The plasmin–antiplasmin system: structural and functional aspects

The plasmin–antiplasmin system plays a key role in blood coagulation and fibrinolysis. Plasmin and α₂-antiplasmin are primarily responsible for a controlled and regulated dissolution of the fibrin polymers into soluble fragments. However, besides plasmin(ogen) and α₂-antiplasmin the system contains a series of specific activators and inhibitors. The main physiological activators of plasminogen are tissue-type plasminogen activator, which is mainly involved in the dissolution of the fibrin polymers by plasmin, and urokinase-type plasminogen activator, which is primarily responsible for the generation of plasmin activity in the intercellular space. Both activators are multidomain serine proteases. Besides the main physiological inhibitor α₂-antiplasmin, the plasmin–antiplasmin system is also regulated by the general protease inhibitor α₂-macroglobulin, a member of the protease inhibitor I39 family. The activity of the plasminogen activators is primarily regulated by the plasminogen activator inhibitors 1 and 2, members of the serine protease inhibitor superfamily.     

Recent progress in understanding the diversity of the human ov-serpin/clade B serpin family

The inhibitory mechanism against proteases is important in the maintenance of homeostasis or health in the body. The human ovalbumin serpin (ovserpin)/ clade B serpin family is one group of the human serpins, a family of serine protease inhibitors. They have acquired diversity in the profiles of target proteases, inhibitory mechanisms, and localization patterns during their evolution. Most serpins target serine proteases, however, some ov-serpins target only cysteine proteases or both serine and cysteine proteases and furthermore, several ov-serpins do not possess inhibitory activities. Although the ov-serpins act primarily as intracellular serpins, some show extracellular and nuclear localizations. Such diversity enables the ov-serpins to play multiple physiological roles in the body. Recent analyses have revealed that the functions of human ov-serpins are more diversified than we previously knew. In this article, we describe recent progress in our understanding of how the human ov-serpin/clade B serpin family demonstrates diversity.     

Matriptase and its putative role in cancer

Tumor progression and metastasis are the pathologic effects of uncontrolled or deregulated invasive growth, a process in which proteases play a fundamental role. They mediate the degradation of extracellular matrix components and intercellular cohesive structures to allow migration of the cells into the extracellular environment and activate growth and angiogenic factors. In addition to metalloproteases and the plasminogen activation system, another protease, matriptase, contributes substantially to these processes. Matriptase is a type II transmembrane trypsin-like serine protease that is expressed by cells of epithelial origin and is overexpressed in a variety of human cancers. It has been suggested that this protease not only facilitates cellular invasiveness but may also activate oncogenic pathways. This review summarizes current knowledge about matriptase, its putative role in tumor initiation and progression, and its potential as a novel target in anti-cancer therapy. Received 29 June 2006; received after revision 1 August 2006; accepted 19 September 2006     

Induction of proteolytic activity in serum by treatment with anionic detergents and organic solvents

Using casein plates as a sensitive assay for proteolytic activity, it was observed that sodium-dodecyl sulfate (SDS) and other anionic detergents induce caseinolysis when mixed with sera and plasma. Caseinolysis was dependent on the presence of plasminogen in the fluids and could be blocked by inhibitors of serine proteases and antibody to plasminogen. Similarly, organic solvents such as isopropanol induced caseinolysis after mixing with plasma, but not normal serum. Isopropanol dissociated complexes of alpha 1-antitrypsin or alpha 2-macroglobulin with trypsin preformed in vitro. As both SDS and organic solvents are widely used in biochemical investigations of biological fluids, attention should be paid to the possible induction of proteolysis.     

Cofactor-induced and mutational activity enhancement of coagulation factor VIIa

Coagulation factor VIIa (FVIIa) is an atypical member of the trypsin family of serine proteases. It fails to attain spontaneously its catalytically competent conformation and requires its protein cofactor tissue factor (TF) to accomplish this. Over a number of years, this unique behaviour of FVIIa has prompted investigations of the TF-induced activation mechanism and the zymogenicity determinants in factor VIIa. Factor VIIa has gained additional interest in the past decade because of its development into a clinically useful haemostatic agent. Here, we present an overview of the current knowledge about the TF-induced allosteric activation of FVIIa and the various molecular approaches to enhance the intrinsic activity and efficacy of FVIIa. Received 18 October 2007; received after revision 12 November 2007; accepted 14 November 2007     

