Profound Memory Deficits Research Articles

Driving Mitochondrial Fission Improves Cognitive, but not Motor Deficits in a Mouse Model of Ataxia of Charlevoix-Saguenay.

Autosomal-recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is caused by loss-of-function mutation in the SACS gene, which encodes sacsin, a putative HSP70-HSP90 co-chaperone. Previous studies with Sacs knock-out (KO) mice and patient-derived fibroblasts suggested that SACSIN mutations inhibit the function of the mitochondrial fission enzyme dynamin-related protein 1 (Drp1). This in turn resulted in mitochondrial hyperfusion and dysfunction. We experimentally tested this hypothesis by genetically manipulating the mitochondrial fission/fusion equilibrium, creating double KO (DKO) mice that also lack positive (PP2A/Bβ2) and negative (PKA/AKAP1) regulators of Drp1. Neither promoting mitochondrial fusion (Bβ2KO) nor fission (Akap1 KO) influenced progression of motor symptoms in Sacs KO mice. However, our studies identified profound learning and memory deficits in aged Sacs KO mice. Moreover, this cognitive impairment was rescued in a gene dose-dependent manner by deletion of the Drp1 inhibitor PKA/Akap1. Our results are inconsistent with mitochondrial dysfunction as a primary pathogenic mechanism in ARSACS. Instead, they imply that promoting mitochondrial fission may be beneficial at later stages of the disease when pathology extends to brain regions subserving learning and memory.

The autism spectrum disorder risk gene NEXMIF over-synchronizes hippocampal CA1 network and alters neuronal coding.

Mutations in autism spectrum disorder (ASD) risk genes disrupt neural network dynamics that ultimately lead to abnormal behavior. To understand how ASD-risk genes influence neural circuit computation during behavior, we analyzed the hippocampal network by performing large-scale cellular calcium imaging from hundreds of individual CA1 neurons simultaneously in transgenic mice with total knockout of the X-linked ASD-risk gene NEXMIF (neurite extension and migration factor). As NEXMIF knockout in mice led to profound learning and memory deficits, we examined the CA1 network during voluntary locomotion, a fundamental component of spatial memory. We found that NEXMIF knockout does not alter the overall excitability of individual neurons but exaggerates movement-related neuronal responses. To quantify network functional connectivity changes, we applied closeness centrality analysis from graph theory to our large-scale calcium imaging datasets, in addition to using the conventional pairwise correlation analysis. Closeness centrality analysis considers both the number of connections and the connection strength between neurons within a network. We found that in wild-type mice the CA1 network desynchronizes during locomotion, consistent with increased network information coding during active behavior. Upon NEXMIF knockout, CA1 network is over-synchronized regardless of behavioral state and fails to desynchronize during locomotion, highlighting how perturbations in ASD-implicated genes create abnormal network synchronization that could contribute to ASD-related behaviors.

Distinct effects of AMPAR subunit depletion on spatial memory

Pharmacological studies established a role for AMPARs in the mammalian forebrain in spatial memory performance. Here we generated global GluA1/3 double knockout mice (Gria1/3-/-) and conditional knockouts lacking GluA1 and GluA3 AMPAR subunits specifically from principal cells across the forebrain (Gria1/3ΔFb). In both models, loss of GluA1 and GluA3 resulted in reduced hippocampal GluA2 and increased levels of the NMDAR subunit GluN2A. Electrically-evoked AMPAR-mediated EPSPs were greatly diminished, and there was an absence of tetanus-induced LTP. Gria1/3-/- mice showed premature mortality. Gria1/3ΔFb mice were viable, and their memory performance could be analyzed. In the Morris water maze (MWM), Gria1/3ΔFb mice showed profound long-term memory deficits, in marked contrast to the normal MWM learning previously seen in single Gria1-/- and Gria3-/- knockout mice. Our results suggest a redundancy of function within the pool of available ionotropic glutamate receptors for long-term spatial memory performance.

Adult-born neurons maintain hippocampal cholinergic inputs and support working memory during aging.

