Caspase-like activity is essential for long-term synaptic plasticity in the terrestrial snail Helix

Cleaved caspase-3 is present in the majority of glial cells in the intact rat spinal cord during postnatal life

Cell death is an essential process that occurs during the development of the central nervous system. Despite the availability of a wide range of commercially produced antibodies against various apoptotic markers, data regarding apoptosis in intact spinal cord during postnatal development and adulthood are mostly missing. We investigated apoptosis in rat spinal cord at different stages of ontogenesis (postnatal days 8, 29, and 90). For this purpose, we applied immunofluorescent detection of two widely used apoptotic markers, cleaved caspase-3 (cC3) and cleaved poly(ADP-ribose) polymerase (cPARP). Surprisingly, we found significant discrepancy between the number of cC3+ cells and PARP+ cells, with a ratio between 500:1 and 5000:1 in rat spinal cord at all postnatal time points. The majority of cC3+ cells were glial cells and did not exhibit an apoptotic phenotype. In contrast with in vivo results, in vitro analysis of primary cell cultures derived from neonatal rat spinal cord and treated with the apoptotic inductor staurosporine revealed a similar onset of occurrence of both cC3 and cPARP in cells subjected to apoptosis. Gene expression analysis of spinal cord revealed elevated expression of the Birc4 (XIAP), Birc2, and Birc5 (Survivin) genes, which are known potent inhibitors of apoptosis. Our data indicate that cC3 is not an exclusive marker of apoptosis, especially in glial cells, owing its possible presence in inhibited forms and/or its participation in other non-apoptotic roles. Therefore, cPARP appears to be a more appropriate marker to detect apoptosis.

Pathological Impact of Tau Proteolytical Process on Neuronal and Mitochondrial Function: a Crucial Role in Alzheimer's Disease

Tau protein plays a pivotal role in the central nervous system (CNS), participating in microtubule stability, axonal transport, and synaptic communication. Research interest has focused on studying the role of post-translational tau modifications in mitochondrial failure, oxidative damage, and synaptic impairment in Alzheimer's disease (AD). Soluble tau forms produced by its pathological cleaved induced by caspases could lead to neuronal injury contributing to oxidative damage and cognitive decline in AD. For example, the presence of tau cleaved by caspase-3 has been suggested as a relevant factor in AD and is considered a previous event before neurofibrillary tangles (NFTs) formation.Interestingly, we and others have shown that caspase-cleaved tau in N- or C- terminal sites induce mitochondrial bioenergetics defects, axonal transport impairment, neuronal injury, and cognitive decline in neuronal cells and murine models. All these abnormalities are considered relevant in the early neurodegenerative manifestations such as memory and cognitive failure reported in AD. Therefore, in this review, we will discuss for the first time the importance of truncated tau by caspases activation in the pathogenesis of AD and how its negative actions could impact neuronal function.

Retina and Brain Display Early and Differential Molecular and Cellular Changes in the 3xTg-AD Mouse Model of Alzheimer's Disease

The concept 'the retina as a window to the brain' has been increasingly explored in Alzheimer´s disease (AD) in recent years, since some patients present visual alterations before the first symptoms of dementia. The retina is an extension of the brain and can be assessed by noninvasive methods. However, assessing the retina for AD diagnosis is still a matter of debate. Using the triple transgenic mouse model of AD (3xTg-AD; males), this study was undertaken to investigate whether the retina and brain (hippocampus and cortex) undergo similar molecular and cellular changes during the early stages (4 and 8 months) of the pathology, and if the retina can anticipate the alterations occurring in the brain. We assessed amyloid-beta (Aβ) and hyperphosphorylated tau (p-tau) levels, barrier integrity, cell death, neurotransmitter levels, and glial changes. Overall, the retina, hippocampus, and cortex of 3xTg-AD are not significantly affected at these early stages. However, we detected a few differential changes in the retina and brain regions, and particularly a different profile in microglia branching in the retina and hippocampus, only at 4 months, where the number and length of the processes decreased in the retina and increased in the hippocampus. In summary, at the early stages of pathology, the retina, hippocampus, and cortex are not significantly affected but already present some molecular and cellular alterations. The retina did not mirror the changes detected in the brain, and these observations should be taking into account when using the retina as a potential diagnostic tool for AD.

