Spine Shrinkage Research Articles

A Hypothesized Role for Dendritic Remodeling in the Etiology of Mood and Anxiety Disorders

A Hypothesized Role for Dendritic Remodeling in the Etiology of Mood and Anxiety Disorders

Postsynaptic PDLIM5/Enigma Homolog binds SPAR and causes dendritic spine shrinkage

Dendritic spine morphology is thought to play important roles in synaptic development and plasticity, and morphological derangements in spines are correlated with several neurological disorders. Here, we identified an interaction between Spine-Associated RapGAP (SPAR), a postsynaptic protein that reorganizes actin cytoskeleton and drives dendritic spine head growth, and PDLIM5/Enigma Homolog (ENH), a PDZ-LIM (postsynaptic density-95/Discs large/zona occludens 1-Lin11/Isl-1/Mec3) family member. PDLIM5 has been implicated in susceptibility to bipolar disorder, major depression, and schizophrenia, but its function in neurological disease is poorly understood. We show that PDLIM5 is present in the postsynaptic density, where it promotes decreased dendritic spine head size and longer, filopodia-like morphology. Conversely, RNA interference against PDLIM5 or loss of PDLIM5 interaction with SPAR caused increased spine head diameter. Furthermore, PKC activation promoted delivery of PDLIM5 into dendritic spines and increased its spine colocalization with SPAR. These data reveal new postsynaptic functions for PDLIM5 in shrinkage of dendritic spines that may be relevant to its association with psychiatric illness.

Epac2 induces synapse remodeling and depression and its disease-associated forms alter spines

Dynamic remodeling of spiny synapses is crucial for cortical circuit development, refinement, and plasticity, while abnormal morphogenesis is associated with neuropsychiatric disorders. Here we show in cultured rat cortical neurons that activation of Epac2, a PKA-independent cAMP target and Rap guanine-nucleotide exchange factor (GEF), induces spine shrinkage, increases spine motility, removes synaptic GluR2/3-containing AMPA receptors, and depresses excitatory transmission, while its inhibition promotes spine enlargement and stabilization. Epac2 is required for dopamine D1-like receptor-dependent spine shrinkage and GluR2 removal from spines. Epac2 interaction with neuroligin promotes its membrane recruitment and enhances its GEF activity. Rare missense mutations in the EPAC2 gene, previously found in individuals with autism, affect basal and neuroligin-stimulated GEF activity, dendritic Rap signaling, synaptic protein distribution, and spine morphology. Thus, we identify a novel mechanism that promotes dynamic remodeling and depression of spiny synapses, whose mutations may contribute to some aspects of disease.

Principles of Long-Term Dynamics of Dendritic Spines

Long-term potentiation of synapse strength requires enlargement of dendritic spines on cerebral pyramidal neurons. Long-term depression is linked to spine shrinkage. Indeed, spines are dynamic structures: they form, change their shapes and volumes, or can disappear in the space of hours. Do all such changes result from synaptic activity, or do some changes result from intrinsic processes? How do enlargement and shrinkage of spines relate to elimination and generation of spines, and how do these processes contribute to the stationary distribution of spine volumes? To answer these questions, we recorded the volumes of many individual spines daily for several days using two-photon imaging of CA1 pyramidal neurons in cultured slices of rat hippocampus between postnatal days 17 and 23. With normal synaptic transmission, spines often changed volume or were created or eliminated, thereby showing activity-dependent plasticity. However, we found that spines changed volume even after we blocked synaptic activity, reflecting a native instability of these small structures over the long term. Such "intrinsic fluctuations" showed unique dependence on spine volume. A mathematical model constructed from these data and the theory of random fluctuations explains population behaviors of spines, such as rates of elimination and generation, stationary distribution of volumes, and the long-term persistence of large spines. Our study finds that generation and elimination of spines are more prevalent than previously believed, and spine volume shows significant correlation with its age and life expectancy. The population dynamics of spines also predict key psychological features of memory.

Synapse elimination accompanies functional plasticity in hippocampal neurons

A critical component of nervous system development is synapse elimination during early postnatal life, a process known to depend on neuronal activity. Changes in synaptic strength in the form of long-term potentiation (LTP) and long-term depression (LTD) correlate with dendritic spine enlargement or shrinkage, respectively, but whether LTD can lead to an actual separation of the synaptic structures when the spine shrinks or is lost remains unknown. Here, we addressed this issue by using concurrent imaging and electrophysiological recording of live synapses. Slices of rat hippocampus were cultured on multielectrode arrays, and the neurons were labeled with genes encoding red or green fluorescent proteins to visualize presynaptic and postsynaptic neuronal processes, respectively. LTD-inducing stimulation led to a reduction in the synaptic green and red colocalization, and, in many cases, it induced a complete separation of the presynaptic bouton from the dendritic spine. This type of synapse loss was associated with smaller initial spine size and greater synaptic depression but not spine shrinkage during LTD. All cases of synapse separation were observed without an accompanying loss of the spine during this period. We suggest that repeated low-frequency stimulation simultaneous with LTD induction is capable of restructuring synaptic contacts. Future work with this model will be able to provide critical insight into the molecular mechanisms of activity- and experience-dependent refinement of brain circuitry during development.

