Maria F. Branca

A Critical Role for TLR4 in the Pathogenesis of Necrotizing Enterocolitis by Modulating Intestinal Injury and Repair

Necrotizing enterocolitis (NEC) is the leading cause of death from gastrointestinal disease in preterm infants and is characterized by translocation of LPS across the inflamed intestine. We hypothesized that the LPS receptor (TLR4) plays a critical role in NEC development, and we sought to determine the mechanisms involved. We now demonstrate that NEC in mice and humans is associated with increased expression of TLR4 in the intestinal mucosa and that physiological stressors associated with NEC development, namely, exposure to LPS and hypoxia, sensitize the murine intestinal epithelium to LPS through up-regulation of TLR4. In support of a critical role for TLR4 in NEC development, TLR4-mutant C3H/HeJ mice were protected from the development of NEC compared with wild-type C3H/HeOUJ littermates. TLR4 activation in vitro led to increased enterocyte apoptosis and reduced enterocyte migration and proliferation, suggesting a role for TLR4 in intestinal repair. In support of this possibility, increased NEC severity in C3H/HeOUJ mice resulted from increased enterocyte apoptosis and reduced enterocyte restitution and proliferation after mucosal injury compared with mutant mice. TLR4 signaling also led to increased serine phosphorylation of intestinal focal adhesion kinase (FAK). Remarkably, TLR4 coimmunoprecipitated with FAK, and small interfering RNA-mediated FAK inhibition restored enterocyte migration after TLR4 activation, demonstrating that the FAK-TLR4 association regulates intestinal healing. These findings demonstrate a critical role for TLR4 in the development of NEC through effects on enterocyte injury and repair, identify a novel TLR4-FAK association in regulating enterocyte migration, and suggest TLR4/FAK as a therapeutic target in this disease.

Relationships between Transforming Growth Factor-β1, Myostatin, and Decorin IMPLICATIONS FOR SKELETAL MUSCLE FIBROSIS

Recent studies have shown that myostatin, first identified as a negative regulator of skeletal muscle growth, may also be involved in the formation of fibrosis within skeletal muscle. In this study, we further explored the potential role of myostatin in skeletal muscle fibrosis, as well as its interaction with both transforming growth factor-β1 and decorin. We discovered that myostatin stimulated fibroblast proliferation in vitro and induced its differentiation into myofibroblasts. We further found that transforming growth factor-β1 stimulated myostatin expression, and conversely, myostatin stimulated transforming growth factor-β1 secretion in C2C12 myoblasts. Decorin, a small leucine-rich proteoglycan, was found to neutralize the effects of myostatin in both fibroblasts and myoblasts. Moreover, decorin up-regulated the expression of follistatin, an antagonist of myostatin. The results of in vivo experiments showed that myostatin knock-out mice developed significantly less fibrosis and displayed better skeletal muscle regeneration when compared with wild-type mice at 2 and 4 weeks following gastrocnemius muscle laceration injury. In wild-type mice, we found that transforming growth factor-β1 and myostatin co-localize in myofibers in the early stages of injury. Recombinant myostatin protein stimulated myofibers to express transforming growth factor-β1 in skeletal muscles at early time points following injection. In summary, these findings define a fibrogenic property of myostatin and suggest the existence of co-regulatory relationships between transforming growth factor-β1, myostatin, and decorin.

Reciprocal Expression and Signaling of TLR4 and TLR9 in the Pathogenesis and Treatment of Necrotizing Enterocolitis

Necrotizing enterocolitis (NEC) is a common and often fatal inflammatory disorder affecting preterm infants that develops upon interaction of indigenous bacteria with the premature intestine. We now demonstrate that the developing mouse intestine shows reciprocal patterns of expression of TLR4 and TLR9, the receptor for bacterial DNA (CpG-DNA). Using a novel ultrasound-guided in utero injection system, we administered LPS directly into the stomachs of early and late gestation fetuses to induce TLR4 signaling and demonstrated that TLR4-mediated signaling within the developing intestine follows its expression pattern. Murine and human NEC were associated with increased intestinal TLR4 and decreased TLR9 expression, suggesting that reciprocal TLR4 and TLR9 signaling may occur in the pathogenesis of NEC. Enteral administration of adenovirus expressing mutant TLR4 to neonatal mice reduced the severity of NEC and increased TLR9 expression within the intestine. Activation of TLR9 with CpG-DNA inhibited LPS-mediated TLR4 signaling in enterocytes in a mechanism dependent upon the inhibitory molecule IRAK-M. Strikingly, TLR9 activation with CpG-DNA significantly reduced NEC severity, whereas TLR9-deficient mice exhibited increased NEC severity. Thus, the reciprocal nature of TLR4 and TLR9 signaling within the neonatal intestine plays a role in the development of NEC and provides novel therapeutic approaches to this disease.

