König H, Matter N, Bader R, Thiele W, Müller F. Splicing Segregation: The Minor Spliceosome Acts outside the Nucleus and Controls Cell Proliferation. Cell 2007 131:718-29.
http://www.cell.com/content/article/abstract?uid=PIIS0092867407012792

Some eukaryotes have two splicing systems that remove introns from pre-mRNA. The predominant system, forming the U2 spliceosome, removes over 99% of introns while the minor system, forming the U12 spliceosome, removes the remaining introns. The major splicing system has long been known to be nuclear; the minor system has now been shown to be predominantly located in the cytoplasm.

The location of the snRNPs that direct splicing was determined by hybridization of labeled LNA probes with their RNA components, snRNA. Locations where RNA still contained introns were assayed by RT-PCR. A functional test of spliceosome activity was performed using Morpholino antisense oligos targeting snRNA of the U12 splicing system; the Morpholinos were conjugated with the nuclear export sequence peptide of HIV Rev. The NES-Morpholino conjugates inhibited minor spliceosome activity (assayed by RT-PCR across intron F of P120 pre-mRNA, normally removed by the U12 splicing system). The same Morpholino sequence coupled with a peptide having an altered NES sequence was far less efficient at blocking P120 intron F splicing.

The endogenous P120 transcript was used to test splicing in cells arrested in mitosis. The introns normally removed by the U2 spliceosome were not removed as efficiently as in cycling cells but intron F was removed, showing that while U2 splicing is downregulated during mitosis the U12 splicing continues.

In zebrafish embryos, interfering with the U2 splicing system halted development before gastrulation, blocking epiboly. Interfering with U12 splicing did not suppress epiboly or early gastrulation; developmental defects appeared during mid-somitogenesis and culminated with “the entire embryo started to disintegrate at about 30 hpf”. Similar phenotype resulted from targeting several minor-spliceosome snRNAs, so the phenotype was very likely to be due to interference with U12 splicing and not due to an off-target oligo effect. In embryos injected with Morpholinos targeting the U12 system snRNAs, entry into S phase of the cell cycle was inhibited starting at the 1-somite stage, followed by increased apoptosis in mid-somitogenesis. These results indicate that minor-class splicing is required for cell-cycle progression in somitogenesis and in its absence increased apoptosis ensues.

The authors suggest that the minor splicing system might play a role in the control of gene expression, particularly during mitosis but also in other situations in which cytoplasmic splicing has been reported, such as in rat neuronal dendrites or in human anucleate platelets. With this paper, techniques have arrived to effectively probe the functions of the minor spliceosome in varied biological systems.