Invertebrate hematopoiesis: an astakine-dependent novel hematopoietic factor.

A novel factor, named crustacean hematopoietic factor (CHF), was identified from a library of suppression subtractive hybridization with the aim to find downstream genes of an invertebrate cytokine, astakine 1, in the freshwater crayfish Pacifastacus leniusculus. CHF is a small cysteine-rich protein (∼9 kDa) with high similarity to the N-terminal region of vertebrate CRIM1 in containing an insulin growth factor binding protein variant motif with unknown function. CHF was found to be induced in primary cell cultures of crayfish hematopoietic tissue (Hpt) cells (precursors of crayfish blood cells) after treatment with astakine 1. Silencing of CHF did not affect the renewal of Hpt cells in vitro, but induced apoptosis of Hpt cells. CHF is exclusively expressed in the blood cell lineage of crayfish (Hpt cells and blood cells), and in vivo RNA interference experiments show that knockdown of this gene results in severe loss of blood cells and a higher apoptotic rate in Hpt. Our data further suggest that crayfish CHF is critical for the survival of hemocytes and Hpt cells by preventing their apoptosis, thus it plays an important role in hemocyte homeostasis in crayfish. Our study of CHF may also shed light on the function of this untypical insulin growth factor binding protein motif located in the N-terminal of vertebrate CRIM1.