Different pain phenotypes are associated with anti-Caspr2 autoantibodies

Autoantibodies against contactin-associated protein 2 (Caspr2) not only induce limbic autoimmune encephalitis but are also associated with pain conditions. Here, we analyzed clinical data on pain in a large cohort of patients included into the German Network for Research in Autoimmune Encephalitis. Out of 102 patients in our cohort, pain was a frequent symptom (36% of all patients), often severe (63.6% of the patients with pain) and/or even the major symptom (55.6% of the patients with pain). Pain phenotypes differed between patients. Cluster analysis revealed two major phenotypes including mostly distal-symmetric burning pain and widespread pain with myalgia and cramps. Almost all patients had IgG4 autoantibodies and some additional IgG1, 2, and/or 3 autoantibodies, but IgG subclasses, titers, and presence or absence of intrathecal synthesis were not associated with the occurrence of pain. However, certain pre-existing risk factors for chronic pain like diabetes mellitus, peripheral neuropathy, or preexisting chronic back pain tended to occur more frequently in patients with anti-Caspr2 autoantibodies and pain. Our data show that pain is a relevant symptom in patients with anti-Caspr2 autoantibodies and support the idea of decreased algesic thresholds leading to pain. Testing for anti-Caspr2 autoantibodies needs to be considered in patients with various pain phenotypes.

Molecular dissection of an immunodominant epitope in Kv1.2-exclusive autoimmunity

Introduction

Subgroups of autoantibodies directed against voltage-gated potassium channel (Kv) complex components have been associated with immunotherapy-responsive clinical syndromes. The high prevalence and the role of autoantibodies directly binding Kv remain, however, controversial. Our objective was to determine Kv autoantibody binding requirements and to clarify their contribution to the observed immune response.

Methods

Binding epitopes were studied in sera (n = 36) and cerebrospinal fluid (CSF) (n = 12) from a patient cohort positive for Kv1.2 but negative for 32 common neurological autoantigens and controls (sera n = 18 and CSF n = 5) by phospho and deep mutational scans. Autoantibody specificity and contribution to the observed immune response were resolved on recombinant cells, cerebellum slices, and nerve fibers.

Results

83% of the patients (30/36) within the studied cohort shared one out of the two major binding epitopes with Kv1.2-3 reactivity. Eleven percent (4/36) of the serum samples showed no binding. Fingerprinting resolved close to identical sequence requirements for both shared epitopes. Kv autoantibody response is directed against juxtaparanodal regions in peripheral nerves and the axon initial segment in central nervous system neurons and exclusively mediated by the shared epitopes.

Discussion

Systematic mapping revealed two shared autoimmune responses, with one dominant Kv1.2-3 autoantibody epitope being unexpectedly prevalent. The conservation of the molecular binding requirements among these patients indicates a uniform autoantibody repertoire with monospecific reactivity. The enhanced sensitivity of the epitope-based (10/12) compared with that of the cell-based detection (7/12) highlights its use for detection. The determined immunodominant epitope is also the primary immune response visible in tissue, suggesting a diagnostic significance and a specific value for routine screening.

Glycine Receptor β-Targeting Autoantibodies Contribute to the Pathology of Autoimmune Diseases

BACKGROUND AND OBJECTIVES

Stiff-person syndrome (SPS) and progressive encephalomyelitis with rigidity and myoclonus (PERM) are rare neurologic disorders of the CNS. Until now, exclusive GlyRα subunit-binding autoantibodies with subsequent changes in function and surface numbers were reported. GlyR autoantibodies have also been described in patients with focal epilepsy. Autoimmune reactivity against the GlyRβ subunits has not yet been shown. Autoantibodies against GlyRα1 target the large extracellular N-terminal domain. This domain shares a high degree of sequence homology with GlyRβ making it not unlikely that GlyRβ-specific autoantibody (aAb) exist and contribute to the disease pathology.

METHODS

In this study, we investigated serum samples from 58 patients for aAb specifically detecting GlyRβ. Studies in microarray format, cell-based assays, and primary spinal cord neurons and spinal cord tissue immunohistochemistry were performed to determine specific GlyRβ binding and define aAb binding to distinct protein regions. Preadsorption approaches of aAbs using living cells and the purified extracellular receptor domain were further used. Finally, functional consequences for inhibitory neurotransmission upon GlyRβ aAb binding were resolved by whole-cell patch-clamp recordings.

RESULTS

Among 58 samples investigated, cell-based assays, tissue analysis, and preadsorption approaches revealed 2 patients with high specificity for GlyRβ aAb. Quantitative protein cluster analysis demonstrated aAb binding to synaptic GlyRβ colocalized with the scaffold protein gephyrin independent of the presence of GlyRα1. At the functional level, binding of GlyRβ aAb from both patients to its target impair glycine efficacy.

DISCUSSION

Our study establishes GlyRβ as novel target of aAb in patients with SPS/PERM. In contrast to exclusively GlyRα1-positive sera, which alter glycine potency, aAbs against GlyRβ impair receptor efficacy for the neurotransmitter glycine. Imaging and functional analyses showed that GlyRβ aAbs antagonize inhibitory neurotransmission by affecting receptor function rather than localization.

Role of the Glycine Receptor β Subunit in Synaptic Localization and Pathogenicity in Severe Startle Disease

Startle disease is due to the disruption of recurrent inhibition in the spinal cord. Most common causes are genetic variants in genes (GLRA1, GLRB) encoding inhibitory glycine receptor (GlyR) subunits. The adult GlyR is a heteropentameric complex composed of α1 and β subunits that localizes at postsynaptic sites and replaces embryonically expressed GlyRα2 homomers. The human GlyR variants of GLRA1 and GLRB, dominant and recessive, have been intensively studied in vitro. However, the role of unaffected GlyRβ, essential for synaptic GlyR localization, in the presence of mutated GlyRα1 in vivo is not fully understood. Here, we used knock-in mice expressing endogenous mEos4b-tagged GlyRβ that were crossed with mouse Glra1 startle disease mutants. We explored the role of GlyRβ under disease conditions in mice carrying a missense mutation (shaky) or resulting from the loss of GlyRα1 (oscillator). Interestingly, synaptic targeting of GlyRβ was largely unaffected in both mouse mutants. While synaptic morphology appears unaltered in shaky animals, synapses were notably smaller in homozygous oscillator animals. Hence, GlyRβ enables transport of functionally impaired GlyRα1 missense variants to synaptic sites in shaky animals, which has an impact on the efficacy of possible compensatory mechanisms. The observed enhanced GlyRα2 expression in oscillator animals points to a compensation by other GlyRα subunits. However, trafficking of GlyRα2β complexes to synaptic sites remains functionally insufficient, and homozygous oscillator mice still die at 3 weeks after birth. Thus, both functional and structural deficits can affect glycinergic neurotransmission in severe startle disease, eliciting different compensatory mechanisms in vivo.

Startle Disease: New Molecular Insights into an Old Neurological Disorder

Startle disease (SD) is characterized by enhanced startle responses, generalized muscle stiffness, unexpected falling, and fatal apnea episodes due to disturbed feedback inhibition in the spinal cord and brainstem of affected individuals. Mutations within the glycine receptor (GlyR) subunit and glycine transporter 2 (GlyT2) genes have been identified in individuals with SD. Impaired inhibitory neurotransmission in SD is due to pre- and/or postsynaptic GlyR or presynaptic GlyT2 dysfunctions. Previous research has focused on mutated GlyRs and GlyT2 that impair ion channel/transporter function or trafficking. With insights provided by recently solved cryo-electron microscopy and X-ray structures of GlyRs, a detailed picture of structural transitions important for receptor gating has emerged, allowing a deeper understanding of SD at the molecular level. Moreover, studies on novel SD mutations have demonstrated a higher complexity of SD, with identification of additional clinical signs and symptoms and interaction partners representing key players for fine-tuning synaptic processes. Although our knowledge has steadily improved during the last years, changes in synaptic localization and GlyR or GlyT2 homeostasis under disease conditions are not yet completely understood. Combined proteomics, interactomics, and high-resolution microscopy techniques are required to reveal alterations in receptor dynamics at the synaptic level under disease conditions.

