Intron-3 retention/splicing controls neuronal expression of apolipoprotein E in the CNS.
Neuronal expression of apolipoprotein (apo) E4 may contribute to the pathogenesis of Alzheimer's disease (AD). In studying how apoE expression is regulated in neurons, we identified a splicing variant of apoE mRNA with intron-3 retention (apoE-I3). ApoE-I3 mRNA was detected in neuronal cell lines and primary neurons, but not in astrocytic cell lines or primary astrocytes, from humans and mice by reverse transcription (RT)-PCR. In both wild-type and human apoE knock-in mice, apoE-I3 was found predominantly in cortical and hippocampal neurons by in situ hybridization. Cell fractionation and quantitative RT-PCR revealed that over 98% of the apoE-I3 mRNA was retained in the nucleus without protein translation. In transfected primary neurons, apoE expression increased dramatically when intron-3 was deleted from a genomic DNA construct and decreased markedly when intron-3 was inserted into a cDNA construct, suggesting that intron-3 retention/splicing controls apoE expression in neurons. In response to excitotoxic challenge, the apoE-I3 mRNA was markedly increased in morphologically normal hippocampal neurons but reduced in degenerating hippocampal neurons in mice; apoE mRNA showed the opposite pattern. This apparent precursor-product relationship between apoE-I3 and apoE mRNA was supported by a transcriptional inhibition study. Thus, neuronal expression of apoE is controlled by transcription of apoE-I3 under normal conditions and by processing of apoE-I3 into mature apoE mRNA in response to injury.
Xu
Qin
Q
Gladstone Institute of Neurological Disease, San Francisco, California 94158, USA.
Walker
David
D
Bernardo
Aubrey
A
Brodbeck
Jens
J
Balestra
Maureen E
ME
Huang
Yadong
Y
D000818:Animals; D001057:Apolipoproteins E / Q000096:biosynthesis* / Q000235:genetics* / Q000378:metabolism; D002460:Cell Line; D002540:Cerebral Cortex / Q000166:cytology / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000502:physiology*; D006624:Hippocampus / Q000166:cytology / Q000378:metabolism*; D006801:Humans; D017403:In Situ Hybridization; D007438:Introns / Q000502:physiology*; D051379:Mice; D008817:Mice, Mutant Strains; D009474:Neurons / Q000378:metabolism*; D012323:RNA Processing, Post-Transcriptional / Q000502:physiology; D012326:RNA Splicing / Q000502:physiology*
D001057:Apolipoproteins E