Acute Fasting Regulates Retrograde Synaptic Enhancement through a 4E-BP-Dependent Mechanism

Using Electrophysiology to Study Homeostatic Plasticity at the Drosophila Neuromuscular Junction

The Drosophila melanogaster neuromuscular junction (NMJ) is a superb system for studying synapse function. Beyond that, the NMJ is also great for studying forms of synaptic plasticity. Over the last 25 years, Drosophila NMJ neuroscientists have pioneered understanding of a form of plasticity called homeostatic synaptic plasticity, which imparts functional stability on synaptic connections. The reason is straightforward: The NMJ has a robust capacity for stability. Moreover, many strategies that the NMJ uses to maintain appropriate levels of function are mirrored at other metazoan synapses. Here, we introduce core approaches that neurophysiologists use to study homeostatic synaptic plasticity at the peripheral Drosophila NMJ. We focus on methods to study a specific form of homeostatic plasticity termed presynaptic homeostatic potentiation (PHP), which is the most well-characterized one. Other forms such as presynaptic homeostatic depression and developmental forms of homeostasis are briefly discussed. Finally, we share lists of several dozen factors and conditions known to influence the execution of PHP.

Glial Sphingosine-Mediated Epigenetic Regulation Stabilizes Synaptic Function in Drosophila Models of Alzheimer's Disease

Destabilization of neural activity caused by failures of homeostatic regulation has been hypothesized to drive the progression of Alzheimer's Disease (AD). However, the underpinning mechanisms that connect synaptic homeostasis and the disease etiology are yet to be fully understood. Here, we demonstrated that neuronal overexpression of amyloid β (Aβ) causes abnormal histone acetylation in peripheral glia and completely blocks presynaptic homeostatic potentiation (PHP) at the neuromuscular junction in Drosophila The synaptic deficits caused by Aβ overexpression in motoneurons are associated with motor function impairment at the adult stage. Moreover, we found that a sphingosine analog drug, Fingolimod, ameliorates synaptic homeostatic plasticity impairment, abnormal glial histone acetylation, and motor behavior defects in the Aβ models. We further demonstrated that perineurial glial sphingosine kinase 2 (Sk2) is not only required for PHP, but also plays a beneficial role in modulating PHP in the Aβ models. Glial overexpression of Sk2 rescues PHP, glial histone acetylation, and motor function deficits that are associated with Aβ in Drosophila Finally, we showed that glial overexpression of Sk2 restores PHP and glial histone acetylation in a genetic loss-of-function mutant of the Spt-Ada-Gcn5 Acetyltransferase complex, strongly suggesting that Sk2 modulates PHP through epigenetic regulation. Both male and female animals were used in the experiments and analyses in this study. Collectively, we provided genetic evidence demonstrating that abnormal glial epigenetic alterations in Aβ models in Drosophila are associated with the impairment of PHP and that the sphingosine signaling pathway displays protective activities in stabilizing synaptic physiology.SIGNIFICANCE STATEMENT Fingolimod, an oral drug to treat multiple sclerosis, is phosphorylated by sphingosine kinases to generate its active form. It is known that Fingolimod enhances the cognitive function in mouse models of Alzheimer's disease (AD), but the role of sphingosine kinases in AD is not clear. We bridge this knowledge gap by demonstrating the relationship between impaired homeostatic plasticity and AD. We show that sphingosine kinase 2 (Sk2) in glial cells is necessary for homeostatic plasticity and that glial Sk2-mediated epigenetic signaling has a protective role in synapse stabilization. Our findings demonstrate the potential of the glial sphingosine signaling as a key player in glia-neuron interactions during homeostatic plasticity, suggesting it could be a promising target for sustaining synaptic function in AD.