Adult neurogenesis is reduced during aging and impaired in disorders of stress, memory, and cognition though its normal function remains unclear. Moreover, a systems level understanding of how a small number of young hippocampal neurons could dramatically influence brain function is lacking. We examined whether adult neurogenesis sustains hippocampal connections cumulatively across the life span. Long-term suppression of neurogenesis as occurs during stress and aging resulted in an accelerated decline in hippocampal acetylcholine signaling and a slow and progressing emergence of profound working memory deficits. These deficits were accompanied by compensatory reorganization of cholinergic dentate gyrus inputs with increased cholinergic innervation to the ventral hippocampus and recruitment of ventrally projecting neurons by the dorsal projection. While increased cholinergic innervation was dysfunctional and corresponded to overall decreases in cholinergic levels and signaling, it could be recruited to correct the resulting memory dysfunction even in old animals. Our study demonstrates that hippocampal neurogenesis supports memory by maintaining the septohippocampal cholinergic circuit across the lifespan. It also provides a systems level explanation for the progressive nature of memory deterioration during normal and pathological aging and indicatesthat the brain connectome is malleable by experience.

Nuclear receptor binding factor 2 (NRBF2) is required for learning and memory

AbstractThe mechanisms which underlie defects in learning and memory are a major area of focus with the increasing incidence of Alzheimer's disease in the aging population. The complex genetically-controlled, age-, and environmentally-dependent onset and progression of the cognitive deficits and neuronal pathology call for better understanding of the fundamental biology of the nervous system function. In this study, we focus on nuclear receptor binding factor-2 (NRBF2) which modulates the transcriptional activities of retinoic acid receptor α and retinoid X receptor α, and the autophagic activities of the BECN1–VPS34 complex. Since both transcriptional regulation and autophagic function are important in supporting neuronal function, we hypothesized that NRBF2 deficiency may lead to cognitive deficits. To test this, we developed a new mouse model with nervous system-specific knockout of Nrbf2. In a series of behavioral assessment, we demonstrate that NRBF2 knockout in the nervous system results in profound learning and memory deficits. Interestingly, we did not find deficits in autophagic flux in primary neurons and the autophagy deficits were minimal in the brain. In contrast, RNAseq analyses have identified altered expression of genes that have been shown to impact neuronal function. The observation that NRBF2 is involved in learning and memory suggests a new mechanism regulating cognition involving the role of this protein in regulating networks related to the function of retinoic acid receptors, protein folding, and quality control.The authors developed nervous-system-specific Nrbf2 knockout mice and found that they exhibited profound learning and memory deficits. RNAseq analyses have identified altered expression of genes related to protein folding and quality control. Aberrant aggregates of some of these key proteins are evident in the hippocampus.

Memory outcomes following hemispherectomy in children

ObjectiveA vast proportion of children with epilepsy exhibit memory impairments. Although numerous studies have examined memory performance following pediatric resective epilepsy surgery, little is known about the memory outcomes following hemispherectomy. The few studies that report on memory performance typically include restricted measures, consist of case reports, or do not report on preoperative performance. Given the current limitations, the aim of this study was to delineate pre- and postoperative memory functioning in youth who underwent a hemispherectomy. MethodParticipants included patients who had undergone a hemispherectomy at the Hospital for Sick Children, between 1999 and 2016. Standardized neuropsychological assessments of verbal (digit span, word list recall, word pair recall, stories) and visual (faces, dot locations) memory abilities prior to and after surgery were reviewed. A Wilcoxon signed rank test and effect sizes was completed to compare patients' memory performance to population norms and to assess pre- to postoperative difference scores. ResultsThirteen participants were included in the study: five patients completed preoperative assessments only and eight patients completed pre- and postoperative assessments. Preoperatively, patients performed substantially worse relative to population norms as indicated by large effect sizes (r: 0.5–0.9). Postoperatively, effect sizes were large for the majority of the memory tests, demonstrating considerable differences in patients' memory scores over time. When clinically meaningful changes were examined, stable performance was evident on 63% of the memory tasks. ConclusionOverall, the results highlight that youth who are eligible for hemispherectomy demonstrate profound memory deficits prior to surgery. In addition, undergoing hemispherectomy was not necessarily associated with declined memory performance, with the majority of patients showing stable scores.

