Joined at the Hip

Cooperation in the cerebellum y crossing knockout mice deficient for two cell adhesion molecules, Sakurai et al. have obtained the first genetic evidence for the molecules' functional overlap, and have explained why earlier tissue culture results were not replicated in single knockout mice (page 1259). Sakurai and colleagues started by engineering mice deficient for NrCAM, which had only mild (‫ف‬ 11%) growth defects in two cerebellar lobes. When the authors crossed the NrCAM-deficient mice with existing mice that are deficient for the related cell adhesion molecule L1, the cerebellum of the double knockout was drastically reduced in size, and the mice were small and never survived later than eight days after birth. The mice probably die because their lack of coordination does not allow them to compete successfully for food. The cerebellar defect may be largely a result of reduced migration or decreased survival of granule cells. In the double knockout, these interneurons initially differentiate and start migrating correctly, but they reach their final destination in vastly reduced numbers. There is no accumulation of these cells in other areas of the cerebellum, suggesting a survival problem. The survival hypothesis would be consistent with tissue culture experiments, in which Sakurai et al. added L1 antibodies to cells isolated from NrCAM-deficient mice, and observed a dramatic reduction in viability at around day 14 of culture. More experiments along these lines should reveal the specific functions of L1, NrCAM, and the two other related CAMs (neurofascin and CHL1) in processes such as cell migration, axon bundling, and synaptogenesis. ᭿ Mice lacking two adhesion molecules (right) have smaller cerebellums. direct link between clathrin cages and actin is described on page 1209 by Engqvist-Goldstein et al. The protein that forms the link is huntingtin interacting protein 1 related (Hip1R), which was characterized previously as an actin-binding protein that associates with clathrin-coated pits and vesicles. The related protein Hip1 has been implicated in Huntington's disease, where loss of an A interaction between normal huntingtin protein and Hip1 may cause problems with membrane– cytoskeletal integrity in the brain. Engqvist-Goldstein et al. now show that Hip1R tightly associates with clathrin during endocytosis Hip1R (gold) bridges actin and clathrin. and that purified Hip1R both binds to purified clathrin cages and stimulates cage assembly in vitro. They also show that clathrin cages assembled in vitro with Hip1R copellet with F-actin at low speed. Other workers have induced the formation of …