Ribosome biogenesis underpins cell growth and division. Disruptions in ribosome biogenesis and translation initiation are deleterious to development and underlie a spectrum of diseases known collectively as ribosomopathies. Here, we describe a novel zebrafish mutant, titania (ttis450), which harbours a recessive lethal mutation in pwp2h, a gene encoding a protein component of the small subunit processome. The biochemical impacts of this lesion are decreased production of mature 18S rRNA molecules, activation of Tp53, and impaired ribosome biogenesis. In ttis450, the growth of the endodermal organs, eyes, brain, and craniofacial structures is severely arrested and autophagy is up-regulated, allowing intestinal epithelial cells to evade cell death. Inhibiting autophagy in ttis450 larvae markedly reduces their lifespan. Somewhat surprisingly, autophagy induction in ttis450 larvae is independent of the state of the Tor pathway and proceeds unabated in Tp53-mutant larvae. These data demonstrate that autophagy is a survival mechanism invoked in response to ribosomal stress. This response may be of relevance to therapeutic strategies aimed at killing cancer cells by targeting ribosome biogenesis. In certain contexts, these treatments may promote autophagy and contribute to cancer cells evading cell death.

Minor class or U12-type splicing is a highly conserved process required to remove a minute fraction of introns from human premRNAs. Defects in this splicing pathway have recently been linked to human disease, including a severe developmental disorder encompassing brain and skeletal abnormalities known as Taybi-Linder syndrome or microcephalic osteodysplastic primordial dwarfism 1, and a hereditary intestinal polyposis condition, Peutz-Jeghers syndrome. Although a key mechanism for regulating gene expression, the impact of impaired U12-type splicing on the transcriptome is unknown. Here, we describe a unique zebrafish mutant, caliban (clbn), with arrested development of the digestive organs caused by an ethylnitrosourea-induced recessive lethal point mutation in the rnpc3 [RNA-binding region (RNP1, RRM) containing 3] gene. rnpc3 encodes the zebrafish ortholog of human RNPC3, also known as the U11/U12 di-snRNP 65-kDa protein, a unique component of the U12-type spliceosome. The biochemical impact of the mutation in clbn is the formation of aberrant U11-and U12-containing small nuclear ribonucleoproteins that impair the efficiency of U12-type splicing. Using RNA sequencing and microarrays, we show that multiple genes involved in various steps of mRNA processing, including transcription, splicing, and nuclear export are disrupted in clbn, either through intron retention or differential gene expression. Thus, clbn provides a useful and specific model of aberrant U12-type splicing in vivo. Analysis of its transcriptome reveals efficient mRNA processing as a critical process for the growth and proliferation of cells during vertebrate development.S plicing, the excision of introns from pre-mRNA, is an essential step in gene expression and a major source of complexity in the transcriptome (1). The process is catalyzed by highly dynamic complexes of small nuclear ribonucleoproteins (snRNPs) called spliceosomes (2). Not widely appreciated is the coexistence of two types of introns in most eukaryotic genomes. The vast majority, the major class or U2-type introns, are marked by GT and AG at their 5′ and 3′ ends, respectively. Minor class or U12-type introns, of which there are ∼700 in the human genome, were initially recognized by the presence of AT and AC in these positions, prompting their original name of AT-AC introns (3). However, we now know that most of these introns contain the same GT-AG termini found in U2-type introns (4, 5) and are instead distinguished from them by two highly conserved motifs: one adjacent to the 5′ splice site (ss) and one corresponding to the branch point sequence (BPS), close to the 3′ ss (6, 7). These introns also lack the 3′ polypyrimidine tract characteristic of U2-type introns. Minor class introns are excised by U12-type spliceosomes, which are analogous in function and similar in composition to U2-type spliceosomes; each comprise five small nuclear RNAs (snRNAs) and hundreds of associated proteins (6). Although the U5 snRNA is shared between the two complexes, the U12-type spliceosome con...

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