Brain actin-associated protein phosphatase 1 holoenzymes containing spinophilin, neurabin, and selected catalytic subunit isoforms

Variants in LRRC7 lead to intellectual disability, autism, aggression and abnormal eating behaviors

Members of the leucine rich repeat (LRR) and PDZ domain (LAP) protein family are essential for animal development and histogenesis. Densin-180, encoded by LRRC7, is the only LAP protein selectively expressed in neurons. Densin-180 is a postsynaptic scaffold at glutamatergic synapses, linking cytoskeletal elements with signalling proteins such as the α-subunit of Ca2+/calmodulin-dependent protein kinase II. We have previously observed an association between high impact variants in LRRC7 and Intellectual Disability; also three individual cases with variants in LRRC7 had been described. We identify here 33 individuals (one of them previously described) with a dominant neurodevelopmental disorder due to heterozygous missense or loss-of-function variants in LRRC7. The clinical spectrum involves intellectual disability, autism, ADHD, aggression and, in several cases, hyperphagia-associated obesity. A PDZ domain variant interferes with synaptic targeting of Densin-180 in primary cultured neurons. Using in vitro systems (two hybrid, BioID, coimmunoprecipitation of tagged proteins from 293T cells) we identified new candidate interaction partners for the LRR domain, including protein phosphatase 1 (PP1), and observed that variants in the LRR reduced binding to these proteins. We conclude that LRRC7 encodes a major determinant of intellectual development and behaviour.

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.

Modulated Start-Up Mode of Cancer Cell Migration Through Spinophilin-Tubular Networks

Spinophilin (SPL) is a multifunctional actin-binding scaffolding protein. Although increased research on SPL in cancer biology has revealed a tumor suppressive role, its modulation in cancer biology, and oncological relevance remains elusive. Thus, we determined the role of SPL in the modulation of the junctional network and cellular migration in A549 lung cancer cell line. Knockdown of SPL promoted cancer cell invasion in agarose spot and scratch wound assays. Attenuation of SPL expression also enhanced invadopodia, as revealed by enhanced vinculin spots, and enhanced sodium bicarbonate cotransporter NBC activity without enhancing membranous expression of NBCn1. Disruption of the tubular structure with nocodazole treatment revealed enhanced SPL expression and reduced NBC activity and A549 migration. SPL-mediated junctional modulation and tubular stability affected bicarbonate transporter activity in A549 cells. The junctional modulatory function of SPL in start-up migration, such as remodeling of tight junctions, enhanced invadopodia, and increased NBC activity, revealed here would support fundamental research and the development of an initial target against lung cancer cell migration.

Spinophilin modulates pain through suppressing dendritic spine morphogenesis via negative control of Rac1-ERK signaling in rat spinal dorsal horn

Both spinophilin (SPN, also known as neurabin 2) and Rac1 (a member of Rho GTPase family) are believed to play key roles in dendritic spine (DS) remodeling and spinal nociception. However, how SPN interacts with Rac1 in the above process is unknown. Here, we first demonstrated natural existence of SPN-protein phosphatase 1-Rac1 complex in the spinal dorsal horn (DH) neurons by both double immunofluorescent labeling and co-immunoprecipitation, then the effects of SPN over-expression and down-regulation on mechanical and thermal pain sensitivity, GTP-bound Rac1-ERK signaling activity, and spinal DS density were studied. Over-expression of SPN in spinal neurons by intra-DH pAAV-CMV-SPN-3FLAG could block both mechanical and thermal pain hypersensitivity induced by intraplantar bee venom injection, however it had no effect on the basal pain sensitivity. Over-expression of SPN also resulted in a significant decrease in GTP-Rac1-ERK activities, relative to naive and irrelevant control (pAAV-MCS). In sharp contrast, knockdown of SPN in spinal neurons by intra-DH pAAV-CAG-eGFP-U6-shRNA[SPN] produced both pain hypersensitivity and dramatic elevation of GTP-Rac1-ERK activities, relative to naive and irrelevant control (pAAV-shRNA [NC]). Moreover, knockdown of SPN resulted in increase in DS density while over-expression of it had no such effect. Collectively, SPN is likely to serve as a regulator of Rac1 signaling to suppress DS morphogenesis via negative control of GTP-bound Rac1-ERK activities at postsynaptic component in rat DH neurons wherein both mechanical and thermal pain sensitivity are controlled.

Mutation of SPINOPHILIN (PPP1R9B) found in human tumors promotes the tumorigenic and stemness properties of cells

Rationale: SPINOPHILIN (SPN, PPP1R9B) is an important tumor suppressor involved in the progression and malignancy of different tumors depending on its association with protein phosphatase 1 (PP1) and the ability of the PP1-SPN holoenzyme to dephosphorylate retinoblastoma (pRB).

Methods: We performed a mutational analysis of SPN in human tumors, focusing on the region of interaction with PP1 and pRB. We explored the effect of the SPN-A566V mutation in an immortalized non-tumorigenic cell line of epithelial breast tissue, MCF10A, and in two different p53-mutated breast cancer cells lines, T47D and MDA-MB-468.

Results: We characterized an oncogenic mutation of SPN found in human tumor samples, SPN-A566V, that affects both the SPN-PP1 interaction and its phosphatase activity. The SPN-A566V mutation does not affect the interaction of the PP1-SPN holoenzyme with pocket proteins pRB, p107 and p130, but it affects its ability to dephosphorylate them during G0/G1 and G1, indicating that the PP1-SPN holoenzyme regulates cell cycle progression. SPN-A566V also promoted stemness, establishing a connection between the cell cycle and stem cell biology via pocket proteins and PP1-SPN regulation. However, only cells with both SPN-A566V and mutant p53 have increased tumorigenic and stemness properties.

Conclusions: SPN-A566V, or other equivalent mutations, could be late events that promote tumor progression by increasing the CSC pool and, eventually, the malignant behavior of the tumor.

Functional repertoire of protein kinases and phosphatases in synaptic plasticity and associated neurological disorders

Protein phosphorylation and dephosphorylation are two essential and vital cellular mechanisms that regulate many receptors and enzymes through kinases and phosphatases. Ca²⁺- dependent kinases and phosphatases are responsible for controlling neuronal processing; balance is achieved through opposition. During molecular mechanisms of learning and memory, kinases generally modulate positively while phosphatases modulate negatively. This review outlines some of the critical physiological and structural aspects of kinases and phosphatases involved in maintaining postsynaptic structural plasticity. It also explores the link between neuronal disorders and the deregulation of phosphatases and kinases.

Modulation of dendritic spines by protein phosphatase-1

Protein phosphatase-1 (PP-1), a highly conserved multifunctional serine/threonine phosphatase, is enriched in dendritic spines where it plays a major role in modulating excitatory synaptic activity. In addition to established functions in spine maturation and development, multi-subunit holoenzyme forms of PP-1 modulate higher-order cognitive functions such learning and memory. Mechanisms involved in regulating PP-1 activity and localization in spines include interactions with neurabin and spinophilin, structurally related synaptic scaffolding proteins associated with the actin cytoskeleton. Since PP-1 is a critical element in synaptic development, signaling, and plasticity, alterations in PP-1 signaling in dendritic spines are implicated in various neurological and psychiatric disorders. The effects of PP-1 depend on its isoform-specific association with regulatory proteins and activation of downstream signaling pathways. Here we review the role of PP-1 and its binding proteins neurabin and spinophilin in both developing and established dendritic spines, as well as some of the disorders that result from its dysregulation.

