Mechanisms and Consequences of Dopamine Depletion-Induced Attenuation of the Spinophilin/Neurofilament Medium Interaction

Mechanisms of spinophilin-dependent pancreas dysregulation in obesity

Spinophilin is an F-actin binding and protein phosphatase 1 (PP1) targeting protein that acts as a scaffold of PP1 to its substrates. Spinophilin knockout (Spino-/-) mice have decreased fat mass, increased lean mass, and improved glucose tolerance, with no difference in feeding behaviors. Although spinophilin is enriched in neurons, its roles in nonneuronal tissues, such as β cells of the pancreatic islets, are unclear. We have corroborated and expanded upon previous studies to determine that Spino-/- mice have decreased weight gain and improved glucose tolerance in two different models of obesity. We have identified multiple putative spinophilin-interacting proteins isolated from intact pancreas and observed increased interactions of spinophilin with exocrine, ribosomal, and cytoskeletal protein classes that normally act to mediate peptide hormone production, processing, and/or release in Leprdb/db and/or high-fat diet-fed (HFF) models of obesity. In addition, we have found that spinophilin interacts with proteins from similar classes in isolated islets, suggesting a role for spinophilin in the pancreatic islet. Consistent with a pancreatic β cell type-specific role for spinophilin, using our recently described conditional spinophilin knockout mice, we found that loss of spinophilin specifically in pancreatic β cells improved glucose tolerance without impacting body weight in chow-fed mice. Our data further support the role of spinophilin in mediating pathophysiological changes in body weight and whole body metabolism associated with obesity. Our data provide the first evidence that pancreatic spinophilin protein interactions are modulated by obesity and that loss of spinophilin specifically in pancreatic β cells impacts whole body glucose tolerance.NEW & NOTEWORTHY To our knowledge, these data are the first to demonstrate that obesity impacts spinophilin protein interactions in the pancreas and identify spinophilin specifically in pancreatic β cells as a modulator of whole body glucose tolerance.

Spinophilin Limits Metabotropic Glutamate Receptor 5 Scaffolding to the Postsynaptic Density and Cell Type Specifically Mediates Excessive Grooming

BACKGROUND

Grooming dysfunction is a hallmark of the obsessive-compulsive spectrum disorder trichotillomania. Numerous preclinical studies have utilized SAPAP3-deficient mice for understanding the neurobiology of repetitive grooming, suggesting that excessive grooming is caused by increased metabotropic glutamate receptor 5 (mGluR5) activity in striatal direct- and indirect-pathway medium spiny neurons (MSNs). However, the MSN subtype-specific signaling mechanisms that mediate mGluR5-dependent adaptations underlying excessive grooming are not fully understood. Here, we investigated the MSN subtype-specific roles of the striatal signaling hub protein spinophilin in mediating repetitive motor dysfunction associated with mGluR5 function.

METHODS

Quantitative proteomics and immunoblotting were utilized to identify how spinophilin impacts mGluR5 phosphorylation and protein interaction changes. Plasticity and repetitive motor dysfunction associated with mGluR5 action were measured using our novel conditional spinophilin mouse model in which spinophilin was knocked out from striatal direct-pathway MSNs and/or indirect-pathway MSNs.

RESULTS

Loss of spinophilin only in indirect-pathway MSNs decreased performance of a novel motor repertoire, but loss of spinophilin in either MSN subtype abrogated striatal plasticity associated with mGluR5 function and prevented excessive grooming caused by SAPAP3 knockout mice or treatment with the mGluR5-specific positive allosteric modulator VU0360172 without impacting locomotion-relevant behavior. Biochemically, we determined that the spinophilin-mGluR5 interaction correlates with grooming behavior and that loss of spinophilin shifts mGluR5 interactions from lipid raft-associated proteins toward postsynaptic density proteins implicated in psychiatric disorders.

CONCLUSIONS

These results identify spinophilin as a novel striatal signaling hub molecule in MSNs that cell subtype specifically mediates behavioral, functional, and molecular adaptations associated with repetitive motor dysfunction in psychiatric disorders.

