A green light for the thrombopoietic program

Comment on Lin et al, page 3803

The opportunity to study the dynamics of thrombopoeisis in real time has long been awaited.

In their new paper, Lin and colleagues have described the generation of transgenic zebrafish with green fluorescent thrombocytes. This fish was created using a zebrafish CD41 gene promoter that drives the expression of jellyfish green fluorescent protein (GFP). Using this fish, the authors have identified that there are 2 populations of thrombocytes that are made in kidney marrow. The authors claim that one population of cells that expresses low levels of GFP seems to be the precursor for the other cells that have high levels of GFP and that this invention will be useful in studying thrombopoiesis. Even though thrombocytes have been identified earlier in zebrafish development and in circulation,^1,2  the current report is important in following thrombocytes in real time in development and in circulation and thus merits attention.

The significance of the above work is the fact that the zebrafish thrombocytes have parallels to megakaryopoiesis and platelet production in mammals. Since almost all mammalian genes exist in fish, it would not be surprising that knockdown of zebrafish genes using anti-sense morpholinos and the above transgenic line could identify novel players in megakaryopoiesis. In this context it is interesting to note that the suggested precursor GFP⁺ cells are large, similar to other known hematopoietic progenitor cells. GFP⁺ cells appear in the ventral region of the aorta that roughly corresponds to aorta-gonads-mesonephros (AGM), which is the site of hematopoiesis in mammalian development. However, these GFP⁺ cells appear after the dissolution of AGM and are too caudal. Thus, the authors may have unearthed yet another novel site for hematopoiesis. All the above point out that there will be novel unidentified programs in thrombocyte development and the transgenic tool will help in exploring such programs. However, one limitation of this study is that it will enhance the information on only precursor thrombocytes that are marked by the expression of CD41 promoter. Thus, whether these large GFP⁺ cells are in fact thrombocyte precursors remains to be established. It is entirely possible that there is expression of CD41 in the nonthrombocytic lineage and the cells could be the multipotent hematopoietic precursors. Thus, caution must be exercised in extrapolating the current findings. In fact, the authors are aware of the expression of CD41 in cell types other than megakaryocytes in birds and mammals. Fluctuations in gene expression during development are not unprecedented and indeed the authors themselves identified expression of CD41 promoter in unfertilized zebrafish eggs. Even though this is not relevant to thrombopoiesis, this observation is novel and raises several questions. Is there maternally derived CD41 mRNA in unfertilized eggs? If so, what is the role for CD41 in early development? Is there a species-specific difference for the role of CD41 since lack of functional CD41 in mice and men seem to have no apparent developmental abnormalities?

In conclusion, the current work will initiate further studies on thrombocyte differentiation and will open new avenues to explore dynamics of thrombopoiesis in real time. ▪