Liviu Aron

REST and stress resistance in ageing and Alzheimer’s disease

Human neurons are functional over an entire lifetime, yet the mechanisms that preserve function and protect against neurodegeneration during ageing are unknown. Here we show that induction of the repressor element 1-silencing transcription factor (REST; also known as neuron-restrictive silencer factor, NRSF) is a universal feature of normal ageing in human cortical and hippocampal neurons. REST is lost, however, in mild cognitive impairment and Alzheimer’s disease. Chromatin immunoprecipitation with deep sequencing and expression analysis show that REST represses genes that promote cell death and Alzheimer’s disease pathology, and induces the expression of stress response genes. Moreover, REST potently protects neurons from oxidative stress and amyloid β-protein toxicity, and conditional deletion of REST in the mouse brain leads to age-related neurodegeneration. A functional orthologue of REST, Caenorhabditis elegans SPR-4, also protects against oxidative stress and amyloid β-protein toxicity. During normal ageing, REST is induced in part by cell non-autonomous Wnt signalling. However, in Alzheimer’s disease, frontotemporal dementia and dementia with Lewy bodies, REST is lost from the nucleus and appears in autophagosomes together with pathological misfolded proteins. Finally, REST levels during ageing are closely correlated with cognitive preservation and longevity. Thus, the activation state of REST may distinguish neuroprotection from neurodegeneration in the ageing brain.

The DNA damage response induces inflammation and senescence by inhibiting autophagy of GATA4

