Abstract
BM88/Cend1 is a neuronal-lineage specific modulator with a pivotal role in coordination of cell cycle exit and differentiation of neuronal precursors. In the current study we identified the signal transduction scaffolding protein Ran-binding protein M (RanBPM) as a BM88/Cend1 binding partner and showed that BM88/Cend1, RanBPM and the dual specificity tyrosine-phosphorylation regulated kinase 1B (Dyrk1B) are expressed in mouse brain as well as in cultured embryonic cortical neurons while RanBPM can form complexes with either of the two other proteins. To elucidate a potential mechanism involving BM88/Cend1, RanBPM and Dyrk1B in cell cycle progression/exit, we transiently co-expressed these proteins in mouse neuroblastoma Neuro 2a cells. We found that the BM88/Cend1-dependent or Dyrk1B-dependent down-regulation of cyclin D1 is reversed following their functional interaction with RanBPM. More specifically, functional interaction of RanBPM with either BM88/Cend1 or Dyrk1B stabilizes cyclin D1 in the nucleus and promotes 5-bromo-2'-deoxyuridine (BrdU) incorporation as a measure of enhanced cell proliferation. However, the RanBPM-dependent Dyrk1B cytosolic retention and degradation is reverted in the presence of Cend1 resulting in cyclin D1 destabilization. Co-expression of RanBPM with either BM88/Cend1 or Dyrk1B also had a negative effect on Neuro 2a cell differentiation. Our results suggest that functional interactions between BM88/Cend1, RanBPM and Dyrk1B affect the balance between cellular proliferation and differentiation in Neuro 2a cells and indicate that a potentially similar mechanism may influence cell cycle progression/exit and differentiation of neuronal precursors.