Microphthalmia due to p53-mediated apoptosis of anterior lens epithelial cells in mice lacking the CREB-2 transcription factor

Disruption of common ocular developmental pathways in patient-derived optic vesicle models of microphthalmia

Genetic perturbations influencing early eye development can result in microphthalmia, anophthalmia, and coloboma (MAC). Over 100 genes are associated with MAC, but little is known about common disease mechanisms. In this study, we generated induced pluripotent stem cell (iPSC)-derived optic vesicles (OVs) from two unrelated microphthalmia patients and healthy controls. At day 20, 35, and 50, microphthalmia patient OV diameters were significantly smaller, recapitulating the "small eye" phenotype. RNA sequencing (RNA-seq) analysis revealed upregulation of apoptosis-initiating and extracellular matrix (ECM) genes at day 20 and 35. Western blot and immunohistochemistry revealed increased expression of lumican, nidogen, and collagen type IV, suggesting ECM overproduction. Increased apoptosis was observed in microphthalmia OVs with reduced phospho-histone 3 (pH3+) cells confirming decreased cell proliferation at day 35. Pharmacological inhibition of caspase-8 activity with Z-IETD-FMK decreased apoptosis in one patient model, highlighting a potential therapeutic approach. These data reveal shared pathophysiological mechanisms contributing to a microphthalmia phenotype.

Possible role of endoplasmic reticulum stress in the pathogenesis of chronic adenoiditis and adenoid hypertrophy: A prospective, parallel-group study

Background

Adenoid tissue is a first-line host defense secondary lymphoid organ, especially in childhood. The endoplasmic reticulum (ER) is required to maintain balanced cellular activity. With impaired ER functions, protein accumulation occurs, resulting in ER stress, which plays a role in the etiopathogenesis of many diseases.

Objective

We aimed to investigate the relationship between ER stress and adenoid tissue disorders, thereby elucidating the mechanisms of immunity-related diseases.

Methods

Fifty-four pediatric patients (>3 years old) who underwent adenoidectomy for chronic adenoiditis (CA) or adenoid hypertrophy (AH) were enrolled in this prospective, parallel-group clinical study. Adenoids were divided into two groups (CA or AH) based on their size and evaluated for ER stress pathway and apoptosis pathway markers by Real-time PCR and Western blot analysis.

Results

ER stress pathway markers significantly differed between the CA and AH groups. Children with CA had higher ER stress marker levels than the AH group (p < .001 for ATF-4, ATF-6, and GRP78, and p < .05 for EDEM1, CHOP, EIF2AK3, ERNI, and GRP94). Apoptosis pathway marker levels (BAX and BCL-2) were not different between groups.

Conclusions

ER stress contributes to the etiopathogenesis of adenoid tissue diseases and the pathogenesis of adenoid tissue disorders, which are part of the immune response. These results may guide the development of new and alternative treatments for immune system disorders.

Signaling plasticity in the integrated stress response

The Integrated Stress Response (ISR) is an essential homeostatic signaling network that controls the cell's biosynthetic capacity. Four ISR sensor kinases detect multiple stressors and relay this information to downstream effectors by phosphorylating a common node: the alpha subunit of the eukaryotic initiation factor eIF2. As a result, general protein synthesis is repressed while select transcripts are preferentially translated, thus remodeling the proteome and transcriptome. Mounting evidence supports a view of the ISR as a dynamic signaling network with multiple modulators and feedback regulatory features that vary across cell and tissue types. Here, we discuss updated views on ISR sensor kinase mechanisms, how the subcellular localization of ISR components impacts signaling, and highlight ISR signaling differences across cells and tissues. Finally, we consider crosstalk between the ISR and other signaling pathways as a determinant of cell health.

ATF4 May Be Essential for Adaption of the Ocular Lens to Its Avascular Environment

The late embryonic mouse lens requires the transcription factor ATF4 for its survival although the underlying mechanisms were unknown. Here, RNAseq analysis revealed that E16.5 Atf4 null mouse lenses downregulate the mRNA levels of lens epithelial markers as well as known markers of late lens fiber cell differentiation. However, a comparison of this list of differentially expressed genes (DEGs) with other known transcriptional regulators of lens development indicated that ATF4 expression is not directly controlled by the previously described lens gene regulatory network. Pathway analysis revealed that the Atf4 DEG list was enriched in numerous genes involved in nutrient transport, amino acid biosynthesis, and tRNA charging. These changes in gene expression likely result in the observed reductions in lens free amino acid and glutathione levels, which would result in the observed low levels of extractable lens protein, finally leading to perinatal lens disintegration. These data demonstrate that ATF4, via its function in the integrated stress response, is likely to play a crucial role in mediating the adaption of the lens to the avascularity needed to maintain lens transparency.

SUMO1-regulated DBC1 promotes p53-dependent stress-induced apoptosis of lens epithelial cells

Deleted in breast cancer 1 (DBC1) was initially identified from a homozygously deleted region in human chromosome 8p21. It has been well established that DBC1 plays a dual role during cancer development. Depending on the physiological context, it can promote or inhibit tumorigenesis. Whether it plays a role in lens pathogenesis remains elusive. In the present study, we demonstrated that DBC1 is highly expressed in lens epithelial cells from different vertebrates and in retina pigment epithelial cells as well. Moreover, DBC1 is SUMOylated through SUMO1 conjugation at K591 residue in human and mouse lens epithelial cells. The SUMOylated DBC1 is localized in the nucleus and plays an essential role in promoting stress-induced apoptosis. Silence of DBC1 attenuates oxidative stress-induced apoptosis. In contrast, overexpression of DBC1 enhances oxidative stress-induced apoptosis, and this process depends on p53. Mechanistically, DBC1 interacts with p53 to regulate its phosphorylation status at multiple sites and the SUMOylation of DBC1 enhances its interaction with p53. Together, our results identify that DBC1 is an important regulator mediating stress-induced apoptosis in lens, and thus participates in control of lens cataractogenesis.

Effects of acute exposure to amisulbrom on retinal development in zebrafish (Danio rerio) embryos

Amisulbrom is an oomycete-specific fungicide that was developed by Nissan Chemical Industries Limited. The exposure of developing zebrafish embryo to amisulbrom caused disorders in the visual phototransduction system. However, the potential toxic mechanisms of amisulbrom on retinal development remains unclear. The research purpose of this study was to evaluate the adverse effects of amisulbrom on retinal development in a model organism, the zebrafish. Zebrafish embryos were treated with 0, 0.0075, 0.075, or 0.75 μM amisulbrom from 3 h post-fertilization (hpf) to 72 hpf. Compared with the control group, amisulbrom-treated zebrafish embryos displayed phenotypic microphthalmia, dysregulation of gene transcription levels (alcama, prox1a, sox2, vsx1, rho, bluops, rdops, uvops, and grops) related to the retinal cell layer differentiation, and increased retinal apoptosis. In addition, the content of glutathione and malondialdehyde increased significantly after exposure to amisulbrom. Overall, our data demonstrate the toxicity of amisulbrom to eye development, which will help to assess the potential ecotoxicological impacts posed by amisulbrom to aquatic species.

