Overexpression of Tau Rescues Nogo-66-Induced Neurite Outgrowth Inhibition In Vitro

Tauopathies: the role of tau in cellular crosstalk and synaptic dysfunctions

Tauopathies are a group of neurodegenerative diseases among which are many of the most prevalent and with higher incidence worldwide, such as Alzheimer's disease (AD). According to the World Health Organization, this set of diseases will continue to increase their incidence, affecting millions of people by 2050. All of them are characterized by aberrant aggregation of tau protein in neurons and glia that are distributed in different brain regions according to their susceptibility. Numerous studies reveal that synaptic regulation not only has a neuronal component, but glia plays a fundamental role in it beyond its neuroinflammatory role. Despite this, it has not been emphasized how the glial inclusions of tau in this cell type directly affect this and many other essential functions, whose alterations have been related to the development of tauopathies. In this way, this review shows how tau inclusions in glia influence the synaptic dysfunctions that result in the cognitive symptoms characteristic of tauopathies. Thus, the mechanisms affected by inclusions in neurons, astrocytes, and oligodendrocytes are unraveled.

The Clinical Significance of Cerebrospinal Fluid Reticulon 4 (RTN4) Levels in the Differential Diagnosis of Neurodegenerative Diseases

Neurodegenerative diseases (NDs) belong to the top global causes of mortality. Diagnostic approaches to improve early diagnosis and differentiation of these diseases are constantly being sought. Therefore, we aimed to assess the cerebrospinal fluid (CSF) concentrations of Reticulon 4 (RTN4) in patients with neurodegenerative diseases and evaluate the potential clinical usefulness of this protein. RTNs are transmembrane proteins mediating neuroanatomical plasticity and functional recovery after central nervous system injury or diseases. According to our best knowledge, this is the first investigation providing the data concerning the dynamic of CSF RTN4 protein levels in patients with different NDs. Methods: Overall, 77 newly diagnosed patients with neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), and multiple sclerosis (MS), as well as 21 controls, were enrolled in the study. The CSF concentrations of tested proteins were assessed using immunological assays. Results: We revealed significantly higher CSF RTN4A levels in patients with AD, PD, and MS in comparison to the controls. Moreover, the comparative analysis of RTN4 concentration between different neurodegenerative diseases revealed the highest concentration of RTN4A in AD patients and a statistically significant difference between AD vs. PD, and AD vs. MS groups. The increased CSF level of the protein correlated with Tau, and pTau181 proteins in AD as well as in PD patients. Conclusions: Our study presents a previously not identified clinical utility of RTN4 in the differential diagnosis of neurodegenerative diseases.

Tau Deletion Prevents Cognitive Impairment and Mitochondrial Dysfunction Age Associated by a Mechanism Dependent on Cyclophilin-D

Aging is an irreversible process and the primary risk factor for the development of neurodegenerative diseases, such as Alzheimer’s disease (AD). Mitochondrial impairment is a process that generates oxidative damage and ATP deficit; both factors are important in the memory decline showed during normal aging and AD. Tau is a microtubule-associated protein, with a strong influence on both the morphology and physiology of neurons. In AD, tau protein undergoes post-translational modifications, which could play a relevant role in the onset and progression of this disease. Also, these abnormal forms of tau could be present during the physiological aging that could be related to memory impairment present during this stage. We previously showed that tau ablation improves mitochondrial function and cognitive abilities in young wild-type mice. However, the possible contribution of tau during aging that could predispose to the development of AD is unclear. Here, we show that tau deletion prevents cognitive impairment and improves mitochondrial function during normal aging as indicated by a reduction in oxidative damage and increased ATP production. Notably, we observed a decrease in cyclophilin-D (CypD) levels in aged tau−/− mice, resulting in increased calcium buffering and reduced mitochondrial permeability transition pore (mPTP) opening. The mPTP is a mitochondrial structure, whose opening is dependent on CypD expression, and new evidence suggests that this could play an essential role in the neurodegenerative process showed during AD. In contrast, hippocampal CypD overexpression in aged tau−/− mice impairs mitochondrial function evidenced by an ATP deficit, increased mPTP opening, and memory loss; all effects were observed in the AD pathology. Our results indicate that the absence of tau prevents age-associated cognitive impairment by maintaining mitochondrial function and reducing mPTP opening through a CypD-dependent mechanism. These findings are novel and represent an important advance in the study of how tau contributes to the cognitive and mitochondrial failure present during aging and AD in the brain.