Controlling Ser/Thr protein phosphatase PP1 activity and function through interaction with regulatory subunits

Protein phosphatase 1 is a major Ser/Thr protein phosphatase activity in eukaryotic cells. It is composed of a catalytic polypeptide (PP1C), with little substrate specificity, that interacts with a large variety of proteins of diverse structure (regulatory subunits). The diversity of holoenzymes that can be formed explain the multiplicity of cellular functions under the control of this phosphatase. In quite a few cases, regulatory subunits have an inhibitory role, downregulating the activity of the phosphatase. In this chapter we shall introduce PP1C and review the most relevant families of PP1C regulatory subunits, with particular emphasis in describing the structural basis for their interaction.

Proteomic approach underlying the hippocampal neurodegeneration caused by low doses of methylmercury after long-term exposure in adult rats

Methylmercury (MeHg) is an important toxicant that causes cognitive dysfunctions in humans. This study aimed to investigate the proteomic and biochemical alterations of the hippocampus associated with behavioural consequences of low doses of MeHg in a long-term exposure model, and to realistically mimic in vivo the result of human exposure to this toxicant. Adult Wistar male rats were exposed to a dose of MeHg at 0.04 mg kg-1 day-1 by gavage for 60 days. Total mercury (Hg) content was significantly increased in the hippocampal parenchyma. The increase in the Hg levels was capable of reducing neuron and astrocyte cell density in the CA1, CA3, hilus and dentate gyrus regions, increasing both malondialdehyde and nitrite levels and decreasing antioxidant capacity against peroxyl radicals. The proteomic analysis detected 1041 proteins with altered expression due to MeHg exposure, including 364 proteins with no expression, 295 proteins with de novo expression and 382 proteins with up- or down-regulated expression. This proteomic approach revealed alterations in pathways related to chemical synapses, metabolism, amino acid transport, cell energy, neurodegenerative processes and myelin maintenance. Therefore, even at low doses of MeHg exposure, it is possible to cause hippocampal damage in adult rats at many organisational levels, triggering oxidative stress and proteome misbalance, featuring a neurodegenerative process and culminating in long- and short-term memory and learning deficits.

Protein phosphatase 1α interacts with a novel ciliary targeting sequence of polycystin-1 and regulates polycystin-1 trafficking

Autosomal dominant polycystic kidney disease (ADPKD) is the most common genetic disorder causing renal failure. Mutations of polycystic kidney disease 1 (PKD1) account for most ADPKD cases. Defective ciliary localization of polycystin-1 (PC1), a large integral membrane protein encoded by PKD1, underlies the pathogenesis of a subgroup of patients with ADPKD. However, the mechanisms by which PC1 and other ciliary proteins traffic to the primary cilium remain poorly understood. A ciliary targeting sequence (CTS) that resides in ciliary receptors is considered to function in the process. It has been reported that the VxP motif in the intracellular C-terminal tail of PC1 functions as a CTS in an ADP ribosylation factor 4 (Arf4)/ArfGAP with SH3 domain, ankyrin repeat and PH domain 1 (ASAP1)-dependent manner. However, other recent studies have revealed that this motif is dispensable for PC1 trafficking to cilia. In this study, we identified a novel CTS consisting of 8 residues (RHKVRFEG) in the PC1 C tail. We found that this motif is sufficient to bind protein phosphatase 1 (PP1)α, a ubiquitously expressed phosphatase in the phosphoprotein phosphatase (PPP) family. Mutations in this CTS motif disrupt binding with PP1α and impair ciliary localization of PC1. Additionally, short hairpin RNA-mediated knockdown of PP1α results in reduced ciliary localization of PC1 and elongated cilia, suggesting a role for PP1α in the regulation of ciliary structure and function.-Luo, C., Wu, M., Su, X., Yu, F., Brautigan, D. L., Chen, J., Zhou, J. Protein phosphatase 1α interacts with a novel ciliary targeting sequence of polycystin-1 and regulates polycystin-1 trafficking.

Mechanisms Regulating the Association of Protein Phosphatase 1 with Spinophilin and Neurabin

Protein phosphorylation is a key mediator of signal transduction, allowing for dynamic regulation of substrate activity. Whereas protein kinases obtain substrate specificity by targeting specific amino acid sequences, serine/threonine phosphatase catalytic subunits are much more promiscuous in their ability to dephosphorylate substrates. To obtain substrate specificity, serine/threonine phosphatases utilize targeting proteins to regulate phosphatase subcellular localization and catalytic activity. Spinophilin and its homologue neurabin are two of the most abundant dendritic spine-localized protein phosphatase 1 (PP1) targeting proteins. The association between spinophilin and PP1 is increased in the striatum of animal models of Parkinson's disease (PD). However, mechanisms that regulate the association of spinophilin and neurabin with PP1 are unclear. Here, we report that the association between spinophilin and PP1α or PP1γ1 was increased by CDK5 expression and activation in a heterologous cell system. This increased association is at least partially due to phosphorylation of PP1. Conversely, CDK5 expression and activation decreased the association of PP1 with neurabin. As with dopamine depletion, methamphetamine (METH) abuse causes persistent alterations in dopamine signaling which influence striatal medium spiny neuron function and biochemistry. Moreover, both METH toxicity and dopamine depletion are associated with deficits in motor control and motor learning. Pathologically, we observed a decreased association of spinophilin with PP1 in rat striatum evaluated one month following a binge METH paradigm. Behaviorally, we found that loss of spinophilin recapitulates rotarod pathology previously observed in dopamine-depleted and METH-treated animals. Together, these data have implications in multiple disease states associated with altered dopamine signaling such as PD and psychostimulant drug abuse and delineate a novel mechanism by which PP1 interactions with spinophilin and neurabin may be differentially regulated.

cAMP regulation of protein phosphatases PP1 and PP2A in brain

Normal functioning of the brain is dependent upon a complex web of communication between numerous cell types. Within neuronal networks, the faithful transmission of information between neurons relies on an equally complex organization of inter- and intra-cellular signaling systems that act to modulate protein activity. In particular, post-translational modifications (PTMs) are responsible for regulating protein activity in response to neurochemical signaling. The key second messenger, cyclic adenosine 3',5'-monophosphate (cAMP), regulates one of the most ubiquitous and influential PTMs, phosphorylation. While cAMP is canonically viewed as regulating the addition of phosphate groups through its activation of cAMP-dependent protein kinases, it plays an equally critical role in regulating removal of phosphate through indirect control of protein phosphatase activity. This dichotomy of regulation by cAMP places it as one of the key regulators of protein activity in response to neuronal signal transduction throughout the brain. In this review we focus on the role of cAMP in regulation of the serine/threonine phosphatases protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A) and the relevance of control of PP1 and PP2A to regulation of brain function and behavior.

