Zn2+-dependent Activation of the Trk Signaling Pathway Induces Phosphorylation of the Brain-enriched Tyrosine Phosphatase STEP

Abstract

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 STEP 61 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 STEP 61 at the PKA sites, Ser-160 and Ser-221 regulates the affinity of STEP 61 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 STEP 61 hyperphosphorylation and ERK2 phosphorylation indicates that loss of function of STEP 61 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 STEP 61 and also offers a molecular basis for the Zn 2؉ -induced sustained activation of ERK2.A growing body of evidence indicates that Zn 2ϩ , the second most abundant transition metal in the brain, is released at high concentrations during excitotoxic neurological conditions and can contribute to neuronal injury (1-7). The proposed mechanisms associated with Zn 2ϩ -dependent neurotoxicity include alteration of activity of a diverse group of post-synaptic receptors like ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), 3 N-methyl-D-aspartic acid (NMDA), voltagegated calcium channels and neurotrophic receptors (7-12). Receptor activation in turn modulates the function of several intracellular kinases including protein kinase C, mitogen-activated protein kinase/extracellular signal-regulated kinase 1/2 (ERK MAPK), and the Src family of tyrosine kinases resulting in impaired energy production, excitability and oxidative stress (8,9,(13)(14)(15). However, the role of phosphatases in Zn 2ϩ -induced neurotoxicity is currently not well understood.The intracellular tyrosine phosphatase, STEP (striatalenriched tyrosine phosphatase, also known as PTPN5) is expressed exclusively in the central nervous system (16,17) and is emerging as a key regulator of neuronal survival and death. The STEP-family of PTPs includes both membrane-assoc...

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Role of Src Family Kinases in BDNF-Mediated Suppression of Cocaine-Seeking and Prevention of Cocaine-Induced ERK, GluN2A, and GluN2B Dephosphorylation in the Prelimbic Cortex

“…One possible mechanism for this effect is release of Zn 2+ from glutamatergic terminals that can induce ERK phosphorylation via NMDA receptors. This Zn 2+ induced ERK activation is accompanied by a corresponding deactivation of STEP (Poddar et al, 2016). In addition, Zn 2+ can stimulate the residues in TrkB that cause autophosphorylation resulting in activation of downstream pathways including ERK, an effect that is SFK-dependent (Huang and McNamara, 2010).…”

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Why and how to investigate the role of protein phosphorylation in ZIP and ZnT zinc transporter activity and regulation

“…The TrkB activation by zinc has been shown to potentiate the hippocampal mossy fiber-CA3 pyramid synapse [132]. Also, Zn 2+ induces the activation of Trk signaling pathways leading to multi-phosphorylation of the striatal-enriched phosphatase (STEP) as well as phosphorylation and activation of Erk2, a STEP substrate involved in Zn 2+ -dependent neurotoxicity [133].…”