Amyloid β peptide toxicity in differentiated PC12 cells: Calpain‐calpastatin, caspase, and membrane damage

JAK2/STAT3 as a new potential target to manage neurodegenerative diseases: An interactive review

Neurodegenerative diseases (NDDs) are a collection of incapacitating disorders in which neuroinflammation and neuronal apoptosis are major pathological consequences due to oxidative stress. Neuroinflammation manifests in the impacted cerebral areas as a result of pro-inflammatory cytokines stimulating the Janus Kinase2 (JAK2)/Signal Transducers and Activators of Transcription3 (STAT3) pathway via neuronal cells. The pro-inflammatory cytokines bind to their respective receptor in the neuronal cells and allow activation of JAK2. Activated JAK2 phosphorylates tyrosines on the intracellular domains of the receptor which recruit the STAT3 transcription factor. The neuroinflammation issues are exacerbated by the active JAK2/STAT3 signaling pathway in conjunction with additional transcription factors like nuclear factor kappa B (NF-κB), and the mammalian target of rapamycin (mTOR). Neuronal apoptosis is a natural process made worse by persistent neuroinflammation and immunological responses via caspase-3 activation. The dysregulation of micro-RNA (miR) expression has been observed in the consequences of neuroinflammation and neuronal apoptosis. Neuroinflammation and neuronal apoptosis-associated gene amplification may be caused by dysregulated miR-mediated aberrant phosphorylation of JAK2/STAT3 signaling pathway components. Therefore, JAK2/STAT3 is an attractive therapeutic target for NDDs. Numerous synthetic and natural small molecules as JAK2/STAT3 inhibitors have therapeutic advances against a wide range of diseases, and many are now in human clinical studies. This review explored the interactive role of the JAK2/STAT3 signaling system with key pathological factors during the reinforcement of NDDs. Also, the clinical trial data provides reasoning evidence about the possible use of JAK2/STAT3 inhibitors to abate neuroinflammation and neuronal apoptosis in NDDs.

Effects of the Hydrophilic N-Terminal Region on Aβ-Mediated Membrane Disruption

Amyloidogenesis of amyloid-β (Aβ) peptides is intimately related to pathological neurodegeneration in Alzheimer's disease. Here, we investigated the membrane damage activity of Aβ40 and its derivatives that contain mutation at the N-terminal charged residues using a membrane leakage assay. A model 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) phospholipid vesicle encapsulating the fluorescent dye carboxyfluorescein was used in the study. Our results show that the mutations of the N-terminal charged residues of Aβ40 significantly affect the peptide-induced membrane leakage. The results suggest that favorable electrostatic interactions of the N-terminal charged residues and the phosphatidylcholine membrane surface are crucial in Aβ-mediated membrane permeation. The flexible and charge-rich N-terminal region may play a critical role in directing Aβ self-association on the membrane surface and in anchoring and stabilizing the peptide aggregates inserted in the phospholipid membrane, which are closely related with membrane disruption activity of Aβ. The results provide new mechanistic insight into the Aβ-mediated membrane damage process, which may be critical for understanding the mechanism of Aβ neurotoxicity in Alzheimer's disease.

[Calpains and their endo- and exogenous regulators in various neurodegeneration models]

On the basis of experimental series with murine models there was obtained the evidence on calcium-dependent protease activity changes in rat brain at induced neurodegeneration. The properties of the proteolytic and regulatory components of calpain system under the effect of neurotoxic stimuli--amyloid beta-peptide or glutamate--were characterized; the basic endogenous regulatory mechanisms of calcium-dependent proteolysis modulation were determined as well. Neuroprotective properties of exogenous calpain regulators differing in the mechanisms of action (sex steroids, calcium regulators) were tested on studied neurodegeneration models.

[Calpain system dysregulation in rat brain at beta-amyloid-induced neurodegeneration]

Experimental evidences of calcium-dependent proteolysis dysregulation in brain of murine model of Alzheimer disease were obtained. Experimental treatment consisted in intra-hippocampal injection of amyloid beta-peptide (AP1-40) promoted activation of main calpain forms in murine brain along with decrease incontent of natural calpain inhibitor, calpastatin. As a result of prognostic experiment on the correction of neurodegeneration induced in murine the neuroprotective properties of steroid hormone estradiol were confirmed and one of the possible protective action mechanisms was suggested. Obtained results allow considering both biochemical modifications in protein facilities of pathology-affected brain and the mechanisms of neurodegeneration and neuroprotection.

