Mechanisms of cell death in neurodegenerative diseases: fashion, fiction, and facts

Induction of C/EBP beta and GADD153 expression by dopamine in human neuroblastoma cells. Relationship with alpha-synuclein increase and cell damage

Expression of CCAAT/enhancer-binding protein beta (C/EBP beta) and growth-arrest DNA damage-inducible 153/C/EBP beta homology protein (GADD153/CHOP) increased after incubation of human neuroblastoma SH-SY5Y cells with a range of dopamine concentrations. Dopamine (100 microM) caused an increase in C/EBP beta expression between 2 and 12 h of treatment, with no evident intracellular morphological changes. Dopamine (500 microM) led to the appearance of autophagic-like vacuoles and a marked increase in GADD153/CHOP between 6 and 24 h of treatment. The expression of alpha-synuclein, the main protein of Lewy bodies in Parkinson's disease and other neurological disorders, increased with a profile similar to C/EBP beta. In addition, overexpression of C/EBP beta caused a concomitant increase in the expression of alpha-synuclein but not of GADD153. In contrast, the overexpression of GADD153 did not alter the expression of alpha-synuclein. Inhibition of JNK by SP600125 reduced increases in C/EBP beta and alpha-synuclein expression, whereas inhibition of both JNK and p38MAPK (with SB203580) blocked the increase in GADD153 expression. We conclude that dopamine, through a mechanism driven by stress-activated MAPKs, triggers C/EBP beta and GADD153 expression in a dose-dependent way. Given that the promoter region of the alpha-synuclein gene contains distinct zones that are susceptible to regulation by C/EBP beta, this factor could be involved in the increased expression of alpha-synuclein after dopamine-induced cell stress. GADD153 increase seems to be related with the endoplasmic reticulum stress, autophagy and cell death observed at high dopamine concentrations.

Role of apoptosis in remodeling after myocardial infarction

The magnitude of an acute myocardial infarction (MI; i.e., number of dead cardiomyocytes) is the most critical determinant of subsequent left ventricular remodeling and heart failure. Also affecting the post-infarction disease process, however, are events occurring during the subacute and chronic stages of the infarction, including late cardiomyocyte death, cardiomyocyte hypertrophy, fibrosis, and expression of various cytokines. Additionally, it has been suggested that apoptosis may be responsible for a significant amount of cardiomyocyte death during the acute ischemic stage, as well as for a progressive loss of surviving cells during the subacute and chronic stages. However, there is very little direct morphological evidence of apoptosis occurring at any stage of MI, despite the availability of much indirect evidence that includes detection of DNA fragmentation and apoptosis-related factors. For that reason, the potential efficacy of therapeutic intervention to prevent apoptosis remains controversial. This review will survey available data from both animals and humans to critically assess the role of cardiomyocyte apoptosis during MI and its relevance to myocardial remodeling and heart failure. Also considered will be nonmyocyte interstitial cells, which have received less attention than myocytes despite definitive evidence of their apoptosis in the infarcted heart and recent studies suggesting that blockade of apoptosis among these cells mitigates post-infarction cardiac remodeling and heart failure. We conclude from our survey that there are many hurdles to surmount before regulation of apoptosis can be clinically applied in the treatment of MI and other heart diseases.

Autophagy is a part of ultrastructural synaptic pathology in Creutzfeldt-Jakob disease: a brain biopsy study

Ultrastructural correlates of synaptic and dendritic spines loss have never been studied in detail in human transmissible spongiform encephalopathies (TSEs)-Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker (GSS) disease and fatal familial insomnia (FFI). In this paper, we describe synaptic alterations as found in brain biopsies from Creutzfeldt-Jakob disease and fatal familial insomnia patients. Our material consisted of brain biopsies obtained by open surgery from one FFI case, one case of variant Creutzfeldt-Jakob disease (vCJD), seven cases of sporadic Creutzfeldt-Jakob disease (sCJD) and one case of iatrogenic (human growth hormone) Creutzfeldt-Jakob disease (iCJD). For electron microscopy, approximately 2mm(3) samples were immersion fixed in 2.5% glutaraldehyde for less than 24h, embedded in Epon and routinely processed. Grids were examined and photographed in a transmission electron microscope. The synaptic alterations were found constantly; in practically every brain biopsy they were frequent. The accumulation of different subcellular organelles (neuroaxonal dystrophy), dark synapses and branching cisterns were the most frequent findings while concentric arrays of membranes were only rarely found. Autophagic vacuoles are formed in many synapses in all categories of human transmissible encephalopathies. We conclude that synaptic autophagy contributes to overall synaptic loss in brains affected in prion diseases.

