Autophagy Is Required for Memory Formation and Reverses Age-Related Memory Decline

Autophagy and the endolysosomal system in presynaptic function

The complex morphology of neurons, the specific requirements of synaptic neurotransmission and the accompanying metabolic demands create a unique challenge for proteostasis. The main machineries for neuronal protein synthesis and degradation are localized in the soma, while synaptic junctions are found at vast distances from the cell body. Sophisticated mechanisms must, therefore, ensure efficient delivery of newly synthesized proteins and removal of faulty proteins. These requirements are exacerbated at presynaptic sites, where the demands for protein turnover are especially high due to synaptic vesicle release and recycling that induces protein damage in an intricate molecular machinery, and where replacement of material is hampered by the extreme length of the axon. In this review, we will discuss the contribution of the two major pathways in place, autophagy and the endolysosomal system, to presynaptic protein turnover and presynaptic function. Although clearly different in their biogenesis, both pathways are characterized by cargo collection and transport into distinct membrane-bound organelles that eventually fuse with lysosomes for cargo degradation. We summarize the available evidence with regard to their degradative function, their regulation by presynaptic machinery and the cargo for each pathway. Finally, we will discuss the interplay of both pathways in neurons and very recent findings that suggest non-canonical functions of degradative organelles in synaptic signalling and plasticity.

Dysfunctional Autophagy and Endolysosomal System in Neurodegenerative Diseases: Relevance and Therapeutic Options

Autophagy and endolysosomal trafficking are crucial in neuronal development, function and survival. These processes ensure efficient removal of misfolded aggregation-prone proteins and damaged organelles, such as dysfunctional mitochondria, thus allowing the maintenance of proper cellular homeostasis. Beside this, emerging evidence has pointed to their involvement in the regulation of the synaptic proteome needed to guarantee an efficient neurotransmitter release and synaptic plasticity. Along this line, an intimate interplay between the molecular machinery regulating synaptic vesicle endocytosis and synaptic autophagy is emerging, suggesting that synaptic quality control mechanisms need to be tightly coupled to neurosecretion to secure release accuracy. Defects in autophagy and endolysosomal pathway have been associated with neuronal dysfunction and extensively reported in Alzheimer's, Parkinson's, Huntington's and amyotrophic lateral sclerosis among other neurodegenerative diseases, with common features and emerging genetic bases. In this review, we focus on the multiple roles of autophagy and endolysosomal system in neuronal homeostasis and highlight how their defects probably contribute to synaptic default and neurodegeneration in the above-mentioned diseases, discussing the most recent options explored for therapeutic interventions.

Autophagy in the Regulation of Tissue Differentiation and Homeostasis

Autophagy is a constitutive pathway that allows the lysosomal degradation of damaged components. This conserved process is essential for metabolic plasticity and tissue homeostasis and is crucial for mammalian post-mitotic cells. Autophagy also controls stem cell fate and defective autophagy is involved in many pathophysiological processes. In this review, we focus on established and recent breakthroughs aimed at elucidating the impact of autophagy in differentiation and homeostasis maintenance of endothelium, muscle, immune system, and brain providing a suitable framework of the emerging results and highlighting the pivotal role of autophagic response in tissue functions, stem cell dynamics and differentiation rates.

HERC1 Ubiquitin Ligase Is Required for Hippocampal Learning and Memory

Mutations in the human HERC1 E3 ubiquitin ligase protein develop intellectual disability. The tambaleante (tbl) mouse carries a HERC1 mutation characterized by cerebellar ataxia due of adult cerebellar Purkinje cells death by extensive autophagy. Our previous studies demonstrated that both the neuromuscular junction and the peripheral nerve myelin sheaths are also affected in this mutant. Moreover, there are signs of dysregulated autophagy in the central nervous system in the tbl mouse, affecting spinal cord motor neurons, and pyramidal neurons of the neocortex and the hippocampal CA3 region. The tbl mutation affects associative learning, with absence of short- and long-term potentiation in the lateral amygdala, altered spinogenesis in their neurons, and a dramatic decrease in their glutamatergic input. To assess whether other brain areas engaged in learning processes might be affected by the tbl mutation, we have studied the tbl hippocampus using behavioral tests, ex vivo electrophysiological recordings, immunohistochemistry, the Golgi-Cox method and transmission electron microscopy. The tbl mice performed poorly in the novel-object recognition, T-maze and Morris water maze tests. In addition, there was a decrease in glutamatergic input while the GABAergic one remains unaltered in the hippocampal CA1 region of tbl mice, accompanied by changes in the dendritic spines, and signs of cellular damage. Moreover, the proportions of immature and mature neurons in the dentate gyrus of the tbl hippocampus differ relative to the control mice. Together, these observations demonstrate the important role of HERC1 in regulating synaptic activity during learning.

