No abstract

To delineate the structural determinants involved in the constitutive activation of the D1 receptor subtypes, we have constructed chimeras between the D1A and D1B receptors. These chimeras harbored a cognate domain corresponding to transmembrane regions 6 and 7 as well as the third extracellular loop (EL3) and cytoplasmic tail, a domain referred herein to as the terminal receptor locus (TRL). A chimeric D1A receptor harboring the D1B-TRL (chimera 1) displays an increased affinity for dopamine that is indistinguishable from the wild-type D1B receptor. Likewise, a chimeric D1B receptor containing the D1A-TRL cassette (chimera 2) binds dopamine with a reduced affinity that is highly reminiscent of the dopamine affinity for the wild-type D1A receptor. Furthermore, we show that the agonist independent activity of chimera 1 is identical to the wildtype D1B receptor whereas the chimera 2 displays a low agonist independent activity that is indistinguishable from the wild-type D1A receptor. Dopamine potencies for the wild-type D1A and D1B receptor were recapitulated in cells expressing the chimera 2 or chimera 1, respectively. However, the differences observed in agonist-mediated maximal activation of adenylyl cyclase elicited by the D1A and D1B receptors remain unchanged in cells expressing the chimeric receptors. To gain further mechanistic insights into the structural determinants of the TRL involved in the activation properties of the D1 receptor subtypes, we have engineered two additional chimeric D1 receptors that contain the EL3 region of their respective cognate wild-type counterparts (hD1A-EL3B and hD1B-EL3A). In marked contrast to chimera 1 and 2, dopamine affinity and constitutive activation were partially modulated by the exchange of the EL3. Meanwhile, hD1A-EL3B and hD1B-EL3A mutant receptors display a full switch in the agonist-mediated maximal activation, which is reminiscent of their cognate wild-type counterparts. Overall, our studies suggest a fundamental role for the TRL in shaping the intramolecular interactions implicated in the constitutive activation and coupling properties of the dopamine D1 receptor subtypes.The classical paradigm for G protein-coupled receptor (GPCR) 1 activation is described by the binding of an agonist to an inactive receptor state (R). This process leads to the formation of an active receptor state (R*) which interacts with heterotrimeric GTP-binding proteins (G proteins) to initiate a variety of intracellular signaling events. In recent years, however, mutagenesis studies have led to a notion asserting that GPCRs exist in an equilibrium between two interchangeable conformational states, R and R* (1-3). In the absence of ligand (agonist), GPCRs are predominantly maintained in an inactive R state by intramolecular constraints that prohibit the interaction with G proteins. These intramolecular constraints are released upon agonist binding or by mutations. Indeed, mutations in the carboxyl end of the third cytoplasmic loop of GPCRs can result in mutant receptors displaying high ...

Men infected with SARS‐CoV‐2 are more likely to be admitted to the intensive care unit (ICU) compared to women. 1 Previously, we have reported that among hospitalized men with COVID‐19, 79% presented with androgenetic alopecia (AA) compared to 31‐53% that would be expected in a similar aged match population. 2 AA is known to be mediated by variations in the androgen receptor (AR) gene. 3 In addition, the only known promoter of the enzyme implicated in SARS‐CoV‐2 infectivity, TMPRSS2, is regulated by an androgen response element. 4 The polyglutamine repeat (CAG repeat) located in the AR gene is associated with androgen sensitivity and AA. 3 These observations led us to hypothesize that variations in the AR gene may predispose male COVID‐19 patients to increased disease severity.