Mark R. Etherton

α-Synuclein Promotes SNARE-Complex Assembly in Vivo and in Vitro

α-Synuclein and Aging Transgenic α-synuclein can reverse the otherwise lethal neurodegeneration of cysteine string protein-α knockout mice via changes in SNARE proteins, which mediate synaptic vesicle release. Using experiments with purified recombinant proteins, triple αβγ-synuclein knockout mice, and studies of mouse aging, Burré et al. (p. 1663, published online 26 August) now demonstrate that α-synuclein directly interacts with the SNARE protein synaptobrevin and functions as a catalyst for SNARE-complex assembly. The role of synucleins is fully dispensable in young animals, but becomes essential late in life, which suggests that α-synuclein maintains normal synaptic function during aging. A protein implicated in neurodegeneration promotes the assembly of membrane fusion complexes. Presynaptic nerve terminals release neurotransmitters repeatedly, often at high frequency, and in relative isolation from neuronal cell bodies. Repeated release requires cycles of soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE)–complex assembly and disassembly, with continuous generation of reactive SNARE-protein intermediates. Although many forms of neurodegeneration initiate presynaptically, only few pathogenic mechanisms are known, and the functions of presynaptic proteins linked to neurodegeneration, such as α-synuclein, remain unclear. Here, we show that maintenance of continuous presynaptic SNARE-complex assembly required a nonclassical chaperone activity mediated by synucleins. Specifically, α-synuclein directly bound to the SNARE-protein synaptobrevin-2/vesicle-associated membrane protein 2 (VAMP2) and promoted SNARE-complex assembly. Moreover, triple-knockout mice lacking synucleins developed age-dependent neurological impairments, exhibited decreased SNARE-complex assembly, and died prematurely. Thus, synucleins may function to sustain normal SNARE-complex assembly in a presynaptic terminal during aging.

Activity-dependent validation of excitatory versus inhibitory synapses by neuroligin-1 versus neuroligin-2

Neuroligins enhance synapse formation in vitro, but surprisingly are not required for the generation of synapses in vivo. We now show that in cultured neurons, neuroligin-1 overexpression increases excitatory, but not inhibitory, synaptic responses, and potentiates synaptic NMDAR/AMPAR ratios. In contrast, neuroligin-2 overexpression increases inhibitory, but not excitatory, synaptic responses. Accordingly, deletion of neuroligin-1 in knockout mice selectively decreases the NMDAR/AMPAR ratio, whereas deletion of neuroligin-2 selectively decreases inhibitory synaptic responses. Strikingly, chronic inhibition of NMDARs or CaM-Kinase II, which signals downstream of NMDARs, suppresses the synapse-boosting activity of neuroligin-1, whereas chronic inhibition of general synaptic activity suppresses the synapse-boosting activity of neuroligin-2. Taken together, these data indicate that neuroligins do not establish, but specify and validate, synapses via an activity-dependent mechanism, with different neuroligins acting on distinct types of synapses. This hypothesis reconciles the overexpression and knockout phenotypes and suggests that neuroligins contribute to the use-dependent formation of neural circuits.

Neuroligin-1 Deletion Results in Impaired Spatial Memory and Increased Repetitive Behavior

Neuroligins (NLs) are a family of neural cell-adhesion molecules that are involved in excitatory/inhibitory synapse specification. Multiple members of the NL family (including NL1) and their binding partners have been linked to cases of human autism and mental retardation. We have now characterized NL1-deficient mice in autism- and mental retardation-relevant behavioral tasks. NL1 knock-out (KO) mice display deficits in spatial learning and memory that correlate with impaired hippocampal long-term potentiation. In addition, NL1 KO mice exhibit a dramatic increase in repetitive, stereotyped grooming behavior, a potential autism-relevant abnormality. This repetitive grooming abnormality in NL1 KO mice is associated with a reduced NMDA/AMPA ratio at corticostriatal synapses. Interestingly, we further demonstrate that the increased repetitive grooming phenotype can be rescued in adult mice by administration of the NMDA receptor partial coagonist d-cycloserine. Broadly, these data are consistent with a role of synaptic cell-adhesion molecules in general, and NL1 in particular, in autism and implicate reduced excitatory synaptic transmission as a potential mechanism and treatment target for repetitive behavioral abnormalities.

