Hematopoietic stem cells are maintained within a microenvironment where signals from a niche are important for the maintenance of quiescence within a precursor population. Lack of such a niche-derived signal will cause loss of “stemness” resulting in increased proliferation and eventual differentiation. We study this phenomenon in the Drosophila hematopoietic organ using genetic technologies available in this model organism (see Martinez-Agosto et al., Genes and Development., 2007 for a review).
We have shown that the “stemness” of these cells is maintained through the combined action of a Niche Signal, generated by Hedgehog (Hh) (Mandal et al., Nature, 2007), a local signal generated by Wingless/Wnt (Sinenko et al., Dev. Cell., 2009) and a reverse signal from the differentiated cells to the stem cells that we have termed the Equilibrium Signal. Several important concepts underlying Drosophila blood development have allowed us to propose this system as an appropriate genetic model for vertebrate hematopoiesis and these molecular mechanisms are being explored in the laboratory.
Myeloid blood cells are ideal for the study of response to many kinds of stresses. Hypoxia-related factors and free radicals known as reactive oxygen species (ROS) play a role both in hematopoietic development and in stress response (Owusu-Ansah and Banerjee, Nature 2009). Similarly, the NF-kB derived inflammatory response plays a major role in the way blood cells respond to injury at distant sites. The emerging view from these studies is that basic developmental mechanisms are co-opted again for stress, injury and inflammatory responses by the myeloid hematopoietic system. Genetic analysis allows us to understand the interrelationship between these important biological phenomena that have great relevance to diseases and disorders in humans.
In the past, our laboratory has identified components of signal transduction pathways that participate in oncogenesis (see Bonfini et al., Science, 1991; Flores et al., Cell 2000 for examples). We have also studied the role of the mitochondrion in controlling cell-cycle (Owusu-Ansah et al., Nature Genetics, 2008; Mandal et al., Developmental Cell, 2005). When cells become transformed, they choose alternate means of metabolism (a phenomenon referred to as the Warburg effect). We study the effect of signal transduction pathways on the control of cellular metabolism and the proper balance between cellular growth and metabolism that goes awry in cancer.
We also explore the control of metabolic pathways and mitochondrial activity and biogenesis during development using early mouse embryo and the embryonic stem cells derived from them as a model. There is relative lack of data on the mechanisms that control transition between various modes of metabolic activity during phases of development. Nuclear and mitochondrial activities constantly modulate each other, and the relationship between signal transduction pathways commonly studied during development, cancer and stem cell maintenance and metabolic pathways such as oxidative phosphorylation, glycolysis, mitochondrial biosynthesis and nutrient sensing are being explored. Immediate projects include determining the metabolic processes that are essential for zygotic transcription, and the switch between mitochondrial and glycolytic processes during early mouse development.
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