In vivo analysis of kallikrein-related peptidase 6 (KLK6) function in oligodendrocyte development and the expression of myelin proteins

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.

Systemic Inflammation Leads to Changes in the Intracellular Localization of KLK6 in Oligodendrocytes in Spinal Cord White Matter

Axonal injury and demyelination occur in demyelinating diseases, such as multiple sclerosis, and the detachment of myelin from axons precedes its degradation. Paranodes are the areas at which each layer of the myelin sheath adheres tightly to axons. The destruction of nodal and paranodal structures during inflammation is an important pathophysiology of various neurological disorders. However, the underlying pathological changes in these structures remain unclear. Kallikrein 6 (KLK6), a serine protease produced by oligodendrocytes, is involved in demyelinating diseases. In the present study, we intraperitoneally injected mice with LPS for several days and examined changes in the localization of KLK6. Transient changes in the intracellular localization of KLK6 to paranodes in the spinal cord were observed during LPS-induced systemic inflammation. However, these changes were not detected in the upper part of brain white matter. LPS-induced changes were suppressed by minocycline, suggesting the involvement of microglia. Moreover, nodal lengths were elongated in LPS-treated wild-type mice, but not in LPS-treated KLK6-KO mice. These results demonstrate the potential involvement of KLK6 in the process of demyelination.

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.

Microglia Impede Oligodendrocyte Generation in Aged Brain

Purpose

Age-related increase in myelin loss may be responsible for brain atrophy, and the mechanism is not completely understood. We aim to comprehensively delineate oligodendrocyte heterogeneity in young and aged mice and to reveal the underlying mechanism for myelin loss during aging.

Methods

Diffusion tensor imaging and immunofluorescent staining were performed to verify the demyelination in the aged brains of both rodents and human. Further, the single-cell RNA sequencing data of all brain cells from young and aged mice were deeply analyzed to identify the subsets of oligodendrocyte lineage cells. Cell-to-cell interaction analysis was performed to detect the mechanism of observed changes in oligodendrocyte generation.

Results

Oligodendrocytes were observed to up-regulate several senescence associated genes in aged brain. Four clusters of oligodendrocyte precursor cells (OPCs) were identified in both young and aged brains. The number of those OPCs in basal state was significantly increased, while the OPCs in the procedure of differentiation were immensely decreased in aged brain. Furthermore, it was identified that activated microglia in the aged brain released inflammatory factors to suppress OPC differentiation. Stat1 might be a potential target to transform senescent microglia into tissue repair type to promote oligodendrocyte generation.

Conclusion

These results provided a perspective on how age activated microglia could impede remyelination and might give a new therapeutic target for age-related remyelinating diseases.

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.

Distinct oligodendrocyte populations have spatial preference and different responses to spinal cord injury

Mature oligodendrocytes (MOLs) show transcriptional heterogeneity, the functional consequences of which are unclear. MOL heterogeneity might correlate with the local environment or their interactions with different neuron types. Here, we show that distinct MOL populations have spatial preference in the mammalian central nervous system (CNS). We found that MOL type 2 (MOL2) is enriched in the spinal cord when compared to the brain, while MOL types 5 and 6 (MOL5/6) increase their contribution to the OL lineage with age in all analyzed regions. MOL2 and MOL5/6 also have distinct spatial preference in the spinal cord regions where motor and sensory tracts run. OL progenitor cells (OPCs) are not specified into distinct MOL populations during development, excluding a major contribution of OPC intrinsic mechanisms determining MOL heterogeneity. In disease, MOL2 and MOL5/6 present different susceptibility during the chronic phase following traumatic spinal cord injury. Our results demonstrate that the distinct MOL populations have different spatial preference and different responses to disease.

The oligodendrocyte lineage is known for its transcriptional heterogeneity, but the functional consequences of this are unclear. Here, the authors show that distinct populations of mature oligodendrocytes have spatial preferences in the brain and spinal cord and show different responses to spinal cord injury.

