Autoantibodies against N-methyl-d-aspartate receptor 1 in health and disease

Beyond the amyloid hypothesis: how current research implicates autoimmunity in Alzheimer's disease pathogenesis

The amyloid hypothesis has so far been at the forefront of explaining the pathogenesis of Alzheimer's Disease (AD), a progressive neurodegenerative disorder that leads to cognitive decline and eventual death. Recent evidence, however, points to additional factors that contribute to the pathogenesis of this disease. These include the neurovascular hypothesis, the mitochondrial cascade hypothesis, the inflammatory hypothesis, the prion hypothesis, the mutational accumulation hypothesis, and the autoimmunity hypothesis. The purpose of this review was to briefly discuss the factors that are associated with autoimmunity in humans, including sex, the gut and lung microbiomes, age, genetics, and environmental factors. Subsequently, it was to examine the rise of autoimmune phenomena in AD, which can be instigated by a blood-brain barrier breakdown, pathogen infections, and dysfunction of the glymphatic system. Lastly, it was to discuss the various ways by which immune system dysregulation leads to AD, immunomodulating therapies, and future directions in the field of autoimmunity and neurodegeneration. A comprehensive account of the recent research done in the field was extracted from PubMed on 31 January 2022, with the keywords "Alzheimer's disease" and "autoantibodies" for the first search input, and "Alzheimer's disease" with "IgG" for the second. From the first search, 19 papers were selected, because they contained recent research on the autoantibodies found in the biofluids of patients with AD. From the second search, four papers were selected. The analysis of the literature has led to support the autoimmune hypothesis in AD. Autoantibodies were found in biofluids (serum/plasma, cerebrospinal fluid) of patients with AD with multiple methods, including ELISA, Mass Spectrometry, and microarray analysis. Through continuous research, the understanding of the synergistic effects of the various components that lead to AD will pave the way for better therapeutic methods and a deeper understanding of the disease.

Factors predisposing to humoral autoimmunity against brain-antigens in health and disease: Analysis of 49 autoantibodies in over 7000 subjects

BACKGROUND

Circulating autoantibodies (AB) against brain-antigens, often deemed pathological, receive increasing attention. We assessed predispositions and seroprevalence/characteristics of 49 AB in > 7000 individuals.

METHODS

Exploratory cross-sectional cohort study, investigating deeply phenotyped neuropsychiatric patients and healthy individuals of GRAS Data Collection for presence/characteristics of 49 brain-directed serum-AB. Predispositions were evaluated through GWAS of NMDAR1-AB carriers, analyses of immune check-point genotypes, APOE4 status, neurotrauma. Chi-square, Fisher's exact tests and logistic regression analyses were used.

RESULTS

Study of N = 7025 subjects (55.8 % male; 41 ± 16 years) revealed N = 1133 (16.13 %) carriers of any AB against 49 defined brain-antigens. Overall, age dependence of seroprevalence (OR = 1.018/year; 95 % CI [1.015-1.022]) emerged, but no disease association, neither general nor with neuropsychiatric subgroups. Males had higher AB seroprevalence (OR = 1.303; 95 % CI [1.144-1.486]). Immunoglobulin class (N for IgM:462; IgA:487; IgG:477) and titers were similar. Abundant were NMDAR1-AB (7.7 %). Low seroprevalence (1.25 %-0.02 %) was seen for most AB (e.g., amphiphysin, KCNA2, ARHGAP26, GFAP, CASPR2, MOG, Homer-3, KCNA1, GLRA1b, GAD65). Non-detectable were others. GWAS of NMDAR1-AB carriers revealed three genome-wide significant SNPs, two intergenic, one in TENM3, previously autoimmune disease-associated. Targeted analysis of immune check-point genotypes (CTLA4, PD1, PD-L1) uncovered effects on humoral anti-brain autoimmunity (OR = 1.55; 95 % CI [1.058-2.271]) and disease likelihood (OR = 1.43; 95 % CI [1.032-1.985]). APOE4 carriers (∼19 %) had lower seropositivity (OR = 0.766; 95 % CI [0.625-0.933]). Neurotrauma predisposed to NMDAR1-AB seroprevalence (IgM: OR = 1.599; 95 % CI [1.022-2.468]).

CONCLUSIONS

Humoral autoimmunity against brain-antigens, frequent across health and disease, is predicted by age, gender, genetic predisposition, and brain injury. Seroprevalence, immunoglobulin class, or titers do not predict disease.