Biochemical and apoptotic changes in the nervous and ovotestis tissues of Biomphalaria alexandrina following infection with Schistosoma mansoni

Infection with trematodes produces physiological and behavioural changes in intermediate snail hosts. One response to infection is parasitic castration, in which energy required for reproduction of the host is thought to be redirected to promote development and multiplication of the parasite. This study investigated some reproductive and biochemical parameters in the nervous (CNS) and ovotestis (OT) tissues of Biomphalaria alexandrina during the course of Schistosoma mansoni infection. Antioxidant and oxidative stress parameters including catalase (CAT), nitric oxide (NO) and lipid peroxidation (MDA) were measured. Levels of steroid hormones, including testosterone, progesterone and estradiol, were also assessed. Finally, flow cytometry was used to compare measures of apoptosis between control snails and those shedding cercariae by examining mitochondrial membrane potential with the stain 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimi-dazolylcarbocyanine iodide (JC-1) and poly(ADP-ribose) polymerase (PARP). Infection with S. mansoni caused a 47.7% reduction in the net reproductive rate (R_o) of B. alexandrina. CAT activity was increased in the CNS at 21 days post infection (dpi) but by 28 dpi it was reduced below control values. Also, CAT activity increased significantly in the OT at 14, 21 and 28 dpi. In CNS tissues, NO levels were reduced at 7 dpi, increased at 14 and 21 dpi, and reduced again at 28 dpi. The overall level of lipid peroxidation gradually increased during the course of infection to reach its highest levels at 28 dpi. Steroid hormone measurements showed that concentrations of testosterone and estradiol were reduced in the CNS tissues at 28 dpi, while those of progesterone were slightly increased in the CNS and OT tissues. The percentage of cells that positively stained with JC-1was significantly increased in CNS and OT tissues of infected snails while the percentage of cells positively stained with PARP was decreased compared to controls. Together, these findings indicate that infection initiates diverse biochemical and hormonal changes leading to loss of cells responsible for egg laying and reproduction in B. alexandrina.

Loss of Non-Apoptotic Role of Caspase-3 in the PINK1 Mouse Model of Parkinson's Disease

Caspases are a family of conserved cysteine proteases that play key roles in multiple cellular processes, including programmed cell death and inflammation. Recent evidence shows that caspases are also involved in crucial non-apoptotic functions, such as dendrite development, axon pruning, and synaptic plasticity mechanisms underlying learning and memory processes. The activated form of caspase-3, which is known to trigger widespread damage and degeneration, can also modulate synaptic function in the adult brain. Thus, in the present study, we tested the hypothesis that caspase-3 modulates synaptic plasticity at corticostriatal synapses in the phosphatase and tensin homolog (PTEN) induced kinase 1 (PINK1) mouse model of Parkinson’s disease (PD). Loss of PINK1 has been previously associated with an impairment of corticostriatal long-term depression (LTD), rescued by amphetamine-induced dopamine release. Here, we show that caspase-3 activity, measured after LTD induction, is significantly decreased in the PINK1 knockout model compared with wild-type mice. Accordingly, pretreatment of striatal slices with the caspase-3 activator α-(Trichloromethyl)-4-pyridineethanol (PETCM) rescues a physiological LTD in PINK1 knockout mice. Furthermore, the inhibition of caspase-3 prevents the amphetamine-induced rescue of LTD in the same model. Our data support a hormesis-based double role of caspase-3; when massively activated, it induces apoptosis, while at lower level of activation, it modulates physiological phenomena, like the expression of corticostriatal LTD. Exploring the non-apoptotic activation of caspase-3 may contribute to clarify the mechanisms involved in synaptic failure in PD, as well as in view of new potential pharmacological targets.

Astrocytes and radial glia-like cells, but not neurons, display a nonapoptotic increase in caspase-3 expression following exercise

BACKGROUND

Exercise induces plasticity in the hippocampus, which includes increases in neurogenesis, the proliferation of new neurons, and angiogenesis, the sprouting of new capillaries from preexisting blood vessels. Following exercise, astrocytes also undergo morphological changes that parallel the events occurring in the neurovascular system. Interestingly, there have also been reports of apoptosis in the hippocampus following aerobic exercise. This experiment aimed to identify which population of hippocampal cells undergoes apoptosis after an acute bout of exercise.

METHODS

Cleaved caspase-3, a terminal protein in the apoptotic cascade, was initially used to identify apoptotic cells in the hippocampus after rats completed an acute bout of exercise. Next, the proportion of immature neurons, adult neurons, astrocytes, or radial glia-like cells expressing cleaved caspase-3 was quantified. TUNEL staining was completed as a second measure of apoptosis.