The plasminogen activation system in tumor growth, invasion, and metastasis

Generation of the serine proteinase plasmin from the extracellular zymogen plasminogen can be catalyzed by either of two other serine proteinases, the urokinase- and tissue-type plasminogen activators (uPA and tPA). The plasminogen activation system also includes the serpins PAI-1 and PAI-2, and the uPA receptor (uPAR). Many findings, gathered over several decades, strongly suggest an important and causal role for uPA-catalyzed plasmin generation in cancer cell invasion through the extracellular matrix. Recent evidence suggests that the uPA system is also involved in cancer cell-directed tissue remodeling. Moreover, the system also supports cell migration and invasion by plasmin-independent mechanisms, including multiple interactions between uPA, uPAR, PAI-1, extracellular matrix proteins, integrins, endocytosis receptors, and growth factors. These interactions seem to allow temporal and spatial reorganizations of the system during cell migration and a selective degradation of extracellular matrix proteins during invasion. The increased knowledge about the plasminogen activation system may allow utilization of its components as targets for anti-invasive therapy.     

Molecular targets of non-steroidal anti-inflammatory drugs in neurodegenerative diseases

During the last decade, interest has grown in the beneficial effects of non-steroidal anti-inflammatory drugs (NSAIDs) in neurodegeneration, particularly in pathologies such as Alzheimer’s (AD) and Parkinson’s (PD) disease. Evidence from epidemiological studies has indicated a decreased risk for AD and PD in patients with a history of chronic NSAID use. However, clinical trials with NSAIDs in AD patients have yielded conflicting results, suggesting that these drugs may be beneficial only when used as preventive therapy or in early stages of the disease. NSAIDs may also have salutary effects in other neurodegenerative diseases with an inflammatory component, such as multiple sclerosis and amyotrophic lateral sclerosis. In this review we analyze the molecular (cyclooxygenases, secretases, NF-κB, PPAR, or Rho-GTPasas) and cellular (neurons, microglia, astrocytes or endothelial cells) targets of NSAIDs that may mediate the therapeutic function of these drugs in neurodegeneration. Received 4 December 2006; received after revision 24 January 2007; accepted 23 February 2007     

Serglycin – Structure and biology

Serglycin is a proteoglycan found in hematopoietic cells and endothelial cells. It has important functions related to formation of several types of storage granules. In connective tissue mast cells the covalently attached glycosaminoglycan is heparin, whereas mucosal mast cells and activated macrophages contain oversulfated chondroitin sulfate (type E). In mast cells, serglycin interact with histamine, chymase, tryptase and carboxypeptidase, in neutrophils with elastase, in cytotoxic T cells with granzyme B, in endothelial cells with tissue-type plasminogen activator and in macrophages with tumor necrosis factor-α. Serglycin is important for the retention of key inflammatory mediators inside storage granules and secretory vesicles. Serglycin can further modulate the activities of partner molecules in different ways after secretion from activated immune cells, through protection, transport, activation and interactions with substrates or target cells. Serglycin is a proteoglycan with important roles in inflammatory reactions. Received 2 October 2007; received after revision 7 November 2007; accepted 12 November 2007     

Neuroserpin: a serpin to think about

Proteinases and their inhibitors play important roles in neural development, homeostasis and disease. Neuroserpin is a member of the serine proteinase inhibitor (serpin) superfamily that is secreted from the growth cones of neurons and inhibits the enzyme tissue-type plasminogen activator (tPA). The temporal and spatial pattern of neuroserpin expression suggests a role in synaptogenesis and is most prominent in areas of the brain that participate in learning, memory and behaviour. Neuroserpin also provides neuronal protection in pathologies such as cerebral ischaemia and epilepsy by preventing excessive activity of tPA. Point mutations in neuroserpin cause aberrant conformational transitions and the formation of loop-sheet polymers that are retained within the endoplasmic reticulum of neurons, forming inclusion bodies that underlie an autosomal dominant dementia that we have called familial encephalopathy with neuroserpin inclusion bodies or FENIB. We review here the role of neuroserpin and other proteinase inhibitors in brain development, function and disease. Received 25 February 2005; received after revision 16 November 2005; accepted 28 November 2005     

Deciphering cryptic proteases

Proteases are deeply involved in physiology and pathology. For most, the mechanism is well defined but several fail to display typical protease features (as is the case of the four proteases contained in fibronectin, the inhibitor-resistant mesotrypsin and the proteosomal deubiquitinating enzyme) or have unclear physiological function (such as calpain-like proteins, transthyretin and factor seven activating protease). In other cases, such as in peroxisomal processing proteases, although substrates are defined, the enzyme remains undiscovered. Furthermore, several proteases were identified in pathological conditions, namely secretases in Alzheimers disease and gross cystic disease fluid protein 15 kDa in breast cancer, when most likely their physiological substrate is still hidden. Lastly, the evolutionary conservation of proteolytic enzymes raises questions related to the origin of biological events, such as the origin of cystein proteases and cell death responses. In this review we will discuss the above cryptic enzymes, as they will probably be relevant in the future.Received 7 December 2004; received after revision 5 January 2005; accepted 10 January 2005 Available online 09 March 2005     