MERCs. The Novel Assistant to Neurotransmission?

In neuroscience, much attention is paid to intercellular interactions, in particular, to synapses. However, many researchers do not pay due attention to the contribution of intracellular contacts to the work of intercellular interactions. Nevertheless, along with synapses, intracellular contacts also have complex organization and a tremendous number of regulatory elements. Mitochondria-endoplasmic reticulum contacts (MERCs) are a specific site of interaction between the two organelles; they provide a basis for a large number of cellular functions, such as calcium homeostasis, lipid metabolism, autophagy, and apoptosis. Despite the presence of these contacts in various parts of neurons and glial cells, it is yet not known whether they fulfill the same functions. There are still many unsolved questions about the work of these intracellular contacts, and one of the most important among them is if MERCs, with their broad implication into synaptic events, can be considered the assistant to neurotransmission?

Mechanisms of neuronal survival safeguarded by endocytosis and autophagy

Multiple aspects of neuronal physiology crucially depend on two cellular pathways, autophagy and endocytosis. During endocytosis, extracellular components either unbound or recognized by membrane-localized receptors (termed "cargo") become internalized into plasma membrane-derived vesicles. These can serve to either recycle the material back to the plasma membrane or send it for degradation to lysosomes. Autophagy also uses lysosomes as a terminal degradation point, although instead of degrading the plasma membrane-derived cargo, autophagy eliminates detrimental cytosolic material and intracellular organelles, which are transported to lysosomes by means of double-membrane vesicles, referred to as autophagosomes. Neurons, like all non-neuronal cells, capitalize on autophagy and endocytosis to communicate with the environment and maintain protein and organelle homeostasis. Additionally, the highly polarized, post-mitotic nature of neurons made them adopt these two pathways for cell-specific functions. These include the maintenance of the synaptic vesicle pool in the pre-synaptic terminal and the long-distance transport of signaling molecules. Originally discovered independently from each other, it is now clear that autophagy and endocytosis are closely interconnected and share several common participating molecules. Considering the crucial role of autophagy and endocytosis in cell type-specific functions in neurons, it is not surprising that defects in both pathways have been linked to the pathology of numerous neurodegenerative diseases. In this review, we highlight the recent knowledge of the role of endocytosis and autophagy in neurons with a special focus on synaptic physiology and discuss how impairments in genes coding for autophagy and endocytosis proteins can cause neurodegeneration.

The ER stress-autophagy axis: implications for cognitive dysfunction in diabetes mellitus

Unfolded protein response (UPR) often coordinates with autophagy to maintain cellular proteostasis. Disturbance of proteostasis correlates with diseases including diabetes and neurological complications. In a recent article in Clinical Science, Kong et al. highlighted the critical role of endoplasmic reticulum (ER) stress-autophagy axis in maintaining cognitive functions and provided pharmacological evidence with respect to cognitive improvements in a diabetic mouse model. These novel findings present new insights into the pathological mechanisms and therapeutic implications with the ER stress modulators in diabetes-related cognitive dysfunction.

Spermidine, an autophagy inducer, as a therapeutic strategy in neurological disorders

Spermidine is a naturally occurring endogenous polyamine synthesized from diamine putrescine. It is a well-known autophagy inducer that maintains cellular and neuronal homeostasis. Healthy brain development and function are dependent on brain polyamine concentration. Polyamines interact with the opioid system, glutamatergic signaling and neuroinflammation in the neuronal and glial compartments. Among the polyamines, spermidine is found highest in the human brain. Age-linked fluctuations in the spermidine levels may possibly contribute to the impairments in neural network and neurogenesis. Exogenously administered spermidine helps in the treatment of brain diseases. Further, current studies highlight the ability of spermidine to promote longevity by inducing autophagy. Still, the causal neuroprotective mechanism of spermidine in neuronal dysfunction remains unidentified. This review aims to summarize various neuroprotective effects of spermidine related to anti-aging/ anti-inflammatory properties and the prevention of neurotoxicity that helps in achieving beneficial effects in age-related neurological disorder. We also expose the signaling cascades modulated by spermidine which might result in therapeutic action. The present review highlights clinical studies along with in-vivo and in-vitro preclinical studies to provide a new dimension for the therapeutic potential of spermidine in neurological disorders.

