Analysis of the cytoprotective role of α-crystallins in cell survival and implication of the αA-crystallin C-terminal extension domain in preventing Bax-induced apoptosis

Therapeutic Effects of Metformin on Central Nervous System Diseases: A Focus on Protection of Neurovascular Unit

Metformin is one of the most commonly used oral hypoglycemic drugs in clinical practice, with unique roles in neurodegeneration and vascular lesions. Neurodegeneration and vasculopathy coexist in many diseases and typically affect the neurovascular unit (NVU), a minimal structural and functional unit in the central nervous system. Its components interact with one another and are indispensable for maintaining tissue homeostasis. This review focuses on retinal (diabetic retinopathy, retinitis pigmentosa) and cerebral (ischemic stroke, Alzheimer's disease) diseases to explore the effects of metformin on the NVU. Metformin has a preliminarily confirmed therapeutic effect on the retinal NUV, affecting many of its components, such as photoreceptors (cones and rods), microglia, ganglion, Müller, and vascular endothelial cells. Since it rapidly penetrates the blood-brain barrier (BBB) and accumulates in the brain, metformin also has an extensively studied neuronal protective effect in neuronal diseases. Its mechanism affects various NVU components, including pericytes, astrocytes, microglia, and vascular endothelial cells, mainly serving to protect the BBB. Regulating the inflammatory response in NVU (especially neurons and microglia) may be the main mechanism of metformin in improving central nervous system related diseases. Metformin may be a potential drug for treating diseases associated with NVU deterioration, however, more trials are needed to validate its timing, duration, dose, clinical effects, and side effects.

HSPB4/CRYAA Protect Photoreceptors during Retinal Detachment in Part through FAIM2 Regulation

Our previous study discussed crystallin family induction in an experimental rat model of retinal detachment. Therefore, we attempted to evaluate the role of α-crystallin in photoreceptor survival in an experimental model of retinal detachment, as well as its association with the intrinsically neuroprotective protein Fas-apoptotic inhibitory molecule 2 (FAIM2). Separation of retina and RPE was induced in rat and mouse eyes by subretinal injection of hyaluronic acid. Retinas were subsequently analyzed for the presence αA-crystallin (HSPB4) and αB-crystallin (HSPB5) proteins using immunohistochemistry and immunoblotting. Photoreceptor death was analyzed using terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling (TUNEL) staining and cell counts. The 661W cells subjected to FasL were used as a cell model of photoreceptor degeneration to assess the mechanisms of the protective effect of αA-crystallin and its dependence on its phosphorylation on T148. We further evaluated the interaction between FAIM2 and αA-crystallin using a co-immunoprecipitation assay. Our results showed that α-crystallin protein levels were rapidly induced in response to retinal detachment, with αA-crystallin playing a particularly important role in protecting photoreceptors during retinal detachment. Our data also show that the photoreceptor intrinsically neuroprotective protein FAIM2 is induced and interacts with α-crystallins following retinal detachment. Mechanistically, our work also demonstrated that the phosphorylation of αA-crystallin is important for the interaction of αA-crystallin with FAIM2 and their neuroprotective effect. Thus, αA-crystallin is involved in the regulation of photoreceptor survival during retinal detachment, playing a key role in the stabilization of FAIM2, serving as an important modulator of photoreceptor cell survival under chronic stress conditions.

Proteostatic Remodeling of Small Heat Shock Chaperones─Crystallins by Ran-Binding Protein 2─and the Peptidyl-Prolyl cis-trans Isomerase and Chaperone Activities of Its Cyclophilin Domain

