Illustration showing the proposed origin of various small bowel epithelial cell types from possible stem cells located at the base of the crypts. Stem cells play an important role in regenerating the intestinal epithelium.

Image courtesy of Dr. Nick Barker. Reprinted from Gastroenterology, 133, Barker N and Clevers H, Tracking down the stem cells of the intestine: strategies to identify adult stem cells, pp. 1755-1760, Copyright 2007, with permission from Elsevier.

0 Opportunities and Challenges in Digestive Diseases Research: Recommendations of the National Commission on Digestive Diseases Intestinal Failure and Regeneration, Nutritional Disorders and Support, Surgically Modified Gut, and Transplantation

SUMMARY OF RESEARCH GOALS

Loss of intestinal function can occur through surgical removal of tissue or diseases that impair digestion or cause tissue death. The Commission recommends research goals that, if pursued, would increase understanding of the natural mechanisms of growth, differentiation, and adaptation in the gastrointestinal (GI) tract and use that information to better treat patients with GI diseases. Research is needed on the development of new treatment strategies for short bowel syndrome (SBS) and intestinal failure, including innovative approaches to optimizing intestinal transplantation and post-transplant survival. GI tract surgeries, including bariatric surgeries for weight loss, are frequently associated with nutritional or hormonal complications. An important research focus is improving nutritional support for surgical patients and others with digestive diseases who rely on parenteral or enteral nutrition to sustain life, including premature infants with necrotizing enterocolitis (NEC). Achieving these research goals would markedly enhance the quality of life and health of many patients with digestive diseases or injury who are unable to properly absorb nutrients through their GI tract.

Intestinal Failure and Regeneration, Nutritional Disorders and Support, Surgically Modified Gut, and Transplantation 0 INTRODUCTION AND BACKGROUND to severe SBS often develop intestinal failure, which results when there is insufficient The collection of topics in this chapter is linked intestine to absorb adequate fluid to maintain by a common interest in the physical integrity hydration and/or to absorb 85 percent of of the gut and strategies to promote natural required nutrients. These patients require repair and regeneration processes in response intravenous fluids, electrolytes, or nutrients to loss of intestinal tissue function through through such means as parenteral nutrition surgery or disease. (PN)—the delivery of nutrients and fluids by vein rather than by ingestion to sustain Intestinal growth and differentiation: At life. Unfortunately, with prolonged use, PN is birth, the human small intestine is typically associated with life-threatening complications. 2-3 meters in length and grows to about 6-7 meters in adults. In addition, the epithelial For fortunate patients without massive gut loss, lining of the intestine is continually renewed SBS is a temporary phenomenon. Intestinal as new cells mature from proliferating stem adaptation can occur by enlargement of cells located at the base of the intestinal the intestinal villi, an increase in crypt cell crypts. This lifelong capacity for growth proliferation, or an increase in the diameter of and differentiation of the complex cellular the small intestine—all of which augment the structure of the intestine suggests the surface area available for nutrient absorption. potential for development of regenerative cures Alternatively, slowing of peristalsis—the for many digestive diseases as researchers movement of food through the digestive learn how to identify, isolate, and manipulate tract—can also help patients adapt to a shorter intestinal stem cells. intestine. These processes can be facilitated with luminal nutrients and growth factors, Short bowel syndrome and intestinal although a much better understanding of these adaptation, repair, and regeneration: factors is needed to optimize this therapy. SBS can occur when half or more of the small Finding ways to promote intestinal adaptation, intestine is missing or not functioning properly. repair, or regeneration could lead to new Infants and children can develop this condition therapies for SBS in both children and adults. for a variety of reasons, including congenital PN failure occurs in some patients from loss defects and NEC. In adults, SBS can result from of venous access as a result of central vein surgical removal of the intestine for treatment thrombosis, recurrent severe septicemias, or of inflammatory, mechanical, and malignant development of irreversible liver disease. These processes, including Crohn’s disease, tumors, patients require small bowel transplantation. volvulus (a twisting of the intestine that causes A combined liver/small bowel transplant tissue death), bowel obstruction, traumatic is necessary when liver failure occurs in injury, or other conditions. Patients with this conjunction with intestinal failure. syndrome develop diarrhea, dehydration, and malnutrition due to the inability of the intestine Intestinal transplantation: For patients to absorb sufficient water, vitamins, minerals, with irreversible intestinal failure or those and other nutrients from ingested food. with progressive complications of PN, small intestine transplantation is the last therapeutic Patients with mild SBS are treated with option. Close to 200 intestinal transplantations, dietary modification (small frequent meals), either alone or in combination with other with or without anti-motility and anti- abdominal organs, are performed each year in secretory medications. Patients with moderate the U.S. The short-term success of intestinal

