Challenges to the GI System Physiology

BI2331: Challenges to the GI System - Physiology

Course Information

• University: Cardiff University (Prifysgol Caerdydd)
• Lecture Number: Lecture 2
• Lecturer: Dr. Julia Gerasimenko

Learning Outcomes and Fundamental Intracellular Pathways

The primary focus of this lecture is the exploration of intracellular pathways that regulate gastrointestinal (GI) function and the subsequent challenges that arise when these systems are compromised, specifically leading to intestinal mucosal inflammation.

Key Learning Areas

• Fundamental intracellular pathways in the regulation of GI function.
• Intestinal Mucosal Inflammation challenges, specifically:     * Inflammatory Bowel Disease (IBD).     * Coeliac disease.

Core Cell Biological Pathways

There are three fundamental cellular pathways central to GI regulation:

1. Autophagy
2. Intracellular bacterial sensing
3. Unfolded protein response (UPR)

Detailed Analysis of Autophagy

Definitions and Types

Autophagy is a conserved biological process characterized as a physiological response to various cellular conditions.

• Macroautophagy: The most well-studied form. It is utilized to:     * Remove damaged organelles and microorganisms (selective autophagy).     * Respond to cellular starvation (non-selective autophagy).
• Other Types: Chaperone-mediated autophagy and microautophagy exist, though they have not yet been explicitly linked to intestinal inflammation in current literature.

The Autophagic Process (Main Steps)

1. Induction: The initiation of the process.
2. Cargo Recognition: Identifying the material (organelles/bacteria) to be degraded.
3. Vesicle Formation: The creation of the autophagosome.
4. Autophagosome-Lysosome Fusion: The merging of the autophagosome with a lysosome to allow for enzymatic degradation.
5. Breakdown of Autophagic Content: Enzymatic digestion of the sequestered material.
6. Release of Degradation Products: The resulting products are released back into the cytosol for reuse.

Physiological Functions

Autophagy serves several critical roles in maintaining cellular homeostasis:

1. Nutrient Provision: Supplies nutrients during catabolism.
2. ATP Generation: Maintains energy levels in stressed or starved cells.
3. Heterophagic Removal: Provides signals for the removal of apoptotic cells.
4. Protein Quality Control: Degradation of misfolded proteins.
5. Organelle Management: Removal of surplus or damaged organelles.
6. Host Defense: Elimination of invasive bacteria.
7. Genomic Stability: Functions in tumor suppression.

Molecular Regulation

• Induction Signals: Autophagy is triggered by starvation or stress. It is inhibited by growth factors, insulin, and high nutrient levels.
• mTOR Complex: A central regulator; Rapamycin acts as an inducer by inhibiting mTOR.
• Atg Proteins: Generated by autophagy ($\text{Atg}$) proteins in a modular interaction cascade.
• Atg8 (LC3-I/II): The recruitment of $\text{Atg8}$ ($\text{LC3-I}$) to the autophagosomal membrane involves membrane anchoring and modification into $\text{Atg8}$ ($\text{LC3-II}$) via phosphatidylethanolamine.

Bacterial Sensing Mechanisms

Intracellular Bacterial Sensor: NOD2

• Definition: Nucleotide-binding Oligomerization Domain 2 ($\text{NOD2}$) is a cytosolic pattern recognition receptor.
• Activation: Activated upon binding to muramyl dipeptide ($\text{MDP}$).
• Source: $\text{MDP}$ is derived from peptidoglycan ($\text{PGN}$), which is present in both gram-positive and gram-negative bacteria.
• Pathway: $\text{NOD2}$ activation induces autophagy. It interacts with $\text{ATG16L1}$ and utilizes $\text{RICK}$ kinase to activate $\text{NF}\kappa\text{B}$.
• Functional Interaction: $\text{NOD2}$ and $\text{ATG16L1}$ work together to control bacteria and facilitate antigen presentation via $\text{MHC-II}$.

Membrane-bound Bacterial Sensor: Toll-like Receptor (TLR)

• Function: Signalling through $\text{TLRs}$ activates $\text{NF}\kappa\text{B}$.
• Ligands:     * Lipopolysaccharide ($\text{LPS}$)     * Flagellin     * Diacyl/Triacyl Lipopeptides

Unfolded Protein Response (UPR)

ER Stress Sensors

The UPR is inactive when the sensors are bound to glucose-regulated protein $\text{grp78}$. There are three major endoplasmic reticulum (ER) stress sensors:

1. ATF6: Activating transcription factor 6.
2. PERK: Protein kinase RNA-like ER kinase.
3. IRE1: Inositol-requiring kinase.

Clinical Implications

• The UPR generates cytosolic transcription factors to adapt to stress.
• IBD Link: Mutations in $\text{IRE1}$ and X Box Protein 1 ($\text{XBP1}$) in the intestinal epithelium lead to the apoptotic death of Paneth cells and Goblet cells, which is characteristic of Inflammatory Bowel Disease.

