A Model of NMDA Receptor Control of F-actin Treadmilling in Synaptic Spines and Their Growth

Can biophysical models of dendritic spines be used to explore synaptic changes associated with addiction?

Effective treatments that prevent or reduce drug relapse vulnerability should be developed to relieve the high burden of drug addiction on society. This will only be possible by enhancing the understanding of the molecular mechanisms underlying the neurobiology of addiction. Recent experimental data have shown that dendritic spines, small protrusions from the dendrites that receive excitatory input, of spiny neurons in the nucleus accumbens exhibit morphological changes during drug exposure and withdrawal. Moreover, these changes relate to the characteristic drug-seeking behavior of addiction. However, due to the complexity of the dendritic spines, we do not yet fully understand the processes underlying their structural changes in response to different inputs. We propose that biophysical models can enhance the current understanding of these processes by incorporating different, and sometimes, discrepant experimental data to identify the shared underlying mechanisms and generate experimentally testable hypotheses. This review aims to give an up-to-date report on biophysical models of dendritic spines, focusing on those models that describe their shape changes, which are well-known to relate to learning and memory. Moreover, it examines how these models can enhance our understanding of the effect of the drugs and the synaptic changes during withdrawal, as well as during neurodegenerative disease progression such as Alzheimer's disease.

Role of actin cytoskeleton in the organization and function of ionotropic glutamate receptors

Neural networks with precise connection are compulsory for learning and memory. Various cellular events occur during the genesis of dendritic spines to their maturation, synapse formation, stabilization of the synapse, and proper signal transmission. The cortical actin cytoskeleton and its multiple regulatory proteins are crucial for the above cellular events. The different types of ionotropic glutamate receptors (iGluRs) present on the postsynaptic density (PSD) are also essential for learning and memory. Interaction of the iGluRs in association of their auxiliary proteins with actin cytoskeleton regulated by actin-binding proteins (ABPs) are required for precise long-term potentiation (LTP) and long-term depression (LTD). There has been a quest to understand the mechanistic detail of synapse function involving these receptors with dynamic actin cytoskeleton. A major, emerging area of investigation is the relationship between ABPs and iGluRs in synapse development. In this review we have summarized the current understanding of iGluRs functioning with respect to the actin cytoskeleton, scaffolding proteins, and their regulators. The AMPA, NMDA, Delta and Kainate receptors need the stable underlying actin cytoskeleton to anchor through synaptic proteins for precise synapse formation. The different types of ABPs present in neurons play a critical role in dynamizing/stabilizing the actin cytoskeleton needed for iGluRs function.

Exploring new roles for actin upon LTP induction in dendritic spines

Dendritic spines, small protrusions of the dendrites, enlarge upon LTP induction, linking morphological and functional properties. Although the role of actin in spine enlargement has been well studied, little is known about its relationship with mechanical membrane properties, such as membrane tension, which is involved in many cell processes, like exocytosis. Here, we use a 3D model of the dendritic spine to investigate how polymerization of actin filaments can effectively elevate the membrane tension to trigger exocytosis in a domain close to the tip of the spine. Moreover, we show that the same pool of actin promotes full membrane fusion after exocytosis and spine stabilization.

Reproducing asymmetrical spine shape fluctuations in a model of actin dynamics predicts self-organized criticality

Dendritic spines change their size and shape spontaneously, but the function of this remains unclear. Here, we address this in a biophysical model of spine fluctuations, which reproduces experimentally measured spine fluctuations. For this, we characterize size- and shape fluctuations from confocal microscopy image sequences using autoregressive models and a new set of shape descriptors derived from circular statistics. Using the biophysical model, we extrapolate into longer temporal intervals and find the presence of 1/f noise. When investigating its origins, the model predicts that the actin dynamics underlying shape fluctuations self-organizes into a critical state, which creates a fine balance between static actin filaments and free monomers. In a comparison against a non-critical model, we show that this state facilitates spine enlargement, which happens after LTP induction. Thus, ongoing spine shape fluctuations might be necessary to react quickly to plasticity events.

Modeling the Shape of Synaptic Spines by Their Actin Dynamics

Dendritic spines are the morphological basis of excitatory synapses in the cortex and their size and shape correlates with functional synaptic properties. Recent experiments show that spines exhibit large shape fluctuations that are not related to activity-dependent plasticity but nonetheless might influence memory storage at their synapses. To investigate the determinants of such spontaneous fluctuations, we propose a mathematical model for the dynamics of the spine shape and analyze it in 2D-related to experimental microscopic imagery-and in 3D. We show that the spine shape is governed by a local imbalance between membrane tension and the expansive force from actin bundles that originates from discrete actin polymerization foci. Experiments have shown that only few such polymerization foci co-exist at any time in a spine, each having limited life time. The model shows that the momentarily existing set of such foci pushes the membrane along certain directions until foci are replaced and other directions may now be affected. We explore these relations in depth and use our model to predict shape and temporal characteristics of spines from the different biophysical parameters involved in actin polymerization. Approximating the model by a single recursive equation we finally demonstrate that the temporal evolution of the number of active foci is sufficient to predict the size of the model-spines. Thus, our model provides the first platform to study the relation between molecular and morphological properties of the spine with a high degree of biophysical detail.

E3 Ubiquitin-Protein Ligase SMURF1 in the Nucleus Accumbens Mediates Cocaine Seeking

BACKGROUND

Substance use disorder is a neurobiological disease characterized by episodes of relapse despite periods of withdrawal. It is thought that neuroadaptations in discrete brain areas of the reward pathway, including the nucleus accumbens, underlie these aberrant behaviors. The ubiquitin-proteasome system degrades proteins and has been shown to be involved in cocaine-induced plasticity, but the role of E3 ubiquitin ligases, which conjugate ubiquitin to substrates, is unknown. Here, we examined E3 ubiquitin-protein ligase SMURF1 (SMURF1) in neuroadaptations and relapse behavior during withdrawal following cocaine self-administration.