The Unfolded Protein Response and Autophagy as Drug Targets in Neuropsychiatric Disorders

Neurons are polarized in structure with a cytoplasmic compartment extending into dendrites and a long axon that terminates at the synapse. The high level of compartmentalization imposes specific challenges for protein quality control in neurons making them vulnerable to disturbances that may lead to neurological dysfunctions including neuropsychiatric diseases. Synapse and dendrites undergo structural modulations regulated by neuronal activity involve key proteins requiring strict control of their turnover rates and degradation pathways. Recent advances in the study of the unfolded protein response (UPR) and autophagy processes have brought novel insights into the specific roles of these processes in neuronal physiology and synaptic signaling. In this review, we highlight recent data and concepts about UPR and autophagy in neuropsychiatric disorders and synaptic plasticity including a brief outline of possible therapeutic approaches to influence UPR and autophagy signaling in these diseases.

Stress and signaling pathways regulating autophagy: From behavioral models to psychiatric disorders

Autophagy is a process of degradation and recycling of cytoplasmatic components by the lysosomes. In the central nervous system (CNS), autophagy is involved in cell surveillance, neuroinflammation, and neuroplasticity. Neuropsychiatric conditions are associated with functional disturbances at molecular and cellular levels, causing significant impairments in cell homeostasis. Additionally, emerging evidence supports that dysfunctions in autophagy contribute to the pathophysiology of neurological diseases. However, the studies on autophagy in psychiatric disorders are highly heterogeneous and have several limitations, mainly to assess causality and determine the autophagy flux in animals and human samples. Besides, the role of this mechanism in non-neuronal cells in the CNS is only recently being explored. Thus, this review summarizes and discusses the changes in the autophagy pathway in animal models of psychiatric disorders and the limitations underlying the significant findings. Moreover, we compared these findings with clinical studies. Understanding the involvement of autophagy in psychiatric conditions, and the limitation of our current models may contribute to the development of more effective research approaches and possibly pharmacological therapies.

Mechanisms by which autophagy regulates memory capacity in ageing

Autophagy agonists have been proposed to slow down neurodegeneration. Spermidine, a polyamine that acts as an autophagy agonist, is currently under clinical trial for the treatment of age‐related memory decline. How Spermidine and other autophagy agonists regulate memory and synaptic plasticity is under investigation. We set up a novel mouse model of mild cognitive impairment (MCI), in which middle‐aged (12‐month‐old) mice exhibit impaired memory capacity, lysosomes engulfed with amyloid fibrils (β‐amyloid and α‐synuclein) and impaired task‐induced GluA1 hippocampal post‐translation modifications. Subchronic treatment with Spermidine as well as the autophagy agonist TAT‐Beclin 1 rescued memory capacity and GluA1 post‐translational modifications by favouring the autophagy/lysosomal‐mediated degradation of amyloid fibrils. These findings provide new mechanistic evidence on the therapeutic relevance of autophagy enhancers which, by improving the degradation of misfolded proteins, slow down age‐related memory decline.

Autophagy in Neuronal Development and Plasticity

Autophagy is a highly conserved intracellular clearance pathway in which cytoplasmic contents are trafficked to the lysosome for degradation. Within neurons, it helps to remove damaged organelles and misfolded or aggregated proteins and has therefore been the subject of intense research in relation to neurodegenerative disease. However, far less is understood about the role of autophagy in other aspects of neuronal physiology. Here we review the literature on the role of autophagy in maintaining neuronal stem cells and in neuronal plasticity in adult life and we discuss how these contribute to structural and functional deficits observed in a range of human disorders.

Responsiveness of dentate neurons generated throughout adult life is associated with resilience to cognitive aging

During aging, some individuals are resilient to the decline of cognitive functions whereas others are vulnerable. These inter-individual differences in memory abilities have been associated with differences in the rate of hippocampal neurogenesis measured in elderlies. Whether the maintenance of the functionality of neurons generated throughout adult life is linked to resilience to cognitive aging remains completely unexplored. Using the immediate early gene Zif268, we analyzed the activation of dentate granule neurons born in adult (3-month-old), middle-aged (12-month-old), or senescent (18-month-old) rats (n = 96) in response to learning when animals reached 21 months of age. The activation of neurons born during the developmental period was also examined. We show that adult-born neurons can survive up to 19 months and that neurons generated 4, 10, or 19 months before learning, but not developmentally born neurons, are activated in senescent rats with good learning abilities. In contrast, aged rats with bad learning abilities do not exhibit activity-dependent regulation of newborn cells, whatever their birthdate. In conclusion, we propose that resilience to cognitive aging is associated with responsiveness of neurons born during adult life. These data add to our current knowledge by showing that the aging of memory abilities stems not only from the number but also from the responsiveness of adult-born neurons.

