(A) In vivo studies showing absorption across intestinal epithelial barriers into blood after gastric administration of the C4:0-GM1-peptide fusion, vehicle alone, or unfused reporter peptide (five independent experiments). (B) Absorption across the intestine into blood 15 min after gastric administration of the C12:0-GM3, C6:0-GM1 peptide fusions or unfused reporter peptide (n = 2). (C) 1 hr after gastric gavage the C4:0-GM1 peptide fusion is absorbed to the liver whereas the unfused reporter peptide is not detected (four independent experiments). (D) Uptake into nasal epithelium. After topical nasal administration, tissue was fixed with 4% formaldehyde and stained with anti-EpCAM to label epithelium (red) and DAPI (blue) for nuclei. Images by two-photon microscopy comparing transport of the unfused reporter peptide (left panels) and C6:0-GM1 peptide fusion (green labeling, right panels) Scale bars 20 um upper panels, 10 um lower panels. (E) Biochemical analysis of blood 30 min after nasal administration shows systemic absorption of C6:0 and C12:0-GM1 peptide fusions (two independent experiments). (A–E) Each data point on graphs represents individual mice and bars represent mean ± s.e.m. (ns = non significant, *p<0.5; ***p<0.0001, Tukey’s multiple comparison test).