METHODS

SMURF1 and downstream targets ras homolog gene family, member A (RhoA), SMAD1/5, and Runt-related transcript factor 2 were examined using Western blotting (n = 9-11/group), quantitative polymerase chain reaction (n = 6-9/group), co-immunoprecipitation (n = 9-11/group), tandem ubiquitin binding entities affinity purification (n = 5-6/group), and quantitative chromatin immunoprecipitation (n = 3-6/group) (2 rats/sample). Viral-mediated gene transfer (n = 7-12/group) and intra-accumbal microinjections (n = 9-10/group) were used to examine causal roles of SMURF1 and substrate RhoA, respectively, in cue-induced cocaine seeking.

RESULTS

SMURF1 protein expression was decreased, while SMURF1 substrates RhoA and SMAD1/5 were increased, in the nucleus accumbens on withdrawal day 7, but not on withdrawal day 1, following cocaine self-administration. Viral-mediated gene transfer of Smurf1 or constitutive activation of RhoA attenuated cue-induced cocaine seeking, while catalytically inactive Smurf1 enhanced cocaine seeking. Furthermore, SMURF1-regulated, SMAD1/5-associated transcription factor Runt-related transcript factor 2 displayed increased binding at promoter regions of genes previously associated with cocaine-induced plasticity.

CONCLUSIONS

SMURF1 is a key mediator of neuroadaptations in the nucleus accumbens following cocaine exposure and mediates cue-induced cocaine seeking during withdrawal.

What does the grey matter decrease in the medial prefrontal cortex reflect in people with chronic pain?

BACKGROUND AND OBJECTIVE

Alterations in the grey matter volume of several brain regions have been reported in people with chronic pain. The most consistent observation is a decrease in grey matter volume in the medial prefrontal cortex. These findings are important as the medial prefrontal cortex plays a critical role in emotional and cognitive processing in chronic pain. Although a logical cause of grey matter volume decrease may be neurodegeneration, this is not supported by the current evidence. Therefore, the purpose of this review was to evaluate the existing literature to unravel what the decrease in medial prefrontal cortex grey matter volume in people with chronic pain may represent on a biochemical and cellular level.

DATABASES AND DATA TREATMENT

A literature search for this topical review was conducted using PubMed and SCOPUS library. Search terms included chronic pain, pain, medial prefrontal cortex, anterior cingulate cortex, grey matter, neurochemistry, spectroscopy, magnetic resonance imaging, positron emission tomography, dendrite, neurodegeneration, glia, astrocyte, microglia, neurotransmitter, glutamate, GABA and different combinations of these terms.

RESULTS

Adopting a stress model of chronic pain, two major pathways are proposed that contribute to grey matter volume decrease in the medial prefrontal cortex: (a) changes in dendritic morphology as a result of hypothalamic-pituitary axis dysfunction and (b) neurotransmitter dysregulation, specifically glutamate and γ-Aminobutyric acid, which affects local microvasculature.

CONCLUSION

Our model proposes new mechanisms in chronic pain pathophysiology responsible for mPFC grey matter loss as alternatives to neurodegeneration.

SIGNIFICANCE

It is unclear what the decrease in medial prefrontal cortex grey matter volume represents in chronic pain. The most attractive reason is neurodegeneration. However, there is no evidence to support this. Our review reveals nondegenerative causes of decreased medial prefrontal grey matter to guide future research into chronic pain pathophysiology.

Excitable neurofibrils and the problem of identifying the structure of central excitatory synapses in the nineteenth century

Neurofibrils, identified after staining with Cajal's reduced silver nitrate, for example, were thought by many senior histologists in the nineteenth and early-twentieth centuries to conduct action potentials. There was no basis for this popular idea, although it was the impetus for intense study of the "neurofibrillar network" within neurons by Golgi, Cajal, Freud, and many others. Here, I trace the way in which this "excitable neurofibrillary" hypothesis led to major problems in the attempt by histologists to identify the central excitatory synapse, postulated by Sherrington on functional grounds and eventually described by Berkley.

Stress-induced grey matter loss determined by MRI is primarily due to loss of dendrites and their synapses

Stress, unaccompanied by signs of post-traumatic stress disorder, is known to decrease grey matter volume (GMV) in the anterior cingulate cortex (ACC) and hippocampus but not the amygdala in humans. We sought to determine if this was the case in stressed mice using high-resolution magnetic resonance imaging (MRI) and to identify the cellular constituents of the grey matter that quantitatively give rise to such changes. Stressed mice showed grey matter losses of 10 and 15 % in the ACC and hippocampus, respectively but not in the amygdala or the retrosplenial granular area (RSG). Concurrently, no changes in the number or volumes of the somas of neurons, astrocytes or oligodendrocytes were detected. A loss of synaptic spine density of up to 60 % occurred on different-order dendrites in the ACC and hippocampus (CA1) but not in the amygdala or RSG. The loss of spines was accompanied by decreases in cumulative dendritic length of neurons of over 40 % in the ACC and hippocampus (CA1) giving rise to decreases in volume of dendrites of 2.6 mm(3) for the former and 0.6 mm(3) for the latter, with no change in the amygdala or RSG. These values are similar to the MRI-determined loss of GMV following stress of 3.0 and 0.8 mm(3) in ACC and hippocampus, respectively, with no changes in the amygdala or RSG. This quantitative study is the first to relate GMV changes in the cortex measured with MRI to volume changes in cellular constituents of the grey matter.