TrkB and Protein Kinase M ζ Regulate Synaptic Localization of PSD-95 in Developing Cortex

Postsynaptic density 95 (PSD-95), the major scaffold at excitatory synapses, is critical for synapse maturation and learning. In rodents, eye opening, the onset of pattern vision, triggers a rapid movement of PSD-95 from visual neuron somata to synapses. We showed previously that the PI3 kinase-Akt pathway downstream of BDNF/TrkB signaling stimulates synaptic delivery of PSD-95 via vesicular transport. However, vesicular transport requires PSD-95 palmitoylation to attach it to a lipid membrane. Also, PSD-95 insertion at synapses is known to require this lipid modification. Here, we show that BDNF/TrkB signaling is also necessary for PSD-95 palmitoylation and its transport to synapses in mouse visual cortical layer 2/3 neurons. However, palmitoylation of PSD-95 requires the activation of another pathway downstream of BDNF/TrkB, namely, signaling through phospholipase C gamma and the brain-specific PKC variant protein kinase M zeta (PKM zeta). We find that PKM zeta selectively regulates phosphorylation of the palmitoylation enzyme ZDHHC8. Inhibition of PKM zeta results in a reduction of synaptic PSD-95 accumulation in vivo, which can be rescued by overexpressing ZDHHC8. Therefore, TrkB and PKM zeta, two critical regulators of synaptic plasticity, facilitate PSD-95 targeting to synapses. These results also indicate that palmitoylation can be regulated by a trophic factor. Our findings have implications for neurodevelopmental disorders as well as aging brains.