ERp5-deficient platelets have increased activation of the PERK pathway. (A) Schematic of the 3 ER stress/unfolded protein response pathways: IRE1, PERK, and ATF6. Proteins included in this study are highlighted in gray. Detection of P-IRE1, IRE1 (B); P-eIF2a, eIF2a (C); and ATF6 (D) in platelet lysates from control (ERp5^fl/fl) and CKO (Pf4Cre⁺/ERp5^fl/fl) mice after treatment of platelets with DMSO (vehicle control) or thapsigargin (TG). Representative immunoblots are shown. Band densitometry of P-IRE1, P-eIF2a, and ATF6 expressed as a ratio to corresponding β-actin bands, in control (gray) and KO (red) platelets after treatment with DMSO (light box) or TG (dark box). (E) Detection of P-eIF2a and eIF2a in platelets after treatment with DMSO or tunicamycin (TN) for 2 hours. Representative immunoblots are shown. Band densitometry of P-eIF2a expressed as a ratio to corresponding β-actin bands, in control (gray) and KO (red) platelets after treatment with DMSO (light box) or TN (dark box). Ratios are presented as fold change over DMSO-treated controls. n = 3 to 4 mice per genotype, mean ± SEM, 1-way analysis of variance (ANOVA) with Dunn post hoc analysis. ∗P < .05, ∗∗P < .005. kDa, kilodalton.