The Ca2+ and Rho GTPase signaling pathways underlying activity-dependent actin remodeling at dendritic spines

Calcium influx and postsynaptic proteins coordinate the dendritic filopodium-spine transition

Dendritic spines are the major locations of excitatory synapses in the mammalian central nervous system. The transformation from dendritic filopodia to dendritic spines has been recognized as one type of spinogenesis. For instance, syndecan-2 (SDC2), a synaptic heparan sulfate proteoglycan, is highly concentrated at dendritic spines and required for spinogenesis. It induces dendritic filopodia formation, followed by spine formation. However, the molecular regulation of the filopodium-spine transition induced by SDC2 is still unclear. In this report, we show that calcium is an important signal downstream of SDC2 in regulation of filopodium-spine transition but not filopodia formation. SDC2 interacted with the postsynaptic proteins calmodulin-dependent serine kinase (CASK) and LIN7 and further recruited NMDAR to the tips of filopodia induced by SDC2. Calcium influx via NMDAR promoted spine maturation because addition of EGTA or AP5 to the culture medium effectively prevented morphological change from dendritic filopodia to dendritic spines. Our data also indicated that F-actin rearrangement regulated by calcium influx is involved in the morphological change, because the knockdown of gelsolin, a calcium-activated F-actin severing molecule, impaired the filopodium-spine transition induced by SDC2. In conclusion, our study demonstrates that postsynaptic proteins coordinate to trigger calcium signalling and cytoskeleton rearrangement and consequently control filopodium-spine transition.

Molecular handoffs in nitrergic neurotransmission

Postsynaptic density (PSD) proteins in excitatory synapses are relatively immobile components, while there is a structured organization of mobile scaffolding proteins lying beneath the PSDs. For example, shank proteins are located further away from the membrane in the cytosolic faces of the PSDs, facing the actin cytoskeleton. The rationale of this organization may be related to important roles of these proteins as “exchange hubs” for the signaling proteins for their migration from the subcortical cytosol to the membrane. Notably, PSD95 have also been demonstrated in prejunctional nerve terminals of nitrergic neuronal varicosities traversing the gastrointestinal smooth muscles. It has been recently reported that motor proteins like myosin Va play important role in transcytosis of nNOS. In this review, the hypothesis is forwarded that nNOS delivered to subcortical cytoskeleton requires interactions with scaffolding proteins prior to docking at the membrane. This may involve significant role of “shank,” named for SRC-homology (SH3) and multiple ankyrin repeat domains, in nitric oxide synthesis. Dynein light chain LC8–nNOS from acto-myosin Va is possibly exchanged with shank, which thereafter facilitates transposition of nNOS for binding with palmitoyl-PSD95 at the nerve terminal membrane. Shank knockout mice, which present with features of autism spectrum disorders, may help delineate the role of shank in enteric nitrergic neuromuscular transmission. Deletion of shank3 in humans is a monogenic cause of autism called Phelan–McDermid syndrome. One fourth of these patients present with cyclical vomiting, which may be explained by junctionopathy resulting from shank deficit in enteric nitrergic nerve terminals.

The role of the actin cytoskeleton in regulating Drosophila behavior

Over the past decade, the function of the cytoskeleton has been studied extensively in developing and mature neurons. Actin, a major cytoskeletal protein, is indispensable for the structural integrity and plasticity of neurons and their synapses. Disruption of actin dynamics has significant consequence for neurons, neuronal circuits, and the functions they govern. In particular, cell adhesion molecules, members of the Rho family of GTPases, and actin-binding proteins are important modulators of actin dynamics and neuronal as well as behavioral plasticity. In this review, we discuss recent advances in Drosophila that highlight the importance of actin regulatory proteins in mediating fly behaviors such as circadian rhythm, courtship behavior, learning and memory, and the development of drug addiction.

Indatraline inhibits Rho- and calcium-mediated glioblastoma cell motility and angiogenesis

Glioblastoma multiforme (GBM) is the most common and lethal primary brain tumor of the central nervous system (CNS). As an attempt to identify drugs for GBM therapeutics, phenotypic assays were used to screen 1000 chemicals from a clinical compound library. GBM subtypes exhibited different capabilities to induce angiogenesis when cultured on Matrigel; proneural cells migrated and formed a tube-like structure without endothelial cells. Among the compounds screened, indatraline, a nonselective monoamine transporter inhibitor, suppressed these morphological changes; it dose dependently inhibited cell spreading, migration, and in vitro/in vivo tube formation. In addition to intracellular calcium concentration, indatraline increased the level of Rho GTPase and its activity. Moreover, indatraline downregulated angiogenesis-related genes such as IGFBP2, PTN, VEGFA, PDGFRA, and VEGFR as well as nestin, a stem cell marker. These findings collectively suggest that the activation of Rho GTPase and the suppression of angiogenesis-related factors mediate the antiangiogenic activity of indatraline in proneural GBM culture.

