The Dopamine/D1 receptor mediates the phosphorylation and inactivation of the protein tyrosine phosphatase STEP via a PKA-dependent pathway

Impairment of neuronal tyrosine phosphatase STEP worsens post-ischemic inflammation and brain injury under hypertensive condition

Hypertension is associated with poor outcome and higher mortality in patients with ischemic stroke. The impairment of adaptive vascular mechanisms under hypertensive condition compromises collateral blood flow after arterial occlusion in patients with acute ischemic stroke resulting in hypoperfusion. The increased oxidative stress caused by hypoperfusion is thought to be a trigger for the rapid evolution of ischemic infarct volume under hypertensive condition. However, the cellular factors and pathways that contribute to the exacerbation of ischemic brain injury under hypertensive condition is not yet understood. The current study reveals that predisposition to hypertension leads to basal loss of function of the neuron-specific tyrosine phosphatase STEP, which plays a crucial role in neuroprotection against excitotoxic insult. The findings further show that a mild ischemic insult in hypertensive rats triggers an early onset and sustained activation of the neuronal extracellular signal regulated kinase (ERK MAPK), a member of the mitogen activated protein kinase family and a substrate of STEP. This leads to rapid increase in the activation of neuronal NF-κB, expression of neuronal cyclooxygenase-2 and subsequent biosynthesis of the pro-inflammatory mediator prostaglandin E2, resulting in rapid morphological transformation of microglia to the pro-inflammatory state and subsequent exacerbation of ischemic brain injury. Restoration of STEP signaling with intravenous administration of a STEP-derived peptide mimetic reduces the pro-inflammatory response in neurons, activation of microglia, and ischemic brain injury. The findings suggest that the basal loss of STEP function under hypertensive condition contributes to the exacerbation of ischemic brain injury by enhancing post-ischemic inflammatory response. The study not only presents a novel role of STEP in regulating neuroimmune communication but also highlights the therapeutic potential of a STEP-mimetic in mitigating ischemic brain damage under hypertensive condition.

An emerging role of STriatal-Enriched protein tyrosine Phosphatase in hyperexcitability-associated brain disorders

STriatal-Enriched protein tyrosine Phosphatase (STEP) is a brain-specific tyrosine phosphatase that is associated with numerous neurological and neuropsychiatric disorders. STEP dephosphorylates and inactivates various kinases and phosphatases critical for neuronal function and health including Fyn, Pyk2, ERK1/2, p38, and PTPα. Importantly, STEP dephosphorylates NMDA and AMPA receptors, two major glutamate receptors that mediate fast excitatory synaptic transmission. This STEP-mediated dephosphorylation leads to their internalization and inhibits both Hebbian synaptic potentiation and homeostatic synaptic scaling. Hence, STEP has been widely accepted to weaken excitatory synaptic strength. However, emerging evidence implicates a novel role of STEP in neuronal hyperexcitability and seizure disorders. Genetic deletion and pharmacological blockade of STEP reduces seizure susceptibility in acute seizure mouse models and audiogenic seizures in a mouse model of Fragile X syndrome. Pharmacologic inhibition of STEP also decreases hippocampal activity and neuronal intrinsic excitability. Here, we will highlight the divergent roles of STEP in excitatory synaptic transmission and neuronal intrinsic excitability, present the potential underlying mechanisms, and discuss their impact on STEP-associated neurologic and neuropsychiatric disorders.

VEGFD signaling balances stability and activity-dependent structural plasticity of dendrites

Mature neurons have stable dendritic architecture, which is essential for the nervous system to operate correctly. The ability to undergo structural plasticity, required to support adaptive processes like memory formation, is still present in mature neurons. It is unclear what molecular and cellular processes control this delicate balance between dendritic structural plasticity and stabilization. Failures in the preservation of optimal dendrite structure due to atrophy or maladaptive plasticity result in abnormal connectivity and are associated with various neurological diseases. Vascular endothelial growth factor D (VEGFD) is critical for the maintenance of mature dendritic trees. Here, we describe how VEGFD affects the neuronal cytoskeleton and demonstrate that VEGFD exerts its effects on dendrite stabilization by influencing the actin cortex and reducing microtubule dynamics. Further, we found that during synaptic activity-induced structural plasticity VEGFD is downregulated. Our findings revealed that VEGFD, acting on its cognate receptor VEGFR3, opposes structural changes by negatively regulating dendrite growth in cultured hippocampal neurons and in vivo in the adult mouse hippocampus with consequences on memory formation. A phosphoproteomic screening identified several regulatory proteins of the cytoskeleton modulated by VEGFD. Among the actin cortex-associated proteins, we found that VEGFD induces dephosphorylation of ezrin at tyrosine 478 via activation of the striatal-enriched protein tyrosine phosphatase (STEP). Activity-triggered structural plasticity of dendrites was impaired by expression of a phospho-deficient mutant ezrin in vitro and in vivo. Thus, VEGFD governs the equilibrium between stabilization and plasticity of dendrites by acting as a molecular brake of structural remodeling.

Fyn Kinase in Alzheimer's Disease: Unraveling Molecular Mechanisms and Therapeutic Implications

Alzheimer's disease, characterized by the accumulation of abnormal protein aggregates and neuronal damage in the brain, leads to a gradual decline in cognitive function and memory. As a complex neurodegenerative disorder, it involves disruptions in various biochemical pathways and neurotransmitter systems, contributing to the progressive loss of neurons and synaptic connections. The complexity of Alzheimer's signaling pathways complicates treatment, presenting a formidable challenge in the quest for effective therapeutic interventions. A member of the Src family of kinases (SFKs), Fyn, is a type of non-receptor tyrosine kinase that has been linked to multiple essential CNS processes, such as myelination and synaptic transmission. Fyn is an appealing target for AD treatments because it is uniquely linked to the two major pathologies in AD by its interaction with tau, in addition to being activated by amyloid-beta (Aβ) through PrPC. Fyn mediates neurotoxicity and synaptic impairments caused by Aβ and is involved in regulating the process of Aβ synthesis.Additionally, the tau protein's tyrosine phosphorylation is induced by Fyn. Fyn is also a challenging target because of its widespread body expression and strong homology with other kinases of the Src family, which could cause unintentional off-target effects. This review emphasizes signaling pathways mediated by Fyn that govern neuronal development and plasticity while also summarizing the most noteworthy recent research relevant to Fyn kinase's function in the brain. Additionally, the therapeutic inhibition of Fyn kinase has been discussed, with a focus on the Fyn kinase inhibitors that are in clinical trials, which presents a fascinating opportunity for targeting Fyn kinase in the creation of possible therapeutic approaches for the management of Alzheimer's disease.

Homocysteine-induced sustained GluN2A NMDA receptor stimulation leads to mitochondrial ROS generation and neurotoxicity

Homocysteine, a sulfur-containing amino acid derived from methionine metabolism, is a known agonist of N-methyl-D-aspartate receptor (NMDAR) and is involved in neurotoxicity. Our previous findings showed that neuronal exposure to elevated homocysteine levels leads to sustained low-level increase in intracellular Ca2+, which is dependent on GluN2A subunit-containing NMDAR (GluN2A-NMDAR) stimulation. These studies further showed a role of ERK MAPK in homocysteine-GluN2A-NMDAR-mediated neuronal death. However, the intracellular mechanisms associated with such sustained GluN2A-NMDAR stimulation and subsequent Ca2+ influx have remained unexplored. Using live-cell imaging with Fluo3-AM and biochemical approaches, we show that homocysteine-GluN2A NMDAR-induced initial Ca2+ influx triggers sequential phosphorylation and subsequent activation of the proline rich tyrosine kinase 2 (Pyk2) and Src family kinases, which in turn phosphorylates GluN2A-Tyr1325 residue of GluN2A-NMDARs to maintain channel activity. The continuity of this cycle of events leads to sustained Ca2+ influx through GluN2A-NMDAR. Our findings also show that lack of activation of the regulatory tyrosine phosphatase STEP, which can limit Pyk2 and Src family kinase activity further contributes to the maintenance of this cycle. Additional studies using live-cell imaging of neurons expressing a redox-sensitive GFP targeted to the mitochondrial matrix show that treatment with homocysteine leads to a progressive increase in mitochondrial reactive oxygen species generation, which is dependent on GluN2A-NMDAR-mediated sustained ERK MAPK activation. This later finding demonstrates a novel role of GluN2A-NMDAR in homocysteine-induced mitochondrial ROS generation and highlights the role of ERK MAPK as the intermediary signaling pathway between GluN2A-NMDAR stimulation and mitochondrial reactive oxygen species generation.