RESULTS

Following exercise, cleaved caspase-3 expression was increased in the CA1 and DG regions of the hippocampus. Cleaved caspase-3 was not highly expressed in neuronal populations, and expression was not increased in these cells postexercise. Instead, cleaved caspase-3 was predominantly expressed in astrocytes. Following exercise, there was an increased number of cleaved caspase-3 positive astrocytes in DG and CA1, and cleaved caspase-3 positive radial glia-like cells located in the subgranular zone. To determine whether cleaved caspase-3 expression in these glial cells was associated with apoptosis, a TUNEL assay was completed. TUNEL staining was negligible in all groups and did not mirror the pattern of caspase-3 labeling.

CONCLUSIONS

Cleaved caspase-3 expression was detected largely in non-neuronal cell populations, and the pattern of cleaved caspase-3 expression did not match that of TUNEL. This suggests that after exercise, cleaved caspase-3 expression may serve a nonapoptotic role in these hippocampal astrocytes and radial glia-like cells. It will be important to identify the function of exercise-induced cleaved caspase-3 expression in the future experiments.

Physiological functions of non-apoptotic caspase activity in the nervous system

Caspases are cysteine proteases that play important and well-defined roles in apoptosis and inflammation. Increasing evidence point to alternative functions of caspases where restricted and localized caspase activation within neurons allows for a variety of non-apoptotic and non-inflammatory processes required for brain development and function. In this review, we highlight sublethal caspase functions in axon and dendrite pruning, neurite outgrowth and dendrite branches formation, as well as in long-term depression and synaptic plasticity. Importantly, as non-apoptotic activity of caspases is often confined in space and time in neurons, we also discuss the mechanisms that restrict caspase activity in order to maintain the neuronal networks in a healthy and functional state.

New Views on the Misconstrued: Executioner Caspases and Their Diverse Non-apoptotic Roles

Initially characterized for their roles in apoptosis, executioner caspases have emerged as important regulators of an array of cellular activities. This is especially true in the nervous system, where sublethal caspase activity has been implicated in axonal pathfinding and branching, axonal degeneration, dendrite pruning, regeneration, long-term depression, and metaplasticity. Here we examine the roles of sublethal executioner caspase activity in nervous system development and maintenance, consider the mechanisms that locally activate and restrain these potential killers, and discuss how their activity be subverted in neurodegenerative disease.

The complexity of apoptotic cell death in mollusks: An update

Apoptosis is a type of programmed cell death that produces changes in cell morphology and in biochemical intracellular processes without inflammatory reactions. The components of the apoptotic pathways are conserved throughout evolution. Caspases are key molecules involved in the transduction of the death signal and are responsible for many of the biochemical and morphological changes associated with apoptosis. Nowadays, It is known that caspases are activated through two major apoptotic pathways (the extrinsic or death receptor pathway and the intrinsic or mitochondrial pathway), but there are also evidences of at least other alternative pathway (the perforin/granzyme pathway). Apoptosis in mollusks seems to be similar in complexity to apoptosis in vertebrates but also has unique features maybe related to their recurrent exposure to environmental changes, pollutants, pathogens and also related to the sedentary nature of some stages in the life cycle of mollusks bivalves and gastropods. As in other animals, apoptotic process is involved in the maintenance of tissue homeostasis and also constitutes an important immune response that can be triggered by a variety of stimuli, including cytokines, hormones, toxic insults, viruses, and protozoan parasites. The main goal of this work is to present the current knowledge of the molecular mechanisms of apoptosis in mollusks and to highlight those steps that need further study.

Caspase-3 modulates regenerative response after stroke

Stroke is a leading cause of long-lasting disability in humans. However, currently there are still no effective therapies available for promoting stroke recovery. Recent studies have shown that the adult brain has the capacity to regenerate neurons after stroke. Although this neurogenic response may be functionally important for brain repair after injury, the mechanisms underlying stroke-induced neurogenesis are not known. Caspase-3 is a major executioner and has been identified as a key mediator of neuronal death in the acute stage of stroke. Recently, however, accumulating data indicate that caspase-3 also participates in various biological processes that do not cause cell death. Here, we show that cleaved caspase-3 was increased in newborn neuronal precursor cells (NPCs) in the subventricular zone (SVZ) and the dentate gyrus during the period of stroke recovery, with no evidence of apoptosis. We observed that cleaved caspase-3 was expressed by NPCs and limited its self-renewal without triggering apoptosis in cultured NPCs from the SVZ of ischemic mice. Moreover, we revealed that caspase-3 negatively regulated the proliferation of NPCs through reducing the phosphorylation of Akt. Importantly, we demonstrated that peptide inhibition of caspase-3 activity significantly promoted the proliferation and migration of SVZ NPCs and resulted in a significant increase in subsequent neuronal regeneration and functional recovery after stroke. Together, our data identify a previously unknown caspase-3-dependent mechanism that constrains stroke-induced endogenous neurogenesis and should revitalize interest in targeting caspase-3 for treatment of stroke.

