disease symptoms in mice is yet to be shown. Moreover, the effects of anti-IgLON5 autoantibodies on neurons and the precise molecular mechanisms in vivo remain controversial.

METHODS

We investigated the effects of anti-IgLON5 antibodies in vivo and evaluated their long-term effects. We used two independent passive-transfer animal models and evaluated the effects of the antibodies on mouse behaviors at different time points from day 1 until day 30 after IgG infusion. A wide range of behaviors, including tests of locomotion, coordination, memory, anxiety, depression and social interactions were established. At termination, brain tissue was analyzed for human IgG, neuronal markers, glial markers, synaptic markers and RNA sequencing.

RESULTS

These experiments showed that patient's anti-IgLON5 antibodies induced progressive and irreversible behavioral deficits in vivo. Notably, cognitive abnormality was supported by impaired average gamma power in the CA1 during novel object recognition testing. Accompanying brain tissue studies showed progressive increase of brain-bound human antibodies in the hippocampus of anti-IgLON5 IgG-injected mice, which persisted 30 days after the injection of patient's antibodies was stopped. Microglial and astrocyte density was increased in the hippocampus of anti-IgLON5 IgG-injected mice at Day 30. Whole-cell voltage clamp recordings proved that anti-IgLON5 antibodies affected synaptic homeostasis. Further western blot investigation of synaptic proteins revealed a reduction of presynaptic (synaptophysin) and post-synaptic (PSD95 and NMDAR1) expression in anti-IgLON5 IgG-injected mice.

CONCLUSIONS

Overall, our findings indicated an irreversible effect of anti-IgLON5 antibodies and supported the pathogenicity of these antibodies in vivo.

Rho GTPase-activating protein 10 (ARHGAP10/GRAF2) is a novel autoantibody target in patients with autoimmune encephalitis

Background

In 2010, we described a novel immunoglobulin G (IgG) autoantibody (termed anti-Ca after the index case) targeting Rho GTPase-activating protein 26 (ARHGAP26, also termed GTPase regulator associated with focal adhesion kinase [GRAF], or oligophrenin-like protein 1 [OPHN1L]) in autoimmune cerebellar ataxia (ACA). Later, ARHGAP26-IgG/anti-Ca was reported in patients with limbic encephalitis/cognitive decline or peripheral neuropathy. In several of the reported cases, the syndrome was associated with cancer. ARHGAP10/GRAF2, which is expressed throughout the central nervous system, shares significant sequence homology with ARHGAP26/GRAF. Mutations in the ARHGAP10 gene have been linked to cognitive and psychiatric symptoms and schizophrenia.

Objective

To assess whether ARHGAP26-IgG/anti-Ca co-reacts with ARHGAP10.

Methods

Serological testing for ARHGAP10/GRAF2 autoantibodies by recombinant cell-based assays and isotype and IgG subclass analyses.

Results

26/31 serum samples (84%) from 9/12 (75%) ARHGAP26-IgG/anti-Ca-positive patients and 4/6 ARHGAP26-IgG/anti-Ca-positive CSF samples from four patients were positive also for ARHGAP10-IgG. ARHGAP10-IgG (termed anti-Ca2) remained detectable in the long-term (up to 109 months) and belonged mainly to the complement-activating IgG1 subclass. Median ARHGAP26-IgG/anti-Ca and median ARHGAP10-IgG/anti-Ca2 serum titres were 1:3200 and 1:1000, respectively, with extraordinarily high titres in some samples (ARHGAP26-IgG/anti-Ca: up to 1:1000,000; ARHGAP10-IgG: up to 1:32,000). ARHGAP26/anti-Ca serum titres exceeded those of ARHGAP10-IgG in all samples but one. A subset of patients was positive also for ARHGAP10-IgM and ARHGAP10-IgA. CSF/serum ratios and antibody index calculation suggested intrathecal production of ARHGAP26-IgG/anti-Ca and anti-ARHGAP10. Of 101 control samples, 100 were completely negative for ARHGAP10-IgG; a single control sample bound weakly (1:10) to the ARHGAP10-transfected cells.

Conclusions

We demonstrate that a substantial proportion of patients with ARHGAP26-IgG/anti-Ca-positive autoimmune encephalitis co-react with ARHGAP10. Further studies on the clinical and diagnostic implications of ARHGAP10-IgG/anti-Ca2 seropositivity in patients with autoimmune encephalitis are warranted.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00415-022-11178-9.

Autoantibodies in neurological disease

The realization that autoantibodies can contribute to dysfunction of the brain has brought about a paradigm shift in neurological diseases over the past decade, offering up important novel diagnostic and therapeutic opportunities. Detection of specific autoantibodies to neuronal or glial targets has resulted in a better understanding of central nervous system autoimmunity and in the reclassification of some diseases previously thought to result from infectious, 'idiopathic' or psychogenic causes. The most prominent examples, such as aquaporin 4 autoantibodies in neuromyelitis optica or NMDAR autoantibodies in encephalitis, have stimulated an entire field of clinical and experimental studies on disease mechanisms and immunological abnormalities. Also, these findings inspired the search for additional autoantibodies, which has been very successful to date and has not yet reached its peak. This Review summarizes this rapid development at a point in time where preclinical studies have started delivering fundamental new data for mechanistic understanding, where new technologies are being introduced into this field, and - most importantly - where the first specifically tailored immunotherapeutic approaches are emerging.