Dual Role of Dysfunctional Asc-1 Transporter in Distinct Human Pathologies, Human Startle Disease, and Developmental Delay

Human startle disease is associated with mutations in distinct genes encoding glycine receptors, transporters or interacting proteins at glycinergic synapses in spinal cord and brainstem. However, a significant number of diagnosed patients does not carry a mutation in the common genes GLRA1, GLRB, and SLC6A5 Recently, studies on solute carrier 7 subfamily 10 (SLC7A10; Asc-1, alanine-serine-cysteine transporter) knock-out (KO) mice displaying a startle disease-like phenotype hypothesized that this transporter might represent a novel candidate for human startle disease. Here, we screened 51 patients from our patient cohort negative for the common genes and found three exonic (one missense, two synonymous), seven intronic, and single nucleotide changes in the 5' and 3' untranslated regions (UTRs) in Asc-1. The identified missense mutation Asc-1G307R from a patient with startle disease and developmental delay was investigated in functional studies. At the molecular level, the mutation Asc-1G307R did not interfere with cell-surface expression, but disrupted glycine uptake. Substitution of glycine at position 307 to other amino acids, e.g., to alanine or tryptophan did not affect trafficking or glycine transport. By contrast, G307K disrupted glycine transport similar to the G307R mutation found in the patient. Structurally, the disrupted function in variants carrying positively charged residues can be explained by local structural rearrangements because of the large positively charged side chain. Thus, our data suggest that SLC7A10 may represent a rare but novel gene associated with human startle disease and developmental delay.

Primary Glial Cell and Glioblastoma Morphology in Cocultures Depends on Scaffold Design and Hydrogel Composition

3D cell cultures better replicate the in vivo environment compared to 2D models. Glioblastoma multiforme, a malignant brain tumor, highly profits from its cellular environment. Here, the U87 glioblastoma cell line in the presence/absence of primary astrocytes is studied. Thiolated hyaluronic acid (HA-SH) hydrogel reinforced with microfiber scaffolds is compared to Matrigel. Hyaluronic acid is a major extracellular matrix (ECM) component in the brain. Poly(ɛ-caprolactone) (PCL) scaffolds are written by meltelectrowriting in a box and triangular shaped design with pore sizes of 200 µm. Scaffolds are composed of 10-layers of PCL microfibers. It is found that scaffold design has an impact on cellular morphology in the absence of hydrogel. Moreover, the used hydrogels have profound influences on cellular morphology resulting in spheroid formation in HA-SH for both the tumor-derived cell line and astrocytes, while cell viability is high. Although cocultures of U87 and astrocytes exhibit cell-cell interactions, polynucleated spheroid formation is still present for U87 cells in HA-SH. Locally restricted ECM production or inability to secrete ECM proteins may underlie the observed cell morphologies. Thus, the 3D reinforced PCL-HA-SH composite with glioma-like cells and astrocytes constitutes a reproducible system to further investigate the impact of hydrogel modifications on cellular behavior and development.

The beer component hordenine inhibits alcohol addiction-associated behaviours in mice

Alcohol consumption is a widespread behaviour that may eventually result in the development of alcohol use disorder (AUD). Alcohol, however, is rarely consumed in pure form but in fruit- or corn-derived preparations, like beer. These preparations add other compounds to the consumption, which may critically modify alcohol intake and AUD risk. We investigated the effects of hordenine, a barley-derived beer compound on alcohol use-related behaviours. We found that the dopamine D2 receptor agonist hordenine (50 mg/kg) limited ongoing alcohol consumption and prophylactically diminished relapse drinking after withdrawal in mice. Although not having reinforcing effects on its own, hordenine blocked the establishment of alcohol-induced conditioned place preference (CPP). However, it independently enhanced alcohol CPP retrieval. Hordenine had a dose-dependent inhibitory effect on locomotor activity. Chronic hordenine exposure enhanced monoamine tissue levels in many brain regions. Further characterization revealed monoaminergic binding sites of hordenine and found a strong binding on the serotonin and dopamine transporters, and dopamine D3 , and adrenergic α1A and α2A receptor activation but no effects on GABAA receptor or glycinergic signalling. These findings suggest that natural ingredients of beer, like hordenine, may work as an inhibitory and use-regulating factor by their modulation of monoaminergic signalling in the brain.

Different binding and pathogenic effect of neurofascin and contactin-1 autoantibodies in autoimmune nodopathies

Introduction

IgG4 autoantibodies against paranodal proteins are known to induce acute-onset and often severe sensorimotor autoimmune neuropathies. How autoantibodies reach their antigens at the paranode in spite of the myelin barrier is still unclear.

Methods

We performed in vitro incubation experiments with patient sera on unfixed and unpermeabilized nerve fibers and in vivo intraneural and intrathecal passive transfer of patient IgG to rats, to explore the access of IgG autoantibodies directed against neurofascin-155 and contactin-1 to the paranodes and their pathogenic effect.

Results

We found that in vitro incubation resulted in weak paranodal binding of anti-contactin-1 autoantibodies whereas anti-neurofascin-155 autoantibodies bound to the nodes more than to the paranodes. After short-term intraneural injection, no nodal or paranodal binding was detectable when using anti-neurofascin-155 antibodies. After repeated intrathecal injections, nodal more than paranodal binding could be detected in animals treated with anti-neurofascin-155, accompanied by sensorimotor neuropathy. In contrast, no paranodal binding was visible in rats intrathecally injected with anti-contactin-1 antibodies, and animals remained unaffected.

Conclusion

These data support the notion of different pathogenic mechanisms of anti-neurofascin-155 and anti-contactin-1 autoantibodies and different accessibility of paranodal and nodal structures.

Glycine receptor autoantibody binding to the extracellular domain is independent from receptor glycosylation

Glycine receptor (GlyR) autoantibodies are associated with stiff-person syndrome and the life-threatening progressive encephalomyelitis with rigidity and myoclonus in children and adults. Patient histories show variability in symptoms and responses to therapeutic treatments. A better understanding of the autoantibody pathology is required to develop improved therapeutic strategies. So far, the underlying molecular pathomechanisms include enhanced receptor internalization and direct receptor blocking altering GlyR function. A common epitope of autoantibodies against the GlyRα1 has been previously defined to residues ¹A-³³G at the N-terminus of the mature GlyR extracellular domain. However, if other autoantibody binding sites exist or additional GlyR residues are involved in autoantibody binding is yet unknown. The present study investigates the importance of receptor glycosylation for binding of anti-GlyR autoantibodies. The glycine receptor α1 harbors only one glycosylation site at the amino acid residue asparagine 38 localized in close vicinity to the identified common autoantibody epitope. First, non-glycosylated GlyRs were characterized using protein biochemical approaches as well as electrophysiological recordings and molecular modeling. Molecular modeling of non-glycosylated GlyRα1 did not show major structural alterations. Moreover, non-glycosylation of the GlyRα1^N38Q did not prevent the receptor from surface expression. At the functional level, the non-glycosylated GlyR demonstrated reduced glycine potency, but patient GlyR autoantibodies still bound to the surface-expressed non-glycosylated receptor protein in living cells. Efficient adsorption of GlyR autoantibodies from patient samples was possible by binding to native glycosylated and non-glycosylated GlyRα1 expressed in living not fixed transfected HEK293 cells. Binding of patient-derived GlyR autoantibodies to the non-glycosylated GlyRα1 offered the possibility to use purified non-glycosylated GlyR extracellular domain constructs coated on ELISA plates and use them as a fast screening readout for the presence of GlyR autoantibodies in patient serum samples. Following successful adsorption of patient autoantibodies by GlyR ECDs, binding to primary motoneurons and transfected cells was absent. Our results indicate that the glycine receptor autoantibody binding is independent of the receptor's glycosylation state. Purified non-glycosylated receptor domains harbouring the autoantibody epitope thus provide, an additional reliable experimental tool besides binding to native receptors in cell-based assays for detection of autoantibody presence in patient sera.

Anti-pan-neurofascin antibodies induce subclass-related complement activation and nodo-paranodal damage

Autoimmune neuropathy associated with antibodies against pan-neurofascin is a new subtype of nodo-paranodopathy. It is relevant because it is associated with high morbidity and mortality. Affected patients often require intensive care unit treatment for several months, and data on the reversibility and long-term prognosis are limited. The pathogenicity including IgG subclass-associated mechanisms has not been unraveled, nor been directly compared to anti-neurofascin-155 IgG4 related pathology. Understanding the underlying pathology might have a direct impact on treatment of these severely affected patients.

