Alzheimer's disease (AD) is a progressive neurological disorder that causes dementia and poses a major public health crisis as the population ages. Aberrant processing of the amyloid precursor protein (APP) is strongly implicated as a proximal event in AD pathophysiology, but the neurochemical signals that regulate APP processing in the brain are not completely understood. Activation of muscarinic acetylcholine receptors (mAChRs) has been shown to affect APP processing and AD pathology, but less is known about the roles of specific mAChR subtypes. In this study, we used M1 mAChR knockout mice (M1KO) to isolate the effects of the M1 mAChR on APP processing in primary neurons and on the development of amyloid pathology in a transgenic mouse model of AD. We demonstrate that the loss of M1 mAChRs increases amyloidogenic APP processing in neurons, as evidenced by decreased agonist-regulated shedding of the neuroprotective APP ectodomain APPsα and increased production of toxic Aβ peptides. Expression of M1 mAChRs on the M1KO background rescued this phenotype, indicating that M1 mAChRs are sufficient to modulate non-amyloidogenic APP processing. In APPSwe/Ind transgenic mice, the loss of M1 mAChRs resulted in increased levels of brain Aβ1-40 and greater accumulation of amyloid plaque pathology. Analysis of APP metabolites in APPSwe/Ind brain tissue indicates that the loss of M1 mAChRs increases amyloidogenic APP processing. These results indicate that the M1 mAChR is an important regulator of amyloidogenesis in the brain and provide strong support for targeting the M1 mAChR as a therapeutic candidate in AD.
Memory deficits in Drosophila nalyot mutants suggest that the Myb family transcription factor Adf-1 is an important regulator of developmental plasticity in the brain. However, the cellular functions for this transcription factor in neurons or molecular mechanisms by which it regulates plasticity remain unknown. Here, we use in vivo 3D reconstruction of identifiable larval motor neuron dendrites to show that Adf-1 is required cell autonomously for dendritic development and activity-dependent plasticity of motor neurons downstream of CaMKII. Adf-1 inhibition reduces dendrite growth and neuronal excitability, and results in motor deficits and altered transcriptional profiles. Surprisingly, analysis by comparative chromatin immunoprecipitation followed by sequencing (ChIP-Seq) of Adf-1, RNA Polymerase II (Pol II), and histone modifications in Kc cells shows that Adf-1 binding correlates positively with high Pol II-pausing indices and negatively with active chromatin marks such as H3K4me3 and H3K27ac. Consistently, the expression of Adf-1 targets Staufen and Fasciclin II (FasII), identified through larval brain ChIP-Seq for Adf-1, is negatively regulated by Adf-1, and manipulations of these genes predictably modify dendrite growth. Our results imply mechanistic interactions between transcriptional and local translational machinery in neurons as well as conserved neuronal growth mechanisms mediated by cell adhesion molecules, and suggest that CaMKII, Adf-1, FasII, and Staufen influence crucial aspects of dendrite development and plasticity with potential implications for memory formation. Further, our experiments reveal molecular details underlying transcriptional regulation by Adf-1, and indicate active interaction between Adf-1 and epigenetic regulators of gene expression during activity-dependent neuronal plasticity.
Huntington's disease (HD) belongs to a family of neurodegenerative diseases caused by misfolded proteins and shares the pathological hallmark of selective accumulation of misfolded proteins in neuronal cells. Polyglutamine expansion in the HD protein, huntingtin (Htt), causes selective neurodegeneration that is more severe in the striatum and cortex than in other brain regions, but the mechanism behind this selectivity is unknown. Here we report that in HD knock-in mice, the expression levels of mutant Htt (mHtt) are higher in brain tissues than in peripheral tissues. However, the expression of N-terminal mHtt via stereotaxic injection of viral vectors in mice also results in greater accumulation of mHtt in the striatum than in muscle. We developed an in vitro assay that revealed that extracts from the striatum and cortex promote the formation of high-molecular weight (HMW) mHtt compared with the relatively unaffected cerebellar and peripheral tissue extracts. Inhibition of ubiquitin-activating enzyme E1 (Ube1) increased the levels of HMW mHtt in the relatively unaffected tissues. Importantly, the expression levels of Ube1 are lower in brain tissues than peripheral tissues and decline in the nuclear fraction with age, which is correlated with the increased accumulation of mHtt in the brain and neuronal nuclei during aging. Our findings suggest that decreased targeting of misfolded Htt to the proteasome for degradation via Ube1 may underlie the preferential accumulation of toxic forms of mHtt in the brain and its selective neurodegeneration.
