Phagocytosis and self-destruction break down dendrites of Drosophila sensory neurons at distinct steps of Wallerian degeneration

Abstract

After injury, severed dendrites and axons expose the “eat-me” signal phosphatidylserine (PS) on their surface while they break down. The degeneration of injured axons is controlled by a conserved Wallerian degeneration (WD) pathway, which is thought to activate neurite self-destruction through Sarm-mediated nicotinamide adenine dinucleotide (NAD+) depletion. While neurite PS exposure is known to be affected by genetic manipulations of NAD+, how the WD pathway coordinates both neurite PS exposure and self-destruction and whether PS-induced phagocytosis contributes to neurite breakdown in vivo remain unknown. Here, we show that in Drosophila sensory dendrites, PS exposure and self-destruction are two sequential steps of WD resulting from Sarm activation. Surprisingly, phagocytosis is the main driver of dendrite degeneration induced by both genetic NAD+ disruptions and injury. However, unlike neuronal Nmnat loss, which triggers PS exposure only and results in phagocytosis-dependent dendrite degeneration, injury activates both PS exposure and self-destruction as two redundant means of dendrite degeneration. Furthermore, the axon-death factor Axed is only partially required for self-destruction of injured dendrites, acting in parallel with PS-induced phagocytosis. Lastly, injured dendrites exhibit a unique rhythmic calcium-flashing that correlates with WD. Therefore, both NAD+-related general mechanisms and dendrite-specific programs govern PS exposure and self-destruction in injury-induced dendrite degeneration in vivo.

TheDrosophilachemokine-like Orion bridges phosphatidylserine and Draper in phagocytosis of neurons

“…The engulfment of injured dendrites by epidermal cells is mediated by PS exposure on dendrites (Ji et al, 2022; Sapar et al, 2018). To test directly if orion is required for PS exposure- induced phagocytosis, we examined two additional paradigms in which PS exposure can be more reliably induced and studied.…”

“…The loss of Nmnat in C4da neurons results in PS exposure and phagocytosis-dependent dendrite degeneration, likely due to NAD + reduction (Ji et al, 2022). Nmnat KO neurons lost most of their dendrites and showed widespread debris in epidermal cells at 120 hrs after egg laying (AEL) (Ji et al, 2022)(Figures 1I, IK, and 1L). In contrast, resembling drpr loss-of-function (LOF) (Ji et al, 2022), orion 1 hemizygosity completely rescued the dendrite degeneration of Nmnat KO neurons, as indicated by the absence of dendrite debris (Figures 1J-IK).…”

“…Because drpr is the only other gene known to be required for epidermal engulfment of PS- exposing dendrites (Ji et al, 2022; Sapar et al, 2018), we wondered if Orion and Drpr act in the same pathway. We found that orion 1 hemizygous and drpr mutant larvae exhibited similar degrees of near complete phagocytosis deficiency in injury-induced dendrite degeneration, as indicated by the portion of unengulfed MApHS-labeled debris (Figures 4A- 4B).…”

“…In this study, we examined Orion’s function in the phagocytosis of Drosophila class IV dendritic arborization (C4da) neurons, a well-established in vivo model for studying PS-mediated phagocytosis (Han et al, 2014; Ji et al, 2022; Sapar et al, 2018). C4da neurons grow elaborate sensory dendrites on larval epidermal cells (Han et al, 2012), which act as the primary phagocytes during dendrite degeneration and remodeling (Han et al, 2014).…”

“…Drpr is involved in the clearance of apoptotic neurons during embryonic development (Freeman et al, 2003; Tung et al, 2013), axon and dendrite pruning during neuronal remodeling (Awasaki et al, 2006; Williams et al, 2006), injury-induced neurite degeneration (MacDonald et al, 2006; Tao and Rolls, 2011), and removal of destabilized boutons at neuromuscular junctions (Fuentes-Medel et al, 2009). Neurons with ectopic PS exposure are engulfed by phagocytes in a Drpr-dependent manner (Ji et al, 2022; Sapar et al, 2018), suggesting that Drpr is critical for PS sensing. However, how Drpr recognizes PS is unclear.…”