The Plasminogen Activation System Promotes Neurorepair in the Ischemic Brain

The Janus face of endogenous neuronal tPA: promoting self-protection and worsening the death of neighboring neurons

Recombinant tissue-type plasminogen activator (r-tPA/Actilyse) stands as the prevailing pharmacological solution for treating ischemic stroke patients, of whom because their endogenous circulating tPA alone is not sufficient to rescue reperfusion and to promote favorable outcome. Beyond the tPA contributed by circulating endothelial cells and hepatocytes, neurons also express tPA, sparking debates regarding its impact on neuronal fate ranging from pro-survival to neurotoxic properties. In order to investigate the role of neuronal tPA during brain injuries, we developed models leading to its conditional deletion in neurons, employing AAV9-pPlat-GFP and AAV9-pPlat-Cre-GFP along with tPA floxed mice. These models were subjected to N-methyl-D-aspartate (NMDA)-induced excitotoxicity or thromboembolic ischemic stroke in mice. Initially, we established that our AAV9 constructs selectively transduce neurons, bypassing other brain cell types. Subsequently, we demonstrated that tPA-expressing neurons exhibit greater resistance against NMDA-induced excitotoxicity compared to tPA negative neurons. The targeted removal of tPA in neurons heightened the susceptibility of these neurons to cell death and prevented a paracrine neurotoxic effect on tPA non-expressing neurons. Under ischemic conditions, the self-neuroprotective influence of tPA encompassed both excitatory (GFP+/Tbr1+) and inhibitory (GFP+/GABA+) neurons. Our data indicate that endogenous neuronal tPA is a protective or deleterious factor against neuronal death in an excitotoxic/ischemic context, depending on whether it acts as an autocrine or a paracrine mediator.

The probable role of tissue plasminogen activator/neuroserpin axis in Alzheimer's disease: a new perspective

Alzheimer's disease (AD) is the most common type of dementia associated with amyloid beta (Aβ) deposition. Dysfunction of the neuronal clearance pathway promotes the accumulation of Aβ. The plasminogen-activating system (PAS) is controlled by various enzymes like tissue plasminogen activators (tPA). Neuronal tPA enhances the conversion of plasminogen to plasmin, which cleaves Aβ; this function is controlled by many inhibitors of PAS, including a plasminogen-activating inhibitor (PAI-1) and neuroserpin. Therefore, the objective of the present narrative review was to explore the potential role of tPA/neuroserpin in the pathogenesis of AD. PAI-1 activity is increased in AD, which is involved in accumulating Aβ. Progressive increase of Aβ level during AD neuropathology is correlated with the over-production of PAI-1 with subsequent reduction of plasmin and tPA activities. Reducing plasmin and tPA activities promote Aβ by reducing Aβ clearance. Neuroserpin plays a critical role in the pathogenesis of AD as it regulates the expression and accumulation of Aβ. Higher expression of neuroserpin inhibits the neuroprotective tPA and the generation of plasmin with subsequent reduction in the clearance of Aβ. These observations raise conflicting evidence on whether neuroserpin is neuroprotective or involved in AD progression. Thus, neuroserpin over-expression with subsequent reduction of tPA may propagate AD neuropathology.

New Wine in an Old Bottle: tPA for Ischemic Stroke Management

As the only clinical thrombolytic drug approved by the FDA, tissue-type plasminogen activator (tPA) is the good standard acute treatment against ischemic stroke (IS) during the super-early stage. tPA forms the active principle of alteplase, a recombinant tissue-type plasminogen activator (rtPA), which is well known for its intravascular thrombolytic activity. However, the multifaceted functions of tPA in the central nervous system (CNS) hold untapped potential. Currently, increasing studies have explored the neuroprotective function of tPA in neurological diseases, particularly in acute ischemic stroke (AIS). A series of studies have indicated that tPA has anti-excitotoxic, neurotrophic, and anti-apoptotic effects on neurons; it is also involved in neuronal plasticity, axonal regeneration, and cerebral inflammatory processes, but how to deeply understand the underlying mechanism and take maximum advantage of tPA seems to be urgent. Therefore, more work is needed to illuminate how tPA performs with more diverse functions after stroke onset. In this comment, we focus on possible hypotheses about why and how tPA promotes ischemic neuronal survival in a comprehensive view. The text provides a holistic picture of the functions of tPA and enlists the considerations for the future, which might attract more attention toward the therapeutic potential of tPA in AIS.