Inflammatory Bowel Disease (IBD)

General Overview

• Nature: Incurable, chronic, relapsing, and remitting.
• Symptoms: Rectal bleeding, severe diarrhea, and weight loss.

Classification

1. Ulcerative Colitis (UC): Diffused mucosal inflammation limited to the colon and rectum. It affects only the mucosa.
2. Crohn's Disease (CD): Patchy, segmental, transmural (affecting the whole bowel wall) inflammation. It may affect any part of the GI tract.
3. Indeterminate Colitis: Cases where clinical features overlap or are not definitive.

Genetic and Molecular Factors

• ATG16L1: Associated with the risk of intestinal inflammation and IBD. Expressed in intestinal epithelium, antigen-presenting cells ($\text{APCs}$), $\text{CD4/8 T-cells}$, $\text{B1 cells}$, and memory $\text{B cells}$. Loss of activity impairs inflammation regulation.
• NOD2: The first susceptibility gene identified for IBD.
• Deficiency Effects: $\text{ATG16L1}$- and $\text{ATG5}$-deficient Paneth cells show abnormalities in granule exocytosis.

Pathophysiology of Ulcerative Colitis

• Decreased mucus layer.
• Cell apoptosis and dysfunction of tight junctions (mediated by Zonulin) disrupt the epithelial barrier.
• Microbiota crosses the barrier, activating $\text{APCs}$ and macrophages, which attract neutrophils.
• Neutrophils form Neutrophil Extracellular Traps (NETs).
• Macrophages produce $\text{TNF}$, $\text{IL-12}$, $\text{IL-23}$, and $\text{IL-6}$, activating $T_h1$ cells.
• Epithelial cells release $\text{IL-36}$, which:     * Inhibits regulatory T ($T_{reg}$) cells.     * Activates $\text{IL-9}$-producing $T_h9$ cells.     * Stimulates fibrogenesis.
• Natural killer ($\text{NK}$) T cells release $\text{IL-13}$, contributing further to barrier dysfunction.

Coeliac Disease (Gluten-Induced Enteropathy)

Clinical Definition and Symptoms

• Definition: An immune-mediated inflammatory disease of the small intestine caused by sensitivity to dietary gluten and related proteins in genetically predisposed individuals.
• Symptoms: Chronic diarrhea, weight loss, and anemia.
• Prevalence: Between $1\%$ and $2\%$, with sharp increases in North America and Europe.
• Pathology: Affects the villi of the absorptive epithelium, leading to malabsorption.

Gluten and Genetics

• Glutens: Proteins found in wheat, barley, rye, spelt, and kamut.
• Heredity: High familial recurrence ($10\text{--}15\%$) and high concordance in monozygotic twins ($75\text{--}80\%$).
• HLA Link: The Human Leukocyte Antigen ($\text{HLA}$) polymorphism is a major risk locus.     * $95\%$ of patients carry HLA-DQ2.     * $5\%$ of patients carry HLA-DQ8.

Diagnostic Criteria

1. Serum concentration: Measuring transglutaminase antibodies.
2. Duodenal Biopsy:     * Flattened villi.     * Crypt hyperplasia.     * Infiltration with lymphocytes and plasma cells.     * Reduced cell differentiation (immature cells ineffective at nutrient absorption).

Pathophysiological Mechanism

1. Zonulin Release: Digested gluten interacts with epithelial cells to release Zonulin ($47\,\text{kDa}$ paracrine protein).
2. Permeability: Zonulin increases intestinal permeability (the "leaky gut" effect).
3. Modification: Gluten peptides translocate to the lamina propria and are modified by tissue transglutaminase (TTG) into deamidated gliadin peptides.
4. Immune Response: Modified peptides are presented by $\text{HLA-DQ2/8}$ on $\text{APCs}$ to specific $CD4^+$ T cells, triggering an adaptive immune response and autoimmunity toward self-antigens.

Summary of Coeliac Consequences

• Villous Atrophy: Stem cells cannot replace enterocytes fast enough.
• Disaccharide Deficiency: Secondary intolerance to lactose and other sugars due to epithelial loss.
• Hormonal Deficiency: Impacts pancreatic secretion stimulation.
• Anemia: Resulting from iron deficiency.
• Malignancy Risk: Predisposition to T cell lymphomas in the small intestine and higher incidence of general GI cancer.

Recommended Reading

• Rochereau et al. (2021). $\text{NOD2}$ deficiency increases retrograde transport of secretory $\text{IgA}$ complexes in Crohn’s disease. Nature Communications.
• Parzanese et al. (2017). Celiac disease: From pathophysiology to treatment. World J Gastrointestinal Pathophysiology.