Actinomycin D-sensitive periods in the differentiation of Drosophila neurons and muscle cells in vitro

Drosophila MelanogasterEmbryo Cultures: AnIn VitroTeratogen Assay

An in vitro teratogen assay has been developed that uses Drosophila embryo cell cultures. The endpoints selected in assessing the teratogenic potential of any agent (physical or chemical) involves detection of interference with normal muscle and/or neuron differentiation, induction of heat stock (stress) proteins, and inhibition of normal neurotransmitter levels. Current studies involve use of reporter gene technology (protein fusions) to identify teratogenicity. Results so far suggest that the Drosophila assay is capable of accurately establishing if a particular agent tested can act as a teratogen by a variety of appropriate endpoints (morphological, biochemical, molecular). Furthermore, this assay can be used not only as a teratogen screen, but also in mechanistic studies of abnormal development, gene involvement in teratogenic resistance, and the possible role of heat shock proteins in preventing birth defects.

Kinesin-1-powered microtubule sliding initiates axonal regeneration in Drosophila cultured neurons

Microtubule sliding drives initial axon regeneration in Drosophila neurons. Axotomy leads to fast calcium influx and subsequent microtubule reorganization. Kinesin-1 heavy chain drives the sliding of antiparallel microtubules to power axonal regrowth, and the JNK pathway promotes axonal regeneration by enhancing microtubule sliding.

Understanding the mechanism underlying axon regeneration is of great practical importance for developing therapeutic treatment for traumatic brain and spinal cord injuries. Dramatic cytoskeleton reorganization occurs at the injury site, and microtubules have been implicated in the regeneration process. Previously we demonstrated that microtubule sliding by conventional kinesin (kinesin-1) is required for initiation of neurite outgrowth in Drosophila embryonic neurons and that sliding is developmentally down-regulated when neurite outgrowth is completed. Here we report that mechanical axotomy of Drosophila neurons in culture triggers axonal regeneration and regrowth. Regenerating neurons contain actively sliding microtubules; this sliding, like sliding during initial neurite outgrowth, is driven by kinesin-1 and is required for axonal regeneration. The injury induces a fast spike of calcium, depolymerization of microtubules near the injury site, and subsequent formation of local new microtubule arrays with mixed polarity. These events are required for reactivation of microtubule sliding at the initial stages of regeneration. Furthermore, the c-Jun N-terminal kinase pathway promotes regeneration by enhancing microtubule sliding in injured mature neurons.

Requirement of RNA synthesis for pathfinding by growing axons

The effects of actinomycin D were studied in cultured grasshopper embryos at different stages of development by following the outgrowth patterns of identified neurones known as aCC, pCC, and Q1. When administered at stages occurring before 31% of embryonic development, actinomycin D (0.05-0.10 microM for 24-48 hours) prevented axon extension, whereas it did not affect the development of the nervous system in embryos older than 34% of development. At 31-34% of development, actinomycin D perturbed pathfinding of aCC without blocking axon extension. Thus, only 22% of the aCCs (n = 271) in embryos treated with actinomycin D extended an axon along the intersegmental nerve as in control embryos. In the remaining embryos, aCC failed to turn into the intersegmental nerve root; its growth cone remained in the longitudinal connective, above or below the turning point. Neurones of the group caudal to the intersegmental nerve root could extend along either the anterior or posterior commissure of the next posterior segment. In contrast to the observations made with aCC, only 1.2% of pCC (n = 166) and 0.0% of Q1 (n = 45) in embryos treated with actinomycin D showed axon growth along aberrant pathways. The position of the growth cones of most pCCs and all Q1s observed were in various points along their normal pathway. Both pCC and Q1, as a population, showed an extension rate significantly lower than that of their control counterparts. The effect of actinomycin D on aCC pathway choice was probably mediated by inhibition of RNA synthesis, because incorporation of uridine into RNA was reduced by 40%. The labelling of several monoclonal antibodies (1C10, 3B11, 7F7) that recognise surface glycoproteins (lachesin, fasciclin I, and REGA-1) involved in nervous system development of grasshopper embryos was suppressed. Our results suggest that the navigation of some axons along different pathways requires the synthesis of new mRNA.

