Glial lipid droplets and neurodegeneration in a Drosophila model of complex I deficiency.

Mitochondrial defects associated with respiratory chain complex I deficiency lead to heterogeneous fatal syndromes. While the role of NDUFS8, an essential subunit of the core assembly of the complex I, is established in mitochondrial diseases, the mechanisms underlying neuropathology are poorly understood. We developed a Drosophila model of NDUFS8 deficiency by knocking down the expression of its fly homologue in neurons or in glial cells. Downregulating ND23 in neurons resulted in shortened lifespan, and decreased locomotion. Although total brain ATP levels were decreased, histological analysis did not reveal any signs of neurodegeneration except for photoreceptors of the retina. Interestingly, ND23 deficiency-associated phenotypes were rescued by overexpressing the glucose transporter hGluT3 demonstrating that boosting glucose metabolism in neurons was sufficient to bypass altered mitochondrial functions and to confer neuroprotection. We then analyzed the consequences of ND23 knockdown in glial cells. In contrast to neuronal knockdown, loss of ND23 in glia did not lead to significant behavioral defects nor to reduced lifespan, but induced brain degeneration, as visualized by numerous vacuoles found all over the nervous tissue. This phenotype was accompanied by the massive accumulation of lipid droplets at the cortex-neuropile boundaries, suggesting an alteration of lipid metabolism in glia. These results demonstrate that complex I deficiency triggers metabolic alterations both in neurons and glial cells which may contribute to the neuropathology.

Brain-derived neurotrophic factor/TrkB signaling regulates daily astroglial plasticity in the suprachiasmatic nucleus: electron-microscopic evidence in mouse.

Synchronization of circadian rhythms to the 24-h light/dark (L/D) cycle is associated with daily rearrangements of the neuronal-glial network of the suprachiasmatic nucleus of the hypothalamus (SCN), the central master clock orchestrating biological functions in mammals. These anatomical plastic events involve neurons synthesizing vasoactive intestinal peptide (VIP), known as major integrators of photic signals in the retinorecipient region of the SCN. Using an analog-sensitive kinase allele murine model (TrkB(F616A) ), we presently show that the pharmacological blockade of the tropomyosin-related kinase receptor type B (TrkB), the high-affinity receptor of brain-derived neurotrophic factor (BDNF), abolished day/night changes in the dendrite enwrapping of VIP neurons by astrocytic processes (glial coverage), used as an index of SCN plasticity on electron-microscopic sections. Therefore, the BDNF/TrkB signaling pathway exerts a permissive role on the ultrastructural rearrangements that occur in SCN under L/D alternance, an action that could be a critical determinant of the well-established role played by BDNF in the photic regulation of the SCN. In contrast, the extent of glial coverage of non-VIP neighboring dendrites was not different at daytime and nighttime in TrkB(F616A) mice submitted to TrkB inactivation or not receiving any pharmacological treatment. These data not only show that BDNF regulates SCN structural plasticity across the 24-h cycle but also reinforce the view that the daily changes in SCN architecture subserve the light synchronization process.

Enhancing Mitofusin/Marf ameliorates neuromuscular dysfunction in Drosophila models of TDP-43 proteinopathies.

Transactive response DNA-binding protein 43 kDa (TDP-43) is considered a major pathological protein in amyotrophic lateral sclerosis and frontotemporal lobar degeneration. The precise mechanisms by which TDP-43 dysregulation leads to toxicity in neurons are not fully understood. Using TDP-43-expressing Drosophila, we examined whether mitochondrial dysfunction is a central determinant in TDP-43 pathogenesis. Expression of human wild-type TDP-43 in Drosophila neurons results in abnormally small mitochondria. The mitochondrial fragmentation is correlated with a specific decrease in the mRNA and protein levels of the Drosophila profusion gene mitofusin/marf. Importantly, overexpression of Marf ameliorates defects in spontaneous walking activity and startle-induced climbing response of TDP-43-expressing flies. Partial inactivation of the mitochondrial profission factor, dynamin-related protein 1, also mitigates TDP-43-induced locomotor deficits. Expression of TDP-43 impairs neuromuscular junction transmission upon repetitive stimulation of the giant fiber circuit that controls flight muscles, which is also ameliorated by Marf overexpression. We show here for the first time that enhancing the profusion gene mitofusin/marf is beneficial in an in vivo model of TDP-43 proteinopathies, serving as a potential therapeutic target.

Development and neuromodulation of spinal locomotor networks in the metamorphosing frog.

