Normatividad clara

Gastrointestinal mucosal epithelia are directly exposed to the hazards of an external environment, the presence of bile salts, acids, digestive enzymes and pathogenic bacteria, making the intestinal lumen a particularly noxious environment (Bevins et al., 1999; Sanderson and Walker, 1999). As a result the mucosal epithelia are rapidly and continuously renewed; a process essential to ensure effective digestion, absorption of nutrients and to reduce the colonisation of pathogenic micro-organisms.

It is thought that exogenous bioactive peptides may influence gastrointestinal epithelia proliferation (Kelly and Coutts, 1997). Certainly bioactive peptides influence the

release of somatostatin, [mentioned previously (Schusdziarra et al., 1983a)] and

gastrin, both of which are known to influence epithelial proliferation in the gut

(Ichikawa et al., 1993). Mucin secretions in the gut contain epidermal growth factor

that promotes epithelial proliferation. The casomorphins are known to affect the

secretion of mucin, suggesting a protective response that also stimulates the production of epidermal growth factor and increased cell growth.

IV.2.9.

The influence of bioactive peptides on mucus secretion

The entire length of the GIT is permanently covered by a strongly adherent layer of high molecular weight glycoproteins, secreted by specialised goblet cells within the mucosa. The mucus is a complex biofilm that contains proteins, fats and bacteria in a gel-like matrix (Lamont, 1992). The matrix may also contain other compounds including: bicarbonate ions, epidermal growth factor, trefoil peptides, bactericidal factors, protease inhibitors, and surface-active lipids (Krause, 2000). Such compounds, when incorporated into the mucus layer, guard against its degradation and protect the underlying mucosa from gastric acid and pancreatic enzymes. The matrix also acts as a barrier to enteric microorganisms and the toxins produced by them, as well as being a diffusion barrier for dissolved compounds of low molecular weight (Perez-Vilar and Hill, 1999).

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In addition to the mucus secreting goblet cells of the crypts and villous epithelium, there are specialised mucus secreting glands in the submucosa of the proximal duodenum called Brunner´s glands. These glands secrete a glutinous alkaline mucin that contains bicarbonate ions to neutralise the acidic gastric fluids and forms a slippery gel that lubricates the mucosa of the proximal intestinal tract. Thought to be the product of the MUC6 gene, the class III mucin from the human Brunner´s gland consists of O-linked oligosaccharides that are attached to a central protein core (Wapnir and Teichberg, 2002). The Brunner´s glands also provide; both active and passive immunological defence mechanisms, promote cellular proliferation and differentiation, as well as raising the pH of the luminal contents by promoting the secretions of the intestinal mucosa, pancreas and gall bladder (Krause, 2000). The most important proteins within the matrix are mucins, a family of polydisperse molecules of high molecular weight and a high proportion of covalently-bound

oligosaccharide side chains (Corfield et al., 2001) which afford high resistance to the

effects of acid and digestive enzymes. They may be characterised as secreted or

membrane-bound. Secreted mucins, up to 2 х 106 Da (Montagne et al., 2004) contain a

central polypeptide core, of 1500 to 4500 amino acids in length, with 100-200

oligosaccharide side chains that contain 1 to 20 or more monosaccharides, oriented in a fashion similar to a bottlebrush; such oligosaccharides may account for 50-80% of the molecule´s mass (Roussel and Delmotte, 2004). The highly glycosylated regions of the

polypeptide are rich in threonine, serine and proline and may account for 70%–80% of

the molecule, the poorly glycosylated regions contain less serine and threonine but are rich in cysteine, which allows the formation of disulphide bridges between mucin molecules to form very high molecular mass mucous polymers (Krause, 2000). The secreted mucins protect the delicate underlying mucosal surfaces by the polymerisation of mucin monomers to form viscoelastic gels (Krause, 2000). Membrane-bound

mucins do not form gels; the glycosylated monomers stretch out from the epithelial surface and form the cell-surface membrane glycocalyx.

The oligosaccharide side chains are strongly hydrophilic, which:

x promotes the binding of water molecules and supports the formation of the

gel matrix,

x prevents the degradation of the polypeptide chain by proteases from the

pancreas and bacteria,

x binds pathogens, parasites and toxins within the gel matrix.

