Germ Line-Specific Expression of Intracisternal A-Particle Retrotransposons in Transgenic Mice

Abstract

Intracisternal A-particle (IAP) sequences are endogenous retrovirus-like mobile elements, or retrotrans-posons, present at 1,000 copies in the mouse genome. These elements transpose in a replicative manner via an RNA intermediate and its reverse transcription, and their transposition should therefore be tightly controlled by their transcription level. To analyze the in vivo pattern of expression of these retrovirus-like elements, we constructed several independent transgenic mice with either a complete IAP element marked with an intron or with the IAP promoter, or long terminal repeat (LTR), alone controlling the expression of a lacZ reporter gene with a nuclear localization signal. For all transgenic lines analyzed, IAP expression as determined by reverse transcription-PCR analysis was found to be essentially restricted to the male germ line. Furthermore, in situ 5-bromo-4-chloro-3-indolyl-␤-D-galactopyranoside (X-Gal) staining of all organs disclosed specific ␤-galacto-sidase-positive blue cells only within the testis, found as patches along the seminiferous tubules and often organized as assemblies of 2, 4, 8, or 16 cells. Histochemical analyses of tissues from 13.5-day-old embryos to adults demonstrated that this LTR activity is restricted to gonocytes and premeiotic undifferentiated sper-matogonia. Finally, analysis of the methylation status of both transgenes and endogenous IAP LTRs demon-strated identical patterns, with methylation in somatic tissues and hypomethylation in the testis. Transgenic mice therefore reveal an intrinsic, highly restricted IAP expression which had escaped detection in previous global Northern (RNA) blot analyses and with possible strong biological relevance, as IAP activation specif-ically within the germ line might be a way to generate diversity at the evolutionary level without being deleterious to individuals. Intracisternal A-particle (IAP) sequences are moderately reiterated transposable elements (approximately 1,000 copies in the mouse genome) which are closely related to retroviruses and transpose via the reverse transcription of an RNA inter-mediate (23, 30). They possess 5Ј and 3Ј long terminal repeats (LTRs) flanking gag-pol open reading frames (30, 42) and encode particles which are intracellular and not infectious. Several classes of IAP elements have been identified, and they differ essentially by internal deletions of various lengths and/or one specific insertion (36, 51). IAPs most probably derive from an ancient retrovirus that invaded the mouse genome and has now reached a status and life cycle compatible with, and pos-sibly beneficial to, the host, as observed in several host-parasite associations and as extensively characterized for Drosophila species (reviewed in reference 38).