Tissue plasminogen activator controls multiple forms of synaptic plasticity and memory Paolo Calabresi, 1,2 Maddalena Napolitano, 3 Diego Centonze, 1,2 Girolama A. Mar®a, 1,2 Paolo Gubellini, 1,4 Martine Ammassari Teule, 2,5 Nicola Berretta, 2 Giorgio Bernardi, 1,2 Luigi Frati, 3,6 Massimo Tolu 1 and Alberto Gulino 3,6 1 Clinica Neurologica, Dip. Neuroscienze, Universita Á di Tor Vergata, Via di Tor Vergata 135, Rome 00133, Italy 2 IRCCS Ospedale Santa Lucia, Rome, Italy 3 Dip. di Medicina Sperimentale e Patologia, Universita Á La Sapienza, Rome, Italy 4 CNR, Istituto di Medicina Sperimentale, Rome, Italy 5 CNR, Istituto di Psicobiologia, Rome, Italy 6 Neuromed Institute, Pozzilli, Italy Keywords: learning, long-term depression, long-term potentiation, synaptic plasticity Abstract Induction of long-term depression (LTD) in rat striatal slices revealed that this form of synaptic plasticity is coupled to an increased expression of tissue-plasminogen activator (t-PA) mRNA, as detected by the mRNA differential display technique. To further investigate the involvement of this gene in synaptic remodelling following striatal LTD, we recorded electrical activity from mice lacking the gene encoding t-PA (t-PA-KO) and from wild-type (WT) mice. Tetanic stimulation induced LTD in the large majority of striatal neurons recorded from WT mice. Conversely, LTD was absent in a signi®cant proportion of striatal neurons obtained from mice lacking t-PA. Electrophysiological recordings obtained from hippocampal slices in the CA1 area showed that mainly the late phase of long-term potentiation (LTP) was reduced in t-PA-KO mice. Learning and memory-related behavioural abnormalities were also found in these transgenic mice. Disruption of the t-PA gene, in fact, altered both the context conditioning test, a hippocampus- related behavioural task, and the two-way active avoidance, a striatum-dependent task. In an open ®eld object exploration task, t-PA- KO mice expressed de®cits in habituation and reactivity to spatial change that are consistent with an altered hippocampal function. Nevertheless,decreasedrearingandpoorinitialobjectexplorationwerealsoobserved,furthersuggestinganalteredstriatalfunction. These data indicate that t-PA plays a critical role in the formation of various forms of synaptic plasticity and memory. Introduction The requirement of protein and messenger RNA synthesis for long- term memory suggests that neuronal activity induced by learning initiates a cascade of gene expression (Bailey et al., 1996). Accordingly, neuronal structural changes, such as new synapse formation and increased number of synaptic release sites, have been postulated to characterize long-term memory storage (Bear & Malenka, 1994; Bartsch et al., 1995). Therefore, it is of crucial importance to identify the target genes which are triggered by learning-related neuronal activity and whose products are involved in cellular events leading to synaptic connection remodelling. Given its role in tissue remodelling, a potential candidate for such a role is the tissue plasminogen activator (t-PA), a serine protease converting plasminogen to plasmin and degrading several other extracellular substrates (Mayer, 1990). Several functions in the nervous system have also been postulated for t-PA. This protease, in fact, has been implicated in neurite outgrowth and tissue remodelling (Sumi et al., 1992), as well as in the modulation of neuronal architecture through the degradation of the extracellular matrix and facilitating axon elongation (Seeds et al., 1990). Three lines of evidence suggest that t-PA activity might be important for synaptic plasticity and learning. First, t-PA mRNA expression is induced by long-term potentiation (LTP) (Qian et al., 1993). Second, an increased level of t-PA protein is found in the cerebellum after motor learning training (Seeds et al., 1995). Third, mice lacking the gene encoding for t-PA (t-PA-KO) show a reduced late phase of hippocampal LTP and exhibit some learning de®cits (Frey et al., 1996; Huang et al., 1996). The role of t-PA in the maintenance of the late phase of the LTP seems to be related to a t- PA-operated control of the synaptic growth in the hippocampal mossy ®bre pathway (Baranes et al., 1998). Although these studies highlight a role of t-PA in the maintenance of the late phase of LTP, nevertheless the possible role of t-PA in additional phases or in forms of synaptic plasticity other than hippocampal LTP has never been addressed. Striatal long-term depression (LTD) has been described as a form of synaptic plasticity playing an important role in motor control and learning (Calabresi et al., 1996, 1997). Little is known, however, about the molecular mechanisms underlying the induction and the maintenance of striatal LTD. In an attempt to identify gene products triggered by LTD- inducing signals, we investigated the mRNA differential display- operated gene expression pro®les by focusing on genes potentially active on striatal synaptic connection remodelling. In this study we Correspondence: Dr Paolo Calabresi, 1 Clinica Neurologica, as above. E-mail: calabre@uniroma2.it Received 6 September 1999, revised 22 November 1999, accepted 10 December 1999 European Journal of Neuroscience, Vol. 12, pp. 1002±1012, 2000 Ó European Neuroscience Association