Nutritional Programming of the Lifespan of Male Drosophila by Activating FOXO on Larval Low-Nutrient Diet

Nutrition during the developmental stages has long-term effects on adult physiology, disease and lifespan, and is termed nutritional programming. However, the underlying molecular mechanisms of nutritional programming are not yet well understood. In this study, we showed that developmental diets could regulate the lifespan of adult Drosophila in a way that interacts with various adult diets during development and adulthood. Importantly, we demonstrated that a developmental low-yeast diet (0.2SY) extended both the health span and lifespan of male flies under nutrient-replete conditions in adulthood through nutritional programming. Males with a low-yeast diets during developmental stages had a better resistance to starvation and lessened decline of climbing ability with age in adulthood. Critically, we revealed that the activity of the Drosophila transcription factor FOXO (dFOXO) was upregulated in adult males under developmental low-nutrient conditions. The knockdown of dFOXO, with both ubiquitous and fat-body-specific patterns, can completely abolish the lifespan-extending effect from the larval low-yeast diet. Ultimately, we identify that the developmental diet achieved the nutritional programming of the lifespan of adult males by modulating the activity of dFOXO in Drosophila. Together, these results provide molecular evidence that the nutrition in the early life of animals could program the health of their later life and their longevity.

A glutamate receptor C-tail recruits CaMKII to suppress retrograde homeostatic signaling

Presynaptic homeostatic plasticity (PHP) adaptively enhances neurotransmitter release following diminished postsynaptic glutamate receptor (GluR) functionality to maintain synaptic strength. While much is known about PHP expression mechanisms, postsynaptic induction remains enigmatic. For over 20 years, diminished postsynaptic Ca2+ influx was hypothesized to reduce CaMKII activity and enable retrograde PHP signaling at the Drosophila neuromuscular junction. Here, we have interrogated inductive signaling and find that active CaMKII colocalizes with and requires the GluRIIA receptor subunit. Next, we generated Ca2+-impermeable GluRs to reveal that both CaMKII activity and PHP induction are Ca2+-insensitive. Rather, a GluRIIA C-tail domain is necessary and sufficient to recruit active CaMKII. Finally, chimeric receptors demonstrate that the GluRIIA tail constitutively occludes retrograde homeostatic signaling by stabilizing active CaMKII. Thus, the physical loss of the GluRIIA tail is sensed, rather than reduced Ca2+, to enable retrograde PHP signaling, highlighting a unique, Ca2+-independent control mechanism for CaMKII in gating homeostatic plasticity.

Madm/NRBP1 mediates synaptic maintenance and neurodegeneration-induced presynaptic homeostatic potentiation

The precise regulation of synaptic connectivity and function is essential to maintain neuronal circuits. Here, we show that the Drosophila pseudo-kinase Madm/NRBP1 (Mlf-1-adapter-molecule/nuclear-receptor-binding protein 1) is required presynaptically to maintain synaptic stability and to coordinate synaptic growth and function. Presynaptic Madm mediates these functions by controlling cap-dependent translation via the target of rapamycin (TOR) effector 4E-BP/Thor (eukaryotic initiation factor 4E binding protein/Thor). Strikingly, at degenerating neuromuscular synapses, postsynaptic Madm induces a compensatory, transsynaptic signal that utilizes the presynaptic homeostatic potentiation (PHP) machinery to offset synaptic release deficits and to delay synaptic degeneration. Madm is not required for canonical PHP but induces a neurodegeneration-specific form of PHP and acts via the regulation of the cap-dependent translation regulators 4E-BP/Thor and S6-kinase. Consistently, postsynaptic induction of canonical PHP or TOR activation can compensate for postsynaptic Madm to alleviate functional and structural synaptic defects. Our results provide insights into the molecular mechanisms underlying neurodegeneration-induced PHP with potential neurotherapeutic applications.

Statins Induce Locomotion and Muscular Phenotypes in Drosophila melanogaster That Are Reminiscent of Human Myopathy: Evidence for the Role of the Chloride Channel Inhibition in the Muscular Phenotypes

