Cldn2-Trpv6 Double Knockout Mice have Hypocalcaemia and Reduced Bone Mineral Densit

• Author / Creator

  Bhullar, Harneet

• Central to calcium homeostasis and bone health is adequate intestinal calcium absorption.
  Calcium from the diet is absorbed via either a transcellular or a paracellular pathway both of
  which are regulated by calciotropic hormones. In cases of low dietary calcium availability, a
  transcellular pathway that relies at least in part on the apical calcium channel Trpv6
  predominates. However, when calcium is abundant in the diet, paracellular calcium absorption
  occurs through tight junction proteins, including claudin-2 (Cldn2). This tight junction protein
  also plays a role in calcium reabsorption from the renal proximal tubule to help maintain overall
  calcium balance. Despite this, Cldn2 knockout mice only display mildly altered calcium
  homeostasis. Specifically, they have hypercalciuria without evidence of altered bone mineral
  density (BMD), plasma calcium or calciotropic hormone levels. Similarly, mice homozygous for
  a non-functional Trpv6 (TRPV6D541A/D541A) channel have reduced intestinal calcium absorption
  without altered urine or plasma calcium levels, yet only display reduced intestinal calcium
  absorption when fed a low calcium diet. We hypothesized that the lack of a more pronounced
  phenotype in these two murine models was due to the compensation of one pathway in the
  absence of the other. To test this hypothesis, we crossed claudin-2 knockout mice with mice
  expressing the mutant Trpv6, generating a functional double knockout (dko, Cldn2-/-/
  TRPV6D541A/D541A mice). The dko mice displayed hypocalcaemia and a significant renal calcium
  leak. In contrast to the single claudin-2 KO mice or single TRPV6D541A/D541A mutant animals the
  dko mice had elevated plasma calcitriol and parathyroid hormone. Moreover, BMD was reduced
  in the dko mice. RT-qPCR of intestinal segments and whole kidney revealed increased calbindin-
  D9k expression in the proximal colon (but not duodenum) and calbindin-D28k in the kidneys,
  iv
  consistent with compensatory increased transcellular absorption from the colon and distal
  nephron. Together these results are consistent with the paracellular pathway compensating for
  the loss of transcellular calcium absorption from the intestine.

• Subjects / Keywords

  • Calcium
  • Claudin 2
  • Trpv6
  • Knockout

• Graduation date

  Fall 2023

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-65pk-vk64

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.