Hormesis

Abstract

Protein folding stress is a salient feature of the most frequent neurodegenerative diseases. Although the accumulation of abnormally folded proteins is a well-characterized event underlying the pathology, the way cells respond to this phenomenon is not well understood. Signs of endoplasmic reticulum (ER) stress are a common marker of neurodegeneration in many diseases, which may represent two contrasting processes: cell protection events due to activation of adaptive programs, or a chronic stress state that culminates in apoptosis to eliminate irreversibly injured cells. Autophagy has been proposed as a protective mechanism to overcome neurodegeneration that is also modulated by ER stress. In this issue of autophagy Bertrand Mollereau’s group provides novel evidence indicating that engagement of nonharmful levels of ER stress protects against experimental Parkinson disease. At the mechanistic level, a homeostatic crosstalk between ER stress signaling and the autophagy pathway was proposed to mediate the therapeutic effects. This study, together with recent findings, supports the involvement of a “hormesis mechanism” to handle degeneration through preconditioning mediated by a dynamic balance between ER stress and autophagy. The implications for aging and future therapeutic development are discussed.

Keywords: :

• autophagy
• ER stress
• Parkinson disease
• neurodegeneration
• protein misfolding

The accumulation of diffusible oligomers or large aggregates of specific misfolded proteins is a salient characteristic of many neurodegenerative diseases including amyotrophic lateral sclerosis (ALS), Alzheimer disease, Parkinson disease (PD), and Huntington disease (HD). Perturbation in protein homeostasis is currently proposed as a key factor underlying the pathological effects of most of the well-characterized brain disease-linked proteins.^1,2 Strategies to either decrease protein aggregation or attenuate proximal downstream signaling are thought to be relevant targets for disease intervention. In this context, autophagy is emerging as an interesting candidate for therapeutic development because it operates as an efficient and selective mechanism for the degradation of abnormally folded proteins.³

Although the molecular mechanisms underlying neurodegeneration in general are poorly understood, accumulating evidence suggests that perturbation in ER function is a common event in many neurodegenerative diseases.¹ ER stress is mitigated through a series of complementary mechanisms that as a whole are known as the unfolded protein response (UPR).⁴ The UPR is a complex signal transduction pathway that selectively modulates the expression of proteins involved in diverse processes including folding, quality control, protein entry into the ER, and organelle biogenesis among others.⁴ Conversely, chronic or irreversible ER stress triggers cell death by apoptosis. The UPR is governed by several stress sensors located at the ER membrane. These sensors transduce information about the protein folding status at the ER lumen to the nucleus and cytosol by controlling expression of specific transcription factors and other proximal signaling components.⁴ The ER stress sensor inositol requiring kinase 1 (ERN1/IRE1α) induces the expression of the transcription factor XBP1, which modulates many genes related to folding and quality control mechanisms. ERN1/IRE1α also controls the activation of c-Jun-N-terminal kinase (MAPK8/JNK), which has been linked to the upregulation of autophagy under ER stress.⁵ Other mechanisms also link ER stress signaling with autophagy (see review in ref. 6). Autophagy is proposed to operate as a mechanism able to eliminate damaged organelles and aggregated proteins under conditions of ER stress.

ER Stress in Neurodegenerative Diseases

Although many reports have linked the occurrence of ER stress with neurodegeneration, genetic and pharmacological manipulations of the pathway have revealed a complex scenario, where the exact contribution of the ER stress to the pathology is difficult to predict a priori. The occurrence of ER stress may represent: (1) a chronic damaging process triggering neuronal loss, (2) the active engagement of prosurvival responses to adapt to the stress and reestablish protein homeostasis, or (3) a late epiphenomenon due to the dramatic alterations in homeostasis. Several studies have reported the expected result where attenuation of ER stress levels protects against neurodegeneration (reviewed in ref. 1). Unexpectedly, genetic inactivation of the UPR transcription factor XBP1 in the nervous system delays degeneration in mouse models of ALS⁷ and HD.⁸ These protective effects were attributed to the induction of autophagy, associated with the clearance of mutant SOD1 or HTT/huntingtin, respectively.^7,8 Nonetheless, although most reports have shown a positive effect of ERN1/MAPK8 signaling in the activation of autophagy, a recent study suggested that this pathway may also inhibit autophagy flux in the context of HD.⁹ Other alternative theoretical mechanisms have been proposed to explain these unexpected results, including the idea that targeting specific UPR components may actually generate low levels (nonlethal) of stress that could then engage a compensatory adaptive response (i.e., autophagy, see below).¹⁰ These observations suggest an interesting concept where the autophagy and UPR pathways operate as supra-autoregulatory networks that integrate information to handle perturbations of protein homeostasis in disease.

