Meningeal Foam Cells and Ependymal Cells in Axolotl Spinal Cord Regeneration

Abstract

A previously unreported population of foam cells (foamy macrophages) accumulates in the invasive fibrotic meninges during gap regeneration of transected adult Axolotl spinal cord (salamander Ambystoma mexicanum) and may act beneficially. Multinucleated giant cells (MNGCs) also occurred in the fibrotic meninges. Actin-label localization and transmission electron microscopy showed characteristic foam cell and MNGC podosome and ruffled border-containing sealing ring structures involved in substratum attachment, with characteristic intermediate filament accumulations surrounding nuclei. These cells co-localized with regenerating cord ependymal cell (ependymoglial) outgrowth. Phase contrast-bright droplets labeled with Oil Red O, DiI, and DyRect polar lipid live cell label showed accumulated foamy macrophages to be heavily lipid-laden, while reactive ependymoglia contained smaller lipid droplets. Both cell types contained both neutral and polar lipids in lipid droplets. Foamy macrophages and ependymoglia expressed the lipid scavenger receptor CD36 (fatty acid translocase) and the co-transporter toll-like receptor-4 (TLR4). Competitive inhibitor treatment using the modified fatty acid Sulfo-N-succinimidyl Oleate verified the role of the lipid scavenger receptor CD36 in lipid uptake studies in vitro. Fluoromyelin staining showed both cell types took up myelin fragments in situ during the regeneration process. Foam cells took up DiI-Ox-LDL and DiI-myelin fragments in vitro while ependymoglia took up only DiI-myelin in vitro. Both cell types expressed the cysteine proteinase cathepsin K, with foam cells sequestering cathepsin K within the sealing ring adjacent to the culture substratum. The two cell types act as sinks for Ox-LDL and myelin fragments within the lesion site, with foamy macrophages showing more Ox-LDL uptake activity. Cathepsin K activity and cellular localization suggested that foamy macrophages digest ECM within reactive meninges, while ependymal cells act from within the spinal cord tissue during outgrowth into the lesion site, acting in complementary fashion. Small MNGCs also expressed lipid transporters and showed cathepsin K activity. Comparison of 3H-glucosamine uptake in ependymal cells and foam cells showed that only ependymal cells produce glycosaminoglycan and proteoglycan-containing ECM, while the cathepsin studies showed both cell types remove ECM. Interaction of foam cells and ependymoglia in vitro supported the dispersion of ependymal outgrowth associated with tissue reconstruction in Axolotl spinal cord regeneration.

Downregulation of Yap1 During Limb Regeneration Results in Defective Bone Formation in Axolotl

“…Enrichment of the BPs related to negative regulation of peptidase activity by the upregulated genes in Yap1_KD samples was noteworthy. Our results are in accordance with earlier investigations which showed that peptidase activity is required for an appropriate regeneration process in both invertebrates and vertebrates (Dolmatov et al, 2019;Dong et al, 2021;Enos et al, 2019;Pasten et al, 2012;Rinkevich et al, 2007).…”

Lipid droplets in the nervous system

“…In fact, ependymal cells are the only cell types in the brain that form significant numbers of lipid droplets under nonpathological conditions, albeit at lower levels in young animals and increasing with age ( Capilla-Gonzalez et al, 2014 ; Bouab et al, 2011 ). Internalization of CSF lipoprotein particles may contribute to lipid droplet formation in ependymal cells, as they express several lipoprotein receptors including CD36 and low-density lipoprotein receptors LRP1 and LRP2 ( Matsumoto et al, 2015 ; Enos et al, 2019 ; Gajera et al, 2010 ). Consistently, in animals fed a high-fat diet, ependymal cells in the hypothalamus that regulate nutrient intake have more lipid droplets ( Zhang et al, 2013 ; Rawish et al, 2020 ).…”

Pathophysiology of Lipid Droplets in Neuroglia

“…The amount of LDs in ependymal cells, however, increases with age [ 173 , 174 ] and also in animals fed with a high-fat diet [ 175 , 176 ]. Consistent with this, the accumulation of LDs in ependymal cells has been linked to high expression levels of lipid scavenger receptors CD36 and LRP (i.e., LDL receptor-related protein) 1 and 2 [ 177 , 178 , 179 ]. The role of LDs in ependymal cells with respect to brain function still needs to be determined in the future.…”