Transcriptional control of cell senescence Senescent cells that have stopped proliferating secrete molecules that influence the cells around them. Prevention of this senescence-activated secretory phenotype seems to slow organismal aging. Kang et al. explored the regulatory process behind cell senescence and found that DNA damage led to stabilization of the transcription factor GATA4 (see the Perspective by Cassidy and Narita). Increased activity of GATA4 in senescent cells stimulated genes encoding secreted factors. GATA4 also accumulates in the brains of aging mice or humans. Science, this issue 10.1126/science.aaa5612; see also p. 1448 The transcription factor GATA4 promotes cell senescence. [Also see Perspective by Cassidy and Narita] INTRODUCTION Cellular senescence is a program of arrested proliferation and altered gene expression triggered by many stresses. Although it is a potent tumor-suppressive mechanism, senescence has been implicated in several pathological processes including aging, age-associated diseases, and (counterintuitively) tumorigenesis. One potential mechanism through which senescent cells exert such pleiotropic effects is the secretion of proinflammatory cytokines, chemokines, growth factors, and proteases, termed the senescence-associated secretory phenotype (SASP), which affects senescent cells and their microenvironment. The mechanism by which the SASP is initiated and maintained is not well characterized beyond the classical regulators of inflammation, including the transcription factors NF-κB and C/EBPβ. RATIONALE In senescence growth arrest, two core senescence-regulating pathways, p53 and p16INK4a/Rb, play a critical role. By contrast, the SASP does not depend on either p53 or p16INK4a, which suggests the existence of an independent senescence regulatory network that controls the SASP. Having observed high levels of induction of microRNA miR-146a during induced senescence in human fibroblasts, we developed a green fluorescent protein–tagged senescence reporter based on a miR-146a promoter fragment. This reporter responded to senescence-inducing stimuli, including replicative exhaustion, DNA damage, and oncogenic RAS activation—all of which activate the SASP. This system allowed us to identify additional regulators of senescence and the SASP. RESULTS Through miR-146a promoter analysis, we mapped the critical region for senescence-induced activity and identified the transcriptional regulator responsible for this regulation, GATA4, previously known as a regulator of embryonic development. Ectopic expression of GATA4 induced senescence, whereas disruption of GATA4 suppressed it, thus establishing GATA4 as a senescence regulator. GATA4 protein abundance, but not mRNA, increased during sene1scence, primarily as a result of increased protein stability. Under normal conditions, GATA4 binds the p62 autophagy adaptor and is degraded by selective autophagy. Upon senescence induction, however, this selective autophagy was suppressed through decreased interaction between GATA4 and p62, thereby stabilizing GATA4. GATA4 in turn induced TRAF3IP2 (tumor necrosis factor receptor–associated factor interacting protein 2) and IL1A (interleukin 1A), which activate NF-κB to initiate and maintain the SASP, thus facilitating senescence. GATA4 pathway activation depends on the key DNA damage response (DDR) kinases ATM (ataxia telangiectasia mutated) and ATR (ataxia telangiectasia and Rad3–related), as does senescence-associated activation of p53 and p16INK4a. However, the GATA4 pathway is independent of p53 and p16INK4a. Finally, GATA4 protein accumulated in multiple tissues in mice treated with senescence-inducing stimuli and during normal mouse and human aging, including many cell types in the brain; these findings raise the possibility that the GATA4 pathway drives age-dependent inflammation. CONCLUSION Our results indicate that GATA4 connects autophagy and the DDR to senescence and inflammation through TRAF3IP2 and IL1A activation of NF-κB. These findings establish GATA4 as a key switch activated by the DDR to regulate senescence, independently of p53 and p16INK4a. Our in vivo data indicate a potential role of GATA4 during aging and its associated inflammation. Because accumulation of senescent cells is thought to promote aging and aging-associated diseases through the resulting inflammatory response, inhibiting the GATA4 pathway may provide an avenue for therapeutic intervention. GATA4 functions as a key switch in the senescence regulatory network to activate the SASP. The nonsenescent state is maintained by inhibitory barriers that prevent cell cycle arrest and inflammation. Upon senescence-inducing signals, ATM and ATR relieve inhibition of the p53 and p16INK4a pathways to induce growth arrest and also block p62-dependent autophagic degradation of GATA4, resulting in NF-κB activation and SASP induction. Cellular senescence is a terminal stress-activated program controlled by the p53 and p16INK4a tumor suppressor proteins. A striking feature of senescence is the senescence-associated secretory phenotype (SASP), a pro-inflammatory response linked to tumor promotion and aging. We have identified the transcription factor GATA4 as a senescence and SASP regulator. GATA4 is stabilized in cells undergoing senescence and is required for the SASP. Normally, GATA4 is degraded by p62-mediated selective autophagy, but this regulation is suppressed during senescence, thereby stabilizing GATA4. GATA4 in turn activates the transcription factor NF-κB to initiate the SASP and facilitate senescence. GATA4 activation depends on the DNA damage response regulators ATM and ATR, but not on p53 or p16INK4a. GATA4 accumulates in multiple tissues, including the aging brain, and could contribute to aging and its associated inflammation.

Repairing the parkinsonian brain with neurotrophic factors

No therapy exists to slow down or prevent Parkinsons disease (PD), a debilitating neurodegenerative disorder. Neurotrophic factors (NTFs) emerged as promising disease-modifying agents in PD and are currently under clinical development. We argue that efforts in three research areas must converge to harness the full therapeutic power of NTFs. First, the physiological roles of NTFs in aging dopaminergic neurons must be comprehensively understood. Second, the mechanisms underlying the neuroprotective, neurorestorative and stimulatory effects of NTFs on diseased neurons need to be defined. Third, improved brain delivery of NTFs and new ways to stimulate NTF signaling are required to achieve clinical benefits. In this review, we discuss progress in these areas and highlight emerging concepts in NTF biology and therapy.

