Targeted deletion of the ATP binding domain of left-right dynein confirms its role in specifying development of left-right asymmetries

Vertebrates develop distinct asymmetries along the left-right axis, which a re consistently aligned with the anteroposterior and dorsoventral axes. The mechanisms that direct this handed development of left-right asymmetries h ave been elusive, but recent studies of mutations that affect left-right de velopment have shed light on the molecules involved. One molecule implicate d in left-right specification is left-right dynein (LRD), a microtubule-bas ed motor protein. In the LRD protein of the inversus viscerum (iv) mouse, t here is a single amino acid difference at a conserved position, and the lrd gene is one of many genes deleted in the legless (Igl) mutation. Both iv a nd Igl mice display randomized left-right development. Here we extend the a nalysis of the lrd gene at the levels of sequence, expression and function, The complete coding sequence of the lrd gene confirms its classification a s an axonemal, or ciliary, dynein, Expression of bd in the node at embryoni c day 7.5 is shown to be symmetric, At embryonic day 8.0, however, a striki ng asymmetric expression pattern is observed in all three germ layers of th e developing headfold, suggesting roles in both the establishment and maint enance of left-right asymmetries. At later times, expression of lrd is also observed in the developing floorplate, gut and limbs, These results sugges t function for LRD protein in both cilitated and non-ciliated cells, despit e its sequence classification as axonemal, In addition, a targeted mutation of lrd was generated that deletes the part of the protein required for ATP binding, and hence motor function. The resulting left-right phenotype, ran domization of laterality, is identical to that of iv and Igl mutants. Gross defects in ciliary structure were not observed in lrd/lrd mutants. Strikin gly, how-ever, the monocilia on mutant embryonic node cells were immotile, These results prove the identity of the iv and lrd genes. Further, they arg ue that LRD motor function, and resulting nodal monocilia movement, are req uired for normal left-right development.