Disturbances in protein phase transitions promote protein aggregation─a neurodegeneration hallmark. The modular Ran-binding protein 2 (Ranbp2) is a cytosolic molecular hub for rate-limiting steps of phase transitions of Ran-GTP-bound protein ensembles exiting nuclear pores. Chaperones also regulate phase transitions and proteostasis by suppressing protein aggregation. Ranbp2 haploinsufficiency promotes the age-dependent neuroprotection of the chorioretina against phototoxicity by proteostatic regulations of neuroprotective substrates of Ranbp2 and by suppressing the buildup of polyubiquitylated substrates. Losses of peptidyl-prolyl cis-trans isomerase (PPIase) and chaperone activities of the cyclophilin domain (CY) of Ranbp2 recapitulate molecular effects of Ranbp2 haploinsufficiency. These CY impairments also stimulate deubiquitylation activities and phase transitions of 19S cap subunits of the 26S proteasome that associates with Ranbp2. However, links between CY moonlighting activity, substrate ubiquitylation, and proteostasis remain incomplete. Here, we reveal the Ranbp2 regulation of small heat shock chaperones─crystallins in the chorioretina by proteomics of mice with total or selective modular deficits of Ranbp2. Specifically, loss of CY PPIase of Ranbp2 upregulates αA-Crystallin, which is repressed in adult nonlenticular tissues. Conversely, impairment of CY's chaperone activity opposite to the PPIase pocket downregulates a subset of αA-Crystallin's substrates, γ-crystallins. These CY-dependent effects cause age-dependent and chorioretinal-selective declines of ubiquitylated substrates without affecting the chorioretinal morphology. A model emerges whereby inhibition of Ranbp2's CY PPIase remodels crystallins' expressions, subdues molecular aging, and preordains the chorioretina to neuroprotection by augmenting the chaperone capacity and the degradation of polyubiquitylated substrates against proteostatic impairments. Further, the druggable Ranbp2 CY holds pan-therapeutic potential against proteotoxicity and neurodegeneration.

Proteostatic remodeling of small heat shock chaperones - crystallins by Ran-binding protein 2 and the peptidyl-prolyl cis-trans isomerase and chaperone activities of its cyclophilin domain

Disturbances in phase transitions and intracellular partitions of nucleocytoplasmic shuttling substrates promote protein aggregation - a hallmark of neurodegenerative diseases. The modular Ran-binding protein 2 (Ranbp2) is a cytosolic molecular hub for rate-limiting steps of disassembly and phase transitions of Ran-GTP-bound protein ensembles exiting nuclear pores. Chaperones also play central roles in phase transitions and proteostasis by suppressing protein aggregation. Ranbp2 haploinsufficiency promotes the age-dependent neuroprotection of the chorioretina against photo-oxidative stress by proteostatic regulations of Ranbp2 substrates and by countering the build-up of poly-ubiquitylated substrates. Further, the peptidyl-prolyl cis-trans isomerase (PPIase) and chaperone activities of the cyclophilin domain (CY) of Ranbp2 modulate the proteostasis of selective neuroprotective substrates, such as hnRNPA2B1, STAT3, HDAC4 or L/M-opsin, while promoting a decline of ubiquitylated substrates. However, links between CY PPIase activity on client substrates and its effect(s) on ubiquitylated substrates are unclear. Here, proteomics of genetically modified mice with deficits of Ranbp2 uncovered the regulation of the small heat shock chaperones - crystallins by Ranbp2 in the chorioretina. Loss of CY PPIase of Ranbp2 up-regulates αA-crystallin proteostasis, which is repressed in non-lenticular tissues. Conversely, the αA-crystallin's substrates, γ-crystallins, are down-regulated by impairment of CY's C-terminal chaperone activity. These CY-dependent effects cause the age-dependent decline of ubiquitylated substrates without overt chorioretinal morphological changes. A model emerges whereby the Ranbp2 CY-dependent remodeling of crystallins' proteostasis subdues molecular aging and preordains chorioretinal neuroprotection by augmenting the chaperone buffering capacity and the decline of ubiquitylated substrates against proteostatic impairments. Further, CY's moonlighting activity holds pan -therapeutic potential against neurodegeneration.

αA-Crystallin Mediated Neuroprotection in the Retinal Neurons Is Independent of Protein Kinase B

Phosphatidylinositol 3-kinase (PI3K)/Akt signal pathway mediates pro-survival function in neurons. In the retina, PI3K/AKT/mTOR signaling pathway is related to the early pathogenesis of diabetic retinopathy. Signaling molecules in the membrane-initiated signaling pathway exhibiting neuroprotective function interacts with the PI3K/Akt pathway as an important survival pathway. Molecular chaperone α-crystallins are known to potentially interact and/or regulate various pro-survival and pro-apoptotic proteins to regulate cell survival. Among these demonstrated mechanisms, they are well-reported to regulate and inhibit apoptosis by interacting and sequestrating the proapoptotic proteins such as Bax and Bcl-Xs. We studied the importance of metabolic stress-induced enhanced Akt signaling and αA-crystallin interdependence for exhibiting neuroprotection in metabolically challenged retinal neurons. For the first time, this study has revealed that αA-crystallin and activated Akt are significantly neuroprotective in the stressed retinal neurons, independent of each other. Furthermore, the study also highlighted that significant inhibition of the PI3K-Akt pathway does not alter the neuroprotective ability of αA-crystallin in stressed retinal neurons. Interestingly, our study also demonstrated that in the absence of Akt activation, αA-crystallin inhibits the translocation of Bax in the mitochondria during metabolic stress, and this function is regulated by the phosphorylation of αA-crystallin on residue 148.

