Exonal elements and factors involved in the depolarization-induced alternative splicing of neurexin 2.

The neurexin genes (NRXN1, NRXN2, and NRXN3) encode polymorphic presynaptic proteins that are implicated in synaptic plasticity and memory processing. In rat brain neurons grown in culture, depolarization induces reversible, calcium-dependent, repression of NRXN2α exon 11 (E11) splicing. Using Neuro2a cells as a model, we explored E11 cis elements and trans-acting factors involved in alternative splicing of NRXN2α E11 pre-mRNA under basal and depolarization conditions. E11 mutation studies revealed two motifs, CTGCCTG (enhancer) and GCACCCA (suppressor) regulating NRXN2α E11 alternative splicing. Subsequent E11 RNA affinity pull-down experiments demonstrated heterogeneous nuclear ribonucleoprotein (hnRNP) K and hnRNP L binding to this exon. Under depolarization, the amount of E11-bound hnRNP L (but not of hnRNP K) increased, in parallel to NRXN2α E11 splicing repression. Depletion of hnRNP K or hnRNP L in the Neuro2a cells by specific siRNAs enhanced NRXN2α E11 splicing and ablated the depolarization-induced repression of this exon. In addition, depolarization suppressed whereas hnRNP K depletion enhanced NRXN2α expression. These results indicate a role for hnRNP K in regulation of NRXN2α expression and of hnRNP L in the activity-dependent alternative splicing of neurexins which may potentially govern trans-synaptic signaling required for memory processing.