Netrin-1 is highly expressed and required in inflammatory infiltrates in wear particle-induced osteolysis

OBJECTIVE

Netrin-1 is a chemorepulsant and matrix protein expressed during and required for osteoclast differentiation, which also plays a role in inflammation by preventing macrophage egress. Because wear particle-induced osteolysis requires osteoclast-mediated destruction of bone, we hypothesised that blockade of Netrin-1 or Unc5b, a receptor for Netrin-1, may diminish this pathological condition.

METHODS

C57BL/6 mice, 6-8 weeks old, had 3 mg of ultrahigh-molecular-weight polyethylene particles implanted over the calvaria and then received 10 µg of monoclonal antibodies for Netrin-1 or its receptors, Unc5b and deleted in colon cancer (DCC), injected intraperitoneally on a weekly basis. After 2 weeks, micro-computed tomography and histology analysis were performed. Netrin-1 expression was analysed in human tissue obtained following primary prosthesis implantation or after prosthesis revision for peri-implant osteolysis and aseptic implant loosening.

RESULTS

Weekly injection of anti-Netrin-1 or anti-Unc5b-antibodies significantly reduced particle-induced bone pitting in calvaria exposed to wear particles (46±4% and 49±3% of control bone pitting, respectively, p<0.001) but anti-DCC antibody did not affect inflammatory osteolysis (80±7% of control bone pitting, p=ns). Anti-Netrin-1 or anti-Unc5b, but not anti-DCC, antibody treatment markedly reduced the inflammatory infiltrate and the number of tartrate resistance acid phosphatase (TRAP)-positive osteoclasts (7±1, 4±1 and 14±1 cells/high power field (hpf), respectively, vs 12±1 cells/hpf for control, p<0.001), with no significant changes in alkaline phosphatase-positive osteoblasts on bone-forming surfaces in any antibody-treated group. Netrin-1 immunostaining colocalised with CD68 staining for macrophages. The peri-implant tissues of patients undergoing prosthesis revision surgery showed an increase in Netrin-1 expression, whereas there was little Netrin-1 expression in soft tissues removed at the time of primary joint replacement.

CONCLUSIONS

These results demonstrate a unique role for Netrin-1 in osteoclast biology and inflammation and may be a novel target for prevention/treatment of inflammatory osteolysis.

Netrin-1 as a novel therapeutic target in cardiovascular disease: to activate or inhibit?

Netrins are a family of laminin-like proteins, which were initially identified for their role in embryonic axonal guidance. Over recent years, it has become apparent that netrin-1 may additionally be involved in the underlying pathology of several multisystem diseases, making it an attractive potential therapeutic target. It is involved in postnatal angiogenesis, particularly in the context of an ischaemic insult, although there are conflicting reports as to whether netrin-1 acts in a pro- or anti-angiogenic capacity. In atherosclerosis, opposing effects have similarly been reported on plaque progression, due to the ability of netrin-1 to inhibit both macrophage egress from and monocyte ingress into lesions. Netrin-1 has also been shown to exert a cardioprotective action in the context of ischaemia-reperfusion injury following myocardial infarction. Moreover, urinary netrin-1 levels rise in response to acute kidney injury and at a faster rate than traditional markers of renal impairment, highlighting a potential clinical role for netrin-1 as a biomarker of renal function. The increased urinary excretion of netrin-1 during kidney disease is paralleled by a down-regulation of its plasma levels, with potential implications at a systemic level. In summary, the role of netrin-1 in cardiovascular disease is an emerging area of research requiring further in-depth study to elucidate its mechanism of action and potential as a therapeutic target, especially in view of its seemingly contradictory actions in certain physiological pathways which serve to highlight its manifold and often opposite effects in numerous physiological and pathophysiological processes.

MicroRNA-150 deletion in mice protects kidney from myocardial infarction-induced acute kidney injury

Despite greater understanding of acute kidney injury (AKI) in animal models, many of the preclinical studies are not translatable. Most of the data were derived from a bilateral renal pedicle clamping model with warm ischemia. However, ischemic injury of the kidney in humans is distinctly different and does not involve clamping of renal vessel. Permanent ligation of the left anterior descending coronary artery model was used to test the role of microRNA (miR)-150 in AKI. Myocardial infarction in this model causes AKI which is similar to human cardiac bypass surgery. Moreover, the time course of serum creatinine and biomarker elevation were also similar to human ischemic injury. Deletion of miR-150 suppressed AKI which was associated with suppression of inflammation and interstitial cell apoptosis. Immunofluorescence staining with endothelial marker and marker of apoptosis suggested that dying cells are mostly endothelial cells with minimal epithelial cell apoptosis in this model. Interestingly, deletion of miR-150 also suppressed interstitial fibrosis. Consistent with protection, miR-150 deletion causes induction of its target gene insulin-like growth factor-1 receptor (IGF-1R) and overexpression of miR-150 in endothelial cells downregulated IGF-1R, suggesting miR-150 may mediate its detrimental effects through suppression of IGF-1R pathways.

Aspirin-induced histone acetylation in endothelial cells enhances synthesis of the secreted isoform of netrin-1 thus inhibiting monocyte vascular infiltration

Background and Purpose

There are conflicting data regarding whether netrin-1 retards or accelerates atherosclerosis progression, as it can lead either to monocyte repulsion from or retention within plaques depending on its cellular source. We investigated the effect of aspirin, which is widely used in cardiovascular prophylaxis, on the synthesis of different isoforms of netrin-1 by endothelial cells under pro-inflammatory conditions, and defined the net effect of aspirin-dependent systemic modulation of netrin-1 on atherosclerosis progression.

Experimental Approach

Netrin-1 synthesis was studied in vitro using human endothelial cells stimulated with TNF-α, with or without aspirin treatment. In vivo experiments were conducted in ApoE^−/− mice fed with a high-fat diet (HFD), receiving either aspirin or clopidogrel.

Key Results

TNF-α-induced NF-κB activation up-regulated the nuclear isoform of netrin-1, while simultaneously reducing secreted netrin-1. Down-regulation of the secreted isoform compromised the chemorepellent action of the endothelium against monocyte chemotaxis. Aspirin counteracted TNF-α-mediated effects on netrin-1 synthesis by endothelial cells through COX-dependent inhibition of NF-κB and concomitant histone hyperacetylation. Administration of aspirin to ApoE^−/− mice on HFD increased blood and arterial wall levels of netrin-1 independently of its effects on platelets, accompanied by reduced plaque size and content of monocytes/macrophages, compared with untreated or clopidogrel-treated mice. In vivo blockade of netrin-1 enhanced monocyte plaque infiltration in aspirin-treated ApoE^−/− mice.

Conclusions and Implications

Aspirin counteracts down-regulation of secreted netrin-1 induced by pro-inflammatory stimuli in endothelial cells. The aspirin-dependent increase of netrin-1 in ApoE^−/− mice exerts anti-atherogenic effects by preventing arterial accumulation of monocytes.

Netrin-1 is a critical autocrine/paracrine factor for osteoclast differentiation

Bone metabolism is a vital process that involves resorption by osteoclasts and formation by osteoblasts, which is closely regulated by immune cells. The neuronal guidance protein Netrin-1 regulates immune cell migration and inflammatory reactions, but its role in bone metabolism is unknown. During osteoclast differentiation, osteoclast precursors increase expression of Netrin-1 and its receptor Unc5b. Netrin-1 binds, in an autocrine and paracrine manner, to Unc5b to promote osteoclast differentiation in vitro, and absence of Netrin-1 or antibody-mediated blockade of Netrin-1 or Unc5b prevents osteoclast differentiation of both murine and human precursors. We confirmed the functional relationship of Netrin-1 in osteoclast differentiation in vivo using Netrin-1-deficient (Ntn1(-/-) ) or wild-type (WT) bone marrow transplanted mice. Notably, Ntn1(-/-) chimeras have markedly diminished osteoclasts, as well as increased cortical and trabecular bone density and volume compared with WT mice. Mechanistic studies revealed that Netrin-1 regulates osteoclast differentiation by altering cytoskeletal assembly. Netrin-1 increases regulator of Rho-GEF subfamily (LARG) and repulsive guidance molecule (RGMa) association with Unc5b, which increases expression and activation of cytoskeletal regulators RhoA and focal adhesion kinase (FAK). Netrin-1 and its receptor Unc5b likely play a role in fusion of osteoclast precursors because Netrin-1 and DC-STAMP are tightly linked. These results identify Netrin-1 as a key regulator of osteoclast differentiation that may be a new target for bone therapies.

