Telomere repeat-binding factor 2 (TRF2) is critical for telomere integrity in dividing stem and somatic cells, but its role in postmitotic neurons is unknown. Apart from protecting telomeres, nuclear TRF2 interacts with the master neuronal gene-silencer repressor element 1-silencing transcription factor (REST), and disruption of this interaction induces neuronal differentiation. Here we report a developmental switch from the expression of TRF2 in proliferating neural progenitor cells to expression of a unique short nontelomeric isoform of TRF2 (TRF2-S) as neurons establish a fully differentiated state. Unlike nuclear TRF2, which enhances REST-mediated gene repression, TRF2-S is located in the cytoplasm where it sequesters REST, thereby maintaining the expression of neuronal genes, including those encoding glutamate receptors, cell adhesion, and neurofilament proteins. In neurons, TRF2-S-mediated antagonism of REST nuclear activity is greatly attenuated by either overexpression of TRF2 or administration of the excitatory amino acid kainic acid. Overexpression of TRF2-S rescues kainic acid-induced REST nuclear accumulation and its gene-silencing effects. Thus, TRF2-S acts as part of a unique developmentally regulated molecular switch that plays critical roles in the maintenance and plasticity of neurons. . REST binds to a conserved 23-bp motif, repressor element 1 (RE1), of numerous neuronal genes (8, 9). During the early stages of neurogenesis, REST is greatly reduced by ubiquitin/proteasome degradation, resulting in derepression of genetic program for the neuronal phenotype (1, 2). Although it is unclear whether REST is present and functional in postmitotic neurons, emerging evidence suggests that REST dysfunction in adult neurons contributes to several neurological disorders, including cerebral ischemia (10) and epilepsy (11-13). In addition, Zuccato and colleagues demonstrated that a wild-type huntingtin protein inhibits the repressive effect of REST by sequestering it in the cytoplasm, whereas mutant huntingtin causes an aberrant nuclear accumulation of REST (14,15). Because huntingtin is also expressed in nonneuronal cells, there is an ongoing search for factors that may regulate the functionality of REST, particularly in mature neurons in the adult brain (4, 16).Alternative pre-mRNA splicing is a major contributor to proteomic diversity during development. From a single pre-mRNA species, multiple mRNA isoforms are generated by the shuffling of exons at intron-exon boundaries (17,18). By containing or excluding certain pre-mRNA sequences, different splicing isoforms of an encoded protein may exhibit distinct subcellular localizations, protein-protein association, and/or enzymatic activities. Recent studies indicate that nearly 95% of human multiexon genes undergo alternative splicing (17). Neurons, in particular, exhibit an unusually large number of functionally relevant alternative splicing events in which specific protein isoforms are produced to regulate a range of neuronal properties, including excitabil...