Tauopathies, including Alzheimer's disease and fronto-temporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), are a group of neurodegenerative disorders characterized by the presence of intraneuronal filamentous inclusions of aberrantly phosphorylated-tau. Tau is a neuronal microtubule-associated protein involved in microtubule assembly and stabilization. Currently, the molecular mechanisms underlying tau-mediated cellular toxicity remain elusive. To address the determinants of tau neurotoxicity, we first characterized the cellular alterations resulting from the over-expression of a mutant form of human tau associated with FTDP-17 (tau V337M) in Drosophila. We found that the over-expression of tau V337M, in Drosophila larval motor neurons, induced disruption of the microtubular network at presynaptic nerve terminals and changes in neuromuscular junctions morphological features. Secondly, we performed a misexpression screen to identify genetic modifiers of the tau V337M-mediated rough eye phenotype. The screening of 1250 mutant Drosophila lines allowed us to identify several components of the cytoskeleton, and particularly from the actin network, as specific modifiers of tau V337M-induced neurodegeneration. Furthermore, we found that numerous tau modulators identified in our screen were involved in the maintenance of synaptic function. Taken together, these findings suggest that disruption of the microtubule network in presynaptic nerve terminals could constitute early events in the pathological process leading to synaptic dysfunction in tau V337M pathology.

Axons contain a smooth tubular endoplasmic reticulum (ER) network that is thought to be continuous with ER throughout the neuron; the mechanisms that form this axonal network are unknown. Mutations affecting reticulon or REEP proteins, with intramembrane hairpin domains that model ER membranes, cause an axon degenerative disease, hereditary spastic paraplegia (HSP). We show that Drosophila axons have a dynamic axonal ER network, which these proteins help to model. Loss of HSP hairpin proteins causes ER sheet expansion, partial loss of ER from distal motor axons, and occasional discontinuities in axonal ER. Ultrastructural analysis reveals an extensive ER network in axons, which shows larger and fewer tubules in larvae that lack reticulon and REEP proteins, consistent with loss of membrane curvature. Therefore HSP hairpin-containing proteins are required for shaping and continuity of axonal ER, thus suggesting roles for ER modeling in axon maintenance and function.DOI: http://dx.doi.org/10.7554/eLife.23882.001

Tau-positive inclusions in neurons are consistent neuropathologic features of the most common causes of dementias such Alzheimer's disease and frontotemporal dementia. Ubiquitinated tau-positive inclusions have been reported in brains of Alzheimer's disease patients, but involvement of the ubiquitin-dependent proteasomal system in tau degradation remains controversial. Before considering the tau degradation in pathologic conditions, it is important to determine whether or not endogenous tau is normally degraded by the proteasome pathway. We therefore investigated this question using two complementary approaches in vitro and in vivo. Firstly, SH-SY5Y human neuroblastoma cells were treated with different proteasome inhibitors, MG132, lactacystin, and epoxomicin. Under these conditions, neither total nor phosphorylated endogenous tau protein levels were increased. Instead, an unexpected decrease of tau protein was observed. Secondly, we took advantage of a temperature-sensitive mutant allele of the 20S proteasome in Drosophila. Genetic inactivation of the proteasome also resulted in a decrease of tau levels in Drosophila. These results obtained in vitro and in vivo demonstrate that endogenous tau is not normally degraded by the proteasome.