A nonlinear meccano for Alzheimer's emergence by amyloid ß-mediated glutamatergic hyperactivity.

The pathophysiological process of Alzheimer's disease (AD) is believed to begin many years before the formal diagnosis of AD dementia. This protracted preclinical phase offers a crucial window for potential therapeutic interventions, yet its comprehensive characterization remains elusive. Accumulating evidence suggests that amyloid-ß (Aß) may mediate neuronal hyperactivity in circuit dysfunction in the early stages of AD. At the same time, neural activity can also facilitate Aß accumulation through intricate feed-forward interactions, complicating elucidating the conditions governing Aß-dependent hyperactivity and its diagnostic utility. In this study, we use biophysical modeling to shed light on such conditions. Our analysis reveals that the inherently nonlinear nature of the underlying molecular interactions can give rise to the emergence of various modes of hyperactivity. This diversity in the mechanisms of hyperactivity may ultimately account for a spectrum of AD manifestations.

Recombinant Integrin ß1 Signal Peptide Blocks Gliosis Induced by Aß Oligomers.

Glial cells participate actively in the early cognitive decline in Alzheimer's disease (AD) pathology. In fact, recent studies have found molecular and functional abnormalities in astrocytes and microglia in both animal models and brains of patients suffering from this pathology. In this regard, reactive gliosis intimately associated with amyloid plaques has become a pathological hallmark of AD. A recent study from our laboratory reports that astrocyte reactivity is caused by a direct interaction between amyloid beta (Aß) oligomers and integrin ß1. Here, we have generated four recombinant peptides including the extracellular domain of integrin ß1, and evaluated their capacity both to bind in vitro to Aß oligomers and to prevent in vivo Aß oligomer-induced gliosis and endoplasmic reticulum stress. We have identified the minimal region of integrin ß1 that binds to Aß oligomers. This region is called signal peptide and corresponds to the first 20 amino acids of the integrin ß1 N-terminal domain. This recombinant integrin ß1 signal peptide prevented Aß oligomer-induced ROS generation in primary astrocyte cultures. Furthermore, we carried out intrahippocampal injection in adult mice of recombinant integrin ß1 signal peptide combined with or without Aß oligomers and we evaluated by immunohistochemistry both astrogliosis and microgliosis as well as endoplasmic reticulum stress. The results show that recombinant integrin ß1 signal peptide precluded both astrogliosis and microgliosis and endoplasmic reticulum stress mediated by Aß oligomers in vivo. We have developed a molecular tool that blocks the activation of the molecular cascade that mediates gliosis via Aß oligomer/integrin ß1 signaling.

Sephin1 Protects Neurons against Excitotoxicity Independently of the Integrated Stress Response.

Sephin1 is a derivative of guanabenz that inhibits the dephosphorylation of the eukaryotic initiation factor 2 alpha (eIF2α) and therefore may enhance the integrated stress response (ISR), an adaptive mechanism against different cellular stresses, such as accumulation of misfolded proteins. Unlike guanabenz, Sephin1 provides neuroprotection without adverse effects on the α2-adrenergic system and therefore it is considered a promising pharmacological therapeutic tool. Here, we have studied the effects of Sephin1 on N-methyl-D-aspartic acid (NMDA) receptor signaling which may modulate the ISR and contribute to excitotoxic neuronal loss in several neurodegenerative conditions. Time-course analysis of peIF2α levels after NMDA receptor overactivation showed a delayed dephosphorylation that occurred in the absence of activating transcription factor 4 (ATF4) and therefore independently of the ISR, in contrast to that observed during endoplasmic reticulum (ER) stress induced by tunicamycin and thapsigargin. Similar to guanabenz, Sephin1 completely blocked NMDA-induced neuronal death and was ineffective against AMPA-induced excitotoxicity, whereas it did not protect from experimental ER stress. Interestingly, both guanabenz and Sephin1 partially but significantly reduced NMDA-induced cytosolic Ca2+ increase, leading to a complete inhibition of subsequent calpain activation. We conclude that Sephin1 and guanabenz share common strong anti-excitotoxic properties with therapeutic potential unrelated to the ISR.

Amyloid ß / PKC-dependent alterations in NMDA receptor composition are detected in early stages of Alzheimer´s disease.

Amyloid beta (Aß)-mediated synapse dysfunction is an early event in Alzheimer's disease (AD) pathogenesis and previous studies suggest that NMDA receptor (NMDAR) dysregulation may contribute to these pathological effects. Although Aß peptides impair NMDAR expression and activity, the mechanisms mediating these alterations in the early stages of AD are unclear. Here, we observed that NMDAR subunit NR2B and PSD-95 levels were aberrantly upregulated and correlated with Aß42 load in human postsynaptic fractions of the prefrontal cortex in early stages of AD patients, as well as in the hippocampus of 3xTg-AD mice. Importantly, NR2B and PSD95 dysregulation was revealed by an increased expression of both proteins in Aß-injected mouse hippocampi. In cultured neurons, Aß oligomers increased the NR2B-containing NMDAR density in neuronal membranes and the NMDA-induced intracellular Ca2+ increase, in addition to colocalization in dendrites of NR2B subunit and PSD95. Mechanistically, Aß oligomers required integrin ß1 to promote synaptic location and function of NR2B-containing NMDARs and PSD95 by phosphorylation through classic PKCs. These results provide evidence that Aß oligomers modify the contribution of NR2B to NMDAR composition and function in the early stages of AD through an integrin ß1 and PKC-dependent pathway. These data reveal a novel role of Aß oligomers in synaptic dysfunction that may be relevant to early-stage AD pathogenesis.

