Caspase-activated ROCK-1 allows erythroblast terminal maturation independently of cytokine-induced Rho signaling.

Stem cell factor (SCF) and erythropoietin are strictly required for preventing apoptosis and stimulating proliferation, allowing the differentiation of erythroid precursors from colony-forming unit-E to the polychromatophilic stage. In contrast, terminal maturation to generate reticulocytes occurs independently of cytokine signaling by a mechanism not fully understood. Terminal differentiation is characterized by a sequence of morphological changes including a progressive decrease in cell size, chromatin condensation in the nucleus and disappearance of organelles, which requires transient caspase activation. These events are followed by nucleus extrusion as a consequence of plasma membrane and cytoskeleton reorganization. Here, we show that in early step, SCF stimulates the Rho/ROCK pathway until the basophilic stage. Thereafter, ROCK-1 is activated independently of Rho signaling by caspase-3-mediated cleavage, allowing terminal maturation at least in part through phosphorylation of the light chain of myosin II. Therefore, in this differentiation system, final maturation occurs independently of SCF signaling through caspase-induced ROCK-1 kinase activation.

Marrow stromal cell recovery after radiation-induced aplasia in mice.

PURPOSE: The identification of fibroblast-like cells of the marrow stroma by means of alkaline phosphatase (ALP) cytochemistry reveals delicate ALP-positive structures interspersed among haematopoietic cells and arranged in a loosely meshed network. These cells are often referred to as 'reticular' cells and the network they form is known as the 'ALP network'. The purpose was to analyse the evolution of this ALP network in relation to haemopoietic regeneration after whole-body irradiation. MATERIALS AND METHODS: The total surface occupied by ALP-positive processes revealed by means of ALP cytochemistry was expressed as a ratio of the total marrow area. ALP-positive cells were counted using nuclei as the defining unit. Cell proliferation was analysed by the detection of bromodeoxyuridine (BrdU) incorporation. Fat cells were identified by oil red O staining and alpha-glycerophosphate dehydrogenase (alpha-GPDH) activity. RESULTS: The ALP network and ALP-positive cell number began to increase 24 h after 4-Gy irradiation to reach a maximum after 72 h, when the bone marrow was almost completely empty of haemopoietic cells. This increase was in advance of haemopoietic recovery and was not due to cell proliferation. A decrease in the ALP network occurred in parallel with an increase in haemopoiesis and was accompanied by a transient increase in fat cells on day 7. CONCLUSIONS: These data indicate that the recovery of the ALP network, which is partially due to the recruitment of ALP- positive cells, occurs in advance of the haemopoietic recovery and that the equilibrium between fat cells and ALP-positive cells seems to be controlled by haemopoietic cells.