Recent transcriptome analysis indicates that >90% of human genes undergoes alternative splicing, underscoring the contribution of differential RNA processing to diverse proteomes in higher eukaryotic cells. The polypyrimidine tract binding protein PTB is a well-characterized splicing repressor, but PTB knockdown causes both exon inclusion and skipping. Genome-wide mapping of PTB-RNA interactions and construction of a functional RNA map now revealed that dominant PTB binding near a competing constitutive splice site generally induces exon inclusion whereas prevalent binding close to an alternative site often causes exon skipping. This positional effect was further demonstrated by disrupting or creating a PTB binding site on minigene constructs and testing their responses to PTB knockdown or overexpression. These findings suggest a mechanism for PTB to modulate splice site competition to produce opposite functional consequences, which may be generally applicable to RNA binding splicing factors to positively or negatively regulate alternative splicing in mammalian cells.

Summary The induction of pluripotency or trans-differentiation of one cell type to another can be accomplished with cell lineage-specific transcription factors. Here we report that repression of a single RNA binding protein PTB, which occurs during normal brain development via the action of miR-124, is sufficient to induce trans-differentiation of fibroblasts into functional neurons. Besides its traditional role in regulated splicing, we show that PTB has a previously undocumented function in the regulation of microRNA functions, suppressing or enhancing microRNA targeting by competitive binding on target mRNA or altering local RNA secondary structure. A key event during neuronal induction is the relief of PTB-mediated blockage of microRNA action on multiple components of the REST complex, thereby de-repressing a large array of neuronal genes, including miR-124 and multiple neuronal-specific transcription factors, in non-neuronal cells. This converts a negative feedback loop to a positive one to elicit cellular reprogramming to the neuronal lineage.

MicroRNAs are well known to mediate translational repression and mRNA degradation in the cytoplasm. Various microRNAs have also been detected in membrane-compartmentalized organelles, but the functional significance has remained elusive. Here we report that miR-1, a microRNA specifically induced during myogenesis, efficiently enters the mitochondria where it unexpectedly stimulates, rather than represses, the translation of specific mitochondrial genome-encoded transcripts. We show that this positive effect requires specific miR:mRNA base-pairing and Ago2, but not its functional partner GW182, which is excluded from the mitochondria. We provide evidence for the direct action of Ago2 in mitochondrial translation by CLIP-seq, functional rescue with mitochondria-targeted Ago2, and selective inhibition of the microRNA machinery in the cytoplasm. These findings unveil a positive function of microRNA in mitochondrial translation and suggest a highly coordinated myogenic program via miR-1 mediated translational stimulation in the mitochondria and repression in the cytoplasm.

SUMMARY Parkinson disease is characterized by loss of dopamine neurons in the substantia nigra 1 . Similar to other major neurodegenerative disorders, no disease-modifying treatment exists. While most treatment strategies aim to prevent neuronal loss or protect vulnerable neuronal circuits, a potential alternative is to replace lost neurons to reconstruct disrupted circuits 2 . Herein we report an efficient single-step conversion of isolated mouse and human astrocytes into functional neurons by depleting the RNA binding protein PTB. Applying this approach to the mouse brain, we demonstrate progressive conversion of astrocytes into new neurons that can innervate into endogenous neural circuits. Astrocytes in different brain regions are found to convert into different neuronal subtypes. Using a chemically induced model of Parkinson’s disease, we show conversion of midbrain astrocytes into dopaminergic neurons whose axons reconstruct the nigro-striatal circuit. Significantly, re-innervation of striatum is accompanied by restoration of dopamine levels and rescue of motor deficits. Similar disease phenotype reversal is also accomplished by converting astrocytes to neurons using antisense oligonucleotides to transiently suppress PTB. These findings identify a potentially powerful and clinically feasible new approach to treating neurodegeneration by replacing lost neurons.

Spermatogenesis in mammals is characterized by two waves of piRNA expression: one corresponds to classic piRNAs responsible for silencing retrotransponsons and the second wave is predominantly derived from nontransposon intergenic regions in pachytene spermatocytes, but the function of these pachytene piRNAs is largely unknown. Here, we report the involvement of pachytene piRNAs in instructing massive mRNA elimination in mouse elongating spermatids (ES). We demonstrate that a piRNA-induced silencing complex (pi-RISC) containing murine PIWI (MIWI) and deadenylase CAF1 is selectively assembled in ES, which is responsible for inducing mRNA deadenylation and decay via a mechanism that resembles the action of miRNAs in somatic cells. Such a highly orchestrated program appears to take full advantage of the enormous repertoire of diversified targeting capacity of pachytene piRNAs derived from nontransposon intergenic regions. These findings suggest that pachytene piRNAs are responsible for inactivating vast cellular programs in preparation for sperm production from ES.

The piRNA machinery is known for its role in mediating epigenetic silencing of transposons. Recent studies suggest that this function also involves piRNA-guided cleavage of transposon-derived transcripts. As many piRNAs also appear to have the capacity to target diverse mRNAs, this raises the intriguing possibility that piRNAs may act extensively as siRNAs to degrade specific mRNAs. To directly test this hypothesis, we compared mouse PIWI (MIWI)-associated piRNAs with experimentally identified cleaved mRNA fragments from mouse testes, and observed cleavage sites that predominantly occur at position 10 from the 5′ end of putative targeting piRNAs. We also noted strong biases for U and A residues at nucleotide positions 1 and 10, respectively, in both piRNAs and mRNA fragments, features that resemble the pattern of piRNA amplification by the 'ping-pong' cycle. Through mapping of MIWI-RNA interactions by CLIP-seq and gene expression profiling, we found that many potential piRNA-targeted mRNAs directly interact with MIWI and show elevated expression levels in the testes of Miwi catalytic mutant mice. Reporter-based assays further revealed the importance of base pairing between piRNAs and mRNA targets and the requirement for both the slicer activity and piRNA-loading ability of MIWI in piRNA-mediated target repression. Importantly, we demonstrated that proper turnover of certain key piRNA targets is essential for sperm formation. Together, these findings reveal the siRNA-like function of the piRNA machinery in mouse testes and its central requirement for male germ cell development and maturation.

SUMMARY Neurodegenerative diseases can occur so early as to affect neurodevelopment. From a cohort of over 2000 consanguineous families with childhood neurological disease, we identified a founder mutation in four independent pedigrees in cleavage and polyadenylation factor I subunit (CLP1). CLP1 is a multifunctional kinase implicated in tRNA, mRNA and siRNA maturation. Kinase activity of the CLP1 mutant protein was defective, and the tRNA endonuclease complex (TSEN) was destabilized, resulting in impaired pre-tRNA cleavage. Germline clp1 null zebrafish showed cerebellar neurodegeneration that was rescued by wild type but not mutant human CLP1 expression. Patient-derived induced neurons displayed both depletion of mature tRNAs and accumulation of unspliced pre-tRNAs. Transfection of partially processed tRNA fragments into patient cells exacerbated an oxidative stress-induced reduction in cell survival. Our data links tRNA maturation to neuronal development and neurodegeneration through defective CLP1 function in humans.

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