Autoantibodies against NMDAR subunit NR1 disappear from blood upon anesthesia

Anesthetics penetrate the blood-brain-barrier (BBB) and - as confirmed preclinically – transiently disrupt it. An analogous consequence in humans has remained unproven. In mice, we previously reported that upon BBB dysfunction, the brain acts as ‘immunoprecipitator’ of autoantibodies against N-methyl-D-aspartate-receptor subunit-NR1 (NMDAR1-AB). We thus hypothesized that during human anesthesia, pre-existing NMDAR1-AB will specifically bind to brain. Screening of N = 270 subjects undergoing general anesthesia during cardiac surgery for serum NMDAR1-AB revealed N = 25 NMDAR1-AB seropositives. Only N = 14 remained positive post-surgery. No changes in albumin, thyroglobulin or CRP were associated with reduction of serum NMDAR1-AB. Thus, upon anesthesia, BBB opening likely occurs also in humans.

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Whether the blood brain barrier opens on general anesthesia in humans is unclear.

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Serum NMDAR1-AB titers drop upon anesthesia during cardiac surgery.

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Drop of serum NMDAR1-AB after anesthesia indicates ‘immunoprecipitation’ by brain.

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Immunoprecipitation needs brain access of NMDAR1-AB, indicating barrier opening.

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Neither hemodilution nor inflammation explain this loss of NMDAR1-AB from serum.

Antibodies against N-Methyl D-Aspartate Receptor in Psychotic Disorders: A Systematic Review

OBJECTIVE

The objective of this study was to provide comprehensive evidence synthesis including all available up-to-date data about the prevalence of N-methyl D-aspartate receptor (NMDAR) antibodies (ABs) in psychotic patients in order to evaluate the clinical relevance of ABs as well as to specify potential explanations of the heterogeneity of the findings and determine areas for further research.

METHODS

A literature search was conducted using the PubMed/Medline, Web of Knowledge, and Scopus databases.

RESULTS

Forty-seven studies and 4 systematic reviews (including 2 meta-analyses) were included in the present review. Studies that used cell-based assays (CBAs) provided heterogeneous results on AB prevalence, obviously depending on the type of detection assay and sample characteristics. Improvement of AB detection methods is necessary to determine the real prevalence of ABs across different groups of patients and healthy people. Live CBAs seem to have better sensitivity but probably poorer specificity than fixed CBAs. Moreover, some links between AB-positive status and acute symptoms are possible. A small amount of data on immunotherapy in AB-positive patients raises the possibility of its effectiveness but obviously require further research.

CONCLUSIONS

NMDAR ABs are definitely present in a subset of psychotic patients. NMDAR ABs might shape psychosis and underlie some symptoms, and immunotherapy might be regarded as a treatment option for patients failing to respond to other therapies.

NMDAR1 autoantibodies amplify behavioral phenotypes of genetic white matter inflammation: a mild encephalitis model with neuropsychiatric relevance

Encephalitis has an estimated prevalence of ≤0.01%. Even with extensive diagnostic work-up, an infectious etiology is identified or suspected in <50% of cases, suggesting a role for etiologically unclear, noninfectious processes. Mild encephalitis runs frequently unnoticed, despite slight neuroinflammation detectable postmortem in many neuropsychiatric illnesses. A widely unexplored field in humans, though clearly documented in rodents, is genetic brain inflammation, particularly that associated with myelin abnormalities, inducing primary white matter encephalitis. We hypothesized that "autoimmune encephalitides" may result from any brain inflammation concurring with the presence of brain antigen-directed autoantibodies, e.g., against N-methyl-D-aspartate-receptor NR1 (NMDAR1-AB), which are not causal of, but may considerably shape the encephalitis phenotype. We therefore immunized young female Cnp^-/- mice lacking the structural myelin protein 2'-3'-cyclic nucleotide 3'-phosphodiesterase (Cnp) with a "cocktail" of NMDAR1 peptides. Cnp^-/- mice exhibit early low-grade inflammation of white matter tracts and blood-brain barrier disruption. Our novel mental-time-travel test disclosed that Cnp^-/- mice are compromised in what-where-when orientation, but this episodic memory readout was not further deteriorated by NMDAR1-AB. In contrast, comparing wild-type and Cnp^-/- mice without/with NMDAR1-AB regarding hippocampal learning/memory and motor balance/coordination revealed distinct stair patterns of behavioral pathology. To elucidate a potential contribution of oligodendroglial NMDAR downregulation to NMDAR1-AB effects, we generated conditional NR1 knockout mice. These mice displayed normal Morris water maze and mental-time-travel, but beam balance performance was similar to immunized Cnp^-/-. Immunohistochemistry confirmed neuroinflammation/neurodegeneration in Cnp^-/- mice, yet without add-on effect of NMDAR1-AB. To conclude, genetic brain inflammation may explain an encephalitic component underlying autoimmune conditions.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

