Divergent Proteome Reactivity Influences Arm-Selective Activation of the Unfolded Protein Response by Pharmacological Endoplasmic Reticulum Proteostasis Regulators.

Pharmacological activation of the activating transcription factor 6 (ATF6) arm of the unfolded protein response (UPR) has proven useful for ameliorating proteostasis deficiencies in cellular and mouse models of numerous etiologically diverse diseases. Previous high-throughput screening efforts identified the small molecule AA147 as a potent and selective ATF6 activating compound that operates through a mechanism involving metabolic activation of its 2-amino-p-cresol substructure affording a quinone methide, which then covalently modifies a subset of endoplasmic reticulum (ER) protein disulfide isomerases (PDIs). Another compound identified in this screen, AA132, also contains a 2-amino-p-cresol moiety; however, this compound showed less transcriptional selectivity, instead globally activating all three arms of the UPR. Here, we show that AA132 activates global UPR signaling through a mechanism analogous to that of AA147, involving metabolic activation and covalent modification of proteins including multiple PDIs. Chemoproteomic-enabled analyses show that AA132 covalently modifies PDIs to a greater extent than AA147. However, the extent of PDI labeling by AA147 approaches a plateau more rapidly than PDI labeling by AA132. These observations together suggest that AA132 can access a larger pool of proteins for covalent modification, possibly because its activated form is less susceptible to quenching than activated AA147. In other words, the lower reactivity of activated AA132 allows it to persist longer and modify more PDIs in the cellular environment. Collectively, these results suggest that AA132 globally activates the UPR through increased engagement of ER PDIs. Consistent with this, reducing the cellular concentration of AA132 decreases PDI modifications and enables selective ATF6 activation. Our results highlight the relationship between metabolically activatable-electrophile stability, ER proteome reactivity, and the transcriptional response observed with the enaminone chemotype of ER proteostasis regulators, enabling continued development of next-generation ATF6 activating compounds.

Divergent Proteome Reactivity Influences Arm-Selective Activation of Pharmacological Endoplasmic Reticulum Proteostasis Regulators.

Pharmacological activation of the activating transcription factor 6 (ATF6) arm of the Unfolded Protein Response (UPR) has proven useful for ameliorating proteostasis deficiencies in a variety of etiologically diverse diseases. Previous high-throughput screening efforts identified the small molecule AA147 as a potent and selective ATF6 activating compound that operates through a mechanism involving metabolic activation of its 2-amino- p -cresol substructure affording a quinone methide, which then covalently modifies a subset of ER protein disulfide isomerases (PDIs). Intriguingly, another compound identified in this screen, AA132, also contains a 2-amino- p -cresol moiety; however, this compound showed less transcriptional selectivity, instead globally activating all three arms of the UPR. Here, we show that AA132 activates global UPR signaling through a mechanism analogous to that of AA147, involving metabolic activation and covalent PDI modification. Chemoproteomic-enabled analyses show that AA132 covalently modifies PDIs to a greater extent than AA147. Paradoxically, activated AA132 reacts slower with PDIs, indicating it is less reactive than activated AA147. This suggests that the higher labeling of PDIs observed with activated AA132 can be attributed to its lower reactivity, which allows this activated compound to persist longer in the cellular environment prior to quenching by endogenous nucleophiles. Collectively, these results suggest that AA132 globally activates the UPR through increased engagement of ER PDIs. Consistent with this, reducing the cellular concentration of AA132 decreases PDI modifications and allows for selective ATF6 activation. Our results highlight the relationship between metabolically activatable-electrophile stability, ER proteome reactivity, and the transcriptional response observed with the enaminone chemotype of ER proteostasis regulators, enabling continued development of next-generation ATF6 activating compounds.

Pharmacologic targeting of plasma cell endoplasmic reticulum proteostasis to reduce amyloidogenic light chain secretion.

Light chain (LC) amyloidosis (AL) involves the toxic aggregation of amyloidogenic immunoglobulin LCs secreted from a clonal expansion of diseased plasma cells. Current AL treatments use chemotherapeutics to ablate the AL plasma cell population. However, no treatments are available that directly reduce the toxic LC aggregation involved in AL pathogenesis. An attractive strategy to reduce toxic LC aggregation in AL involves enhancing endoplasmic reticulum (ER) proteostasis in plasma cells to reduce the secretion and subsequent aggregation of amyloidogenic LCs. Here, we show that the ER proteostasis regulator compound 147 reduces secretion of an amyloidogenic LC as aggregation-prone monomers and dimers in AL patient-derived plasma cells. Compound 147 was established to promote ER proteostasis remodeling by activating the ATF6 unfolded protein response signaling pathway through a mechanism involving covalent modification of ER protein disulfide isomerases (PDIs). However, we show that 147-dependent reductions in amyloidogenic LCs are independent of ATF6 activation. Instead, 147 reduces amyloidogenic LC secretion through the selective, on-target covalent modification of ER proteostasis factors, including PDIs, revealing an alternative mechanism by which this compound can influence ER proteostasis of amyloidogenic proteins. Importantly, compound 147 does not interfere with AL plasma cell toxicity induced by bortezomib, a standard chemotherapeutic used to ablate the underlying diseased plasma cells in AL. This shows that pharmacologic targeting of ER proteostasis through selective covalent modification of ER proteostasis factors is a strategy that can be used in combination with chemotherapeutics to reduce the LC toxicity associated with AL pathogenesis.