Pitx3 Is a Critical Mediator of GDNF-Induced BDNF Expression in Nigrostriatal Dopaminergic Neurons

Pitx3 is a critical homeodomain transcription factor for the proper development and survival of mesodiencephalic dopaminergic (mdDA) neurons in mammals. Several variants of this gene have been associated with human Parkinsons disease (PD), and lack of Pitx3 in mice causes the preferential loss of substantia nigra pars compacta (SNc) mdDA neurons that are most affected in PD. It is currently unclear how Pitx3 activity promotes the survival of SNc mdDA neurons and which factors act upstream and downstream of Pitx3 in this context. Here we show that a transient expression of glial cell line-derived neurotrophic factor (GDNF) in the murine ventral midbrain (VM) induces transcription of Pitx3 via NF-κB-mediated signaling, and that Pitx3 is in turn required for activating the expression of brain-derived neurotrophic factor (BDNF) in a rostrolateral (SNc) mdDA neuron subpopulation during embryogenesis. The loss of BDNF expression correlates with the increased apoptotic cell death of this mdDA neuronal subpopulation in Pitx3−/− mice, whereas treatment of VM cell cultures with BDNF augments the survival of the Pitx3−/− mdDA neurons. Most importantly, only BDNF but not GDNF protects mdDA neurons against 6-hydroxydopamine-induced cell death in the absence of Pitx3. As the feedforward regulation of GDNF, Pitx3, and BDNF expression also persists in the adult rodent brain, our data suggest that the disruption of the regulatory interaction between these three factors contributes to the loss of mdDA neurons in Pitx3−/− mutant mice and perhaps also in human PD.

Inactivation of VCP/ter94 suppresses retinal pathology caused by misfolded rhodopsin in Drosophila.

The most common Rhodopsin (Rh) mutation associated with autosomal dominant retinitis pigmentosa (ADRP) in North America is the substitution of proline 23 by histidine (RhP23H). Unlike the wild-type Rh, mutant RhP23H exhibits folding defects and forms intracellular aggregates. The mechanisms responsible for the recognition and clearance of misfolded RhP23H and their relevance to photoreceptor neuron (PN) degeneration are poorly understood. Folding-deficient membrane proteins are subjected to Endoplasmic Reticulum (ER) quality control, and we have recently shown that RhP23H is a substrate of the ER–associated degradation (ERAD) effector VCP/ter94, a chaperone that extracts misfolded proteins from the ER (a process called retrotranslocation) and facilitates their proteasomal degradation. Here, we used Drosophila, in which Rh1P37H (the equivalent of mammalian RhP23H) is expressed in PNs, and found that the endogenous Rh1 is required for Rh1P37H toxicity. Genetic inactivation of VCP increased the levels of misfolded Rh1P37H and further activated the Ire1/Xbp1 ER stress pathway in the Rh1P37H retina. Despite this, Rh1P37H flies with decreased VCP function displayed a potent suppression of retinal degeneration and blindness, indicating that VCP activity promotes neurodegeneration in the Rh1P37H retina. Pharmacological treatment of Rh1P37H flies with the VCP/ERAD inhibitor Eeyarestatin I or with the proteasome inhibitor MG132 also led to a strong suppression of retinal degeneration. Collectively, our findings raise the possibility that excessive retrotranslocation and/or degradation of visual pigment is a primary cause of PN degeneration.

Pro-Survival Role for Parkinson's Associated Gene DJ-1 Revealed in Trophically Impaired Dopaminergic Neurons

A mouse genetic study reveals a novel cell-survival role for the Parkinsons disease-associated gene DJ-1 in dopaminergic neurons that have reduced support from endogenous survival factors.

ER stress in retinal degeneration: a target for rational therapy?

Mutations that cause rhodopsin misfolding and retention within the endoplasmic reticulum (ER) are a prominent cause of retinitis pigmentosa. Here, we discuss the hypothesis that the failure of photoreceptor neurons to adapt to the stress caused by rhodopsin accumulation in the ER leads to a global collapse of homeostasis and to retinal degeneration. We review the molecular mechanisms underlying the activity of local ER conformational sensors and stress-relaying modules and consider how ER-derived stress signals are amplified and implemented to impact on downstream processes, including rhodopsin clearance and cell fate control. The emerging view is that alterations to the systems responsible for the detection, transduction and implementation of ER stress might be used therapeutically to treat retinitis pigmentosa.

Clearance of Rhodopsin(P23H) aggregates requires the ERAD effector VCP.

