Microecology, intestinal epithelial barrier and necrotizing enterocolitis

Soon after birth, the neonatal intestine is confronted with a massive antigenic challenge of microbial colonization. Microbial signals are required for maturation of several physiological, anatomical, and biochemical functions of intestinal epithelial barrier (IEB) after birth. Commensal bacteria regulate intestinal innate and adaptive immunity and provide stimuli for ongoing repair and restitution of IEB. Colonization by pathogenic bacteria and/or dysmature response to microbial stimuli can result in flagrant inflammatory response as seen in necrotizing enterocolitis (NEC). Characterized by inflammation and hemorrhagic–ischemic necrosis, NEC is a devastating complication of prematurity. Although there is evidence that both prematurity and presence of bacteria, are proven contributing factors to the pathogenesis of NEC, the molecular mechanisms involved in IEB dysfunction associated with NEC have begun to emerge only recently. The metagenomic advances in the field of intestinal microecology are providing insight into the factors that are required for establishment of commensal bacteria that appear to provide protection against intestinal inflammation and NEC. Perturbations in achieving colonization by commensal bacteria such as premature birth or hospitalization in intensive care nursery can result in dysfunction of IEB and NEC. In this article, microbial modulation of functions of IEB and its relationship with barrier dysfunction and NEC are described.     

Mechanisms of gut barrier failure in the pathogenesis of necrotizing enterocolitis: Toll-like receptors throw the switch

Necrotizing enterocolitis (NEC) is the leading cause of death from gastrointestinal causes in premature infants, and its overall survival has not improved in the past three decades. While the precise cause of NEC remains incompletely understood, we and others have shown that a major predisposing factor in the development and propagation of NEC is a breakdown of the intestinal barrier which leads to bacterial translocation and systemic sepsis. In seeking to identify the causes involved, we and others have also determined that activation of the receptor for bacterial endotoxin, namely toll-like receptor 4 (TLR4), is required for the development of intestinal barrier failure leading to NEC. We have also shown that the premature infant is endowed with strategies that can either limit or promote the extent of TLR4 signaling within the gut, which together determine the relative propensity with which NEC develops. In this review, we highlight the evidence for TLR4 signaling in the pathogenesis of NEC through a survey of its effects on gut barrier failure. We identify how TLR4 regulation within the gut can explain the unique susceptibility of the premature infant to the development of NEC, and highlight how strategies to limit the degree of TLR4 signaling can serve as novel therapeutic approaches for this devastating disease.     

Disordered enterocyte signaling and intestinal barrier dysfunction in the pathogenesis of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is the leading cause of death from gastrointestinal disease in neonates, and is characterized by the development of diffuse intestinal necrosis in the stressed, pre-term infant. Systemic stress causes a breakdown in the intestinal mucosal barrier, which leads to translocation of bacteria and endotoxin and the initiation of a signaling response within the enterocyte. This review summarizes recent evidence defining a clear role that defective enterocyte signaling plays in the pathogenesis of NEC through the following mechanisms: 1) The localized production of nitric oxide by villus enterocytes results in an increase in enterocyte apoptosis and impaired proliferation; 2) The translocation of endotoxin results in a PI3K-dependent activation of RhoA-GTPase within the enterocyte leading to decreased enterocyte migration and impaired restitution; 3) Dysregulated sodium-proton exchange within the enterocyte by endotoxin renders the enterocyte monolayer more susceptible to damage in the face of the acidic microenvironment characteristic of systemic sepsis; and 4) Endotoxin causes a p38-dependent release of the pro-inflammatory molecule COX-2 by the enterocyte, which potentiates the systemic inflammatory response. An understanding of the mechanisms by which disordered enterocyte signaling contributes to the pathogenesis of barrier failure and NEC--through these and other mechanisms--may lead to the identification of novel therapeutic approaches for this devastating disease.     