Spinophilin-dependent regulation of GluN2B-containing NMDAR-dependent calcium influx, GluN2B surface expression, and cleaved caspase expression

N-methyl-d-aspartate receptors (NMDARs) are calcium-permeable ion channels that are ubiquitously expressed within the glutamatergic postsynaptic density. Phosphorylation of NMDAR subunits defines receptor conductance and surface localization, two alterations that can modulate overall channel activity. Modulation of NMDAR phosphorylation by kinases and phosphatases regulates the amount of calcium entering the cell and subsequent activation of calcium-dependent processes. The dendritic spine enriched protein, spinophilin, is the major synaptic protein phosphatase 1 (PP1) targeting protein. Depending on the substrate, spinophilin can act as either a PP1 targeting protein, to permit substrate dephosphorylation, or a PP1 inhibitory protein, to enhance substrate phosphorylation. Spinophilin limits NMDAR function in a PP1-dependent manner. Specifically, we have previously shown that spinophilin sequesters PP1 away from the GluN2B subunit of the NMDAR, which results in increased phosphorylation of Ser-1284 on GluN2B. However, how spinophilin modifies NMDAR function is unclear. Herein, we utilize a Neuro2A cell line to detail that Ser-1284 phosphorylation increases calcium influx via GluN2B-containing NMDARs. Moreover, overexpression of spinophilin decreases GluN2B-containing NMDAR activity by decreasing its surface expression, an effect that is independent of Ser-1284 phosphorylation. In hippocampal neurons isolated from spinophilin knockout animals, there is an increase in cleaved caspase-3 levels, a marker of calcium-associated apoptosis, compared with wildtype mice. Taken together, our data demonstrate that spinophilin regulates GluN2B containing NMDAR phosphorylation, channel function, and trafficking and that loss of spinophilin enhances neuronal cleaved caspase-3 expression.

Neurodegeneration Markers in the Cerebrospinal Fluid of 100 Patients with Schizophrenia Spectrum Disorder

BACKGROUND

Schizophrenia spectrum disorders (SSD) can be associated with neurodegenerative processes causing disruption of neuronal, synaptic, or axonal integrity. Some previous studies have reported alterations of neurodegenerative markers (such as amyloid beta [Aβ], tau, or neurofilaments) in patients with SSD. However, the current state of research remains inconclusive. Therefore, the rationale of this study was to investigate established neurodegenerative markers in the cerebrospinal fluid (CSF) of a large group of patients with SSD.

STUDY DESIGN

Measurements of Aβ1-40, Aß1-42, phospho- and total-tau in addition to neurofilament light (NFL), medium (NFM), and heavy (NFH) chains were performed in the CSF of 100 patients with SSD (60 F, 40 M; age 33.7 ± 12.0) and 39 controls with idiopathic intracranial hypertension (33 F, 6 M; age 34.6 ± 12.0) using enzyme-linked immunoassays.

STUDY RESULTS

The NFM levels were significantly increased in SSD patients (P = .009), whereas phospho-tau levels were lower in comparison to the control group (P = .018). No other significant differences in total-tau, beta-amyloid-quotient (Aβ1-42/Aβ1-40), NFL, and NFH were identified.

CONCLUSIONS

The findings argue against a general tauopathy or amyloid pathology in patients with SSD. However, high levels of NFM, which has been linked to regulatory functions in dopaminergic neurotransmission, were associated with SSD. Therefore, NFM could be a promising candidate for further research on SSD.

Impact of Pharmacological and Non-Pharmacological Modulators on Dendritic Spines Structure and Functions in Brain

Dendritic spines are small, thin, hair-like protrusions found on the dendritic processes of neurons. They serve as independent compartments providing large amplitudes of Ca2+ signals to achieve synaptic plasticity, provide sites for newer synapses, facilitate learning and memory. One of the common and severe complication of neurodegenerative disease is cognitive impairment, which is said to be closely associated with spine pathologies viz., decreased in spine density, spine length, spine volume, spine size etc. Many treatments targeting neurological diseases have shown to improve the spine structure and distribution. However, concise data on the various modulators of dendritic spines are imperative and a need of the hour. Hence, in this review we made an attempt to consolidate the effects of various pharmacological (cholinergic, glutamatergic, GABAergic, serotonergic, adrenergic, and dopaminergic agents) and non-pharmacological modulators (dietary interventions, enriched environment, yoga and meditation) on dendritic spines structure and functions. These data suggest that both the pharmacological and non-pharmacological modulators produced significant improvement in dendritic spine structure and functions and in turn reversing the pathologies underlying neurodegeneration. Intriguingly, the non-pharmacological approaches have shown to improve intellectual performances both in preclinical and clinical platforms, but still more technology-based evidence needs to be studied. Thus, we conclude that a combination of pharmacological and non-pharmacological intervention may restore cognitive performance synergistically via improving dendritic spine number and functions in various neurological disorders.