Developmental effects of chemicals and the heat shock response in Drosophila cells

Exposure of prokaryotic and eukaryotic cells to heat shock (hyperthermia) or to a number of diverse environmental stresses such as teratogens, anoxia, and inhibitors of oxidative phosphorylation results in the enhanced synthesis of a number of proteins which have been previously referred to as heat shock proteins (hsps). More recently, in view of the diverse types of agents that can induce these proteins, they have also been referred to as stress proteins. This phenomenon is one of the most basic regulatory mechanisms in living organisms. Exposure of Drosophila embryos, larvae, or pupae to these types of stresses also results in a variety of developmental abnormalities in the ensuing adult. Although the function(s) of these heat shock proteins has yet to be determined, they are widely thought to play an important role in cell survival and protection following some types of environmental stress. In our laboratory, we have developed an in vitro assay for detecting agents that act as teratogens, utilizing Drosophila embryonic cultures. Drosophila embryonic cells differentiate in vitro to a number of functional cell types including myotubes and ganglia. A number of drugs that have been shown to act as teratogens in mammals have also been found to inhibit muscle and/or neuron differentiation in Drosophila embryonic cultures. We have examined, by two-dimensional gel electrophoresis, the effects of such teratogens on protein synthesis in Drosophila embryonic cells. Inhibition of muscle and/or neuron differentiation correlates well with the induction of two proteins of about 20 kilodaltons. These are identical to two of the heat shock proteins (hsp 23, 22) as shown by electrophoretic mobilities and peptide mapping by partial proteolysis. Heat shock and other treatments such as exposure to some of the metal ions and ether induces the entire set of seven major heat shock proteins in the Drosophila embryonic cells. Dose-response studies of several teratogens show a correlation between the degree of inhibition of differentiation and the level of induction of hsps. Since heat shock proteins have been suggested as possibly serving a protecting role, our present studies are aimed at identifying the role of hsps in teratogenesis and investigating the differential regulation of heat shock genes in response to different external stimuli.

Detection of teratogens in the Drosophila embryonic cell culture test: assay of 100 chemicals

An in vitro assay of teratogenesis has been developed that utilizes Drosophila embryonic cell cultures. The endpoint selected in assessing the teratogenic potential of any substance involves detection of interference with normal muscle and/or neuron differentiation. In the validation phase of this project, 100 chemicals were tested. With drugs for which extensive reliable mammalian data are available, the results in the Drosophila assay equate rather favorably with those observed in animals and humans (i.e., a low percentage of false positives and false negatives has been obtained). In an effort to determine if strain differences exist and also to establish that the system shows a dose response, cultures from three wild-type Drosophila strains (Canton S, Canton S109, and Oregon R) were tested. Dose-response differences were observed when diethylstilbestrol, diphenylhydantoin, imipramine, testosterone, and tolbutamide were added to the cultures. These results suggest that the Drosophila assay, with further testing and refinements, might be capable of identifying agents of high teratogenic potential by their effect on neurons and muscle differentiation. Furthermore, sensitive strains might be used to study mechanisms of abnormal development and gene involvement in teratogenic resistance.

