Actin and myosin synthesis during differentiation of neuroblastoma cells

Metabolic switch in energy metabolism mediates the sublethal effects induced by glyphosate-based herbicide on tadpoles of a farmland frog Microhyla fissipes

Glyphosate-based herbicides (GBHs) are widely-used agricultural chemicals, bringing potential detriments to aquatic organisms. Currently, our understanding of sublethal effects and underlying toxicologic mechanisms of GBHs are still limited, especially in amphibians. Here, the sublethal effects of a commercial GBH (KISSUN®) on tadpoles of a farmland dwelling frog, Microhyla fissipes, were investigated. The 10-d LC50 of "KISSUN®" GBH was 77.5 mg/L. Tadpoles exposed to 60-120 mg/L showed increased preference to higher temperature. After 10 days exposure, obvious growth suppression was observed in survived GBH-stressed tadpoles, characterized by dosage depended decrement in body mass, body width, total length, etc. GBH-stressed tadpoles also showed decreased tail length/snout-vent length ratio and smaller tail muscle fiber diameter. Comparative transcriptomics (control, 60 mg/L and 90 mg/L groups) was conducted to analyze the underlying molecular processes. GBH-stressed tadpoles showed downregulated transcription of ribosomal proteins and cytoskeleton proteins, which could explain their suppressed whole body and tail muscle growth. Moreover, GBH-stressed tadpoles showed transcriptional downregulation of carbohydrate and lipid catabolism, but upregulation of amino acid catabolism. It suggested a metabolic switch from carbohydrate and lipid to amino acid in these tadpoles. Accordingly, there was a trade-off between protein synthesis and energy production in respect to amino acid allocation, and it provided a metabolic explanation for why protein synthesis was downregulated and growth was suppressed in GBH-stressed tadpoles. In combination with existing literatures, we speculated that GBH might directly target the enzymes in carbohydrate and lipid catabolism, and this metabolic effect of GBH might be common to fish and amphibians. In conclusion, our study provided a systematic insight into the sublethal symptoms of GBH-stressed tadpoles, and a metabolic switch from carbohydrate and lipid to amino acid likely underlay some common toxic symptoms of GBHs on both fish and tadpoles.

Cytomatrix synthesis in MDCK epithelial cells

Detailed information regarding the synthesis rates of individual protein components is important in understanding the assembly and dynamics of the cytoskeletal matrix of eukaryotic cells. As an approach to this topic, the dual isotope technique of Clark and Zak (J. Biol. Chem., 256:4863-4870, 1981), was employed to measure fractional synthesis rates (FSRs) in growing and quiescent cultures of MDCK epithelial cells. Cell protein was labeled to equilibrium with [14C]leucine over several days and then pulse-labeled for 4 hours with [3H]leucine. FSRs (as percent per hour) were calculated from the 3H/14C ratio of cell extracts or individual proteins separated by two-dimensional polyacrylamide gel electrophoresis and the 3H/14C ratio of free leucine in the medium. Synthesis of total cell protein rose from approximately 1.4%/hour in quiescent cells to 3.5%/hour in the growing cultures. The latter rate was sufficient to account for the rate of protein accumulation and a low level of turnover in the growing cultures. The FSR of the buffered-Triton soluble extract was higher and the cytoskeletal FSR significantly lower than that for total protein in quiescent monolayers. This difference, however, was not observed in growing cultures. A distinct pattern of differences was seen in the FSRs of individual cytoskeletal proteins in the quiescent cultures. Vimentin synthesis was significantly lower than that of the keratins and the keratin FSRs were not obviously matched in pairwise fashion. Unexpectedly, the FSRs of alpha- and beta-tubulin diverged in quiescent cells with alpha-tubulin turnover exceeding beta-tubulin. Likewise, components of the microfilament lattice showed unequal fractional synthesis rates, myosin and alpha-actinin being faster than actin. In addition, the FSR for globular actin exceeded that of the cytoskeletal associated form. The results suggest that metabolic coupling between individual cellular filament systems is not strict. The data are, however, consistent with models that predict that assembly of a subcellular structure influences the turnover of its component proteins.

