Inhibition of ischaemic hippocampal neuronal death in primates with cathepsin B inhibitor CA-074: a novel strategy for neuroprotection based on 'calpain-cathepsin hypothesis'

Protective mechanisms by cystatin C in neurodegenerative diseases

Neurodegeneration occurs in acute pathological conditions such as stroke, ischemia, and head trauma and in chronic disorders such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. While the cause of neuronal death is different and not always known in these varied conditions, hindrance of cell death would be beneficial in the prevention of, slowing of, or halting disease progression. Enhanced cystatin C (CysC) expression in these conditions caused a debate as to whether CysC up-regulation facilitates neurodegeneration or it is an endogenous neuroprotective attempt to prevent the progression of the pathology. However, recent in vitro and in vivo data have demonstrated that CysC plays protective roles via pathways that are dependent on inhibition of cysteine proteases, such as cathepsin B, or by induction of autophagy, induction of proliferation, and inhibition of amyloid-beta aggregation. Here we review the data demonstrating the protective roles of CysC under conditions of neuronal challenge and the protective pathways induced under various conditions. These data suggest that CysC is a therapeutic candidate that can potentially prevent brain damage and neurodegeneration.

Programmed necrosis from molecules to health and disease

During the past decade, cell death researchers have witnessed a gradual but deep conceptual revolution: it has been unequivocally shown that necrosis, which for long had been considered as a purely accidental cell death mode, can also be induced by finely regulated signal transduction pathways. In particular, when caspases are inhibited by pharmacological or genetic means, the ligation of death receptors such as the tumor necrosis factor receptor 1 (TNFR1) can lead to the assembly of a supramolecular complex containing the receptor-interacting protein kinases 1 and 3 (RIP1 and RIP3) that delivers a pronecrotic signal. Such complex has recently been dubbed necrosome and mediates the execution of a specific instance of regulated necrosis, necroptosis. Soon, it turned out that programmed necrosis occurs in nonmammalian model organisms and that it is implicated in human diseases including ischemia and viral infection. In this review, we first describe the historical evolution of the concept of programmed necrosis and the molecular mechanisms that underlie necroptosis initiation and execution. We then provide evidence suggesting that necroptosis represents an ancient and evolutionarily conserved cell death modality that may be targeted for drug development.

Cathepsin B contributes to traumatic brain injury-induced cell death through a mitochondria-mediated apoptotic pathway

It has been reported that lysosomal proteases play important roles in ischemic and excitotoxic neuronal cell death. We have previously reported that cathepsin B expression increased remarkably after traumatic brain injury (TBI). The present study sought to investigate the effects of a selective cathepsin B inhibitor (CBI) [N-L-3-trans-prolcarbamoyloxirane-2-carbonyl)-L-isoleucyl-L-proline] on cell death and behavioral deficits in our model. We examined the levels of cathepsin B enzymatic activity and its expression by double labelling damaged cells in the brain slice with propidium iodide (PI) and anticathepsin B. The results showed an elevated enzymatic activity associated with TBI-induced increase in a mature form of cathepsin B, suggesting that cathepsin B may play a role in TBI-induced cell injury. PI was found to label cells positive for the neuronal-specific nuclear marker NeuN, whereas fewer GFAP-positive cells were labelled by PI, suggesting that neurons are more sensitive to cell death induced by TBI. Additionally, we found that pretreatment with CBI remarkably attenuated TBI-induced cell death, lesion volume, and motor and cognitive dysfunction. To analyze the mechanism of action of cathepsin B in the cell death signaling pathway, we assessed DNA fragmentation by electrophoresis, Bcl-2/Bax protein expression levels, Bid cleavage, cytochrome c release, and caspase-3 activation. The results imply that cathepsin B contributes to TBI-induced cell death through the present programmed cell necrosis and mitochondria-mediated apoptotic pathways.

Erythropoietin prevents nitric oxide and cathepsin-mediated neuronal death in focal brain ischemia

We examined the preventive effect of human recombinant erythropoietin (HrEPO) on nitric oxide (NO)-mediated toxicity to neurons and cysteine protease release into cytoplasm, which is attributed to neuronal death in brain ischemia. Focal cerebral ischemia was induced by permanent occlusion of middle cerebral artery in two sets of rat. The first set was used to monitor NO concentration and cathepsin activity, while the second was used for histological examination with hematoxylin and eosin, and TUNEL staining. A group in both set was administered human recombinant erythropoietin (HrEPO). NO content, cathepsins B and L activity increased significantly in the post-ischemic cerebral tissue (p<0.05). HrEPO treatment reduced NO concentration and cathepsin activity to control level (p>0.05). A significant increase in the number of necrotic and apoptotic neurons was observed in the post-ischemic cerebral cortex (p<0.05). HrEPO treatment was markedly lowered both of these (p<0.05). It is concluded that HrEPO prevents neuronal death by protecting neuronal liposomes from NO-mediated toxicity and suppressing the release of cathepsins.

Roles of zinc and metallothionein-3 in oxidative stress-induced lysosomal dysfunction, cell death, and autophagy in neurons and astrocytes

Zinc dyshomeostasis has been recognized as an important mechanism for cell death in acute brain injury. An increase in the level of free or histochemically reactive zinc in astrocytes and neurons is considered one of the major causes of death of these cells in ischemia and trauma. Although zinc dyshomeostasis can lead to cell death via diverse routes, the major pathway appears to involve oxidative stress.

Recently, we found that a rise of zinc in autophagic vacuoles, including autolysosomes, is a prerequisite for lysosomal membrane permeabilization and cell death in cultured brain cells exposed to oxidative stress conditions. The source of zinc in this process is likely redox-sensitive zinc-binding proteins such as metallothioneins, which release zinc under oxidative conditions. Of the metallothioneins, metallothionein-3 is especially enriched in the central nervous system, but its physiologic role in this tissue is not well established. Like other metallothioneins, metallothionein-3 may function as metal detoxicant, but is also known to inhibit neurite outgrowth and, sometimes, promote neuronal death, likely by serving as a source of toxic zinc release. In addition, metallothionein-3 regulates lysosomal functions. In the absence of metallothionein-3, there are changes in lysosome-associated membrane protein-1 and -2, and reductions in certain lysosomal enzymes that result in decreased autophagic flux. This may have dual effects on cell survival. In acute oxidative injury, zinc dyshomeostasis and lysosomal membrane permeabilization are diminished in metallothionein-3 null cells, resulting in less cell death. But over the longer term, diminished lysosomal function may lead to the accumulation of abnormal proteins and cause cytotoxicity.

