Aberrant regulation and function of Src family tyrosine kinases: Their potential contributions to glutamate-induced neurotoxicity

Role of cytokines and reactive oxygen species in brain aging

Aging is a complex process that produces profound effects on the brain. Although a number of external factors can promote the initiation and progression of brain aging, peripheral and central changes in the immune cells with time, also play an important role. Immunosenescence, which is an age-associated decline in immune function and Inflammaging, a low-grade inflammatory state in the aging brain contribute to an elevation in cytokine and reactive oxygen species production. In this review, we focus on the pro-inflammatory state that is established in the brain as a consequence of these two phenomena and the resulting detrimental changes in brain structure, function and repair that lead to a decline in central and neuroendocrine function.

Bioinformatics and experimental analyses of glutamate receptor and its targets genes in myocardial and cerebral ischemia

BACKGROUND

There is a mutual hemodynamic and pathophysiological basis between the heart and brain. Glutamate (GLU) signaling plays an important role in the process of myocardial ischemia (MI) and ischemic stroke (IS). To further explore the common protective mechanism after cardiac and cerebral ischemic injuries, the relationship between GLU receptor-related genes and MI and IS were analyzed.

RESULTS

A total of 25 crosstalk genes were identified, which were mainly enriched in the Toll-like receptor signaling pathway, Th17 cell differentiation, and other signaling pathways. Protein-protein interaction analysis suggested that the top six genes with the most interactions with shared genes were IL6, TLR4, IL1B, SRC, TLR2, and CCL2. Immune infiltration analysis suggested that immune cells such as myeloid-derived suppressor cells and monocytes were highly expressed in the MI and IS data. Memory B cells and Th17 cells were expressed at low levels in the MI and IS data; molecular interaction network construction suggested that genes such as JUN, FOS, and PPARA were shared genes and transcription factors; FCGR2A was a shared gene of MI and IS as well as an immune gene. Least absolute shrinkage and selection operator logistic regression analysis identified nine hub genes: IL1B, FOS, JUN, FCGR2A, IL6, AKT1, DRD4, GLUD2, and SRC. Receiver operating characteristic analysis revealed that the area under the curve of these hub genes was > 65% in MI and IS for all seven genes except IL6 and DRD4. Furthermore, clinical blood samples and cellular models showed that the expression of relevant hub genes was consistent with the bioinformatics analysis.

CONCLUSIONS

In this study, we found that the GLU receptor-related genes IL1B, FOS, JUN, FCGR2A, and SRC were expressed in MI and IS with the same trend, which can be used to predict the occurrence of cardiac and cerebral ischemic diseases and provide reliable biomarkers to further explore the co-protective mechanism after cardiac and cerebral ischemic injury.

Neuroprotective effect of Src kinase in hypoxia-ischemia: A systematic review

Background

Hypoxic-ischemic encephalopathy (HIE) is a major cause of neonatal morbidity and mortality worldwide. While the application of therapeutic hypothermia has improved neurodevelopmental outcomes for some survivors of HIE, this lone treatment option is only available to a subset of affected neonates. Src kinase, an enzyme central to the apoptotic cascade, is a potential pharmacologic target to preserve typical brain development after HIE. Here, we present evidence of the neuroprotective effects of targeting Src kinase in preclinical models of HIE.

Methods

We performed a comprehensive literature search using the National Library of Medicine's MEDLINE database to compile studies examining the impact of Src kinase regulation on neurodevelopment in animal models. Each eligible study was assessed for bias.

Results

Twenty studies met the inclusion criteria, and most studies had an intermediate risk for bias. Together, these studies showed that targeting Src kinase resulted in a neuroprotective effect as assessed by neuropathology, enzymatic activity, and neurobehavioral outcomes.

Conclusion

Src kinase is an effective neuroprotective target in the setting of acute hypoxic injury. Src kinase inhibition triggers multiple signaling pathways of the sub-membranous focal adhesions and the nucleus, resulting in modulation of calcium signaling and prevention of cell death. Despite the significant heterogeneity of the research studies that we examined, the available evidence can serve as proof-of-concept for further studies on this promising therapeutic strategy.

Obligatory Activation of SRC and JNK by GDNF for Survival and Axonal Outgrowth of Postnatal Intestinal Neurons

The neurotrophin GDNF acts through its co-receptor RET to direct embryonic development of the intestinal nervous system. Since this continues in the post-natal intestine, co-cultures of rat enteric neurons and intestinal smooth muscle cells were used to examine how receptor activation mediates neuronal survival or axonal extension. GDNF-mediated activation of SRC was essential for neuronal survival and axon outgrowth and activated the major downstream signaling pathways. Selective inhibition of individual pathways had little effect on survival but JNK activation was required for axonal maintenance, extension or regeneration. This was localized to axonal endings and retrograde transport was needed for central JUN activation and subsequent axon extension. Collectively, GDNF signaling supports neuronal survival via SRC activation with multiple downstream events, with JNK signaling mediating structural plasticity. These pathways may limit neuron death and drive subsequent regeneration during challenges in vivo such as intestinal inflammation, where supportive strategies could preserve intestinal function.

