The amnionless gene, essential for mouse gastrulation, encodes a visceral-endoderm–specific protein with an extracellular cysteine-rich domain

Abstract

Fate-mapping experiments in the mouse have revealed that the primitive streak can be divided into three functional regions: the proximal region gives rise to germ cells and the extra-embryonic mesoderm of the yolk sac; the distal region generates cardiac mesoderm and node-derived axial mesendoderm; and the middle streak region produces the paraxial, intermediate and lateral plate mesoderm of the trunk. To gain insight into the mechanisms that mediate the assembly of the primitive streak into these functional regions, we have cloned and functionally identified the gene disrupted in the amnionless (amn) mouse, which has a recessive, embryonic lethal mutation that interferes specifically with the formation and/or specification of the middle primitive streak region during gastrulation. Here we report that the gene Amn encodes a novel type I transmembrane protein that is expressed exclusively in the extra-embryonic visceral endoderm layer during gastrulation. The extracellular region of the Amn protein contains a cysteine-rich domain with similarity to bone morphogenetic protein (BMP)-binding cysteine-rich domains in chordin, its Drosophila melanogaster homolog (Short gastrulation) and procollagen IIA (ref. 3). Our findings indicate that Amn may direct the production of trunk mesoderm derived from the middle streak by acting in the underlying visceral endoderm to modulate a BMP signaling pathway.

Luminal expression of cubilin is impaired in Imerslund-Grasbeck syndrome with compound AMN mutations in intron 3 and exon 7

“…10,19 It has been hypothesized that these truncated AMN products were produced for maintaining a proper embryonic development in mice. 8,19 The same group reconsidered this hypothesis after reporting IGS cases with mutations in exons 7 and 11 which were inconsistent with the production of truncated proteins. 10 It seems, therefore, that the role of AMN in embryonic human development, if any, is less critical than that in mice.…”

“…18 It has been suggested that AMN is a 'moonlighting' protein that fulfils 2 functions, one in embryonic development, proved in mice and the other in Cbl absorption, demonstrated in humans. 8,19 In humans, homozygous mutations affecting exons 1-4 of human AMN lead to selective malabsorption of Cbl and no embryonic effect. As a consequence of these mutations, translation initiation occurs downstream and produces truncated AMN proteins in HEK293 transfected cells.…”

“…7 Amnionless (AMN), originally identified as a visceral endoderm specific protein essential for embryonic development, is a 50 kDa transmembrane protein expressed in polarized epithelia that binds tightly to the cubilin N-terminal end. 8,9 Mutations in CUBN and AMN account for 54.8% (23/42) and 23.8% (10/42) of IGS, respectively, whereas mutations in other still unidentified genes account for 21% (9/42). 10 More recently, IGS was caused by a compound heterozygous mutation in CUBN comprising a missense mutation in the paternal allele (c.1010 C>T) and a partial gene deletion in the maternal allele.…”

“…4,5,6 IGS has also been explained as mutations in the cubilin gene (CUBN OMIM # 602997) located on chromosome 10 or in the amnionless gene (AMN OMIM # 605799) located on chromosome 14. [7][8][9][10] These two proteins are parts of the IF-Cbl receptor complex, also called cubam, which is expressed in the ileal mucosa and in the renal proximal tubules. 7,9 Cubilin is a 460 kDa multiple ligand binding protein, which cannot internalize the IF-Cbl complex by itself.…”

Beclin 1, an autophagy gene essential for early embryonic development, is a haploinsufficient tumor suppressor

“…Similar results were obtained from EBs derived from two independent beclin 1 Ϫ/Ϫ ES cell clones. Staining of EBs with antiserum against amnionless (15) revealed that EBs derived from both wt and null mutant ES cells can form VE, although in beclin 1 Ϫ/Ϫ EB, this cell layer was composed of larger, less well organized cells (Fig. 3 a and b, Right).…”