Spinophilin Facilitates Dephosphorylation of Doublecortin by PP1 to Mediate Microtubule Bundling at the Axonal Wrist

Abstract

The axonal shafts of neurons contain bundled microtubules, whereas extending growth cones contain unbundled microtubule filaments, suggesting that localized activation of microtubule-associated proteins (MAP) at the transition zone may bundle these filaments during axonal growth. Dephosphorylation is thought to lead to MAP activation, but specific molecular pathways have remained elusive. We find that Spinophilin, a Protein-phosphatase 1 (PP1) targeting protein, is responsible for the dephosphorylation of the MAP Doublecortin (Dcx) Ser 297 selectively at the "wrist" of growing axons, leading to activation. Loss of activity at the "wrist" is evident as an impaired microtubule cytoskeleton along the shaft. These findings suggest that spatially restricted adaptor-specific MAP reactivation through dephosphorylation is important in organization of the neuronal cytoskeleton.

Phosphorylation of Doublecortin by Protein Kinase A Orchestrates Microtubule and Actin Dynamics to Promote Neuronal Progenitor Cell Migration

“…Phosphorylation of DCX at Ser-297 by Cdk5 decreased the binding affinity of DCX for microtubules in the neurite shaft, and this phosphorylation was canceled by spinophilin-protein phosphatase1 (PP1) complex. Dephosphorylation at Ser-297 is necessary for axon growth (56). We noted that overexpression of DCX-WT, DCX-S47A, and DCX-S47R caused the extension of neurites rather than of lamellipodia (Fig.…”

Antagonistic Effects of Doublecortin and MARK2/Par-1 in the Developing Cerebral Cortex

“…DCX is a substrate of multiple kinases, including Cdk5 (Tanaka et al, 2004b), JNK (Gdalyahu et al, 2004) and MARK2 (Schaar et al, 2004), which are all involved in regulation of neuronal migration. Of particular interest is the role of protein phosphatase 1 (PP1) in centrosomal splitting, since this phosphatase is capable of dephosphorylating DCX in a site-specific manner (Shmueli et al, 2006;Bielas et al, 2007). DCX and MARK2 are capable of mediating a cross talk between the microtubule and actin cytoskeletons (Tsukada et al, 2003(Tsukada et al, , 2005Matenia et al, 2005;Johne et al, 2008).…”

Spinophilin acts as a tumor suppressor by regulating Rb phosphorylation

“…13,16,17 SPN enhances PP1-mediated dephosphorylation of Ser297 in doublecortin 18 and its localization to the cytosol. 19,20 One of the most important PP1 target proteins is Rb, the phosphorylated product of the retinoblastoma gene that is essential in cell cycle regulation. 21 The targeting of Rb by PP1 contributes to the dephosphorylation and subsequent activation of Rb, shutting down the G 1 /S phases of the cell cycle.…”