Active and Inactive Form
Figure 2: Inactive form of the thyrotropin receptor shown in blue. Active form of the thyrotropin receptor shown in green.
The TSHR protein exists in two states: active and inactive (Figure 2). The exists when bound to the . One proposed mechanism for the transition from the active to inactive describes that in a natural state, the TSHR ECD can spontaneously transition to the up state, leading to constitutive activity. In this active state, TSH will bind and keep the active state in the up position because of clash with the cell membrane.[1] Conformational change of ECD allows for signal transduction through the TM and into the cell. The ECD rotates 55 degrees up in the active form. [1]
TSHR Agonists and Antagonists
Chemical agonists are found in many living systems and serve as a way to activate receptors or pathways that are necessary for a wide array of biological processes. Chemical antagonists block or inhibit biological processes. Different types of agonists/antagonists exist within the body including hormones, antibodies, and neurotransmitters. The body naturally produces autoantibodies that can act as agonists and mimic the activating mechanism of the natural hormone.[2].
M22 Agonist
is a
monoclonal antibody that was isolated from a patient with
Graves' Disease. In Graves' disease, autoantibodies mimic TSH function and cause thyroid overactivity.
[2]. The M22
autoantibody activates TSHR by causing a membrane clash with the ECD and cell membrane, keeping the TSHR in the active state by preventing the TSHR from rotating to the inactive state (Figure 3). M22 mimics TSH activation of TSHR because it is a potent activator for TSHR.
[1] Although M22 binds in a similar manner to TSH, M22 does not interact with the hinge region when bound to TSHR, whereas TSH bound to TSHR does.
[1] This finding shows that the hinge region is not necessary for the activation of TSHR, and leads to the discovery of other methods of activation.
Figure 3: Agonist and antagonist drugs for activating or inactivating the TSHR protein.
CS-17 Inverse Agonist
is a monoclonal antibody that acts as an inverse agonist for TSHR constitutive activity. [3]. CS-17 interacts with the ECD of the TSHR protein on the convex side of the LRRD. When bound to TSHR, CS-17 suppresses TSHR function by keeping the receptor in the inactive state (Figure 3). Clash with the cell membrane does not allow the inactive form of TSHR to flip to the active conformation. CS-17 plays a unique role with GPCRs. This type of inhibition is not commonly seen in many biological systems and therefore leads to this method of inhibition being a great target for drug design and future research.[3]. Due to its unique inhibition, CS-17 can be a popular therapy for many thyroid diseases where the thyroid is overactive.
TSH Agonist
Thyroid-stimulating hormone , as previously mentioned, is a hormone that stimulates the thyroid gland to produce proteins that are vital for many metabolic pathways in the body's tissue. TSH activates the TSHR protein by binding to the concave surface of the LRRD and hinge region to keep TSHR in its active state by clashing with the membrane [4]. (Figure 3). This clash is caused by glycosylations of an N52 residue on the . These modifications to the N residue are N-acetyl glucosamine modifications (Figure 4). They stick out from the alpha subunit of TSH to clash with the cell membrane and keep TSH in the active state.
