Endocytosis and autophagy in dying neurons: an ultrastructural study in chick embryos

The Role of Autophagy in Eye Diseases

Autophagy is a catabolic process that ensures homeostasis in the cells of our organism. It plays a crucial role in protecting eye cells against oxidative damage and external stress factors. Ocular pathologies of high incidence, such as age-related macular degeneration, cataracts, glaucoma, and diabetic retinopathy are of multifactorial origin and are associated with genetic, environmental factors, age, and oxidative stress, among others; the latter factor is one of the most influential in ocular diseases, directly affecting the processes of autophagy activity. Alteration of the normal functioning of autophagy processes can interrupt organelle turnover, leading to the accumulation of cellular debris and causing physiological dysfunction of the eye. The aim of this study is to review research on the role of autophagy processes in the main ocular pathologies, which have a high incidence and result in high costs for the health system. Considering the role of autophagy processes in cell homeostasis and cell viability, the control and modulation of autophagy processes in ocular pathologies could constitute a new therapeutic approach.

Mechanisms of action of amyloid-beta and its precursor protein in neuronal cell death

Extracellular senile plaques and intracellular neurofibrillary tangles are the neuropathological findings of the Alzheimer's disease (AD). Based on the amyloid cascade hypothesis, the main component of senile plaques, the amyloid-beta (Aβ) peptide, and its derivative called amyloid precursor protein (APP) both have been found to place their central roles in AD development for years. However, the recent therapeutics have yet to reverse or halt this disease. Previous evidence demonstrates that the accumulation of Aβ peptides and APP can exert neurotoxicity and ultimately neuronal cell death. Hence, we discuss the mechanisms of excessive production of Aβ peptides and APP serving as pathophysiologic stimuli for the initiation of various cell signalling pathways including apoptosis, necrosis, necroptosis and autophagy which lead to neuronal cell death. Conversely, the activation of such pathways could also result in the abnormal generation of APP and Aβ peptides. An elucidation of actions of APP and its metabolite, Aβ, could be vital in suggesting novel therapeutic opportunities.

Cystatin C in aging and in Alzheimer's disease

Under normal conditions, the function of catalytically active proteases is regulated, in part, by their endogenous inhibitors, and any change in the synthesis and/or function of a protease or its endogenous inhibitors may result in inappropriate protease activity. Altered proteolysis as a result of an imbalance between active proteases and their endogenous inhibitors can occur during normal aging, and such changes have also been associated with multiple neuronal diseases, including Amyotrophic Lateral Sclerosis (ALS), rare heritable neurodegenerative disorders, ischemia, some forms of epilepsy, and Alzheimer's disease (AD). One of the most extensively studied endogenous inhibitor is the cysteine-protease inhibitor cystatin C (CysC). Changes in the expression and secretion of CysC in the brain have been described in various neurological disorders and in animal models of neurodegeneration, underscoring a role for CysC in these conditions. In the brain, multiple in vitro and in vivo findings have demonstrated that CysC plays protective roles via pathways that depend upon the inhibition of endosomal-lysosomal pathway cysteine proteases, such as cathepsin B (Cat B), via the induction of cellular autophagy, via the induction of cell proliferation, or via the inhibition of amyloid-β (Aβ) aggregation. We review the data demonstrating the protective roles of CysC under conditions of neuronal challenge and the protective pathways induced by CysC under various conditions. Beyond highlighting the essential role that balanced proteolytic activity plays in supporting normal brain aging, these findings suggest that CysC is a therapeutic candidate that can potentially prevent brain damage and neurodegeneration.

The Evolving Functions of Autophagy in Ocular Health: A Double-edged Sword

Autophagy plays an adaptive role in cell survival, development, differentiation and intracellular homeostasis. Autophagy is recognized as a 'self-cannibalizing' process that is active during stresses such as starvation, chemotherapy, infection, ageing, and oxygen shortage to protect organisms from various irritants and to regenerate materials and energy. However, autophagy can also lead to a form of programmed cell death distinct from apoptosis.

Components of the autophagic pathway are constitutively expressed at a high level in the eye, including in the cornea, lens, retina, and orbit. In addition, the activation of autophagy is directly linked to the development of eye diseases such as age-related macular degeneration (ARMD), cataracts, diabetic retinopathy (DR), glaucoma, photoreceptor degeneration, ocular tumours, ocular infections and thyroid-associated ophthalmopathy (TAO). A high level of autophagy defends against external stress; however, excessive autophagy can result in deterioration, as observed in ocular diseases such as ARMD and DR.

This review summarizes recent developments elucidating the relationship between autophagy and ocular diseases and the potential roles of autophagy in the pathogenesis and treatment of these diseases.

Autophagy in the eye: Development, degeneration, and aging

Autophagy is a catabolic pathway that promotes the degradation and recycling of cellular components. Proteins, lipids, and even whole organelles are engulfed in autophagosomes and delivered to the lysosome for elimination. In response to stress, autophagy mediates the degradation of cell components, which are recycled to generate the nutrients and building blocks required to sustain cellular homeostasis. Moreover, it plays an important role in cellular quality control, particularly in neurons, in which the total burden of altered proteins and damaged organelles cannot be reduced by redistribution to daughter cells through cell division. Research has only begun to examine the role of autophagy in the visual system. The retina, a light-sensitive tissue, detects and transmits electrical impulses through the optic nerve to the visual cortex in the brain. Both the retina and the eye are exposed to a variety of environmental insults and stressors, including genetic mutations and age-associated alterations that impair their function. Here, we review the main studies that have sought to explain autophagy's importance in visual function. We describe the role of autophagy in retinal development and cell differentiation, and discuss the implications of autophagy dysregulation both in physiological aging and in important diseases such as age-associated macular degeneration and glaucoma. We also address the putative role of autophagy in promoting photoreceptor survival and discuss how selective autophagy could provide alternative means of protecting retinal cells. The findings reviewed here underscore the important role of autophagy in maintaining proper retinal function and highlight novel therapeutic approaches for blindness and other diseases of the eye.

An early axonopathy in a hLRRK2(R1441G) transgenic model of Parkinson disease

Mutations in the gene for LRRK2 are the most common cause of familial Parkinson's disease (PD) and patients with these mutations manifest clinical features that are indistinguishable from those of the more common sporadic form. Thus, investigations of disease mechanisms based on disease-causing LRRK2 mutations can be expected to shed light on the more common sporadic form as well as the inherited form. We have shown that as human BAC transgenic hLRRK2(R1441G) mice age, they exhibit two abnormalities in the nigrostriatal dopaminergic system: an axonopathy and a diminished number of dendrites in the substantia nigra (SN). To better understand disease mechanisms it is useful to determine where in the affected neural system the pathology first begins. We therefore examined the nigrostriatal dopaminergic system in young mice to determine the initial site of pathology. Brains from hLRRK2(R1441G) and littermate control mice at 2-4months of age were examined by immunohistochemistry, anterograde fluorescent axon labeling and ultrastructural analysis. SN neurons, their projecting axons and the striatal terminal fields were assessed. The first identifiable abnormality in this system is an axonopathy characterized by giant polymorphic axon spheroids, the presence of intra-axonal autophagic vacuoles and intra-axonal myelin invagination. An initial involvement of axons has also been reported for other genetic models of PD. These observations support the concept that axons are involved early in the course of the disease. We suggest that effective neuroprotective approaches will be aimed at preventing axonal degeneration.

