Control of gap junction formation in early mouse embryos

Wheat germ agglutinin induces compaction- and cavitation-like events in two-cell mouse embryos

The lectin wheat germ agglutinin (WGA) has been observed to induce morphological events similar to compaction and cavitation in 2-cell mouse embryos. In vitro exposure of embryos to WGA results in increased apposition between blastomeres and the subsequent formation of a large intercellular cavity. As is the case for cavitation normally associated with blastocyst formation, WGA-induced cavitation can be inhibited by ouabain, suggesting a requirement for ATPase activity. However, WGA-induced effects are not inhibited by cytoskeletal disruptive agents or inhibitors of a variety of synthetic and metabolic functions. WGA may induce the observed effects by triggering the premature onset of developmental events normally involved in the processes of compaction and cavitation or, perhaps, by inducing morphologically similar changes as a result of the crosslinking of cell surface lectin-binding molecules and regional inhibition of ATPase function.

Timing of transcription and protein synthesis underlying morphogenesis in preimplantation mouse embryos

During preimplantation development of the mouse, embryos pass through a series of morphogenetic events: compaction, fluid accumulation to form the blastocoele (cavitation), and escape from the zona pellucida (hatching). We have used the inhibitors alpha-amanitin and cycloheximide to investigate the timing of transcriptional and translational events underlying these morphogenetic stages. Groups of embryos were transferred from a common pool into medium containing one or the other inhibitor at regular time intervals, and then were scored over the ensuing 24 or more hours for their ability to reach a particular morphogenetic end point. By comparing the time when the control population reached an end point with the time at which embryos had to be transferred into the inhibitor in order to prevent them from reaching that end point, we could determine when in advance of each event the necessary transcription or protein synthesis has been completed. Our results suggest that compaction (as well as cleavage to the eight-cell stage) is an embryonically, rather than maternally, programmed event, although the necessary transcription is completed well in advance, at least by the early four-cell stage. The transcriptional and translational events underlying fluid accumulation, on the other hand, appear to be completed within a few hours of the start of this process. For hatching, there is once again a long delay between the apparent time of completion of the necessary transcriptional events and the process itself, with protein synthesis being completed just a few hours in advance. Our results raise the possibility that post-transcriptional regulatory mechanisms play an important role in the timing of morphogenetic events in early mouse embryos.

Inhibited intercellular communication as a mechanistic link between teratogenesis and carcinogenesis

Teratogenesis and carcinogenesis share many characteristics, leading to the speculation that they may also share pathogenic mechanisms. Direct intercellular communication mediated by membrane junctions is known to occur between a variety of cells and may play an important role in the control of cell growth and differentiation. Inhibition of junctional communication may be a mechanism common to both teratogenesis and carcinogenesis whereby cells and tissues are diverted from their normal differentiation paths. The multistage model of carcinogenesis predicts that the irreversibly initiated cell is at least partially regulated by the surrounding cells of a tissue, and that the initiated cell remains inactive until stimulated to proliferate by a tumor promotor. Tumor promoters may release the initiated cell from control of the surrounding tissue by interrupting intercellular communication, since many tumor promoters have now been shown to interfere with junctional communication in cultured mammalian cells. Furthermore, many tumorigenic cells have compromised junctional communication abilities. Similarly, it has been reasoned that the cells of an embryo must be able to communicate with each other to define tissue specificity and pattern formation, and to coordinate morphogenetic events. Many studies have chronicled alterations in junctional communication that occur coincident with major developmental events and some studies suggest that junctional communication may be modified at boundaries of morphogenetic fields. A recent in vivo study has provided evidence that inhibition of junctional communication may interfere with embryonic development, and several teratogens are known to interrupt junctional communication in mammalian cells in culture. These observations suggest that inhibition of junctional intercellular communication may be a shared mechanism of carcinogenesis and teratogenesis.