Super-resolving Microscopy in Neuroscience

Fluorescence imaging techniques play a pivotal role in our understanding of the nervous system. The emergence of various super-resolution microscopy methods and specialized fluorescent probes enables direct insight into neuronal structure and protein arrangements in cellular subcompartments with so far unmatched resolution. Super-resolving visualization techniques in neurons unveil a novel understanding of cytoskeletal composition, distribution, motility, and signaling of membrane proteins, subsynaptic structure and function, and neuron-glia interaction. Well-defined molecular targets in autoimmune and neurodegenerative disease models provide excellent starting points for in-depth investigation of disease pathophysiology using novel and innovative imaging methodology. Application of super-resolution microscopy in human brain samples and for testing clinical biomarkers is still in its infancy but opens new opportunities for translational research in neurology and neuroscience. In this review, we describe how super-resolving microscopy has improved our understanding of neuronal and brain function and dysfunction in the last two decades.

Stiff-person syndrome: an atypical presentation and a review of the literature

Introduction: Stiff-person syndrome (SPS) is a rare autoimmune neurological disorder associated with muscle rigidity and spasms. A number of antibodies have been associated with disorder, including anti-glutamic acid decarboxylase and anti-amphiphysin.Case report; In this report, we present a rare case of a 79-year-old woman who presented with bilateral lower extremity weakness who was ultimately diagnosed with stiff-limb syndrome, a rare variant of SPS. Extensive laboratory and CSF studies were unrevealing. Electromyography showed significant peroneal motor neuropathy and complex repetitive discharges in the left tibialis anterior muscle. Antibodies to glutamic acid decarboxylase were significantly elevated at 124 units/mL. She was subsequently started on oral diazepam with significant improvement in her symptoms.Conclusion: The presentation of SPS can vary based on epidemiologic factors, clinical symptoms, and associated disorders. These forms can have overlapping features which may make the categorization of patients into one of these forms challenging.

Autoimmune movement disorders with neuronal antibodies - an update

PURPOSE OF REVIEW

The rapid developments in neuroimmunology reflect also on the field of movement disorders, where there is an ever expanding spectrum of new antibodies. This review focuses on the new neuronal antibodies, their clinical spectrum and recent pathophysiological insights. It gives an update on previous work about neuronal antibody-related movement disorders.

RECENT FINDINGS

Phosphodiesterase 10A antibodies are a new marker of paraneoplastic chorea. Seizure-related 6 homolog like 2 antibodies are a differential diagnosis in atypical parkinsonism with cerebellar ataxia and cognitive impairment. mGluR5-antibodies cause various hyperkinetic movement disorders with Ophelia syndrome. Most new antibodies were described in the context of cerebellar ataxia: Kelch-like protein 11 antibodies are a comparatively frequent marker of paraneoplastic cerebellar ataxia with germ cell tumours. Nonparaneoplastic cerebellar ataxia occurs with Septin-5 and neurochondrin antibodies. Studies into the mechanisms of neuronal surface antibodies have shown that there is much pathophysiological heterogeneity, ranging from immediate antagonistic effect to induction of neurodegeneration after weeks.

SUMMARY

The new markers of autoimmune movement disorders are key to identify those patients that may benefit from immunotherapy, and tumour therapy, where appropriate. Insights into the underlying pathophysiology might guide treatment decisions and help tailoring more targeted approaches in the future.

CNS autoimmunity in children: An unwanted wrinkle in a smooth narrative

The emerging paradigm of childhood autoimmune neurological disorders has exploded in recent times due to reliable diagnostic methods and their ease of availability, well-defined diagnostic criteria, and universal awareness about these disorders. The most important aspect of these disorders is a considerable recovery in response to early targeted immunotherapy. If left untreated and/or ill-treated, these can lead to mortality or lifelong morbidity. Autoantibodies can target any part of the central nervous system (CNS), ranging from superficial structures like myelin to deep intracellular ion channels like voltage-gated potassium channels, resulting in contrasting and at times overlapping symptomatology. Though neuroimaging characteristics and serological tests confirm these disorders' diagnosis, it is essential to suspect them clinically and start management before the reports are available for minimizing morbidity and mortality. In the pediatric age group, several metabolic conditions, like mitochondrial disorders and enzyme deficiencies like HMG-CoA-lyase deficiency, can develop neuroimaging patterns similar to those seen in childhood CNS autoimmune disorders and may also show a favorable response to steroids in acute phases. Hence, the clinician must suspect and work up the index patient appropriately. Here, we briefly discuss the pathophysiology, clinical clues, and potential therapeutic targets related to pediatric CNS autoimmune disorders.

Neuronal BIN1 Regulates Presynaptic Neurotransmitter Release and Memory Consolidation

BIN1, a member of the BAR adaptor protein family, is a significant late-onset Alzheimer disease risk factor. Here, we investigate BIN1 function in the brain using conditional knockout (cKO) models. Loss of neuronal Bin1 expression results in the select impairment of spatial learning and memory. Examination of hippocampal CA1 excitatory synapses reveals a deficit in presynaptic release probability and slower depletion of neurotransmitters during repetitive stimulation, suggesting altered vesicle dynamics in Bin1 cKO mice. Super-resolution and immunoelectron microscopy localizes BIN1 to presynaptic sites in excitatory synapses. Bin1 cKO significantly reduces synapse density and alters presynaptic active zone protein cluster formation. Finally, 3D electron microscopy reconstruction analysis uncovers a significant increase in docked and reserve pools of synaptic vesicles at hippocampal synapses in Bin1 cKO mice. Our results demonstrate a non-redundant role for BIN1 in presynaptic regulation, thus providing significant insights into the fundamental function of BIN1 in synaptic physiology relevant to Alzheimer disease.

