Anna B. Ziegler

The brain is highly susceptible to disturbances in lipid metabolism. Among the rare, genetically-linked epilepsies Progressive Myoclonic Epilepsy Type 8 (PME8), associated with the loss of Ceramide Synthase (CerS) activity, causes epileptic symptoms accompanied by early onset of neurodegenerative traits. The function of CerS is embedded in a complex, conserved metabolic pathway, making it difficult to identify the specific disease-relevant alterations. Here, we show that the expression of an enzymatically inactive cerS allele in Drosophila sensory neurons yielded developmental and early onset dendrite loss. Combining lipidomics and refined genetics with quantitative analysis of neuronal morphology in cerS mutants, we identified which lipids species are dysregulated and how they affect neuronal morphology. In cerS mutants, long and very-long acyl-chain C18-C24-ceramides were missing and necessary for dendrite elaboration. In addition, the substrate of CerS, (dh)S, and its metabolite (dh)S1P, increased. Especially increasing (dh)S1P strongly reduces dendritic complexity in cerS mutant neurons. Finally, we performed in vivo experiments to cell-autonomously rescue the morphological defects of cerS mutant neurons and report that a complete rescue can only be achieved if the toxic CerS substrate is converted to produce specific (C18-C24) ceramides. Thus, despite the complex metabolic alterations, our data provides essential information about the metabolic origin of PME8 and delineates a potential therapeutic avenue.

Summary: The brain is susceptible to disturbances in lipid metabolism. Among the rare, genetically-linked epilepsies Progressive Myoclonic Epilepsy Type 8 (PME8), associated with the loss of Ceramide Synthase (CerS) activity, causes epileptic symptoms accompanied by neurodegenerative traits. We show that expression of a disease-causing c erS allele in Drosophila sensory neurons yielded developmental and degenerative dendrite loss. In cerS mutants, C18-C24 ceramides and membrane-forming complex sphingolipids, into which ceramides are converted, were reduced. At the same time bioactive signaling lipids including (dh)Sphingosine-1-P, deriving from the CerS substrate, were increased. To clarifying the etiology of PME8, we thus performed in vivo experiments to cell-autonomously rescue the individual metabolic alterations. We report that restoring specific long-chain ceramides while in parallel decreasing (dh)Sphingosine-1-P fully rescues the cerS mutant phenotype. Thus, despite the complex metabolic alterations, our data provide essential information about the metabolic origin of PME8 and delineate a potential therapy.

The proper functioning of neural circuits that integrate sensory signals is essential for individual adaptation to an ever-changing environment. Many molecules can modulate neuronal activity, including neurotransmitters, receptors, and even amino acids. Here, we ask whether amino acid transporters expressed by neurons can influence neuronal activity. We found that minidiscs (mnd), which encodes a light chain of a heterodimeric amino acid transporter, is expressed in different cell types of the adult Drosophila brain: in mushroom body neurons (MBs) and in glial cells. Using live calcium imaging, we found that MND expressed in α/β MB neurons is essential for sensitivity to the L-amino acids: Leu, Ile, Asp, Glu, Lys, Thr, and Arg. We found that the Target Of Rapamycin (TOR) pathway but not the Glutamate Dehydrogenase (GDH) pathway is involved in the Leucine-dependent response of α/β MB neurons. This study strongly supports the key role of MND in regulating MB activity in response to amino acids.

During development, neurons achieve a stereotyped neuron type-specific morphology, which relies on dynamic support by microtubules (MTs). An important player is the augmin complex (hereafter augmin), which binds to existing MT filaments and recruits the γ-tubulin ring complex (γ-TuRC), to form branched MTs. In cultured neurons, augmin is important for neurite formation. However, little is known about the role of augmin during neurite formation in vivo. Here, we have revisited the role of mammalian augmin in culture and then turned towards the class four Drosophila dendritic arborization (c4da) neurons. We show that MT density is maintained through augmin in cooperation with the γ-TuRC in vivo. Mutant c4da neurons show a reduction of newly emerging higher-order dendritic branches and in turn also a reduced number of their characteristic space-filling higher-order branchlets. Taken together, our data reveal a cooperative function for augmin with the γ-TuRC in forming enough MTs needed for the appropriate differentiation of morphologically complex dendrites in vivo.

Neuronal dendrites receive, integrate, and process numerous inputs and therefore serve as the neuron’s “antennae”. Dendrites display extreme morphological diversity across different neuronal classes to match the neuron’s specific functional requirements. Understanding how this structural diversity is specified is therefore important for shedding light on information processing in the healthy and diseased nervous system. Popular models for in vivo studies of dendrite differentiation are the four classes of dendritic arborization (c1da–c4da) neurons of Drosophila larvae with their class-specific dendritic morphologies. Using da neurons, a combination of live-cell imaging and computational approaches have delivered information on the distinct phases and the time course of dendrite development from embryonic stages to the fully developed dendritic tree. With these data, we can start approaching the basic logic behind differential dendrite development. A major role in the definition of neuron-type specific morphologies is played by dynamic actin-rich processes and the regulation of their properties. This review presents the differences in the growth programs leading to morphologically different dendritic trees, with a focus on the key role of actin modulatory proteins. In addition, we summarize requirements and technological progress towards the visualization and manipulation of such actin regulators in vivo.