MAB21L1 promotes survival of lens epithelial cells through control of αB-crystallin and ATR/CHK1/p53 pathway

The male abnormal gene family 21 (mab21), was initially identified in C. elegans. Since its identification, studies from different groups have shown that it regulates development of ocular tissues, brain, heart and liver. However, its functional mechanism remains largely unknown. Here, we demonstrate that Mab21L1 promotes survival of lens epithelial cells. Mechanistically, Mab21L1 upregulates expression of αB-crystallin. Moreover, our results show that αB-crystallin prevents stress-induced phosphorylation of p53 at S-20 and S-37 through abrogating the activation of the upstream kinases, ATR and CHK1. As a result of suppressing p53 activity by αB-crystallin, Mab21L1 downregulates expression of Bak but upregulates Mcl-1 during stress insult. Taken together, our results demonstrate that Mab21L1 promotes survival of lens epithelial cells through upregulation of αB-crystallin to suppress ATR/CHK1/p53 pathway.

Effects of embryonic exposure to fluxapyroxad on zebrafish (Danio rerio) ocular development

Fluxapyroxad (FLU) is a succinate dehydrogenase inhibitor that protects crops from fungal diseases, however, it has been identified as toxicants to aquatic organisms. The objective of this study is to investigate the potential toxicity and underlying mechanisms of FLU on aquatic organisms. Herein, by using zebrafish embryos as a model organism, we demonstrated that FLU can cause microphthalmia in zebrafish embryos. The cell density in ganglion cell layer (GCL) is increased after exposure. Compared with the control, differentiation of the cells in ganglion cell layer, inner nuclear layer (INL), and outer nuclear layer (ONL) were severely disrupted in response to FLU treatment. The data show clear evidence that FLU exhibits development toxicity to zebrafish embryos by inducing retinal cell apoptosis, which causes microphthalmia. Our study provides comprehensive understanding to the underlying mechanism of FLU toxicity.

Retinal toxicity of isoflucypram to zebrafish (Danio rerio)

Isoflucypram is an active succinate dehydrogenase inhibitor (SDHI) fungicide. Recent studies have demonstrated that isoflucypram is toxic to non-target aquatic organisms such as zebrafish, Danio rerio. However, current knowledge of the potential risks presented by the SDHI to non-target aquatic organism remains limited. To investigate the teratogenic effects of isoflucypram on retinogenesis, zebrafish embryos were exposed to isoflucypram (0.025, 0.25, and 2.5 μM) from the blastula stage (3 h post-fertilization, hpf) to the larval stage (96 hpf). Prolonged exposure to isoflucypram induced abnormalities in retinal development in zebrafish larvae, resulted in the expression of a microphthalmic phenotype, disrupted retinal lamination, and altered the expression levels of retinal markers (opn1sw1, opn1sw2, opn1mw1, opn1lw1, rho, atoh7, vsx1, prox1a, and sox2). Retinal cell apoptosis was also significantly higher in the isoflucypram-exposed larvae than in the control larvae. Catalase activity decreased significantly and malondialdehyde content increased markedly after exposure to isoflucypram. Thus, isoflucypram should be regarded as having retinal neurotoxicity in zebrafish.

A stromal Integrated Stress Response activates perivascular cancer-associated fibroblasts to drive angiogenesis and tumour progression

Bidirectional signalling between the tumour and stroma shapes tumour aggressiveness and metastasis. ATF4 is a major effector of the Integrated Stress Response, a homeostatic mechanism that couples cell growth and survival to bioenergetic demands. Using conditional knockout ATF4 mice, we show that global, or fibroblast-specific loss of host ATF4, results in deficient vascularization and a pronounced growth delay of syngeneic melanoma and pancreatic tumours. Single-cell transcriptomics of tumours grown in Atf4^Δ/Δ mice uncovered a reduction in activation markers in perivascular cancer-associated fibroblasts (CAFs). Atf4^Δ/Δ fibroblasts displayed significant defects in collagen biosynthesis and deposition and a reduced ability to support angiogenesis. Mechanistically, ATF4 regulates the expression of the Col1a1 gene and levels of glycine and proline, the major amino acids of collagen. Analyses of human melanoma and pancreatic tumours revealed a strong correlation between ATF4 and collagen levels. Our findings establish stromal ATF4 as a key driver of CAF functionality, malignant progression and metastasis.

Effects of embryonic exposure to bixafen on zebrafish (Danio rerio) retinal development

Bixafen, a pyrazole-carboxamide fungicide, is a potent toxicant that may elicit multiple adverse effects in non-target organisms. However, knowledge of the mechanisms involved in developmental defects caused by bixafen in aquatic organisms remains limited. In this study, the effects of bixafen on retinal development were evaluated in embryo-larval zebrafish. We exposed zebrafish embryos to 0, 0.1, and 0.3 μM bixafen. Exposure of zebrafish embryos to bixafen caused severe retinal defects, including extreme microphthalmia and a significantly increased cell density of the ganglion cell layer (GCL). Compared with the controls, the expression levels of rod and cone photoreceptor marker genes (rho, opn1sw2, opn1mw1, opn1lw1, and opn1sw1) in the outer nuclear layer (ONL) were significantly downregulated after bixafen exposure. Furthermore, bixafen caused significantly increased expression levels in the GCL marker ath5 and decreased expression levels in the inner nuclear layer (INL) markers prox1a, vsx1, and sox2. Accordingly, we observed a significantly increased rate of cell apoptosis in the retina after bixafen exposure. Taken together, our data demonstrate that bixafen exhibits retinal developmental toxicity to zebrafish embryos/larvae, and thus, it may pose a significant environmental threat to aquatic organisms.

Hepatic mTORC1 signaling activates ATF4 as part of its metabolic response to feeding and insulin

OBJECTIVE

The mechanistic target of rapamycin complex 1 (mTORC1) is dynamically regulated by fasting and feeding cycles in the liver to promote protein and lipid synthesis while suppressing autophagy. However, beyond these functions, the metabolic response of the liver to feeding and insulin signaling orchestrated by mTORC1 remains poorly defined. Here, we determine whether ATF4, a stress responsive transcription factor recently found to be independently regulated by mTORC1 signaling in proliferating cells, is responsive to hepatic mTORC1 signaling to alter hepatocyte metabolism.

