Gastric proteinase digestion of caseins in newborn pups of the mouse

Protein Digestion of Baby Foods: Study Approaches and Implications for Infant Health

Protein digestion is critical for infants. Dissimilarities between infants and adults in food intake and digestive physiology lead to distinct patterns of proteolysis between individuals. However, such differences are not well represented in many studies on protein digestion of baby foods. The complex biological structures of baby foods and the physiology of the infant digestive system are key factors affecting proteolysis during the first two years of life. Well-controlled in vitro studies have demonstrated that varying digestion conditions alter the specificity, rate, and extent of proteolysis of baby foods. Nonetheless, these models do not completely replicate in vivo proteolysis or the complex biogeography of the gastrointestinal tract. Animal and clinical studies have revealed the fate of dietary proteins along the digestive tract and the overall health impact on subjects. Building comprehensive and annotated datasets from human infants will require innovative and standardized measurement. Now, more systematic evaluations of digestion are emerging to advance the knowledge and its translation as food design for effective diet and health management in infants.

Newborns prefer the odor of milk and nipples from females matched in lactation age: Comparison of two mouse strains

Newborn mice are attracted to mammary odor cues carried in murine milk and nipple secretions. However, murine milk odor is not equally attractive along lactation. The present study focuses on the differential response of 2day-old mouse pups of C57Bl/6 (C) and Balb/C (B) strains to the odor of milk (Experiment 1) and nipples (Experiment 2) that are matched/unmatched in terms of pup's age or strain. In Experiment 1, C and B pups were tested in a series of tests simultaneously opposing either murine milk and a blank (water), or two milks collected in early and late lactation (lactation days 2 and 15, respectively) from females belonging to their own or the other strain. Results showed that C and B pups were attracted to the odor of the different milks regardless of the lactation age and the strain of the donor female. Nevertheless, C and B pups preferred the odor conveyed by early- than late-lactation milk of either strain. Moreover, early-lactation milk from C females was more attractive than early-lactation milk from B females for pups of either strain. In Experiment 2, differential nipple grasping response of C and B pups was measured when they were exposed to nipples of females in early or late lactation. The proportion of C pups that grasped a nipple was greater when they were exposed to a nipple in early lactation regardless of the strain of the donor females, whereas the proportion of B pups that grasped a nipple was greater when they were exposed to a nipple in early lactation, but only from own strain. Thus, newborn mice prefer the odor of milk and nipples from females that are matched in lactation age. This result is discussed in terms of reciprocally adaptive mechanisms between lactating females and their newborn offspring.

The response of newly born mice to odors of murine colostrum and milk: unconditionally attractive, conditionally discriminated

It is a general rule that milk conveys chemosensory cues that are attractive to mammalian neonates. This study investigated whether compositional fluctuations in milk along lactation induce variations in newborn mouse pups' (Mus musculus, strain BALB/c) attraction to milk odor. Pups differing in suckling experience were exposed to the odor of milk sampled from females varying in lactational stage. Immediately after birth, suckling-inexperienced (P0) and suckling-experienced (P0suck ) pups were assayed in a series of paired-choice tests contrasting murine milk [of lactation days 0, 3, 15 (abridged L0, L3, L15, respectively)] and a blank (water) to evaluate olfactory detection and attraction of milk odor. Preference tests further paired these milk two-by-two to assess their relative attraction. Results showed first that P0 and P0suck pups detect and positively orient to any milk odor. When L0 is presented against L15 milk, P0 pups orient for a similar duration towards these odor stimuli, whereas P0suck pups spend more time toward the odor of L0 than of L15 milk. Finally, P0suck pups orient similarly to odors of L0 milk collected before/after the first suckling episode (L0 and L0suck , respectively), but the odor of L0 milk was more attractive than that of L3 milk. Thus, mouse pups' positive orientation toward the odors of murine colostrum (assumed to correspond to L0/L0suck milk) and later-lactation milk appears unconditional of previous suckling experience, whereas their ability to discriminate or display preference between milk differing in lactation stage appears conditional on postnatal exposure effects.

