Role of mitochondrial dysfunction in the Ca2+-induced decline of transmitter release at K+-depolarized motor neuron terminals

Axons degenerate in the absence of mitochondria in C. elegans

Many neurodegenerative disorders are associated with mitochondrial defects [1-3]. Mitochondria can play an active role in degeneration by releasing reactive oxygen species and apoptotic factors [4-7]. Alternatively, mitochondria can protect axons from stress and insults, for example by buffering calcium [8]. Recent studies manipulating mitochondria lend support to both of these models [9-13]. Here, we identify a C. elegans mutant, ric-7, in which mitochondria are unable to exit the neuron cell bodies, similar to the kinesin-1/unc-116 mutant. When axons lacking mitochondria are cut with a laser, they rapidly degenerate. Some neurons even spontaneously degenerate in ric-7 mutants. Degeneration can be suppressed by forcing mitochondria into the axons of the mutants. The protective effect of mitochondria is also observed in the wild-type: a majority of axon fragments containing a mitochondrion survive axotomy, whereas those lacking mitochondria degenerate. Thus, mitochondria are not required for axon degeneration and serve a protective role in C. elegans axons.

Mitochondrial potassium channels and uncoupling proteins in synaptic plasticity and neuronal cell death

The function of the nervous system relies upon synaptic transmission, a process in which a neurotransmitter released from pre-synaptic terminals of one neuron (in response to membrane depolarization and calcium influx) activates post-synaptic receptors on dendrites of another neuron. Synapses are subjected to repeated bouts of oxidative and metabolic stress as the result of changing ion gradients and ATP usage. Mitochondria play central roles in meeting the demands of synapses for ATP and in regulating calcium homeostasis, and mitochondrial dysfunction can cause dysfunction and degeneration of synapses, and can trigger cell death. We have identified two types of mitochondrial proteins that serve the function of protecting synapses and neurons against dysfunction and death. Mitochondrial ATP-sensitive potassium (MitoKATP) channels modulate inner membrane potential and oxyradical production; mitochondrial potassium fluxes can affect cytochrome c release and caspase activation and may determine whether neurons live or die in experimental models of stroke and Alzheimer's disease. Uncoupling proteins (UCPs) are a family of mitochondrial membrane proteins that uncouple electron transport from ATP production by transporting protons across the inner membrane. Neurons express at least three UCPs including the widely expressed UCP-2 and the neuron-specific UCP-4 and UCP-5 (BMCP-1). We have found that UCP-4 protects neurons against apoptosis by a mechanism involving suppression of oxyradical production and stabilization of cellular calcium homeostasis. The expression of UCP-4 is itself regulated by changes in energy metabolism. In addition to their roles in neuronal cell survival and death, MitoKATP channels and UCPs may play roles in regulating neuronal differentiation during development and synaptic plasticity in the adult.

Presynaptic function is altered in snake K+-depolarized motor nerve terminals containing compromised mitochondria

Presynaptic function was investigated at K+-stimulated motor nerve terminals in snake costocutaneous nerve muscle preparations exposed to carbonyl cyanide m-chlorophenylhydrazone (CCCP, 2 M), oligomycin (8 g x ml(-1)) or CCCP and oligomycin together. Miniature endplate currents (MEPCs) were recorded at -150 mV with two-electrode voltage clamp. With all three drug treatments, during stimulation by elevated K+ (35 mM), MEPC frequencies initially increased to values > 350 s(-1), but then declined. The decline occurred more rapidly in preparations treated with CCCP or CCCP and oligomycin together than in those treated with oligomycin alone. Staining with FM1-43 indicated that synaptic vesicle membrane endocytosis occurred at some CCCP- or oligomycin-treated nerve terminals after 120 or 180 min of K+ stimulation, respectively. The addition of glucose to stimulate production of ATP by glycolysis during sustained K+ stimulation attenuated the decline in MEPC frequency and increased the percentage of terminals stained by FM1-43 in preparations exposed to either CCCP or oligomycin. We propose that the decline in K+-stimulated quantal release in preparations treated with CCCP, oligomycin or CCCP and oligomycin together could result from a progressive elevation of intracellular calcium concentration ([Ca2+]i). For oligomycin-treated nerve terminals, a progressive elevation of [Ca2+]i could occur as the cytoplasmic ATP/ADP ratio decreases, causing energy-dependent Ca2+ buffering mechanisms to fail. The decline in MEPC frequency could occur more rapidly in preparations treated with CCCP or CCCP and oligomycin together because mitochondrial Ca2+ buffering and ATP production were both inhibited. Therefore, the proposed sustained elevation of [Ca2+]i could occur more rapidly.

