Mice Lacking the Giant Protocadherin mFAT1 Exhibit Renal Slit Junction Abnormalities and a Partially Penetrant Cyclopia and Anophthalmia Phenotype

Abstract

While roles in adhesion and morphogenesis have been documented for classical cadherins, the nonclassical cadherins are much less well understood. Here we have examined the functions of the giant protocadherin FAT by generating a transgenic mouse lacking mFAT1. These mice exhibit perinatal lethality, most probably caused by loss of the renal glomerular slit junctions and fusion of glomerular epithelial cell processes (podocytes). In addition, some mFAT1 ؊/؊ mice show defects in forebrain development (holoprosencephaly) and failure of eye development (anophthalmia). In contrast to Drosophila, where FAT acts as a tumor suppressor gene, we found no evidence for abnormalities of proliferation in two tissues (skin and central nervous system [CNS]) containing stem and precursor cell populations and in which FAT is expressed strongly. Our results confirm a necessary role for FAT1 in the modified adhesion junctions of the renal glomerular epithelial cell and reveal hitherto unsuspected roles for FAT1 in CNS development.Cell adhesion molecules are essential for the regulation of coordinated changes in cell shape and position that occur during morphogenesis and also for the maintenance of cell-cell interactions once development is complete. The best-studied families of cell adhesion molecules are the cadherins. These were originally identified as Ca 2ϩ -dependent cell-cell adhesion proteins characterized by five repeated cadherin-specific motifs in their extracellular domain. This domain is an approximately 110-amino-acid peptide that mediates homophilic interactions with other cadherin molecules, forming dimers which then interact with dimers on neighboring cells (41,45,46). The cytoplasmic domain of these so-called classical fiverepeat cadherins is well conserved between different members of the family, containing motifs required for interaction with catenins that can then mediate linkage to the cytoskeleton (13,17,34,42). In addition to the classical cadherins, at least five other classes of cadherins have been identified: nonclassical cadherins, in which the first extracellular repeat does not contain the HAV (histidine-alanine-valine) cell adhesion sequence present in all classical cadherins, and four additional families. Two of these, the desmosomal adhesion molecules desmocollins and desmogleins, all share the five extracellular cadherin repeat structure with the sole exception of desmocollin-1. Two other families, the flamingo cadherins and protocadherins, have variable numbers of cadherin repeats and additional motifs as well as differing cytoplasmic domain sequences (30,44).

The fat-like Gene of Drosophila Is the True Orthologue of Vertebrate Fat Cadherins and Is Involved in the Formation of Tubular Organs

“…The expression of mouse fat1 at the slit junction suggests a role as both an adhesion molecule and as a spacer of the junction (32). In the mouse knock-out the slit junctions disappear, generating a flattened sheet overlying the basement membrane (14). We thereby infer a similar behavior in Drosophila ftl mutants whereby tracheal epithelia fuse because of degeneration of the luminal space.…”

“…Although these results are not conclusive, they may indicate that Ftl does not play a major role in intercellular adhesion or adhesion to the ECM in S2 cells. Similarly, the adhesion complexes in the skin of mFAT1 knock-out mice were not altered, and the epidermal cells did not show any morphological changes (14).…”

“…The previously proposed tumor suppressor role of vertebrate Fat cadherins was only based on sequence similarity with Fat. Just recently, this issue has been addressed by functional studies to show that mice lacking mFAT1 did not reveal any changes in cellular overgrowth (14).…”

“…We favor the latter alternative, first because classical mutants affecting tube formation also show variable phenotypes, thereby reflecting the view that different processes and various environments interplay with each other in tube development (28). Second, in the mfat1 knock-out mouse, only some mutants showed distinctive morphological defects as holo-prosencephaly (14). In particular, the variation in the eye phenotype reported in the same study was remarkable, often showing severe eye phenotypes on one side and normal eye development on the other side.…”

“…Mutations in cdh-4 are also viable suggesting some redundancy between the two genes. 2 Further clues to the function of Fat cadherins came from the report on the first mouse knock out of the fat1 gene (14). Embryonic lethality in fat1 Ϫ mice is likely to be caused by defects in renal glomerular slit junctions and fusion of epithelial cell processes; however, no tissue overgrowth phenotype was observed.…”