The diverse and complex modes of action of anti-NMDA receptor autoantibodies

NMDA receptors are ligand-gated ion channels that are found throughout the brain and are required for both brain development and many higher order functions. A variety of human patients with diverse clinical phenotypes have been identified that carry autoantibodies directed against NMDA receptor subunits. Here we focus on two general classes of autoantibodies, anti-GluN1 antibodies associated with anti-NMDA receptor encephalitis and anti-GluN2 antibodies associated with systemic lupus erythematosus (SLE). These two general classes of anti-NMDA receptor autoantibodies display a wide range of pathophysiological mechanisms from altering synaptic composition to gating of NMDARs. While we have made progress in understanding how these autoantibodies work at the molecular and cellular level, many unanswered questions remain including their long-term actions on brain function, the significance of clonal variations, and their effects on different NMDA receptor-expressing cell types in local circuits. This information will be needed to define fully the transition from anti-NMDA receptor autoantibodies to a clinical phenotype.

Neuropathobiology of COVID-19: The Role for Glia

SARS-CoV-2, which causes the Coronavirus Disease 2019 (COVID-19) pandemic, has a brain neurotropism through binding to the receptor angiotensin-converting enzyme 2 expressed by neurones and glial cells, including astrocytes and microglia. Systemic infection which accompanies severe cases of COVID-19 also triggers substantial increase in circulating levels of chemokines and interleukins that compromise the blood-brain barrier, enter the brain parenchyma and affect its defensive systems, astrocytes and microglia. Brain areas devoid of a blood-brain barrier such as the circumventricular organs are particularly vulnerable to circulating inflammatory mediators. The performance of astrocytes and microglia, as well as of immune cells required for brain health, is considered critical in defining the neurological damage and neurological outcome of COVID-19. In this review, we discuss the neurotropism of SARS-CoV-2, the implication of neuroinflammation, adaptive and innate immunity, autoimmunity, as well as astrocytic and microglial immune and homeostatic functions in the neurological and psychiatric aspects of COVID-19. The consequences of SARS-CoV-2 infection during ageing, in the presence of systemic comorbidities, and for the exposed pregnant mother and foetus are also covered.

Autoimmunity and NMDA receptor in brain disorders: Where do we stand?

Over the past decades, the identification of autoimmune encephalitis in which patients express autoantibodies directed against neurotransmitter receptors has generated great hope to shed new light on the molecular mechanisms underpinning neurological and psychiatric conditions. Among these autoimmune encephalitides, the discovery of autoantibodies directed against the glutamatergic NMDA receptor (NMDAR-Ab), in the anti-NMDAR encephalitis, has provided some key information on how complex neuropsychiatric symptoms can be caused by a deficit in NMDAR signalling. Yet, NMDAR-Abs have also been detected in several neurological and psychiatric conditions, as well as in healthy individuals. In addition, these various NMDAR-Abs appear to have different molecular properties and pathogenicities onto receptors and synaptic functions. Here, we discuss the current view on the variety of NMDAR-Abs and, in particular, how these autoantibodies can lead to receptor dysfunction in neuronal networks. Since our mechanistic understanding on patients' NMDAR-Abs is still in its infancy, several complementary processes can be proposed and further in-depth molecular and cellular investigations will surely reveal key insights. Autoantibodies represent a great opportunity to gain knowledge on the etiology of neuropsychiatric disorders and pave the way for innovative therapeutic strategies. ONE SENTENCE SUMMARY: Current view on patients' autoantibody against NMDAR.

Cognitive impact of neuronal antibodies: encephalitis and beyond

Cognitive dysfunction is a common feature of autoimmune encephalitis. Pathogenic neuronal surface antibodies are thought to mediate distinct profiles of cognitive impairment in both the acute and chronic phases of encephalitis. In this review, we describe the cognitive impairment associated with each antibody-mediated syndrome and, using evidence from imaging and animal studies, examine how the nature of the impairment relates to the underlying neuroimmunological and receptor-based mechanisms. Neuronal surface antibodies, particularly serum NMDA receptor antibodies, are also found outside of encephalitis although the clinical significance of this has yet to be fully determined. We discuss evidence highlighting their prevalence, and association with cognitive outcomes, in a number of common disorders including cancer and schizophrenia. We consider mechanisms, including blood-brain barrier dysfunction, which could determine the impact of these antibodies outside encephalitis and account for much of the clinical heterogeneity observed.