Protein Kinase A in neurological disorders

Cyclic adenosine 3', 5' monophosphate (cAMP)-dependent Protein Kinase A (PKA) is a multi-functional serine/threonine kinase that regulates a wide variety of physiological processes including gene transcription, metabolism, and synaptic plasticity. Genomic sequencing studies have identified both germline and somatic variants of the catalytic and regulatory subunits of PKA in patients with metabolic and neurodevelopmental disorders. In this review we discuss the classical cAMP/PKA signaling pathway and the disease phenotypes that result from PKA variants. This review highlights distinct isoform-specific cognitive deficits that occur in both PKA catalytic and regulatory subunits, and how tissue-specific distribution of these isoforms may contribute to neurodevelopmental disorders in comparison to more generalized endocrine dysfunction.

Identification of hippocampal area CA2 in hamster and vole brain

Prairie voles (Microtus ochrogaster) and Syrian, or golden, hamsters (Mesocricetus auratus) are closely related to mice (Mus musculus) and rats (Rattus norvegicus, for example) and are commonly used in studies of social behavior including social interaction, social memory, and aggression. The CA2 region of the hippocampus is known to play a key role in social memory and aggression in mice and responds to social stimuli in rats, likely owing to its high expression of oxytocin and vasopressin 1b receptors. However, CA2 has yet to be identified and characterized in hamsters or voles. In this study, we sought to determine whether CA2 could be identified molecularly in vole and hamster. To do this, we used immunofluorescence with primary antibodies raised against known molecular markers of CA2 in mice and rats to stain hippocampal sections from voles and hamsters in parallel with those from mice. Here, we report that, like in mouse and rat, staining for many CA2 proteins in vole and hamster hippocampus reveals a population of neurons that express regulator of G protein signaling 14 (RGS14), Purkinje cell protein 4 (PCP4) and striatal-enriched protein tyrosine phosphatase (STEP), which together delineate the borders with CA3 and CA1. These cells were located at the distal end of the mossy fiber projections, marked by the presence of Zinc Transporter 3 (ZnT-3) and calbindin in all three species. In addition to staining the mossy fibers, calbindin also labeled a layer of CA1 pyramidal cells in mouse and hamster but not in vole. However, Wolframin ER transmembrane glycoprotein (WFS1) immunofluorescence, which marks all CA1 neurons, was present in all three species and abutted the distal end of CA2, marked by RGS14 immunofluorescence. Staining for two stress hormone receptors-the glucocorticoid (GR) and mineralocorticoid (MR) receptors-was also similar in all three species, with GR staining found primarily in CA1 and MR staining enriched in CA2. Interestingly, although perineuronal nets (PNNs) are known to surround CA2 cells in mouse and rat, we found that staining for PNNs differed across species in that both CA2 and CA3 showed staining in voles and primarily CA3 in hamsters with only some neurons in proximal CA2 showing staining. These results demonstrate that, like in mouse, CA2 in voles and hamsters can be molecularly distinguished from neighboring CA1 and CA3 areas, but PNN staining is less useful for identifying CA2 in the latter two species. These findings reveal commonalities across species in molecular profile of CA2, which will facilitate future studies of CA2 in these species. Yet to be determined is how differences in PNNs might relate to differences in social behavior across species.

The Neurobiological Underpinnings of Obsessive-Compulsive Symptoms in Psychosis, Translational Issues for Treatment-Resistant Schizophrenia

Almost 25% of schizophrenia patients suffer from obsessive-compulsive symptoms (OCS) considered a transdiagnostic clinical continuum. The presence of symptoms pertaining to both schizophrenia and obsessive-compulsive disorder (OCD) may complicate pharmacological treatment and could contribute to lack or poor response to the therapy. Despite the clinical relevance, no reviews have been recently published on the possible neurobiological underpinnings of this comorbidity, which is still unclear. An integrative view exploring this topic should take into account the following aspects: (i) the implication for glutamate, dopamine, and serotonin neurotransmission as demonstrated by genetic findings; (ii) the growing neuroimaging evidence of the common brain regions and dysfunctional circuits involved in both diseases; (iii) the pharmacological modulation of dopaminergic, serotoninergic, and glutamatergic systems as current therapeutic strategies in schizophrenia OCS; (iv) the recent discovery of midbrain dopamine neurons and dopamine D1- and D2-like receptors as orchestrating hubs in repetitive and psychotic behaviors; (v) the contribution of N-methyl-D-aspartate receptor subunits to both psychosis and OCD neurobiology. Finally, we discuss the potential role of the postsynaptic density as a structural and functional hub for multiple molecular signaling both in schizophrenia and OCD pathophysiology.

Ferulic Acid Improves Synaptic Plasticity and Cognitive Impairments by Alleviating the PP2B/DARPP-32/PP1 Axis-Mediated STEP Increase and Aβ Burden in Alzheimer's Disease

The burden of Alzheimer's disease, the most prevalent neurodegenerative disease, is increasing exponentially due to the increase in the elderly population worldwide. Synaptic plasticity is the basis of learning and memory, but it is impaired in AD. Uncovering the disease's underlying molecular pathogenic mechanisms involving synaptic plasticity could lead to the identification of targets for better disease management. Using primary neurons treated with Aβ and APP/PS1 animal models, we evaluated the effect of the phenolic compound ferulic acid (FA) on synaptic dysregulations. Aβ led to synaptic plasticity and cognitive impairments by increasing STEP activity and decreasing the phosphorylation of the GluN2B subunit of NMDA receptors, as well as decreasing other synaptic proteins, including PSD-95 and synapsin1. Interestingly, FA attenuated the Aβ-upregulated intracellular calcium and thus resulted in a decrease in PP2B-induced activation of DARPP-32, inhibiting PP1. This cascade event maintained STEP in its inactive state, thereby preventing the loss of GluN2B phosphorylation. This was accompanied by an increase in PSD-95 and synapsin1, improved LTP, and a decreased Aβ load, together leading to improved behavioral and cognitive functions in APP/PS1 mice treated with FA. This study provides insight into the potential use of FA as a therapeutic strategy in AD.

Mechanisms of mGluR-dependent plasticity in hippocampal area CA2

Pyramidal cells in hippocampal area CA2 have synaptic properties that are distinct from the other CA subregions. Notably, this includes a lack of typical long-term potentiation of stratum radiatum synapses. CA2 neurons express high levels of several known and potential regulators of metabotropic glutamate receptor (mGluR)-dependent signaling including Striatal-Enriched Tyrosine Phosphatase (STEP) and several Regulator of G-protein Signaling (RGS) proteins, yet the functions of these proteins in regulating mGluR-dependent synaptic plasticity in CA2 are completely unknown. Thus, the aim of this study was to examine mGluR-dependent synaptic depression and to determine whether STEP and the RGS proteins RGS4 and RGS14 are involved. Using whole cell voltage-clamp recordings from mouse pyramidal cells, we found that mGluR agonist-induced long-term depression (mGluR-LTD) is more pronounced in CA2 compared with that observed in CA1. This mGluR-LTD in CA2 was found to be protein synthesis and STEP dependent, suggesting that CA2 mGluR-LTD shares mechanistic processes with those seen in CA1, but in addition, RGS14, but not RGS4, was essential for mGluR-LTD in CA2. In addition, we found that exogenous application of STEP could rescue mGluR-LTD in RGS14 KO slices. Supporting a role for CA2 synaptic plasticity in social cognition, we found that RGS14 KO mice had impaired social recognition memory as assessed in a social discrimination task. These results highlight possible roles for mGluRs, RGS14, and STEP in CA2-dependent behaviors, perhaps by biasing the dominant form of synaptic plasticity away from LTP and toward LTD in CA2.

Phosphorylation Signals Downstream of Dopamine Receptors in Emotional Behaviors: Association with Preference and Avoidance

Dopamine regulates emotional behaviors, including rewarding and aversive behaviors, through the mesolimbic dopaminergic pathway, which projects dopamine neurons from the ventral tegmental area to the nucleus accumbens (NAc). Protein phosphorylation is critical for intracellular signaling pathways and physiological functions, which are regulated by neurotransmitters in the brain. Previous studies have demonstrated that dopamine stimulated the phosphorylation of intracellular substrates, such as receptors, ion channels, and transcription factors, to regulate neuronal excitability and synaptic plasticity through dopamine receptors. We also established a novel database called KANPHOS that provides information on phosphorylation signals downstream of monoamines identified by our kinase substrate screening methods, including dopamine, in addition to those reported in the literature. Recent advances in proteomics techniques have enabled us to clarify the mechanisms through which dopamine controls rewarding and aversive behaviors through signal pathways in the NAc. In this review, we discuss the intracellular phosphorylation signals regulated by dopamine in these two emotional behaviors.