METHODS

ATF4 protein levels and expression of canonical gene targets were analyzed in the liver following fasting and physiological feeding in the presence or absence of the mTORC1 inhibitor, rapamycin. Primary hepatocytes from wild-type or liver-specific Atf4 knockout (LAtf4KO) mice were used to characterize the effects of insulin-stimulated mTORC1-ATF4 function on hepatocyte gene expression and metabolism. Both unbiased steady-state metabolomics and stable-isotope tracing methods were employed to define mTORC1 and ATF4-dependent metabolic changes. RNA-sequencing was used to determine global changes in feeding-induced transcripts in the livers of wild-type versus LAtf4KO mice.

RESULTS

We demonstrate that ATF4 and its metabolic gene targets are stimulated by mTORC1 signaling in the liver, in a hepatocyte-intrinsic manner by insulin in response to feeding. While we demonstrate that de novo purine and pyrimidine synthesis is stimulated by insulin through mTORC1 signaling in primary hepatocytes, this regulation was independent of ATF4. Metabolomics and metabolite tracing studies revealed that insulin-mTORC1-ATF4 signaling stimulates pathways of nonessential amino acid synthesis in primary hepatocytes, including those of alanine, aspartate, methionine, and cysteine, but not serine.

CONCLUSIONS

The results demonstrate that ATF4 is a novel metabolic effector of mTORC1 in the liver, extending the molecular consequences of feeding and insulin-induced mTORC1 signaling in this key metabolic tissue to the control of amino acid metabolism.

Activating transcription factor-4 promotes neuronal death induced by Parkinson's disease neurotoxins and α-synuclein aggregates

Parkinson's disease (PD) is a neurodegenerative disease characterized by the loss of dopaminergic neurons in the substantia nigra resulting in severe and progressive motor impairments. However, the mechanisms underlying this neuronal loss remain largely unknown. Oxidative stress and ER stress have been implicated in PD and these factors are known to activate the integrated stress response (ISR). Activating transcription factor 4 (ATF4), a key mediator of the ISR, and has been reported to induce the expression of genes involved in cellular homeostasis. However, during prolonged activation ATF4 can also induce the expression of pro-death target genes. Therefore, in the present study, we investigated the role of ATF4 in neuronal cell death in models of PD. We demonstrate that PD neurotoxins (MPP+ and 6-OHDA) and α-synuclein aggregation induced by pre-formed human alpha-synuclein fibrils (PFFs) cause sustained upregulation of ATF4 expression in mouse cortical and mesencephalic dopaminergic neurons. Furthermore, we demonstrate that PD neurotoxins induce the expression of the pro-apoptotic factors Chop, Trb3, and Puma in dopaminergic neurons in an ATF4-dependent manner. Importantly, we have determined that PD neurotoxin and α-synuclein PFF induced neuronal death is attenuated in ATF4-deficient dopaminergic neurons. Furthermore, ectopic expression of ATF4 but not transcriptionally defective ATF4ΔRK restores sensitivity of ATF4-deficient neurons to PD neurotoxins. Finally, we demonstrate that the eIF2α kinase inhibitor C16 suppresses MPP+ and 6-OHDA induced ATF4 activation and protects against PD neurotoxin induced dopaminergic neuronal death. Taken together these results indicate that ATF4 promotes dopaminergic cell death induced by PD neurotoxins and pathogenic α-synuclein aggregates and highlight the ISR factor ATF4 as a potential therapeutic target in PD.

Targeting the integrated stress response in ophthalmology

Purpose: To summarize the Integrated Stress Response (ISR) in the context of ophthalmology, with special interest on the cornea and anterior segment. Results: The ISR is a powerful and conserved signaling pathway that allows for cells to respond to a diverse array of both intracellular and extracellular stressors. The pathway is classically responsible for coordination of the cellular response to amino acid starvation, ultraviolet light, heme dysregulation, viral infection, and unfolded protein. Under normal circumstances, it is considered pro-survival and a necessary mechanism through which protein translation is controlled. However, in cases of severe or prolonged stress the pathway can promote apoptosis, and loss of normal cellular phenotype. The activation of this pathway culminates in the global inhibition of cap-dependent protein translation and the canonical expression of the activating transcription factor 4 (ATF4). Conclusion:The eye is uniquely exposed to ISR responsive stressors due to its environmental exposure and relative isolation from the circulatory system which are necessary for its function. We will discuss how this pathway is critical for the proper function of the tissue, its role in development, as well as how targeting of the pathway could alleviate key aspects of diverse ophthalmic diseases.

Hop2 Interacts with ATF4 to Promote Osteoblast Differentiation

Activating transcription factor 4 (ATF4) is a member of the basic leucine zipper (bZip) transcription factor family required for the terminal differentiation of osteoblasts. Despite its critical importance as one of the three main osteoblast differentiation transcription factors, regulators of osteoblast terminal maturation remain poorly defined. Here we report the identification of homologous pairing protein 2 (Hop2) as a dimerization partner of ATF4 in osteoblasts via the yeast two-hybrid system. Deletional mapping revealed that the Zip domain of Hop2 is necessary and sufficient to bind ATF4 and to enhance ATF4-dependent transcription. Ectopic Hop2 expression in preosteoblasts increased endogenous ATF4 protein content and accelerated osteoblast differentiation. Mice lacking Hop2 (Hop2^-/- ) have a normal stature but exhibit an osteopenic phenotype similar to the one observed in Atf4^-/- mice, albeit milder, which is associated with decreased Osteocalcin mRNA expression and reduced type I collagen synthesis. Compound heterozygous mice (Atf4^+/- :Hop2^+/- ) display identical skeletal defects to those found in Hop2^-/- mice. These results indicate that Hop2 plays a previous unknown role as a determinant of osteoblast maturation via its regulation of ATF4 transcriptional activity. Our work for the first time reveals a function of Hop2 beyond its role in guiding the alignment of homologous chromosomes. © 2019 American Society for Bone and Mineral Research.

The unfolded protein response in metazoan development

Eukaryotic cells respond to an overload of unfolded proteins in the endoplasmic reticulum (ER) by activating signaling pathways that are referred to as the unfolded protein response (UPR). Much UPR research has been conducted in cultured cells that exhibit no baseline UPR activity until they are challenged by ER stress initiated by chemicals or mutant proteins. At the same time, many genes that mediate UPR signaling are essential for the development of organisms ranging from Drosophila and fish to mice and humans, indicating that there is physiological ER stress that requires UPR in normally developing animal tissues. Recent studies have elucidated the tissue-specific roles of all three branches of UPR in distinct developing tissues of Drosophila, fish and mammals. As discussed in this Review, these studies not only reveal the physiological functions of the UPR pathways but also highlight a surprising degree of specificity associated with each UPR branch in development.