Independent Expression of Synaptic and Morphological Plasticity Associated with Long-Term Depression

Physiological and morphological alterations occur with long-term synaptic modifications, such as long-term potentiation (LTP) and long-term depression (LTD), but whether these two processes are independent or interactive is unclear. It is also unknown whether or how morphological modifications, like spine remodeling, may contribute to physiological modifications, such as trafficking of glutamate receptors which underlies, at least partially, the expression of LTP and LTD. In this study, we monitored spine size and synaptic responses simultaneously using combined two photon time-lapse imaging with patch-clamp recording in acute hippocampal slices. We show that spine shrinkage and LTD can occur independently of each other. We further show that changes in spine size are unrelated to trafficking of AMPA receptors (AMPARs) under various conditions: constitutive trafficking of AMPARs, insulin-induced internalization of AMPARs, or lateral movement of AMPARs to extrasynaptic sites. Induction of LTD of NMDA receptor-mediated responses (NMDAR-LTD) is associated with spine shrinkage. Nonetheless, NMDAR-LTD and spine shrinkage diverge in the downstream signaling events, and can occur independently of each other. Thus, spine shrinkage is not caused by or required for trafficking of glutamate receptors. In a broader sense, there is a clear dissociation between physiological and morphological expression of LTD. However, inhibition of actin depolymerization blocked the expression of LTD, suggesting that morphologically silent actin remodeling may be involved in the physiological expression of LTD and different subpopulations of actin filaments undergo changes during LTD.

A model of activity-dependent changes in dendritic spine density and spine structure

Recent evidence indicates that the morphology and density of dendritic spines are regulated during synaptic plasticity. See for instance a review by [1]. High-frequency stimuli that induce long-term potentiation (LTP) have been associated with increases in the number and size of spines. In contrast, low-frequency stimuli that induce long-term depression (LTD) are associated with decreases in the number and size of spines. Decreases in spine density also occur due to excitotoxicity associated with very high levels of activity such as during seizures. In this work, we extend previous modeling studies [2] by combining a model for activity-dependent spine density with one for calcium-mediated spine stem restructuring. The model is based on the standard dimensionless cable equation for the changes in membrane potential in a passive dendrite. An additional equation characterizes the activity-dependent changes in spine density along the dendrite. For this continuum model, a typical Hodgkin-Huxley type current balance equation represents the change in membrane potential in an isopotential compartment representing a spine head. Both the cable equation and the current balance equation rely on the spine stem current to represent current flow between the spines and the dendrite. The model also includes equations for activity-dependent changes in the calcium concentration in spines as well as changes in spine stem resistance that depend on the level of calcium in an individual spine. The calcium-mediated changes in spine density and spine stem resistance are based on a conceptual model proposed by Segal et al. [1] where low calcium concentrations lead to spine shrinkage and pruning, an increase in calcium concentration leads to spine elongation and formation of new spines, and significantly higher values cause spine shrinkage and pruning. We use computational studies to investigate the changes in spine density and structure for differing synaptic inputs and demonstrate the effects of these changes on the input-output properties of the dendritic branch. Moderate amounts of high-frequency synaptic activation to dendritic spines cause an increase in spine stem resistance, which is correlated with spine stem elongation. In addition, the spine density increases both inside and outside the input region. The model is formulated so that this LTP-inducing stimulus eventually leads to structural stability. In contrast, a prolonged low-frequency stimulation paradigm that would typically induce LTD results in a decrease in stem resistance (correlated with spine shortening) and an eventual decrease in spine density.

Double dissociation between long-term depression and dendritic spine morphology in cerebellar Purkinje cells

Experiments in hippocampal area CA1 suggest that long-term potentiation could be associated with spine addition and enlargement, and long-term depression (LTD) with spine shrinkage and loss. Is this a general principle of synaptic plasticity? We used two-photon microscopy to measure dendritic spines in rat cerebellar Purkinje cells. Neither local synaptic induction of LTD nor global chemical induction of LTD changed spine number or size. Conversely, a manipulation that evoked persistent dendritic spine retraction did not alter parallel fiber-evoked excitatory postsynaptic currents.