Deficiency of the serine peptidase Kallikrein 6 does not affect the levels and the pathological accumulation of a-synuclein in mouse brain

Several lines of evidence indicate that the propagation of misfolded α-synuclein (α-syn) plays a central role in the progression and manifestation of Parkinson's disease. Pathogenic α-syn species can be present in the extracellular space. Thus, the identification and modulation of the key enzymes implicated in extracellular α-syn turnover becomes vital. Kallikrein peptidase 6 has been identified as one of the major α-syn degrading enzymes and has been implicated in the clearance of extracellular α-syn. However, the physiological role of this enzyme in regulating α-syn, in vivo, still remains elusive. Here, by utilizing Klk6 knock-out (Klk6^-/- ) mice as our experimental model, we provide insight into the physiologic relevance of endogenous KLK6 expression on α-syn processing. Behavioral phenotyping showed that Klk6^-/- mice display no gross behavioral abnormalities. Further in vivo characterization of this mouse model, in the context of α-syn accumulation, showed that KLK6 deletion had no impact on the protein levels of intracellular or extracellular α-syn. Upon in vivo administration of α-syn pre-formed fibrils (PFF), α-syn pathologic accumulations were evident both in the brains of Klk6^-/- mice and wt mice without significant differences. Intrastriatal delivery of active KLK6, did not affect secreted α-syn levels observed in the A53T α-syn over-expressing mice. These findings suggest that in the in vivo setting of PFF pathology induction, KLK6 alone is not able to modulate pathology transmission. Our study raises implications for the use of recombinant α-syn fibrils in α-syn turnover studies.

Important role of kallikrein 6 for the development of keratinocyte proliferative resistance to topical glucocorticoids

One of the major adverse effects of topical glucocorticoids is cutaneous atrophy often followed by development of resistance to steroids (tachyphylaxis). Previously we showed that after two weeks, interfollicular mouse keratinocytes acquired resistance to anti-proliferative effects of glucocorticoid fluocinolone acetonide (FA). One of the top genes activated by FA during tachyphylaxis was Klk6 encoding kallikrein-related peptidase 6, known to enhance keratinocyte proliferation. KLK6 was also strongly induced by chronic glucocorticoids in human skin. Double immunostaining showed that KLK6⁺ keratinocytes, localized in suprabasal layer of mouse skin, were frequently adjacent to proliferating 5-bromo-2'-deoxyuridine-positive basal keratinocytes. We used KLK6 knockout (KO) mice to evaluate KLK6 role in skin regeneration after steroid-induced atrophy. KLK6 KOs had thinner epidermis and decreased keratinocyte proliferation. The keratinocytes in wild type and KLK6 KO epidermis were equally sensitive to acute anti-proliferative effect of FA. However, the development of proliferative resistance during chronic treatment was reduced in KO epidermis. This was not due to the changes in glucocorticoid receptor (GR) expression or function as GR protein level and induction of GR-target genes were similar in wild type and KLK6 KO skin. Overall, these results suggest a novel mechanism of epidermal regeneration after glucocorticoid-induced atrophy via KLK6 activation.

Kallikrein 6 secreted by oligodendrocytes regulates the progression of experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS), and experimental autoimmune encephalomyelitis (EAE) is a well-established animal model of the disease. Here, we examined the pathophysiological role of Kallikrein 6 (Klk6), a serine protease produced by oligodendrocytes (OLs), in EAE using Klk6 knockout (Klk6-/-) mice. Compared with Klk6+/+ (wild-type) mice, Klk6-/- mice showed milder EAE symptoms, including delayed onset and milder paralysis. Loss of Klk6 suppressed matrix metalloprotease-9 expression and diminished the infiltration of peripheral inflammatory cells into the CNS by decreasing blood-brain barrier (BBB) permeability and reducing expression levels of inflammatory cytokines, chemokines and their receptors. Scanning electron microscopic analysis revealed demyelination characterized by myelin detachment from the axons in the early phase of EAE progression (days 3-7) in Klk6+/+ mice but not in Klk6-/- mice. Interestingly, anti-MOG (myelin oligodendrocyte glycoprotein) autoantibody was also detected in the cerebrospinal fluid (CSF) and spinal cord on day 3 after MOG immunization. Furthermore, treatment of primary cultured OLs with anti-MOG autoantibody induced oligodendroglial morphological changes and increases in myelin basic protein and Klk6 expression. We also developed a novel enzyme-linked immunoabsorbent assay method for detecting activated KLK6 in human CSF. In human autopsy brain samples, expression of active KLK6 was detected in OLs using an antibody that specifically recognizes the protein's activated form. Taken together, our findings demonstrate that Klk6 secreted by OLs plays a critical role in the pathogenesis of EAE/MS and that it might serve as a potential therapeutic target for MS.