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.

A multi-omic analysis of the dorsal striatum in an animal model of divergent genetic risk for alcohol use disorder

The development of selectively bred high and low alcohol-preferring mice (HAP and LAP, respectively) has allowed for an assessment of the polygenetic risk for pathological alcohol consumption and phenotypes associated with alcohol use disorder (AUD). Accumulating evidence indicates that the dorsal striatum (DS) is a central node in the neurocircuitry underlying addictive processes. Therefore, knowledge of differential gene, protein, and phosphorylated protein expression in the DS of HAP and LAP mice may foster new insights into how aberrant DS functioning may contribute to AUD-related phenotypes. To begin to elucidate these basal differences, a complementary and integrated analysis of DS tissue from alcohol-naïve male and female HAP and LAP mice was performed using RNA sequencing, quantitative proteomics, and phosphoproteomics. These datasets were subjected to a thorough analysis of gene ontology, pathway enrichment, and hub gene assessment. Analyses identified 2,108, 390, and 521 significant differentially expressed genes, proteins, and phosphopeptides, respectively between the two lines. Network analyses revealed an enrichment in the differential expression of genes, proteins, and phosphorylated proteins connected to cellular organization, cytoskeletal protein binding, and pathways involved in synaptic transmission and functioning. These findings suggest that the selective breeding to generate HAP and LAP mice may lead to a rearrangement of synaptic architecture which could alter DS neurotransmission and plasticity differentially between mouse lines. These rich datasets will serve as an excellent resource to inform future studies on how inherited differences in gene, protein, and phosphorylated protein expression contribute to AUD-related phenotypes.

Spinophilin regulates phosphorylation and interactions of the GluN2B subunit of the N-methyl-d-aspartate receptor

N-methyl-d-Aspartate receptors (NMDARs) are abundant postsynaptic proteins that are critical for normal synaptic communication. NMDAR channel function is regulated by multiple properties, including phosphorylation. Inhibition of protein phosphatase 1 (PP1) in hippocampal neurons increases NMDAR activity, an effect abrogated by loss of spinophilin, the major PP1-targeting protein in the postsynaptic density. However, how spinophilin regulates PP1-dependent NMDAR function is unclear. We hypothesize that spinophilin regulates PP1 binding to the NMDAR to alter NMDAR phosphorylation. Our data demonstrate that spinophilin interacts with the GluN2B subunit of the NMDAR. In human embryonic kidney 293 FT cells, activation and/or overexpression of protein kinase A increased the association between spinophilin and the GluN2B subunit of the NMDAR. Functionally, we found that spinophilin overexpression decreased PP1 binding to the GluN2B subunit of the NMDAR and attenuated the PP1-dependent dephosphorylation of GluN2B at Ser-1284. Moreover, in P28 hippocampal lysates isolated from spinophilin KO compared to WT mice, there was increased binding of GluN2B to PP1, decreased phosphorylation of GluN2B at Ser-1284, and altered GluN2B protein interactions with postsynaptic density-enriched proteins. Together, our data demonstrate that spinophilin decreases PP1 binding to GluN2B and concomitantly enhances the phosphorylation of GluN2B at Ser-1284. The putative consequences of these spinophilin-dependent alterations in GluN2B phosphorylation and interactions on synaptic GluN2B localization and function are discussed. Open Science: This manuscript was awarded with the Open Materials Badge For more information see: https://cos.io/our-services/open-science-badges/.