The prolyl oligopeptidase family

A group of serine peptidases, the prolyl oligopeptidase family, cannot hydrolyze peptides containing more than about 30 residues. This group is unrelated to the classical trypsin and subtilisin families, and includes dipeptidyl peptidase IV, acylaminoacyl peptidase and oligopeptidase B, in addition to the prototype prolyl oligopeptidase. The recent crystal structure determination of prolyl oligopeptidase (80 kDa) has shown that the enzyme contains a peptidase domain with an α/β hydrolase fold, and its catalytic triad is covered by the central tunnel of an unusual seven-bladed β-propeller. This domain operates as a gating filter, excluding large, structured peptides from the active site. The binding mode of substrates and the catalytic mechanism differ from that of the classical serine peptidases in several features. The members of the family are important targets of drug design. Prolyl oligopeptidase is involved in amnesia, depression and blood pressure control, dipeptidyl peptidase IV in type 2 diabetes and oligopeptidase B in trypanosomiasis. Received 8 August 2001; received after revision 19 September 2001; accepted 21 September 2001     

The role of serpins in the surveillance of the secretory pathway

Serpins (serine protease inhibitors) constitute a class of proteins with an unusually wide spectrum of different functions at extracellular sites and within the nucleocytoplasmic compartment that extends from protease inhibition to hormone transport and regulation of chromatin organization. Recent investigations reveal a growing number of serpins acting in secretory pathway organelles, indicating that they are not simply cargo destined for export, but fulfill distinct roles within the classical organelle-coupled trafficking system. These findings imply that some serpins are part of a quality control system that monitors the export and possibly import routes of eukaryotic cells. The molecular targets of these serpins are often unknown, opening new avenues for future research.     

Substrate specificity of γ-secretase and other intramembrane proteases

γ-Secretase is a promiscuous protease that cleaves bitopic membrane proteins within the lipid bilayer. Elucidating both the mechanistic basis of γ-secretase proteolysis and the precise factors regulating substrate identification is important because modulation of this biochemical degradative process can have important consequences in a physiological and pathophysiological context. Here, we briefly review such information for all major classes of intramembranously cleaving proteases (I-CLiPs), with an emphasis on γ-secretase, an I-CLiP closely linked to the etiology of Alzheimer’s disease. A large body of emerging data allows us to survey the substrates of γ-secretase to ascertain the conformational features that predispose a peptide to cleavage by this enigmatic protease. Because substrate specificity in vivo is closely linked to the relative subcellular compartmentalization of γ-secretase and its substrates, we also survey the voluminous body of literature concerning the traffic of γ-secretase and its most prominent substrate, the amyloid precursor protein. Received 4 October 2007; received after revision 1 December 2007; accepted 7 December 2007     

KIF5C: A new binding partner for protein kinase CK2 with a preference for the CK2α’ subunit

Protein kinase CK2 is a highly conserved serine/threonine kinase that is ubiquitously expressed in eukaryotic cells. CK2 is a constitutively active tetrameric enzyme composed of two catalytic α and/or α’-subunits and two regulatory β-subunits. There is increasing evidence that the individual subunits may have independent functions and that they are asymmetrically distributed inside the cell. To gain a better understanding of the functions of the individual subunits, we employed a yeast-two-hybrid screen with CK2α and CK2α’. We identified the motor neuron protein KIF5C as a new binding partner for CK2. The interaction found in the yeast-two-hybrid screen was confirmed by co-sedimentation analysis on a sucrose density gradient and by co-immunoprecipitation analysis. Pull-down experiments and surface plasmon resonance spectrometry revealed a direct binding of KIF5C to CK2α’. Co-localization studies with neuroblastoma cells, bone marrow and with primary neurons confirmed the biochemical analysis that KIF5C preferentially bound to CK2α’. Received 8 August 2008; received after revision 3 November 2008; accepted 4 November 2008     

Disinhibition of neurite growth to repair the injured adult CNS: Focusing on Nogo