Cellular membrane fluidity in amyloid precursor protein processing

The senile plaque is a pathologic hallmark of Alzheimer's disease (AD). Amyloid-β peptide (Aβ), the main constituent of senile plaques, is neurotoxic especially in its oligomeric form. Aβ is derived from the sequential cleavage of amyloid precursor protein (APP) by β- and γ-secretases in the amyloidogenic pathway. Alternatively, APP can be cleaved by α-secretases within the Aβ domain to produce neurotrophic and neuroprotective α-secretase-cleaved soluble APP (sAPPα) in the nonamyloidogenic pathway. Since APP and α-, β-, and γ-secretases are membrane proteins, APP processing should be highly dependent on the membrane composition and the biophysical properties of cellular membrane. In this review, we discuss the role of the biophysical properties of cellular membrane in APP processing, especially the effects of phospholipases A(2) (PLA(2)s), fatty acids, cholesterol, and Aβ on membrane fluidity in relation to their effects on APP processing.

Active and passive immunization strategies based on the SDPM1 peptide demonstrate pre-clinical efficacy in the APPswePSEN1dE9 mouse model for Alzheimer's disease

Recent clinical and pre-clinical studies suggest that both active and passive immunization strategies targeting Aβ amyloid may have clinical benefit in Alzheimer's disease. Here, we demonstrate that vaccination of APPswePSEN1dE9 mice with SDPM1, an engineered non-native Aβ amyloid-specific binding peptide, lowers brain Aβ amyloid plaque burden and brain Aβ1-40 and Aβ1-42 peptide levels, improves cognitive learning and memory in Morris water maze tests and increases the expression of synaptic brain proteins. This was the case in young mice immunized prior to development of significant brain amyloid burden, and in older mice, where brain amyloid was already present. Active immunization was optimized using ALUM as an adjuvant to stimulate production of anti-SDPM1 and anti-Aβ amyloid antibodies. Intracerebral injection of P4D6, an SDPM1 peptide-mimotope antibody, also lowered brain amyloid plaque burden in APPswePSEN1dE9 mice. Additionally, P4D6 inhibited Aβ amyloid-mediated toxicity in cultured neuronal cells. The protein sequence of the variable domain within the P4D6 heavy chain was found to mimic a multimer of the SDPM1 peptide motif. These data demonstrate the efficacy of active and passive vaccine strategies to target Aβ amyloid oligomers using an engineered peptide-mimotope strategy.

Calpastatin upregulation in Mycoplasma hyorhinis-infected cells is promoted by the mycoplasma lipoproteins via the NF-κB pathway

Mycoplasma hyorhinis frequently contaminates cultured cells, with effects on synthetic and metabolic pathways. We demonstrated for the first time that contamination of cells by a strain of M. hyorhinis (NDMh) results in increased levels of calpastatin (the endogenous inhibitor of the ubiquitous Ca(2+) -dependent protease calpain). We now show that the calpastatin upregulation by NDMh in neuroblastoma SH-SY5Y cells resides in the NDMh lipoprotein fraction (LPP), via the NF-κB transcription pathway. NF-κB activation requires dissociation of the cytoplasmic NF-κB/IκB complex followed by NF-κB translocation to the nucleus. NDMh-LPP induced translocation of the NF-κB RelA subunit to the nucleus and upregulated calpastatin. RelA translocation and calpastatin elevation were prevented when dissociation of the NF-κB/IκB complex was inhibited either by transfection with the non-phosphorylatable IκB mutant ΔNIκBα, or by using PS1145, an inhibitor of the IκB kinase (IKK complex). Increased calpastatin levels attenuate calpain-related amyloid-β-peptide and Ca(2+) -toxicity (these are central to the pathogenesis of Alzheimer's Disease). LPP-induced elevation of calpastatin provides an example of effects on non-inflammatory intracellular proteins, the outcome being significant alterations in host cell functions. Since calpastatin level is important in the control of calpain activity, mycoplasmal LPP may be of interest in treating some pathological processes involving excessive calpain activation.