Netrin 1 regulates blood-brain barrier function and neuroinflammation

Blood-brain barrier function is driven by the influence of astrocyte-secreted factors. During neuroinflammatory responses the blood-brain barrier is compromised resulting in central nervous system damage and exacerbated pathology. Here, we identified endothelial netrin 1 induction as a vascular response to astrocyte-derived sonic hedgehog that promotes autocrine barrier properties during homeostasis and increases with inflammation. Netrin 1 supports blood-brain barrier integrity by upregulating endothelial junctional protein expression, while netrin 1 knockout mice display disorganized tight junction protein expression and barrier breakdown. Upon inflammatory conditions, blood-brain barrier endothelial cells significantly upregulated netrin 1 levels in vitro and in situ, which prevented junctional breach and endothelial cell activation. Finally, netrin 1 treatment during experimental autoimmune encephalomyelitis significantly reduced blood-brain barrier disruption and decreased clinical and pathological indices of disease severity. Our results demonstrate that netrin 1 is an important regulator of blood-brain barrier maintenance that protects the central nervous system against inflammatory conditions such as multiple sclerosis and experimental autoimmune encephalomyelitis.

Therapeutic effects of topical netrin-4 in a corneal acute inflammatory model

AIM

To evaluate the therapeutic effect of netrin-4 on the early acute phase of inflammation in the alkali-burned eye.

METHODS

Eye drops containing netrin-4 or phosphate buffered saline (PBS) were administered to a alkali-burn-induced corneal acute inflammatory model four times daily. The clinical evaluations, including fluorescein staining and inflammatory index, were performed on day 1, 4 and 7 using slit lamp microscopy. Global specimens were collected on day 7 and processed for immunofluorescent staining. The levels of inflammatory mediators in the corneas were determined by real-time polymerase chain reaction (PCR).

RESULTS

Exogenous netrin-4 administered on rat ocular surfaces showed more improvements in decreasing fluorescein staining on day 4 and 7, and resolved alkali burn-induced corneal inflammation index on day 7 (P<0.01). The levels of IL-1β, IL-6, intercellular cell adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), monocyte chemotactic protein-1 (MCP-1) and macrophage inflammatory protein-1 (MIP-1) in corneas were decreased in netrin-4-treated groups (P<0.05). In addition, netrin-4 significantly reduced the expression of leukocyte common antigen 45 (CD45) in the alkali-burn cornea (P<0.001).

CONCLUSION

Topical netrin-4 accelerated wound healing and reduced the inflammation on alkali-burn rat model, suggesting a potential as an anti-inflammatory agent in the clinical to treat the acute inflammation.

Deletion of UNC5B in Kidney Epithelium Exacerbates Diabetic Nephropathy in Mice

BACKGROUND

Guidance cue netrin-1 was shown to have protective effects in diabetic nephropathy. However, the role of its receptor UNC5B in diabetic kidney disease is unknown. Moreover, whether netrin-1 is protective against diabetic kidney disease in a genetic model of nephropathy and in the nephropathy prone DBA background is also unknown. The aim of this study was to determine the significance of UNC5B in tubular epithelial cells in chronic kidney disease due to diabetes and evaluate whether netrin-1 is also protective in the case of a nephropathy-prone mouse.

METHODS

Proximal tubular epithelium-specific UNC5B knockout mice as well as heterozygous UNC5B knockout mice were used to determine the roles of UNC5B in nephropathy. Diabetes was induced in these tissue-specific knockout, heterozygous and WT mice, and albuminuria was then monitored.

RESULTS

WT and heterozygous diabetic mice developed significant albuminuria at 8 weeks after induction of diabetes as compared to buffer-treated control mice. However, albuminuria was significantly more pronounced in mice with proximal tubule specific deletion of UNC5B. Transgenic overexpression of netrin-1 in proximal tubules in the DBA background and administration of recombinant netrin-1 to Ins2Akita mice also significantly reduced diabetes-induced albuminuria and suppressed glomerular and interstitial lesions.

CONCLUSION

Our data suggested that netrin-1 signaling in proximal tubular epithelium may play a critical role in the protection of kidney against diabetic kidney disease.

Maternal age effects on myometrial expression of contractile proteins, uterine gene expression, and contractile activity during labor in the rat

Advanced maternal age of first time pregnant mothers is associated with prolonged and dysfunctional labor and significant risk of emergency cesarean section. We investigated the influence of maternal age on myometrial contractility, expression of contractile associated proteins (CAPs), and global gene expression in the parturient uterus. Female Wistar rats either 8 (YOUNG n = 10) or 24 (OLDER n = 10) weeks old were fed laboratory chow, mated, and killed during parturition. Myometrial strips were dissected to determine contractile activity, cholesterol (CHOL) and triglycerides (TAG) content, protein expression of connexin-43 (GJA1), prostaglandin-endoperoxide synthase 2 (PTGS2), and caveolin 1 (CAV-1). Maternal plasma concentrations of prostaglandins PGE2, PGF2α, and progesterone were determined by RIA. Global gene expression in uterine samples was compared using Affymetrix Genechip Gene 2.0 ST arrays and Ingenuity Pathway analysis (IPA). Spontaneous contractility in myometrium exhibited by YOUNG rats was threefold greater than OLDER animals (P < 0.027) but maternal age had no significant effect on myometrial CAP expression, lipid profiles, or pregnancy-related hormones. OLDER myometrium increased contractile activity in response to PGF2α, phenylephrine, and carbachol, a response absent in YOUNG rats (all P < 0.002). Microarray analysis identified that maternal age affected expression of genes related to immune and inflammatory responses, lipid transport and metabolism, steroid metabolism, tissue remodeling, and smooth muscle contraction. In conclusion YOUNG laboring rat myometrium seems primed to contract maximally, whereas activity is blunted in OLDER animals and requires stimulation to meet contractile potential. Further work investigating maternal age effects on myometrial function is required with focus on lipid metabolism and inflammatory pathways.

Netrin-1 attenuates cardiac ischemia reperfusion injury and generates alternatively activated macrophages

Ischemia reperfusion (IR) injury is a major issue in cardiac transplantation and inflammatory processes play a major role in myocardial IR injury. Netrin-1 is a laminin-related protein identified as a neuronal guidance cue and netrin-1 expressed outside the nervous system inhibits migration of leukocytes in vitro and in vivo and attenuates inflammation-mediated tissue injury. In our study, hearts of C57BL/6 mice were flushed and stored in cold Bretschneider solution for 8 h and then transplanted into syngeneic recipient. We found that netrin-1 decreased cardiomyocyte apoptosis and recruitment of neutrophils and macrophages. Troponin T (TnT) production on 24 h after myocardial IR injury was reduced by netrin-1 administration. Cardiac output at 60 mmHg of afterload pressure was significantly increased in hearts with netrin-1 administration (IR + Netrin-1: 59.9 ± 5.78 ml/min; IR: 26.2 ± 4.3 ml/min; P < 0.05). Netrin-1 treatment increased expression of the alternatively activated macrophage (AAM) markers arginase-1 (Arg-1) and mannose receptor (MR) and promoted proliferator-activated receptor γ (PPARγ) expression in cardiac allograft. Furthermore, decreased TnT expression and reduced allograft infiltration of neutrophils and monocytes/macrophages by netrin-1 was abolished with addition of PPARγ antagonist. In conclusion, netrin-1 attenuates cardiac IR injury and generates AAM which contributes to the protective effect of netrin-1.