Effective Attenuation of Adenosine A1R Signaling by Neurabin Requires Oligomerization of Neurabin

The adenosine A1 receptor (A1R) is a key mediator of the neuroprotective effect by endogenous adenosine. Yet targeting this receptor for neuroprotection is challenging due to its broad expression throughout the body. A mechanistic understanding of the regulation of A1R signaling is necessary for the future design of therapeutic agents that can selectively enhance A1R-mediated responses in the nervous system. In this study, we demonstrate that A1R activation leads to a sustained localization of regulator of G protein signaling 4 (RGS4) at the plasma membrane, a process that requires neurabin (a neural tissue-specific protein). A1R and RGS4 interact with the overlapping regions of neurabin. In addition, neurabin domains required for oligomerization are essential for formation of the A1R/neurabin/RGS4 ternary complex, as well as for stable localization of RGS4 at the plasma membrane and attenuation of A1R signaling. Thus, A1R and RGS4 each likely interact with one neurabin molecule in a neurabin homo-oligomer to form a ternary complex, representing a novel mode of regulation of G protein-coupled receptor signaling by scaffolding proteins. Our mechanistic analysis of neurabin-mediated regulation of A1R signaling in this study will be valuable for the future design of therapeutic agents that can selectively enhance A1R-mediated responses in the nervous system.

Spinophilin-Targeted Protein Phosphatase-1 Alleviated Inflammatory Pain by Negative Control of MEK/ERK Signaling in Spinal Cord Dorsal Horn of Rats

Protein phosphatase-1 (PP1), anchored by regulatory or targeting proteins at excitatory glutamatergic synapses, controls the phosphorylation of postsynaptic substrates and regulates the neurotransmission and plasticity. Here, we found that spinophilin, an actin-binding protein that targets PP1 at postsynaptic density, served as a scaffold for extracellular signal-regulated kinase (ERK) signaling components. Through the C-terminal PDZ domain, spinophilin directly interacted with ERK and its upstream mitogen-activated protein kinase kinase (MEK). PP1, recruited by spinophilin, gained access to and dephosphorylated these kinases, exerting a tonic inhibition of ERK signaling. The removal of PP1 inhibition by disturbing spinophilin/PP1 interaction allowed a restricted activation of MEK/ERK at synapses, which in turn augmented the synaptic transmission specifically mediated by GluN2B subunit-containing N-methyl-d-aspartate subtype of glutamate receptors. We provided evidence that in pain-related spinal cord dorsal horn, the scaffolding function of spinophilin played an important role in the negative control of ERK-dependent and GluN2B-dependent pain sensitization. Expression of wild-type spinophilin produced an effective analgesic action against chronic inflammatory pain induced by complete Freund's adjuvant in rats.

SIGNIFICANCE STATEMENT

Extracellular signal-regulated kinase (ERK) relays the signals from multiple transmembrane receptors to a wide range of downstream effectors critical for the regulation of neuronal excitability and plasticity. The strength and duration of ERK signaling is spatiotemporally controlled by protein phosphatases. Sustained activation of ERK has been implicated in a variety of pathological processes. The current study revealed that spinophilin, a well characterized protein phosphatase 1 (PP1) synaptic targeting protein, was able to scaffold mitogen-activated protein kinase kinase (MEK) and ERK for dephosphorylation and inactivation by PP1. The loss of PP1 inhibition, as a result of spinophilin/PP1 dissociation, led to aberrant activation of MEK/ERK signaling, which had important implications for the exaggeration of NMDA receptor-dependent nociceptive synaptic transmission in spinal cord dorsal horn.

Spinophilin Is Indispensable for the α2B Adrenergic Receptor-Elicited Hypertensive Response

The α₂ adrenergic receptor (AR) subtypes are important for blood pressure control. When activated, the α_2A subtype elicits a hypotensive response whereas the α_2B subtype mediates a hypertensive effect that counteracts the hypotensive response by the α_2A subtype. We have previously shown that spinophilin attenuates the α_2AAR-dependent hypotensive response; in spinophilin null mice, this response is highly potentiated. In this study, we demonstrate that spinophilin impedes arrestin-dependent phosphorylation and desensitization of the α_2BAR subtype by competing against arrestin binding to this receptor subtype. The Del301-303 α_2BAR, a human variation that shows impaired phosphorylation and desensitization and is linked to hypertension in certain populations, exhibits preferential interaction with spinophilin over arrestin. Furthermore, Del301-303 α_2BAR-induced ERK signaling is quickly desensitized in cells without spinophilin expression, showing a profile similar to that induced by the wild type receptor in these cells. Together, these data suggest a critical role of spinophilin in sustaining α_2BAR signaling. Consistent with this notion, our in vivo study reveals that the α_2BAR-elicited hypertensive response is diminished in spinophilin deficient mice. In arrestin 3 deficient mice, where the receptor has a stronger binding to spinophilin, the same hypertensive response is enhanced. These data suggest that interaction with spinophilin is indispensable for the α_2BAR to elicit the hypertensive response. This is opposite of the negative role of spinophilin in regulating α_2AAR-mediated hypotensive response, suggesting that spinophilin regulation of these closely related receptor subtypes can result in distinct functional outcomes in vivo. Thus, spinophilin may represent a useful therapeutic target for treatment of hypertension.

Protein phosphatase 1 catalytic isoforms: specificity toward interacting proteins

The coordinated and reciprocal action of serine-threonine protein kinases and protein phosphatases produces transitory phosphorylation, a fundamental regulatory mechanism for many biological processes. Phosphoprotein phosphatase 1 (PPP1), a major serine-threonine phosphatase, in particular, is ubiquitously distributed and regulates a broad range of cellular functions, including glycogen metabolism, cell cycle progression, and muscle relaxation. PPP1 has evolved effective catalytic machinery but in vitro lacks substrate specificity. In vivo, its specificity is achieved not only by the existence of different PPP1 catalytic isoforms, but also by binding of the catalytic moiety to a large number of regulatory or targeting subunits. Here, we will address exhaustively the existence of diverse PPP1 catalytic isoforms and the relevance of their specific partners and consequent functions.

Low expression of the putative tumour suppressor spinophilin is associated with higher proliferative activity and poor prognosis in patients with hepatocellular carcinoma

BACKGROUND

Spinophilin, a multifunctional intracellular scaffold protein, is reduced in certain types of cancer and is regarded as a novel putative tumour suppressor protein. However, the role of spinophilin in hepatocellular carcinoma (HCC) has never been explored before.

METHODS

In this study, we determined for the first time the expression pattern of spinophilin in human HCC by immunohistochemistry and quantitative reverse transcriptase-PCR analysis. In addition, we performed immunohistochemical analysis of p53, p14(ARF) and the proliferation marker Ki-67. Kaplan-Meier curves and multivariate Cox proportional models were used to study the impact on clinical outcome. Small interfering RNA (siRNA) was used to silence spinophilin and to explore the effects of reduced spinophilin expression on cellular growth.

RESULTS

In our study, complete loss of spinophilin immunoreactivity was found in 44 of 104 HCCs (42.3%) and reduced levels were found in an additional 37 (35.6%) cases. After adjusting for other prognostic factors, multivariate Cox regression analysis identified low expression of spinophilin as an independent prognostic factor with respect to disease-free (hazard ratio (HR)=1.8; 95% confidence interval (CI)=1.04-3.40; P=0.043) and cancer-specific survival (HR=2.0; CI=1.1-3.8; P=0.025). Reduced spinophilin expression significantly correlated with higher Ki-67 index in HCC (P=0.014). Reducing spinophilin levels by siRNA induced a higher cellular growth rate and increased cyclin D2 expression in tumour cells (P<0.05).

