New ways of thinking about and teaching about
New ways of thinking about (and teaching about) intestinal epithelial function Kim E. Barrett, Ph. D. Professor of Medicine University of California, San Diego EB, April 2007
Key features of the intestinal epithelium and its function n Vast surface area Imperative to allow nutrient uptake while restricting passage of undesirable substances/microorganisms Continual turnover ¨ Specialization n of cell function Lifelong symbiotic relationship with a vast commensal microbiota Determinant of luminal fluidity Dynamically regulated in health and disease
Intestinal epithelial functions Na. Cl water nutrients Clwater Commensals (probiotics? ) Barrier for toxins and pathogens; active toxin efflux
Outline of talk Acute regulation of epithelial transport processes n The epithelial barrier – no longer just a static barricade n Links to specific disease states n
Regulation of epithelial transport n A host of transport mechanisms expressed in the GI tract ¨ Major players: chloride secretion, electroneutral Na. Cl absorption, sodium/nutrient absorption, electrogenic sodium absorption n n Acute regulation subserves local needs in the post-prandial period Chronic regulation provides for long term adaptation to environmental changes ¨ Transport functions also specifically modified in the setting of disease
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Cl APICAL CLCA _ Ins. P 4 Other effectors? Ca++ Ca. MK ERK 1/2 Pyk 2 PI 3 -K PKC-e _ Src m 3 r BASOLATERAL CCh EGFr TGFa EGF Erb. B 2 K channel K+
Figure 5 from Donowitz et al. , J. Physiol. 567: 3 -11 (2005) Freely accessible at http: //jp. physoc. org/cgi/content/full/567/1/3
Na+ APICAL ENa. C - Nedd 4 -2 + - Ca++ + [Na+] c. AMP 3 Na+ ~ BASOLATERAL 2 K+ Na+/K+ ATPase Cl. H 2 O
Epithelial barrier function n Multiple new insights into the molecular composition of tight junctions ¨ Pore forming vs. pore sealing claudins ¨ Tight junction proteins stabilized via dynamic phosphorylation events Role of cytoskeleton in tight junction sealing n Transport as a component of barrier function n
Effect of claudins on epithelial barrier function Protein Cldn 1 Resistance PNa Tracer flux - Cldn 2 - Cldn 4 No D Cldn 5 No D Cldn 7 No D Cldn 8 No D Adapted from Van Itallie and Anderson, Annu. Rev. Physiol. 68: 403 -29, 2006
Speculative model of tight junction pores Tight junctions form as continuous contacts at the apical end of adjacent epithelial cells From Van Itallie, C. M. et al. Physiology 19: 331 -338 2004 Freely accessible at http: //physiologyonline. physiology. org/cgi/content/full/19/6/331 Copyright © 2004 American Physiological Society
Barrier dysfunction can be caused by infectious and inflammatory stimuli secondary to occludin endocytosis and actinmyosin contraction Figures 3 and 4 From: Turner, J. R. Molecular Basis of Epithelial Barrier Regulation: From Basic Mechanisms to Clinical Application Am. J. Pathol. 169: 1901 -9 (2006) Under access control at http: //ajp. amjpathol. org/content/vol 169/issue 6/
Epithelial regulation in disease n Molecular experiments of nature provide insights into transporter identities/functions ¨ Cystic fibrosis, congenital chloridorrhea, glucosegalactose malabsorption n More common disease states also associated with alterations in transport and/or barrier function ¨ Inflammatory bowel ¨ Infectious diarrhea n diseases Insights into current and novel therapies
Cystic fibrosis n Defects in epithelial chloride and bicarbonate secretion explain many facets of disease phenotype and illuminate GI transport physiology ¨ Duodenal, n pancreatic, biliary bicarbonate secretion Heterozygote advantage as the basis for relative prevalence of CF ¨ Resistance to cholera vs. typhoid?
Congenital chloridorrhea n Rare congenital diarrheal disease of infancy ¨ Loss n of chloride-rich fluid; fatal if untreated Genetic mutation mapped to downregulated in adenoma ¨ Encodes a chloride/bicarbonate (hydroxyl) exchanger critical for fluid salvage in small and large intestines
Glucose-galactose malabsorption Cloning of responsible gene (SGLT 1) represents first use of expression cloning n A variety of trafficking and missense mutations result in infants that poorly tolerate glucose, galactose or carbohydrates that contain these sugars n Possible role of milder mutations in more common disease states n
Role of chloride secretion in limiting bacterial invasion Control Cholera toxin
“Beneficial” diarrhea? 3. Additional 4. invasion limited Cl-, H 2 O S. typhi 1. Bacterial invasion 2. Secretion Cl 3. enhance d S. typhimurium Cl- 2. Failure to upregulate secretion 3. Disseminated 4. infection
Inflammatory diarrhea n Some surprises with respect to mechanism ¨A variety of models now suggest that defective sodium absorption more important than enhanced chloride secretion n Chloride secretion actively suppressed in many models ¨ Role of crypt secretion in maintaining relative sterility?
Therapeutic implications An understanding of transport regulation provides the basis for teaching about the mechanism of action of anti-diarrheals and other drugs n Molecular insights into transport regulation may suggest novel therapies n Lubiprostone in constipation and IBS n ¨ Epithelial involvement in unexpected settings
Conclusions n n Our knowledge of molecular, cellular and integrated physiology of the gastrointestinal epithelium is rapidly advancing The epithelium is clearly no longer simply a static barrier ¨ The molecular character of the tight junction is an area with particularly rich new insights n Disease correlations enhance understanding for both professional and graduate students
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