What Happens with the Circuit in Alzheimer's Disease in Mice and Humans?

A major mystery of many types of neurological and psychiatric disorders, such as Alzheimer's disease (AD), remains the underlying, disease-specific neuronal damage. Because of the strong interconnectivity of neurons in the brain, neuronal dysfunction necessarily disrupts neuronal circuits. In this article, we review evidence for the disruption of large-scale networks from imaging studies of humans and relate it to studies of cellular dysfunction in mouse models of AD. The emerging picture is that some forms of early network dysfunctions can be explained by excessively increased levels of neuronal activity. The notion of such neuronal hyperactivity receives strong support from in vivo and in vitro cellular imaging and electrophysiological recordings in the mouse, which provide mechanistic insights underlying the change in neuronal excitability. Overall, some key aspects of AD-related neuronal dysfunctions in humans and mice are strikingly similar and support the continuation of such a translational strategy.

Seizure protein 6 controls glycosylation and trafficking of kainate receptor subunits GluK2 and GluK3.

Seizure protein 6 (SEZ6) is required for the development and maintenance of the nervous system, is a major substrate of the protease BACE1 and is linked to Alzheimer's disease (AD) and psychiatric disorders, but its molecular functions are not well understood. Here, we demonstrate that SEZ6 controls glycosylation and cell surface localization of kainate receptors composed of GluK2/3 subunits. Loss of SEZ6 reduced surface levels of GluK2/3 in primary neurons and reduced kainate-evoked currents in CA1 pyramidal neurons in acute hippocampal slices. Mechanistically, loss of SEZ6 in vitro and in vivo prevented modification of GluK2/3 with the human natural killer-1 (HNK-1) glycan, a modulator of GluK2/3 function. SEZ6 interacted with GluK2 through its ectodomain and promoted post-endoplasmic reticulum transport of GluK2 in the secretory pathway in heterologous cells and primary neurons. Taken together, SEZ6 acts as a new trafficking factor for GluK2/3. This novel function may help to better understand the role of SEZ6 in neurologic and psychiatric diseases.

η-Secretase processing of APP inhibits neuronal activity in the hippocampus.

Alzheimer disease (AD) is characterized by the accumulation of amyloid plaques, which are predominantly composed of amyloid-ß peptide. Two principal physiological pathways either prevent or promote amyloid-ß generation from its precursor, ß-amyloid precursor protein (APP), in a competitive manner. Although APP processing has been studied in great detail, unknown proteolytic events seem to hinder stoichiometric analyses of APP metabolism in vivo. Here we describe a new physiological APP processing pathway, which generates proteolytic fragments capable of inhibiting neuronal activity within the hippocampus. We identify higher molecular mass carboxy-terminal fragments (CTFs) of APP, termed CTF-η, in addition to the long-known CTF-α and CTF-ß fragments generated by the α- and ß-secretases ADAM10 (a disintegrin and metalloproteinase 10) and BACE1 (ß-site APP cleaving enzyme 1), respectively. CTF-η generation is mediated in part by membrane-bound matrix metalloproteinases such as MT5-MMP, referred to as η-secretase activity. η-Secretase cleavage occurs primarily at amino acids 504-505 of APP695, releasing a truncated ectodomain. After shedding of this ectodomain, CTF-η is further processed by ADAM10 and BACE1 to release long and short Aη peptides (termed Aη-α and Aη-ß). CTFs produced by η-secretase are enriched in dystrophic neurites in an AD mouse model and in human AD brains. Genetic and pharmacological inhibition of BACE1 activity results in robust accumulation of CTF-η and Aη-α. In mice treated with a potent BACE1 inhibitor, hippocampal long-term potentiation was reduced. Notably, when recombinant or synthetic Aη-α was applied on hippocampal slices ex vivo, long-term potentiation was lowered. Furthermore, in vivo single-cell two-photon calcium imaging showed that hippocampal neuronal activity was attenuated by Aη-α. These findings not only demonstrate a major functionally relevant APP processing pathway, but may also indicate potential translational relevance for therapeutic strategies targeting APP processing.

The Reciprocal Interaction Between Sleep and Alzheimer's Disease.