CONCLUSION

This is the first study of the expression pattern and distribution of spinophilin in HCC. According to our data, the loss of spinophilin is associated with higher proliferation and might be useful as a prognostic marker in patients with HCC.

Protein phosphatase 1γ isoforms linked interactions in the brain

Posttranslational protein modifications, in particular reversible protein phosphorylation, are important regulatory mechanisms involved in cellular signaling transduction pathways. Thousands of human proteins are phosphorylatable and the tight regulation of phosphorylation states is crucial for cell maintenance and development. Protein phosphorylation occurs primarily on serine, threonine, and tyrosine residues, through the antagonistic actions of protein kinases and phosphatases. The catalytic subunit of protein phosphatase 1 (PP1), a major Ser/Thr-phosphatase, associates with a large variety of regulatory subunits that define substrate specificity and determine specific cellular pathway responses. PP1 has been shown to bind to different proteins in the brain in order to execute key and differential functions. This work reports the identification of proteins expressed in the human brain that interact with PP1γ1 and PP1γ2 isoforms by the yeast two-hybrid method. An extensive search of PP1-binding motifs was performed for the proteins identified, revealing already known PP1 regulators but also novel interactors. Moreover, our results were integrated with the data of PP1γ interacting proteins from several public web databases, permitting the development of physical maps of the novel interactions. The PP1γ interactome thus obtained allowed for the identification of novel PP1 interacting proteins, supporting novel functions of PP1γ isoforms in the human brain.

Neurabin scaffolding of adenosine receptor and RGS4 regulates anti-seizure effect of endogenous adenosine

Endogenous adenosine is an essential protective agent against neural damage by various insults to the brain. However, the therapeutic potential of adenosine receptor-directed ligands for neuroprotection is offset by side effects in peripheral tissues and organs. An increase in adenosine receptor responsiveness to endogenous adenosine would enhance neuroprotection while avoiding the confounding effects of exogenous ligands. Here we report novel regulation of adenosine-evoked responses by a neural tissue-specific protein, neurabin. Neurabin attenuated adenosine A(1) receptor (A1R) signaling by assembling a complex between the A1R and the regulator of G-protein signaling 4 (RGS4), a protein known to turn off G-protein signaling. Inactivation of the neurabin gene enhanced A1R signaling and promoted the protective effect of adenosine against excitotoxic seizure and neuronal death in mice. Furthermore, administration of a small molecule inhibitor of RGS4 significantly attenuated seizure severity in mice. Notably, the dose of kainate capable of inducing an ∼50% rate of death in wild-type (WT) mice did not affect neurabin-null mice or WT mice cotreated with an RGS4 inhibitor. The enhanced anti-seizure and neuroprotective effect achieved by disruption of the A1R/neurabin/RGS4 complex is elicited by the on-site and on-demand release of endogenous adenosine, and does not require administration of A1R ligands. These data identify neurabin-RGS4 as a novel tissue-selective regulatory mechanism for fine-tuning adenosine receptor function in the nervous system. Moreover, these findings implicate the A1R/neurabin/RGS4 complex as a valid therapeutic target for specifically manipulating the neuroprotective effects of endogenous adenosine.

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 and validation of novel spinophilin-associated proteins in rodent striatum using an enhanced ex vivo shotgun proteomics approach

Spinophilin regulates excitatory postsynaptic function and morphology during development by virtue of its interactions with filamentous actin, protein phosphatase 1, and a plethora of additional signaling proteins. To provide insight into the roles of spinophilin in mature brain, we characterized the spinophilin interactome in subcellular fractions solubilized from adult rodent striatum by using a shotgun proteomics approach to identify proteins in spinophilin immune complexes. Initial analyses of samples generated using a mouse spinophilin antibody detected 23 proteins that were not present in an IgG control sample; however, 12 of these proteins were detected in complexes isolated from spinophilin knock-out tissue. A second screen using two different spinophilin antibodies and either knock-out or IgG controls identified a total of 125 proteins. The probability of each protein being specifically associated with spinophilin in each sample was calculated, and proteins were ranked according to a chi(2) analysis of the probabilities from analyses of multiple samples. Spinophilin and the known associated proteins neurabin and multiple isoforms of protein phosphatase 1 were specifically detected. Multiple, novel, spinophilin-associated proteins (myosin Va, calcium/calmodulin-dependent protein kinase II, neurofilament light polypeptide, postsynaptic density 95, alpha-actinin, and densin) were then shown to interact with GST fusion proteins containing fragments of spinophilin. Additional biochemical and transfected cell imaging studies showed that alpha-actinin and densin directly interact with residues 151-300 and 446-817, respectively, of spinophilin. Taken together, we have developed a multi-antibody, shotgun proteomics approach to characterize protein interactomes in native tissues, delineating the importance of knock-out tissue controls and providing novel insights into the nature and function of the spinophilin interactome in mature striatum.

Actin and Actin-Binding Proteins: Masters of Dendritic Spine Formation, Morphology, and Function

Dendritic spines are actin-rich protrusions that comprise the postsynaptic sites of synapses and receive the majority of excitatory synaptic inputs in the central nervous system. These structures are central to cognitive processes, and alterations in their number, size, and morphology are associated with many neurological disorders. Although the actin cytoskeleton is thought to govern spine formation, morphology, and synaptic functions, we are only beginning to understand how modulation of actin reorganization by actin-binding proteins (ABPs) contributes to the function of dendritic spines and synapses. In this review, we discuss what is currently known about the role of ABPs in regulating the formation, morphology, motility, and plasticity of dendritic spines and synapses.

Association of protein phosphatase 1 gamma 1 with spinophilin suppresses phosphatase activity in a Parkinson disease model

Sustained nigrostriatal dopamine depletion increases the serine/threonine phosphorylation of multiple striatal proteins that play a role in corticostriatal synaptic plasticity, including Thr(286) phosphorylation of calcium/calmodulin-dependent protein kinase IIalpha (CaMKIIalpha). Mechanisms underlying these changes are unclear, but protein phosphatases play a critical role in the acute modulation of striatal protein phosphorylation. Here we show that dopamine depletion for periods ranging from 3 weeks to 10 months significantly reduces the total activity of protein phosphatase (PP) 1, but not of PP2A, in whole lysates of rat striatum, as measured using multiple substrates, including Thr(286)-autophosphorylated CaMKIIalpha. Striatal PP1 activity is partially inhibited by a fragment of the PP1-binding protein neurabin-I, Nb-(146-493), because of the selective inhibition of the PP1gamma(1) isoform. The fraction of PP1 activity that is insensitive to Nb-(146-493) was unaffected by dopamine depletion, demonstrating that dopamine depletion specifically reduces the activity of PP1 isoforms that are sensitive to Nb-(146-493) (i.e. PP1gamma(1)). However, total striatal levels of PP1gamma(1) or any other PP1 isoform were unaffected by dopamine depletion, and our previous studies showed that total levels of the PP1 regulatory/targeting proteins DARPP-32, spinophilin, and neurabin were also unchanged. Rather, co-immunoprecipitation experiments demonstrated that dopamine depletion increases the association of PP1gamma(1) with spinophilin in striatal extracts. In combination, these data demonstrate that striatal dopamine depletion inhibits a specific synaptic phosphatase by increasing PP1gamma(1) interaction with spinophilin, perhaps contributing to hyperphosphorylation of synaptic proteins and disruptions of synaptic plasticity and/or dendritic morphology.

