Modulation of long interspersed nuclear element-1 in the mouse hippocampus during maturation

Retrotransposition of long interspersed nuclear element-1 (L1-RTP) is proposed to contribute to central nervous system (CNS) plasticity by inducing mosaicism of neuronal cells. Clinical studies have identified increased L1 copy numbers in the brains of patients with psychiatric disorders. These observations implicate that L1-RTP is important for neurogenesis and that its deregulation represents a risk factor for mental disorders. However, no supportive evidence is available for understanding the importance of L1-RTP in CNS function.

To explore the physiological role of L1-RTP in CNS, we examined the L1 copy number during maturation. Interestingly, the L1 copy number increased after birth in the mouse hippocampus, but not the frontal lobe, with maximal copy numbers found in 8-week-old mice. This age-dependent L1 increase was abolished by administration of a reverse-transcriptase inhibitor, stavudine (d4T), which showed no toxic effects. Notably, the age-dependent L1 increase was attenuated by post-weaning social isolation (SI) stress, a well-known intervention for inducing psychiatric disorders in mice, or deletion of the NR2A gene that encodes a subunit of the glutamate receptor. Moreover, the negative effects of SI stress on L1-RTP were partially restored by environmental enrichment with voluntary running, but not by fluoxetine, a commonly used anti-psychiatric drug. Finally, behavioral experiments revealed that learning memory was defective in d4T-treated mice, which was similarly observed in mice raised under SI stress.

We detected the modulation of L1-RTP in the hippocampus during maturation of the CNS. In a recent study, we demonstrated that stimulants such as methamphetamine and cocaine were active in the induction of L1-RTP in neuronal cells, and previous studies have shown that NR2A-deficient mice are susceptible to mental abnormality. Herein, our data support the notion that the age-dependent modulation of L1-RTP is involved in genome differentiation in the hippocampus, and that modulation defects are linked to the development of psychiatric disorders.