Genetic deletion of astrocytic calcineurin B1 prevents cognitive impairment and neuropathology development in acute and chronic mouse models of Alzheimer's disease

Alzheimer's disease (AD) represents an urgent yet unmet challenge for modern society, calling for exploration of innovative targets and therapeutic approaches. Astrocytes, main homeostatic cells in the CNS, represent promising cell-target. Our aim was to investigate if deletion of the regulatory CaNB1 subunit of calcineurin in astrocytes could mitigate AD-related memory deficits, neuropathology, and neuroinflammation. We have generated two, acute and chronic, AD mouse models with astrocytic CaNB1 ablation (ACN-KO). In the former, we evaluated the ability of β-amyloid oligomers (AβOs) to impair memory and activate glial cells once injected in the cerebral ventricle of conditional ACN-KO mice. Next, we generated a tamoxifen-inducible astrocyte-specific CaNB1 knock-out in 3xTg-AD mice (indACNKO-AD). CaNB1 was deleted, by tamoxifen injection, in 11.7-month-old 3xTg-AD mice for 4.4 months. Spatial memory was evaluated using the Barnes maze; β-amyloid plaques burden, neurofibrillary tangle deposition, reactive gliosis, and neuroinflammation were also assessed. The acute model showed that ICV injected AβOs in 2-month-old wild type mice impaired recognition memory and fostered a pro-inflammatory microglia phenotype, whereas in ACN-KO mice, AβOs were inactive. In indACNKO-AD mice, 4.4 months after CaNB1 depletion, we found preservation of spatial memory and cognitive flexibility, abolishment of amyloidosis, and reduction of neurofibrillary tangles, gliosis, and neuroinflammation. Our results suggest that ACN is crucial for the development of cognitive impairment, AD neuropathology, and neuroinflammation. Astrocyte-specific CaNB1 deletion is beneficial for both the abolishment of AβO-mediated detrimental effects and treatment of ongoing AD-related pathology, hence representing an intriguing target for AD therapy.

Overexpression-Induced α-Synuclein Brain Spreading

Interneuronal transfer of pathological α-synuclein species is thought to play an important role in the progressive advancement of Lewy pathology and increasing severity of clinical manifestations in Parkinson's and other diseases commonly referred to as synucleinopathies. Pathophysiological conditions and mechanisms triggering this trans-synaptic spreading bear therefore significant pathogenetic implications but have yet to be fully elucidated. In vivo experimental models support the conclusion that increased expression of intraneuronal α-synuclein can itself induce protein spreading throughout the brain as well as from the brain to peripheral tissues. For example, overexpression of α-synuclein targeted to the rodent dorsal medulla oblongata results in its transfer and accumulation into recipient axons innervating this brain region; through these axons, α-synuclein can then travel caudo-rostrally and reach other brain sites in the pons, midbrain, and forebrain. When protein overexpression is induced in the rodent midbrain, long-distance α-synuclein spreading can be followed over time; spreading-induced α-synuclein accumulation affects lower brain regions, including the dorsal motor nucleus of the vagus, proceeds through efferent axons of the vagus nerve, and is ultimately detected within vagal motor nerve endings in the gastric wall. As discussed in this review, animal models featuring α-synuclein overexpression not only support a relationship between α-synuclein burden and protein spreading but have also provided important clues on conditions/mechanisms capable of promoting interneuronal α-synuclein transfer. Intriguing findings include the relationship between neuronal activity and protein spreading and the role of oxidant stress in trans-synaptic α-synuclein mobility.

Oligomeropathies, inflammation and prion protein binding

The central role of oligomers, small soluble aggregates of misfolded proteins, in the pathogenesis of neurodegenerative disorders is recognized in numerous experimental conditions and is compatible with clinical evidence. To underline this concept, some years ago we coined the term oligomeropathies to define the common mechanism of action of protein misfolding diseases like Alzheimer, Parkinson or prion diseases. Using simple experimental conditions, with direct application of synthetic β amyloid or α-synuclein oligomers intraventricularly at micromolar concentrations, we could detect differences and similarities in the biological consequences. The two oligomer species affected cognitive behavior, neuronal dysfunction and cerebral inflammatory reactions with distinct mechanisms. In these experimental conditions the proposed mediatory role of cellular prion protein in oligomer activities was not confirmed. Together with oligomers, inflammation at different levels can be important early in neurodegenerative disorders; both β amyloid and α-synuclein oligomers induce inflammation and its control strongly affects neuronal dysfunction. This review summarizes our studies with β-amyloid or α-synuclein oligomers, also considering the potential curative role of doxycycline, a well-known antibiotic with anti-amyloidogenic and anti-inflammatory activities. These actions are analyzed in terms of the therapeutic prospects.