Neuronal cell death in transmissible spongiform encephalopathies (prion diseases) revisited: from apoptosis to autophagy

Neuronal autophagy, like apoptosis, is one of the mechanisms of the programmed cell death (PCD). In this review, we summarize the presence of autophagic vacuoles in experimentally induced scrapie, Creutzfeldt-Jakob disease and Gerstmann-Sträussler-Scheinker (GSS) syndrome. Initially, a part of the neuronal cytoplasm was sequestrated by concentric arrays of double membranes; the enclosed cytoplasm appeared relatively normal except that its density was often increased. Next, electron density of the central area dramatically increased. The membranes then proliferated within the cytoplasm in a labyrinth-like manner and the area sequestrated by these membranes enlarged into a more complex structure consisting of vacuoles, electron-dense areas and areas of normally-looking cytoplasm connected by convoluted membranes. Of note, autophagic vacuoles form not only in neuronal perikarya but also in neurites and synapses. Finally, a large area of the cytoplasm was transformed into a collection of autophagic vacuoles of different sizes. On a basis of ultrastructural studies, we suggest that autophagy plays a major role in transmissible spongiform encephalopathies (TSEs) and may even participate in a formation of spongiform change.

Bax deletion prevents neuronal loss but not neurological symptoms in a transgenic model of inherited prion disease

Transgenic Tg(PG14) mice express a mutant prion protein containing 14 octapeptide repeats, whose human homologue is associated with an inherited prion dementia. These mice develop a progressive neurological disorder characterized by ataxia and cerebellar atrophy, with massive apoptotic degeneration of granule neurons. Bax, a proapoptotic gene of the Bcl-2 family, plays a key role in regulating cell death in the nervous system. To analyze the role of Bax in the Tg(PG14) phenotype, we crossed Tg(PG14) mice with Bax(-/-) mice to obtain Tg(PG14)/Bax(-/-) offspring. Bax deletion effectively rescued cerebellar granule neurons from apoptosis, implying that these cells die via a Bax-dependent process. Surprisingly, however, the age at which symptoms began and the duration of the clinical phase of the illness were not altered in Tg(PG14)/Bax(-/-) mice. In addition, Bax deletion failed to prevent shrinkage of the molecular layer of the cerebellum and loss of synaptophysin-positive synaptic endings. Our analysis indicates that synaptic loss makes a critical contribution to the Tg(PG14) phenotype. These results provide insights into the pathogenesis of prion diseases and have important implications for the treatment of these disorders.

The life and death of oligodendrocytes in vanishing white matter disease

Vanishing white matter disease (VWM) is a progressive cavitating disease of central white matter due to a deficiency of the translation initiation factor eIF2B. Oligodendrocytes appear to be numerically increased in some white matter areas, while decreased in others. We compared oligodendrocytes of cerebral, cerebellar, and pontine white matter from 5 VWM patients with those of age-matched controls by light microscopy and immunohistochemistry using antibodies to activated caspase-3, bak, bax, bcl-2, survivin, and Ki-67, as well as by the TUNEL technique. Oligodendrocytes were identified morphologically and quantified using an ocular grid. We observed statistically significant increases in their densities at all sites; Ki-67-labeled oligodendrocytes were identified in 2 of 5 patients. Apoptotic oligodendrocytes were documented in 3 of 5 patients, while bcl-2 and survivin labeling was observed in 2 of 5 and 2 of 2 patients, respectively. There was a trend toward an increase in apoptotic labeling of oligodendrocytes that was strongest in the cerebrum, the major locus of VWM, in the youngest and most severely affected patients. These data conclusively demonstrate increased oligodendrocytic densities in VWM; the increase is not an artifact of white matter contraction. Our data also document that oligodendrocytes undergo apoptosis, perhaps in conjunction with major neurologic crises, and that a subset of oligodendrocytes are able to persist and proliferate. Conflicting proliferative, cell-death, and survival signals impact the oligodendrocytes of VWM.

