Conversion and storage of somatostatin are established before response to secretagogue stimuli in P19 neurons

Ceruloplasmin-induced aggregation of P19 neurons involves a serine protease activity and is accompanied by reelin cleavage

The cytoarchitectural organization of the nervous system depends partly on extracellular serine proteases, including reelin. This 400K protein, which also exists as the N-terminally-derived 300K and 180K fragments, acts through binding to the lipoprotein receptors apolipoprotein E receptor 2 (ApoER2) and very low-density lipoprotein receptor (VLDLR). Ceruloplasmin (CP), a multifunctional protein found in the circulation and also expressed on glial cells, was shown to bind to, and induce aggregation of neurons newly differentiated from P19 embryonic stem cells. This indicated a potential developmental role of CP in neuronal organization, possibly in relation with reelin and other extracellular serine proteases. Therefore, we analysed the effect of cell-impermeant, large spectrum, serine protease inhibitors on CP-induced neuroaggregation and studied reelin expression. Soybean trypsin inhibitor and aprotinin (SBTI+Apro) inhibited CP neuroaggregative action. Undifferentiated and neurally-differentiating cultures secreted the 400K reelin. The 180K fragment was present during and after differentiation whereas the 300K species was barely detectable. However, CP stimulated generation of the 300K in the differentiated neuronal cultures, and SBTI+Apro abolished this CP effect. Time course profiles and function-blocking antibody indicated that neuroaggregation does not depend on the generation of the 300K fragment or on reelin action. CP neuroaggregative action thus involves a pericellular serine protease, different from reelin. On the other hand, the CP stimulation of reelin cleavage is in line with a possible role of CP in nervous system development. Since P19 cells express ApoER2 and VLDLR, they can help understanding relationships existing between CP, reelin and intervening protease(s).

P19 neuronal differentiation and retinoic acid metabolism as criteria to investigate atrazine, nitrite, and nitrate developmental toxicity

Atrazine and nitrogenous fertilizers are agrochemical contaminants frequently detected in water systems in North America. Several studies reported their ability to affect amphibian and mammalian development. Retinoids, supplied in the diet or synthesized by cells, are essential to embryogenesis. Disturbance of their homeostasis may lead to teratogenic effects. Retinoic acid (RA) is a major retinoid regulator of cell proliferation and differentiation. Previous studies reported alterations of retinoid stores in bullfrogs of Yamaska River subwatersheds (Québec, Canada), a region of intensive agricultural activities associated with atrazine, nitrate, and nitrite contaminants. These contaminants could affect RA metabolism and RA-mediated processes. Mouse P19 embryonic stem cells, which can differentiate to neurons in response to RA, were used to test this hypothesis. Cells were cultured in the absence or presence of contaminants during neuroinduction with RA and assayed by flow cytometry for expression of stage-specific embryonic antigen-1 (SSEA1) (embryonic marker) and betaIII-tubulin (neuronal marker). Cell cultures were also analyzed for RA metabolism by high performance liquid chromotagraphy (HPLC). Downregulation of SSEA1 paralleled betaIII-tubulin upregulation in an RA concentration-dependent manner. Atrazine, nitrate, and nitrite did not affect differentiation at environmentally encountered micromolar concentrations. However, low molar nitrite prevented RA-induced SSEA1 downregulation and decreased betaIII-tubulin appearance. Decreased cell viability/proliferation accompanied these differentiation effects. P19 cells metabolized RA to polar retinoids. RA metabolism was not affected at any concentration of atrazine, nitrate, or nitrite. Environmentally relevant levels of these contaminants, thus, had no gross effect on neurodifferentiation and RA catabolism of embryonic stem cells. P19 cell-based bioassays may provide valuable tools in monitoring developmental toxicity.

RhoA/ROCK and Cdc42 regulate cell-cell contact and N-cadherin protein level during neurodetermination of P19 embryonal stem cells

RhoGTPases regulate actin-based signaling cascades and cellular contacts. In neurogenesis, their action modulates cell migration, neuritogenesis, and synaptogenesis. Murine P19 embryonal stem cells differentiate to neurons upon aggregation in the presence of retinoic acid, and we previously showed that RhoA and Cdc42 RhoGTPases are sequentially up-regulated during neuroinduction, suggesting a role at this very early developmental stage. In this work, incubation of differentiating P19 cells with C3 toxin resulted in decreased aggregate cohesion and cadherin protein level. In contrast, C3 effects were not observed in cells overexpressing recombinant dominant active RhoA. On the other hand, C3 did not affect cadherin in uninduced cells and their postmitotic neuronal derivatives, respectively expressing E- and N-cadherin. RhoA is thus influential on cell aggregation and cadherin expression during a sensitive time window that corresponds to the switch of E- to N-cadherin. Cell treatment with Y27632 inhibitor of Rho-associated-kinase ROCK, or advanced overexpression of Cdc42 by gene transfer of a constitutively active form of the protein reproduced C3 effects. RhoA-antisense RNA also reduced cadherin level and the size of cell aggregates, and increased the generation of fibroblast-like cells relative to neurons following neuroinduction. Colchicin, a microtubule disrupter, but not cytochalasin B actin poison, importantly decreased cadherin in neurodifferentiating cells. Overall, our results indicate that the RhoA/ROCK pathway regulates cadherin protein level and cell-cell interactions during neurodetermination, with an impact on the efficiency of the process. The effect on cadherin seems to involve microtubules. The importance of correct timing of RhoA and Cdc42 functional expression in neurogenesis is also raised.

