Shimo Li

Autophagy impairment stimulates PS1 expression and gamma-secretase activity.

γ-Secretase plays an important role in the development of Alzheimer disease (AD). γ-Secretase activity is enriched in autophagic vacuoles and it augments amyloid-β (Aβ) synthesis. Autophagy-lysosomal dysfunction has been implicated in AD, but whether γ-secretase activity is affected by autophagy remains unclear. Here we report that γ-secretase activity is enhanced in basal autophagy-disturbed cells through the α subunit of eukaryotic translation initiation factor 2 (eIF2α) kinase, general control nonderepressible 2 (GCN2). Presenilin-1 (PS1) expression was increased even in the presence of nutrients in autophagy-related 5 knockdown (Atg5KD) human embryonic kidney (HEK293) cells expressing a short hairpin RNA as well as in chloroquine-treated HEK293 cells. However, PS1 expression induction was prevented in GCN2KD and ATF4KD cells. Furthermore, Atg5KD cells showed an increase in Aβ production and Notch1 cleavage. These were reduced by an autophagy inducer, resveratrol. Thus, we conclude that the autophagy-lysosomal system regulates γ-secretase activity through GCN2.

Endoplasmic reticulum stress enhances γ-secretase activity.

The endoplasmic reticulum (ER) copes with unfolded proteins in the lumen (ER stress) by activating three distinct intracellular signaling pathways of unfolded protein response (UPR). ER stress contributes to the pathogenesis of obesity and diabetes, which are risk factors for Alzheimers disease (AD) that accelerate the pathogenesis of AD. However, whether ER stress is involved in the development of AD remains unclear. In this study, we demonstrate that ER stress induces presenilin-1 expression through activating transcription factor 4 (ATF4), resulting in increased amyloid-β (Aβ) secretion by γ-secretase activity, which is suppressed by quercetin by modifying UPR signaling. This result suggests that ER stress may be stimulated in obesity and type 2 diabetes, thereby enhancing γ-secretase activity that is the underlying molecular mechanism affecting the pathogenesis of AD.

Cayman Ataxia-Related Protein is a Presynapse-Specific Caspase-3 Substrate

Caspase plays an important role in apoptosis and physiological processes such as synaptic plasticity. However, the caspase substrate at the synapse is still unknown. Here we used an in vitro cleavage assay with a small-pool human brain cDNA library. We identified the presynaptic protein Caytaxin as a substrate of caspase-3 and caspase-7. Deficiency in Caytaxin causes Cayman ataxia, a disorder characterized by cerebellar dysfunction and mental retardation. Caytaxin cleavage in cerebellar granule neurons is dependent on caspase-3 activation. The cleavage site is upstream of the cellular retinal and the TRIO guanine exchange factor domain, producing a C-terminal fragment that may play an alternative role in inhibiting MEK2 signaling. Thus, we concluded that Caytaxin is a novel substrate of caspase-3 at the presynapse.

Polyglutamine expansion disturbs the endoplasmic reticulum formation, leading to caspase-7 activation through Bax

The endoplasmic reticulum (ER) plays a pivotal role in cellular functions such as the ER stress response. However, the effect of the ER membrane on caspase activation remains unclear. This study reveals that polyglutamine oligomers augmented at ER induce insertion of Bax into the ER membrane, thereby activating caspase-7. In line with the role of ER in cell death induced by polyglutamine expansion, the ER membrane was found to be disrupted and dilated in the brain of a murine model of Huntingtons disease. We can conclude that polyglutamine expansion may drive caspase-7 activation by disrupting the ER membrane.

The expression and localization of Prune2 mRNA in the central nervous system

A family of Bcl-2/adenovirus E1B 19kDa-interacting proteins (BNIPs) plays critical roles in several cellular processes such as cellular transformation, apoptosis, neuronal differentiation, and synaptic function, which are mediated by the BNIP2 and Cdc42GAP homology (BCH) domain. Prune homolog 2 (Drosophila) (PRUNE2) and its isoforms -C9orf65, BCH motif-containing molecule at the carboxyl terminal region 1 (BMCC1), and BNIP2 Extra Long (BNIPXL) - have been shown to be a susceptibility gene for Alzheimers disease, a biomarker for leiomyosarcomas, a proapoptotic protein in neuronal cells, and an antagonist of cellular transformation, respectively. However, precise localization of PRUNE2 in the brain remains unclear. Here, we identified the distribution of Prune2 mRNA in the adult mouse brain. Prune2 mRNA is predominantly expressed in the neurons of the cranial nerve motor nuclei and the motor neurons of the spinal cord. The expression in the dorsal root ganglia (DRG) is consistent with the previously described reports. In addition, we observed the expression in another sensory neuron in the mesencephalic trigeminal nucleus. These results suggest that Prune2 may be functional in these restricted brain regions.

