Critical role for PAR1 in kallikrein 6-mediated oligodendrogliopathy

Kallikrein-related peptidase's significance in Alzheimer's disease pathogenesis: A comprehensive survey

Alzheimer's disease (AD) and related dementias constitute an important global health challenge. Detailed understanding of the multiple molecular mechanisms underlying their pathogenesis constitutes a clue for the management of the disease. Kallikrein-related peptidases (KLKs), a lead family of serine proteases, have emerged as potential biomarkers and therapeutic targets in the context of AD and associated cognitive decline. Hence, KLKs were proposed to display multifaceted impacts influencing various aspects of neurodegeneration, including amyloid-beta aggregation, tau pathology, neuroinflammation, and synaptic dysfunction. We propose here a comprehensive survey to summarize recent findings, providing an overview of the main kallikreins implicated in AD pathophysiology namely KLK8, KLK6 and KLK7. We explore the interplay between KLKs and key AD molecular pathways, shedding light on their significance as potential biomarkers for early disease detection. We also discuss their pertinence as therapeutic targets for disease-modifying interventions to develop innovative therapeutic strategies aimed at halting or ameliorating the progression of AD and associated dementias.

Cytoprotective E-WE thrombin reduces disease severity in a murine model of relapsing-remitting multiple sclerosis

The blood-brain barrier is composed of microvascular endothelial cells, immune cells, and astrocytes that work in concert with the coagulation cascade to control inflammation and immune cell infiltration into the central nervous system. Endothelial cell dysfunction leading to increased permeability and compromised barrier function are hallmarks of neuroinflammatory and autoimmune disorders, including multiple sclerosis (MS). Therapeutic strategies that improve or protect endothelial barrier function may be beneficial in the treatment or prevention of neuroinflammatory diseases. We therefore tested the hypothesis that biasing thrombin toward anticoagulant and cytoprotective activities would provide equivalent or even additive benefit compared with standard-of-care therapeutic strategies, including corticosteroids. In a mouse model of relapsing-remitting MS, treatment with the thrombin mutant, E-WE thrombin, an engineered thrombin mutant with cytoprotective activities that is biased toward anticoagulant and cytoprotective activity, reduced neuroinflammation and extracellular fibrin formation in SJL mice inoculated with proteolipid protein (PLP) peptide. When administered at the onset of detectable disease, E-WE thrombin significantly improved the disease severity of the initial attack as well as the relapse and delayed the onset of relapse to a similar extent as observed with methylprednisolone. Both methylprednisolone and E-WE thrombin reduced demyelination and immune cell recruitment. These results provide rationale for considering engineered forms of thrombin biased toward anticoagulant and cytoprotective activity as a therapeutic strategy and perhaps an effective alternative to high-dose methylprednisolone for the management of acute relapsing MS attacks.NEW & NOTEWORTHY There are limited treatment options for mitigating acute relapsing attacks for patients with multiple sclerosis. We tested the hypothesis that harnessing the cytoprotective activity of the blood coagulation enzyme, thrombin, would provide benefit and protection against relapsing disease in a mouse model of MS. Our results provide rationale for considering engineered forms of thrombin biased toward cytoprotective activity as a therapeutic strategy and perhaps an alternative to steroids for the management of relapsing MS attacks.

E-WE thrombin, a protein C activator, reduces disease severity and spinal cord inflammation in relapsing-remitting murine experimental autoimmune encephalomyelitis

Objective

Relapses in patients with relapsing-remitting multiple sclerosis (RRMS) are typically treated with high-dose corticosteroids including methylprednisolone. However, high-dose corticosteroids are associated with significant adverse effects, can increase the risk for other morbidities, and often do not impact disease course. Multiple mechanisms are proposed to contribute to acute relapses in RRMS patients, including neuroinflammation, fibrin formation and compromised blood vessel barrier function. The protein C activator, E-WE thrombin is a recombinant therapeutic in clinical development for its antithrombotic and cytoprotective properties, including protection of endothelial cell barrier function. In mice, treatment with E-WE thrombin reduced neuroinflammation and extracellular fibrin formation in myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE). We therefore tested the hypothesis that E-WE thrombin could reduce disease severity in a relapsing-remitting model of EAE.

Methods

Female SJL mice were inoculated with proteolipid protein (PLP) peptide and treated with E-WE thrombin (25 μg/kg; iv) or vehicle at onset of detectable disease. In other experiments, E-WE thrombin was compared to methylprednisolone (100 mg/kg; iv) or the combination of both.

Results

Compared to vehicle, administration of E-WE thrombin significantly improved disease severity of the initial attack and relapse and delayed onset of relapse as effectively as methylprednisolone. Both methylprednisolone and E-WE thrombin reduced demyelination and immune cell recruitment, and the combination of both treatments had an additive effect.

Conclusion

The data presented herein demonstrate that E-WE thrombin is protective in mice with relapsing-remitting EAE, a widely used model of MS. Our data indicate that E-WE thrombin is as effective as high-dose methylprednisolone in improving disease score and may exert additional benefit when administered in combination. Taken together, these data suggest that E-WE thrombin may be an effective alternative to high-dose methylprednisolone for managing acute MS attacks.

Thrombin increases the expression of cholesterol 25-hydroxylase in rat astrocytes after spinal cord injury

Astrocytes are important cellular centers of cholesterol synthesis and metabolism that help maintain normal physiological function at the organism level. Spinal cord injury results in aberrant cholesterol metabolism by astrocytes and excessive production of oxysterols, which have profound effects on neuropathology. 25-Hydroxycholesterol (25-HC), the main product of the membrane-associated enzyme cholesterol-25-hydroxylase (CH25H), plays important roles in mediating neuroinflammation. However, whether the abnormal astrocyte cholesterol metabolism induced by spinal cord injury contributes to the production of 25-HC, as well as the resulting pathological effects, remain unclear. In the present study, spinal cord injury-induced activation of thrombin was found to increase astrocyte CH25H expression. A protease-activated receptor 1 inhibitor was able to attenuate this effect in vitro and in vivo. In cultured primary astrocytes, thrombin interacted with protease-activated receptor 1, mainly through activation of the mitogen-activated protein kinase/nuclear factor-kappa B signaling pathway. Conditioned culture medium from astrocytes in which ch25h expression had been knocked down by siRNA reduced macrophage migration. Finally, injection of the protease activated receptor 1 inhibitor SCH79797 into rat neural sheaths following spinal cord injury reduced migration of microglia/macrophages to the injured site and largely restored motor function. Our results demonstrate a novel regulatory mechanism for thrombin-regulated cholesterol metabolism in astrocytes that could be used to develop anti-inflammatory drugs to treat patients with spinal cord injury.

Thrombin acts as inducer of proinflammatory macrophage migration inhibitory factor in astrocytes following rat spinal cord injury

BACKGROUND

The danger-associated molecular patterns (DAMPs) are critical contributors to the progressive neuropathology and thereafter affect the functional outcomes following spinal cord injury (SCI). Up to now, the regulatory mechanisms on their inducible production from the living cells remain elusive, aside from their passive release from the necrotic cells. Thrombin is immediately activated by the damaged or stressed central nervous system (CNS), which potently mediates inflammatory astrocytic responses through proteolytic cleavage of protease-activated receptors (PARs). Therefore, SCI-activated thrombin is conceived to induce the production of DAMPs from astrocytes at lesion site.

METHODS

Rat SCI model was established by the cord contusion at T8-T10. The expression of thrombin and macrophage migration inhibitory factor (MIF) was determined by ELISA and Western blot. The PAR1, PAR3, and PAR4 receptors of thrombin were examined by PCR and immunohistochemistry. Primary astrocytes were isolated and purified from the spinal cord, followed by stimulation with different concentrations of thrombin either for transcriptome sequencing or for analysis of thrombin-mediated expression of MIF and related signal pathways in the presence or absence of various inhibitors. The post-injury locomotor functions were assessed using the Basso, Beattie, and Bresnahan (BBB) locomotor scale.

RESULTS

MIF protein levels were significantly elevated in parallel with those of thrombin induced by SCI. Immunostaining demonstrated that PAR1 receptor, together with MIF, was abundantly expressed in astrocytes. By transcriptome sequencing and bioinformatical analysis of thrombin-stimulated primary astrocytes, MIF was identified to be dynamically regulated by the serine protease. Investigation of the underlying mechanism using various inhibitors revealed that thrombin-activated PAR1 was responsible for the MIF production of astrocytes through modulation of JNK/NFκB pathway. Administration of PAR1 inhibitor at lesion sites following SCI significantly reduced the protein levels of MIF and ameliorated functional deficits of rat locomotion.