Hormesis: What Doesn’t Kill You Makes You Stronger

The idea that low levels of stress may actually protect against a subsequent injury has been around for decades in the toxicology area. In the context of ER stress, this concept has been applied also to the field of neurodegeneration and brain ischemia-reperfusion. Mild or low levels of ER stress selectively engages a subset of UPR signaling events, highlighting the specific induction of XBP1 and not of pro-apoptotic components such as DDIT3/CHOP.^11-13 ER stress preconditioning (i.e., exposure to nonlethal doses of pharmacological ER stressors) protects against brain ischemia.^14,15 Low levels of ER stress also reduce retinal endothelial inflammation¹⁶ and attenuate heart ischemia/reperfusion possibly due to expression of XBP1.¹⁷ The genetic induction of low ER stress also alleviates retinal degeneration in Drosophila models.¹³

Hormesis (from Greek hórmēsis “rapid motion, eagerness,” from ancient Greek hormáein “to set in motion, impel, urge on”) refers to a biological favorable response triggered by subjecting an organism/cell to a low exposure of toxins and other stressors.¹⁸ Conditions that stimulate hormesis can engage adaptive stress signaling events rendering cells resistant against a high dose of the same stimuli. In terms of toxicological definitions, Calabrese and Baldwin considered hormesis as “an adaptive response characterized by biphasic dose responses of generally similar quantitative features with respect to amplitude and range of the stimulatory response that are either directly induced or the result of compensatory biological processes following an initial disruption in homeostasis.”¹⁹ It is becoming evident that mild perturbations of ER function may trigger a hormetic mechanism of protection possibly due to the activation of a prosurvival UPR response that bursters the folding and quality control capacity of the ER.⁴ This phenomenon is actually observed in most physiological systems that requires the UPR for their proper function, such as specialized secretory cells.⁴ New evidence provided in this issue of Autophagy by Fouillet et al., support the concept that one of the main hormetic mechanisms triggered by exposure to nonlethal ER stress is the engagement of autophagy.²⁰

Interplay Between Autophagy and ER Stress in Parkinson Disease: A Complex Hormetic Rheostat

PD is the second most frequent neurodegenerative disease involving the loss of dopaminergic neurons in the substantia nigra pars compacta. One of the most studied PD-related gene products is SNCA/α-synuclein, which is the main component of intracellular inclusions termed Lewy bodies, a key histopathological feature of the disease. Many studies have demonstrated the occurrence of ER stress in animal and cellular models of PD, in addition to postmortem brain tissue from PD patients (see examples in refs. 21–24). The functional contribution of ER stress to PD in vivo has been only recently addressed. It was reported that the accumulation of SΝCΑ occurs inside the ER lumen during early stages of PD,²² associated with a later and increasing chronic ER stress response, that correlates with the occurrence of neurodegeneration. Consistent with this idea, attenuation of ER stress levels with pharmacological or gene therapy approaches delays disease progression.^23,24 Thus, chronic levels of irreversible ER stress were proposed as an important factor underlying neuronal loss in PD. Many other studies have linked ER-related injuries with PD (review in ref. 1).

Mollereau's study indicates that sublethal levels of ER stress generate a preconditioning effect that provides strong neuroprotection against experimental PD in Drosophila and mouse models.²⁰ The neuroprotective effects of nonlethal ER stress are functionally linked to the engagement of autophagy. For example, feeding transgenic flies overexpressing SNCA with tunicamycin (a pharmacological ER stress agent) triggers low levels of stress in the brain reflected in the selective activation of XBP1 splicing and upregulation of autophagy. In this model, genetic inactivation of essential autophagy genes reverses the protective effects of ER stress. Surprisingly, the authors were able to treat mice with tunicamycin through intraperitoneal injections and protect dopaminergic neurons against a PD-inducing neurotoxin, improving motor performance. Experiments in mammalian cells corroborated the role of autophagy in the beneficial effects of mild ER stress preconditioning against experimental PD. This study demonstrates a hormetic mechanism in the control of degeneration in PD where a direct interplay between the UPR and the autophagy pathways generates efficient and long-term neuroprotection (Fig. 1). The molecular link mediating the crosstalk between these two homeostatic pathways in PD remains to be elucidated. A few reports have also suggested that the homeostatic balance between autophagy and ER stress can operate in the opposite direction. For example, in models of ischemia/preconditioning, autophagy was recently shown to regulate ER stress levels.²⁵ MTOR (an upstream regulator of autophagy) signaling also modulates ER stress signaling.²⁶ In addition, autophagy-deficient tumors show a dramatic upregulation of ER chaperones, which may contribute to tumor survival and growth under hypoxic and nutrient starvation conditions.²⁷ Finally, deficiency of ATG7 in the liver triggers ER stress, contributing to insulin resistance in obesity models.²⁸ Thus, the autophagy and UPR signaling network may operate as a delicate rheostat that balances physiological fluctuations and pathological perturbations in protein homeostasis in both directions: UPR signaling controlling autophagy levels, and autophagy signaling/activity controlling ER stress levels.

