Natalie Qishan Zhang

Temporal relationship of autophagy and apoptosis in neurons challenged by low molecular weight β-amyloid peptide

Alzheimers disease (AD) is an aging‐related progressive neurodegenerative disorder. Previous studies suggested that various soluble Aβ species are neurotoxic and able to activate apoptosis and autophagy, the type I and type II programmed cell death, respectively. However, the sequential and functional relationships between these two cellular events remain elusive. Here we report that low molecular weight Aβ triggered cleavage of caspase 3 and poly (ADP‐ribose) polymerase to cause neuronal apoptosis in rat cortical neurons. On the other hand, Aβ activated autophagy by inducing autophagic vesicle formation and autophagy related gene 12 (ATG12), and up‐regulated the lysoso‐mal machinery for the degradation of autophagosomes. Moreover, we demonstrated that activation of autophagy by Aβ preceded that of apoptosis, with death associated protein kinase phosphorylation as the potential molecular link. More importantly, under Aβ toxicity, neurons exhibiting high level of autophagosome formation were absent of apoptotic features, and inhibition of autophagy by 3‐methylade‐nine advanced neuronal apoptosis, suggesting that autophagy can protect neurons from Aβ‐induced apoptosis.

Effects of corticosterone and amyloid-beta on proteins essential for synaptic function: Implications for depression and Alzheimer's disease

The relationship between Alzheimers disease (AD) and depression has been well established in terms of epidemiological and clinical observations. Depression has been considered to be both a symptom and risk factor of AD. Several genetic and neurobiological mechanisms have been described to underlie these two disorders. Despite the accumulating knowledge on this topic, the precise neuropathological mechanisms remain to be elucidated. In this study, we propose that synaptic degeneration plays an important role in the disease progression of depression and AD. Using primary culture of hippocampal neurons treated with oligomeric Aβ and corticosterone as model agents for AD and depression, respectively, we found significant changes in the pre-synaptic vesicle proteins synaptophysin and synaptotagmin. We further investigated whether the observed protein changes affected synaptic functions. By using FM(®)4-64 fluorescent probe, we showed that synaptic functions were compromised in treated neurons. Our findings led us to investigate the involvement of protein degradation mechanisms in mediating the observed synaptic protein abnormalities, namely, the ubiquitin-proteasome system and autophagy. We found up-regulation of ubiquitin-mediated protein degradation, and the preferential signaling for the autophagic-lysosomal degradation pathway. Lastly, we investigated the neuroprotective role of different classes of antidepressants. Our findings demonstrated that the antidepressants Imipramine and Escitalopram were able to rescue the observed synaptic protein damage. In conclusion, our study shows that synaptic degeneration is an important common denominator underlying depression and AD, and alleviation of this pathology by antidepressants may be therapeutically beneficial.

A reciprocal relationship between reactive oxygen species and mitochondrial dynamics in neurodegeneration

Mitochondrial fragmentation due to fission/fusion imbalance has often been linked to mitochondrial dysfunction and apoptosis in neurodegeneration. Conventionally, it is believed that once mitochondrial morphology shifts away from its physiological tubular form, mitochondria become defective and downstream apoptotic signaling pathways are triggered. However, our study shows that beta-amyloid (Aβ) induces morphological changes in mitochondria where they become granular-shaped and are distinct from fragmented mitochondria in terms of both morphology and functions. Accumulation of mitochondrial reactive oxygen species triggers granular mitochondria formation, while mitoTEMPO (a mitochondria-targeted superoxide scavenger) restores tubular mitochondrial morphology within Aβ-treated neurons. Interestingly, modulations of mitochondria fission and fusion by genetic and pharmacological tools attenuated not only the induction of granular mitochondria, but also mitochondrial superoxide levels in Aβ−treated neurons. Our study shows a reciprocal relationship between mitochondrial dynamics and reactive oxygen species and provides a new potential therapeutic target at early stages of neurodegenerative disease pathogenesis.