Altered processing of amyloid precursor protein in cells undergoing apoptosis

Altered proteolysis of amyloid precursor protein is an important determinant of pathology development in Alzheimer's disease. Here, we describe the detection of two novel fragments of amyloid precursor protein in H4 neuroglioma cells undergoing apoptosis. Immunoreactivity of these 25-35 kDa fragments to two different amyloid precursor protein antibodies suggests that they contain the amyloid-β region and an epitope near the C-terminus of amyloid precursor protein. Generation of these fragments is associated with cleavage of caspase-3 and caspase-7, suggesting activation of these caspases. Studies in neurons undergoing DNA damage-induced apoptosis also showed similar results. Inclusion of caspase inhibitors prevented the generation of these novel fragments, suggesting that they are generated by a caspase-dependent mechanism. Molecular weight prediction and immunoreactivity of the fragments generated suggested that such fragments could not be generated by cleavage at any previously identified caspase, secretase, or calpain site on amyloid precursor protein. Bioinformatic analysis of the amino acid sequence of amyloid precursor protein revealed that fragments fitting the observed size and immunoreactivity could be generated by either cleavage at a novel, hitherto unidentified, caspase site or at a previously identified matrix metalloproteinase site in the extracellular domain. Proteolytic cleavage at any of these sites leads to a decrease in the generation of α-secretase cleaved secreted APP, which has both anti-apoptotic and neuroprotective properties, and thus may contribute to neurodegeneration in Alzheimer's disease.

Human coronavirus-induced neuronal programmed cell death is cyclophilin d dependent and potentially caspase dispensable

Human coronaviruses (HCoV) are recognized respiratory pathogens. Some HCoV strains, including HCoV-OC43, can invade the central nervous system, where they infect neurons, with unclear consequences. We have previously reported that HCoV-OC43 infection of human neurons activates the unfolded-protein response and caspase-3 and induces cell death and that the viral spike (S) glycoprotein is involved in the process. We now report on underlying mechanisms associated with the induction of programmed cell death (PCD) after infection by the reference HCoV-OC43 virus (rOC/ATCC) and a more neurovirulent and cytotoxic HCoV-OC43 variant harboring two point mutations in the S glycoprotein (rOC/U(S183-241)). Even though caspase-3 and caspase-9 were both activated after infection, the use of caspase inhibitors neither reduced nor delayed virus-induced PCD, suggesting that these proteases are not essential in the process. On the other hand, the proapoptotic proteins BAX, cytochrome c (CytC), and apoptosis-inducing factor (AIF) were relocalized toward the mitochondria, cytosol, and nucleus, respectively, after infection by both virus variants. Moreover, LA-N-5 neuronal cells treated with cyclosporine (CsA), an inhibitor of the mitochondrial permeabilization transition pore (mPTP), or knocked down for cyclophilin D (CypD) were completely protected from rOC/ATCC-induced neuronal PCD, underlining the involvement of CypD in the process. On the other hand, CsA and CypD knockdown had moderate effects on rOC/U(S183-241)-induced PCD. In conclusion, our results are consistent with mitochondrial AIF and cyclophilin D being central in HCoV-OC43-induced PCD, while caspases appear not to be essential.

Human coronavirus-induced neuronal programmed cell death is cyclophilin d dependent and potentially caspase dispensable

Human coronaviruses (HCoV) are recognized respiratory pathogens. Some HCoV strains, including HCoV-OC43, can invade the central nervous system, where they infect neurons, with unclear consequences. We have previously reported that HCoV-OC43 infection of human neurons activates the unfolded-protein response and caspase-3 and induces cell death and that the viral spike (S) glycoprotein is involved in the process. We now report on underlying mechanisms associated with the induction of programmed cell death (PCD) after infection by the reference HCoV-OC43 virus (rOC/ATCC) and a more neurovirulent and cytotoxic HCoV-OC43 variant harboring two point mutations in the S glycoprotein (rOC/U(S183-241)). Even though caspase-3 and caspase-9 were both activated after infection, the use of caspase inhibitors neither reduced nor delayed virus-induced PCD, suggesting that these proteases are not essential in the process. On the other hand, the proapoptotic proteins BAX, cytochrome c (CytC), and apoptosis-inducing factor (AIF) were relocalized toward the mitochondria, cytosol, and nucleus, respectively, after infection by both virus variants. Moreover, LA-N-5 neuronal cells treated with cyclosporine (CsA), an inhibitor of the mitochondrial permeabilization transition pore (mPTP), or knocked down for cyclophilin D (CypD) were completely protected from rOC/ATCC-induced neuronal PCD, underlining the involvement of CypD in the process. On the other hand, CsA and CypD knockdown had moderate effects on rOC/U(S183-241)-induced PCD. In conclusion, our results are consistent with mitochondrial AIF and cyclophilin D being central in HCoV-OC43-induced PCD, while caspases appear not to be essential.

