Upregulation of TNAP promotes the endocytosis of phosphopeptide assemblies. (A) TEM images of NBD-1p (200 μM) before and after the addition of ALP (0.1 U/mL, 37 °C) for 24 h. The insert is the diameter distribution of the nanoparticles formed by NBD-1p. Scale bar = 100 nm. (B) Immunofluorescence staining of TNAP in the transfected HEK293 cell. The non-transfected cells (pointed by arrows) show little TNAP. (C) Confocal fluorescence images of HEK293_TNAP-RFP cells co-incubated with NBD-1p (200 μM, 5 min, no washing). To visualize NBD-1p at early stage, the laser power here is 5 times stronger than that used in Figure 1D. (D) Confocal fluorescence imaging of TNAP-RFP transfected HEK293 cells treated by NBD-1p (200 μM, 12 h, washed). The non-transfected cells (indicated by arrows) show little intracellular NBD fluorescence.