Synaptic regulation of microtubule dynamics in dendritic spines by calcium, F-actin, and drebrin

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.

Neuronal cytoskeleton in synaptic plasticity and regeneration

During development, dynamic changes in the axonal growth cone and dendrite are necessary for exploratory movements underlying initial axo-dendritic contact and ultimately the formation of a functional synapse. In the adult central nervous system, an impressive degree of plasticity is retained through morphological and molecular rearrangements in the pre- and post-synaptic compartments that underlie the strengthening or weakening of synaptic pathways. Plasticity is regulated by the interplay of permissive and inhibitory extracellular cues, which signal through receptors at the synapse to regulate the closure of critical periods of developmental plasticity as well as by acute changes in plasticity in response to experience and activity in the adult. The molecular underpinnings of synaptic plasticity are actively studied and it is clear that the cytoskeleton is a key substrate for many cues that affect plasticity. Many of the cues that restrict synaptic plasticity exhibit residual activity in the injured adult CNS and restrict regenerative growth by targeting the cytoskeleton. Here, we review some of the latest insights into how cytoskeletal remodeling affects neuronal plasticity and discuss how the cytoskeleton is being targeted in an effort to promote plasticity and repair following traumatic injury in the central nervous system.

Application of FRET probes in the analysis of neuronal plasticity

Breakthroughs in imaging techniques and optical probes in recent years have revolutionized the field of life sciences in ways that traditional methods could never match. The spatial and temporal regulation of molecular events can now be studied with great precision. There have been several key discoveries that have made this possible. Since green fluorescent protein (GFP) was cloned in 1992, it has become the dominant tracer of proteins in living cells. Then the evolution of color variants of GFP opened the door to the application of Förster resonance energy transfer (FRET), which is now widely recognized as a powerful tool to study complicated signal transduction events and interactions between molecules. Employment of fluorescent lifetime imaging microscopy (FLIM) allows the precise detection of FRET in small subcellular structures such as dendritic spines. In this review, we provide an overview of the basic and practical aspects of FRET imaging and discuss how different FRET probes have revealed insights into the molecular mechanisms of synaptic plasticity and enabled visualization of neuronal network activity both in vitro and in vivo.

Abl2/Arg controls dendritic spine and dendrite arbor stability via distinct cytoskeletal control pathways

Rho family GTPases coordinate cytoskeletal rearrangements in neurons, and mutations in their regulators are associated with mental retardation and other neurodevelopmental disorders (Billuart et al., 1998; Kutsche et al., 2000; Newey et al., 2005; Benarroch, 2007). Chromosomal microdeletions encompassing p190RhoGAP or its upstream regulator, the Abl2/Arg tyrosine kinase, have been observed in cases of mental retardation associated with developmental defects (Scarbrough et al., 1988; James et al., 1996; Takano et al., 1997; Chaabouni et al., 2006; Leal et al., 2009). Genetic knock-out of Arg in mice leads to synapse, dendritic spine, and dendrite arbor loss accompanied by behavioral deficits (Moresco et al., 2005; Sfakianos et al., 2007). To elucidate the cell-autonomous mechanisms by which Arg regulates neuronal stability, we knocked down Arg in mouse hippocampal neuronal cultures. We find that Arg knockdown significantly destabilizes dendrite arbors and reduces dendritic spine density by compromising dendritic spine stability. Inhibiting RhoA prevents dendrite arbor loss following Arg knockdown in neurons, but does not block spine loss. Interestingly, Arg-deficient neurons exhibit increased miniature EPSC amplitudes, and their remaining spines exhibit larger heads deficient in the actin stabilizing protein cortactin. Spine destabilization in Arg knockdown neurons is prevented by blocking NMDA receptor-dependent relocalization of cortactin from spines, or by forcing cortactin into spines via fusion to an actin-binding region of Arg. Thus, Arg employs distinct mechanisms to selectively regulate spine and dendrite stability: Arg dampens activity-dependent disruption of cortactin localization to stabilize spines and attenuates Rho activity to stabilize dendrite arbors.