The proper intracellular distribution of mitochondria is assumed to be critical for normal physiology of neuronal cells, but direct evidence for this idea is lacking. Extension or movement of mitochondria into dendritic protrusions correlates with the development and morphological plasticity of spines. Molecular manipulations of dynamin-like GTPases Drp1 and OPA1 that reduce dendritic mitochondria content lead to loss of synapses and dendritic spines, whereas increasing dendritic mitochondrial content or mitochondrial activity enhances the number and plasticity of spines and synapses. Thus, the dendritic distribution of mitochondria is essential and limiting for the support of synapses. Reciprocally, synaptic activity modulates the motility and fusion/fission balance of mitochondria and controls mitochondrial distribution in dendrites.

Summary NMDA receptor-dependent synaptic modifications, such as long-term potentiation (LTP) and long-term depression (LTD), are essential for brain development and function. LTD occurs mainly by the removal of AMPA receptors from the postsynaptic membrane, but the underlying molecular mechanisms remain unclear. Here we show that activation of caspase-3 via mitochondria is required for LTD and AMPA receptor internalization in hippocampal neurons. LTD and AMPA receptor internalization are blocked by peptide inhibitors of caspase-3 and -9. In hippocampal slices from caspase-3 knockout mice, LTD is abolished whereas LTP remains normal. LTD is also prevented by overexpression of the anti-apoptotic proteins XIAP or Bcl-xL, and by a mutant Akt1 protein that is resistant to caspase-3 proteolysis. NMDA receptor stimulation that induces LTD transiently activates caspase-3 in dendrites, without causing cell death. These data indicate an unexpected causal link between the molecular mechanisms of apoptosis and LTD.

Summary It has recently been found that caspases not only function in apoptosis, but are also crucial for non-apoptotic processes such as NMDA receptor-dependent long-term depression (LTD) of synaptic transmission. It remains unknown, however, how caspases are activated and how neurons escape death in LTD. Here we show that caspase-3 is activated by the BAD-BAX cascade for LTD induction. This cascade is required specifically for NMDA receptor-dependent LTD but not for mGluR-LTD, and its activation is sufficient to induce synaptic depression. In contrast to apoptosis, however, BAD is activated only moderately and transiently and BAX is not translocated to mitochondria, resulting in only modest caspase-3 activation. We further demonstrate that the intensity and duration of caspase-3 activation determin whether it leads to cell death or LTD, thus fine-tuning of caspase-3 activation is critical in distinguishing between these two pathways.