Review Article
Porcine models of digestive disease: the future of large animal translational research

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There is increasing interest in nonrodent translational models for the study of human disease. The pig, in particular, serves as a useful animal model for the study of pathophysiological conditions relevant to the human intestine. This review assesses currently used porcine models of gastrointestinal physiology and disease and provides a rationale for the use of these models for future translational studies. The pig has proven its utility for the study of fundamental disease conditions such as ischemia-reperfuion injury, stress-induced intestinal dysfunction, and short bowel syndrome. Pigs have also shown great promise for the study of intestinal barrier function, surgical tissue manipulation and intervention, as well as biomaterial implantation and tissue transplantation. Advantages of pig models highlighted by these studies include the physiological similarity to human intestine and mechanisms of human disease. Emerging future directions for porcine models of human disease include the fields of transgenics and stem cell biology, with exciting implications for regenerative medicine.

Introduction

Digestive disease results in more than 230,000 deaths annually in the United States, with colorectal cancer as the leading cause of mortality in adults from digestive disease.1 Animal models are imperative for translational research targeted at improving human health. Because of the limitations of directly studying human disease in a clinical setting, animal models have been used extensively to expand basic science knowledge. Rodents, particularly mice, have been commonly used animal models of disease because of their relatively low cost, ease of maintenance, and rapid reproduction rate.2, 3 This has been facilitated by the creation of inbred strains that represent spontaneous models of disease. Examples include TNFΔARE and SAMP1/YitFc mice strains that exhibit Crohn's disease–like ileitis as is seen in human patients.4, 5 They also make for effective models of cancer because of their high susceptibility to developing chemically induced malignancies.6 Furthermore, they are highly amenable to genetic manipulation.2, 7 The use of transgenic and knockout mice has provided invaluable insight into the impact of genetic mutations and specific genes on disease etiology and progression.8, 9, 10 However, murine models often lack key clinical signs or pathologic changes representative of human gastrointestinal disease, which are essential to improve translational studies and drug discovery (Table I).7, 11, 12, 13, 14, 15, 16, 17, 18, 19 Therefore, there is renewed interest in large animal models that more closely resemble human disease processes20, 21 and provide a nonrodent model for drug discovery. Aside from physiological considerations, the larger size of pigs is advantageous for models requiring surgical manipulation, such as Thiry-Vella loops in which an isolated cannulated segment of intestine is studied in vivo,22 or where research involves tissue transplantation.23 Of other large animals used in biomedical research, dogs have been used extensively, particularly for the study of ischemia-reperfusion injury. However, with increasing social pressures to limit use of dogs as experimental animals and the high mortality rate associated with some disease models, the use of dogs is declining.18, 23 However, dogs are particularly well suited and are increasingly used for the study of spontaneous naturally occurring diseases that also affect humans, such as cancer. In fact, clinical trials in veterinary oncology have been used to inform drug efficacy and safety in humans.24

The pig has a number of distinct advantages that has made this species a useful translational research animal model (Table I). In particular, there are important anatomic and physiological similarities to human beings.19, 25 The pig has a comparably sized genome with extensive homology to humans. The pig genome has a 60% sequence homology to humans compared with rodents with only 40% homology.26, 27 Additionally, compared with mouse, rat, dog, cat, or horse, the pig chromosomal structure is more like humans.28, 29 Pigs, like humans, are omnivores and share similar metabolic and intestinal physiological processes.19, 30, 31 For example, a comparison of the recommended daily allowances of vitamins and minerals in the human diet and the daily nutrient requirement of pigs reveal striking similarities between the 2 species in infancy, growth, reproduction, and lactation.19, 32 This likely contributes to their comparable mucosal barrier physiology and enteric microbiota, as well as susceptibility to select enteric pathogens.19, 33 The role of the intestinal microbiota in maintaining intestinal health has been highlighted in recent years, and disturbances in microbial composition have been associated with important human diseases such as diarrhea, neonatal necrotizing enterocolitis (NEC), and obesity. Conversely, studies focusing on the relationship between the composition of the gut microbiota and disease have shown widely diverging results when comparing mice with humans.33, 34, 35 This has been addressed most recently by using humanized germ-free mice transplanted with human microbiota.36, 37, 38, 39 However, similarities in the intestinal microbial ecology between pigs and humans have made the pig a useful nonprimate animal model for studies of dietary modulation of microbiota.40 Furthermore, a humanized germ-free pig model has also been established.33, 39 Under natural conditions, both human and porcine gut microbiota consist mainly of Firmicutes and Bacteroidetes phyla in spite of the fact that their overall gastrointestinal microbial diversity is affected by diet, age, and environ–mental conditions.40, 41, 42 Additionally, pharmaceutical bioavailability and nutrient digestibility in pigs closely resemble that of humans.19, 25, 43 These characteristics have led to the use of pigs for the development of pig models of a number of gastrointestinal diseases including NEC,16, 44, 45, 46, 47, 48 acute mesenteric ischemia (AMI),18, 49, 50, 51, 52 short bowel syndrome (SBS),43, 53, 54, 55, 56, 57 Acquired Immune Deficiency Syndrome-associated cryptosporidium infection,17, 58, 59 stress-induced intestinal dysfunction,60, 61, 62, 63 cystic fibrosis (CF),64, 65 and familial adenomatous polyposis (FAP).14 This review will highlight strengths and limitations of pig models of intestinal ischemia-reperfusion injury, stress-induced intestinal dysfunction, and SBS. In addition, we have reviewed information that will extend the discussion on animal models in the fields of transplantation, bioengineering, and transgenics.