CONCLUSION

SCI-activated thrombin is a robust inducer of MIF production from astrocytes. Exploring the roles of thrombin in promoting the production of DAMPs from astrocytes at lesion site will provide an alternative strategy for the clinical therapy of CNS inflammation.

Thrombin induces morphological and inflammatory astrocytic responses via activation of PAR1 receptor

Spinal cord injury (SCI) will result in the significant elevation of thrombin production at lesion site via either breakage of blood-spinal cord barrier or upregulated expression within nerve cells. Thrombin-induced activation of the protease activated receptors (PARs) evokes various pathological effects that deteriorate the functional outcomes of the injured cord. The cellular consequences of thrombin action on the astrocytes, as well as the underlying mechanism are not fully elucidated by far. In the present study, SCI model of rats was established by contusion, and primary astrocytes were isolated for culture from newborn rats. The expression levels of thrombin and PAR1 receptor at lesion sites of the spinal cord were determined. The primary astrocytes cultured in vitro were stimulated with different concentration of thrombin, and the resultant morphological changes, inflammatory astrocytic responses, as well as PAR1-activated signal pathway of astrocytes were accordingly examined using various agonists or antagonists of the receptor. Thrombin was found to reverse astrocytic stellation, promote proliferation but inhibit migration of astrocytes. Furthermore, the serine protease was shown to facilitate inflammatory response of astrocytes through regulation of MAPKs/NFκB pathway. Our results have provided the morphological evidence of astrocytic reactivity in response to thrombin stimulation and its neuroinflammatory effects following SCI, which will be indicative for the fundamental insights of thrombin-induced neuropathology.

Blocking Kallikrein 6 promotes developmental myelination

Kallikrein related peptidase 6 (Klk6) is a secreted serine protease highly expressed in oligodendrocytes and implicated in demyelinating conditions. To gain insights into the significance of Klk6 to oligodendrocyte biology, we investigated the impact of global Klk6 gene knockout on CNS developmental myelination using the spinal cord of male and female mice as a model. Results demonstrate that constitutive loss of Klk6 expression accelerates oligodendrocyte differentiation developmentally, including increases in the expression of myelin proteins such as MBP, PLP and CNPase, in the number of CC-1+ mature oligodendrocytes, and myelin thickness by the end of the first postnatal week. Co-ordinate elevations in the pro-myelinating signaling pathways ERK and AKT, expression of fatty acid 2-hydroxylase, and myelin regulatory transcription factor were also observed in the spinal cord of 7d Klk6 knockouts. LC/MS/MS quantification of spinal cord lipids showed sphingosine and sphingomyelins to be elevated in Klk6 knockouts at the peak of myelination. Oligodendrocyte progenitor cells (OPCs)-derived from Klk6 knockouts, or wild type OPCs-treated with a Klk6 inhibitor (DFKZ-251), also showed increased MBP and PLP. Moreover, inhibition of Klk6 in OPC cultures enhanced brain derived neurotrophic factor-driven differentiation. Altogether, these findings suggest that oligodendrocyte-derived Klk6 may operate as an autocrine or paracrine rheostat, or brake, on pro-myelinating signaling serving to regulate myelin homeostasis developmentally and in the adult. These findings document for the first time that inhibition of Klk6 globally, or specifically in oligodendrocyte progenitors, is a strategy to increase early stages of oligodendrocyte differentiation and myelin production in the CNS.

Neurocoagulation from a Mechanistic Point of View in the Central Nervous System

Coagulation mechanisms are critical for maintaining homeostasis in the central nervous system (CNS). Thrombin, an important player of the coagulation cascade, activates protease activator receptors (PARs), members of the G-protein coupled receptor family. PAR1 is located on neurons and glia. Following thrombin activation, PAR1 signals through the extracellular signal-regulated kinase pathway, causing alterations in neuronal glutamate release and astrocytic morphological changes. Similarly, the anticoagulation factor activated protein C (aPC) can cleave PAR1, following interaction with the endothelial protein C receptor. Both thrombin and aPC are expressed on endothelial cells and pericytes in the blood-brain barrier (BBB). Thrombin-induced PAR1 activation increases cytosolic Ca²⁺ concentration in brain vessels, resulting in nitric oxide release and increasing F-actin stress fibers, damaging BBB integrity. aPC also induces PAR1 activation and preserves BBB vascular integrity via coupling to sphingosine 1 phosphate receptors. Thrombin-induced PAR1 overactivation and BBB disruption are evident in CNS pathologies. During epileptic seizures, BBB disruption promotes thrombin penetration. Thrombin induces PAR1 activation and potentiates N-methyl-D-aspartate receptors, inducing glutamate-mediated hyperexcitability. Specific PAR1 inhibition decreases status epilepticus severity in vivo. In stroke, the elevation of brain thrombin levels further compromises BBB integrity, with direct parenchymal damage, while systemic factor Xa inhibition improves neurological outcomes. In multiple sclerosis (MS), brain thrombin inhibitory capacity correlates with clinical presentation. Both thrombin inhibition by hirudin and the use of recombinant aPC improve disease severity in an MS animal model. This review presents the mechanisms underlying the effects of coagulation on the physiology and pathophysiology of the CNS.

Translatable pathways classification (TransPath-C) for inferring processes germane to human biology from animal studies data: example application in neurobiology

How to translate insights gained from studies in one organismal species for what is most likely to be germane in another species, such as from mice to humans, is a ubiquitous challenge in basic biology as well as biomedicine. This is an especially difficult problem when there are few molecular features that are obviously important in both species for a given phenotype of interest. Neuropathologies are a prominent realm of this complication. Schizophrenia is complex psychiatric disorder that affects 1% of the population. Many genetic factors have been proposed to drive the development of schizophrenia, and the 22q11 microdeletion (MD) syndrome has been shown to dramatically increase this risk. Due to heterogeneity of presentation of symptoms, diagnosis and formulation of treatment options for patients can often be delayed, and there is an urgent need for novel therapeutics directed toward the treatment of schizophrenia. Here, we present a novel computational approach, Translational Pathways Classification (TransPath-C), that can be used to identify shared pathway dysregulation between mouse models and human schizophrenia cohorts. This method uses variation of pathway activation in the mouse model to predict both mouse and human disease phenotype. Analysis of shared dysregulated pathways called out by both the mouse and human classifiers of TransPath-C can identify pathways that can be targeted in both preclinical and human cohorts of schizophrenia. In application to the 22q11 MD mouse model, our findings suggest that PAR1 pathway activation found upregulated in this mouse phenotype is germane for the corresponding human schizophrenia cohort such that inhibition of PAR1 may offer a novel therapeutic target.

Protease Activated Receptor 1 and Its Ligands as Main Regulators of the Regeneration of Peripheral Nerves

In contrast with the brain and spinal cord, peripheral nerves possess a striking ability to regenerate after damage. This characteristic of the peripheral nervous system is mainly due to a specific population of glial cells, the Schwann cells. Schwann cells promptly activate after nerve injury, dedifferentiate assuming a repair phenotype, and assist axon regrowth. In general, tissue injury determines the release of a variety of proteases which, in parallel with the degradation of their specific targets, also activate plasma membrane receptors known as protease-activated receptors (PARs). PAR1, the prototypical member of the PAR family, is also known as thrombin receptor and is present at the Schwann cell plasma membrane. This receptor is emerging as a possible regulator of the pro-regenerative capacity of Schwann cells. Here, we summarize the most recent literature data describing the possible contribution of PAR1 and PAR1-activating proteases in regulating the regeneration of peripheral nerves.

