Synaptic Regulation of Microtubule Dynamics in Dendritic Spines by Calcium, F-Actin, and Drebrin

Abstract

Dendritic spines are actin-rich compartments that protrude from the microtubule-rich dendritic shafts of principal neurons. Spines contain receptors and postsynaptic machinery for receiving the majority of glutamatergic inputs. Recent studies have shown that microtubules polymerize from dendritic shafts into spines and that signaling through synaptic NMDA receptors regulates this process. However, the mechanisms regulating microtubule dynamics in dendrites and spines remain unclear. Here we show that in hippocampal neurons from male and female mice, the majority of microtubules enter spines from highly localized sites at the base of spines. These entries occur in response to synapse-specific calcium transients that promote microtubule entry into active spines. We further document that spine calcium transients promote local actin polymerization, and that F-actin is both necessary and sufficient for microtubule entry. Finally, we show that drebrin, a protein known to mediate interactions between F-actin and microtubules, acts as a positive regulator of microtubule entry into spines. Together these results establish for the first time the essential mechanisms regulating microtubule entry into spines and contribute importantly to our understanding of the role of microtubules in synaptic function and plasticity.

The role of the drebrin/EB3/Cdk5 pathway in dendritic spine plasticity, implications for Alzheimer’s disease

“…These effects of drebrin on spine activity are dependent on F-actin binding (Ivanov et al, 2009b). Over-expression of drebrin in mature neurons in culture produces dendritic spines with abnormally long necks Ivanov et al, 2009b;Merriam et al, 2013) consistent with the idea that drebrin stabilises F-actin in the spine neck.…”

“…Dynamic microtubules in dendrite shafts can transiently enter dendritic spines by polymerizing from their plus-ends, the end at which tubulin assembly occurs preferentially (Gu et al 2008;Hu et al 2008;Jaworski et al 2009;Wagner et al, 2011;Merriam et al, 2013). The frequency of invasion is low, with approximately 1% of dendritic protrusions containing a microtubule at any one time (Hu et al 2008) and the dwell time is in the order of minutes but, importantly, dwell time and the frequency of invasion are enhanced by pharmacological interventions that mimic LTP (Gu et al 2008, Hu et al 2008Mitsuyama et al 2008;Jaworski et al 2009;Merriam et al 2011) and supressed by eliciting LTD (Kapitein et al 2011).…”

“…Exposure of dendritic spines to BDNF induces an increase of the post-synaptic density protein PSD-95 in the PSD along with spine maturation and this has also been shown to depend on invasion of dendritic spines by microtubules . Importantly, the frequency of microtubule invasion into dendritic spines and the number of spines invaded is enhanced by over-expression of drebrin and, conversely, reduced by siRNA-mediated knock-down of drebrin (Merriam et al, 2013). Whether drebrin and EB3 are acting independently of each other or are interacting in the drebrin/EB3 pathway in spine maturation has yet to be determined.…”

“…Future studies will also have to address the question of whether or not Aβ can also induce drebrin proteolysis. In contrast, brief and transient activation of NMDA receptors in vitro has been reported to accompany a transient shift of drebrin A from dendritic spines to dendritic shafts followed by a re-entry into spines without changes in overall drebrin levels (Sekino et al, 2006;Bosch and Hayashi, 2012;Merriam et al, 2013;Mizui et al, 2014). LTP in vivo is also associated with an influx of drebrin into the spine (Fukazawa et al, 2003;Bosch et al, 2014).…”

“…Thus, the drebrin/EB3 interaction cross-links dynamic microtubules to F-actin in filopodia and this likely forms part of the cellular mechanism underlying the response to axon growth and growth cone guidance cues (Cammarata et al, 2016). The drebrin/EB3/Cdk5 pathway has been independently confirmed in other cellular contexts where dynamic microtubules interacting with F-actin underlies cell shape changes, suggesting that the pathway is canonical (Bazelliéres et al, 2012;Merriam et al, 2013;Ketschek et al, 2016).…”