Protein inhibitors of proteinases from brain

Attenuation of early brain injury and learning deficits following experimental subarachnoid hemorrhage secondary to Cystatin C: possible involvement of the autophagy pathway

Cystatin C (CysC) is a cysteine protease inhibitor and previous studies have demonstrated that increasing endogenous CysC expression has therapeutic implications on brain ischemia, Alzheimer's disease, and other neurodegenerative disorders. Our previous reports have demonstrated that the autophagy pathway was activated in the brain after experimental subarachnoid hemorrhage (SAH), and it may play a beneficial role in early brain injury (EBI). This study investigated the effects of exogenous CysC on EBI, cognitive dysfunction, and the autophagy pathway following experimental SAH. All SAH animals were subjected to injections of 0.3 ml fresh arterial, nonheparinized blood into the prechiasmatic cistern in 20 s. As a result, treatment with CysC with low and medial concentrations significantly ameliorated the degree of EBI when compared with vehicle-treated SAH rats. Microtubule-associated protein light chain-3 (LC3), a biomarker of autophagosomes, and beclin-1, a Bcl-2-interacting protein required for autophagy, were significantly increased in the cortex 48 h after SAH and were further up-regulated after CysC therapy. By ultrastructural observation, there was a marked increase in autophagosomes and autolysosomes in neurons of CysC-treated rats. Learning deficits induced by SAH were markedly alleviated after CysC treatment with medial doses. In conclusion, pre-SAH CysC administration may attenuate EBI and neurobehavioral dysfunction in this SAH model, possibly through activating autophagy pathway.

Cysteine cathepsins in neurological disorders

Increased proteolytic activity is a hallmark of several pathological processes, including neurodegeneration. Increased expression and activity of cathepsins, lysosomal cysteine proteases, during degeneration of the central nervous system is frequently reported. Recent studies reveal that a disturbed balance of their enzymatic activities is the first insult in brain aging and age-related diseases. Leakage of cathepsins from lysosomes, due to their membrane permeability, and activation of pro-apoptotic factors additionally contribute to neurodegeneration. Furthermore, in inflammation-induced neurodegeneration the cathepsins expressed in activated microglia play a pivotal role in neuronal death. The proteolytic activity of cysteine cathepsins is controlled by endogenous protein inhibitors-the cystatins-which evidently fail to perform their function in neurodegenerative processes. Exogenous synthetic inhibitors, which may augment their inhibitory potential, are considered as possible therapeutic tools for the treatment of neurological disorders.

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.

Cystatin C in macular and neuronal degenerations: implications for mechanism(s) of age-related macular degeneration

Cystatin C is a strong inhibitor of cysteine proteinases expressed by diverse cells. Variant B cystatin C, which was associated with increased risk of developing age-related macular degeneration, differs from the wild type protein by a single amino acid (A25T) in the signal sequence responsible for its targeting to the secretory pathway. The same variant conveys susceptibility to Alzheimer disease. Our investigations of the trafficking and processing of variant B cystatin C in living RPE cells highlight impaired secretion of extracellular modulators and inappropriate protein retention in RPE cells as potential molecular mechanisms underpinning macular, and possibly neuronal, degeneration.

The role of cystatin C in cerebral amyloid angiopathy and stroke: cell biology and animal models

A variant of the cysteine protease inhibitor, cystatin C, forms amyloid deposited in the cerebral vasculature of patients with hereditary cerebral hemorrhage with amyloidosis, Icelandic type (HCHWA-I), leading to cerebral hemorrhages early in life. However, cystatin C is also implicated in neuronal degenerative diseases in which it does not form the amyloid protein, such as Alzheimer disease (AD). Accumulating data suggest involvement of cystatin C in the pathogenic processes leading to amyloid deposition in cerebral vasculature and most significantly to cerebral hemorrhage in patients with cerebral amyloid angiopathy (CAA). This review focuses on cell culture and animal models used to study the role of cystatin C in these processes.