Critical Role of Astrocyte NAD+ Glycohydrolase in Myelin Injury and Regeneration

Western-style diets cause disruptions in myelinating cells and astrocytes within the mouse CNS. Increased CD38 expression is present in the cuprizone and experimental autoimmune encephalomyelitis models of demyelination and CD38 is the main nicotinamide adenine dinucleotide (NAD⁺)-depleting enzyme in the CNS. Altered NAD⁺ metabolism is linked to both high fat consumption and multiple sclerosis (MS). Here, we identify increased CD38 expression in the male mouse spinal cord following chronic high fat consumption, after focal toxin [lysolecithin (LL)]-mediated demyelinating injury, and in reactive astrocytes within active MS lesions. We demonstrate that CD38 catalytically inactive mice are substantially protected from high fat-induced NAD⁺ depletion, oligodendrocyte loss, oxidative damage, and astrogliosis. A CD38 inhibitor, 78c, increased NAD⁺ and attenuated neuroinflammatory changes induced by saturated fat applied to astrocyte cultures. Conditioned media from saturated fat-exposed astrocytes applied to oligodendrocyte cultures impaired myelin protein production, suggesting astrocyte-driven indirect mechanisms of oligodendrogliopathy. In cerebellar organotypic slice cultures subject to LL-demyelination, saturated fat impaired signs of remyelination effects that were mitigated by concomitant 78c treatment. Significantly, oral 78c increased counts of oligodendrocytes and remyelinated axons after focal LL-induced spinal cord demyelination. Using a RiboTag approach, we identified a unique in vivo brain astrocyte translatome profile induced by 78c-mediated CD38 inhibition in mice, including decreased expression of proinflammatory astrocyte markers and increased growth factors. Our findings suggest that a high-fat diet impairs oligodendrocyte survival and differentiation through astrocyte-linked mechanisms mediated by the NAD⁺ase CD38 and highlights CD38 inhibitors as potential therapeutic candidates to improve myelin regeneration.SIGNIFICANCE STATEMENT Myelin disturbances and oligodendrocyte loss can leave axons vulnerable, leading to permanent neurologic deficits. The results of this study suggest that metabolic disturbances, triggered by consumption of a diet high in fat, promote oligodendrogliopathy and impair myelin regeneration through astrocyte-linked indirect nicotinamide adenine dinucleotide (NAD⁺)-dependent mechanisms. We demonstrate that restoring NAD⁺ levels via genetic inactivation of CD38 can overcome these effects. Moreover, we show that therapeutic inactivation of CD38 can enhance myelin regeneration. Together, these findings point to a new metabolic targeting strategy positioned to improve disease course in multiple sclerosis and other conditions in which the integrity of myelin is a key concern.

The thrombin receptor modulates astroglia-neuron trophic coupling and neural repair after spinal cord injury

Excessive activation of the thrombin receptor, protease activated receptor 1 (PAR1) is implicated in diverse neuropathologies from neurodegenerative conditions to neurotrauma. PAR1 knockout mice show improved outcomes after experimental spinal cord injury (SCI), however information regarding the underpinning cellular and molecular mechanisms is lacking. Here we demonstrate that genetic blockade of PAR1 in female mice results in improvements in sensorimotor co-ordination after thoracic spinal cord lateral compression injury. We document improved neuron preservation with increases in Synapsin-1 presynaptic proteins and GAP43, a growth cone marker, after a 30 days recovery period. These improvements were coupled to signs of enhanced myelin resiliency and repair, including increases in the number of mature oligodendrocytes, their progenitors and the abundance of myelin basic protein. These significant increases in substrates for neural recovery were accompanied by reduced astrocyte (Serp1) and microglial/monocyte (CD68 and iNOS) pro-inflammatory markers, with coordinate increases in astrocyte (S100A10 and Emp1) and microglial (Arg1) markers reflective of pro-repair activities. Complementary astrocyte-neuron co-culture bioassays suggest astrocytes with PAR1 loss-of-function promote both neuron survival and neurite outgrowth. Additionally, the pro-neurite outgrowth effects of switching off astrocyte PAR1 were blocked by inhibiting TrkB, the high affinity receptor for brain derived neurotrophic factor. Altogether, these studies demonstrate unique modulatory roles for PAR1 in regulating glial-neuron interactions, including the capacity for neurotrophic factor signaling, and underscore its position at neurobiological intersections critical for the response of the CNS to injury and the capacity for regenerative repair and restoration of function.

Identification of First-in-Class Inhibitors of Kallikrein-Related Peptidase 6 That Promote Oligodendrocyte Differentiation

Multiple sclerosis (MS) is an autoimmune demyelinating disease of the central nervous system (CNS) that causes severe motor, sensory, and cognitive impairments. Kallikrein-related peptidase (KLK)6 is the most abundant serine protease secreted in the CNS, mainly by oligodendrocytes, the myelin-producing cells of the CNS, and KLK6 is assumed to be a robust biomarker of MS, since it is highly increased in the cerebrospinal fluid (CSF) of MS patients. Here, we report the design and biological evaluation of KLK6's low-molecular-weight inhibitors, para-aminobenzyl derivatives. Interestingly, selected hit compounds were selective of the KLK6 proteolytic network encompassing KLK1 and plasmin that also participate in the development of MS physiopathology. Moreover, hits were found noncytotoxic on primary cultures of murine neurons and oligodendrocyte precursor cells (OPCs). Among them, two compounds (32 and 42) were shown to promote the differentiation of OPCs into mature oligodendrocytes in vitro constituting thus emerging leads for the development of regenerative therapies.

The thrombin receptor links brain derived neurotrophic factor to neuron cholesterol production, resiliency and repair after spinal cord injury

Despite concerted efforts to identify CNS regeneration strategies, an incomplete understanding of how the needed molecular machinery is regulated limits progress. Here we use models of lateral compression and FEJOTA clip contusion-compression spinal cord injury (SCI) to identify the thrombin receptor (Protease Activated Receptor 1 (PAR1)) as an integral facet of this machine with roles in regulating neurite growth through a growth factor- and cholesterol-dependent mechanism. Functional recovery and signs of neural repair, including expression of cholesterol biosynthesis machinery and markers of axonal and synaptic integrity, were all increased after SCI in PAR1 knockout female mice, while PTEN was decreased. Notably, PAR1 differentially regulated HMGCS1, a gene encoding a rate-limiting enzyme in cholesterol production, across the neuronal and astroglial compartments of the intact versus injured spinal cord. Pharmacologic inhibition of cortical neuron PAR1 using vorapaxar in vitro also decreased PTEN and promoted neurite outgrowth in a cholesterol dependent manner, including that driven by suboptimal brain derived neurotrophic factor (BDNF). Pharmacologic inhibition of PAR1 also augmented BDNF-driven HMGCS1 and cholesterol production by murine cortical neurons and by human SH-SY5Y and iPSC-derived neurons. The link between PAR1, cholesterol and BDNF was further highlighted by demonstrating that the deleterious effects of PAR1 over-activation are overcome by supplementing cultures with BDNF, cholesterol or by blocking an inhibitor of adenylate cyclase, Gαi. These findings document PAR1-linked neurotrophic coupling mechanisms that regulate neuronal cholesterol metabolism as an important component of the machinery regulating CNS repair and point to new strategies to enhance neural resiliency after injury.

Thrombin generation and activity in multiple sclerosis

The coagulation cascade and immune system are intricately linked, highly regulated and respond cooperatively in response to injury and infection. Increasingly, evidence of hyper-coagulation has been associated with autoimmune disorders, including multiple sclerosis (MS). The pathophysiology of MS includes immune cell activation and recruitment to the central nervous system (CNS) where they degrade myelin sheaths, leaving neuronal axons exposed to damaging inflammatory mediators. Breakdown of the blood-brain barrier (BBB) facilitates the entry of peripheral immune cells. Evidence of thrombin activity has been identified within the CNS of MS patients and studies using animal models of experimental autoimmune encephalomyelitis (EAE), suggest increased thrombin generation and activity may play a role in the pathogenesis of MS as well as inhibit remyelination processes. Thrombin is a serine protease capable of cleaving multiple substrates, including protease activated receptors (PARs), fibrinogen, and protein C. Cleavage of all three of these substrates represent pathways through which thrombin activity may exert immuno-regulatory effects and regulate permeability of the BBB during MS and EAE. In this review, we summarize evidence that thrombin activity directly, through PARs, and indirectly, through fibrin formation and activation of protein C influences neuro-immune responses associated with MS and EAE pathology.

Extracellular proteolysis in glioblastoma progression and therapeutics

Gliomas encompass highly invasive primary central nervous system (CNS) tumours of glial cell origin with an often-poor clinical prognosis. Of all gliomas, glioblastoma is the most aggressive form of primary brain cancer. Current treatments in glioblastoma are insufficient due to the invasive nature of brain tumour cells, which typically results in local tumour recurrence following treatment. The latter represents the most important cause of mortality in glioblastoma and underscores the necessity for an in-depth understanding of the underlying mechanisms. Interestingly, increased synthesis and secretion of several proteolytic enzymes within the tumour microenvironment, such as matrix metalloproteinases, lysosomal proteases, cathepsins and kallikreins for extracellular-matrix component degradation may play a major role in the aforementioned glioblastoma invasion mechanisms. These proteolytic networks are key players in establishing and maintaining a tumour microenvironment that promotes tumour cell survival, proliferation, and migration. Indeed, the targeted inhibition of these proteolytic enzymes has been a promisingly useful therapeutic strategy for glioblastoma management in both preclinical and clinical development. We hereby summarize current advances on the biology of the glioblastoma tumour microenvironment, with a particular emphasis on the role of proteolytic enzyme families in glioblastoma invasion and progression, as well as on their subsequent prognostic value as biomarkers and their therapeutic targeting in the era of precision medicine.

