Fig. 2: Small peptides bypass S1 and are reabsorbed in S2.

a Enzymatically-digested lactoglobulin peptide fragments of different sizes were labeled with Atto 532 and purified, and their renal handling post intravenous injection was investigated with intravital imaging (mAU milli-absorbance unit). b The small peptide ALK^At532 (331 Da) was reabsorbed in S2 (representative of three independent experiments). Scale bar = 20 μm. c Conversely, a larger peptide VYVEELK^At532PTPEGDLEILLQK (2313 Da) triggered S1 uptake (representative of three independent experiments). Scale bars = 20 μm. d, e ROIs were drawn around tubular segments and the plots depict fluorescence intensity over time post injection (mean value ± SEM; n = 3 mice per group). f, g Subsequent injection of intact lysozyme^Atto647N showed a clearly distinct uptake pattern from the small peptide, but not the larger species (representative of three independent experiments; arrowheads = S1). h, i Single experiments showing uptake intensities along the proximal tubule 20 min post injection, with individual ROIs ordered according to lysozyme^Atto647N signal intensity (from highest to lowest). j–l Overview of the kidney on cross-section post fixation showing uptake of lysozyme^At647N and ALK^At532 in early and late segments of the proximal tubule, respectively. Scale bars = 500 μm. Single example image planes are depicted (representative of three independent experiments). m Higher resolution image of cortical region showing lysozyme^At647N in S1 segments leaving the glomerulus (G), and ALK^At532 in segments staining positive for the S2 marker OAT1. Nuclei were labeled with Hoechst (blue). Scale bar = 20 μm. Single example image planes are depicted (representative of three independent experiments).