Dominant mutations in the visual pigment Rhodopsin (Rh) cause retinitis pigmentosa (RP) characterized by progressive blindness and retinal degeneration. The most common Rh mutation, Rh(P23H) forms aggregates in the endoplasmic reticulum (ER) and impairs the proteasome; however, the mechanisms linking Rh aggregate formation to proteasome dysfunction and photoreceptor cell loss remain unclear. Using mammalian cell cultures, we provide the first evidence that misfolded Rh(P23H) is a substrate of the ERAD effector VCP, an ATP-dependent chaperone that extracts misfolded proteins from the ER and escorts them for proteasomal degradation. VCP co-localizes with misfolded Rh(P23H) in retinal cells and requires functional N-terminal and D1 ATPase domains to form a complex with Rh(P23H) aggregates. Furthermore, VCP uses its D2 ATPase activity to promote Rh(P23H) aggregate retrotranslocation and proteasomal delivery. Our results raise the possibility that modulation of VCP and ERAD activity might have potential therapeutic significance for RP.

Altered social behavior and neuronal development in mice lacking the Uba6-Use1 ubiquitin transfer system

The Uba6 (E1)-Use1 (E2) ubiquitin transfer cascade is a poorly understood alternative arm of the ubiquitin proteasome system (UPS) and is required for mouse embryonic development, independent of the canonical Uba1-E2-E3 pathway. Loss of neuronal Uba6 during embryonic development results in altered patterning of neurons in the hippocampus and the amygdala, decreased dendritic spine density, and numerous behavioral disorders. The levels of the E3 ubiquitin ligase Ube3a (E6-AP) and Shank3, both linked with dendritic spine function, are elevated in the amygdala of Uba6-deficient mice, while levels of the Ube3a substrate Arc are reduced. Uba6 and Use1 promote proteasomal turnover of Ube3a in mouse embryo fibroblasts (MEFs) and catalyze Ube3a ubiquitylation in vitro. These activities occur in parallel with an independent pathway involving Uba1-UbcH7, but in a spatially distinct manner in MEFs. These data reveal an unanticipated role for Uba6 in neuronal development, spine architecture, mouse behavior, and turnover of Ube3a.

Proteomic Survey Reveals Altered Energetic Patterns and Metabolic Failure Prior to Retinal Degeneration

Inherited mutations that lead to misfolding of the visual pigment rhodopsin (Rho) are a prominent cause of photoreceptor neuron (PN) degeneration and blindness. How Rho proteotoxic stress progressively impairs PN viability remains unknown. To identify the pathways that mediate Rho toxicity in PNs, we performed a comprehensive proteomic profiling of retinas from Drosophila transgenics expressing Rh1P37H, the equivalent of mammalian RhoP23H, the most common Rho mutation linked to blindness in humans. Profiling of young Rh1P37H retinas revealed a coordinated upregulation of energy-producing pathways and attenuation of energy-consuming pathways involving target of rapamycin (TOR) signaling, which was reversed in older retinas at the onset of PN degeneration. We probed the relevance of these metabolic changes to PN survival by using a combination of pharmacological and genetic approaches. Chronic suppression of TOR signaling, using the inhibitor rapamycin, strongly mitigated PN degeneration, indicating that TOR signaling activation by chronic Rh1P37H proteotoxic stress is deleterious for PNs. Genetic inactivation of the endoplasmic reticulum stress-induced JNK/TRAF1 axis as well as the APAF-1/caspase-9 axis, activated by damaged mitochondria, dramatically suppressed Rh1P37H-induced PN degeneration, identifying the mitochondria as novel mediators of Rh1P37H toxicity. We thus propose that chronic Rh1P37H proteotoxic stress distorts the energetic profile of PNs leading to metabolic imbalance, mitochondrial failure, and PN degeneration and therapies normalizing metabolic function might be used to alleviate Rh1P37H toxicity in the retina. Our study offers a glimpse into the intricate higher order interactions that underlie PN dysfunction and provides a useful resource for identifying other molecular networks that mediate Rho toxicity in PNs.