A data-entrained computational model for testing the regulatory logic of the vertebrate unfolded protein response

The vertebrate unfolded protein response (UPR) is characterized by multiple interacting nodes among its three pathways, yet the logic underlying this regulatory complexity is unclear. To begin to address this issue, we created a computational model of the vertebrate UPR that was entrained upon and then validated against experimental data. As part of this validation, the model successfully predicted the phenotypes of cells with lesions in UPR signaling, including a surprising and previously unreported differential role for the eIF2α phosphatase GADD34 in exacerbating severe stress but ameliorating mild stress. We then used the model to test the functional importance of a feedforward circuit within the PERK/CHOP axis and of cross-regulatory control of BiP and CHOP expression. We found that the wiring structure of the UPR appears to balance the ability of the response to remain sensitive to endoplasmic reticulum stress and to be deactivated rapidly by improved protein-folding conditions. This model should serve as a valuable resource for further exploring the regulatory logic of the UPR.

Physiological/pathological ramifications of transcription factors in the unfolded protein response

Numerous environmental, physiological, and pathological insults disrupt protein-folding homeostasis in the endoplasmic reticulum (ER), referred to as ER stress. Eukaryotic cells evolved a set of intracellular signaling pathways, collectively termed the unfolded protein response (UPR), to maintain a productive ER protein-folding environment through reprogramming gene transcription and mRNA translation. The UPR is largely dependent on transcription factors (TFs) that modulate expression of genes involved in many physiological and pathological conditions, including development, metabolism, inflammation, neurodegenerative diseases, and cancer. Here we summarize the current knowledge about these mechanisms, their impact on physiological/pathological processes, and potential therapeutic applications.

The integrated stress response

In response to diverse stress stimuli, eukaryotic cells activate a common adaptive pathway, termed the integrated stress response (ISR), to restore cellular homeostasis. The core event in this pathway is the phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α) by one of four members of the eIF2α kinase family, which leads to a decrease in global protein synthesis and the induction of selected genes, including the transcription factor ATF4, that together promote cellular recovery. The gene expression program activated by the ISR optimizes the cellular response to stress and is dependent on the cellular context, as well as on the nature and intensity of the stress stimuli. Although the ISR is primarily a pro-survival, homeostatic program, exposure to severe stress can drive signaling toward cell death. Here, we review current understanding of the ISR signaling and how it regulates cell fate under diverse types of stress.

Lens ER-stress response during cataract development in Mip-mutant mice

Major intrinsic protein (MIP) is a functional water-channel (AQP0) that also plays a key role in establishing lens fiber cell architecture. Genetic variants of MIP have been associated with inherited and age-related forms of cataract; however, the underlying pathogenic mechanisms are unclear. Here we have used lens transcriptome profiling by microarray-hybridization and qPCR to identify pathogenic changes during cataract development in Mip-mutant (Lop/+) mice. In postnatal Lop/+ lenses (P7) 99 genes were up-regulated and 75 were down-regulated (>2-fold, p=<0.05) when compared with wild-type. A pathway analysis of up-regulated genes in the Lop/+ lens (P7) was consistent with endoplasmic reticulum (ER)-stress and activation of the unfolded protein response (UPR). The most up-regulated UPR genes (>4-fold) in the Lop/+ lens included Chac1>Ddit3>Atf3>Trib3>Xbp1 and the most down-regulated genes (>5-fold) included two anti-oxidant genes, Hspb1 and Hmox1. Lop/+ lenses were further characterized by abundant TUNEL-positive nuclei within central degenerating fiber cells, glutathione depletion, free-radical overproduction, and calpain hyper-activation. These data suggest that Lop/+ lenses undergo proteotoxic ER-stress induced cell-death resulting from prolonged activation of the Eif2ak3/Perk-Atf4-Ddit3-Chac1 branch of the UPR coupled with severe oxidative-stress.

C/EBPγ Is a Critical Regulator of Cellular Stress Response Networks through Heterodimerization with ATF4

The integrated stress response (ISR) controls cellular adaptations to nutrient deprivation, redox imbalances, and endoplasmic reticulum (ER) stress. ISR genes are upregulated in stressed cells, primarily by the bZIP transcription factor ATF4 through its recruitment to cis-regulatory C/EBP:ATF response elements (CAREs) together with a dimeric partner of uncertain identity. Here, we show that C/EBPγ:ATF4 heterodimers, but not C/EBPβ:ATF4 dimers, are the predominant CARE-binding species in stressed cells. C/EBPγ and ATF4 associate with genomic CAREs in a mutually dependent manner and coregulate many ISR genes. In contrast, the C/EBP family members C/EBPβ and C/EBP homologous protein (CHOP) were largely dispensable for induction of stress genes. Cebpg(-/-) mouse embryonic fibroblasts (MEFs) proliferate poorly and exhibit oxidative stress due to reduced glutathione levels and impaired expression of several glutathione biosynthesis pathway genes. Cebpg(-/-) mice (C57BL/6 background) display reduced body size and microphthalmia, similar to ATF4-null animals. In addition, C/EBPγ-deficient newborns die from atelectasis and respiratory failure, which can be mitigated by in utero exposure to the antioxidant, N-acetyl-cysteine. Cebpg(-/-) mice on a mixed strain background showed improved viability but, upon aging, developed significantly fewer malignant solid tumors than WT animals. Our findings identify C/EBPγ as a novel antioxidant regulator and an obligatory ATF4 partner that controls redox homeostasis in normal and cancerous cells.

ER stress: Autophagy induction, inhibition and selection

An accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) leads to stress conditions. To mitigate such circumstances, stressed cells activate a homeostatic intracellular signaling network cumulatively called the unfolded protein response (UPR), which orchestrates the recuperation of ER function. Macroautophagy (hereafter autophagy), an intracellular lysosome-mediated bulk degradation pathway for recycling and eliminating wornout proteins, protein aggregates, and damaged organelles, has also emerged as an essential protective mechanism during ER stress. These 2 systems are dynamically interconnected, and recent investigations have revealed that ER stress can either stimulate or inhibit autophagy. However, the stress-associated molecular cues that control the changeover switch between induction and inhibition of autophagy are largely obscure. This review summarizes the crosstalk between ER stress and autophagy and their signaling networks mainly in mammalian-based systems. Additionally, we highlight current knowledge on selective autophagy and its connection to ER stress.

Knockdown of 15-kDa selenoprotein (Sep15) increases hLE cells' susceptibility to tunicamycin-induced apoptosis

In this work, we investigated the effect of Sep15 gene knockdown on apoptosis in human lens epithelial (hLE) cells, trying to understand the relevance of Sep15 to cataract formation in the Sep15 knockout (KO) mice. The results showed that sole knockdown of Sep15 by RNA interference did not result in apoptosis; however, reduction of Sep15 expression aggravated tunicamycin (Tm)-induced cell apoptosis and caspases activation. Furthermore, Tm-induced mitochondrial dysfunction was also exacerbated under the Sep15 knockdown condition by measurement of mitochondrial membrane potential decrease and human cytochrome c release into cytosol. Interestingly, the knockdown of Sep15 exacerbated Tm-induced oxidative stress while endoplasmic reticulum (ER) stress was not correspondingly elevated. These results suggest that the protective role of Sep15 against Tm-induced apoptosis in hLE cells is operated via inhibiting oxidative stress rather than regulating Tm-induced ER stress, and the protective role becomes dependent on Sep15 only in acute stress condition.

