M. Shioya, S. Obayashi, H. Tabunoki, K. Arima, Y. Saito, T. Ishida and J. Satoh (2010) Neuropathology and Applied Neurobiology36, 320–330 Aberrant microRNA expression in the brains of neurodegenerative diseases: miR‐29a decreased in Alzheimer disease brains targets neurone navigator 3 Aims: MicroRNAs (miRNAs) are small non‐coding RNAs that regulate translational repression of target mRNAs. Accumulating evidence indicates that various miRNAs, expressed in a spatially and temporally controlled that manner in the brain plays a key role in neuronal development. However, at present, the pathological implication of aberrant miRNA expression in neurodegenerative events remains largely unknown. To identify miRNAs closely associated with neurodegeneration, we performed miRNA expression profiling of brain tissues of various neurodegenerative diseases. Methods: We initially studied the frontal cortex derived from three amyotrophic lateral sclerosis patients by using a microarray of 723 human miRNAs. This was followed by enlargement of study population with quantitative RT‐PCR analysis (n = 21). Results: By microarray analysis, we identified up‐regulation of miR‐29a, miR‐29b and miR‐338‐3p in amyotrophic lateral sclerosis brains, but due to a great interindividual variation, we could not validate these results by quantitative RT‐PCR. However, we found significant down‐regulation of miR‐29a in Alzheimer disease (AD) brains. The database search on TargetScan, PicTar and miRBase Target identified neurone navigator 3 (NAV3), a regulator of axon guidance, as a principal target of miR‐29a, and actually NAV3 mRNA levels were elevated in AD brains. MiR‐29a‐mediated down‐regulation of NAV3 was verified by the luciferase reporter assay. By immunohistochemistry, NAV3 expression was most evidently enhanced in degenerating pyramidal neurones in the cerebral cortex of AD. Conclusions: These observations suggest the hypothesis that underexpression of miR‐29a affects neurodegenerative processes by enhancing neuronal NAV3 expression in AD brains.

MicroRNAs (miRNAs) mediate posttranscriptional regulation of protein-coding genes by binding to the 3' untranslated region of target mRNAs, leading to translational inhibition, mRNA destabilization or degradation, depending on the degree of sequence complementarity. In general, a single miRNA concurrently downregulates hundreds of target mRNAs. Thus, miRNAs play a key role in fine-tuning of diverse cellular functions, such as development, differentiation, proliferation, apoptosis and metabolism. However, it remains to be fully elucidated whether a set of miRNA target genes regulated by an individual miRNA in the whole human microRNAome generally constitute the biological network of functionally-associated molecules or simply reflect a random set of functionally-independent genes. The complete set of human miRNAs was downloaded from miRBase Release 16. We explored target genes of individual miRNA by using the Diana-microT 3.0 target prediction program, and selected the genes with the miTG score ≧ 20 as the set of highly reliable targets. Then, Entrez Gene IDs of miRNA target genes were uploaded onto KeyMolnet, a tool for analyzing molecular interactions on the comprehensive knowledgebase by the neighboring network-search algorithm. The generated network, compared side by side with human canonical networks of the KeyMolnet library, composed of 430 pathways, 885 diseases, and 208 pathological events, enabled us to identify the canonical network with the most significant relevance to the extracted network. Among 1,223 human miRNAs examined, Diana-microT 3.0 predicted reliable targets from 273 miRNAs. Among them, KeyMolnet successfully extracted molecular networks from 232 miRNAs. The most relevant pathway is transcriptional regulation by transcription factors RB/E2F, the disease is adult T cell lymphoma/leukemia, and the pathological event is cancer. The predicted targets derived from approximately 20% of all human miRNAs constructed biologically meaningful molecular networks, supporting the view that a set of miRNA targets regulated by a single miRNA generally constitute the biological network of functionally-associated molecules in human cells.