The facial nerve axotomy model

Experimental models such as the facial nerve axotomy paradigm in rodents allow the systematic and detailed study of the response of neurones and their microenvironment to various types of challenges. Well-studied experimental examples include peripheral nerve trauma, the retrograde axonal transport of neurotoxins and locally enhanced inflammation following the induction of experimental autoimmune encephalomyelitis in combination with axotomy. These studies have led to novel insights into the regeneration programme of the motoneurone, the role of microglia and astrocytes in synaptic plasticity and the biology of glial cells. Importantly, many of the findings obtained have proven to be valid in other functional systems and even across species barriers. In particular, microglial expression of major histocompatibility complex molecules has been found to occur in response to various types of neuronal damage and is now regarded as a characteristic component of "glial inflammation". It is found in the context of numerous neurodegenerative disorders including Parkinson's and Alzheimer's disease. The detachment of afferent axonal endings from the surface membrane of regenerating motoneurones and their subsequent displacement by microglia ("synaptic stripping") and long-lasting insulation by astrocytes have also been confirmed in humans. The medical implications of these findings are significant. Also, the facial nerve system of rats and mice has become the best studied and most widely used test system for the evaluation of neurotrophic factors.

Effects of 6-hydroxydopamine on primary cultures of substantia nigra: specific damage to dopamine neurons and the impact of glial cell line-derived neurotrophic factor

6-Hydroxydopamine (6-OHDA)-induced loss of dopamine (DA) neurons has served to produce an animal model of DA neuron loss in Parkinson's disease. We report here the use of 6-OHDA to produce an in vitro model of this phenomena using dissociated cultures prepared from neonatal rat mesencephalon. Cultures were exposed to 6-OHDA (40-100 microm, 15 min) in an antioxidant medium, and DA and GABA neurons evaluated by immunocytochemistry. 6-OHDA induced morphological and biochemical signs of cell death in DA neurons within 3 h, followed by loss of tyrosine hydroxylase immunoreactive neurons within 2 days. In substantia nigra (SN) cultures, DA neurons were much more affected by 6-OHDA than were GABA neurons. In contrast, DA neurons from the ventral tegmental area were only lost at higher, non-specific concentrations of 6-OHDA. The effects of 6-OHDA on nigral DA neurons were blocked by inhibitors of high affinity DA transport and by z-DEVD-fmk (150 microm), a caspase inhibitor. Glial cell line-derived neurotrophic factor (GDNF) treatment reduced TUNEL labeling 3 h after 6-OHDA exposure, but did not prevent loss of DA neurons at 48 h. Thus, 6-OHDA can selectively destroy DA neurons in post-natal cultures of SN, acting at least in part by initiating caspase-dependent apoptosis, and this effect can be attenuated early but not late by GDNF.

Selectivity and types of cell death in the neuronal ceroid lipofuscinoses

Cloning of the individual genes that are mutated in the neuronal ceroid lipofuscinoses (NCLs), or Batten disease, has opened up new avenues of research into the pathogenesis of these fatal autosomal recessive storage disorders. Genetically accurate mouse models have now been generated for each major form of the disorder, together with several variant forms. Ongoing analysis of these mice is revealing significant new data about the staging and progression of disease phenotypes. Combined with data from human autopsy tissues and large animal models, it is now clear that neurodegeneration is initially selective in the NCL CNS, targeting specific regions and particular cell populations. There is also evidence of selective glial activation that appears to precede obvious neurodegeneration, becoming more widespread with disease progression. Currently, there is debate over the mechanisms of cell death that operate in each form of NCL, with evidence of both apoptosis and autophagy. It is likely that these mechanisms may encompass a spectrum of cell death events, depending upon the specific context of each neuronal population. Taken together, these data have significant clinical implications for the development and targeting of appropriate therapeutic strategies, and for providing the landmarks to judge their efficacy.

The focality of the global Alzheimer brain process: is the selective vulnerability of neurons a specific phenomenon of primary neuronal pathobiology?

A focality in the development of a global series of predisposing factors that conditions a progressiveness in the neurodegenerative process of Alzheimer type would appear to arise as a specific lesion of the neuron. Such a neuronal lesion would perhaps disrupt functional connectivity of neuronal networks in a process involving loss of neuronal viability. Indeed, a strict concept of selective vulnerability of neurons in the Alzheimer brain might be simply a preconditioning by microenvironmental factors that interacts with the individual neuron in terms of cellular component depletion or in terms of plasmalemmal disruption. In a final analysis, perhaps, the individual neuron would appear as the essential focus of a process that would account for a conditioning globality of the Alzheimer process that promotes both progressiveness and irreversibility of the brain pathology.