Cryptotanshinione inhibits β-amyloid aggregation and protects damage from β-amyloid in SH-SY5Y cells

The deposition of amyloid β-protein (Aβ) fibrils into plaques within the brain parenchyma and along cerebral blood vessels is a hallmark of Alzheimer's disease (AD). Aβ42 oligomers and fibrils cause the breakdown of neural circuits, neuronal death and eventually dementia. Drugs that inhibit Aβ42 aggregation may be a novel direction in AD drug discovery. Cryptotanshinone (CTS), an active component of the medicinal herb Salvia miltiorrhiza, has been shown to improve learning and memory in several pharmacological models of AD. However, the effects of CTS on the Aβ aggregation and toxicity are unclear. The current work shows the effectiveness of CTS on the inhibition of Aβ42 aggregation and toxicity to human neuroblastoma cells. In this study, we demonstrated that CTS can inhibit Aβ42 spontaneous aggregation using thioflavin T fluorescence assay and transmission electron microscopy. Furthermore, we investigated the effects of CTS on Aβ-induced oxidative cell death in cultured SH-SY5Y cells. MTT and lactate dehydrogenase assays showed that CTS reduced the cytotoxicity induced by Aβ42. CTS also dramatically reduced Aβ42-induced cellular apoptosis and increased level of reactive oxygen species in these cells. Our study suggests that CTS may be useful in the inhibition or prevention of AD development and progression.

Impacts of membrane biophysics in Alzheimer's disease: from amyloid precursor protein processing to aβ Peptide-induced membrane changes

An increasing amount of evidence supports the notion that cytotoxic effects of amyloid-β peptide (Aβ), the main constituent of senile plaques in Alzheimer's disease (AD), are strongly associated with its ability to interact with membranes of neurons and other cerebral cells. Aβ is derived from amyloidogenic cleavage of amyloid precursor protein (AβPP) by β- and γ-secretase. In the nonamyloidogenic pathway, AβPP is cleaved by α-secretases. These two pathways compete with each other, and enhancing the non-amyloidogenic pathway has been suggested as a potential pharmacological approach for the treatment of AD. Since AβPP, α-, β-, and γ-secretases are membrane-associated proteins, AβPP processing and Aβ production can be affected by the membrane composition and properties. There is evidence that membrane composition and properties, in turn, play a critical role in Aβ cytotoxicity associated with its conformational changes and aggregation into oligomers and fibrils. Understanding the mechanisms leading to changes in a membrane's biophysical properties and how they affect AβPP processing and Aβ toxicity should prove to provide new therapeutic strategies for prevention and treatment of AD.

Molecular determinants of survival motor neuron (SMN) protein cleavage by the calcium-activated protease, calpain

Spinal muscular atrophy (SMA) is a leading genetic cause of childhood mortality, caused by reduced levels of survival motor neuron (SMN) protein. SMN functions as part of a large complex in the biogenesis of small nuclear ribonucleoproteins (snRNPs). It is not clear if defects in snRNP biogenesis cause SMA or if loss of some tissue-specific function causes disease. We recently demonstrated that the SMN complex localizes to the Z-discs of skeletal and cardiac muscle sarcomeres, and that SMN is a proteolytic target of calpain. Calpains are implicated in muscle and neurodegenerative disorders, although their relationship to SMA is unclear. Using mass spectrometry, we identified two adjacent calpain cleavage sites in SMN, S192 and F193. Deletion of small motifs in the region surrounding these sites inhibited cleavage. Patient-derived SMA mutations within SMN reduced calpain cleavage. SMN(D44V), reported to impair Gemin2 binding and amino-terminal SMN association, drastically inhibited cleavage, suggesting a role for these interactions in regulating calpain cleavage. Deletion of A188, a residue mutated in SMA type I (A188S), abrogated calpain cleavage, highlighting the importance of this region. Conversely, SMA mutations that interfere with self-oligomerization of SMN, Y272C and SMNΔ7, had no effect on cleavage. Removal of the recently-identified SMN degron (Δ268-294) resulted in increased calpain sensitivity, suggesting that the C-terminus of SMN is important in dictating availability of the cleavage site. Investigation into the spatial determinants of SMN cleavage revealed that endogenous calpains can cleave cytosolic, but not nuclear, SMN. Collectively, the results provide insight into a novel aspect of the post-translation regulation of SMN.