Sound categorization is essential for auditory behaviors like acoustic communication, but its genesis within the auditory pathway is not well understood—especially for learned natural categories like vocalizations, which often share overlapping acoustic features that must be distinguished (e.g., speech). We use electrophysiological mapping and single-unit recordings in mice to investigate how representations of natural vocal categories within core auditory cortex are modulated when one category acquires enhanced behavioral relevance. Taking advantage of a maternal mouse model of acoustic communication, we found no long-term auditory cortical map expansion to represent a behaviorally relevant pup vocalization category—contrary to expectations from the cortical plasticity literature on conditioning with pure tones. Instead, we observed plasticity that improved the separation between acoustically similar pup and adult vocalization categories among a physiologically defined subset of late-onset, putative pyramidal neurons, but not among putative interneurons. Additionally, a larger proportion of these putative pyramidal neurons in maternal animals compared with nonmaternal animals responded to the individual pup call exemplars having combinations of acoustic features most typical of that category. Together, these data suggest that higher-order representations of acoustic categories arise from a subset of core auditory cortical pyramidal neurons that become biased toward the combination of acoustic features statistically predictive of membership to a behaviorally relevant sound category.
The GluN2D subunit of the NMDA receptor is prominently expressed in the basal ganglia and associated brainstem nuclei, including the subthalamic nucleus (STN), globus pallidus, striatum, and substantia nigra. However, little is known about how GluN2D-containing NMDA receptors contribute to synaptic activity in these regions. Using Western blotting of STN tissue punches, we demonstrated that GluN2D is expressed in the rat STN throughout development [age postnatal day 7 (P7)-P60] and in the adult (age P120). Immunoelectron microscopy of the adult rat brain showed that GluN2D is predominantly expressed in dendrites, unmyelinated axons, and axon terminals within the STN. Using subunit-selective allosteric modulators of NMDA receptors (TCN-201, ifenprodil, CIQ, and DQP-1105), we provide evidence that receptors containing the GluN2B and GluN2D subunits mediate responses to exogenously applied NMDA and glycine, as well as synaptic NMDA receptor activation in the STN of rat brain slices. EPSCs in the STN were mediated primarily by AMPA and NMDA receptors and GluN2D-containing NMDA receptors controlled the slow deactivation time course of EPSCs in the STN. In vivo recordings from the STN of anesthetized adult rats demonstrated that the spike firing rate was increased by the GluN2C/D potentiator CIQ and decreased by the GluN2C/D antagonist DQP-1105, suggesting that NMDA receptor activity can influence STN output. These data indicate that the GluN2B and GluN2D NMDA receptor subunits contribute to synaptic activity in the STN and may represent potential therapeutic targets for modulating subthalamic neuron activity in neurological disorders such as Parkinson’s disease.
Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by progressive cognitive decline and neuropathological changes, including the deposition of amyloid β (Aβ) in senile plaques. The mechanisms causing the disease and Aβ accumulation are not well understood, but important genetic associations with apolipoprotein E genotype and involvement of lipoprotein receptors have become apparent. LR11 (also known as SorLA), a member of the low-density lipoprotein receptor family, has been identified previously as an altered transcript in microarray analyses of samples from human AD cases. Here, we show neuronal expression of the lipoprotein receptor LR11 in control brain in regions vulnerable to AD neuropathology and marked reduction of LR11 expression in these regions in AD brains before cell death. Overexpression of LR11 drastically reduces levels of extracellular Aβ and also lowers levels of total cellular amyloid precursor protein (APP). LR11 colocalizes with APP and regulates its trafficking in endocytic compartments, which are important intracellular sites for APP processing and Aβ generation. Endogenous LR11 localizes to neuronal multivesicular bodies in both rat and human brain. The robust correlation between reduced LR11 expression and AD neuropathology and its potent effects on extracellular Aβ levels suggest that this neuronal lipoprotein receptor could play an important role in AD pathogenesis.