Olfaxin as a novel Prune2 isoform predominantly expressed in olfactory system.

Prune homolog 2 (Drosophila) (PRUNE2) encodes a BCH motif-containing protein that shares homology with the Cayman ataxia-related protein Caytaxin. Caytaxin is a substrate of caspase-3 and is specifically expressed at the presynapse of vesicular-type glutamate transporter (VGLUT)-positive neurons, where it plays a role in glutamate neurotransmission primarily in the cerebellum and hippocampus. Here, we showed that a novel Prune2 isoform contains a BCH motif and localizes predominantly to the synaptic cytosol, similar to Caytaxin. However, the isoform is expressed predominantly in the olfactory bulb and layer Ia of the piriform cortex, where Caytaxin is scarcely expressed. The isoform expression is upregulated during development, similar to that in the presynaptic-localizing proteins Synapsin I and Bassoon. Prune2 and its previously identified isoforms have been shown to be a susceptibility gene for Alzheimers disease, a biomarker for leiomyosarcomas, a proapoptotic protein, and an antagonist of cellular transformation. In addition, a novel isoform may develop new roles for Prune2 at the synapse in olfactory systems.

Improvement of memory recall by quercetin in rodent contextual fear conditioning and human early-stage Alzheimer's disease patients.

Patients with Alzheimer’s disease (AD) experience a wide array of cognitive deficits, which typically include the impairment of explicit memory. In previous studies, the authors reported that a flavonoid, quercetin, reduces the expression of ATF4 and delays memory deterioration in an early-stage AD mouse model. In the present study, the effects of long-term quercetin intake on memory recall were assessed using contextual fear conditioning in aged wild-type mice. In addition, the present study examined whether memory recall was affected by the intake of quercetin-rich onion (a new cultivar of hybrid onion ‘Quergold’) powder in early-stage AD patients. In-vivo analysis indicated that memory recall was enhanced in aged mice fed a quercetin-containing diet. Memory recall in early-stage AD patients, determined using the Revised Hasegawa Dementia Scale, was significantly improved by the intake of quercetin-rich onion (Quergold) powder for 4 weeks compared with the intake of control onion (‘Mashiro’ white onion) powder. These results indicate that quercetin might influence memory recall.

An alternative spliced mouse presenilin-2 mRNA encodes a novel γ-secretase inhibitor

MINT‐7025631: APH1 (uniprotkb:Q96BI3) physically interacts (MI:0218) with NCT (uniprotkb:Q92542), PEN2 (uniprotkb:Q9NZ42) and PS1 (uniprotkb:P49769) by anti tag coimmunoprecipitation (MI:0007)

Expanded polyglutamine embedded in the endoplasmic reticulum causes membrane distortion and coincides with Bax insertion

The endoplasmic reticulum (ER) is important in various cellular functions, such as secretary and membrane protein biosynthesis, lipid synthesis, and calcium storage. ER stress, including membrane distortion, is associated with many diseases such as Huntingtons disease. In particular, nuclear envelope distortion is related to neuronal cell death associated with polyglutamine. However, the mechanism by which polyglutamine causes ER membrane distortion remains unclear. We used electron microscopy, fluorescence protease protection assay, and alkaline treatment to analyze the localization of polyglutamine in cells. We characterized polyglutamine embedded in the ER membrane and noted an effect on morphology, including the dilation of ER luminal space and elongation of ER-mitochondria contact sites, in addition to the distortion of the nuclear envelope. The polyglutamine embedded in the ER membrane was observed at the same time as Bax insertion. These results demonstrated that the ER membrane may be a target of polyglutamine, which triggers cell death through Bax.

CED-4 is an mRNA-binding protein that delivers ced-3 mRNA to ribosomes.

Cell death abnormal (ced)-3 and ced-4 genes regulate apoptosis to maintain tissue homeostasis in Caenorhabditis elegans. Apoptosome formation and CED-4 translocation drive CED-3 activation. However, the precise role of CED-4 translocation is not yet fully understood. In this study, using a combination of immunoprecipitation and reverse transcription-polymerase chain reaction methods in cells and a glutathione-S-transferase pull down assay in a cell-free system, we show that CED-4 binds ced-3 mRNA. In the presence of ced-3 mRNA, CED-4 protein is enriched in the microsomal fraction and interacts with ribosomal protein L10a in mammalian cells, increasing the levels of CED-3. These results suggest that CED-4 forms a complex with ced-3 mRNA and delivers it to ribosomes for translation.