The Journey of the Synaptic Autophagosome: A Cell Biological Perspective

Autophagy is a key cellular degradative pathway, important for neuronal homeostasis and function. Disruption of autophagy is associated with neuronal dysfunction and neurodegeneration. Autophagy is compartmentalized in neurons, with specific stages of the pathway occurring in distinct subcellular compartments. Coordination of these stages drives progression of autophagy and enables clearance of substrates. Yet, we are only now learning how these distributed processes are integrated across the neuron. In this review, we focus on the cell biological course of autophagy in neurons, from biogenesis at the synapse to degradation in the soma. We describe how the steps of autophagy are distributed across neuronal subcellular compartments, how local machinery regulates autophagy, and the impact of coordinated regulation on neuronal physiology and disease. We also discuss how recent advances in our understanding of neuronal autophagic mechanisms have reframed how we think about the role of local regulation of autophagy in all tissues.

Intraperitoneal injection of IFN-γ restores microglial autophagy, promotes amyloid-β clearance and improves cognition in APP/PS1 mice

Autophagy is a major self-degradative process that maintains cellular homeostasis and function in mammalian cells. Autophagic dysfunction occurs in the early pathogenesis of Alzheimer’s disease (AD) and directly regulates amyloid-β (Aβ) metabolism. Although it has been proven that the cytokine IFN-γ enhances autophagy in macrophage cell lines, whether the signaling cascade is implicated in Aβ degradation in AD mouse models remains to be elucidated. Here, we found that 9 days of the intraperitoneal administration of IFN-γ significantly increased the LC3II/I ratio and decreased the level of p62 in APP/PS1 mice, an AD mouse model. In vitro, IFN-γ protected BV2 cells from Aβ toxicity by upregulating the expressions of Atg7 and Atg5 and the LC3II/I ratio, whereas these protective effects were ablated by interference with Atg5 expression. Moreover, IFN-γ enhanced autophagic flux, probably through suppressing the AKT/mTOR pathway both in vivo and in vitro. Importantly, using intravital two-photon microscopy and fluorescence staining, we found that microglia interacted with exogenous IFN-γ and Aβ, and surrounded Aβ in APP/PS1;CX3CR1-GFP^+/− mice. In addition, IFN-γ treatment decreased the Aβ plaque load in the cortex and hippocampus and rescued cognitive deficits in APP/PS1 mice. Our data suggest a possible mechanism by which the peripheral injection of IFN-γ restores microglial autophagy to induce the phagocytosis of cerebral Aβ, which represents a potential therapeutic approach for the use of exogenous IFN-γ in AD.

The involvement of stress granules in aging and aging-associated diseases

Stress granules (SGs) are nonmembrane assemblies formed in cells in response to stress conditions. SGs mainly contain untranslated mRNA and a variety of proteins. RNAs and scaffold proteins with intrinsically disordered regions or RNA-binding domains are essential for the assembly of SGs, and multivalent macromolecular interactions among these components are thought to be the driving forces for SG assembly. The SG assembly process includes regulation through post-translational modification and involvement of the cytoskeletal system. During aging, many intracellular bioprocesses become disrupted by factors such as cellular environmental changes, mitochondrial dysfunction, and decline in the protein quality control system. Such changes could lead to the formation of aberrant SGs, as well as alterations in their maintenance, disassembly, and clearance. These aberrant SGs might in turn promote aging and aging-associated diseases. In this paper, we first review the latest progress on the molecular mechanisms underlying SG assembly and SG functioning under stress conditions. Then, we provide a detailed discussion of the relevance of SGs to aging and aging-associated diseases.