ATF4 plays a pivotal role in the development of functional hematopoietic stem cells in mouse fetal liver

The fetal liver (FL) serves as a predominant site for expansion of functional hematopoietic stem cells (HSCs) during mouse embryogenesis. However, the mechanisms for HSC development in FL remain poorly understood. In this study, we demonstrate that deletion of activating transcription factor 4 (ATF4) significantly impaired hematopoietic development and reduced HSC self-renewal in FL. In contrast, generation of the first HSC population in the aorta-gonad-mesonephros region was not affected. The migration activity of ATF4(-/-) HSCs was moderately reduced. Interestingly, the HSC-supporting ability of both endothelial and stromal cells in FL was significantly compromised in the absence of ATF4. Gene profiling using RNA-seq revealed downregulated expression of a panel of cytokines in ATF4(-/-) stromal cells, including angiopoietin-like protein 3 (Angptl3) and vascular endothelial growth factor A (VEGFA). Addition of Angptl3, but not VEGFA, partially rescued the repopulating defect of ATF4(-/-) HSCs in the culture. Furthermore, chromatin immunoprecipitation assay in conjunction with silencing RNA-mediated silencing and complementary DNA overexpression showed transcriptional control of Angptl3 by ATF4. To summarize, ATF4 plays a pivotal role in functional expansion and repopulating efficiency of HSCs in developing FL, and it acts through upregulating transcription of cytokines such as Angptl3 in the microenvironment.

Attenuation of oxygen fluctuation-induced endoplasmic reticulum stress in human lens epithelial cells

Cataractogenic stresses are associated with the induction of endoplasmic reticulum (ER) stress. However, little is known about oxygen (O₂)-induced ER stress in the lens. Cataract research has focused on elevated levels of O₂ in lens epithelial cells (LECs). Excessive levels or a lack of O₂ are known to induce ER stress whereas chronic ER stress activates the unfolded protein response (UPR). The present study investigated the hypothesis that the fluctuation of O₂ levels induces a UPR, and may be controlled by maintaining human LECs (hLECs) in a specific concentration of O₂. Human LECs were cultured in different atmospheric levels of O₂. Hypoxic conditions were determined by the level of hypoxia-inducible factor (HIF)-1α. 2',7'-Dichlorodihydrofluorescein diacetate and ethidium homodimer-1 staining were conducted to detect reactive oxygen species (ROS) and cell death, respectively. Protein blot analyses were performed with antibodies specific to antioxidant and UPR-specific proteins. Reverse transcription-quantitatative polymerase chain reaction assays were performed to quantify the mRNA levels of activated NF-E2-related factor 2 (Nrf2) and kelch-like ECH-associated protein 1 (Keap1). The treatment of human LECs with 0 and 20% atmospheric O₂ activated Nrf2/Keap1. The LECs shifted to 1% atmospheric O₂ from 0, 4 or 20% for 24 h showed decreased levels of Keap1. By contrast, hLECs cultured in 1% atmospheric O₂ for 24 h and then shifted to 0, 4 or 20% O₂ exhibited a significant upregulation of Nrf2. These results suggest that oxidative stress proteins were not expressed in a 1% O₂ environment. The O₂ levels in the culture medium were equilibrated within 2 h in the cell culture plates. These results showed that an appropriate oxygen environment for the culture of LECs is ~1 % atmospheric O₂. Either 0 or 20% of atmospheric O₂ activated the UPR and the Nrf2/Keap1-mediated antioxidant system in LECs and chronic exposure to O₂ fluctuation led to ROS production and cell death. This study revealed that O₂ fluctuation-induced UPR/ER stress could be prevented by maintaining the cells in a 1% O₂ environment.

A stochastic model of eye lens growth

The size and shape of the ocular lens must be controlled with precision if light is to be focused sharply on the retina. The lifelong growth of the lens depends on the production of cells in the anterior epithelium. At the lens equator, epithelial cells differentiate into fiber cells, which are added to the surface of the existing fiber cell mass, increasing its volume and area. We developed a stochastic model relating the rates of cell proliferation and death in various regions of the lens epithelium to deposition of fiber cells and radial lens growth. Epithelial population dynamics were modeled as a branching process with emigration and immigration between proliferative zones. Numerical simulations were in agreement with empirical measurements and demonstrated that, operating within the strict confines of lens geometry, a stochastic growth engine can produce the smooth and precise growth necessary for lens function.

Oxidative stress, unfolded protein response, and apoptosis in developmental toxicity

Physiological development requires precise spatiotemporal regulation of cellular and molecular processes. Disruption of these key events can generate developmental toxicity in the form of teratogenesis or mortality. The mechanism behind many developmental toxicants remains unknown. While recent work has focused on the unfolded protein response (UPR), oxidative stress, and apoptosis in the pathogenesis of disease, few studies have addressed their relationship in developmental toxicity. Redox regulation, UPR, and apoptosis are essential for physiological development and can be disturbed by a variety of endogenous and exogenous toxicants to generate lethality and diverse malformations. This review examines the current knowledge of the role of oxidative stress, UPR, and apoptosis in physiological development as well as in developmental toxicity, focusing on studies and advances in vertebrates model systems.

Peroxidasin is essential for eye development in the mouse

Mutations in Peroxidasin (PXDN) cause severe inherited eye disorders in humans, such as congenital cataract, corneal opacity and developmental glaucoma. The role of peroxidasin during eye development is poorly understood. Here, we describe the first Pxdn mouse mutant which was induced by ENU (N-ethyl-N-nitrosourea) and led to a recessive phenotype. Sequence analysis of cDNA revealed a T3816A mutation resulting in a premature stop codon (Cys1272X) in the peroxidase domain. This mutation causes severe anterior segment dysgenesis and microphthalmia resembling the manifestations in patients with PXDN mutations. The proliferation and differentiation of the lens is disrupted in association with aberrant expression of transcription factor genes (Pax6 and Foxe3) in mutant eyes. Additionally, Pxdn is involved in the consolidation of the basement membrane and lens epithelium adhesion in the ocular lens. Lens material including γ-crystallin is extruded into the anterior and posterior chamber due to local loss of structural integrity of the lens capsule as a secondary damage to the anterior segment development leading to congenital ocular inflammation. Moreover, Pxdn mutants exhibited an early-onset glaucoma and progressive retinal dysgenesis. Transcriptome profiling revealed that peroxidasin affects the transcription of developmental and eye disease-related genes at early eye development. These findings suggest that peroxidasin is necessary for cell proliferation and differentiation and for basement membrane consolidation during eye development. Our studies provide pathogenic mechanisms of PXDN mutation-induced congenital eye diseases.

