Direct measurements of neurosteroid binding to specific sites on GABAA receptors.

BACKGROUND AND PURPOSE: Neurosteroids are allosteric modulators of GABAA currents, acting through several functional binding sites although their affinity and specificity for each site are unknown. The goal of this study was to measure steady-state binding affinities of various neurosteroids for specific sites on the GABAA receptor. EXPERIMENTAL APPROACH: Two methods were developed to measure neurosteroid binding affinity: (1) quenching of specific tryptophan residues in neurosteroid binding sites by the neurosteroid 17-methylketone group, and (2) FRET between MQ290 (an intrinsically fluorescent neurosteroid) and tryptophan residues in the binding sites. The assays were developed using ELIC-α1GABAAR, a chimeric receptor containing transmembrane domains of the α1-GABAA receptor. Tryptophan mutagenesis was used to identify specific interactions. KEY RESULTS: Allopregnanolone (3α-OH neurosteroid) was shown to bind at intersubunit and intrasubunit sites with equal affinity, whereas epi-allopregnanolone (3ß-OH neurosteroid) binds at the intrasubunit site. MQ290 formed a strong FRET pair with W246, acting as a site-specific probe for the intersubunit site. The affinity and site-specificity of several neurosteroid agonists and inverse agonists was measured using the MQ290 binding assay. The FRET assay distinguishes between competitive and allosteric inhibition of MQ290 binding and demonstrated an allosteric interaction between the two neurosteroid binding sites. CONCLUSIONS AND IMPLICATIONS: The affinity and specificity of neurosteroid binding to two sites in the ELIC-α1GABAAR were directly measured and an allosteric interaction between the sites was revealed. Adaptation of the MQ290 FRET assay to a plate-reader format will enable screening for high affinity agonists and antagonists for neurosteroid binding sites.

The Mechanism of Enantioselective Neurosteroid Actions on GABAA Receptors.

The neurosteroid allopregnanolone (ALLO) and pregnanolone (PREG), are equally effective positive allosteric modulators (PAMs) of GABAA receptors. Interestingly, the PAM effects of ALLO are strongly enantioselective, whereas those of PREG are not. This study was aimed at determining the basis for this difference in enantioselectivity. The oocyte electrophysiology studies showed that ent-ALLO potentiates GABA-elicited currents in α1ß3 GABAA receptors with lower potency and efficacy than ALLO, PREG or ent-PREG. The small PAM effect of ent-ALLO was prevented by the α1(Q242L) mutation in the intersubunit neurosteroid binding site between the ß3 and α1 subunits. Consistent with this result, neurosteroid analogue photolabeling with mass spectrometric readout, showed that ent-ALLO binds weakly to the ß3-α1 intersubunit binding site in comparison to ALLO, PREG and ent-PREG. Rigid body docking predicted that ent-ALLO binds in the intersubunit site with a preferred orientation 180° different than ALLO, PREG or ent-PREG, potentially explaining its weak binding and effect. Photolabeling studies did not identify differences between ALLO and ent-ALLO binding to the α1 or ß3 intrasubunit binding sites that also mediate neurosteroid modulation of GABAA receptors. The results demonstrate that differential binding of ent-ALLO and ent-PREG to the ß3-α1 intersubunit site accounts for the difference in enantioselectivity between ALLO and PREG.

Searching for autoimmune encephalitis: Beware of normal CSF.

OBJECTIVE: To determine the prevalence of cerebrospinal fluid (CSF) markers associated with inflammation (i.e., elevated white blood cell count, protein concentration, and CSF-specific oligoclonal bands) in patients with early active autoimmune encephalitis (AE). METHODS: CSF characteristics, including WBC count, protein concentration, and oligoclonal banding, were analyzed in patients diagnosed with AE at two tertiary care centers. RESULTS: Ninety-five patients were included in the study. CSF white blood cell counts and protein levels were within normal limits for 27% (CI95%: 19-37) of patients with AE. When results of oligoclonal banding were added, 14% (CI95%: 6-16) of patients with AE had "normal" CSF. The median CSF white blood cell count was 8 cells/mm3 (range: 0-544) and the median CSF protein concentration was 0.42 g/L (range: 0.15-3.92). CONCLUSIONS: White blood cell counts and protein levels were within normal limits in the CSF of a substantial proportion of patients with early active AE. Inclusion of CSF oligoclonal banding identified a higher proportion of patients with an inflammatory CSF profile, especially when CSF was sampled early in the disease process.

