Continuous Flow Synthesis of A2E Guided by Design of Experiments and High-Throughput Studies.

N-Retinylidene-N-retinylethanolamine (A2E) is the most studied lipid bisretinoid. It forms lipofuscin deposits in the retinal pigment epithelium (RPE), causing vision impairment and blindness in eye conditions, such as Stargardt's disease, cone-rod dystrophy, Best's macular dystrophy, and potentially age-related macular degeneration. Synthetic A2E is often used for inducing the accumulation of lipofuscins within the lysosomes of RPE cells in culture as an in vitro surrogate of retinal lipofuscin buildup, providing insights into the mechanisms of these eye conditions. Many reports describing the use of synthetic A2E employ material that has been prepared using a one-pot reaction of all-trans-retinal (ATR) and ethanolamine at room temperature for 48 h. We have revisited this synthesis by performing a design of experiments (DoE) and high-throughput experimentation workflow that was tailored to identify the most productive combination of the variables (temperature, solvent, and reagent equivalences) for optimization of A2E yield. Our DoE findings revealed that the interaction of ethanolamine with acetic acid and ATR was pivotal for the formation of A2E in high yield, indicating that imine formation is the critical step in the reaction. Armed with these results, we were able to optimize the method using a microfluidic reactor system before upscaling those conditions for continuous flow synthesis of A2E. This revised method enabled a more efficient production of material, from a reaction time of 48 h to a residence time of 33 min, with an accompanying yield improvement from 49 to 78%. Furthermore, we implemented a simple method to evaluate the quality of the A2E produced using optical spectroscopy and LC-MS characteristics to assure that the biological properties observed with A2E samples are not confounded by the presence of oxidized impurities that are commonly present in conventional A2E samples.

Lipofuscin causes atypical necroptosis through lysosomal membrane permeabilization.

Lipofuscin granules enclose mixtures of cross-linked proteins and lipids in proportions that depend on the tissue analyzed. Retinal lipofuscin is unique in that it contains mostly lipids with very little proteins. However, retinal lipofuscin also presents biological and physicochemical characteristics indistinguishable from conventional granules, including indigestibility, tendency to cause lysosome swelling that results in rupture or defective functions, and ability to trigger NLRP3 inflammation, a symptom of low-level disruption of lysosomes. In addition, like conventional lipofuscins, it appears as an autofluorescent pigment, considered toxic waste, and a biomarker of aging. Ocular lipofuscin accumulates in the retinal pigment epithelium (RPE), whereby it interferes with the support of the neuroretina. RPE cell death is the primary cause of blindness in the most prevalent incurable genetic and age-related human disorders, Stargardt disease and age-related macular degeneration (AMD), respectively. Although retinal lipofuscin is directly linked to the cell death of the RPE in Stargardt, the extent to which it contributes to AMD is a matter of debate. Nonetheless, the number of AMD clinical trials that target lipofuscin formation speaks for the potential relevance for AMD as well. Here, we show that retinal lipofuscin triggers an atypical necroptotic cascade, amenable to pharmacological intervention. This pathway is distinct from canonic necroptosis and is instead dependent on the destabilization of lysosomes. We also provide evidence that necroptosis is activated in aged human retinas with AMD. Overall, this cytotoxicity mechanism may offer therapeutic targets and markers for genetic and age-related diseases associated with lipofuscin buildups.

Correction to: Integrin-FAK signaling rapidly and potently promotes mitochondrial function through STAT3.

An amendment to this paper has been published and can be accessed via the original article.

Reduced FAK-STAT3 signaling contributes to ER stress-induced mitochondrial dysfunction and death in endothelial cells.

