A candidate chimeric mammalian mRNA transcript is derived from distinct chromosomes and is associated with nonconsensus splice junction motifs.

The process of creating chimeric mRNA transcripts derived from separately transcribed genes via known spliceosome mechanisms is termed trans-splicing, and has been primarily described in lower eukaryotes. Isolation of cDNA clones containing sequences from distinct genes has raised the possibility of trans-splicing across distinct genes in mammalian systems; however, the possibility of cloning artifacts or splicing via nonspliceosome mechanisms has been difficult to rule out. In most cases, the absence of corresponding genomic clones has limited assessment of splice donor and acceptance sites and associated intronic elements that would be expected to participate in spliceosome-based reactions. We have previously reported a cDNA clone encoding the rat Leukocyte Common Antigen-Related (LAR) tyrosine phosphatase receptor that contains an alternative 3' UTR. In the present study Northern, RT-PCR and RNase protection assays verified the existence of developmentally regulated 3' UTR alternative splicing of LAR transcripts in vivo. FISH and radiation hybrid mapping demonstrated that loci encoding LAR and its alternative 3' UTR are present on distinct chromosomes, raising the possibility that alternatively spliced transcripts resulted from trans-splicing. Exon/intron analysis of corresponding genomic clones revealed nonconsensus splice junctions along with elements known to promote both cis- and trans-splicing. Verification in a mammalian in vivo system of chimeric transcripts derived from distinct genes along with identification of atypical nonconsensus-associated genomic elements points to the novel possibilities of atypical spliceosome-based trans-splicing or nonconventional, nonspliceosome-based mechanisms leading to chimeric transcripts.

Modulation of Alzheimer-like synaptic and cholinergic deficits in transgenic mice by human apolipoprotein E depends on isoform, aging, and overexpression of amyloid beta peptides but not on plaque formation.

The most frequent human apolipoprotein (apo) E isoforms, E3 and E4, differentially affect Alzheimer's disease (AD) risk (E4 > E3) and age of onset (E4 < E3). Compared with apoE3, apoE4 promotes the cerebral deposition of amyloid beta (Abeta) peptides, which are derived from the amyloid precursor protein (APP) and play a central role in AD. However, it is uncertain whether Abeta deposition into plaques is the main mechanism by which apoE isoforms affect AD. We analyzed murine apoE-deficient transgenic mice expressing in their brains human APP (hAPP) and Abeta together with apoE3 or apoE4. Because cognitive decline in AD correlates better with decreases in synaptophysin-immunoreactive presynaptic terminals, choline acetyltransferase (ChAT) activity, and ChAT-positive fibers than with plaque load, we compared these parameters in hAPP/apoE3 and hAPP/apoE4 mice and singly transgenic controls at 6-7, 12-15, and 19-24 months of age. Brain aging in the context of high levels of nondeposited human Abeta resulted in progressive synaptic/cholinergic deficits. ApoE3 delayed the synaptic deficits until old age, whereas apoE4 was not protective at any of the ages analyzed. Old hAPP/apoE4 mice had more plaques than old hAPP/apoE3 mice, but synaptic/cholinergic deficits preceded plaque formation in hAPP/apoE4 mice. Moreover, despite their different plaque loads, old hAPP/apoE4 and hAPP/apoE3 mice had comparable synaptic/cholinergic deficits, and these deficits were found not only in the hippocampus but also in the neocortex, which in most mice contained no plaques. Thus, apoE3, but not apoE4, delays age- and Abeta-dependent synaptic deficits through a plaque-independent mechanism. This difference could contribute to the differential effects of apoE isoforms on the risk and onset of AD.