The identification of a second actin-binding region in spinophilin/neurabin II

SPINOPHILIN: a multiplayer tumor suppressor

SPINOPHILIN (SPN, PPP1R9B or NEURABIN-2) is a multifunctional protein that regulates protein-protein interactions in different cell signaling pathways. SPN is also one of the regulatory subunits of protein phosphatase 1 (PP1), implicated in the dephosphorylation of retinoblastoma protein (pRB) during cell cycle. The SPN gene has been described as a tumor suppressor in different human tumor contexts, in which low levels of SPN are correlated with a higher grade and worse prognosis. In addition, mutations of the SPN protein have been reported in human tumors. Recently, an oncogenic mutation of SPN, A566V, was described, which affects both the SPN-PP1 interaction and the phosphatase activity of the holoenzyme, and promotes p53-dependent tumorigenesis by increasing the cancer stem cell (CSC) pool in breast tumors. Thus, the loss or mutation of SPN could be late events that promotes tumor progression by increasing the CSC pool and, eventually, the malignant behavior of the tumor.

Role of the Holoenzyme PP1-SPN in the Dephosphorylation of the RB Family of Tumor Suppressors During Cell Cycle

Simple Summary

Cell cycle progression is highly regulated by modulating the phosphorylation status of retinoblastoma (RB) family proteins. This process is controlled by a balance in the action of kinases, such as the complexes formed by cyclin-dependent kinases (CDKs) and cyclins, and phosphatases, mainly the protein phosphatase 1 (PP1). However, while the phosphorylation of the RB family has been largely studied, its dephosphorylation is less known. Recently, the PP1-Spinophilin (SPN) holoenzyme has been described as the main phosphatase responsible for the dephosphorylation of RB proteins during the G0/G1 transition and at the end of G1. Here, we describe the regulation of the phosphorylation status of RB family proteins, giving importance not only to their inactivation by phosphorylation but also to their dephosphorylation to restore the cell cycle.

Abstract

Cell cycle progression is highly regulated by modulating the phosphorylation status of the retinoblastoma protein (pRB) and the other two members of the RB family, p107 and p130. This process is controlled by a balance in the action of kinases, such as the complexes formed by cyclin-dependent kinases (CDKs) and cyclins, and phosphatases, mainly the protein phosphatase 1 (PP1). However, while the phosphorylation of the RB family has been largely studied, its dephosphorylation is less known. Phosphatases are holoenzymes formed by a catalytic subunit and a regulatory protein with substrate specificity. Recently, the PP1-Spinophilin (SPN) holoenzyme has been described as the main phosphatase responsible for the dephosphorylation of RB proteins during the G0/G1 transition and at the end of G1. Moreover, SPN has been described as a tumor suppressor dependent on PP1 in lung and breast tumors, where it promotes tumorigenesis by increasing the cancer stem cell pool. Therefore, a connection between the cell cycle and stem cell biology has also been proposed via SPN/PP1/RB proteins.

Regulation of Synaptic Transmission and Plasticity by Protein Phosphatase 1

Protein phosphatases, by counteracting protein kinases, regulate the reversible phosphorylation of many substrates involved in synaptic plasticity, a cellular model for learning and memory. A prominent phosphatase regulating synaptic plasticity and neurologic disorders is the serine/threonine protein phosphatase 1 (PP1). PP1 has three isoforms (α, β, and γ, encoded by three different genes), which are regulated by a vast number of interacting subunits that define their enzymatic substrate specificity. In this review, we discuss evidence showing that PP1 regulates synaptic transmission and plasticity, as well as presenting novel models of PP1 regulation suggested by recent experimental evidence. We also outline the required targeting of PP1 by neurabin and spinophilin to achieve substrate specificity at the synapse to regulate AMPAR and NMDAR function. We then highlight the role of inhibitor-2 in regulating PP1 function in plasticity, including its positive regulation of PP1 function in vivo in memory formation. We also discuss the distinct function of the three PP1 isoforms in synaptic plasticity and brain function, as well as briefly discuss the role of inhibitory phosphorylation of PP1, which has received recent emphasis in the regulation of PP1 activity in neurons.

