Morphological, cytochemical, and cytofluorimetric features of supramedullary neurons of the fish Solea ocellata

DNA endoreplication in the brain neurons during body growth of an adult slug

Endoreplication is DNA synthesis without cell division. Giant neurons observed in the brains of mollusks are thought to be generated as a result of DNA endoreplication. It has been hypothesized that neuronal size becomes larger in parallel with an increase in body size and that DNA endoreplication is involved in this process to meet the increasing demand for macromolecules in neurons. There is, however, no experimental evidence for this hypothesis to date. In the present study, we investigated the following quantitatively: (1) the size of the brain and each ganglion, (2) the size of identified neurons, (3) the total number of neurons undergoing DNA endoreplication, (4) the total number of the neurons containing a cardioexcitatory peptide, and (5) the gene expression level per neuron, using terrestrial slugs whose body growth was regulated through the amount of food supplied in the laboratory. The body growth was accompanied by increases in the sizes of both neurons and ganglia and triggered more frequent DNA endoreplication events in each ganglion of the growth-promoted slugs, without increasing the total number of neurons. Increase in the neuronal size also involved the increase in the amount of transcripts expressed in a single neuron. This is the first quantitative evidence showing that the DNA endoreplication, neuronal size, and gene expression are increased concomitantly with body growth in adult mollusks.

Endoreplication: a molecular trick during animal neuron evolution

The occurrence of endoreplication has been repeatedly reported in many organisms, including protists, plants, worms, arthropods, molluscs, fishes, and mammals. As a general rule, cells possessing endoreplicated genomes are large-sized and highly metabolically active. Endoreplication has not been frequently reported in neuronal cells that are typically considered to be fully differentiated and non-dividing, and which normally contain a diploid genome. Despite this general statement, various papers indicate that giant neurons in molluscs, as well as supramedullary and hypothalamic magnocellular neurons in fishes, contain DNA amounts larger than 2C. In order to study this issue in greater detail here, we review the available data about endoreplication in invertebrate and vertebrate neurons, and discuss its possible functional significance. As a whole, endoreplication seems to be a sort of molecular trick used by neurons in response to the high functional demands that they experience during evolution.