Astaxanthine secured apoptotic death of PC12 cells induced by beta-amyloid peptide 25-35: its molecular action targets

Astaxanthine (ASTx) is a novel carotenoid nutraceutical occurring in many crustaceans and red yeasts. It has potent antioxidant, photoprotective, hepatodetoxicant, and anti-inflammatory activities. Documented effect of ASTx on treatment of neurodegenerative disease is still lacking. We used the beta-amyloid peptide (Abeta) 25-35-treated PC12 model to investigate the neuron-protective effect of ASTx. The parameters examined included cell viability, caspase activation, and various apoptotic biomarkers that play their critical roles in the transduction pathways independently or synergistically. Results indicated that Abeta25-35 at 30 microM suppressed cell viability by 55%, whereas ASTx was totally nontoxic below a dose of 5.00 microM. ASTx at 0.1 microM protected PC12 cells from damaging effects of Abeta25-35 in several ways: (1) by securing the cell viability; (2) by partially down-regulating the activation of caspase 3; (3) by inhibiting the expression of Bax; (4) by completely eliminating the elevation of interleukin-1beta and tumor necrosis factor-alpha; (5) by inhibiting the nuclear translocation of nuclear factor kappaB; (6) by completely suppressing the phosphorylation of p38 mitogen-activated protein kinase; (7) by completely abolishing the calcium ion influx to effectively maintain calcium homeostasis; and (8) by suppressing the majority (about 75%) of reactive oxygen species production. Conclusively, ASTx may have merit to be used as a very potential neuron protectant and an anti-early-stage Alzheimer's disease adjuvant therapy.

Protective effects of galantamine against Abeta-induced PC12 cell apoptosis by preventing mitochondrial dysfunction and endoplasmic reticulum stress

Amyloid beta (Abeta) is considered to be responsible for the pathogenesis of Alzheimer's disease (AD). Mitochondrial and ER apoptotic pathways are considered to be involved in this process. Galantamine is an acetylcholinesterase (AChE) inhibitor widely used for patients with AD. In this study, we investigated the neuroprotective effects of galantamine on Abeta(25-35)-induced apoptosis in PC12 cells and the underlying mechanisms. Exposure of PC12 cells to 20 microM Abeta(25-35) caused significant cell viability loss and apoptosis, Abeta aggregation, mitochondrial and ER morphological changes, as well as mitochondrial membrane potential dissipation, reactive oxygen species (ROS) production, intracellular calcium elevation, and cytochrome c release from mitochondria. Pretreatment with 10 microM galantamine for 24 h prior to Abeta(25-35) exposure significantly reduced Abeta(25-35)-induced apoptosis not only by preventing Abeta aggregation, mitochondrial and ER morphological changes, mitochondrial membrane potential dissipation, ROS production, intracellular calcium elevation, and cytochrome c release, but also via reversing Bcl-2/Bax ratio and suppressing the activity of GADD153, Grp78/94, caspase-9, caspase-12, and caspase-3. All these data indicate that galantamine protects PC12 cells against Abeta(25-35)-induced apoptosis by preventing mitochondrial dysfunction and endoplasmic reticulum (ER) stress.

Tyk2/STAT3 signaling mediates beta-amyloid-induced neuronal cell death: implications in Alzheimer's disease

One of the pathological hallmarks of Alzheimer's disease (AD) is deposition of extracellular amyloid-beta (Abeta) peptide, which is generated from the cleavage of amyloid precursor protein (APP). Accumulation of Abeta is thought to associate with the progressive neuronal death observed in AD. However, the precise signaling mechanisms underlying the action of Abeta in AD pathophysiology are not completely understood. Here, we report the involvement of the transcription factor signal transducer and activator of transcription 3 (STAT3) in mediating Abeta-induced neuronal death. We find that tyrosine phosphorylation of STAT3 is elevated in the cortex and hippocampus of APP/PS1 transgenic mice. Treatment of cultured rat neurons with Abeta or intrahippocampal injection of mice with Abeta both induces tyrosine phosphorylation of STAT3 in neurons. Importantly, reduction of either the expression or activation of STAT3 markedly attenuates Abeta-induced neuronal apoptosis, suggesting that STAT3 activation contributes to neuronal death after Abeta exposure. We further identify Tyk2 as the tyrosine kinase that acts upstream of STAT3, as Abeta-induced activation of STAT3 and caspase-3-dependent neuronal death can be inhibited in tyk2(-/-) neurons. Finally, increased tyrosine phosphorylation of STAT3 is also observed in postmortem brains of AD patients. Our observations collectively reveal a novel role of STAT3 in Abeta-induced neuronal death and suggest the potential involvement of Tyk2/STAT3 signaling in AD pathophysiology.