Early Effects of Aß Oligomers on Dendritic Spine Dynamics and Arborization in Hippocampal Neurons.

Alzheimer's disease (AD) is a neurodegenerative disorder that leads to impaired memory and cognitive deficits. Spine loss as well as changes in spine morphology correlates with cognitive impairment in this neurological disorder. Many studies in animal models and ex vivo cultures indicate that amyloid ß-peptide (Aß) oligomers induce synaptic damage early during the progression of the disease. Here, in order to determine the events that initiate synaptic alterations, we acutely applied oligomeric Aß to primary hippocampal neurons and an ex vivo model of organotypic hippocampal cultures from a mouse after targeted expression of EGFP to allow high-resolution imaging and algorithm-based evaluation of spine changes. Dendritic spines were classified as thin, stubby or mushroom, based on morphology. In vivo, time-lapse imaging showed that the three spine types were relatively stable, although their stability significantly decreased after treatment with Aß oligomers. Unexpectedly, we observed that the density of total dendritic spines increased in organotypic hippocampal slices treated with Aß compared to control cultures. Specifically, the fraction of stubby spines significantly increased, while mushroom and thin spines remained unaltered. Pharmacological tools revealed that acute Aß oligomers induced spine changes through mechanisms involving CaMKII and integrin ß1 activities. Additionally, analysis of dendritic complexity based on a 3D reconstruction of the whole neuron morphology showed an increase in the apical dendrite length and branching points in CA1 organotypic hippocampal slices treated with Aß. In contrast to spines, the morphological changes were affected by integrin ß1 but not by CaMKII inhibition. Altogether, these data indicate that the Aß oligomers exhibit early dual effects by acutely enhancing dendritic complexity and spine density.

Aß oligomers promote oligodendrocyte differentiation and maturation via integrin ß1 and Fyn kinase signaling.

Alzheimer´s disease (AD) is characterized by a progressive cognitive decline that correlates with the levels of amyloid ß-peptide (Aß) oligomers. Strong evidences connect changes of oligodendrocyte function with the onset of neurodegeneration in AD. However, the mechanisms controlling oligodendrocyte responses to Aß are still elusive. Here, we tested the role of Aß in oligodendrocyte differentiation, maturation, and survival in isolated oligodendrocytes and in organotypic cerebellar slices. We found that Aß peptides specifically induced local translation of 18.5-kDa myelin basic protein (MBP) isoform in distal cell processes concomitant with an increase of process complexity of MBP-expressing oligodendrocytes. Aß oligomers required integrin ß1 receptor, Src-family kinase Fyn and Ca2+/CaMKII as effectors to modulate MBP protein expression. The pharmacological inhibition of Fyn kinase also attenuated oligodendrocyte differentiation and survival induced by Aß oligomers. Similarly, using ex vivo organotypic cerebellar slices Aß promoted MBP upregulation through Fyn kinase, and modulated oligodendrocyte population dynamics by inducing cell proliferation and differentiation. Importantly, application of Aß to cerebellar organotypic slices enhanced remyelination and oligodendrocyte lineage recovery in lysolecithin (LPC)-induced demyelination. These data reveal an important role of Aß in oligodendrocyte lineage function and maturation, which may be relevant to AD pathogenesis.

Mangiferin and Morin Attenuate Oxidative Stress, Mitochondrial Dysfunction, and Neurocytotoxicity, Induced by Amyloid Beta Oligomers.

Amyloid beta- (Aß-) mediated ROS overproduction disrupts intraneuronal redox balance and exacerbates mitochondrial dysfunction which leads to neuronal injury. Polyphenols have been investigated as therapeutic agents that promote neuroprotective effects in experimental models of brain injury and neurodegenerative diseases. The aim of this study was to identify the neuroprotective effects of morin and mangiferin against Aß oligomers in cultured cortical neurons and organotypic slices as well as their mechanisms of action. Cell death caused by Aß oligomers in neuronal cultures was decreased in the presence of micromolar concentrations of mangiferin or morin, which in turn attenuated oxidative stress. The neuroprotective effects of antioxidants against Aß were associated with the reduction of Aß-induced calcium load to mitochondria; mitochondrial membrane depolarization; and release of cytochrome c from mitochondria, a key trigger of apoptosis. Additionally, we observed that both polyphenols activated the endogenous enzymatic antioxidant system and restored oxidized protein levels. Finally, Aß induced an impairment of energy homeostasis due to a decreased respiratory capacity that was mitigated by morin and mangiferin. Overall, the beneficial effects of polyphenols in preventing mitochondrial dysfunction and neuronal injury in AD cell models suggest that morin and mangiferin hold promise for the treatment of this neurological disorder.