Střevní bariéry
Barrier
- Intestinal barrier covers a surface of about 400 m2 and requires approximately 40% of the body’s energy expenditure [10]
- Lower oesophageal sphincter to the anus
- Intestinal epithelium
- Most extensive and important mucosal surface of body
- Single layer of cells
- Selective filter
- Selective permeability [1]
- části střevní bariéry
- Vascular endothelium
- Epithelial cell lining
- Spojení endotelu
- Mucus layer
Transepithelial or transcellular permeability
- Specific transport of solutes across the epithelial cells
- Regulated by the activities of specialised transporters
- Electrolytes
- Amino acids
- sugars
- Short chain fatty acids [1]
- Other
- Epithelial damage or increased rates of apoptosis
- Provide alternate routes for the permeation of larger molecules such as lactulose [4]
- Cca 1% makromolekul abs. z GIT
- Manitol
- L-rhamóza
Epitelial cells
- Enterocytes
- Goblet cells
- Paneth cells
- Enteroendocrine cells
- Immunity cells
- Intraepithelial lymphocytes
- Dendritic cells [16]
Lipid bilayer of the enterocyte brush
Transport systems within this membrane
- For the water soluble compounds
Aqueous pores
- Size of disaccharides (for example, lactulose) are restricted from moving across the villus tip
- Mannitol can do so with relative freedom
- Distributed along the crypt-villus axis of the small intestine
- Tips of the villus
- Relatively abundant small channels (radius <6 A)
- Increased in number by addition of glucose [3]
- Susceptible to solvent drag effects
- In the crypts t
- Much larger channels (50–60 A) in low abundance
- At the base of the villus
- Intermediate sized (10–15 A) channels
- Seem to be unaffected by solvent drag effects
- Perhaps as under physiological conditions this part of the villus is not exposed to luminal contents [4]
Endocytoza
- Lipofilní látky
M-buňky (epiteliální)
- 10x nižsíš mikroklky než enterocyty
- Antigen prezentující buňky (APC)
- Lymfocytům v Peyerových placích
- Volně v submukóze (GALT - Gut associated Lymphatic Tissue)
Paracellular permeability
- Regulated by cellular junctions
- In the laminal membranes of the cells
- Main route of passive flow
- water
- Solutes
- Depends on the intercellular tight junctions
- Most influence on paracellular transport [1]
- Series of intercellular junctions along their lateral margins
Spojovací komplex mezi enterocyty - junctional complex :
- 1) zonulae occludentes - tight junctions (TJs)
- 2) zonulae adherentes - adherence junctions (Ajs)
- 3) maculae adherentes - desmozomy
Póry v paracelulárních spojeních
- Velikost se zmenšuje aborálním směrem
- 0,7 um ileum
- 0,5 um colon [7]
- Hydrofilní stěna
- Negativní náboj
- Hlavní cesta resrpce makromolekulárních látek a antigenů
- Hydrofilní látky
Tight junction (TJ)
- Closest to the luminal surface
- Regulate the flow of water ions and small molecules
- Through the composition of claudins and other proteins in the junctional complex [5]
- Consist of intra-membrane proteins
- Occludin
- Different members of the claudin family
- Depending on the tissue and location
- Occludin and claudins and tricellulin
- Link adjacent cells to the actin cytoskeleton through cytoplasmatic scaffolding proteins [5]
Adherens junction (AJ)
- Underneath the tight junctions
- Epithelial restitution
- Cell-cell adhesion and intracellular signaling [11]
- TJ and adherens junctions = apical junctional complex
- Associated to the actin cytoskeleton [11]
Gap junctions
- Cell-cell adhesion and intracellular communication [11]
Desmosomy
- Desmosomes supporting epithelial stability [5]
- Cell-cell adhesion and intracellular communication [11]
Transmembrane proteins - Fibrils
- Fibrils between cells
- Underneath the junctional complex lies a ring of actin microfilaments
- Contraction has been proposed to regulate paracellular permeability [3]
- Cross the plasma membrane
- Interact with proteins from the adjoining cell
- Interact intracellularly with the actomyosin ring
- Encircles the enterocyte at the level of the tight junction [3]
- Two types of tetraspanning membrane proteins
- Occludin
- Members of the claudin family
- 19 different but related proteins [3]
Occludin
- 1st integral membrane TJ protein