The tractable contribution of synapses and their component molecules to individual differences in learning

Though once of central importance to psychologists and neurophysiologists alike, the elucidation of neural substrates for individual differences in learning no longer attracts a broad research effort and occupies a place of largely historical interest to the contemporary disciplines. The decline in interest in this subject ensued in part from the perception, arrived at decades ago, that individual differences in learning were not quantified as easily as had once been presumed. Furthermore, the dominant hypotheses in the field defied testing within the constraints imposed by the complex and largely inaccessible vertebrate nervous system. Using a 'model systems' approach where the individual cells and synaptic interactions that comprise a neural network can be identified, we have returned to this question and have established a framework by which we can begin to discern the basis for much of the variability between individuals in their capacity to learn. In the marine mollusc Hermissenda, we have found that a common influence on transmitter exocytosis is expressed homogeneously throughout the nervous system regardless of transmitter system or receptor class. Though uniformly expressed within an individual, this influence on synaptic efficacy is differentially expressed between animals. Importantly, the basal efficiency of exocytosis expressed in an individual nervous system is strongly correlated with the degree to which activity-dependent forms of neuronal/synaptic facilitation can be induced in that nervous system, and predicts the capacity for the intact animal to learn a Pavlovian association. Furthermore, we have established that a decline in basal synaptic efficacy in aged animals, arising from chronic presynaptic Ca(2+) 'leak', may contribute to age-related learning impairments. Because certain fundamental components of the exocytotic cascade are conserved widely across cell types, transmitter systems and species, the principles that we describe may have broad implications for understanding normal variability in learning, but also, in the development of specific strategies to compensate for mild learning deficits and age-related cognitive decline.

Recycling and refilling of transmitter quanta at the frog neuromuscular junction

1. Fluorescent dyes have been used at the frog neuromuscular junction to label synaptic vesicular membrane. Retrieved membrane is reformed into vesicles, which are released along with pre-existing vesicles. Consequently, if vesicular refilling with acetylcholine (ACh) is depressed by inhibitors, two sizes of quanta should be released: normal and smaller. As recycling continues the fraction of smaller size quanta should increase exponentially. 2. We enhanced the rate of quantal release by elevating the K+ concentration. The principal inhibitors were (-)-vesamicol (VES), hemicholinium-3 (HC3), and NH4+. Quantal size measurements were fitted to one and to two cumulative lognormal probability distribution functions. When two fitted better, the statistical significance assessment took into account the three additional parameters used in calculating the fit. 3. After recycling in the presence of inhibitor, many sets were fitted better by two lognormal functions. As recycling continued, the fraction of the miniature endplate potential voltage-time integrals ( MEPPs) in the larger sub-population decreased exponentially. 4. The size of the releasable pool was estimated by counting the quanta released by carbonyl cyanide m-chlorophenylhydrazone (CCCP). This was compared to pool sizes calculated from the inhibitor experiments. The two estimates of pool size were indistinguishable, with mean values ranging from about 170,000 to 270,000. 5. With all of the treatments tested, the means of the sizes in the smaller sub-population of MEPPs were about 1/3 those of the larger sub-populations. 6. Recycling synaptic vesicles appear to be incorporated into the releasable pool from which they have roughly the same probability of release as the pre-existing vesicles.