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The Unfolded Protein Response Regulator, ATF6, is Essential for Cone Photoreceptor Development

ABSTRACT: The Unfolded Protein Response is a conserved intracellular signal transduction mechanism that maintains endoplasmic reticulum homeostasis and may contribute to the pathogenesis and progression of human diseases associated with ER stress. Genetic mutations in the UPR regulator, ATF6, lead to vision loss in heritable retinal diseases. Our prior studies found that disease mutations disrupt ATF6 function, but the pathomechanism by which loss of ATF6 activity causes vision loss in people is unknown. To investigate this, we developed retinal organoids from patient iPSCs carrying ATF6 disease mutations and from hESCs bearing CRISPR-edited ATF6 null alleles. Unexpectedly, we found that retinal organoids lacking functional ATF6 failed to form cone photoreceptors while rod photoreceptors were unaffected. We used adaptive optics imaging of the retinas in patients with ATF6 mutations and saw pronounced loss of cones and preservation of rods that closely mirrored our in vitro organoid phenotypes. Last, we found that a small molecule proteostasis agonist partially restored cone photoreceptor outer segment formation and gene expression in ATF6 mutant retinal organoids. Our results reveal a surprising and novel function for ATF6 in human cone photoreceptor development. Our findings identify a potential therapeutic strategy for patients based on small molecule reprogramming of the proteostasis network in the retina. Our study suggests that the UPR guides intrinsic developmental processes in specialized metazoan cell types in addition to its role in responding to extrinsic pathologic events.

ORGANISM(S): Homo sapiens

PROVIDER: GSE153302 | GEO | 2021/06/25

REPOSITORIES: GEO

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Project description:The loss of cone photoreceptor cells, which are critical for optimal daylight vision, have the great impact on vision during retinal degenerations. Retinal differentiation of human induced pluripotent stem cell (hiPSC) sources could provide a renewable source of cone photoreceptors towards developing a cone cell replacement therapy to treat blindness. Demonstration of comparable gene expression profiles between human foetal and stem cell-derived cones at equivalent stages is required to progress the cell transplantation approach into the patient, as it is hypothesised stem cell-derived cones are required to show high levels of developmental recapitulation of the in vivo generated cones. In this study, the AAV2/9.pR2.1.GFP reporter was used to specifically label L/M-opsin cone photoreceptors in human foetal retinal samples, obtained from the MRC-Wellcome Trust Human Developmental Biology Resource, at a range of developmental stages. The L/M-opsin cone population represent the majority cone cell types in the adult human retina and are the only photoreceptors present within the fovea. Using fluorescence activated cell sorting, L/M-opsin GFP+ cones and GFP- retinal populations, alongside total foetal retinal samples containing all retinal cell tytpes, were isolated and processed for bulk RNA sequencing and downstream comparative analysis. Using DESeq2 differential gene expression analyses, statistically significant genes enriched within the GFP+ human foetal LM-opsin cone populations were determined which led to the identification of a cone enriched gene signature of human L/M-opsin cone photoreceptors. The AAV2/9.pR2.1.GFP reporter was applied to hiPSC-derived retinal cultures to isolate and process cone-like cell populations for RNA sequencing using the same strategy developed within the human foetal retina. Applying the cone enriched gene signature to the transcriptome of hiPSC-derived GFP+ samples at equivalent developmental stages revealed some expression similarities in genes found to be enriched within the late foetal L/M-opsin cone photoreceptors. This analysis overall revealed an intermediate stage of cone differentiation was achieved within the hiPSC-derived samples and the comparison to human foetal L/M-opsin gene express profiles suggesting further differentiation of hiPSC-derived sample is required.

Project description:The unfolded protein response (UPR) maintains endoplasmic reticulum (ER) proteostasis through the activation of transcription factors such as XBP1s and ATF6. The functional consequences of these transcription factors for ER proteostasis remain poorly defined. Here, we describe methodology that enables orthogonal, small molecule-mediated activation of the UPR-associated transcription factors XBP1s and/or ATF6 in the same cell independent of stress. We employ transcriptomics and quantitative proteomics to evaluate ER proteostasis network remodeling owing to the XBP1s and/or ATF6 transcriptional programs. Furthermore, we demonstrate that the three ER proteostasis environments accessible by activating XBP1s and/or ATF6 differentially influence the folding, trafficking, and degradation of destabilized ER client proteins without globally affecting the endogenous proteome. Our data reveal how the ER proteostasis network is remodeled by the XBP1s and/or ATF6 transcriptional programs at the molecular level and demonstrate the potential for selectively restoring aberrant ER proteostasis of pathologic, destabilized proteins through arm-selective UPR-activation. The unfolded protein response adapts endoplasmic reticulum (ER) proteostasis via stress-responsive transcription factors including XBP1s and ATF6. Here, R. Luke Wiseman and colleagues implement technology for the orthogonal, ligand-dependent activation of XBP1s and/or ATF6 in a single cell. They characterize how XBP1s and/or ATF6 activation impacts ER proteostasis pathway composition and function. Adapted ER environments influence the proteostasis of destabilized protein variants without affecting the endogenous proteome. The work informs the development of proteostasis environment-adapting therapeutics for protein misfolding-related diseases. In order to activate both XBP1s and ATF6 in the same cell, we incorporated DHFR.ATF6 and tet-inducible XBP1s into a HEK293T-REx cell line stably expressing the tet-repressor. The HEK293DYG control cell line expresses tet-inducible eGFP and DHFR.YFP and is used as a control to demonstrate that the addition of doxycycline (dox) and trimethoprim (TMP) do not induce UPR genes. HEK293DYG cells were treated for 12 h with vehicle or 1 μg/mL dox and 10 μM TMP in biological triplicate. Cells were harvested and RNA was extracted using the RNeasy Mini Kit (Qiagen). Genomic DNA was removed by on-column digestion using the RNase-free DNase Set (Qiagen). Data from HEK293DYG cells showed no significant overlap in the ligand-treated transcriptomes obtained from HEK293DAX cells.