The guanine nucleotide exchange factor (GEF) Asef2 promotes dendritic spine formation via Rac activation and spinophilin-dependent targeting

Dendritic spines are actin-rich protrusions that establish excitatory synaptic contacts with surrounding neurons. Reorganization of the actin cytoskeleton is critical for the development and plasticity of dendritic spines, which is the basis for learning and memory. Rho family GTPases are emerging as important modulators of spines and synapses, predominantly through their ability to regulate actin dynamics. Much less is known, however, about the function of guanine nucleotide exchange factors (GEFs), which activate these GTPases, in spine and synapse development. In this study we show that the Rho family GEF Asef2 is found at synaptic sites, where it promotes dendritic spine and synapse formation. Knockdown of endogenous Asef2 with shRNAs impairs spine and synapse formation, whereas exogenous expression of Asef2 causes an increase in spine and synapse density. This effect of Asef2 on spines and synapses is abrogated by expression of GEF activity-deficient Asef2 mutants or by knockdown of Rac, suggesting that Asef2-Rac signaling mediates spine development. Because Asef2 interacts with the F-actin-binding protein spinophilin, which localizes to spines, we investigated the role of spinophilin in Asef2-promoted spine formation. Spinophilin recruits Asef2 to spines, and knockdown of spinophilin hinders spine and synapse formation in Asef2-expressing neurons. Furthermore, inhibition of N-methyl-d-aspartate receptor (NMDA) activity blocks spinophilin-mediated localization of Asef2 to spines. These results collectively point to spinophilin-Asef2-Rac signaling as a novel mechanism for the development of dendritic spines and synapses.

Aging-induced proteostatic changes in the rat hippocampus identify ARP3, NEB2 and BRAG2 as a molecular circuitry for cognitive impairment

Disturbed proteostasis as a particular phenotype of the aging organism has been advanced in C. elegans experiments and is also conceived to underlie neurodegenerative diseases in humans. Here, we investigated whether particular changes in non-disease related proteostasis can be identified in the aged mammalian brain, and whether a particular signature of aberrant proteostasis is related to behavioral performance of learning and memory. Young (adult, n = 30) and aged (2 years, n = 50) Wistar rats were tested in the Morris Water Maze (MWM) to distinguish superior and inferior performers. For both young and old rats, the best and worst performers in the MWM were selected and the insoluble proteome, termed aggregome, was purified from the hippocampus as evidence for aberrant proteostasis. Quantitative proteomics (iTRAQ) was performed. The aged inferior performers were considered as a model for spontaneous, age-associated cognitive impairment. Whereas variability of the insoluble proteome increased with age, absolute changes in the levels of insoluble proteins were small compared to the findings in the whole C. elegans insoluble proteome. However, we identified proteins with aberrant proteostasis in aging. For the cognitively impaired rats, we identified a changed molecular circuitry of proteins selectively involved in F-actin remodeling, synapse building and long-term depression: actin related protein 3 (ARP3), neurabin II (NEB2) and IQ motif and SEC7 domain-containing protein 1 (BRAG2). We demonstrate that aberrant proteostasis is a specific phenotype of brain aging in mammals. We identify a distinct molecular circuitry where changes in proteostasis are characteristic for poor learning and memory performance in the wild type, aged rat. Our findings 1. establish the search for aberrant proteostasis as a successful strategy to identify neuronal dysfunction in deficient cognitive behavior, 2. reveal a previously unknown functional network of proteins (ARP3, NEB2, BRAG2) involved in age-associated cognitive dysfunction.

