Natural Compounds as Protease Inhibitors in Therapeutic Focus on Cancer Therapy.

Proteases are implicated in every hallmark of cancer and have complicated functions. For cancer cells to survive and thrive, the process of controlling intracellular proteins to keep the balance of the cell proteome is essential. Numerous natural compounds have been used as ligands/ small molecules to target various proteases that are found in the lysosomes, mitochondria, cytoplasm, and extracellular matrix, as possible anticancer therapeutics. Promising protease modulators have been developed for new drug discovery technology through recent breakthroughs in structural and chemical biology. The protein structure, function of significant tumor-related proteases, and their natural compound inhibitors have been briefly included in this study. This review highlights the most current frontiers and future perspectives for novel therapeutic approaches associated with the list of anticancer natural compounds targeting protease and the mode and mechanism of proteinase-mediated molecular pathways in cancer.

Extracellular Matrix Structure and Interaction with Immune Cells in Adult Astrocytic Tumors.

The extracellular matrix (ECM) is a dynamic set of molecules produced by the cellular component of normal and pathological tissues of the embryo and adult. ECM acts as critical regulator in various biological processes such as differentiation, cell proliferation, angiogenesis, and immune control. The most frequent primary brain tumors are gliomas and by far the majority are adult astrocytic tumors (AATs). The prognosis for patients with these neoplasms is poor and the treatments modestly improves survival. In the literature, there is a fair number of studies concerning the composition of the ECM in AATs, while the number of studies relating the composition of the ECM with the immune regulation is smaller. Circulating ECM proteins have emerged as a promising biomarker that reflect the general immune landscape of tumor microenvironment and may represent a useful tool in assessing disease activity. Given the importance it can have for therapeutic and prognostic purposes, the aim of our study is to summarize the biological properties of ECM components and their effects on the tumor microenvironment and to provide an overview of the interactions between major ECM proteins and immune cells in AATs. As the field of immunotherapy in glioma is quickly expanding, we retain that current data together with future studies on ECM organization and functions in glioma will provide important insights into the tuning of immunotherapeutic approaches.

Intron retention in PI-PLC γ1 mRNA as a key mechanism affecting MMP expression in human primary fibroblast-like synovial cells.

The intron retention (IR) is a phenomenon utilized by cells to allow diverse fates at the same mRNA, leading to a different pattern of synthesis of the same protein. In this study, we analyzed the modulation of phosphoinositide-specific phospholipase C (PI-PLC) enzymes by Harpagophytum procumbens extract (HPE) in synoviocytes from joins of osteoarthritis (OA) patients. In some samples, the PI-PLC γ1 isoform mature mRNA showed the IR and, in these synoviocytes, the HPE treatment increased the phenomenon. Moreover, we highlighted that as a consequence of IR, a lower amount of PI-PLC γ1 was produced. The decrease of PI-PLC γ1 was associated with the decrease of metalloprotease-3 (MMP-3), and MMP-13, and ADAMTS-5 after HPE treatment. The altered expression of MMPs is a hallmark of the onset and progression of OA, thus substances able to decrease their expression are very desirable. The interesting outcomes of this study are that 35% of analyzed synovial tissues showed the IR phenomenon in the PI-PLC γ1 mRNA and that the HPE treatment increased this phenomenon. For the first time, we found that the decrease of PI-PLC γ1 protein in synoviocytes interferes with MMP production, thus affecting the pathways involved in the MMP expression. This finding was validated by the silencing of PI-PLC γ1 in synoviocytes where the IR phenomenon was not present. Our results shed new light on the biochemical mechanisms involved in the degrading enzyme production in the joint of OA patients, suggesting a new therapeutic target and highlighting the importance of personalized medicine.

Ectodomain shedding of PLA2R1 is mediated by the metalloproteases ADAM10 and ADAM17.