Multiple interacting cell death mechanisms in the mediation of excitotoxicity and ischemic brain damage: a challenge for neuroprotection

There is currently no approved neuroprotective pharmacotherapy for acute conditions such as stroke and cerebral asphyxia. One of the reasons for this may be the multiplicity of cell death mechanisms, because inhibition of a particular mechanism leaves the brain vulnerable to alternative ones. It is therefore essential to understand the different cell death mechanisms and their interactions. We here review the multiple signaling pathways underlying each of the three main morphological types of cell death--apoptosis, autophagic cell death and necrosis--emphasizing their importance in the neuronal death that occurs during cerebral ischemia and hypoxia-ischemia, and we analyze the interactions between the different mechanisms. Finally, we discuss the implications of the multiplicity of cell death mechanisms for the design of neuroprotective strategies.

Autophagy and neuronal cell death in neurological disorders

Autophagy is implicated in the pathogenesis of major neurodegenerative disorders although concepts about how it influences these diseases are still evolving. Once proposed to be mainly an alternative cell death pathway, autophagy is now widely viewed as both a vital homeostatic mechanism in healthy cells and as an important cytoprotective response mobilized in the face of aging- and disease-related metabolic challenges. In Alzheimer's, Parkinson's, Huntington's, amyotrophic lateral sclerosis, and other diseases, impairment at different stages of autophagy leads to the buildup of pathogenic proteins and damaged organelles, while defeating autophagy's crucial prosurvival and antiapoptotic effects on neurons. The differences in the location of defects within the autophagy pathway and their molecular basis influence the pattern and pace of neuronal cell death in the various neurological disorders. Future therapeutic strategies for these disorders will be guided in part by understanding the manifold impact of autophagy disruption on neurodegenerative diseases.

Using comparative anatomy in the axotomy model to identify distinct roles for microglia and astrocytes in synaptic stripping

The synaptic terminals' withdrawal from the somata and proximal dendrites of injured motoneuron by the processes of glial cells following facial nerve axotomy has been the subject of research for many years. This phenomenon is referred to as synaptic stripping, which is assumed to help survival and regeneration of neurons via reduction of synaptic inputs. Because there is no disruption of the blood-brain barrier or infiltration of macrophages, the axotomy paradigm has the advantage of being able to selectively investigate the roles of resident glial cells in the brain. Although there have been numerous studies of synaptic stripping, the detailed mechanisms are still under debate. Here we suggest that the species and strain differences that are often present in previous work might be related to the current controversies of axotomy studies. For instance, the survival ratios of axotomized neurons were generally found to be higher in rats than in mice. However, some studies have used the axotomy paradigm to follow the glial reactions and did not assess variations in neuronal viability. In the first part of this article, we summarize and discuss the current knowledge on species and strain differences in neuronal survival, glial augmentation and synaptic stripping. In the second part, we focus on our recent findings, which show the differential involvement of microglia and astrocytes in synaptic stripping and neuronal survival. This article suggests that the comparative study of the axotomy paradigm across various species and strains may provide many important and unexpected discoveries on the multifaceted roles of microglia and astrocytes in injury and repair.

Autophagy is induced by 3β-O-succinyl-lupeol (LD9-4) in A549 cells via up-regulation of Beclin 1 and down-regulation mTOR pathway

The purpose of this study is to investigate the antitumor activity of a new derivative of lupeol-3β-O-succinyl-lupeol (LD9-4) and the molecular mechanism underlying cell death in human non-small cell lung cancer A549 cells. The results revealed that LD9-4 inhibited A549 cell proliferation in a time- and dose-dependent manner, with an IC(50) value of 5.78 ± 0.48 μM after cells exposed to LD9-4 for 72 h. Markers indicative of apoptosis (cell cycle arrest, phosphatidylserine externalization and Hoechst33258 staining) were uniformly negative in LD9-4 exposed cells. Interestingly, transmission electron microscope, MDC staining and LC3 level determination all confirmed that autophagy was induced in LD9-4 treated A549 cells. Furthermore, we found that LD9-4-induced autophagy in A549 cells was associated with the increase of intracellular reactive oxygen species and the decrease of phosphorylated mTOR and p70S6K levels. In the meanwhile, both mRNA and protein levels of Beclin 1 were up-regulated in a time-dependent manner. Our data suggest that autophagy is induced by LD9-4 in A549 cells, and the accumulating reactive oxygen species, up-regulation of Beclin 1 and inhibition of the mTOR signaling pathway are involved in this process.

Activation of autophagy in a rat model of retinal ischemia following high intraocular pressure

Acute primary open angle glaucoma is an optic neuropathy characterized by the elevation of intraocular pressure, which causes retinal ischemia and neuronal death. Rat ischemia/reperfusion enhances endocytosis of both horseradish peroxidase (HRP) or fluorescent dextran into ganglion cell layer (GCL) neurons 24 h after the insult. We investigated the activation of autophagy in GCL-neurons following ischemia/reperfusion, using acid phosphatase (AP) histochemistry and immunofluorescence against LC3 and LAMP1. Retinal I/R lead to the appearance of AP-positive granules and LAMP1-positive vesicles 12 and 24 h after the insult, and LC3 labelling at 24 h, and induced a consistent retinal neuron death. At 48 h the retina was negative for autophagic markers. In addition, Western Blot analysis revealed an increase of LC3 levels after damage: the increase in the conjugated, LC3-II isoform is suggestive of autophagic activity. Inhibition of autophagy by 3-methyladenine partially prevented death of neurons and reduces apoptotic markers, 24 h post-lesion. The number of neurons in the GCL decreased significantly following I/R (I/R 12.21±1.13 vs controls 19.23±1.12 cells/500 µm); this decrease was partially prevented by 3-methyladenine (17.08±1.42 cells/500 µm), which potently inhibits maturation of autophagosomes. Treatment also prevented the increase in glial fibrillary acid protein immunoreactivity elicited by I/R. Therefore, targeting autophagy could represent a novel and promising treatment for glaucoma and retinal ischemia.

Neuronal autophagy as a mediator of life and death: contrasting roles in chronic neurodegenerative and acute neural disorders

Autophagy is a cellular mechanism for degrading proteins and organelles. It was first described as a physiological process essential for cellular health and survival, and this is its role in most cells. However, it can also be a mediator of cell death, either by the triggering of apoptosis or by an independent "autophagic" cell death mechanism. This duality is important in the central nervous system, where the activation of autophagy has recently been shown to be protective in certain chronic neurodegenerative diseases but deleterious in acute neural disorders such as stroke and hypoxic/ischemic injury. The authors here discuss these distinct roles of autophagy in the nervous system with a focus on the role of autophagy in mediating neuronal death. The development of new therapeutic strategies based on the manipulation of autophagy will need to take into account these opposing roles of autophagy.