A Western diet impairs CNS energy homeostasis and recovery after spinal cord injury: Link to astrocyte metabolism

A diet high in fat and sucrose (HFHS), the so-called Western diet promotes metabolic syndrome, a significant co-morbidity for individuals with spinal cord injury (SCI). Here we demonstrate that the spinal cord of mice consuming HFHS expresses reduced insulin-like growth factor 1 (IGF-1) and its receptor and shows impaired tricarboxylic acid cycle function, reductions in PLP and increases in astrogliosis, all prior to SCI. After SCI, Western diet impaired sensorimotor and bladder recovery, increased microgliosis, exacerbated oligodendrocyte loss and reduced axon sprouting. Direct and indirect neural injury mechanisms are suggested since HFHS culture conditions drove parallel injury responses directly and indirectly after culture with conditioned media from HFHS-treated astrocytes. In each case, injury mechanisms included reductions in IGF-1R, SIRT1 and PGC-1α and were prevented by metformin. Results highlight the potential for a Western diet to evoke signs of neural insulin resistance and injury and metformin as a strategy to improve mechanisms of neural neuroprotection and repair.

Involvement of Kallikrein-Related Peptidases in Nervous System Disorders

Kallikrein-related peptidases (KLKs) are a family of serine proteases that when dysregulated may contribute to neuroinflammation and neurodegeneration. In the present review article, we describe what is known about their physiological and pathological roles with an emphasis on KLK6 and KLK8, two KLKs that are highly expressed in the adult central nervous system (CNS). Altered expression and activity of KLK6 have been linked to brain physiology and the development of multiple sclerosis. On the other hand, altered levels of KLK6 in the brain and serum of people affected by Alzheimer's disease and Parkinson's disease have been documented, pointing out to its function in amyloid metabolism and development of synucleinopathies. People who have structural genetic variants of KLK8 can suffer mental illnesses such as intellectual and learning disabilities, seizures, and autism. Increased expression of KLK8 has also been implicated in schizophrenia, bipolar disorder, and depression. Also, we discuss the possible link that exists between KLKs activity and certain viral infections that can affect the nervous system. Although little is known about the exact mechanisms that mediate KLKs function and their participation in neuroinflammatory and neurodegenerative disorders will open a new field to develop novel therapies to modulate their levels and/or activity and their harmful effects on the CNS.

Effect of Sox10 on remyelination of the hippocampus in cuprizone-induced demyelinated mice

OBJECTIVE

The low number of oligodendrocytes (OLs) in the hippocampus of patients with schizophrenia suggests that hippocampal demyelination is changed in this condition. Sox10 is expressed throughout OL development. The effect of Sox10 on myelin regeneration is unknown. This study aimed to analyze changes in Sox10 expression in the hippocampus and its regulatory role in hippocampal myelin regeneration in a mouse model of demyelination.

METHODS

Mice were fed 0.2% cuprizone (CPZ) for six weeks to establish the acute demyelinating model (CPZ mice). Behavioral changes of these mice were assessed via open field and tail suspension tests. The ultrastructure of the myelin sheaths in the hippocampus was observed by transmission electron microscopy. The expression levels of myelin sheath-related proteins and the transcription factor Sox10 were detected via immunohistochemistry and Western blots. Furthermore, Sox10-overexpressing adeno-associated virus was injected into the hippocampus after establishing the demyelinating model to investigate effects of Sox10 on remyelination.

RESULTS

CPZ mice showed abnormal behavioral changes, a large number of pathological changes in the myelin sheaths, and significantly reduced protein expression of the myelin sheath markers myelin basic protein and proteolipid protein. This confirmed that the demyelinating model was successfully established. Meanwhile, the protein expression of the oligodendrocyte precursor cell marker neural/glial antigen 2 (NG2) increased, whereas Sox10 expression decreased. After Sox10 overexpression in the hippocampus, the abnormal behavior was improved, the ultrastructure of the myelin sheaths was restored, and the expression of myelin sheath protein was reversed. NG2 expression was upregulated.

CONCLUSION

Overexpression of Sox10 promotes hippocampal remyelination after CPZ-induced acute demyelination.

Protein Degradome of Spinal Cord Injury: Biomarkers and Potential Therapeutic Targets

Degradomics is a proteomics sub-discipline whose goal is to identify and characterize protease-substrate repertoires. With the aim of deciphering and characterizing key signature breakdown products, degradomics emerged to define encryptic biomarker neoproteins specific to certain disease processes. Remarkable improvements in structural and analytical experimental methodologies as evident in research investigating cellular behavior in neuroscience and cancer have allowed the identification of specific degradomes, increasing our knowledge about proteases and their regulators and substrates along with their implications in health and disease. A physiologic balance between protein synthesis and degradation is sought with the activation of proteolytic enzymes such as calpains, caspases, cathepsins, and matrix metalloproteinases. Proteolysis is essential for development, growth, and regeneration; however, inappropriate and uncontrolled activation of the proteolytic system renders the diseased tissue susceptible to further neurotoxic processes. In this article, we aim to review the protease-substrate repertoires as well as emerging therapeutic interventions in spinal cord injury at the degradomic level. Several protease substrates and their breakdown products, essential for the neuronal structural integrity and functional capacity, have been characterized in neurotrauma including cytoskeletal proteins, neuronal extracellular matrix glycoproteins, cell junction proteins, and ion channels. Therefore, targeting exaggerated protease activity provides a potentially effective therapeutic approach in the management of protease-mediated neurotoxicity in reducing the extent of damage secondary to spinal cord injury.

Matrix metalloproteinases shape the oligodendrocyte (niche) during development and upon demyelination

The oligodendrocyte lineage cell is crucial to proper brain function. During central nervous system development, oligodendrocyte progenitor cells (OPCs) migrate and proliferate to populate the entire brain and spinal cord, and subsequently differentiate into mature oligodendrocytes that wrap neuronal axons in an insulating myelin layer. When damage occurs to the myelin sheath, OPCs are activated and recruited to the demyelinated site, where they differentiate into oligodendrocytes that remyelinate the denuded axons. The process of OPC attraction and differentiation is influenced by a multitude of factors from the cell's niche. Matrix metalloproteinases (MMPs) are powerful and versatile enzymes that do not only degrade extracellular matrix proteins, but also cleave cell surface receptors, growth factors, signaling molecules, proteases and other precursor proteins, leading to their activation or degradation. MMPs are markedly upregulated during brain development and upon demyelinating injury, where their broad functions influence the behavior of neural progenitor cells (NPCs), OPCs and oligodendrocytes. In this review, we focus on the role of MMPs in (re)myelination. We will start out in the developing brain with describing the effects of MMPs on NPCs, OPCs and eventually oligodendrocytes. Then, we will outline their functions in oligodendrocyte process extension and developmental myelination. Finally, we will review their potential role in demyelination, describe their significance in remyelination and discuss the evidence for a role of MMPs in remyelination failure, focusing on multiple sclerosis. In conclusion, MMPs shape the oligodendrocyte (niche) both during development and upon demyelination, and thus are important players in directing the fate and behavior of oligodendrocyte lineage cells throughout their life cycle.

A Definition of "Multitargeticity": Identifying Potential Multitarget and Selective Ligands Through a Vector Analysis

The design of multitarget drugs is an essential area of research in Medicinal Chemistry since they have been proposed as potential therapeutics for the management of complex diseases. However, defining a multitarget drug is not an easy task. In this work, we propose a vector analysis for measuring and defining "multitargeticity." We developed terms, such as order and force of a ligand, to finally reach two parameters: multitarget indexes 1 and 2. The combination of these two indexes allows discrimination of multitarget drugs. Several training sets were constructed to test the usefulness of the indexes: an experimental training set, with real affinities, a docking training set, within theoretical values, and an extensive database training set. The indexes proved to be useful, as they were used independently in silico and experimental data, identifying actual multitarget compounds and even selective ligands in most of the training sets. We then applied these indexes to evaluate a virtual library of potential ligands for targets related to multiple sclerosis, identifying 10 compounds that are likely leads for the development of multitarget drugs based on their in silico behavior. With this work, a new milestone is made in the way of defining multitargeticity and in drug design.

