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Inhibition of microtubule formation by metabotropic glutamate receptors
1Faculty of Pharmacy and Department of Pharmacology, University of Toronto, Ontario, Canada.
Abstract:
Activation of glutamate receptors is known to alter the biophysical state of the cytoskeleton of neurons in the developing brain. In this study, we examined the ability of G protein-coupled metabotropic glutamate receptors (mGluRs) to inhibit the formation of processes induced by the expression of the microtubule-associated protein MAP2c. The infection of insect MG-1 cells with a recombinant baculovirus (BV) encoding MAP2c induced the formation of fine filamentous processes. The binding of MAPs to tubulin promotes tubulin polymerization and the formation of microtubules. Co-infection with BVs for the phosphoinositide (PI)-linked mGluR1a or mGluR1b receptor subtypes inhibited the formation of processes induced by MAP2c, whereas co-infection with BVs encoding the mGluR4a or mGluR4b subtypes that couple to adenylyl cyclase did not inhibit the formation of processes. The biochemical pathways responsible for producing the inhibitory effect of mGluR1 were investigated. Inhibitors of protein kinase C, calcium/calmodulin-dependent kinase, and protein tyrosine kinases did not block the inhibitory effect of mGluR1a. The calcium chelator BAPTA and the calcium depletor thapsigargin also did not affect the ability of mGluR1a to inhibit process formation. In contrast, inhibitors of phospholipase C reversed the effect of mGluR1 on process formation, suggesting that one or more metabolites in the PI pathway were responsible for the inhibitory effect. These findings indicate that PIs generated by activation of mGluRs inhibit the binding of MAPs to tubulin and reduce tubulin polymerization and microtubule stability.
Insights
Metabotropic glutamate receptors (mGluRs) linked to phosphoinositide pathways can inhibit neuronal process formation. This inhibition occurs by affecting microtubule-associated proteins (MAPs) and tubulin polymerization, crucial for cytoskeletal development.
Area of Science:
- Neuroscience
- Cell Biology
- Molecular Biology
Background:
- Glutamate receptor activation influences neuronal cytoskeleton dynamics.
- Microtubule-associated protein MAP2c expression induces filamentous process formation in insect cells.
- Metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors involved in neuronal signaling.
Purpose of the Study:
- To investigate the inhibitory effects of specific mGluR subtypes on MAP2c-induced process formation.
- To elucidate the biochemical pathways mediating the inhibitory action of mGluR1.
Main Methods:
- Utilized recombinant baculovirus to express MAP2c and mGluR subtypes in insect MG-1 cells.
- Assessed process formation in response to co-expression of MAP2c with different mGluRs.
- Employed biochemical inhibitors and pathway modulators (e.g., phospholipase C inhibitors, calcium chelators) to dissect signaling mechanisms.
Main Results:
- Phosphoinositide (PI)-linked mGluR1a and mGluR1b inhibited MAP2c-induced process formation.
- Adenylyl cyclase-coupled mGluR4a and mGluR4b did not show inhibitory effects.
- Inhibition by mGluR1 was reversed by phospholipase C inhibitors, implicating the PI pathway.
- Protein kinase C, calcium/calmodulin-dependent kinase, and calcium signaling pathways were not involved.
Conclusions:
- Activation of PI-linked mGluRs inhibits neuronal process formation.
- The mechanism involves inhibition of MAPs binding to tubulin, reducing tubulin polymerization and microtubule stability.
- This highlights a novel role for mGluRs in regulating cytoskeletal dynamics via the PI pathway.