Tau protein, which binds to and stabilizes microtubules, is critical for neuronal survival and function. In the human brain, tau pre-mRNA splicing is regulated to maintain a delicate balance of exon 10-containing and exon 10-skipping isoforms. Splicing mutations affecting tau exon 10 alternative splicing lead to tauopathies, a group of neurodegenerative disorders including dementia. Molecular mechanisms regulating tau alternative splicing remain to be elucidated. In this study, we have developed an expression cloning strategy to identify splicing factors that stimulate tau exon 10 inclusion. Using this expression cloning approach, we have identified a previously unknown tau exon 10 splicing regulator, RBM4 (RNA binding motif protein 4). In cells transfected with a tau minigene, RBM4 overexpression leads to an increased inclusion of exon 10, whereas RBM4 down-regulation decreases exon 10 inclusion. The activity of RBM4 in stimulating tau exon 10 inclusion is abolished by mutations in its RNA-binding domain. A putative intronic splicing enhancer located in intron 10 of the tau gene is required for the splicing stimulatory activity of RBM4. Immunohistological analyses reveal that RBM4 is expressed in the human brain regions affected in tauopathy, including the hippocampus and frontal cortex. Our study demonstrates that RBM4 is involved in tau exon 10 alternative splicing. Our work also suggests that down-regulating tau exon 10 splicing activators, such as RBM4, may be of therapeutic potential in tauopathies involving excessive tau exon 10 inclusion.Microtubule-associated protein tau regulates the organization and stability of microtubules (MTs) 2 in the neurons. In humans, the Tau protein is encoded by a single gene on chromosome 17. The tau gene is expressed at a high level in neurons and at lower levels in glia and certain nonneuronal cells. Involved in maintaining cell morphology, axonal extension, and vesicle transport, Tau is critical for the formation and function of neurons (1-3) (for recent reviews, see . The expression of the tau gene is under complex regulation at multiple steps, including both post-transcriptional and post-translational levels. In the human brain, six tau isoforms are expressed as a result of alternative splicing of exons 2, 3, and 10 (10-12). Alternative splicing of exon 10 (Ex10), which encodes for one of the four MT-binding domains, gives rise to tau isoforms containing either four MT-binding repeats (Tau4R, Ex10+) or three * This work was supported by National Institutes of Health Grants AG17518, EY014576, and GM070967 (to J. Y. W.), the Society for Progressive Supranuclear Palsy, Muscular Dystrophy Association (to J. Y. W.), and by a scholar award from the Leukemia Society of America (to J. Y. W.).1To whom correspondence should be addressed: Northwestern University Feinberg School of Medicine, Center for Genetic Medicine, 303 E. Superior St., Chicago,; E-mail: jane-wu@northwestern.edu. 2 The abbreviations used are: MT, microtubule; Ex10, exon 10; FTDP-17, frontotemporal dem...