Netrin-1 controls sympathetic arterial innervation

Autonomic sympathetic nerves innervate peripheral resistance arteries, thereby regulating vascular tone and controlling blood supply to organs. Despite the fundamental importance of blood flow control, how sympathetic arterial innervation develops remains largely unknown. Here, we identified the axon guidance cue netrin-1 as an essential factor required for development of arterial innervation in mice. Netrin-1 was produced by arterial smooth muscle cells (SMCs) at the onset of innervation, and arterial innervation required the interaction of netrin-1 with its receptor, deleted in colorectal cancer (DCC), on sympathetic growth cones. Function-blocking approaches, including cell type-specific deletion of the genes encoding Ntn1 in SMCs and Dcc in sympathetic neurons, led to severe and selective reduction of sympathetic innervation and to defective vasoconstriction in resistance arteries. These findings indicate that netrin-1 and DCC are critical for the control of arterial innervation and blood flow regulation in peripheral organs.

Guidance cue netrin-1 and the regulation of inflammation in acute and chronic kidney disease

Acute kidney injury (AKI) is a common problem in the hospital setting and intensive care unit. Despite improved understanding, there are no effective therapies available to treat AKI. A large body of evidence strongly suggests that ischemia reperfusion injury is an inflammatory disease mediated by both adaptive and innate immune systems. Cell migration also plays an important role in embryonic development and inflammation, and this process is highly regulated to ensure tissue homeostasis. One such paradigm exists in the developing nervous system, where neuronal migration is mediated by a balance between chemoattractive and chemorepulsive signals. The ability of the guidance molecule netrin-1 to repulse or abolish attraction of neuronal cells expressing the UNC5B receptor makes it an attractive candidate for the regulation of inflammatory cell migration. Recent identification of netrin-1 as regulators of immune cell migration has led to a large number of studies looking into how netrin-1 controls inflammation and inflammatory cell migration. This review will focus on recent advances in understanding netrin-1 mediated regulation of inflammation during acute and chronic kidney disease and whether netrin-1 and its receptor activation can be used to treat acute and chronic kidney disease.

Semaphorin 3A inactivation suppresses ischemia-reperfusion-induced inflammation and acute kidney injury

Recent studies show that guidance molecules that are known to regulate cell migration during development may also play an important role in adult pathophysiologic states. One such molecule, semaphorin3A (sema3A), is highly expressed after acute kidney injury (AKI) in mice and humans, but its pathophysiological role is unknown. Genetic inactivation of sema3A protected mice from ischemia-reperfusion-induced AKI, improved tissue histology, reduced neutrophil infiltration, prevented epithelial cell apoptosis, and increased cytokine and chemokine excretion in urine. Pharmacological-based inhibition of sema3A receptor binding likewise protected against ischemia-reperfusion-induced AKI. In vitro, sema3A enhanced toll-like receptor 4-mediated inflammation in epithelial cells, macrophages, and dendritic cells. Moreover, administration of sema3A-treated, bone marrow-derived dendritic cells exacerbated kidney injury. Finally, sema3A augmented cisplatin-induced apoptosis in kidney epithelial cells in vitro via expression of DFFA-like effector a (cidea). Our data suggest that the guidance molecule sema3A exacerbates AKI via promoting inflammation and epithelial cell apoptosis.

Netrin-1 promotes medulloblastoma cell invasiveness and angiogenesis, and demonstrates elevated expression in tumor tissue and urine of patients with pediatric medulloblastoma

Invasion and dissemination of medulloblastoma within the central nervous system is the principal factor predicting medulloblastoma treatment failure and death. Netrin-1 is an axon guidance factor implicated in tumor and vascular biology, including in invasive behaviors. We found that exogenous netrin-1 stimulated invasion of human medulloblastoma cells and endothelial cells in contrast to VEGF-A, which promoted invasion of endothelial cells but not medulloblastoma cells. Furthermore, medulloblastoma cells expressed endogenous netrin-1 along with its receptors, neogenin and UNC5B. Blockades in endogenous netrin-1, neogenin, or UNC5B reduced medulloblastoma invasiveness. Neogenin blockade inhibited netrin-1-induced endothelial cells tube formation and recruitment of endothelial cells into Matrigel plugs, two hallmarks of angiogenesis. In patients with pediatric medulloblastoma, netrin-1 mRNA levels were increased 1.7-fold in medulloblastoma tumor specimens compared with control specimens from the same patient. Immunohistochemical analyses showed that netrin-1 was elevated in medulloblastoma tumors versus cerebellum controls. Notably, urinary levels of netrin-1 were 9-fold higher in patients with medulloblastoma compared with control individuals. Moreover, urinary netrin-1 levels were higher in patients with invasive medulloblastoma compared with patients with noninvasive medulloblastoma. Finally, we noted that urinary netrin-1 levels diminished after medulloblastoma resection in patients. Our results suggest netrin-1 is a candidate biomarker capable of detecting an invasive, disseminated phenotype in patients with medulloblastoma and predicting their disease status.

UNC5B receptor deletion exacerbates DSS-induced colitis in mice by increasing epithelial cell apoptosis

The netrin-1 administration or overexpression is known to protect colon from acute colitis. However, the receptor that mediates netrin-1 protective activities in the colon during colitis remains unknown. We tested the hypothesis that UNC5B receptor is a critical mediator of protective function of netrin-1 in dextran sodium sulfate (DSS)-induced colitis using mice with partial deletion of UNC5B receptor. DSS colitis was performed in mice with partial genetic UNC5B deficiency (UNC5B^+/− mice) or wild-type mice to examine the role of endogenous UNC5B. These studies were supported by in vitro models of DSS-induced apoptosis in human colon epithelial cells. WT mice developed colitis in response to DSS feeding as indicated by reduction in bw, reduction in colon length and increase in colon weight. These changes were exacerbated in heterozygous UNC5B knockout mice treated with DSS. Periodic Acid-Schiff stained section shows damages in colon epithelium and mononuclear cell infiltration in WT mice, which was further increased in UNC5B heterozygous knockout mice. This was associated with large increase in inflammatory mediators such as cytokine and chemokine expression and extensive apoptosis of epithelial cells in heterozygous knockout mice as compared to WT mice. Overexpression of UNC5B human colon epithelial cells suppressed DSS-induced apoptosis and caspase-3 activity. Moreover, DSS induced large amount of netrin-1 and shRNA mediated knockdown of netrin-1 induction exacerbated DSS-induced epithelial cell apoptosis. Our results suggest that UNC5B is a critical mediator of cell survival in response to stress in colon.

Netrin-1 guides inflammatory cell migration to control mucosal immune responses during intestinal inflammation

The intestinal epithelium is a dynamic barrier playing an active role in intestinal homeostasis and inflammation. Intestinal barrier function is dysregulated during inflammatory bowel disease (IBD), with epithelial cells playing a significant part in generating an inflammatory milieu through the release of signals that attract leukocytes to the intestinal lamina propria. However, it is increasingly appreciated that the intestinal epithelium mediates a counterbalancing response to drive resolution. Drawing analogies with neuronal development, where the balance of chemoattractive and chemorepellent signals is key to directed neuronal movement it has been postulated that such secreted cues play a role in leukocyte migration. Netrin-1 is one of the best-described neuronal guidance molecules, which has been shown to play a significant role in directed migration of leukocytes. Prior to our study the potential role of netrin-1 in IBD was poorly characterized. We defined netrin-1 as an intestinal epithelial-derived protein capable of limiting neutrophil recruitment to attenuate acute colitis. Our study highlights that the intestinal epithelium releases factors during acute inflammation that are responsible for fine-tuning the immune response. Exploration of these epithelial-mediated protective mechanisms will shed light on the complexity of the intestinal epithelial barrier in health and disease.

Netrin-1 promotes adipose tissue macrophage retention and insulin resistance in obesity

During obesity, macrophage accumulation in adipose tissue propagates the chronic inflammation and insulin resistance associated with type 2 diabetes. The factors, however, that regulate the accrual of macrophages in adipose tissue are not well understood. Here we show that the neuroimmune guidance cue netrin-1 is highly expressed in obese but not lean adipose tissue of humans and mice, where it directs the retention of macrophages. Netrin-1, whose expression is induced in macrophages by the saturated fatty acid palmitate, acts via its receptor Unc5b to block their migration. In a mouse model of diet-induced obesity, we show that adipose tissue macrophages exhibit reduced migratory capacity, which can be restored by blocking netrin-1. Furthermore, hematopoietic deletion of Ntn1 facilitates adipose tissue macrophage emigration, reduces inflammation and improves insulin sensitivity. Collectively, these findings identify netrin-1 as a macrophage retention signal in adipose tissue during obesity that promotes chronic inflammation and insulin resistance.