Enhanced cystatin C and lysosomal protease expression following 6-hydroxydopamine exposure

6-Hydroxydopamine (6-OHDA) is a selective neurotoxin used to induce apoptosis in catecholamine-containing neurons. Although biochemical products and reactive oxygen species (ROS) of 6-OHDA have been well documented, the activation of cellular pathways following exposure are not well understood. Apoptosis in PC12 (Pheochromocytoma) cells was induced by 6-OHDA in a dose (10-150 microM) and time-dependent (24-72 h) manner compared to experimental controls (no treatment). PC 12 cells exposed to 50 microM 6-OHDA demonstrated the involvement of caspase 3 and lysosomal protease alterations. Following 6-OHDA exposure, the caspase 3-like inhibitor Ac-DEVD-CHO significantly decreased 6-OHDA induced cell death. In addition, alterations in expression of the lysosomal cysteine and aspartic proteases, cathepsin B (CB) and cathepsin D (CD) and the endogenous cysteine protease inhibitor cystatin C were observed utilizing immunocytochemical analysis at 24, 48, and 72 h following 6-OHDA exposure. Furthermore, CB and CD and cystatin C immuno-like reactivity was more pronounced in TUNEL positive cells. Moreover, Western blot analysis confirmed a significant increase in protein expression for CB and CD at 72 h and a temporal and concentration dependent increase in cystatin C in response to 6-OHDA. Cells treated with pepstatin A, an inhibitor for CD, showed a significant decrease in cell death, however, CA-074ME, a specific inhibitor for CB, failed to protect cells from 6-OHDA induced cell death. Thus, these results suggest that apoptosis induced by 6-OHDA exposure is mediated in part through caspase 3 activation and lysosomal protease CD.

Cystatin C prevents degeneration of rat nigral dopaminergic neurons: in vitro and in vivo studies

Destruction of nigrostriatal dopaminergic (DA) pathway triggers various persistent responses, such as inflammation and increased synthesis of neural growth factors, both in striatum and in substantia nigra. The pathological processes involved in such responses are poorly characterized and could contribute to secondary damage and/or regeneration in the central nervous system (CNS). Cystatin C was previously implicated in the process of neurodegeneration. However, its biological role during neurodegeneration is not understood and remains controversial. The present study identified an increased cystatin C mRNA level in the DA-depleted rat striatum, starting from the second week following a 6-OHDA-induced lesion. Immunohistochemical analysis confirmed the increase in cystatin C protein level in the striatum following DA depletion. Double-labeled fluorescence immunohistochemistry revealed that nigrostriatal neurons, astrocytes, and microglia contributed to the elevated level of cystatin C. Exposure to 6-hydroxydopamine, a DA-specific neurotoxin, resulted in DA neurons loss in the fetal mesencephalic cultures, an effect which could be partially reversed by treatment with cystatin C. Moreover, in vivo DA neurons survival study showed that administration of cystatin C in rats with 6-OHDA-induced lesion partially rescued the nigral DA neurons. The results indicate that the 6-OHDA lesioning induced a relatively slow but sustained up-regulation of cystatin C expression and suggest that the inhibitor may exert a neuroprotective action on DA neurons. The findings raise the possibility that cysteine proteinase inhibitors may be new candidates for neuroprotective treatment of Parkinson's disease. Cystatin C may be useful therapeutically in limiting neuropathy in Parkinson's disease.

Binding of cystatin C to Alzheimer’s amyloid β inhibits in vitro amyloid fibril formation

The colocalization of cystatin C, an inhibitor of cysteine proteases, with amyloid beta (Abeta) in parenchymal and vascular amyloid deposits in brains of Alzheimer's disease (AD) patients may reflect cystatin C involvement in amyloidogenesis. We therefore sought to determine the association of cystatin C with Abeta. Immunofluorescence analysis of transfected cultured cells demonstrated colocalization of cystatin C and beta amyloid precursor protein (betaAPP) intracellularly and on the cell surface. Western blot analysis of immunoprecipitated cell lysate or medium proteins revealed binding of cystatin C to full-length betaAPP and to secreted betaAPP (sbetaAPP). Deletion mutants of betaAPP localized the cystatin C binding site within betaAPP to the Abeta region. Cystatin C association with betaAPP resulted in increased sbetaAPP but did not affect levels of secreted Abeta. Analysis of the association of cystatin C and Abeta demonstrated a specific, saturable and high affinity binding between cystatin C and both Abeta(1-42) and Abeta(1-40). Notably, cystatin C association with Abeta results in a concentration-dependent inhibition of Abeta fibril formation.

