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.

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

Synaptic dysfunction is considered the primary substrate for the functional declines observed within the nervous system during age-related neurodegenerative disease. Dietary restriction (DR), which extends lifespan in numerous species, has been shown to have beneficial effects on many neurodegenerative disease models. Existing data sets suggest that the effects of DR during disease include the amelioration of synaptic dysfunction but evidence of the beneficial effects of diet on the synapse is lacking. Dynactin mutant flies have significant increases in mortality rates and exhibit progressive loss of motor function. Using a novel fly motor disease model, we demonstrate that mutant flies raised on a low calorie diet have enhanced motor function and improved survival compared to flies on a high calorie diet. Neurodegeneration in this model is characterized by an early impairment of neurotransmission that precedes the deterioration of neuromuscular junction (NMJ) morphology. In mutant flies, low calorie diet increases neurotransmission, but has little effect on morphology, supporting the hypothesis that enhanced neurotransmission contributes to the effects of diet on motor function. Importantly, the effects of diet on the synapse are not because of the reduction of mutant pathologies, but by the increased release of synaptic vesicles during activity. The generality of this effect is demonstrated by the observation that diet can also increase synaptic vesicle release at wild-type NMJs. These studies reveal a novel presynaptic mechanism of diet that may contribute to the improved vigor observed in mutant flies raised on low calorie diet.

Cell type-specific requirements for heparan sulfate biosynthesis at the Drosophila neuromuscular junction: effects on synapse function, membrane trafficking, and mitochondrial localization.

Heparan sulfate proteoglycans (HSPGs) are concentrated at neuromuscular synapses in many species, including Drosophila. We have established the physiological and patterning functions of HSPGs at the Drosophila neuromuscular junction by using mutations that block heparan sulfate synthesis or sulfation to compromise HSPG function. The mutant animals showed defects in synaptic physiology and morphology suggesting that HSPGs function both presynaptically and postsynaptically; these defects could be rescued by appropriate transgene expression. Of particular interest were selective disruptions of mitochondrial localization, abnormal distributions of Golgi and endoplasmic reticulum markers in the muscle, and a markedly increased level of stimulus-dependent endocytosis in the motoneuron. Our data support the emerging view that HSPG functions are not limited to the cell surface and matrix environments, but also affect a diverse set of cellular processes including membrane trafficking and organelle distributions.

The heparan sulfate proteoglycans Dally-like and Syndecan have distinct functions in axon guidance and visual-system assembly in Drosophila.

Heparan sulfate proteoglycans (HSPGs), a class of glycosaminoglycan-modified proteins, control diverse patterning events via their regulation of growth-factor signaling and morphogen distribution. In C. elegans, zebrafish, and the mouse, heparan sulfate (HS) biosynthesis is required for normal axon guidance, and mutations affecting Syndecan (Sdc), a transmembrane HSPG, disrupt axon guidance in Drosophila embryos. Glypicans, a family of glycosylphosphatidylinositol (GPI)-linked HSPGs, are expressed on axons and growth cones in vertebrates, but their role in axon guidance has not been determined. We demonstrate here that the Drosophila glypican Dally-like protein (Dlp) is required for proper axon guidance and visual-system function. Mosaic studies revealed that Dlp is necessary in both the retina and the brain for different aspects of visual-system assembly. Sdc mutants also showed axon guidance and visual-system defects, some that overlap with dlp and others that are unique. dlp+ transgenes were able to rescue some sdc visual-system phenotypes, but sdc+ transgenes were ineffective in rescuing dlp abnormalities. Together, these findings suggest that in some contexts HS chains provide the biologically critical component, whereas in others the structure of the protein core is also essential.

Drosophila neuromuscular synapse assembly and function require the TGF-beta type I receptor saxophone and the transcription factor Mad.

Transforming growth factor-betas (TGF-beta) comprise a superfamily of secreted proteins with diverse functions in patterning and cell division control. TGF-beta signaling has been implicated in synapse assembly and plasticity in both vertebrate and invertebrate systems. Recently, wishful thinking, a Drosophila gene that encodes a protein related to BMP type II receptors, has been shown to be required for the normal function and development of the neuromuscular junction (NMJ). These findings suggest that a TGF-beta-related ligand activates a signaling cascade involving type I and II receptors and the Smad family of transcription factors to orchestrate the assembly of the NMJ. Here we demonstrate that the TGF-beta type I receptor Saxophone and the downstream transcription factor Mothers against dpp (Mad) are essential for the normal structural and functional development of the Drosophila NMJ, a synapse that displays activity-dependent plasticity.