Netrin-1, a urinary proximal tubular injury marker, is elevated early in the time course of human diabetes

BACKGROUND

Netrin-1 was recently identified as an early diagnostic biomarker of chronic kidney disease (CKD) in an experimental animal model. However, its usefulness for early diagnosis of CKD in humans is unknown. The current study evaluated whether netrin-1 is increased in urine from human diabetic patients.

METHODS

Spot urine samples from healthy volunteers, diabetes without microalbuminuria, diabetes with microalbuminuria and diabetes with macroalbuminuria were collected after receiving consent. Netrin-1 in urine was quantified by enzyme-linked immunosorbent assay and the data analyzed to determine whether urinary netrin-1 significantly correlates with disease progression.

RESULTS

Urinary netrin-1 levels were significantly increased in normoalbuminuric diabetic patients compared to healthy controls, and still further elevated in patients with microalbuminuria and overt nephropathy. Urinary netrin-1 was significantly associated with albuminuria and estimated glomerular filtration rate, independently of age and sex.

CONCLUSION

Netrin-1 is detectable in urine from diabetic patients and may serve as a useful early diagnostic biomarker predicting the development of CKD in diabetes.

Netrin-1 overexpression promotes white matter repairing and remodeling after focal cerebral ischemia in mice

Damage of oligodendrocytes after ischemia has negative impact on white matter integrity and neuronal function. In this work, we explore whether Netrin-1 (NT-1) overexpression facilitates white matter repairing and remodeling. Adult CD-1 mice received stereotactic injection of adeno-associated virus carrying NT-1 gene (AAV-NT-1). One week after gene transfer, mice underwent 60 minutes of middle cerebral artery occlusion. The effect of NT-1 on neural function was evaluated by neurobehavioral tests. Proliferated oligodendrocyte progenitor cells (OPCs), newly matured oligodendrocytes, and remyelination were semi-quantified by immunohistochemistry. The role of NT-1 in oligodendrogenesis was further explored by examining specific NT-1 receptors and their function. Netrin-1 overexpression was detected in neurons and astrocytes 2 weeks after AAV-NT-1 gene transfer and significantly improved the neurobehavioral outcomes compared with the control (P<0.05). In comparison with the control, proliferated OPCs, newly matured oligodendrocytes, and remyelination were greatly increased in the ipsilateral hemisphere of AAV-NT-1-transduced mice. Furthermore, both NT-1 receptors deleted in colorectal carcinoma and UNC5H2 were expressed on OPCs whereas only UNC5H2 was expressed in myelinated axons. Our study indicated that NT-1 promoted OPC proliferation, differentiation, and increased remyelination, suggesting that NT-1 is a promising factor for white matter repairing and remodeling after ischemia.

UNC5B receptor deletion exacerbates tissue injury in response to AKI

Netrin-1 regulates cell survival and apoptosis by activation of its receptors, including UNC5B. However, the in vivo role of UNC5B in cell survival during cellular stress and tissue injury is unknown. We investigated the role of UNC5B in cell survival in response to stress using mice heterozygously expressing the UNC5B gene (UNC5B(-/flox)) and mice with targeted homozygous deletion of UNC5B in kidney epithelial cells (UNC5B(-/flox/GGT-cre)). Mice were subjected to two different models of organ injury: ischemia reperfusion injury of the kidney and cisplatin-induced nephrotoxicity. Both mouse models of UNC5B depletion had normal organ function and histology under basal conditions. After AKI, however, UNC5B(-/flox/GGT-cre) mice exhibited significantly worse renal function and damage, increased tubular apoptosis, enhanced p53 activation, and exacerbated inflammation compared with UNC5B(-/flox) and wild-type mice. shRNA-mediated suppression of UNC5B expression in cultured tubular epithelial cells exacerbated cisplatin-induced cell death in a p53-dependent manner and blunted Akt phosphorylation. Inhibition of PI3 kinase similarly exacerbated cisplatin-induced apoptosis; in contrast, overexpression of UNC5B reduced cisplatin-induced apoptosis in these cells. Taken together, these results show that the netrin-1 receptor UNC5B plays a critical role in cell survival and kidney injury through Akt-mediated inactivation of p53 in response to stress.

The uncoordinated-5 homolog B (UNC5B) receptor increases myocardial ischemia-reperfusion injury

The UNC5 receptor family are chemorepulsive neuronal guidance receptors with additional functions outside the central nervous system. Previous studies have implicated that the UNC5B receptor influences the migration of leukocytes into sites of tissue inflammation. Given that this process is a critical step during the pathophysiology of myocardial ischemia followed by reperfusion (IR) we investigated the role of UNC5B during myocardial IR. In initial in-vitro experiments, the functional inhibition of UNC5B resulted in a significant reduction of chemotactic migration of neutrophils. In-vivo, using a model of acute myocardial ischemia in UNC5B^+/− and wild type (WT) animals, we found a significant reduction of infarct sizes in UNC5B^+/− animals. This was associated with significantly reduced levels of troponin-I and IL-6 in UNC5B^+/− mice. The repression of UNC5B using siRNA and the functional inhibition of UNC5B significantly dampened the extent of myocardial IR injury. Following depletion of neutrophils, we were not able to observe any further reduction in infarct size through functional inhibition of UNC5B in WT and UNC5B^+/− mice. In summary our studies demonstrate an important role for UNC5B during myocardial IR injury, and that UNC5B might be a potential therapeutic target to control reperfusion injury in the future.

Full-length and fragmented netrin-1 in multiple sclerosis plaques are inhibitors of oligodendrocyte precursor cell migration

Oligodendrocytes exhibit a limited capacity to remyelinate in multiple sclerosis. Factors present in multiple sclerosis lesions are thought to inhibit oligodendrocyte precursor cell migration, limiting their recruitment to axons requiring remyelination; however, few inhibitors have been identified. A candidate inhibitor is netrin-1, a secreted protein that repels migrating oligodendrocyte precursor cells during neural development and is expressed by myelinating oligodendrocytes in the mature rodent central nervous system. Herein, we examined the distribution of netrin-1 in adult human white matter and multiple sclerosis lesions. We detected full-length netrin-1 protein and shorter netrin-1 fragments in samples of normal white matter and of multiple sclerosis lesions from adult human brain. We demonstrate that peptides corresponding to amino terminal domains VI and V of netrin-1 repel migrating oligodendrocyte precursor cells, but lack the chemoattractant activity of full-length netrin-1. Furthermore, recombinant domains VI-V of netrin-1 disrupt the chemoattractant activity of full-length netrin-1, consistent with a competitive mechanism of action. These findings indicate that full-length and fragmented forms of netrin-1, found in multiple sclerosis lesions, have the capacity to inhibit oligodendrocyte precursor migration, identifying netrin-1 as a potential target for therapies that promote remyelination.

Galectin-1-mediated suppression of Pseudomonas aeruginosa-induced corneal immunopathology

Corneal infection with Pseudomonas aeruginosa leads to a severe immunoinflammatory lesion, often causing vision impairment and blindness. Although past studies have indicated a critical role for CD4(+) T cells, particularly Th1 cells, in corneal immunopathology, the relative contribution of recently discovered Th17 and regulatory T cells is undefined. In this study, we demonstrate that after corneal P. aeruginosa infection, both Th1 and Th17 cells infiltrate the cornea with increased representation of Th17 cells. In addition to Th1 and Th17 cells, regulatory T cells also migrate into the cornea during early as well as late stages of corneal pathology. Moreover, using galectin-1 (Gal-1), an immunomodulatory carbohydrate-binding molecule, we investigated whether shifting the balance among various CD4(+) T cell subsets can modulate P. aeruginosa-induced corneal immunopathology. We demonstrate in this study that local recombinant Gal-1 (rGal-1) treatment by subconjunctival injections significantly diminishes P. aeruginosa-mediated corneal inflammation through multiple mechanisms. Specifically, in our study, rGal-1 treatment significantly diminished corneal infiltration of total CD45(+) T cells, neutrophils, and CD4(+) T cells. Furthermore, rGal-1 treatment significantly reduced proinflammatory Th17 cell response in the cornea as well as local draining lymph nodes. Also, rGal-1 therapy promoted anti-inflammatory Th2 and IL-10 response in secondary lymphoid organs. Collectively, our results indicate that corneal P. aeruginosa infection induces a strong Th17-mediated corneal pathology, and treatment with endogenously derived protein such as Gal-1 may be of therapeutic value for the management of bacterial keratitis, a prevalent cause of vision loss and blindness in humans worldwide.