The hydrophobic areas may bind fats and promote protein–protein interactions. The

most important mucosal produced mucins are rich in cysteine, threonine, proline and serine, and substantial changes in mucus secretion therefore have a measurable effect on the cysteine and threonine requirements of an animal (Reeds et al., 1999).

Studies suggest that mucin production is regulated by the same hormonal control as

other digestive processes, e.g. insulin (Tabuchi et al., 1997) secretin (Tani et al., 1997)

and gastrin. (Ichikawa et al., 1993) The physiological role of gastrointestinal mucins is

summarised in Table 7.

Mucins are continuously being degraded, by both proteolysis and physical erosion at the apical surface; this is compensated for by continuous secretions by goblet cells of the mucosa. The density of goblet cells increases from the proximal duodenum to the distal rectum.

Table 7 Thephysiological role of gastrointestinal mucin.

Function in relation to gut physiology Function in relation to gut health

Protection of the epithelium within the gut against acidic environment

Fixation of commensal bacteria permitting colonisation resistance Selective diffusion barrier permeable to

nutrients but not to macromolecules

Fixation of pathogens; bacteria, viruses, and parasites

Protection against endogenous and bacterial proteases

Component of the gut-associated lymphoid tissue

Lubrication of the gut epithelium Substrate for bacterial fermentation

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Within the GIT, a dynamic equilibrium exists between the rate at which mucin is synthesised and secreted and the rate at which it is degraded. Moreover, the mucus is continuously transported through the GIT including the cellular fats, DNA, proteins, sloughed epithelial cells and micro-organisms captured by it (Bannink et al., 2006). The quantity of mucins secreted into the GIT may total 50% to 65% of the endogenous protein secreted into the gut (Montagne et al., 2004). Although the synthesis of mucus is a smaller metabolic burden than ion and nutrient transport and tissue turnover, the costs are nevertheless still significant [estimated to be 10% to 20% of the total energy costs of gastrointestinal wall metabolism (Allen and Flemstrom, 2005)]. Being

continually degraded mucins are a significant proportion of endogenous protein losses

and may total 5% to11% of the endogenous protein leaving the terminal ileum (Lien et

al., 1997b; Montagne et al., 2004). Because of the importance of mucin in the

protection of the mucosa any mechanism that alters this defensive barrier has important physiological implications, especially in the control and management of

inflammatory diseases of the bowel (Barcelo et al., 2000).

The secretion of mucus may be increased in response to a variety of physiological and pathological stimuli, e.g. bacteria, bacterial toxins, inflammation mediators, chemical stimuli or neural stimuli (Bannink et al., 2006). A number of dietary factors may also affect the secretion of mucin including the amount of fibre and anti-nutritional factors

(Claustre et al., 2002; Lien et al., 1996; Montagne et al., 2004; Satchithanandam et al.,

1996). It has also been reported that certain dietary components may alter the composition of mucin secreted into the GIT (Montagne et al., 2004).

The hypothesis that dietary proteins and their hydrolysates, containing bioactive peptides, may affect mucin secretion has been studied by a number of researchers

(Claustre et al., 2002; Han et al., 2008; Lien et al., 2001; Montagne et al., 2000). The

effect of opioid peptides, specifically the β-casomorphins -7, -6, -4, -4NH2 and -3 and

a number of neuropeptides, on mucin secretion has been reported by Trompette et al.

(2003) where the intra-luminal administration of the β-casomorhin-7 provoked a 500%

increase (over the controls), in the secretion of mucin. β-casomorphin-7 seems unique

in this respect as little or no increase in mucin secretion was observed from the other opioid peptides tested.

Perhaps here lies the key to the variability in the observed changes in endogenous protein secretion that have been reported by so many researchers (Claustre et al., 2002; Han et al., 2008; Lien et al., 2001; Montagne et al., 2000; Trompette et al., 2003). The analysis of digesta samples taken from the terminal ileum of animals given casein hydrolysate compared to those given a milk protein or protein-free diets may reveal clearer information regarding the effects of opioid peptides on endogenous protein losses.