By a multicenter combined pro- and retrospective approach, we provide clinical data of a large cohort of patients with anti-neurofascin associated neuropathy (n = 18) including longitudinal titre and neurofilament light chain assessment via Ella® and relate clinical data to in vitro pathogenicity studies of anti-neurofascin antibodies. We assessed antibody binding characteristics and the pathogenic effects of anti-pan-neurofascin vs. neurofascin-155 antibodies on living myelinating dorsal root ganglia co-cultures. Additionally, we analyzed the IgG subclass profile and the complement binding capacity and effector functions considering the effects of intravenous immunoglobulin preparations via ELISA and cell-based assays.

In contrast to chronic, neurofascin-155 IgG4 associated neuropathy, anti-pan-neurofascin associated disease presented with a high morbidity and mortality, but as a monophasic and potentially reversible disorder. During follow-up, antibodies were no longer detectable in 8/11 patients. Anti-pan-neurofascin had direct access to the nodes of Ranvier in myelinating cultures titre-dependently, most probably inducing this severe phenotype. Antibody preincubation lead to impaired paranode formation, destruction of paranodal architecture and alterations on paranodal myelin and sensory neurons in the cultures, with more severe effects than neurofascin-155 antibodies. Besides IgG4, subclass IgG3 was detected and associated with complement binding and cytotoxic effects in vitro. As a possible correlate of axonal damage in vivo, we detected highly increased serum neurofilament light chain levels (sNF-L), correlating to serum C3a. Still, sNF-L was not identified as a marker for poor prognosis, but rather as an inter-individual and intra-individual marker for acuteness, severity, and course, with a strong decrease during recovery.

Our data provide evidence that anti-pan-neurofascin antibodies directly attack the node and induce severe and acute, but potentially reversible nodo-paranodal pathology, possibly involving complement-mediated mechanisms. Screening for autoantibodies thus is crucial to identify this subset of patients who benefit from early antibody-depleting therapy. Titre and sNF-L might serve as valuable follow-up parameters. The prospect of a favourable outcome has high relevance for physicians, patients and relatives during months of critical care.

Reinforced Hyaluronic Acid-Based Matrices Promote 3D Neuronal Network Formation

3D neuronal cultures attempt to better replicate the in vivo environment to study neurological/neurodegenerative diseases compared to 2D models. A challenge to establish 3D neuron culture models is the low elastic modulus (30-500 Pa) of the native brain. Here, an ultra-soft matrix based on thiolated hyaluronic acid (HA-SH) reinforced with a microfiber frame is formulated and used. Hyaluronic acid represents an essential component of the brain extracellular matrix (ECM). Box-shaped frames with a microfiber spacing of 200 µm composed of 10-layers of poly(ɛ-caprolactone) (PCL) microfibers (9.7 ± 0.2 µm) made via melt electrowriting (MEW) are used to reinforce the HA-SH matrix which has an elastic modulus of 95 Pa. The neuronal viability is low in pure HA-SH matrix, however, when astrocytes are pre-seeded below this reinforced construct, they significantly support neuronal survival, network formation quantified by neurite length, and neuronal firing shown by Ca²⁺ imaging. The astrocyte-seeded HA-SH matrix is able to match the neuronal viability to the level of Matrigel, a gold standard matrix for neuronal culture for over two decades. Thus, this 3D MEW frame reinforced HA-SH composite with neurons and astrocytes constitutes a reliable and reproducible system to further study brain diseases.

A Versatile Synthetic Affinity Probe Reveals Inhibitory Synapse Ultrastructure and Brain Connectivity**

Visualization of inhibitory synapses requires protocol tailoring for different sample types and imaging techniques, and usually relies on genetic manipulation or the use of antibodies that underperform in tissue immunofluorescence. Starting from an endogenous ligand of gephyrin, a universal marker of the inhibitory synapse, we developed a short peptidic binder and dimerized it, significantly increasing affinity and selectivity. We further tailored fluorophores to the binder, yielding “Sylite”—a probe with outstanding signal‐to‐background ratio that outperforms antibodies in tissue staining with rapid and efficient penetration, mitigation of staining artifacts, and simplified handling. In super‐resolution microscopy Sylite precisely localizes the inhibitory synapse and enables nanoscale measurements. Sylite profiles inhibitory inputs and synapse sizes of excitatory and inhibitory neurons in the midbrain and combined with complimentary tracing techniques reveals the synaptic connectivity.

Insulin-like growth factor 5 associates with human Aß plaques and promotes cognitive impairment

Risk factors such as dysregulation of Insulin-like growth factor (IGF) signaling have been linked to Alzheimer's disease. Here we show that Insulin-like Growth Factor Binding Protein 5 (Igfbp5), an inhibitory binding protein for insulin-like growth factor 1 (Igf-1) accumulates in hippocampal pyramidal neurons and in amyloid plaques in brains of Alzheimer patients. We investigated the pathogenic relevance of this finding with transgenic mice overexpressing Igfbp5 in pyramidal neurons of the brain. Neuronal overexpression of Igfbp5 prevents the training-induced increase of hippocampal and cortical Bdnf expression and reduces the effects of exercise on memory retention, but not on learning acquisition. Hence, elevated IGFBP5 expression could be responsible for some of the early cognitive deficits that occur during the course of Alzheimer's disease.

Loss, Gain and Altered Function of GlyR α2 Subunit Mutations in Neurodevelopmental Disorders

Glycine receptors (GlyRs) containing the α2 subunit govern cell fate, neuronal migration and synaptogenesis in the developing cortex and spinal cord. Rare missense variants and microdeletions in the X-linked GlyR α2 subunit gene (GLRA2) have been associated with human autism spectrum disorder (ASD), where they typically cause a loss-of-function via protein truncation, reduced cell-surface trafficking and/or reduced glycine sensitivity (e.g., GLRA2Δex8-9 and extracellular domain variants p.N109S and p.R126Q). However, the GlyR α2 missense variant p.R323L in the intracellular M3-M4 domain results in a gain-of-function characterized by slower synaptic decay times, longer duration active periods and increases in channel conductance. This study reports the functional characterization of four missense variants in GLRA2 associated with ASD or developmental disorders (p.V-22L, p.N38K, p.K213E, p.T269M) using a combination of bioinformatics, molecular dynamics simulations, cellular models of GlyR trafficking and electrophysiology in artificial synapses. The GlyR α2^V–22L variant resulted in altered predicted signal peptide cleavage and a reduction in cell-surface expression, suggestive of a partial loss-of-function. Similarly, GlyR α2^N38K homomers showed reduced cell-surface expression, a reduced affinity for glycine and a reduced magnitude of IPSCs in artificial synapses. By contrast, GlyR α2^K213E homomers showed a slight reduction in cell-surface expression, but IPSCs were larger, with faster rise/decay times, suggesting a gain-of-function. Lastly, GlyR α2^T269M homomers exhibited a high glycine sensitivity accompanied by a substantial leak current, suggestive of an altered function that could dramatically enhance glycinergic signaling. These results may explain the heterogeneity of clinical phenotypes associated with GLRA2 mutations and reveal that missense variants can result in a loss, gain or alteration of GlyR α2 function. In turn, these GlyR α2 missense variants are likely to either negatively or positively deregulate cortical progenitor homeostasis and neuronal migration in the developing brain, leading to changes in cognition, learning, and memory.

Genetic Code Expansion and Click-Chemistry Labeling to Visualize GABA-A Receptors by Super-Resolution Microscopy

Fluorescence labeling of difficult to access protein sites, e.g., in confined compartments, requires small fluorescent labels that can be covalently tethered at well-defined positions with high efficiency. Here, we report site-specific labeling of the extracellular domain of γ-aminobutyric acid type A (GABA-A) receptor subunits by genetic code expansion (GCE) with unnatural amino acids (ncAA) combined with bioorthogonal click-chemistry labeling with tetrazine dyes in HEK-293-T cells and primary cultured neurons. After optimization of GABA-A receptor expression and labeling efficiency, most effective variants were selected for super-resolution microscopy and functionality testing by whole-cell patch clamp. Our results show that GCE with ncAA and bioorthogonal click labeling with small tetrazine dyes represents a versatile method for highly efficient site-specific fluorescence labeling of proteins in a crowded environment, e.g., extracellular protein domains in confined compartments such as the synaptic cleft.

