Chemically induced growth inhibition and cell cycle perturbations in cultures of differentiating rodent embryonic cells

Cell cycle inhibition by sodium arsenite in primary embryonic rat midbrain neuroepithelial cells

Arsenite (As3+) exposure during development has been associated with neural tube defects and other structural malformations, and with behavioral alterations including altered locomotor activity and operant learning. The molecular mechanisms underlying these effects are uncertain. Because arsenic can cross the placenta and accumulate in the developing neuroepithelium, we examined cell cycling effects of sodium arsenite (As3+ 0, 0.5, 1, 2, and 4 microM) on embryonic primary rat midbrain (gestational day [GD] 12) neuroepithelial cells over 48 h. There was a concentration- and time-dependent As3+-induced reduction in cell viability assessed by neutral red dye uptake assay but minimal apoptosis at concentrations below 4 microM. Morphologically, apoptosis was not apparent until 4 microM at 24 h, which was demonstrated by a marginal but statistically significant increase in cleaved caspase-3/7 activity. Cell cycling effects over several rounds of replication were determined by continuous 5-bromo-2'-deoxyuridine (BrdU) labeling and bivariate flow cytometric Hoechst-Propidium Iodide analysis. We observed a time- and concentration-dependent inhibition of cell cycle progression as early as 12 h after exposure (> or =0.5 microM). In addition, data demonstrated a concentration-dependent increase in cytostasis within all cell cycle phases, a decreased proportion of cells able to reach the second cell cycle, and a reduced cell cycle entry from gap 1 phase (G1). The proportion of affected cells and the severity of the cell cycle perturbation, which ranged from a decreased transition probability to complete cytostasis in all cell cycle phases, were also found to be concentration-dependent. Together, these data support a role for perturbed cell cycle progression in As3+ mediated neurodevelopmental toxicity.

Micromass cultures in teratology

This unit describes methods for culture of undifferentiated midbrain (mesencephalon) and limb bud cells from gestation day 12 rat embryos. When grown over 5 days in vitro, these mixed cell populations express many morphological, biochemical, molecular, and immunophenotypic characteristics observed during in vivo differentiation. These cultures can be used in a wide variety of studies designed to investigate normal cellular ontogeny, the teratogenic potential of test agents, or the mechanisms underlying the cellular response to environmental stress.

Induction of growth arrest and DNA damage-inducible genes Gadd45 and Gadd153 in primary rodent embryonic cells following exposure to methylmercury

Methylmercury (MeHg) is recognized as a significant environmental hazard, particularly to the development of the nervous system. Studies on the mechanism of MeHg-induced toxicity reveal that inhibition of cell cycle progression may be one way by which MeHg interferes with normal development. In this study, we utilized primary rodent embryonic neuronal cell (CNS) and limb bud (LB) cultures to determine the mRNA expression level of two genes involved in cell cycle arrest, Gadd45 and Gadd153, both during cellular differentiation and in response to MeHg exposure. A differential expression pattern of Gadd45 and Gadd153 was observed during CNS and LB differentiation in culture. However, both CNS and LB cells responded to MeHg exposure with a concentration-dependent increase in Gadd45 and Gadd153 mRNA. Previous studies have shown that MeHg exposure (2 microm) of CNS cells for 24 hr causes a fourfold decrease in the number of cells passing through the cell cycle. The present study shows that at the same exposure concentration, a five- to eightfold increase in Gadd45 mRNA levels and a two- to fourfold increase of Gadd153 was observed. Induction of Gadd45 was also noted in adult female mice chronically exposed to 10 ppm MeHg, a dose that caused developmental toxicity in vivo. Based on the known involvement of the Gadd genes in cell cycle arrest, activation of these genes could be one mechanism by which MeHg interferes with the cell cycle in adult and developing organisms.

Characterization of cytoskeletal and neuronal markers in micromass cultures of rat embryonic midbrain cells

Micromass cultures of rat embryonic midbrain cells were characterized with regard to the immunolocalization of neuronal and cytoskeletal markers. Cells taken from gestational day-12 embryos and cultured for 5 days in vitro comprise at least two morphologically distinct cells types: fibroblast-like cells and neurons. Antibodies to the following markers yielded preferential staining of neuronal cells: A2B5 (GQ ganglioside), gamma-aminobutyric acid (GABA), microtubule-associated protein 2 (MAP2), MAP5, neuron-specific enolase (NSE), neural cell adhesion molecule (N-CAM), and tau. Antibodies to beta-tubulin, c-neu, MAP1, and neurofilament (NF-H) stained both neuronal and fibroblast-like cells. Antibodies to glial fibrillary acidic protein (GFAP) and vimentin failed to immunoreact with any cells in day-5 CNS cultures. SDS-PAGE and Western analysis were employed to determine the specificity of the antibodies and determine the electrophoretic profiles of the markers. We conclude that the pattern of neuronal differentiation in CNS micromass cultures exhibits certain similarities to that observed in vivo. In addition, certain markers identified in this study may be of potential utility as (1) biomarkers of chemically-induced developmental neurotoxicity, and (2) indicators of differential toxicity toward the diverse cell types that comprise the mammalian central nervous system.

Effects of benzimidazole analogs on cultures of differentiating rodent embryonic cells

Micromass cell culture systems for rat embryo midbrain (CNS) and limb bud (LB) cells were employed to assess the in vitro developmental toxicity of the benzimidazole analogs, mebendazole (MBZ), thiabendazole (TBZ), and nocodazole (NCZ), in addition to the classic microtubule inhibitor, colchicine. Comparison was made to albendazole (ABZ), a previously studied benzimidazole anthelmintic. Two parameters for assessing developmental toxicity were measured: differentiation and cytotoxicity. The relative potencies of the benzimidazole analogs in the micromass system (NCZ greater than MBZ approximately ABZ much greater than TBZ) mirrored their effectiveness in an assay for in vitro inhibition of mammalian tubulin polymerization. Colchicine also exhibits a high affinity for mammalian tubulin and was a potent inhibitor of cell proliferation, chondrogenesis, and neuronal differentiation. Immunofluorescent staining of Day 1 LB cultures with a monoclonal antibody to beta-tubulin revealed that these agents elicited mitotic arrest. Many anti-tubulin agents are teratogenic in rats and their in vivo developmental toxicity may reflect perturbation of microtubular structure or function. With the exception of TBZ, these agents should be considered potential developmental toxicants since they inhibit cell growth and differentiation of micromass cultures at nanomolar concentrations.

Effects of albendazole and albendazole sulfoxide on cultures of differentiating rodent embryonic cells

Micromass cell culture systems for rat embryo midbrain (CNS) and limb bud (LB) cells were employed to assess the in vitro developmental toxicity of the human and veterinary anthelmintic albendazole (ABZ) and its sulfoxide metabolite (SOABZ). ABZ is reported to be teratogenic in rats, and is extensively metabolized to the sulfoxide derivative. It has been postulated that SOABZ is the reactive metabolite responsible for albendazole's developmental toxicity and anthelmintic activity in vivo. Three parameters for assessing developmental toxicity were measured: cell growth, differentiation, and cytotoxicity. CNS and LB cultures were equivalent in their sensitivities to both ABZ and SOABZ. ABZ was approximately 50-fold more potent than SOABZ. Immunohistochemical determinations of tubulin organization revealed that both ABZ and its sulfoxide metabolite elicit an accumulation of cells in the mitotic phase of the cell cycle. Since ABZ is one of the most potent agents tested in the micromass system to date, this anthelmintic should be considered a potential developmental toxicant.