In vitro attenuation of acrolein-induced toxicity by phloretin, a phenolic compound from apple

In the current study, the protective effects of phloretin were investigated in acrolein-challenged amino acid, protein, and cell models. It was found that the formation of FDP-lysine (a typical acrolein-lysine adduct) was strongly inhibited in the presence of phloretin and the remaining electrophilic site in FDP-lysine was also blocked by phloretin. Moreover, direct trapping of acrolein by phloretin was found to be responsible for inhibiting the incorporation of carbonyl groups into BSA and oligomerisation in RNase A. Subsequently, the reduction of LDH release in human neuroblastoma SH-SY5Y cells under acrolein challenge suggested the cytoprotective effects of phloretin. Such protection might be mediated through inhibiting the increased cellular protein carbonyl level as revealed by Western blotting analysis. The present study highlighted an apple phenolic compound, phloretin as a promising candidate in prevention or treatment of acrolein-associated human diseases.

Disruptions of endoplasmic reticulum and mitochondria prime mTOR suppression in low molecular weight Aβ-induced autophagy

Background: Recent studies have demonstrated that soluble Ab dimers isolated from AD brains inhibit long-term potentiation (LTP), reduce dendritic spine density in vitro and upon direct injection into the brain of normal rats disrupt memory (Shankar et al., 2008). In the current study we evaluated the acquisition performance of PSAPP mice in a Morris water maze, and searched for Ab oligomers in brains of PSAPP transgenic mice to determine whether the soluble Ab in the transgenic mice has similar biochemical properties to human Ab extracted from AD brains. Methods: Wild type (wt) and heterozygous PSAPP mice were trained in the Morris water maze (4 trials/day for 9 days) and the latency to reach the platform was recorded for each trial. Soluble Ab oligomers were measured from the brains of the PSAPP transgenic mice using a sensitive immunoprecipitation/Western blotting protocol after homogenizing the whole cerebra in Tris-buffered saline. Results: Aged mice (>12 months) exhibit an increase in plaque load and have deficits in the Morris water maze. We observe high levels of Ab dimers in young transgenic mice (2-3 months of age) which decline before increasing again from 7-12 months. No overt cognitive deficits were seen in the 3 and 9 month old mice suggesting that dimers need to reach a critical threshold before memory deficits, in this specific cognitive paradigm, are observed. Both Ab1-40 and Ab1-42 are present in soluble extracts from PSAPP brains. However, in human AD brains Ab1-42 is the predominant species in the soluble extract. Conclusions: These studies demonstrate that the biochemical and, potentially, the pathophysiological properties of Ab in young PSAPP mice are different from that of Ab extracted from elderly human AD patients.

Aggregation of the endoplasmic reticulum triggered by oligomeric beta-amyloid peptides could initiate autophagy

Background: Autophagic vacuoles have been demonstrated from postmortem humanAlzheimer’s brains. However, the underlyingmechanisms of initiating autophagic vacuoles or autophagosomes remain unclear. We have shown that oligomeric Abeta peptides trigger aggregation of the endoplasmic reticulum (ER). Following this event, the number of autophagosomes is increased. The aim of this study is to investigate the mechanisms of forming autophagosomes from the aggregated ER.Methods: Primary cultures of hippocampal neurons were treated with oligomeric Abeta. Neurons were then transfected with different DNA constructs to express GFP-DFCP1 for protein docking to Phosphatidylinositol phosphate (PI(3)K), DsRedKDEL for protein expressing in the ER, GFP/AsRed-Atg14L, mRFPVps34 and mCherryAmbra-1 for proteins involving in the nucleation of autophagosomes. Results:With the use of live cell imaging by multiphoton microscopy, we found that Atg14L was localized on the ER. Aggregation of the ER facilitated them to be clustered which can then attract Vps34 together with Beclin-1 to form a nucleation complex. Ambra-1 is not the protein to initiate the nucleation complex. Consequently, an intermediate form of vesicles called omegasomes was formed and will be furthered matured to be autophagosomes. All these initiation steps were not triggered by mTOR signaling pathway. Conclusions: Our results are among the first to demonstrate the initiation factors for formation of autophagosomes. Aggregation of the ER may be the starting point for forming autophagosomes. In addition, the initiation processes for formation of autophagosomes are different to other systems which are regulated by mTOR. The results can answer a long-term question of how autophagy is initiated.