Activity-dependent release of tissue plasminogen activator from the dendritic spines of hippocampal neurons revealed by live-cell imaging.
Tissue plasminogen activator (tPA) has been implicated in a variety of important cellular functions, including learning-related synaptic plasticity and potentiating N-methyl-D-aspartate (NMDA) receptor-dependent signaling. These findings suggest that tPA may localize to, and undergo activity-dependent secretion from, synapses; however, conclusive data supporting these hypotheses have remained elusive. To elucidate these issues, we studied the distribution, dynamics, and depolarization-induced secretion of tPA in hippocampal neurons, using fluorescent chimeras of tPA. We found that tPA resides in dense-core granules (DCGs) that traffic to postsynaptic dendritic spines and that can remain in spines for extended periods. We also found that depolarization induced by high potassium levels elicits a slow, partial exocytotic release of tPA from DCGs in spines that is dependent on extracellular Ca(+2) concentrations. This slow, partial release demonstrates that exocytosis occurs via a mechanism, such as fuse-pinch-linger, that allows partial release and reuse of DCG cargo and suggests a mechanism that hippocampal neurons may rely upon to avoid depleting tPA at active synapses. Our results also demonstrate release of tPA at a site that facilitates interaction with NMDA-type glutamate receptors, and they provide direct confirmation of fundamental hypotheses about tPA localization and release that bear on its neuromodulatory functions, for example, in learning and memory.
Department of Chemistry, Lewis & Clark College, Portland, OR 97219, USA. Lochner@LClark.edu
Janis E
JE
Lochner
Leah S
LS
Honigman
Wilmon F
WF
Grant
Sarah K
SK
Gessford
Alexis B
AB
Hansen
Michael A
MA
Silverman
Bethe A
BA
Scalettar
D000818:Animals; D001426:Bacterial Proteins; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D049229:Dendritic Spines / Q000378:metabolism*; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology; D006624:Hippocampus / Q000378:metabolism*; D008164:Luminescent Proteins; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008568:Memory / Q000502:physiology; D008856:Microscopy, Fluorescence / Q000379:methods; D018715:Microscopy, Video / Q000379:methods; D050506:Mutant Chimeric Proteins / Q000378:metabolism; D011188:Potassium / Q000378:metabolism / Q000494:pharmacology; D021381:Protein Transport / Q000502:physiology; D017966:Pyramidal Cells / Q000378:metabolism*; D051381:Rats; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism; D022142:Secretory Vesicles / Q000378:metabolism; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*; D013997:Time Factors; D010959:Tissue Plasminogen Activator / Q000235:genetics / Q000378:metabolism*
D001426:Bacterial Proteins; D008164:Luminescent Proteins; D050506:Mutant Chimeric Proteins; D016194:Receptors, N-Methyl-D-Aspartate; C054562:yellow fluorescent protein, Bacteria; D010959:Tissue Plasminogen Activator; D011188:Potassium