We also determined the spatial and temporal expression pattern of mOPA1 in the developing mouse retina, which may provide fundamental clues to the in vivo function of OPA1 and hence help our understanding of the pathogenesis of OPA1-type ADOA. In our study, the earliest ocular expression of mOPA1 was detected at E15. Between E16 and E17, mOPA1 was found to be expressed very strongly in the prospective GCL of the retina and weakly in the lens
(Fig. 2) . No expression was detected in the developing photoreceptor layer, the pigmented epithelium, or other ocular tissues at these stages. Expression was also detected in the lung, liver, heart, skeletal muscle, and brain of the developing embryo. After birth, strong mOPA1 expression continued in the GCL of the retina and in the optic nerve until P2 and then declined sharply
(Fig. 3) . Between P0 and P2, mOPA1 was localized both within the GCL and the optic fiber layer of the retina and the optic nerve. Between P4 and P12, although mOPA1 levels decreased in the GCL of the retina, its expression domain was found to spread to the IPL and INL. Beyond this stage, mOPA1 was expressed at a weak level in the retina and optic nerve throughout life. However, we detected a high level of mOPA1 expression in the adult mouse liver, and Misaka et al.
20 have reported high levels of
mOPA1 transcripts in the adult mouse brain, heart, lung, liver, kidney, ovary, and skeletal muscle. Thus, the downregulation in the mOPA1 signal that we observed in the eye in the postnatal stages is in contrast to its continued high level of expression in other body tissues. This may explain why the pathologic effects of ADOA are seen only in the eye, despite the ubiquitous presence of OPA1 in almost all body tissues. It may be that the sharp decline in mOPA1 expression beyond P2 merely reflects a natural reduction in the number of RGCs by apoptosis.
35 However, the fact that OPA1 expression gradually increases before birth and peaks shortly after birth suggests another role for OPA1. In the mouse, RGCs are generated between E11 and E18, and the latter age coincides with the onset of naturally occurring RGC death.
36 After E12, the RGCs extend their axons along defined paths and establish topographically ordered connections in the central nervous system. The first RGC axons reach the optic chiasm at E12 and their most distal target in the midbrain, the superior colliculus, at E14 but additional RGC axons continue to grow throughout the optic tract and into the superior colliculus until birth.
37 38 By P14, the mature retinocollicular topographic map is normally established.
37 The appearance of OPA1 after E14 suggests that it may play a role in RGC axon growth, pathfinding, and/or target innervation.