Expression and distribution of microtubule-associated protein 2 (MAP2) in neuroblastoma and primary neuronal cells

We examined the expression and distribution of microtubule-associated protein 2 (MAP2) during the differentiation in culture of both mouse NB2a neuroblastoma and primary embryonic rat neurons. The differentiation of NB2a cells was induced with retinoic acid (RA) which stimulated the extension of a highly branched neuritic network and dibutyryl cAMP which stimulated the outgrowth of long bipolar or monopolar processes. We found that although monoclonal antibodies to MAP2 stained the cell bodies of control and differentiated cells, only the RA-induced neurites were positive for this antigen. These data support our ultrastructural studies indicating that the RA-induced neurites were dendrite-like and that the dibutyryl cAMP-induced processes were axon-like. Studies on the biosynthesis of MAP2 indicated that RA induced a 2-3-fold increase in MAP2 synthesis in 24 h; however, this effect was transient, with the synthesis of MAP2 in RA-treated cells returning to control level by 72 h. Although biosynthetic studies suggested the synthesis of species at 250-300 kdalton, the major molecular weight form in the neuroblastoma cells was 230 kdalton. Immunocytochemical analysis of primary neurons showed staining of neuronal cell bodies and of short processes, but virtually no staining of the long axon-like processes. The staining of neuronal cell bodies and processes was evident at all stages of cell differentiation. This finding was corroborated by immunoblots which showed significant amounts of MAP2 throughout cell development. The molecular weight of the immunoreactive material was ca. 300 kdalton in both primary neurons and rat brain. Immunoblots also revealed that embryonic neurons expressed only MAP2B as they differentiated in culture for 14 days. Biosynthesis studies suggested that early in culture there was a modest increase in MAP2 synthesis, but no detectable change was observed thereafter. We concluded therefore that both neuroblastoma cells and primary neurons can differentiate neuritic processes, which show dendritic properties in terms of morphology and preferential distribution of MAP2.

Synthesis of cytoskeletal and contractile proteins by cultured IMR-90 fibroblasts

Models of the assembly of cytoskeletal and contractile proteins of eukaryotic cells require quantitative information about the rates of synthesis of individual component proteins. We applied the dual isotope technique of Clark and Zak (1981, J. Biol. Chem., 256:4863-4870) to measure the synthesis rates of cytoskeletal and contractile proteins in stationary and growing cultures of IMR-90 fibroblasts. Fibroblast proteins were labeled to equilibrium with [14C]leucine over several days, at the end of which there was a 4-h pulse with [3H]leucine. Fractional synthesis rates (percent per hour) were calculated from the 3H/14C ratio of cell protein extracts or protein purified by one- or two-dimensional polyacrylamide gel electrophoresis and the 3H/14C ratio of medium-free leucine. The average fractional synthesis rate for total, SDS- or urea-soluble; Triton-soluble; and cytoskeletal protein extracts in stationary cells each was approximately 4.0%/h. The range of values for the synthesis of individual proteins from total cell extracts or cytoskeletal extracts sliced from one-dimensional gels was similar, though this range was greater than that for major proteins of Triton-soluble protein extracts. Three specific cytoskeletal proteins--actin, vimentin, and tubulin--were synthesized at similar rates that were significantly slower than the average fractional synthesis rate for total protein. Myosin, on the other hand, was synthesized faster than average. Synthesis rates were the same for beta-and gamma-actin and polymerized (cytoskeletal extract) vs. Triton-soluble actin. The same was true for alpha- and beta-tubulin and two different forms of vimentin. Synthesis rates were uniformly higher in growing cells, though the same pattern of differential rates was observed as for stationary cells. Synthesis rates in growing cells were higher than the rate necessary to maintain the growth rate, even for those cytoskeletal proteins being synthesized slowly. Therefore, there appears to be some turnover of these cytoskeletal elements even during growth. We conclude that proteins in cytoskeletal extracts may have nonuniform rates of synthesis, but at least one important subclass of cytoskeletal proteins that comprise filament subunits have the same synthesis rates.

Identification of myosin in isolated synaptic junctions

A monospecific antibody prepared against chicken gizzard myosin reacted with only one peptide corresponding to myosin heavy chain (Mr = 200,000) in gels of synaptic plasma membranes (SPM) and synaptic junctions (SJ) prepared from several species. Preadsorption of antisera with purified brain myosin eliminated antibody reactivity to SPMs and SJs. SJs were found to contain approximately 3 times the concentration of myosin found in SPMs when assayed by an indirect immunoradiometric assay. Postsynaptic density and myelin fractions contained no myosin detectable by immunoradiometric assay, antibody binding to gels, or Coomassie blue staining. The band identified as myosin in SJ fraction yielded peptide fingerprints indistinguishable from fingerprints of purified brain myosin but distinct from fingerprints of purified smooth and skeletal muscle myosins. The distribution of exogenous [125I]myosin during subcellular fractionation indicated that myosin in isolated synaptic junction could not have resulted from artifactual re-distribution of soluble myosin. Together these results show that a non-muscle myosin is an endogenous component of CNS asymmetric synapses.