The roles of zinc and metallothionein-3 in autophagy and/or lysosomal function have just begun to be investigated. In light of evidence that autophagy and lysosomes may play significant roles in the pathogenesis of various neurological diseases, further insight into the contribution of zinc dynamics and metallothionein-3 function may help provide ways to effectively regulate these processes in brain cells.

Characterization of Puma-dependent and Puma-independent neuronal cell death pathways following prolonged proteasomal inhibition

Proteasomal stress and the accumulation of polyubiquitinated proteins are key features of numerous neurodegenerative disorders. Previously we demonstrated that stabilization of p53 and activation of its target gene, puma (p53-upregulated mediator of apoptosis), mediated proteasome inhibitor-induced apoptosis in cancer cells. Here we demonstrated that Puma also contributed to proteasome inhibitor-induced apoptosis in mouse neocortical neurons. Although protection afforded by puma gene deletion was incomplete, we found little evidence indicating contributions from other proapoptotic BH3-only proteins. Attenuation of bax expression did not further reduce Puma-independent apoptosis, suggesting that pathways other than the mitochondrial apoptosis pathway were activated. Real-time imaging experiments in wild-type and puma-deficient neurons using a fluorescence resonance energy transfer (FRET)-based caspase sensor confirmed the involvement of a second cell death pathway characterized by caspase activation prior to mitochondrial permeabilization and, more prominently, a third, caspase-independent and Puma-independent pathway characterized by rapid cell shrinkage and nuclear condensation. This pathway involved lysosomal permeabilization in the absence of autophagy activation and was sensitive to cathepsin but not autophagy inhibition. Our data demonstrate that proteasomal stress activates distinct cell death pathways in neurons, leading to both caspase-dependent and caspase-independent apoptosis, and demonstrate independent roles for Puma and lysosomal permeabilization in this model.

Molecular mechanisms of necroptosis: an ordered cellular explosion

For a long time, apoptosis was considered the sole form of programmed cell death during development, homeostasis and disease, whereas necrosis was regarded as an unregulated and uncontrollable process. Evidence now reveals that necrosis can also occur in a regulated manner. The initiation of programmed necrosis, 'necroptosis', by death receptors (such as tumour necrosis factor receptor 1) requires the kinase activity of receptor-interacting protein 1 (RIP1; also known as RIPK1) and RIP3 (also known as RIPK3), and its execution involves the active disintegration of mitochondrial, lysosomal and plasma membranes. Necroptosis participates in the pathogenesis of diseases, including ischaemic injury, neurodegeneration and viral infection, thereby representing an attractive target for the avoidance of unwarranted cell death.

Differential expression of FABP 3, 5, 7 in infantile and adult monkey cerebellum

To clarify the involvement of fatty acid binding proteins (FABPs) in cerebellar development and function, we explored the distribution of three brain-expressed FABPs, FABP 3, 5 and 7, by comparing three animal groups--infantile, normal and postischemic adult monkeys. Immunoblotting analysis revealed intense expression of FABP 3 and 7, but not of FABP5, in the control and postischemic adult cerebellum. The protein levels of FABP7, but not of FABP 3 or 5, gradually increased until 2 weeks after the insult. Immunohistochemical analysis showed that cerebellar FABP3-positive cells were Purkinje cells and Bergmann glia. FABP5-positive cells were found only in the postischemic cerebellum, and were identified as activated microglia. Interestingly, in the infantile cerebellum, both the granule cell progenitors in the external granular layer (EGL) and the oligodendrocyte progenitors in the internal granular layer (IGL) expressed FABP5. In the adult cerebellum, FABP7 was expressed in Purkinje cells and basket interneurons, while in the infantile cerebellum it was also found in Bergmann glia. These results showed differential expression of FABPs in cerebellar neuronal and glial cell types; FABP 3 and 7 were predominantly expressed in normal cerebellum, FABP5 after ischemic injury, while FABP 3, 5 and 7 were expressed during cerebellar development.

Lysosomal rupture, necroapoptotic interactions and potential crosstalk between cysteine proteases in neurons shortly after focal ischemia

Ischemic cell death is a complex process and the initial distinction between apoptosis and necrosis appears to be an oversimplification. We previously reported that in ischemic neurons with disrupted plasmalemma, apoptotic mechanisms were also active. In the present study, we investigated cellular co-localization of another necrotic feature, lysosomal rupture, with apoptotic mechanisms in the mouse brain and assessed the potential interactions between cysteine proteases. The lysosomal enzymes were spilled into the cytoplasm 1-4h after ischemia/reperfusion, suggesting that lysosomal membrane integrity was rapidly lost, as occurs in necrosis. The same neurons also exhibited caspase-3 and Bid cleavage, and cytochrome-c release. Caspase-3 activity preceded cathepsin-B leakage in most neurons, and declined by 12h, while lysosomal leakage continued to increase. Concurrent inhibition of cathepsin-B and caspase-3 provided significantly better neuroprotection than obtained with separate use of each inhibitor. These data suggest that necrotic and apoptotic mechanisms may act both in concert as well as independently within the same cell beginning at the onset of ischemia to ensure the demise of damaged neurons. Therefore, combined inhibition of cysteine proteases may abrogate potential shifts between alternative death pathways and improve the success of stroke treatments.

Regulation of apoptosis-associated lysosomal membrane permeabilization

Lysosomal membrane permeabilization (LMP) occurs in response to a large variety of cell death stimuli causing release of cathepsins from the lysosomal lumen into the cytosol where they participate in apoptosis signaling. In some settings, apoptosis induction is dependent on an early release of cathepsins, while under other circumstances LMP occurs late in the cell death process and contributes to amplification of the death signal. The mechanism underlying LMP is still incompletely understood; however, a growing body of evidence suggests that LMP may be governed by several distinct mechanisms that are likely engaged in a death stimulus- and cell-type-dependent fashion. In this review, factors contributing to permeabilization of the lysosomal membrane including reactive oxygen species, lysosomal membrane lipid composition, proteases, p53, and Bcl-2 family proteins, are described. Potential mechanisms to safeguard lysosomal integrity and confer resistance to lysosome-dependent cell death are also discussed.