Cellular Mechanisms of Endoplasmic Reticulum Stress Signaling in Health and Disease. 1. An overview

Increased demand on the protein folding capacity of the endoplasmic reticulum (ER) engages an adaptive reaction known as the unfolded protein response (UPR). The UPR regulates protein translation and the expression of numerous target genes that contribute to restore ER homeostasis or induce apoptosis of irreversibly damaged cells. UPR signaling is highly regulated and dynamic and integrates information about the type, intensity, and duration of the stress stimuli, thereby determining cell fate. Recent advances highlight novel physiological outcomes of the UPR beyond specialized secretory cells, particularly in innate immunity, metabolism, and cell differentiation. Here we discuss studies on the fine-tuning of the UPR and its physiological role in diverse organs and diseases.

Aberrant activation of p53 due to loss of MDM2 or MDMX causes early lens dysmorphogenesis

Although forming a heterodimer or heterooligomer is essential for MDM2 and MDMX to fully control p53 during early embryogenesis, deletion of either MDM2 or MDMX in specific tissues using the loxp-Cre system reveals phenotypic diversity during organ morphogenesis, which can be completely rescued by loss of p53, suggesting the spatiotemporal independence and specificity of the regulation of p53 by MDM2 and MDMX. In this study, we investigated the role of the MDM2-MDMX-p53 pathway in the developing lens that is a relatively independent region integrating cell proliferation, differentiation and apoptosis. Using the mice expressing Cre recombinase specifically in the lens epithelial cells (LECs) beginning at E9.5, we demonstrated that deletion of either MDM2 or MDMX induces apoptosis of LEC and reduces cell proliferation, resulting in lens developmental defect that finally progresses into aphakia. Specifically, the lens defect caused by MDM2 deletion was evident at E10, occurring earlier than that caused by MDMX deletion. These lens defects were completely rescued by loss of two alleles of p53, but not one allele of p53. These results demonstrate that both MDM2 and MDMX are required for monitoring p53 activity during lens development, and they may function independently or synergistically to control p53 and maintain normal lens morphogenesis.

Activation of the amino acid response modulates lineage specification during differentiation of murine embryonic stem cells

In somatic cells, a collection of signaling pathways activated by amino acid limitation have been identified and referred to as the amino acid response (AAR). Despite the importance of possible detrimental effects of nutrient limitation during in vitro culture, the AAR has not been investigated in embryonic stem cells (ESC). AAR activation caused the expected increase in transcription factors that mediate specific AAR pathways, as well as the induction of asparagine synthetase, a terminal AAR target gene. Neither AAR activation nor stable knockdown of activating transcription factor (Atf) 4, a transcriptional mediator of the AAR, adversely affected ESC self-renewal or pluripotency. Low-level induction of the AAR over a 12-day period of embryoid body differentiation did alter lineage specification such that the primitive endodermal, visceral endodermal, and endodermal lineages were favored, whereas mesodermal and certain ectodermal lineages were suppressed. Knockdown of Atf4 further enhanced the AAR-induced increase in endodermal formation, suggesting that this phenomenon is mediated by an Atf4-independent mechanism. Collectively, the results indicate that, during differentiation of mouse embryoid bodies in culture, the availability of nutrients, such as amino acids, can influence the formation of specific cell lineages.

ER-stress-induced transcriptional regulation increases protein synthesis leading to cell death

Protein misfolding in the endoplasmic reticulum (ER) leads to cell death through PERK-mediated phosphorylation of eIF2α, although the mechanism is not understood. ChIP-seq and mRNA-seq of activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP), key transcription factors downstream of p-eIF2α, demonstrated that they interact to directly induce genes encoding protein synthesis and the unfolded protein response, but not apoptosis. Forced expression of ATF4 and CHOP increased protein synthesis and caused ATP depletion, oxidative stress and cell death. The increased protein synthesis and oxidative stress were necessary signals for cell death. We show that eIF2α-phosphorylation-attenuated protein synthesis, and not Atf4 mRNA translation, promotes cell survival. These results show that transcriptional induction through ATF4 and CHOP increases protein synthesis leading to oxidative stress and cell death. The findings suggest that limiting protein synthesis will be therapeutic for diseases caused by protein misfolding in the ER.

Functional interaction of protein kinase CK2 and activating transcription factor 4 (ATF4), a key player in the cellular stress response

Protein kinase CK2 is a pleiotropic enzyme, which is implicated in the regulation of numerous biological processes. It seems to regulate the various functions by binding to other proteins and by phosphorylation of many different substrates. Here, we identified the activating transcription factor 4 (ATF4), an essential component of the ER stress signaling, as a new binding partner and a new substrate of CK2 in vitro and in vivo. Bifluorescence complementation analysis (BiFC) revealed that CK2α and ATF4 associate in the nucleus. By using mutants of ATF4 we identified serine 215 as the main CK2 phosphorylation site. The ATF4 S215A mutant turned out to be more stable than the wild-type form. We further noticed that an inhibition of CK2 caused an increased transcription of the ATF4 gene. Analyses of the transcription factor activity revealed an impaired activity of the CK2 phosphorylation mutant of ATF4. Thus, we show that (i) ATF4 is a binding partner of CK2α (ii) ATF4 is a substrate of CK2, (iii) the phosphorylation of ATF4 by CK2 influences the stability of ATF4, (iv) the transcription of ATF4 is regulated by CK2 and (v) the transcription factor activity of ATF4 is regulated by the CK2 phosphorylation of ATF4. Thus, CK2 plays an essential role in the regulation of the ER-stress induced signaling pathway.

Eukaryotic initiation factor 2 phosphorylation and translational control in metabolism

Regulation of mRNA translation is a rapid and effective means to couple changes in the cellular environment with global rates of protein synthesis. In response to stresses, such as nutrient deprivation and accumulation of misfolded proteins in the endoplasmic reticulum, phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α~P) reduces general translation initiation while facilitating the preferential translation of select transcripts, such as that encoding activating transcription factor 4 (ATF4), a transcriptional activator of genes subject to the integrated stress response (ISR). In this review, we highlight the translational control processes regulated by nutritional stress, with an emphasis on the events triggered by eIF2α~P, and describe the family of eukaryotic initiation factor 2 kinases and the mechanisms by which each sense different stresses. We then address 3 questions. First, what are the mechanisms by which eIF2α~P confers preferential translation on select mRNA and what are the consequences of the gene expression induced by the ISR? Second, what are the molecular processes by which certain stresses can differentially activate eIF2α~P and ATF4 expression? The third question we address is what are the modes of cross-regulation between the ISR and other stress response pathways, such as the unfolded protein response and mammalian target of rapamycin, and how do these regulatory schemes provide for gene expression programs that are tailored for specific stresses? This review highlights recent advances in each of these areas of research, emphasizing how eIF2α~P and the ISR can affect metabolic health and disease.