Metamorphosis in the anuran frog, Xenopus laevis, involves profound structural and functional transformations in most of the organism's physiological systems as it encounters a complete alteration in body plan, habitat, mode of respiration and diet. The metamorphic process also involves a transition in locomotory strategy from axial-based undulatory swimming using alternating contractions of left and right trunk muscles, to bilaterally-synchronous kicking of the newly developed hindlimbs in the young adult. At critical stages during this behavioural switch, functional larval and adult locomotor systems co-exist in the same animal, implying a progressive and dynamic reconfiguration of underlying spinal circuitry and neuronal properties as limbs are added and the tail regresses. To elucidate the neurobiological basis of this developmental process, we use electrophysiological, pharmacological and neuroanatomical approaches to study isolated in vitro brain stem/spinal cord preparations at different metamorphic stages. Our data show that the emergence of secondary limb motor circuitry, as it supersedes the primary larval network, spans a developmental period when limb circuitry is present but not functional, functional but co-opted into the axial network, functionally separable from the axial network, and ultimately alone after axial circuitry disappears with tail resorption. Furthermore, recent experiments on spontaneously active in vitro preparations from intermediate metamorphic stage animals have revealed that the biogenic amines serotonin (5-HT) and noradrenaline (NA) exert short-term adaptive control over circuit activity and inter-network coordination: whereas bath-applied 5-HT couples axial and appendicular rhythms into a single unified pattern, NA has an opposite decoupling effect. Moreover, the progressive and region-specific appearance of spinal cord neurons that contain another neuromodulator, nitric oxide (NO), suggests it plays a role in the maturation of limb locomotor circuitry. In summary, during Xenopus metamorphosis the network responsible for limb movements is progressively segregated from an axial precursor, and supra- and intra-spinal modulatory inputs are likely to play crucial roles in both its functional flexibility and maturation.

Intercostal and abdominal respiratory motoneurons in the neonatal rat spinal cord: spatiotemporal organization and responses to limb afferent stimulation.

Respiration requires the coordinated rhythmic contractions of diverse muscles to produce ventilatory movements adapted to organismal requirements. During fast locomotion, locomotory and respiratory movements are coordinated to reduce mechanical conflict between these functions. Using semi-isolated and isolated in vitro brain stem-spinal cord preparations from neonatal rats, we have characterized for the first time the respiratory patterns of all spinal intercostal and abdominal motoneurons and explored their functional relationship with limb sensory inputs. Neuroanatomical and electrophysiological procedures were initially used to locate intercostal and abdominal motoneurons in the cord. Intercostal motoneuron somata are distributed rostrocaudally from C(7)-T(13) segments. Abdominal motoneuron somata lie between T(8) and L(2). In accordance with their soma distributions, inspiratory intercostal motoneurons are recruited in a rostrocaudal sequence during each respiratory cycle. Abdominal motoneurons express expiratory-related discharge that alternates with inspiration. Lesioning experiments confirmed the pontine origin of this expiratory activity, which was abolished by a brain stem transection at the rostral boundary of the VII nucleus, a critical area for respiratory rhythmogenesis. Entrainment of fictive respiratory rhythmicity in intercostal and abdominal motoneurons was elicited by periodic low-threshold dorsal root stimulation at lumbar (L(2)) or cervical (C(7)) levels. These effects are mediated by direct ascending fibers to the respiratory centers and a combination of long-projection and polysynaptic descending pathways. Therefore the isolated brain stem-spinal cord in vitro generates a complex pattern of respiratory activity in which alternating inspiratory and expiratory discharge occurs in functionally identified spinal motoneuron pools that are in turn targeted by both forelimb and hindlimb somatic afferents to promote locomotor-respiratory coupling.

Rare and spatially segregated release sites mediate a synaptic interaction between two identified network neurons.

Laser-scanning confocal microscopy (LSCM), electron microcopy (EM), and cellular electrophysiology were used in combination to study the structural basis of an inhibitory synapse between two identified neurons of the same network. To achieve this, we examined the chemical inhibitory synapse between identified neurons belonging to the lobster (Homarus gammarus) pyloric network: the pyloric dilator (PD) and the lateral pyloric (LP) neurons. In order to visualize simultaneously these two neurons, we used intrasomatic injection of Lucifer Yellow (LY) in one and rhodamine/horseradish peroxydase (HRP) in the other. Under LSCM, we found only two zones of close apposition in a restricted part of the neuritic tree of the two network neurons. Then, within these two zones, the synaptic release sites were searched using EM. To this end, photoconversion of LY with immunogold and development of HRP with DAB were performed on the previously observed preparations. Structural evidence was found for only one release site per zone. To confirm this result, and because the zones of contact were always segregated in a restricted part of the dendrites, we used laser photoablation to selectively delete, either pre- or postsynaptically, the branches on which the release sites were located. In both cases, such restrictive ablation completely abolished the functional interaction between these neurons. Our results therefore demonstrate that an inhibitory synapse that is essential for the operation of a neural network relies on only very few sites of contact localized in a highly restricted part of each neuron's dendritic arbor.