Toll-Like Receptor-4 Inhibits Enterocyte Proliferation via Impaired β-Catenin Signaling in Necrotizing Enterocolitis

BACKGROUND & AIMS Necrotizing enterocolitis (NEC), the leading cause of gastrointestinal death from gastrointestinal disease in preterm infants, is characterized by exaggerated TLR4 signaling and decreased enterocyte proliferation through unknown mechanisms. Given the importance of beta-catenin in regulating proliferation of many cell types, we hypothesize that TLR4 impairs enterocyte proliferation in NEC via impaired beta-catenin signaling. METHODS Enterocyte proliferation was detected in IEC-6 cells or in ileum or colon from wild-type, TLR4-mutant, or TLR4(-/-) mice after induction of NEC or endotoxemia. beta-Catenin signaling was assessed by cell fractionation or immunoconfocal microscopy to detect its nuclear translocation. Activation and inhibition of beta-catenin were achieved via cDNA or small interfering RNA, respectively. TLR4 in the intestinal mucosa was inhibited with adenoviruses expressing dominant-negative TLR4. RESULTS TLR4 activation significantly impaired enterocyte proliferation in the ileum but not colon in newborn but not adult mice and in IEC-6 enterocytes. beta-Catenin activation reversed these effects in vitro. To determine the mechanisms involved, TLR4 activation phosphorylated the upstream inhibitory kinase GSK3beta, causing beta-catenin degradation. NEC in both mouse and humans was associated with decreased beta-catenin and increased mucosal GSK3beta expression. Strikingly, the inhibition of enterocyte beta-catenin signaling in NEC could be reversed, and enterocyte proliferation restored, through adenoviral-mediated inhibition of TLR4 signaling in the small intestinal mucosa. CONCLUSION We now report a novel pathway linking TLR4 with inhibition of beta-catenin signaling via GSK3beta activation, leading to reduced enterocyte proliferation in vitro and in vivo. These data provide additional insights into the pathogenesis of diseases of intestinal inflammation such as NEC.

Hypoxia causes an increase in phagocytosis by macrophages in a HIF-1α-dependent manner

Phagocytosis is the process by which microbial pathogens are engulfed by macrophages and neutrophils and represents the first line of defense against bacterial infection. The importance of phagocytosis for bacterial clearance is of particular relevance to systemic inflammatory diseases, which are associated with the development of hypoxia, yet the precise effects of hypoxia on phagocytosis remain largely unexplored. We now hypothesize that hypoxia inhibits phagocytosis in macrophages and sought to determine the mechanisms involved. Despite our initial prediction, hypoxia significantly increased the phagocytosis rate of particles in vitro by RAW264.7 and primary peritoneal macrophages and increased phagocytosis of labeled bacteria in vivo by hypoxic mice compared with normoxic controls. In understanding the mechanisms involved, hypoxia caused no changes in RhoA‐GTPase signaling but increased the phosphorylation of p38‐MAPK significantly. Inhibition of p38 reversed the effects of hypoxia on phagocytosis, suggesting a role for p38 in the hypoxic regulation of phagocytosis. Hypoxia also significantly increased the expression of hypoxia‐inducible factor‐1α (HIF‐1α) in macrophages, which was reversed after p38 inhibition, suggesting a link between p38 activation and HIF‐1α expression. It is striking that small interfering RNA knockdown of HIF‐1α reversed the effects of hypoxia on phagocytosis, and overexpression of HIF‐1α caused a surprising increase in phagocytosis compared with nontransfected controls, demonstrating a specific role for HIF‐1α in the regulation of phagocytosis. These data indicate that hypoxia enhances phagocytosis in macrophages in a HIF‐1α‐dependent manner and shed light on an important role for HIF‐1α in host defense.

Intestinal epithelial Toll-like receptor 4 regulates goblet cell development and is required for necrotizing enterocolitis in mice.