Protective role for netrin-1 during diabetic nephropathy

Recent studies implicate neuronal guidance molecules in the orchestration of inflammatory events. For example, previous studies demonstrate a functional role for netrin-1 in attenuating acute kidney injury. Here, we hypothesized a kidney-protective role for netrin-1 during chronic kidney disease, such as occurs during diabetic nephropathy. To study the role of netrin-1 during diabetic nephropathy, we induced diabetes in mice at the age of 8 weeks by streptocotozin (STZ) treatment. Sixteen weeks after STZ treatment, we examined the kidneys. Initial studies in wild-type mice demonstrated robust induction of renal, urinary, and plasma netrin-1 protein levels during diabetic nephropathy. Subsequent genetic studies in mice with partial netrin-1 deficiency (Ntrn1(+/-) mice) revealed a more severe degree of diabetic nephropathy, including more severe loss of kidney function (albuminuria, glomerular filtration rate, histology). We subsequently performed pharmacologic studies with recombinant netrin-1 treatment given continuously via osmotic pump. Indeed, netrin-1 treatment was associated with attenuated albuminuria and improved histologic scores for diabetic nephropathy compared to controls. Consistent with previous studies implicating purinergic signaling in netrin-1-elicited tissue protection, mice deficient in the Adora2b adenosine receptor were not protected. Taken together, these studies demonstrate a functional role for endogenous netrin-1 in attenuating diabetic kidney disease.

Hypoxia induces netrin-1 and Unc5b in atherosclerotic plaques: mechanism for macrophage retention and survival

OBJECTIVE

Hypoxia is intimately linked to atherosclerosis and has become recognized as a primary impetus of inflammation. We recently demonstrated that the neuroimmune guidance cue netrin-1 (Ntn1) inhibits macrophage emigration from atherosclerotic plaques, thereby fostering chronic inflammation. However, the mechanisms governing netrin-1 expression in atherosclerosis are not well understood. In this study, we investigate the role of hypoxia in regulating expression of netrin-1 and its receptor uncoordinated-5-B receptor (Unc5b) in plaque macrophages and its functional consequences on these immune cells.

APPROACH AND RESULTS

We show by immunostaining that netrin-1 and Unc5b are expressed in macrophages in hypoxia-rich regions of human and mouse plaques. In vitro, Ntn1 and Unc5b mRNA are upregulated in macrophages treated with oxidized low-density lipoprotein or inducers of oxidative stress (CoCl2, dimethyloxalylglycine, 1% O2). These responses are abrogated by inhibiting hypoxia-inducible transcription factor (HIF)-1α, indicating a causal role for this transcription factor in regulating Ntn1 and Unc5b expression in macrophages. Indeed, using promoter-luciferase reporter genes, we show that Ntn1- and Unc5b-promoter activities are induced by oxidized low-density lipoprotein and require HIF-1α. Correspondingly, J774 macrophages overexpressing active HIF-1α show increased netrin-1 and Unc5b expression and reduced migratory capacity compared with control cells, which was restored by blocking the effects of netrin-1. Finally, we show that netrin-1 protects macrophages from apoptosis under hypoxic conditions in a HIF-1α-dependent manner.

CONCLUSIONS

These findings provide a molecular mechanism by which netrin-1 and its receptor Unc5b are expressed in atherosclerotic plaques and implicate hypoxia and HIF-1α-induced netrin-1/Unc5b in sustaining inflammation by inhibiting the emigration and promoting the survival of lesional macrophages.

PKCα is involved in the progression of kidney carcinoma through regulating netrin-1/UNC5B signaling pathway

With a special interest towards a better understanding of signal pathways, we attempted to discover a safer and more effective therapeutic strategy for kidney carcinoma. Recent studies had suggested a role mediated by PKCα for netrin-1 and its receptors in the initiation and progression of tumors. Real-time PCR and western blotting were used to determine the expression levels of netrin-1 and UNC5B. We made use of the agonist of PKCα (phorbol-12-myristate 13-acetate-PMA) and the inhibitor of PKCα (calphostin C) to treat renal cell carcinoma (RCC) cells, and MTT assays were used to measure cell proliferation. By immunofluorescence, we identified the localization of netrin-1 and UNC5B in RCC cell lines 769-P and ACHN. The expression of UNC5B in tumor tissues was significantly downregulated compared to the corresponding normal tissues in which netrin-1 was upregulated. In low grade tumors, UNC5B expression was more prominent while netrin-1 expression was the opposite when compared with high grade ones. Proliferation of ACHN cells was concentration dependent in the presence of PMA and calphostin C. Netrin-1 and UNC5B expressions were upregulated in cells treated with PMA while calphostin C reversed this upregulation. By immunofluorescence, we identified that netrin-1 was highly expressed in the nuclear but none of UNC5B. Our data highly suggested that PMA-induced upregulation and calphostin C-induced reversion of netrin-1 and UNC5B in kidney carcinoma were accompanied by the activation of the netrin-1/UNC5B pathways.

Neuronal ER stress impedes myeloid-cell-induced vascular regeneration through IRE1α degradation of netrin-1

In stroke and proliferative retinopathy, despite hypoxia driven angiogenesis, delayed revascularization of ischemic tissue aggravates the loss of neuronal function. What hinders vascular regrowth in the ischemic central nervous system remains largely unknown. Using the ischemic retina as a model of neurovascular interaction in the CNS, we provide evidence that the failure of reparative angiogenesis is temporally and spatially associated with endoplasmic reticulum (ER) stress. The canonical ER stress pathways of protein kinase RNA-like ER kinase (PERK) and inositol-requiring enzyme-1α (IRE1α) are activated within hypoxic/ischemic retinal ganglion neurons, initiating a cascade that results in angiostatic signals. Our findings demonstrate that the endoribonuclease IRE1α degrades the classical guidance cue netrin-1. This neuron-derived cue triggers a critical reparative-angiogenic switch in neural macrophage/microglial cells. Degradation of netrin-1, by persistent neuronal ER stress, thereby hinders vascular regeneration. These data identify a neuronal-immune mechanism that directly regulates reparative angiogenesis.

Semaphorin 3A is a new early diagnostic biomarker of experimental and pediatric acute kidney injury

BACKGROUND

Semaphorin 3A is a secreted protein that regulates cell motility and attachment in axon guidance, vascular growth, immune cell regulation and tumor progression. However, nothing is known about its role in kidney pathophysiology. Here, we determined whether semaphorin3A is induced after acute kidney injury (AKI) and whether urinary semaphorin 3A can predict AKI in humans undergoing cardiopulmonary bypass (CPB).

METHODS AND PRINCIPAL FINDINGS

In animals, semaphorin 3A is localized in distal tubules of the kidney and excretion increased within 3 hr after reperfusion of the kidney whereas serum creatinine was significantly raised at 24 hr. In humans, using serum creatinine, AKI was detected on average only 48 hours after CPB. In contrast, urine semaphorin increased at 2 hours after CPB, peaked at 6 hours (2596±591 pg/mg creatinine), and was no longer significantly elevated 12 hours after CPB. The predictive power of semaphorin 3A as demonstrated by area under the receiver-operating characteristic curve for diagnosis of AKI at 2, 6, and 12 hours after CPB was 0.88, 0.81, and 0.74, respectively. The 2-hour urine semaphorin measurement strongly correlated with duration and severity of AKI, as well as length of hospital stay. Adjusting for CPB time and gender, the 2-hour semaphorin remained an independent predictor of AKI, with an odds ratio of 2.19.