Mouse neurexin-1α deletion causes correlated electrophysiological and behavioral changes consistent with cognitive impairments

Deletions in the neurexin-1α gene were identified in large-scale unbiased screens for copy-number variations in patients with autism or schizophrenia. To explore the underlying biology, we studied the electrophysiological and behavioral phenotype of mice lacking neurexin-1α. Hippocampal slice physiology uncovered a defect in excitatory synaptic strength in neurexin-1α deficient mice, as revealed by a decrease in miniature excitatory postsynaptic current (EPSC) frequency and in the input-output relation of evoked postsynaptic potentials. This defect was specific for excitatory synaptic transmission, because no change in inhibitory synaptic transmission was observed in the hippocampus. Behavioral studies revealed that, compared with littermate control mice, neurexin-1α deficient mice displayed a decrease in prepulse inhibition, an increase in grooming behaviors, an impairment in nest-building activity, and an improvement in motor learning. However, neurexin-1α deficient mice did not exhibit any obvious changes in social behaviors or in spatial learning. Together, these data indicate that the neurexin-1α deficiency induces a discrete neural phenotype whose extent correlates, at least in part, with impairments observed in human patients.

Autism-linked neuroligin-3 R451C mutation differentially alters hippocampal and cortical synaptic function

Multiple independent mutations in neuroligin genes were identified in patients with familial autism, including the R451C substitution in neuroligin-3 (NL3). Previous studies showed that NL3R451C knock-in mice exhibited modestly impaired social behaviors, enhanced water maze learning abilities, and increased synaptic inhibition in the somatosensory cortex, and they suggested that the behavioral changes in these mice may be caused by a general shift of synaptic transmission to inhibition. Here, we confirm that NL3R451C mutant mice behaviorally exhibit social interaction deficits and electrophysiologically display increased synaptic inhibition in the somatosensory cortex. Unexpectedly, however, we find that the NL3R451C mutation produced a strikingly different phenotype in the hippocampus. Specifically, in the hippocampal CA1 region, the NL3R451C mutation caused an ∼1.5-fold increase in AMPA receptor-mediated excitatory synaptic transmission, dramatically altered the kinetics of NMDA receptor-mediated synaptic responses, induced an approximately twofold up-regulation of NMDA receptors containing NR2B subunits, and enhanced long-term potentiation almost twofold. NL3 KO mice did not exhibit any of these changes. Quantitative light microscopy and EM revealed that the NL3R451C mutation increased dendritic branching and altered the structure of synapses in the stratum radiatum of the hippocampus. Thus, in NL3R451C mutant mice, a single point mutation in a synaptic cell adhesion molecule causes context-dependent changes in synaptic transmission; these changes are consistent with the broad impact of this mutation on murine and human behaviors, suggesting that NL3 controls excitatory and inhibitory synapse properties in a region- and circuit-specific manner.

An autism-associated point mutation in the neuroligin cytoplasmic tail selectively impairs AMPA receptor-mediated synaptic transmission in hippocampus

