Decapentaplegic Acutely Defines the Connectivity of Central Pacemaker Neurons in Drosophila

Suprachiasmatic nucleus VIPergic fibers show a circadian rhythm of expansion and retraction

In animals, overt circadian rhythms of physiology and behavior are centrally regulated by a circadian clock located in specific brain regions. In the fruit fly Drosophila and in mammals, these clocks rely on single-cell oscillators, but critical for their function as central circadian pacemakers are network properties that change dynamically throughout the circadian cycle as well as in response to environmental stimuli.1,2,3 In the fly, this plasticity involves circadian rhythms of expansion and retraction of clock neuron fibers.4,5,6,7,8,9,10,11,12,13,14 Whether these drastic structural changes are a universal property of central neuronal pacemakers is unknown. To address this question, we studied neurons of the mouse suprachiasmatic nucleus (SCN) that express vasoactive intestinal polypeptide (VIP), which are critical for the SCN to function as a central circadian pacemaker. By targeting the expression of the fluorescent protein tdTomato to these neurons and using tissue clearing techniques to visualize all SCN VIPergic neurons and their fibers, we show that, similar to clock neurons in the fly, VIPergic fibers undergo a daily rhythm of expansion and retraction, with maximal branching during the day. This rhythm is circadian, as it persists under constant environmental conditions and is present in both males and females. We propose that circadian structural remodeling of clock neurons represents a key feature of central circadian pacemakers that is likely critical to regulate network properties, the response to environmental stimuli, and the regulation of circadian outputs.

A rapid and dynamic role for FMRP in the plasticity of adult neurons

Fragile X syndrome (FXS) is a neuro-developmental disorder caused by silencing Fmr1, which encodes the RNA-binding protein FMRP. Although Fmr1 is expressed in adult neurons, it has been challenging to separate acute from chronic effects of loss of Fmr1 in models of FXS. We have used the precision of Drosophila genetics to test if Fmr1 acutely affects adult neuronal plasticity in vivo, focusing on the s-LNv circadian pacemaker neurons that show 24 hour rhythms in structural plasticity. We found that over-expressing Fmr1 for only 4 hours blocks the activity-dependent expansion of s-LNv projections without altering the circadian clock or activity-regulated gene expression. Conversely, acutely reducing Fmr1 expression prevented s-LNv projections from retracting. One FMRP target that we identified in s-LNvs is sif, which encodes a Rac1 GEF. Our data indicate that FMRP normally reduces sif mRNA translation at dusk to reduce Rac1 activity. Overall, our data reveal a previously unappreciated rapid and direct role for FMRP in acutely regulating neuronal plasticity in adult neurons, and underscore the importance of RNA-binding proteins in this process.

Identification of atlastin genetic modifiers in a model of hereditary spastic paraplegia in Drosophila

Hereditary spastic paraplegias (HSPs) are a group of neurodegenerative disorders characterized by progressive dysfunction of corticospinal motor neurons. Mutations in Atlastin1/Spg3, a small GTPase required for membrane fusion in the endoplasmic reticulum, are responsible for 10% of HSPs. Patients with the same Atlastin1/Spg3 mutation present high variability in age at onset and severity, suggesting a fundamental role of the environment and genetic background. Here, we used a Drosophila model of HSPs to identify genetic modifiers of decreased locomotion associated with atlastin knockdown in motor neurons. First, we screened for genomic regions that modify the climbing performance or viability of flies expressing atl RNAi in motor neurons. We tested 364 deficiencies spanning chromosomes two and three and found 35 enhancer and four suppressor regions of the climbing phenotype. We found that candidate genomic regions can also rescue atlastin effects at synapse morphology, suggesting a role in developing or maintaining the neuromuscular junction. Motor neuron-specific knockdown of 84 genes spanning candidate regions of the second chromosome identified 48 genes required for climbing behavior in motor neurons and 7 for viability, mapping to 11 modifier regions. We found that atl interacts genetically with Su(z)2, a component of the Polycomb repressive complex 1, suggesting that epigenetic regulation plays a role in the variability of HSP-like phenotypes caused by atl alleles. Our results identify new candidate genes and epigenetic regulation as a mechanism modifying neuronal atl pathogenic phenotypes, providing new targets for clinical studies.

The Drosophila circadian clock circuit is a nonhierarchical network of peptidergic oscillators

The relatively simple Drosophila circadian clock circuit consists of 150 clock neurons that coordinate rhythmic behavior and physiology, which are generally classified based on neuroanatomical location. Transcriptional and connectomic studies have identified novel subdivisions of these clock neuron populations, and identified neuropeptides not previously known to be expressed in the fly clock circuit. An additional feature of fly clock neurons is daily axonal remodeling, first noted in small ventrolateral neurons, but more recently also found in additional clock neuron groups. These findings raise new questions about the functional roles of clock neuron subpopulations and daily remodeling of network architecture in regulating circadian behavior and physiology.