Involvement of Enteric Glia in Small Intestine Neuromuscular Dysfunction of Toll-Like Receptor 4-Deficient Mice

Enteric glial cells (EGCs) influence nitric oxide (NO)⁻ and adenosine diphosphate (ADP)⁻ mediated signaling in the enteric nervous system (ENS). Since Toll-like receptor 4 (TLR4) participates to EGC homoeostasis, this study aimed to evaluate the possible involvement of EGCs in the alterations of the inhibitory neurotransmission in TLR4^−/− mice. Ileal segments from male TLR4^−/− and wild-type (WT) C57BL/6J mice were incubated with the gliotoxin fluoroacetate (FA). Alterations in ENS morphology and neurochemical coding were investigated by immunohistochemistry whereas neuromuscular responses were determined by recording non-adrenergic non-cholinergic (NANC) relaxations in isometrically suspended isolated ileal preparations. TLR4^−/− ileal segments showed increased iNOS immunoreactivity associated with enhanced NANC relaxation, mediated by iNOS-derived NO and sensitive to P2Y1 inhibition. Treatment with FA diminished iNOS immunoreactivity and partially abolished NO⁻ and ADP⁻ mediated relaxation in the TLR4^−/− mouse ileum, with no changes of P2Y1 and connexin-43 immunofluorescence distribution in the ENS. After FA treatment, S100β and GFAP immunoreactivity in TLR4^−/− myenteric plexus was reduced to levels comparable to those observed in WT. Our findings show the involvement of EGCs in the alterations of ENS architecture and in the increased purinergic and nitrergic-mediated relaxation, determining gut dysmotility in TLR4^−/− mice.

Blocking the Thrombin Receptor Promotes Repair of Demyelinated Lesions in the Adult Brain

Myelin loss limits neurological recovery and myelin regeneration and is critical for restoration of function. We recently discovered that global knock-out of the thrombin receptor, also known as Protease Activated Receptor 1 (PAR1), accelerates myelin development. Here we demonstrate that knocking out PAR1 also promotes myelin regeneration. Outcomes in two unique models of myelin injury and repair, that is lysolecithin or cuprizone-mediated demyelination, showed that PAR1 knock-out in male mice improves replenishment of myelinating cells and remyelinated nerve fibers and slows early axon damage. Improvements in myelin regeneration in PAR1 knock-out mice occurred in tandem with a skewing of reactive astrocyte signatures toward a prorepair phenotype. In cell culture, the promyelinating effects of PAR1 loss of function are consistent with possible direct effects on the myelinating potential of oligodendrocyte progenitor cells (OPCs), in addition to OPC-indirect effects involving enhanced astrocyte expression of promyelinating factors, such as BDNF. These findings highlight previously unrecognized roles of PAR1 in myelin regeneration, including integrated actions across the oligodendrocyte and astroglial compartments that are at least partially mechanistically linked to the powerful BDNF-TrkB neurotrophic signaling system. Altogether, findings suggest PAR1 may be a therapeutically tractable target for demyelinating disorders of the CNS.SIGNIFICANCE STATEMENT Replacement of oligodendroglia and myelin regeneration holds tremendous potential to improve function across neurological conditions. Here we demonstrate Protease Activated Receptor 1 (PAR1) is an important regulator of the capacity for myelin regeneration across two experimental murine models of myelin injury. PAR1 is a G-protein-coupled receptor densely expressed in the CNS, however there is limited information regarding its physiological roles in health and disease. Using a combination of PAR1 knock-out mice, oligodendrocyte monocultures and oligodendrocyte-astrocyte cocultures, we demonstrate blocking PAR1 improves myelin production by a mechanism related to effects across glial compartments and linked in part to regulatory actions toward growth factors such as BDNF. These findings set the stage for development of new clinically relevant myelin regeneration strategies.

Neuron-generated thrombin induces a protective astrocyte response via protease activated receptors

Astrocytes protect neurons during cerebral injury through several postulated mechanisms. Recent therapeutic attention has focused on enhancing or augmenting the neuroprotective actions of astrocytes but in some instances astrocytes can assume a neurotoxic phenotype. The signaling mechanisms that drive astrocytes toward a protective versus toxic phenotype are not fully known but cell-cell signaling via proteases acting on cell-specific receptors underlies critical mechanistic steps in neurodevelopment and disease. The protease activated receptor (PAR), resides in multiple brain cell types, and most PARs are found on astrocytes. We asked whether neuron-generated thrombin constituted an important astrocyte activation signal because our previous studies have shown that neurons contain prothrombin gene and transcribed protein. We used neuron and astrocyte mono-cell cultures exposed to oxygen-glucose deprivation and a model of middle cerebral artery occlusion. We found that ischemic neurons secrete thrombin into culture media, which leads to astrocyte activation; such astrocyte activation can be reproduced with low doses of thrombin. Media from prothrombin-deficient neurons failed to activate astrocytes and adding thrombin to such media restored activation. Astrocytes lacking PAR1 did not respond to neuron-generated thrombin. Induced astrocyte activation was antagonized dose-dependently with thrombin inhibitors or PAR1 antagonists. Ischemia-induced astrocyte activation in vivo was inhibited after neuronal prothrombin knockout, resulting in larger strokes. Restoring prothrombin to neurons with a lentiviral gene vector restored astrocyte activation and reduced stroke damage. We conclude that neuron-generated thrombin, released during ischemia, acts via PAR1 and may cause astrocyte activation and paracrine neuroprotection.

Kallikrein-related peptidases 6 and 10 are elevated in cerebrospinal fluid of patients with Alzheimer's disease and associated with CSF-TAU and FDG-PET

Background

Alterations in the expression of human kallikrein-related peptidases (KLKs) have been described in patients with Alzheimer’s disease (AD). We elucidated the suitability of KLK6, KLK8 and KLK10 to distinguish AD from NC and explored associations with established AD biomarkers.

Methods

KLK levels in cerebrospinal fluid (CSF), as determined by ELISA, were compared between 32 AD patients stratified to A/T/(N) system with evidence for amyloid pathology and of 23 normal controls with normal AD biomarkers. Associations between KLK levels and clinical severity, CSF and positron emission tomography (PET) based AD biomarkers were tested for.

Results

Levels of KLK6 and KLK10 were significantly increased in AD. KLK6 differed significantly between AD A+/T+/N+ and AD A+/T−/N+ or NC with an AUC of 0.922. CSF pTau and tTau levels were significantly associated with KLK6 in AD.

Conclusions

KLK6 deserves further investigations as a potential biomarker of Tau pathology in AD.

Electronic supplementary material

The online version of this article (10.1186/s40035-019-0168-6) contains supplementary material, which is available to authorized users.

Kallikrein-related peptidase 6 orchestrates astrocyte form and function through proteinase activated receptor-dependent mechanisms

Kallikrein-related peptidase 6 (Klk6) is the most abundant serine proteinase in the adult central nervous system (CNS), yet we know little regarding its physiological roles or mechanisms of action. Levels of Klk6 in the extracellular environment are dynamically regulated in CNS injury and disease positioning this secreted enzyme to affect cell behavior by potential receptor dependent and independent mechanisms. Here we show that recombinant Klk6 evokes increases in intracellular Ca2+ in primary astrocyte monolayer cultures through activation of proteinase activated receptor 1 (PAR1). In addition, Klk6 promoted a condensation of astrocyte cortical actin leading to an elongated stellate shape and multicellular aggregation in a manner that was dependent on the presence of either PAR1 or PAR2. Klk6-evoked changes in astrocyte shape were accompanied by translocation of β-catenin from the plasma membrane to the cytoplasm. These data are exciting because they demonstrate that Klk6 can influence astrocyte plasticity through receptor-dependent mechanisms. Furthermore, this study expands our understanding of the mechanisms by which kallikreins can contribute to neural homeostasis and remodeling and point to both PAR1 and PAR2 as new therapeutic targets to modulate astrocyte form and function.

The Thrombin Receptor Restricts Subventricular Zone Neural Stem Cell Expansion and Differentiation

Thrombin is frequently increased in the CNS after injury yet little is known regarding its effects on neural stem cells. Here we show that the subventricular zone (SVZ) of adult mice lacking the high affinity receptor for thrombin, proteinase activated receptor 1 (PAR1), show increased numbers of Sox2+ and Ki-67+ self-renewing neural stem cells (NSCs) and Olig2+ oligodendrocyte progenitors. SVZ NSCs derived from PAR1-knockout mice, or treated with a PAR1 small molecule inhibitor (SCH79797), exhibited enhanced capacity for self-renewal in vitro, including increases in neurosphere formation and BrdU incorporation. PAR1-knockout SVZ monolayer cultures contained more Nestin, NG2+ and Olig2+ cells indicative of enhancements in expansion and differentiation towards the oligodendrocyte lineage. Cultures of NSCs lacking PAR1 also expressed higher levels of myelin basic protein, proteolipid protein and glial fibrillary acidic protein upon differentiation. Complementing these findings, the corpus callosum and anterior commissure of adult PAR1-knockout mice contained greater numbers of Olig2+ progenitors and CC1+ mature oligodendrocytes. Together these findings highlight PAR1 inhibition as a means to expand adult SVZ NSCs and to promote an increased number of mature myelinating oligodendrocytes in vivo that may be of particular benefit in the context of neural injury where PAR1 agonists such as thrombin are deregulated.