Emerging Role of the Autophagy/Lysosomal Degradative Pathway in Neurodevelopmental Disorders With Epilepsy

Autophagy is a highly conserved degradative process that conveys dysfunctional proteins, lipids, and organelles to lysosomes for degradation. The post-mitotic nature, complex and highly polarized morphology, and high degree of specialization of neurons make an efficient autophagy essential for their homeostasis and survival. Dysfunctional autophagy occurs in aging and neurodegenerative diseases, and autophagy at synaptic sites seems to play a crucial role in neurodegeneration. Moreover, a role of autophagy is emerging for neural development, synaptogenesis, and the establishment of a correct connectivity. Thus, it is not surprising that defective autophagy has been demonstrated in a spectrum of neurodevelopmental disorders, often associated with early-onset epilepsy. Here, we discuss the multiple roles of autophagy in neurons and the recent experimental evidence linking neurodevelopmental disorders with epilepsy to genes coding for autophagic/lysosomal system-related proteins and envisage possible pathophysiological mechanisms ranging from synaptic dysfunction to neuronal death.

Regulation of autophagy by inhibitory CSPG interactions with receptor PTPσ and its impact on plasticity and regeneration after spinal cord injury

Chondroitin sulfate proteoglycans (CSPGs), extracellular matrix molecules that increase dramatically following a variety of CNS injuries or diseases, have long been known for their potent capacity to curtail cell migrations as well as axon regeneration and sprouting. The inhibition can be conferred through binding to their major cognate receptor, Protein Tyrosine Phosphatase Sigma (PTPσ). However, the precise mechanisms downstream of receptor binding that mediate growth inhibition have remained elusive. Recently, CSPGs/PTPσ interactions were found to regulate autophagic flux at the axon growth cone by dampening the autophagosome-lysosomal fusion step. Because of the intense interest in autophagic phenomena in the regulation of a wide variety of critical cellular functions, we summarize here what is currently known about dysregulation of autophagy following spinal cord injury, and highlight this critical new mechanism underlying axon regeneration failure. Furthermore, we review how CSPGs/PTPσ interactions influence plasticity through autophagic regulation and how PTPσ serves as a switch to execute either axon outgrowth or synaptogenesis. This has exciting implications for the role CSPGs play not only in axon regeneration failure after spinal cord injury, but also in neurodegenerative diseases where, again, inhibitory CSPGs are upregulated.

Autophagy in Rare (NonLysosomal) Neurodegenerative Diseases

Neurodegenerative diseases (NDDs) comprise conditions with impaired neuronal function and loss and may be associated with a build-up of aggregated proteins with altered physicochemical properties (misfolded proteins). There are many disorders, and causes include gene mutations, infections, or exposure to toxins. The autophagy pathway is involved in the removal of unwanted proteins and organelles through lysosomes. While lysosomal storage disorders have been described for many years, it is now recognised that perturbations of the autophagy pathway itself can also lead to neurodegenerative disease. These include monogenic disorders of key proteins involved in the autophagy pathway, and disorders within pathways that critically control autophagy through monitoring of the supply of nutrients (mTORC1 pathway) or of energy supply in cells (AMPK pathway). This review focuses on childhood-onset neurodegenerative disorders with perturbed autophagy, due to defects in the autophagy pathway, or in upstream signalling via mTORC1 and AMPK. The review first provides a short description of autophagy, as related to neurons. It then examines the extended role of autophagy in neuronal function, plasticity, and memory. There follows a description of each step of the autophagy pathway in greater detail, illustrated with examples of diseases grouped by the stage of their perturbation of the pathway. Each disease is accompanied by a short clinical description, to illustrate the diversity but also the overlap of symptoms caused by perturbation of key proteins necessary for the proper functioning of autophagy. Finally, there is a consideration of current challenges that need addressing for future therapeutic advances.

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Autophagy is an important pathway for the removal of misfolded proteins from terminally differentiated neurons.

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Monogenic defects in autophagy cause childhood-onset neurodegeneration.

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Defects in different stages of the pathway may present with overlapping clinical features.

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Increasing autophagic flux may be a therapeutic strategy to treat many autophagic disorders.

How autophagy can restore proteostasis defects in multiple diseases?