Blinded potency comparison of transthyretin kinetic stabilisers by subunit exchange in human plasma.

Transthyretin (TTR) tetramer dissociation is rate limiting for aggregation and subunit exchange. Slowing of TTR tetramer dissociation via kinetic stabiliser binding slows cardiomyopathy progression. Quadruplicate subunit exchange comparisons of the drug candidate AG10, and the drugs tolcapone, diflunisal, and tafamidis were carried out at 1, 5, 10, 20 and 30 µM concentrations in 4 distinct pooled wild type TTR (TTRwt) human plasma samples. These experiments reveal that the concentration dependence of the efficacy of each compound at inhibiting TTR dissociation was primarily determined by the ratio between the stabiliser's dissociation constants from TTR and albumin, which competes with TTR to bind kinetic stabilisers. The best stabilisers, tafamidis (80 mg QD), AG10 (800 mg BID), and tolcapone (3 x 100 mg over 12 h), exhibit very similar kinetic stabilisation at the plasma concentrations resulting from these doses. At a 10 µM plasma concentration, AG10 is slightly more potent as a kinetic stabiliser vs. tolcapone and tafamidis (which are similar), which are substantially more potent than diflunisal. Dissociation of TTR can be limited to 10% of its normal rate at concentrations of 5.7 µM AG10, 10.3 µM tolcapone, 12.0 µM tafamidis, and 188 µM diflunisal. The potency similarities revealed by our study suggest that differences in safety, adsorption and metabolism, pharmacokinetics, and tissue distribution become important for kinetic stabiliser clinical use decisions.

Pharmacologic IRE1/XBP1s activation confers targeted ER proteostasis reprogramming.

Activation of the IRE1/XBP1s signaling arm of the unfolded protein response (UPR) is a promising strategy to correct defects in endoplasmic reticulum (ER) proteostasis implicated in diverse diseases. However, no pharmacologic activators of this pathway identified to date are suitable for ER proteostasis remodeling through selective activation of IRE1/XBP1s signaling. Here, we use high-throughput screening to identify non-toxic compounds that induce ER proteostasis remodeling through IRE1/XBP1s activation. We employ transcriptional profiling to stringently confirm that our prioritized compounds selectively activate IRE1/XBP1s signaling without activating other cellular stress-responsive signaling pathways. Furthermore, we demonstrate that our compounds improve ER proteostasis of destabilized variants of amyloid precursor protein (APP) through an IRE1-dependent mechanism and reduce APP-associated mitochondrial toxicity in cellular models. These results establish highly selective IRE1/XBP1s activating compounds that can be widely employed to define the functional importance of IRE1/XBP1s activity for ER proteostasis regulation in the context of health and disease.

Pharmacologic ATF6 activation confers global protection in widespread disease models by reprograming cellular proteostasis.

Pharmacologic activation of stress-responsive signaling pathways provides a promising approach for ameliorating imbalances in proteostasis associated with diverse diseases. However, this approach has not been employed in vivo. Here we show, using a mouse model of myocardial ischemia/reperfusion, that selective pharmacologic activation of the ATF6 arm of the unfolded protein response (UPR) during reperfusion, a typical clinical intervention point after myocardial infarction, transcriptionally reprograms proteostasis, ameliorates damage and preserves heart function. These effects were lost upon cardiac myocyte-specific Atf6 deletion in the heart, demonstrating the critical role played by ATF6 in mediating pharmacologically activated proteostasis-based protection of the heart. Pharmacological activation of ATF6 is also protective in renal and cerebral ischemia/reperfusion models, demonstrating its widespread utility. Thus, pharmacologic activation of ATF6 represents a proteostasis-based therapeutic strategy for ameliorating ischemia/reperfusion damage, underscoring its unique translational potential for treating a wide range of pathologies caused by imbalanced proteostasis.

Modulating protein quality control.

Small molecules that modulate the unfolded protein response have the potential to treat a variety of human protein misfolding diseases.

Small molecule proteostasis regulators that reprogram the ER to reduce extracellular protein aggregation.

Imbalances in endoplasmic reticulum (ER) proteostasis are associated with etiologically-diverse degenerative diseases linked to excessive extracellular protein misfolding and aggregation. Reprogramming of the ER proteostasis environment through genetic activation of the Unfolded Protein Response (UPR)-associated transcription factor ATF6 attenuates secretion and extracellular aggregation of amyloidogenic proteins. Here, we employed a screening approach that included complementary arm-specific UPR reporters and medium-throughput transcriptional profiling to identify non-toxic small molecules that phenocopy the ATF6-mediated reprogramming of the ER proteostasis environment. The ER reprogramming afforded by our molecules requires activation of endogenous ATF6 and occurs independent of global ER stress. Furthermore, our molecules phenocopy the ability of genetic ATF6 activation to selectively reduce secretion and extracellular aggregation of amyloidogenic proteins. These results show that small molecule-dependent ER reprogramming, achieved through preferential activation of the ATF6 transcriptional program, is a promising strategy to ameliorate imbalances in ER function associated with degenerative protein aggregation diseases.