Postsynaptic Targeting and Mobility of Membrane Surface-Localized hASIC1a

Acid-sensing ion channels (ASICs), the main H+ receptors in the central nervous system, sense extracellular pH fluctuations and mediate cation influx. ASIC1a, the major subunit responsible for acid-activated current, is widely expressed in brain neurons, where it plays pivotal roles in diverse functions including synaptic transmission and plasticity. However, the underlying molecular mechanisms for these functions remain mysterious. Using extracellular epitope tagging and a novel antibody recognizing the hASIC1a ectodomain, we examined the membrane targeting and dynamic trafficking of hASIC1a in cultured cortical neurons. Surface hASIC1a was distributed throughout somata and dendrites, clustered in spine heads, and co-localized with postsynaptic markers. By extracellular pHluorin tagging and fluorescence recovery after photobleaching, we detected movement of hASIC1a in synaptic spine heads. Single-particle tracking along with use of the anti-hASIC1a ectodomain antibody revealed long-distance migration and local movement of surface hASIC1a puncta on dendrites. Importantly, enhancing synaptic activity with brain-derived neurotrophic factor accelerated the trafficking and lateral mobility of hASIC1a. With this newly-developed toolbox, our data demonstrate the synaptic location and high dynamics of functionally-relevant hASIC1a on the surface of excitatory synapses, supporting its involvement in synaptic functions.

Tau Post-translational Modifications: Dynamic Transformers of Tau Function, Degradation, and Aggregation

Post-translational modifications (PTMs) on tau have long been recognized as affecting protein function and contributing to neurodegeneration. The explosion of information on potential and observed PTMs on tau provides an opportunity to better understand these modifications in the context of tau homeostasis, which becomes perturbed with aging and disease. Prevailing views regard tau as a protein that undergoes abnormal phosphorylation prior to its accumulation into the toxic aggregates implicated in Alzheimer's disease (AD) and other tauopathies. However, the phosphorylation of tau may, in fact, represent part of the normal but interrupted function and catabolism of the protein. In addition to phosphorylation, tau undergoes another forms of post-translational modification including (but not limited to), acetylation, ubiquitination, glycation, glycosylation, SUMOylation, methylation, oxidation, and nitration. A holistic appreciation of how these PTMs regulate tau during health and are potentially hijacked in disease remains elusive. Recent studies have reinforced the idea that PTMs play a critical role in tau localization, protein-protein interactions, maintenance of levels, and modifying aggregate structure. These studies also provide tantalizing clues into the possibility that neurons actively choose how tau is post-translationally modified, in potentially competitive and combinatorial ways, to achieve broad, cellular programs commensurate with the distinctive environmental conditions found during development, aging, stress, and disease. Here, we review tau PTMs and describe what is currently known about their functional impacts. In addition, we classify these PTMs from the perspectives of protein localization, electrostatics, and stability, which all contribute to normal tau function and homeostasis. Finally, we assess the potential impact of tau PTMs on tau solubility and aggregation. Tau occupies an undoubtedly important position in the biology of neurodegenerative diseases. This review aims to provide an integrated perspective of how post-translational modifications actively, purposefully, and dynamically remodel tau function, clearance, and aggregation. In doing so, we hope to enable a more comprehensive understanding of tau PTMs that will positively impact future studies.

Reduction of NgR in perforant path decreases amyloid-β peptide production and ameliorates synaptic and cognitive deficits in APP/PS1 mice

BACKGROUND

Amyloid beta (Aβ) which is recognized as a main feature of Alzheimer's disease (AD) has been proposed to "spread" through anatomically and functionally connected brain regions. The entorhinal cortex and perforant path are the earliest affected brain regions in AD. The perforant path is the most vulnerable circuit in the cortex with respect to both aging and AD. Previous data show that the origins and terminations of the perforant path are susceptible to amyloid deposition at the younger age in AD. Nogo receptor (NgR) plays an essential role in limiting injury-induced axonal growth and experience-dependent plasticity in the adult brain. It has been suggested that NgR is involved in AD pathological features, but the results have been conflicting and the detailed mechanism needs further investigation. In this study, the effect of NgR in the perforant path on the pathological and functional phenotype of APP/PS1 transgenic mice was studied.

METHODS

To genetically manipulate NgR expression, adeno-associated virus (AAV) with short hairpin (shRNA) against NgR was injected into the perforant path of APP/PS1 transgenic mice, followed by an assessment of behavioral, synaptic plasticity and neuropathological phenotypes. NgR was overexpressed or knockdown in neuroblastoma N2a cells and APPswe/HEK293 cells to investigate the interaction between NgR and amyloid precursor protein (APP).

