Effects of diet on synaptic vesicle release in dynactin complex mutants: a mechanism for improved vitality during motor disease

Keeping Neurons Young and Foxy: FoxOs Promote Neuronal Plasticity

Any adult who has tried to take up the piano or learn a new language is faced with the sobering realization that acquiring such skills is more challenging as an adult than as a child. Neuronal plasticity, or the malleability of brain circuits, declines with age. Young neurons tend to be more adaptable and can alter the size and strength of their connections more readily than can old neurons. Myriad circuit- and synapse-level mechanisms that shape plasticity have been identified. Yet, molecular mechanisms setting the overall competence of young neurons for distinct forms of plasticity remain largely obscure. Recent studies indicate evolutionarily conserved roles for FoxO proteins in establishing the capacity for cell-fate, morphological, and synaptic plasticity in neurons.

Transcriptomics of aged Drosophila motor neurons reveals a matrix metalloproteinase that impairs motor function

The neuromuscular junction (NMJ) is responsible for transforming nervous system signals into motor behavior and locomotion. In the fruit fly Drosophila melanogaster, an age‐dependent decline in motor function occurs, analogous to the decline experienced in mice, humans, and other mammals. The molecular and cellular underpinnings of this decline are still poorly understood. By specifically profiling the transcriptome of Drosophila motor neurons across age using custom microarrays, we found that the expression of the matrix metalloproteinase 1 (dMMP1) gene reproducibly increased in motor neurons in an age‐dependent manner. Modulation of physiological aging also altered the rate of dMMP1 expression, validating dMMP1 expression as a bona fide aging biomarker for motor neurons. Temporally controlled overexpression of dMMP1 specifically in motor neurons was sufficient to induce deficits in climbing behavior and cause a decrease in neurotransmitter release at neuromuscular synapses. These deficits were reversible if the dMMP1 expression was shut off again immediately after the onset of motor dysfunction. Additionally, repression of dMMP1 enzymatic activity via overexpression of a tissue inhibitor of metalloproteinases delayed the onset of age‐dependent motor dysfunction. MMPs are required for proper tissue architecture during development. Our results support the idea that matrix metalloproteinase 1 is acting as a downstream effector of antagonistic pleiotropy in motor neurons and is necessary for proper development, but deleterious when reactivated at an advanced age.

Ciberial Muscle 9 (CM9) Electrophysiological Recordings in Adult Drosophila melanogaster

The complexity surrounding presynaptic recordings in mammals is a significant barrier to the study of presynaptic mechanisms during neurotransmission in the mammalian central nervous system (CNS). Here we describe an adult fly neuromuscular junction (NMJ), the ciberial muscle 9 (CM9) NMJ, which allows for the recording of both evoked (EPSPs) and spontaneous postsynaptic excitatory potentials (mEPSPs) at a mature glutamatergic synapse. Combined with CM9-specific genetic technologies, the CM9 NMJ provides a powerful experimental system to better understand the regulation of neurotransmitter release at a mature synapse.

Extension of Health Span and Life Span in Drosophila by S107 Requires the calstabin Homologue FK506-BP2

The accumulation of oxidative damage is strongly linked to age-dependent declines in cell function, but the contribution of oxidative damage to morbidity is still debated. Many organisms seem to tolerate oxidative damage, and the extension of health span and life span by augmenting antioxidant activity has been inconsistent. Here we use the Drosophila model system to investigate the relationship among oxidative stress, health span, and life span. The oxidation-dependent dissociation of the Calstabin protein from the ryanodine receptor has been shown to result in reduced muscle function in mammals. The S107 molecule is able to reestablish this binding resulting in improved muscle function. We find that S107 is able to restore motor function in aging Drosophila to young levels, and this effect of S107 is absent in calstabin (FK506-BP2) mutants. Interestingly, FK506-BP2 mutant flies have reduced sensitivity to the effects of age and oxidative stress on motor function between 7 and 35 days of age. Muscle expression of FK506-BP2 in FK506-BP2 mutants completely restores the sensitivity of motor function to both age and oxidative stress, supporting the idea that the age-dependent decline in motor function in Drosophila requires FK506-BP2 function within the muscle. Although FK506-BP2 mutant flies are found to have less sensitivity to oxidative stress, FK506-BP2 mutants do not live longer than wild type. These results demonstrate that the deleterious effects of oxidation on motor function early in life are the result of a singular event that does not compromise survival.

Insulin signaling controls neurotransmission via the 4eBP-dependent modification of the exocytotic machinery

Altered insulin signaling has been linked to widespread nervous system dysfunction including cognitive dysfunction, neuropathy and susceptibility to neurodegenerative disease. However, knowledge of the cellular mechanisms underlying the effects of insulin on neuronal function is incomplete. Here, we show that cell autonomous insulin signaling within the Drosophila CM9 motor neuron regulates the release of neurotransmitter via alteration of the synaptic vesicle fusion machinery. This effect of insulin utilizes the FOXO-dependent regulation of the thor gene, which encodes the Drosophila homologue of the eif-4e binding protein (4eBP). A critical target of this regulatory mechanism is Complexin, a synaptic protein known to regulate synaptic vesicle exocytosis. We find that the amounts of Complexin protein observed at the synapse is regulated by insulin and genetic manipulations of Complexin levels support the model that increased synaptic Complexin reduces neurotransmission in response to insulin signaling.