CONCLUSION

Our results suggest that semaphorin 3A is an early, predictive biomarker in experimental and pediatric AKI, and may allow for the reliable early diagnosis and prognosis of AKI after CPB, much before the rise in serum creatinine.

Netrin-1 regulates colon-kidney cross talk through suppression of IL-6 function in a mouse model of DSS-colitis

Organ cross talk is increasingly appreciated in human disease, and inflammatory mediators are shown to mediate distant organ injury in many disease models. Colitis and intestinal injury are known to be mediated by infiltrating immune cells and their secreted cytokines. However, its effect on other organs, such as the kidney, has never been studied. In the current study, we examined the effect of dextran sulfate sodium (DSS)-colitis on kidney injury and inflammation. In addition, we hypothesized that netrin-1 could modulate colon-kidney cross talk through regulation of inflammation and apoptosis. Consistent with our hypothesis, DSS-colitis induced acute kidney injury in mice. Epithelial-specific overexpression of netrin-1 suppressed both colitis and colitis-induced acute kidney injury, which was associated with reduced weight loss, neutrophil infiltration into colon mucosa, intestinal permeability, epithelial cell apoptosis, and cytokine and chemokine production in netrin-1 transgenic mice colon and kidney. To determine whether netrin-1-protective effects were mediated through suppression of IL-6, IL-6 knockout mice were treated with DSS and acute kidney injury was determined. IL-6 knockout was resistant to colitis and acute kidney injury. Moreover, administration of IL-6 to netrin-1 transgenic mice did not affect the netrin-1-protective effects on the colon and kidney, suggesting that netrin-1 may reduce both IL-6 production and its activity. The present study identifies previously unrecognized cross talk between the colon and kidney, and netrin-1 may limit distant organ injury by suppressing inflammatory mediators and apoptosis.

Netrin-1 regulates the inflammatory response of neutrophils and macrophages, and suppresses ischemic acute kidney injury by inhibiting COX-2-mediated PGE2 production

Netrin-1 regulates inflammation but the mechanism by which this occurs is unknown. Here we explore the role of netrin-1 in regulating the production of the prostanoid metabolite PGE2 from neutrophils in in vitro and in vivo disease models. Ischemia reperfusion in wild-type and RAG-1 knockout mice induced severe kidney injury that was associated with a large increase in neutrophil infiltration and COX-2 expression in the infiltrating leukocytes. Administration of netrin-1 suppressed COX-2 expression, PGE2 and thromboxane production, and neutrophil infiltration into the kidney. This was associated with reduced apoptosis, inflammatory cytokine and chemokine expression, and improved kidney function. Treatment with the PGE2 receptor EP4 agonist enhanced neutrophil infiltration and renal injury which was not inhibited by netrin-1. Consistent with in vivo data, both LPS and IFNγ-induced inflammatory cytokine production in macrophages and IL-17-induced IFNγ production in neutrophils were suppressed by netrin-1 in vitro by suppression of COX-2 expression. Moreover, netrin-1 regulates COX-2 expression at the transcriptional level through the regulation of NFκB activation. Thus, netrin-1 regulates the inflammatory response of neutrophils and macrophages through suppression of COX-2 mediated PGE2 production. This could be a potential drug for treating many inflammatory immune disorders.

Netrin-1-treated macrophages protect the kidney against ischemia-reperfusion injury and suppress inflammation by inducing M2 polarization

Improper macrophage activation is pathogenically linked to various metabolic, inflammatory, and immune disorders. Therefore, regulatory proteins controlling macrophage activation have emerged as important new therapeutic targets. We recently demonstrated that netrin-1 regulates inflammation and infiltration of monocytes and ameliorates ischemia-reperfusion-induced kidney injury. However, it was not known whether netrin-1 regulates the phenotype of macrophages and the signaling mechanism through which it might do this. In this study, we report novel mechanisms underlying netrin-1's effects on macrophages using in vivo and in vitro studies. Overexpression of netrin-1 in spleen and kidney of transgenic mice increased expression of arginase-1, IL-4, and IL-13 and decreased expression of COX-2, indicating a phenotypic switch in macrophage polarization toward an M2-like phenotype. Moreover, flow cytometry analysis showed a significant increase in mannose receptor-positive macrophages in spleen compared with wild type. In vitro, netrin-1 induced the expression of M2 marker expression in bone marrow-derived macrophages, peritoneal macrophages, and RAW264.7 cells, and suppressed IFNγ-induced M1 polarization and production of inflammatory mediators. Adoptive transfer of netrin-1-treated macrophages suppressed inflammation and kidney injury against ischemia-reperfusion. Netrin-1 activated PPAR pathways and inhibition of PPAR activation abolished netrin-1-induced M2 polarization and suppression of cytokine production. Consistent with in vitro studies, administration of PPAR antagonist to mice abolished the netrin-1 protective effects against ischemia-reperfusion injury of the kidney. These findings illustrate that netrin-1 regulates macrophage polarization through PPAR pathways and confers anti-inflammatory actions in inflammed kidney tissue.

Proximal tubule-specific overexpression of netrin-1 suppresses acute kidney injury-induced interstitial fibrosis and glomerulosclerosis through suppression of IL-6/STAT3 signaling

Acute kidney injury-induced organ fibrosis is recognized as a major risk factor for the development of chronic kidney disease, which remains one of the leading causes of death in the developed world. However, knowledge on molecules that may suppress the fibrogenic response after injury is lacking. In ischemic models of acute kidney injury, we demonstrate a new function of netrin-1 in regulating interstitial fibrosis. Acute injury was promptly followed by a rise in serum creatinine in both wild-type and netrin-1 transgenic animals. However, the wild-type showed a slow recovery of kidney function compared with netrin-1 transgenic animals and reached baseline by 3 wk. Histological examination showed increased infiltration of interstitial macrophages, extensive fibrosis, reduction of capillary density, and glomerulosclerosis. Collagen IV and α-smooth muscle actin expression was absent in sham-operated kidneys; however, their expression was significantly increased at 2 wk and peaked at 3 wk after reperfusion. These changes were reduced in the transgenic mouse kidney, which overexpresses netrin-1 in proximal tubular epithelial cells. Fibrosis was associated with increased expression of IL-6 and extensive and chronic activation of STAT3. Administration of IL-6 exacerbated fibrosis in vivo in wild-type, but not in netrin-1 transgenic mice kidney and increased collagen I expression and STAT3 activation in vitro in renal epithelial cells subjected to hypoxia-reoxygenation, which was suppressed by netrin-1. Our data suggest that proximal tubular epithelial cells may play a prominent role in interstitial fibrosis and that netrin-1 could be a useful therapeutic agent for treating kidney fibrosis.

Netrin-1 promotes glioblastoma cell invasiveness and angiogenesis by multiple pathways including activation of RhoA, cathepsin B, and cAMP-response element-binding protein

Glioblastomas are very difficult tumors to treat because they are highly invasive and disseminate within the normal brain, resulting in newly growing tumors. We have identified netrin-1 as a molecule that promotes glioblastoma invasiveness. As evidence, netrin-1 stimulates glioblastoma cell invasion directly through Matrigel-coated transwells, promotes tumor cell sprouting and enhances metastasis to lymph nodes in vivo. Furthermore, netrin-1 regulates angiogenesis as shown in specific angiogenesis assays such as enhanced capillary endothelial cells (EC) sprouting and by increased EC infiltration into Matrigel plugs in vivo, as does VEGF-A. This netrin-1 signaling pathway in glioblastoma cells includes activation of RhoA and cyclic AMP response element-binding protein (CREB). A novel finding is that netrin-1-induced glioblastoma invasiveness and angiogenesis are mediated by activated cathepsin B (CatB), a cysteine protease that translocates to the cell surface as an active enzyme and co-localizes with cell surface annexin A2 (ANXA2). The specific CatB inhibitor CA-074Me inhibits netrin-1-induced cell invasion, sprouting, and Matrigel plug angiogenesis. Silencing of CREB suppresses netrin-1-induced glioblastoma cell invasion, sprouting, and CatB expression. It is concluded that netrin-1 plays an important dual role in glioblastoma progression by promoting both glioblastoma cell invasiveness and angiogenesis in a RhoA-, CREB-, and CatB-dependent manner. Targeting netrin-1 pathways may be a promising strategy for brain cancer therapy.

