The prion protein regulates glutamate-mediated Ca2+ entry and mitochondrial Ca2+ accumulation in neurons

Abstract

The cellular prion protein (PrP) whose conformational misfolding leads to the production of deadly prions, has a still-unclarified cellular function despite decades of intensive research. Following our recent finding that PrP limits Ca entry via store-operated Ca channels in neurons, we investigated whether the protein could also control the activity of ionotropic glutamate receptors (iGluRs). To this end, we compared local Ca movements in primary cerebellar granule neurons and cortical neurons transduced with genetically encoded Ca probes and expressing, or not expressing, PrP Our investigation demonstrated that PrP downregulates Ca entry through each specific agonist-stimulated iGluR and after stimulation by glutamate. We found that, although PrP-knockout (KO) mitochondria were displaced from the plasma membrane, glutamate addition resulted in a higher mitochondrial Ca uptake in PrP-KO neurons than in their PrP-expressing counterpart. This was because the increased Ca entry through iGluRs in PrP-KO neurons led to a parallel increase in Ca-induced Ca release via ryanodine receptor channels. These data thus suggest that PrP takes part in the cell apparatus controlling Ca homeostasis, and that PrP is involved in protecting neurons from toxic Ca overloads.

Neuronal pathophysiology featuring PrPC and its control over Ca2+ metabolism

“…Ser845 is phosphorylated by protein kinase A (PKA) [30], whose activity depends on the interplay between the formation and hydrolysis of cyclic adenosine monophosphate (cAMP). In line with our expectations, extracellular signal-regulated kinase 1 and 2, which potentiates PKA activity by its inhibitory phosphorylation of (cAMP-hydrolyzing) phosphodiesterase [31], was less active (»50%) in PrP C -expressing than in PrP-KO CGN [24]. This finding adds a further molecular tile for explaining why the surface expression of AMPAR GluR1 was diminished in the presence of PrP C .…”

“…Our findings corroborated in these neurons what had been previously observed for hippocampal NMDAR activity. However, data showing that PrP C equally downregulated AMPAR-and kainateR-mediated Ca 2+ entry suggested a much wider action of PrP C in diminishing neuronal vulnerability by glutamate over-stimulation [24], in accord with results obtained in PrP-KO animals exposed to different excitotoxic insults [25][26][27][28][29].…”

“…Specifically, by controlling PM pathways PrP C contributes to prevent potentially noxious high Ca 2+ entry [27]. Intriguingly, however, we also found that, despite PrP C sits on the cell surface, its beneficial action strategically extends beyond the PM by protecting mitochondria from an equally deleterious Ca 2+ overload [24].…”

“…As to the mechanism allowing PrP C to regulate AMPAR-dependent Ca 2+ entry, we focused on AMPAR trafficking based on the notion that regulation of this process goes through Ser845 phosphorylation on the GluR1 subunit [30]. Surface biotinylation of CGN with the two PrP genotypes clearly demonstrated that the surface expression of GluR1 was more abundant (»70%) in PrP-KO neurons than in PrP C -expressing counterparts, a result highly suggestive that the diminished presence of the receptor in PrP C -expressing CGNs could have contributed to the restricted AMPAR-mediated Ca 2+ entry [24]. Ser845 is phosphorylated by protein kinase A (PKA) [30], whose activity depends on the interplay between the formation and hydrolysis of cyclic adenosine monophosphate (cAMP).…”

“…However, if this displacement protected PrP-KO mitochondria from increased Ca 2+ uptake following the selective stimulation of a single iGluR sub-type, this behavior no longer held when neurons were exposed to glutamate. Such an apparent paradox was eventually resolved by demonstrating that the higher glutamate-mediated Ca 2+ influx in PrP-KO neurons was capable to better stimulate the process of Ca 2+ -induced Ca 2+ release via ryanodine receptor channels [10], which, in turn, remarkably increased mitochondrial Ca 2+ uptake [24]. This set of data suggests that (at least in CGN) the protection by PrP C against Ca 2+ overload is integrated towards a few regions of the cell, which may have a crucial relevance under pathologic settings.…”