Robert K. K. Lee

Interleukin-1 beta converting enzyme (ICE) is a mammalian homolog of CED-3, a protein required for programmed cell death in the nematode Caenorhabditis elegans. The activity of ICE can be specifically inhibited by the product of crmA, a cytokine response modifier gene encoded by cowpox virus. Microinjection of the crmA gene into chicken dorsal root ganglion neurons was found to prevent cell death induced by deprivation of nerve growth factor. Thus, ICE is likely to participate in neuronal death in vertebrates.

We have examined the zebrafish embryo to ascertain the location of endocardial and myocardial progenitors prior to gastrulation, in an attempt to define the earliest stages of cardiac patterning. Currently there is uncertainty as to the spatial and lineage relationship of the progenitors for these two phenotypically distinct cell types that form the two concentric layers of the primitive heart tube. By single-cell injection and tracking, we distinguish a region in the early and midblastula which has the properties of a heart field, in that it defines a zone of cardiac progenitors within which there is a spatial gradient of propensity to generate heart cells, and which regulates, in the sense of adapting to the transplantation of pluripotential cells. This zone extends from the future ventral axis dorsally along the margin, with cardiogenic propensity tapering off laterally and dorsally. Myocardial progenitors are spread throughout this region, but endocardial precursors are restricted to the ventral marginal region. The cardiovascular progeny of the ventral cells include, in addition to endocardium and myocardium, cells in the endothelium and blood.

Reticulospinal neurons of the larval zebrafish Brachydanio rerio have been categorized into 27 different types (Kimmel et al.: Journal of Comparative Neurology 205:112-127, 1982; Metcalfe et al.: Journal of Comparative Neurology 251:147-159, 1986). Nineteen of these occur as bilateral pairs which are individually identifiable. Since considerable remolding of brain structures (e.g., cell death and modifications of neuronal architecture) occurs during development, we ask if these cells are preserved in the adult zebrafish and the extent to which neuronal morphology of the larva is conserved during ontogeny. In our analysis, we studied reticular neurons from 84 brains retrogradely labelled from the spinal cord with HRP. We show that all reticulospinal types of the larva are retained without considerable change in morphology in the adult. Many neurons, including the Mauthner cell and two of its serial homologues, MiD2cm and MiD3cm, can be individually and unambiguously identified. In addition, the appearance of later developing (tertiary) neurons leads to an increase in the numbers of some neuron types. Although tertiary neurons are often isomorphic with neighboring cells, they can have unique morphologies of their own and, therefore, are also individually identifiable. We suggest that the appearance of tertiary neurons may serve to extend the behavioural repertoire of the embryo. Moreover, morphological repetitions in adjacent segments of the otic region (level of VIIIth nerve entry) may represent the replication of a functional motif, perhaps involving the C-type escape response which is known to involve the Mauthner cell.

The hindbrain is evolutionarily conserved among diverse vertebrate phyla. In vertebrate embryos, the hindbrain is segmentally organized as a series of overt swellings known as rhombomeres. In the larval zebrafish Brachydanio rerio, conspicuous and identifiable reticulospinal neurons are positioned in the center of rhombomeres. Segmentally homologous reticulospinal neurons that share a range of morphological, developmental, and biochemical features occupy adjacent rhombomeres. We have recently shown that reticulospinal neurons of the zebrafish survive ontogeny without considerable morphological modification and we suggested that homologous neurons may share similar functions at different stages of development (Lee and Eaton: Journal of Comparative Neurology 304:34-52, 1991). The goldfish Carassius auratus, a related cyprinid, is especially suited for neurophysiological and behavioral studies. However, it is not yet known if the various reticulospinal neurons of zebrafish are generalizable to other species such as the goldfish. Therefore, we sought to examine the extent to which reticulospinal neurons of the zebrafish are also present in the adult goldfish. Analysis of 45 brains retrogradely labeled with horseradish peroxidase (HRP) from the spinal cord showed that reticulospinal neurons are arranged as a series of seven segments within the hindbrain; a regular interval of approximately 200 microns separates adjacent segments. Although the goldfish reticulospinal system has more neurons than the zebrafish, many reticulospinal neuron types continue to be identifiable. Moreover, comparisons of dendritic arborizations and axon paths between the two species showed that the morphology between various neuron types is virtually identical. The cross-taxonomic similarities between the reticulospinal systems of these related cyprinids make it possible to pursue functional considerations of segmentally homologous neurons in the goldfish hindbrain.

Amyloid plaques that accumulate in the brains of patients with Alzheimer's disease (AD) are primarily composed of aggregates of amyloid peptides that are derived from the amyloid precursor protein (APP). Overexpression of APP in cell cultures increases the formation of amyloidogenic peptides and causes neurodegeneration and cognitive dysfunction in transgenic mice. We now report that activation of prostaglandin E2 (PGE2) receptors increases cAMP formation and stimulates overexpression of APP mRNA and holoprotein in primary cultures of cortical astrocytes. Levels of glial fibrillary acidic protein were also increased by PGE2 treatment, suggesting that these cultured astrocytes resemble reactive astrocytes found in vivo. The stimulation by PGE2 of APP synthesis was mimicked or blocked by activators or inhibitors, respectively, of protein kinase A. Actinomycin D or cycloheximide also inhibited the increase in APP holoprotein stimulated by PGE2. Treatment of astrocytes with 8-Bromo-cAMP or forskolin for 24 hr also stimulated APP overexpression in cultured astrocytes. The immunosuppressants cyclosporin A and FK-506 inhibited the increase in APP mRNA and holoprotein levels caused by PGE2 or by other treatments that elevated cellular cAMP levels; the inhibitory effect of FK-506 but not of cyclosporin A was attenuated by rapamycin. These results suggest that prostaglandins produced by brain injury or inflammation can activate APP transcription in astrocytes and that immunosuppressants may be used to prevent APP overexpression and possibly the pathophysiological processes underlying AD.