The underlying mechanisms for statin-induced myopathy (SIM) are still equivocal. In this study, we employ Drosophila melanogaster to dissect possible underlying mechanisms for SIM. We observe that chronic fluvastatin treatment causes reduced general locomotion activity and climbing ability. In addition, transmission microscopy of dissected skeletal muscles of fluvastatin-treated flies reveals strong myofibrillar damage, including increased sarcomere lengths and Z-line streaming, which are reminiscent of myopathy, along with fragmented mitochondria of larger sizes, most of which are round-like shapes. Furthermore, chronic fluvastatin treatment is associated with impaired lipid metabolism and insulin signalling. Mechanistically, knockdown of the statin-target Hmgcr in the skeletal muscles recapitulates fluvastatin-induced mitochondrial phenotypes and lowered general locomotion activity; however, it was not sufficient to alter sarcomere length or elicit myofibrillar damage compared to controls or fluvastatin treatment. Moreover, we found that fluvastatin treatment was associated with reduced expression of the skeletal muscle chloride channel, ClC-a (Drosophila homolog of CLCN1), while selective knockdown of skeletal muscle ClC-a also recapitulated fluvastatin-induced myofibril damage and increased sarcomere lengths. Surprisingly, exercising fluvastatin-treated flies restored ClC-a expression and normalized sarcomere lengths, suggesting that fluvastatin-induced myofibrillar phenotypes could be linked to lowered ClC-a expression. Taken together, these results may indicate the potential role of ClC-a inhibition in statin-associated muscular phenotypes. This study underlines the importance of Drosophila melanogaster as a powerful model system for elucidating the locomotion and muscular phenotypes, promoting a better understanding of the molecular mechanisms underlying SIM.

Evaluating the beneficial effects of dietary restrictions: A framework for precision nutrigeroscience

Dietary restriction (DR) has long been viewed as the most robust nongenetic means to extend lifespan and healthspan. Many aging-associated mechanisms are nutrient responsive, but despite the ubiquitous functions of these pathways, the benefits of DR often vary among individuals and even among tissues within an individual, challenging the aging research field. Furthermore, it is often assumed that lifespan interventions like DR will also extend healthspan, which is thus often ignored in aging studies. In this review, we provide an overview of DR as an intervention and discuss the mechanisms by which it affects lifespan and various healthspan measures. We also review studies that demonstrate exceptions to the standing paradigm of DR being beneficial, thus raising new questions that future studies must address. We detail critical factors for the proposed field of precision nutrigeroscience, which would utilize individualized treatments and predict outcomes using biomarkers based on genotype, sex, tissue, and age.

Synaptic homeostats: latent plasticity revealed at the Drosophila neuromuscular junction

Homeostatic signaling systems are fundamental forms of biological regulation that maintain stable functionality in a changing environment. In the nervous system, synapses are crucial substrates for homeostatic modulation, serving to establish, maintain, and modify the balance of excitation and inhibition. Synapses must be sufficiently flexible to enable the plasticity required for learning and memory but also endowed with the stability to last a lifetime. In response to the processes of development, growth, remodeling, aging, and disease that challenge synapses, latent forms of adaptive plasticity become activated to maintain synaptic stability. In recent years, new insights into the homeostatic control of synaptic function have been achieved using the powerful Drosophila neuromuscular junction (NMJ). This review will focus on work over the past 10 years that has illuminated the cellular and molecular mechanisms of five homeostats that operate at the fly NMJ. These homeostats adapt to loss of postsynaptic neurotransmitter receptor functionality, glutamate imbalance, axonal injury, as well as aberrant synaptic growth and target innervation. These diverse homeostats work independently yet can be simultaneously expressed to balance neurotransmission. Growing evidence from this model glutamatergic synapse suggests these ancient homeostatic signaling systems emerged early in evolution and are fundamental forms of plasticity that also function to stabilize mammalian cholinergic NMJs and glutamatergic central synapses.

Epileptic Adolescents and Ramadan Fasting: A Prospective Cohort Study

This study aimed to evaluate the effect of Ramadan fasting on seizure control in epileptic adolescents. A prospective cohort study that included 56 epileptic patients aged 11 to 18 years. Patients were followed-up in the pediatric neurology clinic at Tanta University Children's Hospital during Ramadan 2019. Seizures frequency and severity, measured by Chalfont seizure severity scale, during Ramadan fasting were compared with those in the month before Ramadan. Before Ramadan, 43/56 patients were controlled and seizure free. Meanwhile, 13/56 patients were uncontrolled and had seizure frequency of one to six seizures in the month before Ramadan. There was no significant difference in seizure frequency during Ramadan compared with before Ramadan (p = 0.132), whereas seizure severity was significantly reduced during Ramadan (p = 0.028). Both seizure frequency and severity were significantly lower during Ramadan in uncontrolled epileptic patients compared with before Ramadan (p = 0.02 and 0.005). Ramadan fasting is safe in adolescents with epilepsy. Fasting is not a precipitating factor for seizures in epileptic adolescents, provided compliance to antiepileptic drugs, and could be beneficial for seizure control in uncontrolled epilepsy.