MLL5 is involved in retinal photoreceptor maturation through facilitating CRX-mediated photoreceptor gene transactivation

Histone methylation, particularly at the H3K4 position, is thought to contribute to the specification of photoreceptor cell fate; however, the mechanisms linking histone methylation with transcription factor transactivation and photoreceptor gene expression have not yet been determined. Here, we demonstrate that MLL5 is abundantly expressed in the mouse retina. Mll5 deficiency impaired electroretinogram responses, alongside attenuated expression of a number of retina genes. Mechanistic studies revealed that MLL5 interacts with the retina-specific transcription factor, CRX, contributing to its binding to photoreceptor-specific gene promoters. Moreover, depletion of MLL5 impairs H3K4 methylation and H3K79 methylation, which subsequently compromises CRX-CBP assembly and H3 acetylation on photoreceptor promoters. Our data support a scenario in which recognition of H3K4 methylation by MLL5 is required for photoreceptor-specific gene transcription through maintaining a permissive chromatin state and proper CRX-CBP recruitment at promoter sites.

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MLL5 is essential for the expression of critical photoreceptor genes

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MLL5 depletion reduces H3K4/K79 methylation at photoreceptor gene promoters

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MLL5 interacts with CRX via its CD4 domain

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Recognition of H3K4me2/3 by MLL5 is a prerequisite for CRX recruitment to chromatin

HspB5 protects mouse neural stem/progenitor cells from paraquat toxicity

INTRODUCTION

HspB5 (αB-crystallin) is known to be involved in a variety of cellular functions, including, protection of cells from oxidative damage and inhibiting apoptosis. Neural stem/progenitor cells (NSPCs) have significant therapeutic value, especially in the NSC/NPC transplantation therapy. However, the viability of the transplanted NSPCs remains low because of various factors, including oxidative stress.

OBJECTIVE

The current investigation explored the possible role of HspB5 in the protection of mouse NSPCs (mNSPCs) against paraquat-induced toxicity.

METHODS

The recombinant human HspB5 was expressed in E.coli and was purified using gel filtration and Ion-exchange chromatography. The biophysical characterization of HspB5 was carried out using DLS, CD, and Analytical Ultracentrifugation (SV); the chaperone activity of HspB5 was determined by alcohol dehydrogenase aggregation assay. We have subjected the mNSPCs to paraquat-induced oxidative stress and monitored the protective ability of HspB5 by MTT assay and Hoechst-PI staining. Furthermore, increase in the expression of the anti-apoptotic protein, procaspase-3 was monitored using western blotting.

RESULTS

The recombinant HspB5 was purified to its homogeneity and was characterized using various biophysical techniques. The externally added FITC-labeled HspB5 was found to be localized within the cytoplasm of mNSPCs. Our Immunocytochemistry results showed that the externally added FITC-labeled HspB5 not only entered the cells but also conferred cytoprotection against paraquat-induced toxicity. The protective events were monitored by a decrease in the PI-positive cells and an increase in the procaspase-3 expression through Immunocytochemistry and Western blotting respectively.

CONCLUSION

Our results clearly demonstrate that exogenously added recombinant human HspB5 enters the mNSPCs and confers protection against paraquat toxicity.

The Brain Microenvironment Induces DNMT1 Suppression and Indolence of Metastatic Cancer Cells

Brain metastasis is an ineffective process, and many cancer cells enter into an indolent state following extravasation in the brain. Single cell RNA sequencing of melanoma brain metastases reveals that non-proliferating brain metastatic melanoma cells exhibit a pattern of gene expression associated with inhibition of DNA methyltransferase 1 (DNMT1). The brain microenvironment, specifically the combination of reactive astrocytes and mechanically soft surroundings, suppressed DNMT1 expression in various cancer types and caused cell cycle delay. Somewhat unexpectedly, we find that DNMT1 suppression not only induces cell cycle delay but also activates pro-survival signals in brain metastatic cancer cells, including L1CAM and CRYAB. Our results demonstrate that transcriptional changes triggered by DNMT1 suppression is a key step for cancer cells to survive in the brain microenvironment and that they also restrict cancer cell proliferation. The dual consequences of DNMT1 suppression can explain the persistence of indolent cancer cells in the brain microenvironment.

