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

The Role of Cysteine Protease Cathepsins B, H, C, and X/Z in Neurodegenerative Diseases and Cancer

Papain-like cysteine proteases are composed of 11 human cysteine cathepsins, originally located in the lysosomes. They exhibit broad specificity and act as endopeptidases and/or exopeptidases. Among them, only cathepsins B, H, C, and X/Z exhibit exopeptidase activity. Recently, cysteine cathepsins have been found to be present outside the lysosomes and often participate in various pathological processes. Hence, they have been considered key signalling molecules. Their potentially hazardous proteolytic activities are tightly regulated. This review aims to discuss recent advances in understanding the structural aspects of these four cathepsins, mechanisms of their zymogen activation, regulation of their activities, and functional aspects of these enzymes in neurodegeneration and cancer. Neurodegenerative effects have been evaluated, particularly in Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, multiple sclerosis, and neuropsychiatric disorders. Cysteine cathepsins also participate in tumour progression and metastasis through the overexpression and secretion of proteases, which trigger extracellular matrix degradation. To our knowledge, this is the first review to provide an in-depth analysis regarding the roles of cysteine cathepsins B, H, C, and X in neurodegenerative diseases and cancer. Further advances in understanding the functions of cysteine cathepsins in these conditions will result in the development of novel, targeted therapeutic strategies.

Lysosomal peptidases – Intriguing roles in cancer progression and neurodegeneration

Lysosomal peptidases are hydrolytic enzymes capable of digesting waste proteins that are targeted to lysosomes via endocytosis and autophagy. Besides intracellular protein catabolism, they play more specific roles in several other cellular processes and pathologies, either within lysosomes, upon secretion into the cell cytoplasm or extracellular space, or bound to the plasma membrane. In cancer, lysosomal peptidases are generally associated with disease progression, as they participate in crucial processes leading to changes in cell morphology, signaling, migration, and invasion, and finally metastasis. However, they can also enhance the mechanisms resulting in cancer regression, such as apoptosis of tumor cells or antitumor immune responses. Lysosomal peptidases have also been identified as hallmarks of aging and neurodegeneration, playing roles in oxidative stress, mitochondrial dysfunction, abnormal intercellular communication, dysregulated trafficking, and the deposition of protein aggregates in neuronal cells. Furthermore, deficiencies in lysosomal peptidases may result in other pathological states, such as lysosomal storage disease. The aim of this review was to highlight the role of lysosomal peptidases in particular pathological processes of cancer and neurodegeneration and to address the potential of lysosomal peptidases in diagnosing and treating patients.

Upregulation of Cysteine Protease Cathepsin X in the 6-Hydroxydopamine Model of Parkinson's Disease

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by loss of midbrain dopaminergic neurons in the substantia nigra pars compacta (SNc). In vitro, a contribution to neuroinflammation and neurotoxicity has been shown for the lysosomal protease cathepsin X; however, its expression and its role in PD remain unknown. Therefore, the current study was designed to address the regional, cellular, and subcellular localization and activity of cathepsin X in hemi-parkinsonian rats with 6-hydroxydopamine (6-OHDA)-induced excitotoxicity in the unilateral medial forebrain bundle (MFB) lesion. We report for the first time that cathepsin X expression and activity are rapidly increased in the ipsilateral SNc after injection of 6-OHDA into the MFB reaching a maximum after 12 h but seem to stay strongly upregulated after 4 weeks after injection. At early time points of 6-OHDA injection into the MFB, the increased cathepsin X is localized in the lysosomes in the neuronal, predominantly tyrosine hydroxylase-positive dopaminergic cells. After 12 h of 6-OHDA induced lesion, only a few activated microglial cells are positive for cathepsin X whereas, in 4 weeks post-lesion accompanied with complete loss of dopaminergic neurons, there is persistent cathepsin X upregulation restricted to activated glia cells. Taken together, our results demonstrate that cathepsin X upregulation in the lesioned dopaminergic system may play a role as a pathogenic factor in PD. Moreover, inhibition of cathepsin X expression or activity may be useful in protecting the nigrostriatal dopaminergic projection in the PD.

Lysosomal cell death mechanisms in aging

Lysosomes are degradative organelles essential for cell homeostasis that regulate a variety of processes, from calcium signaling and nutrient responses to autophagic degradation of intracellular components. Lysosomal cell death is mediated by the lethal effects of cathepsins, which are released into the cytoplasm following lysosomal damage. This process of lysosomal membrane permeabilization and cathepsin release is observed in several physiopathological conditions and plays a role in tissue remodeling, the immune response to intracellular pathogens and neurodegenerative diseases. Many evidences indicate that aging strongly influences lysosomal activity by altering the physical and chemical properties of these organelles, rendering them more sensitive to stress. In this review we focus on how aging alters lysosomal function and increases cell sensitivity to lysosomal membrane permeabilization and lysosomal cell death, both in physiological conditions and age-related pathologies.

Cystatin C is a disease-associated protein subject to multiple regulation

A protease inhibitor, cystatin C (Cst C), is a secreted cysteine protease inhibitor abundantly expressed in body fluids. Clinically, it is mostly used to measure glomerular filtration rate as a marker for kidney function due to its relatively small molecular weight and easy detection. However, recent findings suggest that Cst C is regulated at both transcriptional and post‐translational levels, and Cst C production from haematopoietic cell lineages contributes significantly to the systematic pools of Cst C. Furthermore, Cst C is directly linked to many pathologic processes through various mechanisms. Thus fluctuation of Cst C levels might have serious clinical implications rather than a mere reflection of kidney functions. Here, we summarize the pathophysiological roles of Cst C dependent and independent on its inhibition of proteases, outline its change of expression by various stimuli, and elucidate the regulatory mechanisms to control this disease‐related protease inhibitor. Finally, we discuss the clinical implications of these findings for translational gains.