- Long C-terminal domain of occludin interacts with several intracellular TJ proteins
- ZO proteins
- Link occludin to the actin cytoskeleton
- Knockdown of occludin
- Induces an increase in paracellular permeability to macromolecules
- Role in the maintenance and assembly of TJs
- Intact epithelium
- Phosphorylation has a role in the maintenance and assembly of the TJ structure
- Tyrosine phosphorylation is caused during disassembly by various stimuli [9]
- Tyrosine phosphorylation of occludin attenuates the interaction with ZO-1
- Dissociation from the junctional complex
Claudins
- Structural backbone of TJ [11]
- Do not have any sequence similarity to occludin
- Key component and backbone of TJs
- Expressed in fibroblast
- Incorporated into TJ strands
- Claudin-1 knockout mice
- Die within 24 hours of birth
- Dramatic loss of fluid and electrolytes
- Multigene family with at least 24 members in humans and mice
- Phosphorylation is associated with
- Localization
- Paracellular permeability [9]
Defects in claudins
- Transmembrane proteins mainly responsible for the intestinal barrier function [5]
- Egulator of this epithelial barrier is the intestinal microbiota [5]
- Mutation in claudin 16
- Renal hypomagnesaemia
- Massive renal loss of magnesium [3]
- Claudins 3 and 4 are
- Targets for Clostridia perfringens enterotoxin
- Dramatically increases tight junctional permeability [3]
- Relative abundance of the claudins can change by up to 1000 x
- Increased expression of claudin 2
- Increase permeability
- Homodimers of this protein are not as “tight” as heterodimers with other claudins
- Upregulation of claudin 2 expression has been reported in Crohn's disease and ulcerative colitis [4]
- Claudin-1, -3, -4, -5, and -8
- Tighten TJ (decrease paracellular permeability) [11]
- Claudin-2 forms
- Charge-selective paracellular pores [11]
- Claudins-7, -12, and -15
- Unclear
- Deletion of claudin-7 initiates colonic inflammation [16]
Junctional adhesion molecule (JAM)
- Single span transmembrane proteins [11]
- Ig superfamily
- Characterized by 2 extracellular Ig domains
- Interactions function in cell-cell adhesion
- Association between
- Leukocytes and epithelial/endothelial cells
- Platelet activation
- Virus recognition [9]
- Expressed in various cell types
- Epithelial
- Endothelial
- Immune cells
- Intestinal epithelial cells
- JAM-A and JAM-4
- Involved in TJ regulation
- Monoclonal JAM-A antibodies
- Inhibited the resealing of the TJs
- Delays in transepithelial electrical resistance (TER) recovery
- Delays in occludin assembly
- JAM-A knockout mice exhibit
- Higher permeability to dextran and myeloperoxidase activity in the colon
- Colonic injury and inflammation induced by dextran sodium sulfate more severe [9]
- Coxsackie and adenovirus receptor
- Regulate epithelial barrier function
- Expression leads to a reduction in paracellular permeability
- Implicated in TJ assembly [11]
- JAM-A and coxsackie and adenovirus receptor
- Recruit specific TJ proteins
- Promote their localization at cell boundaries [11]
- Inhibition of JAM-A
- Prevents TJ reassembly
- X recovery of TEER after disruption by calcium depletion in cultured T84 epithelial monolayers [11]
Intracelulární proteiny
- Cytosolic scaffold proteins
Tricelluin
- Vazba na occludin
- Barrier at the junctions between 3 epithelial cells is reinforced by tricellulin
- Suppression of tricellulin gene expression
- Impairs epithelial barrier integrity [11]
ZO-1, ZO-2, ZO-3
- Anchor the transmembrane proteins to the F-actin cytoskeleton [9]
- Vital to the maintenance of TJ structure
- Permits the cytoskeletal regulation of TJ barrier integrity [9]
- On the intracellular side of the membrane
- Tight junction proteins [3]
- Membrane associated guanylate kinase superfamily
- Enzymatically inactive guanylate kinase-like domain
- Connections of microfibrilal ring to the junctional complex
- Series of actin filaments(ZO family members) [3]
- Zonula occludens proteins
- Important intracellular tight junction proteins
- Linking the cell cytoskeleton to the transmembrane tight junction proteins [5]
- TJ-specific proteins
- Members of the membrane-associated guanylate kinase homolog family
- Intracellular scaffold in the TJs