A brain-targeted, modified neurosin (kallikrein-6) reduces α-synuclein accumulation in a mouse model of multiple system atrophy

Background

Multiple system atrophy (MSA) is a progressive, neurodegenerative disease characterized by parkinsonism, resistance to dopamine therapy, ataxia, autonomic dysfunction, and pathological accumulation of α-synuclein (α-syn) in oligodendrocytes. Neurosin (kallikrein-6) is a serine protease capable of cleaving α-syn in the CNS, and we have previously shown that lentiviral (LV) vector delivery of neurosin into the brain of a mouse model of dementia with Lewy body/ Parkinson’s disease reduces the accumulation of α-syn and improves neuronal synaptic integrity.

Results

In this study, we investigated the ability of a modified, systemically delivered neurosin to reduce the levels of α-syn in oligodendrocytes and reduce the cell-to-cell spread of α-syn to glial cells in a mouse model of MSA (MBP-α-syn). We engineered a viral vector that expresses a neurosin genetically modified for increased half-life (R80Q mutation) that also contains a brain-targeting sequence (apoB) for delivery into the CNS. Peripheral administration of the LV-neurosin-apoB to the MBP-α-syn tg model resulted in accumulation of neurosin-apoB in the CNS, reduced accumulation of α-syn in oligodendrocytes and astrocytes, improved myelin sheath formation in the corpus callosum and behavioral improvements.

Conclusion

Thus, the modified, brain-targeted neurosin may warrant further investigation as potential therapy for MSA.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-015-0043-6) contains supplementary material, which is available to authorized users.

Comparison of the transcriptomes of mouse skin derived precursors (SKPs) and SKP-derived fibroblasts (SFBs) by RNA-Seq

Skin-derived precursors (SKPs) from dermis possess the capacities of self-renewal and multipotency. In vitro and in vivo studies demonstrated that they can differentiate into fibroblasts. However, little is known about the molecular mechanism of the differentiation of SKPs into fibroblasts. Here we compare the transcriptomes of mouse SKPs and SKP-derived fibroblasts (SFBs) by RNA-Seq analysis, trying to find differences in gene expression between the two kinds of cells and then elucidate the candidate genes that may play important roles in the differentiation of SKPs into fibroblasts. A total of 1971 differentially expressed genes (DEGs) were identified by RNA-Seq, which provided abundant data for further analysis. Gene Ontology enrichment analysis revealed that genes related to cell differentiation, cell proliferation, protein binding, transporter activity and membrane were significantly enriched. The most significantly up-regulated genes Wnt4, Wisp2 and Tsp-1 and down-regulated genes Slitrk1, Klk6, Agtr2, Ivl, Msx1, IL15, Atp6v0d2, Kcne1l and Thbs4 may play important roles in the differentiation of SKPs into fibroblasts. KEGG analysis showed that DEGs were significantly enriched in the TGF-β signaling pathway, Wnt signaling pathway and Notch signaling pathway, which have been previously proven to regulate the differentiation and self-renewal of various stem cells. These identified DEGs and pathways could facilitate further investigations of the detailed molecular mechanisms, making it possible to take advantage of the potential therapeutic applications of SKPs in skin regeneration in the future.

Interferon regulatory factor 7 participates in the M1-like microglial polarization switch

Microglia are generally considered the immune cells of the central nervous system. Recent studies have demonstrated that under specific polarization conditions, microglia develop into two different phenotypes, termed M1-like and M2-like microglia. However, the phenotypic characteristics of M1-like- and M2-like-polarized microglia and the mechanisms that regulate polarization are largely unknown. In this study, we characterized lipopolysaccharide-treated M1-like and IL-4-treated M2-like microglia and investigated the mechanisms that regulate phenotypic switching. The addition of M2-like microglial conditioned medium (CM) to primary neurons resulted in an increase in neurite length when compared with neurons treated with M1-like microglial CM, possibly because of the enhanced secretion of neurotrophic factors by M2-like microglia. M1-like microglia were morphologically characterized by larger soma, whereas M2-like microglia were characterized by long processes. M2-like microglia exhibited greater phagocytic capacity than M1-like microglia. These features switched in response to polarization cues. We found that expression of interferon regulatory factor 7 (IRF7) increased during the M2-like to M1-like switch in microglia in vitro and in vivo. Knockdown of IRF7 using siRNA suppressed the expression of M1 marker mRNA and reduced phosphorylation of STAT1. Our findings suggest that IRF7 signaling may play an important role in microglial polarization switching.