Age-dependent targeting of protein phosphatase 1 to Ca2+/calmodulin-dependent protein kinase II by spinophilin in mouse striatum

Mechanisms underlying age-dependent changes of dendritic spines on striatal medium spiny neurons are poorly understood. Spinophilin is an F-actin- and protein phosphatase 1 (PP1)-binding protein that targets PP1 to multiple downstream effectors to modulate dendritic spine morphology and function. We found that calcium/calmodulin-dependent protein kinase II (CaMKII) directly and indirectly associates with N- and C-terminal domains of spinophilin, but F-actin can displace CaMKII from the N-terminal domain. Spinophilin co-localizes PP1 with CaMKII on the F-actin cytoskeleton in heterologous cells, and spinophilin co-localizes with synaptic CaMKII in neuronal cultures. Thr286 autophosphorylation enhances the binding of CaMKII to spinophilin in vitro and in vivo. Although there is no change in total levels of Thr286 autophosphorylation, maturation from postnatal day 21 into adulthood robustly enhances the levels of CaMKII that co-immunoprecipitate with spinophilin from mouse striatal extracts. Moreover, N- and C-terminal domain fragments of spinophilin bind more CaMKII from adult vs. postnatal day 21 striatal lysates. Total levels of other proteins that interact with C-terminal domains of spinophilin decrease during maturation, perhaps reducing competition for CaMKII binding to the C-terminal domain. In contrast, total levels of α-internexin and binding of α-internexin to the spinophilin N-terminal domain increases with maturation, perhaps bridging an indirect interaction with CaMKII. Moreover, there is an increase in the levels of myosin Va, α-internexin, spinophilin, and PP1 in striatal CaMKII immune complexes isolated from adult and aged mice compared to those from postnatal day 21. These changes in spinophilin/CaMKII interactomes may contribute to changes in striatal dendritic spine density, morphology, and function during normal postnatal maturation and aging.

Identification of a putative network of actin-associated cytoskeletal proteins in glomerular podocytes defined by co-purified mRNAs

The glomerular podocyte is a highly specialized and polarized kidney cell type that contains major processes and foot processes that extend from the cell body. Foot processes from adjacent podocytes form interdigitations with those of adjacent cells, thereby creating an essential intercellular junctional domain of the renal filtration barrier known as the slit diaphragm. Interesting parallels have been drawn between the slit diaphragm and other sites of cell-cell contact by polarized cells. Notably mutations in several genes encoding proteins localized to the foot processes can lead to proteinuria and kidney failure. Mutations in the Wilm's tumor gene (WT1) can also lead to kidney disease and one isoform of WT1, WT1(+KTS), has been proposed to regulate gene expression post-transcriptionally. We originally sought to identify mRNAs associated with WT1(+KTS) through an RNA immunoprecipitation and microarray approach, hypothesizing that the proteins encoded by these mRNAs might be important for podocyte morphology and function. We identified a subset of mRNAs that were remarkably enriched for transcripts encoding actin-binding proteins and other cytoskeletal proteins including several that are localized at or near the slit diaphragm. Interestingly, these mRNAs included those of α-actinin-4 and non-muscle myosin IIA that are mutated in genetic forms of kidney disease. However, isolation of the mRNAs occurred independently of the expression of WT1, suggesting that the identified mRNAs were serendipitously co-purified on the basis of co-association in a common subcellular fraction. Mass spectroscopy revealed that other components of the actin cytoskeleton co-purified with these mRNAs, namely actin, tubulin, and elongation factor 1α. We propose that these mRNAs encode a number of proteins that comprise a highly specialized protein interactome underlying the slit diaphragm. Collectively, these gene products and their interactions may prove to be important for the structural integrity of the actin cytoskeleton in podocytes as well as other polarized cell types.

Structure-function analysis of the filamentous actin binding domain of the neuronal scaffolding protein spinophilin

Spinophilin, a neuronal scaffolding protein, is essential for synaptic transmission, and functions to target protein phosphatase-1 to distinct subcellular locations in dendritic spines. It is vital for the regulation of dendritic spine formation and motility, and functions by regulating glutamatergic receptors and binding to filamentous actin. To investigate its role in regulating actin cytoskeletal structure, we initiated structural studies of the actin binding domain of spinophilin. We demonstrate that the spinophilin actin binding domain is intrinsically unstructured, and that, with increasing C-terminal length, the domain shows augmented secondary structure content. Further characterization confirmed the previously known crosslinking activity and uncovered a novel filamentous actin pointed-end capping activity. Both of these functions seem to be fully contained within residues 1-154 of spinophilin.