Akt suppresses retrograde degeneration of dopaminergic axons by inhibition of macroautophagy

Axon degeneration is a hallmark of neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. Such degeneration is not a passive event but rather an active process mediated by mechanisms that are distinct from the canonical pathways of programmed cell death that mediate destruction of the cell soma. Little is known of the diverse mechanisms involved, particularly those of retrograde axon degeneration. We have previously observed in living animal models of degeneration in the nigrostriatal projection that a constitutively active form of the kinase, myristoylated Akt (Myr-Akt), demonstrates an ability to suppress programmed cell death and preserve the soma of dopamine neurons. Here, we show in both neurotoxin and physical injury (axotomy) models that Myr-Akt is also able to preserve dopaminergic axons due to suppression of acute retrograde axon degeneration. This cellular phenotype is associated with increased mammalian target of rapamycin (mTor) activity and can be recapitulated by a constitutively active form of the small GTPase Rheb, an upstream activator of mTor. Axon degeneration in these models is accompanied by the occurrence of macroautophagy, which is suppressed by Myr-Akt. Conditional deletion of the essential autophagy mediator Atg7 in adult mice also achieves striking axon protection in these acute models of retrograde degeneration. The protection afforded by both Myr-Akt and Atg7 deletion is robust and lasting, because it is still observed as protection of both axons and dopaminergic striatal innervation weeks after injury. We conclude that acute retrograde axon degeneration is regulated by Akt/Rheb/mTor signaling pathways.

Autophagic pathways in Parkinson disease and related disorders

Macroautophagy and chaperone-mediated autophagy (CMA) are the two main mammalian lysosomal proteolytic systems. In macroautophagy, double-membrane structures engulf organelles and other intracellular constituents through a highly regulated process that involves the formation of autophagic vacuoles and their fusion with lysosomes. In CMA, selected proteins are targeted through a nonvesicular pathway to a transport complex at the lysosomal membrane, through which they are threaded into the lysosomes and degraded. Autophagy is important in development, differentiation, cellular remodelling and survival during nutrient starvation. Increasing evidence suggests that autophagic dysregulation causes accumulation of abnormal proteins or damaged organelles, which is a characteristic of chronic neurodegenerative conditions, such as Parkinson disease (PD). Evidence from post-mortem material, transgenic mice, and animal and cellular models of PD suggests that both major autophagic pathways are malfunctioning. Numerous connections exist between proteins genetically linked to autosomal dominant PD, in particular α-synuclein and LRRK2, and autophagic pathways. However, proteins involved in recessive PD, such as PINK1 and Parkin (PINK2), function in the process of mitophagy, whereby damaged mitochondria are selectively engulfed by macroautophagy. This wealth of new data suggests that both autophagic pathways are potential targets for therapeutic intervention in PD and other related neurodegenerative conditions.

Induction of autophagy by cystatin C: a mechanism that protects murine primary cortical neurons and neuronal cell lines

Cystatin C (CysC) expression in the brain is elevated in human patients with epilepsy, in animal models of neurodegenerative conditions, and in response to injury, but whether up-regulated CysC expression is a manifestation of neurodegeneration or a cellular repair response is not understood. This study demonstrates that human CysC is neuroprotective in cultures exposed to cytotoxic challenges, including nutritional-deprivation, colchicine, staurosporine, and oxidative stress. While CysC is a cysteine protease inhibitor, cathepsin B inhibition was not required for the neuroprotective action of CysC. Cells responded to CysC by inducing fully functional autophagy via the mTOR pathway, leading to enhanced proteolytic clearance of autophagy substrates by lysosomes. Neuroprotective effects of CysC were prevented by inhibiting autophagy with beclin 1 siRNA or 3-methyladenine. Our findings show that CysC plays a protective role under conditions of neuronal challenge by inducing autophagy via mTOR inhibition and are consistent with CysC being neuroprotective in neurodegenerative diseases. Thus, modulation of CysC expression has therapeutic implications for stroke, Alzheimer's disease, and other neurodegenerative disorders.

Oxidative modifications, mitochondrial dysfunction, and impaired protein degradation in Parkinson's disease: how neurons are lost in the Bermuda triangle

While numerous hypotheses have been proposed to explain the molecular mechanisms underlying the pathogenesis of neurodegenerative diseases, the theory of oxidative stress has received considerable support. Although many correlations have been established and encouraging evidence has been obtained, conclusive proof of causation for the oxidative stress hypothesis is lacking and potential cures have not emerged. Therefore it is likely that other factors, possibly in coordination with oxidative stress, contribute to neuron death. Using Parkinson's disease (PD) as the paradigm, this review explores the hypothesis that oxidative modifications, mitochondrial functional disruption, and impairment of protein degradation constitute three interrelated molecular pathways that execute neuron death. These intertwined events are the consequence of environmental exposure, genetic factors, and endogenous risks and constitute a "Bermuda triangle" that may be considered the underlying cause of neurodegenerative pathogenesis.

All-you-can-eat: autophagy in neurodegeneration and neuroprotection

Autophagy is the major pathway involved in the degradation of proteins and organelles, cellular remodeling, and survival during nutrient starvation. Autophagosomal dysfunction has been implicated in an increasing number of diseases from cancer to bacterial and viral infections and more recently in neurodegeneration. While a decrease in autophagic activity appears to interfere with protein degradation and possibly organelle turnover, increased autophagy has been shown to facilitate the clearance of aggregation-prone proteins and promote neuronal survival in a number of disease models. On the other hand, too much autophagic activity can be detrimental as well and lead to cell death, suggesting the regulation of autophagy has an important role in cell fate decisions. An increasing number of model systems are now available to study the role of autophagy in the central nervous system and how it might be exploited to treat disease. We will review here the current knowledge of autophagy in the central nervous system and provide an overview of the various models that have been used to study acute and chronic neurodegeneration.

How autophagy is related to programmed cell death during the development of the nervous system

Programmed cell death, together with proliferation and differentiation, is an essential process during the development of the nervous system. During neurogenesis, neurons and glia are generated in large numbers and, subsequently, they die in a process that depends on trophic signalling that refines the cytoarchitecture and connectivity of the nervous system. In addition, programmed cell death also affects proliferating neuroepithelial cells and recently differentiated neuroblasts. Autophagy is a lysosomal degradative pathway that allows the recycling of cell constituents, and seems to be able to play a dual role. It may serve to protect the cell by preventing the accumulation of deleterious products and organelles and supplying energy and amino acids. On the other hand, it has been considered a type of cell death. The role of autophagy during development is little characterized. The retina provides an excellent model system to study autophagy in the context of neural development, and to establish its relationship with proliferation, differentiation and cell death. In the present review, we summarize recent findings showing that autophagy contributes to the development of the nervous system by providing energy for cell corpse removal after physiological cell death, a process associated with retinal neurogenesis.

The role of autophagy in mammalian development: cell makeover rather than cell death

Autophagy is important for the degradation of bulk cytoplasm, long-lived proteins, and entire organelles. In lower eukaryotes, autophagy functions as a cell death mechanism or as a stress response during development. However, autophagy's significance in vertebrate development, and the role (if any) of vertebrate-specific factors in its regulation, remains unexplained. Through careful analysis of the current autophagy gene mutant mouse models, we propose that in mammals, autophagy may be involved in specific cytosolic rearrangements needed for proliferation, death, and differentiation during embryogenesis and postnatal development. Thus, autophagy is a process of cytosolic "renovation," crucial in cell fate decisions.

Neuronal cell death in neurodegenerative diseases: recurring themes around protein handling

Neuronal cell death plays a role in many chronic neurodegenerative diseases with the loss of particular subsets of neurons. The loss of the neurons occurs during a period of many years, which can make the mode(s) of cell death and the initiating factors difficult to determine. In vitro and in vivo models have proved invaluable in this regard, yielding insight into cell death pathways. This review describes the main mechanisms of neuronal cell death, particularly apoptosis, necrosis, excitotoxicity and autophagic cell death, and their role in neurodegenerative diseases such as ischaemia, Alzheimer's, Parkinson's and Huntington's diseases. Crosstalk between these death mechanisms is also discussed. The link between cell death and protein mishandling, including misfolded proteins, impairment of protein degradation, protein aggregation is described and finally, some pro-survival strategies are discussed.