The Signaling and Pharmacology of the Dopamine D1 Receptor

The dopamine D1 receptor (D1R) is a Gα_s/olf-coupled GPCR that is expressed in the midbrain and forebrain, regulating motor behavior, reward, motivational states, and cognitive processes. Although the D1R was initially identified as a promising drug target almost 40 years ago, the development of clinically useful ligands has until recently been hampered by a lack of suitable candidate molecules. The emergence of new non-catechol D1R agonists, biased agonists, and allosteric modulators has renewed clinical interest in drugs targeting this receptor, specifically for the treatment of motor impairment in Parkinson's Disease, and cognitive impairment in neuropsychiatric disorders. To develop better therapeutics, advances in ligand chemistry must be matched by an expanded understanding of D1R signaling across cell populations in the brain, and in disease states. Depending on the brain region, the D1R couples primarily to either Gα_s or Gα_olf through which it activates a cAMP/PKA-dependent signaling cascade that can regulate neuronal excitability, stimulate gene expression, and facilitate synaptic plasticity. However, like many GPCRs, the D1R can signal through multiple downstream pathways, and specific signaling signatures may differ between cell types or be altered in disease. To guide development of improved D1R ligands, it is important to understand how signaling unfolds in specific target cells, and how this signaling affects circuit function and behavior. In this review, we provide a summary of D1R-directed signaling in various neuronal populations and describe how specific pathways have been linked to physiological and behavioral outcomes. In addition, we address the current state of D1R drug development, including the pharmacology of newly developed non-catecholamine ligands, and discuss the potential utility of D1R-agonists in Parkinson's Disease and cognitive impairment.

High-Content Single-Cell Förster Resonance Energy Transfer Imaging of Cultured Striatal Neurons Reveals Novel Cross-Talk in the Regulation of Nuclear Signaling by Protein Kinase A and Extracellular Signal-Regulated Kinase 1/2

Genetically encoded biosensors can be used to track signaling events in living cells by measuring changes in fluorescence emitted by one or more fluorescent proteins. Here, we describe the use of genetically encoded biosensors based on Förster resonance energy transfer (FRET), combined with high-content microscopy, to image dynamic signaling events simultaneously in thousands of neurons in response to drug treatments. We first applied this approach to examine intercellular variation in signaling responses among cultured striatal neurons stimulated with multiple drugs. Using high-content FRET imaging and immunofluorescence, we identified neuronal subpopulations with unique responses to pharmacological manipulation and used nuclear morphology to identify medium spiny neurons within these heterogeneous striatal cultures. Focusing on protein kinase A (PKA) and extracellular signal-regulated kinase 1/2 (ERK1/2) signaling in the cytoplasm and nucleus, we noted pronounced intercellular differences among putative medium spiny neurons, in both the magnitude and kinetics of signaling responses to drug application. Importantly, a conventional "bulk" analysis that pooled all cells in culture yielded a different rank order of drug potency than that revealed by single-cell analysis. Using a single-cell analytical approach, we dissected the relative contributions of PKA and ERK1/2 signaling in striatal neurons and unexpectedly identified a novel role for ERK1/2 in promoting nuclear activation of PKA in striatal neurons. This finding adds a new dimension of signaling crosstalk between PKA and ERK1/2 with relevance to dopamine D1 receptor signaling in striatal neurons. In conclusion, high-content single-cell imaging can complement and extend traditional population-level analyses and provides a novel vantage point from which to study cellular signaling. SIGNIFICANCE STATEMENT: High-content imaging revealed substantial intercellular variation in the magnitude and pattern of intracellular signaling events driven by receptor stimulation. Since individual neurons within the same population can respond differently to a given agonist, interpreting measures of intracellular signaling derived from the averaged response of entire neuronal populations may not always reflect what happened at the single-cell level. This study uses this approach to identify a new form of cross-talk between PKA and ERK1/2 signaling in the nucleus of striatal neurons.

Tyrosine phosphatase STEP is a key regulator of glutamate-induced prostaglandin E2 release from neurons

The neuron-specific tyrosine phosphatase striatal-enriched phosphatase (STEP) is emerging as a key regulator of excitotoxicity, which is involved in the pathogenesis of both acute and chronic neurological diseases. However, the intracellular mechanisms that are regulated by STEP to confer neuroprotection against excitotoxic insults are not well understood. The present study investigates the role of STEP in regulating neuronal release of the proinflammatory prostanoid prostaglandin E2 (PGE2), which is associated with a wide range of pathological conditions. The findings show that glutamate-mediated activation of the N-methyl-D-aspartic acid receptor in STEP-deficient neurons leads to rapid and sustained increase in the phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK), a signaling molecule involved in the production of inflammatory mediators. Such sustained p38 MAPK activation increases the activity of cytosolic phospholipase A2, which catalyzes the release of arachidonic acid, the initial substrate for PGE2 biosynthesis. Sustained p38 MAPK activation also induces nuclear factor-κB-mediated increase in expression of cyclooxygenase-2 that is involved in the conversion of arachidonic acid to prostanoids, resulting in enhanced biosynthesis and release of PGE2 from neurons. Restoration of STEP function with a STEP mimetic (TAT-STEP-myc peptide) significantly decreases the activation of p38 MAPK-mediated cytosolic phospholipase A2/cyclooxygenase-2/PGE2 signaling cascade. This study identifies an important mechanism involved in the neuronal release of the proinflammatory mediator PGE2 after excitotoxic insult and highlights for the first time the immunomodulatory ability of a neuronal tyrosine phosphatase.

PTPN5 promotes follicle-stimulating hormone secretion through regulating intracellular calcium homeostasis

Protein tyrosine phosphatase non-receptor type 5 (PTPN5), also called striatal-enriched protein tyrosine phosphatase (STEP), is highly expressed in neurons of the basal ganglia, hippocampus, cortex, and related structures, also in the pituitary. Gonadotropins are the key regulator of the reproduction in mammals. In this study, PTPN5 is detected to express in murine pituitary in a developmental manner. Moreover, the expression of PTPN5 in the pituitary is heavily reduced after ovary removal. Follicle-stimulating hormone (FSH) secretion in gonadotropes is regulated by PTPN5 via binding GnRH to GnRH-R. Two parallel signaling pathways, Gs-protein kinase A (PKA)-PTPN5 and Gq-phospholipases C (PLC)-p38 MAPK-PTPN5, cooperatively regulate GnRH-induced FSH secretion. We also show that influx of Ca²⁺ activates the Ca²⁺ -dependent phosphatase calcineurin, leading to the phosphorylation and activation of PTPN5. The intracellular release of Ca²⁺ is reduced via TC2153. In conclusion, blocking or knocking out of PTPN5 reduces the release of FSH in whole pituitary. Mechanically, PTPN5 regulates gonadotropes' function through regulating intracellular calcium homeostasis.

The Implication of STEP in Synaptic Plasticity and Cognitive Impairments in Alzheimer's Disease and Other Neurological Disorders

STriatal-Enriched protein tyrosine Phosphatase (STEP) is a tyrosine phosphatase that has been implicated in Alzheimer’s disease (AD), the most common form of dementia, and many other neurological diseases. The protein level and activity of STEP have been found to be elevated in most of these disorders, and specifically in AD as a result of dysregulation of different pathways including PP2B/DARPP32/PP1, PKA as well as impairments of both proteasomal and lysosomal systems. The upregulation in STEP leads to increased binding to, and dephosphorylation of, its substrates which are mainly found to be synaptic plasticity and thus learning and memory related proteins. These proteins include kinases like Fyn, Pyk2, ERK1/2 and both NMDA and AMPA receptor subunits GluN2B and GluA2. The dephosphorylation of these molecules results in inactivation of these kinases and internalization of NMDA and AMPA receptor complexes leading to synapse loss and cognitive impairments. In this study, we aim to review STEP regulation and its implications in AD as well as other neurological disorders and then summarize data on targeting STEP as therapeutic strategy in these diseases.

Development of a Robust High-Throughput Screening Platform for Inhibitors of the Striatal-Enriched Tyrosine Phosphatase (STEP)

Many human diseases are the result of abnormal expression or activation of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Not surprisingly, more than 30 tyrosine kinase inhibitors (TKIs) are currently in clinical use and provide unique treatment options for many patients. PTPs on the other hand have long been regarded as “undruggable” and only recently have gained increased attention in drug discovery. Striatal-enriched tyrosine phosphatase (STEP) is a neuron-specific PTP that is overactive in Alzheimer’s disease (AD) and other neurodegenerative and neuropsychiatric disorders, including Parkinson’s disease, schizophrenia, and fragile X syndrome. An emergent model suggests that the increase in STEP activity interferes with synaptic function and contributes to the characteristic cognitive and behavioral deficits present in these diseases. Prior efforts to generate STEP inhibitors with properties that warrant clinical development have largely failed. To identify novel STEP inhibitor scaffolds, we developed a biophysical, label-free high-throughput screening (HTS) platform based on the protein thermal shift (PTS) technology. In contrast to conventional HTS using STEP enzymatic assays, we found the PTS platform highly robust and capable of identifying true hits with confirmed STEP inhibitory activity and selectivity. This new platform promises to greatly advance STEP drug discovery and should be applicable to other PTP targets.

