Unfolded protein response IRE1/XBP1 signaling is required for healthy mammalian brain aging

The physiological role of the unfolded protein response in the nervous system

The unfolded protein response (UPR) is a cellular stress response pathway activated when the endoplasmic reticulum, a crucial organelle for protein folding and modification, encounters an accumulation of unfolded or misfolded proteins. The UPR aims to restore endoplasmic reticulum homeostasis by enhancing protein folding capacity, reducing protein biosynthesis, and promoting protein degradation. It also plays a pivotal role in coordinating signaling cascades to determine cell fate and function in response to endoplasmic reticulum stress. Recent research has highlighted the significance of the UPR not only in maintaining endoplasmic reticulum homeostasis but also in influencing various physiological processes in the nervous system. Here, we provide an overview of recent findings that underscore the UPR's involvement in preserving the function and viability of neuronal and myelinating cells under physiological conditions, and highlight the critical role of the UPR in brain development, memory storage, retinal cone development, myelination, and maintenance of myelin thickness.

Less is better: various means to reduce protein load in the endoplasmic reticulum

The endoplasmic reticulum (ER) is an important organelle that controls the intracellular and extracellular environments. The ER is responsible for folding almost one-third of the total protein population in the eukaryotic cell. Disruption of ER-protein folding is associated with numerous human diseases, including metabolic disorders, neurodegenerative diseases, and cancer. During ER perturbations, the cells deploy various mechanisms to increase the ER-folding capacity and reduce ER-protein load by minimizing the number of substrates entering the ER to regain homeostasis. These mechanisms include signaling pathways, degradation mechanisms, and other processes that mediate the reflux of ER content to the cytosol. In this review, we will discuss the recent discoveries of five different ER quality control mechanisms, including the unfolded protein response (UPR), ER-associated-degradation (ERAD), pre-emptive quality control, ER-phagy and ER to cytosol signaling (ERCYS). We will discuss the roles of these processes in decreasing ER-protein load and inter-mechanism crosstalk.

Newsights of endoplasmic reticulum in hypoxia

The endoplasmic reticulum (ER) is important to cells because of its essential functions, including synthesizing three major nutrients and ion transport. When cellular homeostasis is disrupted, ER quality control (ERQC) system is activated effectively to remove misfolded and unfolded proteins through ER-phagy, ER-related degradation (ERAD), and molecular chaperones. When unfolded protein response (UPR) and ER stress are activated, the cell may be suffering a huge blow, and the most probable consequence is apoptosis. The membrane contact points between the ER and sub-organelles contribute to communication between the organelles. The decrease in oxygen concentration affects the morphology and structure of the ER, thereby affecting its function and further disrupting the stable state of cells, leading to the occurrence of disease. In this study, we describe the functions of ER-, ERQC-, and ER-related membrane contact points and their changes under hypoxia, which will help us further understand ER and treat ER-related diseases.

A Youthful Touch: Reversal of Aging Hallmarks by Cell Reprogramming

BACKGROUND

With the elderly population projected to double by 2050, there is an urgent need to address the increasing prevalence of age-related debilitating diseases and ultimately minimize discrepancies between the rising lifespan and stagnant health span. Cellular reprogramming by overexpression of Oct3/4, Klf4, Sox2, and cMyc (OKSM) transcription factors is gaining attention in this context thanks to demonstrated rejuvenating effects in human cell cultures and live mice, many of which can be uncoupled from dedifferentiation and loss of cell identity.

SUMMARY

Here, we review current evidence of the impact of cell reprogramming on established aging hallmarks and the underlying mechanisms that mediate these effects. We also provide a critical assessment of the challenges in translating these findings and, overall, cell reprogramming technologies into clinically translatable antiaging interventions.

KEY MESSAGES

Cellular reprogramming has the potential to reverse at least partially some key hallmarks of aging. However, further research is necessary to determine the biological significance and duration of such changes and to ensure the safety of cell reprogramming as a rejuvenation approach. With this review, we hope to stimulate new research directions in the quest to extend health span effectively.