Neonatal necrotizing enterocolitis, a disease of the immature intestinal mucosal barrier

Necrotizing enterocolitis (NEC) is an enigmatic process in that one single etiologic factor has been sought and not found. Epidemiologic studies suggest that immaturity of the host plays a very important role. This article reviews the intestinal host defense system and its immature nature early in life in animal models and humans and suggests that it is this immaturity, along with other factors, which allows for the proliferation and invasion of antigens and organism, and the subsequent development of NEC. Data are presented which support the efficacy of pharmacologic maturation of the intestinal barrier with growth factors, either prenatally or postnatally, to decrease the incidence of NEC or, potentially, to provide a more benign course for the disease.     

肠屏障功能障碍与新生儿坏死性小肠结肠炎

坏死性小肠结肠炎( necrotizing enterocolitis,NEC)是严重危及新生儿生命的消化系统疾病,是导致新生儿,尤其是早产儿死亡的重要病因之一。新生儿,尤其是早产儿维持肠屏障功能的作用元件发育不成熟,极易受损,不能有效形成上皮细胞间的紧密连接,无法早期形成正常肠道蠕动以及分泌型IgA的减少,因此各种致病因素极易诱发肠屏障功能障碍,导致菌群移位和败血症,造成严重的肠道损害甚至并发症。缺氧缺血、炎症反应、病原体感染均可造成肠机械屏障损害,微生态屏障建立延迟、免疫屏障发育的不成熟以及病理情况下的肠微循环障碍均参与NEC的发生。此外,miRNA在肠上皮细胞的分化、结构和屏障功能调控中也发挥重要作用。 NEC的组织病理改变是肠屏障功能障碍的结果,而肠屏障功能的损害则加重NEC的病理改变。因此,认识肠屏障功能障碍在 NEC发病过程中的作用,对于防治NEC意义重大。     

Bugs and the barrier: A review of the gut microbiome and intestinal barrier in necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a devastating gastrointestinal disease that affects premature neonates. It frequently results in significant morbidity and mortality for those affected. Years of research into the pathophysiology of NEC have revealed it to be a variable and multifactorial disease. However, there are risk factors associated with NEC including low birth weight, prematurity, intestinal immaturity, alterations in microbial colonization, and history of rapid or formula based enteral feeds (Fig. 1).1, 2, 3 An accepted generalization of the pathogenesis of NEC includes a hyperresponsive immune reaction to insults such as ischemia, starting formula feeds, or alterations in the microbiome with pathologic bacterial colonization and translocation. This reaction causes a hyperinflammatory response disrupting the normal intestinal barrier, allowing abnormal bacterial translocation and ultimately sepsis.^1,2,4 This review will focus specifically on the interactions with the microbiome and intestinal barrier function in NEC.     

新生鼠坏死性小肠结肠炎肠组织一氧化氮和一氧化氮合酶变化

目的动态观察新生大鼠坏死性小肠结肠炎(NEC)发病过程中肠组织一氧化氮(NO)含量、一氧化氮合酶(NOS)活性变化及其与肠损伤关系,为进一步阐明NEC发病机制、寻找新的治疗方法提供实验依据。方法40只新生SD大鼠按简单随机法分成模型组(M)32只,对照组(C)8只。模型组大鼠出生48h开始鼠配方奶人工喂养,并予以3次缺氧和冷刺激建立NEC模型,缺氧冷刺激开始后24h(M24)、48h(M48)、72h(造模结束,M72)及最后一次缺氧和冷刺激后24h(M96)分别空腹断头处死8只;实验结束时处死对照组大鼠,分别留取肠管进行肠组织损伤评分、肠组织中NO含量和NOS活性检测。结果建模后,模型组出现腹泻、腹胀、萎靡、活动减少。M24、M48、M72、M96及对照组肠组织损伤评分分别为(1.25±0.56)、(1.46±0.31)、(2.79±0.40)、(3.33±0.59)和(0.08±0.15)分,肠组织NO含量分别为(2.07±0.38)、(2.88±0.32)、(3.09±0.40)、(3.98±1.15)和(0.94±0.44)μmol/gprot,总NOS活性分别为(2.21±0.42)、(2.77±0...     