Neuronal death in the lateral geniculate nucleus of young ferrets following a cortical lesion: time-course, age dependence and involvement of caspases

In humans and many other mammalian species, the behavioural consequences of a cortical lesion tend to be milder when it occurs early in life, and there is evidence that an important factor contributing to the behavioural sparing in the young is the formation of new thalamo-cortical connections by thalamic neurons initially connected with the lesioned area. However, this plasticity may be hindered by the secondary death of many of these neurons owing to the elimination by the primary lesion of their trophic support from the cortex. With the long-term aim of preventing this neuronal death, we have here characterised its timing in the lateral geniculate nucleus of ferrets following lesions of the visual cortex on postnatal days 5, 10, 20 or 35. After the earliest lesions (P5 or P10), this cell death began rapidly and occurred synchronously, being maximal at 48 h and declining to zero over the next few days. Following later lesions the cell death began more slowly and continued for longer. The dying neurons contained activated caspase-3 and fragmented DNA and their number 2 days after a P5 lesion was reduced by the broad-band caspase inhibitor z-VAD.fmk. These experiments open the way for a concerted effort to enhance adaptive plasticity by neuroprotection in the hours or days following a cortical lesion.

Excitotoxicity-related endocytosis in cortical neurons

Recent studies showed that endocytosis is enhanced in neurons exposed to an excitototoxic stimulus. We here confirm and analyze this new phenomenon using dissociated cortical neuronal cultures. NMDA-induced uptake (FITC-dextran or FITC or horseradish peroxidase) occurs in these cultures and is due to endocytosis, not to cell entry through damaged membranes; it requires an excitotoxic dose of NMDA and is dependent on extracellular calcium, but occurs early, while the neuron is still intact and viable. It involves two components, NMDA-induced and constitutive, with different characteristics. Neither component involves specific binding of the endocytosed molecules to a saturable receptor. Strikingly, molecules internalized by the NMDA-induced component are targeted to neuronal nuclei. This component, but not the constitutive one, is blocked by a c-Jun N-terminal protein kinase inhibitor. In conclusion, an excitotoxic dose of NMDA triggers c-Jun N-terminal protein kinase-dependent endocytosis in cortical neuronal cultures, providing an in vitro model of the excitotoxicity-induced endocytosis reported in intact tissues.

Motor neuron degeneration in amyotrophic lateral sclerosis mutant superoxide dismutase-1 transgenic mice: mechanisms of mitochondriopathy and cell death

The mechanisms of human mutant superoxide dismutase-1 (mSOD1) toxicity to motor neurons (MNs) are unresolved. We show that MNs in G93A-mSOD1 transgenic mice undergo slow degeneration lacking similarity to apoptosis structurally and biochemically. It is characterized by somal and mitochondrial swelling and formation of DNA single-strand breaks prior to double-strand breaks occurring in nuclear and mitochondrial DNA. p53 and p73 are activated in degenerating MNs, but without nuclear import. The MN death is independent of activation of caspases-1, -3, and -8 or apoptosis-inducing factor within MNs, with a blockade of apoptosis possibly mediated by Aven up-regulation. MN swelling is associated with compromised Na,K-ATPase activity and aggregation. mSOD1 mouse MNs accumulate mitochondria from the axon terminals and generate higher levels of superoxide, nitric oxide, and peroxynitrite than MNs in control mice. Nitrated and aggregated cytochrome c oxidase subunit-I and alpha-synuclein as well as nitrated SOD2 accumulate in mSOD1 mouse spinal cord. Mitochondria in mSOD1 mouse MNs accumulate NADPH diaphorase and inducible nitric oxide synthase (iNOS)-like immunoreactivity, and iNOS gene deletion extends significantly the life span of G93A-mSOD1 mice. Prior to MN loss, spinal interneurons degenerate. These results identify novel mechanisms for mitochondriopathy and MN degeneration in amyotrophic lateral sclerosis (ALS) mice involving blockade of apoptosis, accumulation of MN mitochondria with enhanced toxic potential from distal terminals, NOS localization in MN mitochondria and peroxynitrite damage, and early degeneration of alpha-synuclein(+) interneurons. The data support roles for oxidative stress, protein nitration and aggregation, and excitotoxicity as participants in the process of MN degeneration caused by mSOD1.

Autophagy in neurodegenerative disease: friend, foe or turncoat?

Autophagy, a lysosomal pathway for degrading organelles and long-lived proteins, is becoming recognized as a key adaptive response that can preclude death in stressed or diseased cells. However, during development strong induction of autophagy in specific cell populations mediates a type of programmed cell death that has distinctive 'autophagic' morphology and a requirement for autophagy activity. The recent identification of autophagosomes in neurons in a growing number of neurodegenerative disorders has, therefore, sparked controversy about whether these structures are contributing to neuronal cell death or protecting against it. Emerging evidence supports the view that induction of autophagy is a neuroprotective response and that inadequate or defective autophagy, rather than excessive autophagy, promotes neuronal cell death in most of these disorders. In this review, we consider possible mechanisms underlying autophagy-associated cell death and their relationship to pathways mediating apoptosis and necrosis.

Role of phosphoinositide 3-kinase in the autophagic death of serum-deprived PC12 cells

The death of serum-deprived undifferentiated PC12 cells shows both autophagic and apoptotic features. Since it is still controversial whether the autophagy is instrumental in the cell death or a mere epiphenomenon, we tested the effects of inhibiting the autophagy by a variety of phosphoinositide 3-kinase inhibitors, and provided evidence that the autophagy, or a related trafficking event, is indeed instrumental in the cell death. Furthermore, by comparing the effects of PI3-K inhibition and caspase-inhibition on autophagic and apoptotic cellular events, we showed that in this case the autophagic and apoptotic mechanisms mediate cell death by parallel pathways and do not act in series.

Cell death in early neural life

Programmed cell death is a relevant process in the physiology and pathology of the nervous system. Neuronal cell death during development is well characterized, and studies of this process have provided valuable information regarding the regulatory mechanisms of cell death in the nervous system. In the last few years, cell death occurring at earlier developmental stages and affecting proliferating neuroepithelial cells and recently born neuroblasts has been recognized. In this review we cover the observations on cell death in the early, proliferating stages of vertebrate neural development. Genetically modified mouse model systems and complementary in vivo approaches in other vertebrates have provided a solid basis for its relevance and contribution to normal neural development, as well as for the pathological consequences of its deregulation. However, the precise functional role of cell death remains a topic of debate.

