Abstract
Precision targeting imaging agents and/or treatment agents to select cells or organs in the body remains a significant need and is an area of intense research. It has been hypothesized that the vitamin B12 (B12) dietary pathway, or components thereof, may be exploitable in this area. The question of whether gastric Intrinsic factor (IF), critical for B12 absorption in the GI tract via the cubilin receptor, could be used as a targeting moiety for the cubilin receptor systemically, has not been investigated. Cubilin is the only known receptor for holo-IF and is found primarily in the kidney and ear (outside of the ileum of the GI) offering significant scope for specific targeting. We utilized plant derived human gastric IF in fluorescent cell and PET based in vivo imaging and biodistribution studies and demonstrated that plant derived IF primarily targets the liver, likely a consequence of the unique glycosylation profile of the IF, and is not affected by endogenous B12 levels.
Highlights
A basic understanding of the dietary pathway is in place[20]
Since this differs from a human glycosylation profile we wanted to investigate the hypothesis that the glycosylation profile might alter Intrinsic factor (IF) recognition in the body and be recognized by the CD206
We investigated uptake in J774.A1 macrophage cells (CUBN- and CD206+), which indicated that IF-B12-Cy5 recognition is IF specific and supports the GC-MS sugar profile
Summary
A basic understanding of the dietary pathway is in place[20]. Transport and delivery of B12 utilizes three primary carrier proteins: haptocorrin (HC; Kd = 0.01 pM), intrinsic factor (IF; Kd = 1 pM), and transcobalamin (TC; Kd = 0.005 pM), each responsible for carrying a single B12 molecule[20]. Once B12 is bound to IF, it facilitates intestinal transport and passage across the ileal enterocyte. This passage occurs via receptor-mediated endocytosis through the IF-B12 receptor cubilin (CUBN) combined with a transmembrane protein, amnionless (CUBAM)[21,22]. The first outcome postulated would be that IF pre-binding would facilitate targeting the only known holo-IF receptor, CUBN, located in the ileum in the enterocyte, as described for dietary uptake, and in the proximal tubules (PT) of the kidney, where it partners with megalin and plays a role in reabsorption of such ligands as albumin, transferrin, vitamin D binding protein, apolipoprotein AI, amongst others[24]. Questions to be explored with A. thaliana produced IF included the glycosylation profile of such a protein and the effects of such glycosylation on receptor targeting in vivo, as noted above, and whether this profile negated, complemented or refocused the CUBN targeting hypothesis noted above
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