Inflammation and Parkinson's disease pathogenesis: Mechanisms and therapeutic insight

After Alzheimer's disease, Parkinson's disease is the most frequent neurodegenerative disorder. Although numerous treatments have been developed to control the disease symptomatology, with some successes, an efficacious therapy affecting the causes of PD is still a goal to pursue. The genetic evidence and the identification of α-synuclein as the main component of intracellular Lewy bodies, the neuropathological hallmark of PD and related disorders, have changed the approach to these disorders. More recently, the detrimental role of α-synuclein has been further extended to explain the wide spread of cerebral pathology through its oligomers. To emphasize the central pathogenic role of these soluble aggregates, we have defined synucleinopathies and other neurodegenerative disorders associated with protein misfolding as oligomeropathies. Another common element in the pathogenesis of oligomeropathies is the role played by inflammation, both at the peripheral and cerebral levels. In the brain parenchyma, inflammatory reaction has been considered an obvious consequence of neuronal degeneration, but recent observations indicate a direct contribution of glial alteration in the early phase of the disease. Furthermore, systemic inflammation also influences the development of neuronal dysfunction caused by specific elements, β amyloid, α-synuclein, tau or prion. However, each disorder has its own specific pathological process and within the same pathological condition, it is possible to find inter-individual differences. This heterogeneity might explain the difficulties developing efficacious therapeutic approaches, even though the possibility of intervention is supported by robust biological evidence. We have recently demonstrated that peripheral inflammation can amplify the neuronal dysfunction induced by α-synuclein oligomers and the neuropathological consequences observed in a Parkinson's disease model. In both cases, activation of microglia was incremented by the "double hit" process, compared to the single treatment. In contrast, astrocyte activation was attenuated and these cells appeared damaged when chronic inflammation was combined with α-synuclein exposure. This evidence might indicate a more specific anti-inflammatory strategy rather than the generic anti-inflammatory treatment.

Flavonoid-Derived Human Phenyl-γ-Valerolactone Metabolites Selectively Detoxify Amyloid-β Oligomers and Prevent Memory Impairment in a Mouse Model of Alzheimer's Disease

SCOPE

Amyloid-β oligomers (AβO) are causally related to Alzheimer's disease (AD). Dietary natural compounds, especially flavonoids and flavan-3-ols, hold great promise as potential AD-preventive agents but their host and gut microbiota metabolism complicates identification of the most relevant bioactive species. This study aims to investigate the ability of a comprehensive set of phenyl-γ-valerolactones (PVL), the main circulating metabolites of flavan-3-ols and related dietary compounds in humans, to prevent AβO-mediated toxicity.

METHODS AND RESULTS

The anti-AβO activity of PVLs is examined in different cell model systems using a highly toxic β-oligomer-forming polypeptide (β23) as target toxicant. Multiple PVLs, and particularly the monohydroxylated 5-(4'-hydroxyphenyl)-γ-valerolactone metabolite [(4'-OH)-PVL], relieve β-oligomer-induced cytotoxicity in yeast and mammalian cells. As revealed by atomic force microscopy (AFM) and other in vitro assays, (4'-OH)-PVL interferes with AβO (but not fibril) assembly and actively remodels preformed AβOs into nontoxic amorphous aggregates. In keeping with the latter mode of action, treatment of AβOs with (4'-OH)-PVL prior to brain injection strongly reduces memory deterioration as well as neuroinflammation in a mouse model of AβO-induced memory impairment.

CONCLUSION

PVLs, which have been validated as biomarkers of the dietary intake of flavan-3-ols, lend themselves as novel AβO-selective, candidate AD-preventing compounds.