Morphological comparison of apoptotic with non-apoptotic dying cells in the developing inner ear of mouse embryos

Dying cells studied by the TdT-mediated dUTP nick end-labeling (TUNEL) method have been classified as "apoptotic" and "non-apoptotic" cells. In this study, in which 12-day-old mouse embryos were used because of a high frequency of "natural cell death" due to changing inner ear morphology [Kaufman, M.H., 1992. The Atlas of Mouse Development, first ed., Academic Press, London, p. 147], the percentages of "apoptotic" and "non-apoptotic" dying cells (ADC and NADC) among total dying cells in the inner ear were calculated. Observation of consecutive paraffin sections showed about 90% of the dying inner ear cells to be ADC and about 10% to be NADC. ADC and NADC TUNEL positive dying cells in resin sections observed by light microscopy were examined again by transmission electron microscopy using a re-embedding procedure. ADC and NADC were then analyzed based on the classification of dying cells (types 1, 2, 3A, and 3B) as described by Clarke [Anat. Embryol. 181 (1990) 195]. It was clear that ADC were the equivalent of type 1 (apoptotic) dying cells and NADC were the equivalent of type 2 (autophagic) dying cells. We consider these findings to be important baselines for determining the process underlying abnormal development of the inner ear and its functional disorders such as hearing loss.

Extensive involvement of autophagy in Alzheimer disease: an immuno-electron microscopy study

The accumulation of lysosomes and their hydrolases within neurons is a well-established neuropathologic feature of Alzheimer disease (AD). Here we show that lysosomal pathology in AD brain involves extensive alterations of macroautophagy, an inducible pathway for the turnover of intracellular constituents, including organelles. Using immunogold labeling with compartmental markers and electron microscopy on neocortical biopsies from AD brain, we unequivocally identified autophagosomes and other prelysosomal autophagic vacuoles (AVs), which were morphologically and biochemically similar to AVs highly purified from mouse liver. AVs were uncommon in brains devoid of AD pathology but were abundant in AD brains particularly, within neuritic processes, including synaptic terminals. In dystrophic neurites, autophagosomes, multivesicular bodies, multilamellar bodies, and cathepsin-containing autophagolysosomes were the predominant organelles and accumulated in large numbers. These compartments were distinguishable from lysosomes and lysosomal dense bodies, previously shown also to be abundant in dystrophic neurites. Autophagy was evident in the perikarya of affected neurons, particularly in those with neurofibrillary pathology where it was associated with a relative depletion of mitochondria and other organelles. These observations provide the first evidence that macroautophagy is extensively involved in the neurodegenerative/regenerative process in AD. The striking accumulations of immature AV forms in dystrophic neurites suggest that the transport of AVs and their maturation to lysosomes may be impaired, thereby impeding the suspected neuroprotective functions of autophagy.

Degradative organelles containing mislocalized alpha-and beta-synuclein proliferate in presenilin-1 null neurons

Presenilin-1 null mutation (PS1 -/-) in mice is associated with morphological alterations and defects in cleavage of transmembrane proteins. Here, we demonstrate that PS1 deficiency also leads to the formation of degradative vacuoles and to the aberrant translocation of presynaptic alpha- and beta-synuclein proteins to these organelles in the perikarya of primary neurons, concomitant with significant increases in the levels of both synucleins. Stimulation of autophagy in control neurons produced a similar mislocalization of synucleins as genetic ablation of PS1. These effects were not the result of the loss of PS1 gamma-secretase activity; however, dysregulation of calcium channels in PS1 -/- cells may be involved. Finally, colocalization of alpha-synuclein and degradative organelles was observed in brains from patients with the Lewy body variant of AD. Thus, aberrant accumulation of alpha- and beta-synuclein in degradative organelles are novel features of PS1 -/- neurons, and similar events may promote the formation of alpha-synuclein inclusions associated with neurodegenerative diseases.

Alix, a Protein Regulating Endosomal Trafficking, Is Involved in Neuronal Death

Alix/AIP1 is a cytoplasmic protein, which was first characterized as an interactor of ALG-2, a calcium-binding protein necessary for cell death. Alix has also recently been defined as a regulator of the endo-lysosomal system. Here we have used post-mitotic cerebellar neurons to test Alix function in caspase-dependent and -independent cell death. Indeed, these neurons survived when cultured in 25 mm potassium-containing medium but underwent apoptosis soon after the extracellular potassium was lowered to 5 mm. In agreement with other studies, we show that caspases are activated after K+ deprivation, but that inhibition of these proteases, using the pancaspase inhibitor boc-aspartyl(OMe)-fluoromethylketone, has no effect on cell survival. Transfection experiments demonstrated that Alix overexpression is sufficient to induce caspase activation, whereas overexpression of its C-terminal half, Alix-CT, blocks caspase activation and cell death after K+ deprivation. We also define a 12-amino acid PXY repeat of the C-terminal proline-rich domain necessary for binding ALG-2. Deletion of this domain in Alix or in Alix-CT abolished the effects of the overexpressed proteins on neuronal survival, demonstrating that the ALG-2-binding region is crucial for the death-modulating function of Alix. Overall, these findings define the Alix/ALG-2 complex as a regulator of cell death controlling both caspase-dependent and -independent pathways. They also suggest a molecular link between the endo-lysosomal system and the effectors of the cell death machinery.

Kainate-induced endocytosis in retinal amacrine cells

Endocytosis is enhanced in some cases of neuronal death. We report for the first time that intraocular injections, in chick embryos, of excitotoxic doses of kainate induce strong endocytosis in retinal amacrine cells destined to die and that even subtoxic doses can induce some degree of endocytosis. That the uptake was due to endocytosis rather than passive diffusion through the plasma membrane was shown ultrastructurally. The endocytosis was demonstrated by using three unrelated tracers--horseradish peroxidase, microperoxidase, and 4.4-kDa fluorescein isothiocyanate (FITC)-labeled dextran--suggesting that it does not depend on the binding of the tracers to a particular receptor. However, it appears to be surprisingly sensitive to the size of the ligand, because a heavier (42-kDa) FITC-dextran was not endocytosed. The induction of endocytosis by kainate can occur even when protein synthesis is blocked. These results indicate that toxic or near-toxic doses of kainate induce endocytosis, raising the question of whether this is mechanistically implicated in causing or preventing excitotoxic neuronal death.

The mouse APG10 homologue, an E2-like enzyme for Apg12p conjugation, facilitates MAP-LC3 modification

Autophagy is a process for the bulk degradation of cytosolic compartments by lysosomes/vacuoles. The formation of autophagosomes involves a dynamic rearrangement of the membrane for which two ubiquitin-like modifications (the conjugation of Apg12p and the modification of a soluble form of MAP-LC3 to a membrane-bound form) are essential. In yeast, Apg10p is an E2-like enzyme essential for Apg12p conjugation. The isolated mouse APG10 gene product interacts with mammalian Apg12p dependent on mammalian Apg7p (E1-like enzyme), and facilitates Apg12p conjugation. The interaction of Apg10p with Apg12p is dependent on the carboxyl-terminal glycine of Apg12p. Mutational analysis of the predicted active site cysteine (Cys161) within mouse Apg10p shows that mutant Apg10pC161S, which can form a stable intermediate with Apg12p, inhibits Apg12p conjugation even in the presence of Apg7p, while overexpression of Apg7p facilitates formation of an Apg12p-Apg5p conjugate. Furthermore, the coexpression of Apg10p with Apg7p facilitates the modification of a soluble form of MAP-LC3 to a membrane-bound form, a second modification essential for autophagy. Mouse Apg10p interacts with MAP-LC3 in HEK293 cells, while no mutant Apg10pC161S forms any intermediate with MAP-LC3. Direct interaction between Apg10p and MAP-LC3 is also demonstrated by yeast two-hybrid analysis. The inability of mutant Apg10pC161S to form any intermediate with MAP-LC3 has ruled out the possibility that MAP-LC3 interacts with Apg10p as a substrate.