Drosophila Uri, a PP1alpha binding protein, is essential for viability, maintenance of DNA integrity and normal transcriptional activity

BACKGROUND

Protein phosphatase 1 (PP1) is involved in diverse cellular processes, and is targeted to substrates via interaction with many different protein binding partners. PP1 catalytic subunits (PP1c) fall into PP1alpha and PP1beta subfamilies based on sequence analysis, however very few PP1c binding proteins have been demonstrated to discriminate between PP1alpha and PP1beta.

RESULTS

URI (unconventional prefoldin RPB5 interactor) is a conserved molecular chaperone implicated in a variety of cellular processes, including the transcriptional response to nutrient signalling and maintenance of DNA integrity. We show that Drosophila Uri binds PP1alpha with much higher affinity than PP1beta, and that this ability to discriminate between PP1c forms is conserved to humans. Most Uri is cytoplasmic, however we found some protein associated with active RNAPII on chromatin. We generated a uri loss of function allele, and show that uri is essential for viability in Drosophila. uri mutants have transcriptional defects, reduced cell viability and differentiation in the germline, and accumulate DNA damage in their nuclei.

CONCLUSION

Uri is the first PP1alpha specific binding protein to be described in Drosophila. Uri protein plays a role in transcriptional regulation. Activity of uri is required to maintain DNA integrity and cell survival in normal development.

Selective targeting of the gamma1 isoform of protein phosphatase 1 to F-actin in intact cells requires multiple domains in spinophilin and neurabin

Protein phosphatase 1 (PP1) catalytic subunits dephosphorylate specific substrates in discrete subcellular compartments to modulate many cellular processes. Canonical PP1-binding motifs (R/K-V/I-X-F) in a family of proteins mediate subcellular targeting, and the amino acids that form the binding pocket for the canonical motif are identical in all PP1 isoforms. However, PP1gamma1 but not PP1beta is selectively localized to F-actin-rich dendritic spines in neurons. Although the F-actin-binding proteins neurabin I and spinophilin (neurabin II) also bind PP1, their role in PP1 isoform selective targeting in intact cells is poorly understood. We show here that spinophilin selectively targets PP1gamma1, but not PP1beta, to F-actin-rich cortical regions of intact cells. Mutation of a PP1gamma1 selectivity determinant (N(464)EDYDRR(470) in spinophilin: conserved as residues 473-479 in neurabin) to VKDYDTW severely attenuated PP1gamma1 interactions with neurabins in vitro and in cells and disrupted PP1gamma1 targeting to F-actin. This domain is not involved in the weaker interactions of neurabins with PP1beta. In contrast, mutation of the canonical PP1-binding motif attenuated interactions of neurabins with both isoforms. Thus, selective targeting of PP1gamma1 to F-actin by neurabins in intact cells requires both the canonical PP1-binding motif and an auxiliary PP1gamma1-selectivity determinant.

Essential, overlapping and redundant roles of the Drosophila protein phosphatase 1 alpha and 1 beta genes

Protein serine/threonine phosphatase type 1 (PP1) has been found in all eukaryotes examined to date and is involved in the regulation of many cellular functions, including glycogen metabolism, muscle contraction, and mitosis. In Drosophila, four genes code for the catalytic subunit of PP1 (PP1c), three of which belong to the PP1 alpha subtype. PP1 beta 9C (flapwing) encodes the fourth PP1c gene and has a specific and nonredundant function as a nonmuscle myosin phosphatase. PP1 alpha 87B is the major form and contributes approximately 80% of the total PP1 activity. We describe the first mutant alleles of PP1 alpha 96A and show that PP1 alpha 96A is not an essential gene, but seems to have a function in the regulation of nonmuscle myosin. We show that overexpression of the PP1 alpha isozymes does not rescue semilethal PP1 beta 9C mutants, whereas overexpression of either PP1 alpha 96A or PP1 beta 9C does rescue a lethal PP1 alpha 87B mutant combination, showing that the lethality is due to a quantitative reduction in the level of PP1c. Overexpression of PP1 beta 9C does not rescue a PP1 alpha 87B, PP1 alpha 96A double mutant, suggesting an essential PP1 alpha-specific function in Drosophila.

Proteomic characterization of protein phosphatase 1 complexes in ischemia-reperfusion and ischemic tolerance

Serine/threonine protein phosphatase 1 (PP1) regulates multiple cellular processes. Protein phosphorylation-dephosphorylation is largely altered during ischemia and subsequent reperfusion. The brain is particularly vulnerable to stress resulting from ischemia-reperfusion (IR), however, the acquisition of ischemic tolerance (IT) protects against IR stress. We studied PP1 complexes in response to IR stress and IT in brain using proteomic characterization of PP1 complexes in animal models of IR and IT. PP1alpha and PP1gamma were immunoprecipitated and resolved by 2-D. DIGE analysis detected 14 different PP1-interacting proteins that exhibited significant changes in their association with PP1alpha or PP1gamma. These proteins were identified by MALDI-TOF MS. Seven had the PP1-binding RVxF motif. IR altered the interaction of heat shock cognate 71 kDa-protein, creatine kinase B, and dopamine- and cAMP-regulated phosphoprotein 32 kDa (DARPP32) with both PP1alpha and PP1gamma, and the interaction of phosphodiesterase-6B, transitional ER ATPase, lamin-A, glucose-regulated 78 kDa-protein, dihydropyrimidinase-related protein-2, gamma-enolase, neurofilament-L, and ubiquitin ligase SIAH2 with PP1gamma. IT prevented most of the IR-induced effects. This study identifies novel PP1alpha- and PP1gamma-interacting proteins and reveals an in vivo modularity of PP1 holoenzymes in response to physiological ischemic stress. It supports a potential role of PP1 in IR stress and as a target of the endogenous protective mechanisms induced by IT.

Structural and functional analysis of the apoptosis-associated tyrosine kinase (AATYK) family

Apoptosis-associated tyrosine kinase (AATYK) is a protein kinase that is predominantly expressed in the nervous system and is involved in apoptosis and neurite growth of cerebellar granule cells. In this study, we cloned three new members of the mouse AATYK family, AATYK1B, AATYK2 and AATYK3. AATYK1B is a splicing variant of the previously reported AATYK1 (referred to as AATYK1A hereafter). In comparison with AATYK1A, these three AATYK members were characterized by having an extra N-terminal region that consists of a signal peptide-like sequence and a predicted transmembrane (TM) region, which is followed by a kinase domain and a long C-terminal domain. Both TM-containing AATYK isoforms (AATYK(+)TM: AATYK1B, 2, and 3) and TM-lacking isoform (AATYK(-)TM: AATYK1A) were recovered in membrane fractions, suggesting that AATYK(+)TM and AATYK(-)TM are transmembrane- and peripheral-membrane protein kinases, respectively. AATYK1A was recovered in the soluble fraction when the cells were treated with 2-bromo palmitate, suggesting that AATYK1A associates with membrane via palmitoylation. The kinase domain was highly conserved among all AATYK members and was shown to be catalytically active. Three AATYK family members were predominantly expressed in adult mouse brains with almost similar expression profiles: widespread distribution over the various brain regions, especially in the cerebellum and hippocampus, and up-regulated expression during development of the cerebellum. In cultured cerebellar granule cells, AATYK1 was abundantly localized in both soma and axons, AATYK2 distribution was restricted to soma, and AATYK3 was punctately present over the cells. AATYK1 was concentrated in the central domain of growth cones of dorsal root ganglion neurons. Our results indicate that AATYK family members are brain-dominant and membrane-associated kinases with slightly different distribution patterns in the developing and adult mouse brain, which may be involved in fine regulation of neuronal functions including neurite extension and apoptosis.