新生鼠坏死性小肠结肠炎肠组织一氧化氮和一氧化氮合酶变化

目的 动态观察新生大鼠坏死性小肠结肠炎(NEC)发病过程中肠组织一氧化氮(NO)含量、一氧化氮合酶(NOS)活性变化及其与肠损伤关系,为进一步阐明NEC发病机制、寻找新的治疗方法提供实验依据.方法 40只新生SD大鼠按简单随机法分成模型组(M)32只,对照组(C)8只.模型组大鼠出生48 h开始鼠配方奶人工喂养,并予以3次缺氧和冷刺激建立NEC模型,缺氧冷刺激开始后24 h(M24)、48 h(M48)、72 h(造模结束,M72)及最后一次缺氧和冷刺激后24h(M96)分别空腹断头处死8只;实验结束时处死对照组大鼠,分别留取肠管进行肠组织损伤评分、肠组织中NO含量和NOS活性检测.结果 建模后,模型组出现腹泻、腹胀、萎靡、活动减少.M24、M48、M72、M96及对照组肠组织损伤评分分别为(1.25±0.56)、(1.46±0.31)、(2.79±0.40)、(3.33±0.59)和(0.08±0.15)分,肠组织NO含量分别为(2.07±0.38)、(2.88±0.32)、(3.09±0.40)、(3.98±1.15)和(0.94±0.44) μmol/gprot,总NOS活性分别为(2.21±0.42)、(2.77±0.58)、(2.95±0.32)、(3.80±1.08)和(1.49±0.25)U/mgprot,诱导型NOS活性为(1.25±0.27)、(1.94±0.46)、(2.06±0.18)、(2.86±1.07)和(0.55±0.23)U/mgprot.随缺氧和冷刺激时间延长,模型组肠组织损伤评分、肠组织中NO、总NOS、诱导型NOS的含量逐渐增加,均高于对照组(P均<0.05),肠组织NO、总NOS、诱导型NOS含量与肠组织损伤程度均呈正相关(r分别为0.865、0.743、0.807,P均<0.05).结论 NO可能是参与肠道屏障损伤过程的重要介质,在NEC肠道屏障损伤发病机制中起重要作用.     

Effects of nitric oxide synthase inhibition on intestinal damage in rats with experimental necrotizing enterocolitis.

In the inflamed intestinal mucosa of necrotizing enterocolitis (NEC), nitric oxide (NO) generated by inducible nitric oxide synthase (iNOS) may contribute to the pathogenesis of local intestinal damage. To study the importance of iNOS for the pathogenesis of NEC, the effects of selective (aminoguanidine, AG) and nonselective (L-nitroarginine methyl ester, L-NAME) iNOS inhibitors on intestinal morphologic changes were assessed in neonatal rats with experimental NEC. The neonatal rats were randomized into one of the five treatment groups. The control group consisted of rats that were breast-fed. The NEC group, consisting of neonates separated from their mothers, were gavaged with a special rodent formula to produce NEC. Rats in the sham, the AG, and the L-NAME groups were gavaged in a similar fashion to those in the NEC group; in addition, they were treated with 0.9 % saline, 10 mg/kg/day AG, and 10 mg/kg/day L-NAME, respectively. The rats were sacrificed on day 4, and the last 4 cm of terminal ileum was harvested for morphological studies and detection of nitrite and nitrate levels in tissue. The animals in the NEC and sham groups showed various degrees of intestinal inflammatory changes and increased tissue levels of nitrite and nitrate compared to those in the control group. Both AG and L-NAME treatment decreased the tissue levels of these nitrogen oxides, but the inflammatory changes of the intestine appeared to be attenuated only in the AG treated animals. L-NAME treatment did not improve the intestinal damage and increased mortality. These results may indicate that NO synthesized by iNOS plays a pathogenic role in formula-fed induced NEC and that inhibition of iNOS improves intestinal inflammatory damage.     