The 'middle-aging' brain

Middle age has historically been an understudied period of life compared to older age, when cognitive and brain health decline are most pronounced, but the scope for intervention may be limited. However, recent research suggests that middle age could mark a shift in brain aging. We review emerging evidence on multiple levels of analysis indicating that midlife is a period defined by unique central and peripheral processes that shape future cognitive trajectories and brain health. Informed by recent developments in aging research and lifespan studies in humans and animal models, we highlight the utility of modeling non-linear changes in study samples with wide subject age ranges to distinguish life stage-specific processes from those acting linearly throughout the lifespan.

Potential Role of Endoplasmic Reticulum Stress in Modulating Protein Homeostasis in Oligodendrocytes to Improve White Matter Injury in Preterm Infants

Preterm white matter injury (WMI) is a demyelinating disease with high incidence and mortality in premature infants. Oligodendrocyte cells (OLs) are a specialized glial cell that produces myelin proteins and adheres to the axons providing energy and metabolic support which susceptible to endoplasmic reticulum protein quality control. Disruption of cellular protein homeostasis led to OLs dysfunction and cell death, immediately, the unfolded protein response (UPR) activated to attempt to restore the protein homeostasis via IRE1/XBP1s, PERK/eIF2α and ATF6 pathway that reduced protein translation, strengthen protein-folding capacity, and degraded unfolding/misfolded protein. Moreover, recent works have revealed the conspicuousness function of ER signaling pathways in regulating influenced factors such as calcium homeostasis, mitochondrial reactive oxygen generation, and autophagy activation to regulate protein hemostasis and improve the myelination function of OLs. Each of the regulation modes and their corresponding molecular mechanisms provides unique opportunities and distinct perspectives to obtain a deep understanding of different actions of ER stress in maintaining OLs' health and function. Therefore, our review focuses on summarizing the current understanding of ER stress on OLs' protein homeostasis micro-environment in myelination during white matter development, as well as the pathophysiology of WMI, and discussing the further potential experimental therapeutics targeting these factors that restore the function of the UPR in OLs myelination function. Potential Role of ER Stress in Modulating Protein Homeostasis in OLs. OLs, produce myelin proteins and provide energy and metabolic support which are susceptible to cellular protein homeostasis and ER protein quality control. 1) UPR plays a different role in activating IRE1/XBP1s, PERK/eIF2α, and ATF6 pathways not only in attempting to restore protein homeostasis to promote cell survival but also aggravating disruption of cellular protein homeostasis to accelerate cell death. 2) PERK pathway facilitated the protein secretion, amino acid metabolism, and stress response to promote cell survival via phosphorylating eIF2α level and strengthening ATF4 expression; Nevertheless, the prolonged activating of the PERK pathway could up-regulate CHOP, GADD34, and other pro-apoptotic factors to further aggravates cell injury. 3) IRE1 and ATF6 pathways enhanced various gene transcription associated with protein folding, secreting, EARD, and ERQC to prompt cell protein homeostasis micro-environment; However, sustained IRE1 and/or ATF6 activity could prompt cell survival toward apoptosis via the pro-apoptotic pathway, inflammation, and other patterns.

Dual RNase activity of IRE1 as a target for anticancer therapies

The unfolded protein response (UPR) is a cellular mechanism that protects cells during stress conditions in which there is an accumulation of misfolded proteins in the endoplasmic reticulum (ER). UPR activates three signaling pathways that function to alleviate stress conditions and promote cellular homeostasis and cell survival. During unmitigated stress conditions, however, UPR activation signaling changes to promote cell death through apoptosis. Interestingly, cancer cells take advantage of this pathway to facilitate survival and avoid apoptosis even during prolonged cell stress conditions. Here, we discuss different signaling pathways associated with UPR and focus specifically on one of the ER signaling pathways activated during UPR, inositol-requiring enzyme 1α (IRE1). The rationale is that the IRE1 pathway is associated with cell fate decisions and recognized as a promising target for cancer therapeutics. Here we discuss IRE1 inhibitors and how they might prove to be an effective cancer therapeutic.

