tissue-type plasminogen activator in neurodegeneration

Tissue-type plasminogen activator (t-PA) is well known for its ability to remove blood clots from the circulation. Emerging reports have also provided clear evidence for a previously unsuspected role for t-PA within the central nervous system (CNS). In this context, t-PA has been shown to participate in neuronal plasticity, memory development, visual processing, and in the host response to stress. Although these are positive features of t-PA within the CNS, t-PA has also been shown to mediate excitotoxic injury. This has been concluded from results using t-PA -/- mice since these mice are resistant to excitotoxic injury mediated by the glutamate analogue, kainic acid. These results could also have severe implications for the use of t-PA in patients with acute ischaemic stroke. Clinical studies have shown that t-PA is of benefit in removing blood clots from patients with ischaemic stroke, provided that it is administered within 3 hours after stroke onset. Later use is accompanied with an increase in intracerebral haemorrhage. It has been speculated that the neurotoxic effects of t-PA seen in mice may play a role in the development of intracerebral bleeding and the use of t-PA as a treatment for ischaemic stroke has come under question. A number of in vitro studies have also indicated an important role for t-PA in promoting excitotoxic and other forms of injury including haemoglobin-induced injury and oxygen deprivation, while other reports have shown that t-PA can have a negative impact on ischaemic injury. However, if t-PA is beneficial or deleterious in the treatment of ischaemic stroke is still a matter of controversy and contradictory results in animal models have been obtained by other groups. Thus much is still to be learnt concerning the mechanisms by which t-PA implements its toxic effects on neurons. In some studies, the effect of t-PA is dependent upon its ability to produce plasmin, while other reports have shown that t-PA can cause cell death independent of its catalytic activity. A recent study has suggested that t-PA implements its neurotoxic action by directly cleaving the R1 subunit of the NMDA receptor, causing calcium influx and neuronal death. We will try to delineate the role of t-PA in promoting excitotoxic injury on a cellular and molecular level in vitro using cultures of primary murine cortical neurons and microglial cells. We will prepare these cultures from wild- type mice, t-PA -/- mice and transgenic mice that express the human t-PA gene promoter fused to the LacZ reporter gene.