Increasing evidence suggests that neuronal synchronization in the gamma band (30–100 Hz) may play an important role in mediating cognitive processes. Gamma-band synchronization provides for the optimal temporal relationship between two signals to produce the long-term synaptic changes that have been theorized to underlie memory formation. While neuronal populations in the hippocampus oscillate in the gamma range, the role of these oscillations in memory formation is still unclear. In order to address this issue, we recorded neuronal activity in the hippocampus while macaque monkeys performed a visual recognition memory task. During the encoding phase of this task, hippocampal neurons displayed gamma-band synchronization. Additionally, enhanced gamma-band synchronization during encoding predicted greater subsequent recognition memory performance. These changes in synchronization reflect enhanced coordination among hippocampal neurons and may facilitate synaptic changes necessary for successful memory encoding.
Oxytocin receptors in the nucleus accumbens have been implicated in the regulation of alloparental behavior and pair bond formation in the socially monogamous prairie vole. Oxytocin receptor density in the nucleus accumbens is positively correlated with alloparenting in juvenile and adult female prairie voles, and oxytocin receptor antagonist infused into the nucleus accumbens blocks this behavior. Furthermore, prairie voles have higher densities of oxytocin receptors in the accumbens than non-monogamous rodent species, and blocking accumbal oxytocin receptors prevents mating-induced partner preference formation. Here we used adeno-associated viral vector gene transfer to examine the functional relationship between accumbal oxytocin receptor density and social behavior in prairie and meadow voles. Adult female prairie voles that over-express oxytocin receptor in the nucleus accumbens displayed accelerated partner preference formation after cohabitation with a male, but did not display enhanced alloparental behavior. However, partner preference was not facilitated in non-monogamous meadow voles by introducing oxytocin receptor into the nucleus accumbens. These data confirm a role for oxytocin receptor in the accumbens in the regulation of partner preferences in female prairie voles, and suggest that oxytocin receptor expression in the accumbens is not sufficient to promote partner preferences in non-monogamous species. These data are the first to demonstrate a direct relationship between oxytocin receptor density in the nucleus accumbens and variation in social attachment behaviors. Thus, individual variation in oxytocin receptor expression in the striatum may contribute to natural diversity in social behaviors.
Spines are dendritic protrusions that receive most of the excitatory input in the brain. Early after the onset of cerebral ischemia dendritic spines in the peri-infarct cortex are replaced by areas of focal swelling, and their re-emergence from these varicosities is associated with neurological recovery after acute ischemic stroke (AIS). Urokinase-type plasminogen activator (uPA) is a serine proteinase that plays a central role in tissue remodeling via binding to the urokinase plasminogen activator receptor (uPAR). We report that cerebral cortical neurons release uPA during the recovery phase from ischemic stroke in vivo or hypoxia in vitro. Although uPA does not have an effect on ischemia- or hypoxia-induced neuronal death, genetic deficiency of uPA (uPA-/-) or uPAR (uPAR-/-) abrogates functional recovery after AIS. Treatment with recombinant uPA after ischemic stroke induces neurological recovery in wild-type and uPA-/- but not in uPAR-/- mice. Diffusion tensor imaging studies indicate that uPA-/- mice have increased water diffusivity and decreased anisotropy associated with impaired dendritic spine recovery and decreased length of distal neurites in the peri-infarct cortex. We found that the excitotoxic injury induces the clustering of uPAR in dendritic varicosities, and that the binding of uPA to uPAR promotes the reorganization of the actin cytoskeleton and re-emergence of dendritic filopodia from uPAR-enriched varicosities. This effect is independent of uPA’s proteolytic properties and instead is mediated by Rac-regulated profilin expression and cofilin phosphorylation. Our data indicate that binding of uPA to uPAR promotes dendritic spine recovery and improves functional outcome following AIS.