Phospholipase A2 receptor 1 (PLA2R1) is a 180-kDa transmembrane protein that plays a role in inflammation and cancer, and is the major autoantigen in membranous nephropathy (MN), a rare but severe autoimmune kidney disease. A soluble form of PLA2R1 has been detected in mouse and human serum. It is likely produced by proteolytic shedding of membrane-bound PLA2R1 but the mechanism is unknown. Here, we show that human PLA2R1 is cleaved by A Disintegrin And Metalloprotease 10 (ADAM10) and ADAM17 in HEK293 cells, mouse embryonic fibroblasts and human podocytes. By combining site-directed mutagenesis and sequencing, we determined the exact cleavage site within the extracellular juxtamembrane stalk of human PLA2R1. Orthologs and paralogs of PLA2R1 are also shed. By using pharmacological inhibitors and genetic approaches with RNA interference and knock-out cellular models, we identified a major role of ADAM10 in the constitutive shedding of PLA2R1, and a dual role of ADAM10 and ADAM17 in the stimulated shedding. We did not observe evidence for cleavage by ß- or γ-secretase, suggesting that PLA2R1 may not be a substrate for Regulated Intramembrane Proteolysis. PLA2R1 shedding occurs constitutively and can be triggered by the calcium ionophore ionomycin, the protein kinase C inducer PMA, cytokines and lipopolysaccharides, in vitro and in vivo. Altogether, our results show that PLA2R1 is a novel substrate for ADAM10 and ADAM17, producing a soluble form that is increased in inflammatory conditions and likely exerts various functions in physiological and pathophysiological conditions including inflammation, cancer and MN.

Investigation of diabetes-related molecular changes in embryo-endometrium crosstalk.

The primary aim of this study was to explore the impact of diabetes on matrix metalloproteases and tissue inhibitors, crucial factors for successful implantation, and to elucidate the molecular mechanisms that undergo changes in the endometrium and the embryo during diabetic pregnancies. In this investigation, we established a streptozotocin-induced diabetic pregnant rat model. Microarray analysis followed by RT-PCR was utilized to identify gene regions exhibiting expression alterations. Subsequently, we assessed the effects of MMPs and tissue inhibitors using ELISA and immunohistochemistry techniques, in addition to analyzing changes at the genetic level. Diabetes led to the upregulation of MMP3, MMP9, and MMP20 on the 6.5th day of pregnancy, while causing the downregulation of MMP3, MMP9, and MMP11 on the 8.5th day of pregnancy. TIMP1 expression was downregulated on the 8.5th day compared to the control group. No statistically significant differences were observed between the groups regarding other TIMP expressions. KEGG pathway analysis revealed that diabetes induced alterations in the expression of genes associated with certain microRNAs, as well as signaling pathways such as cAMP, calcium, BMP, p53, MAPK, PI3K-Akt, Jak-STAT, Hippo, Wnt, and TNF. Additionally, gene ontology analysis unveiled changes in membrane structures, extracellular matrix, signaling pathways, ion binding, protein binding, cell adhesion molecule binding, and receptor-ligand activity. This study serves as a valuable guide for investigating the mechanisms responsible for complications in diabetic pregnancies. By revealing the early-stage effects of diabetes, it offers insight into the development of new diagnostic and treatment approaches, ultimately contributing to improved patient care.

Low Rate of Postoperative Pterygium Recurrence in Patients under Treatment with Low-Dose Oral Doxycycline for Chronic Blepharitis: A First Report.

