Tissue‐type plasminogen activator requires a co‐receptor to enhance NMDA receptor function

Abstract

Glutamate is the main excitatory neurotransmitter of the central nervous system. Tissue-type plasminogen activator (tPA) is recognized as a modulator of glutamatergic neurotransmission. This attribute is exemplified by its ability to potentiate calcium signaling following activation of the glutamate-binding N-methyl-D-aspartate receptor (NMDAR). It has been hypothesized that tPA can directly cleave the NR1 subunit of the NMDAR and thereby potentiate NMDA-induced calcium influx. In contrast, here we show that this increase in NMDAR signaling requires tPA to be proteolytically active, but does not involve cleavage of the NR1 subunit or plasminogen. Rather, we demonstrate that enhancement of NMDAR function by tPA is mediated by a member of the Low-Density Lipoprotein Receptor (LDLR) family. Hence, this study proposes a novel functional relationship between tPA, the NMDAR, a LDLR and an unknown substrate which we suspect to be a serpin. Interestingly, whilst tPA alone failed to cleave NR1, cell-surface NMDARs did serve as an efficient and discrete proteolytic target for plasmin. Hence, plasmin and tPA can affect the NMDAR via distinct avenues. Altogether, we find that plasmin directly proteolyses the NMDAR whilst tPA functions as an indirect modulator of NMDA-induced events via LDLR engagement.

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

“…[11][12][13] In a previous study, 22 we have demonstrated that although tPA promoted NMDAR-mediated neuronal death in cortical neurons, both in vitro and in vivo, it failed in hippocampic neurons, due to their lack of gluN2D subunit. Moreover, in agreement with our own data, Samson et al 30 reported that tPA potentiated NMDA-induced calcium influx in cortical neurons in a plasminindependent manner. Here, we performed parallel NMDAmediated neurotoxicity assays using neuronal cultures maintained in the presence of EACA (e-amino caproic acid) or aprotinin.…”

“…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.…”

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

“…1,[10][11][12] These interactions mediate several potentially damaging effects of tPA, including potentiation of NMDAR-mediated signalling and excitotoxicity. 6,7,10 During excitotoxic conditions, tPA has been shown to promote NMDA-induced calcium influx in cortical neurons and subsequent neuronal death through the binding to and cleavage of the NR1 subunit of the NMDAR, either directly 10,11 or through the recruitment of LRP. 7 The interaction between tPA and the NR1 subunit was shown in vivo to be involved in both excitotoxic and memory paradigms in mice.…”

“…[1][2][3] However, tPA not only activates plasminogen, but rather acts through several modalities 4 by interacting with the low-density lipoproteinrelated receptor protein (LRP), [5][6][7][8] annexin-II 9 or N-methyl-Daspartate receptors (NMDAR). 1,[10][11][12] These interactions mediate several potentially damaging effects of tPA, including potentiation of NMDAR-mediated signalling and excitotoxicity.…”