Transthyretin Internalization by Sensory Neurons Is Megalin Mediated and Necessary for Its Neuritogenic Activity

Abstract

Mutated transthyretin (TTR) causes familial amyloid polyneuropathy, a neurodegenerative disorder characterized by TTR deposition in the peripheral nervous system (PNS). The origin/reason for TTR deposition in the nerve is unknown. Here we demonstrate that both endogenous mouse TTR and TTR injected intravenously have access to the mouse sciatic nerve. We previously determined that in the absence of TTR, both neurite outgrowth in vitro and nerve regeneration in vivo were impaired. Reinforcing this finding, we now show that local TTR delivery to the crushed sciatic nerve rescues the regeneration phenotype of TTR knock-out (KO) mice. As the absence of TTR was unrelated to neuronal survival, we further evaluated the Schwann cell and inflammatory response to injury, as well as axonal retrograde transport, in the presence/absence of TTR. Only retrograde transport was impaired in TTR KO mice which, in addition to the neurite outgrowth impairment, might account for the decreased regeneration in this strain. Moreover, we show that in vitro, in dorsal root ganglia neurons, clathrin-dependent megalin-mediated TTR internalization is needed for TTR neuritogenic activity. Supporting this observation, we demonstrate that in vivo, decreased levels of megalin lead to decreased nerve regeneration and that megalin's action as a regeneration enhancer is dependent on TTR. In conclusion, our work unravels the mechanism of TTR action during nerve regeneration. Additionally, TTR presence in the nerve, as is here shown, may underlie its preferential deposition in the PNS of familial amyloid polyneuropathy patients.

Aboard transthyretin: From transport to cleavage

“…Recently, megalin-TTR interaction was also described in sensory neurons where TTR effect in neurite outgrowth and nerve regeneration was shown to be mediated by megalindependent internalization, further supporting the relevance of this interaction (35).…”

“…Although no further study described the impact of TTR-MT interaction, more common features of the proteins were identified. TTR has neuritogenic activity dependent on megalin-mediated internalization (35). Interestingly, MT stimulation of neurite outgrowth is also mediated by megalin (47).…”

Transthyretin binds to glucose-regulated proteins and is subjected to endocytosis by the pancreatic β-cell

“…Membrane associated proteins of 90, 100, and 115 kDa have earlier been proposed as receptor proteins involved in endocytosis of TTR, although they have not been identified [8][9][10]15]. Megalin is a member of the low-density lipoprotein receptor family [16] and it has been shown to be involved in renal and sensory neuronal uptake of TTR [17,18]. Megalin is an endocytotic receptor expressed in clathrin-coated pits and binds the receptor-associated protein (RAP) which functions as a chaperone.…”

Fibroblasts endocytose and degrade transthyretin aggregates in transthyretin-related amyloidosis

“…24 Fibroblasts, together with macrophages, also have a central role in degrading and remodelling the ECM. Although, different types of cells reportedly exhibit varying degrees of TTR internalization activity in vitro, [13][14][15] fibroblasts and macrophages have higher migratory potential and more accessibility to TTR deposits in ECM. Previous ultrastructural analyses revealed amyloid deposits at the outer layer of the basement membrane, which occurs at the base of all epithelial linings, underlying endothelia of capillaries and venules, and around Schwann cells, adipocytes, and muscle cells.…”

“…12 Previous in vitro studies showed that WT and mutant TTR are internalized by different types of cells, including hepatocytes, mouse embryonic fibroblasts, yolk sac cells, and sensory neurons. [13][14][15] Whether these cells could endocytose and degrade TTR aggregates in vivo needs investigation.…”