Thinking globally, acting locally: steroid hormone regulation of the dendritic architecture, synaptic connectivity and death of an individual neuron

Steroid hormones act via evolutionarily conserved nuclear receptors to regulate neuronal phenotype during development, maturity and disease. Steroid hormones exert 'global' effects in organisms to produce coordinated physiological responses whereas, at the 'local' level, individual neurons can respond to a steroidal signal in highly specific ways. This review focuses on two phenomena-the loss of dendritic processes and the programmed cell death (PCD) of neurons-that can be regulated by steroid hormones (e.g. during sexual differentiation in vertebrates). In insects such as the moth, Manduca sexta, and fruit fly, Drosophila melanogaster, ecdysteroids orchestrate a reorganization of neural circuits during metamorphosis. In Manduca, accessory planta retractor (APR) motoneurons undergo dendritic loss at the end of larval life in response to a rise in 20-hydroxyecdysone (20E). Dendritic regression is associated with a decrease in the strength of monosynaptic inputs, a decrease in the number of contacts from pre-synaptic neurons, and the loss of a behavior mediated by these synapses. The APRs in different abdominal segments undergo segment-specific PCD at pupation and adult emergence that is triggered directly and cell-autonomously by a genomic action of 20E, as demonstrated in cell culture. The post-emergence death of APRs provides a model for steroid-mediated neuroprotection. APR death occurs by autophagy, not apoptosis, and involves caspase activation and the aggregation and ultracondensation of mitochondria. Manduca genes involved in segmental identity, 20E signaling and PCD are being sought by suppressive subtractive hybridization (SSH) and cDNA microarrays. Experiments utilizing Drosophila as a complementary system have been initiated. These insect model systems contribute toward understanding the causes and functional consequences of dendritic loss and neurodegeneration in human neurological disorders.

N-methyl-d-aspartate-triggered neuronal death in organotypic hippocampal cultures is endocytic, autophagic and mediated by the c-Jun N-terminal kinase pathway

Acute excitotoxic neuronal death was studied in rat organotypic hippocampal slices exposed to 100 micro mN-methyl-d-aspartate. Fulgurant death of pyramidal neurons occurred in the CA1 and CA3 regions and was already detectable within 2 h of the N-methyl-d-aspartate administration. Morphologically, the neuronal death was neither apoptotic nor necrotic but had the hallmarks of autophagic neuronal death, as shown by acid phosphatase histochemistry in both CA1 and CA3 and by electron microscopy in CA1. The dying neurons also manifested strong endocytosis of horseradish peroxidase or microperoxidase, occurring probably by a fluid phase mechanism, and followed, surprisingly, by nuclear entry. In addition to these autophagic and endocytic characteristics, there were indications that the c-Jun N-terminal kinase pathway was activated. Its target c-Jun was selectively phosphorylated in CA1, CA3 and the dentate gyrus and c-Fos, the transcription of which is under the positive control of c-Jun N-terminal kinase target Elk1, was selectively up-regulated in CA1 and CA3. All these effects, the neuronal death itself and the associated autophagy and endocytosis, were totally prevented by a cell-permeable inhibitor of the interaction between c-Jun N-terminal kinase and certain of its targets. These results show that pyramidal neurons undergoing excitotoxic death in this situation are autophagic and endocytic and that both the cell death and the associated autophagy and endocytosis are under the control of the c-Jun N-terminal kinase pathway.

Steroid-triggered programmed cell death of a motoneuron is autophagic and involves structural changes in mitochondria

Neuronal death occurs during normal development and disease and can be regulated by steroid hormones. In the hawkmoth, Manduca sexta, individual accessory planta retractor (APR) motoneurons undergo a segment-specific pattern of programmed cell death (PCD) at pupation that is triggered directly and cell autonomously by the steroid hormone 20-hydroxyecdysone (20E). APRs from abdominal segment six [APR(6)s] die by 48 hours after pupal ecdysis (PE; entry into the pupal stage), whereas APR(4)s survive until adulthood. Cell culture experiments showed previously that 20E acts directly on APRs to trigger PCD, with intrinsic segmental identity determining which APRs die. The APR(6) death pathway includes caspase activation and loss of mitochondrial function. We used transmission electron microscopy to investigate the ultrastructure of APR somata before and during PCD. APR(4)s showed normal ultrastructure at all stages examined, as did APR(6)s until approximately stage PE. During APR(6) death, there was massive accumulation of autophagic bodies and vacuoles, mitochondria became ultracondensed and aggregated into compact clusters, and ribosomes aggregated in large blocks. Nuclear ultrastructure remained normal, without chromatin condensation, until the nuclear envelope fragmented late in the death process. Light microscopic immunocytochemistry showed that dying APR(6)s were TUNEL-positive, which is diagnostic of fragmented DNA. These observations indicate that the steroid-induced, caspase-dependent, cell-autonomous PCD of APR(6)s is autophagic, not apoptotic, and support an early role for mitochondrial alterations during PCD. This system permits the study of neuronal death in response to its bona fide developmental signal, the rise in a steroid hormone.

Autophagy: a barrier or an adaptive response to cancer

Macroautophagy or autophagy is a degradative pathway terminating in the lysosomal compartment after the formation of a cytoplasmic vacuole that engulfs macromolecules and organelles. The recent discovery of the molecular controls of autophagy that are common to eukaryotic cells from yeast to human suggests that the role of autophagy in cell functioning is far beyond its nonselective degradative capacity. The involvement of proteins with properties of tumor suppressor and oncogenic properties at different steps of the pathway implies that autophagy must be considered in tumor progression. Autophagy as a stress response mechanism protects cancer cells from low nutrient supply or therapeutic insults. Autophagy is also involved in the elimination of cancer cells by triggering a non-apoptotic cell death program, suggesting a negative role in tumor development. These two aspects of autophagy will be discussed in this review.

Ultrastructure of retinal ganglion cell death after axotomy in chick embryos

Axotomy often leads to neuronal death, which occurs after a particularly short delay in immature animals. Tectal lesions were made in embryonic day (E) 12 chick embryos, thereby axotomizing the retinal ganglion cells of the contralateral eye, which then died within 3 days. We here describe the ultrastructural changes in the axotomized ganglion cells. The main changes were nuclear invagination and type 3B (cytoplasmic type) cell death characterized by dilation of the perinuclear space, endoplasmic reticulum, and Golgi apparatus. However, nuclear invagination was never seen in type 3B dying cells. All the axotomy-induced retinal ganglion cell death appears to have been of type 3B; apoptosis was not induced by axotomy, as was confirmed by additional light microscopic experiments showing that it did not increase the frequency of apoptotic markers revealed by terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (the TUNEL method) labeling and immunoreactivity for activated caspase-3. However, the latter methods did show small numbers of apoptotic cells dying naturally even in control retinas. After the death of the axotomized ganglion cells, they were phagocytosed mainly in Müller processes. The present findings open up the chick tectal lesion model as a system for analyzing type 3B neuronal death in vivo.