Cellular evolution develops several conserved mechanisms by which cells can tolerate various difficult conditions and overall maintain homeostasis. Autophagy is a well-developed and evolutionarily conserved mechanism of catabolism, which endorses the degradation of foreign and endogenous materials via autolysosome. To decrease the burden of the ubiquitin-proteasome system (UPS), autophagy also promotes the selective degradation of proteins in a tightly regulated way to improve the physiological balance of cellular proteostasis that may get perturbed due to the accumulation of misfolded proteins. However, the diverse as well as selective clearance of unwanted materials and regulations of several cellular mechanisms via autophagy is still a critical mystery. Also, the failure of autophagy causes an increase in the accumulation of harmful protein aggregates that may lead to neurodegeneration. Therefore, it is necessary to address this multifactorial threat for in-depth research and develop more effective therapeutic strategies against lethal autophagy alterations. In this paper, we discuss the most relevant and recent reports on autophagy modulations and their impact on neurodegeneration and other complex disorders. We have summarized various pharmacological findings linked with the induction and suppression of autophagy mechanism and their promising preclinical and clinical applications to provide therapeutic solutions against neurodegeneration. The conclusion, key questions, and future prospectives sections summarize fundamental challenges and their possible feasible solutions linked with autophagy mechanism to potentially design an impactful therapeutic niche to treat neurodegenerative diseases and imperfect aging.

Decreased ovarian function and autophagy gene methylation in aging rats

Background

Degeneration of ovarian function is an obvious feature of female aging. In addition, studies have shown that autophagy decreases with age, and DNA methylation is a hallmark epigenetic pattern during aging. However, it is not clear whether the expression and DNA methylation of autophagy genes are involved in the declines in ovarian function that occur during aging.

Results

Three groups of rats were used: 6-month-old (6 M) rats, 12-month-old (12 M) rats and 24-month-old (24 M) rats. Serum E₂ levels and the mRNA and protein expression levels of Atg5, Atg12, Atg16L, Beclin1 and Lc3B were significantly decreased in aged rats. In addition, the methylation levels of the Atg5 gene were significantly increased in aged rats. The expression of the Dnmt1 and Dnmt2 genes decreased with aging; however, the expression of the Dnmt3A and Dnmt3B genes gradually increased with aging.

Conclusions

Decreased autophagic activity was involved in the declines in ovarian function in aging rats. Upregulation of the DNA methyltransferases Dnmt3A and Dnmt3B may have led to methylation of the autophagy genes Atg5 and Lc3B to ultimately cause the observed decreases in autophagic activity.

Cell-type-specific regulation of neuronal intrinsic excitability by macroautophagy

The basal ganglia are a group of subcortical nuclei that contribute to action selection and reinforcement learning. The principal neurons of the striatum, spiny projection neurons of the direct (dSPN) and indirect (iSPN) pathways, maintain low intrinsic excitability, requiring convergent excitatory inputs to fire. Here, we examined the role of autophagy in mouse SPN physiology and animal behavior by generating conditional knockouts of Atg7 in either dSPNs or iSPNs. Loss of autophagy in either SPN population led to changes in motor learning but distinct effects on cellular physiology. dSPNs, but not iSPNs, required autophagy for normal dendritic structure and synaptic input. In contrast, iSPNs, but not dSPNs, were intrinsically hyperexcitable due to reduced function of the inwardly rectifying potassium channel, Kir2. These findings define a novel mechanism by which autophagy regulates neuronal activity: control of intrinsic excitability via the regulation of potassium channel function.

Differential regulation of autophagy during metabolic stress in astrocytes and neurons

Macroautophagy/autophagy is a key homeostatic process that targets cytoplasmic components to the lysosome for breakdown and recycling. Autophagy plays critical roles in glia and neurons that affect development, functionality, and viability of the nervous system. The mechanisms that regulate autophagy in glia and neurons, however, are poorly understood. Here, we define the molecular underpinnings of autophagy in primary cortical astrocytes in response to metabolic stress, and perform a comparative study in primary hippocampal neurons. We find that inducing metabolic stress by nutrient deprivation or pharmacological inhibition of MTOR (mechanistic target of rapamycin kinase) robustly activates autophagy in astrocytes. While both paradigms of metabolic stress dampen MTOR signaling, they affect the autophagy pathway differently. Further, we find that starvation-induced autophagic flux is dependent on the buffering system of the starvation solution. Lastly, starvation conditions that strongly activate autophagy in astrocytes have less pronounced effects on autophagy in neurons. Combined, our study reveals the complexity of regulating autophagy in different paradigms of metabolic stress, as well as in different cell types of the brain. Our findings raise important implications for how neurons and glia may collaborate to maintain homeostasis in the brain.

ABBREVIATIONS

ACSF: artificial cerebrospinal fluid; baf A₁: bafilomycin A₁; EBSS: earle's balanced salt solution; GFAP: glial fibrillary acidic protein; Glc: glucose; GM: glial media; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; p-RPS6: phospho-RPS6; p-ULK1: phospho-ULK1; RPS6: ribosomal protein S6; SQSTM1/p62: sequestosome 1; ULK1: unc-51-like kinase 1.