PURPOSE: Low doses of systemic doxycycline (LD-SD) inhibit angiogenesis and the expression of matrix metalloproteases, which are determinants of pterygium progression. This study aimed to compare the recurrence rate and visual outcome of pterygium excision in patients undergoing chronic treatment with LD-SD for chronic refractory blepharitis and LD-SD-naive patients. METHODS: A retrospective analysis of patients that underwent surgical excision and conjunctival graft apposition was conducted. Patients were divided in a TETRA group (under LD-SD treatment at the moment of surgery) and a control group. The main outcome was the rate of recurrence at 1 year postoperatively. Secondary outcomes were the comparisons of surface regularity, visual quality, and dry-eye symptoms at 6-week, 6-month, and 1-year follow-up in the two groups. RESULTS: The TETRA group showed a significantly lower rate of 1-year recurrence both in primary (p = 0.034) and recurrent (p < 0.001) pterygia. The best corrected visual acuity (BCVA), astigmatic error, corneal total root mean square (RMS), and ocular surface disease index (OSDI) significantly reduced during the follow-up in both groups. The surface asymmetry index and high-order aberrations (HOAs) significantly reduced only in the TETRA group. The final BCVA was significantly higher, while the OSDI score and total RMS and HOAs were significantly lower in the TETRA group compared to the control. CONCLUSIONS: Patients under treatment with LD-SD showed a lower rate of recurrence at 1-year follow-up compared to controls. These patients also experienced higher BCVA and surface regularity and less dry-eye symptoms.

Decoding Tumor Angiogenesis for Therapeutic Advancements: Mechanistic Insights.

Tumor angiogenesis, the formation of new blood vessels within the tumor microenvironment, is considered a hallmark of cancer progression and represents a crucial target for therapeutic intervention. The tumor microenvironment is characterized by a complex interplay between proangiogenic and antiangiogenic factors, regulating the vascularization necessary for tumor growth and metastasis. The study of angiogenesis involves a spectrum of techniques, spanning from biomarker assessment to advanced imaging modalities. This comprehensive review aims to provide insights into the molecular intricacies, regulatory dynamics, and clinical implications of tumor angiogenesis. By delving into these aspects, we gain a deeper understanding of the processes driving vascularization in tumors, paving the way for the development of novel and effective antiangiogenic therapies in the fight against cancer.

Plasmodium falciparum proteases as new drug targets with special focus on metalloproteases.

Malaria poses a significant global health threat particularly due to the prevalence of Plasmodium falciparum infection. With the emergence of parasite resistance to existing drugs including the recently discovered artemisinin, ongoing research seeks novel therapeutic avenues within the malaria parasite. Proteases are promising drug targets due to their essential roles in parasite biology, including hemoglobin digestion, merozoite invasion, and egress. While exploring the genomic landscape of Plasmodium falciparum, it has been revealed that there are 92 predicted proteases, with only approximately 14 of them having been characterized. These proteases are further distributed among 26 families grouped into five clans: aspartic proteases, cysteine proteases, metalloproteases, serine proteases, and threonine proteases. Focus on metalloprotease class shows further role in organelle processing for mitochondria and apicoplasts suggesting the potential of metalloproteases as viable drug targets. Holistic understanding of the parasite intricate life cycle and identification of potential drug targets are essential for developing effective therapeutic strategies against malaria and mitigating its devastating global impact.

Comparison of Tissue Factors in the Ontogenetic Aspects of Human Cholesteatoma.

BACKGROUND: An acquired cholesteatoma is a benign but locally aggressive lesion in the middle ear. It is characterized by chronic inflammation and the destruction of surrounding bone. Therefore, the aim of this study was to compare defensins HßD-2 and HßD-4; pro- and anti-inflammatory cytokines IL-1α and IL-10; proliferation marker Ki-67; transcription factor NF-κß; angiogenetic factor VEGF; Sonic hedgehog gene protein SHH; and remodeling factors MMP-2, MMP-9, TIMP-2, and TIMP-4 in adult and pediatric cholesteatoma tissue, and to compare these groups with control skin tissue. METHODS: The study included 25 cholesteatoma tissue material samples from children, 25 from adults, and 7 deep external ear canal skin samples from cadavers. The tissues were stained immunohistochemically and evaluated using semi-quantitative methods. Nonparametric tests, such as the Kruskal-Wallis test and Spearman rank correlation, were used. RESULTS: There were no statistically discernible differences between the adult and children groups when comparing the relative numbers of factor-positive cells. CONCLUSIONS: There are no histopathological differences between adult and children cholesteatoma tissues.