Altered Hippocampal Place Cell Representation and Theta Rhythmicity following Moderate Prenatal Alcohol Exposure

Prenatal alcohol exposure (PAE) leads to profound deficits in spatial memory and synaptic and cellular alterations to the hippocampus that last into adulthood. Neurons in the hippocampus called place cells discharge as an animal enters specific places in an environment, establish distinct ensemble codes for familiar and novel places, and are modulated by local theta rhythms. Spatial memory is thought to critically depend on the integrity of hippocampal place cell firing. Therefore, we tested the hypothesis that hippocampal place cell firing is impaired after PAE by performing invivo recordings from the hippocampi (CA1 and CA3) of moderate PAE and control adult rats. Our results show that hippocampal CA3 neurons from PAE rats have reduced spatial tuning. Second, CA1 and CA3 neurons from PAE rats are less likely to orthogonalize their firing between directions of travel on a linear track and between changes in contextual stimuli in an open arena compared to control neurons. Lastly, reductions in the number of hippocampal place cells exhibiting significant theta rhythmicity and phase precession were observed, which may suggest changes to hippocampal microcircuit function. Together, the reduced spatial tuning and sensitivity to contextual changes provide a neural systems-level mechanism to explain spatial memory impairment after moderate PAE.

Neonatal Monosodium Glutamate Administration Disrupts Place Learning and Alters Hippocampal–Prefrontal Learning-Related Theta Activity in the Adult Rat

Neonatal treatment with monosodium glutamate causes profound deficits in place learning and memory in adult rats evaluated in the Morris maze. Theta activity has been related to hippocampal learning, and increased high-frequency theta activity occurs through efficient place learning training in the Morris maze. We wondered whether the place learning deficits observed in adult rats that had been neonatally treated with monosodium glutamate (MSG), were related to altered theta patterns in the hippocampus and prelimbic cortex, which were recorded during place learning training in the Morris maze. The MSG-treated group had a profound deficit in place learning ability, with a marginal reduction in escape latencies during the final days of training. Learning-related changes were observed in the relative power distribution in control and MSG-treated groups in the hippocampal EEG, but not in the prelimbic cortex. Increased prefrontal and reduced hippocampal absolute power that appeared principally during the final days of training, and reduced coherence between regions throughout the training (4–12 Hz), were observed in the MSG-treated rats, thereby suggesting a misfunction of the circuits rather than a hyperexcitable general state. In conclusion, neonatal administration of MSG, which caused a profound deficit in place learning at the adult age, also altered the theta pattern both in the hippocampus and prelimbic cortex.

Tracking progressive pathological and functional decline in the rTg4510 mouse model of tauopathy

BackgroundThe choice and appropriate use of animal models in drug discovery for Alzheimer’s disease (AD) is pivotal to successful clinical translation of novel therapeutics, yet true alignment of research is challenging. Current models do not fully recapitulate the human disease, and even exhibit various degrees of regional pathological burden and diverse functional alterations. Given this, relevant pathological and functional endpoints must be determined on a model-by-model basis. The present work explores the rTg4510 mouse model of tauopathy as a case study to define best practices for the selection and validation of cognitive and functional endpoints for the purposes of pre-clinical AD drug discovery.MethodsMale rTg4510 mice were first tested at an advanced age, 12 months, in multiple behavioural assays (step 1). Severe tau pathology and neurodegeneration was associated with profound locomotor hyperactivity and spatial memory deficits. Four of these assays were then selected for longitudinal assessment, from 4 to 12 months, to investigate whether behavioural performance changes as a function of accumulation of tau pathology (step 2). Experimental suppression of tau pathology—via doxycycline administration—was also investigated for its effect on functional performance.ResultsProgressive behavioural changes were detected where locomotor activity and rewarded alternation were found to most closely correlate with tau burden and neurodegeneration. Doxycycline initiated at 4 months led to a 50% suppression of transgene expression, which was sufficient to prevent subsequent increases in tau pathology and arrest related functional decline.ConclusionsThis two-step approach demonstrates the importance of selecting assays most sensitive to the phenotype of the model. A robust relationship was observed between pathological progression, development of phenotype, and their experimental manipulation—three crucial factors for assessing the translational relevance of future pre-clinical findings.

