MYCT1 restricts endothelial endocytosis and IFITM2/3-dependent mTORC1 activation, related to Figs. 6 and 7. (A) IFITM2/3 antibody and siRNA validation for identification of endogenous human IFITM2/3 proteins. IFITM2/3 knockdown reduces MYCT1 protein levels. Staining of ECs for MYCT1 (gray), IFITM2/3 (green), and DNA (blue). Scale bar, 20 µm. (B) Quantification of MYCT1 protein levels in control and IFITM2/3^KD cells. n = 4 independent experiments; mean ± SD; Welch’s t test, P = 0.0014 (*). (C and D)MYCT1 knockdown does not affect IFITM2 (C) nor IFITM3 (D) mRNA levels in ECs. n = 3 independent experiments; mean ± SD; Welch’s t test, P > 0.05. (E)MYCT1 knockdown does not impact RAB7⁺ late endosomes nor LAMP1⁺ endolysosomes. Staining of ECs for RAB7 (gray), LAMP1 (green), VE-cadherin (magenta), and DNA (blue). Scale bar, 20 µm. (F and G) Quantification of RAB7⁺ (F) and LAMP1⁺ (G) areas per cell in control and MYCT1^KD cells. n = 3 independent experiments; 20–50 cells were analyzed per condition for each experiment; mean ± SD; Welch’s t test, P > 0.05. (H)MYCT1 knockdown increased FITC-dextran uptake. 2 days after siRNA transfection, cells were starved for 1 h in PBS, followed by a 30-min induction with amino acid solution together with 10-kDa FITC dextran. Detection of 10-kDa FITC-dextran (gray) and staining of ECs for VE-cadherin (magenta) and DAPI (blue). Arrow, dextran⁺ puncta. Scale bar, 10 μm. (I) Quantification of the number of dextran⁺ puncta per cell in control and MYCT1^KD cells. n = 3 independent experiments; 30–50 cells were analyzed per condition for each experiment; mean ± SD; Welch’s t test, P = 0.0016 (*). (J) Example of gating strategy (7-AAD^neg CD45^neg CD31⁺) of ECs from gonadal fat pad by flow cytometry. (K) WAT ECs take up higher amounts of labeled plasma proteins compared with colon ECs. Quantification of labeled plasma protein uptake in ECs from s.c. and visceral WAT and colon normalized to plasma Atto-647 signal. n = 10 mice per organ; Friedman test with Dunn’s multiple comparisons test, P > 0.05 for scFAT versus visFAT, P = 0.0052 for scFAT versus colon, and P = 0.001 (*) for visFAT versus colon. (L) Endocytosis inhibition with dynasore rescues mTORC1 hyperactivation caused by knockdown of MYCT1. Staining for p-S6 (gray), β-catenin (magenta), and DAPI (blue). Scale bar, 50 μm. (M) Quantification of mTORC1 activation by amino acid supplementation in control and MYCT1^KD cells in the absence or presence of dynasore. The percentage of p-S6⁺ cells was quantified in the indicated conditions. n = 3 independent experiments; 1,500–6,000 cells were analyzed per condition for each experiment; mean ± SD; two-way ANOVA with Tukey’s multiple comparisons test, P = 0.004 (*) for MYCT1 knockdown effect in control conditions and P < 0.001 (*) for its rescue by dynasore treatment. (N)RAB5 knockdown rescues mTORC1 hyperactivation in MYCT1^KD cells. Staining of ECs for p-S6 (gray), β-catenin (magenta), and DAPI (blue). Scale bar, 50 μm. (O) Quantification of mTORC1 activation in control, MYCT1^KD, and MYCT1-RAB5^KD cells. The percentage of p-S6⁺ cells was quantified in the indicated conditions. n = 3 independent experiments; 7,000-15,000 cells were analyzed per condition for each experiment; mean ± SD; one-way ANOVA with Tukey’s multiple comparisons test, P = 0.0021 (*) for MYCT1 knockdown effect and P = 0.0292 (*) for its rescue by RAB5 double knockdown. (P)IFITM2/3 knockdown rescues mTORC1 hyperactivation in MYCT1-deficient human adipose ECs. Staining for p-S6 (gray), β-catenin (magenta), and DAPI (blue). Scale bar, 100 μm. (Q) Quantification of mTORC1 activation in control, MYCT1^KD, IFITM2/3^KD, and MYCT1–IFITM2/3^KD cells. The percentage of p-S6⁺ cells was quantified in the indicated conditions. n = 2 independent experiments; 1,500–3,000 cells were analyzed per condition for each experiment; mean ± SD; one-way ANOVA with Tukey’s multiple comparisons test, P = 0.0168 (*) for MYCT1 knockdown effect and P = 0.0123 (*) for rescue effect by IFITM2/3 double knockdown.