Transcriptomic analysis reveals distinct adaptive molecular mechanism in the hippocampal CA3 from rats susceptible or not-susceptible to hyperthermia-induced seizures

Febrile seizures during early childhood are a relevant risk factor for the development of mesial temporal lobe epilepsy. Nevertheless, the molecular mechanism induced by febrile seizures that render the brain susceptible or not-susceptible to epileptogenesis remain poorly understood. Because the temporal investigation of such mechanisms in human patients is impossible, rat models of hyperthermia-induced febrile seizures have been used for that purpose. Here we conducted a temporal analysis of the transcriptomic and microRNA changes in the ventral CA3 of rats that develop (HS group) or not-develop (HNS group) seizures after hyperthermic insult on the eleventh postnatal day. The selected time intervals corresponded to acute, latent, and chronic phases of the disease. We found that the transcriptional differences between the HS and the HNS groups are related to inflammatory pathways, immune response, neurogenesis, and dendritogenesis in the latent and chronic phases. Additionally, the HNS group expressed a greater number of miRNAs (some abundantly expressed) as compared to the HS group. These results indicate that HNS rats were able to modulate their inflammatory response after insult, thus presenting better tissue repair and re-adaptation. Potential therapeutic targets, including genes, miRNAs and signaling pathways involved in epileptogenesis were identified.

Insight into Crosstalk Between Mitophagy and Apoptosis/Necroptosis: Mechanisms and Clinical Applications in Ischemic Stroke

Ischemic stroke is a serious cerebrovascular disease with high morbidity and mortality. As a result of ischemia-reperfusion, a cascade of pathophysiological responses is triggered by the imbalance in metabolic supply and demand, resulting in cell loss. These cellular injuries follow various molecular mechanisms solely or in combination with this disorder. Mitochondria play a driving role in the pathophysiological processes of ischemic stroke. Once ischemic stroke occurs, damaged cells would respond to such stress through mitophagy. Mitophagy is known as a conservatively selective autophagy, contributing to the removal of excessive protein aggregates and damaged intracellular components, as well as aging mitochondria. Moderate mitophagy may exert neuroprotection against stroke. Several pathways associated with the mitochondrial network collectively contribute to recovering the homeostasis of the neurovascular unit. However, excessive mitophagy would also promote ischemia-reperfusion injury. Therefore, mitophagy is a double-edged sword, which suggests that maximizing the benefits of mitophagy is one of the direction of future efforts. This review emphasized the role of mitophagy in ischemic stroke, and highlighted the crosstalk between mitophagy and apoptosis/necroptosis.

Review of evidence implicating the plasminogen activator system in blood-brain barrier dysfunction associated with Alzheimer's disease

Elucidating the pathogenic mechanisms of Alzheimer’s disease (AD) to identify therapeutic targets has been the focus of many decades of research. While deposition of extracellular amyloid-beta plaques and intraneuronal neurofibrillary tangles of hyperphosphorylated tau have historically been the two characteristic hallmarks of AD pathology, therapeutic strategies targeting these proteinopathies have not been successful in the clinics. Neuroinflammation has been gaining more attention as a therapeutic target because increasing evidence implicates neuroinflammation as a key factor in the early onset of AD disease progression. The peripheral immune response has emerged as an important contributor to the chronic neuroinflammation associated with AD pathophysiology. In this context, the plasminogen activator system (PAS), also referred to as the vasculature’s fibrinolytic system, is emerging as a potential factor in AD pathogenesis. Evolving evidence suggests that the PAS plays a role in linking chronic peripheral inflammatory conditions to neuroinflammation in the brain. While the PAS is better known for its peripheral functions, components of the PAS are expressed in the brain and have been demonstrated to alter neuroinflammation and blood-brain barrier (BBB) permeation. Here, we review plasmin-dependent and -independent mechanisms by which the PAS modulates the BBB in AD pathogenesis and discuss therapeutic implications of these observations.