Presenilins regulate synaptic plasticity and mitochondrial calcium homeostasis in the hippocampal mossy fiber pathway

BackgroundPresenilins play a major role in the pathogenesis of Alzheimer’s disease, in which the hippocampus is particularly vulnerable. Previous studies of Presenilin function in the synapse, however, focused exclusively on the hippocampal Schaffer collateral (SC) pathway. Whether Presenilins play similar or distinct roles in other hippocampal synapses is unknown.MethodsTo investigate the role of Presenilins at mossy fiber (MF) synapses we performed field and whole-cell electrophysiological recordings and Ca2+ imaging using acute hippocampal slices of postnatal forebrain-restricted Presenilin conditional double knockout (PS cDKO) and control mice at 2 months of age. We also performed quantitative electron microscopy (EM) analysis to determine whether mitochondrial content is affected at presynaptic MF boutons of PS cDKO mice. We further conducted behavioral analysis to assess spatial learning and memory of PS cDKO and control mice at 2 months in the Morris water maze.ResultsWe found that long-term potentiation and short-term plasticity, such as paired-pulse and frequency facilitation, are impaired at MF synapses of PS cDKO mice. Moreover, post-tetanic potentiation (PTP), another form of short-term plasticity, is also impaired at MF synapses of PS cDKO mice. Furthermore, blockade of mitochondrial Ca2+ efflux mimics and occludes the PTP deficits at MF synapses of PS cDKO mice, suggesting that mitochondrial Ca2+ homeostasis is impaired in the absence of PS. Quantitative EM analysis showed normal number and area of mitochondria at presynaptic MF boutons of PS cDKO mice, indicating unchanged mitochondrial content. Ca2+ imaging of dentate gyrus granule neurons further revealed that cytosolic Ca2+ increases induced by tetanic stimulation are reduced in PS cDKO granule neurons in acute hippocampal slices, and that inhibition of mitochondrial Ca2+ release during high frequency stimulation mimics and occludes the Ca2+ defects observed in PS cDKO neurons. Consistent with synaptic plasticity impairment observed at MF and SC synapses in acute PS cDKO hippocampal slices, PS cDKO mice exhibit profound spatial learning and memory deficits in the Morris water maze.ConclusionsOur findings demonstrate the importance of PS in the regulation of synaptic plasticity and mitochondrial Ca2+ homeostasis in the hippocampal MF pathway.

Adenosine Kinase Deficiency in the Brain Results in Maladaptive Synaptic Plasticity.

A novel human genetic condition (OMIM #614300) that is based on mutations in the adenosine kinase (Adk) gene has been discovered recently. Affected patients develop hepatic encephalopathy, seizures, and severe cognitive impairment. To model and understand the neurological phenotype of the human mutation, we generated a new conditional knock-out mouse with a brain-specific deletion of Adk (AdkΔbrain). Similar to ADK-deficient patients, AdkΔbrain mice develop seizures and cognitive deficits. We identified increased basal synaptic transmission and enhanced adenosine A2A receptor (A2AR)-dependent synaptic plasticity as the underlying mechanisms that govern these phenotypes. Our data show that neurological phenotypes in ADK-deficient patients are intrinsic to ADK deficiency in the brain and that blocking A2AR activity therapeutically can attenuate neurological symptoms in ADK deficiency.

Naïve beliefs about the natural world in a case of childhood onset amnesia

The individual profiled here (M.S.) suffered an episode of severe oxygen deprivation (anoxia) at the age of eight, damaging memory relevant structures in the mid-temporal lobes, including the hippocampus bilaterally. The resulting anterograde amnesia was characterized by profound deficits in autobiographical memory, but also a compromised ability to acquire new facts and information (semantic memory), resulting in the formation of idiosyncratic and naïve beliefs about the natural world that have persisted into his adult years. This article presents an interview with M.S. in which many of these idiosyncratic beliefs are detailed, and argues that they can be broadly viewed as the interaction of; 1) intact frontal lobe functioning that supports the application of rational analysis to his lived experience, and 2) an impoverished factual knowledge base upon which to construct sophisticated and evidence-based models of his lived experience and of natural world processes.