Induction of cell death in neuroblastoma by inhibition of cathepsins B and L

A specific irreversible inhibitor of both cathepsins B and L, Fmoc-Tyr-Ala-CHN(2) (FYAD) induced apoptosis of neuroblastoma cells but not other tumor cells. Cysteine protease inhibitors that were not efficient inhibitors of both proteases did not cause death of any cell line tested. Apoptosis was preceded by accumulation of large electron dense vesicles and multivesicular bodies in the cytoplasm. Exposure of cells to the cathepsin D inhibitor, pepstatin, failed to rescue cells from FYAD-induced death. These results indicate that inhibition of cathepsins B and L may provide a unique mechanism for selectively inducing death of neuroblastoma with limited toxicity to normal cells and tissues.

Lipotoxicity-mediated cell dysfunction and death involve lysosomal membrane permeabilization and cathepsin L activity

Lipotoxicity, which is triggered when cells are exposed to elevated levels of free fatty acids, involves cell dysfunction and apoptosis and is emerging as an underlying factor contributing to various pathological conditions including disorders of the central nervous system and diabetes. We have shown that palmitic acid (PA)-induced lipotoxicity (PA-LTx) in nerve growth factor-differentiated PC12 (NGFDPC12) cells is linked to an augmented state of cellular oxidative stress (ASCOS) and apoptosis and that these events are inhibited by docosahexanoic acid (DHA). The mechanisms of PA-LTx in nerve cells are not well understood, but our previous findings indicate that it involves ROS generation, mitochondrial membrane permeabilization (MMP), and caspase activation. The present study used nerve growth factor differentiated PC12 cells (NGFDPC12 cells) and found that lysosomal membrane permeabilization (LMP) is an early event during PA-induced lipotoxicity that precedes MMP and apoptosis. Cathepsin L, but not cathepsin B, is an important contributor in this process since its pharmacological inhibition significantly attenuated LMP, MMP, and apoptosis. In addition, co-treatment of NGFDPC12 cells undergoing lipotoxicity with DHA significantly reduced LMP, suggesting that DHA acts by antagonizing upstream signals leading to lysosomal dysfunction. These results suggest that LMP is a key early mediator of lipotoxicity and underscore the value of interventions targeting upstream signals leading to LMP for the treatment of pathological conditions associated with lipotoxicity.

Endostatin expression in neurons during the early stage of cerebral ischemia is associated with neuronal apoptotic cell death in adult hypertensive rat model of stroke

Endostatin (ES) has been recognized as a potent anti-angiogenic factor. We here investigated the expression of ES in ischemic brain and the consequence of cells expressing ES after stroke in adult stroke-prone renovascular hypertensive rats. A single dose of Ca-074ME, a membrane-permeable cathepsin B (CB) specific inhibitor, or vehicle was given by intraperitoneal injection immediately after distal middle cerebral artery occlusion (dMCAO), ES expression was evaluated using fluorescent immunohistochemistry staining, and CB enzyme activity was tested by measuring the free 7-amino-4-methylcoumarin (AMC) released by CB from its' specific substrate, the Z-Arg-Arg-7-amido-4-methylcoumarin. ES immunoreactivity (IR) was significantly up-regulated as early as 6 h and returned to baseline level at 3 days in peri-infarct area following dMCAO. Double-staining experiment revealed that the majority of ischemia-induced ES positive cells were neurons. Furthermore, ES was co-labeled with CB and Cleaved Caspase-3(Asp175) whereas treatment with Ca-074ME reduced up-regulation of ES expression and attenuated apoptosis in peri-infarct neurons. Collectively, our data suggest that peri-infarct neurons express ES during the early stage of cerebral ischemia and treatment with Ca-074ME attenuates ES expression and apoptosis in peri-infarct neurons.

Increased activity and altered subcellular distribution of lysosomal enzymes determine neuronal vulnerability in Niemann-Pick type C1-deficient mice

Niemann-Pick disease type C (NPC), caused by mutations in the Npc1 or Npc2 genes, is a progressive neurodegenerative disorder characterized by intracellular accumulation/redistribution of cholesterol in a number of tissues including the brain. This is accompanied by a severe loss of neurons in selected brain regions. In this study, we evaluated the role of lysosomal enzymes, cathepsins B and D, in determining neuronal vulnerability in NPC1-deficient (Npc1(-/-)) mouse brains. Our results showed that Npc1(-/-) mice exhibit an age-dependent degeneration of neurons in the cerebellum but not in the hippocampus. The cellular level/expression and activity of cathepsins B and D are increased more predominantly in the cerebellum than in the hippocampus of Npc1(-/-) mice. In addition, the cytosolic levels of cathepsins, cytochrome c, and Bax2 are higher in the cerebellum than in the hippocampus of Npc1(-/-) mice, suggesting a role for these enzymes in the degeneration of neurons. This suggestion is supported by our observation that degeneration of cultured cortical neurons treated with U18666A, which induces an NPC1-like phenotype at the cellular level, can be attenuated by inhibition of cathepsin B or D enzyme activity. These results suggest that the increased level/activity and altered subcellular distribution of cathepsins may be associated with the underlying cause of neuronal vulnerability in Npc1(-/-) brains. Therefore, their inhibitors may have therapeutic potential in attenuating NPC pathology.