Enhanced Locomotor Activity Is Required to Exert Dietary Restriction-Dependent Increase of Stress Resistance in Drosophila

Dietary restriction (DR) is known to be one of the most effective interventions to increase stress resistance, yet the mechanisms remain elusive. One of the most obvious DR-induced changes in phenotype is an increase in locomotor activity. Although it is conceptually perceivable that nutritional scarcity should prompt enhanced foraging behavior to garner additional dietary resources, the significance of enhanced movement activity has not been associated with the DR-dependent increase of stress resistance. In this study, we confirmed that flies raised on DR exhibited enhanced locomotive activity and increased stress resistance. Excision of fly wings minimized the DR-induced increase in locomotive activity, which resulted in attenuation of the DR-dependent increase of stress resistance. The possibility that wing clipping counteracts the DR by coercing flies to have more intake was ruled out since it did not induce any weight gain. Rather it was found that elimination of reactive oxygen species (ROS) that is enhanced by DR-induced upregulation of expression of antioxidant genes was significantly reduced by wing clipping. Collectively, our data suggests that DR increased stress resistance by increasing the locomotor activity, which upregulated expression of protective genes including, but not limited to, ROS scavenger system.

An age-dependent change in the set point of synaptic homeostasis

Homeostatic plasticity functions within the nervous system to maintain normal neural functions, such as neurotransmission, within predefined optimal ranges. The defined output of these neuronal processes is referred to as the set point, which is the value that the homeostatic system defends against fluctuations. Currently, it is unknown how stable homeostatic set points are within the nervous system. In the present study we used the CM9 neuromuscular junctions (NMJs) in the adult Drosophila to investigate the stability of the set point of synaptic homeostasis across the lifespan of the fly. At the fly NMJ, it is believed that the depolarization of the muscle by neurotransmitter during an action potential, represented by the EPSP, is a homeostatic set point that is precisely maintained via changes in synaptic vesicle release. We find that the amplitude of the EPSP abruptly increases during middle age and that this enhanced EPSP is maintained into late life, consistent with an age-dependent change to the homeostatic set point of the synapse during middle age. In support of this, comparison of the homeostatic response at the young versus the old synapse shows that the magnitude of the homeostatic response at the older synapse is significantly larger than the response at the young NMJ, appropriate for a synapse at which the set point has been increased. Our data demonstrate that the amplitude of the EPSP at the Drosophila NMJ increases during aging and that the homeostatic signaling system adjusts its response to accommodate the new set point.

Long-term analyses of innervation and neuromuscular integrity in the Trembler-J mouse model of Charcot-Marie-Tooth disease

A large fraction of hereditary demyelinating neuropathies, classified as Charcot-Marie-Tooth disease type 1A, is associated with misexpression of peripheral myelin protein 22. In this study, we characterized morphologic and biochemical changes that occur with diseaseprogression in neuromuscular tissue of Trembler-J mice, a spontaneous rodent model of Charcot-Marie-Tooth disease type 1A. Using age-matched, 2- and 10-month-old, wild-type and Trembler-J mice, we observed neuromuscular deficits that progress from distal to proximal regions. The impairments in motor performance are underlined by degenerative events at distal nerve segments and structural alterations at nerve-muscle synapses. Furthermore, skeletal muscle of affected mice showed reduced myofiber diameter, increased expression of the muscle atrophy marker muscle ring-finger protein 1, and fiber type switching. A dietary intervention of intermittent fasting attenuated these progressive changes and supported distal nerve myelination and neuromuscular junction integrity. In addition to the well-characterized demyelination aspects of this model, our investigations identified distinct degenerative events in distal nerves and muscle of affected neuropathic mice. Therefore, therapeutic studies aimed at slowing or reversing the neuropathic features of these disorders should include the examination of muscle tissue, as well as neuromuscular contact sites.

Dynein interacts with the neural cell adhesion molecule (NCAM180) to tether dynamic microtubules and maintain synaptic density in cortical neurons

Cytoplasmic dynein is well characterized as an organelle motor, but dynein also acts to tether and stabilize dynamic microtubule plus-ends in vitro. Here we identify a novel and direct interaction between dynein and the 180-kDa isoform of the neural cell adhesion molecule (NCAM). Optical trapping experiments indicate that dynein bound to beads via the NCAM180 interaction domain can tether projecting microtubule plus-ends. Live cell assays indicate that the NCAM180-dependent recruitment of dynein to the cortex leads to the selective stabilization of microtubules projecting to NCAM180 patches at the cell periphery. The dynein-NCAM180 interaction also enhances cell-cell adhesion in heterologous cell assays. Dynein and NCAM180 co-precipitate from mouse brain extract and from synaptosomal fractions, consistent with an endogenous interaction in neurons. Thus, we examined microtubule dynamics and synaptic density in primary cortical neurons. We find that depletion of NCAM, inhibition of the dynein-NCAM180 interaction, or dampening of microtubule dynamics with low dose nocodazole all result in significantly decreased in synaptic density. Based on these observations, we propose a working model for the role of dynein at the synapse, in which the anchoring of the motor to the cortex via binding to an adhesion molecule mediates the tethering of dynamic microtubule plus-ends to potentiate synaptic stabilization.

Dietary restriction supports peripheral nerve health by enhancing endogenous protein quality control mechanisms

The peripheral nervous system (PNS) comprises of an extensive network of connections that convey information between the central nervous system (CNS) and peripheral organs. Long myelinated nerve fibers are particularly susceptible to age-related changes, as maintenance of the insulating glial membrane requires extensive synthesis and processing of many proteins. In rodent models, peripheral demyelination caused by genetic risk factors or by normal aging are attenuated by intermittent fasting (IF) or calorie restriction (CR) supporting a role for dietary intervention in preserving neural function. This review will summarize recent studies examining mechanisms by which life-long CR or extended IF supports peripheral nerve health.