Excessive endoplasmic reticulum (ER) stress leads to cell loss in many diseases, e.g., contributing to endothelial cell loss after spinal cord injury. Here, we determined whether ER stress-induced mitochondrial dysfunction could be explained by interruption of the focal adhesion kinase (FAK)-mitochondrial STAT3 pathway we recently discovered. ER stress was induced in brain-derived mouse bEnd5 endothelial cells by thapsigargin or tunicamycin and caused apoptotic cell death over a 72h period. In concert, ER stress caused mitochondrial dysfunction as shown by reduced bioenergetic function, loss of mitochondrial membrane potential and increased mitophagy. ER stress caused a reduction in mitochondrial phosphorylated S727-STAT3, known to be important for maintaining mitochondrial function. Normal activation or phosphorylation of the upstream cytoplasmic FAK was also reduced, through mechanisms that involve tyrosine phosphatases and calcium signaling, as shown by pharmacological inhibitors, bisperoxovanadium (bpV) and 2-aminoethoxydiphenylborane (APB), respectively. APB mitigated the reduction in FAK and STAT3 phosphorylation, and improved endothelial cell survival caused by ER stress. Transfection of cells rendered null for STAT3 using CRISPR technology with STAT3 mutants confirmed the specific involvement of S727-STAT3 inhibition in ER stress-mediated cell loss. These data suggest that loss of FAK signaling during ER stress causes mitochondrial dysfunction by reducing the protective effects of mitochondrial STAT3, leading to endothelial cell death. We propose that stimulation of the FAK-STAT3 pathway is a novel therapeutic approach against pathological ER stress.

Integrin-FAK signaling rapidly and potently promotes mitochondrial function through STAT3.

BACKGROUND: STAT3 is increasingly becoming known for its non-transcriptional regulation of mitochondrial bioenergetic function upon activation of its S727 residue (S727-STAT3). Lengthy mitochondrial dysfunction can lead to cell death. We tested whether an integrin-FAK-STAT3 signaling pathway we recently discovered regulates mitochondrial function and cell survival, and treatments thereof. METHODS: Cultured mouse brain bEnd5 endothelial cells were treated with integrin, FAK or STAT3 inhibitors, FAK siRNA, as well as integrin and STAT3 activators. STAT3 null cells were transfected with mutant STAT3 plasmids. Outcome measures included oxygen consumption rate for mitochondrial bioenergetics, Western blotting for protein phosphorylation, mitochondrial membrane potential for mitochondrial integrity, ROS production, and cell counts. RESULTS: Vitronectin-dependent mitochondrial basal respiration, ATP production, and maximum reserve and respiratory capacities were suppressed within 4 h by RGD and αvß3 integrin antagonist peptides. Conversely, integrin ligands vitronectin, laminin and fibronectin stimulated mitochondrial function. Pharmacological inhibition of FAK completely abolished mitochondrial function within 4 h while FAK siRNA treatments confirmed the specificity of FAK signaling. WT, but not S727A functionally dead mutant STAT3, rescued bioenergetics in cells made null for STAT3 using CRISPR-Cas9. STAT3 inhibition with stattic in whole cells rapidly reduced mitochondrial function and mitochondrial pS727-STAT3. Stattic treatment of isolated mitochondria did not reduce pS727 whereas more was detected upon phosphatase inhibition. This suggests that S727-STAT3 is activated in the cytoplasm and is short-lived upon translocation to the mitochondria. FAK inhibition reduced pS727-STAT3 within mitochondria and reduced mitochondrial function in a non-transcriptional manner, as shown by co-treatment with actinomycin. Treatment with the small molecule bryostatin-1 or hepatocyte growth factor (HGF), which indirectly activate S727-STAT3, preserved mitochondrial function during FAK inhibition, but failed in the presence of the STAT3 inhibitor. FAK inhibition induced loss of mitochondrial membrane potential, which was counteracted by bryostatin, and increased superoxide and hydrogen peroxide production. Bryostatin and HGF reduced the substantial cell death caused by FAK inhibition over a 24 h period. CONCLUSION: These data suggest that extracellular matrix molecules promote STAT3-dependent mitochondrial function and cell survival through integrin-FAK signaling. We furthermore show a new treatment strategy for cell survival using S727-STAT3 activators.