It is becoming increasingly recognized that patients with a variety of neurodegenerative diseases exhibit disordered sleep/wake patterns. While sleep impairments have typically been thought of as sequelae of underlying neurodegenerative processes in sleep-wake cycle regulating brain regions, including the brainstem, hypothalamus, and basal forebrain, emerging evidence now indicates that sleep deficits may also act as pathophysiological drivers of brain-wide disease progression. Specifically, recent work has indicated that impaired sleep can impact on neuronal activity, brain clearance mechanisms, pathological build-up of proteins, and inflammation. Altered sleep patterns may therefore be novel (potentially reversible) dynamic functional markers of proteinopathies and modifiable targets for early therapeutic intervention using non-invasive stimulation and behavioral techniques. Here we highlight research describing a potentially reciprocal interaction between impaired sleep and circadian patterns and the accumulation of pathological signs and features in Alzheimer's disease, the most prevalent neurodegenerative disease in the elderly.

BACE inhibition-dependent repair of Alzheimer's pathophysiology.

Amyloid-ß (Aß) is thought to play an essential pathogenic role in Alzheimer´s disease (AD). A key enzyme involved in the generation of Aß is the ß-secretase BACE, for which powerful inhibitors have been developed and are currently in use in human clinical trials. However, although BACE inhibition can reduce cerebral Aß levels, whether it also can ameliorate neural circuit and memory impairments remains unclear. Using histochemistry, in vivo Ca2+ imaging, and behavioral analyses in a mouse model of AD, we demonstrate that along with reducing prefibrillary Aß surrounding plaques, the inhibition of BACE activity can rescue neuronal hyperactivity, impaired long-range circuit function, and memory defects. The functional neuronal impairments reappeared after infusion of soluble Aß, mechanistically linking Aß pathology to neuronal and cognitive dysfunction. These data highlight the potential benefits of BACE inhibition for the effective treatment of a wide range of AD-like pathophysiological and cognitive impairments.

Neuronal hyperactivity--A key defect in Alzheimer's disease?

Traditionally, the impairment of cognitive functions in Alzheimer's disease (AD) is thought to result from a reduction in neuronal and synaptic activities, and ultimately cell death. Here, we review recent in vivo evidence from mouse models and human patients indicating that, particularly in early stages of AD, neuronal circuits are hyperactive instead of hypoactive. Functional analyses at many levels, from single neurons to neuronal populations to large-scale networks, with a variety of electrophysiological and imaging techniques have revealed two forms of AD-related hyperactivity and provided first insights into the synaptic mechanisms. The unexpected finding that hyperactivity is an early neuronal dysfunction represents a major conceptual shift in our understanding of AD that may have important implications for the development of therapeutic approaches.

Critical role of soluble amyloid-ß for early hippocampal hyperactivity in a mouse model of Alzheimer's disease.

Alzheimer's disease (AD) is characterized by a progressive dysfunction of central neurons. Recent experimental evidence indicates that in the cortex, in addition to the silencing of a fraction of neurons, other neurons are hyperactive in amyloid-ß (Aß) plaque-enriched regions. However, it has remained unknown what comes first, neuronal silencing or hyperactivity, and what mechanisms might underlie the primary neuronal dysfunction. Here we examine the activity patterns of hippocampal CA1 neurons in a mouse model of AD in vivo using two-photon Ca(2+) imaging. We found that neuronal activity in the plaque-bearing CA1 region of older mice is profoundly altered. There was a marked increase in the fractions of both silent and hyperactive neurons, as previously also found in the cortex. Remarkably, in the hippocampus of young mice, we observed a selective increase in hyperactive neurons already before the formation of plaques, suggesting that soluble species of Aß may underlie this impairment. Indeed, we found that acute treatment with the γ-secretase inhibitor LY-411575 reduces soluble Aß levels and rescues the neuronal dysfunction. Furthermore, we demonstrate that direct application of soluble Aß can induce neuronal hyperactivity in wild-type mice. Thus, our study identifies hippocampal hyperactivity as a very early functional impairment in AD transgenic mice and provides direct evidence that soluble Aß is crucial for hippocampal hyperactivity.