108 Opportunities and Challenges in Digestive Diseases Research: Recommendations of the National Commission on Digestive Diseases transplantation procedures is now similar to micronutrients to dehydration and starvation that for other solid organs (close to 80 percent in extreme cases. Some patients, such as those 1-year survival for host and graft) due to recent with anorexia or dysfunction of the upper advances in immunosuppression, and recent GI tract, can be treated by enteral feeding studies suggest successful transplantation is through a tube placed directly into the GI associated with improved quality of life. Long- tract. If all gut function is lost, as for patients term survival, however, remains suboptimal. with moderate to severe SBS and intestinal failure or in premature infants with NEC, PN Metabolic and nutritional consequences supports survival. Specialized enteral diets of surgically modified gut: With the and gut peptide analogues, such as glucagon- increasing prevalence of obesity in the U.S. like peptide-2 (GLP-2) and growth hormone, and worldwide, bariatric surgical procedures can maximize mucosal adaptation and are becoming more common in both adults regeneration. However, complications of PN for and adolescents. Different surgical procedures patients needing long-term nutritional support are used, with the most frequent being a can include infections, chronic liver failure, reduction in stomach size, bypass of a portion loss of kidney function, metabolic bone disease, of the small intestine, or a combination and blood clots. of both strategies. Many patients achieve significant weight loss in response to the surgery, although serious complications at RECENT RESEARCH ADVANCES the time of surgery, such as anastomotic leakage, pneumonia, deep vein thrombosis Mechanisms regulating mucosal function and embolism, or death, can occur in rare and growth cases. Serious chronic side effects also may follow bariatric surgery procedures, including The mechanisms responsible for regulating intestinal infections, food intolerance, hernia, mucosal function and growth have been and the need for surgical revisions or surgery clarified at the cellular and molecular levels, for treatment of complications, occasionally allowing for manipulation of the intestinal resulting in intestinal loss. Nutritional milieu in order to augment intestinal deficiencies can occur due to poor absorption of adaptation. For example, growth factors have food and vitamins or minerals in the modified been shown to enhance villus growth, stimulate gut. Weight loss after bariatric surgery is not enterocyte proliferation, and attenuate wholly explained by restricted food intake, enterocyte apoptosis in the remnant gut but may also involve metabolic and hormonal following massive intestinal resection. Animal changes resulting from the surgery that are investigations, as well as preliminary studies in not yet fully understood. humans, suggest that growth factors, including GLP-2, insulin-like growth factor-1 (IGF-1), and Nutritional support of patients with epidermal growth factor, may help stimulate GI disorders :12 Patients with GI disorders intestinal growth and development and lead often develop nutritional deficiencies due to improved fluid and nutrientabsorption. to interference with the normal digestion Collectively, these growth-stimulating and absorption of food, ranging from mild phenomena in animal models are termed post- deficiencies resulting from poor absorption of resectional adaptation. Work over the last two

12 The Commission considered the issue of nutritional support and its consequences for patients with gastrointestinal diseases. However, the broader topic of nu- tritional research planning is overseen by an existing group, the NIH Nutrition Coordinating Committee within the Division of Nutrition Research Coordination.