BACKGROUND & AIMS Little is known about factors that regulate intestinal epithelial differentiation; microbial recognition receptors such as Toll-like receptor (TLR)4 might be involved. We investigated whether intestinal TLR4 regulates epithelial differentiation and is involved in development of necrotizing enterocolitis (NEC) of the immature intestine. METHODS Mice with conditional disruption of TLR4 in the intestinal epithelium and TLR4 knockout (TLR4(-/-)) mice were generated by breeding TLR4(loxp/loxp) mice with villin-cre and Ella-cre, respectively. Enterocytes that did not express or overexpressed TLR4 were created by lentiviral or adenoviral transduction. Intestinal organoids were cultured on tissue matrices. Bile acids were measured by colorimetric assays, and microbial composition was determined by 16S pyrosequencing. NEC was induced in 7- to 10-day-old mice by induction of hypoxia twice daily for 4 days. RESULTS TLR4(-/-) mice and mice with enterocyte-specific deletion of TLR4 were protected from NEC; epithelial differentiation into goblet cells was increased via suppressed Notch signaling in the small intestinal epithelium. TLR4 also regulates differentiation of goblet cells in intestinal organoid and enterocyte cell cultures; differentiation was increased on deletion of TLR4 and restored when TLR4 was expressed ectopically. TLR4 signaling via Notch was increased in intestinal tissue samples from patients with NEC, and numbers of goblet cells were reduced. 16S pyrosequencing revealed that wild-type and TLR4-deficient mice had similar microbial profiles; increased numbers of goblet cells were observed in mice given antibiotics. TLR4 deficiency reduced levels of luminal bile acids in vivo, and addition of bile acids to TLR4-deficient cell cultures prevented differentiation of goblet cells. CONCLUSIONS TLR4 signaling and Notch are increased in intestinal tissues of patients with NEC and required for induction of NEC in mice. TLR4 prevents goblet cell differentiation, independently of the microbiota. Bile acids might initiate goblet cell development.

Amniotic fluid inhibits Toll-like receptor 4 signaling in the fetal and neonatal intestinal epithelium

The fetal intestinal mucosa is characterized by elevated Toll-like receptor 4 (TLR4) expression, which can lead to the development of necrotizing enterocolitis (NEC)—a devastating inflammatory disease of the premature intestine—upon exposure to microbes. To define endogenous strategies that could reduce TLR4 signaling, we hypothesized that amniotic fluid can inhibit TLR4 signaling within the fetal intestine and attenuate experimental NEC, and we sought to determine the mechanisms involved. We show here that microinjection of amniotic fluid into the fetal (embryonic day 18.5) gastrointestinal tract reduced LPS-mediated signaling within the fetal intestinal mucosa. Amniotic fluid is abundant in EGF, which we show is required for its inhibitory effects on TLR4 signaling via peroxisome proliferator-activated receptor, because inhibition of EGF receptor (EGFR) with cetuximab or EGF-depleted amniotic fluid blocked the inhibitory effects of amniotic fluid on TLR4, whereas amniotic fluid did not prevent TLR4 signaling in EGFR- or peroxisome proliferator-activated receptor γ–deficient enterocytes or in mice deficient in intestinal epithelial EGFR, and purified EGF attenuated the exaggerated intestinal mucosal TLR4 signaling in wild-type mice. Moreover, amniotic fluid-mediated TLR4 inhibition reduced the severity of NEC in mice through EGFR activation. Strikingly, NEC development in both mice and humans was associated with reduced EGFR expression that was restored upon the administration of amniotic fluid in mice or recovery from NEC in humans, suggesting that a lack of amniotic fluid-mediated EGFR signaling could predispose to NEC. These findings may explain the unique susceptibility of premature infants to the development of NEC and offer therapeutic approaches to this devastating disease.

Endothelial TLR4 activation impairs intestinal microcirculatory perfusion in necrotizing enterocolitis via eNOS–NO–nitrite signaling

Necrotizing enterocolitis (NEC) is a devastating disease of premature infants characterized by severe intestinal necrosis and for which breast milk represents the most effective protective strategy. Previous studies have revealed a critical role for the lipopolysaccharide receptor toll-like receptor 4 (TLR4) in NEC development through its induction of mucosal injury, yet the reasons for which intestinal ischemia in NEC occurs in the first place remain unknown. We hypothesize that TLR4 signaling within the endothelium plays an essential role in NEC development by regulating perfusion to the small intestine via the vasodilatory molecule endothelial nitric oxide synthase (eNOS). Using a unique mouse system in which we selectively deleted TLR4 from the endothelium, we now show that endothelial TLR4 activation is required for NEC development and that endothelial TLR4 activation impairs intestinal perfusion without effects on other organs and reduces eNOS expression via activation of myeloid differentiation primary response gene 88. NEC severity was significantly increased in eNOS−/− mice and decreased upon administration of the phosphodiesterase inhibitor sildenafil, which augments eNOS function. Strikingly, compared with formula, human and mouse breast milk were enriched in sodium nitrate—a precursor for enteral generation of nitrite and nitric oxide—and repletion of formula with sodium nitrate/nitrite restored intestinal perfusion, reversed the deleterious effects of endothelial TLR4 signaling, and reduced NEC severity. These data identify that endothelial TLR4 critically regulates intestinal perfusion leading to NEC and reveal that the protective properties of breast milk involve enhanced intestinal microcirculatory integrity via augmentation of nitrate–nitrite–NO signaling.

