Chapter 17: Transthyretin: an enhancer of nerve regeneration.

Transthyretin (TTR), a plasma and cerebrospinal fluid protein secreted by the liver and choroid plexus, is mainly known as the physiological carrier of thyroxine (T(4)) and retinol. Under pathological conditions, various TTR mutations are related to familial amyloid polyneuropathy (FAP), a neurodegenerative disorder characterized by deposition of TTR amyloid fibrils, particularly in the peripheral nervous system (PNS), leading to axonal loss and neuronal death. Recently, a number of TTR functions in neurobiology have been described; these may explain the preferential TTR deposition, when mutated, in the PNS of FAP patients. In this respect, and with a particular relevance in the PNS, TTR has been shown to have the ability to enhance neurite outgrowth in vitro and nerve regeneration following injury, in vivo. In the following pages, this novel TTR function, as well as its importance in nerve biology and repair will be discussed.

Transthyretin internalization by sensory neurons is megalin mediated and necessary for its neuritogenic activity.

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.

Substrate specificity of transthyretin: identification of natural substrates in the nervous system.

Besides functioning as the plasma transporter of retinol and thyroxine, TTR (transthyretin) is a protease, cleaving apoA-I (apolipoprotein A-I) after a phenylalanine residue. In the present study, we further investigated TTR substrate specificity. By using both P-diverse libraries and a library of phosphonate inhibitors, a TTR preference for a lysine residue in P1 was determined, suggesting that TTR might have a dual specificity and that, in addition to apoA-I, other TTR substrates might exist. Previous studies revealed that TTR is involved in the homoeostasis of the nervous system, as it participates in neuropeptide maturation and enhances nerve regeneration. We investigated whether TTR proteolytic activity is involved in these functions. Both wild-type TTR and TTR(prot-) (proteolytically inactive TTR) had a similar effect in the expression of peptidylglycine alpha-amidating mono-oxygenase, the rate-limiting enzyme in neuropeptide amidation, excluding the involvement of TTR proteolytic activity in neuropeptide maturation. However, TTR was able to cleave amidated NPY (neuropeptide Y), probably contributing to the increased NPY levels reported in TTR-knockout mice. To assess the involvement of TTR proteolytic activity in axonal regeneration, neurite outgrowth of cells cultivated with wild-type TTR or TTR(prot-), was measured. Cells grown with TTR(prot-) displayed decreased neurite length, thereby suggesting that TTR proteolytic activity is important for its function as a regeneration enhancer. By showing that TTR is able to cleave NPY and that its proteolytic activity affects axonal growth, the present study shows that TTR has natural substrates in the nervous system, establishing further its relevance in neurobiology.

Transthyretin enhances nerve regeneration.

Mutations in transthyretin (TTR) are associated with familial amyloid polyneuropathy, a neurodegenerative disorder characterized by TTR deposition in the PNS. The aim of this study was to unravel whether TTR has a role in nerve physiology that could account for its preferential accumulation in the PNS, when mutated. The sensorimotor performance of wild-type and TTR knockout (KO) littermate mice was compared and showed impairment in mice lacking TTR. Given the possibility that, upon regeneration, the consequences arising from TTR absence might be exacerbated, nerve crush was performed in both strains. TTR KO mice presented delayed functional recovery resulting from decreased number of myelinated and unmyelinated fibers. Moreover, in transgenic mice in a TTR KO background, expressing human TTR in neurons, this phenotype was rescued, reinforcing that TTR enhances nerve regeneration. In vitro assays showed that neurite outgrowth and extension were decreased in the absence of TTR, probably underlying the decreased number of regenerating axons in TTR KO mice. Our findings demonstrate that TTR participates in nerve physiology and that it enhances nerve regeneration. Moreover, the assignment of a TTR function in nerve biology and repair, may explain its preferential deposition, when mutated, in the PNS of familial amyloid polyneuropathy patients.