Figure 1. Hormesis mechanism to avoid degeneration in PD. During presymptomatic stages of PD, low levels of stress (i.e., oxidative and ER stress) may trigger the activation of autophagy as a survival pathway to reduce stress levels. This preconditioning may effect protect against a subsequent toxic stimuli that prevents or delays degeneration of dopaminergic neurons through a hormesis mechanism. The appearance of the clinical manifestation of PD may be due in part to a gradual increase in stress levels to an irreversible point where hormesis mechanisms (i.e., UPR prosurvival/adaptive signaling and upregulation of autophagy) are not sufficient to recover cellular homeostasis.

We also speculate that a hormesis-related mechanism may underlie the notion that most neurodegenerative diseases are clinically manifested during aging despite the fact the pathological events are operating since very early presymptomatic stages. This is especially evident in familial forms of the disease where the patient is born with a genetic mutation. In this scenario, low or fluctuating perturbations of ER function during early presymptomatic phases of the disease may actually promote adaptive programs that in the long-term protect neurons against subsequent and stronger pathological stimuli (Fig. 2). Then, only when stress levels reach an irreversible point, neurological impairment is expressed, which is in agreement with recent finding by Lee’s group.^22,23 In this context, aging-related events (the major risk factor to developing neurodegenerative diseases) may constitute a crucial factor in ablating hormetic mechanisms of survival due to the known attenuation of the adaptive capacity of the cell against protein homeostasis alterations (Fig. 2).³ In fact, chronic treatment with low doses of rapamycin (an autophagy inducer) or exposure to organismal stress (i.e., nonlethal heat shock or oxidative stress) have a tremendous effect on prolonging life span in animal models.³ The impairment of adaptive mechanisms during aging may dramatically burster ER and oxidative stress levels in a vicious cycle, leading to the irreversible damage of the cell.² Based on increasing evidence suggesting a causative role of autophagy defects and ER perturbations in neurodegenerative diseases, these new studies set an interesting scenario where targeting one particular component of a stress pathway may compensate or alleviate defects in other points of the protein homeostasis network through a hormetic mechanism.

Figure 2. The aging and hormesis rehostat in neurodegenerative diseases. During presymptomatic stages of disease, low or fluctuating perturbations of ER function trigger adaptive programs that protect neurons against subsequent stress episodes. When stress levels reach an irreversible point, neurological impairment is expressed. The gradual decrease of autophagy activity and increased oxidative stress during aging ablates the hormetic capacity of the cell, further increasing stress/injury levels in a vicious cycle, culminating in irreversible neuronal dysfunction and possibly neuronal loss.

Abbreviations:
ALS	=	amyotrophic lateral sclerosis
PD	=	Parkinson disease
HD	=	Huntington disease
ER	=	endoplasmic reticulum
ERN1/IRE1α	=	inositol requiring kinase 1
UPR	=	unfolded protein response
MAPK8/JNK	=	c-Jun-N terminal kinase
XBP1	=	X-Box Binding protein-1
SNCA	=	α-synuclein

Acknowledgments

This article was funded by the Michael J. Fox Foundation for Parkinson’s Research (C.H.). In addition we received support from FONDECYT no. 1100176, FONDAP grant no. 15010006, Millennium Institute no. P09-015-F, Muscular Dystrophy Association, ALS Therapy Alliance, North American Spine Society and Alzheimer’s Disease Foundation (C.H.), Insercion de Capital Humano Avanzado CONICYT no. 79100007 (S.M.) and FONDECYT no. 3100112 (K.C.).