Metallothionein and a peptide modeled after metallothionein, EmtinB, induce neuronal differentiation and survival through binding to receptors of the low‐density lipoprotein receptor family

Abstract

Accumulating evidence suggests that metallothionein (MT)‐I and ‐II promote neuronal survival and regeneration in vivo. The present study investigated the molecular mechanisms underlying the differentiation and survival‐promoting effects of MT and a peptide modeled after MT, EmtinB. Both MT and EmtinB directly stimulated neurite outgrowth and promoted survival in vitro using primary cultures of cerebellar granule neurons. In addition, expression and surface localization of megalin, a known MT receptor, and the related lipoprotein receptor‐related protein‐1 (LRP) are demonstrated in cerebellar granule neurons. By means of surface plasmon resonance MT and EmtinB were found to bind to both megalin and LRP. The bindings were abrogated in the presence of receptor‐associated protein‐1, an antagonist of the low‐density lipoprotein receptor family, which also inhibited MT‐ and EmtinB‐induced neurite outgrowth and survival. MT‐mediated neurite outgrowth was furthermore inhibited by an anti‐megalin serum. EmtinB‐mediated inhibition of apoptosis occurred without a reduction of caspase‐3 activity, but was associated with reduced expression of the pro‐apoptotic B‐cell leukemia/lymphoma‐2 interacting member of cell death (BimS). Finally, evidence is provided that MT and EmtinB activate extracellular signal‐regulated kinase, protein kinase B, and cAMP response element binding protein. Altogether, these results strongly suggest that MT and EmtinB induce their neuronal effects through direct binding to surface receptors belonging to the low‐density lipoprotein receptor family, such as megalin and LRP, thereby activating signal transduction pathways resulting in neurite outgrowth and survival.

Redefining the Role of Metallothionein within the Injured Brain

“…Taken together, the results support our observations of neuronal MT-I/-II uptake, at least some of which is mediated by the megalin receptor, which can then in turn promote neurite outgrowth. We note that at the time of writing a recent report has been published indicating that interaction with megalin is required for MT to promote neurite outgrowth of cerebellar granule neurons (26), further supporting our hypothesis.…”

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

“…Regarding the nervous system, megalin has been described as an important protein for the development of the forebrain (Willnow et al, 1996;Spoelgen et al, 2005) and spinal cord (Wicher and Aldskogius, 2008). Furthermore, megalin was thought to be expressed exclusively by epithelial cells; however, very recently, several reports have shown that megalin is expressed by other cell types, namely oligodendrocytes (Wicher et al, 2006), astrocytes (Bento-Abreu et al, 2008), and neurons, including retinal ganglion cells (Fitzgerald et al, 2007), cortical neurons (Chung et al, 2008), and cerebellar granule neurons (Ambjørn et al, 2008). We now show that DRG neurons also express megalin and that TTR neuritogenic activity depends on its internalization by this receptor.…”

“…Given that one endocytic TTR receptor, megalin, has been identified, being important for preventing TTR filtration through the glomerulus (Sousa et al, 2000) and since megalin was recently implicated in metallothionein uptake by neurons (Fitzgerald et al, 2007;Ambjørn et al, 2008), we hypothesized that megalin might be involved in the uptake of TTR. To verify whether DRG neurons express megalin, RT-PCR and immunohistochemistry were performed.…”

The Metallothionein/Thionein System: An Oxidoreductive Metabolic Zinc Link

“…If so, this would initiate signal transduction pathways resulting in neurite outgrowth and survival. [87] The neural functions of MT are complicated by the observation that MT-3, which predominates in the central nervous system, is isolated with copper and zinc bound to separate A C H T U N G T R E N N U N G domains. While the precise metal composition of MT-3 in vivo remains to be identified, the protein's rapid-dynamic structure might be central to its various roles in neurodegenerative diseases, in which both zinc and copper have been implicated.…”