Aß42 Protofibrils Interact with and Are Trafficked through Microglial-Derived Microvesicles.

Microvesicles (MVs) and exosomes comprise a class of cell-secreted particles termed extracellular vesicles (EVs). These cargo-holding vesicles mediate cell-to-cell communication and have recently been implicated in neurodegenerative diseases such as Alzheimer's disease (AD). The two types of EVs are distinguished by the mechanism of cell release and their size, with the smaller exosomes and the larger MVs ranging from 30 to 100 nm and 100 nm to 1 µm in diameter, respectively. MV numbers are increased in AD and appear to interact with amyloid-ß peptide (Aß), the primary protein component of the neuritic plaques in the AD brain. Because microglial cells play such an important role in AD-linked neuroinflammation, we sought to characterize MVs shed from microglial cells, better understand MV interactions with Aß, and determine whether internalized Aß may be incorporated into secreted MVs. Multiple strategies were used to characterize MVs shed from BV-2 microglia after ATP stimulation. Confocal images of isolated MVs bound to fluorescently labeled annexin-V via externalized phosphatidylserine revealed a polydisperse population of small spherical structures. Dynamic light scattering measurements yielded MV diameters ranging from 150 to 600 nm. Electron microscopy of resin-embedded MVs cut into thin slices showed well-defined uranyl acetate-stained ring-like structures in a similar diameter range. The use of a fluorescently labeled membrane insertion probe, NBD C6-HPC, effectively tracked MVs in binding experiments, and an Aß ELISA confirmed a strong interaction between MVs and Aß protofibrils but not Aß monomers. Despite the lesser monomer interaction, MVs had an inhibitory effect on monomer aggregation. Primary microglia rapidly internalized Aß protofibrils, and subsequent stimulation of the microglia with ATP resulted in the release of MVs containing the internalized Aß protofibrils. The role of MVs in neurodegeneration and inflammation is an emerging area, and further knowledge of MV interaction with Aß may shed light on extracellular spread and influence on neurotoxicity and neuroinflammation.

The conformational epitope for a new Aß42 protofibril-selective antibody partially overlaps with the peptide N-terminal region.

Aggregation and accumulation of amyloid-ß peptide (Aß) is a key component of Alzheimer's disease (AD). While monomeric Aß appears to be benign, oligomers adopt a biologically detrimental structure. These soluble structures can be detected in AD brain tissue by antibodies that demonstrate selectivity for aggregated Aß. Protofibrils are a subset of soluble oligomeric Aß species and are described as small (< 100 nm) curvilinear assemblies enriched in ß-sheet structure. Our own in vitro studies demonstrate that microglial cells are much more sensitive to soluble Aß42 protofibrils compared to Aß42 monomer or insoluble Aß42 fibrils. Protofibrils interact with microglia, trigger Toll-like receptor signaling, elicit cytokine transcription and expression, and are rapidly taken up by the cells. Because of the importance of this Aß species, we sought to develop an antibody that selectively recognizes protofibrils over other Aß species. Immunization of rabbits with isolated Aß42 protofibrils generated a high-titer anti serum with a strong affinity for Aß42 protofibrils. The antiserum, termed AbSL, was selective for Aß42 protofibrils over Aß42 monomers and Aß42 fibrils. AbSL did not react with amyloid precursor protein and recognized distinct pathological features in AD transgenic mouse brain slices. Competition studies with an Aß antibody that targets residues 1-16 indicated that the conformational epitope for AbSL involved the N-terminal region of protofibrils in some manner. The newly developed antibody may have potential diagnostic and therapeutic uses in AD tissue and patients, and targeting of protofibrils in AD may have beneficial effects. Read the Editorial Highlight for this article on page 621. Cover Image for this issue: doi. 10.1111/jnc.13827.