Intestinal Failure and Regeneration, Nutritional Disorders and Support, Surgically Modified Gut, and Transplantation 0 decades has demonstrated that this process is to minimize malabsorption, it has been influenced by a number of factors, including demonstrated that intestinal lengthening specific luminal nutrients, such as fiber, as well procedures, including serial transverse as a variety of GI and systemic hormones and enteroplasty (STEP) and the Bianchi peptides. These studies have demonstrated that procedure to remove non-functional and luminal nutrients and bacteria are capable of dilated loops of the intestine, lead to improved altering gene expression profiles andabsorption intestinal function, including absorption of and digestion in enterocytes. The availability nutrients and liquids. and study of isolated enterocytes and enterocyte cell lines have clarified the specific role of Intestinal transplant registry peptides, hormones, and matrix factors on these growth and differentiation processes. Advances in intestinal transplantation have been documented by data from establishment The chronology of intestinal adaptation has of a voluntary international intestinal demonstrated that the gut is most responsive to transplant registry. The registry includes stimulation and augmented growth immediately virtually all intestinal, intestine/liver, and following the loss of intestinal surface area. multivisceral transplants performed around Both animal and human models demonstrate the world. Expansion of the registry has that growth hormone, but not glutamine, may allowed accurate appraisal of patient survival, enhance intestinal adaptation and improve fluid graft survival, impact on survival of PN use, and nutrient absorption, leading to the ability to and other outcome data. The collaborations reduce PN requirements. Recent translational that contribute to the ongoing registry project studies in patients with SBS-intestinal failure have facilitated improved management of have shown promise for efficacy of novel agents patients and development of new collaborative and medications not originally developed for research projects by international centers of GI conditions. The adaptive processes of villus excellence in intestinal transplantation. hypertrophy and improved fluidabsorption, with the reduced need for PN, can be enhanced with Intestinal transplantation GLP-2 and GLP-2 analogues. Improved chloride and fluidabsorption has been reported with Intestinal transplantation, with or without orally administered or transdermal clonidine. the liver, has become progressively more successful in the major transplant centers, with The identification of a stem cell niche with first-year survival rates similar to orthotopic specific responsiveness to growth factors, gut liver transplantation alone. The improvement peptides, and paracrine factors has enhanced in quality of life is substantial for patients our understanding of specific molecular with intestinal failure who are dependent on features of this growth adaptive process. permanent PN. This occurs in the majority of, but not all, graft recipients. Definition of Surgical modification of thesmall intestine factors contributing to graft survival, optimal management of immunosuppressive regimens, Intestinal lengthening procedures have led improved methods to monitor rejection, to improved management of infants and and factors contributing to adaptation of children with refractory SBS. Intestinal the transplanted gut remain areas of active dilation, bacterial overgrowth, and luminal investigation. It has also been observed that stasis are hallmarks of chronic SBS in infants intestinal transplantation can not only prevent, and children. Refractory to medical strategies but also reverse, early PN-induced liver

110 Opportunities and Challenges in Digestive Diseases Research: Recommendations of the National Commission on Digestive Diseases dysfunction, thus avoiding the eventual need for Effect of parenteral nutrition on combined small bowel/liver transplantation. GI development

Candidate markers for intestinal transplant Several strategies to reduce the negative rejection without the need for tissue biopsy impact of PN on developing GI organs have been identified, including 3-0-methyl have been identified. PN may cause choline glucose absorption, serum citrulline, and deficiency, which has been implicated in calprotectin. Each may potentially serve as a fatty liver, an early step in liver disease. surrogate for intestinal mass and/or rejection Intravenous choline supplementation may and, thus, avoid the need for frequent intestinal ameliorate this process. Fish-based emulsions, biopsies to identify early reversible rejection. tumor necrosis factor (TNF) blockade, cycling Tolerance to the intestinal graft develops in of PN, and ursodeoxycholic acid administration some patients, allowing a reduction in immune have been reported to possibly be of benefit in suppression to a few times per week. Factors the treatment of PN-associated liver disease. responsible for the development of tolerance are The timing of introduction of enteral feedings unclear and are being investigated. or PN in neonates and premature infants has demonstrated that there are critical Regenerative medicine for treatment of windows to optimally introduce these factors to intestinal failure maximize GI development, infant weight gain, and growth. Mucosal plugs from the intestinal stem cell niche have been successfully grown on Prevention and treatment of NEC bioartificial scaffolds. Placed in continuity with the native intestinal tract, these mucosal NEC remains one of the most lethal perinatal plugs have demonstrated normal proliferative conditions of premature, low birth weight patterns and the capacity to expand to infants. Prevention is the key objective fill gaps in the intestinal mucosal surface. for, once established, this condition is the Given the dense lymphatic tissue burden lead cause of intestinal failure in children. in intestinal allografts, the ideal long-term Preliminary data from trials in premature solution for patients with intestinal failure infants suggest that probiotics may be will be a regenerative medicine approach in beneficial in the prevention of NEC, and which native intestinal tissue is expanded granulocyte stimulating factor may reduce on a suitable scaffold and grown to a size progression to more severe NEC. Surgical and surface area sufficient to support enteral approaches have also been introduced nutrition when placed in continuity in the GI to minimize the role of resection in the tract. Identification of the stem cell niche and management of these patients while ensuring expansion of this population into a mature and adequate management of abdominal sepsis in differentiated mucosal surface is an important these critically ill infants. first step in this process. Identification of appropriate matrix, manipulation of the Intestinal microflora growth and differentiated environment, and strategies to induce vascularization sufficient The application of DNA methodology to to incorporate the tissue into the native GI assess the resident bacteria of the gut has tract will be required to achieve a tissue- revealed tremendous diversity and mass of the engineered solution. microflora. These studies have opened