Insight into the Role of the STriatal-Enriched Protein Tyrosine Phosphatase (STEP) in A2A Receptor-Mediated Effects in the Central Nervous System

The STriatal-Enriched protein tyrosine phosphatase STEP is a brain-specific tyrosine phosphatase that plays a pivotal role in the mechanisms of learning and memory, and it has been demonstrated to be involved in several neuropsychiatric diseases. Recently, we found a functional interaction between STEP and adenosine A_2A receptor (A_2AR), a subtype of the adenosine receptor family widely expressed in the central nervous system, where it regulates motor behavior and cognition, and plays a role in cell survival and neurodegeneration. Specifically, we demonstrated the involvement of STEP in A_2AR-mediated cocaine effects in the striatum and, more recently, we found that in the rat striatum and hippocampus, as well as in a neuroblastoma cell line, the overexpression of the A_2AR, or its stimulation, results in an increase in STEP activity. In the present article we will discuss the functional implication of this interaction, trying to examine the possible mechanisms involved in this relation between STEP and A_2ARs.

Regulation of post-ischemic inflammatory response: A novel function of the neuronal tyrosine phosphatase STEP

The neuron-specific tyrosine phosphatase STEP is emerging as a key neuroprotectant against acute ischemic stroke. However, it remains unclear how STEP impacts the outcome of stroke. We find that the exacerbation of ischemic brain injury in STEP deficient mice involves an early onset and sustained activation of neuronal p38 mitogen activated protein kinase, a substrate of STEP. This leads to rapid increase in the expression of neuronal cyclooxygenase-2 and synthesis of prostaglandin E₂, causing change in microglial morphology to an amoeboid activated state, activation of matrix metalloproteinase-9, cleavage of tight junction proteins and extravasation of IgG into the ischemic brain. Restoration of STEP signaling with intravenous administration of a STEP-derived peptide mimetic reduces the post-ischemic inflammatory response and attenuates brain injury. The findings identify a unique role of STEP in regulating post-ischemic neuroinflammation and further emphasizes the therapeutic potential of the STEP-mimetic in neurological disorders where inflammation contributes to brain damage.

Emerging neuroprotective strategies for the treatment of ischemic stroke: An overview of clinical and preclinical studies

Stroke is the leading cause of disability and thesecond leading cause of death worldwide. With the global population aged 65 and over growing faster than all other age groups, the incidence of stroke is also increasing. In addition, there is a shift in the overall stroke burden towards younger age groups, particularly in low and middle-income countries. Stroke in most cases is caused due to an abrupt blockage of an artery (ischemic stroke), but in some instances stroke may be caused due to bleeding into brain tissue when a blood vessel ruptures (hemorrhagic stroke). Although treatment options for stroke are still limited, with the advancement in recanalization therapy using both pharmacological and mechanical thrombolysis some progress has been made in helping patients recover from ischemic stroke. However, there is still a substantial need for the development of therapeutic agents for neuroprotection in acute ischemic stroke to protect the brain from damage prior to and during recanalization, extend the therapeutic time window for intervention and further improve functional outcome. The current review has assessed the past challenges in developing neuroprotective strategies, evaluated the recent advances in clinical trials, discussed the recent initiative by the National Institute of Neurological Disorders and Stroke in USA for the search of novel neuroprotectants (Stroke Preclinical Assessment Network, SPAN) and identified emerging neuroprotectants being currently evaluated in preclinical studies. The underlying molecular mechanism of each of the neuroprotective strategies have also been summarized, which could assist in the development of future strategies for combinational therapy in stroke treatment.

Molecular causes of sex-specific deficits in rodent models of neurodevelopmental disorders

Neurodevelopmental disorders (NDDs) such as intellectual disability and autism spectrum disorder consistently show a male bias in prevalence, but it remains unclear why males and females are affected with different frequency. While many behavioral studies of transgenic NDD models have focused only on males, the requirement by the National Institutes of Health to consider sex as a biological variable has promoted the comparison of male and female performance in wild-type and mutant animals. Here, we review examples of rodent models of NDDs in which sex-specific deficits were identified in molecular, physiological, and/or behavioral responses, showing sex differences in susceptibility to disruption of genes mutated in NDDs. Haploinsufficiency in genes involved in mechanisms such as synaptic function (GABRB3 and NRXN1), chromatin remodeling (CHD8, EMHT1, and ADNP), and intracellular signaling (CC2D1A and ERK1) lead to more severe behavioral outcomes in males. However, in the absence of behavioral deficits, females can still present with cellular and electrophysiological changes that could be due to compensatory mechanisms or differential allocation of molecular and cellular functions in the two sexes. By contrasting these findings with mouse models where females are more severely affected (MTHFR and AMBRA1), we propose a framework to approach the study of sex-specific deficits possibly leading to sex bias in NDDs.

The activity of the Striatal-enriched protein tyrosine phosphatase in neuronal cells is modulated by adenosine A2A receptor

We recently demonstrated that a tonic activation of adenosine A_2A receptors (A_2A Rs) is required for cocaine-induced synaptic depression and increase in the activity of STriatal-Enriched protein tyrosine Phosphatase (STEP). In this study, we elaborated on the relationship between A_2A R and STEP using genetic, pharmacological, and cellular tools. We found that the activities of protein tyrosine phosphatases (PTPs), and in particular of STEP, are significantly increased in the striatum and hippocampus of a transgenic rat strain over-expressing the neuronal A_2A R (NSEA_2A ) with respect to wild-type (WT) rats. Moreover the selective A_2A R agonist 4-[2-[[6-Amino-9-(N-ethyl-β-d-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride up-regulates PTPs and STEP activities in WT but not in NSEA_2A rats, while the selective A_2A R antagonist 4-(-2-[7-amino-2-{2-furyl}{1,2,4}triazolo{2,3-a} {1,3,5}triazin-5-yl-amino]ethyl)phenol restores the tyrosine phosphatase activities in NSEA_2A , having no effects in WT rats. In addition, while cocaine induced the activation of PTP and STEP in WT rats, it failed to increase phosphatase activity in NSEA_2A rats. A_2A Rs modulate STEP activity also in the SH-SY5Y neuroblastoma cell line, where a calcium-dependent calcineurin/PP1 pathway was found to play a major role. In summary, the present study identified a novel interaction between A_2A R and STEP that could have important clinical implications, since STEP has emerged as key regulator of signaling pathways involved in neurodegenerative and neuropsychiatric diseases and A_2A Rs are considered a promising target for the development of therapeutic strategies for different diseases of the central nervous system. Read the Editorial Highlight for this article on page 270.

Changes in neuronal immunofluorescence in the C- versus N-terminal domains of hnRNP H following D1 dopamine receptor activation

RNA binding proteins are a diverse class of proteins that regulate all aspects of RNA metabolism. Accumulating studies indicate that heterogeneous nuclear ribonucleoproteins are associated with cellular adaptations in response to drugs of abuse. We recently mapped and validated heterogeneous nuclear ribonucleoprotein H1 (Hnrnph1) as a quantitative trait gene underlying differential behavioral sensitivity to methamphetamine. The molecular mechanisms by which hnRNP H1 alters methamphetamine behaviors are unknown but could involve pre- and/or post-synaptic changes in protein localization and function. Methamphetamine initiates post-synaptic D1 dopamine receptor signaling indirectly by binding to pre-synaptic dopamine transporters and vesicular monoamine transporters of midbrain dopaminergic neurons which triggers reverse transport and accumulation of dopamine at the synapse. Here, we examined changes in neuronal localization of hnRNP H in primary rat cortical neurons that express dopamine receptors that can be modulated by the D1 or D2 dopamine receptor agonists SKF38393 and (-)-Quinpirole HCl, respectively. Basal immunostaining of hnRNP H was localized primarily to the nucleus. D1 dopamine receptor activation induced an increase in hnRNP H nuclear immunostaining as detected by immunocytochemistry with a C-domain directed antibody containing epitope near the glycine-rich domain but not with an N-domain specific antibody. Although there was no change in hnRNP H protein in the nucleus or cytoplasm, there was a decrease in Hnrnph1 transcript following D1 receptor stimulation. Taken together, these results suggest that D1 receptor activation increases availability of the hnRNP H C-terminal epitope, which could potentially reflect changes in protein-protein interactions. Thus, D1 receptor signaling could represent a key molecular post-synaptic event linking Hnrnph1 polymorphisms to drug-induced behavior.