N-Methyl-D-Aspartate Receptor Signaling-Protein Kinases Crosstalk in Cerebral Ischemia

Although stroke is very often the cause of death worldwide, the burden of ischemic and hemorrhagic stroke varies between regions and over time regarding differences in prognosis, prevalence of risk factors, and treatment strategies. Excitotoxicity, oxidative stress, dysfunction of the blood-brain barrier, neuroinflammation, and lysosomal membrane permeabilization, sequentially lead to the progressive death of neurons. In this process, protein kinases-related checkpoints tightly regulate N-methyl-D-aspartate (NMDA) receptor signaling pathways. One of the major hallmarks of cerebral ischemia is excitotoxicity, characterized by overactivation of glutamate receptors leading to intracellular Ca²⁺ overload and ultimately neuronal death. Thus, reduced expression of postsynaptic density-95 protein and increased protein S-nitrosylation in neurons is responsible for neuronal vulnerability in cerebral ischemia. In this chapter death-associated protein kinases, cyclin-dependent kinase 5, endoplasmic reticulum stress-induced protein kinases, hyperhomocysteinemia-related NMDA receptor overactivation, ephrin-B-dependent amplification of NMDA-evoked neuronal excitotoxicity and lysosomocentric hypothesis have been discussed.Consequently, ample evidences have demonstrated that enhancing extrasynaptic NMDA receptor activity triggers cell death after stroke. In this context, considering the dual roles of NMDA receptors in both promoting neuronal survival and mediating neuronal damage, selective augmentation of NR2A-containing NMDA receptor activation in the presence of NR2B antagonist may constitute a promising therapy for stroke.

Activation of Phosphotyrosine-Mediated Signaling Pathways in the Cortex and Spinal Cord of SOD1G93A, a Mouse Model of Familial Amyotrophic Lateral Sclerosis

Degeneration of cortical and spinal motor neurons is the typical feature of amyotrophic lateral sclerosis (ALS), a progressive neurodegenerative disease for which a pathogenetic role for the Cu/Zn superoxide dismutase (SOD1) has been demonstrated. Mice overexpressing a mutated form of the SOD1 gene (SOD1^G93A) develop a syndrome that closely resembles the human disease. The SOD1 mutations confer to this enzyme a “gain-of-function,” leading to increased production of reactive oxygen species. Several oxidants induce tyrosine phosphorylation through direct stimulation of kinases and/or phosphatases. In this study, we analyzed the activities of src and fyn tyrosine kinases and of protein tyrosine phosphatases in synaptosomal fractions prepared from the motor cortex and spinal cord of transgenic mice expressing SOD1^G93A. We found that (i) protein phosphotyrosine level is increased, (ii) src and fyn activities are upregulated, and (iii) the activity of tyrosine phosphatases, including the striatal-enriched tyrosine phosphatase (STEP), is significantly decreased. Moreover, the NMDA receptor (NMDAR) subunit GluN2B tyrosine phosphorylation was upregulated in SOD1^G93A. Tyrosine phosphorylation of GluN2B subunits regulates the NMDAR function and the recruitment of downstream signaling molecules. Indeed, we found that proline-rich tyrosine kinase 2 (Pyk2) and ERK1/2 kinase are upregulated in SOD1^G93A mice. These results point out an involvement of tyrosine kinases and phosphatases in the pathogenesis of ALS.

Pannexin-1 is involved in neuronal apoptosis and degeneration in experimental intracerebral hemorrhage in rats

Pannexins serve an important role in the regulation of extracellular neuronal regenerative currents and cellular signal transduction of glial cells; however, the effects of pannexins in various cerebrovascular diseases have not been reported. The present study focused on the expression and influence of pannexins in a rat model of intracerebral hemorrhage (ICH), and confirmed that pannexins (including Pannexin‑1, Pannexin‑2 and Pannexin‑3) are expressed in rat brain tissues. However, only the expression of Pannexin‑1 was significantly increased and peaked 48 h post‑ICH. Following treatment with carbenoxolone (CBX), which is an inhibitor of Pannexin‑1, apoptosis and neuronal degeneration in the brain tissues around the ICH hematoma decreased. The extent of secondary brain injury due to ICH was also alleviated. Compared with rats in the ICH‑only group, recovery of neurocognitive functions improved significantly in the CBX‑treated groups. Results from the present study suggested that the upregulation of Pannexin‑1 expression may be involved in apoptosis and degeneration of neurons in the rat brain following ICH, and may contribute to subsequent cognitive dysfunction.