Neuroprotective Effects of Doxycycline in the R6/2 Mouse Model of Huntington's Disease

Mechanisms of tissue damage in Huntington’s disease involve excitotoxicity, mitochondrial damage, and inflammation, including microglia activation. Immunomodulatory and anti-protein aggregation properties of tetracyclines were demonstrated in several disease models. In the present study, the neuroprotective and anti-inflammatory effects of the tetracycline doxycycline were investigated in the mouse model of HD disease R6/2. Transgenic mice were daily treated with doxycycline 20 mg/kg, starting from 4 weeks of age. After sacrifice, histological and immunohistochemical studies were performed. We found that doxycycline-treated R6/2 mice survived longer and displayed less severe signs of neurological dysfunction than the saline-treated ones. Primary outcome measures such as striatal atrophy, neuronal intranuclear inclusions, and the negative modulation of microglial reaction revealed a neuroprotective effect of the compound. Doxycycline provided a significantly increase of activated CREB and BDNF in the striatal neurons, along with a down modulation of neuroinflammation, which, combined, might explain the beneficial effects observed in this model. Our findings show that doxycycline treatment could be considered as a valid therapeutic approach for HD.

Biophysical and in Vivo Studies Identify a New Natural-Based Polyphenol, Counteracting Aβ Oligomerization in Vitro and Aβ Oligomer-Mediated Memory Impairment and Neuroinflammation in an Acute Mouse Model of Alzheimer's Disease

In this study natural-based complex polyphenols, obtained through a smart synthetic approach, have been evaluated for their ability to inhibit the formation of Aβ₄₂ oligomers, the most toxic species causing synaptic dysfunction, neuroinflammation, and neuronal death leading to the onset and progression of Alzheimer's disease. In vitro neurotoxicity tests on primary hippocampal neurons have been employed to select nontoxic candidates. Solution NMR and molecular docking studies have been performed to clarify the interaction mechanism of Aβ₄₂ with the synthesized polyphenol derivatives, and highlight the sterical and chemical requirements important for their antiaggregating activity. NMR results indicated that the selected polyphenolic compounds target Aβ₄₂ oligomeric species. Combined NMR and docking studies indicated that the Aβ₄₂ central hydrophobic core, namely, the 17-31 region, is the main interaction site. The length of the peptidomimetic scaffold and the presence of a guaiacol moiety were identified as important requirements for the antiaggregating activity. In vivo experiments on an Aβ₄₂ oligomer-induced acute mouse model highlighted that the most promising polyphenolic derivative (PP04) inhibits detrimental effects of Aβ₄₂ oligomers on memory and glial cell activation. NMR kinetic studies showed that PP04 is endowed with the chemical features of true inhibitors, strongly affecting both the Aβ₄₂ nucleation and growth rates, thus representing a promising candidate to be further developed into an effective drug against neurodegenerative diseases of the amyloid type.

Doxycycline counteracts neuroinflammation restoring memory in Alzheimer's disease mouse models

β-Amyloid oligomers (AβOs) and neuroinflammation are 2 main culprits to counteract in Alzheimer's disease (AD). Doxycycline (DOXY) is a second generation antibiotic of the tetracycline class that are promising drugs tested in many clinical trials for a number of different pathologies. DOXY is endowed with antiamyloidogenic properties and better crosses the blood-brain barrier, but its efficacy has never been tested in AD mice. We herein show that 15- to 16-month-old APP/PS1dE9 (APP/PS1) AD mice receiving DOXY under different treatment regimens recovered their memory without plaque reduction. An acute DOXY treatment was, also, sufficient to improve APP/PS1 mouse memory, suggesting an action against soluble AβOs. This was confirmed in an AβO-induced mouse model, where the AβO-mediated memory impairment was abolished by a DOXY pretreatment. Although AβOs induce memory impairment through glial activation, assessing the anti-inflammatory action of DOXY, we found that in both the AβO-treated and APP/PS1 mice, the memory recovery was associated with a lower neuroinflammation. Our data promote DOXY as a hopeful repositioned drug counteracting crucial neuropathological AD targets.

