Phosphorylation of unique C‐terminal sites of the mu‐opioid receptor variants 1B2 and 1C1 influences their Gs association following chronic morphine

Abstract

We recently demonstrated in rat spinal cord that a regimen of escalating doses of systemic morphine, analogous to regimens used clinically for chronic pain management, selectively up‐regulates the mu‐opioid receptor (MOR) splice variants MOR‐1B2 and MOR‐1C1 mRNA and functional protein. This study investigated the potential relevance of up‐regulating MOR‐1B2 and MOR‐1C1 to the ability of chronic morphine to shift MOR signaling from predominantly Gi/Go inhibitory to Gs stimulatory. Specifically, we tested the hypotheses that chronic morphine induces phosphorylation of carboxyl terminal sites unique to MOR‐1B2 and MOR‐1C1, and that this phosphorylation is causally related to augmented association of these variants with Gsα. Hypotheses were validated by (i) abolition of the chronic morphine‐induced increment in MOR‐1C1 and MOR‐1B2 association with Gsα by inhibitors of protein kinase A and Casein kinase 2, respectively; (ii) failure of chronic morphine to augment MOR variant Gsα interactions in Chinese hamster ovary cells transiently transfected with either rat MOR‐1C1 or MOR‐1B2 in which targeted protein kinase A and Casein kinase 2 serine phosphorylation sites, respectively, were mutated to alanine; (iii) abrogation of chronic morphine‐induced augmented MOR Gsα association in spinal cord of male rats following intrathecal administration of dicer substrate small interfering RNAs targeting MOR‐1B2/MOR‐1C1 mRNA. The ability of chronic morphine to not only up‐regulate‐specific MOR variants but also their carboxyl terminal phosphorylation and consequent augmented association with Gsα may represent a novel component of opioid tolerance mechanisms, suggesting novel potential targets for tolerance abatement.

Molecular mechanisms of opioid tolerance: From opioid receptors to inflammatory mediators (Review)

“…Within-system drug tolerance occurs when opposite reactions are elicited within the same system. A recent study has indicated that chronic morphine use induced a shift in MOR signalling from the predominantly inhibitory Gi/Go adenylyl cyclase to the stimulatory Gs adenylyl cyclase through the upregulation of different MOR receptor variants ( 68 ). Specifically, chronic morphine use induced the phosphorylation of the carboxyl terminal sites on MOR-1B2 and MOR-1C1, enhancing Gs protein association ( 68 ).…”

“…A recent study has indicated that chronic morphine use induced a shift in MOR signalling from the predominantly inhibitory Gi/Go adenylyl cyclase to the stimulatory Gs adenylyl cyclase through the upregulation of different MOR receptor variants ( 68 ). Specifically, chronic morphine use induced the phosphorylation of the carboxyl terminal sites on MOR-1B2 and MOR-1C1, enhancing Gs protein association ( 68 ). In between-system adaptation, different drug-sensitive systems are linked to the drugs' primary action system.…”

Brief Overview about Opioid Use Disorder: Review Article

“…Recent research has shown that chronic morphine use leads to an increase in the expression of a subset of the MOR receptors that signal via the stimulatory Gs adenylyl cyclase rather than the predominantly inhibitory Gi/Go adenylyl cyclase. In particular, MOR-1B2 and MOR-1C1 had their carboxyl-terminal regions phosphorylated due to chronic morphine usage, which increased their interaction with Gs proteins (10) .…”

Casein Kinase 2 Mediates HIV- and Opioid-Induced Pathologic Phosphorylation of TAR DNA Binding Protein 43 in the Basal Ganglia

“…There are also factors unrelated to HIV that can worsen neuroHIV pathology. One such comorbid insult is opioid use disorder (OUD) (Fitting et al, 2010(Fitting et al, , 2020Hauser et al, 2006;Murphy et al, 2019), which can by itself dysregulate TDP-43 kinase (Chakrabarti et al, 2020;Wager et al, 2014). Hence, we also evaluated the impact of morphine, a prototypical opioid of abuse/misuse, in some outcome measures.…”