KIF1Bβ is responsible for axonal elongation. (A and B) Fluorescent micrographs of the hippocampal neurons of Kif1b^+/+ and Kif1b^−/− mice at DIV3 (A). The neurons were plated at 8.6 × 10⁴ cells/cm² and transfected with TagRFP accompanied by the quantification of axon lengths at DIV2 and 3 (B). The fluorescence microscopy data were collected using LSM710 or 780 confocal microscopes using Plan Apochromat 40×/1.4 or 40×/1.3 objectives, and the data are represented as the mean ± SEM. Bars, 25 µm. n = 21–25. **, P < 0.01; ***, P < 0.001, one-sided unpaired Welch’s t test. (C and D) Fluorescent micrographs of DIV4 hippocampal neurons electroporated with the indicated miRNA vectors fused with TurboRFP at DIV0 using a Plan Apochromat 20×/0.8 objective (C) accompanied with the statistics of the longest neurite lengths (D). Note that KIF1Bβ knockdown significantly reduced neurite elongation. Bars, 20 µm. n = 55–90. ***, P < 0.001; ns, P > 0.05; one-sided Dunnett’s method. (E) IB verifying the effects of KIF1Bα and KIF1Bβ knockdown vectors. N1E-115 cells were transfected with the respective miRNA vectors, lysed 2 d after transfection, and subjected to IB. Scrambled (SC) miRNA was used as a negative control. (F and G) TUNEL staining of the dissociated hippocampal neurons of Kif1b^+/+ and Kif1b^−/− mice at DIV3 counterstained with an anti–β3-tubulin antibody (F) and quantified (G). Despite the significantly higher apoptosis rate in Kif1b^−/− neurons (G), note that the cells with long neurites are not largely apoptotic (F). Bar, 25 µm. n = 5–6 fields of vision. *, P < 0.05, one-sided unpaired Welch’s t test.