Genetic inactivation of mTORC1 or mTORC2 in neurons reveals distinct functions in glutamatergic synaptic transmission

Although mTOR signaling is known as a broad regulator of cell growth and proliferation, in neurons it regulates synaptic transmission, which is thought to be a major mechanism through which altered mTOR signaling leads to neurological disease. Although previous studies have delineated postsynaptic roles for mTOR, whether it regulates presynaptic function is largely unknown. Moreover, the mTOR kinase operates in two complexes, mTORC1 and mTORC2, suggesting that mTOR's role in synaptic transmission may be complex-specific. To better understand their roles in synaptic transmission, we genetically inactivated mTORC1 or mTORC2 in cultured mouse glutamatergic hippocampal neurons. Inactivation of either complex reduced neuron growth and evoked EPSCs (eEPSCs), however, the effects of mTORC1 on eEPSCs were postsynaptic and the effects of mTORC2 were presynaptic. Despite postsynaptic inhibition of evoked release, mTORC1 inactivation enhanced spontaneous vesicle fusion and replenishment, suggesting that mTORC1 and mTORC2 differentially modulate postsynaptic responsiveness and presynaptic release to optimize glutamatergic synaptic transmission.

Skeletal muscle mTORC1 regulates neuromuscular junction stability

BACKGROUND

Skeletal muscle is a plastic tissue that can adapt to different stimuli. It is well established that Mammalian Target of Rapamycin Complex 1 (mTORC1) signalling is a key modulator in mediating increases in skeletal muscle mass and function. However, the role of mTORC1 signalling in adult skeletal muscle homeostasis is still not well defined.

METHODS

Inducible, muscle-specific Raptor and mTOR k.o. mice were generated. Muscles at 1 and 7 months after deletion were analysed to assess muscle histology and muscle force.

RESULTS

We found no change in muscle size or contractile properties 1 month after deletion. Prolonging deletion of Raptor to 7 months, however, leads to a very marked phenotype characterized by weakness, muscle regeneration, mitochondrial dysfunction, and autophagy impairment. Unexpectedly, reduced mTOR signalling in muscle fibres is accompanied by the appearance of markers of fibre denervation, like the increased expression of the neural cell adhesion molecule (NCAM). Both muscle-specific deletion of mTOR or Raptor, or the use of rapamycin, was sufficient to induce 3-8% of NCAM-positive fibres (P < 0.01), muscle fibrillation, and neuromuscular junction (NMJ) fragmentation in 24% of examined fibres (P < 0.001). Mechanistically, reactivation of autophagy with the small peptide Tat-beclin1 is sufficient to prevent mitochondrial dysfunction and the appearance of NCAM-positive fibres in Raptor k.o. muscles.

CONCLUSIONS

Our study shows that mTOR signalling in skeletal muscle fibres is critical for maintaining proper fibre innervation, preserving the NMJ structure in both the muscle fibre and the motor neuron. In addition, considering the beneficial effects of exercise in most pathologies affecting the NMJ, our findings suggest that part of these beneficial effects of exercise are through the well-established activation of mTORC1 in skeletal muscle during and after exercise.

Sarcopenia: Tilting the Balance of Protein Homeostasis

Sarcopenia, defined as age-associated decline of muscle mass and function, is a risk factor for mortality and disability, and comorbid with several chronic diseases such as type II diabetes and cardiovascular diseases. Clinical trials showed that nutritional supplements had positive effects on muscle mass, but not on muscle function and strength, demonstrating our limited understanding of the molecular events involved in the ageing muscle. Protein homeostasis, the equilibrium between protein synthesis and degradation, is proposed as the major mechanism underlying the development of sarcopenia. As the key central regulator of protein homeostasis, the mammalian target of rapamycin (mTOR) is proposed to be essential for muscle hypertrophy. Paradoxically, sustained activation of mTOR complex 1 (mTORC1) is associated with a loss of sensitivity to extracellular signaling in the elderly. It is not understood why sustained mTORC1 activity, which should induce muscle hypertrophy, instead results in muscle atrophy. Here, recent findings on the implications of disrupting protein homeostasis on muscle physiology and sarcopenia development in the context of mTOR/protein kinase B (AKT) signaling are reviewed. Understanding the role of these molecular mechanisms during the ageing process will contribute towards the development of targeted therapies that will improve protein metabolism and reduce sarcopenia.