Inhibition of STEP61 ameliorates deficits in mouse and hiPSC-based schizophrenia models

The brain-specific tyrosine phosphatase, STEP (STriatal-Enriched protein tyrosine Phosphatase) is an important regulator of synaptic function. STEP normally opposes synaptic strengthening by increasing N-methyl D-aspartate glutamate receptor (NMDAR) internalization through dephosphorylation of GluN2B and inactivation of the kinases extracellular signal-regulated kinase 1/2 and Fyn. Here we show that STEP₆₁ is elevated in the cortex in the Nrg1^+/- knockout mouse model of schizophrenia (SZ). Genetic reduction or pharmacological inhibition of STEP prevents the loss of NMDARs from synaptic membranes and reverses behavioral deficits in Nrg1^+/- mice. STEP₆₁ protein is also increased in cortical lysates from the central nervous system-specific ErbB2/4 mouse model of SZ, as well as in human induced pluripotent stem cell (hiPSC)-derived forebrain neurons and Ngn2-induced excitatory neurons, from two independent SZ patient cohorts. In these selected SZ models, increased STEP₆₁ protein levels likely reflect reduced ubiquitination and degradation. These convergent findings from mouse and hiPSC SZ models provide evidence for STEP₆₁ dysfunction in SZ.

Male-specific deficits in natural reward learning in a mouse model of neurodevelopmental disorders

Neurodevelopmental disorders, including autism spectrum disorders, are highly male biased, but the underpinnings of this are unknown. Striatal dysfunction has been strongly implicated in the pathophysiology of neurodevelopmental disorders, raising the question of whether there are sex differences in how the striatum is impacted by genetic risk factors linked to neurodevelopmental disorders. Here we report male-specific deficits in striatal function important to reward learning in a mouse model of 16p11.2 hemideletion, a genetic mutation that is strongly associated with the risk of neurodevelopmental disorders, particularly autism and attention-deficit hyperactivity disorder. We find that male, but not female, 16p11.2 deletion animals show impairments in reward-directed learning and maintaining motivation to work for rewards. Male, but not female, deletion animals overexpress mRNA for dopamine receptor 2 and adenosine receptor 2a in the striatum, markers of medium spiny neurons signaling via the indirect pathway, associated with behavioral inhibition. Both sexes show a 50% reduction of mRNA levels of the genes located within the 16p11.2 region in the striatum, including the kinase extracellular-signal related kinase 1 (ERK1). However, hemideletion males show increased activation in the striatum for ERK1, both at baseline and in response to sucrose, a signaling change associated with decreased striatal plasticity. This increase in ERK1 phosphorylation is coupled with a decrease in the abundance of the ERK phosphatase striatum-enriched protein-tyrosine phosphatase in hemideletion males. In contrast, females do not show activation of ERK1 in response to sucrose, but notably hemideletion females show elevated protein levels for ERK1 as well as the related kinase ERK2 over what would be predicted by mRNA levels. These data indicate profound sex differences in the impact of a genetic lesion linked with neurodevelopmental disorders, including mechanisms of male-specific vulnerability and female-specific resilience impacting intracellular signaling in the brain.

Striatal-enriched phosphatase 61 inhibited the nociceptive plasticity in spinal cord dorsal horn of rats

Striatal-enriched phosphatase 61 (STEP61) is a member of intracellular protein tyrosine phosphatases, which is involved in the regulation of synaptic plasticity and a line of neuropsychiatric disorders. This protein tyrosine phosphatase is also abundant in pain-related spinal cord dorsal horn neurons. However, whether and how this tyrosine phosphatase modulates the nociceptive plasticity and behavioral hypersensitivity remain largely unknown. The present study recorded the long-term potentiation (LTP) of primary afferent C fiber-evoked field potentials in vivo in superficial dorsal horn of rats, and tested the possible role of STEP61 in spinal LTP. We found that LTP induction significantly increased STEP61 phosphorylation at Ser221 residue, a key molecular event that has been shown to impair the phosphatase activity. The STEP61 hypoactivity allowed for the activation of three substrates, GluN2B subunit-containing N-methyl-d-aspartate-subtype glutamate receptors, Src-family protein tyrosine kinase member Fyn and extracellular signal-regulated kinase 1/2, through which the thresholds for LTP induction were noticeably decreased. To reinstate STEP61 activity, we overexpressed wild-type STEP61 [STEP61(WT)] in spinal dorsal horn, finding that STEP61(WT) completely blunted LTP induction. Behavioral tests showed that LTP blockade by STEP61(WT) correlated with a long-lasting alleviation of thermal hypersensitivity and mechanical allodynia induced by chronic constriction injury of sciatic nerves. These data implicated that STEP61 exerted a negative control over spinal nociceptive plasticity, which might have therapeutic benefit in pathological pain.

STEP activation by Gαq coupled GPCRs opposes Src regulation of NMDA receptors containing the GluN2A subunit

N-methyl-D-aspartate receptors (NMDARs) are necessary for the induction of synaptic plasticity and for the consolidation of learning and memory. NMDAR function is tightly regulated by functionally opposed families of kinases and phosphatases. Herein we show that the striatal-enriched protein tyrosine phosphatase (STEP) is recruited by Gα_q-coupled receptors, including the M1 muscarinic acetylcholine receptor (M1R), and opposes the Src tyrosine kinase-mediated increase in the function of NMDARs composed of GluN2A. STEP activation by M1R stimulation requires IP₃Rs and can depress NMDA-evoked currents with modest intracellular Ca²⁺ buffering. Src recruitment by M1R stimulation requires coincident NMDAR activation and can augment NMDA-evoked currents with high intracellular Ca²⁺ buffering. Our findings suggest that Src and STEP recruitment is contingent on differing intracellular Ca²⁺ dynamics that dictate whether NMDAR function is augmented or depressed following M1R stimulation.

Striatal-enriched protein tyrosine phosphatase modulates nociception: evidence from genetic deletion and pharmacological inhibition

The information from nociceptors is processed in the dorsal horn of the spinal cord by complex circuits involving excitatory and inhibitory interneurons. It is well documented that GluN2B and ERK1/2 phosphorylation contributes to central sensitization. Striatal-enriched protein tyrosine phosphatase (STEP) dephosphorylates GluN2B and ERK1/2, promoting internalization of GluN2B and inactivation of ERK1/2. The activity of STEP was modulated by genetic (STEP knockout mice) and pharmacological (recently synthesized STEP inhibitor, TC-2153) approaches. STEP(61) protein levels in the lumbar spinal cord were determined in male and female mice of different ages. Inflammatory pain was induced by complete Freund's adjuvant injection. Behavioral tests, immunoblotting, and electrophysiology were used to analyze the effect of STEP on nociception. Our results show that both genetic deletion and pharmacological inhibition of STEP induced thermal hyperalgesia and mechanical allodynia, which were accompanied by increased pGluN2B(Tyr1472) and pERK1/2(Thr202/Tyr204)levels in the lumbar spinal cord. Striatal-enriched protein tyrosine phosphatase heterozygous and knockout mice presented a similar phenotype. Furthermore, electrophysiological experiments showed that TC-2153 increased C fiber-evoked spinal field potentials. Interestingly, we found that STEP(61) protein levels in the lumbar spinal cord inversely correlated with thermal hyperalgesia associated with age and female gender in mice. Consistently, STEP knockout mice failed to show age-related thermal hyperalgesia, although gender-related differences were preserved. Moreover, in a model of inflammatory pain, hyperalgesia was associated with increased phosphorylation-mediated STEP(61) inactivation and increased pGluN2B(Tyr1472) and pERK1/2(Thr202/Tyr204)levels in the lumbar spinal cord. Collectively, the present results underscore an important role of spinal STEP activity in the modulation of nociception.

Higher levels of phosphorylated Y1472 on GluN2B subunits in the frontal cortex of aged mice are associated with good spatial reference memory, but not cognitive flexibility

The N-methyl-D-aspartate receptor (NMDAr) is particularly vulnerable to aging. The GluN2B subunit of the NMDAr, compared to other NMDAr subunits, suffers the greatest losses of expression in the aging brain, especially in the frontal cortex. While expression levels of GluN2B mRNA and protein in the aged brain are well documented, there has been little investigation into age-related posttranslational modifications of the subunit. In this study, we explored some of the mechanisms that may promote differences in the NMDAr complex in the frontal cortex of aged animals. Two ages of mice, 3 and 24 months, were behaviorally tested in the Morris water maze. The frontal cortex and hippocampus from each mouse were subjected to differential centrifugation followed by solubilization in Triton X-100. Proteins from Triton-insoluble membranes, Triton-soluble membranes, and intracellular membranes/cytosol were examined by Western blot. Higher levels of GluN2B tyrosine 1472 phosphorylation in frontal cortex synaptic fractions of old mice were associated with better reference learning but poorer cognitive flexibility. Levels of GluN2B phosphotyrosine 1336 remained steady, but there were greater levels of the calpain-induced 115 kDa GluN2B cleavage product on extrasynaptic membranes in these old good learners. There was an age-related increase in calpain activity, but it was not associated with better learning. These data highlight a unique aging change for aged mice with good spatial learning that might be detrimental to cognitive flexibility. This study also suggests that higher levels of truncated GluN2B on extrasynaptic membranes are not deleterious to spatial memory in aged mice.