Low glucose under hypoxic conditions induces unfolded protein response and produces reactive oxygen species in lens epithelial cells

Aging is enhanced by hypoxia and oxidative stress. As the lens is located in the hypoglycemic environment under hypoxia, aging lens with diabetes might aggravate these stresses. This study was designed to examine whether low glucose under hypoxic conditions induces the unfolded protein response (UPR), and also if the UPR then generates the reactive oxygen species (ROS) in lens epithelial cells (LECs). The UPR was activated within 1 h by culturing the human LECs (HLECs) and rat LECs in <1.5 mM glucose under hypoxic conditions. These conditions also induced the Nrf2-dependent antioxidant-protective UPR, production of ROS, and apoptosis. The rat LECs located in the anterior center region were the least susceptible to the UPR, whereas the proliferating LECs in the germinative zone were the most susceptible. Because the cortical lens fiber cells are differentiated from the LECs after the onset of diabetes, we suggest that these newly formed cortical fibers have lower levels of Nrf2, and are then oxidized resulting in cortical cataracts. Thus, low glucose and oxygen conditions induce the UPR, generation of ROS, and expressed the Nrf2 and Nrf2-dependent antioxidant enzymes at normal levels. But these cells eventually lose reduced glutathione (GSH) and induce apoptosis. The results indicate a new link between hypoglycemia under hypoxia and impairment of HLEC functions.

The eIF2 kinase PERK and the integrated stress response facilitate activation of ATF6 during endoplasmic reticulum stress

This study shows that the eIF2 kinase PERK is required not only for translational control but also for activation of ATF6 and its target genes in the unfolded protein response. The PERK pathway facilitates both the synthesis of ATF6 and trafficking of ATF6 from the endoplasmic reticulum to the Golgi for intramembrane proteolysis and activation of ATF6.

Disruptions of the endoplasmic reticulum (ER) that perturb protein folding cause ER stress and elicit an unfolded protein response (UPR) that involves translational and transcriptional changes in gene expression aimed at expanding the ER processing capacity and alleviating cellular injury. Three ER stress sensors (PERK, ATF6, and IRE1) implement the UPR. PERK phosphorylation of the α subunit of eIF2 during ER stress represses protein synthesis, which prevents further influx of ER client proteins. Phosphorylation of eIF2α (eIF2α∼P) also induces preferential translation of ATF4, a transcription activator of the integrated stress response. In this study we show that the PERK/eIF2α∼P/ATF4 pathway is required not only for translational control, but also for activation of ATF6 and its target genes. The PERK pathway facilitates both the synthesis of ATF6 and trafficking of ATF6 from the ER to the Golgi for intramembrane proteolysis and activation of ATF6. As a consequence, liver-specific depletion of PERK significantly reduces both the translational and transcriptional phases of the UPR, leading to reduced protein chaperone expression, disruptions of lipid metabolism, and enhanced apoptosis. These findings show that the regulatory networks of the UPR are fully integrated and help explain the diverse biological defects associated with loss of PERK.

A global view of porcine transcriptome in three tissues from a full-sib pair with extreme phenotypes in growth and fat deposition by paired-end RNA sequencing

Background

Elucidation of the pig transcriptome is essential for interpreting functional elements of the genome and understanding the genetic architecture of complex traits such as fat deposition, metabolism and growth.

Results

Here we used massive parallel high-throughput RNA sequencing to generate a high-resolution map of the porcine mRNA and miRNA transcriptome in liver, longissimus dorsi and abdominal fat from two full-sib F₂ hybrid pigs with segregated phenotypes on growth, blood physiological and biochemical parameters, and fat deposition. We obtained 8,508,418-10,219,332 uniquely mapped reads that covered 78.0% of the current annotated transcripts and identified 48,045-122,931 novel transcript fragments, which constituted 17,085-29,499 novel transcriptional active regions in six tested samples. We found that about 18.8% of the annotated genes showed alternative splicing patterns, and alternative 3' splicing is the most common type of alternative splicing events in pigs. Cross-tissue comparison revealed that many transcriptional events are tissue-differential and related to important biological functions in their corresponding tissues. We also detected a total of 164 potential novel miRNAs, most of which were tissue-specifically identified. Integrated analysis of genome-wide association study and differential gene expression revealed interesting candidate genes for complex traits, such as IGF2, CYP1A1, CKM and CES1 for heart weight, hemoglobin, pork pH value and serum cholesterol, respectively.

Conclusions

This study provides a global view of the complexity of the pig transcriptome, and gives an extensive new knowledge about alternative splicing, gene boundaries and miRNAs in pigs. Integrated analysis of genome wide association study and differential gene expression allows us to find important candidate genes for porcine complex traits.

A novel null allele of mouse DSCAM survives to adulthood on an inbred C3H background with reduced phenotypic variability

DSCAMs are cell adhesion molecules that play several important roles in neurodevelopment. Mouse alleles of Dscam identified to date do not survive on an inbred C57BL/6 background, complicating analysis of DSCAM-dependent developmental processes because of phenotypic variability related to the segregating backgrounds needed for postnatal survival. A novel spontaneous allele of Dscam, hereafter referred to as Dscam²(J), has been identified. This allele contains a four base pair duplication in exon 19, leading to a frameshift and truncation of the open reading frame. Mice homozygous for the Dscam²(J) mutant allele survive into adulthood on the C3H/HeJ background on which the mutation was identified. Using the Dscam²(J) allele, retinal phenotypes that have variable severity on a segregating background were examined. A neurite lamination defect similar to that described in chick was discovered in mice. These results indicate that, in the retina, additional DSCAM-dependent processes can be found by analysis of mutations on different genetic backgrounds.

Transcriptome characterization uncovers the molecular response of hematopoietic cells to ionizing radiation

Ionizing radiation causes rapid and acute suppression of hematopoietic cells that manifests as the hematopoietic syndrome. However, the roles of molecules and regulatory pathways induced in vivo by irradiation of different hematopoietic cells have not been completely elaborated. Using a strategy that combined different microarray bioinformatics tools, we identified gene networks that might be involved in the early response of hematopoietic cells radiation response in vivo. The grouping of similar time-ordered gene expression profiles using quality threshold clustering enabled the successful identification of common binding sites for 56 transcription factors that may be involved in the regulation of the early radiation response. We also identified novel genes that are responsive to the transformation-related protein 53; all of these genes were biologically validated in p53-transgenic null mice. Extension of the analysis to purified bone marrow cells including highly purified long-term hematopoietic stem cells, combined with functional classification, provided evidence of gene expression modifications that were largely unknown in this primitive population. Our methodology proved particularly useful for analyzing the transcriptional regulation of the complex ionizing radiation response of hematopoietic cells. Our data may help to elucidate the molecular mechanisms involved in tissue radiosensitivity and to identify potential targets for improving treatment in radiation emergencies.