An odor timer in milk? Synchrony in the odor of milk effluvium and neonatal chemosensation in the mouse

Mammalian newborns exhibit avid responsiveness to odor compounds emanating from conspecific milk. Milk is however developmentally heterogeneous in composition as a function of both evolved constraints and offspring demand. The present study aimed to verify whether milk odor attractivity for neonates is equally distributed along lactation in Mus musculus (Balb-c strain). Therefore, we exposed pups varying in age to milk samples collected from females in different lactational stages. The pups were assayed at postnatal days 2 (P2), 6 (P6) and 15 (P15) in a series of paired-choice tests opposing either murine milk and a blank (water), or two samples of milk collected in different stages of lactation [lactation days 2 (L2), 6 (L6), and 15 L15)]. Pups of any age were able to detect, and were attracted to, the odor of the different milk. When milk from different lactational stages were simultaneously presented, P2 pups oriented for a similar duration to the odors of L2 and of L6 milk, but significantly less to the odor of L15 milk. Next, P6 pups roamed equivalently over L2 and L6 milk odors, but still less over the odor of L15 milk. Finally, P15 pups explored as much L15 milk odor as the odors of both L2 and L6 milk. This developmental shift in milk attractivity is discussed in terms of changing chemosensory properties of milk and of shifting chemosensory abilities/experience of pups.

Lactational performance of Quackenbush Swiss line 5 mice

We evaluated 2 strains of mice for their utility in the investigation of nutritional and molecular regulatory mechanisms of lactation. The lactational performance and milk composition were characterized for an inbred mouse strain, inbred Quackenbush Swiss line 5 (QSi5) selected persistently for fecundity, and a nonselected strain, CBA. The milk yield assessed by changes in BW in response to suckling of sustainable litter sizes for each strain was 3-fold greater (P < 0.001) in QSi5 mice than the CBA strain. The QSi5 mice also produced milk more efficiently (P < 0.001) than CBA mice, despite having the same quantity of mammary tissue per unit of BW. Milk composition did not vary between strains or by stage of lactation, with the exception of lactose concentration, which was greater (P = 0.003) in QSi5 mice. Expression of epsilon-casein was > or = 10-fold greater, and alpha(S1)-casein was > or = 3-fold greater, during mid and late lactation compared with early lactation in both strains, whereas kappa-casein underwent an apparent alteration in posttranslational modifications in both strains from early to mid lactation. Changes in casein composition coincided with an increased susceptibility to proteolytic degradation; hence milk from early lactation may be more readily degraded to facilitate digestion in the neonate. The greater milk synthetic capacity of QSi5 mice over the lactation cycle provides a useful model for studies of nutritional and molecular regulation of lactation.

Application of two-dimensional electrophoresis for monitoring gastrointestinal digestion of milk

Two-dimensional electrophoresis (2-DE) was used for tracing in vivo gastrointestinal digestion of milk proteins in a rapid model system with rats. Contents of stomach and small intestine from digestion trials with rats given a single dose of milk powder were recovered after 1 hour. They were then subjected to 2-DE (IEF and SDS-PAGE). 2-DE showed undigested proteins in a MW range 13.0-66.0 kDa in stomach and 13.0-25.0 kDa in the small intestine, thus indicating that milk proteins are slowly digested. This approach may shed light on pattern of protein digestion and mechanism of amino acid and peptide assimilation.

The casein mRNA decay changes in parallel with the poly(A) tail length in the mouse mammary gland

Using beta- and gamma-casein mRNAs, the relationship between poly(A) tail length and half-life of mRNA is determined in the mouse mammary gland during pregnancy and lactation. beta- and gamma-Casein mRNAs increase before and after parturition, respectively. The poly(A) tail as well as the half-life of casein mRNA becomes longer upon the active casein mRNA synthesis. The poly(A) tail is shortened gradually as lactation progresses. The half-life of mRNA decreases approximately from 20 h at early to 4 h at late lactation. Northern blot analysis reveals that nuclear RNA has the same poly(A) tail length as casein mRNA in the cytoplasm does. Thus, the mammary gland changes the poly(A) tail length of casein mRNA. The poly(A) tail length changes in parallel with the level of poly(A) polymerase (PAP) mRNA during pregnancy and lactation, suggesting that the mammary gland determines the poly(A) tail length of casein mRNA through the change in the PAP gene expression. As the half-life of casein mRNA is related with the degree of polyadenylation, we conclude that the poly(A) tail elongation and shortening is a mechanism in regulating the mRNA decay.