Differential regulation of protein phosphatase-1(I) by neurabin

Neurabin is a brain-specific actin and protein phosphatase-1 (PP-1) binding protein that inhibits the purified catalytic subunit of protein phosphatase-1 (PP-1(C)). However, endogenous PP-1 exists primarily as multimeric complexes of PP-1(C) bound to various regulatory proteins that determine its activity, substrate specificity, subcellular localization and function. The major form of endogenous PP-1 in brain is protein phosphatase-1(I) (PP-1(I)), a Mg(2+)/ATP-dependent form of PP-1 that consists of PP-1(C), the inhibitor-2 regulatory subunit, an activating protein kinase and other unidentified proteins. We have identified four PP-1(I) holoenzyme fractions (PP-1(IA), PP-1(IB), PP-1(IC), and PP-1(ID)) in freshly harvested pig brain separable by poly-L-lysine chromatography. Purified recombinant neurabin (amino acid residues 1-485) inhibited PP-1(IB) (IC(50)=1.1 microM), PP-1(IC) (IC(50)=0.1 microM), and PP-1(ID) (IC(50)=0.2 microM), but activated PP-1(IA) by up to threefold (EC(50)=40 nM). The PP-1(IA) activation domain was localized to neurabin(1-210). Our results indicate a novel mechanism of PP-1 regulation by neurabin as both an inhibitor and an activator of distinct forms of PP-1(I) in brain.

Spinophilin facilitates dephosphorylation of doublecortin by PP1 to mediate microtubule bundling at the axonal wrist

The axonal shafts of neurons contain bundled microtubules, whereas extending growth cones contain unbundled microtubule filaments, suggesting that localized activation of microtubule-associated proteins (MAP) at the transition zone may bundle these filaments during axonal growth. Dephosphorylation is thought to lead to MAP activation, but specific molecular pathways have remained elusive. We find that Spinophilin, a Protein-phosphatase 1 (PP1) targeting protein, is responsible for the dephosphorylation of the MAP Doublecortin (Dcx) Ser 297 selectively at the "wrist" of growing axons, leading to activation. Loss of activity at the "wrist" is evident as an impaired microtubule cytoskeleton along the shaft. These findings suggest that spatially restricted adaptor-specific MAP reactivation through dephosphorylation is important in organization of the neuronal cytoskeleton.

Ectopic doublecortin gene expression suppresses the malignant phenotype in glioblastoma cells

Doublecortin (DCX) is one of the three genes found from Affymetrix gene chip analysis related to glioma patient survival. Two other genes (e.g., osteonectin and semaphorin 3B) are well characterized as antioncogenic and tumor suppressor genes. However, there is no report about the involvement of DCX in cancer. Here, we show that gene transfer technology into DCX-deficient glioblastoma cell lines, such as A172, U87, U251N, RG2, and 9L, with DCX cDNA significantly suppressed growth of these glioma cells. U87 cells with ectopic expression of DCX exhibit a marked suppression of the transformed phenotype as growth arrested in the G(2) phase of the cell cycle progression, small colony formation in soft agar, and no tumor formation in nude rats. This transformed phenotype can be restored by knocking down DCX expression with DCX small interfering RNA. DCX was highly phosphorylated in glioma cells. Phosphorylation in the glioma cells was greater than in noncancer cells such as mouse NIH 3T3 and human embryonic kidney 293T cells. Coimmunoprecipitation of the phosphorylated DCX and spinophilin/neurabin II from DCX-synthesizing glioma cells indicated their interaction. This interaction would lead to a block of anchorage-independent growth as neurabin II is a synergistic inhibitor of anchorage-independent growth with p14ARF (ARF). Interaction between phosphorylated DCX and neurabin II may induce the association of the protein phosphatase 1 catalytic subunit (PP1) with neurabin II and inactivate PP1 and block mitosis during G(2) and M phases of the cell cycle progression. Thus, DCX seems to be a tumor suppressor of glioma.

Regulation of alpha2AR trafficking and signaling by interacting proteins

The continuing discovery of new G protein-coupled receptor (GPCR) interacting proteins and clarification of the functional consequences of these interactions has revealed multiple roles for these events. Some of these interactions serve to scaffold GPCRs to particular cellular micro-compartments or to tether them to defined signaling molecules, while other GPCR-protein interactions control GPCR trafficking and the kinetics of GPCR-mediated signaling transduction. This review provides a general overview of the variety of GPCR-protein interactions reported to date, and then focuses on one prototypical GPCR, the alpha(2)AR, and the in vitro and in vivo significance of its reciprocal interactions with arrestin and spinophilin. It seems appropriate to recognize the life and career of Arthur Hancock with a summary of studies that both affirm and surprise our preconceived notions of how nature is designed, as his career-long efforts similarly affirmed the complexity of human biology and attempted to surprise pathological changes in that biology with novel, discovery-based therapeutic interventions. Dr. Hancock's love of life, of family, and of commitment to making the world a better place are a model of the life well lived, and truly missed by those who were privileged to know, and thus love, him.

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.

Actin-associated neurabin-protein phosphatase-1 complex regulates hippocampal plasticity

Protein phosphatase-1 (PP1) has been implicated in the control of long-term potentiation (LTP) and depression (LTD) in rat hippocampal CA1 neurons. PP1 catalytic subunits associate with multiple postsynaptic regulatory subunits, but the PP1 complexes that control hippocampal LTP and LTD in the rat hippocampus remain unidentified. The neuron-specific actin-binding protein, neurabin-I, is enriched in dendritic spines, and tethers PP1 to actin-rich postsynaptic density to regulate morphology and maturation of spines. The present studies utilized Sindbis virus-mediated expression of wild-type and mutant neurabin-I polypeptides in organotypic cultures of rat hippocampal slices to investigate their role in synaptic plasticity. While wild-type neurabin-I elicited no change in basal synaptic transmission, it enhanced LTD and inhibited LTP in CA1 pyramidal neurons. By comparison, mutant neurabins, specifically those unable to bind PP1 or F-actin, decreased basal synaptic transmission, attenuated LTD and increased LTP in slice cultures. Biochemical and cell biological analyses suggested that, by mislocalizing synaptic PP1, the mutant neurabins impaired the functions of endogenous neurabin-PP1 complexes and modulated LTP and LTD. Together, these studies provided the first biochemical and physiological evidence that a postsynaptic actin-bound neurabin-I-PP1 complex regulates synaptic transmission and bidirectional changes in hippocampal plasticity.

The evolving doublecortin (DCX) superfamily

Background

Doublecortin (DCX) domains serve as protein-interaction platforms. Mutations in members of this protein superfamily are linked to several genetic diseases. Mutations in the human DCX gene result in abnormal neuronal migration, epilepsy, and mental retardation; mutations in RP1 are associated with a form of inherited blindness, and DCDC2 has been associated with dyslectic reading disabilities.