Neuroligins are evolutionarily conserved postsynaptic cell‐adhesion molecules that function, at least in part, by forming trans‐synaptic complexes with presynaptic neurexins. Different neuroligin isoforms perform diverse functions and exhibit distinct intracellular localizations, but contain similar cytoplasmic sequences whose role remains largely unknown. Here, we analysed the effect of a single amino‐acid substitution (R704C) that targets a conserved arginine residue in the cytoplasmic sequence of all neuroligins, and that was associated with autism in neuroligin‐4. We introduced the R704C mutation into mouse neuroligin‐3 by homologous recombination, and examined its effect on synapses in vitro and in vivo. Electrophysiological and morphological studies revealed that the neuroligin‐3 R704C mutation did not significantly alter synapse formation, but dramatically impaired synapse function. Specifically, the R704C mutation caused a major and selective decrease in AMPA receptor‐mediated synaptic transmission in pyramidal neurons of the hippocampus, without similarly changing NMDA or GABA receptor‐mediated synaptic transmission, and without detectably altering presynaptic neurotransmitter release. Our results suggest that the cytoplasmic tail of neuroligin‐3 has a central role in synaptic transmission by modulating the recruitment of AMPA receptors to postsynaptic sites at excitatory synapses.

HIV-associated Neurocognitive Disorders and Antiretroviral Therapy: Current Concepts and Controversies

Antiretroviral drugs may help prevent neurological decline in individuals with HIV infection by suppressing viral replication and associated chronic immune activation in the central nervous system. However, HIV control in the brain may come at the price of drug-induced neurotoxicity. Herein, we review recent advances in the balance between adequate viral suppression in the nervous system and adverse effects of the medications used in HIV treatment.

Recent Advances in Leukoaraiosis: White Matter Structural Integrity and Functional Outcomes after Acute Ischemic Stroke

Leukoaraiosis, a radiographic marker of cerebral small vessel disease detected on T2-weighted brain magnetic resonance imaging (MRI) as white matter hyperintensity (WMH), is a key contributor to the risk and severity of acute cerebral ischemia. Prior investigations have emphasized the pathophysiology of WMH development and progression; however, more recently, an association between WMH burden and functional outcomes after stroke has emerged. There is growing evidence that WMH represents macroscopic injury to the white matter and that the extent of WMH burden on MRI influences functional recovery in multiple domains following acute ischemic stroke (AIS). In this review, we discuss the current understanding of WMH pathogenesis and its impact on AIS and functional recovery.

Diffuse microvascular dysfunction and loss of white matter integrity predict poor outcomes in patients with acute ischemic stroke.

We sought to investigate the relationship between blood–brain barrier (BBB) permeability and microstructural white matter integrity, and their potential impact on long-term functional outcomes in patients with acute ischemic stroke (AIS). We studied 184 AIS subjects with perfusion-weighted MRI (PWI) performed <9 h from last known well time. White matter hyperintensity (WMH), acute infarct, and PWI-derived mean transit time lesion volumes were calculated. Mean BBB leakage rates (K2 coefficient) and mean diffusivity values were measured in contralesional normal-appearing white matter (NAWM). Plasma matrix metalloproteinase-2 (MMP-2) levels were studied at baseline and 48 h. Admission stroke severity was evaluated using the NIH Stroke Scale (NIHSS). Modified Rankin Scale (mRS) was obtained at 90-days post-stroke. We found that higher mean K2 and diffusivity values correlated with age, elevated baseline MMP-2 levels, greater NIHSS and worse 90-day mRS (all p < 0.05). In multivariable analysis, WMH volume was associated with mean K2 (p = 0.0007) and diffusivity (p = 0.006) values in contralesional NAWM. In summary, WMH severity measured on brain MRI of AIS patients is associated with metrics of increased BBB permeability and abnormal white matter microstructural integrity. In future studies, these MRI markers of diffuse cerebral microvascular dysfunction may improve prediction of cerebral tissue infarction and functional post-stroke outcomes.

Fungal Infections of the Central Nervous System

Fungal infections of the central nervous system have manifold presentations and courses that depend largely on both host and organism characteristics. Although subjects with impaired immunity are generally at higher risk for severe disease, several fungal organisms are considered primary pathogens and can also cause disease in otherwise immunocompetent individuals. Herein, we describe the epidemiology, presentation, diagnosis, and management of central nervous system complications of several fungal pathogens.