Regulation of Embryonic Hematopoiesis by a Cytokine-Inducible SH2 Domain Homolog in Zebrafish

Cytokine-inducible SH2 domain-containing protein (CISH), a member of the suppressor of cytokine signaling family of negative feedback regulators, is induced by cytokines that activate STAT5 and can inhibit STAT5 signaling in vitro. However, demonstration of a definitive in vivo role for CISH during development has remained elusive. This study employed expression analysis and morpholino-mediated knockdown in zebrafish in concert with bioinformatics and biochemical approaches to investigate CISH function. Two zebrafish CISH paralogs were identified, cish.a and cish.b, with high overall conservation (43-46% identity) with their mammalian counterparts. The cish. a gene was maternally derived, with transcripts present throughout embryogenesis, and increasing at 4-5 d after fertilization, whereas cish. b expression commenced at 8 h after fertilization. Expression of cish. a was regulated by the JAK2/STAT5 pathway via conserved tetrameric STAT5 binding sites (TTCN(3)GAA) in its promoter. Injection of morpholinos targeting cish. a, but not cish. b or control morpholinos, resulted in enhanced embryonic erythropoiesis, myelopoiesis, and lymphopoiesis, including a 2-3-fold increase in erythrocytic markers. This occurred concomitantly with increased activation of STAT5. This study indicates that CISH functions as a conserved in vivo target and regulator of STAT5 in the control of embryonic hematopoiesis.