Role of Striatal-Enriched Tyrosine Phosphatase in Neuronal Function

Striatal-enriched protein tyrosine phosphatase (STEP) is a CNS-enriched protein implicated in multiple neurologic and neuropsychiatric disorders. STEP regulates key signaling proteins required for synaptic strengthening as well as NMDA and AMPA receptor trafficking. Both high and low levels of STEP disrupt synaptic function and contribute to learning and behavioral deficits. High levels of STEP are present in human postmortem samples and animal models of Alzheimer's disease, Parkinson's disease, and schizophrenia and in animal models of fragile X syndrome. Low levels of STEP activity are present in additional disorders that include ischemia, Huntington's chorea, alcohol abuse, and stress disorders. Thus the current model of STEP is that optimal levels are required for optimal synaptic function. Here we focus on the role of STEP in Alzheimer's disease and the mechanisms by which STEP activity is increased in this illness. Both genetic lowering of STEP levels and pharmacological inhibition of STEP activity in mouse models of Alzheimer's disease reverse the biochemical and cognitive abnormalities that are present. These findings suggest that STEP is an important point for modulation of proteins required for synaptic plasticity.

STEP signaling pathway mediates psychomotor stimulation and morphine withdrawal symptoms, but not for reward, analgesia and tolerance

Striatal-enriched protein tyrosine phosphatase (STEP) is abundantly expressed in the striatum, which strongly expresses dopamine and opioid receptors and mediates the effects of many drugs of abuse. However, little is known about the role of STEP in opioid receptor function. In the present study, we generated STEP-targeted mice carrying a nonsense mutation (C230X) in the kinase interaction domain of STEP by screening the N-ethyl-N-nitrosourea (ENU)-driven mutant mouse genomic DNA library and subsequent in vitro fertilization. It was confirmed that the C230X nonsense mutation completely abolished functional STEP protein expression in the brain. STEP(C230X-/-) mice showed attenuated acute morphine-induced psychomotor activity and withdrawal symptoms, whereas morphine-induced analgesia, tolerance and reward behaviors were unaffected. STEP(C230X-/-) mice displayed reduced hyperlocomotion in response to intrastriatal injection of the μ-opioid receptor agonist DAMGO, but the behavioral responses to δ- and κ-opioid receptor agonists remained intact. These results suggest that STEP has a key role in the regulation of psychomotor action and physical dependency to morphine. These data suggest that STEP inhibition may be a critical target for the treatment of withdrawal symptoms associated with morphine.

Aging is associated with dimerization and inactivation of the brain-enriched tyrosine phosphatase STEP

The STriatal-Enriched tyrosine Phosphatase (STEP) is involved in the etiology of several age-associated neurologic disorders linked to oxidative stress and is also known to play a role in neuroprotection by modulating glutamatergic transmission. However, the possible effect of aging on STEP level and activity in the brain is still unclear. In this study, using young (1 month), adult (4 months), and aged (18 months) rats, we show that aging is associated with increase in dimerization and loss of activity of STEP. Increased dimerization of STEP is primarily observed in the cortex and hippocampus and is associated with depletion of both reduced and total glutathione levels, suggesting an increase in oxidative stress. Consistent with this interpretation, studies in cell culture models of glutathione depletion and oxidative stress also demonstrate formation of dimers and higher order oligomers of STEP that involve intermolecular disulfide bond formation between multiple cysteine residues. Conversely, administration of N-acetyl cysteine, a major antioxidant that enhances glutathione biosynthesis, attenuates STEP dimerization both in the cortex and hippocampus. The findings indicate that loss of this intrinsic protective response pathway with age-dependent increase in oxidative stress may be a contributing factor for the susceptibility of the brain to age-associated neurologic disorders.

Dopamine agonists rescue Aβ-induced LTP impairment by Src-family tyrosine kinases

Soluble forms of oligomeric amyloid beta (AβO) are involved in the loss of synaptic plasticity and memory, especially in early phases of Alzheimer's disease. Stimulation of dopamine D1/D5 receptors (D1R/D5R) is known to increase surface expression of synaptic α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate subtype glutamate and N-methyl-D-aspartate subtype glutamate receptors and facilitates the induction of the late phase of long-term potentiation (LTP), probably via a related mechanism. In this study, we show that the D1/D5R agonist SKF38393 protects LTP of hippocampal CA1 synapses from the deleterious action of oligomeric amyloid beta. Unexpectedly, the D1R/D5R-mediated recovery of LTP is independent of protein kinase A or phospholipase C pathways. Instead, we found that the inhibition of Src-family tyrosine kinases completely abolished the protective effects of D1R/D5R stimulation in a cellular model of learning and memory.

Dopamine D2 receptors are involved in the regulation of Fyn and metabotropic glutamate receptor 5 phosphorylation in the rat striatum in vivo

Fyn, a major Src family kinase (SFK) member that is densely expressed in striatal neurons, is actively involved in the regulation of cellular and synaptic activities in local neurons. This SFK member is likely regulated by dopamine signaling through a receptor mechanism involving dopamine D2 receptors (D2Rs). This study characterizes the D2R-dependent regulation of Fyn in the rat striatum in vivo. Moreover, we explore whether D2Rs regulate metabotropic glutamate receptor 5 (mGluR5) in its tyrosine phosphorylation and whether the D2R-SFK pathway modulates trafficking of mGluR5. We found that blockade of D2Rs by systemic administration of a D2R antagonist, eticlopride, substantially increased SFK phosphorylation in the striatum. This increase was a transient and reversible event. The eticlopride-induced SFK phosphorylation occurred predominantly in immunopurified Fyn but not in another SFK member, Src. Eticlopride also elevated tyrosine phosphorylation of mGluR5. In parallel, eticlopride enhanced synaptic delivery of active Fyn and mGluR5. Pretreatment with an SFK inhibitor blocked the eticlopride-induced tyrosine phosphorylation and synaptic trafficking of mGluR5. These results indicate that D2Rs inhibit SFK (mainly Fyn) phosphorylation in the striatum. D2Rs also inhibit tyrosine phosphorylation and synaptic recruitment of mGluR5 through a signaling mechanism likely involving Fyn.

Zn2+-dependent Activation of the Trk Signaling Pathway Induces Phosphorylation of the Brain-enriched Tyrosine Phosphatase STEP: MOLECULAR BASIS FOR ZN2+-INDUCED ERK MAPK ACTIVATION

Excessive release of Zn(2+) in the brain is implicated in the progression of acute brain injuries. Although several signaling cascades have been reported to be involved in Zn(2+)-induced neurotoxicity, a potential contribution of tyrosine phosphatases in this process has not been well explored. Here we show that exposure to high concentrations of Zn(2+) led to a progressive increase in phosphorylation of the striatal-enriched phosphatase (STEP), a component of the excitotoxic-signaling pathway that plays a role in neuroprotection. Zn(2+)-mediated phosphorylation of STEP61 at multiple sites (hyperphosphorylation) was induced by the up-regulation of brain-derived neurotropic factor (BDNF), tropomyosin receptor kinase (Trk) signaling, and activation of cAMP-dependent PKA (protein kinase A). Mutational studies further show that differential phosphorylation of STEP61 at the PKA sites, Ser-160 and Ser-221 regulates the affinity of STEP61 toward its substrates. Consistent with these findings we also show that BDNF/Trk/PKA mediated signaling is required for Zn(2+)-induced phosphorylation of extracellular regulated kinase 2 (ERK2), a substrate of STEP that is involved in Zn(2+)-dependent neurotoxicity. The strong correlation between the temporal profile of STEP61 hyperphosphorylation and ERK2 phosphorylation indicates that loss of function of STEP61 through phosphorylation is necessary for maintaining sustained ERK2 phosphorylation. This interpretation is further supported by the findings that deletion of the STEP gene led to a rapid and sustained increase in ERK2 phosphorylation within minutes of exposure to Zn(2+). The study provides further insight into the mechanisms of regulation of STEP61 and also offers a molecular basis for the Zn(2+)-induced sustained activation of ERK2.