Involvement of cystatin C in oxidative stress-induced apoptosis of cultured rat CNS neurons

Oxidative stress is involved in neuronal degeneration in cerebrovascular injury, neuropathology and aging. When rat CNS neurons were cultured in a high (50%) oxygen atmosphere, the neurons died. This high oxygen-induced cell death showed features of apoptotic cell death, characterized by DNA fragmentation, and was blocked by inhibitor of protein synthesis. We found that cystatin C and HuC mRNA, the products of which are an inhibitor of cysteine proteases and an RNA binding protein, respectively, were up-regulated in neurons cultured in the high oxygen atmosphere. In the present study, we focused on cystatin C. Cystatin C protein levels were also increased in neurons cultured in the high oxygen atmosphere. In situ hybridization with an RNA probe for rat cystatin C and immunocytochemistry with anti-human cystatin C antibody showed that microtubule-associated protein 2 (MAP2)-positive neurons expressed cystatin C mRNA and protein, respectively, in the high oxygen atmosphere. These results indicated that oxidative stress stimulates an increase in cystatin C expression in cultured neurons, and that cystatin C might have important roles in regulation of apoptosis elicited by oxidative stress.

Immunoreactivity, Ultrastructural Localization, and Transcript Expression of Prostate-specific Antigen in Human Neuroblastoma Cell Lines

Prostate-specific antigen (PSA) is considered a highly specific biochemical marker of the human prostate gland, and it currently is used for prostate cancer diagnosis and monitoring. Recently, PSA production and secretion were found in nondiseased and diseased cells, tissues, and fluids from women. In this study, we characterized the presence of PSA in two human neuroblastoma cell lines with biochemical, ultrastructural, and molecular approaches. Using reverse transcription-PCR, we identified PSA mRNA, and Western blotting revealed a substantial amount of complexed form of PSA protein, which is localized mainly in free ribosomes. Although the role of PSA in human neuroblastoma cell lines is still unknown, our study supports the hypothesis that this serine protease may be involved in controlling the growth of human brain tumor cells, adding more support to the notion that PSA is a widespread kallikrein-like protease with biological functions much more complex than recently thought.

Amyloid precursor proteins protect neurons of transgenic mice against acute and chronic excitotoxic injuries in vivo

The beta-amyloid protein precursor (APP) is well conserved across different species and may fulfill important physiological functions within the CNS. While high-level neuronal expression of amyloidogenic forms of human APP results in beta-amyloid production and neurodegeneration, lower levels of neuronal human APP expression in neurons of transgenic mice may primarily accentuate physiological functions of this molecule. To assess the neuroprotective potential of human APP in vivo, mice from seven distinct transgenic lines expressing different human APP isoforms from the neuron-specific enolase promoter were challenged with systemic kainate injections (n=30) or transgene-mediated glial expression of gp120 (n=32), an HIV-1 protein capable of inducing excitotoxic neuronal damage. To quantitate human APP-mediated neuroprotection. the area of neuropil occupied by presynaptic terminals and neuronal dendrites in the neocortex and hippocampus of each mouse was determined using laser scanning confocal microscopy of double-immunolabelled brain sections and computer-aided image analysis. Compared with gp120 singly transgenic controls, mice from three of three human APP751gp120 bigenic lines expressing the 751 amino acid form of human APP at low levels showed significant protection against degeneration of presynaptic terminals; two of these lines also showed significantly less damage to neuronal dendrites. Two of three human APP695/gp120 bigenic lines expressing human APP695 at low levels were protected against presynaptic and dendritic damage, whereas one low expressor line and a human APP695/gp120 bigenic line expressing human APP695 at higher levels showed no significant protection. In the corresponding human APP singly transgenic lines, overexpressing only specific human APP isoforms, significant protection against kainate-induced degeneration of presynaptic terminals and neuronal dendrites was found in two of three human APP751 lines and not in any of the four human APP695 lines tested. These results indicate that human APP can protect neurons against chronic and acute excitotoxic insults in vivo and that human APP isoforms differ in their neuroprotective potential, at least with respect to specific forms of neural injury. It is therefore possible that impairments of neuroprotective human APP functions or aberrant shifts in human APP isoform ratios could contribute to neurodegeneration.