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.

Cathepsin X promotes 6-hydroxydopamine-induced apoptosis of PC12 and SH-SY5Y cells

The cysteine carboxypeptidase cathepsin X is an important player in degenerative processes under normal ageing and pathological conditions. In the present study, we investigated the potential role of cathepsin X in 6-hydroxydopamine (6-OHDA)-induced toxicity in the pheochromocytoma cell line PC12 and neuroblastoma cell line SH-SY5Y. Cells exposed to 6-OHDA demonstrated alterations in the protein level of cathepsin X and activity of cathepsin X. Downregulation of cathepsin X expression by siRNA attenuated the neuronal death caused by 6-OHDA. Treatment with specific cathepsin X inhibitor AMS36 protected cells against 6-OHDA mediated cytotoxicity, resulting in reduced cell death and apoptosis. Furthermore, AMS36 reversed 6-OHDA-induced loss of tyrosine hydroxylase and attenuated 6-OHDA-induced activation of caspase-3, triggering apoptosis, intracellular generation of reactive oxygen species and mitochondrial dysfunction, including the release of cytochrome c and an imbalanced Bax/Bcl-2 ratio. Moreover, AMS36 interfered with NF-κB activation by blocking degradation of IκBα, preventing NF-κB translocation to the nucleus. Our data provide the first evidence that inhibition of cathepsin X protects both, PC12 and SH-SY5Y cells against 6-OHDA toxicity and indicate that cathepsin X may be responsible for dopamine neuron death, involved in the pathogenic cascade event for the neurodegenerative disorders, such as Parkinson's disease.

Predominant release of lysosomal enzymes by newborn rat microglia after LPS treatment revealed by proteomic studies

Growing evidence suggest that microglia may play an important role in the pathogenesis of neurodegenerative disease including Parkinson's disease, Alzheimer's disease, and so forth. The activation of microglia may cause neuronal damage through the release of reactive oxygen species and proinflammatory cytokines. However, the early response of microglial cells remains unclear before cells can secrete the proinflammatory cytokines. Here, a time course analysis showed the earliest expression of inducible nitric oxide synthase and cyclooxygenase-2 at 3 and 24 h following lipopolysaccharide (LPS) treatment. To further define initial response proteins of microglia after LPS treatment, we utilized a novel mass spectrometry-based quantitative proteomic technique termed SILAC (for stable isotope labeling by amino acids in cell culture) to compare the protein profiles of the cell culture-conditioned media of 1 h LPS-treated microglia as compared with controls. The proteomic analysis identified 77 secreted proteins using SignalP; of these, 28 proteins were associated with lysosome of cells and 13 lysosome-related proteins displayed significant changes in the relative abundance after 1 h LPS treatment. Four proteins were further evaluated with Western blot, demonstrating good agreement with quantitative proteomic data. These results suggested that microglia first released some lysosomal enzymes which may be involved in neuronal damage process. Furthermore, ammonium chloride, which inhibits microglia lysosomal enzyme activity, could prevent microglia from causing neuronal injury. Hence, in addition to the numerous novel proteins that are potentially important in microglial activation-mediated neurodegeneration revealed by the search, the study has indicated that the early release of lysosomal enzymes in microglial cells would contribute to LPS-activated inflammatory response.

Hydrogen peroxide induces lysosomal protease alterations in PC12 cells

Alterations in lysosomal proteases have been implicated in many neurodegenerative diseases. The current study demonstrates a concentration-dependent decrease in PC12 cell viability and transient changes in cystatin C (CYSC), cathepsin B (CATB), cathepsin D (CATD) and caspase-3 following exposure to H2O2. Furthermore, activation of CATD occurred following exposure to H2O2 and cysteine protease suppression, while inhibition of CATD with pepstatin A significantly improved cell viability. Additionally, significant PARP cleavage, suggestive of caspase-3-like activity, was observed following H2O2 exposure, while inhibition of caspase-3 significantly increased cell viability compared to H2O2 administration alone. Collectively, our data suggest that H2O2 induced cell death is regulated at least in part by caspase-3 and CATD. Furthermore, cysteine protease suppression increases CATD expression and activity. These studies provide insight for alternate pathways and potential therapeutic targets of cell death associated with oxidative stress and lysosomal protease alterations.

Neurotoxins and neurotoxicity mechanisms. an overview

Neurotoxins represent unique chemical tools, providing a means to 1) gain insight into cellular mechanisms of apopotosis and necrosis, 2) achieve a morphological template for studies otherwise unattainable, 3) specifically produce a singular phenotype of denervation, and 4) provide the starting point to delve into processes and mechanisms of nerve regeneration and sprouting. There are many other notable uses of neurotoxins in neuroscience research, and ever more being discovered each year. The objective of this review paper is to highlight the broad areas of neuroscience in which neurotoxins and neurotoxicity mechanism come into play. This shifts the focus away from neurotoxins per se, and onto the major problems under study today. Neurotoxins broadly defined are used to explore neurodegenerative disorders, psychiatric disorders and substance use disorders. Neurotoxic mechanisms relating to protein aggregates are indigenous to Alzheimer disease, Parkinson's disease. NeuroAIDS is a disorder in which microglia and macrophages have enormous import. The gap between the immune system and nervous system has been bridged, as neuroinflammation is now considered to be part of the neurodegenerative process. Related mechanisms now arise in the process of neurogenesis. Accordingly, the entire spectrum of neuroscience is within the purview of neurotoxins and neurotoxicity mechanisms. Highlights on discoveries in the areas noted, and on selective neurotoxins, are included, mainly from the past 2 to 3 years.