Does MAPT have anything new to say? Discovery of novel nonaggregative Tau isoforms that are decreased in Alzheimer’s disease.

Introduction: Tauopathies, including Alzheimer's disease (AD), are a group of neurodegenerative disorders characterised by Tau hyperphosphorylation. Post-translational modifications of Tau such as phosphorylation and truncation have been demonstrated to be an essential step in the molecular pathogenesis of tauopathies. Alternative splicing is the process by which a single gene can produce multiple transcripts and, potentially, as many different proteins. Tau isoforms are generated by alternative splicing from a single gene, MAPT. In this work, we demonstrate the existence of a new, human- specific truncated form of Tau generated by intron 12 retention. Materials and methods: Intron-12-retaining MAPT transcripts were detected using qPCR with SH-SY5Y neuroblastoma human cells and confirmed in human brain samples. Results were validated on a database of human RNA-seq samples (363 samples from three brain regions). Functional assays were carried out after cloning the isoform on the eukaryotic and prokariotic expression vectors pSG5 and pRK172. Functional analysis evaluated sarkosyl solubility, heparin-induced self-aggregation, microtubule stabilisation and binding affinity and epitope phosphorylation; all of them evaluated by Western blot. Heparin-induced aggregation and microtubule stabilisation were also analysed by electron microscopy. Protein levels on human samples were analysed by Western blot on Alzheimer’s brain classified according to Braak stages I (n = 3), II (n = 6), III (n = 4), IV (n = 1), V (n = 10) and VI (n = 8), and non-demented control subjects (n = 10). Results and conclusions: In this work, we demonstrated the existence of a previously undescribed, human-specific truncated tau isoform generated by intron 12 retention. RNA transcripts retaining intron 12 were detected in SHSY5Y cells and, to a greater extent, in human brains, which were then further confirmed on a larger RNAseq public database. Functional analysis demonstrated that, while this new Tau isoform exhibits similar post-transcriptional modifications by phosphorylation and affinity for microtubule binding, it is less prone to aggregate than other Tau isoforms. Importantly, diminished protein levels of this new Tau isoform are found in Alzheimer's patients' brains with respect to non-demented control subjects, suggesting that the lack of this truncated isoform may play an important role in the pathology. Our results open up new research avenues focused on the exploration of MAPT alternative splicing and the striking characteristics of novel isoforms, that may help develop future therapies for Alzheimer’s disease and other tauopathies