Regulation of STEP61 and tyrosine-phosphorylation of NMDA and AMPA receptors during homeostatic synaptic plasticity

Background

Sustained changes in network activity cause homeostatic synaptic plasticity in part by altering the postsynaptic accumulation of N-methyl-D-aspartate receptors (NMDAR) and α-amino-3-hydroxyle-5-methyl-4-isoxazolepropionic acid receptors (AMPAR), which are primary mediators of excitatory synaptic transmission. A key trafficking modulator of NMDAR and AMPAR is STriatal-Enriched protein tyrosine Phosphatase (STEP₆₁) that opposes synaptic strengthening through dephosphorylation of NMDAR subunit GluN2B and AMPAR subunit GluA2. However, the role of STEP₆₁ in homeostatic synaptic plasticity is unknown.

Findings

We demonstrate here that prolonged activity blockade leads to synaptic scaling, and a concurrent decrease in STEP₆₁ level and activity in rat dissociated hippocampal cultured neurons. Consistent with STEP₆₁ reduction, prolonged activity blockade enhances the tyrosine phosphorylation of GluN2B and GluA2 whereas increasing STEP₆₁ activity blocks this regulation and synaptic scaling. Conversely, prolonged activity enhancement increases STEP₆₁ level and activity, and reduces the tyrosine phosphorylation and level of GluN2B as well as GluA2 expression in a STEP₆₁–dependent manner.

Conclusions

Given that STEP₆₁-mediated dephosphorylation of GluN2B and GluA2 leads to their internalization, our results collectively suggest that activity-dependent regulation of STEP₆₁ and its substrates GluN2B and GluA2 may contribute to homeostatic stabilization of excitatory synapses.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0148-4) contains supplementary material, which is available to authorized users.

BDNF Induces Striatal-Enriched Protein Tyrosine Phosphatase 61 Degradation Through the Proteasome

Brain-derived neurotrophic factor (BDNF) promotes synaptic strengthening through the regulation of kinase and phosphatase activity. Conversely, striatal-enriched protein tyrosine phosphatase (STEP) opposes synaptic strengthening through inactivation or internalization of signaling molecules. Here, we investigated whether BDNF regulates STEP levels/activity. BDNF induced a reduction of STEP61 levels in primary cortical neurons, an effect that was prevented by inhibition of tyrosine kinases, phospholipase C gamma, or the ubiquitin-proteasome system (UPS). The levels of pGluN2B(Tyr1472) and pERK1/2(Thr202/Tyr204), two STEP substrates, increased in BDNF-treated cultures, and blockade of the UPS prevented STEP61 degradation and reduced BDNF-induced GluN2B and ERK1/2 phosphorylation. Moreover, brief or sustained cell depolarization reduced STEP61 levels in cortical neurons by different mechanisms. BDNF also promoted UPS-mediated STEP61 degradation in cultured striatal and hippocampal neurons. In contrast, nerve growth factor and neurotrophin-3 had no effect on STEP61 levels. Our results thus indicate that STEP61 degradation is an important event in BDNF-mediated effects.

Striatal-enriched protein tyrosine phosphatase regulates the PTPα/Fyn signaling pathway

The tyrosine kinase Fyn has two regulatory tyrosine residues that when phosphorylated either activate (Tyr(420)) or inhibit (Tyr(531)) Fyn activity. Within the central nervous system, two protein tyrosine phosphatases (PTPs) target these regulatory tyrosines in Fyn. PTPα dephosphorylates Tyr(531) and activates Fyn, while STEP (STriatal-Enriched protein tyrosine Phosphatase) dephosphorylates Tyr(420) and inactivates Fyn. Thus, PTPα and STEP have opposing functions in the regulation of Fyn; however, whether there is cross talk between these two PTPs remains unclear. Here, we used molecular techniques in primary neuronal cultures and in vivo to demonstrate that STEP negatively regulates PTPα by directly dephosphorylating PTPα at its regulatory Tyr(789). Dephosphorylation of Tyr(789) prevents the translocation of PTPα to synaptic membranes, blocking its ability to interact with and activate Fyn. Genetic or pharmacologic reduction in STEP61 activity increased the phosphorylation of PTPα at Tyr(789), as well as increased translocation of PTPα to synaptic membranes. Activation of PTPα and Fyn and trafficking of GluN2B to synaptic membranes are necessary for ethanol (EtOH) intake behaviors in rodents. We tested the functional significance of STEP61 in this signaling pathway by EtOH administration to primary cultures as well as in vivo, and demonstrated that the inactivation of STEP61 by EtOH leads to the activation of PTPα, its translocation to synaptic membranes, and the activation of Fyn. These findings indicate a novel mechanism by which STEP61 regulates PTPα and suggest that STEP and PTPα coordinate the regulation of Fyn. STEP61 , PTPα, Fyn, and NMDA receptor (NMDAR) have been implicated in ethanol intake behaviors in the dorsomedial striatum (DMS) in rodents. Here, we report that PTPα is a novel substrate for STEP61. Upon ethanol exposure, STEP61 is phosphorylated and inactivated by protein kinase A (PKA) signaling in the DMS. As a result of STEP61 inhibition, there is an increase in the phosphorylation of PTPα, which translocates to lipid rafts and activates Fyn and subsequent NMDAR signaling. The results demonstrate a synergistic regulation of Fyn-NMDAR signaling by STEP61 and PTPα, which may contribute to the regulation of ethanol-related behaviors. NMDA, N-methyl-D-aspartate; PTPα, receptor-type protein tyrosine phosphatase alpha; STEP, STriatal-Enriched protein tyrosine Phosphatase.

GABAergic Inhibition Regulated Pain Sensitization through STEP61 Signaling in Spinal Dorsal Horn of Mice

Background:

The reduction of γ-aminobutyric acid (GABA) type A receptor–mediated inhibition has long been implicated in spinal sensitization of nociceptive responses. However, it is largely unknown which signaling cascades in spinal dorsal horn neurons are initiated by the reduced inhibition to trigger pain hypersensitivity.

Methods:

GABAergic inhibition was manipulated by intrathecal application of GABA type A receptor antagonist bicuculline in intact mice or by GABA type A receptor agonist muscimol in complete Freund’s adjuvant–injected mice. Immunoblotting, coimmunoprecipitation, immunohistochemistry, and behavioral tests were used to explore the signaling pathways downstream of the altered GABAergic tone.

Results:

The study data revealed that the 61-kD isoform of striatal-enriched protein phosphatase (STEP61) was a key molecule that relayed the signals from GABAergic neurotransmission. The authors found that STEP61 was highly expressed in dorsal horn neurons. Under physiological conditions, STEP61 tonically interacted with and negatively controlled the activities of extracellular signal–regulated kinase and Src-family protein tyrosine kinases member Fyn, two critical kinases involved in spinal sensitization. Once GABAergic inhibition was impaired, STEP61 interaction with its substrates was substantially disturbed, allowing for activation of extracellular signal–regulated kinase and Fyn (n = 4 to 6). The hyperactivities of extracellular signal–regulated kinase and Fyn, along with STEP61 dysregulation, caused the tyrosine phosphorylation and synaptic accumulation of GluN2B subunit-containing N-methyl-d-aspartate subtype of glutamate receptors (n = 6), leading to GluN2B receptor-dependent pain hypersensitivity. Overexpression of wild-type STEP61 to resume its enzymatic activity significantly blocked the mechanical allodynia evoked by bicuculline and more importantly, alleviated chronic inflammatory pain (n = 6 in each group).

Conclusion:

These data identified STEP61 as a key intermediary for GABAergic inhibition to regulate pain sensitization.

STEP61 is a substrate of the E3 ligase parkin and is upregulated in Parkinson's disease

Parkinson's disease (PD) is characterized by the degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc). The loss of SNc dopaminergic neurons affects the plasticity of striatal neurons and leads to significant motor and cognitive disabilities during the progression of the disease. PARK2 encodes for the E3 ubiquitin ligase parkin and is implicated in genetic and sporadic PD. Mutations in PARK2 are a major contributing factor in the early onset of autosomal-recessive juvenile parkinsonism (AR-JP), although the mechanisms by which a disruption in parkin function contributes to the pathophysiology of PD remain unclear. Here we demonstrate that parkin is an E3 ligase for STEP61 (striatal-enriched protein tyrosine phosphatase), a protein tyrosine phosphatase implicated in several neuropsychiatric disorders. In cellular models, parkin ubiquitinates STEP61 and thereby regulates its level through the proteasome system, whereas clinically relevant parkin mutants fail to do so. STEP61 protein levels are elevated on acute down-regulation of parkin or in PARK2 KO rat striatum. Relevant to PD, STEP61 accumulates in the striatum of human sporadic PD and in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned mice. The increase in STEP61 is associated with a decrease in the phosphorylation of its substrate ERK1/2 and the downstream target of ERK1/2, pCREB [phospho-CREB (cAMP response element-binding protein)]. These results indicate that STEP61 is a novel substrate of parkin, although further studies are necessary to determine whether elevated STEP61 levels directly contribute to the pathophysiology of PD.

