Spine neck plasticity regulates compartmentalization of synapses

Abstract

Dendritic spines have been proposed to transform synaptic signals through chemical and electrical compartmentalization. However, the quantitative contribution of spine morphology to synapse compartmentalization and its dynamic regulation are still poorly understood.We used time-lapse superresolution STED imaging in combination with FRAP measurements, 2-photon glutamate uncaging, electrophysiology and simulations to investigate the dynamic link between nanoscale anatomy and compartmentalization in live spines of CA1 neurons in mouse brain slices.We report a diversity of spine morphologies that argues against common categorization schemes, and establish a close link between compartmentalization and spine morphology, where spine neck width is the most critical morphological parameter. We demonstrate that spine necks are plastic structures that become wider and shorter after LTP. These morphological changes are predicted to lead to a substantial drop in spine head EPSP, while leaving overall biochemical compartmentalization preserved. 3 Dendritic spines form the postsynaptic component of most excitatory synapses, whose plasticity is essential for brain development and higher brain functions 1,2 . In addition to the molecular composition of the synapse, the morphology of spines is thought to be critical for synaptic function, as spine head size correlates with synaptic strength 3,4 and undergoes changes during synaptic plasticity [5][6][7][8] . Even so, our understanding of how spine structure shapes synapse function remains fragmented.It is well established that spines compartmentalize biochemical signals 9 . By contrast, the quantitative contribution of spine morphology to compartmentalization is still unknown, and only moderate correlations between spine neck length or head volume and chemical diffusion have been reported [9][10][11] . It is an open question to what extent biochemical compartmentalization is determined primarily by spine geometry or intracellular factors such as organelles or protein assemblies.Concerning electrical compartmentalization, it is not clear how electrical signals are transformed by the spine neck 9,[12][13][14] . This is an important question because synaptic strength may be adjusted through structural changes in spine necks, which has been a long-standing hypothesis 15,16 . An early electron microscopy study reported that the average spine head becomes larger and the neck wider and shorter after the induction of long-term plasticity (LTP) 17 , which was corroborated more recently by work based on 2-photon microscopy 6,18,19 . However, it is not known how these structural changes might affect biochemical and electrical compartmentalization, because 2-photon microscopy does not have sufficient spatial resolution to properly resolve spines and electron microscopy cannot be combined with functional assays. 4 Here, we combined stimulated emission depletion (STED) microscopy, fluorescence recovery after photo-bleaching (FRAP) experiments, 2-photon glutamate uncaging, and patch-clam...

Stimulated Emission Depletion (STED) Microscopy Reveals Nanoscale Defects in the Developmental Trajectory of Dendritic Spine Morphogenesis in a Mouse Model of Fragile X Syndrome

“…*p Ͻ 0.05; **p Ͻ 0.01. Oh et al, 2013). In addition, recent studies show that spine necks exert a strong influence on electrochemical compartmentalization of dendritic spines (Harnett et al, 2012;Tonnesen et al, 2014).…”

“…Moreover, we cannot exclude the possibility that the loss of FMRP leads to defects in the specificity of synaptic connections or affects dynamic aspects of the structure-function relationship of synapses, such as activity-dependent structural plasticity. In support of this possibility, studies have reported a change in spine turnover in Fmr1KO mice (Cruz-Martin, 2010;Pan et al, 2010).…”

“…For example, while Galvez and Greenough (2005) found an increase in spine length for L5 pyramidal cells at P73-P76, no difference was observed at P25. Similarly, no differences in spine length were seen at 1 month (Pan et al, 2010) or at P21 (Nimchinsky et al, 2001). That said, in direct contrast to our findings, Nimchinsky et al (2001) found an increase in spine length at P14.…”

Morphological plasticity of astroglia: Understanding synaptic microenvironment

“…Although such evidence has been essential in advancing our understanding of astroglia‐neuron communication, it leaves unresolved some critical questions regarding molecular physiology of the synaptic microenvironment, and in particular, local astroglial protrusions, on the nanoscale. Single‐molecule tracking of mGluRs and glutamate transporters in cultured astroglia has provided first glimpses on what could be the dynamic molecular micro‐organization of astrocytes (Arizono et al, 2012; Murphy‐Royal et al, 2015; Shrivastava et al, 2013), and STED imaging in organised brain tissue (Tonnesen et al, 2014) has opened up the nanoscopic world of live astroglial architecture in situ . Clearly, further implementation of super‐resolution techniques in organised brain tissue should help to bridge the knowledge gap in our understanding of the molecular micro‐physiology of astroglia and its role in information processing of brain networks.…”

“…The STED methodology has successfully been used to monitor the fine structure of dendritic spines (Bethge et al, 2013; Ding et al, 2009; Tonnesen et al, 2014, 2011), and it has recently been applied to image astroglia (Panatier et al, 2014) (Fig. 5A).…”