Identification of a stereotypic molecular arrangement of endogenous glycine receptors at spinal cord synapses

Precise quantitative information about the molecular architecture of synapses is essential to understanding the functional specificity and downstream signaling processes at specific populations of synapses. Glycine receptors (GlyRs) are the primary fast inhibitory neurotransmitter receptors in the spinal cord and brainstem. These inhibitory glycinergic networks crucially regulate motor and sensory processes. Thus far, the nanoscale organization of GlyRs underlying the different network specificities has not been defined. Here, we have quantitatively characterized the molecular arrangement and ultra-structure of glycinergic synapses in spinal cord tissue using quantitative super-resolution correlative light and electron microscopy. We show that endogenous GlyRs exhibit equal receptor-scaffold occupancy and constant packing densities of about 2000 GlyRs µm^-2 at synapses across the spinal cord and throughout adulthood, even though ventral horn synapses have twice the total copy numbers, larger postsynaptic domains, and more convoluted morphologies than dorsal horn synapses. We demonstrate that this stereotypic molecular arrangement is maintained at glycinergic synapses in the oscillator mouse model of the neuromotor disease hyperekplexia despite a decrease in synapse size, indicating that the molecular organization of GlyRs is preserved in this hypomorph. We thus conclude that the morphology and size of inhibitory postsynaptic specializations rather than differences in GlyR packing determine the postsynaptic strength of glycinergic neurotransmission in motor and sensory spinal cord networks.

Spinal Cord Neuronal Network Formation in a 3D Printed Reinforced Matrix-A Model System to Study Disease Mechanisms

3D cell cultures allow a better mimicry of the biological and mechanical environment of cells in vivo compared to 2D cultures. However, 3D cell cultures have been challenging for ultrasoft tissues such as the brain. The present study uses a microfiber reinforcement approach combining mouse primary spinal cord neurons in Matrigel with melt electrowritten (MEW) frames. Within these 3D constructs, neuronal network development is followed for 21 days in vitro. To evaluate neuronal development in 3D constructs, the maturation of inhibitory glycinergic synapses is analyzed using protein expression, the complex mechanical properties by assessing nonlinearity, conditioning, and stress relaxation, and calcium imaging as readouts. Following adaptation to the 3D matrix-frame, mature inhibitory synapse formation is faster than in 2D demonstrated by a steep increase in glycine receptor expression between days 3 and 10. The 3D expression pattern of marker proteins at the inhibitory synapse and the mechanical properties resemble the situation in native spinal cord tissue. Moreover, 3D spinal cord neuronal networks exhibit intensive neuronal activity after 14 days in culture. The spinal cord cell culture model using ultrasoft matrix reinforced by MEW fibers provides a promising tool to study and understand biomechanical mechanisms in health and disease.

Novel Functional Properties of Missense Mutations in the Glycine Receptor β Subunit in Startle Disease

Startle disease is a rare disorder associated with mutations in GLRA1 and GLRB, encoding glycine receptor (GlyR) α1 and β subunits, which enable fast synaptic inhibitory transmission in the spinal cord and brainstem. The GlyR β subunit is important for synaptic localization via interactions with gephyrin and contributes to agonist binding and ion channel conductance. Here, we have studied three GLRB missense mutations, Y252S, S321F, and A455P, identified in startle disease patients. For Y252S in M1 a disrupted stacking interaction with surrounding aromatic residues in M3 and M4 is suggested which is accompanied by an increased EC50 value. By contrast, S321F in M3 might stabilize stacking interactions with aromatic residues in M1 and M4. No significant differences in glycine potency or efficacy were observed for S321F. The A455P variant was not predicted to impact on subunit folding but surprisingly displayed increased maximal currents which were not accompanied by enhanced surface expression, suggesting that A455P is a gain-of-function mutation. All three GlyR β variants are trafficked effectively with the α1 subunit through intracellular compartments and inserted into the cellular membrane. In vivo, the GlyR β subunit is transported together with α1 and the scaffolding protein gephyrin to synaptic sites. The interaction of these proteins was studied using eGFP-gephyrin, forming cytosolic aggregates in non-neuronal cells. eGFP-gephyrin and β subunit co-expression resulted in the recruitment of both wild-type and mutant GlyR β subunits to gephyrin aggregates. However, a significantly lower number of GlyR β aggregates was observed for Y252S, while for mutants S321F and A455P, the area and the perimeter of GlyR β subunit aggregates was increased in comparison to wild-type β. Transfection of hippocampal neurons confirmed differences in GlyR-gephyrin clustering with Y252S and A455P, leading to a significant reduction in GlyR β-positive synapses. Although none of the mutations studied is directly located within the gephyrin-binding motif in the GlyR β M3-M4 loop, we suggest that structural changes within the GlyR β subunit result in differences in GlyR β-gephyrin interactions. Hence, we conclude that loss- or gain-of-function, or alterations in synaptic GlyR clustering may underlie disease pathology in startle disease patients carrying GLRB mutations.

Anti-contactin-1 Antibodies Affect Surface Expression and Sodium Currents in Dorsal Root Ganglia

Background and Objectives

As autoantibodies to contactin-1 from patients with chronic inflammatory demyelinating polyradiculoneuropathy not only bind to the paranodes where they are supposed to cause conduction failure but also bind to other neuronal cell types, we aimed to investigate the effect of anti–contactin-1 autoantibodies on contactin-1 surface expression in cerebellar granule neurons, dorsal root ganglion neurons, and contactin-1–transfected human embryonic kidney 293 cells.

Methods

Immunocytochemistry including structured illumination microscopy and immunoblotting was used to determine expression levels of contactin-1 and/or sodium channels after long-term exposure to autoantibodies from 3 seropositive patients. For functional analysis of sodium channels, whole-cell recordings of sodium currents were performed on dorsal root ganglion neurons incubated with anti–contactin-1 autoantibodies.

Results

We found a reduction in contactin-1 expression levels on dorsal root ganglion neurons, cerebellar granule neurons, and contactin-1–transfected human embryonic kidney 293 cells and decreased dorsal root ganglion sodium currents after long-term exposure to anti–contactin-1 autoantibodies. Sodium channel density did not decrease.

Discussion

Our results demonstrate a direct effect of anti–contactin-1 autoantibodies on the surface expression of contactin-1 and sodium currents in dorsal root ganglion neurons. This may be the pathophysiologic correlate of sensory ataxia reported in these patients.

Sesquiterpenes and sesquiterpenoids harbor modulatory allosteric potential and affect inhibitory GABAA receptor function in vitro

Naturally occurring compounds such as sesquiterpenes and sesquiterpenoids (SQTs) have been shown to modulate GABA_A receptors (GABA_A Rs). In this study, the modulatory potential of 11 SQTs at GABA_A Rs was analyzed to characterize their potential neurotropic activity. Transfected HEK293 cells and primary hippocampal neurons were functionally investigated using electrophysiological whole-cell recordings. Significantly different effects of β-caryophyllene and α-humulene, as well as their respective derivatives β-caryolanol and humulol, were observed in the HEK293 cell system. In neurons, the concomitant presence of phasic and tonic GABA_A R configurations accounts for differences in receptor modulation by SQTs. The in vivo presence of the γ₂ and δ subunits is important for SQT modulation. While phasic GABA_A receptors in hippocampal neurons exhibited significantly altered GABA-evoked current amplitudes in the presence of humulol and guaiol, negative allosteric potential at recombinantly expressed α₁ β₂ γ₂ receptors was only verified for humolol. Modeling and docking studies provided support for the binding of SQTs to the neurosteroid-binding site of the GABA_A R localized between transmembrane segments 1 and 3 at the (⁺ α)-(^- α) interface. In sum, differences in the modulation of GABA_A R isoforms between SQTs were identified. Another finding is that our results provide an indication that nutritional digestion affects the neurotropic potential of natural compounds.