BIN1 is a significant risk factor for late-onset Alzheimer disease. BIN1 has a general role in endocytosis and membrane dynamics in non-neuronal cells. De Rossi et al. show that BIN1 localizes to presynaptic terminals and plays an indispensable role in excitatory synaptic transmission by regulating neurotransmitter vesicle dynamics.

The search for an autoimmune origin of psychotic disorders: Prevalence of autoantibodies against hippocampus antigens, glutamic acid decarboxylase and nuclear antigens

The etiology of psychotic disorders is still unknown, but in a subgroup of patients symptoms might be caused by an autoimmune reaction. In this study, we tested patterns of autoimmune reactivity against potentially novel hippocampal antigens. Serum of a cohort of 621 individuals with psychotic disorders and 257 controls were first tested for reactivity on neuropil of rat brain sections. Brain reactive sera (67 diseased, 27 healthy) were further tested for antibody binding to glutamic acid decarboxylase (GAD) isotype 65 and 67 by cell-based assay (CBA). A sub-cohort of 199 individuals with psychotic disorders and 152 controls was tested for the prevalence of anti-nuclear antibodies (ANA) on HEp2-substrate as well as for reactivity to double-stranded DNA, ribosomal P (RPP), and cardiolipin (CL). Incubation of rat brain with serum resulted in unidentified hippocampal binding patterns in both diseased and control groups. Upon screening with GAD CBA, one of these patterns was identified as GAD65 in one individual with schizophrenia and also in one healthy individual. Two diseased and two healthy individuals had low antibody levels targeting GAD67 by CBA. Antibody reactivity on HEp-2-substrate was increased in patients with schizoaffective disorder, but only in 3 patients did antibody testing hint at a possible diagnosis of systemic lupus erythematosus. Although reactivity of serum to intracellular antigens might be increased in patients with psychotic disorder, no specific targets could be identified. GAD antibodies are very rare and do not seem increased in serum of patients with psychotic disorders.

Antibodies From Children With PANDAS Bind Specifically to Striatal Cholinergic Interneurons and Alter Their Activity

OBJECTIVE

Pediatric obsessive-compulsive disorder (OCD) sometimes appears rapidly, even overnight, often after an infection. Pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections, or PANDAS, describes such a situation after infection with Streptococcus pyogenes. PANDAS may result from induced autoimmunity against brain antigens, although this remains unproven. Pilot work suggests that IgG antibodies from children with PANDAS bind to cholinergic interneurons (CINs) in the striatum. CIN deficiency has been independently associated with tics in humans and with repetitive behavioral pathology in mice, making it a plausible locus of pathology. The authors sought to replicate and extend earlier work and to investigate the cellular effects of PANDAS antibodies on cholinergic interneurons.

METHODS

Binding of IgG to specific neurons in human and mouse brain slices was evaluated ex vivo after incubation with serum from 27 children with rigorously characterized PANDAS, both at baseline and after intravenous immunoglobulin (IVIG) treatment, and 23 matched control subjects. Binding was correlated with symptom measures. Neural activity after serum incubation was assessed in mouse slices using molecular markers and electrophysiological recording.

RESULTS

IgG from children with PANDAS bound to CINs, but not to several other neuron types, more than IgG from control subjects, in three independent cohorts of patients. Post-IVIG serum had reduced IgG binding to CINs, and this reduction correlated with symptom improvement. Baseline PANDAS sera decreased activity of striatal CINs, but not of parvalbumin-expressing GABAergic interneurons, and altered their electrophysiological responses, in acute mouse brain slices. Post-IVIG PANDAS sera and IgG-depleted baseline sera did not alter the activity of striatal CINs.

CONCLUSIONS

These findings provide strong evidence for striatal CINs as a critical cellular target that may contribute to pathophysiology in children with rapid-onset OCD symptoms, and perhaps in other conditions.

Autoimmune encephalitis: novel therapeutic targets at the preclinical level

INTRODUCTION

Antibody-mediated encephalitides (AE) with pathogenic autoantibodies (aAB) against neuronal surface antigens are a growing group of diseases characterized by antineuronal autoimmunity in the brain. AE patients typically present with rapidly progressive encephalitis and characteristic disease symptoms dependent on the target antigen. Current treatment consists of an escalating immunotherapy strategy including plasma exchange, steroid application, and B cell depletion.

AREAS COVERED

For this review, we searched Medline database and google scholar with inclusive dates from 2000. We summarize current treatment strategies and present novel therapeutic approaches of target-specific interventions at the pre-clinical level as well as immunotherapy directed at antibody-induced pathology. Treatment options include modulation of target proteins, intervention with downstream pathways, antibody modification, and depletion of antibody-secreting cells.

EXPERT OPINION

Although current therapies in AE are effective in many patients, recovery is often prolonged and relapses as well as persistent deficits can occur. Specific immunotherapy together with supportive target-specific therapy may provide faster control of severe symptoms, shorten the disease course, and lead to long-lasting disease stability. Among the various novel therapeutic approaches, modulation of targeted receptors by small molecules crossing the blood-brain barrier as well as prevention of aAB binding is of particular interest.