Bcl-2 family regulation of neuronal development and neurodegeneration

Neuronal cell death is a key feature of both normal nervous system development and neuropathological conditions. The Bcl-2 family, via its regulation of both caspase-dependent and caspase-independent cell death pathways, is uniquely positioned to critically control neuronal cell survival. Targeted gene disruptions of specific bcl-2 family members and the generation of transgenic mice overexpressing anti- or pro-apoptotic Bcl-2 family members have confirmed the importance of the Bcl-2 family in the nervous system. Data from studies of human brain tissue and experimental animal models of neuropathological conditions support the hypothesis that the Bcl-2 family regulates cell death in the mature nervous system and suggest that pharmacological manipulation of Bcl-2 family action could prove beneficial in the treatment of human neurological conditions such as stroke and neurodegenerative diseases.

Histopathological alterations and functional brain deficits after transient hypoxia in the newborn rat pup: a long term follow-up

To assess temporal brain deficits consecutive to severe birth hypoxia, newborn rats were exposed for 20 min to 100% N2. This treatment induced a long-term growth retardation and a delayed, but only transient, neuronal loss (approximately 25%) in the CA1 hippocampus and parietal cortex, starting from 3 days and peaking at 6 days post-hypoxia. The expression profiles of various apoptosis-regulating proteins (including Bcl-2, Bax, p53 and caspase-3) were well correlated to the alterations of nuclear morphology depicted by 4,6-diamidino-2-phenylindole (DAPI). Whereas they confirmed a gradual histological recovery, specific DNA fragmentation patterns suggested that birth hypoxia may transiently reactivate the developmental programme of neuronal elimination. Although they successfully achieved various behavioral tests such as the righting reflex, negative geotaxis, locomotor coordination, and the eight-arm maze tasks, both developing and adult hypoxic rats were repeatedly slower than controls, suggesting that birth hypoxia is associated to moderate but persistent impairments of functional capacities.

Hypoxia-induced caspase-3 activation and DNA fragmentation in cortical neurons of newborn piglets: role of nitric oxide

Hypoxia results in generation of nitric oxide (NO) free radicals, activation of caspase-3, and genomic DNA fragmentation. The present study tests the hypothesis that hypoxia-induced caspase-3 activation and DNA fragmentation are nitric oxide mediated. Studies were conducted in newborn piglets, divided into normoxic (n = 5), hypoxic (n = 5), and hypoxic-7-NINA (n = 6). Hypoxic-7-NINA group received the neuronal nitric oxide synthase inhibitor, 7-Nitroindazole (7-NINA). Caspase-3 activity was determined spectrofluorometrically using enzyme-specific substrates. Sections from the neocortex were stained with an antiserum recognizing active caspase-3. Purified DNA was separated by gel electrophoresis. Administration of 7-NINA resulted in decreased immunoreactivity of caspase-3 (mean LI: 20.2%) as compared to the untreated hypoxia group (mean LI: 57.5%) (P < 0.05). 7-NINA attenuated caspase-3 enzymatic activity as well in comparison to the untreated hypoxia group (P < 0.05). Furthermore, multiple low molecular weight bands corresponding to DNA fragments were present in the hypoxic but not in the normoxic or hypoxic-7-NINA groups. Inhibition of nNOS abates the hypoxia-induced increase in active caspase-3 immunoreactivity, as well as enzymatic activity in cortical neurons, and DNA fragmentation in brain homogenates. We conclude that the coordinate increase of capase-3 activity and fragmentation of nuclear DNA in the hypoxic newborn piglet brain are NO mediated.

Dopamine induces autophagic cell death and alpha-synuclein increase in human neuroblastoma SH-SY5Y cells