Intestinal Failure and Regeneration, Nutritional Disorders and Support, Surgically Modified Gut, and Transplantation 111 up research on the role of bacteria in the indicate that substantial changes in metabolic prevention and causation of intestinal and GI hormones occur. Identification of disorders, including those that may lead to the mechanisms underlying surgically SBS-intestinal failure. induced weight loss in animal models could result in development of medical means to Animal models of bariatric surgery produce significant and durable weight loss, which is currently achievable only through A rat model of gastric banding has been surgery. Animal models may also allow better developed, and bariatric surgical mouse understanding of the long-term metabolic models have also been developed. While sequelae of bariatric procedures, including mechanical mechanisms were once considered specific nutrient deficiencies, metabolic and the primary modality of weight loss, recent bone disorders, management of bypassed advances in measurement of gut hormones, segments, and other issues. including ghrelin, polypeptide Y, and others,

GOALS FOR RESEARCH 13

Research Goal 6.1: Define mechanisms of Objectives: intestinal growth and differentiation. (See also ß Isolate, characterize, manipulate, and expand Goal 1.6.) human intestinal stem cells in vitro. ß Define optimal growth factors, nutrients, The intestine has the capacity to grow during extracellular matrix, and milieu to enhance childhood, renew its lining throughout life, and post-resectional adaptation in human patients. adapt to loss of mucosal surface area due to ß Develop an optimal bioartificialscaffold for surgical resection or disease. By understanding neomucosal growth. these processes at a molecular and cellular level, it ß Develop artificial intestinal constructs for might be possible to develop new pharmaceutical replacement of diseased bowel. or cell-based therapies to enhance these natural ß Conduct a clinical trial of exogenous factors to phenomena, either to (1) effect total remission optimize post-resectional adaptation. or cure of disease by replacing sections of the intestine with functional tissue; or (2) promote recovery from surgery or injury by stimulating Research Goal 6.2: Develop new strategies to endogenous repair pathways. Researchers are treat short bowel syndrome and intestinal failure. focused on characterizing the mechanisms that govern lineage selection of cell phenotypes Surgical bowel resection for conditions such from intestinal stem cells and understanding as Crohn’s disease or injury can lead to the the molecular pathways involved in intestinal development of SBS or intestinal failure, although adaptation. some adaptation of the remaining tissue is possible.

13 Research Goals are numbered for ease of reference only; the numbers do not indicate prioritization of scientific topics.

112 Opportunities and Challenges in Digestive Diseases Research: Recommendations of the National Commission on Digestive Diseases Goals for Research

Studying the nutritional, hormonal, or other Moreover, new techniques are needed to monitor factors that promote adaptation could reveal organ rejection that would be less invasive than new strategies for enhancing bowel recovery conventional endoscopic biopsy. from surgical resection. Avoidance of SBS and intestinal failure would represent a significant Objectives: therapeutic advance and relieve the significant ß Determine the role of exogenous growth factors medical and economic burden of these conditions. and micronutrients post-transplantation. Further research is needed to understand why PN ß Improve methods for donor bowel preservation complications arise and how they can be prevented pre-transplant. and/or treated. ß Identify new targeted pathways for novel immunosuppressive therapies. Objectives: ß Develop artificial intestinal conduits from native ß Evaluate the effect of specific micronutrients and tissues and cells for autotransplantation. diet on post-operative intestinal adaptation. ß Identify biomarkers for noninvasive diagnosis of ß Develop and validate noninvasive markers of intestinal transplant rejection. intestinal growth and adaptation in models of SBS. ß Identify factors that diminish long-term post- ß Develop reliable, noninvasive methods to measure transplant survival and develop appropriate intestinal growth and adaptation in patients. countermeasures. ß Develop more effective techniques and strategies to reduce septic, metabolic, thrombotic, and hepatic complications of PN and intestinal failure. Research Goal 6.4: Understand and treat ß Define the molecular basis of radiation enteritis the metabolic and nutritional consequences of and of potential approaches to prevent and bariatric procedures and other surgical modifications treat radiation enteritis and proctitis. of the gut. ß Conduct a clinical trial of optimal growth factor (or synergistic combination) therapy following Bariatric surgery for weight loss and other massive intestinal resection. surgical modifications of the gut have metabolic ß Develop prognostic indicators for PN failure and hormonal consequences that were not to guide the timing of intestinal transplant originally predicted based on simple resection of evaluation in optimal candidates. tissue. Researchers are working to understand the molecular bases for these phenomena and use these insights to develop non-surgical Research Goal 6.3: Improve the success of interventions to achieve the same result. Further, intestinal transplantation. knowledge of these pathways could aid in the identification of biologic markers that predict Intestinal transplantation can be a life-saving which patients are most likely to benefit from treatment for some patients with intestinal bariatric or other surgeries. failure who have developed potentially life- threatening complications. The success of this Objectives: procedure could be improved by developing novel ß Identify pre-operative biomarkers to predict immunosuppressive drugs that are tailored for weight loss and metabolic correction. the unique immunological milieu of the intestine ß Characterize the neuroendocrine, hormonal, or by optimizing organ selection and preparation cytokine, and proteomic responses to bariatric to minimize the risk of rejection and infection. procedures in animal models and humans.