The Unfolded Protein Response: A Double-Edged Sword for Brain Health

Efficient brain function requires as much as 20% of the total oxygen intake to support normal neuronal cell function. This level of oxygen usage, however, leads to the generation of free radicals, and thus can lead to oxidative stress and potentially to age-related cognitive decay and even neurodegenerative diseases. The regulation of this system requires a complex monitoring network to maintain proper oxygen homeostasis. Furthermore, the high content of mitochondria in the brain has elevated glucose demands, and thus requires a normal redox balance. Maintaining this is mediated by adaptive stress response pathways that permit cells to survive oxidative stress and to minimize cellular damage. These stress pathways rely on the proper function of the endoplasmic reticulum (ER) and the activation of the unfolded protein response (UPR), a cellular pathway responsible for normal ER function and cell survival. Interestingly, the UPR has two opposing signaling pathways, one that promotes cell survival and one that induces apoptosis. In this narrative review, we discuss the opposing roles of the UPR signaling pathways and how a better understanding of these stress pathways could potentially allow for the development of effective strategies to prevent age-related cognitive decay as well as treat neurodegenerative diseases.

Olfactory chemosensation extends lifespan through TGF-β signaling and UPR activation

Animals rely on chemosensory cues to survive in pathogen-rich environments. In Caenorhabditis elegans, pathogenic bacteria trigger aversive behaviors through neuronal perception and activate molecular defenses throughout the animal. This suggests that neurons can coordinate the activation of organism-wide defensive responses upon pathogen perception. In this study, we found that exposure to volatile pathogen-associated compounds induces activation of the endoplasmic reticulum unfolded protein response (UPRER) in peripheral tissues after xbp-1 splicing in neurons. This odorant-induced UPRER activation is dependent upon DAF-7/transforming growth factor beta (TGF-β) signaling and leads to extended lifespan and enhanced clearance of toxic proteins. Notably, rescue of the DAF-1 TGF-β receptor in RIM/RIC interneurons is sufficient to significantly recover UPRER activation upon 1-undecene exposure. Our data suggest that the cell non-autonomous UPRER rewires organismal proteostasis in response to pathogen detection, pre-empting proteotoxic stress. Thus, chemosensation of particular odors may be a route to manipulation of stress responses and longevity.

The unfolded protein response transcription factor XBP1s ameliorates Alzheimer's disease by improving synaptic function and proteostasis

Alteration in the buffering capacity of the proteostasis network is an emerging feature of Alzheimer's disease (AD), highlighting the occurrence of endoplasmic reticulum (ER) stress. The unfolded protein response (UPR) is the main adaptive pathway to cope with protein folding stress at the ER. Inositol-requiring enzyme-1 (IRE1) operates as a central ER stress sensor, enabling the establishment of adaptive and repair programs through the control of the expression of the transcription factor X-box binding protein 1 (XBP1). To artificially enforce the adaptive capacity of the UPR in the AD brain, we developed strategies to express the active form of XBP1 in the brain. Overexpression of XBP1 in the nervous system using transgenic mice reduced the load of amyloid deposits and preserved synaptic and cognitive function. Moreover, local delivery of XBP1 into the hippocampus of an 5xFAD mice using adeno-associated vectors improved different AD features. XBP1 expression corrected a large proportion of the proteomic alterations observed in the AD model, restoring the levels of several synaptic proteins and factors involved in actin cytoskeleton regulation and axonal growth. Our results illustrate the therapeutic potential of targeting UPR-dependent gene expression programs as a strategy to ameliorate AD features and sustain synaptic function.