Section snippets

Comparative Gastrointestinal Anatomy: Similarities and Differences Between Humans and Pigs

Pigs have significant anatomic and physiological similarity with human beings, with some key comparisons noted in Table II and Fig 1. The structure of the small intestine is very similar in humans and pigs, including macroscopic features such as the ratio of intestinal length per kilogram bodyweight (Table II).19 Other gross similarities include the presence of sacculations and tenia (bands of longitudinal muscle) extending along most of the colonic length in both human and porcine colons.30, 66

Psychological Stress-Induced Intestinal Dysfunction

Stress plays a central role in the onset and exacerbation of clinical symptoms in several gastrointestinal diseases of humans. Progress in the field of stress-related gastrointestinal disease has been hampered by the lack of relevant animal models of stress that recapitulate the chronic and complex biology of stress and the clinically relevant outcomes (eg, diarrhea and weight loss) that are observed in humans. Much of the work in this field has been done on rodent models of short term and

Ischemia-Reperfusion Injury

Intestinal ischemic injury occurs when the blood supply to a segment of intestine is compromised (Fig 2). Paradoxically, tissue damage may continue when blood flow is re-established and this is referred to as reperfusion injury. Injury attributed to ischemia-reperfusion injury is associated with many disease states including intestinal volvulus,90 AMI,91 intestinal transplantation,92 and systemic disease that lowers tissue perfusion such as hemorrhagic shock or cardiopulmonary disease.93, 94 In

Mucosal Repair

The slow progress in translating basic science findings on reperfusion injury to clinical trials has been reviewed elsewhere.123, 124 More information on mechanisms of mucosal repair is critical. Three important stages of intestinal epithelial repair have been identified. First, the immediate postinjury or acute stage of repair is when barrier function is re-established by villous contraction and cellular restitution. The process of epithelial restitution has been re-examined in porcine models

Necrotizing Enterocolitis

NEC is an abdominal emergency that afflicts approximately 10% of infants born with low birth weight and has a mortality rate of approximately 30%.16, 98 Severe, irreversible damage to the intestine often necessitates extensive surgical resection with resultant SBS. This syndrome is associated with debilitating malnutrition, diarrhea, abdominal pain, and fatigue.98, 140 The piglet is the only animal model that develops NEC under the same conditions that predispose human infants, namely

Short Bowel Syndrome

Although the preterm piglet is an ideal model of SBS in NEC patients, variations between infant, juvenile, and adult growth rates as well as developmental physiology preclude extrapolation of knowledge based on neonatal models of SBS to older patients. Here again, pigs provide a translational advantage because developmental and age-related changes in the gut are similar between pigs and humans.43, 149, 150, 151 SBS as a result of extensive resection of ischemic-injured or Crohn's

Transplantation Studies

Ultimately, when extensive and irreversible injury to the intestine occurs and when TPN therapy fails to effectively maintain patients with SBS, intestinal transplantation is indicated.140 Unfortunately, there is an unacceptably high mortality after intestinal transplantation. For example, death occurs in 40% of patients within 5 years of receiving an allograft.166 Acute rejection, chronic allograft dysfunction and ischemia-reperfusion injury within the transplanted bowel are the most

Transgenic Pigs

The impetus for advancement in the field of transgenic pig development has been because of limitations in rodent models when their small size, short life span, or inadequate representation of a disease phenotype prevents successful translational research.12 Most transgenic pig work has been in the field of xenotransplantation, as previously discussed. However, in recent years there have been significant improvements in the technology used to create genetically modified pigs to model human

Future Promise for Translational Research

The need for large animal models to improve translational research is well accepted.20, 21 The limited availability of human-derived tissues requires that information gained from animal models is extrapolated to humans. The fact that mice and rodents are the most commonly used preclinical models has been implicated in the “pipeline problem.”20, 186 One of the reasons for this problem is that rodent injury models fail to recapitulate human disease. For instance, murine injury models used in the

Acknowledgments

Conflicts of Interest: All authors have read the journal's policy on disclosure of potential conflicts of interest and have none to declare.

Authorship agreement: All authors have read the journal's authorship agreement.

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