The role of non-receptor protein tyrosine kinases in the excitotoxicity induced by the overactivation of NMDA receptors

Protein tyrosine phosphorylation is one of the primary modes of regulation of N-methyl-d-aspartate (NMDA) receptors. The non-receptor tyrosine kinases are one of the two types of protein tyrosine kinases that are involved in this process. The overactivation of NMDA receptors is a primary reason for neuron death following cerebral ischemia. Many studies have illustrated the important role of non-receptor tyrosine kinases in ischemia insults. This review introduces the roles of Src, Fyn, focal adhesion kinase, and proline-rich tyrosine kinase 2 in the excitotoxicity induced by the overactivation of NMDA receptors following cerebral ischemia.

Inhibition of N1-Src kinase by a specific SH3 peptide ligand reveals a role for N1-Src in neurite elongation by L1-CAM

In the mammalian brain the ubiquitous tyrosine kinase, C-Src, undergoes splicing to insert short sequences in the SH3 domain to yield N1- and N2-Src. We and others have previously shown that the N-Srcs have altered substrate specificity and kinase activity compared to C-Src. However, the exact functions of the N-Srcs are unknown and it is likely that N-Src signalling events have been misattributed to C-Src because they cannot be distinguished by conventional Src inhibitors that target the kinase domain. By screening a peptide phage display library, we discovered a novel ligand (PDN1) that targets the unique SH3 domain of N1-Src and inhibits N1-Src in cells. In cultured neurons, PDN1 fused to a fluorescent protein inhibited neurite outgrowth, an effect that was mimicked by shRNA targeting the N1-Src microexon. PDN1 also inhibited L1-CAM-dependent neurite elongation in cerebellar granule neurons, a pathway previously shown to be disrupted in Src^−/− mice. PDN1 therefore represents a novel tool for distinguishing the functions of N1-Src and C-Src in neurons and is a starting point for the development of a small molecule inhibitor of N1-Src.

HIV-1 Tat causes cognitive deficits and selective loss of parvalbumin, somatostatin, and neuronal nitric oxide synthase expressing hippocampal CA1 interneuron subpopulations

Memory deficits are characteristic of HIV-associated neurocognitive disorders (HAND) and co-occur with hippocampal pathology. The HIV-1 transactivator of transcription (Tat), a regulatory protein, plays a significant role in these events, but the cellular mechanisms involved are poorly understood. Within the hippocampus, diverse populations of interneurons form complex networks; even subtle disruptions can drastically alter synaptic output, resulting in behavioral dysfunction. We hypothesized that HIV-1 Tat would impair cognitive behavior and injure specific hippocampal interneuron subtypes. Male transgenic mice that inducibly expressed HIV-1 Tat (or non-expressing controls) were assessed for cognitive behavior or had hippocampal CA1 subregions evaluated via interneuron subpopulation markers. Tat exposure decreased spatial memory in a Barnes maze and mnemonic performance in a novel object recognition test. Tat reduced the percentage of neurons expressing neuronal nitric oxide synthase (nNOS) without neuropeptide Y immunoreactivity in the stratum pyramidale and the stratum radiatum, parvalbumin in the stratum pyramidale, and somatostatin in the stratum oriens, which are consistent with reductions in interneuron-specific interneuron type 3 (IS3), bistratified, and oriens-lacunosum-moleculare interneurons, respectively. The findings reveal that an interconnected ensemble of CA1 nNOS-expressing interneurons, the IS3 cells, as well as subpopulations of parvalbumin- and somatostatin-expressing interneurons are preferentially vulnerable to HIV-1 Tat. Importantly, the susceptible interneurons form a microcircuit thought to be involved in feedback inhibition of CA1 pyramidal cells and gating of CA1 pyramidal cell inputs. The identification of vulnerable CA1 hippocampal interneurons may provide novel insight into the basic mechanisms underlying key functional and neurobehavioral deficits associated with HAND.

Raised Intracellular Calcium Contributes to Ischemia-Induced Depression of Evoked Synaptic Transmission

Oxygen-glucose deprivation (OGD) leads to depression of evoked synaptic transmission, for which the mechanisms remain unclear. We hypothesized that increased presynaptic [Ca²⁺]_i during transient OGD contributes to the depression of evoked field excitatory postsynaptic potentials (fEPSPs). Additionally, we hypothesized that increased buffering of intracellular calcium would shorten electrophysiological recovery after transient ischemia. Mouse hippocampal slices were exposed to 2 to 8 min of OGD. fEPSPs evoked by Schaffer collateral stimulation were recorded in the stratum radiatum, and whole cell current or voltage clamp recordings were performed in CA1 neurons. Transient ischemia led to increased presynaptic [Ca²⁺]_i, (shown by calcium imaging), increased spontaneous miniature EPSP/Cs, and depressed evoked fEPSPs, partially mediated by adenosine. Buffering of intracellular Ca²⁺ during OGD by membrane-permeant chelators (BAPTA-AM or EGTA-AM) partially prevented fEPSP depression and promoted faster electrophysiological recovery when the OGD challenge was stopped. The blocker of BK channels, charybdotoxin (ChTX), also prevented fEPSP depression, but did not accelerate post-ischemic recovery. These results suggest that OGD leads to elevated presynaptic [Ca²⁺]_i, which reduces evoked transmitter release; this effect can be reversed by increased intracellular Ca²⁺ buffering which also speeds recovery.