Investigations into mechanisms that restrict the recovery of functions after an injury to the brain or the spinal cord have led to the discovery of specific neurite growth inhibitory factors in the adult central nervous system (CNS) of mammals. Blocking their growth-suppressive function resulted in disinhibition of axonal growth, i.e. growth of cultured neurons on inhibitory CNS tissue in vitro and regeneration of injured axons in vivo. The enhanced regenerative and compensatory fibre growth was often accompanied by a substantial improvement in the functional recovery after CNS injury. The first clinical studies to assess the therapeutic potential of compounds that neutralize growth inhibitors or interfere with their downstream signalling are currently in progress. This review discusses recent advances in the understanding of how the ‘founder molecule’ Nogo-A and other glialderived growth inhibitors restrict the regeneration and repair of disrupted neuronal circuits, thus limiting the functional recovery after CNS injuries. Received 5 April 2007; received after revision 28 September 2007; accepted 1 October 2007     

Neuroactive steroids: State of the art and new perspectives

Neuroactive steroids include synthetic steroidal compounds and endogenous steroids, produced by endocrine glands (hormonal steroids) or the nervous tissue (neurosteroids), which regulate neural functions. These steroids bind to nuclear receptors or act through the activation of membrane-associated signaling pathways to modulate various important processes including the development of the nervous system, neural plasticity and the adaptive responses of neurons and glial cells under pathological conditions. Reviewed and updated in the present paper are the pleiotropic and protective abilities of neuroactive steroids. The fundamental evidence and knowledge gained constitute a profound background that offers interesting possibilities for developing effective strategies against several disorders of the nervous system. Received 3 September 2007; received after revision 24 October 2007; accepted 29 October 2007     

Aminoglycoside antibiotics: old drugs and new therapeutic approaches

Aminoglycoside antibiotics kill bacteria by binding to the ribosomal decoding site and reducing fidelity of protein synthesis. Since the discovery of these natural products over 50 years ago, aminoglycosides have provided a mainstay of antibacterial therapy of serious Gram-negative infections. In recent years, aminoglycosides have become important tools to study molecular recognition of ribonucleic acid (RNA). In an ingenious exploitation of the aminoglycosides’ mechanism of action, it has been speculated that drug-induced readthrough of premature stop codons in mutated messenger RNAs might be used to treat patients suffering from certain heritable genetic disorders. Received 23 January 2007; received after revision 25 February 2007; accepted 29 March 2007     

Understanding BACE1: essential protease for amyloid-β production in Alzheimer’s disease

The identification of the aspartic protease BACE1 (β-secretase) was a defining event in research aimed at understanding the molecular mechanisms that underlie Alzheimer’s disease (AD) pathogenesis. This is because BACE1 catalyses the rate limiting step in the production of amyloid-β (Aβ) the principal component of plaque pathology in AD, the excessive production of which is believed to be a primary cause of neurodegeneration, and cognitive dysfunction in AD. Subsequent discoveries showed that genetic deletion of BACE1 completely abolishes Aβ production and deposition in vivo, and that BACE1 activity is significantly increased in AD brain. In this review we present current knowledge on BACE1, discussing its structure, function and complex regulation with a view to understanding BACE1 function in the brain, and BACE1 as a target in blocking aberrant Aβ production in AD. Received 15 May 2008; received after revision 13 June 2008; accepted 18 June 2008     

Metabolic syndrome as a risk factor for neurological disorders

The metabolic syndrome is a cluster of common pathologies: abdominal obesity linked to an excess of visceral fat, insulin resistance, dyslipidemia and hypertension. At the molecular level, metabolic syndrome is accompanied not only by dysregulation in the expression of adipokines (cytokines and chemokines), but also by alterations in levels of leptin, a peptide hormone released by white adipose tissue. These changes modulate immune response and inflammation that lead to alterations in the hypothalamic ‘bodyweight/appetite/satiety set point,’ resulting in the initiation and development of metabolic syndrome. Metabolic syndrome is a risk factor for neurological disorders such as stroke, depression and Alzheimer’s disease. The molecular mechanism underlying the mirror relationship between metabolic syndrome and neurological disorders is not fully understood. However, it is becoming increasingly evident that all cellular and biochemical alterations observed in metabolic syndrome like impairment of endothelial cell function, abnormality in essential fatty acid metabolism and alterations in lipid mediators along with abnormal insulin/leptin signaling may represent a pathological bridge between metabolic syndrome and neurological disorders such as stroke, Alzheimer’s disease and depression. The purpose of this review is not only to describe the involvement of brain in the pathogenesis of metabolic syndrome, but also to link the pathogenesis of metabolic syndrome with neurochemical changes in stroke, Alzheimer’s disease and depression to a wider audience of neuroscientists with the hope that this discussion will initiate more studies on the relationship between metabolic syndrome and neurological disorders.