Programmed neurite degeneration in human central nervous system neurons driven by changes in NAD⁺ metabolism

Neurite degeneration (ND) precedes cell death in many neurodegenerative diseases. However, it remains unclear how this compartmentalized cell death process is orchestrated in the central nervous system (CNS). The establishment of a CNS axotomy model (using modified 3D LUHMES cultures) allowed us to study metabolic control of ND in human midbrain-derived neurons without the use of toxicants or other direct disturbance of cellular metabolism. Axotomy lead to a loss of the NAD+ synthesis enzyme NMNAT2 within 2 h and a depletion of NAD+ within 4-6 h. This process appeared specific, as isolated neurites maintained ATP levels and a coupled mitochondrial respiration for at least 6 h. In the peripheral nervous system (PNS) many studies observed that NAD+ metabolism, in particular by the NADase SARM1, plays a major role in the ND occurring after axotomy. Since neither ferroptosis nor necroptosis, nor caspase-dependent apoptosis seemed to be involved in neurite loss, we investigated SARM1 as potential executioner (or controller). Knock-down or expression of a dominant-negative isoform of SARM1 indeed drastically delayed ND. Various modifications of NAD+ metabolism known to modulate SARM1 activity showed the corresponding effects on ND. Moreover, supplementation with NAD+ attenuated ND. As a third approach to investigate the role of altered NAD+ metabolism, we made use of the WLD(s) protein, which has been found in a mutant mouse to inhibit Wallerian degeneration of axons. This protein, which has a stable NMNAT activity, and thus can buffer the loss of NMNAT2, protected the neurites by stabilizing neurite NAD+ levels. Thus CNS-type ND was tightly linked to neurite metabolism in multiple experimental setups. Based on this knowledge, several new strategies for treating neurodegenerative diseases can be envisaged.publishe

Fate of Planktothrix-derived toxins in aquatic food webs: A case study in Lake Mindelsee (Germany)

Blooms of the red, filamentous cyanobacterium Planktothrix rubescens occur frequently in pre-alpine lakes in Europe, often with concomitant toxic microcystin (MC) production. Trophic transfer of MCs has been observed in bivalves, fish, and zooplankton species, while uptake of MCs into Diptera species could facilitate distribution of MCs into terrestrial food webs and habitats. In this study, we characterized a Planktothrix bloom in summer 2019 in Lake Mindelsee and tracked possible trophic transfer and/or bioaccumulation of MCs via analysis of phytoplankton, zooplankton (Daphnia) and emergent aquatic insects (Chaoborus, Chironomidae and Trichoptera). Using 16 S rRNA gene amplicon sequencing, we found that five sequence variants of Planktothrix spp. were responsible for bloom formation in September and October of 2019, and these MC-producing variants, provisionally identified as P. isothrix and/or P. serta, occurred exclusively in Lake Mindelsee (Germany), while other variants were also detected in nearby Lake Constance. The remaining cyanobacterial community was dominated by Cyanobiaceae species with high species overlap with Lake Constance, suggesting a well-established exchange of cyanobacteria species between the adjacent lakes. With targeted LC–HRMS/MS we identified two MC-congeners, MC-LR and [Asp3]MC-RR with maximum concentrations of 45 ng [Asp3]MC-RR/L in lake water in September. Both MC congeners displayed different predominance patterns, suggesting that two different MC-producing species occurred in a time-dependent manner, whereby [Asp3]MC-RR was clearly associated with the Planktothrix spp. bloom. We demonstrate an exclusive transfer of MC-LR, but not [Asp3]MC-RR, from phytoplankton into zooplankton reaching a 10-fold bioconcentration, yet complete absence of these MC congeners or their conjugates in aquatic insects. The latter demonstrated a limited trophic transfer of MCs from zooplankton to zooplanktivorous insect larvae (e.g., Chaoborus), or direct transfer into other aquatic insects (e.g. Chironomidae and Trichoptera), whether due to avoidance or limited uptake and/or rapid excretion of MCs by higher trophic emergent aquatic insects