Neurotrophic and neuroprotective effects of hAPP in transgenic mice

To better understand the role the human amyloid precursor protein (hAPP) plays in Alzheimer's disease (AD), it is essential to define its primary function(s). Here we expressed different hAPPs in neurons of transgenic (tg) mice to characterize their effects on the intact central nervous system (CNS). Immunolabeled brain sections of tg and non-tg mice were compared quantitatively by microdensitometry and computer-aided analysis of laser scanning confocal digitized images. Compared with non-tg mice, tg mice overexpressing hAPPs showed an increase in the number of synaptophysin immunoreactive presynaptic terminals as well as in the expression of the growth-associated marker GAP-43. While non-tg controls and tg mice expressing hAPP751 at moderate levels displayed a normal pattern of reinnervation of the dentate gyrus following perforant pathway transection, tg mice expressing hAPP695 at severalfold higher levels showed an accentuation of the synaptic loss and no sprouting reaction. In addition, expression of hAPP751 at moderate levels effectively protected neurons against excitotoxic injury induced either acutely by systemic injection of kainic acid or chronically by transgene-driven glial production of the soluble HIV-1 protein gp120. Neuronal expression of hAPP695 at higher levels provided less excitoprotection. Our findings are consistent with the postulate that APP plays a role in the formation/maintenance of synapses and that processes which affect this function could contribute to the synaptic pathology seen in AD. Our study also revealed that hAPPs can exert important excitoprotective functions in vivo and that the efficiency of this protection may depend on the hAPP isoform expressed as well as on the level of neuronal hAPP expression. Neuronal overexpression of hAPP beyond a certain level may have detrimental effects on the CNS, particularly in the context of secondary neural injuries.

Cystatin C, a protease inhibitor, in degenerating rat hippocampal neurons following transient forebrain ischemia

Cystatin C, a cysteine protease inhibitor produced by the choroid plexus and found in CSF at high concentrations, may have an important role in brain injury. We used the two-vessel occlusion model with hypotension to induce transient forebrain ischemia (TFI) in rats for 10 min and then examined cystatin C immuno-like reactivity (CC-IR) after 1, 3, 7 and 14 days of recovery. Our results reveal that CC-IR was minimal or absent in the hippocampus of normal and 1 day post-ischemic animals. However, CC-IR was present in CA1 pyramidal cells and a small number of reactive glia of the stratum radiatum (SR) and stratum oriens (SO) at 3, 7 and 14 days post-ischemia. Histological assessment of the hippocampus indicates that CC-IR was localized in morphologically degenerative neurons. This distinct temporal expression of cystatin C in the rat hippocampus is concurrent with delayed neuronal death following TFI. Thus, these results indicate that cystatin C and/or its substrates may be important components of the post-ischemic neurodegenerative and repair process.

Protection against HIV-1 gp120-induced brain damage by neuronal expression of human amyloid precursor protein

Expression of the HIV-1 envelope protein gp120 in brains of transgenic (tg) mice induces extensive neurodegeneration (Toggas, S. M., E. Masliah, E. M. Rockenstein, G. F. Rall, C. R. Abraham, and L. Mucke. 1994. Nature [Lond.]. 367:188-193.). To further analyze the pathogenesis of gp120-induced neurotoxicity and to assess the neuroprotective potential of human amyloid precursor proteins (hAPPs) in vivo, different hAPP isoforms were expressed in neurons of gp120/hAPP-bigenic mice: hAPP751, which contains a Kunitz-type protease inhibitor domain, or hAPP695, which lacks this domain. Bigenic mice overexpressing hAPP751 at moderate levels showed significantly less neuronal loss, synapto-dendritic degeneration, and gliosis than singly tg mice expressing gp120 alone. In contrast, higher levels of hAPP695 expression in bigenic mice failed to prevent gp120-induced brain damage. These data indicate that hAPP can exert important neuroprotective functions in vivo and that the efficiency of this protection may depend on the hAPP isoform expressed and/or on the level of neuronal hAPP expression. Hence, molecules that mimic beneficial APP activities may be useful in the prevention/treatment of HIV-1-associated nervous system damage and, perhaps, also of other types of neural injury.