The role of the intestinal microcirculation in necrotizing enterocolitis

Necrotizing enterocolitis (NEC) continues to be a devastating inflammatory disease of the newborn intestine. Despite advances in management, morbidity and mortality remain high. While it is clear that intestinal ischemia plays a large role in disease pathogenesis, attempts to link NEC to intestinal macrovascular derangement have been largely unsuccessful. More recently, there has been a concerted effort to characterize the pathologic changes of the intestinal microcirculation in response to intestinal injury, including NEC. This microcirculatory regulation is controlled by a balance of vasoconstrictor and vasodilator forces. Vasoconstriction is mediated primarily by endothelin-1 (ET-1), while vasodilation is mediated primarily by nitric oxide (NO). These chemical mediators have been implicated in many aspects of intestinal ischemic injury and NEC, with the balance shifting toward increased vasoconstriction associated with intestinal injury. With a proper understanding of these antagonistic forces, potential therapeutic avenues may result from improving this pathologic microcirculatory dysregulation.     

Late intestinal strictures following successful treatment of necrotizing enterocolitis

Between 1975 and 1992, in 16 infants (14%) out of 113 neonates with previous necrotizing enterocolitis (NEC) a total of 25 intestinal strictures had to be treated. Four (16%) were found in the ileum and 21 (84%) in the colon, and in 50% multiple strictures were present. In these 16 patients initial treatment for acute NEC included conservative treatment in 5, primary resection and enterostomies in 6 and proximal diverting enterostomies in 5. Therefore, the incidence of late strictures was 11% after conservative therapy, 11% after primary resection and 55% after primary proximal diverting enterostomies. An average of 49 days elapses between the recovery from NEC and the diagnosis of late strictures in conservatively treated patients. After initial surgical treatment, late strictures were detected on contrast studies on an average of 80 days. In pathologic specimens, marked fibrosis in the submucosa was consistently present in all strictures, whereas inflammatory changes in the mucosa, disruption or hypertrophy of the muscle layers or absence of ganglion cells were seen less frequently. All strictures were resected and primary end-to-end anastomosis was performed. But despite the development of late intestinal strictures, bowel preservation was improved after initial restrictive surgical therapy and aggressive medical treatment.     

Comparative study between isolated intestinal perforation and necrotizing enterocolitis.

INTRODUCTION: Intestinal perforations in the neonatal period are usually related to necrotizing enterocolitis (NEC) or intestinal occlusion. Intestinal perforation in the absence of these conditions is called isolated perforation (IP). Several risk factors and pathogenic mechanisms have been suggested, and most of them are common to those classically attributed to NEC. AIM: To identify and compare the clinical and pathological features of IP and NEC. MATERIAL AND METHODS: We reviewed all cases of neonatal intestinal perforation and NEC in the last five years. Thirty-three patients were retrospectively classified into Group NEC: 24 cases, and Group IP: 9 cases. We collected multiple data as study variables: 1) General features; 2) Obstetric history; 3) Neonatal treatment; 4) Comorbidity; 5) Perforation features; 6) Treatment and outcome. RESULTS: Comparing the groups, we found statistical significant differences in isolated perforation cases with these risk factors: extreme prematurity, very low birth weight, abruptio placenta, intubation and neonatal mechanical ventilation, umbilical catheterization, precocious sepsis, and indomethacin therapy. A more precocious operation and a good prognosis also reached statistical significance. In the other hand, we found statistically significant differences in NEC with congenital cardiopathy (excluding isolated patent ductus arteriosus), with intestinal pneumatosis, with diffuse bowel involvement and a worse prognosis. Risk factors and pathologic findings seem to support an ischaemic pathogenesis in both diseases.     