Free cytoplasmic dopamine may be involved in the genesis of neuronal degeneration in Parkinson's disease and other such diseases. We used SH-SY5Y human neuroblastoma cells to study the effect of dopamine on cell death, activation of stress-induced pathways, and expression of alpha-synuclein, the characteristic protein accumulated in Lewy bodies. We show that 100 and 500 microM dopamine causes a 40% and 60% decrease of viability, respectively, and triggers autophagy after 24 hr of exposure, characterized by the presence of numerous cytoplasmic vacuoles with inclusions. Dopamine causes mitochondrial aggregation in adherent cells prior to the loss of functionality. Plasma membrane and nucleus also maintain their integrity. Cell viability is protected by the dopamine transporter blocker nomifensine and the antioxidants N-acetylcysteine and ascorbic acid. Dopamine activates the stress-response kinases, SAPK/JNK and p38, but not ERK/MAPK or MEK, and increases alpha-synuclein expression. Both cell viability and the increase in alpha-synuclein expression are prevented by antioxidants; by the specific inhibitors of p38 and SAPK/JNK, SB203580 and SP600125, respectively; and by the inhibitor of autophagy 3-methyladenine. This indicates that oxidative stress, stress-activated kinases, and factors involved in autophagy up-regulate alpha-synuclein content. The results show that nonapoptotic death pathways are triggered by dopamine, leading to autophagy. These findings should be taken into account in the search for strategies to protect dopaminergic neurons from degeneration.

Huntington's disease: prospects for neuroprotective therapy 10 years after the discovery of the causative genetic mutation

PURPOSE OF REVIEW

Ten years of intensive research are now beginning to bring candidate neuroprotective therapies to clinical trials. This review describes recent progress in basic, preclinical, and clinical research that underlies current and potential neuroprotective trials.

RECENT FINDINGS

Basic research continues to elucidate the proteolytic processing of huntingtin into toxic fragments and has examined the toxic potential of huntingtin monomers versus oligomers versus insoluble aggregates. Energy depletion has been reinvigorated as a therapeutic target by studies identifying very early mitochondrial alterations. Toxic interactions between mutant huntingtin and a variety of transcription factors have emerged as a major focus with a variety of studies suggesting transcriptional dysfunction to be a central mechanism in Huntington's disease. Progress in preclinical research included therapeutic leads identified by compound library screens, by designing polypeptides that can interact with huntingtin, and by testing compounds in transgenic mice with the potential for affecting some of the mechanisms thought to underlie neurodegeneration. While early results of neurotransplantation are generating increasing controversy, a variety of compounds discovered to benefit transgenic mice are working their way into clinical trials in symptomatic patients. Studies in presymptomatic individuals at risk for developing Huntington's disease are underway to enable the testing of agents with the potential for delaying or preventing onset of symptoms.

SUMMARY

While laboratory research continues to advance and provide therapeutic leads, clinical trials are needed to test existing leads and guide further progress. With any luck, some of these tests will begin to identify treatments that make a difference for families with the disease.

Apoptotic mitochondrial pathway in neurones and astrocytes after neonatal hypoxia-ischaemia in the rat brain

Neuronal apoptosis plays an essential role in early brain development and contributes to secondary neuronal loss after acute ischaemia. Recent studies have provided evidence that caspase-3 is an important downstream event after hypoxia-ischaemia in the immature brain, but a minor event in the adult brain. Our investigations have focused on cell populations that expressed apoptotic effectors in the enzymatic death pathway including cytochrome c, caspase-9 and caspase-3. Expression, activation and cellular localization of these proteins were studied using cleavage of fluorogenic substrate and immunohistochemistry in neonatal rat brain after unilateral focal ischaemia. Caspase-3 enzyme activity was elevated in brain homogenate between 6 and 48 h after reperfusion. This activation was preceded by that of caspase-9, between 3 and 24 h. Apoptotic cell death was finally accomplished by poly-ADP-ribose polymerase cleavage, an endogenous caspase-3 substrate. In addition, immunodetection demonstrated that cytochrome c and activated caspase-9 and caspase-3 were expressed not only in the neurones, the primarily affected cells, but also within the astrocytes, which constituted a dense network delineating the infarct. These results suggested that glial injury may promote the formation of cystic lesions such as those observed clinically in the newborn brain.

Dependence on electron transport chain function and intracellular signaling of genomic responses in SH-SY5Y cells to the mitochondrial neurotoxin MPP(+)