Animal models in the study of the unfolded protein response

The endoplasmic reticulum, a highly dynamic and complex organelle, is the site for synthesis, folding, and modification of transmembrane and secretory proteins. Any disruptions to the endoplasmic reticulum such as an accumulation of misfolded or unfolded proteins results in activation of the unfolded protein response (UPR). The UPR is comprised of three distinct signal transduction pathways that work to restore homeostasis to the endoplasmic reticulum. This review summarizes select mouse models available to study the UPR and the information learned from the analyses of these models.

Morpholino artifacts provide pitfalls and reveal a novel role for pro-apoptotic genes in hindbrain boundary development

Morpholino antisense oligonucleotides (MOs) are widely used as a tool to achieve loss of gene function, but many have off-target effects mediated by activation of Tp53 and associated apoptosis. Here, we re-examine our previous MO-based loss-of-function studies that had suggested that Wnt1 expressed at hindbrain boundaries in zebrafish promotes neurogenesis and inhibits boundary marker gene expression in the adjacent para-boundary regions. We find that Tp53 is highly activated and apoptosis is frequently induced by the MOs used in these studies. Co-knockdown of Tp53 rescues the decrease in proneural and neuronal marker expression, which is thus an off-target effect of MOs. While loss of gene expression can be attributed to cell loss through apoptotic cell death, surprisingly we find that the ectopic expression of hindbrain boundary markers is also dependent on Tp53 activity and its downstream apoptotic effectors. We examine whether this non-specific activation of hindbrain boundary gene expression provides insight into the endogenous mechanisms underlying boundary cell specification. We find that the pro-apoptotic Bcl genes puma and bax-a are required for hindbrain boundary marker expression, and that gain of function of the Bcl-caspase pathway leads to ectopic boundary marker expression. These data reveal a non-apoptotic role for pro-apoptotic genes in the regulation of gene expression at hindbrain boundaries. In light of these findings, we discuss the precautions needed in performing morpholino knockdowns and in interpreting the data derived from their use.

Control of activating transcription factor 4 (ATF4) persistence by multisite phosphorylation impacts cell cycle progression and neurogenesis

Organogenesis is a highly integrated process with a fundamental requirement for precise cell cycle control. Mechanistically, the cell cycle is composed of transitions and thresholds that are controlled by coordinated post-translational modifications. In this study, we describe a novel mechanism controlling the persistence of the transcription factor ATF4 by multisite phosphorylation. Proline-directed phosphorylation acted additively to regulate multiple aspects of ATF4 degradation. Stabilized ATF4 mutants exhibit decreased β-TrCP degron phosphorylation, β-TrCP interaction, and ubiquitination, as well as elicit early G₁ arrest. Expression of stabilized ATF4 also had significant consequences in the developing neocortex. Mutant ATF4 expressing cells exhibited positioning and differentiation defects that were attributed to early G₁ arrest, suggesting that neurogenesis is sensitive to ATF4 dosage. We propose that precise regulation of the ATF4 dosage impacts cell cycle control and impinges on neurogenesis.

Stabilization of ATF4 protein is required for the regulation of epithelial-mesenchymal transition of the avian neural crest

Epithelial-mesenchymal transition (EMT) permits neural crest cells to delaminate from the epithelial ectoderm and to migrate extensively in the embryonic environment. In this study, we have identified ATF4, a basic-leucine-zipper transcription factor, as one of the neural crest EMT regulators. Although ATF4 alone was not sufficient to drive the formation of migratory neural crest cells, ATF4 cooperated with Sox9 to induce neural crest EMT by controlling the expression of cell-cell and cell-extracellular matrix adhesion molecules. This was likely, at least in part, by inducing the expression of Foxd3, which encodes another neural crest transcription factor. We also found that the ATF4 protein level was strictly regulated by proteasomal degradation and p300-mediated stabilization, allowing ATF4 protein to accumulate in the nuclei of neural crest cells undergoing EMT. Thus, our results emphasize the importance of the regulation of protein stability in the neural crest EMT.

Lens fiber cell differentiation and denucleation are disrupted through expression of the N-terminal nuclear receptor box of NCOA6 and result in p53-dependent and p53-independent apoptosis

Nuclear receptor coactivator 6 (NCOA6) is a multifunctional protein implicated in embryonic development, cell survival, and homeostasis. An 81-amino acid fragment, dnNCOA6, containing the N-terminal nuclear receptor box (LXXLL motif) of NCOA6, acts as a dominant-negative (dn) inhibitor of NCOA6. Here, we expressed dnNCOA6 in postmitotic transgenic mouse lens fiber cells. The transgenic lenses showed reduced growth; a wide spectrum of lens fiber cell differentiation defects, including reduced expression of gamma-crystallins; and cataract formation. Those lens fiber cells entered an alternate proapoptotic pathway, and the denucleation (karyolysis) process was stalled. Activation of caspase-3 at embryonic day (E)13.5 was followed by double-strand breaks (DSBs) formation monitored via a biomarker, gamma-H2AX. Intense terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) signals were found at E16.5. Thus, a window of approximately 72 h between these events suggested prolonged though incomplete apoptosis in the lens fiber cell compartment that preserved nuclei in its cells. Genetic experiments showed that the apoptotic-like processes in the transgenic lens were both p53-dependent and p53-independent. Lens-specific deletion of Ncoa6 also resulted in disrupted lens fiber cell differentiation. Our data demonstrate a cell-autonomous role of Ncoa6 in lens fiber cell differentiation and suggest novel insights into the process of lens fiber cell denucleation and apoptosis.

FIAT, the factor-inhibiting ATF4-mediated transcription, also represses the transcriptional activity of the bZIP factor FRA-1

FIAT is a leucine zipper protein whose name was coined for its interaction with ATF4 and subsequent blockage of ATF4-directed osteocalcin gene transcription. FIAT lacks a basic DNA-binding domain but contains three leucine zippers; it heterodimerizes with ATF4 to prohibit binding to DNA. FIAT could also potentially interact with additional basic domain-leucine zipper transcriptional regulators of osteoblast activity, such as FRA-1. We have found that FIAT inhibits transcriptional activation by a FRA-1/c-JUN heterodimer without affecting transcription mediated by a c-JUN homodimer. The repressor effect of FIAT on FRA-1-dependent transcription was measured using reporter constructs for the natural FRA-1 targets, Mmp-9 and Mgp. The FIAT-FRA-1 interaction is mediated through the second leucine zipper of FIAT. These data confirm an additional target of the FIAT transcriptional repressor activity and suggest that FIAT can both modulate early osteoblast activity by interacting with ATF4 and regulate later osteoblast function through inhibition of FRA-1.