Nexilin Regulates Oligodendrocyte Progenitor Cell Migration and Remyelination and Is Negatively Regulated by Protease-Activated Receptor 1/Ras-Proximate-1 Signaling Following Subarachnoid Hemorrhage

Progressive white matter (WM) impairments caused by subarachnoid hemorrhage (SAH) contribute to cognitive deficits and poor clinical prognoses; however, their pathogenetic mechanisms are poorly understood. We investigated the role of nexilin and oligodendrocyte progenitor cell (OPC)-mediated repair in a mouse model of experimental SAH generated via left endovascular perforation. Nexilin expression was enhanced by the elevated migration of OPCs after SAH. Knocking down nexilin by siRNA reduced OPC migration both in vitro and in vivo and abridged WM repair. In contrast, the protease-activated receptor 1 (PAR1), Ras-proximate-1 (RAP1) and phosphorylated RAP1 (pRAP1) levels in WM were elevated after SAH. The genetic inhibition of PAR1 reduced RAP1 and pRAP1 expression, further enhancing nexilin expression. When delivered at an early stage at a concentration of 25 µg/kg, thrombin receptor antagonist peptide along with PAR1 knockdown rescued the down-regulation of myelin basic protein and improved remyelination at the later stage of SAH. Our results suggest that nexilin is required for OPC migration and remyelination following SAH, as it negatively regulates PAR1/RAP1 signaling, thus providing a promising therapeutic target in WM repair and functional recovery.

The progeroid gene BubR1 regulates axon myelination and motor function

Myelination, the process by which oligodendrocytes form the myelin sheath around axons, is key to axonal signal transduction and related motor function in the central nervous system (CNS). Aging is characterized by degenerative changes in the myelin sheath, although the molecular underpinnings of normal and aberrant myelination remain incompletely understood. Here we report that axon myelination and related motor function are dependent on BubR1, a mitotic checkpoint protein that has been linked to progeroid phenotypes when expressed at low levels and healthy lifespan when overabundant. We found that oligodendrocyte progenitor cell proliferation and oligodendrocyte density is markedly reduced in mutant mice with low amounts of BubR1 (BubR1^H/H mice), causing axonal hypomyelination in both brain and spinal cord. Expression of essential myelin-related genes such as MBP and PLP1 was significantly reduced in these tissues. Consistent with defective myelination, BubR1^H/H mice exhibited various motor deficits, including impaired motor strength, coordination, and balance, irregular gait patterns and reduced locomotor activity. Collectively, these data suggest that BubR1 is a key determinant of oligodendrocyte production and function and provide a molecular entry point to understand age-related degenerative changes in axon myelination.

Protease activated receptor 2 controls myelin development, resiliency and repair

Oligodendrocytes are essential regulators of axonal energy homeostasis and electrical conduction and emerging target cells for restoration of neurological function. Here we investigate the role of protease activated receptor 2 (PAR2), a unique protease activated G protein-coupled receptor, in myelin development and repair using the spinal cord as a model. Results demonstrate that genetic deletion of PAR2 accelerates myelin production, including higher proteolipid protein (PLP) levels in the spinal cord at birth and higher levels of myelin basic protein and thickened myelin sheaths in adulthood. Enhancements in spinal cord myelin with PAR2 loss-of-function were accompanied by increased numbers of Olig2- and CC1-positive oligodendrocytes, as well as in levels of cyclic adenosine monophosphate (cAMP), and extracellular signal related kinase 1/2 (ERK1/2) signaling. Parallel promyelinating effects were observed after blocking PAR2 expression in purified oligodendrocyte cultures, whereas inhibiting adenylate cyclase reversed these effects. Conversely, PAR2 activation reduced PLP expression and this effect was prevented by brain derived neurotrophic factor (BDNF), a promyelinating growth factor that signals through cAMP. PAR2 knockout mice also showed improved myelin resiliency after traumatic spinal cord injury and an accelerated pattern of myelin regeneration after focal demyelination. These findings suggest that PAR2 is an important controller of myelin production and regeneration, both in the developing and adult spinal cord.

Astrocyte heterogeneity across the brain and spinal cord occurs developmentally, in adulthood and in response to demyelination

Astrocytes have emerged as essential regulators of function and response to injury in the brain and spinal cord, yet very little is known about regional differences that exist. Here we compare the expression of key astroglial markers (glial fibrillary acidic protein (GFAP) and Aldehyde Dehydrogenase-1 Family Member L1 (ALDH1L1)) across these disparate poles of the neuraxis, tracking their expression developmentally and in the context of demyelination. In addition, we document changes in the astrocyte regulatory cytokine interleukin 6 (IL-6), and its signaling partner signal transducer and activator of transcription 3 (STAT3), in vivo and in vitro. Results demonstrate that GFAP expression is higher in the developing and adult spinal cord relative to brain. Comparisons between GFAP and ALDH1L1 expression suggest elevations in spinal cord GFAP during the early postnatal period reflect an accelerated appearance of astrocytes, while elevations in adulthood reflect higher expression by individual astrocytes. Notably, increases in spinal cord compared to whole brain GFAP were paralleled by higher levels of IL-6 and STAT3. Equivalent elevations in GFAP, GFAP/ALDH1L1 ratios, and in IL-6, were observed in primary astrocyte cultures derived from spinal cord compared to cortex. Also, higher levels of GFAP were observed in the spinal cord compared to the brain after focal demyelinating injury. Altogether, these studies point to key differences in astrocyte abundance and the expression of GFAP and IL-6 across the brain and spinal cord that are positioned to influence regional specialization developmentally and responses occurring in the context of injury and disease.

Kallikrein-related peptidase 6 exacerbates disease in an autoimmune model of multiple sclerosis

Kallikrein-related peptidase 6 (Klk6) is elevated in the serum of multiple sclerosis (MS) patients and is hypothesized to participate in inflammatory and neuropathogenic aspects of the disease. To test this hypothesis, we investigated the impact of systemic administration of recombinant Klk6 on the development and progression of MOG35-55-induced experimental autoimmune encephalomyelitis (EAE). First, we determined that Klk6 expression is elevated in the spinal cord of mice with EAE at the peak of clinical disease and in immune cells upon priming with the disease-initiating peptide in vitro. Systemic administration of recombinant Klk6 to mice during the priming phase of disease resulted in an exacerbation of clinical symptoms, including earlier onset of disease and higher levels of spinal cord inflammation and pathology. Treatment of MOG35-55-primed immune cells with Klk6 in culture enhanced expression of pro-inflammatory cytokines, interferon-γ, tumor necrosis factor, and interleukin-17, while reducing anti-inflammatory cytokines interleukin-4 and interleukin-5. Together these findings provide evidence that elevations in systemic Klk6 can bias the immune system towards pro-inflammatory responses capable of exacerbating the development of neuroinflammation and paralytic neurological deficits. We suggest that Klk6 represents an important target for conditions in which pro-inflammatory responses play a critical role in disease development, including MS.

Protease-activated receptor-1 activation by granzyme B causes neurotoxicity that is augmented by interleukin-1β

Background

The cause of neurodegeneration in progressive forms of multiple sclerosis is unknown. We investigated the impact of specific neuroinflammatory markers on human neurons to identify potential therapeutic targets for neuroprotection against chronic inflammation.

Methods

Surface immunocytochemistry directly visualized protease-activated receptor-1 (PAR1) and interleukin-1 (IL-1) receptors on neurons in human postmortem cortex in patients with and without neuroinflammatory lesions. Viability of cultured neurons was determined after exposure to cerebrospinal fluid from patients with progressive multiple sclerosis or purified granzyme B and IL-1β. Inhibitors of PAR1 activation and of PAR1-associated second messenger signaling were used to elucidate a mechanism of neurotoxicity.

Results

Immunohistochemistry of human post-mortem brain tissue demonstrated cells expressing higher amounts of PAR1 near and within subcortical lesions in patients with multiple sclerosis compared to control tissue. Human cerebrospinal fluid samples containing granzyme B and IL-1β were toxic to human neuronal cultures. Granzyme B was neurotoxic through activation of PAR1 and subsequently the phospholipase Cβ-IP3 second messenger system. Inhibition of PAR1 or IP3 prevented granzyme B toxicity. IL-1β enhanced granzyme B-mediated neurotoxicity by increasing PAR1 expression.

Conclusions

Neurons within the inflamed central nervous system are imperiled because they express more PAR1 and are exposed to a neurotoxic combination of both granzyme B and IL-1β. The effects of these inflammatory mediators may be a contributing factor in the progressive brain atrophy associated with neuroinflammatory diseases. Knowledge of how exposure to IL-1β and granzyme B act synergistically to cause neuronal death yields potential novel neuroprotective treatments for neuroinflammatory diseases.

