Dendritic spine loss is recognized as an early feature of Alzheimer's disease (AD), but the underlying mechanisms are poorly understood. Dendritic spine structure is defined by filamentous actin (F-actin) and we observed depolymerization of synaptosomal F-actin accompanied by increased globular-actin (G-actin) at as early as 1 month of age in a mouse model of AD (APPswe/PS1ΔE9, male mice). This led to recall deficit after contextual fear conditioning (cFC) at 2 months of age in APPswe/PS1ΔE9 male mice, which could be reversed by the actin-polymerizing agent jasplakinolide. Further, the F-actin-depolymerizing agent latrunculin induced recall deficit after cFC in WT mice, indicating the importance of maintaining F-/G-actin equilibrium for optimal behavioral response. Using direct stochastic optical reconstruction microscopy (dSTORM), we show that F-actin depolymerization in spines leads to a breakdown of the nano-organization of outwardly radiating F-actin rods in cortical neurons from APPswe/PS1ΔE9 mice. Our results demonstrate that synaptic dysfunction seen as F-actin disassembly occurs very early, before onset of pathological hallmarks in AD mice, and contributes to behavioral dysfunction, indicating that depolymerization of F-actin is causal and not consequent to decreased spine density. Further, we observed decreased synaptosomal F-actin levels in postmortem brain from mild cognitive impairment and AD patients compared with subjects with normal cognition. F-actin decrease correlated inversely with increasing AD pathology (Braak score, Aβ load, and tangle density) and directly with performance in episodic and working memory tasks, suggesting its role in human disease pathogenesis and progression.SIGNIFICANCE STATEMENT Synaptic dysfunction underlies cognitive deficits in Alzheimer's disease (AD). The cytoskeletal protein actin plays a critical role in maintaining structure and function of synapses. Using cultured neurons and an AD mouse model, we show for the first time that filamentous actin (F-actin) is lost selectively from synapses early in the disease process, long before the onset of classical AD pathology. We also demonstrate that loss of synaptic F-actin contributes directly to memory deficits. Loss of synaptosomal F-actin in human postmortem tissue correlates directly with decreased performance in memory test and inversely with AD pathology. Our data highlight that synaptic cytoarchitectural changes occur early in AD and they may be targeted for the development of therapeutics.

We present observational evidence from studies on primary cortical cultures from AD transgenic mice, APPSwe/PS1ΔE9 (APP/PS1) mice, for significant decrease in total spine density at DIV-15 and onward. This indicates reduction in potential healthy synapses and strength of connections among neurons. Based on this, a network model of neurons is developed, that explains the consequent loss of coordinated activity and transmission efficiency among neurons that manifests over time. The critical time when structural connectivity in the brain undergoes a phase-transition, from initial robustness to irreparable breakdown, is estimated from this model. We also show how the global efficiency of signal transmission in the network decreases over time. Moreover, the number of multiple paths of high efficiency decreases rapidly as the disease progresses, indicating loss of structural plasticity and inefficiency in choosing alternate paths or desired paths for any pattern of activity. Thus loss of spines caused by β -Amyloid (A β ) peptide results in disintegration of the neuronal network over time with consequent cognitive dysfunctions in Alzheimer’s Disease (AD).

Aims: Reactive oxygen species (ROS) generated during Alzheimer's disease (AD) pathogenesis through multiple sources are implicated in synaptic pathology observed in the disease. We have previously shown Factin disassembly in dendritic spines in early AD (34). The actin cytoskeleton can be oxidatively modified resulting in altered F-actin dynamics. Therefore, we investigated whether disruption of redox signaling could contribute to actin network disassembly and downstream effects in the amyloid precursor protein/presenilin-1 double transgenic (APP/PS1) mouse model of AD. Results: Synaptosomal preparations from 1-month-old APP/PS1 mice showed an increase in ROS levels, coupled with a decrease in the reduced form of F-actin and increase in glutathionylated synaptosomal actin. Furthermore, synaptic glutaredoxin 1 (Grx1) and thioredoxin levels were found to be lowered. Overexpressing Grx1 in the brains of these mice not only reversed F-actin loss seen in APP/PS1 mice but also restored memory recall after contextual fear conditioning. F-actin levels and F-actin nanoarchitecture in spines were also stabilized by Grx1 overexpression in APP/PS1 primary cortical neurons, indicating that glutathionylation of Factin is a critical event in early pathogenesis of AD, which leads to spine loss. Innovation: Loss of thiol/disulfide oxidoreductases in the synapse along with increase in ROS can render Factin nanoarchitecture susceptible to oxidative modifications in AD. Conclusions: Our findings provide novel evidence that altered redox signaling in the form of S-glutathionylation and reduced Grx1 levels can lead to synaptic dysfunction during AD pathogenesis by directly disrupting the Factin nanoarchitecture in spines. Increasing Grx1 levels is a potential target for novel disease-modifying therapies for AD.

A BS TRACT: Background: Parkinson's disease (PD) is characterized by a severe loss of the dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc). Perturbation of protein thiol redox homeostasis has been shown to play a role in the dysregulation of cell death and cell survival signaling pathways in these neurons. Glutaredoxin 1 (Grx1) is a thiol/disulfide oxidoreductase that catalyzes the deglutathionylation of proteins and is important for regulation of cellular protein thiol redox homeostasis. Objectives: We evaluated if the downregulation of Grx1 could lead to dopaminergic degeneration and PDrelevant motor deficits in mice. Methods: Grx1 was downregulated unilaterally through viral vector-mediated transduction of short hairpin RNA against Grx1 into the SNpc. Behavioral assessment was performed through rotarod and elevated body swing test. Stereological analysis of tyrosine hydroxylase-positive and Nissl-positive neurons was carried out to evaluate neurodegeneration. Results: Downregulation of Grx1 resulted in contralateral bias of elevated body swing and reduced latency to fall off, accelerating rotarod. This was accompanied by a loss of tyrosine hydroxylase-positive neurons in the SNpc and their DA projections in the striatum. Furthermore, there was a loss Nissl-positive neurons in the SNpc, indicating cell death. This was selective to the SNpc neurons because DA neurons in the ventral tegmental area were unaffected akin to that seen in human PD. Furthermore, Grx1 mRNA expression was substantially decreased in the SNpc from PD patients. Conclusions: Our study indicates that Grx1 is critical for the survival of SNpc DA neurons and that it is downregulated in human PD.