BACKGROUND: Tubulin is a major substrate of the cytoplasmic class II histone 
deacetylase HDAC6. Inhibition of HDAC6 results in higher levels of acetylated 
tubulin and enhanced binding of the motor protein kinesin-1 to tubulin, which 
promotes transport of cargoes along microtubules. Microtubule-dependent 
intracellular trafficking may therefore be regulated by modulating the activity 
of HDAC6. We have shown previously that the neuromodulator serotonin increases 
mitochondrial movement in hippocampal neurons via the Akt-GSK3beta signaling 
pathway. Here, we demonstrate a role for HDAC6 in this signaling pathway.
METHODOLOGY/PRINCIPAL FINDINGS: We found that the presence of tubacin, a 
specific HDAC6 inhibitor, dramatically enhanced mitochondrial movement in 
hippocampal neurons, whereas niltubacin, an inactive tubacin analog, had no 
effect. Compared to control cultures, higher levels of acetylated tubulin were 
found in neurons treated with tubacin, and more kinesin-1 was associated with 
mitochondria isolated from these neurons. Inhibition of GSK3beta decreased 
cytoplasmic deacetylase activity and increased tubulin acetylation, whereas 
blockade of Akt, which phosphorylates and down-regulates GSK3beta, increased 
cytoplasmic deacetylase activity and decreased tubulin acetylation. 
Concordantly, the administration of 5-HT, 8-OH-DPAT (a specific 5-HT1A receptor 
agonist), or fluoxetine (a 5-HT reuptake inhibitor) increased tubulin 
acetylation. GSK3beta was found to co-localize with HDAC6 in hippocampal 
neurons, and inhibition of GSK3beta resulted in decreased binding of antibody to 
phosphoserine-22, a potential GSK3beta phosphorylation site in HDAC6. GSK3beta 
may therefore regulate HDAC6 activity by phosphorylation.
CONCLUSIONS/SIGNIFICANCE: This study demonstrates that HDAC6 plays an important 
role in the modulation of mitochondrial transport. The link between HDAC6 and 
GSK3beta, established here, has important implications for our understanding of 
neurodegenerative disorders. In particular, abnormal mitochondrial transport, 
which has been observed in such disorders as Alzheimer's disease and Parkinson's 
disease, could result from the misregulation of HDAC6 by GSK3beta. HDAC6 may 
therefore constitute an attractive target in the treatment of these disorders.