Disrupted SOX10 function causes spongiform neurodegeneration in gray tremor mice

Mice homozygous for the gray tremor (gt) mutation have a pleiotropic phenotype that includes pigmentation defects, megacolon, whole body tremors, sporadic seizures, hypo- and dys-myelination of the central nervous system (CNS) and peripheral nervous system, vacuolation of the CNS, and early death. Vacuolation similar to that caused by prions was originally reported to be transmissible, but subsequent studies showed the inherited disease was not infectious. The gt mutation mapped to distal mouse chromosome 15, to the same region as Sox10, which encodes a transcription factor with essential roles in neural crest survival and differentiation. As dominant mutations in mouse or human SOX10 cause white spotting and intestinal aganglionosis, we screened the Sox10 coding region for mutations in gt/gt DNA. An adenosine to guanine transversion was identified in exon 2 that changes a highly conserved glutamic acid residue in the SOX10 DNA binding domain to glycine. This mutant allele was not seen in wildtype mice, including the related GT/Le strain, and failed to complement a Sox10 null allele. Gene expression analysis revealed significant down-regulation of genes involved in myelin lipid biosynthesis pathways in gt/gt brains. Knockout mice for some of these genes develop CNS vacuolation and/or myelination defects, suggesting that their down-regulation may contribute to these phenotypes in gt mutants and could underlie the neurological phenotypes associated with peripheral demyelinating neuropathy-central dysmyelinating leukodystrophy-Waardenburg syndrome-Hirschsprung disease, caused by mutations in human SOX10.

What have we learned about the kallikrein-kinin and renin-angiotensin systems in neurological disorders?

The kallikrein-kinin system (KKS) is an intricate endogenous pathway involved in several physiological and pathological cascades in the brain. Due to the pathological effects of kinins in blood vessels and tissues, their formation and degradation are tightly controlled. Their components have been related to several central nervous system diseases such as stroke, Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, epilepsy and others. Bradykinin and its receptors (B1R and B2R) may have a role in the pathophysiology of certain central nervous system diseases. It has been suggested that kinin B1R is up-regulated in pathological conditions and has a neurodegenerative pattern, while kinin B2R is constitutive and can act as a neuroprotective factor in many neurological conditions. The renin angiotensin system (RAS) is an important blood pressure regulator and controls both sodium and water intake. AngII is a potent vasoconstrictor molecule and angiotensin converting enzyme is the major enzyme responsible for its release. AngII acts mainly on the AT1 receptor, with involvement in several systemic and neurological disorders. Brain RAS has been associated with physiological pathways, but is also associated with brain disorders. This review describes topics relating to the involvement of both systems in several forms of brain dysfunction and indicates components of the KKS and RAS that have been used as targets in several pharmacological approaches.

IKAP deficiency in an FD mouse model and in oligodendrocyte precursor cells results in downregulation of genes involved in oligodendrocyte differentiation and myelin formation

The splice site mutation in the IKBKAP gene coding for IKAP protein leads to the tissue-specific skipping of exon 20, with concomitant reduction in IKAP protein production. This causes the neurodevelopmental, autosomal-recessive genetic disorder - Familial Dysautonomia (FD). The molecular hallmark of FD is the severe reduction of IKAP protein in the nervous system that is believed to be the main reason for the devastating symptoms of this disease. Our recent studies showed that in the brain of two FD patients, genes linked to oligodendrocyte differentiation and/or myelin formation are significantly downregulated, implicating IKAP in the process of myelination. However, due to the scarcity of FD patient tissues, these results awaited further validation in other models. Recently, two FD mouse models that faithfully recapitulate FD were generated, with two types of mutations resulting in severely low levels of IKAP expression. Here we demonstrate that IKAP deficiency in these FD mouse models affects a similar set of genes as in FD patients' brains. In addition, we identified two new IKAP target genes involved in oligodendrocyte cells differentiation and myelination, further underscoring the essential role of IKAP in this process. We also provide proof that IKAP expression is needed cell-autonomously for the regulation of expression of genes involved in myelin formation since knockdown of IKAP in the Oli-neu oligodendrocyte precursor cell line results in similar deficiencies. Further analyses of these two experimental models will compensate for the lack of human postmortem tissues and will advance our understanding of the role of IKAP in myelination and the disease pathology.