A Case Report of Pediatric Paraneoplastic Dysautonomia

We present the case of a 16-year-old girl who presented with severe refractory orthostatic hypotension secondary to pandysautonomia. Initially, she was treated for Guillain-Barré syndrome given clinical symptoms and increased protein on cerebrospinal fluid, but the severity of symptoms and lack of response to intravenous immunoglobulin prompted further evaluation for an autoimmune etiology. She was ultimately diagnosed with paraneoplastic neuropathy secondary to Hodgkin lymphoma. Paraneoplastic neurologic phenomena are rare, occurring in just 0.01% of cancers, and prompt recognition is crucial for initiating appropriate therapy. Rapid progression of severe disabling symptoms should raise suspicion for an underlying malignancy. The patient had limited response to splanchnic vasoconstrictors in addition to α-agonists, anticholinergics, and mineralocorticoids until initiation of modified Hodgkin lymphoma directed chemotherapy plus rituximab.

Experimental Models of Neuroimmunological Disorders: A Review

Immune-mediated inflammatory diseases of the central nervous system (CNS) are a group of neurological disorders in which inflammation and/or demyelination are induced by cellular and humoral immune responses specific to CNS antigens. They include diseases such as multiple sclerosis (MS), neuromyelitis optica spectrum disorders (NMOSD), acute disseminated encephalomyelitis (ADEM) and anti-NMDA receptor encephalitis (NMDAR encephalitis). Over the years, many in vivo and in vitro models were used to study clinical, pathological, physiological and immunological features of these neuroimmunological disorders. Nevertheless, there are important aspects of human diseases that are not fully reproduced in the experimental models due to their technical limitations. In this review, we describe the preclinical models of neuroimmune disorders, and how they contributed to the understanding of these disorders and explore potential treatments. We also describe the purpose and limitation of each one, as well as the recent advances in this field.

Distinct Effects of Stereotactically Injected Human Cerebrospinal Fluid Containing Glutamic Acid Decarboxylase Antibodies into the Hippocampus of Rats on the Development of Spontaneous Epileptic Activity

Background: The conversion of glutamic acid into γ-aminobutyric acid (GABA) is catalyzed by the glutamic acid decarboxylase (GAD). Antibodies against this enzyme have been described in neurological disorders, but the pathophysiological role of these antibodies is still poorly understood. We hypothesized that anti-GAD autoantibodies could diminish the GABA content in the slice and facilitate epileptic activity. Methods: Cerebrospinal fluids (CSF) from two patients containing anti-GAD (A and B) were injected into the rat hippocampus in vivo. Hippocampal slices were prepared for electrophysiological field potential recordings in order to record recurrent epileptic discharges (REDs) in the CA1 region induced by the removal of Mg2+ and/or by adding gabazine. As control groups, we injected an anti-GAD-negative human CSF or saline solution, and we used non-operated naive animals. Results: RED frequencies were significantly higher in the Mg2+-free solution than in the gabazine-containing solution. The average frequency of REDs in the last 10 min and the average duration of REDs in the last 5 min did not show significant differences between the anti-GAD-B-treated and the control slices, but in the Mg2+-free solution, anti-GAD-A had significantly higher epileptic activity than anti-GAD-B. Conclusions: These results indicate that anti-GAD has distinct effects on the development of spontaneous epileptic activity.

Autoantibodies to synapsin I sequestrate synapsin I and alter synaptic function

Synapsin I is a phosphoprotein that coats the cytoplasmic side of synaptic vesicles and regulates their trafficking within nerve terminals. Autoantibodies against Syn I have been described in sera and cerebrospinal fluids of patients with numerous neurological diseases, including limbic encephalitis and clinically isolated syndrome; however, the effects and fate of autoantibodies in neurons are still unexplored. We found that in vitro exposure of primary hippocampal neurons to patient’s autoantibodies to SynI decreased the density of excitatory and inhibitory synapses and impaired both glutamatergic and GABAergic synaptic transmission. These effects were reproduced with a purified SynI antibody and completely absent in SynI knockout neurons. Autoantibodies to SynI are internalized by FcγII/III-mediated endocytosis, interact with endogenous SynI, and promote its sequestration and intracellular aggregation. Neurons exposed to human autoantibodies to SynI display a reduced density of SVs, mimicking the SynI loss-of-function phenotype. Our data indicate that autoantibodies to intracellular antigens such as SynI can reach and inactivate their targets and suggest that an antibody-mediated synaptic dysfunction may contribute to the evolution and progression of autoimmune-mediated neurological diseases positive for SynI autoantibodies.

Combining super-resolution microscopy with neuronal network recording using magnesium fluoride thin films as cover layer for multi-electrode array technology

We present an approach for fabrication of reproducible, chemically and mechanically robust functionalized layers based on MgF₂ thin films on thin glass substrates. These show great advantages for use in super-resolution microscopy as well as for multi-electrode-array fabrication and are especially suited for combination of these techniques. The transparency of the coated substrates with the low refractive index material is adjustable by the layer thickness and can be increased above 92%. Due to the hydrophobic and lipophilic properties of the thin crystalline MgF₂ layers, the temporal stable adhesion needed for fixation of thin tissue, e.g. cryogenic brain slices is given. This has been tested using localization-based super-resolution microscopy with currently highest spatial resolution in light microscopy. We demonstrated that direct stochastic optical reconstruction microscopy revealed in reliable imaging of structures of central synapses by use of double immunostaining of post- (homer1 and GluA2) and presynaptic (bassoon) marker structure in a 10 µm brain slice without additional fixing of the slices. Due to the proven additional electrical insulating effect of MgF₂ layers, surfaces of multi-electrode-arrays were coated with this material and tested by voltage-current-measurements. MgF₂ coated multi-electrode-arrays can be used as a functionalized microscope cover slip for combination with live-cell super-resolution microscopy.