ß-amyloid monomer scavenging by an anticalin protein prevents neuronal hyperactivity in mouse models of Alzheimer's Disease.

Hyperactivity mediated by synaptotoxic ß-amyloid (Aß) oligomers is one of the earliest forms of neuronal dysfunction in Alzheimer's disease. In the search for a preventive treatment strategy, we tested the effect of scavenging Aß peptides before Aß plaque formation. Using in vivo two-photon calcium imaging and SF-iGluSnFR-based glutamate imaging in hippocampal slices, we demonstrate that an Aß binding anticalin protein (Aß-anticalin) can suppress early neuronal hyperactivity and synaptic glutamate accumulation in the APP23xPS45 mouse model of ß-amyloidosis. Our results suggest that the sole targeting of Aß monomers is sufficient for the hyperactivity-suppressing effect of the Aß-anticalin at early disease stages. Biochemical and neurophysiological analyses indicate that the Aß-anticalin-dependent depletion of naturally secreted Aß monomers interrupts their aggregation to neurotoxic oligomers and, thereby, reverses early neuronal and synaptic dysfunctions. Thus, our results suggest that Aß monomer scavenging plays a key role in the repair of neuronal function at early stages of AD.

PolyGR and polyPR knock-in mice reveal a conserved neuroprotective extracellular matrix signature in C9orf72 ALS/FTD neurons.

Dipeptide repeat proteins are a major pathogenic feature of C9orf72 amyotrophic lateral sclerosis (C9ALS)/frontotemporal dementia (FTD) pathology, but their physiological impact has yet to be fully determined. Here we generated C9orf72 dipeptide repeat knock-in mouse models characterized by expression of 400 codon-optimized polyGR or polyPR repeats, and heterozygous C9orf72 reduction. (GR)400 and (PR)400 knock-in mice recapitulate key features of C9ALS/FTD, including cortical neuronal hyperexcitability, age-dependent spinal motor neuron loss and progressive motor dysfunction. Quantitative proteomics revealed an increase in extracellular matrix (ECM) proteins in (GR)400 and (PR)400 spinal cord, with the collagen COL6A1 the most increased protein. TGF-ß1 was one of the top predicted regulators of this ECM signature and polyGR expression in human induced pluripotent stem cell neurons was sufficient to induce TGF-ß1 followed by COL6A1. Knockdown of TGF-ß1 or COL6A1 orthologues in polyGR model Drosophila exacerbated neurodegeneration, while expression of TGF-ß1 or COL6A1 in induced pluripotent stem cell-derived motor neurons of patients with C9ALS/FTD protected against glutamate-induced cell death. Altogether, our findings reveal a neuroprotective and conserved ECM signature in C9ALS/FTD.

The ß-Secretase Substrate Seizure 6-Like Protein (SEZ6L) Controls Motor Functions in Mice.

The membrane protein seizure 6-like (SEZ6L) is a neuronal substrate of the Alzheimer's disease protease BACE1, and little is known about its physiological function in the nervous system. Here, we show that SEZ6L constitutive knockout mice display motor phenotypes in adulthood, including changes in gait and decreased motor coordination. Additionally, SEZ6L knockout mice displayed increased anxiety-like behaviour, although spatial learning and memory in the Morris water maze were normal. Analysis of the gross anatomy and proteome of the adult SEZ6L knockout cerebellum did not reveal any major differences compared to wild type, indicating that lack of SEZ6L in other regions of the nervous system may contribute to the phenotypes observed. In summary, our study establishes physiological functions for SEZ6L in regulating motor coordination and curbing anxiety-related behaviour, indicating that aberrant SEZ6L function in the human nervous system may contribute to movement disorders and neuropsychiatric diseases.

Neuronal excitation/inhibition imbalance: core element of a translational perspective on Alzheimer pathophysiology.