On the use of alpha-amanitin as a transcriptional blocking agent in mouse embryos: a cautionary note

We have tested the effect of alpha-amanitin at 10, 50 and 100 micrograms/ml, on precursor uptake and incorporation into poly(A)+ RNA and poly(A)- RNA of mouse embryos on days 2, 3 and 4 of gestation. Embryos were pretreated with the inhibitor for 2 hr, then labeled for 2 hr in its continued presence. RNA fractions were separated by affinity chromatography on oligo(dT)-cellulose. alpha-Amanitin did not suppress uptake of RNA precursors at any of the concentrations tested in any stage. At 10 micrograms/ml, we could not detect any effect on incorporation into either RNA fraction in any stage. Only the highest concentration tested, 100 micrograms/ml, was effective in all stages in substantially suppressing incorporation into poly(A)+ RNA within 2 hr. Longer treatments increased the level of suppression to a maximum of about 80%. Incorporation into poly(A)- RNA was suppressed to roughly the same extent. Despite previously reported data, it cannot be assumed that alpha-amanitin at concentrations less than 100 micrograms/ml brings about a quick interruption of mRNA synthesis in preimplantation mouse embryos.

Junction formation in aggregated embryonal carcinoma cells

Under the action of supplemental calcium, H6 mouse embryonal carcinoma cell aggregates undergo compaction, a morphological phenomenon similar to mouse embryonic compaction. Formation of various types of cell junctions, especially gap junctions, is associated with compaction of the embryo and we sought to analyze the pattern of junction formation during aggregation and compaction of H6 cells. At 24 hr of aggregation, gap junctions were abundant in both uncompacted and compacted aggregates but quantitative analysis of freeze fracture replicas of these junctions showed a 20-fold increase in the size of the largest gap junctions in compacted aggregates. Such a difference in size could even be detected at 12 hr of aggregation. Tight junctions were not normally formed in 12 hr aggregates but initial stages of tight junction formation could be noticed in 12 hr compacted aggregates. More definitive tight junctions and desmosomes were evident only after 48 hr of aggregation. Thus we have observed that both uncompacted and compacted aggregates can form gap junctions at similar frequencies, suggesting that cell flattening, which contributes to the compacted morphology, is not a requisite for gap junctions. Likewise, generation of the compacted morphology seems to be independent of gap junction formation. This supports the idea that compaction in embryonal carcinoma cells results from calcium-induced cell flattening, probably through the mobilization of cytoskeletal elements. Calcium-dependent features of H6 cell aggregation and compaction enables the independent analysis of separate steps in compaction.

Cell-to-cell communication in monolayers of epithelioid cells (MDCK) as a function of the age of the monolayer

We explore the existence of cell-to-cell communication in monolayers of MDCK cells plated at high densities so that they form a continuous monolayer in a few minutes. Lucifer Yellow CH is injected in the cytoplasm of a given cell by using a glass microelectrode with a fine tip (ca. 100 M omega) and passing square pulses of current of 1.0 nA that last 10 msec, every 20 msec, during 1 to 3 min. We then examine the monolayer with fluorescence microscopy. In 27 out of 111 cells injected during the first 4 to 15 hr after plating, the dye was transferred to neighboring cells. Electron micrographs of freeze-fracture replicas prepared at this time, show that 20 to 25% of the lateral surfaces present the aggregates of intramembrane particles typical of gap junctions. These early hours correspond to the formation of occluding junctions and polarization into an apical and a basolateral domain of the plasma membrane (Cereijido, Meza & Martínez-Palomo, 1981). Cell-to-cell coupling then decreases sharply and, in the period between the 1st and 3rd day (mature monolayers), only 4 out of 49 injected cells were able to transfer the dye to their neighbors in the monolayers. No image of gap junctions was found in freeze-fracture replicas of mature monolayers. The degree of coupling between cells, as well as the number of cells coupled to the injected one, were highly variable. The lack of coupling between cells in mature monolayers observed in this article with Lucifer Yellow CH and electron microscopy is in keeping with the absence of electrical coupling observed in a previous work (Stefani & Cereijido, 1983). The transient existence of communicating junctions observed in monolayers of MDCK cells is similar to that described in the literature for embryo tissues during development.