| Amn was first discovered as the disrupted gene underlying the gastrulation defects of the amn mouse mutant. The mutation leads to developmental arrest between embryonic day (E) 6.5 and E8.5, and defects appear to be largely restricted to the embryonic portion of the conceptus. The few mutant embryos that proceed through gastrulation survive until E10.5 and reach a developmental stage equivalent to early somite E8.5 wild type embryos. They possess readily visible headfolds, beating heart and allantois, but have a severely shortened trunk with limited tissue between the head/heart region and the allantois. Additionally the E10.5 amn mutant lacks any recognizable somites. Despite these gross morphological defects in epiblast-derived embryonic tissues, during gastrulation Amn is expressed exclusively in the visceral endoderm, an extra-embryonic layer. Amn is also expressed in other resorptive polarized epithelia later in development and in adulthood. Recent genetic studies on Imeslund-Grasbeck Syndrome/hereditary Megaloblastic Anemia 1 (MGA1) have shed light on the cellular function of Amn and its possible role in absorption and secretion. MGA1 is a rare inherited vitamin B12 deficiency caused by malabsorption of the vitamin from the diet. In addition to mutations in the B12 cofactor (Intrinsic Factor, IF ) and the IF-B12 receptor (Cubilin), MGA1 disease-causing mutations have been identified in AMN. Previous work in the Lacy lab and by others has shown that Amn directs apical cell surface expression of Cubilin in intestinal enterocytes, kidney proximal tubules, and visceral endoderm. This thesis has focused on both cell culture assays and yeast two-hybrid screens to define Amn's role in the internalization and trafficking of Cubilin. These studies will contribute insights and define future experimental avenues toward the question of whether Amn supports gastrulation by specifically targeting Cubilin and its ligands to a transcytotic pathway. |