Our incomplete understanding of the link between Alzheimer's Disease pathology and symptomatology is a crucial obstacle for therapeutic success. Recently, translational studies have begun to connect the dots between protein alterations and deposition, brain network dysfunction and cognitive deficits. Disturbance of neuronal activity, and in particular an imbalance in underlying excitation/inhibition (E/I), appears early in AD, and can be regarded as forming a central link between structural brain pathology and cognitive dysfunction. While there are emerging (non-)pharmacological options to influence this imbalance, the complexity of human brain dynamics has hindered identification of an optimal approach. We suggest that focusing on the integration of neurophysiological aspects of AD at the micro-, meso- and macroscale, with the support of computational network modeling, can unite fundamental and clinical knowledge, provide a general framework, and suggest rational therapeutic targets.

Aη-α and Aη-ß peptides impair LTP ex vivo within the low nanomolar range and impact neuronal activity in vivo.

BACKGROUND: Amyloid precursor protein (APP) processing is central to Alzheimer's disease (AD) etiology. As early cognitive alterations in AD are strongly correlated to abnormal information processing due to increasing synaptic impairment, it is crucial to characterize how peptides generated through APP cleavage modulate synapse function. We previously described a novel APP processing pathway producing η-secretase-derived peptides (Aη) and revealed that Aη-α, the longest form of Aη produced by η-secretase and α-secretase cleavage, impaired hippocampal long-term potentiation (LTP) ex vivo and neuronal activity in vivo. METHODS: With the intention of going beyond this initial observation, we performed a comprehensive analysis to further characterize the effects of both Aη-α and the shorter Aη-ß peptide on hippocampus function using ex vivo field electrophysiology, in vivo multiphoton calcium imaging, and in vivo electrophysiology. RESULTS: We demonstrate that both synthetic peptides acutely impair LTP at low nanomolar concentrations ex vivo and reveal the N-terminus to be a primary site of activity. We further show that Aη-ß, like Aη-α, inhibits neuronal activity in vivo and provide confirmation of LTP impairment by Aη-α in vivo. CONCLUSIONS: These results provide novel insights into the functional role of the recently discovered η-secretase-derived products and suggest that Aη peptides represent important, pathophysiologically relevant, modulators of hippocampal network activity, with profound implications for APP-targeting therapeutic strategies in AD.

Synergy between amyloid-ß and tau in Alzheimer's disease.

Patients with Alzheimer's disease (AD) present with both extracellular amyloid-ß (Aß) plaques and intracellular tau-containing neurofibrillary tangles in the brain. For many years, the prevailing view of AD pathogenesis has been that changes in Aß precipitate the disease process and initiate a deleterious cascade involving tau pathology and neurodegeneration. Beyond this 'triggering' function, it has been typically presumed that Aß and tau act independently and in the absence of specific interaction. However, accumulating evidence now suggests otherwise and contends that both pathologies have synergistic effects. This could not only help explain negative results from anti-Aß clinical trials but also suggest that trials directed solely at tau may need to be reconsidered. Here, drawing from extensive human and disease model data, we highlight the latest evidence base pertaining to the complex Aß-tau interaction and underscore its crucial importance to elucidating disease pathogenesis and the design of next-generation AD therapeutic trials.

Tipping the Scales: Peptide-Dependent Dysregulation of Neural Circuit Dynamics in Alzheimer's Disease.

Identifying effective treatments for Alzheimer's disease (AD) has proven challenging and has instigated a shift in AD research focus toward the earliest disease-initiating cellular mechanisms. A key insight has been an increase in soluble Aß oligomers in early AD that is causally linked to neuronal and circuit hyperexcitability. However, other accumulating AD-related peptides and proteins, including those derived from the same amyloid precursor protein, such as Aη or sAPPα, and autonomously, such as tau, exhibit surprising opposing effects on circuit dynamics. We propose that the effects of these on neuronal circuits have profound implications for our understanding of disease complexity and heterogeneity and for the development of personalized diagnostic and therapeutic strategies in AD. Here, we highlight important peptide-specific mechanisms of dynamic pathological disequilibrium of cellular and circuit activity in AD and discuss approaches in which these may be further understood, and theoretically and experimentally leveraged, to elucidate AD pathophysiology.

Tau impairs neural circuits, dominating amyloid-ß effects, in Alzheimer models in vivo.

