Infants and children must maintain a positive calcium balance for appropriate growth. Evidence suggests that adaptations in intestinal absorption of calcium are the major determinant of this net balance. However, the molecular details of the pathways mediating this increased absorption at a young age and in different segments have not been delineated. The aim of this must maintain a positive calcium balance for appropriate growth. Evidence suggests that adaptations in intestinal absorption of calcium are the major determinant of this net balance. However, the molecular details of the pathways mediating this increased absorption at a young age and in different segments have not been delineated. The aim of this thesis work was, therefore, to delineate the molecular details of intestinal absorption contributing to a positive calcium balance during postnatal development. Using mice from birth through adult ages, we initially examined gene expression and protein abundance of known contributors to calcium absorption pathways in the duodenum, jejunum, and ileum of the small intestine as well as the proximal colon. The results of these studies implicated weaning as an age of alterations in the expression of these pathways. We, therefore, employed ex vivo and in vitro Ussing chamber functional studies, micro-CT of bones and gene expression to test our specific hypotheses that specific patterns of intestinal absorption maximize uptake of calcium during postnatal development. Transient receptor potential vanilloid 6 (TRPV6) mediated transcellular calcium absorption was only observed after weaning in the duodenum. However, we identified transcellular calcium absorption mediated by TRPV6 and Ca_v1.3 across the jejunum and nifedipine inhibitable absorption across the ileum in suckling mice which was absent after weaning. We also observed 2-fold greater intestinal calcium permeability, which contributed increased paracellular calcium absorption across the jejunum and ileum in the suckling mice. This permeability is conferred by increased claudin-2 expression in young animals, which is the result of upregulation by epidermal growth factor contained in breast milk.

Despite identifying significant transcellular calcium absorption and paracellular calcium permeability in the proximal colon of older animals, as has been previously reported, our results did not implicate these pathways in younger animals. Specifically, the genetic or pharmacological loss of TRPV6, L-type calcium channels, claudin-2 and claudin-12 did not alter proximal colon calcium absorption or permeability. However, these studies led to our discovery that claudin-2 and claudin-12 form independent Ca²⁺ permeable pores in the proximal colon and proximal renal tubule. Loss of both of these claudins results in an inability to maintain a normal serum calcium level in adult mice, due to decreased calcium permeability across the colon, decreased net intestinal calcium absorption and decreased renal calcium reabsorption. These alterations in transepithelial calcium absorption contributed to reduced bone mineralization and to altered bone microarchitecture. Thus, this work highlights the role the colon plays in overall calcium balance.

This thesis has ultimately identified novel pathways mediating calcium absorption during postnatal development when requirements for this mineral are greatest and discovered a potential therapeutic modality for infants and adults at risk of poor bone mineralization.