Role of the complement system in kidney cell death induced by Loxosceles venom Sphingomyelinases D.

Envenomation by Loxosceles spiders can result in local and systemic pathologies. Systemic loxoscelism, which can lead to death, is characterized by intravascular hemolysis, platelet aggregation, and acute kidney injury. Sphingomyelinase D (SMase D) in Loxosceles spider venom is responsible for both local and systemic pathologies, and has been shown to induce metalloprotease activity. As the complement system is involved in many renal pathologies and is involved in hemolysis in systemic loxoscelism, the aim of this study was to investigate its role and the role of complement regulators and metalloproteases in an in vitro model of Loxosceles venom induced renal pathology. We investigated the effects of the venom/SMase D and the complement system on the HK-2 kidney cell line. Using cell viability assays, western blotting, and flow cytometry, we show that human serum, as a source of complement, enhanced the venom/SMase D induced cell death and the deposition of complement components and properdin. Inhibitors for ADAM-10 and ADAM-17 prevented the venom induced release of the of the complement regulator MCP/CD46 and reduced the venom/SMase D induced cell death. Our results show that the complement system can contribute to Loxosceles venom induced renal pathology. We therefore suggest that patients experiencing systemic loxoscelism may benefit from treatment with metalloproteinase inhibitors and complement inhibitors, but this proposition should be further analyzed in future pre-clinical and clinical assays.

Identification, Biochemical Characterization, and In Vivo Detection of a Zn-Metalloprotease with Collagenase Activity from Mannheimia haemolytica A2.

Respiratory diseases in ruminants are a main cause of economic losses to farmers worldwide. Approximately 25% of ruminants experience at least one episode of respiratory disease during the first year of life. Mannheimia haemolytica is the main etiological bacterial agent in the ruminant respiratory disease complex. M. haemolytica can secrete several virulence factors, such as leukotoxin, lipopolysaccharide, and proteases, that can be targeted to treat infections. At present, little information has been reported on the secretion of M. haemolytica A2 proteases and their host protein targets. Here, we obtained evidence that M. haemolytica A2 proteases promote the degradation of hemoglobin, holo-lactoferrin, albumin, and fibrinogen. Additionally, we performed biochemical characterization for a specific 110 kDa Zn-dependent metalloprotease (110-Mh metalloprotease). This metalloprotease was purified through ion exchange chromatography and characterized using denaturing and chaotropic agents and through zymography assays. Furthermore, mass spectrometry identification and 3D modeling were performed. Then, antibodies against the 110 kDa-Mh metalloprotease were produced, which achieved great inhibition of proteolytic activity. Finally, the antibodies were used to perform immunohistochemical tests on postmortem lung samples from sheep with suggestive histology data of pneumonic mannheimiosis. Taken together, our results strongly suggest that the 110-Mh metalloprotease participates as a virulence mechanism that promotes damage to host tissues.

Redefining metalloproteases specificity through network proteolysis.

Proteolytic processes on cell surfaces and extracellular matrix (ECM) sustain cell behavior and tissue integrity in health and disease. Matrix metalloproteases (MMPs) and a disintegrin and metalloproteases (ADAMs) remodel cell microenvironments through irreversible proteolysis of ECM proteins and cell surface bioactive molecules. Pan-MMP inhibitors in inflammation and cancer clinical trials have encountered challenges due to promiscuous activities of MMPs. Systems biology advances revealed that MMPs initiate multifactorial proteolytic cascades, creating new substrates, activating or suppressing other MMPs, and generating signaling molecules. This review highlights the intricate network that underscores the role of MMPs beyond individual substrate-enzyme activities. Gaining insight into MMP function and tissue specificity is crucial for developing effective drug discovery strategies and novel therapeutics. This requires considering the dynamic cellular processes and consequences of network proteolysis.