Targeting the thrombin receptor modulates inflammation and astrogliosis to improve recovery after spinal cord injury

The deregulation of serine protease activity is a common feature of neurological injury, but little is known regarding their mechanisms of action or whether they can be targeted to facilitate repair. In this study we demonstrate that the thrombin receptor (Protease Activated Receptor 1, (PAR1)) serves as a critical translator of the spinal cord injury (SCI) proteolytic microenvironment into a cascade of pro-inflammatory events that contribute to astrogliosis and functional decline. PAR1 knockout mice displayed improved locomotor recovery after SCI and reduced signatures of inflammation and astrogliosis, including expression of glial fibrillary acidic protein (GFAP), vimentin, and STAT3 signaling. SCI-associated elevations in pro-inflammatory cytokines such as IL-1β and IL-6 were also reduced in PAR1-/- mice and co-ordinate improvements in tissue sparing and preservation of NeuN-positive ventral horn neurons, and PKCγ corticospinal axons, were observed. PAR1 and its agonist's thrombin and neurosin were expressed by perilesional astrocytes and each agonist increased the production of IL-6 and STAT3 signaling in primary astrocyte cultures in a PAR1-dependent manner. In turn, IL-6-stimulated astrocytes increased expression of PAR1, thrombin, and neurosin, pointing to a model in which PAR1 activation contributes to increased astrogliosis by feedforward- and feedback-signaling dynamics. Collectively, these findings identify the thrombin receptor as a key mediator of inflammation and astrogliosis in the aftermath of SCI that can be targeted to reduce neurodegeneration and improve neurobehavioral recovery.

SERPINE2/Protease Nexin-1 in vivo multiple functions: Does the puzzle make sense?

Cultures of glial cells and fibroblasts allowed and lead to the identification SERPINE2/Protease Nexin-1 (SERPINE2/PN-1). Cellular, biochemical, immunological and molecular characterization substantiated its variable expression in many organs as a function of development, adult stages, pathological situations or following injury. It is not a circulating serpin, but as other members of the family, its target specificity is influenced by components of the extracellular matrix. The challenges are to identify where and when SERPINE2/PN-1 modulatory action becomes crucial or even possibly specific in a mosaic of feasible in vivo impacts. Data providing correlations are not sufficient to satisfy this aim. Genetically modified mice, or tissue derived thereof, provide interesting in vivo models to identify and study the relevance of this serpin. This review will highlight sometimes-intriguing results indicating a crucial impact of SERPINE2/PN-1, especially in the vasculature, the nervous system or the behavior of cancer cells in vivo. Data presently available will be discussed in an attempt to define general trends in the diversity of SERPINE2/PN-1 modes of action in vivo.

Identification of brain-enriched proteins in the cerebrospinal fluid proteome by LC-MS/MS profiling and mining of the Human Protein Atlas

Background

Cerebrospinal fluid (CSF) is a proximal fluid which communicates closely with brain tissue, contains numerous brain-derived proteins and thus represents a promising fluid for discovery of biomarkers of central nervous system (CNS) diseases. The main purpose of this study was to generate an extensive CSF proteome and define brain-related proteins identified in CSF, suitable for development of diagnostic assays.

Methods

Six non-pathological CSF samples from three female and three male individuals were selected for CSF analysis. Samples were first subjected to strong cation exchange chromatography, followed by LC-MS/MS analysis. Secreted and membrane-bound proteins enriched in the brain tissues were retrieved from the Human Protein Atlas.

Results

In total, 2615 proteins were identified in the CSF. The number of proteins identified per individual sample ranged from 1109 to 1421, with inter-individual variability between six samples of 21 %. Based on the Human Protein Atlas, 78 brain-specific proteins found in CSF samples were proposed as a signature of brain-enriched proteins in CSF.

Conclusion

A combination of Human Protein Atlas database and experimental search of proteins in specific body fluid can be applied as an initial step in search for disease biomarkers specific for a particular tissue. This signature may be of significant interest for development of novel diagnostics of CNS diseases and identification of drug targets.

Electronic supplementary material

The online version of this article (doi:10.1186/s12014-016-9111-3) contains supplementary material, which is available to authorized users.

Kallikrein Promotes Inflammation in Human Dental Pulp Cells Via Protease-Activated Receptor-1

Plasma kallikrein (KLKB1), a serine protease, cleaves high-molecular weight kininogen to produce bradykinin, a potent vasodilator and pro-inflammatory peptide. In addition, KLKB1 activates plasminogen and other leukocyte and blood coagulation factors and processes pro-enkephalin, prorenin, and C3. KLKB1 has also been shown to cleave protease-activated receptors in vascular smooth muscle cells to regulate the expression of epidermal growth factor receptor. In this study, we investigated KLKB1-dependent inflammation and activation of protease-activated receptor-1 in human dental pulp cells. These cells responded to KLKB1 stimulation by increasing intracellular Ca(2+) , upregulating cyclooxygenase-2, and secreting prostaglandin E2 . Remarkably, SCH79797, an antagonist of protease-activated receptor-1, blocked these effects. Thus, these data indicate that KLKB1 induces inflammatory reactions in human dental tissues via protease-activated receptor 1. J. Cell. Biochem. 117: 1522-1528, 2016. © 2015 Wiley Periodicals, Inc.

Interplay between exercise and dietary fat modulates myelinogenesis in the central nervous system

Here we show that the interplay between exercise training and dietary fat regulates myelinogenesis in the adult central nervous system. Mice consuming high fat with coordinate voluntary running wheel exercise for 7weeks showed increases in the abundance of the major myelin membrane proteins, proteolipid (PLP) and myelin basic protein (MBP), in the lumbosacral spinal cord. Expression of MBP and PLP RNA, as well that for Myrf1, a transcription factor driving oligodendrocyte differentiation were also differentially increased under each condition. Furthermore, expression of IGF-1 and its receptor IGF-1R, known to promote myelinogenesis, were also increased in the spinal cord in response to high dietary fat or exercise training. Parallel increases in AKT signaling, a pro-myelination signaling intermediate activated by IGF-1, were also observed in the spinal cord of mice consuming high fat alone or in combination with exercise. Despite the pro-myelinogenic effects of high dietary fat in the context of exercise, high fat consumption in the setting of a sedentary lifestyle reduced OPCs and mature oligodendroglia. Whereas 7weeks of exercise training alone did not alter OPC or oligodendrocyte numbers, it did reverse reductions seen with high fat. Evidence is presented suggesting that the interplay between exercise and high dietary fat increase SIRT1, PGC-1α and antioxidant enzymes which may permit oligodendroglia to take advantage of diet and exercise-related increases in mitochondrial activity to yield increases in myelination despite higher levels of reactive oxygen species.

Involvement of Kallikrein-Related Peptidases in Normal and Pathologic Processes

Human kallikrein-related peptidases (KLKs) are a subgroup of serine proteases that participate in proteolytic pathways and control protein levels in normal physiology as well as in several pathological conditions. Their complex network of stimulatory and inhibitory interactions may induce inflammatory and immune responses and contribute to the neoplastic phenotype through the regulation of several cellular processes, such as proliferation, survival, migration, and invasion. This family of proteases, which includes one of the most useful cancer biomarkers, kallikrein-related peptidase 3 or PSA, also has a protective effect against cancer promoting apoptosis or counteracting angiogenesis and cell proliferation. Therefore, they represent attractive therapeutic targets and may have important applications in clinical oncology. Despite being intensively studied, many gaps in our knowledge on several molecular aspects of KLK functions still exist. This review aims to summarize recent data on their involvement in different processes related to health and disease, in particular those directly or indirectly linked to the neoplastic process.

Kallikreins - The melting pot of activity and function

The human tissue kallikrein and kallikrein-related peptidases (KLKs), encoded by the largest contiguous cluster of protease genes in the human genome, are secreted serine proteases with diverse expression patterns and physiological roles. Because of the broad spectrum of processes that are modulated by kallikreins, these proteases are the subject of extensive investigations. This review brings together basic information about the biochemical properties affecting enzymatic activity, with highlights on post-translational modifications, especially glycosylation. Additionally, we present the current state of knowledge regarding the physiological functions of KLKs in major human organs and outline recent discoveries pertinent to the involvement of kallikreins in cell signaling and in viral infections. Despite the current depth of knowledge of these enzymes, many questions regarding the roles of kallikreins in health and disease remain unanswered.