The role of lysosomal rupture in neuronal death

Apoptosis research in the past two decades has provided an enormous insight into its role in regulating cell death. However, apoptosis is only part of the story, and inhibition of neuronal necrosis may have greater impact than apoptosis, on the treatment of stroke, traumatic brain injury, and neurodegenerative diseases. Since the "calpain-cathepsin hypothesis" was first formulated, the calpain- and cathepsin-mediated regulation of necrotic cascades observed in monkeys, has been demonstrated to be a common neuronal death mechanism occurring from simpler organisms to humans. However, the detailed mechanism inducing lysosomal destabilization still remains poorly understood. Heat-shock protein-70 (Hsp70) is known to stabilize lysosomal membrane and protect cells from oxidative stress and apoptotic stimuli in many cell death pathways. Recent proteomics approach comparing pre- and post-ischemic hippocampal CA1 neurons as well as normal and glaucoma-suffered retina of primates, suggested that the substrate protein upon which activated calpain acts at the lysosomal membrane of neurons might be Hsp70. Understanding the interaction between activated calpains and Hsp70 will help to unravel the mechanism that destabilizes the lysosomal membrane, and will provide new insights into clarifying the whole cascade of neuronal necrosis. Although available evidence is circumferential, it is hypothesized that activated calpain cleaves oxidative stress-induced carbonylated Hsp70.1 (a major human Hsp70) at the lysosomal membrane, which result in lysosomal rupture/permeabilization. This review aims at highlighting the possible mechanism of lysosomal rupture in neuronal death by a modified "calpain-cathepsin hypothesis". As the autophagy-lysosomal degradation pathway is a target of oxidative stress, the implication of autophagy is also discussed.

Proteomic identification of carbonylated proteins in the monkey hippocampus after ischemia-reperfusion

Reactive oxygen species (ROS) are known to participate in neurodegeneration after ischemia-reperfusion. With the aid of ROS, the calpain-induced lysosomal rupture provokes ischemic neuronal death in the cornu Ammonis (CA) 1 of the hippocampus; however, the target proteins of ROS still remain unknown. Here a proteomic analysis was done to identify and characterize ROS-induced carbonyl modification of proteins in the CA1 of the macaque monkey after transient whole-brain ischemia followed by reperfusion. We found that carbonyl modification of heat shock 70-kDa protein 1 (Hsp70-1), a major stress-inducible member of the Hsp70 family, was extensively increased before the neuronal death in the CA1 sector, and the carbonylation site was identified to be Arg469 of Hsp70-1. The CA1 neuronal death conceivably occurs by calpain-mediated cleavage of carbonylated Hsp70 that becomes prone to proteolysis with the resultant lysosomal rupture. In addition, the carbonyl levels of dihydropyrimidinase-like 2 isoform 2, glial fibrillary acidic protein, and beta-actin were remarkably increased in the postischemic CA1. Therefore, ischemia-reperfusion-induced oxidative damage to these proteins in the CA1 may lead to loss of the neuroprotective function, which contributes to neuronal death.

Medical bioremediation of age-related diseases

Catabolic insufficiency in humans leads to the gradual accumulation of a number of pathogenic compounds associated with age-related diseases, including atherosclerosis, Alzheimer's disease, and macular degeneration. Removal of these compounds is a widely researched therapeutic option, but the use of antibodies and endogenous human enzymes has failed to produce effective treatments, and may pose risks to cellular homeostasis. Another alternative is "medical bioremediation," the use of microbial enzymes to augment missing catabolic functions. The microbial genetic diversity in most natural environments provides a resource that can be mined for enzymes capable of degrading just about any energy-rich organic compound. This review discusses targets for biodegradation, the identification of candidate microbial enzymes, and enzyme-delivery methods.

HIV-1 transforms the monocyte plasma membrane proteome

How HIV-1 affects the monocyte proteome is incompletely understood. We posit that one functional consequence of virus-exposure to the monocyte is the facilitation of protein transformation from the cytosol to the plasma membrane (PM). To test this, cell surface labeling with CyDye fluorophores followed by 2 dimensional differential in-gel electrophoresis (2D DIGE) and liquid chromatography tandem mass spectrometry (LC-MS/MS) was performed. Fifty three percent of HIV-1 induced proteins were PM associated. These were linked, in large measure, to cellular activation and oxidative stress. They included, but not limited to, biliverdin reductase, leukotriene hydrolase A(4), heat shock protein 70, and cystatin B. HIV-1 induced PM protein translocation was associated with cathepsin B- and caspase 9, 3-dependent apoptosis. In contrast, PMA-treated monocytes bypassed caspase 3, 9 pathways and lead to cathepsin B-dependent necrosis. These results demonstrate that HIV-1 uniquely affects monocyte activation and oxidative stress. These do not affect viral infection dynamics but are linked to stress-induced cell death.

High susceptibility of cortical neural progenitor cells to trimethyltin toxicity: involvement of both caspases and calpain in cell death

Neural progenitor cells play an essential role in both the developing embryonic nervous system and in the adult brain, where the capacity for self-renewal would be important for normal brain functions. In the present study, we used embryonic cortical neural progenitor cells to investigate the effects of trimethyltin chloride (TMT) on the survival of neural progenitor cells. In cultures of cortical neural progenitor cells, the formation of round neurospheres was observed in the presence of epidermal growth factor and basic fibroblast growth factor within 9 days in vitro. The neurospheres were then harvested for subsequent replating and culturing for assessment of cell viability in either the presence or absence of TMT at the concentration of 5microM. Lasting exposure to TMT produced not only nuclear condensation in the cells in a time-dependent manner over a period of 6-24h, but also the release of lactate dehydrogenase into the culture medium. Immunoblot and immunocytochemical analyses revealed that TMT had the ability to activate both caspase-3 and calpain, as well as to cause nuclear translocation of deoxyribonuclease II, which is located within cytoplasm in intact cells. Additionally, treatment with a calpain inhibitor [trans-epoxysuccinyl-l-leucylamido-(4-guanidino) butane] and a caspase inhibitor [Z-Val-Ala-Asp(OMe)-CH2F] produced a significant reduction in damaged cells induced by TMT. Taken together, our data indicate that neural progenitor cells are highly susceptible to TMT in undergoing cell death via the activation of 2 parallel pathways, ones involving calpain and the other, caspase-3.