Homeostatic control of Drosophila neuromuscular junction function

The ability to adapt to changing internal and external conditions is a key feature of biological systems. Homeostasis refers to a regulatory process that stabilizes dynamic systems to counteract perturbations. In the nervous system, homeostatic mechanisms control neuronal excitability, neurotransmitter release, neurotransmitter receptors, and neural circuit function. The neuromuscular junction (NMJ) of Drosophila melanogaster has provided a wealth of molecular information about how synapses implement homeostatic forms of synaptic plasticity, with a focus on the transsynaptic, homeostatic modulation of neurotransmitter release. This review examines some of the recent findings from the Drosophila NMJ and highlights questions the field will ponder in coming years.

Cul3 and insomniac are required for rapid ubiquitination of postsynaptic targets and retrograde homeostatic signaling

At the Drosophila neuromuscular junction, inhibition of postsynaptic glutamate receptors activates retrograde signaling that precisely increases presynaptic neurotransmitter release to restore baseline synaptic strength. However, the nature of the underlying postsynaptic induction process remains enigmatic. Here, we design a forward genetic screen to discover factors in the postsynaptic compartment necessary to generate retrograde homeostatic signaling. This approach identified insomniac (inc), a putative adaptor for the Cullin-3 (Cul3) ubiquitin ligase complex, which together with Cul3 is essential for normal sleep regulation. Interestingly, we find that Inc and Cul3 rapidly accumulate at postsynaptic compartments following acute receptor inhibition and are required for a local increase in mono-ubiquitination. Finally, we show that Peflin, a Ca²⁺-regulated Cul3 co-adaptor, is necessary for homeostatic communication, suggesting a relationship between Ca²⁺ signaling and control of Cul3/Inc activity in the postsynaptic compartment. Our study suggests that Cul3/Inc-dependent mono-ubiquitination, compartmentalized at postsynaptic densities, gates retrograde signaling and provides an intriguing molecular link between the control of sleep and homeostatic plasticity at synapses.

The authors use a forward genetic screen to discover postsynaptic factors required for homeostatic synaptic plasticity at the Drosophila neuromuscular junction. They identify insomniac and the ubiquitin ligase Cul3, genes involved in sleep regulation, to be necessary for retrograde homeostatic signalling at this synapse.

Maintenance of homeostatic plasticity at the Drosophila neuromuscular synapse requires continuous IP3-directed signaling

Synapses and circuits rely on neuroplasticity to adjust output and meet physiological needs. Forms of homeostatic synaptic plasticity impart stability at synapses by countering destabilizing perturbations. The Drosophila melanogaster larval neuromuscular junction (NMJ) is a model synapse with robust expression of homeostatic plasticity. At the NMJ, a homeostatic system detects impaired postsynaptic sensitivity to neurotransmitter and activates a retrograde signal that restores synaptic function by adjusting neurotransmitter release. This process has been separated into temporally distinct phases, induction and maintenance. One prevailing hypothesis is that a shared mechanism governs both phases. Here, we show the two phases are separable. Combining genetics, pharmacology, and electrophysiology, we find that a signaling system consisting of PLCβ, inositol triphosphate (IP₃), IP₃ receptors, and Ryanodine receptors is required only for the maintenance of homeostatic plasticity. We also find that the NMJ is capable of inducing homeostatic signaling even when its sustained maintenance process is absent.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

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.