Spermidine protects from age-related synaptic alterations at hippocampal mossy fiber-CA3 synapses

Aging is associated with functional alterations of synapses thought to contribute to age-dependent memory impairment (AMI). While therapeutic avenues to protect from AMI are largely elusive, supplementation of spermidine, a polyamine normally declining with age, has been shown to restore defective proteostasis and to protect from AMI in Drosophila. Here we demonstrate that dietary spermidine protects from age-related synaptic alterations at hippocampal mossy fiber (MF)-CA3 synapses and prevents the aging-induced loss of neuronal mitochondria. Dietary spermidine rescued age-dependent decreases in synaptic vesicle density and largely restored defective presynaptic MF-CA3 long-term potentiation (LTP) at MF-CA3 synapses (MF-CA3) in aged animals. In contrast, spermidine failed to protect CA3-CA1 hippocampal synapses characterized by postsynaptic LTP from age-related changes in function and morphology. Our data demonstrate that dietary spermidine attenuates age-associated deterioration of MF-CA3 synaptic transmission and plasticity. These findings provide a physiological and molecular basis for the future therapeutic usage of spermidine.

The Synaptic Autophagy Cycle

Macroautophagy (hereafter referred to as autophagy) is an evolutionarily conserved pathway in which proteins and organelles are delivered to the lysosome for degradation. In neurons, autophagy was originally described as associated with disease states and neuronal survival. Over the last decade, however, evidence has accumulated that autophagy controls synaptic function in both the axon and dendrite. Here, we review this literature, highlighting the role of autophagy in the pre- and postsynapse, synaptic plasticity, and behavior. We end by discussing open questions in the field of synaptic autophagy.

Iron Overload Impairs Autophagy: Effects of Rapamycin in Ameliorating Iron-Related Memory Deficits

Over the years, iron accumulation in specific brain regions has been observed in normal aging and related to the pathogenesis of neurodegenerative disorders. Many neurodegenerative diseases may involve cognitive dysfunction, and we have previously shown that neonatal iron overload induces permanent cognitive deficits in adult rats and exacerbates age-associated memory decline. Autophagy is a catabolic pathway involved in the removal of toxic protein aggregates, which are a hallmark of neurodegenerative events. In the present study, we investigated whether iron accumulation would interfere with autophagy and also sought to determine the effects of rapamycin-induced stimulation of autophagy in attenuating iron-related cognitive deficits. Male Wistar rats received a single daily oral dose of vehicle or iron carbonyl (30 mg/kg) at postnatal days 12-14. In adulthood, they received daily intraperitoneal injections of vehicle or rapamycin (0.25 mg/kg) for 14 days. Results showed that iron given in the neonatal period impaired inhibitory avoidance memory and induced a decrease in proteins critically involved in the autophagy pathway, Beclin-1 and LC3, in the hippocampus. Rapamycin in the adulthood reversed iron-induced memory deficits, decreased the ratio phospho-mTOR/total mTOR, and recovered LC3 II levels in iron-treated rats. Our results suggest that iron accumulation, as observed in neurodegenerative disorders, hinders autophagy, which might play a role in iron-induced neurotoxicity. Rapamycin, by inducing authophagy, was able to ameliorate iron-induced cognitive impairments. These findings support the use of rapamycin as a potential neuroprotective treatment against the cognitive decline associated to neurodegenerative disorders.

Autophagy in Neurons

Autophagy is the major cellular pathway to degrade dysfunctional organelles and protein aggregates. Autophagy is particularly important in neurons, which are terminally differentiated cells that must last the lifetime of the organism. There are both constitutive and stress-induced pathways for autophagy in neurons, which catalyze the turnover of aged or damaged mitochondria, endoplasmic reticulum, other cellular organelles, and aggregated proteins. These pathways are required in neurodevelopment as well as in the maintenance of neuronal homeostasis. Here we review the core components of the pathway for autophagosome biogenesis, as well as the cell biology of bulk and selective autophagy in neurons. Finally, we discuss the role of autophagy in neuronal development, homeostasis, and aging and the links between deficits in autophagy and neurodegeneration.

