Reduced availability of a selective human milk oligosaccharide during lactation impairs post-weaning executive functions

Maternal milk is considered the ideal source of early life nutrition. It contains nutritional, immunological and signalling factors that are essential for the correct development of the new-born, including development of cognitive capabilities, gastrointestinal tract, and immune functions. Thus, breast-feeding has been associated with improved cognitive capabilities and increased resistance to infection. Specifically, there is an association between duration of breast feeding and verbal IQ. Human milk oligosaccharides (HMOs) represent the third most abundant component of breast milk; many of them are composed by sialylated molecules that represent the principal source of sialic acid (Sia) for the infant. Sia is an essential component of glycolipids (gangliosides) and glycoproteins (polySia-NCAM), two highly represented molecules in the brain. Since endogenous synthesis of Sia is insufficient to meet the nutritional needs of the neonate, its dietary provision is fundamental. Among the components of the breast-milk, sialylated-HMOs have been indicated as possible mediators of its beneficial effects on neurodevelopment and brain function. This study investigated the effects of one specific HMO, sialyl(alpha2,3)lactose (3’SL), on cognitive development. Importantly, 3’SL is abundant in maternal milk, and absent in infant formula. We hypothesised that 3’SL during lactation has permanent effects on executive functions in adulthood and that variations in this HMO during lactation related to a differential development of learning, memory, attention and locomotion. Methods: To modulate the availability of 3’SL during lactation, we performed a cross-fostering study in which wild-type mice were reared to knock-out mice constitutively deficient of 3’SL. Specifically, the study involves C57BL/6J wild-type and C57BL/6-St3gal4tm1.1Jxm/J transgenic male mice. The deletion of the ST3GAL4 gene (B6-129 St3gal4-/-) resulted in an 80% decrease of 3’SL in milk. The cross-fostering design resulted in the following groups: WT mice reared to WT dams (WT to WT, N=11), WT mice reared to KO dams (WT to KO, N=12), KO mice reared to WT dams (KO to WT, N=12) and KO mice reared to KO dams (KO to KO, N=12). Adult subjects were tested for spatial memory (Barnes maze), recognition memory (Novel object recognition task), attention (Attentional set shifting task, ASST), impulsivity (T-maze), sensorimotor gating (Prepulse inhibition) and anxiety (Elevated plus maze). Data was analysed using a factorial ANOVA using 2 between subject factors (dams genotype and pups genotype). Results: compared to control subjects, all experimental groups exhibited a reduction of retention memory in the Barnes maze one day after training, but not in the novel object recognition task, indicating that early life presence of 3’SL may support spatial memory. When evaluating attentional capabilities, mice that received 3’SL deficient-milk exhibited poor performance in several stages of the ASST. Furthermore, the same experimental group showed increased impulsivity. The experimental groups did not differ in anxiety-like behaviour, evaluated through the elevated plus maze. Conclusion: reduced availability of 3’SL during lactation caused long-lasting deficits in cognitive abilities, thus confirming the importance of the presence of this breast-milk component during the crucial period of lactation. Since this study was the first to evaluate this deficiency, further studies will be needed to confirm these findings and deepen the mechanism underlying the effect of 3’SL.