DNA Endoreplication in the Brain Neurons during Body Growth of an Adult Slug

Abstract

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.

The microRNA bantam regulates a developmental transition in epithelial cells that restricts sensory dendrite growth

“…In some contexts, programmed polyploidy facilitates neuronal growth. For example, in the terrestrial slug Limax valentianus endoreplication occurs throughout the nervous system in proportion to animal growth, presumably to facilitate neuron expansion (Yamagishi et al, 2011). Neuronal polyploidy has been documented in vertebrates as well; tetraploid neurons exist in the retina and cortex (López-Sánchez and Frade, 2013;Morillo et al, 2010), and Purkinje neurons may be polyploid (Lapham, 1968;Mann et al, 1978).…”

p27Kip1participates in the regulation of endoreplication in differentiating chick retinal ganglion cells

“…37 In contrast, neurons with an elevated degree of ploidy have been observed both in invertebrates and in lower vertebrates. 33,34 Polyploidization in these latter neurons takes place through successive endoreplicative cycles that seem to be abolished in higher vertebrates, in which neurons are maintained with 4C DNA content. Our results suggest that p27 Kip1 , a CKI known to prevent Cdk2 activity and block G1/S transition, 3,9,58 is likely involved in the mechanism that prevents tetraploid neurons to undergo successive rounds of endoreplication in the chick retina.…”

“…[29][30][31][32] Somatic polyploid neurons generated by endoreplication are very common in the nervous system of lower vertebrates and invertebrates. 33,34 For instance, the nucleus of giant neurons from Aplysia californica has been shown to contain 200,000-fold the normal amount of haploid DNA (i.e., 200,000C). 33 These neurons have routinely been subjected to electrophysiological analyses, 35 proving that they are fully functional.…”

Target innervation is necessary for neuronal polyploidization in the terrestrial slugLimax

“…In situ hybridization of Limax nitric oxide synthase 2 (lim-NOS2), vesicular glutamate transporter (vGluT), light-yellow cell prepropeptide (LYCp), and achatin-I was performed as described previously (Matsuo and Ito, 2009;Yamagishi et al, 2011). The images of the sections were obtained with a BX-51 light microscope, equipped with a DP70 CCD camera and an objective (43, NA 0.16).…”

“…We have recently reported that these brain neurons are generated as a result of repeated whole genome duplication (polyploidization) in a body growth-dependent manner using the gastropod slug Limax valentianus (Yamagishi et al, 2011Matsuo et al, 2012). However, it remains unclear what induces genome duplication in neurons when the slug's body grows, that is, what aspects of the body's growth are detected by neurons.…”