Neuronal Antibodies and Associated Syndromes

INTRODUCTION

Multiple well-recognized conditions, such as Lambert-Eaton myasthenic syndrome (LEMS) and myasthenia gravis (MG), have been associated with neuronal antibodies.

MATERIALS AND METHODS

A search was performed using Embase, PubMed, and CINAHL. An initial search of each database was conducted using keywords and terms related to the aim of the current review. Additional articles were obtained by examining the reference lists and citations in the selected records.

RESULTS

The studies identified through the search process used different designs and methods to explore neuronal antibodies and associated syndromes. Previous studies have shown that neurological and psychiatric disorders can be mediated and influenced by various antibodies. The identification of autoantibodies can help with the accurate diagnosis of conditions and commencement of early treatment.

DISCUSSION

A review of selected studies identified in the literature implicated that classic anti-neuronal antibodies, such as anti-Ri and anti-Hu, play a role in the development of neurological diseases. More recent studies have indicated that other novel antibodies act on neuronal cell surface antigens to contribute to the development of neurological disorders.

CONCLUSION

Existing research provides evidence revealing a spectrum of antibodies linked to the development and progression of neurological diseases. However, further antibody testing and studies should be performed to validate the relationship between conditions and antibodies.

Movement disorders associated with neuronal antibodies

Movement disorders are one of the common clinical features of neurological disease associated with neuronal antibodies which is a group of potentially reversible disorder. They can present with hypokinetic or hyperkinetic types of involuntary movements and may have other associated neurological symptoms. The spectrum of abnormal movements associated with neuronal antibodies is widening. Some specific phenomenology of movement disorders are likely to give clue about the type of antibody, for instance, presence of paroxysmal dystonia (facio-brachial dystonic seizures) are a pointer toward presence of LGI-1 antibodies, and orofacial lingual dyskinesia is associated with NMDAR associated encephalitis. The presence of specific type of movement disorder allows high suspicion of testing of certain specific type of antibodies. In this review, we have discussed the various antibodies and the spectrum of movement disorder associated with them, highlighting if any distinct movement disorder allows the clinician to suspect type of antibody in a certain clinical context. We have also reviewed the treatment of the movement disorder associated with the neuronal antibodies. Physicians should have high index of suspicion of these disorders, as early institution of treatment options can lead to better outcome.

Epitope specificity of anti-synapsin autoantibodies: Differential targeting of synapsin I domains

OBJECTIVE

To identify the specific domains of the presynaptic protein synapsin targeted by recently described autoantibodies to synapsin.

METHODS

Sera of 20 and CSF of two patients with different psychiatric and neurological disorders previously tested positive for immunoglobulin (Ig)G antibodies to full-length synapsin were screened for IgG against synapsin I domains using HEK293 cells transfected with constructs encoding different domains of rat synapsin Ia. Additionally, IgG subclasses were determined using full-length synapsin Ia. Serum and CSF from one patient were also screened for IgA autoantibodies to synapsin I domains. Sera from nine and CSF from two healthy subjects were analyzed as controls.

RESULTS

IgG in serum from 12 of 20 IgG synapsin full-length positive patients, but from none of the healthy controls, bound to synapsin domains. Of these 12 sera, six bound to the A domain, five to the D domain, and one to the B- (and possibly A-), D-, and E-domains of synapsin I. IgG antibodies to the D-domain were also detected in one of the CSF samples. Determination of IgG subclasses detected IgG1 in two sera and one CSF, IgG2 in none of the samples, IgG3 in two sera, and IgG4 in eight sera. One patient known to be positive for IgA antibodies to full-length synapsin had IgA antibodies to the D-domain in serum and CSF.

CONCLUSIONS

Anti-synapsin autoantibodies preferentially bind to either the A- or the D-domain of synapsin I.

LGI1 antibodies alter Kv1.1 and AMPA receptors changing synaptic excitability, plasticity and memory

Leucine-rich glioma-inactivated 1 (LGI1) is a secreted neuronal protein that forms a trans-synaptic complex that includes the presynaptic disintegrin and metalloproteinase domain-containing protein 23 (ADAM23), which interacts with voltage-gated potassium channels Kv1.1, and the postsynaptic ADAM22, which interacts with AMPA receptors. Human autoantibodies against LGI1 associate with a form of autoimmune limbic encephalitis characterized by severe but treatable memory impairment and frequent faciobrachial dystonic seizures. Although there is evidence that this disease is immune-mediated, the underlying LGI1 antibody-mediated mechanisms are unknown. Here, we used patient-derived immunoglobulin G (IgG) antibodies to determine the main epitope regions of LGI1 and whether the antibodies disrupt the interaction of LGI1 with ADAM23 and ADAM22. In addition, we assessed the effects of patient-derived antibodies on Kv1.1, AMPA receptors, and memory in a mouse model based on cerebroventricular transfer of patient-derived IgG. We found that IgG from all patients (n = 25), but not from healthy participants (n = 20), prevented the binding of LGI1 to ADAM23 and ADAM22. Using full-length LGI1, LGI3, and LGI1 constructs containing the LRR1 domain (EPTP1-deleted) or EPTP1 domain (LRR3-EPTP1), IgG from all patients reacted with epitope regions contained in the LRR1 and EPTP1 domains. Confocal analysis of hippocampal slices of mice infused with pooled IgG from eight patients, but not pooled IgG from controls, showed a decrease of total and synaptic levels of Kv1.1 and AMPA receptors. The effects on Kv1.1 preceded those involving the AMPA receptors. In acute slice preparations of hippocampus, patch-clamp analysis from dentate gyrus granule cells and CA1 pyramidal neurons showed neuronal hyperexcitability with increased glutamatergic transmission, higher presynaptic release probability, and reduced synaptic failure rate upon minimal stimulation, all likely caused by the decreased expression of Kv1.1. Analysis of synaptic plasticity by recording field potentials in the CA1 region of the hippocampus showed a severe impairment of long-term potentiation. This defect in synaptic plasticity was independent from Kv1 blockade and was possibly mediated by ineffective recruitment of postsynaptic AMPA receptors. In parallel with these findings, mice infused with patient-derived IgG showed severe memory deficits in the novel object recognition test that progressively improved after stopping the infusion of patient-derived IgG. Different from genetic models of LGI1 deficiency, we did not observe aberrant dendritic sprouting or defective synaptic pruning as potential cause of the symptoms. Overall, these findings demonstrate that patient-derived IgG disrupt presynaptic and postsynaptic LGI1 signalling, causing neuronal hyperexcitability, decreased plasticity, and reversible memory deficits.