Expanding the phenotype of CRYAA nucleotide variants to a complex presentation of anterior segment dysgenesis

Background

Mutations in CRYAA, which encodes the α-crystallin protein, are associated with a spectrum of congenital cataract–microcornea syndromes.

Results

In this study, we performed clinical examination and subsequent genetic analysis in two unrelated sporadic cases of different geographical origins presenting with a complex phenotype of ocular malformation. Both cases manifested bilateral microphthalmia and severe anterior segment dysgenesis, primarily characterized by congenital aphakia, microcornea, and iris hypoplasia/aniridia. NGS-based analysis revealed two novel single nucleotide variants occurring de novo and affecting the translation termination codon of the CRYAA gene, c.520T > C and c.521A > C. Both variants are predicted to elongate the C-terminal protein domain by one-third of the original length.

Conclusions

Our report not only expands the mutational spectrum of CRYAA but also identifies the genetic cause of the unusual ocular phenotype described in this report.

Functional Rescue of Cataract-Causing αA-G98R-Crystallin by Targeted Compensatory Suppressor Mutations in Human αA-Crystallin

The G98R mutation in αA-crystallin is associated with the onset of presenile cataract and is characterized biochemically by an increased oligomeric mass, altered chaperone function, and loss of structural stability over time. Thus, far, it is not known whether the inherent instability caused by gain-of-charge mutation could be rescued by a compensatory loss of charge mutation elsewhere on the protein. To answer this question, we investigated whether αA-G98R-mediated instability could be rescued through suppressor mutations by introducing site-specific "compensatory" mutations in αA-G98R-crystallin, αA-R21Q/G98R, αA-G98R/R116C, and αA-R157Q/G98R. The recombinant proteins were expressed, purified, characterized, and evaluated by circular dichroism (CD), intrinsic fluorescence, and bis-ANS-binding studies. Chaperone-like activities of recombinant proteins were assessed using alcohol dehydrogenase (ADH) and insulin as unfolding substrates. Far-UV CD studies revealed an increased α-helical content in αA-G98R in comparison to αA-WT, αA-R21Q, R157Q, and the double mutants, αA-R21Q/G98R, and αA-R157Q/G98R. Compared to αA-WT, αA-R21Q, and αA-G98R, the double mutants showed an increased intrinsic tryptophan fluorescence, whereas the highest hydrophobicity (bis-ANS-binding) was shown by αA-G98R. Introduction of a second mutation in αA-G98R reduced its bis-ANS-binding activity. Both αA-R21Q/G98R and αA-R157Q/G98R showed greater chaperone-like activity against ADH aggregation than αA-G98R. However, among the three G98R mutants, only αA-R21Q/G98R protected ARPE-19 cells from H₂O₂-induced cytotoxicity. These results suggest that the lost chaperone-like activity of αA-G98R-crystallin can be rescued by another targeted mutation and that substitution of αA-R21Q-crystallin at the N-terminal region can rescue a deleterious mutation in the conserved α-crystallin domain of the protein.

Rescue of Retinal Degeneration in rd1 Mice by Intravitreally Injected Metformin

Retinitis pigmentosa (RP) is a progressive hereditary retinal degenerative disease in which photoreceptor cells undergo degeneration and apoptosis, eventually resulting in irreversible loss of visual function. Currently, no effective treatment exists for this disease. Neuroprotection and inflammation suppression have been reported to delay the development of RP. Metformin is a well-tested drug used to treat type 2 diabetes, and it has been reported to exert beneficial effects in neurodegenerative diseases, such as Parkinson’s disease and Alzheimer’s disease. In the present study, we used immunofluorescence staining, electroretinogram (ERG) recordings and RNA-Seq to explore the effects of metformin on photoreceptor degeneration and its mechanism in rd1 mice. We found that metformin significantly reduced apoptosis in photoreceptors and delayed the degeneration of photoreceptors and rod bipolar cells in rd1 mice, thus markedly improving the visual function of rd1 mice at P14, P18, and P22 when tested with a light/dark transition test and ERG. Microglial activation in the outer nuclear layer (ONL) of the retina of rd1 mice was significantly suppressed by metformin. RNA-Seq showed that metformin markedly downregulated inflammatory genes and upregulated the expression of crystallin proteins, which have been demonstrated to be important neuroprotective molecules in the retina, revealing the therapeutic potential of metformin for RP treatment. αA-crystallin proteins were further confirmed to be involved in the neuroprotective effects of metformin in a Ca²⁺ ionophore-damaged 661W photoreceptor-like cell line. These data suggest that metformin exerts a protective effect in rd1 mice via both immunoregulatory and new neuroprotective mechanisms.