Autoantibodies to cortactin and agrin in sera of patients with myasthenia gravis

Autoantibodies against agrin and cortactin have been described in patients with myasthenia gravis. To further validate and characterize these autoantibodies, sera and/or plasma exchange material of 135 patients with myasthenia gravis were screened for anti-agrin or anti-cortactin autoantibodies. Autoantibodies against cortactin were detected in three patients and two controls and could be confirmed by cell-based assays using cortactin-transfected human embryonic kidney cells in both controls and one patient, but were not detectable in follow-up sera of the three patients. We did not detect any autoantibodies against agrin. The clinical phenotype of anti-cortactin-positive patients varied, arguing against a relevant pathogenic role.

C-2-Linked Dimeric Strychnine Analogues as Bivalent Ligands Targeting Glycine Receptors

Strychnine is the prototypic antagonist of glycine receptors, a family of pentameric ligand-gated ion channels. Recent high-resolution structures of homomeric glycine receptors have confirmed the presence of five orthosteric binding sites located in the extracellular subunit interfaces of the receptor complex that are targeted by strychnine. Here, we report the synthesis and extensive pharmacological evaluation of bivalent ligands composed of two strychnine pharmacophores connected by appropriate spacers optimized toward simultaneous binding to two adjacent orthosteric sites of homomeric α1 glycine receptors. In all bivalent ligands, the two strychnine units were linked through C-2 by amide spacers of various lengths ranging from 6 to 69 atoms. Characterization of the compounds in two functional assays and in a radioligand binding assay indicated that compound 11a, with a spacer consisting of 57 atoms, may be capable of bridging the homomeric α1 GlyRs by simultaneous occupation of two adjacent strychnine-binding sites. The findings are supported by docking experiments to the crystal structure of the homomeric glycine receptor. Based on its unique binding mode, its relatively high binding affinity and antagonist potency, and its slow binding kinetics, the bivalent strychnine analogue 11a could be a valuable tool to study the functional properties of glycine receptors.

Melt Electrowritten In Vitro Radial Device to Study Cell Growth and Migration

The development of in vitro assays for 3D microenvironments is essential for understanding cell migration processes. A 3D-printed in vitro competitive radial device is developed to identify preferred Matrigel concentration for glioblastoma migration. Melt electrowriting (MEW) is used to fabricate the structural device with defined and intricate radial structures that are filled with Matrigel. Controlling the printing path is necessary to account for the distance lag in the molten jet, the applied electric field, and the continuous direct-writing nature of MEW. Circular printing below a diameter threshold results in substantial inward tilting of the MEW fiber wall. An eight-chamber radial device with a diameter of 9.4 mm is printed. Four different concentrations of Matrigel are dispensed into the radial chambers. Glioblastoma cells are seeded into the center and grow into all chambers within 8 days. The cell spreading area demonstrates that 6 and 8 mg mL^-1 of Matrigel are preferred over 2 and 4 mg mL^-1 . Furthermore, topographical cues via the MEW fiber wall are observed to promote migration even further away from the cell seeding depot. Previous studies implement MEW to fabricate cell invasive scaffolds whereas here it is applied to 3D-print in vitro tools to study cell migration.

Anxiety and Startle Phenotypes in Glrb Spastic and Glra1 Spasmodic Mouse Mutants

A GWAS study recently demonstrated single nucleotide polymorphisms (SNPs) in the human GLRB gene of individuals with a prevalence for agoraphobia. GLRB encodes the glycine receptor (GlyRs) β subunit. The identified SNPs are localized within the gene flanking regions (3' and 5' UTRs) and intronic regions. It was suggested that these nucleotide polymorphisms modify GlyRs expression and phenotypic behavior in humans contributing to an anxiety phenotype as a mild form of hyperekplexia. Hyperekplexia is a human neuromotor disorder with massive startle phenotypes due to mutations in genes encoding GlyRs subunits. GLRA1 mutations have been more commonly observed than GLRB mutations. If an anxiety phenotype contributes to the hyperekplexia disease pattern has not been investigated yet. Here, we compared two mouse models harboring either a mutation in the murine Glra1 or Glrb gene with regard to anxiety and startle phenotypes. Homozygous spasmodic animals carrying a Glra1 point mutation (alanine 52 to serine) displayed abnormally enhanced startle responses. Moreover, spasmodic mice exhibited significant changes in fear-related behaviors (freezing, rearing and time spent on back) analyzed during the startle paradigm, even in a neutral context. Spastic mice exhibit reduced expression levels of the full-length GlyRs β subunit due to aberrant splicing of the Glrb gene. Heterozygous animals appear normal without an obvious behavioral phenotype and thus might reflect the human situation analyzed in the GWAS study on agoraphobia and startle. In contrast to spasmodic mice, heterozygous spastic animals revealed no startle phenotype in a neutral as well as a conditioning context. Other mechanisms such as a modulatory function of the GlyRs β subunit within glycinergic circuits in neuronal networks important for fear and fear-related behavior may exist. Possibly, in human additional changes in fear and fear-related circuits either due to gene-gene interactions e.g., with GLRA1 genes or epigenetic factors are necessary to create the agoraphobia and in particular the startle phenotype.

Antiparanodal antibodies and IgG subclasses in acute autoimmune neuropathy

Objective

To determine whether IgG subclasses of antiparanodal autoantibodies are related to disease course and treatment response in acute- to subacute-onset neuropathies, we retrospectively screened 161 baseline serum/CSF samples and 66 follow-up serum/CSF samples.

Methods

We used ELISA and immunofluorescence assays to detect antiparanodal IgG and their subclasses and titers in serum/CSF of patients with Guillain-Barré syndrome (GBS), recurrent GBS (R-GBS), Miller-Fisher syndrome, and acute- to subacute-onset chronic inflammatory demyelinating polyradiculoneuropathy (A-CIDP). We evaluated clinical data retrospectively.

Results

We detected antiparanodal autoantibodies with a prevalence of 4.3% (7/161), more often in A-CIDP (4/23, 17.4%) compared with GBS (3/114, 2.6%). Longitudinal subclass analysis in the patients with GBS revealed IgG2/3 autoantibodies against Caspr-1 and against anti–contactin-1/Caspr-1, which disappeared at remission. At disease onset, patients with A-CIDP had IgG2/3 anti–Caspr-1 and anti–contactin-1/Caspr-1 or IgG4 anti–contactin-1 antibodies, IgG3 being associated with good response to IV immunoglobulins (IVIg). In the chronic phase of disease, IgG subclass of one patient with A-CIDP switched from IgG3 to IgG4.

Conclusion

Our data (1) confirm and extend previous observations that antiparanodal IgG2/3 but not IgG4 antibodies can occur in acute-onset neuropathies manifesting as monophasic GBS, (2) suggest association of IgG3 to a favorable response to IVIg, and (3) lend support to the hypothesis that in some patients, an IgG subclass switch from IgG3 to IgG4 may be the correlate of a secondary progressive or relapsing course following a GBS-like onset.

Comparison of biophysical properties of α1β2 and α3β2 GABAA receptors in whole-cell patch-clamp electrophysiological recordings

In the present study we have characterized the biophysical properties of wild-type (WT) α1β2 and α3β2 GABA_A receptors and probed the molecular basis for the observed differences. The activation and desensitization behavior and the residual currents of the receptors expressed in HEK293 cells were determined in whole-cell patch clamp recordings. Kinetic parameters of α1β2 and α3β2 activation differed significantly, with α1β2 and α3β2 exhibiting rise times (10–90%) of 24 ± 2 ms and 51 ± 7 ms, respectively. In contrast, the two receptors exhibited largely comparable desensitization behavior with decay currents that could be fitted to exponential functions with two or three components. Most notably, the two receptor compositions displayed different degrees of desentization, with the residual currents of α1β2 and α3β2 constituting 34 ± 2% and 21 ± 2% of the peak current, respectively. The respective contributions of the extracellular domains and the transmembrane/intracellular domains of the α-subunit to these physiological profiles were next assessed in recordings from cells expressing αβ2 receptors comprising chimeric α-subunits. The rise times displayed by α1^ECD/α3^TMDβ2 and α3^ECD/α1^TMDβ2 receptors were intermediate to those of WT α1β2 and WT α3β2, and the distribution of the different components of the current decays exhibited by the two chimeric receptors followed the same pattern as the two WT receptors. The residual current exhibited by α1^ECD/α3^TMDβ2 (23 ± 3%) was similar to that of α3β2 but significantly different from that of α1β2, whereas the residual current displayed by α3^ECD/α1^TMDβ2 (27 ± 2%) was intermediate to and did not differ significantly from either of the WT receptors. This points to molecular differences in the transmembrane/intracellular domains of the α-subunit as the main determinants of the observed differences in receptor physiology between α1β2 and α3β2 receptors.

