Significant Functional Differences Between Dopamine D4 Receptor Polymorphic Variants Upon Heteromerization with α1A Adrenoreceptors.

The functional role of the dopamine D4 receptor (D4R) and its main polymorphic variants has become more evident with the demonstration of heteromers of D4R that control the function of frontal cortico-striatal neurons. Those include heteromers with the α2A adrenoceptor (α2AR) and with the D2R, localized in their cortical somato-dendritic region and striatal nerve terminals, respectively. By using biophysical and cell-signaling methods and heteromer-disrupting peptides in mammalian transfected cells and rat brain slice preparations, here we provide evidence for a new functionally relevant D4R heteromer, the α1AR-D4R heteromer, which is also preferentially localized in cortico-striatal glutamatergic terminals. Significant differences in allosteric modulations between heteromers of α1AR with the D4.4R and D4.7R polymorphic variants could be evidenced with the analysis of G protein-dependent and independent signaling. Similar negative allosteric modulations between α1AR and D4R ligands could be demonstrated for both α1AR-D4.4R and α1AR-D4.7R heteromers on G protein-independent signaling, but only for α1AR-D4.4R on G protein-dependent signaling. From these functional differences, it is proposed that the D4.4R variant provides a gain of function of the α1AR-mediated noradrenergic stimulatory control of cortico-striatal glutamatergic neurotransmission, which could result in a decrease in the vulnerability for impulse control-related neuropsychiatric disorders and increase in the vulnerability for posttraumatic stress disorder.

Heteromerization between α2A adrenoceptors and different polymorphic variants of the dopamine D4 receptor determines pharmacological and functional differences. Implications for impulsive-control disorders.

Polymorphic alleles of the human dopamine D4 receptor gene (DRD4) have been consistently associated with individual differences in personality traits and neuropsychiatric disorders, particularly between the gene encoding dopamine D4.7 receptor variant and attention deficit hyperactivity disorder (ADHD). The α2A adrenoceptor gene has also been associated with ADHD. In fact, drugs targeting the α2A adrenoceptor (α2AR), such as guanfacine, are commonly used in ADHD treatment. In view of the involvement of dopamine D4 receptor (D4R) and α2AR in ADHD and impulsivity, their concurrent localization in cortical pyramidal neurons and the demonstrated ability of D4R to form functional heteromers with other G protein-coupled receptors, in this study we evaluate whether the α2AR forms functional heteromers with D4R and weather these heteromers show different properties depending on the D4R variant involved. Using cortical brain slices from hD4.7R knock-in and wild-type mice, here, we demonstrate that α2AR and D4R heteromerize and constitute a significant functional population of cortical α2AR and D4R. Moreover, in cortical slices from wild-type mice and in cells transfected with α2AR and D4.4R, we detect a negative crosstalk within the heteromer. This negative crosstalk is lost in cortex from hD4.7R knock-in mice and in cells expressing the D4.7R polymorphic variant. We also show a lack of efficacy of D4R ligands to promote G protein activation and signaling only within the α2AR-D4.7R heteromer. Taken together, our results suggest that α2AR-D4R heteromers play a pivotal role in catecholaminergic signaling in the brain cortex and are likely targets for ADHD pharmacotherapy.

Therapeutic targeting of HER2-CB2R heteromers in HER2-positive breast cancer.

Although human epidermal growth factor receptor 2 (HER2)-targeted therapies have dramatically improved the clinical outcome of HER2-positive breast cancer patients, innate and acquired resistance remains an important clinical challenge. New therapeutic approaches and diagnostic tools for identification, stratification, and treatment of patients at higher risk of resistance and recurrence are therefore warranted. Here, we unveil a mechanism controlling the oncogenic activity of HER2: heteromerization with the cannabinoid receptor CB2R. We show that HER2 physically interacts with CB2R in breast cancer cells, and that the expression of these heteromers correlates with poor patient prognosis. The cannabinoid Δ9-tetrahydrocannabinol (THC) disrupts HER2-CB2R complexes by selectively binding to CB2R, which leads to (i) the inactivation of HER2 through disruption of HER2-HER2 homodimers, and (ii) the subsequent degradation of HER2 by the proteasome via the E3 ligase c-CBL. This in turn triggers antitumor responses in vitro and in vivo. Selective targeting of CB2R transmembrane region 5 mimicked THC effects. Together, these findings define HER2-CB2R heteromers as new potential targets for antitumor therapies and biomarkers with prognostic value in HER2-positive breast cancer.