Long-lasting aberrant tubulovesicular membrane inclusions accumulate in developing motoneurons after a sublethal excitotoxic insult: a possible model for neuronal pathology in neurodegenerative disease

We have previously shown that chronic treatment of chick embryos [from embryonic day 5 (E5) to E9] with NMDA rescues spinal cord motoneurons (MNs) from programmed cell death. In this situation, MNs exhibit a reduced vulnerability to acute excitotoxic lesions and downregulate NMDA and AMPA-kainate receptors. Here, we report that this treatment results in long-lasting sublethal structural changes in MNs. In Nissl-stained sections from the spinal cord of NMDA-treated embryos, MNs display an area adjacent to an eccentrically positioned nucleus in which basophilia is excluded. Ultrastructurally, MNs accumulate tubulovesicular structures surrounded by Golgi stacks. Thiamine pyrophosphatase but not acid phosphatase was detected inside the tubulovesicular structures, which are resistant to disruption by brefeldin A or monensin. Immunocytochemistry reveals changes in the content and distribution of calcitonin gene-related peptide, the KDEL receptor, the early endosomal marker EEA1, and the recycling endosome marker Rab11, indicating that a dysfunction in membrane trafficking and protein sorting occurs in these MNs. FM1-43, a marker of the endocytic pathway, strongly accumulates in MNs from isolated spinal cords after chronic NMDA treatment. Changes in the distribution of cystatin C and presenilin-1 and an accumulation of amyloid precursor protein and beta-amyloid product were also observed in NMDA-treated MNs. None of these alterations involve an interruption of MN-target (muscle) connections, as detected by the retrograde tracing of MNs with cholera toxin B subunit. These results demonstrate that chronic NMDA treatment induces severe changes in the motoneuronal endomembrane system that may be related to some neuropathological alterations described in human MN disease.

Huntingtin expression stimulates endosomal-lysosomal activity, endosome tubulation, and autophagy

An expansion of polyglutamines in the N terminus of huntingtin causes Huntington's disease (HD) and results in the accrual of mutant protein in the nucleus and cytoplasm of affected neurons. How mutant huntingtin causes neurons to die is unclear, but some recent observations suggest that an autophagic process may occur. We showed previously that huntingtin markedly accumulates in endosomal-lysosomal organelles of affected HD neurons and, when exogenously expressed in clonal striatal neurons, huntingtin appears in cytoplasmic vacuoles causing cells to shrink. Here we show that the huntingtin-enriched cytoplasmic vacuoles formed in vitro internalized the lysosomal enzyme cathepsin D in proportion to the polyglutamine-length in huntingtin. Huntingtin-labeled vacuoles displayed the ultrastructural features of early and late autophagosomes (autolysosomes), had little or no overlap with ubiquitin, proteasome, and heat shock protein 70/heat shock cognate 70 immunoreactivities, and altered the arrangement of Golgi membranes, mitochondria, and nuclear membranes. Neurons with excess cytoplasmic huntingtin also exhibited increased tubulation of endosomal membranes. Exogenously expressed human full-length wild-type and mutant huntingtin codistributed with endogenous mouse huntingtin in soluble and membrane fractions, whereas human N-terminal huntingtin products were found only in membrane fractions that contained lysosomal organelles. We speculate that mutant huntingtin accumulation in HD activates the endosomal-lysosomal system, which contributes to huntingtin proteolysis and to an autophagic process of cell death.

Two waves of programmed cell death occur during formation and development of somatic embryos in the gymnosperm, Norway spruce

In the animal life cycle, the earliest manifestations of programmed cell death (PCD) can already be seen during embryogenesis. The aim of this work was to determine if PCD is also involved in the elimination of certain cells during plant embryogenesis. We used a model system of Norway spruce somatic embryogenesis, which represents a multistep developmental pathway with two broad phases. The first phase is represented by proliferating proembryogenic masses (PEMs). The second phase encompasses development of somatic embryos, which arise from PEMs and proceed through the same sequence of stages as described for their zygotic counterparts. Here we demonstrate two successive waves of PCD, which are implicated in the transition from PEMs to somatic embryos and in correct embryonic pattern formation, respectively. The first wave of PCD is responsible for the degradation of PEMs when they give rise to somatic embryos. We show that PCD in PEM cells and embryo formation are closely interlinked processes, both stimulated upon withdrawal or partial depletion of auxins and cytokinins. The second wave of PCD eliminates terminally differentiated embryo-suspensor cells during early embryogeny. During the dismantling phase of PCD, PEM and embryo-suspensor cells exhibit progressive autolysis, resulting in the formation of a large central vacuole. Autolytic degradation of the cytoplasm is accompanied by lobing and budding-like segmentation of the nucleus. Nuclear DNA undergoes fragmentation into both large fragments of about 50 kb and multiples of approximately 180 bp. The tonoplast rupture is delayed until lysis of the cytoplasm and organelles, including the nucleus, is almost complete. The protoplasm then disappears, leaving a cellular corpse represented by only the cell wall. This pathway of cell dismantling suggests overlapping of apoptotic and autophagic types of PCD during somatic embryogenesis in Norway spruce.

Response of macrophage/microglial cells to experimental neuronal degeneration in the avian isthmo-optic nucleus during development

Blockade of the retrograde axonal transport of isthmo-optic nucleus (ION) neurons in the avian embryo results in their massive degeneration. We used this system to investigate the response of macrophage/microglial cells to neuronal degeneration in the embryonic brain. Colchicine was injected into the right eye of quail or chick embryos at a time when the survival of ION neurons depends on retrograde trophic support from the retina, and the chronology of the subsequent macrophage/microglial response in the ION was analyzed. This response was restricted to the ION contralateral to the injected eye; no modifications of the normal state were observed in the surrounding parenchyma or in the opposite ION, used as control. The response was first detected 18 hours after the colchicine injection (18 hours pi), when an increase of the macrophage/microglial cell number was evident. The number of these cells in the affected ION increased, peaking at 40-48 hours pi. At later survival times, macrophage/microglial cells were progressively less abundant in the affected ION, which gradually diminished in size. At 120 hours pi the only remnant of the ION was a small cluster of macrophage/microglial cells, surrounded by a clear area with scarce nonmicroglial cells, in the region formerly occupied by the ION. This study reveals that a strong macrophage/microglial response occurs in the embryonic brain in response to neuronal degeneration but that these cells do not trigger the neuronal death, as they only appear after pyknotic fragments are already observable.

Neuronal death in the central nervous system during development

About half the neurons in the brain die at the time when their connections are being formed. This neuronal death is regulated by anterograde and retrograde signals that reflect both electrical activity and the uptake of trophic factors. Our recent data on the isthmo-optic projection indicate that there are in fact two different retrograde signals: a slow-acting survival signal mediated by a neurotrophin, and a fast-acting death signal mediated by calcium entry due to electrical activity in the presynaptic terminals. The developmental roles of the cell death are not well understood, but they appear to include the elimination of aberrant connections. The intracellular mechanisms of the cell death may not always correspond to the apoptotic ones so thoroughly investigated in vitro, because only one of the three morphological types occurring regularly in vivo resembles apoptosis. However, our experiments on retinal ganglion cells indicate that several apoptotic mechanisms apply in this particular in vivo situation: these include an involvement of oxygenated free radicals and glutathione, cell cycle-related events, and probably the synthesis of proteins promoting neuroprotection or cell death.

