The Role of the In Vivo Microenvironment in Human Stem Cell Fate Decisions

Abstract

<p>Years of research in the field of stem cell biology have resulted in only modest gains in our ability to purify human stem cells and manipulate their function <em>ex vivo, </em>suggesting that stem cell fate decisions are highly dependent upon non-cell-autonomous parameters in their physiologic <em>in vivo </em>setting. A number of non-human model systems have now revealed that stem cell function appears to be regulated by a specific <em>in vivo</em> microenvironment, also known as the niche. Prior to the work undertaken in this thesis, the role of the niche in the function of normal and transformed human stem cells had not been investigated, existing only as a theoretical concept originally proposed by the biologist Raymond Schofield. We therefore hypothesized that <em>the in vivo microenvironment is an essential regulator of human stem cell fate decisions, and that the niche is a determinant of both functional heterogeneity in the human stem cell compartment as well as the process of transformation.</em></p> <p>Initially, we postulated that if human stem cell fate decisions are dependent upon an <em>in vivo </em>niche, then we could identify novel molecular regulators of human stem cell fate decisions within the context of the <em>in vivo </em>microenvironment, and show that these regulators function uniquely within this setting. Our findings revealed the Bcl-2 family member MCL-1 as a novel molecular regulator of human hematopoietic stem cell (hHSC) self-renewal <em>in vivo</em>, and showed that the role of MCL-1 is unique to the <em>in vivo</em> setting as opposed to <em>in vitro</em> culture systems.</p> <p>Subsequently, we sought to characterize the anatomical and molecular parameters that define the human stem cell microenvironment and regulate stem cell function <em>in vivo</em>. We identified a specific <em>in vivo</em> niche in the trabecular bone region of the marrow, which regulates hHSC fate decisions through a Notch/Notch-ligand axis. We further showed that <em>in vivo</em> niche propensity underlies functional heterogeneity in the hHSC compartment, and that this niche propensity could be exploited to prospectively isolate hHSCs that are enhanced for <em>in vivo</em> regenerative function.</p> <p>Finally, we investigated whether human cancer stem cells (CSCs) are dependent upon the same niches as their normal tissue counterparts <em>in vivo </em>as part of the transformation process. We found that transformed leukemic stem cells (LSCs) dynamically compete with normal human HSCs for niche occupancy <em>in vivo</em> to support their stem cell self-renewal function, and further by replacing a transformed LSC with a normal human HSC in the <em>in vivo</em> niche, we could eradicate self-renewing LSCs and reduce the leukemic burden <em>in vivo</em>.</p> <p>Overall, this thesis has demonstrated that normal and transformed human stem cell fate decisions are controlled in a non-cell-autonomous manner by their <em>in vivo</em> microenvironment. The insights presented here support the theory originally proposed by Raymond Schofield that stem cells are not independent functional entities, but rather function as fixed tissue cells <em>in vivo</em>. These findings warrant a novel approach in the field to both stem cell-based therapies in regenerative medicine, and to targeting human malignancies at the stem cell level.</p>