The clinical features, underlying immunology, and treatment of autoantibody-mediated movement disorders

An increasing number of movement disorders are associated with autoantibodies. Many of these autoantibodies target the extracellular domain of neuronal surface proteins and associate with highly specific phenotypes, suggesting they have pathogenic potential. Below, we describe the phenotypes associated with some of these commoner autoantibody‐mediated movement disorders, and outline increasingly well‐established mechanisms of autoantibody pathogenicity which include antigen downregulation and complement fixation. Despite these advances, and the increasingly robust evidence for improved clinical outcomes with early escalation of immunotherapies, the underlying cellular immunology of these conditions has received little attention. Therefore, here, we outline the likely roles of T cells and B cells in the generation of autoantibodies, and reflect on how these may guide both current immunotherapy regimes and our future understanding of precision medicine in the field. In addition, we summarise potential mechanisms by which these peripherally‐driven immune responses may reach the central nervous system. We integrate this with the immunologically‐relevant clinical observations of preceding infections, tumours and human leucocyte antigen‐associations to provide an overview of the therapeutically‐relevant underlying adaptive immunology in the autoantibody‐mediated movement disorders. © 2018 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Human Autoantibodies against the AMPA Receptor Subunit GluA2 Induce Receptor Reorganization and Memory Dysfunction

AMPA receptors are essential for fast excitatory transmission in the CNS. Autoantibodies to AMPA receptors have been identified in humans with autoimmune encephalitis and severe defects of hippocampal function. Here, combining electrophysiology and high-resolution imaging with neuronal culture preparations and passive-transfer models in wild-type and GluA1-knockout mice, we analyze how specific human autoantibodies against the AMPA receptor subunit GluA2 affect receptor function and composition, synaptic transmission, and plasticity. Anti-GluA2 antibodies induce receptor internalization and a reduction of synaptic GluA2-containing AMPARs followed by compensatory ryanodine receptor-dependent incorporation of synaptic non-GluA2 AMPARs. Furthermore, application of human pathogenic anti-GluA2 antibodies to mice impairs long-term synaptic plasticity in vitro and affects learning and memory in vivo. Our results identify a specific immune-neuronal rearrangement of AMPA receptor subunits, providing a framework to explain disease symptoms.

Autoimmune septin-5 cerebellar ataxia

Objective

To report a form of autoimmune cerebellar ataxia in which antibodies target septin-5, a guanosine triphosphate (GTP)-binding neural protein involved in neurotransmitter exocytosis.

Methods

Archived sera and CSF specimens with unclassified synaptic antibodies were re-evaluated by tissue-based indirect immunofluorescence assay. Autoantigens were identified by Western blot and mass spectrometry. Recombinant protein assays (Western blot, cell based, and protein screening array) confirmed antigen specificity.

Results

Serum and CSF from 6 patients produced identical synaptic immunoglobulin G (IgG) staining patterns of synaptic regions (neuropil) of the mouse cerebrum and cerebellum. The molecular layer of the cerebellum and the thalamus demonstrated stronger immunoreactivity than the midbrain, hippocampus, cortex, and basal ganglia. The antigen revealed by mass spectrometry analysis of immunoprecipitated cerebellar proteins and confirmed by recombinant protein assays was septin-5. All 4 patients with records available had subacute onset of cerebellar ataxia with prominent eye movement symptoms (oscillopsia or vertigo). None had cancer detected. Improvements occurred after immunotherapies (2) or spontaneously (1). One patient died.

Conclusion

Septin-5 IgG represents a biomarker for a potentially fatal but treatable autoimmune ataxia.

Paraneoplastic movement disorders

Paraneoplastic movement disorders are rare, autoimmune-mediated, nonmetastatic complications of malignant neoplasms. Common paraneoplastic movement disorders include paraneoplastic chorea, dystonia, cerebellar degeneration, different types of encephalitis, opsoclonus-myoclonus syndrome, stiff person syndrome, and neuromyotonia. Syndromes usually develop before tumor diagnosis, have subacute onset, and are associated with serum or cerebrospinal fluid antibodies. Two types of antibodies can be distinguished: antibodies against nuclear and cytoplasmic neuronal antigens (anti-Hu, anti-Ri, anti-Yo, anti-Ma, anti-CV2/CRMP5, anti-Gephrin, and anti-GABATRAP) and antibodies recently identified against cell surface and synaptic proteins (anti-NMDAR, anti-LGI1, and anti-Caspr2). These two types differ from each other in a few important aspects. Antibodies against cell surface and synaptic protein disrupt cell-surface antigens. Clinical symptoms are related to the disruption of antigens and potentially can be reversed by immunotherapy. The association between these antibodies and malignancy is much less consistent. On the other hand, antibodies against nuclear and cytoplasmic neuronal antigens seem to be not pathogenic; however, they most likely indicate a T-cell-mediated immune response against neurons. Due to T-cell-mediated neuronal loss, response to immunotherapy is generally disappointing. Early recognition of all these diseases is crucial because it may lead to the disclosure of occult cancer. This review is focused on paraneoplastic movement disorders with emphasis on clinical presentations, investigational findings, and therapeutic results.