Neurodegeneration in excitotoxicity, global cerebral ischemia, and target deprivation: A perspective on the contributions of apoptosis and necrosis

In the human brain and spinal cord, neurons degenerate after acute insults (e.g., stroke, cardiac arrest, trauma) and during progressive, adult-onset diseases [e.g., amyotrophic lateral sclerosis, Alzheimer's disease]. Glutamate receptor-mediated excitotoxicity has been implicated in all of these neurological conditions. Nevertheless, effective approaches to prevent or limit neuronal damage in these disorders remain elusive, primarily because of an incomplete understanding of the mechanisms of neuronal death in in vivo settings. Therefore, animal models of neurodegeneration are crucial for improving our understanding of the mechanisms of neuronal death. In this review, we evaluate experimental data on the general characteristics of cell death and, in particular, neuronal death in the central nervous system (CNS) following injury. We focus on the ongoing controversy of the contributions of apoptosis and necrosis in neurodegeneration and summarize new data from this laboratory on the classification of neuronal death using a variety of animal models of neurodegeneration in the immature or adult brain following excitotoxic injury, global cerebral ischemia, and axotomy/target deprivation. In these different models of brain injury, we determined whether the process of neuronal death has uniformly similar morphological characteristics or whether the features of neurodegeneration induced by different insults are distinct. We classified neurodegeneration in each of these models with respect to whether it resembles apoptosis, necrosis, or an intermediate form of cell death falling along an apoptosis-necrosis continuum. We found that N-methyl-D-aspartate (NMDA) receptor- and non-NMDA receptor-mediated excitotoxic injury results in neurodegeneration along an apoptosis-necrosis continuum, in which neuronal death (appearing as apoptotic, necrotic, or intermediate between the two extremes) is influenced by the degree of brain maturity and the subtype of glutamate receptor that is stimulated. Global cerebral ischemia produces neuronal death that has commonalities with excitotoxicity and target deprivation. Degeneration of selectively vulnerable populations of neurons after ischemia is morphologically nonapoptotic and is indistinguishable from NMDA receptor-mediated excitotoxic death of mature neurons. However, prominent apoptotic cell death occurs following global ischemia in neuronal groups that are interconnected with selectively vulnerable populations of neurons and also in nonneuronal cells. This apoptotic neuronal death is similar to some forms of retrograde neuronal apoptosis that occur following target deprivation. We conclude that cell death in the CNS following injury can coexist as apoptosis, necrosis, and hybrid forms along an apoptosis-necrosis continuum. These different forms of cell death have varying contributions to the neuropathology resulting from excitotoxicity, cerebral ischemia, and target deprivation/axotomy. Degeneration of different populations of cells (neurons and nonneuronal cells) may be mediated by distinct or common causal mechanisms that can temporally overlap and perhaps differ mechanistically in the rate of progression of cell death.

Early retrograde effects of blocking axoplasmic transport in the axons of developing neurons

Depriving developing neurons of retrograde trophic support may disrupt their development and often causes them to die. We here report the effects, in chick embryos, of eliminating retrograde support in the isthmo-optic projection by blocking axoplasmic transport in the terminal parts of the axons, which is known ultimately to kill the isthmo-optic neurons. Within only 9 h, this had perturbed the process of cellular reorganisation that eventually leads to the laminated appearance of the mature isthmo-optic nucleus. Neuron survival in the isthmo-optic nucleus was affected even more quickly, but the earliest change, occurring in as little as 3 h, was not an increase in the number of dying neurons, but a decrease below control values. This novel effect was still present at 6 and 9 h after the injection, but at longer survival times the number of dying neurons increased well above control values as expected. Our interpretation of the transient decrease in neuronal death is that retrograde trophic signals include both death-promoting and life-promoting components, and that the former act faster in this system.

Apoptotic features of selective neuronal death in ischemia, epilepsy and gp 120 toxicity

The occurrence of physiological cell death has been known for decades, but interest in the subject was renewed in 1972 when Kerr, Wyllie and Currie described in detail the ultrastructural changes characteristic of dying cells and coined the term apoptosis to describe the process. Cells display a wide variety of morphological changes when dying during development or following a toxic insult. A binary classification scheme suggests that physiologically appropriate death is due to apoptosis and that pathological mechanisms involve necrosis. However, recent studies indicate a potential involvement of apoptotic cell death in ischemia, status epilepticus and HIV-1 infection.

Ultrastructure of excitotoxic neuronal death in murine cortical culture

Ischemic and traumatic brain injury are likely to involve neuronal injury triggered by glutamate receptor overactivation. Although excitotoxic neuronal injury has been widely studied in the setting of primary culture, the extent to which these in vitro injury paradigms resemble in vivo ischemic injury morphologically has not previously been well studied. We studied glutamate receptor mediated neuronal death by transmission electron microscopy and light microscopy. Morphologic characteristics of neurons injured by 10 min exposure to 500 microM glutamate include rapid swelling of mitochondria and endoplasmic reticulum, and cytoplasmic and nuclear lucency. Both alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid and kainic acid caused vacuolation, dilatation of the endoplasmic reticulum, cytoplasmic condensation and random condensation of chromatin with preserved mitochondria. None of these injuries was ameliorated by cycloheximide or actinomycin D; all were significantly lessened by aurintricarboxylic acid. Gel electrophoresis showed no increase in DNA fragmentation over control. The morphologic changes seen with alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid and kainate are distinct from the changes induced by glutamate. Excitotoxic injury in this system due to high concentrations of glutamate resembles necrosis while the other agonists produce a different form of cell death which is neither necrosis nor apoptosis.

Induction of DNA fragmentation after 10 to 120 minutes of focal cerebral ischemia in rats

BACKGROUND AND PURPOSE

The induction of neuronal necrosis has been studied after various durations of transient middle cerebral artery (MCA) occlusion in the rat. The objective of the present study was to measure the numbers and anatomic distribution of cells exhibiting apoptotic bodies as an indication of DNA fragmentation and apoptotic cell death as a function of duration of transient MCA occlusion in the rat.

METHODS

The MCA of male Wistar rats (n = 24) was occluded for 10, 20, 30, 60, 90, and 120 minutes (n = 4 per group) with the use of an intraluminal monofilament, and reperfusion was instituted for 48 hours. DNA fragmentation was measured in paraffin sections with the use of a terminal deoxynucleotidyl-transferase (TdT)-mediated dUTP-biotin nick end-labeling (TUNEL) method. Adjacent sections were stained with hematoxylin and eosin for analysis of ischemic cell damage, and immunohistochemical double staining methods were used for cell identification. Sham-operated rats (n = 4) and normal rats not subjected to any surgical procedure (n = 4) were used as controls for apoptosis detection.

RESULTS

Within 5-microns-thick coronal sections, DNA fragmentation was present in 0 to 3 apoptotic cells in each hemisphere of normal, sham-operated rats as well as in the contralateral hemisphere of ischemic rats. After 10 to 20 minutes of MCA occlusion, apoptotic cells exhibiting DNA fragmentation (10 to 20) increased in the regions of selective neuronal necrosis in the preoptic area and in the striatum. After 30 to 60 minutes of ischemia, scattered apoptotic cells (30 to 60) exhibited DNA fragmentation and expanded into areas of selective neuronal necrosis in the cortex. After 90 to 120 minutes of occlusion, groups of apoptotic cells (70 to 200, > 95% neurons) were primarily localized to the inner boundary zone of the infarct.

CONCLUSIONS

A range of mild to severe ischemia-reperfusion stimuli induce internucleosomal DNA cleavage. The presence and anatomic location of apoptotic cells exhibiting DNA fragmentation after transient cerebral occlusion indicate that apoptosis accompanies neuronal necrosis.