EETs alleviate alveolar epithelial cell senescence by inhibiting endoplasmic reticulum stress through the Trim25/Keap1/Nrf2 axis

Alveolar epithelial cell (AEC) senescence is a key driver of a variety of chronic lung diseases. It remains a challenge how to alleviate AEC senescence and mitigate disease progression. Our study identified a critical role of epoxyeicosatrienoic acids (EETs), downstream metabolites of arachidonic acid (ARA) by cytochrome p450 (CYP), in alleviating AEC senescence. In vitro, we found that 14,15-EET content was significantly decreased in senescent AECs. Exogenous EETs supplementation, overexpression of CYP2J2, or inhibition of EETs degrading enzyme soluble epoxide hydrolase (sEH) to increase EETs alleviated AECs' senescence. Mechanistically, 14,15-EET promoted the expression of Trim25 to ubiquitinate and degrade Keap1 and promoted Nrf2 to enter the nucleus to exert an anti-oxidant effect, thereby inhibiting endoplasmic reticulum stress (ERS) and alleviating AEC senescence. Furthermore, in D-galactose (D-gal)-induced premature aging mouse model, inhibiting the degradation of EETs by Trifluoromethoxyphenyl propionylpiperidin urea (TPPU, an inhibitor of sEH) significantly inhibited the protein expression of p16, p21, and γH2AX. Meanwhile, TPPU reduced the degree of age-related pulmonary fibrosis in mice. Our study has confirmed that EETs are novel anti-senescence substances for AECs, providing new targets for the treatment of chronic lung diseases.

Deletion of the Unfolded Protein Response Transducer IRE1α Is Detrimental to Aging Photoreceptors and to ER Stress-Mediated Retinal Degeneration

Purpose

The unfolded protein response (UPR) is triggered when the protein folding capacity of the endoplasmic reticulum (ER) is overwhelmed and misfolded proteins accumulate in the ER, a condition referred to as ER stress. IRE1α is an ER-resident protein that plays major roles in orchestrating the UPR. Several lines of evidence implicate the UPR and its transducers in neurodegenerative diseases, including retinitis pigmentosa (RP), a group of inherited diseases that cause progressive dysfunction and loss of rod and cone photoreceptors. This study evaluated the contribution of IRE1α to photoreceptor development, homeostasis, and degeneration.

Methods

We used a conditional gene targeting strategy to selectively inactivate Ire1α in mouse rod photoreceptors. We used a combination of optical coherence tomography (OCT) imaging, histology, and electroretinography (ERG) to assess longitudinally the effect of IRE1α deficiency in retinal development and function. Furthermore, we evaluated the IRE1α-deficient retina responses to tunicamycin-induced ER stress and in the context of RP caused by the rhodopsin mutation RhoP23H.

Results

OCT imaging, histology, and ERG analyses did not reveal abnormalities in IRE1α-deficient retinas up to 3 months old. However, by 6 months of age, the Ire1α mutant animals showed reduced outer nuclear layer thickness and deficits in retinal function. Furthermore, conditional inactivation of Ire1α in rod photoreceptors accelerated retinal degeneration caused by the RhoP23H mutation.

Conclusions

These data suggest that IRE1α is dispensable for photoreceptor development but important for photoreceptor homeostasis in aging retinas and for protecting against ER stress-mediated photoreceptor degeneration.

A Year at the Forefront of Proteostasis and Aging

During aging, animals experience a decline in proteostasis activity, including loss of stress-response activation, culminating in the accumulation of misfolded proteins and toxic aggregates, which are causal in the onset of some chronic diseases. Finding genetic and pharmaceutical treatments that can increase organismal proteostasis and lengthen life is an ongoing goal of current research. The regulation of stress responses by cell non-autonomous mechanisms appears to be a potent way to impact organismal healthspan. In this Review, we cover recent findings in the intersection of proteostasis and aging, with a special focus on articles and preprints published between November 2021 and October 2022. A significant number of papers published during this time increased our understanding of how cells communicate with each other during proteotoxic stress. Finally, we also draw attention to emerging datasets that can be explored to generate new hypotheses that explain age-related proteostasis collapse.