RNA synthesis dependence of action potential development in spinal cord neurones

The role of transcription in the development of electrical properties of neuronal membranes has been largely unexplored. To study the molecular events which result in the expression of these properties it is useful to describe the timing of the underlying RNA synthetic events. For example, the timing of the transcription involved in denervation-induced action potentials in frog slow muscle fibres and brain extract-induced sodium channels in chick muscle cells has been investigated. Previous studies of Xenopus laevis spinal neurones have established that the timing of the development of the neuronal action potential ionic dependence in dissociated cell cultures parallels that seen in vivo. This culture system, therefore, allows the determination of transcription-dependent periods necessary for the development of membrane properties known to have in vivo relevance. In the study described here, actinomycin D was used to examine the timing of the RNA synthetic events necessary for (1) neurite outgrowth and (2) development of the ionic dependence of the action potential. I report that inhibition of transcription at an early stage specifically blocks the appearance of the mature sodium-dependent action potential without affecting either neurite outgrowth or the development of delayed rectification.

Use of Drosophila embryo cell cultures as an in vitro teratogen assay

An in vitro assay has been developed for detecting teratogens by adding them to primary cultures of embryonic Drosophila cells and analyzing the degree of change in cell differentiation and tissue formation. Cultures are scored by an automated image analysis system that counts the number of myotubes and ganglia in culture. A decrease in their number compared to controls is taken as an indication of teratogenicity. In the group of 100 drugs and chemicals examined thus far in this assay, a high correlation with known teratogenic activity has been obtained with few false-positives or false-negatives. Procedures also have been developed for testing metabolic products of ingested compounds for teratogenicity. Mice and rats are fed the particular agent to be tested, and their serum is then added to the differentiating culture. Preliminary trials with human serum from patients receiving chemotherapy have also been performed. With further testing, validation, and incorporation of a metabolic activation system, it is hoped that this assay can be used, along with a battery of other in vitro assays, as a screen for the large number of agents awaiting comprehensive testing of their teratogenic potential. We also see the use of this assay as means to gain further information on the basic biochemical and developmental aspects of teratogenesis.

Ultrastructural differentiation during Drosophila neurogenesis in vitro

Drosophila melanogaster field neuroblasts differentiate in vitro, and each gives rise to a cluster of about 18 daughter neurons. Electron microscopic observations of single clusters show that axons from daughter neurons form a neuropile within the cluster of cell bodies. The neuropile increases in size and complexity for several hours, during which time chemical, and probably electrotonic, synapses form between neurites. Clear vesicles with diameters of about 35 nm and dense core vesicles with diameters of about 60 and 160 nm were detected. The development of the neuropile indicates that the prerequisite cell recognition phenomena were manifested during differentiation in vitro, and the complexity of the neuropile suggests it may have attained the capacity to process information.

Ultrastructural differentiation during embryonic Drosophila myogenesis in vitro

Cultures of embryonic Drosophila melanogaster cells were examined by electron microscopy and events in myogenesis were recorded. Thick and thin myofilaments, T-tubules and sarcoplasmic reticulum all appeared at about the same time, 10.5 hr. This was about 5 hr after the final division of myoblasts and about the time that muscle cells were elongating, aligning and fusing. Sarcoplasm typical of insect muscle was detected by 18.5 hr, as were myotendonal and tendocuticular junctions. Two populations of myocytes were detected, the cytoplasm of one more electron-dense than the other. The only previous report of myofibrilogenesis in invertebrate embryos had described novel mechanisms. In Drosophila embryonic material, however, the sequence of myofibrilogenesis resembled that in postembryonic insect or vertebrate material.

A 5-bromodeoxyuridine-sensitive interval during drosophila myogenesis

Drosophila myogenesis was monitored in vitro and the cells were treated with 5-bromodeoxyuridine (BrdU) or with thymidine at certain intervals. Muscle cells were scored for survival, contractility, and the uptake of thymidine and BrdU. Results indicated that the final S period for myoblasts takes place in vitro between 1.3 and 3.3 h following the initiation of gastrulation in the donor embryos. Treatment with 10(-4) M BrdU during this internal inhibited myogenesis, but later treatment did not. Thymidine reversed or prevented the BrdU effect if given before the final myoblast division, but not if given after wards. All results support the hypothesis that BrdU inhibits Drosophila myogenesis through its incorporation into DNA.