Cortical Neurons form a Functional Neuronal Network in a 3D Printed Reinforced Matrix

Impairments in neuronal circuits underly multiple neurodevelopmental and neurodegenerative disorders. 3D cell culture models enhance the complexity of in vitro systems and provide a microenvironment closer to the native situation than with 2D cultures. Such novel model systems will allow the assessment of neuronal network formation and their dysfunction under disease conditions. Here, mouse cortical neurons are cultured from embryonic day E17 within in a fiber-reinforced matrix. A soft Matrigel with a shear modulus of 31 ± 5.6 Pa is reinforced with scaffolds created by melt electrowriting, improving its mechanical properties and facilitating the handling. Cortical neurons display enhance cell viability and the neuronal network maturation in 3D, estimated by staining of dendrites and synapses over 21 days in vitro, is faster in 3D compared to 2D cultures. Using functional readouts with electrophysiological recordings, different firing patterns of action potentials are observed, which are absent in the presence of the sodium channel blocker, tetrodotoxin. Voltage-gated sodium currents display a current-voltage relationship with a maximum peak current at -25 mV. With its high customizability in terms of scaffold reinforcement and soft matrix formulation, this approach represents a new tool to study neuronal networks in 3D under normal and, potentially, disease conditions.

11-Aminostrychnine and N-(Strychnine-11-yl)propionamide: Synthesis, Configuration, and Pharmacological Evaluation at Glycine Receptors

(11S)-11-Aminostrychnine (1) and N-[(11S)-strychnine-11-yl]propionamide (2) were synthesized and characterized as antagonists of homomeric α1 and heteromeric α1β glycine receptors in a functional fluorescence-based assay and a patch-clamp assay and in radioligand binding studies. The absolute configuration at C-11 of 1 was determined based on vicinal coupling constants and NOESY data. Docking experiments to the orthosteric binding site of the α3 glycine receptor showed a binding mode of compound 2 analogous to that of strychnine, explaining its high antagonistic potency. The findings identify the C-11 amide function of strychnine as a suitable linker group for the future development of dimeric strychnine analogues targeting glycine receptors. The findings extend the SAR of strychnine at glycine receptors.

Anti-pan-neurofascin IgG3 as a marker of fulminant autoimmune neuropathy

Objective

To identify and characterize patients with autoantibodies against different neurofascin (NF) isoforms.

Methods

Screening of a large cohort of patient sera for anti-NF autoantibodies by ELISA and further characterization by cell-based assays, epitope mapping, and complement binding assays.

Results

Two different clinical phenotypes became apparent in this study: The well-known clinical picture of subacute-onset severe sensorimotor neuropathy with tremor that is known to be associated with IgG4 autoantibodies against the paranodal isoform NF-155 was found in 2 patients. The second phenotype with a dramatic course of disease with tetraplegia and almost locked-in syndrome was associated with IgG3 autoantibodies against nodal and paranodal isoforms of NF in 3 patients. The epitope against which these autoantibodies were directed in this second phenotype was the common Ig domain found in all 3 NF isoforms. In contrast, anti–NF-155 IgG4 were directed against the NF-155–specific Fn3Fn4 domain. The description of a second phenotype of anti–NF-associated neuropathy is in line with some case reports of similar patients that were published in the last year.

Conclusions

Our results indicate that anti–pan-NF-associated neuropathy differs from anti–NF-155-associated neuropathy, and epitope and subclass play a major role in the pathogenesis and severity of anti–NF-associated neuropathy and should be determined to correctly classify patients, also in respect to possible differences in therapeutic response.

Anti-CNTN1 IgG3 induces acute conduction block and motor deficits in a passive transfer rat model

Background

Autoantibodies against the paranodal protein contactin-1 have recently been described in patients with severe acute-onset autoimmune neuropathies and mainly belong to the IgG4 subclass that does not activate complement. IgG3 anti-contactin-1 autoantibodies are rare, but have been detected during the acute onset of disease in some cases. There is evidence that anti-contactin-1 prevents adhesive interaction, and chronic exposure to anti-contactin-1 IgG4 leads to structural changes at the nodes accompanied by neuropathic symptoms. However, the pathomechanism of acute onset of disease and the pathogenic role of IgG3 anti-contactin-1 is largely unknown.

Methods

In the present study, we aimed to model acute autoantibody exposure by intraneural injection of IgG of patients with anti-contacin-1 autoantibodies to Lewis rats. Patient IgG obtained during acute onset of disease (IgG3 predominant) and IgG from the chronic phase of disease (IgG4 predominant) were studied in comparison.

Results

Conduction blocks were measured in rats injected with the “acute” IgG more often than after injection of “chronic” IgG (83.3% versus 35%) and proved to be reversible within a week after injection. Impaired nerve conduction was accompanied by motor deficits in rats after injection of the “acute” IgG but only minor structural changes of the nodes. Paranodal complement deposition was detected after injection of the “acute IgG”. We did not detect any inflammatory infiltrates, arguing against an inflammatory cascade as cause of damage to the nerve. We also did not observe dispersion of paranodal proteins or sodium channels to the juxtaparanodes as seen in patients after chronic exposure to anti-contactin-1.

Conclusions

Our data suggest that anti-contactin-1 IgG3 induces an acute conduction block that is most probably mediated by autoantibody binding and subsequent complement deposition and may account for acute onset of disease in these patients. This supports the notion of anti-contactin-1-associated neuropathy as a paranodopathy with the nodes of Ranvier as the site of pathogenesis.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1462-z) contains supplementary material, which is available to authorized users.

3D Electrophysiological Measurements on Cells Embedded within Fiber-Reinforced Matrigel

2D electrophysiology is often used to determine the electrical properties of neurons. In the brain however, neurons form extensive 3D networks. Thus, performing electrophysiology in a 3D environment provides a closer situation to the physiological condition and serves as a useful tool for various applications in the field of neuroscience. In this study, 3D electrophysiology is established within a fiber-reinforced matrix to enable fast readouts from transfected cells, which are often used as model systems for 2D electrophysiology. Using melt electrowriting (MEW) of scaffolds to reinforce Matrigel, 3D electrophysiology is performed on a glycine receptor-transfected Ltk-11 mouse fibroblast cell line. The glycine receptor is an inhibitory ion channel associated when mutated with impaired neuromotor behavior. The average thickness of the MEW scaffold is 141.4 ± 5.7 µm, using 9.7 ± 0.2 µm diameter fibers, and square pore spacings of 100, 200, and 400 µm. For the first time, the electrophysiological characterization of glycine receptor-transfected cells is demonstrated with respect to agonist efficacy and potency in a 3D matrix. With the MEW scaffold reinforcement not interfering with the electrophysiological measurement, this approach can now be further adapted and developed for different kinds of neuronal cultures to study and understand pathological mechanisms under disease conditions.

