U4 and Alzheimer disease: The phosphopeptide and phosphosite analyses revealed that biological processes such as neurotransmission (e.g., glutamine synthetase, glutamate ionotropic receptor) and mRNA splicing (e.g., U4/U6.U5 tri-snRNP-associated protein 2), including a specific repressor of the MAPT/Tau exon 10 splicing (e.g., splicing factor U2AF 65 kDa subunit) are down-regulated in AD.