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From Proteopedia
Human metabotropic glutamate receptor 5 transmembrane domain
Receiving and responding to extracellular messages is critical to the proper function of the nervous system. Glutamate is the primary excitory neurotransmitter of the CNS, and metabotropic glutamate receptor 5 will play a major role in glutamate signaling[1]. Metabotropic glutamate receptor 5 transmembrane domain is a homodimeric GPCR that resides in the cellular membrane [1]. This domain is a member of the Class C GPCR family and can further be categorized into the Group I subgroup[2]. The transmembrane domain will signal through a Gq/11 pathway[1]. mGlu5 will bind glutamate to the extracellular Venus flytrap domain and the signal will be transduced across the membrane to a heterotrimeric G protein, which will ultimately lead to calcium release and the activation of PKC[2]. This will elicit a excitory post-synaptic response and modulate long term potentiation[2]. Human metabotropic glutamate receptor 5 is found throughout the central nervous system. Areas containing high concentrations of this protein are often involved in emotional processing and higher cognition[3]. The localization of mGlu5 in the CNS and the presence of multiple domains makes mGlu5 an excellent target for treating schizophrenia,Fragile X, depression, anxiety, and Alzheimer's disease[2].
DiscoveryThe mGlu family of receptors was the first of the Class C GPCR to be extensively studied[2]. The first regions of the protein crystallized and studied were the Venus fly trap domain and the cystiene-rich domain on the extracellular region of the receptor[1]. The hydrophobic nature and flexibility of the transmembrane domain made it difficult to crystallize. Recently, the human metabotropic glutamate receptor 5 transmembrane domain (TMD) was crystallized and a structure elucidated[1]. There were several necessary modifications made to the TMD for it to successfully crystallize. The protein was thermostabilized and flexible domains were removed[1]. In total residue 2-568 and residues 837-1153 were excised from the structure. Also, a T4 - was inserted into ICL-2 to add stability[1]. Structure OverviewThe mGlu5 TMD contains 7 that span the membrane. The protein was crystallized with Oleic acid and MES. On the superior portion of the protein there are several critical extracellular loops. The binding pocket can be found near the middle of the protein. Inserted into the biding pocket is the negative allosteric modulator mavoglurant. It is important to note that the TMD as illustrated is in an inactive conformation. On the intracellular portion of the protein there exist several ionic locks whose positions will determine the activity of the protein. Extracellular DomainThese are the with extracellular loops (ECLs) 1, 2, and 3 highlighted in purple. Additionally in the ECL domain, a is attached to both Helix 3 and the amino acid chain between Helix 5 and the N terminus. The disulfide bond is highlighted in yellow, and it is conserved in all classes of glutamate receptor 5 transmembrane domains[2]. Binding Pocket The binding pocket represents an interesting source of regulatory control. The binding pocket is only accessible by a relatively narrow (7 angstrom) [1]. This small entrance severely restricts the access of both positive and negative allosteric regulators. Important Amino Acids[1]:
Once bound to mavoglurant, transmembrane helix 7 undergoes a conformational change[1]. The shifting of TM7 will lead to a more global conformational change, which we leave the receptor incapable of signaling[1]. Variation can be seen in positioning of alpha helices across receptor class. Class C receptors have seemingly less space for mavoglurant to enter as compared to Class A and F receptors[2]. The position of the ligand binding site is also varied between different classes of mGlu receptors[1]. Ionic LocksAnother important structural feature is the series of on the intracellular side of the protein. Interactions between amino acids will form a salt bridge, which will stabilize the inactive conformation[1]. The primary ionic lock forms between Glu770, Lys665, and Ser613[1]. A secondary ionic lock occurs between Ser614 and Arg668[1]. The purpose of these ionic locks is analogous to the ionic interactions that stabilize the T state in Hemoglobin. In the case of the TMD, when the NAM mavoglurant is bound the ionic lock is formed. This stabilizes the inactive state, where the intracellular loops are stabilized inwards[2]. This will effectively block the crevice that is involved in binding the G-protein[2]. Models have suggested that, even in a glutamate bound state, the mavoglurant bound receptor would be dimerized but incapable of signaling[2]. This helps maintain the readiness of the pathway, while still decreasing signal response. PathwayIt all begins with glutamate binding to the Venus flytrap domain. The signal is transduced across the cystine-rich domain to the TMD. Next, the dimerization of the TMD occurs. This activates the Gq/11 pathway, which activates phspholipase Cβ[3]. The active phospholipase Cβ hydrolyzes phosphotinositides and generates inositol 1,4,5-trisphosphate and diacyl-glycerol[4]. This results in calcium mobilization and activation of PKC[3]. DiseaseFragile XFragile X syndrome is the most common genetic cause of mental disability, and is a member of the Autism spectrum disorder family[5]. The severity of intellectual disability can vary from patient to patient, but symptoms stem from a misregulation of the mGlu1 and MGlu5 pathways[5]. This leads to over potentiation in neural cells. Mavoglurant and other allosteric regulators like fenobam have shown promise in treating Fragile X. One positive characteristic of ligands that target the TMD is they tend to be more specific, thus interacting less with brain proteins[6]. Mavoglurant would act to down regulate glutamate signaling in an attempt to decrease potentiation. Unfortunately, recent Phase 2 clinical trials have proven mavoglurant ineffective [5]. Novartis the company who developed the drug has stopped clinical trials of mavoglurant [5]. However, modulators of mGlu5 TMD are still being researched to treat Parkinson's, Alzheimer's disease, and various addictions[3].
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