Comparative Effects of Domain-Specific Human Monoclonal Antibodies Against LGI1 on Neuronal Excitability.

BACKGROUND AND OBJECTIVES: Autoantibodies to leucine-rich glioma inactivated protein 1 (LGI1) cause an autoimmune limbic encephalitis with frequent focal seizures and anterograde memory dysfunction. LGI1 is a neuronal secreted linker protein with 2 functional domains: the leucine-rich repeat (LRR) and epitempin (EPTP) regions. LGI1 autoantibodies are known to interfere with presynaptic function and neuronal excitability; however, their epitope-specific mechanisms are incompletely understood. METHODS: We used patient-derived monoclonal autoantibodies (mAbs), which target either LRR or EPTP domains of LGI1 to investigate long-term antibody-induced alteration of neuronal function. LRR- and EPTP-specific effects were evaluated by patch-clamp recordings in cultured hippocampal neurons and compared with biophysical neuron modeling. Kv1.1 channel clustering at the axon initial segment (AIS) was quantified by immunocytochemistry and structured illumination microscopy techniques. RESULTS: Both EPTP and LRR domain-specific mAbs decreased the latency of first somatic action potential firing. However, only the LRR-specific mAbs increased the number of action potential firing together with enhanced initial instantaneous frequency and promoted spike-frequency adaptation, which were less pronounced after the EPTP mAb. This also led to an effective reduction in the slope of ramp-like depolarization in the subthreshold response, suggesting Kv1 channel dysfunction. A biophysical model of a hippocampal neuron corroborated experimental results and suggests that an isolated reduction of the conductance of Kv1-mediated K+ currents largely accounts for the antibody-induced alterations in the initial firing phase and spike-frequency adaptation. Furthermore, Kv1.1 channel density was spatially redistributed from the distal toward the proximal site of AIS under LRR mAb treatment and, to a lesser extant, under EPTP mAb. DISCUSSION: These findings indicate an epitope-specific pathophysiology of LGI1 autoantibodies. The pronounced neuronal hyperexcitability and SFA together with dropped slope of ramp-like depolarization after LRR-targeted interference suggest disruption of LGI1-dependent clustering of K+ channel complexes. Moreover, considering the effective triggering of action potentials at the distal AIS, the altered spatial distribution of Kv1.1 channel density may contribute to these effects through impairing neuronal control of action potential initiation and synaptic integration.

ATR regulates neuronal activity by modulating presynaptic firing.

Ataxia Telangiectasia and Rad3-related (ATR) protein, as a key DNA damage response (DDR) regulator, plays an essential function in response to replication stress and controls cell viability. Hypomorphic mutations of ATR cause the human ATR-Seckel syndrome, characterized by microcephaly and intellectual disability, which however suggests a yet unknown role for ATR in non-dividing cells. Here we show that ATR deletion in postmitotic neurons does not compromise brain development and formation; rather it enhances intrinsic neuronal activity resulting in aberrant firing and an increased epileptiform activity, which increases the susceptibility of ataxia and epilepsy in mice. ATR deleted neurons exhibit hyper-excitability, associated with changes in action potential conformation and presynaptic vesicle accumulation, independent of DDR signaling. Mechanistically, ATR interacts with synaptotagmin 2 (SYT2) and, without ATR, SYT2 is highly upregulated and aberrantly translocated to excitatory neurons in the hippocampus, thereby conferring a hyper-excitability. This study identifies a physiological function of ATR, beyond its DDR role, in regulating neuronal activity.

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.

Autoimmune encephalitis with anti-IgLON5 and anti-GABAB-receptor antibodies: A case report.

RATIONALE: Anti-IgLON5 disease is a complex neurological illness which is characterized by progressive sleep and movement disorders and defined by specific autoantibodies to IgLON5. We here describe the first case of a patient with coexisting anti-IgLON5 as well as anti-γ-aminobutyric acid B (GABAB)-receptor antibodies and predominant clinical features of anti-IgLON5 disease. PATIENT CONCERNS: The patient initially presented with subacute symptoms of severe sleep disorder, gait stability, dysarthria, cognitive impairment, depressive episode and hallucinations. DIAGNOSES: The patient was diagnosed with autoimmune encephalitis, based on clinical features and positive anti-IgLON5 antibodies in serum as well as in cerebrospinal fluid and anti-GABAB-receptor antibodies in serum only. INTERVENTIONS: Initially, the patient was treated with high dosages of methylprednisolone and subsequently with plasmapheresis. Due to the lack of clinical improvement immunosuppressive treatment with intravenous cyclophosphamide was initiated. OUTCOMES: Following the first year of cyclophosphamide treatment, neurological examination revealed an improvement in gait instability, visual and acoustic hallucinations and sleep disorder. LESSONS: The case report demonstrates that anti-IgLON5 and anti-GABAB-receptor antibodies can coexist in the same patient whereas clinical leading symptoms are determined by those antibodies that were tested positive in cerebrospinal fluid.

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.