Role of MIR-7 in neural development of the ascidian ciona intestinalis

MicroRNAs are small, non-coding RNAs of 21-23 nucleotides that regulate gene expression at post-transcriptional level1. miR-7 mature sequence is evolutionary conserved across Bilateria2. During embryo development miR-7 neural expression was reported in mouse and teleosts as well as in animals distantly related as annelids. The majority of studies involving this microRNA were focused on its role in cancer, where miR-7 seemed to be context-specific as evidence exists for both oncogenic and tumor suppressor roles3. The first in vivo studies were performed on fly and demonstrated that miR-7 mutant flies develop defects in sensory structures only under stressful conditions4. In human neural stem cells miR-7 overexpression induced an increase of synapsin gene expression in the derived neurons playing an important role in synaptic formation5. A miR-7 sequence, identical to human one, is also present in the ascidian Ciona intestinalis genome. We investigated miR-7 expression during C. intestinalis development by in situ hybridization with LNA probes. Mature transcripts were diffuse in early embryos from gastrula to neurula stage. In late tailbud stage expression was restricted into a population of mesenchyme cells of the trunk while in late larva miR-7 was present in tail peripheral neurons. miR-7 function was studied by injecting anti-miR-7 Peptide Nucleic Acids (PNAs) in unfertilized eggs. PNAs are synthetic pseudopeptides bearing the four nucleobases mimicking the antisense oligonucleotide basematching properties. The hatching ratio of control and injected samples was not affected by anti miR-7 PNA injections but morphological alterations were observed. Knockdown embryos displayed rounded trunk region, smaller than controls. In situ hybridization experiments revealed that although the nervous system was differentiated properly the expression of Ci-syn was drastically reduced in injected embryos. Synapsins are neuronal proteins involved in neurotransmitter release, neurite elongation and synapse formation6. Our first results are therefore consistent with what reported in human rising the hypothesis that neural role of miR-7 is conserved between vertebrates and ascidians. References 1. Ambros V. Nature 2004;431:350-5. 2. Prochnik SE, et al. Dev Genes Evol 2007;217:73-7. 3. Sanek NA, Young WS. microRNA 2012;1:11-8. 4. Li X ,et al. Cell 2009;137:273-82. 5. Liu J, et al. Stem Cell Rev Rep 2012;8:1129-37. 6. Candiani S, et al. BMC Evol Biol 2010;10:32.