Early Induction of Neurotrophin Receptor and miRNA Genes in Mouse Brain after Pentilenetetrazole-Induced Neuronal Activity

Neurotrophin receptors regulate neuronal survival and network formation, as well as synaptic plasticity in the brain via interaction with their ligands. Here, we examined early changes in the expression of neurotrophin receptor genes Ntk1 (TrkA), Ntrk2 (TrkB), Ntrk3 (TrkC), Ngfr (p75NTR) and miRNAs that target theses gens in the mouse brain after induction of seizure activity by pentylenetetrazol. We found that expression of Ntrk3 and Ngfr was upregulated in the cortex and the hippocampus 1-3 hours after the seizures, while Ntrk2 expression increased after 3-6 hours in the anterior cortex and after 1 and 6 hours in the hippocampus. At the same time, the ratio of Bcl-2/Bax signaling proteins increased in the anterior and posterior cortex, but not in the hippocampus, suggesting the activation of anti-apoptotic signaling. Expression of miRNA-9 and miRNA-29a, which were predicted to target Ntrk3, was upregulated in the hippocampus 3 hours after pentylenetetrazol injection. Therefore, early cellular response to seizures in the brain includes induction of the Ntrk2, Ntrk3, Ngfr, miRNA-9, and miRNA-29a expression, as well as activation of Bcl-2 and Bax signaling pathways, which may characterize them as important mediators of neuronal adaptation and survival upon induction of the generalized brain activity.

Research Progress on the Mechanism of Mitochondrial Autophagy in Cerebral Stroke

Mitochondrial autophagy is an early defense and protection process that selectively clears dysfunctional or excessive mitochondria through a distinctive mechanism to maintain intracellular homeostasis. Mitochondrial dysfunction during cerebral stroke involves metabolic disbalance, oxidative stress, apoptosis, endoplasmic reticulum stress, and abnormal mitochondrial autophagy. This article reviews the research progress on the mechanism of mitochondrial autophagy in ischemic stroke to provide a theoretical basis for further research on mitochondrial autophagy and the treatment of ischemic stroke.

FUNDC1-dependent mitophagy induced by tPA protects neurons against cerebral ischemia-reperfusion injury

Autophagy of mitochondria, termed mitophagy, plays an important role in cerebral ischemia-reperfusion (IR) injury, but the mechanism is not yet clear. Tissue-type plasminogen activator (tPA) is the most important thrombolytic drug in the clinical treatment of ischemic stroke and has neuroprotective effects. Here, we explored the effects of tPA on neuronal apoptosis and mitophagy following IR. We found that knocking out the tPA gene significantly aggravated brain injury and increased neuronal apoptosis and mitochondrial damage. Exposure of neurons to tPA reduced injury severity and protected mitochondria. Further studies demonstrated that this protective effect of tPA was achieved via regulation of FUNDC1-mediated mitophagy. Furthermore, we found that tPA enhanced the expression level of FUNDC1 by activating the phosphorylation of AMPK. In summary, our results confirm that tPA exerts neuroprotective effects by increasing the phosphorylation of AMPK and the expression of FUNDC1, thereby inhibiting apoptosis and improving mitochondrial function.

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After cerebral ischemia, tPA released by neurons has a neuroprotective effect.

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tPA modulates mitophagy to decrease oxidative stress and inhibit apoptosis.

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The mitochondrial membrane protein FUNDC1 and the AMPK signaling pathway are involved in the neuroprotective process of tPA.