Kidney proximal tubular epithelial-specific overexpression of netrin-1 suppresses inflammation and albuminuria through suppression of COX-2-mediated PGE2 production in streptozotocin-induced diabetic mice

Inflammation plays a key role in the development and progression of diabetic kidney disease; however, the role of the anti-inflammatory molecule netrin-1 in diabetic kidney disease is unknown. We examined the role of netrin-1 in diabetes-induced kidney inflammation and injury using tubule-specific netrin-1 transgenic mice. Diabetes was induced using streptozotocin in wild-type and netrin-1 transgenic animals. Kidney function, fibrosis, glucose excretion, albuminuria, and inflammation were evaluated. The mechanism of netrin-1-induced suppression of inflammation was studied in vitro using a proximal tubular epithelial cell line. Diabetes was associated with increased infiltration of neutrophils and macrophages, chemokine expression, and tubular epithelial cell apoptosis in kidney. These changes were minimal in kidney of netrin-1 transgenic mice. In addition, diabetes induced a large increase in the excretion of prostaglandin E2 (PGE2) in urine, which was suppressed in netrin-1 transgenic mice. Netrin-1-induced suppression of PGE2 production was mediated through suppression of NFκB-mediated cyclooxygenase-2 (COX-2) in renal tubular epithelial cells. Furthermore, netrin-1 also increased albumin uptake by proximal tubular epithelial cells through the PI3K and ERK pathways without increasing glucose uptake. These findings suggest that netrin-1 is a major regulator of inflammation and apoptosis in diabetic nephropathy and may be a useful therapeutic molecule for treating chronic kidney diseases such as diabetic nephropathy.

Netrin-1 protects against L-Arginine-induced acute pancreatitis in mice

Acute pancreatitis (AP) is a common inflammatory disease mediated by damage to acinar cells and subsequent pancreatic inflammation with infiltration of leukocytes. The neuronal guidance protein, netrin-1, has been shown to control leukocyte trafficking and modulate inflammatory responses in several inflammation-based diseases. The present study was aimed toward investigating the effects of netrin-1 in an in vivo model of AP in mice. AP was induced in C57BL/6 mice by administration of two intraperitoneal injections of L-Arginine (4 g/kg). Mice were treated with recombinant mouse netrin-1 at a dose of 1 µg/mouse or vehicle (0.1% BSA) intravenously through the tail vein immediately after the second injection of L-Arginine, and every 24 h thereafter. Mice were sacrificed at several time intervals from 0 to 96 h after the induction of pancreatitis. Blood and tissue samples of pancreas and lung were collected and processed to determine the severity of pancreatitis biochemically and histologically. Immunohistochemical staining demonstrated that netrin-1 was mainly expressed in the islet cells of the normal pancreas and the AP model pancreas, and the pancreatic expression of netrin-1 was down-regulated at both the mRNA and protein levels during the course of AP. Exogenous netrin-1 administration significantly reduced plasma amylase levels, myeloperoxidase activity, pro-inflammatory cytokine production, and pancreas and lung tissue damages. Furthermore, netrin-1 administration did not cause significant inhibition of nuclear factor-kappa B activation in the pancreas of L-Arginine-induced AP. In conclusion, our novel data suggest that netrin-1 is capable of improving damage of pancreas and lung, and exerting anti-inflammatory effects in mice with severe acute pancreatitis. Thus, our results indicate that netrin-1 may constitute a novel target in the management of AP.

Guidance for life, cell death, and colorectal neoplasia by netrin dependence receptors

This review is focusing on a critical mediator of embryonic and postnatal development with multiple implications in inflammation, neoplasia, and other pathological situations in brain and peripheral tissues. These morphogenetic guidance and dependence processes are involved in several malignancies targeting the epithelial and immune systems including the progression of human colorectal cancers. We consider the most important findings and their impact on basic, translational, and clinical cancer research. Expected information can bring new cues for innovative, efficient, and safe strategies of personalized medicine based on molecular markers, protagonists, signaling networks, and effectors inherent to the Netrin axis in pathophysiological states.

The uncoordinated-5 homolog B receptor affects hepatic ischemia reperfusion injury

Recent evidence has demonstrated additional roles for the neuronal guidance protein receptor UNC5B outside the nervous system. Given the fact that ischemia reperfusion injury (IRI) of the liver is a common source of liver dysfunction and the role of UNC5B during an acute inflammatory response we investigated the role of UNC5B on acute hepatic IRI. We report here that UNC5B(+/-) mice display reduced hepatic IRI and neutrophil (PMN) infiltration compared to WT controls. This correlated with serum levels of lactate dehydrogenase (LDH), aspartate- (AST) and alanine- (ALT) aminotransferase, the presence of PMN within ischemic hepatic tissue, and serum levels of inflammatory cytokines. Moreover, injection of an anti-UNC5B antibody resulted in a significant reduction of hepatic IR injury. This was associated with reduced parameters of liver injury (LDH, ALT, AST) and accumulation of PMN within the injured hepatic tissue. In conclusion our studies demonstrate a significant role for UNC5B in the development of hepatic IRI and identified UNC5B as a potential drug target to prevent liver dysfunction in the future.

Pentraxin 3(PTX 3): an endogenous modulator of the inflammatory response

Inflammatory or anti-inflammatory? That is the question as far as the acute-phase response and its mediators, the pentraxins, are concerned. Only some ten years ago, the classical or short pentraxin C-reactive protein and the newly discovered long pentraxin PTX3 were considered to exert most of the detrimental effects of acute inflammation, whether microbial or sterile in origin. However, accumulating evidence suggests an at least dichotomous, context-dependent outcome attributable to the pentraxins, if not a straightforward anti-inflammatory nature of the acute-phase response. This paper is focused on the inherent effects of pentraxin 3 in inflammatory responses, mainly in coronary artery disease and in Aspergillus fumigatus infection. Both are examples of inflammatory reactions in which PTX3 is substantially involved; the former sterile, the latter infectious in origin. Apart from different inducing noxae, similarities in the pathogenesis of the two are striking. All the same, the introductory question still persists: is the ultimate impact of PTX3 in these conditions inflammatory or anti-inflammatory, paradoxical as the latter might appear? We try to provide an answer such as it emerges in the light of recent findings.

The axonal guidance receptor neogenin promotes acute inflammation

Neuronal guidance proteins (NGP) were originally described in the context of axonal growth and migration. Yet recent work has demonstrated that NGPs also serve as guidance cues for immune competent cells. A crucial target receptor for NGPs during embryonic development is the neogenin receptor, however its role during acute inflammation is unknown. We report here that neogenin is abundantly expressed outside the nervous system and that animals with endogenous repression of neogenin (Neo1^−/−) demonstrate attenuated changes of acute inflammation. Studies using functional inhibition of neogenin resulted in a significant attenuation of inflammatory peritonitis. In studies employing bone marrow chimeric animals we found the hematopoietic presence of Neo1^−/− to be responsible for the attenuated inflammatory response. Taken together our studies suggest that the guidance receptor neogenin holds crucial importance for the propagation of an acute inflammatory response and further define mechanisms shared between the nervous and the immune system.