Genetic targeting of protease activated receptor 2 reduces inflammatory astrogliosis and improves recovery of function after spinal cord injury

Inflammatory-astrogliosis exacerbates damage in the injured spinal cord and limits repair. Here we identify Protease Activated Receptor 2 (PAR2) as an essential regulator of these events with mice lacking the PAR2 gene showing greater improvements in motor coordination and strength after compression-spinal cord injury (SCI) compared to wild type littermates. Molecular profiling of the injury epicenter, and spinal segments above and below, demonstrated that mice lacking PAR2 had significantly attenuated elevations in key hallmarks of astrogliosis (glial fibrillary acidic protein (GFAP), vimentin and neurocan) and in expression of pro-inflammatory cytokines (interleukin-6 (IL-6), tumor necrosis factor (TNF) and interleukin-1 beta (IL-1β)). SCI in PAR2-/- mice was also accompanied by improved preservation of protein kinase C gamma (PKCγ)-immunopositive corticospinal axons and reductions in GFAP-immunoreactivity, expression of the pro-apoptotic marker BCL2-interacting mediator of cell death (BIM), and in signal transducer and activator of transcription 3 (STAT3). The potential mechanistic link between PAR2, STAT3 and astrogliosis was further investigated in primary astrocytes to reveal that the SCI-related serine protease, neurosin (kallikrein 6) promotes IL-6 secretion in a PAR2 and STAT3-dependent manner. Data point to a signaling circuit in primary astrocytes in which neurosin signaling at PAR2 promotes IL-6 secretion and canonical STAT3 signaling. IL-6 promotes expression of GFAP, vimentin, additional IL-6 and robust increases in both neurosin and PAR2, thereby driving the PAR2-signaling circuit forward. Given the significant reductions in astrogliosis and inflammation as well as superior neuromotor recovery observed in PAR2 knockout mice after SCI, we suggest that this receptor and its agonists represent new drug targets to foster neuromotor recovery.

Astrocyte barriers to neurotoxic inflammation

Astrocytes form borders (glia limitans) that separate neural from non-neural tissue along perivascular spaces, meninges and tissue lesions in the CNS. Transgenic loss-of-function studies reveal that astrocyte borders and scars serve as functional barriers that restrict the entry of inflammatory cells into CNS parenchyma in health and disease. Astrocytes also have powerful pro-inflammatory potential. Thus, astrocytes are emerging as pivotal regulators of CNS inflammatory responses. This Review discusses evidence that astrocytes have crucial roles in attracting and restricting CNS inflammation, with important implications for diverse CNS disorders.

Differential expression of multiple kallikreins in a viral model of multiple sclerosis points to unique roles in the innate and adaptive immune response

Recent studies provide a functional link between kallikrein 6 (Klk6) and the development and progression of disease in patients with multiple sclerosis (MS) and in its murine models. To evaluate the involvement of additional kallikrein family members, we compared Klk6 expression with four other kallikreins (Klk1, Klk7, Klk8, and Klk10) in the brain and spinal cord of mice infected with Theiler's murine encephalomyelitis virus, an experimental model of progressive MS. The robust upregulation of Klk6 and Klk8 in the brain during the acute phase of viral encephalitis and in the spinal cord during disease development and progression points to their participation in inflammation, demyelination, and progressive axon degeneration. More limited changes in Klk1, Klk7, and Klk10 were also observed. In addition, Klk1, Klk6, and Klk10 were dynamically regulated in T cells in vitro as a recall response to viral antigen and in activated monocytes, pointing to their activities in the development of adaptive and innate immune function. Together, these results point to overlapping and unique roles for multiple kallikreins in the development and progression of virus-mediated central nervous system inflammatory demyelinating disease, including activities in the development of the adaptive and innate immune response, in demyelination, and in progressive axon degeneration.

Interference with protease-activated receptor 1 does not reduce damage to subventricular zone cells of immature rodent brain following exposure to blood or blood plasma

BACKGROUND

Prior work showed that whole blood, plasma, and serum injections are damaging to the neonatal rodent brain in a model of intracerebral/periventricular hemorrhage. Thrombin alone is also damaging. In adult animal models of hemorrhagic stroke, the protease-activated (thrombin) receptor PAR1 mediates some of the brain damage. We hypothesized that PAR1 interference will reduce the adverse effects of blood products on immature rodent brain and cells.

RESULTS

Cultured oligodendrocyte precursor cells from rats and mice were exposed to blood plasma with and without the PAR1 antagonists SCH-79797 or BMS-200261. In concentrations previously shown to have activity on brain cells, neither drug showed evidence of protection against the toxicity of blood plasma. Newborn mice (wild type, heterozygous, and PAR1 knockout) were subjected to intracerebral injection of autologous whole blood into the periventricular region of the frontal lobe. Cell proliferation, measured by Ki67 immunoreactivity in the subventricular zone, was suppressed at 1 and 2 days, and was not normalized in the knockout mice. Cell apoptosis, measured by activated caspase 3 immunoreactivity, was not apparent in the subventricular zone. Increased apoptosis in periventricular striatal cells was not normalized in the knockout mice.

CONCLUSION

Interference with the thrombin-PAR1 system does not reduce the adverse effects of blood on germinal cells of the immature rodent brain. PAR1 interference is unlikely to be a useful treatment for reducing the brain damage that accompanies periventricular (germinal matrix) hemorrhage, a common complication of premature birth.

The thrombin receptor is a critical extracellular switch controlling myelination

Hemorrhagic white matter injuries in the perinatal period are a growing cause of cerebral palsy yet no neuroprotective strategies exist to prevent the devastating motor and cognitive deficits that ensue. We demonstrate that the thrombin receptor (protease-activated receptor 1, PAR1) exhibits peak expression levels in the spinal cord at term and is a critical regulator of the myelination continuum from initiation to the final levels achieved. Specifically, PAR1 gene deletion resulted in earlier onset of spinal cord myelination, including substantially more Olig2-positive oligodendrocytes, more myelinated axons, and higher proteolipid protein (PLP) levels at birth. In vitro, the highest levels of PAR1 were observed in oligodendrocyte progenitor cells (OPCs), being reduced with differentiation. In parallel, the expression of PLP and myelin basic protein (MBP), in addition to Olig2, were all significantly higher in cultures of PAR1-/- oligodendroglia. Moreover, application of a small molecule inhibitor of PAR1 (SCH79797) to OPCs in vitro increased PLP and MBP expression. Enhancements in myelination associated with PAR1 genetic deletion were also observed in adulthood as evidenced by higher amounts of MBP and thickened myelin sheaths across large, medium, and small diameter axons. Enriched spinal cord myelination in PAR1-/- mice was coupled to increases in extracellular-signal-regulated kinase 1/2 and AKT signaling developmentally. Nocturnal ambulation and rearing activity were also elevated in PAR1-/- mice. These studies identify the thrombin receptor as a powerful extracellular regulatory switch that could be readily targeted to improve myelin production in the face of white matter injury and disease.

Reactive gliosis and the multicellular response to CNS damage and disease

The CNS is prone to heterogeneous insults of diverse etiologies that elicit multifaceted responses. Acute and focal injuries trigger wound repair with tissue replacement. Diffuse and chronic diseases provoke gradually escalating tissue changes. The responses to CNS insults involve complex interactions among cells of numerous lineages and functions, including CNS intrinsic neural cells, CNS intrinsic nonneural cells, and CNS extrinsic cells that enter from the circulation. The contributions of diverse nonneuronal cell types to outcome after acute injury, or to the progression of chronic disease, are of increasing interest as the push toward understanding and ameliorating CNS afflictions accelerates. In some cases, considerable information is available, in others, comparatively little, as examined and reviewed here.

Kallikrein cascades in traumatic spinal cord injury: in vitro evidence for roles in axonopathy and neuron degeneration

Kallikreins (KLKs) are a family of 15 secreted serine proteases with emerging roles in neurologic diseases. To illuminate their contributions to the pathophysiology of spinal cord injury (SCI), we evaluated acute through chronic changes in the immunohistochemical appearance of 6 KLKs (KLK1, KLK5, KLK6, KLK7, KLK8, and KLK9) in postmortem human traumatic SCI cases, quantified their RNA expression levels in experimental murine SCI, and assessed the impact of recombinant forms of each enzyme toward murine cortical neurons in vitro. Temporally and spatially distinct changes in KLK expression were observed with partially overlapping patterns between human and murine SCI, including peak elevations (or reductions) during the acute and subacute periods. Kallikrein 9 showed the most marked changes and remained chronically elevated. Importantly, a subset of KLKs (KLK1, KLK5, KLK6, KLK7, and KLK9) were neurotoxic toward primary neurons in vitro. Kallikrein immunoreactivity was also observed in association with swollen axons and retraction bulbs in the human SCI cases examined. Together, these findings demonstrate that elevated levels of a significant subset of KLKs are positioned to contribute to neurodegenerative changes in cases of CNS trauma and disease and, therefore, represent new potential targets for the development of neuroprotective strategies.