Assessing Efficacy of Afatinib toward Elastic Matrix Repair in Aortic Aneurysms.

Abdominal aortic aneurysm (AAA) is a critical, multifactorial cardiovascular disorder marked by localized dilatation of the abdominal aorta. A major challenge to countering the pathophysiology of AAAs lies in the naturally irreversible breakdown of elastic fibers in the aorta wall, which is linked to the poor elastogenicity of adult and diseased vascular smooth muscle cells (SMCs) and their impaired ability to assemble mature elastic fibers in a chronic proteolytic tissue milieu. We have previously shown that these are downstream effects of neutrophil elastase-induced activation of the epidermal growth factor receptor (EGFR) activity in aneurysmal SMCs. The novelty of this study lies in investigating the benefits of an EGFR inhibitor drug, afatinib (used to treat nonsmall cell lung cancer), for proelastogenic and antiproteolytic stimulation of aneurysmal SMCs. In in vitro cell cultures, we have shown that safe doses of 0.5 and 1 nM afatinib inhibit EGFR and p-extracellular signal-regulated kinases 1/2 protein expression by 50-70% and downstream elastolytic matrix metalloprotease 2 (MMP2) versus untreated control cultures. In addition, elastin production on a per cell basis was significantly upregulated by afatinib doses within the 0.1-1 nM dose range, which was further validated through transmission electron microscopy showing significantly increased presence of tropoelastin coacervates and maturing elastic fibers upon afatinib treatment at the above doses. Therefore, our studies for the first time demonstrate the therapeutic benefits of afatinib toward use for elastic matrix repair in small AAAs.

Role of the complement system in kidney cell death induced by Loxosceles venom Sphingomyelinases D

Envenomation by Loxosceles spiders can result in local and systemic pathologies. Systemic loxoscelism, which can lead to death, is characterized by intravascular hemolysis, platelet aggregation, and acute kidney injury. Sphingomyelinase D (SMase D) in Loxosceles spider venom is responsible for both local and systemic pathologies, and has been shown to induce metalloprotease activity. As the complement system is involved in many renal pathologies and is involved in hemolysis in systemic loxoscelism, the aim of this study was to investigate its role and the role of complement regulators and metalloproteases in an in vitro model of Loxosceles venom induced renal pathology. We investigated the effects of the venom/SMase D and the complement system on the HK-2 kidney cell line. Using cell viability assays, western blotting, and flow cytometry, we show that human serum, as a source of complement, enhanced the venom/SMase D induced cell death and the deposition of complement components and properdin. Inhibitors for ADAM-10 and ADAM-17 prevented the venom induced release of the of the complement regulator MCP/CD46 and reduced the venom/SMase D induced cell death. Our results show that the complement system can contribute to Loxosceles venom induced renal pathology. We therefore suggest that patients experiencing systemic loxoscelism may benefit from treatment with metalloproteinase inhibitors and complement inhibitors, but this proposition should be further analyzed in future pre-clinical and clinical assays.

Correlation of MR-Based Metabolomics and Molecular Profiling in the Tumor Microenvironment of Temozolomide-Treated Orthotopic GL261 Glioblastoma in Mice.