Results

The DCX-repeat gene family is composed of eleven paralogs in human and in mouse. Its evolution was followed across vertebrates, invertebrates, and was traced to unicellular organisms, thus enabling following evolutionary additions and losses of genes or domains. The N-terminal and C-terminal DCX domains have undergone sub-specialization and divergence. Developmental in situ hybridization data for nine genes was generated. In addition, a novel co-expression analysis for most human and mouse DCX superfamily-genes was performed using high-throughput expression data extracted from Unigene. We performed an in-depth study of a complete gene superfamily using several complimentary methods.

Conclusion

This study reveals the existence and conservation of multiple members of the DCX superfamily in different species. Sequence analysis combined with expression analysis is likely to be a useful tool to predict correlations between human disease and mouse models. The sub-specialization of some members due to restricted expression patterns and sequence divergence may explain the successful addition of genes to this family throughout evolution.

Site-specific dephosphorylation of doublecortin (DCX) by protein phosphatase 1 (PP1)

Mutations in doublecortin (DCX) cause X-linked lissencephaly ("smooth brain") and double cortex syndrome in humans. DCX is highly phosphorylated in migrating neurons. Here, we demonstrate that dephosphorylation of specific sites phosphorylated by JNK is mediated by Neurabin II, which recruits the phosphatase PP1. During cortical development, the expression pattern of PP1 is widespread, while the expression of DCX and Neurabin II is dynamic, and they are coexpressed in migrating neurons. In vitro, DCX is site-specific dephosphorylated by PP1 without the presence of Neurabin II, this dephosphorylation requires an intact RVXF motif in DCX. Overexpression of the coiled-coil domain of Neurabin II, which is sufficient for interacting with DCX and recruiting the endogenous Neurabin II with PP1, induced dephosphorylation of DCX on one of the JNK-phosphorylated sites. We hypothesize that the transient recruitment of DCX to different scaffold proteins, JIP-1/2, which will regulate its phosphorylation by JNK, and Neurabin II, which will regulate its dephosphorylation by PP1, plays an important role in normal neuronal migration.

Bifocal and PP1 interaction regulates targeting of the R-cell growth cone in Drosophila

Bifocal is a putative cytoskeletal regulator and a Protein phosphatase-1 (PP1) interacting protein that mediates normal photoreceptor morphology in Drosophila. We show here that Bif and PP1-87B as well as their ability to interact with each other are required for photoreceptor growth cone targeting in the larval visual system. Single mutants for bif or PP1-87B show defects in axonal projections in which the axons of the outer photoreceptors bypass the lamina, where they normally terminate. The data show that the functions of bif and PP1-87B in either stabilizing R-cell morphology (for Bif) or regulating the cell cycle (for PP1-87B) can be uncoupled from their function in visual axon targeting. Interestingly, the axon targeting phenotypes are observed in both PP1-87B mutants and PP1-87B overexpression studies, suggesting that an optimal PP1 activity may be required for normal axon targeting. bif mutants also display strong genetic interactions with receptor tyrosine phosphatases, dptp10d and dptp69d, and biochemical studies demonstrate that Bif interacts directly with F-actin in vitro. We propose that, as a downstream component of axon signaling pathways, Bif regulates PP1 activity, and both proteins influence cytoskeleton dynamics in the growth cone of R cells to allow proper axon targeting.

Dopamine depletion alters phosphorylation of striatal proteins in a model of Parkinsonism

Nigrostriatal dopamine depletion disrupts striatal medium spiny neuron morphology in Parkinson's disease and modulates striatal synaptic plasticity in animal models of parkinsonism. We demonstrate that long-term nigrostriatal dopamine depletion in the rat induces evolving changes in the phosphorylation of striatal proteins critical for synaptic plasticity. Dopamine depletion increased the phosphorylation of the alpha isoform of calcium-calmodulin-dependent protein kinase II (CaMKIIalpha) at Thr286, a site associated with enhanced autonomous kinase activity, but did not alter total levels of CaMKIIalpha or other synaptic proteins. Dopamine depletion decreased CaMKIIalpha levels in postsynaptic density-enriched fractions without significant changes in other proteins. The activity of protein phosphatase 1 (PP1), a postsynaptic phosphatase that dephosphorylates CaMKII, is regulated by DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of 32 kDa). Dopamine depletion had no effect on DARPP-32 phosphorylation at Thr34, but increased DARPP-32 phosphorylation at Thr75. Levodopa administration reversed the increased phosphorylation of both CaMKIIalpha and DARPP-32. Normal ageing increased the levels of PP1(gamma1 isoform) but decreased levels of the PP1gamma1-targeting proteins spinophilin and neurabin. Elevated phosphorylations of CaMKIIalpha and DARPP-32 were maintained for up to 20 months after dopamine depletion. However, phosphorylation of the CaMKII-PP1 substrate, Ser831 in the glutamate receptor GluR1 subunit, was increased only after sustained (9-20 months) dopamine depletion. Interaction of ageing-related changes in PP1 with the dopamine depletion-induced changes in CaMKIIalpha may account for enhanced GluR1 phosphorylation only after long-term dopamine depletion. These evolving changes may impact striatal synaptic plasticity, Parkinson's disease progression and the changing efficacy and side-effects associated with dopamine replacement therapy.

The Rho-specific GEF Lfc interacts with neurabin and spinophilin to regulate dendritic spine morphology

Neurabin and spinophilin are homologous protein phosphatase 1 and actin binding proteins that regulate dendritic spine function. A yeast two-hybrid analysis using the coiled-coil domain of neurabin revealed an interaction with Lfc, a Rho GEF. Lfc was highly expressed in brain, where it interacted with either neurabin or spinophilin. In neurons, Lfc was largely found in the shaft of dendrites in association with microtubules but translocated to spines upon neuronal stimulation. Moreover, expression of Lfc resulted in reduction in spine length and size. Both the translocation and the effect on spine morphology depended on the coiled-coil domain of Lfc. Coexpression of neurabin or spinophilin with Lfc resulted in their clustering together with F-actin, a process that depended on Rho activity. Thus, interaction between Lfc and neurabin/spinophilin selectively regulates Rho-dependent organization of F-actin in spines and is a link between the microtubule and F-actin cytoskeletons in dendrites.

CaMKIIα enhances the desensitization of NR2B-containing NMDA receptors by an autophosphorylation-dependent mechanism

Long-term potentiation or depression of synaptic function often requires Ca2+ influx via NMDA-type glutamate receptors (NMDARs) and changes in the autophosphorylation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) at Thr286. Autophosphorylated CaMKII binds directly to NMDAR subunits, co-localizes with NMDARs in the postsynaptic density, and phosphorylates NR2B subunits at Ser1303. Here, we demonstrate that CaMKIIalpha enhances the extent and/or rate of desensitization of NMDA-induced macroscopic currents in HEK293 cells co-expressing NR2B with either the NR1(011) or NR1(101) splice variants, without significantly changing other current parameters. In contrast, the extent of desensitization of NMDARs containing NR2A in place of NR2B is significantly decreased by co-expression of CaMKIIalpha. Kinases harboring K42R (inactive kinase) or T286A (autophosphorylation-deficient) mutations are defective in enhancing the desensitization of NR1/NR2B channels. In addition, the CaMKII-dependent enhancement of NR1/NR2B channel desensitization is abrogated by intracellular loading with BAPTA. These data suggest a novel mechanism for Ca2+-dependent negative-feedback regulation of NR2B-containing NMDARs in a CaMKII activity- and autophosphorylation-dependent manner that may modulate NMDAR-mediated synaptic plasticity.

