Christopher Brown

• Cytokine receptors
• hematopoiesis
• hematopoietic stem cell transplantation

Our lab is interested in hematopoietic development and function at several levels, both physiological and pathological. The spectrum of investigation is broadening and includes cytokine and cytokine receptor biology in myeloid cells, the immune system and leukemia, erythrocyte membrane biology. This focus arises from my clinical duties as a hematologist and marrow transplant physician. The foundations of the lab lie in cytokine receptor biology. We have been trying to understand the mechanisms by which cytokines deliver their messages via the multisubunit cytokine receptor complexes. The receptor for the myeloid cytokine GM-CSF is the model for our work (see figure below). What we really want to achieve is an understanding of the assembly of this receptor complex and a clear picture of how this assembly results in the transmission of an intracellular signal.

In the early stages of the pursuit of this goal we cloned a splice variant of one subunit of the receptor which encodes a soluble isoform. We have devoted substantial energy to the biochemical and biological characterization of this soluble receptor both in-vitro and in-vivo (see references below) and continue on this path in ongoing experiments. Currently this falls into two broad experimental areas. In one we are using retroviral infection of factor-dependent cell lines with the various receptor component cDNAs to understand the full spectrum of the influence of the soluble receptor on GM-CSF signaling. We are particularly interested in a situation in which the soluble receptor becomes anchored to the cell surface by a ligand-independent covalent bond with the GM-CSF receptor signaling subunit. In another general initiative we are examining the regulation of production of the soluble receptor in-vivo, particularly in human monocyte/macorphages which seem to be the principal cellular source of the soluble receptor. In a parallel project we are culturing freshly isolated human leukemic blasts from patient samples to investigate our observation that the level of the soluble GM-CSF receptor is dysregulated in roughly 40% of all adult acute leukemias.

Learning about the intricacies of soluble cytokine receptors has prompted us to become curious about the more general notion that the lines between our classic understandings of a ligand as being a soluble entity and a receptor as a fixed, cell-surface docking station are being blurred. In pursuit of this we are in the very early stages of seeking evidence that, complementary to the fact that there is a soluble GM-CSF receptor, there may also be a cell-surface anchored GM-CSF ligand. We have tantalizing early data to suggest that this may indeed be the case.

In a separate initiative in my lab, initiated and directed by Dr. Mike Brain, we have observed that human erythrocytes have the ability to direct their own destruction which we hypothesize represents the normal mechanism of red blood cell senescence. This process seems to be a ligand-driven process involving the ubiquitous erythrocyte membrane protein glycophorin A acting as the receptor. We have demonstrated that, in-vitro, the destruction is mediated by the formation of a cation-permeable pore on the red cell surface allowing a rapid Na+ influx with osmotic lysis as the outcome. Dr. Brain has refined his hypothesis to suggest that, under proper conditions, glycophorin A was capable of undergoing an ordered homomeric assembly to form the cation pore and preliminary experiments seemed to support this idea. We surmise that in-vivo, the cation pore initiates more than just the osmotic process and that a Ca++ influx through the pore induces a cascade complex cellular events.

Finally our lab is branching out into more translational types of research with an eye to developing novel therapeutic strategies for the treatment of acute leukemia. Our current venture is in collaboration with Dr. Patrick Lee and the Reovirus Working Group of the University of Calgary using an oncolytic virus in fresh leukemic blast cultures in-vitro.