Toll-like Receptor 4 Is Expressed on Intestinal Stem Cells and Regulates Their Proliferation and Apoptosis via the p53 Up-regulated Modulator of Apoptosis

Background: Factors that regulate intestinal stem cell (ISC) proliferation and apoptosis are unknown. Results: Toll-like receptor 4 (TLR4) is expressed on ISCs and regulates their proliferation and apoptosis, which is critical in the pathogenesis of necrotizing enterocolitis (NEC). Conclusion: TLR4 regulates ISC proliferation and apoptosis. Significance: This is the first study showing that ISC regulation by microbial receptors contributes to NEC pathogenesis. Factors regulating the proliferation and apoptosis of intestinal stem cells (ISCs) remain incompletely understood. Because ISCs exist among microbial ligands, immune receptors such as toll-like receptor 4 (TLR4) could play a role. We now hypothesize that ISCs express TLR4 and that the activation of TLR4 directly on the intestinal stem cells regulates their ability to proliferate or to undergo apoptosis. Using flow cytometry and fluorescent in situ hybridization for the intestinal stem cell marker Lgr5, we demonstrate that TLR4 is expressed on the Lgr5-positive intestinal stem cells. TLR4 activation reduced proliferation and increased apoptosis in ISCs both in vivo and in ISC organoids, a finding not observed in mice lacking TLR4 in the Lgr5-positive ISCs, confirming the in vivo significance of this effect. To define molecular mechanisms involved, TLR4 inhibited ISC proliferation and increased apoptosis via the p53-up-regulated modulator of apoptosis (PUMA), as TLR4 did not affect crypt proliferation or apoptosis in organoids or mice lacking PUMA. In vivo effects of TLR4 on ISCs required TIR-domain-containing adapter-inducing interferon-β (TRIF) but were independent of myeloid-differentiation primary response-gene 88 (MYD88) and TNFα. Physiological relevance was suggested, as TLR4 activation in necrotizing enterocolitis led to reduced proliferation and increased apoptosis of the intestinal crypts in a manner that could be reversed by inhibition of PUMA, both globally or restricted to the intestinal epithelium. These findings illustrate that TLR4 is expressed on ISCs where it regulates their proliferation and apoptosis through activation of PUMA and that TLR4 regulation of ISCs contributes to the pathogenesis of necrotizing enterocolitis.

Nucleotide-binding Oligomerization Domain-2 Inhibits Toll Like Receptor-4 Signaling in the Intestinal Epithelium

BACKGROUND & AIMS Factors that regulate enterocyte apoptosis in necrotizing enterocolitis (NEC) remain incompletely understood, although Toll-like receptor-4 (TLR4) signaling in enterocytes plays a major role. Nucleotide-binding oligomerization domain-2 (NOD2) is an immune receptor that regulates other branches of the immune system, although its effects on TLR4 in enterocytes and its role in NEC remain unknown. We now hypothesize that activation of NOD2 in the newborn intestine inhibits TLR4, and that failure of NOD2 signaling leads to NEC through increased TLR4-mediated enterocyte apoptosis. METHODS The effects of NOD2 on enterocyte TLR4 signaling and intestinal injury and repair were assessed in enterocytes lacking TLR4 or NOD2, in mice with intestinal-specific wild-type or dominant-negative TLR4 or NOD2, and in mice with NEC. A protein array was performed on NOD2-activated enterocytes to identify novel effector molecules involved. RESULTS TLR4 activation caused apoptosis in newborn but not adult small intestine or colon, and its intestinal expression was influenced by NOD2. NOD2 activation inhibited TLR4 in enterocytes, but not macrophages, and reversed the effects of TLR4 on intestinal mucosal injury and repair. Protection from TLR4-induced enterocyte apoptosis by NOD2 required a novel pathway linking NOD2 with the apoptosis mediator second mitochondria-derived activator of caspase/direct inhibitor of apoptosis-binding protein with low PI (SMAC-DIABLO), both in vitro and in vivo. Strikingly, activation of NOD2 reduced SMAC-DIABLO expression, attenuated the extent of enterocyte apoptosis, and reduced the severity of NEC. CONCLUSIONS These findings reveal a novel inhibitory interaction between TLR4 and NOD2 signaling in enterocytes leading to the regulation of enterocyte apoptosis and suggest a therapeutic role for NOD2 in the protection of intestinal diseases such as NEC.