Plasma cells (PCs) are the final B-cell differentiation stage. Recent evidence reveals relevant functional differences within the PC compartment. In rodents, early PCs formed in secondary lymphoid tissues show enhanced apoptosis and short life span, whereas PCs present in a final destination organ, such as the bone marrow (BM), have reached a stable prolonged survival state. BM PCs arrive at this organ as a circulating precursor whose cellular nature remains uncertain. An initial aim of this study was to characterize this circulating cell. We hypothesized that antibody-secreting cells detectable in the human blood after immunization might be a candidate precursor. These cells were obtained from the blood of volunteers immunized 6 days earlier with tetanus toxoid (tet), and they were unambiguously identified as PCs, as demonstrated by their expression of the CD38^h phenotype, by morphology, by immunoglobulin (Ig) intracytoplasmic staining, and by IgG-tet–secreting capacity in vitro. In addition, by using the common CD38^h feature, human PCs from tonsil (as a possible source of early PCs), from blood from tet-immunized donors (as the putative precursors of BM PCs), and from BM (as a deposit organ) have been purified and their phenotypes compared. The results show that a variety of differentiation molecules, proteins involved in the control of apoptosis, the B-cell transcription factors, positive regulatory domain I-binding factor 1/B lymphocyte-induced maturation protein 1 and B cell–specific activating protein and, at least partially, the chemokine receptor CXCR4 were expressed by human PCs following a gradient of increasing maturity in the direction: tonsil→blood→BM. However, PCs from these different organs showed a local pattern of adhesion molecule expression. These observations are discussed in light of the complex physiology of the human PC compartment.

Nitric oxide (NO) inhibits proliferation of subventricular zone (SVZ) neural precursor cells in adult mice in vivo under physiological conditions. The mechanisms underlying this NO effect have now been investigated using SVZ-derived neural stem cells, which generate neurospheres in vitro when stimulated by epidermal growth factor (EGF). In these cultures, NO donors decreased the number of newly formed neurospheres as well as their size, which indicates that NO was acting on the neurosphere-forming neural stem cells and the daughter neural progenitors. The effect of NO was cytostatic, not proapoptotic, and did not involve cGMP synthesis. Neurosphere cells expressed the neuronal and endothelial isoforms of NO synthase (NOS) and produced NO in culture. Inhibition of NOS activity by Nω-nitro-l-arginine methylester (l-NAME) promoted neurosphere formation and growth, thus revealing an autocrine/paracrine action of NO on the neural precursor cells. Both exogenous and endogenous NO impaired the EGF-induced activation of the EGF receptor (EGFR) tyrosine kinase and prevented the EGF-induced Akt phosphorylation in neurosphere cells. Inhibition of the phosphoinositide-3-kinase (PI3-K)/Akt pathway by LY294002 significantly reduced the number of newly formed neurospheres, which indicates that this is an essential pathway for neural stem cell self-renewal. Chronic administration of l-NAME to adult mice enhanced phospho-Akt staining in the SVZ and reduced nuclear p27Kip1 in the SVZ and olfactory bulb. The inhibition of EGFR and PI3-K pathway by NO explains, at least in part, its antimitotic effect on neurosphere cells and may be a mechanism involved in the physiological role of NO as a negative regulator of SVZ neurogenesis in adult mice.

Neuronal nicotinic acetylcholine receptors from bovine adrenomedullary chromaffin cells play a primary role in triggering catecholamine secretion. In the present study, their constituent subunits were characterized. In addition to the α3 subunit, which we have previously cloned, the presence of α5 and β4 but not of β2 subunits was detected by reverse transcription‐PCR analysis of mRNA from adrenal medulla. In situ hybridization indicated that α3, α5, and β4 subunits are coexpressed in all chromaffin cells. The primary structure of α5 and β4 subunits was determined and functional receptors were obtained upon coinjection of subunit cRNAs into Xenopus oocytes. In contrast to other β4‐containing nicotinic receptors, the ones formed by the bovine β4 subunit are insensitive to the agonist cytisine. Finally, we characterized the intergenic region of α3 and α5 subunits, which together with the β4 subunit, form a gene cluster in rats and chickens. RNase assays and the existence of overlapping cDNAs indicate that, in the bovine genome, the α3 and α5 genes overlap at their 3′ ends. This fact is probably due to inefficient transcription termination, as a result of weak polyadenylation signals.

Binding of agonists to nicotinic acetylcholine receptors generates a sequence of changes that activate a cation-selective conductance. By measuring electrophysiological responses in chimeric alpha7/alpha3 receptors expressed in Xenopus oocytes, we have showed the involvement of the M2-M3 loop in coupling agonist binding to the channel gate. An aspartate residue therein, Asp-266 in the alpha7 subunit, was identified by site-directed mutagenesis as crucial, since mutants at this position exhibited very poor functional responses to three different nicotinic agonists. We have extended this investigation to another neuronal nicotinic receptor (alpha3/beta4), and found that a homologous residue in the beta4 subunit, Asp-268, played a similar role in coupling. These findings are consistent with a hypothesis that the aspartate residue in the M2-M3 loop, which is conserved in all homomer-forming alpha-type subunits and all neuronal beta-type subunits that combine to form functional receptors, is a major determinant of information transmission from binding site to channel gate in all neuronal nicotinic receptors.

The present study shows that reimmunization with tetanus toxoid (tet) caused a transient increase of the human blood plasma cell (PC) pool, detectable from 6th to 15th day postboost, as well as the temporal alteration of several PC features. Labeling of specific PC with FITC-tet C fragment (tetC) allowed kinetics analysis of the tetC⁺ and tetC⁻ PC, and revealed remarkable differences between them: 1) the kinetics of tetC⁺ PC occurrence was exponential, and most of them appeared in a narrow time frame (5th to 8th day postboost), whereas the tetC⁻ PC increase was lower (three to five times) and more prolonged (4th to 15th day postboost). 2) The tetC⁺ PC subset contained a fraction of cycling cells, expressed high levels of DR, CD138, and CD126, and responded to IL-6 by improving their survival and Ig secretion; in contrast, the tetC⁻ PC showed higher CXCR4 and lower DR and CD138, did not respond to IL-6, and contained a fraction of apoptotic cells. 3) Sequential phenotypic analysis revealed maturational changes within the tetC⁺, but not tetC⁻, PC subset; sequencing of tetC⁺ PC IgVH genes showed clear features of Ag selection. 4) The tetC⁺ PC expressed several times more positive regulatory domain I- binding factor 1/B lymphocyte-induced maturation protein 1 transcription factor than the tetC⁻ PC. 5) The tetC⁻ PC and bone marrow resident PC similarly expressed low DR and high CXCR4, but differed in that the latter exhibited higher levels of CD31, CD138, and positive regulatory domain I- binding factor 1/B lymphocyte-induced maturation protein 1. These findings support the view that tetC⁺ PC contain bone marrow PC precursors, and tetC⁻ PC probably belong to a removable compartment of aged PC.