Intestinal Failure and Regeneration, Nutritional Disorders and Support, Surgically Modified Gut, and Transplantation 113 GOALS FOR RESEARCH

ß Characterize the long-term metabolic (vitamins, understand the impact of nutritional support calcium, minerals, other) sequelae and changes protocols on patients’ daily lives, to improve the in anorexic and orexigenic hormones in bariatric nutritional value of these treatments, and to reduce surgical patients. the risks of adverse events. ß Develop non-surgical therapy that “mimics” neurohumoral sequelae of bariatric procedures. Objectives: ß Develop specific dietary guidelines for patients ß Develop and validate quality-of-life measures undergoing bariatric and other surgical for patients with chronic GI dysfunction to modifications of the gut that can effectively allow assessment of the efficacy of different prevent adverse metabolic and nutritional treatments. consequences based on newly identified ß Evaluate the effect of specific micronutrients hormonal and absorptive abnormalities. and diet on GI absorption, motility, and immunity. ß Evaluate the importance of the gut microflora in Research Goal 6.5: Optimize nutritional the prevention and causation of GI diseases. support of patients with GI disorders. ß Determine optimal micronutrient requirements for patients that require long-term PN, as well as Many patients with severe GI dysfunction, including for those with catabolic illness that require PN. premature infants, rely on enteral or parenteral ß Assess the safety and potential efficacy of nutritional support to sustain life. Although these prebiotics, probiotics, and symbiotics in the procedures are indispensable for many patients, prevention of NEC and catheter-related sepsis. they carry the risk of severe side effects and do not ß Design and test diet formulations to prevent perfectly replicate normal digestion and absorption neonatal feeding intolerance and NEC. of nutrients. Further research is warranted to

MAJOR CHALLENGES AND STEPS with small intestine allografts would facilitate TO ACHIEVE the RESEARCH GOALS recruitment of patients for clinical research and intervention trials. Finally, centralized National research resources: Translational tissue banks of biosamples from patients and clinical research on intestinal failure with different GI disorders or those who are and regeneration and related issues are undergoing bariatric surgery and follow-up hampered by the small numbers of patients would enable researchers to readily access at any single institution. In addition, many human tissues for research, regardless of the investigators have difficulty accessing human location where the patients received care. intestinal tissue at the time of resection or at regular intervals after adaptation. The Standardized clinical definitions: establishment of multicenter clinical and basic Development of a standardized system to research networks would promote progress characterize SBS and intestinal failure in in the field by fostering collaboration and terms of anatomy, nutritional support, and sharing of resources. A national registry for complications is an important challenge for SBS-intestinal failure patients and for those the field. Having such a system would enable

114 Opportunities and Challenges in Digestive Diseases Research: Recommendations of the National Commission on Digestive Diseases researchers to directly compare data and Advanced technologies: The difficulty in outcomes across studies and patient groups. accessing the small bowel with repetitive Achieving consensus on data points, definitions, surgical or endoscopic procedures hampers and outcome measures would facilitate both clinical research and patient care. understanding of the relative effectiveness of The development of novel, less invasive medical, nutritional, and surgical intervention technologies to access the intestinal lumen strategies. ICD-9 codes should be created and would stimulate research on human disease. implemented to assist in the tracking of afflicted Furthermore, identification of serum or patients. Creation of a health outcomes research other surveillance markers would enhance consortium is one step that could be taken to the ability to care for patients with small promote standardization. intestinal disorders, including SBS and intestinal failure, as well as recipients of intestinal transplants.

Intestinal Failure and Regeneration, Nutritional Disorders and Support, Surgically Modified Gut, and Transplantation 115