Recently, overlapping molecular phenotypes of hematopoietic and neuropoietic cells were described in mice. Here, we examined primary human CD34 ؉ hematopoietic stem and progenitor cells applying specialized cDNA arrays, real-time reverse-transcriptase-polymerase chain reaction (RT-PCR), and fluorescent-activated cell sorter (FACS) analysis focusing on genes involved in neurobiologic functions. We found expression of vesicle fusion and motility genes, ligand-and voltage-gated ion channels, receptor kinases and phosphatases, and, most interestingly, mRNA as well as protein expression of G protein-coupled receptors of neuromediators (corticotropin-releasing hormone 1 [CRH 1] and CRH 2 receptors, orexin/ hypocretin 1 and 2 receptors, GABA B receptor, adenosine A 2 B receptor, opioid 1 and 1 receptors, and 5-HT 1F receptor). As shown by 2-color immunofluorescence, the protein expression of these receptors was higher in the more immature CD38 dim than in the CD38 bright subset within the CD34 ؉ population, and completely absent in fully differentiated blood cells, suggesting that those receptors play a role in developmentally early CD34 ؉ stem and progenitor cells. IntroductionHuman CD34 ϩ hematopoietic stem and progenitor cells ensure lifelong production of mature blood cells according to the varying needs of the individual. Hematopoiesis is a precisely regulated process based upon a balance of self-renewal and commitment to differentiation along the different hematopoietic lineages. The restorative capacity of human CD34 ϩ cells is clinically used in the autologous and allogeneic transplantation setting to reconstitute hematopoiesis following cytotoxic therapy for the treatment of patients with malignant or autoimmune diseases. [1][2][3] Beyond that, data of recent studies suggest that hematopoietic progenitors might also be able to transdifferentiate into nonhematopoietic cells, which could open novel therapeutic avenues in the treatment of diseases such as myocardial or cerebral infarction as well as other degenerative disorders. [4][5][6][7] However, novel data have challenged the transdifferentiation model by suggesting cell fusion rather than plasticity of stem cells. [8][9][10] A better molecular understanding of the signal perception pathways of hematopoietic stem and progenitor cells seems to be required to understand the conditions under which transdifferentiation of hematopoietic cells may occur. 11 Studies in animal models showed the presence of sensory and autonomic nerves in the bone marrow as a morphologic correlate of a possible neural regulation of hematopoiesis. [12][13][14] However, the idea that neuromediators might directly influence hematopoietic progenitors is controversially discussed. [15][16][17][18][19] Recently, several investigators described partly overlapping genetic programs of hematopoietic and neuropoietic cells in mice. 20,21 Those findings prompted us to examine human hematopoietic cells by means of specialized cDNA arrays, quantitative real-time reversetranscription-polymerase ch...