Target validation: Weak selectivity of LY341495 for mGluR2 over mGluR4 makes glutamate a less selective agonist

The role of glutamate receptors in the regulation of the tumor microenvironment

Glutamate, as one of the most important carbon sources in the TCA cycle, is central in metabolic processes that will subsequently influence tumor progression. Several factors can affect the expression of glutamate receptors, playing either a tumor-promoting or tumor-suppressor role in cancer. Thus, the activation of glutamate receptors by the ligand could play a role in tumor development as ample studies have demonstrated the expression of glutamate receptors in a broad range of tumor cells. Glutamate and its receptors are involved in the regulation of different immune cells' development and function, as suggested by the receptor expression in immune cells. The activation of glutamate receptors can enhance the effectiveness of the effector's T cells, or decrease the cytokine production in immunosuppressive myeloid-derived suppressor cells, increasing the antitumor immune response. These receptors are essential for the interaction between tumor and immune cells within the tumor microenvironment (TME) and the regulation of antitumor immune responses. Although the role of glutamate in the TCA cycle has been well studied, few studies have deeply investigated the role of glutamate receptors in the regulation of cancer and immune cells within the TME. Here, by a systematic review of the available data, we will critically assess the physiopathological relevance of glutamate receptors in the regulation of cancer and immune cells in the TME and provide some unifying hypotheses for futures research on the role of glutamate receptors in the immune modulation of the tumor.

Expression of group II metabotropic glutamate receptors in rat superior cervical ganglion

BACKGROUND

The superior cervical ganglion (SCG) plays critical roles in the regulation of blood pressure and cardiac output. Metabotropic glutamate receptors (mGluRs) in the SCG are not clearly elucidated yet. Most studies on the expression and functions of mGluRs in the SCG focused on the cultured SCG neurons, and yet little information has been reported in the SCG tissue. Chronic intermittent hypoxia (CIH), one of the major clinical features of obstructive sleep apnea (OSA) patients, is a critical pathological cause of secondary hypertension in OSA patients, but its impact on the level of mGluRs in the SCG is unknown.

OBJECTIVE

To explore the expression and localization of mGluR2/3 and the effect of CIH on mGluR2/3 level in rat SCG tissue.

METHODS

RT-PCR and immunostaining were conducted to examine the mRNA and protein expression of mGluR2/3 in rat SCG. Immunofluorescence staining was conducted to examine the distribution of mGluR2/3. Rats were divided into control and CIH group which the rats were exposed to CIH for 6 weeks. Western blots were performed to examine the level of mGluR2/3 in rat SCG.

RESULTS

mRNAs of mGluR2/3 expressed in rat SCG. mGluR2 distributed in principal neurons and small intensely fluorescent cells but not in satellite glial cells, nerve fibers, and vascular endothelial cells; mGluR3 was detected in nerve fibers rather than in the cells mentioned above. CIH exposure reduced the protein level of mGluR2/3 in rat SCG.

CONCLUSION

mGluR2/3 exists in rat SCG with diverse distribution patterns, and may be involved in CIH-induced hypertension.

The evidence for and consequences of metabotropic glutamate receptor heterodimerization

Metabotropic glutamate receptors (mGluRs) are an essential component of the mammalian central nervous system. These receptors modulate neuronal excitability in response to extracellular glutamate through the activation of intracellular heterotrimeric G proteins. Like most other class C G protein-coupled receptors, mGluRs function as obligate dimer proteins, meaning they need to form dimer complexes before becoming functional receptors. All mGluRs possess the ability to homodimerize, but studies over the past ten years have demonstrated these receptors are also capable of forming heterodimers in specific patterns. These mGluR heterodimers appear to have their own unique biophysical behavior and pharmacology with both native and synthetic compounds with few rules having been identified that allow for prediction of the consequences of any particular mGluR pair forming heterodimers. Here, we review the relevant literature demonstrating the existence and consequences of mGluR heterodimerization. By collecting biophysical and pharmacological data of several mGluR heterodimers we demonstrate the lack of generalizable behavior of these complexes indicating that each individual dimeric pair needs to be investigated independently. Additionally, by combining sequence alignment and structural analysis, we propose that interactions between the β4-A Helix Loop and the D Helix in the extracellular domain of these receptors are the structural components that dictate heterodimerization compatibility. Finally, we discuss the potential implications of mGluR heterodimerization from the viewpoints of further developing our understanding of neuronal physiology and leveraging mGluRs as a therapeutic target for the treatment of pathophysiology.

Negative allosteric modulators of group II metabotropic glutamate receptors: A patent review (2015 - present)

INTRODUCTION

Group II metabotropic glutamate (mGlu) receptors have emerged as an attractive potential target for the development of novel CNS therapeutics in areas such as Alzheimer's disease (AD), anxiety, cognitive disorders, depression, and others. Several small molecules that act as negative allosteric modulators (NAMs) on these receptors have demonstrated efficacy and/or target engagement in animal models, and one molecule (decoglurant) has been advanced into clinical trials.

AREAS COVERED

This review summarizes patent applications published between January 2015 and November 2020. It is divided into three sections: (1) small molecule nonselective mGlu_2/3 NAMs, (2) small molecule selective mGlu₂ NAMs, and (3) small molecule selective mGlu₃ NAMs.

EXPERT OPINION

Much progress has been made in the discovery of novel small molecule mGlu₂ NAMs. Still, chemical diversity remains somewhat limited and room for expansion remains. Progress with mGlu₃ NAMs has been more limited; however, some promising molecules have been disclosed. The process of elucidating the precise role of each receptor in the diseases associated with group II receptors has begun. Continued studies in animals with selective NAMs for both receptors will be critical in the coming years to inform researchers on the right compound profile and patient population for clinical development.