Megumu Ogawa

Available evidence indicates that qualitative changes in hematopoietic stem cells and progenitors, such as the decision of stem cells to self-renew or differentiate, or selection of lineage potentials by the multipotential progenitors during differentiation (commitment), are intrinsic properties of the progenitors and are stochastic in nature. In-contrast, proliferative kinetics of the progenitors, namely survival and expansion of the progenitors, appear to be controlled by a number of interacting cytokines. While proliferation and maturation of committed progenitors is controlled by late-acting lineage-specific factors such as Ep, M-CSF, G-CSF, and IL-5, progenitors at earlier stages of development are controlled by a group of several overlapping cytokines. IL-3, GM-CSF, and IL-4 regulate proliferation of multipotential progenitors only after they exit from G0 and begin active cell proliferation. Triggering of cycling by dormant primitive progenitors and maintenance of B-cell potential of the primitive progenitors appears to require interactions of early acting cytokines including IL-6, G-CSF, IL-11, IL-12, LIF, and SF. Currently, this simple model fits our understanding of the interactions of growth factors with hematopoietic progenitors. Naturally the model risks oversimplification of a very complex process. However, because the model is testable, it will hopefully challenge investigators to design new experiments to examine its validity.

Caveolae are invaginations of the plasma membrane involved in many cellular processes, including clathrin-independent endocytosis, cholesterol transport, and signal transduction. They are characterized by the presence of caveolin proteins. Mutations that cause deficiency in caveolin-3, which is expressed exclusively in skeletal and cardiac muscle, have been linked to muscular dystrophy. Polymerase I and transcript release factor (PTRF; also known as cavin) is a caveolar-associated protein suggested to play an essential role in the formation of caveolae and the stabilization of caveolins. Here, we identified PTRF mutations in 5 nonconsanguineous patients who presented with both generalized lipodystrophy and muscular dystrophy. Muscle hypertrophy, muscle mounding, mild metabolic complications, and elevated serum creatine kinase levels were observed in these patients. Skeletal muscle biopsies revealed chronic dystrophic changes, deficiency and mislocalization of all 3 caveolin family members, and reduction of caveolae structure. We generated expression constructs recapitulating the human mutations; upon overexpression in myoblasts, these mutations resulted in PTRF mislocalization and disrupted physical interaction with caveolins. Our data confirm that PTRF is essential for formation of caveolae and proper localization of caveolins in human cells and suggest that clinical features observed in the patients with PTRF mutations are associated with a secondary deficiency of caveolins.

We have used a serum-free culture system for enriched human hemopoietic progenitors to analyze the developmental stages and lineage specificities of the human hemopoietic colony-stimulating factors. None of the individual factors alone efficiently supported hemopoietic colony formation. Neither interleukin 3 nor granulocyte/macrophage-colony-stimulating factor alone or in combination effectively supported proliferation of progenitor cells. However, when combined with granulocyte-colony-stimulating factor or erythropoietin, these factors yielded neutrophil colonies or erythroid bursts, respectively. Serial observations of interleukin 3-supported cultures revealed sequential emergence and subsequent degeneration of clusters of cells. These observations suggest that the primary targets of interleukin 3 and granulocyte/macrophage-colony-stimulating factor are multipotent progenitors at the early stages of development rather than cells in the terminal process of maturation.