Movement disorders with neuronal antibodies: syndromic approach, genetic parallels and pathophysiology

Movement disorders are a prominent and common feature in many autoantibody-associated neurological diseases, a group of potentially treatable conditions that can mimic infectious, metabolic or neurodegenerative disease. Certain movement disorders are likely to associate with certain autoantibodies; for example, the characteristic dyskinesias, chorea and dystonia associated with NMDAR antibodies, stiff person spectrum disorders with GAD, glycine receptor, amphiphysin or DPPX antibodies, specific paroxysmal dystonias with LGI1 antibodies, and cerebellar ataxia with various anti-neuronal antibodies. There are also less-recognized movement disorder presentations of antibody-related disease, and a considerable overlap between the clinical phenotypes and the associated antibody spectra. In this review, we first describe the antibodies associated with each syndrome, highlight distinctive clinical or radiological 'red flags', and suggest a syndromic approach based on the predominant movement disorder presentation, age, and associated features. We then examine the underlying immunopathophysiology, which may guide treatment decisions in these neuroimmunological disorders, and highlight the exceptional interface between neuronal antibodies and neurodegeneration, such as the tauopathy associated with IgLON5 antibodies. Moreover, we elaborate the emerging pathophysiological parallels between genetic movement disorders and immunological conditions, with proteins being either affected by mutations or targeted by autoantibodies. Hereditary hyperekplexia, for example, is caused by mutations of the alpha subunit of the glycine receptor leading to an infantile-onset disorder with exaggerated startle and stiffness, whereas antibodies targeting glycine receptors can induce acquired hyperekplexia. The spectrum of such immunological and genetic analogies also includes cerebellar ataxias and some encephalopathies. Lastly, we discuss how these pathophysiological considerations could reflect on possible future directions regarding antigen-specific immunotherapies or targeting the pathophysiological cascades downstream of the antibody effects.

Nodding syndrome may be an autoimmune reaction to the parasitic worm Onchocerca volvulus

Nodding syndrome is an epileptic disorder of unknown etiology that occurs in children in East Africa. There is an epidemiological association with Onchocerca volvulus, the parasitic worm that causes onchocerciasis (river blindness), but there is limited evidence that the parasite itself is neuroinvasive. We hypothesized that nodding syndrome may be an autoimmune-mediated disease. Using protein chip methodology, we detected autoantibodies to leiomodin-1 more abundantly in patients with nodding syndrome compared to unaffected controls from the same village. Leiomodin-1 autoantibodies were found in both the sera and cerebrospinal fluid of patients with nodding syndrome. Leiomodin-1 was found to be expressed in mature and developing human neurons in vitro and was localized in mouse brain to the CA3 region of the hippocampus, Purkinje cells in the cerebellum, and cortical neurons, structures that also appear to be affected in patients with nodding syndrome. Antibodies targeting leiomodin-1 were neurotoxic in vitro, and leiomodin-1 antibodies purified from patients with nodding syndrome were cross-reactive with O. volvulus antigens. This study provides initial evidence supporting the hypothesis that nodding syndrome is an autoimmune epileptic disorder caused by molecular mimicry with O. volvulus antigens and suggests that patients may benefit from immunomodulatory therapies.

Defective synaptic transmission causes disease signs in a mouse model of juvenile neuronal ceroid lipofuscinosis

Juvenile neuronal ceroid lipofuscinosis (JNCL or Batten disease) caused by mutations in the CLN3 gene is the most prevalent inherited neurodegenerative disease in childhood resulting in widespread central nervous system dysfunction and premature death. The consequences of CLN3 mutation on the progression of the disease, on neuronal transmission, and on central nervous network dysfunction are poorly understood. We used Cln3 knockout (Cln3^Δex1-6) mice and found increased anxiety-related behavior and impaired aversive learning as well as markedly affected motor function including disordered coordination. Patch-clamp and loose-patch recordings revealed severely affected inhibitory and excitatory synaptic transmission in the amygdala, hippocampus, and cerebellar networks. Changes in presynaptic release properties may result from dysfunction of CLN3 protein. Furthermore, loss of calbindin, neuropeptide Y, parvalbumin, and GAD65-positive interneurons in central networks collectively support the hypothesis that degeneration of GABAergic interneurons may be the cause of supraspinal GABAergic disinhibition.

Autoimmune Movement Disorders: a Clinical and Laboratory Approach

Autoimmune movement disorders are caused by an aberrant immune response to neural self-antigens. These disorders may be paraneoplastic, parainfectious, or (most commonly) idiopathic. The neurological presentations are diverse, and sometimes multifocal. Movement disorders can occur as part of the spectrum with phenotypes including chorea, myoclonus, ataxia, CNS hyperexcitability (including stiff-person syndrome), dystonia, and parkinsonism. Symptoms are subacute in onset and may have a fluctuating course. The best characterized disorders are unified by neural autoantibodies identified in serum or cerebrospinal fluid. The antibody specificity may predict the association with cancer and the response to immunotherapy. In this article, we review autoimmune-mediated movement disorders, associated cancers, diagnosis, and treatment.

