Adenylate Kinase 2 Regulates Zebrafish Primitive and Definitive Hematopoiesis

The adenylate kinase (AK) gene family consists of 7 different members that contribute to energy cell metabolism by converting ATP + AMP to 2 molecules of ADP. AKs are critical players in ensuring cellular energy homeostasis in all tissues and are generally involved in a broad range of cellular functions. Among AKs, AK2 has unique features such as its location in the mitochondrial intermembrane space and critical role in human lymphopoiesis and granulopoiesis. Indeed, mutations of the AK2 gene cause reticular dysgenesis (RD), an autosomal recessive form of severe combined immunodeficiency (SCID) characterized by an early differentiation arrest in the granulocyte lineage and impaired lymphoid maturation. The mechanisms underlying the pathophysiology of RD remain unclear. The phenotype of AK2 deficient animals has not been reported in the literature, but murine lines carrying homozygous inactivating retroviral insertions are embryonically lethal (our personal observations).

To study the role of AK2 in hematopoietic system development and define the effects of AK2 deficiency, we set out to generate a zebrafish model of RD.

We injected zebrafish embryos with morpholino oligomers specific for the two zebrafish AK2 isoforms and analyzed the serial expression pattern of several hematopoietic markers in developing AK2 morphants. To confirm our observations in AK2 knockdown embryos, we screened a zebrafish DNA library of ENU-induced mutations and recovered a mutant fish line carrying a T371C/L124P missense mutation within the exon 4 of AK2 gene that is predicted to be deleterious for protein stability and function.

The downregulation of zebrafish AK2 expression phenocopied the human disease and resulted in strong reduction of developing lymphocytes. In addition, in situ hybridization for GATA1, alpha-globin 1, L-plastin and Odianisidine staining indicated that erythroid development was affected in AK2 morphants during primitive hematopoiesis, while myeloid development was conserved. Furthermore, in situ hybridization studies of the expression of markers of zebrafish definitive hematopoiesis showed abnormalities distributed among all hematopoietic lineages suggesting a broad role of AK2 in zebrafish hematopoiesis. Importantly, the ENU-induced Ak2 mutant recapitulated all the primitive and definitive hematopoietic phenotypes observed in AK2 morphants. Finally, preliminary data suggest that AK2 deficiency (both in morphant and mutant embryos) induces an increased level of reactive oxygen species (ROS) triggering oxidative stress and consequent apoptosis in hematopoietic progenitor cells.

Our data provide new insights into the AK2 function and indicate that zebrafish represents a good model for studying the molecular mechanisms involved in RD. To date, our mutant line represents the first example of animal model of this rare and unique human disease.

No relevant conflicts of interest to declare.