Cognitive benefits of memantine in Alzheimer's 5XFAD model mice decline during advanced disease stages

Memantine, a noncompetitive NMDA receptor antagonist with neuroprotective properties, has been used for the treatment of Alzheimer's disease (AD). Administration of memantine to various transgenic AD mice has been reported to improve cognitive deficits, very often completely back to normal wild-type control levels. However, such great benefits of memantine in preclinical studies do not translate into clinical results of this drug, showing only marginal and transient efficacy in moderate to severe AD. To further address in vivo efficacy, we compared the effects of memantine at different disease stages in 5XFAD mice, one of the rapid-onset and most aggressive amyloid models. Specifically, we administered memantine once daily for 30days to 5XFAD mice, which showed moderate (6–7months of age) and robust (12–15months) β-amyloid (Aβ) accumulation. Treatments with memantine (10mg/kg, i.p.) reversed memory impairments in the younger 5XFAD mice, as tested by the contextual fear conditioning and spontaneous alternation Y-maze paradigms. Memantine had no effects on soluble Aβ oligomer or total Aβ42 levels in 5XFAD mouse brains. In contrast, subchronic treatments with memantine showed no behavioral benefits in the older 5XFAD group, which exhibited more profound memory deficits concomitant with highly increased concentrations of Aβ as compared with those of the younger 5XFAD group. Since subchronic memantine at the higher dose (30mg/kg) impaired memory performances in wild-type controls, we further tested acute administration of 50mg/kg memantine, which was reported to enhance hippocampal adult neurogenesis and memory function. However, this treatment also failed to rescue memory deficits in 12–15-month-old 5XFAD mice. Collectively, our results demonstrate that cognitive benefits of memantine independent of Aβ reductions were no longer observed in the 5XFAD Alzheimer mouse model during advanced stages, which may be reflective of the limited efficacy of memantine in clinical settings.

Early postnatal GABAA receptor modulation reverses deficits in neuronal maturation in a conditional neurodevelopmental mouse model of DISC1.

Exploring drug targets based on disease-associated molecular mechanisms during development is crucial for the generation of novel prevention and treatment strategies for neurodevelopmental psychiatric conditions. We report that prefrontal cortex-specific postnatal knockdown of DISC1 via in utero electroporation combined with an inducible knockdown expression system drives deficits in synaptic GABAA function and dendritic development in pyramidal neurons, as well as abnormalities in sensorimotor gating, albeit without profound memory deficits. We show for the first time that DISC1 is specifically involved in regulating cell surface expression of α2 subunit-containing GABAA receptors in immature developing neurons, but not after full maturation. Notably, pharmacological intervention with α2/3 subtype-selective GABAA receptor positive allosteric modulators during the early postnatal period ameliorates dendritic deficits and behavioral abnormalities induced by knockdown of DISC1. These findings highlight a critical role of DISC1-mediated disruption of postnatal GABA signaling in aberrant prefrontal cortex maturation and function.

Cognitive Impairment, Neuroimaging, and Alzheimer Neuropathology in Mouse Models of Down Syndrome.

Down syndrome (DS) is the most common non-lethal genetic condition that affects approximately 1 in 700 births in the United States of America. DS is characterized by complete or segmental chromosome 21 trisomy, which leads to variable intellectual disabilities, progressive memory loss, and accelerated neurodegeneration with age. During the last three decades, people with DS have experienced a doubling of life expectancy due to progress in treatment of medical comorbidities, which has allowed this population to reach the age when they develop early onset Alzheimer's disease (AD). Individuals with DS develop cognitive and pathological hallmarks of AD in their fourth or fifth decade, and are currently lacking successful prevention or treatment options for dementia. The profound memory deficits associated with DS-related AD (DS-AD) have been associated with degeneration of several neuronal populations, but mechanisms of neurodegeneration are largely unexplored. The most successful animal model for DS is the Ts65Dn mouse, but several new models have also been developed. In the current review, we discuss recent findings and potential treatment options for the management of memory loss and AD neuropathology in DS mouse models. We also review agerelated neuropathology, and recent findings from neuroimaging studies. The validation of appropriate DS mouse models that mimic neurodegeneration and memory loss in humans with DS can be valuable in the study of novel preventative and treatment interventions, and may be helpful in pinpointing gene-gene interactions as well as specific gene segments involved in neurodegeneration.

