Programming and re-programming of haemopoietic cell fate

The blood is composed of a variety of different types of cells that each have specialised and distinct functions. As these cells die they are replenished by a small pool of long-lived cells known as stem cells which have the capacity both to produce more stem cells (a process termed self-renewal) or give rise to all the specialized blood cell types of the blood (a process termed differentiation). Our research programme attempts to understand, at the molecular level, how stem cells decide whether to self-renew, or to differentiate and in the latter case, into which cell type (a process termed lineage-specification). These working decisions that stem cells make are ultimately effected at the level of differential gene activity or usage, and gaining an understanding of how this process works is an important problem in developmental biology, transplantation medicine, stem cell based gene therapy and haematological malignancy. We study this process by genetically altering the activities of candidate molecular regulators of these processes. We anticipate that these studies will illuminate how normal cell fate decisions are instigated and inform approaches aimed at manipulating the expansion, directed-differentiation or reprogramming of stem cell fate for therapeutic or commercial advantage.

All the different specialised cell types that constitute the blood are derived from a multipotential haemopoietic stem cell (HSC). HSCs are long-lived and in addition to having the capacity to give rise, through differentiation, to all blood lineages, HSC are capable of extensive self-renewal. It remains unclear at the molecular level how the balance between self-renewal and differentiation is regulated and how the different blood lineages are specified. However we can presume that the process of cell type restriction involves the formation of stable transcription factor complexes that initiate and consolidate (or repress) exclusive programmes of gene expression. Transcription factors have therefore been intensively studied as candidate instigators of cell fate decisions. This capacity has been emphasised by recent studies that indicate that individual transcription factors can reprogramme the development fate of already committed cell types. In our own laboratory we have shown that the erythroid-affiliated transcription factor GATA-1 can re-specify the fate of freshly isolated, bone marrow-derived, committed neutrophil-monocyte progenitors. In response to exogenous GATA-1 activity these cells adopt erythroid or other GATA-1-affiliated cell fates. These observations provide a mechanism for reprogramming of committed haematopoietic progenitors and an experimental entry point for directly identifying the key loci involved. Since manipulation of GATA-1 activity can also alter cell fate in multipotential progenitors that express endogenous GATA-1 and retain both erythroid and neutrophil monocyte potential, the same mechanisms and loci are likely to also be directly involved in normal uni-lineage differentiation from multipotent progenitor cells. Thus our approach is to examine transcription factor-mediated alterations in cell fate primarily in the experimental context of reprogramming and apply our results to the more normal context of cell fate programming from the multipotential uncommitted state. We will ask the following questions: 1) Which cells can be reprogrammed (lineage/stage of differentiation) by a single transcription factor. 2) Which transcription factors can reprogram. 3) What is the mechanism of reprogramming, is it de-differentiation or trans-differentiation? 4) How complete is the reprogramming i.e. do the cells show residual features of their original lineage and are they fully functional? 5) To what extent is the mechanism the same regardless of the lineage/stage being reprogrammed and to what extent is it lineage/stage specific? 6) What is the detailed mechanism of reprogramming in respect of target genes, chromatin re-modelling etc. 7) Does reprogramming require cell division. 8) Can human cells be reprogrammed? 9) To what extent to the processes which mediate reprogramming also function to programme the cell fate output of multipotent cells. We anticipate that these studies will contribute to our understanding of how normal cell fate decisions are instigated and inform approaches aimed at manipulating the expansion, directed-differentiation or reprogramming of stem cell fate for therapeutic or commercial advantage.