Ethambutol-induced toxicity is mediated by zinc and lysosomal membrane permeabilization in cultured retinal cells

Ethambutol, an efficacious antituberculosis agent, can cause irreversible visual loss in a small but significant fraction of patients. However, the mechanism of ocular toxicity remains to be established. We previously reported that ethambutol caused severe vacuole formation in cultured retinal cells, and that the addition of zinc along with ethambutol aggravated vacuole formation whereas addition of the cell-permeable zinc chelator, N,N,N',N'-tetrakis (2-pyridylmethyl) ethylenediamine (TPEN), reduced vacuole formation. To investigate the origin of vacuoles and to obtain an understanding of drug toxicity, we used cultured primary retinal cells from newborn Sprague-Dawley rats and imaged ethambutol-treated cells stained with FluoZin-3, zinc-specific fluorescent dye, under a confocal microscope. Almost all ethambutol-induced vacuoles contained high levels of labile zinc. Double staining with LysoTracker or MitoTracker revealed that almost all zinc-containing vacuoles were lysosomes and not mitochondria. Intracellular zinc chelation with TPEN markedly blocked both vacuole formation and zinc accumulation in the vacuole. Immunocytochemistry with antibodies to lysosomal-associated membrane protein-2 (LAMP-2) and cathepsin D, an acid lysosomal hydrolase, disclosed lysosomal activation after exposure to ethambutol. Immunoblotting after 12 h exposure to ethambutol showed that cathepsin D was released into the cytosol. In addition, cathepsin inhibitors attenuated retinal cell toxicity induced by ethambutol. This is consistent with characteristics of lysosomal membrane permeabilization (LMP). TPEN also inhibited both lysosomal activation and LMP. Thus, accumulation of zinc in lysosomes, and eventual LMP, may be a key mechanism of ethambutol-induced retinal cell death.

Cathepsins: key modulators of cell death and inflammatory responses

Apoptosis is a key mechanism in the build up and maintenance of both innate and adaptive immunity as well as in the regulation of cellular homeostasis in almost every organ and tissue. Central to the apoptotic process is a family of intracellular cysteine proteases with aspartate-specificity, called caspases. Nevertheless, there is growing evidence that other non-caspase proteases, in particular lysosomal cathepsins, can play an important role in the regulation of apoptosis. In this review, the players and the molecular mechanisms involved in the lysosomal apoptotic pathways will be discussed as well as the importance of these pathways in the immune system and the pathogenesis of diseases.

Calpain-mediated signaling mechanisms in neuronal injury and neurodegeneration

Calpain is a ubiquitous calcium-sensitive protease that is essential for normal physiologic neuronal function. However, alterations in calcium homeostasis lead to persistent, pathologic activation of calpain in a number of neurodegenerative diseases. Pathologic activation of calpain results in the cleavage of a number of neuronal substrates that negatively affect neuronal structure and function, leading to inhibition of essential neuronal survival mechanisms. In this review, we examine the mechanistic underpinnings of calcium dysregulation resulting in calpain activation in the acute neurodegenerative diseases such as cerebral ischemia and in the chronic neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, prion-related encephalopathy, and amylotrophic lateral sclerosis. The premise of this paper is that analysis of the signaling and transcriptional consequences of calpain-mediated cleavage of its various substrates for any neurodegenerative disease can be extrapolated to all of the neurodegenerative diseases vulnerable to calcium dysregulation.

Possible involvement of cathepsin B released by microglia in methylmercury-induced cerebellar pathological changes in the adult rat

There is increasing evidence that cathepsin B (CB), a lysosomal cysteine protease, is one of the toxic molecules that are secreted by activated microglia. We herein provide evidence that CB released by activated microglia may play a role in the methylmercury (MeHg)-induced pathological changes observed in the cerebellum of the adult rat. Pathological changes tended to progress slowly after treatment with MeHg (5 mg/kg) for 12 consecutive days. At 5 days after the final treatment of MeHg, there was a mild pyknotic change of the granule cells, whereas a marked accumulation of activated microglia was observed in the granule cell layer of the lingual and central lobe. At 8 days after the final treatment, intense pyknotic changes of the granule cells and the accumulation of activated microglia were observed throughout the cerebellar vermis. CB first significantly increased at 3 days after the final treatment of MeHg as the mature form. CB mainly increased in activated microglia which accumulated in the granule cell layer. The coadministration of CA074, an irreversible CB inhibitor, with MeHg significantly reduced the severity of pyknotic changes of the granule cells. Furthermore, primary cultured microglia secreted the mature CB in the culture medium following cellular activation. These observations strongly suggest that CB secreted by activated microglia is thus closely associated with the MeHg-induced severe pyknotic changes of the cerebellar granule cells. The treatment of CA074 could be a potentially effective therapeutic intervention to prevent the pathological changes in the cerebellum caused by ingestion of MeHg-contaminated food.

In vivo acute treatment with trimethyltin chloride causes neuronal degeneration in the murine olfactory bulb and anterior olfactory nucleus by different cascades in each region

Our earlier study demonstrated that in vivo acute treatment with trimethyltin chloride (TMT) produces severe neuronal damage in the dentate gyrus and cognition impairment in mice. In the present study, we assessed whether TMT was capable of causing neuronal degeneration in the olfactory bulb (OB) and anterior olfactory nucleus (AON) of the mouse brain. An intraperitoneal injection of TMT at the dose of 2.8 mg/kg led to a dramatic increase in the number of degenerating cells, which were reactive with antibody against single-stranded DNA, in the granule cell layer (GCL) of the OB and AON 1 day and 2 days later, respectively. TMT treatment produced a marked translocation of phospho-c-Jun-N-terminal kinase from the cytoplasm to the nucleus in the AON. Expectedly, a marked increase in phospho-c-Jun-positive cells was seen in the AON after the treatment. In addition to the AON, the mitral cell layer of the olfactory bulb showed the presence of phospho-c-Jun-positive cells after the treatment. However, the GCL had no cells positive for either phospho-c-Jun-N-terminal kinase or phospho-c-Jun at any time after the treatment with TMT. Similarly, TMT-induced nuclear translocation of the lysosomal enzyme deoxyribonuclease II was seen in the AON, but not in the GCL. On the other hand, TMT elicited the expression of activated caspase 3 in the GCL but not in the AON. Taken together, our results suggest that TMT is capable of causing neuronal degeneration in the murine OB and AON through different cascades in the two structures.

