Generation of dhx15^?/? zebrafish. A, DNA sequencing results of dhx15^+/+ , dhx15^+/? , and dhx15^?/? embryos. Arrows indicate a 13 bp deletion (CTGAACCGCCATC) at dhx15 exon 2. B, B?, WISH analysis of dhx15 in dhx15^+/+ and dhx15^?/? embryos at 24 hpf. C, Western blot analysis of dhx15 in dhx15^+/+ and dhx15^?/? embryos at 3 dpf. D?F?, Bright?field microscopy images of dhx15^+/+ and dhx15^?/? zebrafish. dpf, days post fertilization; hpf, hours post fertilization; WISH, whole?mount in situ hybridization

Dhx15^?/? zebrafish displayed defective definitive hematopoiesis in CHT. A?B?, WISH analysis of cmyb expression in dhx15^+/+ and dhx15^?/? at 3 dpf (A, A?) and 4 dpf (B, B?). C?F?, WISH analysis of mpx (C, C?), l?plastin (D, D?), hbae1 (E, E?), rag1 (F, F?) expression in dhx15^+/+ and dhx15^?/? zebrafish. G?I?, WISH analysis of cmyb expression in dhx15^+/+ and dhx15^?/? zebrafish at 36 hpf (G, G?), 48 hpf (H, H?), 60 hpf (I, I?). Black arrows indicate CHT; red arrows indicate ventral wall of dorsal aorta; red arrowheads indicate thymus. CHT, caudal hematopoietic tissue; dpf: days post fertilization; hpf, hours post fertilization; WISH, whole?mount in situ hybridization

Abated proliferation but not excessive apoptosis of HSPCs is observed in dhx15^?/? zebrafish. A, Schematic indicates the area of the imaging in the tail (CHT), outlined by the black box. B, C, Fluorescence images of the cmyb and apoptotic cells in CHT of dhx15^+/+; Tg(cmyb:gfp) and dhx15^?/?; Tg(cmyb:gfp) zebrafish embryos at 48 hours post fertilization (hpf) (B) and 60 hpf (C). D, E, Fluorescence images of pH3 and cmyb positive cells in CHT of dhx15^+/+; Tg(cmyb:gfp) and dhx15^?/?; Tg(cmyb:gfp) zebrafish embryos at 48 hpf (D) and 60 hpf (E). F, Number of apoptotic cells in CHT at 48 hpf (wildtype, 1.2 ± 0.2494, n = 10; dhx15^?/? embryos, 1.25 ± 0.1637, n = 8; P > .05, Student?s t test) and 60 hpf (wildtype, 2.063 ± 0.3923, n = 16; dhx15^?/? embryos, 10.91 ± 1.944, n = 11; P < .005, Student?s t test). G, Number of apoptotic HSPCs in CHT at 48 hpf (wildtype, 0.2 ± 0.1333, n = 10; dhx15^?/? embryos, 0.375 ± 0.183, n = 8; P > .05, Student?s t test) and 60 hpf (wildtype, 0.125 ± 0.08539, n = 16; dhx15^?/? embryos, 0.1818 ± 0.122, n = 11; P > .05, Student?s t test). H, Percentages of proliferating HSPCs in CHT at 48 hpf (wildtype, 11.34 ± 2.379, n = 9; dhx15^?/? embryos, 13.98 ± 1.829, n = 11; P > .05, Student?s t test) and 60 hpf (wildtype, 7.464 ± 0.9413, n = 11; dhx15^?/? embryos, 2.367 ± 0.6178, n = 9; P < .0005, Student?s t test). Data are represented as mean ± SEM. *P < .05; **P < .005; ***P < .0005. Scale bar = 40 ?m. CHT, caudal hematopoietic tissue; hpf, hours post fertilization; HSPCs, hematopoietic stem/progenitor cells; ns, not significant; pH3, pospho?histone H3

UPR pathway mediates HSPC failure in dhx15^?/? zebrafish. A, qRT?PCR analysis of CHOP, JUN, and ATF3 in dhx15^+/+ and dhx15^?/? zebrafish at 3 dpf. Each qRT?PCR experiment was carried out in triplicates, and repeated at least three times. B?E, WISH analysis of cmyb in dhx15^+/+ and dhx15^?/? zebrafish treated with 4?PBA. P < .005, ?² test. F?I, WISH analysis of cmyb in dhx15^+/+ and dhx15^?/? zebrafish treated with toyocamycin. P < .005, ?² test. Black arrow indicates caudal hematopoietic tissue. ***P < .0005. ATF3, activating transcription factor 3; dpf, days post fertilization; HSPC, hematopoietic stem/progenitor cell; qRT?PCR, quantitative real?time PCR; UPR, unfolded protein response pathway; WISH, whole?mount in situ hybridization; 4?PBA, 4?phenylbutyric acid

Definitive hematopoiesis in zebrafish is normal when DHX15 or DHX15 R222G is overexpressed. WISH analysis of definitive hematopoietic markers: cmyb (A?A??), hbae1 (B?B??), mpx (C?C??), and rag1 (D?D??). Embryos from wildtype, Tg(hsp: DHX15), and Tg(hsp: DHX15 R222G) zebrafish were heat?shocked at 38? for 2 h every day since 25 hours post fertilization (hpf). DHX15, Tg(hsp: DHX15); DHX15 R222G, Tg(hsp: DHX15 R222G). Black arrows indicate caudal hematopoietic tissue; red arrowheads indicate thymus. hpf, hours post fertilization; WISH, whole?mount in situ hybridization

DHX15 R222G is not as effective as DHX15 in restoring HSPC failure in dhx15^?/? zebrafish. A, cmyb expression of wildtype zebrafish at 3 dpf. B, B?, cmyb expression of Dhx15^?/?; Tg(hsp: DHX15) zebrafish at 3 dpf with (B?) or without (B) heat?shock treatment. HSPC failure was rescued after DHX15 overexpression in dhx15^?/? zebrafish (P < .0001, ?² test). C?C??, cmyb expression of Dhx15^?/?; Tg(hsp: DHX15 R222G) zebrafish at 3 dpf with (C?, C??) or without (C) HS treatment. HSPC failure was rescued after DHX15 R222G overexpression in dhx15^?/? zebrafish (P < .05, ?² test). DHX15 R222G is not as effective as DHX15 in restoring HSPC failure in dhx15^?/? zebrafish. (No HS vs HS, P < .05, ?² test; No HS vs HSL, P < .0001, ?² test; HS vs HSL, P < .005, ?² test). D, Western blot analysis of DHX15 protein expression in DHX15/DHX15 R222G transgenic zebrafish with or without heat activation. DHX15, Dhx15^?/?; Tg(hsp: DHX15); DHX15 R222G, Dhx15^?/? ; Tg(hsp: DHX15 R222G); dpf, days post fertilization; hpf, hours post fertilization; HS, heat?shocked at 38? for 1 h and 45 min once a day since 25 hpf; HSL, heat?shocked at 38? for 2 h once a day since 25 hpf; HSPC, hematopoietic stem/progenitor cell; No HS, without heat?shock treatment. Black arrow indicates caudal hematopoietic tissue