The role of the intestinal microbiota in the pathogenesis of necrotizing enterocolitis

Development of necrotizing enterocolitis (NEC) requires a susceptible host, typically a premature infant or an infant with congenital heart disease, enteral feedings and bacterial colonization. Although there is little doubt that microbes are critically involved in the pathogenesis of NEC, the identity of specific causative pathogens remains elusive. Unlike established normal adult gut microbiota, which is quite complex, uniform, and stable, early postnatal bacterial populations are simple, diverse, and fluid. These properties complicate studies aimed at elucidating characteristics of the gut microbiome that may play a role in the pathogenesis of NEC. A broad variety of bacterial, viral, and fungal species have been implicated in both clinical and experimental NEC. Frequently, however, the same species have also been found in physiologically matched healthy individuals. Clustered outbreaks of NEC, in which the same strain of a suspected pathogen is detected in several patients suggest, but do not prove, a causative relationship between the specific pathogen and the disease. Studies in Cronobacter sakazakii, the best characterized NEC pathogen, have demonstrated that virulence is not a property of a bacterial species as a whole, but rather a characteristic of certain strains, which may explain why the same species can be pathogenic or non-pathogenic. The fact that a given microbe may be innocuous in a full-term, yet pathogenic in a pre-term infant has led to the idea of opportunistic pathogens in NEC. Progress in understanding the infectious nature of NEC may require identifying specific pathogenic strains and unambiguously establishing their virulence in animal models.     

肠细胞凋亡在新生鼠坏死性小肠结肠炎肠损伤中动态变化

目的 动态观察新生大鼠坏死性小肠结肠炎(NEC)发病过程中肠细胞凋亡率变化及其与肠损伤关系.方法 40只新生SD大鼠随机分成对照组(C)和模型组(M).对照组8只;模型组32只,在出生48 h开始给予鼠配方奶人工喂养,100%氮气缺氧90 s,4℃冷刺激10 min,每天2次,连续3 d,建立新生大鼠NEC模型;模型组开始造模后24 h(M24)、48 h(M48)、72 h(造模结束,M72)及造模结束后24 h(M96)分别处死8只,留取肠管进行肠组织损伤评分和肠细胞凋亡率检测(流式细胞仪).组织学评分≥2确定为NEC.各组随机选取1份回盲部近端小肠标本进行肠黏膜透射电镜检查.采用SPSS 11.0统计学软件进行统计分析,α =0.05为显著性检验标准.结果 透射电镜显示模型组大鼠肠黏膜出现大量凋亡细胞,形成凋亡小体.对照组、M24、M48、M72和M96肠组织损伤评分分别为(0.08±0.15)、(1.38±0.42)、(1.46±0.69)、(1.58±0.30)分和(3.33±0.59)分,肠细胞凋亡率分别为4.8%±2.9%、12.8%±6.3%、14.9%±5.5%、17.7%±5.5%和27.6%±9.9%.肠损伤程度与肠细胞凋亡率呈显著正相关(r＜凋亡率=0.853,P＜0.01).结论 新生鼠肠细胞凋亡增加是NEC肠组织损伤起始事件;随时间延长,肠细胞凋亡增加程度进一步加重;肠细胞凋亡增加是造成新生鼠NEC肠道进行性损伤的病理基础.     

肠细胞凋亡在新生鼠坏死性小肠结肠炎肠损伤中动态变化

目的动态观察新生大鼠坏死性小肠结肠炎(NEC)发病过程中肠细胞凋亡率变化及其与肠损伤关系。方法40只新生SD大鼠随机分成对照组(C)和模型组(M)。对照组8只;模型组32只,在出生48h开始给予鼠配方奶人工喂养,100%氮气缺氧90s,4℃冷刺激10min,每天2次,连续3d,建立新生大鼠NEC模型;模型组开始造模后24h(M24)、48h(M48)、72h(造模结束,M72)及造模结束后24h(M96)分别处死8只,留取肠管进行肠组织损伤评分和肠细胞凋亡率检测(流式细胞仪)。组织学评分≥2确定为NEC。各组随机选取1份回盲部近端小肠标本进行肠黏膜透射电镜检查。采用SPSS11.0统计学软件进行统计分析,α=0.05为显著性检验标准。结果透射电镜显示模型组大鼠肠黏膜出现大量凋亡细胞,形成凋亡小体。对照组、M24、M48、M72和M96肠组织损伤评分分别为(0.08±0.15)、(1.38±0.42)、(1.46±0.69)、(1.58±0.30)分和(3.33±0.59)分,肠细胞凋亡率分别为4.8%±2.9%、12.8%±6.3%、14.9%±5.5%、17.7%±5.5%和27.6%±9.9%。肠损伤程度与肠细胞凋亡率呈显著正相关(r凋亡率=0.853,P<0.01)。结论新生鼠肠细胞凋亡增加是NEC肠组织损伤起始事件;随时间延长,肠细胞凋亡增加程度进一步加重;肠细胞凋亡增加是造成新生鼠NEC肠道进行性损伤的病理基础。     