Lysosomal release of cathepsins causes ischemic damage in the rat hippocampal slice and depends on NMDA-mediated calcium influx, arachidonic acid metabolism, and free radical production

NMDA-mediated calcium entry and reactive oxygen species (ROS) production are well-recognized perpetrators of ischemic neuronal damage. The current studies show that these events lead to the release of the protein hydrolase, cathepsin B, from lysosomes 2 h following 5-min oxygen-glucose deprivation in the rat hippocampal slice. This release reflects a lysosomal membrane permeabilization (LMP) and was measured as the appearance of diffuse immunolabeled cathepsin B in the cytosol of CA1 pyramidal neurons. Necrotic neuronal damage begins after the release of cathepsins and is prevented by inhibitors of either cathepsin B or D indicating that the release of cathepsins is an important mediator of severe damage. There was an increase in superoxide levels, measured by dihydroethidium fluorescence, at the same time as LMP and reducing ROS levels with antioxidants, Trolox or N-tert-butyl-alpha-phenyl nitrone, blocked LMP. Both LMP and ROS production were blocked by an NMDA channel blocker (MK-801) and by inhibitors of mitogen-activated protein kinase kinase (U0126), calcium-dependent/independent phospholipases A2 (methyl arachidonyl fluorophosphonate) but not calcium-independent phospholipases A2 (bromoenol lactone) and cyclooxygenase-2 (NS398). A cell-permeant specific inhibitor of calpain (PD150606) prevented LMP, but not ROS production. It is concluded that LMP results in part from calcium-initiated and extracellular signal-regulated kinase-initiated arachidonic acid metabolism, which produces free radicals; it also requires the action of calpain.

The effect of intracerebroventricular application of the caspase-3 inhibitor zDEVD-FMK on neurological outcome and neuronal cell death after global cerebral ischaemia due to cardiac arrest in rats

BACKGROUND

Global cerebral ischaemia after cardiac arrest (CA) leads to programmed cell death (PCD) with characteristic signs of apoptosis in selectively vulnerable areas of the brain. The activation of caspase-3, an executioner caspase, plays a key role in the apoptotic cascade. We, therefore, studied the effects of the application of the specific caspase-3 inhibitor zDEVD-FMK on neurological outcome and neuronal cell death after experimental CA in rats.

METHODS

A 6-min CA was induced in anaesthetised and mechanically ventilated male Wistar rats. After cardiopulmonary resuscitation (CPR) and restoration of spontaneous circulation (ROSC) the animals were randomised to two groups to receive a continuous intracerebroventricular (i.c.v.) infusion for 7 days of zDEVD-FMK or placebo (artificial cerebrospinal fluid, CSF). At 24h, 3 and 7 days after ROSC, animals were tested according to a neurological deficit score (NDS). Seven days after ROSC, coronal sections of the brain were taken at the dorsal hippocampal level and analysed with cresyl-violet staining, the TUNEL technique and a caspase activity assay. Viable and TUNEL-positive neurons were counted in the hippocampal CA-1 sector.

RESULTS

The NDS demonstrated severe deficits 1 and 3 days after ROSC, which resolved by 7 days with no difference between the two groups. At 7 days after ROSC neuronal death could be detected using cresyl-violet and TUNEL staining with no difference between the groups.

CONCLUSION

We conclude that zDEVD-FMK administration has no effect on neurological outcome and PCD after global cerebral ischaemia following CA in rats. Other mechanisms or pathways must be identified in the pathophysiology of PCD after CA.

Secondary necrosis in multicellular animals: an outcome of apoptosis with pathogenic implications

In metazoans apoptosis is a major physiological process of cell elimination during development and in tissue homeostasis and can be involved in pathological situations. In vitro, apoptosis proceeds through an execution phase during which cell dismantling is initiated, with or without fragmentation into apoptotic bodies, but with maintenance of a near-to-intact cytoplasmic membrane, followed by a transition to a necrotic cell elimination traditionally called “secondary necrosis”. Secondary necrosis involves activation of self-hydrolytic enzymes, and swelling of the cell or of the apoptotic bodies, generalized and irreparable damage to the cytoplasmic membrane, and culminates with cell disruption. In vivo, under normal conditions, the elimination of apoptosing cells or apoptotic bodies is by removal through engulfment by scavengers prompted by the exposure of engulfment signals during the execution phase of apoptosis; if this removal fails progression to secondary necrosis ensues as in the in vitro situation. In vivo secondary necrosis occurs when massive apoptosis overwhelms the available scavenging capacity, or when the scavenger mechanism is directly impaired, and may result in leakage of the cell contents with induction of tissue injury and inflammatory and autoimmune responses. Several disorders where secondary necrosis has been implicated as a pathogenic mechanism will be reviewed.

The roles of therapy-induced autophagy and necrosis in cancer treatment

Metabolic and therapeutic stresses activate several signal transduction pathways that regulate cell death and cell survival in cancer cells. Although decades of research unraveled the pathways that regulate apoptosis and allowed the development of novel diagnostic and therapeutic modalities in cancer treatment, only recently has the regulation and significance of tumor cell autophagy and necrosis become the focus of investigations. Necrosis is an irreversible inflammatory form of cell death. In contrast, autophagy is a reversible process that can contribute both to tumor cell death and survival. This review describes recent advances in understanding the regulation of autophagy and necrosis and their implications for cancer therapy. Currently available methods to measure autophagy and necrosis are highlighted. The effect of tumor cell autophagy and necrosis on host immunity is explored. Finally, therapeutic approaches that target autophagy and necrosis in cancer are described.

Effects of cathepsins B and L inhibition on postischemic protein alterations in the brain

The effects of selective inhibition of cathepsins B and L on postischemic protein alterations in the brain were investigated in a rat model of middle cerebral artery occlusion (MCAO). Cathepsin B activity increased predominantly in the subcortical region of the ischemic hemisphere where the levels of collapsing mediator response protein 2, heat shock cognate 70 kDa protein, 60 kDa heat shock protein, protein disulfide isomerase A3 and albumin, were found to be significantly elevated. Postischemic treatment with Cbz-Phe-Ser(OBzl)-CHN(2), cysteine protease inhibitor 1 (CP-1), reduced infarct volume, neurological deficits and cathepsin B activity as well as the amount of heat shock proteins and albumin found in the brain. Our data strongly suggests that the decrease in heat shock protein levels and the significant reduction of serum albumin leakage into the brain following acute treatment with CP-1 is indicative of less secondary ischemic damage, which ultimately, is related to less cerebral tissue loss and improved neurological recovery of the animals.