RESULTS

It is shown that reduction of NgR in the perforant path rescued cognitive and synaptic deficits in APP/PS1 transgenic mice. Concurrently, Aβ production in the perforant path and levels of soluble Aβ and amyloid plaques in the hippocampus were significantly decreased. There was a positive correlation between the total APP protein level and NgR expression both in transgenic mice and in cultured cells, where the α-secretase and β-secretase cleavage products both changed with APP level in parallel. Finally, NgR might inhibit APP degradation through lysosome by Rho/Rho-associated protein kinases (ROCK) signaling pathway.

CONCLUSIONS

Our findings demonstrate that perforant path NgR plays an important role in regulating APP/Aβ level and cognitive functions in AD transgenic mice, which might be related to the suppression of APP degradation by NgR. Our study suggests that NgR in the perforant path could be a potential target for modulating AD progression.

LncRNA FTX Involves in the Nogo-66-Induced Inhibition of Neurite Outgrowth Through Regulating PDK1/PKB/GSK-3β Pathway

Nogo-66 can inhibit neurite outgrowth, while its regulation mechanisms have not been fully elucidated. Recent studies prove that lncRNAs are involved in neurite outgrowth. This study was aimed to investigate whether lncRNA FTX was involved in Nogo-66-induced inhibition of neurite outgrowth and explore the potential mechanism. The expression of relative genes was detected by qRT-PCR and western blot. The function of FTX was determined by overexpression and knockdown techniques. The interaction between FTX and PDK1 was evaluated by RIP and RNA pull-down assays. FTX expression was downregulated by Nogo-66 in PC12 cells. Nogo-66-induced inhibition of neurite outgrowth was relieved by FTX overexpression. FTX bound to PDK1 protein to disturb the interaction between PDK1 and E3 ubiquitin ligase RNF126, thereby blocked the ubiquitination degradation of PDK1 and elevated PDK1 protein level. Mechanically, FTX involved in the Nogo-66-induced inhibition of neurite outgrowth through the PDK1/PKB/GSK-3β pathway. In SCI rats, FTX knockdown inhibited neurite outgrowth induced by the receptor antagonist of Nogo-66. The present results suggested that FTX took part in Nogo-66-inhibited neurite outgrowth, and FTX exerted its function through regulating PDK1/PKB/GSK-3β pathway.

Genetic ablation of tau improves mitochondrial function and cognitive abilities in the hippocampus

Tau is a key protein for microtubule stability; however, post-translationally modified tau contributes to neurodegenerative diseases by forming tau aggregates in the neurons. Previous reports from our group and others have shown that pathological forms of tau are toxic and impair mitochondrial function, whereas tau deletion is neuroprotective. However, the effects of tau ablation on brain structure and function in young mice have not been fully elucidated. Therefore, the aim of this study was to investigate the implications of tau ablation on the mitochondrial function and cognitive abilities of a litter of young mice (3 months old). Our results showed that tau deletion had positive effects on hippocampal cells by decreasing oxidative damage, favoring a mitochondrial pro-fusion state, and inhibiting mitochondrial permeability transition pore (mPTP) formation by reducing cyclophilin D (Cyp-D) protein. More importantly, tau deletion increased ATP production and improved the recognition memory and attentive capacity of juvenile mice. Therefore, the absence of tau enhanced brain function by improving mitochondrial health, which supplied more energy to the synapses. Thus, our work opens the possibility that preventing negative tau modifications could enhance brain function through the improvement of mitochondrial health.

The ProNGF/p75NTR pathway induces tau pathology and is a therapeutic target for FTLD-tau

Tau pathology is characterized as a form of frontotemporal lobar degeneration (FTLD) known as FTLD-tau. The underlying pathogenic mechanisms are not known and no therapeutic interventions are currently available. Here, we report that the neurotrophin receptor p75NTR plays a critical role in the pathogenesis of FTLD-tau. The expression of p75NTR and the precursor of nerve growth factor (proNGF) were increased in the brains of FTLD-tau patients and mice (P301L transgenic). ProNGF-induced tau phosphorylation via p75NTR in vitro, which was associated with the AKT/glycogen synthase kinase (GSK)3β pathway. Genetic reduction of p75NTR in P301L mice rescued the memory deficits, alleviated tau hyperphosphorylation and restored the activity of the AKT/GSK3β pathway. Treatment of the P301L mice with the soluble p75NTR extracellular domain (p75ECD-Fc), which can antagonize neurotoxic ligands of p75NTR, effectively improved memory behavior and suppressed tau pathology. This suggests that p75NTR plays a crucial role in tau paGSKthology and represents a potential druggable target for FTLD-tau and related tauopathies.