Anna Dunaevsky

Bidirectional Regulation of Hippocampal Mossy Fiber Filopodial Motility by Kainate Receptors: A Two-Step Model of Synaptogenesis

The rapid motility of axonal filopodia and dendritic spines is prevalent throughout the developing CNS, although the function of this motility remains controversial. Using two-photon microscopy, we imaged hippocampal mossy fiber axons in slice cultures and discovered that filopodial extensions are highly motile. Axonal filopodial motility is actin based and is downregulated with development, although it remains in mature cultures. This motility is correlated with free extracellular space yet is inversely correlated with contact with postsynaptic targets, indicating a potential role in synaptogenesis. Filopodial motility is differentially regulated by kainate receptors: synaptic stimulation of kainate receptors enhances motility in younger slices, but it inhibits it in mature slices. We propose that neuronal activity controls filopodial motility in a developmentally regulated manner, in order to establish synaptic contacts in a two-step process. A two-step model of synaptogenesis can also explain the opposite effects of neuronal activity on the motility of dendritic protrusions.

Amyloid β peptide adversely affects spine number and motility in hippocampal neurons

Elevated levels of amyloid-beta peptide (Abeta) are found in Downs syndrome patients and alter synaptic function during the early stages of Alzheimers disease. Dendritic spines, sites of most excitatory synaptic contacts, are considered to be an important locus for encoding synaptic plasticity. We used time-lapse two-photon imaging of hippocampal pyramidal neurons in organotypic slices to study the effects of Abeta on the development of dendritic spines. We report that exposure of hippocampal neurons to sub-lethal levels of Abeta decreased spine density, increased spine length and subdued spine motility. The effect of Abeta on spine density was reversible. Moreover, Abetas effect on dendritic spine density was blocked by rolipram, a phosphodiesterase type IV inhibitor, suggesting the involvement of a cAMP dependent pathway. These findings raise the possibility that Abeta-induced spine alterations could underlie the cognitive defects in Alzheimers disease and Down syndrome.

Spine motility with synaptic contact.

Dendritic protrusions, including filopodia and spines, are highly dynamic, but the extent to which their motility depends on afferent innervation or synaptic activity is under debate. By monitoring dendritic spines of labeled Purkinje cells in cerebellar slices by two-photon microscopy, followed by ultrastructural analysis of the same imaged spines, we show that dendritic spines can exhibit morphological rearrangements even when they are contacted by presynaptic terminals.

Dendritic spine plasticity: Looking beyond development

Most excitatory synapses in the CNS form on dendritic spines, tiny protrusions from the dendrites of excitatory neurons. As such, spines are likely loci of synaptic plasticity. Spines are dynamic structures, but the functional consequences of dynamic changes in these structures in the mature brain are unclear. Changes in spine density, morphology, and motility have been shown to occur with paradigms that induce synaptic plasticity, as well as altered sensory experience and neuronal activity. These changes potentially lead to an alteration in synaptic connectivity and strength between neuronal partners, affecting the efficacy of synaptic communication. Here, we review the formation and modification of excitatory synapses on dendritic spines as it relates to plasticity in the central nervous system after the initial phase of synaptogenesis. We will also discuss some of the molecular links that have been implicated in both synaptic plasticity and the regulation of spine morphology.

Purkinje cell dendrites grow in alignment with Bergmann glia

The pattern of growth of Purkinje cell dendrites has been analyzed and related to their interactions with Bergmann glial radial processes. In cerebellar slice cultures from mice expressing green fluorescent protein (GFP) under the glial fibrillary acidic protein (GFAP) promoter, Purkinje cells were transfected and imaged with two‐photon microscopy over 2 days. We report that while the Purkinje cell dendritic tree grows, individual dendrites increase or decrease in length. Importantly, we demonstrate that vertical growth of Purkinje cell dendrites occurs primarily in alignment with radial glial processes. These findings suggest that radial glial processes provide a structural substrate for the directional growth of Purkinje cell dendrites, thus influencing the shape of the dendritic tree.

Transient Spine Expansion and Learning-Induced Plasticity in Layer 1 Primary Motor Cortex

Experience-dependent regulation of synaptic strength in the horizontal connections in layer 1 of the primary motor cortex is likely to play an important role in motor learning. Dendritic spines, the primary sites of excitatory synapses in the brain, are known to change shape in response to various experimental stimuli. We used a rat motor learning model to examine connection strength via field recordings in slices and confocal imaging of labeled spines to explore changes induced solely by learning a simple motor task. We report that motor learning increases response size, while transiently occluding long-term potentiation (LTP) and increasing spine width in layer 1. This demonstrates learning-induced changes in behavior, synaptic responses, and structure in the same animal, suggesting that an LTP-like process in the motor cortex mediates the initial learning of a skilled task.