Proteomic Analysis of the Spinophilin Interactome in Rodent Striatum Following Psychostimulant Sensitization

Glutamatergic projections from the cortex and dopaminergic projections from the substantia nigra or ventral tegmental area synapse on dendritic spines of specific GABAergic medium spiny neurons (MSNs) in the striatum. Direct pathway MSNs (dMSNs) are positively coupled to protein kinase A (PKA) signaling and activation of these neurons enhance specific motor programs whereas indirect pathway MSNs (iMSNs) are negatively coupled to PKA and inhibit competing motor programs. An imbalance in the activity of these two programs is observed following increased dopamine signaling associated with exposure to psychostimulant drugs of abuse. Alterations in MSN signaling are mediated by changes in MSN protein post-translational modifications, including phosphorylation. Whereas direct changes in specific kinases, such as PKA, regulate different effects observed in the two MSN populations, alterations in the specific activity of serine/threonine phosphatases, such as protein phosphatase 1 (PP1) are less well known. This lack of knowledge is due, in part, to unknown, cell-specific changes in PP1 targeting proteins. Spinophilin is the major PP1-targeting protein in striatal postsynaptic densities. Using proteomics and immunoblotting approaches along with a novel transgenic mouse expressing hemagglutainin (HA)-tagged spinophilin in dMSNs and iMSNs, we have uncovered cell-specific regulation of the spinophilin interactome following a sensitizing regimen of amphetamine. These data suggest regulation of spinophilin interactions in specific MSN cell types and may give novel insight into putative cell-specific, phosphatase-dependent signaling pathways associated with psychostimulants.

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.

CX-4945 Induces Methuosis in Cholangiocarcinoma Cell Lines by a CK2-Independent Mechanism

Cholangiocarcinoma is a disease with a poor prognosis and increasing incidence and hence there is a pressing unmet clinical need for new adjuvant treatments. Protein kinase CK2 (previously casein kinase II) is a ubiquitously expressed protein kinase that is up-regulated in multiple cancer cell types. The inhibition of CK2 activity using CX-4945 (Silmitasertib) has been proposed as a novel treatment in multiple disease settings including cholangiocarcinoma. Here, we show that CX-4945 inhibited the proliferation of cholangiocarcinoma cell lines in vitro. Moreover, CX-4945 treatment induced the formation of cytosolic vacuoles in cholangiocarcinoma cell lines and other cancer cell lines. The vacuoles contained extracellular fluid and had neutral pH, features characteristic of methuosis. In contrast, simultaneous knockdown of both the α and α' catalytic subunits of protein kinase CK2 using small interfering RNA (siRNA) had little or no effect on the proliferation of cholangiocarcinoma cell lines and failed to induce the vacuole formation. Surprisingly, low doses of CX-4945 increased the invasive properties of cholangiocarcinoma cells due to an upregulation of matrix metallopeptidase 7 (MMP-7), while the knockdown of CK2 inhibited cell invasion. Our data suggest that CX-4945 inhibits cell proliferation and induces cell death via CK2-independent pathways. Moreover, the increase in cell invasion brought about by CX-4945 treatment suggests that this drug might increase tumor invasion in clinical settings.

The association of spinophilin with disks large-associated protein 3 (SAPAP3) is regulated by metabotropic glutamate receptor (mGluR) 5

Spinophilin is the most abundant protein phosphatase 1 targeting protein in the postsynaptic density of dendritic spines. Spinophilin associates with myriad synaptic proteins to regulate normal synaptic communication; however, the full complement of spinophilin interacting proteins and mechanisms regulating spinophilin interactions are unclear. Here we validate an association between spinophilin and the scaffolding protein, disks large-associated protein 3 (SAP90/PSD-95 associated protein 3; SAPAP3). Loss of SAPAP3 leads to obsessive-compulsive disorder (OCD)-like behaviors due to alterations in metabotropic glutamate receptor (mGluR) signaling. Here we report that spinophilin associates with SAPAP3 in the brain and in a heterologous cell system. Moreover, we have found that expression or activation of group I mGluRs along with activation of the mGluR-dependent kinase, protein kinase C β, enhances this interaction. Functionally, global loss of spinophilin attenuates amphetamine-induced hyperlocomotion, a striatal behavior associated with dopamine dysregulation and OCD. Together, these data delineate a novel link between mGluR signaling, spinophilin, and SAPAP3 in striatal pathophysiology.