Mild mitochondrial metabolic deficits by α-ketoglutarate dehydrogenase inhibition cause prominent changes in intracellular autophagic signaling: Potential role in the pathobiology of Alzheimer's disease.

Brain activities of the mitochondrial enzyme α-ketoglutarate dehydrogenase complex (KGDHC) are reduced in Alzheimer's disease and other age-related neurodegenerative disorders. The goal of the present study was to test the consequences of mild impairment of KGDHC on the structure, protein signaling and dynamics (mitophagy, fusion, fission, biogenesis) of the mitochondria. Inhibition of KGDHC reduced its in situ activity by 23-53% in human neuroblastoma SH-SY5Y cells, but neither altered the mitochondrial membrane potential nor the ATP levels at any tested time-points. The attenuated KGDHC activity increased translocation of dynamin-related protein-1 (Drp1) and microtubule-associated protein 1A/1B-light chain 3 (LC3) from the cytosol to the mitochondria, and promoted mitochondrial cytochrome c release. Inhibition of KGDHC also increased the negative surface charges (anionic phospholipids as assessed by Annexin V binding) on the mitochondria. Morphological assessments of the mitochondria revealed increased fission and mitophagy. Taken together, our results suggest the existence of the regulation of the mitochondrial dynamism including fission and fusion by the mitochondrial KGDHC activity via the involvement of the cytosolic and mitochondrial protein signaling molecules. A better understanding of the link among mild impairment of metabolism, induction of mitophagy/autophagy and altered protein signaling will help to identify new mechanisms of neurodegeneration and reveal potential new therapeutic approaches.

Abnormal Glucose Metabolism in Alzheimer's Disease: Relation to Autophagy/Mitophagy and Therapeutic Approaches.

Diminished glucose metabolism accompanies many neurodegenerative diseases including Alzheimer's disease. An understanding of the relation of these metabolic changes to the disease will enable development of novel therapeutic strategies. Following a metabolic challenge, cells generally conserve energy to preserve viability. This requires activation of many cellular repair/regenerative processes such as mitophagy/autophagy and fusion/fission. These responses may diminish cell function in the long term. Prolonged fission induces mitophagy/autophagy which promotes repair but if prolonged progresses to mitochondrial degradation. Abnormal glucose metabolism alters protein signaling including the release of proteins from the mitochondria or migration of proteins from the cytosol to the mitochondria or nucleus. This overview provides an insight into the different mechanisms of autophagy/mitophagy and mitochondrial dynamics in response to the diminished metabolism that occurs with diseases, especially neurodegenerative diseases such as Alzheimer's disease. The review discusses multiple aspects of mitochondrial responses including different signaling proteins and pathways of mitophagy and mitochondrial biogenesis. Improving cellular bioenergetics and mitochondrial dynamics will alter protein signaling and improve cellular/mitochondrial repair and regeneration. An understanding of these changes will suggest new therapeutic strategies.

Dopamine Cytotoxicity Involves Both Oxidative and Nonoxidative Pathways in SH-SY5Y Cells: Potential Role of Alpha-Synuclein Overexpression and Proteasomal Inhibition in the Etiopathogenesis of Parkinson's Disease.

Background. The cytotoxic effects of dopamine (DA) on several catecholaminergic cell lines involve DA oxidation products like reactive oxygen species (ROS) and toxic quinones and have implications in the pathogenesis of sporadic Parkinson's disease (PD). However, many molecular details are yet to be elucidated, and the possible nonoxidative mechanism of dopamine cytotoxicity has not been studied in great detail. Results. Cultured SH-SY5Y cells treated with DA (up to 400 µM) or lactacystin (5 µM) or DA (400 µM) plus N-acetylcysteine (NAC, 2.5 mM) for 24 h are processed accordingly to observe the cell viability, mitochondrial dysfunctions, oxidative stress parameters, proteasomal activity, expression of alpha-synuclein gene, and intracellular accumulation of the protein. DA causes mitochondrial dysfunction and extensive loss of cell viability partially inhibited by NAC, potent inhibition of proteasomal activity marginally prevented by NAC, and overexpression with accumulation of intracellular alpha-synuclein partially preventable by NAC. Under similar conditions of incubation, NAC completely prevents enhanced production of ROS and increased formation of quinoprotein adducts in DA-treated SH-SY5Y cells. Separately, proteasomal inhibitor lactacystin causes accumulation of alpha-synuclein as well as mitochondrial dysfunction and cell death. Conclusions. DA cytotoxicity includes both oxidative and nonoxidative modes and may involve overexpression and accumulation of alpha-synuclein as well as proteasomal inhibition.

