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.
Lochner
Janis E
JE
Department of Chemistry, Lewis & Clark College, Portland, OR 97219, USA. Lochner@LClark.edu
Honigman
Leah S
LS
Grant
Wilmon F
WF
Gessford
Sarah K
SK
Hansen
Alexis B
AB
Silverman
Michael A
MA
Scalettar
Bethe A
BA
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