The presence of novel amino acids in the cytoplasmic domain of stem cell factor results in hematopoietic defects in Steel(17H) mice

Stem cell factor (SCF) is expressed as an integral membrane growth factor t hat may be differentially processed to produce predominantly soluble (S) (S CF248) Or membrane-associated (MA) (SCF220) protein. A critical role for me mbrane presentation of SCF in the hematopoietic microenvironment (HM) has b een suggested from the phenotype of the Steel-dickie(Sl(d)) mice, which lac k MA SCF, and by studies performed in our laboratory land by others) using long-term bone marrow cultures and transgenic mice expressing different SCF isoforms, Steel(17H) (Sl(17H)) is an SCF mutant that demonstrates melanocy te defects and sterility in males but not in females. The Sl(17H) allele co ntains a intronic mutation resulting in the substitution of 36 amino acids (aa's) in the SCF cytoplasmic domain with 28 novel aa's, This mutation, whi ch affects virtually the entire cytoplasmic domain of SCF, could be expecte d to alter membrane SCF presentation. To investigate this possibility, we e xamined the biochemical and biologic properties of the Sl(17H)-encoded prot ein and its impact in vivo and in vitro on hematopoiesis and on c-Kit signa ling. We demonstrate that compound heterozygous Sl/Sl(17H) mice manifest mu ltiple hematopoietic abnormalities in vivo, including red blood cell defici ency, bone marrow hypoplasia, and defective thymopoiesis, In vitro, both S and MA Sl(17H) isoforms of SCF exhibit reduced cell surface expression on s tromal cells and diminished biological activity in comparison to wild-type (wt) SCF isoforms, These alterations in presentation and biological activit y are associated with a significant reduction in the proliferation of an SC F-responsive erythroid progenitor cell line and in the activation of phosph atidylinositol 3-Kinase/ Akt and mitogen-activated protein-Kinase signaling pathways. In vivo, transgene expression of the membrane-restricted (MR) (S CFX9/D3) SCF in Sl/Sl(17H) mutants results in a significant improvement in peripheral red blood cell counts in comparison to Sl/Sl(17H) mice. (C) 1999 by The American Society of Hematology.