Extracellular signal-regulated protein kinases 1 and 2 activation by addictive drugs: a signal toward pathological adaptation

Addiction is a chronic and relapsing psychiatric disorder that is thought to occur in vulnerable individuals. Synaptic plasticity evoked by drugs of abuse in the so-called neuronal circuits of reward has been proposed to underlie behavioral adaptations that characterize addiction. By increasing dopamine in the striatum, addictive drugs alter the balance of dopamine and glutamate signals converging onto striatal medium-sized spiny neurons (MSNs) and activate intracellular events involved in long-term behavioral alterations. Our laboratory contributed to the identification of salient molecular changes induced by administration of addictive drugs to rodents. We pioneered the observation that a common feature of addictive drugs is to activate, by a double tyrosine/threonine phosphorylation, the extracellular signal-regulated kinases 1 and 2 (ERK1/2) in the striatum, which control a plethora of substrates, some of them being critically involved in cocaine-mediated molecular and behavioral adaptations. Herein, we review how the interplay between dopamine and glutamate signaling controls cocaine-induced ERK1/2 activation in MSNs. We emphasize the key role of N-methyl-D-aspartate receptor potentiation by D1 receptor to trigger ERK1/2 activation and its subsequent nuclear translocation where it modulates both epigenetic and genetic processes engaged by cocaine. We discuss how cocaine-induced long-term synaptic and structural plasticity of MSNs, as well as behavioral adaptations, are influenced by ERK1/2-controlled targets. We conclude that a better knowledge of molecular mechanisms underlying ERK1/2 activation by drugs of abuse and/or its role in long-term neuronal plasticity in the striatum may provide a new route for therapeutic treatment in addiction.

Disruption of striatal-enriched protein tyrosine phosphatase (STEP) function in neuropsychiatric disorders

Striatal-enriched protein tyrosine phosphatase (STEP) is a brain-specific tyrosine phosphatase that plays a major role in the development of synaptic plasticity. Recent findings have implicated STEP in several psychiatric and neurological disorders, including Alzheimer's disease, schizophrenia, fragile X syndrome, Huntington's disease, stroke/ischemia, and stress-related psychiatric disorders. In these disorders, STEP protein expression levels and activity are dysregulated, contributing to the cognitive deficits that are present. In this review, we focus on the most recent findings on STEP, discuss how STEP expression and activity are maintained during normal cognitive function, and how disruptions in STEP activity contribute to a number of illnesses.

Alterations in STriatal-Enriched protein tyrosine Phosphatase expression, activation, and downstream signaling in early and late stages of the YAC128 Huntington's disease mouse model

Striatal neurodegeneration and synaptic dysfunction in Huntington's disease are mediated by the mutant huntingtin (mHtt) protein. MHtt disrupts calcium homeostasis and facilitates excitotoxicity, in part by altering NMDA receptor (NMDAR) trafficking and function. Pre-symptomatic (excitotoxin-sensitive) transgenic mice expressing full-length human mHtt with 128 polyglutamine repeats (YAC128 Huntington's disease mice) show increased calpain activity and extrasynaptic NMDAR (Ex-NMDAR) localization and signaling. Furthermore, Ex-NMDAR stimulation facilitates excitotoxicity in wild-type cortical neurons via calpain-mediated cleavage of STriatal-Enriched protein tyrosine Phosphatase 61 (STEP61). The cleavage product, STEP33, cannot dephosphorylate p38 mitogen-activated protein kinase (MAPK), thereby augmenting apoptotic signaling. Here, we show elevated extrasynaptic calpain-mediated cleavage of STEP61 and p38 phosphorylation, as well as STEP61 inactivation and reduced extracellular signal-regulated protein kinase 1/2 phosphorylation (ERK1/2) in the striatum of 6-week-old, excitotoxin-sensitive YAC128 mice. Calpain inhibition reduced basal and NMDA-induced STEP61 cleavage. However, basal p38 phosphorylation was normalized by a peptide disrupting NMDAR-post-synaptic density protein-95 (PSD-95) binding but not by calpain inhibition. In 1-year-old excitotoxin-resistant YAC128 mice, STEP33 levels were not elevated, but STEP61 inactivation and p38 and ERK 1/2 phosphorylation levels were increased. These results show that in YAC128 striatal tissue, enhanced NMDAR-PSD-95 interactions contributes to elevated p38 signaling in early, excitotoxin-sensitive stages, and suggest that STEP61 inactivation enhances MAPK signaling at late, excitotoxin-resistant stages. The YAC128 Huntington's disease mouse model shows early, enhanced susceptibility to NMDA receptor-mediated striatal apoptosis, progressing to late-stage excitotoxicity resistance. This study shows that elevated NMDA receptor-PSD-95 interactions as well as decreased extrasynaptic STriatal-Enriched protein tyrosine Phosphatase 61 (STEP61) activation may contribute to early enhanced apoptotic signaling. In late-stage YAC128 mice, reduced STEP61 levels and activity correlate with elevated MAPK signaling, consistent with excitotoxicity resistance. Solid and dotted arrows indicate conclusions drawn from the current study and other literature, respectively.

Inhibition of striatal-enriched tyrosine phosphatase 61 in the dorsomedial striatum is sufficient to increased ethanol consumption

The STriatal-Enriched protein tyrosine Phosphatase 61 (STEP61 ) inhibits the activity of the tyrosine kinase Fyn and dephosphorylates the GluN2B subunit of the NMDA receptor, whereas the protein kinase A phosphorylation of STEP61 inhibits the activity of the phosphatase (Pharmacol. Rev., 64, , p. 65). Previously, we found that ethanol activates Fyn in the dorsomedial striatum (DMS) leading to GluN2B phosphorylation, which, in turn, underlies the development of ethanol intake (J. Neurosci., 30, , p. 10187). Here, we tested the hypothesis that inhibition of STEP61 by ethanol is upstream of Fyn/GluN2B. We show that exposure of mice to ethanol increased STEP61 phosphorylation in the DMS, which was maintained after withdrawal and was not observed in other striatal regions. Specific knockdown of STEP61 in the DMS of mice enhanced ethanol-mediated Fyn activation and GluN2B phosphorylation, and increased ethanol intake without altering the level of water, saccharine, quinine consumption or spontaneous locomotor activity. Together, our data suggest that blockade of STEP61 activity in response to ethanol is sufficient for the activation of the Fyn/GluN2B pathway in the DMS. Being upstream of Fyn and GluN2B, inactive STEP61 in the DMS primes the induction of ethanol intake. We show that ethanol-mediated inhibition of STEP61 in the DMS leads to Fyn activation and GluN2B phosphorylation. (a) Under basal conditions, active STEP61 inhibits Fyn activity and dephosphorylates GluN2B. (b) Ethanol leads to the phosphorylation of STEP61 on a specific inhibitory site. The inhibition of STEP61 activity contributes to the activation of Fyn in response to ethanol, which, in turn, phosphorylates GluN2B. These molecular adaptations in the DMS promote ethanol drinking.

Interconnected network motifs control podocyte morphology and kidney function

Podocytes are kidney cells with specialized morphology that is required for glomerular filtration. Diseases, such as diabetes, or drug exposure that causes disruption of the podocyte foot process morphology results in kidney pathophysiology. Proteomic analysis of glomeruli isolated from rats with puromycin-induced kidney disease and control rats indicated that protein kinase A (PKA), which is activated by adenosine 3',5'-monophosphate (cAMP), is a key regulator of podocyte morphology and function. In podocytes, cAMP signaling activates cAMP response element-binding protein (CREB) to enhance expression of the gene encoding a differentiation marker, synaptopodin, a protein that associates with actin and promotes its bundling. We constructed and experimentally verified a β-adrenergic receptor-driven network with multiple feedback and feedforward motifs that controls CREB activity. To determine how the motifs interacted to regulate gene expression, we mapped multicompartment dynamical models, including information about protein subcellular localization, onto the network topology using Petri net formalisms. These computational analyses indicated that the juxtaposition of multiple feedback and feedforward motifs enabled the prolonged CREB activation necessary for synaptopodin expression and actin bundling. Drug-induced modulation of these motifs in diseased rats led to recovery of normal morphology and physiological function in vivo. Thus, analysis of regulatory motifs using network dynamics can provide insights into pathophysiology that enable predictions for drug intervention strategies to treat kidney disease.