Characterization of an N-terminal mutant of αA-crystallin αA-R21Q associated with congenital cataract

Several mutations associated with congenital cataracts in human beings target conserved arginine residues in αA-crystallin. The N-terminal region of αA-crystallin is a "mutational hotspot," with multiple cataract-related mutations reported in this region. Two mutations at arginine 21 in the N-terminal domain of αA-crystallin - αA-R21L and αA-R21W have been associated with congenital cataract. A third mutant of R21, αA-R21Q, was recently identified to be associated with congenital cataract in a South Australian family. The point mutation was reported to compromise the quaternary structure of αA-crystallin by preventing its assembly into higher ordered oligomers. To assess the effect of the αA-R21Q mutation on αA-crystallin function, recombinant αA-R21Q was expressed, purified and characterized in vitro. Compared to wild-type αA-crystallin, the recombinant αA-R21Q exhibits enhanced chaperone-like activity, increased surface hydrophobicity, lesser stability in urea and increased susceptibility to digestion by trypsin. αA-R21Q demonstrated increased binding affinity towards unfolding ADH and bovine lens fiber cell membranes. αA-R21Q homo-oligomers and hetero-oligomers also prevented H₂O₂-induced apoptosis in ARPE-19 cells. Taken together, αA-R21Q exhibited a gain of function despite subtle structural differences as compared to wild-type αA-crystallin. This study further validates the involvement of arginine 21 in regulating αA-crystallin structure and function.

The small heat shock proteins, especially HspB4 and HspB5 are promising protectants in neurodegenerative diseases

Small heat shock proteins (sHsps) are a group of proteins with molecular mass between 12 and 43 kDa. Currently, 11 members of this family have been classified, namely HspB1 to HspB11. HspB1, HspB2, HspB5, HspB6, HspB7, and HspB8, which are expressed in brain have been observed to be related to the pathology of neurodegenerative diseases, including Parkinson's, Alzheimer's, Alexander's disease, multiple sclerosis, and human immunodeficiency virus-associated dementia. Specifically, sHsps interact with misfolding and damaging protein aggregates, like Glial fibrillary acidic protein in AxD, β-amyloid peptides aggregates in Alzheimer's disease, Superoxide dismutase 1 in Amyotrophic lateral sclerosis and cytosine-adenine-guanine/polyglutamine (CAG/PolyQ) in Huntington's disease, Spinocerebellar ataxia type 3, Spinal-bulbar muscular atrophy, to reduce the toxicity or increase the clearance of these protein aggregates. The degree of HspB4 expression in brain is still debated. For neuroprotective mechanisms, sHsps attenuate mitochondrial dysfunctions, reduce accumulation of misfolded proteins, block oxidative/nitrosative stress, and minimize neuronal apoptosis and neuroinflammation, which are molecular mechanisms commonly accepted to mirror the progression and development of neurodegenerative diseases. The increasing incidence of the neurodegenerative diseases enhanced search for effective approaches to rescue neural tissue from degeneration with minimal side effects. sHsps have been found to exert neuroprotective functions. HspB5 has been emphasized to reduce the paralysis in a mouse model of experimental autoimmune encephalomyelitis, providing a therapeutic basis for the disease. In this review, we discuss the current understanding of the properties and the mechanisms of protection orchestrated by sHsps in the nervous system, highlighting the promising therapeutic role of sHsps in neurodegenerative diseases.

