Tissue plasminogen activator mediates amyloid-induced neurotoxicity via Erk1/2 activation

Abstract

Tissue plasminogen activator (tPA) is the main activator of plasminogen into plasmin in the brain where it may have beneficial roles but also neurotoxic effects that could be plasmin dependent or not. Little is known about the substrates and pathways that mediate plasmin-independent tPA neurotoxicity. Here we show in primary hippocampal neurons that tPA promotes a catalytic-independent activation of the extracellular regulated kinase (Erk)1/2 signal transduction pathway through the N-methyl-Daspartate receptor, G-proteins and protein kinase C. This results in GSK3 activation in a process that requires de novo synthesis of proteins, and leads to tau aberrant phosphorylation, microtubule destabilization and apoptosis. Similar effects are produced by amyloid aggregates in a tPA-dependent manner, as demonstrated by pharmacological treatments and in wt and tPAÀ/À mice neurons. Consistently, in Alzheimer's disease (AD) patients' brains, high levels of tPA colocalize with amyloid-rich areas, activated Erk1/2 and phosphorylated tau. This is the first demonstration of an intracellular pathway by which tPA triggers kinase activation, tau phosphorylation and neurotoxicity, suggesting a key role for this molecule in AD pathology.

NR2D-containing NMDA receptors mediate tissue plasminogen activator-promoted neuronal excitotoxicity

“…Accordingly, in primary cultures of pure cortical neurons maintained either 7 days in vitro (DIV) (Supplementary Figure 1a In the presence of MK-801 (10 mM), an NMDAR-channel blocker with high affinity and a very slow off rate, Erk( 1 2 ) activation induced by NMDA alone, or NMDA plus tPA (20 mg/ml) was dramatically reduced in neurons maintained for 12-14 DIV (Figure 1c), arguing that Erk( 1 2 ) activation is dependent on NMDAR activation. tPA alone also led to an increased activation of Erk( 1 2 ), which was not blocked by the addition of MK-801 (10 mM) (Figure 1c), suggesting as shown earlier 15 that tPA could mediate some of its effects through other receptors.…”

“…Erk( 1 2 ), a member of the mitogen-activated kinase (MAPK) family, was earlier reported to be recruited in response to NMDAR activation. 15 We thus used phosphorylation of Erk ( 1 2 ) as an index of NMDAR-dependent signalling. Accordingly, in primary cultures of pure cortical neurons maintained either 7 days in vitro (DIV) (Supplementary Figure 1a In the presence of MK-801 (10 mM), an NMDAR-channel blocker with high affinity and a very slow off rate, Erk( 1 2 ) activation induced by NMDA alone, or NMDA plus tPA (20 mg/ml) was dramatically reduced in neurons maintained for 12-14 DIV (Figure 1c), arguing that Erk( 1 2 ) activation is dependent on NMDAR activation.…”

“…12 The intimate link between tPA and NMDAR is reinforced by other observations, such as the modulation of the phosphorylation state of the NR2B subunit of the NMDAR in conditions of chronic alcohol consumption and contextual fear conditioning 13,14 or the promotion of NMDAR-dependent extracellular signal-regulated kinase (Erk( 1 2 )) signalling in hippocampal neurons. 6,15 Thus, there is now accumulating evidence that tPA must be considered as a positive neuromodulator of NMDAR-mediated glutamatergic transmission. 11,16 NMDAR have a crucial function in brain development, plasticity and survival.…”

Unveiling an exceptional zymogen: the single-chain form of tPA is a selective activator of NMDA receptor-dependent signaling and neurotoxicity

“…The present data point out a mechanism that is dependent on the structure of tPA itself and may therefore be independent of the experimental setting. However, we do not exclude the possibility that other well-described mechanisms may explain the ability of tPA to influence NMDAR signaling: (i) a proteolytic-independent activation leading to an increase in Erk(½)-GSK3 signal transduction pathway 29,31 and (ii) a plasmin-dependent pathway including a proteolytic cleavage of the NMDA receptor 30 leading to an increase in calcium influx. 28 It is nevertheless important to note that experiments in which plasminogen was proposed as the effector of tPA neurotoxicity were performed in the hippocampus and not in the cortex, and that kainate was used in place of a specific NMDAR agonist.…”

“…Several mechanisms have been proposed to explain the ability of tPA to influence NMDAR signaling, 6,[28][29][30] in accordance with the region of the CNS studied and the experimental paradigm, but in no case a selective effect of sctPA has been evoked. The present data point out a mechanism that is dependent on the structure of tPA itself and may therefore be independent of the experimental setting.…”