The tumor microenvironment in glioblastoma (GB) is considered to be "cold", i.e., the fraction of cytotoxic T cells, for instance, is low. Instead, macrophages are the major immune cell population in GB, which stem either from tissue response (resident microglia) or recruitment of macrophages from the periphery, thereby undergoing tumor-dependent "imprinting" mechanisms by which macrophages can adapt a tumor-supportive phenotype. In this regard, it is important to describe the nature of macrophages associated with GB, in particular under therapy conditions using the gold standard chemotherapy drug temozolomide (TMZ). Here, we explored the suitability of combining information from in vivo magnetic resonance spectroscopic (MRS) approaches (metabolomics) with in vitro molecular analyses to assess therapy response and characterize macrophage populations in mouse GB using an isogenic GL261 model. For macrophage profiling, expression levels of matrix metalloproteinases (MMPs) and A disintegrin and metalloproteinases (ADAMs) were determined, since their gene products affect macrophage-tumor cell communication by extensive cleavage of immunomodulatory membrane proteins, such as PD-L1. In tumor mice with an overall therapy response, expression of genes encoding the proteases ADAM8, ADAM10, and ADAM17 was increased and might contribute to the immunosuppressive phenotype of GB and immune cells. In tumors responding to therapy, expression levels of ADAM8 were upregulated by TMZ, and higher levels of PD-L1 were correlated significantly. Using a CRISPR/Cas9 knockout of ADAM8 in GL261 cells, we demonstrated that soluble PD-L1 (sPD-L1) is only generated in the presence of ADAM8. Moreover, primary macrophages from WT and ADAM8-deficient mice showed ADAM8-dependent release of sPD-L1, independent of the macrophage polarization state. Since ADAM8 expression is induced in responding tumors and PD-L1 shedding is likely to decrease the anti-tumor activities of T-cells, we conclude that immunotherapy resistance is caused, at least in part, by the increased presence of proteases, such as ADAM8.

Inhibition of MMP-2 and MMP-9 attenuates surgery-induced cognitive impairment in aged mice.

BACKGROUND: The inhibition of matrix metalloproteinases (MMPs) has shown potential in the treatment of various neurodegenerative diseases, and perioperative neurocognitive disorders (PND) is accompanied by the increased expression of MMP-2 and MMP-9 in the hippocampus. However, the effect of inhibiting MMP-2 and MMP-9 on PND is not clear. In this study we aimed to evaluate the effects of inhibiting MMP-2 and MMP-9 on cognitive function in the aged mice after surgery, in order to find a possible target for the prevention and treatment of PND METHODS: In this study, 14-month-old C57BL/6 mice were used to establish a PND model by tibial fracture surgery and sevoflurane anesthesia. Three days later, part of the mice were subjected to cognitive assessment and the other was sacrificed for biochemical analysis. We used the Novel object recognition test and Fear conditioning test to evaluate the postoperative cognitive function of mice. The expression of mmp-2 and MMP-9 was detected by western blotting. We also examined the expression of claudin-5 and occludin using Western blotting, and the activation of microglia and astrocytes using immunofluorescence. RESULTS: The results showed that surgery increased the expression of MMP-2 and MMP-9 in the hippocampus of mice, accompanied by cognitive impairment, decreased expression of claudin-5 and occludin, and increased activation of microglia and astrocytes. However, inhibition of MMP-2 and MMP-9 expression by SB-3CT reversed these changes. CONCLUSIONS: Our study shows that inhibition of MMP-2 and MMP-9 alleviates anesthesia/surgery-induced cognitive decline by increasing BBB integrity and inhibiting glial cell activation.

Novel glycosyl prodrug of RXP03 as MMP-11 prodrug: design, synthesis and virtual screening.

Like most phosphinic acids, the potent and selective RXP03 inhibitor of different MMPs exhibited moderate absorption and low bioavailability, which impaired its use. In an unprecedented attempt, we present an interesting synthetic approach to a new class of phosphinate prodrug, glycosyl ester of RXP03, to provide a potentially improved blood-brain barrier (BBB) behavior compared to the former lead compound RXP03. To validate this speculation, a predictive study for permeability enhancer of glycosyl ester of RXP03 showed encouraging insights to improve drug delivery across biological barriers.

Matrix Metalloproteases 8 Polymorphism as Risk Factor for Rotator Cuff Tear.