Dual role of Src kinase in governing neuronal survival

BACKGROUND

Src-family kinases (SFKs) are involved in neuronal survival and their aberrant regulation contributes to neuronal death. However, how they control neuronal survival and death remains unclear.

OBJECTIVE

To define the effect of inhibition of Src activity and expression on neuronal survival.

RESULTS

In agreement with our previous findings, we demonstrated that Src was cleaved by calpain to form a 52-kDa truncated fragment in neurons undergoing excitotoxic cell death, and expression of the recombinant truncated Src fragment induced neuronal death. The data confirm that the neurotoxic signaling pathways are intact in the neurons we used for our study. To define the functional role of neuronal SFKs, we treated these neurons with SFK inhibitors and discovered that the treatment induced cell death, suggesting that the catalytic activity of one or more of the neuronal SFKs is critical to neuronal survival. Using small hairpin RNAs that suppress Src expression, we demonstrated that Src is indispensable to neuronal survival. Additionally, we found that neuronal death induced by expression of the neurotoxic truncated Src mutant, treatment of SFK inhibitors or knock-down of Src expression caused inhibition of the neuroprotective protein kinases Erk1/2, or Akt.

CONCLUSIONS

Src is critical to both neuronal survival and death. Intact Src sustains neuronal survival. However, in the excitotoxic condition, calpain cleavage of Src generates a neurotoxic truncated Src fragment. Both intact Src and the neurotoxic truncated Src fragment exert their biological actions by controlling the activities of neuroprotective protein kinases.

Alpha-synuclein overexpression increases phospho-protein phosphatase 2A levels via formation of calmodulin/Src complex

Alpha-synuclein (α-Syn) is the principal protein component of Lewy bodies, a pathological hallmark of Parkinson's disease (PD). This protein may regulate protein phosphatase 2A (PP2A) activity, although the molecular mechanisms for α-Syn-mediated regulation of PP2A and the potential neuroprotective actions of PP2A against PD-associated pathology remain largely unexplored. We found that α-Syn gene overexpression in SK-N-SH cells and primary neurons led to PP2A/C phosphorylation at Y307, a known target of Src kinase, and consequent phosphatase inhibition. In addition, phospho-activated Src (p-Y416 Src, pSrc) was higher in SK-N-SH cells and primary neurons overexpressing α-Syn. Thus, α-Syn may promote Src activation and PP2A inactivation, leading to hyperphosphorylation of proteins. Immunoprecipitation revealed higher calmodulin/Src complex formation in α-Syn-overexpressing cells and α-Syn transgenic mice. A TUNEL apoptosis assay and an MTT cell viability assay demonstrated that the PP2A activator C2-ceramide protected neurons against α-Syn-induced cell injury. Buffering the Ca(2+) elevations induced by α-Syn overexpression ameliorated the cytotoxicity of α-Syn. Our findings define a potential molecular mechanism for α-Syn-mediated regulation of PP2A through formation of the calmodulin/Src complex, activation of Src, and Src-mediated phospho-inhibition of PP2A. Overexpression of α-Syn may lead to neurodegeneration in PD in part by suppressing the endogenous neuroprotective activity of PP2A.

Excitatory and Mitogenic Signaling in Cell Death, Blood-brain Barrier Breakdown, and BBB Repair after Intracerebral Hemorrhage

Intracerebral hemorrhage (ICH) results in the release of a large number of endogenous molecules, including glutamate, Ca(2+), ROS, thrombin, heme, iron, TNF-α, and others. These molecules participate in excitatory and mitogenic signaling transduction in which N-methyl-D-aspartate (NMDA) receptors and Src family kinases (SFKs) are implicated. Mitogenic signaling initiates the cell cycle for normal cell division of microglia and neural progenitor cells, whereas aberrant mitogenic signaling causes toxicity, killing neurons, astrocytes, and brain microvascular endothelial cells in neurological diseases including ICH. In this review, we summarize (1) how SFKs modulate NMDA receptors to kill neurons following ICH and (2) how SFKs modulate mitogenic signaling transduction to kill neurons and play a role in disrupting the blood-brain barrier (BBB) immediately following ICH and in repairing the BBB during the recovery phases weeks following ICH.