Putting heterosynaptic plasticity in its place - as easy as PIE.

Since Milner and Scoville published their observations of Henry Molaison's profound memory deficits following bilateral removal of the hippocampal formation (Scoville & Milner, 1957), generations of neuroscientists have sought to understand how the hippocampus executes its essential role in learning and memory. Central to that understanding is elaboration of the functions performed by the circuitry of the hippocampal CA3 region. A signature feature of CA3 is that its principal cells, the pyramidal cells (PCs), generate a highly recurrent network, by giving off and receiving excitatory collateral inputs to and from myriad other CA3 PCs. Inputs to this recurrent network come primarily from layer 2 neurons in nearby entorhinal cortex (EC) via the perforant pathway (PP), which bifurcates into two distinct excitatory pathways: a monosynaptic pathway onto the distal apical dendrites of the PCs; and a disynaptic pathway, where EC inputs synapse onto granule cells in the dentate gyrus, which, in turn, synapse onto the proximal apical dendrites of CA3 PCs via mossy fibre (MF) axons. Owing to their strength and location, the MF inputs strongly drive activity in the PCs despite there being only several dozen of these inputs per neuron (as opposed to thousands of inputs from the PP and recurrent collaterals).

EPA-0690 – Housing conditions modulate the cognitive performance in transgenic mice overexpressing the schizophrenia susceptibility gene Tcf4

Introduction The basic helix-loop-helix (bHLH) transcription factor TCF4 was confirmed in the combined analysis of several large genomewide association studies (GWAS) as one of the most significant schizophrenia (SZ) susceptibility genes. TCF4 influences verbal learning and memory in humans and modulates sensorimotor gating in SZ patients. We showed recently that transgenic mice overexpressing Tcf4 in forebrain (Tcf4tg) display profound deficits in fear memory and sensorimotor gating. Methods We investigated the influence of different environmental conditions on the behavior and cognitive performance of Tcf4tg mice by exposing 4 weeks old animals for 4 weeks to single cage housing (SH) or to group housing in enriched environment (EE). In SH, animals were housed under poor conditions without any enrichment. In contrast, EE cages contained two separate compartments: the first with one-way entrance and one-way exit for drinking/feeding, and the second compartment with tubes and a running wheel enabling physical exercise. Animals underwent a profound analysis addressing basic behavior and cognitive performance. Results We demonstrated that SH (isolation stress) increases whereas housing in EE (combination of social support with physical exercise) rescues the cognitive impairment of Tcf4tg mice. We provided evidence that the manifestation of the phenotype of Tcf4tg mice depends strongly on environmental factors. This substantiates the importance of G×E interactions in the manifestation of schizophrenic phenotypes. Conclusions The observed phenotype of Tcf4tg mice may resemble the situation in patients where symptoms can be elicited by the ‘second hit’ (social stress) and can be ameliorated by social support or physiotherapy.

Reversal learning and associative memory impairments in a BACHD rat model for Huntington disease.

Chorea and psychiatric symptoms are hallmarks of Huntington disease (HD), a neurodegenerative disorder, genetically characterized by the presence of expanded CAG repeats (>35) in the HUNTINGTIN (HTT) gene. HD patients present psychiatric symptoms prior to the onset of motor symptoms and we recently found a similar emergence of non motor and motor deficits in BACHD rats carrying the human full length mutated HTT (97 CAG-CAA repeats). We evaluated cognitive performance in reversal learning and associative memory tests in different age cohorts of BACHD rats. Male wild type (WT) and transgenic (TG) rats between 2 and 12 months of age were tested. Learning and strategy shifting were assessed in a cross-maze test. Associative memory was evaluated in different fear conditioning paradigms (context, delay and trace). The possible confound of a fear conditioning phenotype by altered sensitivity to a ‘painful’ stimulus was assessed in a flinch-jump test. In the cross maze, 6 months old TG rats showed a mild impairment in reversal learning. In the fear conditioning tasks, 4, 6 and 12 months old TG rats showed a marked reduction in contextual fear conditioning. In addition, TG rats showed impaired delay conditioning (9 months) and trace fear conditioning (3 months). This phenotype was unlikely to be affected by a change in ‘pain’ sensitivity as WT and TG rats showed no difference in their threshold response in the flinch-jump test. Our results suggest that BACHD rats have a profound associative memory deficit and, possibly, a deficit in reversal learning as assessed in a cross maze task. The time course for the emergence of these symptoms (i.e., before the occurrence of motor symptoms) in this rat model for HD appears similar to the time course in patients. These data suggest that BACHD rats may be a useful model for preclinical drug discovery.