Two novel mutations identified in ADCC families impair crystallin protein distribution and induce apoptosis in human lens epithelial cells

Congenital cataract (CC) is a clinical and genetically heterogeneous eye disease that primarily causes lens disorder and even amblyopic blindness in children. As the mechanism underlying CC is genetically inherited, identification of CC-associated gene mutations and their role in protein distribution are topics of both pharmacological and biological research. Through physical and ophthalmic examinations, two Chinese pedigrees with autosomal dominant congenital cataract (ADCC) were recruited for this study. Mutation analyses of CC candidate genes by next-generation sequencing (NGS) and Sanger sequencing revealed a novel missense mutation in CRYBB2 (p.V146L) and a deletion mutation in CRYAA (p.116_118del). Both mutations fully co-segregated were not observed in unaffected family members or in 100 unrelated healthy controls. The CRYBB2 missense mutation disrupts the distribution of CRYBB2 in human lens epithelial cells (HLEpiCs), and the CRYAA deletion mutation causes hyperdispersion of CRYAA. Furthermore, these two crystallin mutations result in aberrant expression of unfolded protein response (UPR) marker genes as well as apoptosis in HLEpiCs. Collectively, these findings broaden the genetic spectrum of ADCC.

Heat shock factor 1 induces crystallin-αB to protect against cisplatin nephrotoxicity

Cisplatin, a wildly used chemotherapy drug, induces nephrotoxicity that is characterized by renal tubular cell apoptosis. In response to toxicity, tubular cells can activate cytoprotective mechanisms, such as the heat shock response. However, the role and regulation of the heat shock response in cisplatin-induced nephrotoxicity remain largely unclear. In the present study, we demonstrated the induction of heat shock factor (Hsf)1 and the small heat shock protein crystallin-αB (CryAB) during cisplatin nephrotoxicity in mice. Consistently, cisplatin induced Hsf1 and CryAB in a cultured renal proximal tubular cells (RPTCs). RPTCs underwent apoptosis during cisplatin treatment, which was increased when Hsf1 was knocked down. Transfection or restoration of Hsf1 into Hsf1 knockdown cells suppressed cisplatin-induced apoptosis, further supporting a cytoprotective role of Hsf1 and its associated heat shock response. Moreover, Hsf1 knockdown increased Bax translocation to mitochondria and cytochrome c release into the cytosol. In RPTCs, Hsf1 knockdown led to a specific downregulation of CryAB. Transfection of CryAB into Hsf1 knockdown cells diminished their sensitivity to cisplatin-induced apoptosis, suggesting that CryAB may be a key mediator of the cytoprotective effect of Hsf1. Taken together, these results demonstrate a heat shock response in cisplatin nephrotoxicity that is mediated by Hsf1 and CryAB to protect tubular cells against apoptosis.

Biological Characterization of Gene Response to Insulin-Induced Hypoglycemia in Mouse Retina

Glucose is the most important metabolic substrate of the retina and maintenance of normoglycemia is an essential challenge for diabetic patients. Chronic, exaggerated, glycemic excursions could lead to cardiovascular diseases, nephropathy, neuropathy and retinopathy. We recently showed that hypoglycemia induced retinal cell death in mouse via caspase 3 activation and glutathione (GSH) decrease. Ex vivo experiments in 661W photoreceptor cells confirmed the low-glucose induction of death via superoxide production and activation of caspase 3, which was concomitant with a decrease of GSH content. We evaluate herein retinal gene expression 4 h and 48 h after insulin-induced hypoglycemia. Microarray analysis demonstrated clusters of genes whose expression was modified by hypoglycemia and we discuss the potential implication of those genes in retinal cell death. In addition, we identify by gene set enrichment analysis, three important pathways, including lysosomal function, GSH metabolism and apoptotic pathways. Then we tested the effect of recurrent hypoglycemia (three successive 4h periods of hypoglycemia spaced by 48 h recovery) on retinal cell death. Interestingly, exposure to multiple hypoglycemic events prevented GSH decrease and retinal cell death, or adapted the retina to external stress by restoring GSH level comparable to control situation. We hypothesize that scavenger GSH is a key compound in this apoptotic process, and maintaining "normal" GSH level, as well as a strict glycemic control, represents a therapeutic challenge in order to avoid side effects of diabetes, especially diabetic retinopathy.

