Environmental influence on L1 retrotransposons in the adult hippocampus

Abstract

It is well established that neuronal circuits can be shaped by experience. Neuronal plasticity can be achieved by synaptic competitive interactions and the addition of new neuronal units in neurogenic regions of the adult brain. Recent data have suggested that neuronal progenitor cells can accommodate somatic LINE-1 (Long Interspersed Nuclear Elements-1 or L1) retrotransposition. Genomic L1 insertions may up- or down-regulate transcriptional control of gene expression. Here, we show that exercise has a positive effect on a L1-EGFP reporter in vivo. We found that neurons from mice that experience voluntary exercise are more likely to activate an EGFP reporter marker, representing L1 insertions in the brain, when compared with sedentary animals. In the hippocampus, a neurogenic region of the adult brain, EGFP expression is mainly found in cells localized in the subgranular layer of the dentate gyrus. This observation implies that neuronal progenitor cells may support de novo retrotransposition upon exposure to a new environment. Such evidence suggests that experience-dependent L1 retrotransposition may contribute to the physiological consequences of neuronal plasticity.

FIGURE 1

Neurogenesis in the adult hippocampus. Images of new neurons from sedentary (A, C) and running mice (B, D). New neurons were labeled by retrovirus-mediated expression of GFP (A, B) and BrdU incorporation (C, D). The number of Ki67+ cells (E, F) remains elevated in running mice (F) when compared with sedentary mice (E). Green = GFP (A, B), BrdU (C, D) and Ki67 (E, F). Blue = NeuN. Scale bar: 50 μm. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com].

FIGURE 2

Running promotes the survival of newborn neurons. Running significantly increases the total number of newborn cells labeled by retrovirus (A) and BrdU (B). In addition, the proliferative activity (Ki67+ cells) remains high in mice after more than 2 weeks of continuous access to running wheels (C). Cell numbers shown in the figure are from 1/6 of one hemisphere. GFP cells were counted from the right hemispheres, into which the retrovirus was injected. BrdU+ and Ki67+ cells were counted from the left hemispheres.

FIGURE 3

Detection of L1 retrotransposition. The structure of the L1-EGFP transgene is indicated at the top of the figure. The retrotransposition-competent human L1 contains a 5′ untranslated region (UTR) that harbors an internal promoter, two open reading frames (ORF1 and ORF2; not drawn to scale), and a 3′ UTR that ends in a poly (A) tail. The EGFP retrotransposition indicator cassette consists of a backward copy of the EGFP reporter gene whose expression is controlled by the human cytomegalovirus major immediate early promoter (pCMV) and the herpes simplex virus thymidine kinase polyadenylation sequence (pA). This arrangement ensures that EGFP expression will only become activated upon L1 retrotransposition. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com].

FIGURE 4

Somatic L1 retrotransposition in the adult hippocampus. EGFP expression in granular cells in the dentate gyrus of the hippocampus is increased in L1-EGFP transgenic mice previously exposed to voluntary exercise (B) when compared with sedentary animals (A). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com].

FIGURE 5

Quantification of EGFP-positive cells in the hippocampus. L1-EGFP transgenic animals housed with running wheels (runners) displayed 3-fold more granular cells expressing EGFP than nonrunners.