Allelic variation on murine chromosome 11 modifies host inflammatory responses and resistance to Bacillus anthracis

Anthrax is a potentially fatal disease resulting from infection with Bacillus anthracis. The outcome of infection is influenced by pathogen-encoded virulence factors such as lethal toxin (LT), as well as by genetic variation within the host. To identify host genes controlling susceptibility to anthrax, a library of congenic mice consisting of strains with homozygous chromosomal segments from the LT-responsive CAST/Ei strain introgressed on a LT-resistant C57BL/6 (B6) background was screened for response to LT. Three congenic strains containing CAST/Ei regions of chromosome 11 were identified that displayed a rapid inflammatory response to LT similar to, but more severe than that driven by a LT-responsive allele of the inflammasome constituent NRLP1B. Importantly, increased response to LT in congenic mice correlated with greater resistance to infection by the Sterne strain of B. anthracis. The genomic region controlling the inflammatory response to LT was mapped to 66.36–74.67 Mb on chromosome 11, a region that encodes the LT-responsive CAST/Ei allele of Nlrp1b. However, known downstream effects of NLRP1B activation, including macrophage pyroptosis, cytokine release, and leukocyte infiltration could not fully explain the response to LT or the resistance to B. anthracis Sterne in congenic mice. Further, the exacerbated response in congenic mice is inherited in a recessive manner while the Nlrp1b-mediated response to LT is dominant. Finally, congenic mice displayed increased responsiveness in a model of sepsis compared with B6 mice. In total, these data suggest that allelic variation of one or more chromosome 11 genes in addition to Nlrp1b controls the severity of host response to multiple inflammatory stimuli and contributes to resistance to B. anthracis Sterne. Expression quantitative trait locus analysis revealed 25 genes within this region as high priority candidates for contributing to the host response to LT.

We show that genetic variation within an 8.3 Mb region on mouse chromosome 11 controls host response to anthrax lethal toxin (LT) and resistance to infection by the Sterne strain of Bacillus anthracis. Specifically, congenic C57BL/6 mice in which this region of chromosome 11 is derived from a genetically divergent CAST/Ei strain presented with a rapid and strong innate immune response to LT and displayed increased survival following infection with Sterne spores. CAST/Ei chromosome 11 encodes a dominant LT-responsive allele of Nlrp1b that may partially account for the severe response to LT. However, the strength of this response was attenuated in mice with only one copy of chromosome 11 derived from CAST/Ei indicating the existence of a recessive modifier of the inflammatory response to LT. In addition, congenic mice displayed a pronounced immune response using an experimental model of sepsis, indicating that one or more genes within the chromosome 11 region control host response to multiple inflammatory stimuli. Analyzing the influence of allelic variation on gene expression identified 25 genes as candidates for controlling these responses. In summary, we report a genetic model to study inflammatory responses beneficial to the host during anthrax.

Netrin-1 overexpression in kidney proximal tubular epithelium ameliorates cisplatin nephrotoxicity

Netrin-1, a multifunctional laminin-related protein is widely expressed in various tissues, including kidney. The pathophysiological roles of netrin-1 in toxic acute kidney injury are unknown. To determine the role of netrin-1 in cisplatin-induced nephrotoxicity, we used netrin-1 transgenic mice that overexpress netrin-1 in the proximal tubular epithelium using the fatty acid binding protein promoter. Administration of cisplatin caused severe renal injury in WT mice but not in netrin-1 transgenic mice. Functional improvement was associated with better preservation of morphology, reduced cytokine expression and oxidative stress in the kidney, and reduced serum and urine cytokine and chemokine levels of transgenic mice as compared with WT mice. Cisplatin induced an increase in neutrophil infiltration into the kidney of WT mice, which was not significantly reduced in netrin-1 transgenic mice. Interestingly, ischemia reperfusion induced a large increase in apoptosis in WT mice but not in netrin-1 transgenic mice (215 ± 40 vs 94 ± 20 cells/5 HPF ( × 400), P < 0.0001), which was associated with reduced caspase-3 and p53 activation in the transgenic kidney. These results suggest that netrin-1 protects renal tubular epithelial cells against cisplatin-induced kidney injury by suppressing apoptosis and inflammation.

Intracellular kinases mediate increased translation and secretion of netrin-1 from renal tubular epithelial cells

Background

Netrin-1 is a laminin-related secreted protein, is highly induced after tissue injury, and may serve as a marker of injury. However, the regulation of netrin-1 production is not unknown. Current study was carried out in mouse and mouse kidney cell line (TKPTS) to determine the signaling pathways that regulate netrin-1 production in response to injury.

Methods and Principal Findings

Ischemia reperfusion injury of the kidney was induced in mice by clamping renal pedicle for 30 minutes. Cellular stress was induced in mouse proximal tubular epithelial cell line by treating with pervanadate, cisplatin, lipopolysaccharide, glucose or hypoxia followed by reoxygenation. Netrin-1 expression was quantified by real time RT-PCR and protein production was quantified using an ELISA kit. Cellular stress induced a large increase in netrin-1 production without increase in transcription of netrin-1 gene. Mitogen activated protein kinase, ERK mediates the drug induced netrin-1 mRNA translation increase without altering mRNA stability.

Conclusion

Our results suggest that netrin-1 expression is suppressed at the translational level and MAPK activation leads to rapid translation of netrin-1 mRNA in the kidney tubular epithelial cells.

Netrins: versatile extracellular cues with diverse functions

Netrins are secreted proteins that were first identified as guidance cues, directing cell and axon migration during neural development. Subsequent findings have demonstrated that netrins can influence the formation of multiple tissues, including the vasculature, lung, pancreas, muscle and mammary gland, by mediating cell migration, cell-cell interactions and cell-extracellular matrix adhesion. Recent evidence also implicates the ongoing expression of netrins and netrin receptors in the maintenance of cell-cell organisation in mature tissues. Here, we review the mechanisms involved in netrin signalling in vertebrate and invertebrate systems and discuss the functions of netrin signalling during the development of neural and non-neural tissues.

Partial netrin-1 deficiency aggravates acute kidney injury

The netrin family of secreted proteins provides migrational cues in the developing central nervous system. Recently, netrins have also been shown to regulate diverse processes beyond their functions in the brain, incluing the ochrestration of inflammatory events. Particularly netrin-1 has been implicated in dampening hypoxia-induced inflammation. Here, we hypothesized an anti-inflammatory role of endogenous netrin-1 in acute kidney injury (AKI). As homozygous deletion of netrin-1 is lethal, we studied mice with partial netrin-1 deletion (Ntn-1^+/− mice) as a genetic model. In fact, Ntn-1^+/− mice showed attenuated Ntn-1 levels at baseline and following ischemic AKI. Functional studies of AKI induced by 30 min of renal ischemia and reperfusion revealed enhanced kidney dysfunction in Ntn-1^+/− mice as assessed by measurements of glomerular filtration, urine flow rate, urine electrolytes, serum creatinine and creatinine clearance. Consistent with these findings, histological studies indicated a more severe degree kidney injury. Similarly, elevations of renal and systemic inflammatory markers were enhanced in mice with partial netrin-1 deficiency. Finally, treatment of Ntn-1^+/− mice with exogenous netrin-1 restored a normal phenotype during AKI. Taking together, these studies implicate endogenous netrin-1 in attenuating renal inflammation during AKI.

Regulation of leukocyte function by adenosine receptors

The immune system responds to cues in the microenvironment to make acute and chronic adaptations in response to inflammation and injury. Locally produced purine nucleotides and adenosine provide receptor-mediated signaling to all bone-marrow derived cells of the immune system to modulate their responses. This review summarizes recent advances in our understanding of the effects of adenosine signaling through G protein-coupled adenosine receptors on cells of the immune system. Adenosine A(2A) receptors (A(2A)Rs) have a generally suppressive effect on the activation of immune cells. Moreover, their transcription is strongly induced by signals that activate macrophages or dendritic cells through toll-like receptors, or T cells through T cell receptors. A(2A)R induction is responsible for producing a gradual dissipation of inflammatory responses. A(2A)R activation is particularly effective in limiting the activation of invariant NKT (iNKT) cells that play a central role in acute reperfusion injury. A(2A) agonists have clinical promise for the treatment of vaso-occlusive tissue injury. Blockade of A(2A) receptors may be useful to enhance immune-mediated killing of cancer cells. A(2B)R expression also is transcriptionally regulated by hypoxia, cytokines, and oxygen radicals. Acute A(2B)R activation attenuates the production of proinflammatory cytokines from macrophages, but sustained activation facilitates macrophage and dendritic cell remodeling and the production of acute phase proteins and angiogenic factors that may participate in evoking insulin resistance and tissue fibrosis. A(2B)R activation also influences macrophage and neutrophil function by influencing expression of the anti-inflammatory netrin receptor, UNC5B. The therapeutic significance of adenosine-mediated effects on the immune system is discussed.