Hippocampal CA1 cell loss in a non-human primate model of transient global ischemia: a pilot study

We exposed adult Rhesus (Macaca mulatta) to a transient global ischemia, which was induced by clipping the innominate and subclavian arteries that originated from the aortic arch. NHP1 received 20-min, while NHP2 and NHP3, were exposed to a 15-min transient global ischemia and were euthanized at day 1 (NHP1), day 5 (NHP2) or day 30 (NHP3) after ischemia, respectively. NHP1 displayed severe paralysis and rigidity, and intermittent convulsions over the next 24 h. Although histological examination of the brain revealed no detectable gross brain damage (i.e., swelling) and only minimal cell loss in the hippocampus, the acute survival time after surgery likely prevented the cerebral ischemia to fully develop and to be morphologically manifested. Nonetheless, the 20-min ischemia might have been too severe and caused a systemic multiple organ collapse that produced the abnormal behavioral symptoms. On the other hand, NHP2 and NHP3 which received 15-min ischemia only exhibited minor hindlimb paralysis. Indeed, by 48 h after ischemia, both animals appeared fully recovered with only fine motor deficits. Immunohistochemical examination revealed that NHP2 and 3, but not NHP1, had a marked neuronal cell loss in the hippocampal region, specifically the cornu Ammonis (CA1) region. The cell loss in these two ischemic NHP hippocampi was further confirmed by direct comparison with a normal Rhesus brain. These findings replicate the brain pathology seen in Japanese macaques exposed to the same ischemia model [T. Tsukada, M. Watanabe, T. Yamashima, Implications of CAD and DNase II in ischemic neuronal necrosis specific for the primate hippocampus, J. Neurochem. 79 (2001) 1196-1206; T. Yamashima, Implication of cysteine proteases calpain, cathepsin and caspase in ischemic neuronal death of primates, Prog. Neurobiol. 62 (2000) 273-295; T. Yamashima, Y. Kohda, K. Tsuchiya, T. Ueno, J. Yamashita, T. Yoshioka, E. Kominami, Inhibition of ischemic hippocampal neuronal death in primates with cathepsin B inhibitor CA-074: a novel strategy for neuroprotection based on calpain-cathepsin hypothesis, Eur. J. Neurosci. 10 (1998) 1723-1733; T. Yamashima, T.C. Saido, M. Takita, A. Miyazawa, J. Yamano, A. Miyakawa, H. Nishijyo, J. Yamashita, S. Kawashima, T. Ono, T. Yoshioka, Transient brain ischemia provokes Ca2+, PIP2 and calpain responses prior to delayed neuronal death in monkeys, Eur. J. Neurosci. 8 (1996) 1932-1944; T. Yamashima, A.B. Tonchey, T. Tsukada, T.C. Saido, S. Imajoh-Ohmi, T. Momoi, E. Kominami, Sustained calpain activation associated with lysosomal rupture executes necrosis of the postischemic CA1 neurons in primates, Hippocampus 13 (2003) 791-800]. The present minimally invasive transient global ischemia model using Rhesus shows many histopathological symptoms seen in human patients who experienced global ischemia, and should allow translational validation of experimental therapeutics for ischemic injury. Additional studies are warranted to reveal behavioral deficits associated with this ischemia model.

The cysteine protease inhibitors cystatins inhibit herpes simplex virus type 1-induced apoptosis and virus yield in HEp-2 cells

The role of cystatins in herpes simplex virus (HSV)-induced apoptosis and viral replication has been studied. Human epithelial (HEp-2) cells infected with wild-type HSV-1 (F), with a deletion virus lacking the anti-apoptotic gene Us3 (R7041) or with a deletion virus lacking the anti-apoptotic genes Us3 and ICP4 (d120) were treated with cystatin A, C or D. Cells and culture media were studied at different time points for replicating HSV-1 and for apoptosis. Cystatins C and D inhibited the yield of replicative HSV-1 significantly in HEp-2 cells. In addition, cystatin D inhibited R7041 and d120 virus-induced apoptosis. Moreover, cystatin A inhibited R7041-induced apoptosis. These inhibitory effects of cystatins on virus replication and apoptosis are likely to be separate functions. Cystatin D treatment decreased cellular cathepsin B activity in HSV-1 infection, suggesting that cathepsin B is involved in virus-induced apoptosis.

Altered levels and distribution of IGF-II/M6P receptor and lysosomal enzymes in mutant APP and APP + PS1 transgenic mouse brains

The insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptor participates in the trafficking of lysosomal enzymes from the trans-Golgi network or the cell surface to lysosomes. In Alzheimer's disease (AD) brains, marked up-regulation of the lysosomal system in vulnerable neuronal populations has been correlated with altered metabolic functions. To establish whether IGF-II/M6P receptors and lysosomal enzymes are altered in the brain of transgenic mice harboring different familial AD mutations, we measured the levels and distribution of the receptor and lysosomal enzymes cathepsins B and D in select brain regions of transgenic mice overexpressing either mutant presenilin 1 (PS1; PS1(M146L+L286V)), amyloid precursor protein (APP; APP(KM670/671NL+V717F)) or APP+PS1 (APP(KM670/671NL+V717F)+PS1(M146L+L286V)) transgenes. Our results revealed that levels and expression of the IGF-II/M6P receptor and lysosomal enzymes are increased in the hippocampus and frontal cortex of APP and APP+PS1, but not in PS1, transgenic mouse brains compared with wild-type controls. The changes were more prominent in APP+PS1 than in APP single transgenic mice. Additionally, all beta-amyloid-containing neuritic plaques in the hippocampal and cortical regions of APP and APP+PS1 transgenic mice were immunopositive for both lysosomal enzymes, whereas only a subset of the plaques displayed IGF-II/M6P receptor immunoreactivity. These results suggest that up-regulation of the IGF-II/M6P receptor and lysosomal enzymes in neurons located in vulnerable regions reflects an altered functioning of the endosomal-lysosomal system which may be associated with the increased intracellular and/or extracellular A beta deposits observed in APP and APP+PS1 transgenic mouse brains.