Morphogenesis and regulation of Bergmann glial processes during Purkinje cell dendritic spine ensheathment and synaptogenesis

Astrocytes have an important role in synaptic formation and function but how astrocytic processes become associated with synaptic structures during development is not well understood. Here we analyzed the pattern of growth of the processes extending off the main Bergmann glial (BG) shafts during synaptogenesis in the cerebellum. We found that during this period, BG process outgrowth was correlated with increased ensheathment of dendritic spines. In addition, two‐photon time‐lapse imaging revealed that BG processes were highly dynamic, and processes became more stable as the period of spine ensheathment progressed. While process motility was dependent on actin polymerization, activity of cytoskeletal regulators Rac1 and RhoG did not play a role in glial process dynamics or density, but was critical for maintaining process length. We extended this finding to probe the relationship between process morphology and ensheathment, finding that shortened processes result in decreased coverage of the spine. Furthermore, we found that areas in which BG expressed dn‐Rac1, and therefore had a lower level of synaptic ensheathment, showed an overall increase in synapse number. These analyses reveal how BG processes grow to surround synaptic structures, elucidate the importance of BG process structure for proper development of synaptic ensheathment, and reveal a role for ensheathment in synapse formation.

Spine motility: a means towards an end?

From the first glimpse of moving spines half a decade ago, the prevailing view has been that spine contortions and wiggling, especially during development, maximize encounters with presynaptic growth cones or synaptic boutons. Other new evidence has revealed that spines continue to be motile even after they settle on a presynaptic partner and form a synapse. We present the evidence for each view, and discuss how spines with synapses could move relative to their apparently stable presynaptic partners. Thus, spine motility might not simply be a means towards an end of synapse formation, but could continue, albeit at a lower rate, during synapse turnover after development ends.

Altered Structural and Functional Synaptic Plasticity with Motor Skill Learning in a Mouse Model of Fragile X Syndrome

Fragile X syndrome (FXS) is the most common inherited intellectual disability. FXS results from a mutation that causes silencing of the FMR1 gene, which encodes the fragile X mental retardation protein. Patients with FXS exhibit a range of neurological deficits, including motor skill deficits. Here, we have investigated motor skill learning and its synaptic correlates in the fmr1 knock-out (KO) mouse. We find that fmr1 KO mice have impaired motor skill learning of a forelimb-reaching task, compared with their wild-type (WT) littermate controls. Electrophysiological recordings from the forelimb region of the primary motor cortex demonstrated reduced, training-induced synaptic strengthening in the trained hemisphere. Moreover, long-term potentiation (LTP) is impaired in the fmr1 KO mouse, and motor skill training does not occlude LTP as it does in the WT mice. Whereas motor skill training induces an increase of synaptic AMPA-type glutamate receptor subunit 1 (GluA1), there is a delay in GluA1 increase in the trained hemisphere of the fmr1 KO mice. Using transcranial in vivo multiphoton microscopy, we find that fmr1 KO mice have similar spine density but increased dendritic spine turnover compared with WT mice. Finally, we report that motor skill training-induced formation of dendritic spines is impaired in fmr1 KO mice. We conclude that FMRP plays a role in motor skill learning and that reduced functional and structural synaptic plasticity might underlie the behavioral deficit in the fmr1 KO mouse.

Impaired synaptic development in a maternal immune activation mouse model of neurodevelopmental disorders

Both genetic and environmental factors are thought to contribute to neurodevelopmental and neuropsychiatric disorders with maternal immune activation (MIA) being a risk factor for both autism spectrum disorders and schizophrenia. Although MIA mouse offspring exhibit behavioral impairments, the synaptic alterations in vivo that mediate these behaviors are not known. Here we employed in vivo multiphoton imaging to determine that in the cortex of young MIA offspring there is a reduction in number and turnover rates of dendritic spines, sites of majority of excitatory synaptic inputs. Significantly, spine impairments persisted into adulthood and correlated with increased repetitive behavior, an ASD relevant behavioral phenotype. Structural analysis of synaptic inputs revealed a reorganization of presynaptic inputs with a larger proportion of spines being contacted by both excitatory and inhibitory presynaptic terminals. These structural impairments were accompanied by altered excitatory and inhibitory synaptic transmission. Finally, we report that a postnatal treatment of MIA offspring with the anti-inflammatory drug ibudilast, prevented both synaptic and behavioral impairments. Our results suggest that a possible altered inflammatory state associated with maternal immune activation results in impaired synaptic development that persists into adulthood but which can be prevented with early anti-inflammatory treatment.