Drosophila Primary Neuronal Cultures as a Useful Cellular Model to Study and Image Axonal Transport

Oxidative Stress Promotes Axonal Atrophy through Alterations in Microtubules and EB1 Function

Axons are crucial for transmitting neurochemical signals. As organisms age, the ability of neurons to maintain their axons declines; hence, aged axons are more susceptible to damage or dysfunction. Understanding how aging causes axonal vulnerability is crucial for developing strategies to enhance overall resilience of neurons and prevent neuronal deterioration during aging and in age-related neurodegenerative diseases. Increasing levels of reactive oxygen species (ROS) causes oxidative stress - a hallmark of aging and age-related diseases. Despite this association, a causal relationship between oxidative stress and neuronal aging remains unclear, particularly in how subcellular physiology may be affected by ROS. By using Drosophila-derived primary neuronal cultures and a recently developed in vivo neuronal model of aging, which involves the visualisation of Drosophila medulla neurons, we investigated the interplay between oxidative stress, neuronal aging and the microtubule cytoskeleton. Our results showed that oxidative stress is a key driver of axonal and synaptic decay, as shown by an enhanced appearance of axonal swellings, microtubule alterations (in both axons and synapses) and morphological transformation of axonal terminals during aging. We demonstrated that increasing the levels of ROS sensitises microtubule plus end-binding protein 1 (EB1), leading to microtubule defects that effect neuronal integrity. Furthermore, manipulating EB1 proved to be a valuable therapeutic strategy to prevent aging hallmarks enhanced in conditions of elevated ROS. In summary, we demonstrate a mechanistic pathway linking cellular oxidative stress with changes in the microtubule cytoskeleton leading to axonal deterioration during aging and provide evidence of the therapeutic potential of enhancing microtubule plus-end physiology to improve the resilience of axons.

Sniffer restricts arboviral brain infections by regulating ROS levels and protecting blood-brain barrier integrity in Drosophila and mosquitoes

Arthropod-borne viruses (arboviruses) are transmitted to humans by arthropod vectors and pose a serious threat to global public health. Neurotropic arboviruses including Sindbis virus (SINV) persistently infect the central nervous system (CNS) of vector insects without causing notable pathological changes or affecting their behavior or lifespan. However, the mechanisms by which vector insects evade these viral infections in the brains are poorly understood. In this study, we found that loss of the carbonyl reductase Sniffer (Sni) led to a significant increase in SINV infection in the Drosophila brain. Sni regulates reactive oxygen species (ROS) levels, and its depletion leads to elevated ROS, which in turn disrupts the septate junctions (SJs) between subperineurial glia (SPG) cells, compromising the integrity and barrier function of the blood-brain barrier (BBB). Genetic and pharmacological reduction of ROS restored BBB integrity and reduced viral load in the brains of Sni-depleted flies. Additionally, we identified Sni homologs and revealed that the antiviral function of Sni is highly conserved in mosquitoes, where it regulates ROS and protects BBB integrity. Our results revealed an evolutionarily conserved antiviral mechanism in which Sni acts as an antioxidant that protects BBB integrity and restricts viral infection in the vector insect brain.

Handling Difficult Cryo-ET Samples: A Study with Primary Neurons from Drosophila melanogaster

Cellular neurobiology has benefited from recent advances in the field of cryo-electron tomography (cryo-ET). Numerous structural and ultrastructural insights have been obtained from plunge-frozen primary neurons cultured on electron microscopy grids. With most primary neurons having been derived from rodent sources, we sought to expand the breadth of sample availability by using primary neurons derived from 3rd instar Drosophila melanogaster larval brains. Ultrastructural abnormalities were encountered while establishing this model system for cryo-ET, which were exemplified by excessive membrane blebbing and cellular fragmentation. To optimize neuronal samples, we integrated substrate selection, micropatterning, montage data collection, and chemical fixation. Efforts to address difficulties in establishing Drosophila neurons for future cryo-ET studies in cellular neurobiology also provided insights that future practitioners can use when attempting to establish other cell-based model systems.

Handling difficult cryo-ET samples: A study with primary neurons from Drosophila melanogaster

Cellular neurobiology has benefited from recent advances in the field of cryo-electron tomography (cryo-ET). Numerous structural and ultrastructural insights have been obtained from plunge-frozen primary neurons cultured on electron microscopy grids. With most primary neurons been derived from rodent sources, we sought to expand the breadth of sample availability by using primary neurons derived from 3rd instar Drosophila melanogaster larval brains. Ultrastructural abnormalities were encountered while establishing this model system for cryo-ET, which were exemplified by excessive membrane blebbing and cellular fragmentation. To optimize neuronal samples, we integrated substrate selection, micropatterning, montage data collection, and chemical fixation. Efforts to address difficulties in establishing Drosophila neurons for future cryo-ET studies in cellular neurobiology also provided insights that future practitioners can use when attempting to establish other cell-based model systems.

In vivo imaging of axonal transport in peripheral nerves of rodent forelimbs

Axonal transport is the essential process by which neurons actively traffic a variety of cargoes between the cell soma and axon terminals. Accordingly, dysfunctional axonal transport is linked to many nervous system conditions. Therefore, being able to image and quantify this dynamic process in live neurons of animal disease models is beneficial for understanding neuropathology and testing new therapies at the preclinical level. As such, intravital approaches have been developed to assess cargo movement in the hindlimb sciatic nerves of live, anaesthetised mice. Here, we describe an adapted method for in vivo imaging of axonal transport in intact median and ulnar nerves of the rodent forelimb. Injection of a fluorescently labelled and non-toxic fragment of tetanus neurotoxin (H_CT) into the mouse forepaw permits the identification of signalling endosomes in intact axons of median and ulnar nerves. Through immunofluorescent analysis of forelimb lumbrical muscles and median/ulnar nerves, we confirmed that H_CT is taken up at motor nerve terminals and predominantly locates to motor axons. We then showed that the baseline trafficking of signalling endosomes is similar between the median/ulnar nerves and the sciatic nerve in adult wild-type mice. Importantly, this adapted method can be readily tailored for assessment of additional cargoes, such as mitochondria. By measuring transport in forelimb and hindlimb nerves, comparative anatomical and functional analyses can be performed in rodent disease models to aid our understanding of peripheral nerve disease pathogenesis and response to injury.