PKC epsilon‐dependent calcium oscillations associated with metabotropic glutamate receptor 5 prevent agonist‐mediated receptor desensitization in astrocytes

Abstract

A critical role has been assigned to protein kinase C (PKC)ε in the control of intracellular calcium oscillations triggered upon activation of type 5 metabotropic glutamate receptor (mGluR5) in cultured astrocytes. Nevertheless, the physiological significance of this particular signalling profile in the response of astrocytes to glutamate remains largely unknown. Considering that kinases are frequently involved in the regulation of G protein-coupled receptors, we have examined a putative link between the nature of the calcium signals and the response regulation upon repeated exposures of astrocytes to the agonist (S)-3,5-dihydroxyphenylglycine. We show that upon repeated mGluR5 activations, a robust desensitization was observed in astrocytes grown in culture conditions favouring the peak-plateau-type response. At variance, in cell cultures where calcium oscillations were predominating, the response was fully preserved even during repeated challenges with the agonist. Pharmacological inhibition of PKCε or genetic suppression of this isoform using shRNA was found to convert an oscillatory calcium profile to a sustained calcium mobilization and this latter profile was subject to desensitization upon repetitive mGluR5 activation. Our results suggest a yet undocumented scheme in which the activity of PKCε contributes to preserve the receptor sensitivity upon repeated or sustained activations. Cover Image for this issue: doi: 10.1111/jnc.13797.

Constitutive downregulation protein kinase C epsilon in hSOD1G93A astrocytes influences mGluR5 signaling and the regulation of glutamate uptake

“…Data from our previous studies (Vergouts et al, 2017) and the present observations suggest that PKCE indirectly acts as a key regulator in the maintenance of mGluR5-mediated regulation of glutamate clearance by astroglial transporters. A major observation with implications for future research on ALS is that rescuing PKCE expression in hSOD1 G93A astrocytes permits the restoration of a dynamic mGluR5dependent control of glutamate clearance by these cells.…”

“…Therefore, the expression of PKCE was determined in total cell extracts of astrocytes derived from both WT and hSOD1 G93A rats. As previously shown (Vergouts et al, 2017), pre-treatment of WT astrocytes with the pan-PKC inhibitor Ro 31-8220 considerably reduced the proportion of cells showing Ca 21 oscillations in response to DHPG exposure and almost all cells showed a peak-plateau type response (Figure 2b,c). The implication of reduced PKCE expression in the altered response to DHPG was examined through pharmacological manipulation of its activity.…”

“…3.2 | PKCE activity and expression influence the mGluR5-mediated Ca 21 signaling in astrocytes derived from both WT and hSOD1 G93A rats A direct role of PKCE in the control of mGluR5-mediated Ca 21 oscillations has been described and we have recently confirmed that a reduction of expression in WT astrocytes causes a loss of Ca 21 oscillations in response to DHPG (Bradley & Challiss, 2011;Vergouts et al, 2017). Therefore, the expression of PKCE was determined in total cell extracts of astrocytes derived from both WT and hSOD1 G93A rats.…”

“…In particular, a critical role has been assigned to PKC epsilon (PKCE) in the control of mGluR5mediated Ca 21 oscillations, both in transfected cells and in cultured astrocytes (Bradley & Challiss, 2011). We also identified a role for this PKC isoform and the associated Ca 21 oscillations in the prevention of mGluR5 desensitization upon repeated activation (Vergouts et al, 2017). We also identified a role for this PKC isoform and the associated Ca 21 oscillations in the prevention of mGluR5 desensitization upon repeated activation (Vergouts et al, 2017).…”

“…Using diverse pharmacological inhibitors or genetic tools, the authors have demonstrated that expression of PKCE and its activity are essential for the maintenance of mGluR5-mediated Ca 21 oscillations. We also identified a role for this PKC isoform and the associated Ca 21 oscillations in the prevention of mGluR5 desensitization upon repeated activation (Vergouts et al, 2017). Considering the importance of mGluR5 in the control of astrocytic functions, alteration in its signaling through molecular partners, such as PKCE could indirectly contribute to an impairment of neuroprotective support, as observed in ALS.…”