Disparate Postsynaptic Induction Mechanisms Ultimately Converge to Drive the Retrograde Enhancement of Presynaptic Efficacy

Retrograde signaling systems are fundamental modes of communication synapses utilize to dynamically and adaptively modulate activity. However, the inductive mechanisms that gate retrograde communication in the postsynaptic compartment remain enigmatic. We have investigated retrograde signaling at the Drosophila neuromuscular junction, where three seemingly disparate perturbations to the postsynaptic cell trigger a similar enhancement in presynaptic neurotransmitter release. We show that the same presynaptic genetic machinery and enhancements in active zone structure are utilized by each inductive pathway. However, all three induction mechanisms differ in temporal, translational, and CamKII activity requirements to initiate retrograde signaling in the postsynaptic cell. Intriguingly, pharmacological blockade of postsynaptic glutamate receptors, and not calcium influx through these receptors, is necessary and sufficient to induce rapid retrograde homeostatic signaling through CamKII. Thus, three distinct induction mechanisms converge on the same retrograde signaling system to drive the homeostatic strengthening of presynaptic neurotransmitter release.

Distinct homeostatic modulations stabilize reduced postsynaptic receptivity in response to presynaptic DLK signaling

Synapses are constructed with the stability to last a lifetime, yet sufficiently flexible to adapt during injury. Although fundamental pathways that mediate intrinsic responses to neuronal injury have been defined, less is known about how synaptic partners adapt. We have investigated responses in the postsynaptic cell to presynaptic activation of the injury-related Dual Leucine Zipper Kinase pathway at the Drosophila neuromuscular junction. We find that the postsynaptic compartment reduces neurotransmitter receptor levels, thus depressing synaptic strength. Interestingly, this diminished state is stabilized through distinct modulations to two postsynaptic homeostatic signaling systems. First, a retrograde response normally triggered by reduced receptor levels is silenced, preventing a compensatory enhancement in presynaptic neurotransmitter release. However, when global presynaptic release is attenuated, a postsynaptic receptor scaling mechanism persists to adaptively stabilize this diminished neurotransmission state. Thus, the homeostatic set point of synaptic strength is recalibrated to a reduced state as synapses acclimate to injury.

A postsynaptic PI3K-cII dependent signaling controller for presynaptic homeostatic plasticity

Presynaptic homeostatic plasticity stabilizes information transfer at synaptic connections in organisms ranging from insect to human. By analogy with principles of engineering and control theory, the molecular implementation of PHP is thought to require postsynaptic signaling modules that encode homeostatic sensors, a set point, and a controller that regulates transsynaptic negative feedback. The molecular basis for these postsynaptic, homeostatic signaling elements remains unknown. Here, an electrophysiology-based screen of the Drosophila kinome and phosphatome defines a postsynaptic signaling platform that includes a required function for PI3K-cII, PI3K-cIII and the small GTPase Rab11 during the rapid and sustained expression of PHP. We present evidence that PI3K-cII localizes to Golgi-derived, clathrin-positive vesicles and is necessary to generate an endosomal pool of PI(3)P that recruits Rab11 to recycling endosomal membranes. A morphologically distinct subdivision of this platform concentrates postsynaptically where we propose it functions as a homeostatic controller for retrograde, trans-synaptic signaling.

The Maintenance of Synaptic Homeostasis at the Drosophila Neuromuscular Junction Is Reversible and Sensitive to High Temperature

Homeostasis is a vital mode of biological self-regulation. The hallmarks of homeostasis for any biological system are a baseline set point of physiological activity, detection of unacceptable deviations from the set point, and effective corrective measures to counteract deviations. Homeostatic synaptic plasticity (HSP) is a form of neuroplasticity in which neurons and circuits resist environmental perturbations and stabilize levels of activity. One assumption is that if a perturbation triggers homeostatic corrective changes in neuronal properties, those corrective measures should be reversed upon removal of the perturbation. We test the reversibility and limits of HSP at the well-studied Drosophila melanogaster neuromuscular junction (NMJ). At the Drosophila NMJ, impairment of glutamate receptors causes a decrease in quantal size, which is offset by a corrective, homeostatic increase in the number of vesicles released per evoked presynaptic stimulus, or quantal content. This process has been termed presynaptic homeostatic potentiation (PHP). Taking advantage of the GAL4/GAL80^TS/UAS expression system, we triggered PHP by expressing a dominant-negative glutamate receptor subunit at the NMJ. We then reversed PHP by halting expression of the dominant-negative receptor. Our data show that PHP is fully reversible over a time course of 48–72 h after the dominant-negative glutamate receptor stops being genetically expressed. As an extension of these experiments, we find that when glutamate receptors are impaired, neither PHP nor NMJ growth is reliably sustained at high culturing temperatures (30–32°C). These data suggest that a limitation of homeostatic signaling at high temperatures could stem from the synapse facing a combination of challenges simultaneously.