Aging promotes amyloid-ß peptide induced mitochondrial dysfunctions in rat brain: a molecular link between aging and Alzheimer's disease.

The entangled relationship of brain aging, mitochondrial dysfunction, and amyloid-ß peptide (Aß42) toxicity occupies the center stage in the pathogenesis of Alzheimer's disease (AD). The present study examines some of the toxic effects of Aß42 on brain mitochondria and provides evidence that aged brain mitochondria are significantly more vulnerable to Aß42 toxicity. In particular, the study has shown that the aggregated, but not the monomeric, form of Aß42 in varying concentrations (10-40 µM) during in vitro incubation causes a loss of mitochondrial membrane potential, a decrease in phosphorylation capacity and ATP synthesis, and the release of cytochrome c from the mitochondria but without any noticeable change in the activities of respiratory chain complexes. Such effects of Aß42 are strikingly more conspicuous on aged rat (22-24 months) brain mitochondria compared to that on brain mitochondria of young rats (4-6 months). More interestingly is the observation that in contrast to young rat brain mitochondria, a significantly higher level of Aß42 remains associated with aged brain mitochondria under basal incubation condition as well as after exposure to exogenously added peptide. Extrapolated to an in vivo scenario, the results have clear implications in AD pathogenesis and also partly explain why brain aging is a dominant risk factor for this disease condition.

Mitochondrial dysfunction mediated by quinone oxidation products of dopamine: Implications in dopamine cytotoxicity and pathogenesis of Parkinson's disease.

The study has demonstrated that dopamine induces membrane depolarization and a loss of phosphorylation capacity in dose-dependent manner in isolated rat brain mitochondria during extended in vitro incubation and the phenomena are not prevented by oxyradical scavengers or metal chelators. Dopamine effects on brain mitochondria are, however, markedly prevented by reduced glutathione and N-acetyl cysteine and promoted by tyrosinase present in the incubation medium. The results imply that quinone oxidation products of dopamine are involved in mitochondrial damage under this condition. When PC12 cells are exposed to dopamine in varying concentrations (100-400µM) for up to 24h, a pronounced impairment of mitochondrial bio-energetic functions at several levels is observed along with a significant (nearly 40%) loss of cell viability with features of apoptotic nuclear changes and increased activities of caspase 3 and caspase 9 and all these effects of dopamine are remarkably prevented by N-acetyl cysteine. N-acetyl cysteine also blocks nearly completely the dopamine induced increase in reactive oxygen species production and the formation of quinoprotein adducts in mitochondrial fraction within PC12 cells and also the accumulation of quinone products in the culture medium. Clorgyline, an inhibitor of MAO-A, markedly decreases the formation of reactive oxygen species in PC12 cells upon dopamine exposure but has only mild protective actions against quinoprotein adduct formation, mitochondrial dysfunctions, cell death and caspase activation induced by dopamine. The results have indicated that quinone oxidation products and not reactive oxygen species are primarily involved in cytotoxic effects of dopamine and the mitochondrial impairment plays a central role in the latter process. The data have clear implications in the pathogenesis of Parkinson's disease.

Mitochondrial Dysfunction during Brain Aging: Role of Oxidative Stress and Modulation by Antioxidant Supplementation.