The stability of a stochastic CaMKII switch: dependence on the number of enzyme molecules and protein turnover

Molecular switches have been implicated in the storage of information in biological systems. For small structures such as synapses, these switches are composed of only a few molecules and stochastic fluctuations are therefore of importance. Such fluctuations could potentially lead to spontaneous switch reset that would limit the lifetime of information storage. We have analyzed a model of the calcium/calmodulin-dependent protein kinase II (CaMKII) switch implicated in long-term memory in the nervous system. The bistability of this switch arises from autocatalytic autophosphorylation of CaMKII, a reaction that is countered by a saturable phosphatase-1-mediated dephosphorylation. We sought to understand the factors that control switch stability and to determine the functional relationship between stability and the number of molecules involved. Using Monte Carlo simulations, we found that the lifetime of states of the switch increase exponentially with the number of CaMKII holoenzymes. Switch stability requires a balance between the kinase and phosphatase rates, and the kinase rate must remain high relative to the rate of protein turnover. Thus, a critical limit on switch stability is set by the observed turnover rate (one per 30 h on average). Our computational results show that, depending on the timescale of fluctuations in enzyme numbers, for a switch composed of about 15 CaMKII holoenzymes, the stable persistent activation can span from a few years to a human lifetime.

Computational modeling indicates that autophosphorylation of CaMKII can create stable persistent activation lasting several years

Protein Phosphatase Regulation of Na+/H+ Exchanger Isoform I

We investigated regulation of Na(+)/H(+) exchanger isoform 1 (NHE1) by dephosphorylation. Treatment of primary cultures of cardiomyocytes with the phosphatase inhibitor okadaic acid increased the rate of recovery from an acid load, suggesting that the okadaic acid sensitive PP1 may be involved in NHE1 regulation in vivo. We examined the ability of purified protein phosphatases PP1, PP2A, and PP2B to dephosphorylate the regulatory cytoplasmic tail. NHE1 was completely dephosphorylated by PP1, poorly dephosphorylated by PP2A, and not dephosphorylated by PP2B. Examination of NHE1 binding to PP1 or PP2B revealed that an association occurs between NHE1 and PP1 both in vitro and in vivo, but NHE1 did not associate with full-length PP2B. We expressed PP1 or inhibitor 2, a specific PP1 inhibitor, in cell lines to examine the effect of PP1 on NHE1 activity in vivo. Overexpression of PP1 causes a decrease in NHE1 activity but does not affect stimulation by thrombin. Cell lines expressing the specific PP1 inhibitor, inhibitor 2, had elevated proton efflux rates and could not be further stimulated by the Na(+)/H(+) exchanger agonist thrombin. The results suggest that PP1 is an important regulatory phosphatase of NHE1, that it can bind to and dephosphorylate the protein, and that it regulates NHE1 activity in vivo.

Differential localization of protein phosphatase-1alpha, beta and gamma1 isoforms in primate prefrontal cortex

Prefrontal cortical functioning depends on D1 family receptors and their complex signal transduction cascade, including protein phosphatase-1 (PP1). Three PP1 isoforms are prominent in the brain: PP1alpha, PP1beta and PP1gamma1. PP1 localization by a variety of scaffolding proteins is critical for dopamine-mediated modulation of glutamatergic neurotransmission. We have quantified the subcellular distribution of each isoform in primate prefrontal cortex using immunoelectron microscopy. All three are found in spines, dendrites, axon terminals, axons and glia. However, PP1alpha and PP1gamma1 labeling is enriched in spines, whereas PP1beta label is enriched in dendrites. Using post-embedding immunogold labeling, we further examined the distribution of PP1alpha and PP1gamma1 within spines. PP1gamma1 is highly and specifically concentrated in the postsynaptic density (PSD) of these spines, while PP1alpha is enriched in the PSD but also found subjacent to the PSD in moderate amounts. Thus, PP1 isoforms are heterogeneously distributed in the cortical neuropil and within spines. These results suggest that each PP1 isoform has access to a different set of substrates and, furthermore, they demonstrate that the composition of signal transduction proteins varies in different parts of the neuron and even in different regions of a dendritic spine in the primate PFC.

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.

Induction of spine growth and synapse formation by regulation of the spine actin cytoskeleton

We explored the relationship between regulation of the spine actin cytoskeleton, spine morphogenesis, and synapse formation by manipulating expression of the actin binding protein NrbI and its deletion mutants. In pyramidal neurons of cultured rat hippocampal slices, NrbI is concentrated in dendritic spines by binding to the actin cytoskeleton. Expression of one NrbI deletion mutant, containing the actin binding domain, dramatically increased the density and length of dendritic spines with synapses. This hyperspinogenesis was accompanied by enhanced actin polymerization and spine motility. Synaptic strengths were reduced to compensate for extra synapses, keeping total synaptic input per neuron constant. Our data support a model in which synapse formation is promoted by actin-powered motility.

Localization of myosin phosphatase target subunit 1 in rat brain and in primary cultures of neuronal cells

Myosin phosphatase (PP1M) is composed of the delta isoform of the PP1 catalytic subunit (PP1cdelta), the myosin phosphatase target subunit (MYPT), and a 20 kDa subunit. Western blots detected higher amounts of the MYPT1 isoform compared to MYPT2 in whole brain extracts. The localization of MYPT1 was studied in rat brain and in primary cell cultures of neurons using specific antibodies. Analysis of lysates of brain regions for MYPT1 and PP1M by Western blots using anti-MYPT1 antibodies and by phosphatase assays with myosin as substrate suggested a ubiquitous distribution. Immunohistochemistry of tissue sections revealed that MYPT1 was distributed in all areas of the brain, with staining observed in many different cell types. Depending on the method used for fixation, the MYPT1 appeared with varying intensity in nuclei, in nucleoli, and in the cytoplasm. In primary hippocampal cultures, MYPT1 was identified by confocal microscopy in the cytoplasm and in the nucleus, whereas a predominantly cytoplasmic localization was found in cochlear nucleus cells. In cultured cells, MYPT1 and PP1cdelta colocalized with synaptophysin. PP1M activity was high in synaptosomes isolated from the cerebral cortex, but was relatively low in the postsynaptic densities. The interaction of MYPT1 with synaptophysin and with known partners (Rho-kinase, PP1cdelta) in brain extracts was shown by immunoprecipitation with anti-MYPT1. Pull-down assays from synaptosomes, using GST-MYPT1, also confirmed these interactions. In conclusion, the widespread cellular and subcellular localization of MYPT1 implies that PP1M may play an important role in the dephosphorylation of key regulatory proteins in neuronal cells.

Functional plasticity in the organization of signaling complexes in the striatum

Dopamine plays a prominent role in regulating fast synaptic transmission in the striatum. Following dopamine receptor stimulation, various signal transduction pathways are activated, leading to the altered phosphorylation state and functional activity of substrate proteins, including glutamate-gated ion channels. Protein phosphatase 1 (PP1) plays a central role in these events. Recent studies have revealed a system for targeting PP1 to specific substrates in dendritic spines, via association with the cytoskeletal scaffolding proteins, spinophilin and neurabin. Interactions between these proteins and the actin cytoskeleton are dynamically regulated by the cAMP pathway, and thus play a role in dopamine-mediated striatal plasticity.