DMF treatment disrupts lysosomes in human pancreatic beta cells after loading with LAMP1-SPIONs. (A) Immunostaining of human islet beta cells with or without DMF treatment. Lysosomes were stained with the anti-LAMP1 antibody (red), SPIONs with TRITC (orange), and islet beta cells with an anti-insulin antibody (green), respectively. Scale bars = 2 μm. (B) The SPIONs are located in the membrane of a lysosome. The intensity profile (right) was derived along the red line in the image (left). (C) Same as (B), except after treatment with DMF. (D) Differences in size distribution of lysosomes before (gray bars) and after (black bars) DMF treatment. (E) Average size of lysosomes without and with DMF treatment. ***p < 0.001. (F) Transmission electron microscopy (TEM) images of the intracellular distribution of SPIONs in INS-1 cells. Images on the bottom are magnified versions of the areas indicated with white boxes. While without DMF treatment the LAMP1-SPIONs are clustered in vesicular structures, their distribution is scattered throughout the cytosol after DMF treatment.