Mitochondrial dysfunction and oxidative stress are two interdependent and reinforcing damage mechanisms that play a central role in brain aging. Oxidative stress initiated and propagated by active oxyradicals and various other free radicals in the presence of catalytic metal ions not only can damage the phospholipid, protein and DNA molecules within the cell but can also modulate cell signalling pathways and gene expression pattern and all these processes may be of critical importance in the aging of brain. The present article describes the mechanism of formation of reactive oxyradicals within mitochondria and then explains how these can initiate mitochondrial biogenesis program and activate various transcriptional factors in the cytosol to boost up the antioxidative capacity of the mitochondria and the cell. However, a high level of oxidative stress finally inflicts critical damage to the oxidative phosphorylation machinery and mitochondrial DNA (mtDNA). The latter part of the article is a catalogue showing the accumulating evidence in favour of oxidative inactivation of mitochondrial functions in aged brain and the detailed reports of various studies with antioxidant supplementation claiming variable success in preventing the age-related brain mitochondrial decay and cognitive decline. The antioxidant supplementation approach may be of potential help in the management of neurodegenerative diseases like Alzheimer's disease. The newly developed mitochondria-targeted antioxidants have brought a new direction to experimental studies related to oxidative damage and they may provide potential drugs in near future for a variety of diseases or degenerative conditions including brain aging and neurodegenerative disorders.

Alpha-synuclein induced membrane depolarization and loss of phosphorylation capacity of isolated rat brain mitochondria: implications in Parkinson's disease.

This study demonstrates that in vitro incubation of isolated rat brain mitochondria with recombinant human alpha-synuclein leads to dose-dependent loss of mitochondrial transmembrane potential and phosphorylation capacity. However, alpha-synuclein does not seem to have any significant effect on the activities of respiratory chain complexes under similar conditions of incubation suggesting that the former may impair mitochondrial bioenergetics by direct effect on mitochondrial membranes. Moreover, the recombinant wild type alpha-synuclein and different mutant forms (A30P, A53T and E46K) have essentially similar effects on rat brain isolated mitochondria. The results are significant in view of the fact that alpha-synucleinopathy is involved in the pathogenesis of Parkinson's disease.

Quinone and oxyradical scavenging properties of N-acetylcysteine prevent dopamine mediated inhibition of Na+, K+-ATPase and mitochondrial electron transport chain activity in rat brain: implications in the neuroprotective therapy of Parkinson's disease.

Dopamine oxidation products such as H2O2 and reactive quinones have been held responsible for various toxic actions of dopamine, which have implications in the aetiopathogenesis of Parkinson's disease. This study has shown that N-acetylcysteine (0.25-1 mm) is a potent scavenger of both H2O2 and toxic quinones derived from dopamine and it further prevents dopamine mediated inhibition of Na+,K+-ATPase activity and mitochondrial respiratory chain function. The quinone scavenging ability of N-acetylcysteine is presumably related to its protective effect against dopamine mediated inhibition of mitochondrial respiratory chain activity. However, both H2O2 scavenging and quinone scavenging properties of N-acetylcysteine probably account for its protective effect against Na+,K+-ATPase inhibition induced by dopamine. The results have important implications in the neuroprotective therapy of sporadic Parkinson's disease since inactivation of mitochondrial respiratory activity and Na+,K+-ATPase may trigger intracellular damage pathways leading to the death of nigral dopaminergic neurons.

Dopamine but not 3,4-dihydroxy phenylacetic acid (DOPAC) inhibits brain respiratory chain activity by autoxidation and mitochondria catalyzed oxidation to quinone products: implications in Parkinson's disease.

This study reveals that, in contrast to dopamine (DA), 3,4 dihydroxyphenylacetic acid (DOPAC) during in vitro incubation up to 2 h causes only marginal inhibition of rat brain mitochondrial respiratory chain activity, a minimal loss of protein free thiols and very little quinoprotein adduct formation. The damaging effects of DA on brain mitochondria are, however, conspicuous and apparently mediated by quinone oxidation products generated by autoxidation of DA as well as catalyzed by a mitochondrial activity, inhibitable by clorgyline (2.5-10 microM) and cyanide (1 mM).