Objectives: Rotator Cuff Tear (RCT) is a multifactorial disease, but an important one is the increased collagen degradation that would lead to a higher chance of tear. MMP-8 is a protein that degrades type I collagen, and it is known that MMP-8 has a polymorphism in which a T allele in the gene promoter region increases its transcription activity. This study aims to investigate the association between MMP-8 polymorphism g.-799 C>T (rs11225394) and RCT. Methods: To do that, we collected DNA samples from buccal epithelial cells of 128 patients (separated into RCT group and control group in a proportion 1:1) and genotyped the DNA using PCR. The statistical analyses were done using the ARLEQUIN Version 2.0, and the data normality was tested with the Shapiro-Wilk test. Results: The results showed a significantly higher frequency of T/T genotype in the test group (29% in the control group and 39% in the test group, p=0.0417), and that would represent a risk factor for increased collagen degradation. Conclusion: The MMP-8 g.-799 C>T (rs11225394) SNP was associated with RCT. With the description of a new risk factor, future research can be done to analyze how to prevent RCT or develop new treatment strategies since the disease's failure index is currently high.

Assessment of MMP14, CAV2, CLU and SPARCL1 expression profiles in endometriosis.

Endometriotic cells exhibit a notable degree of invasiveness and some characteristics of tissue remodeling underlying lesion formation. In this regard, do matrix metalloproteinases 14 (MMP14) and other related genes such as SPARC-like protein 1 (SPARCL1), caveolin 2 (CAV2), and clusterin (CLU) exert any significant influence in the processes of endometriosis development and pathophysiology is not apparent. We aim to assess whether these genes could serve as potential diagnostic biomarkers in endometriosis. Microarray-based gene expression analysis was performed on total RNA extracted from endometriotic tissue samples treated with and without gonadotropin-releasing hormone agonist (GnRHa). The GnRHa untreated patients were considered the control group. The validation of genes was performed using quantitative real-time polymerase chain reaction (qRT-PCR). qRT-PCR analysis showed significant downregulation in the expression of MMP14 (p = 0.024), CAV2 (p = 0.017), and upregulation of CLU (p = 0.005) in endometriosis patients treated with GnRHa. SPARCL1 did not show any significant (p = 0.30) change in the expression compared to the control group. These data have the potential to contribute to the comprehension of the molecular pathways implicated in the remodeling of the extracellular matrix, which is a vital step for the physiology of the endometrium. Based on the result, it is concluded that changes in the expression of MMP14, CAV2, and CLU post-treatment imply their role in the pathophysiology of endometriosis and may serve as a potential diagnostic biomarker of endometriosis in response to GnRHa treatment in patients with ovarian endometrioma.

Age-dependent changes on fractalkine forms and their contribution to neurodegenerative diseases.

The chemokine fractalkine (FKN, CX3CL1), a member of the CX3C subfamily, contributes to neuron-glia interaction and the regulation of microglial cell activation. Fractalkine is expressed by neurons as a membrane-bound protein (mCX3CL1) that can be cleaved by extracellular proteases generating several sCX3CL1 forms. sCX3CL1, containing the chemokine domain, and mCX3CL1 have high affinity by their unique receptor (CX3CR1) which, physiologically, is only found in microglia, a resident immune cell of the CNS. The activation of CX3CR1contributes to survival and maturation of the neural network during development, glutamatergic synaptic transmission, synaptic plasticity, cognition, neuropathic pain, and inflammatory regulation in the adult brain. Indeed, the various CX3CL1 forms appear in some cases to serve an anti-inflammatory role of microglia, whereas in others, they have a pro-inflammatory role, aggravating neurological disorders. In the last decade, evidence points to the fact that sCX3CL1 and mCX3CL1 exhibit selective and differential effects on their targets. Thus, the balance in their level and activity will impact on neuron-microglia interaction. This review is focused on the description of factors determining the emergence of distinct fractalkine forms, their age-dependent changes, and how they contribute to neuroinflammation and neurodegenerative diseases. Changes in the balance among various fractalkine forms may be one of the mechanisms on which converge aging, chronic CNS inflammation, and neurodegeneration.