Neurofilament light as an immune target for pathogenic antibodies

Antibodies to neuronal antigens are associated with many neurological diseases including paraneoplastic neurological disorders, epilepsy, amyotrophic lateral sclerosis and multiple sclerosis. Immunization with neuronal antigens such as neurofilament light (NF-L), a neuronal intermediate filament in axons, has been shown to induce neurological disease and spasticity in mice. Also, although antibodies to NF-L are widely used as surrogate biomarkers of axonal injury in amyotrophic lateral sclerosis and multiple sclerosis, it remains to be elucidated if antibodies to NF-L contribute to neurodegeneration and neurological disease. To address this, we examined the pathogenic role of antibodies directed to NF-L in vitro using spinal cord co-cultures and in vivo in experimental autoimmune encephalomyelitis (EAE) and optic neuritis animal models of multiple sclerosis. Here we show that peripheral injections of antibodies to NF-L augmented clinical signs of neurological disease in acute EAE, increased retinal ganglion cell loss in experimental optic neuritis and induced neurological signs following intracerebral injection into control mice. The pathogenicity of antibodies to NF-L was also observed in spinal cord co-cultures where axonal loss was induced. Taken together, our results reveal that as well as acting as reliable biomarkers of neuronal damage, antibodies to NF-L exacerbate neurological disease, suggesting that antibodies to NF-L generated during disease may also be pathogenic and play a role in the progression of neurodegeneration.

Anti-glutamic acid decarboxylase antibody positive neurological syndromes

A rare kind of antibody, known as anti-glutamic acid decarboxylase (GAD) autoantibody, is found in some patients. The antibody works against the GAD enzyme, which is essential in the formation of gamma aminobutyric acid (GABA), an inhibitory neurotransmitter found in the brain. Patients found with this antibody present with motor and cognitive problems due to low levels or lack of GABA, because in the absence or low levels of GABA patients exhibit motor and cognitive symptoms. The anti-GAD antibody is found in some neurological syndromes, including stiff-person syndrome, paraneoplastic stiff-person syndrome, Miller Fisher syndrome (MFS), limbic encephalopathy, cerebellar ataxia, eye movement disorders, and epilepsy. Previously, excluding MFS, these conditions were calledhyperexcitability disorders. However, collectively, these syndromes should be known as "anti-GAD positive neurological syndromes." An important limitation of this study is that the literature is lacking on the subject, and why patients with the above mentioned neurological problems present with different symptoms has not been studied in detail. Therefore, it is recommended that more research is conducted on this subject to obtain a better and deeper understanding of these anti-GAD antibody induced neurological syndromes.

Synaptic Vesicle-Recycling Machinery Components as Potential Therapeutic Targets

Presynaptic nerve terminals are highly specialized vesicle-trafficking machines. Neurotransmitter release from these terminals is sustained by constant local recycling of synaptic vesicles independent from the neuronal cell body. This independence places significant constraints on maintenance of synaptic protein complexes and scaffolds. Key events during the synaptic vesicle cycle-such as exocytosis and endocytosis-require formation and disassembly of protein complexes. This extremely dynamic environment poses unique challenges for proteostasis at synaptic terminals. Therefore, it is not surprising that subtle alterations in synaptic vesicle cycle-associated proteins directly or indirectly contribute to pathophysiology seen in several neurologic and psychiatric diseases. In contrast to the increasing number of examples in which presynaptic dysfunction causes neurologic symptoms or cognitive deficits associated with multiple brain disorders, synaptic vesicle-recycling machinery remains an underexplored drug target. In addition, irrespective of the involvement of presynaptic function in the disease process, presynaptic machinery may also prove to be a viable therapeutic target because subtle alterations in the neurotransmitter release may counter disease mechanisms, correct, or compensate for synaptic communication deficits without the need to interfere with postsynaptic receptor signaling. In this article, we will overview critical properties of presynaptic release machinery to help elucidate novel presynaptic avenues for the development of therapeutic strategies against neurologic and neuropsychiatric disorders.

Hello from the Other Side: How Autoantibodies Circumvent the Blood-Brain Barrier in Autoimmune Encephalitis

Antibodies against neuronal receptors and synaptic proteins are associated with autoimmune encephalitides (AE) that produce movement and psychiatric disorders. In order to exert their pathological effects on neural circuits, autoantibodies against central nervous system (CNS) targets must gain access to the brain and spinal cord by crossing the blood–brain barrier (BBB), a tightly regulated gateway formed by endothelial cells lining CNS blood vessels. To date, the pathogenic mechanisms that underlie autoantibody-triggered encephalitic syndromes are poorly understood, and how autoantibodies breach the barrier remains obscure for almost all AE syndromes. The relative importance of cellular versus humoral immune mechanisms for disease pathogenesis also remains largely unexplored. Here, we review the proposed triggers for various autoimmune encephalopathies and their animal models, as well as basic structural features of the BBB and how they differ among various CNS regions, a feature that likely underlies some regional aspects of autoimmune encephalitis pathogenesis. We then discuss the routes that antibodies and immune cells employ to enter the CNS and their implications for AE. Finally, we explore future therapeutic strategies that may either preserve or restore barrier function and thereby limit immune cell and autoantibody infiltration into the CNS. Recent mechanistic insights into CNS autoantibody entry indicate promising future directions for therapeutic intervention beyond current, short-lived therapies that eliminate circulating autoantibodies.