αB-crystallin: Portrait of a malignant chaperone as a cancer therapeutic target

αB-crystallin is a widely expressed member of the small heat shock protein family that protects cells from stress by its dual function as a molecular chaperone to preserve proteostasis and as a cell death antagonist that negatively regulates components of the conserved apoptotic cell death machinery. Deregulated expression of αB-crystallin occurs in a broad array of solid tumors and has been linked to tumor progression and poor clinical outcomes. This review will focus on new insights into the molecular mechanisms by which oncogenes, oxidative stress, matrix detachment and other tumor microenvironmental stressors deregulate αB-crystallin expression. We will also review accumulating evidence pointing to an essential role for αB-crystallin in the multi-step metastatic cascade whereby tumor cells colonize distant organs by circumventing a multitude of barriers to cell migration and survival. Finally, we will evaluate emerging strategies to therapeutically target αB-crystallin and/or interacting proteins to selectively activate apoptosis and/or derail the metastatic cascade in an effort to improve outcomes for patients with metastatic disease.

Exposure to the complement C5b-9 complex sensitizes 661W photoreceptor cells to both apoptosis and necroptosis

The loss of photoreceptors is the defining characteristic of many retinal degenerative diseases, but the mechanisms that regulate photoreceptor cell death are not fully understood. Here we have used the 661W cone photoreceptor cell line to ask whether exposure to the terminal complement complex C5b-9 induces cell death and/or modulates the sensitivity of these cells to other cellular stressors. 661W cone photoreceptors were exposed to complete normal human serum following antibody blockade of CD59. Apoptosis induction was assessed morphologically, by flow cytometry, and on western blotting by probing for cleaved PARP and activated caspase-3. Necroptosis was assessed by flow cytometry and Sirtuin 2 inhibition using 2-cyano-3-[5-(2,5-dichlorophenyl)-2-furyl]-N-5-quinolinylacrylamide (AGK2). The sensitivity of 661W cells to ionomycin, staurosporine, peroxide and chelerythrine was also investigated, with or without prior formation of C5b-9. 661W cells underwent apoptotic cell death following exposure to C5b-9, as judged by poly(ADP-ribose) polymerase 1 cleavage and activation of caspase-3. We also observed apoptotic cell death in response to staurosporine, but 661W cells were resistant to both ionomycin and peroxide. Interestingly, C5b-9 significantly increased 661W sensitivity to staurosporine-induced apoptosis and necroptosis. These studies show that low levels of C5b-9 on 661W cells can induce apoptosis, and that C5b-9 specifically sensitizes 661W cells to certain apoptotic and necroptotic pathways. Our observations provide new insight into the potential role of the complement system in photoreceptor loss, with implications for the molecular aetiology of retinal disease.

Crystallins and neuroinflammation: The glial side of the story

BACKGROUND

There is an abundance of evidence to support the association of damaging neuroinflammation and neurodegeneration across a multitude of diseases. One of the links between these pathological phenomena is the role of chaperone proteins as both neuroprotective and immune-regulatory agents.

SCOPE OF REVIEW

Chaperone proteins are highly expressed at sites of neuroinflammation both in glial cells and in the injured neurons that initiate the immune response. For this reason, the use of chaperones as treatment for various diseases associated with neuroinflammation is a highly active area of investigation. This review explores the various ways that the small heat shock protein chaperones, α-crystallins, can affect glial cell function with a specific focus on their implication in the inflammatory response associated with neurodegenerative disorders, and their potential as therapeutic treatment.

MAJOR CONCLUSIONS

Although the mechanisms are still under investigation, a clear link has now been established between alpha-crystallins and neuroinflammation, especially through their roles in microglial and macroglial cells. Interestingly, similar to inflammation in itself, crystallins can have a beneficial or detrimental impact on the CNS based on the context and duration of the condition.

GENERAL SIGNIFICANCE

Overall this review points out the novel roles that chaperones such as alpha-crystallins can play outside of the classical protein folding pathways, and their potential in the development of new therapies for the treatment of neuroinflammatory/neurodegenerative diseases. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