新生儿坏死性小肠结肠炎血浆肠脂肪酸结合蛋白水平变化的意义

目的 探讨血浆肠脂肪酸结合蛋白（I-FABP）水平变化在指导新生儿坏死性小肠结肠炎（NEC）诊断及治疗中的意义.方法 选择2011年5月至2012年12月我院新生儿科收治的患儿,按入院先后顺序,以明确诊断NEC的50例新生儿为NEC组,其中NECⅡ期30例,NECⅢ期20例,以非NEC新生儿50例为对照组.NEC组在确诊后24 h内、对照组在相应日龄取血,采用酶联免疫吸附法（ELISA）检测血浆I-FABP水平,根据NEC患儿病情转归分为存活组及病死组,按治疗方法分为保守治疗组和手术治疗组,比较不同组间血浆I-FABP水平、新生儿危重病例评分（NCIS）分值、脓毒症的发生率及病死率.结果 NECⅡ期组、NECⅢ期组和对照组血浆I-FABP水平分别为（95.6±18.5） μmol/L、（151.2±10.8）μmol/L和（1.2±2.3）μmol/L,组间比较差异有统计学意义（P＜0.05）;NECⅡ期组和NECⅢ期组NCIS评分明显低于对照组,脓毒症发生率和病死率均高于对照组,差异有统计学意义（P＜0.05）,NECⅡ期组和NECⅢ期组差异无统计学意义（P＞0.05）.病死组血浆I-FABP水平、脓毒症发生率高于存活组,NCIS评分低于存活组;保守治疗组I-FABP水平低于手术治疗组,NCIS评分高于手术治疗组,差异均有统计学意义（P＜0.05）.结论 血浆I-FABP水平可较敏感地反映NEC患儿的病情变化,可作为预测NEC病情严重程度及指导采取内外科治疗的指标之一.     

Interdisciplinary treatment of necrotizing enterocolitis and spontaneous intestinal perforations in preterm infants

From January 1986 to December 1992, 13 patients with necrotizing enterocolitis (NEC) (Grade II-III; Bell) were treated. The incidence was highest in the very immature infants with birth weight < 1000 g: 6/148 (4%). From onset, NEC was associated with clinical symptoms such as abdominal distension, bloody stools, retained gastric contents and septicemia. Indications of inflammation were seen in only 6 out of 13 patients at the time of diagnosis. No complications were seen in 10 patients during the acute phase. Two infants developed a bowel perforation and another one a gangrene. Immediate surgery was performed. In three other infants, elective surgery was performed because of colonic strictures. Twelve (92%) patients survived NEC. Five other VLBW infants developed spontaneous perforations of the bowel. The clinical presentation, laboratory and radiological findings differed greatly from those with NEC. Four infants survived. A primarily conservative therapeutic regime with close cooperation between the surgeon and pediatrician may be an alternative to early surgical intervention in NEC.     

The role of growth factors in intestinal regeneration and repair in necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a devastating intestinal disease resulting in major neonatal morbidity and mortality. The pathology is poorly understood, and the means of preventing and treating NEC are limited. Several endogenous growth factors have been identified as having important roles in intestinal growth as well as aiding intestinal repair from injury or inflammation. In this review, we will discuss several growth factors as mediators of intestinal regeneration and repair as well as potential therapeutic agents for NEC.