Postsynaptic localization of PSD-95 is regulated by all three pathways downstream of TrkB signaling.

Akira Yoshii,Martha Constantine-Paton

doi:10.3389/fnsyn.2014.00006

Abstract

Brain-derived neurotrophic factor (BDNF) and its receptor TrkB regulate synaptic plasticity. TrkB triggers three downstream signaling pathways; Phosphatidylinositol 3-kinase (PI3K), Phospholipase Cγ (PLCγ) and Mitogen activated protein kinases/Extracellular signal-regulated kinases (MAPK/ERK). We previously showed two distinct mechanisms whereby BDNF-TrkB pathway controls trafficking of PSD-95, which is the major scaffold at excitatory synapses and is critical for synapse maturation. BDNF activates the PI3K-Akt pathway and regulates synaptic delivery of PSD-95 via vesicular transport (Yoshii and Constantine-Paton, 2007). BDNF-TrkB signaling also triggers PSD-95 palmitoylation and its transport to synapses through the phosphorylation of the palmitoylation enzyme ZDHHC8 by a protein kinase C (PKC; Yoshii etal., 2011). The second study used PKC inhibitors chelerythrine as well as a synthetic zeta inhibitory peptide (ZIP) which was originally designed to block the brain-specific PKC isoform protein kinase Mϖ (PKMϖ). However, recent studies raise concerns about specificity of ZIP. Here, we assessed the contribution of TrkB and its three downstream pathways to the synaptic distribution of endogenous PSD-95 in cultured neurons using chemical and genetic interventions. We confirmed that TrkB, PLC, and PI3K were critical for the postsynaptic distribution of PSD-95. Furthermore, suppression of MAPK/ERK also disrupted PSD-95 expression. Next, we examined the contribution of PKC. While both chelerythrine and ZIP suppressed the postsynaptic localization of PSD-95, RNA interference for PKMϖ did not have a significant effect. This result suggests that the ZIP peptide, widely used as the “specific” PKMϖ antagonist by many investigators may block a PKC variant other than PKMϖ such as PKCλ/ι. Our results indicate that TrkB regulates postsynaptic localization of PSD-95 through all three downstream pathways, but also recommend further work to identify other PKC variants that regulate palmitoylation and synaptic localization of PSD-95.

Highlights

IntroductionThe Brain-derived neurotrophic factor (BDNF) and its receptor TrkB are critical for maturation of both excitatory (Gorski et al, 2003; Wirth et al, 2003; Chakravarthy et al, 2006; Tanaka et al, 2008; Kaneko et al, 2012) and inhibitory neurons (Hanover et al, 1999; Huang et al, 1999)
The current results suggest that a protein kinase C (PKC) variant other than protein kinase Mζ (PKMζ) should be present in the developing visual cortex and is likely to phosphorylate the palmitoylation enzyme ZDHHC8
It is clear that the increases in PSD-95 at synapses are mediated by all three signaling pathways downstream of TrkB

Summary

Introduction

The Brain-derived neurotrophic factor (BDNF) and its receptor TrkB are critical for maturation of both excitatory (Gorski et al, 2003; Wirth et al, 2003; Chakravarthy et al, 2006; Tanaka et al, 2008; Kaneko et al, 2012) and inhibitory neurons (Hanover et al, 1999; Huang et al, 1999). In the visual pathway of rodents upon eye-opening, PSD-95, which is the major scaffolding protein at mature glutamate synapses (Yoshii and Constantine-Paton, 2007; Yoshii et al, 2011), is transported to young synaptic contacts by BDNF/TrkB. BDNF-TrkB signaling is necessary for PSD-95’s initial association with membranes

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