GNE Myopathy and Cell Apoptosis: A Comparative Mutation Analysis

In a number of genetic disorders such as GNE myopathy, it is not clear how mutations in target genes result in disease phenotype. GNE myopathy is a progressive neuro-degenerative disorder associated with homozygous or compound heterozygous missense mutations in either epimerase or kinase domain of UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE). This bifunctional enzyme catalyses the rate limiting step in sialic acid biosynthesis. Many mechanisms have been suggested as possible cause of muscle degeneration. These include hyposialylation of critical proteins, defects in cytoskeletal network, sarcomere organization and apoptosis. In order to elucidate the role of GNE in cell apoptosis, we have used HEK cell-based model system overexpressing pathologically relevant GNE mutations. These cells display a reduction in the levels of sialic acid-bound glycoconjugates. These mutants GNE overexpressing cells have defect in cell proliferation as compared to vector or wild-type GNE (wtGNE) controls. Moreover, effect of different GNE mutations on cell apoptosis was also observed using staining with annexin V-FITC and TUNEL assay. The downstream apoptosis signalling pathway involving activation of caspases and increased PARP cleavage were observed in all GNE mutant cell lines. In addition, morpho-structural changes in mitochondria in cells overexpressing different GNE mutants were noticed by transmission electron microscopy, and mitochondrial transmembrane potential was found to be altered in absence of functional GNE. Our results clearly indicate role of GNE in mitochondria-dependent cell apoptosis and provide insights into the pathomechanism of GNE myopathy.

Therapeutic potential of α-crystallin

BACKGROUND

The findings that α-crystallins are multi-functional proteins with diverse biological functions have generated considerable interest in understanding their role in health and disease. Recent studies have shown that chaperone peptides of α-crystallin could be delivered into cultured cells and in experimental animals with beneficial effects against protein aggregation, oxidation, inflammation and apoptosis.

SCOPE OF REVIEW

In this review, we will summarize the latest developments on the therapeutic potential of α-crystallins and their functional peptides.

MAJOR CONCLUSIONS

α-Crystallins and their functional peptides have shown significant favorable effects against several diseases. Their targeted delivery to tissues would be of great therapeutic benefit. However, α-crystallins can also function as disease-causing proteins. These seemingly contradictory functions must be carefully considered prior to their therapeutic use.

GENERAL SIGNIFICANCE

αA and αB-Crystallin are members of the small heat shock protein family. These proteins exhibit molecular chaperone and anti-apoptotic activities. The core crystallin domain within these proteins is largely responsible for these prosperities. Recent studies have identified peptides within the crystallin domain of both α- and αB-crystallins with remarkable chaperone and anti-apoptotic activities. Administration of α-crystallin or their functional peptides has shown substantial inhibition of pathologies in several diseases. However, α-crystallins have been shown to promote disease-causing pathways. These two sides of the proteins are discussed in this review. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Addition of αA-crystallin sequence 164-173 to a mini-chaperone DFVIFLDVKHFSPEDLT alters the conformation but not the chaperone-like activity

It has been shown that αA-mini-chaperone, a peptide representing the chaperone binding site in αA-crystallin, prevents destabilized protein aggregation. αA-Mini-chaperone has been shown to form amyloid fibrils. This study was undertaken to improve the stability of αA-mini-chaperone while preserving its anti-aggregation activity by fusing the flexible and solvent-exposed C-terminal 164-173 region of αA-crystallin to the mini-chaperone sequence DFVIFLDVKHFSPEDLT. The resulting chimeric chaperone peptide, DFVIFLDVKHFSPEDLTEEKPTSAPSS (designated CP1), was characterized. Circular dichroism studies showed that unlike αA-mini-chaperone with its β-sheet structure, the CP1 peptide exhibited a random structure. Transmission electron microscopy (TEM) examination of the CP1 peptide incubated in a shaker at 37 °C for 72 h did not reveal amyloid fibrils, whereas αA-mini-chaperone showed distinct fibrils. Consistent with TEM observation, the thioflavin T binding assay showed an increased level of dye binding in the mini-chaperone incubated at 37 °C and subjected to shaking but not of the CP1 peptide incubated under similar conditions. The chaperone activity of the CP1 peptide was comparable to that of αA-mini-chaperone against denaturing alcohol dehydrogenase, citrate synthase, and α-lactalbumin. Transduction of both peptide chaperones to COS-7 cells showed no cytotoxic effects. The antioxidation assay involving the H2O2 treatment of COS-7 cells revealed that αA-mini-chaperone and the CP1 peptide have comparable cytoprotective properties against H2O2-induced oxidative damage in COS-7 cells. This study therefore shows that the addition of C-terminal sequence 164-173 of αA-crystallin to αA-mini-chaperone influences the conformation of αA-mini-chaperone without affecting its chaperone function or cytoprotective activity.