Effect of dietary protein level on intestinal aminopeptidase activity and mRNA level in rats

Rapid and parallel changes in activity and mRNA of intestinal peptidase to match altered dietary protein levels in juvenile house sparrows (Passer domesticus)

Although dietary flexibility in digestive enzyme activity (i.e. reaction rate) is widespread in vertebrates, mechanisms are poorly understood. When laboratory rats are switched to a higher protein diet, the activities of apical intestinal peptidases increase within 15 h, in some cases by rapid increase in enzyme transcription followed by rapid translation and translocation to the intestine's apical, brush-border membrane (BBM). Focusing on aminopeptidase-N (APN), we studied intestinal digestive enzyme flexibility in birds, relying on activity and mRNA data from the same animals. Our model was nestling house sparrows (Passer domesticus), already known to modulate intestinal peptidase activity when switching between lower and higher protein diets. Twenty-four hours after a switch from an adequate, lower protein diet to a higher protein diet, APN activity was increased in both whole intestinal tissue homogenates and in isolated BBM, but not at 12 h post-diet switch. Twenty-four hours after a reverse switch back to the lower protein diet, APN activity was decreased, but not at 12 h post-diet switch. Changes in APN activity in both diet switch experiments were associated with parallel changes in APN mRNA. Although transcriptional changes seem to be an important mechanism underlying dietary modulation of intestinal peptidase in both nestling house sparrows and laboratory rodents, the time course for modulation in nestlings seemed slower (taking approximately twice as long) compared with laboratory rodents. It may be ecologically advantageous if nestlings biochemically restructure their gut in response to a sustained increase in insects and protein intake rather than one or a few lucky insect meals.

Dietary adaptation to high starch involves increased relative abundance of sucrase-isomaltase and its mRNA in nestling house sparrows

Dietary flexibility in digestive enzyme activity is widespread in vertebrates but mechanisms are poorly understood. When laboratory rats are switched to a higher carbohydrate diet, the activities of the apical intestinal α-glucosidases (AGs) increase within 6-12 h, mainly by rapid increase in enzyme transcription, followed by rapid translation and translocation to the intestine's apical, brush-border membrane (BBM). We performed the first unified study of the overall process in birds, relying on activity, proteomic, and transcriptomic data from the same animals. Our avian model was nestling house sparrows (Passer domesticus), which switch naturally from a low-starch insect diet to a higher starch seed diet and in whom the protein sucrase-isomaltase (SI) is responsible for all maltase and sucrase intestinal activities. Twenty-four hours after the switch to a high-starch diet, SI activity was increased but not at 12 h post diet switch. SI was the only hydrolase increased in the BBM, and its relative abundance and activity were positively correlated. Twenty-four hours after a reverse switch back to the lower starch diet, SI activity was decreased but not at 12 h post diet switch. Parallel changes in SI mRNA relative abundance were associated with the changes in SI activity in both diet-switch experiments, but our data also revealed an apparent diurnal rhythm in SI mRNA. This is the first demonstration that birds may rely on rapid increase in abundance of SI and its mRNA when adjusting to high-starch diet. Although the mechanisms underlying dietary induction of intestinal enzymes seem similar in nestling house sparrows and laboratory rodents, the time course for modulation in nestlings seemed half as fast compared with laboratory rodents. Before undertaking modulation, an opportunistic forager facing limited resources might rely on more extensive or prolonged environmental sampling, because the redesign of the intestine's hydrolytic capacity shortly after just one or a few meals of a new substrate might be a costly mistake.

Intestinal digestive enzyme modulation in house sparrow nestlings occurs within 24 hours of a change in diet composition

Nestling house sparrows near fledging age (12 days) were previously found to reversibly modulate the activity of their intestinal digestive enzymes in response to changes in diet composition. However, it is not known how quickly nestlings can adjust to new diets with different substrate compositions, nor is it known how early in life nestlings can modulate their enzyme activity in response to changes in diet. In the current study, 3-day-old nestlings were captured from the wild and fed and switched among contrasting diets – one high in protein and low in carbohydrate and another higher in carbohydrate and with lower but adequate protein – in order to determine (1) how quickly house sparrow nestlings could adjust to changes in diet composition, (2) how early in life nestlings could modulate their digestive enzyme activity in response to these changes, and (3) which digestive enzymes could be modulated in house sparrow nestlings earlier in life. We found that house sparrow nestlings as young as three days post-hatch were capable of modulating their intestinal disaccharidase activity within 24 hours of a change in diet composition, and nestlings gained the ability to modulate aminopeptidase-N by six or seven days of age. To our knowledge, this is the first evidence of digestive enzyme modulation completed within 24 hours of a change in diet in an avian species and the first study to show intestinal digestive enzyme modulation in response to changes in diet composition in any animal this early in development.

The Secretion and Action of Brush Border Enzymes in the Mammalian Small Intestine

Microvilli are conventionally regarded as an extension of the small intestinal absorptive surface, but they are also, as latterly discovered, a launching pad for brush border digestive enzymes. Recent work has demonstrated that motor elements of the microvillus cytoskeleton operate to displace the apical membrane toward the apex of the microvillus, where it vesiculates and is shed into the periapical space. Catalytically active brush border digestive enzymes remain incorporated within the membranes of these vesicles, which shifts the site of BB digestion from the surface of the enterocyte to the periapical space. This process enables nutrient hydrolysis to occur adjacent to the membrane in a pre-absorptive step. The characterization of BB digestive enzymes is influenced by the way in which these enzymes are anchored to the apical membranes of microvilli, their subsequent shedding in membrane vesicles, and their differing susceptibilities to cleavage from the component membranes. In addition, the presence of active intracellular components of these enzymes complicates their quantitative assay and the elucidation of their dynamics. This review summarizes the ontogeny and regulation of BB digestive enzymes and what is known of their kinetics and their action in the peripheral and axial regions of the small intestinal lumen.