Membrane biophysics and mechanics in Alzheimer's disease

Alzheimer's disease is a chronic neurodegenerative disorder characterized by neuronal loss, cerebrovascular inflammation, and accumulation of senile plaques in the brain parenchyma and cerebral blood vessels. Amyloid-beta peptide (Abeta), a major component of senile plaques, has been shown to exert multiple toxic effects to neurons, astrocytes, glial cells, and brain endothelium. Oligomeric Abeta can disturb the structure and function of cell membranes and alter membrane mechanical properties, such as membrane fluidity and molecular order. Much of these effects are attributed to their capability to trigger oxidative stress and inflammation. In this review, we discuss the effects of Abeta on neuronal cells, astrocytes, and cerebral endothelial cells with special emphasis on cell membrane properties and cell functions.

Calpain activates caspase-8 in neuron-like differentiated PC12 cells via the amyloid-beta-peptide and CD95 pathways

The neurotoxic amyloid-beta-peptide (Abeta) is important in the pathogenesis of Alzheimer's disease (AD). Calpain (Ca(2+)-dependent protease) and caspase-8 (the initiating caspase for the extrinsic, receptor-mediated apoptosis pathway) have been implicated in AD/Abeta toxicity. We previously found that Abeta promoted degradation of calpastatin (the specific endogenous calpain inhibitor); calpastatin degradation was prevented by inhibitors of either calpain or caspase-8. The results implied a cross-talk between the two proteases and suggested that one protease was responsible for the activity of the other one. We now report on the previously unrecognized caspase-8 activation by calpain. In neuron-like differentiated PC12 cells, calpain promotes active caspase-8 formation from procaspase-8 via the Abeta and CD95 pathways, along with degradation of the procaspase-8 processing inhibitor caspase-8 (FLICE)-like inhibitory protein, short isoform (FLIP(S)). Inhibition of calpain (by pharmacological inhibitors and by overexpression of calpastatin) prevents the cleavage of procaspase-8 to mature, active caspase-8, and inhibits FLIP(S) degradation in the Abeta-treated and CD95-triggered cells. Increased cellular Ca(2+) per se results in calpain activation but does not lead to caspase-8 activation or FLIP(S) degradation. The results suggest that procaspase-8 and FLIP(S) association with cell membrane receptor complexes is required for calpain-induced caspase-8 activation. The results presented here add to the understanding of the roles of calpain, caspase-8, and CD95 pathway in AD/Abeta toxicity. Calpain-promoted activation of caspase-8 may have implications for other types of CD95-induced cell damage, and for nonapoptotic functions of caspase-8. Inhibition of calpain may be useful for modulating certain caspase-8-dependent processes.

Calpastatin overexpression attenuates amyloid-beta-peptide toxicity in differentiated PC12 cells

Amyloid beta peptide (Abeta) plays a major role in the pathogenesis of Alzheimer's disease (AD). Abeta is toxic to neurons, possibly through causing initial synaptic dysfunction and neuronal membrane dystrophy, promoted by increased cellular Ca(2+). Calpain (Ca(2+)-dependent protease) and caspase have been implicated in AD. Previously, we used calpain and caspase pharmacological inhibitors to study effects of Abeta25-35 (sAbeta) on neuronal-like differentiated PC12 cells. We reported that sAbeta-treated cells exhibited calpain activation and protein degradation (due to both calpain and caspase-8). We have now found that overexpression of the calpain specific inhibitor calpastatin in differentiated PC12 cells significantly inhibited the sAbeta-induced calpain activation and decreased the protease activity. Calpastatin overexpression inhibited the sAbeta-promoted degradation of fodrin, protein kinase Cepsilon, beta-catenin (membrane structural proteins and proteins involved in signal transduction pathways), and prevented the sAbeta-induced alteration of neurite structure (manifested by varicosities). Overexpression of calpastatin also inhibited Ca(2+)-promoted calpain activation and protein degradation; this is consistent with the notion that the Abeta-induced increase in calpain activity results from a rise in cellular Ca(2+), provided the calpastatin level is not so high as to strongly inhibit calpain. Carrying out transfection without selection allowed the comparison in the same culture of calpastatin-overexpressing with non-overexpressing cells. In cultures transfected with green fluorescent protein (GFP)-calpastatin plasmid, calpastatin overexpression (indicated by GFP-labeling) led to inhibition in sAbeta-induced membrane propidium iodide (PI) permeability, whereas non-transfected, GFP-unlabeled cells exhibited PI permeability. Overall, the results demonstrate that the effects of Abeta-toxicity studied here were attenuated to a large extent by calpastatin overexpression, indicating that the protease calpain is involved in Abeta-toxicity (obviating a primary, direct role for caspases). Increased expression of calpastatin and/or decrease in calpain may serve as one of the means for ameliorating some of the early symptoms of AD.