Spinophilin: from partners to functions

Spinophilin/neurabin 2 has been isolated independently by two laboratories as a protein interacting with protein phosphatase 1 (PP1) and F-actin. Gene analysis and biochemical approaches have contributed to define a number of distinct modular domains in spinophilin that govern protein-protein interactions such as two F-actin-, three potential Src homology 3 (SH3)-, a receptor- and a PP1-binding domains, a PSD95/DLG/zo-1 (PDZ) and three coiled-coil domains, and a potential leucine/isoleucine zipper (LIZ) motif. More than 30 partner proteins of spinophilin have been discovered, including cytoskeletal and cell adhesion molecules, enzymes, guanine nucleotide exchange factors (GEF) and regulator of G-protein signalling protein, membrane receptors, ion channels and others proteins like the tumour suppressor ARF. The physiological relevance of some of these interactions remains to be demonstrated. However, spinophilin structure suggests that the protein is a multifunctional protein scaffold that regulates both membrane and cytoskeletal functions. Spinophilin plays important functions in the nervous system where it is implicated in spine morphology and density regulation, synaptic plasticity and neuronal migration. Spinophilin regulates also seven-transmembrane receptor signalling and may provide a link between some of these receptors and intracellular mitogenic signalling events dependent on p70(S6) kinase and Rac G protein-GEF. Strikingly a role for spinophilin in cell growth was demonstrated and this effect was enhanced by its interaction with ARF. Here we review the current knowledge of the protein partners of spinophilin and present the available data that are contributing to the appreciation of spinophilin functions.

A regulatory role for actin in dendritic spine proliferation

Dendritic spines are small protrusions that receive 90% of excitatory cortical synapses and are critically important to neural function. Each dendritic spine is supported by a dynamic actin cytoskeleton that responds to internal and external cues to allow spine development, elongation, retraction and movement. Multiple proteins have roles in spinogenesis, but until now, a regulatory role for actin itself has not been established. Here, we show that, in the acute slice preparation, actin expression increases during a period of rapid spinogenesis. Furthermore, actin overexpression in organotypic hippocampal cultures leads to a significant increase in spine density on CA1 pyramidal cells. Specifically, the number of filopodia (long, thin protrusions without heads) increases by 38% on secondary apical dendrites and 88% on basal dendrites and the number of elongated spines with heads increases by 162% on secondary apical dendrites and 113% on basal dendrites. Synapsin-I immunostaining demonstrated that the majority of filopodia and elongated spines are apposed by axon terminals. Additionally, we show that overexpressed actin enters both new and established spines within 24 h. These data demonstrate that neurons undertaking spinogenesis upregulate actin expression, that actin overexpression per se increases spine density, and that both new and established spines incorporate exogenous actin.

Clozapine and haloperidol differentially regulate dendritic spine formation and synaptogenesis in rat hippocampal neurons

Antipsychotic drugs are the primary therapeutic treatment for schizophrenia. In addition to their dopaminergic/serotonergic function, atypical antipsychotics differ from conventional antipsychotics in the way they affect glutamatergic receptor function. A cellular correlate of this may be the modulation of dendritic spines (DS). Here, we demonstrate that in rat dissociated hippocampal neurons 1.0 microM clozapine administration increased DS-enriched protein spinophilin by 70%, increased post-synaptic protein shank1a puncta density by 26% and increased overall primary dendrite DS density by 59%. Filopodia and mushroom DS were particularly affected by clozapine. Conversely, 0.1 microM haloperidol decreased spinophilin protein by 40%, caused a 25% decrease in shank1a puncta and reduced the numbers of filopodia. In contrast, neither haloperidol nor clozapine induced any change in the levels of the pre-synaptic protein synapsin. This indicates that clozapine and haloperidol differentially regulate DS and post-synaptic plasticity. These findings may provide a molecular and cellular correlate to the superior therapeutic profile of clozapine when compared with haloperidol.