A perspective on neuronal cell death signaling and neurodegeneration

Although neuronal cell death through apoptotic pathways represents a common feature of dysferopathies, the canonical apoptotic changes familiar from nonneuronal cells are late events. Loss of neuronal function occurs at a much early time, when synaptic-based neuronal connectivity fails. In this context, apoptotic pathways may normally serve a cleanup role, rather than a pathogenic one. Reframing the consideration of cell death in the nervous system to include the early stages of axonal degeneration provides a better understanding of the roles played by various apoptotic signaling pathways in neurodegenerative diseases. Focusing on disease-specific mechanisms that initiate the sequence that eventually leads to neuronal loss should facilitate development of therapies that preserve neuronal function and neuronal numbers.

PACAP has anti-apoptotic effect in the salivary gland of an invertebrate species, Helix pomatia

Pituitary adenylate cyclase activating polypeptide (PACAP) shows a remarkable sequence similarity among species and several studies provide evidence that the functions of PACAP have also been conserved among vertebrate species. Relatively little is known about its presence and functions in invertebrates. The aim of the present study was to investigate whether the well-known anti-apoptotic effect of PACAP can also be demonstrated in invertebrates. This effect was studied in the salivary gland of a molluscan species, Helix pomatia. In this work, we first showed the presence of PACAP-like immunoreactivity in the Helix salivary gland by means of immunohistochemistry. Radioimmunoassay measurements showed that PACAP38-like immunoreactivity dominated in the salivary gland of both active and inactive snails and its concentration was higher in active than in inactive animals in contrast to PACAP27-like immunoreactivity, which did not show activity-dependent changes. PACAP induced a significant elevation of cAMP level in salivary gland extracts. Application of apoptosis-inducing agents, dopamine and colchicine, led to a marked increase in the number of terminal uridine deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive apoptotic cells in the salivary gland, which was significantly attenuated by PACAP treatment. In a similar manner, the number of caspase-positive cells was reduced after co-application of dopamine and PACAP. Taken together, the data indicate that PACAP activates cAMP in a molluscan species and we show, for the first time, that PACAP is anti-apoptotic in the invertebrate Helix pomatia.

Cell death in rat cerebellar granule neurons induced by hydrogen peroxide in vitro: mechanisms and protection by adenosine receptor ligands

Oxidative stress, resulting from excessive production of reactive oxygen species (ROS), is a pathological state that causes profound cellular damage and eventual death resulting from the overactivation of glutamate receptors, and the generation of nitric oxide, superoxide and hydrogen peroxide (H(2)O(2)). As such, H(2)O(2) represents an important model for studying the neuropathology of oxidative stress in a variety of CNS disorders. The effects of H(2)O(2) on the viability of post-natal cerebellar granule neurons (CGNs), the nature of the cell death involved and the potential protection by adenosine receptors against the damage were examined in the current study. Hydrogen peroxide (10-400 microM) reduced CGN viability in a concentration- and time-dependent manner. The addition of catalase (100 U/ml) prevented this effect, and the non-specific COX inhibitor aspirin (1 mM) also alleviated the damage. A combination of H(2)O(2) (5 microM) and Cu(2+) (0.5 mM) resulted in a significant damage that was not prevented by the hydroxyl radical scavenger mannitol (50 mM). The permeability transition pore blocker cyclosporin A, the caspase-3 inhibitor Z-DEVD-fmk (40 microM) and the PARP-1 inhibitor DPQ (10 microM) each significantly protected against peroxide damage. While the A(1) adenosine receptor agonist CPA and the A(2A) receptor antagonist ZM241385 (each at 100 nM) elicited protection, the A(1) adenosine receptor blocker DPCPX and the A(2A) receptor agonist CGS21680 (each at 100 nM) showed no effect. The data demonstrate that H(2)O(2) induced oxidative stress in CGNs, involving both apoptotic and necrotic death, and this can be ameliorated by A(1) receptor activation or A(2A) receptor blockade.