Differential response to ischemia in adjacent hippocampalsectors: neuronal death in CA1versus neurogenesis in dentate gyrus

Two hippocampal sectors show distinct responses to transient ischemia: the cornu Ammonis (CA)1 sector undergoes a delayed neuronal death followed by a lack of neuronal generation, while the dentate gyrus (DG) shows slight postischemic damage followed by an increased neurogenesis. Using the monkey experimental paradigm of transient whole brain global ischemia, the 'calpain-cathepsin hypothesis' was formulated in 1998. This hypothesis proposes that following ischemia calpain compromises the integrity of lysosomal membrane, causing a leakage of degrading hydrolytic enzymes--cathepsins--into the cytoplasm. Ischemia induces Ca(2+) mobilization, calpain activation, lysosomal membrane disruption, and cathepsin release, which all occur specifically in the CA1 sector and cause neuronal death. In the postischemic DG, a vascular niche has been implicated in adult neurogenesis, in that adventitial cells of the DG microvascular environment provoke postischemic up-reguation of neurogenesis with the aid of brain-derived neurotrophic factor and polysialylated form of the neural cell adhesion molecule. In parallel, Down's syndrome cell adhesion molecule has recently been shown to be expressed specifically in the neural progenitor cells of DG. In this review, we focus on the monkey experimental paradigm to reveal the remarkable contrasts between CA1 and DG in response to the ischemic insult.

A novel dual inhibitor of calpains and lipid peroxidation (BN82270) rescues the cochlea from sound trauma

Free radical and calcium buffering mechanisms are implicated in cochlear cell damage that has been induced by sound trauma. Thus in this study we evaluated the therapeutic effect of a novel dual inhibitor of calpains and of lipid peroxidation (BN 82270) on the permanent hearing and hair cell loss induced by sound trauma. Perfusion of BN 82270 into the scala tympani of the guinea pig cochlea prevented the formation of calpain-cleaved fodrin, translocation of cytochrome c, DNA fragmentation and hair cell degeneration caused by sound trauma. This was confirmed by functional tests in vivo, showing a clear dose-dependent reduction of permanent hearing loss (ED50 = 4.07 microM) with almost complete protection at 100 microM. Furthermore, BN82270 still remained effective even when applied onto the round window membrane after sound trauma had occurred, within a therapeutic window of 24 h. This indicates that BN 82270 may be of potential therapeutic value in treating the cochlea after sound trauma.

Expression of angiogenic and neurotrophic factors in the progenitor cell niche of adult monkey subventricular zone

The subventricular zone along the anterior horn (SVZa) of the cerebral lateral ventricle of adult mammals contains multipotent progenitor cells, which supposedly exist in an angiogenic niche. Numerous signals are known to modulate the precursor cell proliferation, migration or differentiation, in rodent models. In contrast, the data on signals regulating the primate SVZa precursors in vivo are scarce. We analyzed the expression at protein level of a panel of angiogenic and/or neurotrophic factors and their receptors in SVZa of adult macaque monkeys, under normal condition or after transient global ischemia which enhances endogenous progenitor cell proliferation. We found that fms-like tyrosine kinase 1 (Flt1), a receptor for vascular endothelial cell growth factor, was expressed by over 30% of the proliferating progenitors, and the number of Flt1-positive precursors was significantly increased by the ischemic insult. Smaller fractions of mitotic progenitors were positive for the neurotrophin receptor tropomyosin-related kinase (Trk) B or the hematopoietic receptor Kit, while immature neurons expressed Flt1 and the neurotrophin receptor TrkA. Further, SVZa astroglia, ependymal cells and blood vessels were positive for distinctive sets of ligands/receptors, which we characterized. The presented data provide a molecular phenotypic analysis of cell types comprising adult monkey SVZa, and suggest that a complex network of angiogenic/neurotrophic signals operating in an autocrine or paracrine manner may regulate SVZa neurogenesis in the adult primate brain.

Up-regulation of cation-independent mannose 6-phosphate receptor and endosomal-lysosomal markers in surviving neurons after 192-IgG-saporin administrations into the adult rat brain

The cation-independent mannose 6-phosphate receptor (CI-MPR) is a single transmembrane domain glycoprotein that plays a major role in the trafficking of lysosomal enzymes from the trans-Golgi network to the endosomal-lysosomal (EL) system. Because dysfunction of EL system is associated with a variety of neurodegenerative disorders, it is possible that the CI-MPR may have a role in regulating neuronal viability after toxicity/injury. In the present study, we report that 192-IgG-saporin-induced loss of basal forebrain cholinergic neurons causes a transient up-regulation of CI-MPR protein levels in surviving neurons of the basal forebrain and frontal cortex but not in the brainstem region, which was relatively spared by the immunotoxin. This was accompanied by a parallel time-dependent increase in other EL markers, ie, cathepsin D, Rab5, and LAMP2 in the basal forebrain region, whereas in the frontal cortex the levels of cathepsin D, and to some extent Rab5, were increased. Given the critical role of the EL system in the clearance of abnormal proteins in response to changing conditions, it is likely that the observed increase in the CI-MPR and components of the EL system in surviving neurons after 192-IgG-saporin treatment represents an adaptive mechanism to restore the metabolic/structural abnormalities induced by the loss of cholin-ergic neurons.

Extensive degradation of myelin basic protein isoforms by calpain following traumatic brain injury

Axonal injury is one of the key features of traumatic brain injury (TBI), yet little is known about the integrity of the myelin sheath. We report that the 21.5 and 18.5-kDa myelin basic protein (MBP) isoforms degrade into N-terminal fragments (of 10 and 8 kDa) in the ipsilateral hippocampus and cortex between 2 h and 3 days after controlled cortical impact (in a rat model of TBI), but exhibit no degradation contralaterally. Using N-terminal microsequencing and mass spectrometry, we identified a novel in vivo MBP cleavage site between Phe114 and Lys115. A MBP C-terminal fragment-specific antibody was then raised and shown to specifically detect MBP fragments in affected brain regions following TBI. In vitro naive brain lysate and purified MBP digestion showed that MBP is sensitive to calpain, producing the characteristic MBP fragments observed in TBI. We hypothesize that TBI-mediated axonal injury causes secondary structural damage to the adjacent myelin membrane, instigating MBP degradation. This could initiate myelin sheath instability and demyelination, which might further promote axonal vulnerability.