Distinct roles for spinophilin and neurabin in dopamine-mediated plasticity

Protein phosphatase 1 plays a major role in the governance of excitatory synaptic activity, and is subject to control via the neuromodulatory actions of dopamine. Mechanisms involved in regulating protein phosphatase 1 activity include interactions with the structurally related cytoskeletal elements spinophilin and neurabin, synaptic scaffolding proteins that are highly enriched in dendritic spines. The requirement for these proteins in dopamine-related neuromodulation was tested using knockout mice. Dopamine D1-mediated regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor activity was deficient in both striatal and prefrontal cortical neurons from neurabin knockout mice; in spinophilin knockout mice this deficit was manifest only in striatal neurons. At corticostriatal synapses long-term potentiation was deficient in neurabin knockout mice, but not in spinophilin knockout mice, and was rescued by a D1 receptor agonist. In contrast, long-term depression was deficient in spinophilin knockout mice but not in neurabin knockout mice, and was rescued by D2 receptor activation. Spontaneous excitatory post-synaptic current frequency was increased in neurabin knockout mice, but not in spinophilin knockout mice, and this effect was normalized by D2 receptor agonist application. Both knockout strains displayed increased induction of GluR1 Ser(845) phosphorylation in response to D1 receptor stimulation in slices, and also displayed enhanced locomotor activation in response to cocaine administration. These effects could be dissociated from cocaine reward, which was enhanced only in spinophilin knockout mice, and was accompanied by increased immediate early gene induction. These data establish a requirement for synaptic scaffolding in dopamine-mediated responses, and further indicate that spinophilin and neurabin play distinct roles in dopaminergic signal transduction and psychostimulant response.

Neurabin/protein phosphatase-1 complex regulates dendritic spine morphogenesis and maturation

The majority of excitatory synapses in the mammalian brain form on filopodia and spines, actin-rich membrane protrusions present on neuronal dendrites. The biochemical events that induce filopodia and remodel these structures into dendritic spines remain poorly understood. Here, we show that the neuronal actin- and protein phosphatase-1-binding protein, neurabin-I, promotes filopodia in neurons and nonneuronal cells. Neurabin-I actin-binding domain bundled F-actin, promoted filopodia, and delayed the maturation of dendritic spines in cultured hippocampal neurons. In contrast, dimerization of neurabin-I via C-terminal coiled-coil domains and association of protein phosphatase-1 (PP1) with neurabin-I through a canonical KIXF motif inhibited filopodia. Furthermore, the expression of a neurabin-I polypeptide unable to bind PP1 delayed the maturation of neuronal filopodia into spines, reduced the synaptic targeting of AMPA-type glutamate (GluR1) receptors, and decreased AMPA receptor-mediated synaptic transmission. Reduction of endogenous neurabin levels by interference RNA (RNAi)-mediated knockdown also inhibited the surface expression of GluR1 receptors. Together, our studies suggested that disrupting the functions of a cytoskeletal neurabin/PP1 complex enhanced filopodia and impaired surface GluR1 expression in hippocampal neurons, thereby hindering the morphological and functional maturation of dendritic spines.

Phosphorylation of spinophilin by ERK and cyclin-dependent PK 5 (Cdk5)

Spinophilin is a protein that binds to protein phosphatase-1 and actin and modulates excitatory synaptic transmission and dendritic spine morphology. We have identified three sites phosphorylated by ERK2 (Ser-15 and Ser-205) and cyclin-dependent PK 5 (Cdk5) (Ser-17), within the actin-binding domain of spinophilin. Cdk5 and ERK2 both phosphorylated spinophilin in intact cells. However, in vitro, phosphorylation by ERK2, but not by Cdk5, was able to modulate the ability of spinophilin to bind to and bundle actin filaments. In neurons and HEK293 cells expressing GFP-tagged variants of spinophilin, imaging studies demonstrated that introduction of a phospho-site mimic (Ser-15 to glutamate) was associated with increased filopodial density. These results support a role for spinophilin phosphorylation by ERK2 in the regulation of spine morphogenesis.