Alpha-synuclein oligomers impair memory through glial cell activation and via Toll-like receptor 2

Alpha-synuclein oligomers (α-synOs) are emerging as crucial factors in the pathogenesis of synucleinopathies. Although the connection between neuroinflammation and α-syn still remains elusive, increasing evidence suggests that extracellular moieties activate glial cells leading to neuronal damage. Using an acute mouse model, we explored whether α-synOs induce memory impairment in association to neuroinflammation, addressing Toll-like receptors 2 and 4 (TLR2 and TLR4) involvement. We found that α-synOs abolished mouse memory establishment in association to hippocampal glial activation. On brain slices α-synOs inhibited long-term potentiation. Indomethacin and Ibuprofen prevented the α-synOs-mediated detrimental actions. Furthermore, while the TLR2 functional inhibitor antibody prevented the memory deficit, oligomers induced memory deficits in the TLR4 knockout mice. In conclusion, solely α-synOs induce memory impairment likely inhibiting synaptic plasticity. α-synOs lead to hippocampal gliosis that is involved in memory impairment. Moreover, while the oligomer-mediated detrimental actions are TLR2 dependent, the involvement of TLR4 was ruled out.

Toll-like receptor 4-dependent glial cell activation mediates the impairment in memory establishment induced by β-amyloid oligomers in an acute mouse model of Alzheimer's disease

BACKGROUND

Amyloid-β oligomers (AβO) are species mainly involved in the synaptic and cognitive dysfunction in Alzheimer's disease. Although their action has been described mainly at neuronal level, it is now clear that glial cells govern synaptic activity in their resting state, contributing to new learning and memory establishment. In contrast, when activated, they may lead to synaptic and cognitive dysfunction. Using a reliable acute AβO-mediated mouse model of AD, we explored whether the memory alteration AβOs induce relies on the activation of glial cells, and if Toll-like receptor 4 (TLR4), pivotal in the initiation of an immune response, is involved.

METHODS

C57 naïve mice were given a single intracerebroventricular injection of synthetic AβO-containing solution (1μM), which induces substantial impairment in the establishment of recognition memory. Then, first we assessed glial cell activation at different times post-injection by western blot, immunohistochemistry and ELISA in the hippocampus. After that we explored the efficacy of pre-treatment with anti-inflammatory drugs (indomethacin and an IL-1β receptor antagonist) to prevent impairment in the novel object recognition task, and compared AβO's effects in TLR4 knockout mice.

RESULTS

A single AβO injection rapidly activated glial cells and increased pro-inflammatory cytokine expression. Both anti-inflammatory drugs prevented the AβO-mediated impairment in memory establishment. A selective TLR4 receptor antagonist abolished AβO's action on memory, and in TLR4 knockout mice it had no effect on either memory or glial activation.

CONCLUSIONS

These data provide new information on AβO's mechanism of action, indicating that besides direct action at the synapses, they also act through the immune system, with TLR4 playing a major role. This suggests that in a potential therapeutic setting inflammation must be considered as well.

An N-terminal fragment of the prion protein binds to amyloid-β oligomers and inhibits their neurotoxicity in vivo

A hallmark of Alzheimer disease (AD) is the accumulation of the amyloid-β (Aβ) peptide in the brain. Considerable evidence suggests that soluble Aβ oligomers are responsible for the synaptic dysfunction and cognitive deficit observed in AD. However, the mechanism by which these oligomers exert their neurotoxic effect remains unknown. Recently, it was reported that Aβ oligomers bind to the cellular prion protein with high affinity. Here, we show that N1, the main physiological cleavage fragment of the cellular prion protein, is necessary and sufficient for binding early oligomeric intermediates during Aβ polymerization into amyloid fibrils. The ability of N1 to bind Aβ oligomers is influenced by positively charged residues in two sites (positions 23–31 and 95–105) and is dependent on the length of the sequence between them. Importantly, we also show that N1 strongly suppresses Aβ oligomer toxicity in cultured murine hippocampal neurons, in a Caenorhabditis elegans-based assay, and in vivo in a mouse model of Aβ-induced memory dysfunction. These data suggest that N1, or small peptides derived from it, could be potent inhibitors of Aβ oligomer toxicity and represent an entirely new class of therapeutic agents for AD.