Development of a tissue-specific ribosome profiling approach in Drosophila enables genome-wide evaluation of translational adaptations

Recent advances in next-generation sequencing approaches have revolutionized our understanding of transcriptional expression in diverse systems. However, measurements of transcription do not necessarily reflect gene translation, the process of ultimate importance in understanding cellular function. To circumvent this limitation, biochemical tagging of ribosome subunits to isolate ribosome-associated mRNA has been developed. However, this approach, called TRAP, lacks quantitative resolution compared to a superior technology, ribosome profiling. Here, we report the development of an optimized ribosome profiling approach in Drosophila. We first demonstrate successful ribosome profiling from a specific tissue, larval muscle, with enhanced resolution compared to conventional TRAP approaches. We next validate the ability of this technology to define genome-wide translational regulation. This technology is leveraged to test the relative contributions of transcriptional and translational mechanisms in the postsynaptic muscle that orchestrate the retrograde control of presynaptic function at the neuromuscular junction. Surprisingly, we find no evidence that significant changes in the transcription or translation of specific genes are necessary to enable retrograde homeostatic signaling, implying that post-translational mechanisms ultimately gate instructive retrograde communication. Finally, we show that a global increase in translation induces adaptive responses in both transcription and translation of protein chaperones and degradation factors to promote cellular proteostasis. Together, this development and validation of tissue-specific ribosome profiling enables sensitive and specific analysis of translation in Drosophila.

Recent advances in next-generation sequencing approaches have revolutionized our understanding of transcriptional expression in diverse systems. However, transcriptional expression alone does not necessarily report gene translation, the process of ultimate importance in understanding cellular function. Ribosome profiling is a powerful approach to quantify the number of ribosomes associated with each mRNA to determine rates of gene translation. However, ribosome profiling requires large quantities of starting material, limiting progress in developing tissue-specific approaches. Here, we have developed the first tissue-specific ribosome profiling system in Drosophila to reveal genome-wide changes in translation. We first demonstrate successful ribosome profiling from muscle cells that exhibit superior resolution compared to other translational profiling methods. We then use transcriptional and ribosome profiling to define whether transcriptional or translational mechanisms are necessary for synaptic signaling at the neuromuscular junction. Finally, we utilize ribosome profiling to reveal adaptive changes in cellular translation following cellular stress to muscle tissue. Together, this now enables the power of Drosophila genetics to be leveraged with ribosome profiling in specific tissues.

Combining Constitutively Active Rheb Expression and Chondroitinase Promotes Functional Axonal Regeneration after Cervical Spinal Cord Injury

After spinal cord injury (SCI), severed axons in the adult mammalian CNS are unable to mount a robust regenerative response. In addition, the glial scar at the lesion site further restricts the regenerative potential of axons. We hypothesized that a combinatorial approach coincidentally targeting these obstacles would promote axonal regeneration. We combined (1) transplantation of a growth-permissive peripheral nerve graft (PNG) into an incomplete, cervical lesion cavity; (2) transduction of neurons rostral to the SCI site to express constitutively active Rheb (caRheb; a Ras homolog enriched in brain), a GTPase that directly activates the growth-promoting pathway mammalian target of rapamycin (mTOR) via AAV-caRheb injection; and (3) digestion of growth-inhibitory chondroitin sulfate proteoglycans within the glial scar at the distal PNG interface using the bacterial enzyme chondroitinase ABC (ChABC). We found that expressing caRheb in neurons post-SCI results in modestly yet significantly more axons regenerating across a ChABC-treated distal graft interface into caudal spinal cord than either treatment alone. Excitingly, we found that caRheb+ChABC treatment significantly potentiates the formation of synapses in the host spinal cord and improves the animals' ability to use the affected forelimb. Thus, this combination strategy enhances functional axonal regeneration following a cervical SCI.

Fasting, translation control, and seizures

Acute fasting, a therapy that helps some epilepsy patients, inhibits protein translation and dampens synaptic activity in neuromuscular junctions in flies.