The blue copper ceruloplasmin induces aggregation of newly differentiated neurons: a potential modulator of nervous system organization

Ceruloplasmin (CP) is a copper-dependent ferroxidase. It regulates iron metabolism and is involved in inflammation, angiogenesis, and protection against oxidative stress. CP also modulates K(+) channel activity in neuroblastoma cells and affects cardiodynamics of isolated hearts. Considering the presence of CP in the nervous system and the importance of iron ions and K(+) channels in neuronal activity, we postulated a role of CP in neuronal development. This hypothesis was tested using the P19 mouse embryonal carcinoma cell line, a model of neuronal differentiation. Addition of CP to the culture medium of newly differentiated P19 neurons induced cell aggregation within 24 h. This effect was concentration-dependent half-maximal at 50 nM, and not associated with necrosis, apoptosis or changes in secretory function. Deglycosylated CP was aggregative but not denatured CP, copper salts, His(2)Cu complex, or other copper enzymes or serum proteins. CP-induced aggregation was less pronounced with aging neurons and seemed not to involve K(+) channels. Immunocytofluorescence analysis demonstrated that digoxigenin-labeled CP bound to P19 neurons and the proportion of responding neurons decreased with aging. The interaction of digoxigenin-labeled CP with neurons was half-maximal at 120 nM by enzyme-linked immunosorbent assay and displaced by unlabeled CP. Our data indicate a specific aggregative action of CP on young neurons in vitro, possibly involving CP receptors. A potential developmental role of CP in nervous system organization is thus demonstrated.

Oxytocin induces differentiation of P19 embryonic stem cells to cardiomyocytes

We recently discovered the existence of the oxytocin/oxytocin receptor (OT/OTR) system in the heart. Activation of cardiac OTR stimulates the release of atrial natriuretic peptide (ANP), which is involved in regulation of blood pressure and cell growth. Having observed elevated OT levels in the fetal and newborn heart at a stage of intense cardiomyocyte hyperplasia, we hypothesized a role for OT in cardiomyocyte differentiation. We used mouse P19 embryonic stem cells to substantiate this potential role. P19 cells give rise to the formation of cell derivatives of all germ layers. Treatment of P19 cell aggregates with dimethyl sulfoxide (DMSO) induces differentiation to cardiomyocytes. In this work, P19 cells were allowed to aggregate from day 0 to day 4 in the presence of 0.5% DMSO, 10(-7) M OT and/or 10(-7) M OT antagonist (OTA), and then cultured in the absence of these factors until day 14. OT alone stimulated the production of beating cell colonies in all 24 independently growing cultures by day 8 of the differentiation protocol, whereas the same result was obtained in cells induced by DMSO only after 12 days. Cells induced with OT exhibited increased ANP mRNA, had abundant mitochondria (i.e., they strongly absorbed rhodamine 123), and expressed sarcomeric myosin heavy chain and dihydropyridine receptor-alpha 1, confirming a cardiomyocyte phenotype. In addition, OT as well as DMSO increased OTR protein and OTR mRNA, and OTA completely inhibited the formation of cardiomyocytes in OT- and DMSO-supplemented cultures. These results suggest that the OT/OTR system plays an important role in cardiogenesis by promoting cardiomyocyte differentiation.

RhoB expression is induced after the transient upregulation of RhoA and Cdc42 during neuronal differentiation and influenced by culture substratum and microtubule integrity

RhoGTPases are important intracellular signalling switches in the regulation of cytoskeleton organization. They likely have an important role in ontogenesis because cytoskeletal rearrangements accompany cell differentiation and specialization. Western blotting showed that protein expression of RhoA, RhoB and Cdc42 RhoGTPases dramatically increased, in a programmed manner, during neuronal differentiation of P19 mouse embryonal carcinoma cells with retinoic acid. RhoA and Cdc42 expression were sequentially upregulated and peaked during the commitment period while that of RhoB was induced in post-mitotic neurons. Although RhoB had a higher expression on matrices allowing cell spreading and neurite elongation, it was distributed throughout cell volume by immunocytofluorescence and associated with various cell compartments by centrifugal subfractionation, suggesting a role not restricted at neurites at this stage of differentiation. RhoA and Cdc42 were mainly cytosolic and RhoB particulate in the P19 cell model. Treatment of cells with cytoskeleton disruptors showed that poisons of microtubules but not of actin filaments or neurofilaments increased the cytosolic level of RhoB. The results indicate that RhoA, Cdc42 and RhoB must intervene at specific stages of neuronal development and there exists a relationship between RhoB expression/distribution, the microtubule network and the extracellular matrix during this process.