The Inflamed Brain in Schizophrenia: The Convergence of Genetic and Environmental Risk Factors That Lead to Uncontrolled Neuroinflammation

Schizophrenia is a disorder with a heterogeneous etiology involving complex interplay between genetic and environmental risk factors. The immune system is now known to play vital roles in nervous system function and pathology through regulating neuronal and glial development, synaptic plasticity, and behavior. In this regard, the immune system is positioned as a common link between the seemingly diverse genetic and environmental risk factors for schizophrenia. Synthesizing information about how the immune-brain axis is affected by multiple factors and how these factors might interact in schizophrenia is necessary to better understand the pathogenesis of this disease. Such knowledge will aid in the development of more translatable animal models that may lead to effective therapeutic interventions. Here, we provide an overview of the genetic risk factors for schizophrenia that modulate immune function. We also explore environmental factors for schizophrenia including exposure to pollution, gut dysbiosis, maternal immune activation and early-life stress, and how the consequences of these risk factors are linked to microglial function and dysfunction. We also propose that morphological and signaling deficits of the blood-brain barrier, as observed in some individuals with schizophrenia, can act as a gateway between peripheral and central nervous system inflammation, thus affecting microglia in their essential functions. Finally, we describe the diverse roles that microglia play in response to neuroinflammation and their impact on brain development and homeostasis, as well as schizophrenia pathophysiology.

Cerebrospinal Fluid Findings of 36 Adult Patients with Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a common neurodevelopmental disorder characterized by difficulties with social interaction, repetitive behavior, and additional features, such as special interests. Its precise etiology is unclear. Recently, immunological mechanisms, such as maternal autoantibodies/infections, have increasingly been the subject of discussion. Cerebrospinal fluid (CSF) investigations play a decisive role in the detection of immunological processes in the brain. This study therefore retrospectively analyzed the CSF findings of adult patients with ASD. CSF basic measures (white blood cell count, total protein, albumin quotient, immunoglobulin G (IgG) index, and oligoclonal bands) and various antineuronal antibody findings of 36 adult patients with ASD, who had received lumbar puncture, were compared with an earlier described mentally healthy control group of 39 patients with idiopathic intracranial hypertension. CSF protein concentrations and albumin quotients of patients with ASD were significantly higher as compared to controls (age corrected: p = 0.003 and p = 0.004, respectively); 17% of the patients with ASD showed increased albumin quotients. After correction for age and gender, the group effect for total protein remained significant (p = 0.041) and showed a tendency for albumin quotient (p = 0.079). In the CSF of two ASD patients, an intrathecal synthesis of anti-glutamate decarboxylase 65 (GAD65) antibodies was found. In total, more of the ASD patients (44%) presented abnormal findings in CSF basic diagnostics compared to controls (18%; p = 0.013). A subgroup of the patients with adult ASD showed indication of a blood-brain barrier dysfunction, and two patients displayed an intrathecal synthesis of anti-GAD65 antibodies; thus, the role of these antibodies in patients with ASD should be further investigated. The results of the study are limited by its retrospective and open design. The group differences in blood-brain barrier markers could be influenced by a different gender distribution between ASD patients and controls.

Receptor autoimmunity: diagnostic and therapeutic implications

Receptor autoimmunity is one of the ways in which autoimmune diseases appear in humans. Graves' disease, myasthenia gravis, idiopathic membranous nephropathy, and autoimmune acute encephalitis are the major autoimmune diseases belonging to this particular group. Receptor autoimmune disease are dependent on the presence of autoantibodies directed against cell-surface antigens, namely TSH receptor in thyrocytes, acetylcholine receptor in neuromuscular junction, phospholipase 2 receptor in podocytes, and NMDA receptor in cortical neurons. In this article we outline the distinctive features of receptor autoimmunity and the specific relationship between the autoimmunology laboratory and the presence/concentration of autoantibodies. Some immunological features distinguish receptor autoimmunity. Anti-receptor autoantibody pathologies are considered T cell-dependent, B-cell-mediated autoimmune disorders: the knowledge about the presence of circulating and/or localized autoantibodies to target organs and identification of autoantigens involved in the autoimmune reaction is of paramount importance. Due to the close correlation between the concentration of anti-receptor autoantibodies, the autoimmune target of some cell-surface receptors and the intensity of symptoms, the measurement of these immunoglobulins has become central to diagnose autoimmune diseases in all affected patients, not just in clinically dubious cases. The measurement of autoantibodies is also relevant for differential diagnosis of autoimmune and non-autoimmune forms with similar symptoms. From the methodological point of view, quantitative immunoassay methods of measurement should be preferred over semi-quantitative ones, for the capacity of the first class of methods to define precisely the reference ranges and decision levels overcoming the measurement uncertainty of semi-quantitative methods.