SummaryLysosomes have traditionally been viewed as degradative organelles, although a growing body of evidence suggests that they can function as Ca2+ stores. Here we examined the function of these stores in hippocampal pyramidal neurons. We found that back-propagating action potentials (bpAPs) could elicit Ca2+ release from lysosomes in the dendrites. This Ca2+ release triggered the fusion of lysosomes with the plasma membrane, resulting in the release of Cathepsin B. Cathepsin B increased the activity of matrix metalloproteinase 9 (MMP-9), an enzyme involved in extracellular matrix (ECM) remodelling and synaptic plasticity. Inhibition of either lysosomal Ca2+ signaling or Cathepsin B release prevented the maintenance of dendritic spine growth induced by Hebbian activity. This impairment could be rescued by exogenous application of active MMP-9. Our findings suggest that activity-dependent exocytosis of Cathepsin B from lysosomes regulates the long-term structural plasticity of dendritic spines by triggering MMP-9 activation and ECM remodelling.

No abstract

1 Miniature inhibitory postsynaptic currents (mIPSCs) were recorded in mouse Purkinje cells in the presence of 1 mM tetrodotoxin (TTX). Under these conditions, which eliminated Ca 2+ in¯ux through voltage-dependent Ca 2+ channels (VDCCs), the contribution of Ca 2+ stores to spontaneous GABA release was examined. 2 The plant alkaloid ryanodine acts as an inhibitor of endoplasmic reticulum ryanodine-sensitive Ca 2+ release channels (ryanodine receptors) at low micromolar concentrations. Ryanodine eects were con®ned to a subpopulation of cells tested. At 10 mM ryanodine, 4/12 cells showed a signi®cant increase in mean mIPSC frequency of +19.6+4.0% (n=4). 3 The sarco/endoplasmic reticulum Ca 2+ -ATPase (SERCA) pump inhibitor cyclopiazonic acid (CPA) produced a more robust eect. In 8/10 cells, 25 mM CPA caused a signi®cant increase in mean mIPSC frequency; the mean increase being +26.0+3.0% (n=8). Similar results were seen with thapsigargin (1 ± 2 mM), another SERCA pump inhibitor. 4 Ruthenium red (RuR) has been proposed to either act directly on the release machinery or block Ca 2+ pumps on internal stores. At 10 mM RuR, all cells showed a rapid, large increase in mean mIPSC frequency of +90.4+16.4% (n=9). This increase was greater than that seen by agents known to modulate Ca 2+ stores and was more consistent with a direct action. At this concentration, RuR also occluded the eects of CPA. 5 For all reagents, there were no obvious eects on mean mIPSC amplitude. However, the eects on mIPSC frequency were consistent with a presynaptic action and indicate that Ca 2+ stores may contribute to spontaneous GABA release onto mouse Purkinje cells.

No abstract