The coexistence of amyloid-ß (Aß) plaques and tau neurofibrillary tangles in the neocortex is linked to neural system failure and cognitive decline in Alzheimer's disease. However, the underlying neuronal mechanisms are unknown. By employing in vivo two-photon Ca2+ imaging of layer 2/3 cortical neurons in mice expressing human Aß and tau, we reveal a dramatic tau-dependent suppression of activity and silencing of many neurons, which dominates over Aß-dependent neuronal hyperactivity. We show that neurofibrillary tangles are neither sufficient nor required for the silencing, which instead is dependent on soluble tau. Surprisingly, although rapidly effective in tau mice, suppression of tau gene expression was much less effective in rescuing neuronal impairments in mice containing both Aß and tau. Together, our results reveal how Aß and tau synergize to impair the functional integrity of neural circuits in vivo and suggest a possible cellular explanation contributing to disappointing results from anti-Aß therapeutic trials.

In Vivo Two-Photon Calcium Imaging of Hippocampal Neurons in Alzheimer Mouse Models.

The use of in vivo two-photon microscopy in mouse models of Alzheimer's disease (AD) has propelled studies of disease mechanisms and treatments. For instance, this approach allowed for the first time to study in the intact brain the dynamics of individual amyloid plaques, and the effects of anti-amyloid therapies on plaque formation and growth. Moreover, by combining two-photon microscopy with fluorescent calcium indicators, an amyloid-dependent abnormal hyperactivity of cortical and hippocampal neurons was revealed as a primary neuronal impairment, which was not predicted from previous in vitro analyses. Here, a method for in vivo two-photon calcium imaging with single-cell and single-action potential accuracy in the hippocampus of Alzheimer mouse models is presented.

Impairments of neural circuit function in Alzheimer's disease.

An essential feature of Alzheimer's disease (AD) is the accumulation of amyloid-ß (Aß) peptides in the brain, many years to decades before the onset of overt cognitive symptoms. We suggest that during this very extended early phase of the disease, soluble Aß oligomers and amyloid plaques alter the function of local neuronal circuits and large-scale networks by disrupting the balance of synaptic excitation and inhibition (E/I balance) in the brain. The analysis of mouse models of AD revealed that an Aß-induced change of the E/I balance caused hyperactivity in cortical and hippocampal neurons, a breakdown of slow-wave oscillations, as well as network hypersynchrony. Remarkably, hyperactivity of hippocampal neurons precedes amyloid plaque formation, suggesting that hyperactivity is one of the earliest dysfunctions in the pathophysiological cascade initiated by abnormal Aß accumulation. Therapeutics that correct the E/I balance in early AD may prevent neuronal dysfunction, widespread cell loss and cognitive impairments associated with later stages of the disease.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Rescue of long-range circuit dysfunction in Alzheimer's disease models.

Alzheimer's disease (AD) is associated with defects of synaptic connectivity. Such defects may not be restricted to local neuronal interactions but may extend to long-range brain activities, such as slow-wave oscillations that are particularly prominent during non-rapid eye movement (non-REM) sleep and are important for integration of information across distant brain regions involved in memory consolidation. There is increasing evidence that sleep is often impaired in AD, but it is unclear whether this impairment is directly related to amyloid-ß (Aß) pathology. Here we demonstrate that slow-wave activity is severely altered in the neocortex, thalamus and hippocampus in mouse models of AD amyloidosis. Most notably, our results reveal an Aß-dependent impairment of slow-wave propagation, which causes a breakdown of the characteristic long-range coherence of slow-wave activity. The finding that the impairment can be rescued by enhancing GABAAergic inhibition identifies a synaptic mechanism underlying Aß-dependent large-scale circuit dysfunction.

Decreased amyloid-ß and increased neuronal hyperactivity by immunotherapy in Alzheimer's models.

Among the most promising approaches for treating Alzheimer's disease is immunotherapy with amyloid-ß (Aß)-targeting antibodies. Using in vivo two-photon imaging in mouse models, we found that two different antibodies to Aß used for treatment were ineffective at repairing neuronal dysfunction and caused an increase in cortical hyperactivity. This unexpected finding provides a possible cellular explanation for the lack of cognitive improvement by immunotherapy in human studies.