My latest love with evolution is an exciting and adrenaline-rich experience. We are trying to develop neutrophil based drug delivery at the inflammation sites. Neutrophils are white blood cells with dumbbell shaped nucleus and are the first ones to reach the site of inflammation to seek and destroy foreign bodies. It turns out that neutrophils are good at reaching for not only invading virus or bacteria but also cancer cells. The first striking property of neutrophils is their multi-lobled nucleus though continuous. What are the evolutionary benefits of such nucleus in constructing the first line of defense? Is it possible that chromosomes are separated from each other so that they can function independently without genetic interference? Can one set of genes switch on/off another set of genes? Or, are we evolving towards neutrophils without nucleus like red blood cells (RBCs) as both of them get differentiated from hematopoietic stem cells (HSCs)?
The missing nucleus of RBC has baffled the scientific community for ages. Several possible explanations are given, maximizing oxygen carrying capacity through disc shaped structure is the most notable explanation. We, now, know that HSC differentiates into two daughter RBCs, one with nucleus and the other without nucleus. The daughter cell with nucleus is phagocytized by macrophages leaving behind the RBC without nucleus. Mature RBCs cannot divide due to lack of nuclei and hence, no chance of cancer of RBCs. Hemoglobin is the oxygen carrying protein interacting with four porphyrin-Fe molecules present in the RBC membrane. If RBC doesn't have any genetic material, how does globin protein bind to the cell membrane of RBC? It is the HSC from which RBC differentiates, supplies the membrane bound protein. We have evolved with RBCs which circulate for about 120 days and then perish. HSCs generate RBCs continuously in the bone marrow. Removing nucleus from RBC kills the prospect of making mistake in genetic level and therefore, in the protein synthesis level extending mammalian life. You can counter-argue the survival of other vertebrates with nucleated RBCs! More questions need to be answered for complete understanding of this "weird" phenomenon.
References:
1. Ji, Peng; Jayapal, Senthil Raja; Lordish, Harvey, F. Enucleation of cultured mouse fetal erythroblasts requires Rac GTPases and mDia2. Nature Cell Biology, 2008, 10, 314-321
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