Autophagy in the mammalian nervous system: a primer for neuroscientists

Autophagy refers to the lysosomal degradation of damaged or superfluous components and is essential for metabolic plasticity and tissue integrity. This evolutionarily conserved process is particularly vital to mammalian post-mitotic cells such as neurons, which face unique logistical challenges and must sustain homoeostasis over decades. Defective autophagy has pathophysiological importance, especially for human neurodegeneration. The present-day definition of autophagy broadly encompasses two distinct yet related phenomena: non-selective and selective autophagy. In this minireview, we focus on established and emerging concepts in the field, paying particular attention to the physiological significance of macroautophagy and the burgeoning world of selective autophagy pathways in the context of the vertebrate nervous system. By highlighting established basics and recent breakthroughs, we aim to provide a useful conceptual framework for neuroscientists interested in autophagy, in addition to autophagy enthusiasts with an eye on the nervous system.

Autophagy Promotes Memory Formation

Autophagy is traditionally known to be a stress response and a quality control mechanism for protecting cells from injury and disease. In addition to its housekeeping functions, autophagy also has specialized functions including regulation of synaptic activity and neurotransmission. Decreased autophagy is commonly associated with aging; however, the functional importance of autophagy in regulating cognitive function and its decline during aging were previously not known. A recent study showed that the induction of hippocampal autophagy improves cognition by enhancing memory formation and reverses memory decline during aging. Here, we discuss the findings of that study and explore the scope of the physiological process of autophagy in the development of treatments for age-related cognitive decline and neurodegenerative diseases.

Balanced actions of protein synthesis and degradation in memory formation

Storage of long-term memory requires not only protein synthesis but also protein degradation. In this article, we overview recent publications related to this issue, stressing that the balanced actions of protein synthesis and degradation are critical for long-term memory formation. We particularly focused on the brain-derived neurotrophic factor signaling that leads to protein synthesis; proteasome- and autophagy-dependent protein degradation that removes molecular constraints; the role of Fragile X mental retardation protein in translational suppression; and epigenetic modifications that control gene expression at the genomic level. Numerous studies suggest that an imbalance between protein synthesis and degradation leads to intellectual impairment and cognitive disorders.

Proteasome and Autophagy-Mediated Impairment of Late Long-Term Potentiation (l-LTP) after Traumatic Brain Injury in the Somatosensory Cortex of Mice

Traumatic brain injury (TBI) can lead to impaired cognition and memory consolidation. The acute phase (24–48 h) after TBI is often characterized by neural dysfunction in the vicinity of the lesion, but also in remote areas like the contralateral hemisphere. Protein homeostasis is crucial for synaptic long-term plasticity including the protein degradation systems, proteasome and autophagy. Still, little is known about the acute effects of TBI on synaptic long-term plasticity and protein degradation. Thus, we investigated TBI in a controlled cortical impact (CCI) model in the motor and somatosensory cortex of mice ex vivo-in vitro. Late long-term potentiation (l-LTP) was induced by theta-burst stimulation in acute brain slices after survival times of 1–2 days. Protein levels for the plasticity related protein calcium/calmodulin-dependent protein kinase II (CaMKII) was quantified by Western blots, and the protein degradation activity by enzymatical assays. We observed missing maintenance of l-LTP in the ipsilateral hemisphere, however not in the contralateral hemisphere after TBI. Protein levels of CaMKII were not changed but, interestingly, the protein degradation revealed bidirectional changes with a reduced proteasome activity and an increased autophagic flux in the ipsilateral hemisphere. Finally, LTP recordings in the presence of pharmacologically modified protein degradation systems also led to an impaired synaptic plasticity: bath-applied MG132, a proteasome inhibitor, or rapamycin, an activator of autophagy, both administered during theta burst stimulation, blocked the induction of LTP. These data indicate that alterations in protein degradation pathways likely contribute to cognitive deficits in the acute phase after TBI, which could be interesting for future approaches towards neuroprotective treatments early after traumatic brain injury.

Autophagy: Supporting cellular and organismal homeostasis by self-eating

Autophagy is a conserved catabolic process that delivers cytoplasmic components and organelles to lysosomes for degradation and recycling. This pathway serves to degrade nonfunctional organelles and aggregate-prone proteins, as well as to produce substrates for energy production and biosynthesis. Autophagy is especially important for the maintenance of stem cells, and for the survival and homeostasis of post-mitotic cells like neurons. Functional autophagy promotes longevity in several model organisms. Autophagy regulates immunity and inflammation at several levels and has both anti- and pro-tumorigenic roles in cancer. This review provides a concise overview of autophagy and its importance in cellular and organismal homeostasis, with emphasis on aging, stem cells, neuronal cells, immunity, inflammation, and cancer.