Multi-Scale Analysis of Granulopoiesis: A Bottoms-Up Approach.

Granulopoiesis constitutes a dynamical system, able to respond acutely to internal or external stimuli, with production of neutrophils. Because neutrophils are short-lived (<24 hr) in the peripheral circulation, a major component of bone marrow activity is devoted to their generation. The hematopoietic growth factor, granulocyte colony-stimulating factor (G-CSF), is the most important growth factor for the production of granulocytes. When it or its cognate receptor, the G-CSF Receptor, is genetically ablated in mice, severe neutropenia results. The two most commonly expressed isoforms are the normal full-length G-CSF Receptor (class I) and a truncated, differentiation-defective receptor (class IV). We hypothesized that expression of these two receptor isoforms critically control granulopoiesis. We have developed a quantitative PCR assay and have found that class IV expression is variably upregulated in primary AML and leukemic cell lines. Receptor endocytosis rates differ between class I and class IV, which contributes to proximal phosphoprotein signaling profile (e.g. Lyn, Erk1/2, Akt). Aberrant expression of class IV leads to profound changes in intracellular signaling of both positive and negative effectors, resulting in proliferation without differentiation. Using a multi-scale approach, we have developed a mathematical model containing ordinary differential equations (ODE) to explain the kinetic properties of this dynamical system. This model involves three major steps: probability of receptor dimerization, single cell fate decision making, and population distribution of differentiated and non-differentiated cells. Computer simulation of receptor isoform expression and endocytosis revealed a decreased ratio of class I to class IV over time until steady state (∼120 min). Further validation is underway. This model can be applied to better understand disease states such as congenital neutropenia and acute myeloid leukemia, where aberrant G-CSF Receptor signaling can occur.