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

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.

Differential mRNA Expression Profiling Reveals the Role of MiR-375 in Inflammation of Bovine Mammary Epithelial Cells

Simple Summary

Bovine mammary epithelial cells (bMECs) are often used as cell models for mammary gland research. They are the most important cells for mammary gland function and the first line of defense for pathogen identification. MicroRNAs (miRNAs) are important regulatory factors involved in many physiological and pathological processes. Here, we examined a transcriptome profile of bovine mammary epithelial cell lines transfected with miR-375 inhibitor or negative control (NC) inhibitor, and further reveal the potential role of miR-375 in bMECs by differentially expressed mRNA analysis. We found that miR-375 potentially promotes inflammation in the mammary gland through the MAPK signaling pathway.

Abstract

MicroRNAs (miRNAs) are a class of small non-coding RNAs that regulate post-transcriptional gene expression and several biological processes. Bovine mammary epithelial cells (bMECs) mediate critical immune responses in the mammary gland and the occurrence of mastitis. Current research focuses on miRNA regulation of bMECs, but the miR-375 regulatory mechanism in bMECs is unclear. This study explored the role of miR-375 by profiling the transcriptome of miR-375-silenced bMECs using RNA-seq and identifying differentially expressed mRNAs (DIE-mRNAs). There were 63 DIE-mRNAs, including 48 down-regulated and 15 up-regulated mRNAs between miR-375-silenced bMECs and the controls. The Kyoto encyclopedia of genes and genomes (KEGG) and Gene Ontology (GO) functional analysis showed that the DIE-mRNAs enriched nuclear receptor subfamily 4 group A member 1 (NR4A1) and protein tyrosine phosphatase non-receptor type 5 (PTPN5) anti-inflammatory genes of the mitogen-activated protein kinase (MAPK) signaling pathway. However, they showed an opposite trend to the expression of miR-375 silencing, suggesting that miR-375 promotes bMEC inflammation through the MAPK signaling pathway. The findings of this study provide a new reference for understanding the regulation of bMEC inflammation and cow mastitis.

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.