Invasion of microglia/macrophages and granulocytes into the Mauthner axon myelin sheath following spinal cord injury of the adult goldfish, Carassius auratus

Abstract

Rapid activation of resident glia occurs after spinal cord injury. Somewhat later, innate and adaptive immune responses occur with the invasion of peripheral immune cells into the wound site. The activation of resident and peripheral immune cells has been postulated to play harmful as well as beneficial roles in the regenerative process. Mauthner cells, large identifiable neurons located in the hindbrain of most fish and amphibians, provided the opportunity to study the morphological relationship between reactive cells and Mauthner axons (M‐axons) severed by spinal cord crush or by selective axotomy. After crossing in the hindbrain, the M‐axons of adult goldfish, Carassius auratus, extend the length of the spinal cord. Following injury, the M‐axon undergoes retrograde degeneration within its myelin sheath creating an axon‐free zone (proximal dieback zone). Reactive cells invade the wound site, enter the axon‐free dieback zone and are observed in the vicinity of the retracted M‐axon tip as early as 3 hr postinjury. Transmission electron microscopy allowed the detection of microglia/macrophages and granulocytes, some of which appear to be neutrophil‐like, at each of these locations. We believe that this is the first report of the invasion of such cells within the myelin sheath of an identifiable axon in the vertebrate central nervous system (CNS). We speculate that microglia/macrophages and granulocytes that are attracted within a few hours to the damaged M‐axon are part of an inflammatory response that allows phagocytosis of debris and plays a role in the regenerative process. Our results provide the baseline from which to utilize immunohistochemical and genetic approaches to elucidate the role of non‐neuronal cells in the regenerative process of a single axon in the vertebrate CNS.

Bone Marrow Mesenchymal Stem Cell-Derived Exosomes Accelerate Functional Recovery After Spinal Cord Injury by Promoting the Phagocytosis of Macrophages to Clean Myelin Debris

“…Acute spinal cord trauma damages the vasculature, leading to rupture of the blood–brain barrier, induces acute mechanical compression of the myelin sheath, generates myelin debris, and triggers a cascade of pathological processes, including immune cell infiltration and activation ( Kopper and Gensel, 2018 ; Koganti et al, 2020 ). The sustained presence of myelin debris at the injury site of the spinal cord leads to neurological functional recovery failure because the accumulation of myelin debris containing inhibitor molecules resists axon regeneration ( Cunha et al, 2020 ; Liu Y. et al, 2021 ).…”

“…Targeting the phagocytic capacity of macrophages could be a potential therapeutic strategy for SCI treatment. A few transcriptome analysis studies demonstrated that microglial cells could express high levels of certain specific genes, including Sall-1, Siglec-H, TMEM119, and P2RY12, which could be considered potential microglia-specific markers ( Doring et al, 2015 ; Guo et al, 2016 ; Kucharova and Stallcup, 2017 ; Koganti et al, 2020 ). However, there is still no widely accepted cell marker to distinguish between BMDMs and microglia in the CNS, which needs further exploration.…”

Survival and Axonal Outgrowth of the Mauthner Cell Following Spinal Cord Crush Does Not Drive Post-injury Startle Responses

“…The tissue was then dehydrated, and cleared in methyl salicylate before observing the whole brain ("brain wholemount") under the fluorescent microscope. An example of filled, uninjured M-axons in the cleared brain wholemounts can be seen in Figure 1 of Koganti et al (2020). Occasionally other axons were inadvertently filled with dye.…”

“…This technique was used prior to Lucifer yellow injections of the proximal and distal segments of the M-cell (see Figure 9). A more detailed account of this technique can be found in Koganti et al (2020).…”

“…M-axons in adult fish have been shown to traverse a spinal cord wound in some studies (Becker et al, 1998;Becker and Becker, 2001) but not in others (Sharma et al, 1993;Becker et al, 1997). Intracellular labeling of M-axons in the adult goldfish damaged by spinal cord crush initiate sprouting in a few days (Koganti et al, 2020) but choose aberrant pathways between 30 and 42 days postoperatively (Zottoli et al, 1994;Zottoli and Faber, 2000). Little is known whether these sprouts are maintained or re-routed and form functional synapses over long post-operative intervals.…”