Project description:The unfolded protein response (UPR) maintains endoplasmic reticulum (ER) proteostasis through the activation of transcription factors such as XBP1s and ATF6. The functional consequences of these transcription factors for ER proteostasis remain poorly defined. Here, we describe methodology that enables orthogonal, small molecule-mediated activation of the UPR-associated transcription factors XBP1s and/or ATF6 in the same cell independent of stress. We employ transcriptomics and quantitative proteomics to evaluate ER proteostasis network remodeling owing to the XBP1s and/or ATF6 transcriptional programs. Furthermore, we demonstrate that the three ER proteostasis environments accessible by activating XBP1s and/or ATF6 differentially influence the folding, trafficking, and degradation of destabilized ER client proteins without globally affecting the endogenous proteome. Our data reveal how the ER proteostasis network is remodeled by the XBP1s and/or ATF6 transcriptional programs at the molecular level and demonstrate the potential for selectively restoring aberrant ER proteostasis of pathologic, destabilized proteins through arm-selective UPR-activation. The unfolded protein response adapts endoplasmic reticulum (ER) proteostasis via stress-responsive transcription factors including XBP1s and ATF6. Here, R. Luke Wiseman and colleagues implement technology for the orthogonal, ligand-dependent activation of XBP1s and/or ATF6 in a single cell. They characterize how XBP1s and/or ATF6 activation impacts ER proteostasis pathway composition and function. Adapted ER environments influence the proteostasis of destabilized protein variants without affecting the endogenous proteome. The work informs the development of proteostasis environment-adapting therapeutics for protein misfolding-related diseases. In order to activate both XBP1s and ATF6 in the same cell, we incorporated DHFR.ATF6 and tet-inducible XBP1s into a HEK293T-REx cell line stably expressing the tet-repressor. Selection of a single colony resulted in the HEK293DAX cell line in which XBP1s is induced by doxycycline and DHFR.ATF6 is activated by trimethoprim (TMP; TMP-dependent DHFR.ATF6 activation in HEK293DAX cells will henceforth be referred to as ATF6 activation for simplicity). HEK293DAX cells were treated for 12 h with vehicle, 1 ?g/mL dox, 10 ?M TMP, or both in biological triplicate. Cells were harvested and RNA was extracted using the RNeasy Mini Kit (Qiagen). Genomic DNA was removed by on-column digestion using the RNase-free DNase Set (Qiagen). Data from HEK293DYG cells showed no significant overlap in the ligand-treated transcriptomes obtained from the control HEK293DYG cells.

Project description:Human cone photoreceptors represent a rare cell population of the human retina and are largely outnumbered by other retinal cell types. Despite their rarity, cone photoreceptors are critical for optimal daylight vision and their loss has the greatest impact on sight during the progression of retinal dystrophies. This is most applicable to the L/M-opsin cone populations which form the largest proportion of cones in the human retina and are the only cone types present within the fovea, which is responsible for fine detailed vision. Better understanding of the early genesis of these cells within the human developing retina, particularly in relation to the expression of genes that could be involved in regulating early events such as cell fate decision, is required. These molecular determinants could be exploited in vitro to generate more cone photoreceptors within human stem cell-derived retinal differentiation culture systems, suitable for use in cell transpantation therapies or disease modelling purposes. Additionally, the nature of this cell population in terms of their gene expression heterogeneity remains to be defined. State of the art technology has now advanced so that mRNA sequencing can be performed on individual cells, which means the transcriptome of rare cell populations such as cone photoreceptor cells can be interrogated without the need for ample material. Using the AAV2/9 pR2.1:GFP reporter, containing a synthetic promoter region previously characterised to drive expression in cone photoreceptors, we label and isolate human foetal L/M-opsin cone photoreceptors within the 15pcw human retinal retina. Isolated GFP+ cells were separated into individual cells using the Fluidigm C1 microfludics system, before RNA extraction and cDNA library preparation was performed. In total, 74 single cells were captured and processed for RNA sequencing; quality control of the data led to the inclusion of 65 individual cells for the downstream analysis. Principal component analysis revealed a very subtle heterogeneity across the cells, with the highest variability observed according to the first principal component (PC1). Differential gene expression analysis correlated to PC1 led to the identification of 503 significantly differentially expressed genes. Notably, some genes increase in expression across the cells when ordered according to PC1 which have previous associations with photoreceptor maturation, whereas other genes associated with developmental processes became downregulated across the cells. This led to the hypothesis that at this single timepoint in the human foetal retina, developing L/M-opsin cone photoreceptors exist at different stages of maturation. Overlapping genes were identified from comparing RNA seq data from 15pcw single cells and population samples at early and late timepoints of pR2.1:GFP+ cells, which showed the same gene expression trend between experiments. This suggests differences in cell maturation at a single timepoint of development could be representative of their true developmental trajectory. This overlapping gene dataset is also highly useful to define new gene that could be involved in cone photoreceptor development.

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The Unfolded Protein Response Regulator, ATF6, is Essential for Cone Photoreceptor Development

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