Impaired Glycine Receptor Trafficking in Neurological Diseases

Ionotropic glycine receptors (GlyRs) enable fast synaptic neurotransmission in the adult spinal cord and brainstem. The inhibitory GlyR is a transmembrane glycine-gated chloride channel. The immature GlyR protein undergoes various processing steps, e.g., folding, assembly, and maturation while traveling from the endoplasmic reticulum to and through the Golgi apparatus, where post-translational modifications, e.g., glycosylation occur. The mature receptors are forward transported via microtubules to the cellular surface and inserted into neuronal membranes followed by synaptic clustering. The normal life cycle of a receptor protein includes further processes like internalization, recycling, and degradation. Defects in GlyR life cycle, e.g., impaired protein maturation and degradation have been demonstrated to underlie pathological mechanisms of various neurological diseases. The neurological disorder startle disease is caused by glycinergic dysfunction mainly due to missense mutations in genes encoding GlyR subunits (GLRA1 and GLRB). In vitro studies have shown that most recessive forms of startle disease are associated with impaired receptor biogenesis. Another neurological disease with a phenotype similar to startle disease is a special form of stiff-person syndrome (SPS), which is most probably due to the development of GlyR autoantibodies. Binding of GlyR autoantibodies leads to enhanced receptor internalization. Here we focus on the normal life cycle of GlyRs concentrating on assembly and maturation, receptor trafficking, post-synaptic integration and clustering, and GlyR internalization/recycling/degradation. Furthermore, this review highlights findings on impairment of these processes under disease conditions such as disturbed neuronal ER-Golgi trafficking as the major pathomechanism for recessive forms of human startle disease. In SPS, enhanced receptor internalization upon autoantibody binding to the GlyR has been shown to underlie the human pathology. In addition, we discuss how the existing mouse models of startle disease increased our current knowledge of GlyR trafficking routes and function. This review further illuminates receptor trafficking of GlyR variants originally identified in startle disease patients and explains changes in the life cycle of GlyRs in patients with SPS with respect to structural and functional consequences at the receptor level.

Functional Consequences of the Postnatal Switch From Neonatal to Mutant Adult Glycine Receptor α1 Subunits in the Shaky Mouse Model of Startle Disease

Mutations in GlyR α1 or β subunit genes in humans and rodents lead to severe startle disease characterized by rigidity, massive stiffness and excessive startle responses upon unexpected tactile or acoustic stimuli. The recently characterized startle disease mouse mutant shaky carries a missense mutation (Q177K) in the β8-β9 loop within the large extracellular N-terminal domain of the GlyR α1 subunit. This results in a disrupted hydrogen bond network around K177 and faster GlyR decay times. Symptoms in mice start at postnatal day 14 and increase until premature death of homozygous shaky mice around 4–6 weeks after birth. Here we investigate the in vivo functional effects of the Q177K mutation using behavioral analysis coupled to protein biochemistry and functional assays. Western blot analysis revealed GlyR α1 subunit expression in wild-type and shaky animals around postnatal day 7, a week before symptoms in mutant mice become obvious. Before 2 weeks of age, homozygous shaky mice appeared healthy and showed no changes in body weight. However, analysis of gait and hind-limb clasping revealed that motor coordination was already impaired. Motor coordination and the activity pattern at P28 improved significantly upon diazepam treatment, a pharmacotherapy used in human startle disease. To investigate whether functional deficits in glycinergic neurotransmission are present prior to phenotypic onset, we performed whole-cell recordings from hypoglossal motoneurons (HMs) in brain stem slices from wild-type and shaky mice at different postnatal stages. Shaky homozygotes showed a decline in mIPSC amplitude and frequency at P9-P13, progressing to significant reductions in mIPSC amplitude and decay time at P18-24 compared to wild-type littermates. Extrasynaptic GlyRs recorded by bath-application of glycine also revealed reduced current amplitudes in shaky mice compared to wild-type neurons, suggesting that presynaptic GlyR function is also impaired. Thus, a distinct, but behaviorally ineffective impairment of glycinergic synapses precedes the symptoms onset in shaky mice. These findings extend our current knowledge on startle disease in the shaky mouse model in that they demonstrate how the progression of GlyR dysfunction causes, with a delay of about 1 week, the appearance of disease symptoms.

PKA and PKC Modulators Affect Ion Channel Function and Internalization of Recombinant Alpha1 and Alpha1-Beta Glycine Receptors

Glycine receptors (GlyRs) are important mediators of fast inhibitory neurotransmission in the mammalian central nervous system. Their function is controlled by multiple cellular mechanisms, including intracellular regulatory processes. Modulation of GlyR function by protein kinases has been reported for many cell types, involving different techniques, and often yielding contradictory results. Here, we studied the effects of protein kinase C (PKC) and cAMP-dependent protein kinase A (PKA) on glycine induced currents in HEK293 cells expressing human homomeric α1 and heteromeric α1-β GlyRs using whole-cell patch clamp techniques as well as internalization assays. In whole-cell patch-clamp measurements, modulators were applied in the intracellular buffer at concentrations between 0.1 μM and 0.5 μM. EC₅₀ of glycine increased upon application of the protein kinase activators Forskolin and phorbol-12-myristate-13-acetate (PMA) but decreased in the presence of the PKC inhibitor Staurosporine aglycon and the PKA inhibitor H-89. Desensitization of recombinant α1 receptors was significantly increased in the presence of Forskolin. Staurosporine aglycon, on the other hand decreased desensitization of heteromeric α1-β GlyRs. The time course of receptor activation was determined for homomeric α1 receptors and revealed two simultaneous effects: cells showed a decrease of EC₅₀ after 3–6 min of establishing whole-cell configuration. This effect was independent of protein kinase modulators. All modulators of PKA and PKC, however, produced an additional shift of EC₅₀, which overlay and eventually exceeded the cells intrinsic variation of EC₅₀. The effect of kinase activators was abolished if the corresponding inhibitors were co-applied, consistent with PKA and PKC directly mediating the modulation of GlyR function. Direct effects of PKA- and PKC-modulators on receptor expression on transfected HEK cells were monitored within 15 min of drug application, showing a significant increase of receptor internalization with PKA and PKC activators, while the corresponding inhibitors had no significant effect on receptor surface expression or internalization. Our results confirm the observation that phosphorylation via PKA and PKC has a direct effect on the GlyR ion channel complex and plays an important role in the fine-tuning of glycinergic signaling.

Structural changes at the myrtenol backbone reverse its positive allosteric potential into inhibitory GABAA receptor modulation

GABAA receptors are ligand-gated anion channels that form pentameric arrangements of various subunits. Positive allosteric modulators of GABAA receptors have been reported as being isolated either from plants or synthesized analogs of known GABAA receptor targeting drugs. Recently, we identified monoterpenes, e.g. myrtenol as a positive allosteric modulator at α1β2 GABAA receptors. Here, along with pharmacophore-based virtual screening studies, we demonstrate that scaffold modifications of myrtenol resulted in the loss of modulatory activity. Two independent approaches, fluorescence-based compound analysis and electrophysiological recordings in whole-cell configurations were used for analysis of transfected cells. C-atoms 1 and 2 of the myrtenol backbone were identified as crucial to preserve positive allosteric potential. A modification at C-atom 2 and lack of the hydroxyl group at C-atom 1 exhibited significantly reduced GABAergic currents at α1β2, α1β2γ, α2β3, α2β3γ and α4β3δ receptors. This effect was independent of the γ2 subunit. A sub-screen with side chain length and volume differences at the C-atom 1 identified two compounds that inhibited GABAergic responses but without receptor subtype specificity. Our combined approach of pharmacophore-based virtual screening and functional readouts reveals that side chain modifications of the bridged six-membered ring structure of myrtenol are crucial for its modulatory potential at GABAA receptors.

The GlyR Extracellular β8-β9 Loop - A Functional Determinant of Agonist Potency

Ligand-binding of Cys-loop receptors results in rearrangements of extracellular loop structures which are further translated into the tilting of membrane spanning helices, and finally opening of the ion channels. The cryo-EM structure of the homopentameric α1 glycine receptor (GlyR) demonstrated an involvement of the extracellular β8–β9 loop in the transition from ligand-bound receptors to the open channel state. Recently, we identified a functional role of the β8–β9 loop in a novel startle disease mouse model shaky. The mutation of residue GlyRα1^Q177 to lysine present in shaky mice resulted in reduced glycine potency, reduced synaptic expression, and a disrupted hydrogen network at the structural level around position GlyRα1^Q177. Here, we investigated the role of amino acid volume, side chain length, and charge at position Q177 to get deeper insights into the functional role of the β8–β9 loop. We used a combined approach of in vitro expression analysis, functional electrophysiological recordings, and GlyR modeling to describe the role of Q177 for GlyR ion channel function. GlyRα1^Q177 variants do not disturb ion channel transport to the cellular surface of transfected cells, neither in homomeric nor in heteromeric GlyR configurations. The EC₅₀ values were increased for all GlyRα1^Q177 variants in comparison to the wild type. The largest decrease in glycine potency was observed for the variant GlyRα1^Q177R. Potencies of the partial agonists β-alanine and taurine were also reduced. Our data are further supported by homology modeling. The GlyRα1^Q177R variant does not form hydrogen bonds with the surrounding network of residue Q177 similar to the substitution with a basic lysine present in the mouse mutant shaky. Among all investigated Q177 mutants, the neutral exchange of glutamine to asparagine as well as the introduction of the closely related amino acid glutamic acid preserve the hydrogen bond network. Introduction of amino acids with small side chains or larger volume resulted in a loss of their hydrogen bonds to neighboring residues. The β8–β9 loop is thus an important structural and functional determinant of the inhibitory GlyR.