SH-SY5Y neuroblastoma cells exposed to the complex I inhibitor/parkinsonian neurotoxin methylpyridinium ion (MPP(+)) activate both survival and death-promoting signaling pathways and undergo MEK/ERK-dependent, phosphatidylinositol-3 kinase-dependent, and c-Jun kinase-dependent cell death. Because genomic responses to MPP(+) are not extensively characterized, we used nylon cDNA arrays to measure gene expression following exposure to an apoptosis-producing [MPP(+)]. Many changes occurred within 5 min, and all gene expression changes appeared before biochemical and morphological markers of apoptosis. The majority of gene expression changes in SY5Y were not found in rho(0) cells, indicating dependence of these changes on intact electron transport activity. rho(0) cells exposed to MPP(+) produced different expression profiles, indicating the potential for responses independent of complex I inhibition. MPP(+)-induced gene expression patterns in normal SY5Y cells were sensitive to inhibitors of MEK/ERK (UO 126) or phosphatidylinositol-3 kinase (LY 294002), demonstrating regulation of gene expression by these survival-promoting signaling pathways. The primary signaling molecules mediating these MPP(+)-induced gene expression changes are unknown but ultimately utilize MEK/ERK and phosphatidylinositol-3 kinase signaling. Genes suppressed by UO 126 or LY 294002 during MPP(+) exposure may mediate cell survival; those expressed in the presence of UO 126 or LY 294002 may mediate cell death in this in vitro model of Parkinson's disease.

Prospects for antiapoptotic drug therapy of neurodegenerative diseases

The evidence for a role of apoptosis in the neurodegenerative diseases, Alzheimer's disease (AD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS), and in the more acute conditions of cerebral ischemia, traumatic brain injury (TBI), and spinal cord injury (SCI) is reviewed with regard to potential intervention by means of small antiapoptotic molecules. In addition, the available animal models for these diseases are discussed with respect to their relevance for testing small antiapoptotic molecules in the context of what is known about the apoptotic pathways involved in the diseases and the models. The principal issues related to pharmacotherapy by apoptosis inhibition, i.e., functionality of rescued neurons and potential interference with physiologically occurring apoptosis, are pointed out. Finally, the properties of a number of small antiapoptotic molecules currently under clinical investigation are summarized. It is concluded that the evidence for a role of apoptosis at present is more convincing for PD and ALS than for AD. In PD, damage to dopaminergic neurons may occur through oxidative stress and/or mitochondrial impairment and culminate in activation of an apoptotic, presumably p53-dependent cascade; some neurons experiencing energy failure may not be able to complete apoptosis, end up in necrosis and give rise to inflammatory processes. These events are reasonably well reflected in some of the PD animal models, notably those involving 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and rotenone. In sporadic ALS, an involvement of pathways involving p53 and Bcl-2 family members appears possible if not likely, but is not established. The issue is important for the development of antiapoptotic compounds for the treatment of this disease because of differential involvement of p53 in different mutant superoxide dismutase (SOD) mice. Most debated is the role of apoptosis in AD; this implies that little is known about potentially involved pathways. Moreover, there is a lack of suitable animal models for compound evaluation. Apoptosis or related phenomena are likely involved in secondary cell death in cerebral ischemia, TBI, and SCI. Most of the pertinent information comes from animal experiments, which have provided some evidence for prevention of cell death by antiapoptotic treatments, but little for functional benefit. Much remains to be done in this area to explore the potential of antiapoptotic drugs. There is a small number of antiapoptotic compounds in clinical development. With some of them, evidence for maintenance of functionality of the rescued neurons has been obtained in some animal models, and the fact that they made it to phase II studies in patients suggests that interference with physiological apoptosis is not an obligatory problem. The prospect that small antiapoptotic molecules will have an impact on the therapy of neurodegenerative diseases, and perhaps also of ischemia and trauma, is therefore judged cautiously positively.

Hypothesis for a common basis for neuroprotection in glaucoma and Alzheimer's disease: anti-apoptosis by alpha-2-adrenergic receptor activation

Recent studies have suggested glaucomatous loss of retinal ganglion cells and their axons in Alzheimer's disease. Amyloid beta peptides and phosphorylated tau protein have been implicated in the selective regional neuronal loss and protein accumulations characteristic of Alzheimer's disease. Similar protein accumulations are not present on glaucomatous retinal ganglion cells. Neurons die in both Alzheimer's disease and glaucoma by apoptosis, although the signaling pathways for neuronal degradation appear to differ in the two diseases. Alzheimer's disease features a loss of locus ceruleus noradrenergic neurons, which send axon terminals to the brain regions suffering neuronal apoptosis and results in reductions in noradrenaline in those regions. Activation of alpha-2 adrenergic receptors reduces neuronal apoptosis, in part through a protein kinase B (Akt)-dependent signaling pathway. Loss of noradrenaline innervation facilitates neuronal apoptosis in Alzheimer's disease models and may act similarly in glaucoma. Alpha-2 adrenergic receptor agonists offer the potential to slow the neuronal loss in both diseases by compensating for lost noradrenaline innervation.