- Regulation and maintenance of TJ structure
- May mediate the early assembly of TJ proteins into cell-cell contacts
- ZO-1 deficient cells
- Still able to form normal TJ structures
- Show normal permeability
- Delay in the assembly of other TJ proteins
- Including occludin and claudins into the TJ [9]
- Spojují occludin/klaudin s F-aktinem [11]
- Tetra-span proteins = 4 vazebné domény
Myosin light chain (MLC)
- Regulate circumferential contraction and tension in the actin
- Induction of MLC phosphorylation by kinases
- MLC kinase (MLCK)
- Rho-associated kinase
- Causes the contraction of the actin
- Opening of the paracellular pathways [9]
Signaling molecules
- Assembly, disassembly, and maintenance of TJ
Protein kinase C
Mitogen-activated protein kinases
Myosin light chain kinase
Rho GTPases
Chemical barrier - Secreted products
Vodní vrstva
Immunoglobulin
Mucous from Goblet cells
- Mucus covers the surface of intestinal epithelium
- Composed of:
- Ulcerative Colitis - reduced number of goblet cells
- Reduced thickness of the mucus layer
- Altered mucus composition
- Mucins
- Phosphatidylcholine
- Glycosylation [16]
Mucin
- Trefoil peptides [9] [10]
- Surfactant lipids
- MUC2 - nejdůležitější [22]
- MUC2 synthesis, sulfation and secretion
- Decreased in active UC
- Restored while UC is in remission [22]
Trefoil factor 3 (TFF3)
- Bioactive peptide
- Involved in epithelial repair
- By Goblest cells [22]
Glycocalyx
- Protects apical surface of the epithelium
- Membrane tethered mucins
- Glycosylated
- Released upon binding by microorganisms
- Defense mechanism to prevent colonization [5]
Defensins by Paneth cells
- Antimicrobial products
- Mainly produced
- In the crypts of the small intestine [5]
- Altered Paneth cell distribution and function in IBD
- Normally restricted to the small intestines
- Crypts of Lieberkühn
- In IBD metaplastic Paneth cells may be detected in colonic mucosa
- Subsequent secretion of defensins also in the large intestine
- Expression of defensins is inducible by colonic inflammation in UC
- Is reduced in Crohn's disease (CD) [16]
- May be a physiological response to mucosal damage [16]
- Diminished Paneth cell antimicrobial function
- Might be a primary pathogenic factor in CD, particularly ileal CD [16]
Digestive secretions
ALP
- Enterocytes in the small intestine and in the colon
- Involved in the epithelial defense
- By the expression of alkaline phosphatase at their apical membranes
- Has detoxifying capacities [22]
Cytokines, inflammatory mediators
Microbiota
- Can be considered as a barrier
Propustnost zdravého střeva
- Particles < 4 A in radius
- Migrate through tight junction claudin pore pathways [1]
- Particles up to 10-15 A (3.5 kDa)
- Transit through the paracellular space uptake route [1]
Regenerace střevní výstelky
- The intestinal epithelium is renewed cca every 5 days in humans
- Proliferation
- Differentiation of multipotential stem cells
- Located in the crypts of Lieberkühn
- Tips of the villus and epithelial surface in the colon
- Fully differentiated cells undergo apoptosis
- Extruded into the lumen
Střevní epitel -intestinal epithelium
- Is the most vigorously self-renewing tissue of adult mammals [22]
- Excessive stem cell production can result in cancer
- At the end of their lifespan, i.e. 3-5 days
- Migrating cells reach
- Tips of the villi (in the small intestine)
- Surface epithelium (in the colon)
- And undergo apoptosis and/or are shed into the lumen [21]
Buněčné linie
- Major differentiated epithelial lineages
- Frequently cycling stem cells
- Lgr5(+)
- More quiescent
- Bmi1(+) (14), mTert(+) (15), Hopx(+) (16) and Lrig1(+)
- ISC-enriched populations
- Musashi-1 (21) [21]
Lymphoid tissue
- Essential role in controlling
- Proliferation
- Differentiation of intestinal epithelial cells
- Maintenance of gut integrity [18]
Modulace střevní propustnosti
- Dietary state
- Humoral or neuronal signals
- Inflammatory mediators
- Mast cell products
- Variety of cellular pathways
- Microbial
- Viral [3]
- Transcellular absorptive processes [3]
- Concentration gradient across the barrier.
- Surface area of the epithelium.
- Time available for permeation
- Intrinsic permeability properties of the barrier [4]