Phospholipase C Is Required for Changes in Postsynaptic Structure and Function Associated with NMDA Receptor-Dependent Long-Term Depression

NMDA receptor (NMDAR)-dependent hippocampal synaptic plasticity underlying learning and memory coordinately regulates dendritic spine structure and AMPA receptor (AMPAR) postsynaptic strength through poorly understood mechanisms. Induction of long-term depression (LTD) activates protein phosphatase 2B/calcineurin (CaN), leading to dendritic spine shrinkage through actin depolymerization and AMPAR depression through receptor dephosphorylation and internalization. The scaffold proteins A-kinase-anchoring protein 79/150 (AKAP79/150) and postsynaptic density 95 (PSD95) form a complex that controls the opposing actions of the cAMP-dependent protein kinase (PKA) and CaN in regulation of AMPAR phosphorylation. The AKAP79/150-PSD95 complex is disrupted in hippocampal neurons during LTD coincident with internalization of AMPARs, decreases in PSD95 levels, and loss of AKAP79/150 and PKA from spines. AKAP79/150 is targeted to spines through binding F-actin and the phospholipid phosphatidylinositol-(4,5)-bisphosphate (PIP2). Previous electrophysiological studies have demonstrated that inhibition of phospholipase C (PLC)-catalyzed hydrolysis of PIP2 inhibits NMDAR-dependent LTD; however, the signaling mechanisms that link PLC activation to alterations in dendritic spine structure and AMPAR function in LTD are unknown. We show here that NMDAR stimulation of PLC in cultured hippocampal neurons is necessary for AKAP79/150 loss from spines and depolymerization of spine actin. Importantly, we demonstrate that NMDAR activation of PLC is also necessary for decreases in spine PSD95 levels and AMPAR internalization. Thus, PLC signaling is required for structural and functional changes in spine actin, PSD scaffolding, and AMPAR trafficking underlying postsynaptic expression of LTD.

Shrinkage of Dendritic Spines Associated with Long-Term Depression of Hippocampal Synapses

Activity-induced modification of neuronal connections is essential for the development of the nervous system and may also underlie learning and memory functions of mature brain. Previous studies have shown an increase in dendritic spine density and/or enlargement of spines after the induction of long-term potentiation (LTP). Using two-photon time-lapse imaging of dendritic spines in acute hippocampal slices from neonatal rats, we found that the induction of long-term depression (LTD) by low-frequency stimulation is accompanied by a marked shrinkage of spines, which can be reversed by subsequent high-frequency stimulation that induces LTP. The spine shrinkage requires activation of NMDA receptors and calcineurin, similar to that for LTD. However, spine shrinkage is mediated by cofilin, but not by protein phosphatase 1 (PP1), which is essential for LTD, suggesting that different downstream pathways are involved in spine shrinkage and LTD. This activity-induced spine shrinkage may contribute to activity-dependent elimination of synaptic connections.

Lumbar spine mechanical response to axial compression load in vivo.

Lumbar spine kinematic response to a 1.0 body weight compressive load was measured in vivo by comparison of relaxed and loaded magnetic resonance image sets in the sagittal plane. To identify and measure acute response mechanisms of the lumbar spine during compression loading. The isolated ligamentous spine buckles under small loads (88 N); yet, the spine supports >10 times that load in daily activities. Mechanical function of the lumbar spine in vivo is not well understood, and only a few studies examined the spine during in vivo loading. Magnetic resonance imaging scans of subjects were taken while subjects were relaxed and while supporting a 1.0 body weight compressive load. Vertebral bodies and disc perimeters were digitized, and relative centroid positions were measured and compared between conditions. Lumbar rotation, bending, compression, and disc translation were determined. Two parameter ensembles were analyzed to describe mechanisms of "spine shrinkage" (decrease of projected spine length) and lumbosacral response. All subjects underwent spine shrinkage (-3.9 +/- 1.2 mm) dominated by cumulative bending, except in three subjects where the rotation component dominated. Levels L2-L4 extended, while L5 flexed, and dL2 through dL4 translated anterior, while dL5 translated posterior. Significant segmental deformations were as follows: L3 extension (-3.3 +/- 3.1 degrees ), dL5 disc translation (-1.4 +/- 1.4 mm), and posterior sacral rotation (3.2 +/- 4.7 degrees ). Spine shrinkage occurred mainly from spine bending and rotation, with only small contribution from spine compression (shortening along the spine curvature). Response pattern groupings indicated at least two unique subgroups, but the cause remains to be determined.

Stadiometry: on measurement technique to reduce variability in spine shrinkage measurement

Objective. To test the effect of two measurement techniques for repeated measures of spine height using stadiometry following five experimental activity conditions. Design. Six subjects were repeatedly measured while they stepped in and out of the stadiometer for each pair of measures and again on another day when they remained in place in the stadiometer for all 10 measures. Results. There was much greater variability in height measures with the “in–out” method while the “in place” method demonstrated a steady shrinkage over the 3–3.5 min required to obtain the repeated measures. Relevance Contrary to popular practice, leaving a subject in the stadiometer during repeated measures includes the shrinkage that occurs over the 3–3.5 min of measurement when standing and reduces random variation due to posture change.