Mechanisms that lessen benefits of β-secretase reduction in a mouse model of Alzheimer's disease.

The β-secretase enzyme BACE1 (β-site amyloid precursor protein-cleaving enzyme 1), which initiates amyloid-β (Aβ) production, is an excellent therapeutic target for Alzheimer's disease (AD). However, recent evidence raises concern that BACE1-inhibiting approaches may encounter dramatic declines in their abilities to ameliorate AD-like pathology and memory deficits during disease progression. Here, we used BACE1 haploinsufficiency as a therapeutic relevant model to evaluate the efficacy of partial inhibition of this enzyme. Specifically, we crossed BACE1+/− mice with 5XFAD transgenic mice and investigated the mechanisms by which Aβ accumulation and related memory impairments become less sensitive to rescue by BACE1+/− reduction. Haploinsufficiency lowered BACE1 expression by ∼50% in 5XFAD mice regardless of age in concordance with reduction in gene copy number. However, profound Aβ plaque pathology and memory deficits concomitant with BACE1 equivalent to wild-type control levels remained in BACE1+/−·5XFAD mice with advanced age (15–18 months old). Therefore, BACE1 haploinsufficiency is not sufficient to block the elevation of BACE1 expression (approximately twofold), which is also reported to occur during human AD progression, in 5XFAD mice. Our investigation revealed that PERK (PKR-endoplasmic reticulum-related kinase)-dependent activation of eIF2α (eukaryotic translation initiation factor-2α) accounts for the persistent BACE1 upregulation in BACE1+/−·5XFAD mouse brains at 15–18 months of age. Moreover, BACE1 haploinsufficiency was also no longer able to prevent reduction in the expression of neprilysin, a crucial Aβ-degrading enzyme, in 5XFAD mice with advanced age. These findings demonstrate that partial BACE1 suppression cannot attenuate deleterious BACE1-elevating or neprilysin-reducing mechanisms, limiting its capabilities to reduce cerebral Aβ accumulation and rescue memory defects during the course of AD development.

Combining Small-Volume Metabolomic and Transcriptomic Approaches for Assessing Brain Chemistry

The integration of disparate data types provides a more complete picture of complex biological systems. Here we combine small-volume metabolomic and transcriptomic platforms to determine subtle chemical changes and to link metabolites and genes to biochemical pathways. Capillary electrophoresis–mass spectrometry (CE–MS) and whole-genome gene expression arrays, aided by integrative pathway analysis, were utilized to survey metabolomic/transcriptomic hippocampal neurochemistry. We measured changes in individual hippocampi from the mast cell mutant mouse strain, C57BL/6 KitW-sh/W-sh. These mice have a naturally occurring mutation in the white spotting locus that causes reduced c-Kit receptor expression and an inability of mast cells to differentiate from their hematopoietic progenitors. Compared with their littermates, the mast cell-deficient mice have profound deficits in spatial learning, memory, and neurogenesis. A total of 18 distinct metabolites were identified in the hippocampus that discriminated between the C57BL/6 KitW-sh/W-sh and control mice. The combined analysis of metabolite and gene expression changes revealed a number of altered pathways. Importantly, results from both platforms indicated that multiple pathways are impacted, including amino acid metabolism, increasing the confidence in each approach. Because the CE–MS and expression profiling are both amenable to small-volume analysis, this integrated analysis is applicable to a range of volume-limited biological systems.