Disruption of a Structurally Important Extracellular Element in the Glycine Receptor Leads to Decreased Synaptic Integration and Signaling Resulting in Severe Startle Disease

Functional impairments or trafficking defects of inhibitory glycine receptors (GlyRs) have been linked to human hyperekplexia/startle disease and autism spectrum disorders. We found that a lack of synaptic integration of GlyRs, together with disrupted receptor function, is responsible for a lethal startle phenotype in a novel spontaneous mouse mutant shaky, caused by a missense mutation, Q177K, located in the extracellular β8-β9 loop of the GlyR α1 subunit. Recently, structural data provided evidence that the flexibility of the β8-β9 loop is crucial for conformational transitions during opening and closing of the ion channel and represents a novel allosteric binding site in Cys-loop receptors. We identified the underlying neuropathological mechanisms in male and female shaky mice through a combination of protein biochemistry, immunocytochemistry, and both in vivo and in vitro electrophysiology. Increased expression of the mutant GlyR α1Q177K subunit in vivo was not sufficient to compensate for a decrease in synaptic integration of α1Q177Kβ GlyRs. The remaining synaptic heteromeric α1Q177Kβ GlyRs had decreased current amplitudes with significantly faster decay times. This functional disruption reveals an important role for the GlyR α1 subunit β8-β9 loop in initiating rearrangements within the extracellular-transmembrane GlyR interface and that this structural element is vital for inhibitory GlyR function, signaling, and synaptic clustering.SIGNIFICANCE STATEMENT GlyR dysfunction underlies neuromotor deficits in startle disease and autism spectrum disorders. We describe an extracellular GlyR α1 subunit mutation (Q177K) in a novel mouse startle disease mutant shaky Structural data suggest that during signal transduction, large transitions of the β8-β9 loop occur in response to neurotransmitter binding. Disruption of the β8-β9 loop by the Q177K mutation results in a disruption of hydrogen bonds between Q177 and the ligand-binding residue R65. Functionally, the Q177K change resulted in decreased current amplitudes, altered desensitization decay time constants, and reduced GlyR clustering and synaptic strength. The GlyR β8-β9 loop is therefore an essential regulator of conformational rearrangements during ion channel opening and closing.

Metabolic Products of Linalool and Modulation of GABAA Receptors

Terpenoids are major subcomponents in aroma substances which harbor sedative physiological potential. We have demonstrated that various monoterpenoids such as the acyclic linalool enhance GABAergic currents in an allosteric manner in vitro upon overexpression of inhibitory α1β2 GABA_A receptors in various expression systems. However, in plants or humans, i.e., following intake via inhalation or ingestion, linalool undergoes metabolic modifications including oxygenation and acetylation, which may affect the modulatory efficacy of the generated linalool derivatives. Here, we analyzed the modulatory potential of linalool derivatives at α1β2γ2 GABA_A receptors upon transient overexpression. Following receptor expression control, electrophysiological recordings in a whole cell configuration were used to determine the chloride influx upon co-application of GABA EC₁₀₋₃₀ together with the modulatory substance. Our results show that only oxygenated linalool metabolites at carbon 8 positively affect GABAergic currents whereas derivatives hydroxylated or carboxylated at carbon 8 were rather ineffective. Acetylated linalool derivatives resulted in non-significant changes of GABAergic currents. We can conclude that metabolism of linalool reduces its positive allosteric potential at GABA_A receptors compared to the significant potentiation effects of the parent molecule linalool itself.

The role of charged residues in independent glycine receptor folding domains for intermolecular interactions and ion channel function

Glycine receptor (GlyR) truncations in the intracellular TM3-4 loop, documented in patients suffering from hyperekplexia and in the mouse mutant oscillator, lead to non-functionality of GlyRs. The missing part that contains the TM3-4 loop, TM4 and C-terminal sequences is essential for pentameric receptor arrangements. In vitro co-expressions of GlyRα1-truncated N-domains and C-domains were able to restore ion channel function. An ionic interaction between both domains was hypothesized as the underlying mechanism. Here, we analysed the proposed ionic interaction between GlyR N- and C-domains using C-terminal constructs with either positively or negatively charged N-termini. Charged residues at the N-terminus of the C-domain did interfere with receptor surface expression and ion channel function. In particular, presence of negatively charged residues at the N-terminus led to significantly decreased ion channel function. Presence of positive charges resulted in reduced maximal currents possibly as a result of repulsion of both domains. If the C-domain was tagged by a myc-epitope, low maximal current amplitudes were detected. Intrinsic charges of the myc-epitope and charged N-terminal ends of the C-domain most probably induce intramolecular interactions. These interactions might hinder the close proximity of C-domains and N-domains, which is a prerequisite for functional ion channel configurations. The remaining basic subdomains close to TM3 and 4 were sufficient for domain complementation and functional ion channel formation. Thus, these basic subdomains forming α-helical elements or an intracellular portal represent attractants for incoming negatively charged chloride ions and interact with the phospholipids thereby stabilizing the GlyR in a conformation that allows ion channel opening.

Oxime Ethers of (E)-11-Isonitrosostrychnine as Highly Potent Glycine Receptor Antagonists

A series of (E)-11-isonitrosostrychnine oxime ethers, 2-aminostrychnine, (strychnine-2-yl)propionamide, 18-oxostrychnine, and N-propylstrychnine bromide were synthesized and evaluated pharmacologically at human α1 and α1β glycine receptors in a functional fluorescence-based and a whole-cell patch-clamp assay and in [³H]strychnine binding studies. 2-Aminostrychnine and the methyl, allyl, and propargyl oxime ethers were the most potent α1 and α1β antagonists in the series, displaying IC₅₀ values similar to those of strychnine at the two receptors. Docking experiments to the strychnine binding site of the crystal structure of the α3 glycine receptor indicated the same orientation of the strychnine core for all analogues. For the most potent oxime ethers, the ether substituent was accommodated in a lipophilic receptor binding pocket. The findings identify the oxime hydroxy group as a suitable attachment point for linking two strychnine pharmacophores by a polymethylene spacer and are, therefore, important for the design of bivalent ligands targeting glycine receptors.

Three-Step Test System for the Identification of Novel GABAA Receptor Modulating Food Plants

Potentiation of γ-amino butyric acid (GABA)-induced GABA_A receptor (GABA_AR) activation is a common pathway to achieve sedative, sleep-enhancing, anxiolytic, and antidepressant effects. Presently, a three-component test system was established for the identification of novel GABA_AR modulating food plants. In the first step, potentiation of GABA-induced response of the GABA_AR was analysed by two-electrode voltage clamp (TEVC) for activity on human α1β2-GABA_AR expressed in Xenopus laevis oocytes. Positively tested food plants were then subjected to quantification of GABA content by high-performance liquid chromatography with fluorescence detection (HPLC-FLD) to exclude test foods, which evoke a TEVC-response by endogenous GABA. In the third step, specificity of GABA_A-modulating activity was assessed by TEVC analysis of Xenopus laevis oocytes expressing the homologous glycine receptor (GlyR). The three-component test was then applied to screen 10 aqueous extracts of food plants for their GABA_AR activity. Thus, hop cones (Humulus lupulus) and Sideritis sipylea were identified as the most potent specific GABA_AR modulators eliciting significant potentiation of the current by 182 ± 27 and 172 ± 19 %, respectively, at the lowest concentration of 0.5 μg/mL. The extracts can now be further evaluated by in vivo studies and by structural evaluation of the active components.