Localization of the CGRP family of peptides in relation to the TGV system.
CGRP is expressed in a granular pattern in small (<30 µm) to medium (30-60 µm) sized TG neurons (Eftekhari, Salvatore, Calamari, Kane, Tajti & Edvinsson, 2010; Lennerz et al., 2008) where CGRP is packed in vesicles that are surrounded by the Golgi apparatus. In addition, C-type of sensory unmyelinated nerves appear as pearl-like CGRP immunoreactivity (ir) in boutons (also known as varicosities) (Figure 1). The myelinated fibres do not contain CGRP as sometimes has been proposed (Eftekhari, Warfvinge, Blixt & Edvinsson, 2013; Lennerz et al., 2008). The CGRP receptor components, CLR and RAMP1, are co-expressed in medium to large (>60 µm) diameter neurons (Eftekhari et al., 2016; Eftekhari, Salvatore, Calamari, Kane, Tajti & Edvinsson, 2010). In addition, CLR and RAMP1 have been observed in satellite glial cells (SGC) and in thinly myelinated fibres, typical for Aδ-fibres in various parts of the TGV system such as the TG, dura mater, and root entry zone (Figure 1) (Eftekhari, Salvatore, Calamari, Kane, Tajti & Edvinsson, 2010; Lennerz et al., 2008; Miller et al., 2016). Early findings, which have today been confirmed by many researchers, has stood the test of time, and served a solid base for the successful development of mAbs and small molecule gepants (Edvinsson, Haanes, Warfvinge & Krause, 2018).
CT is a hormone produced mainly by C cells in the thyroid gland with a role to reduce plasma calcium and to promote bone formation (Findlay & Sexton, 2004). In the clinic, CT is used in treatment of bone disorders characterized by increased bone resorption, osteoporosis and hypercalcemia due to malignancy, with some pain relief (Findlay & Sexton, 2004). Thus, CT and its receptor CTR are seen in different cell types and tissues which suggests multiple physiological roles, including bone metabolism (Findlay & Sexton, 2004). CT has not been shown to be expressed in the nervous system, however binding sites for CT are found in many brain structures (Hendrikse, Bower, Hay & Walker, 2018). In addition, a recent study has revealed the presence of the CT receptor in the human brainstem (Bower et al., 2016).
Recently, CT was found to be expressed in the trigeminal system; CT the peptide was found in vesicles that are surrounded by the endoplasmic reticulum/Golgi apparatus and occur in addition as pearl-like CT-immunoreactivity (ir) in the C-type of sensory fibres (Figure 1B) (Edvinsson, Grell & Warfvinge, 2020).
. The CT-ir was expressed in a pattern like that of CGRP; granular staining of small to medium sized neurons and pearl-like staining of fibres in the TG. In addition, many SGCs were CT positive (Edvinsson, Grell & Warfvinge, 2020). In a subpopulation of TG neurons there is granular and cytosolic expression of CT-ir (Figure 1).
AMY is currently in focus of much migraine research, due to its affinity for the CGRP receptor that pharmacologically is almost equal to that of CGRP and because CGRP has strong affinity for the AMY1 receptor (Hendrikse, Bower, Hay & Walker, 2018). Comparing the binding affinity of olcegepant and CGRP8-37 to the AMY1 and CGRP receptors revealed similarities in binding characteristics as well as in cAMP production (Walker et al., 2015). Human AMY was first isolated in 1987 (Cooper, Willis, Clark, Turner, Sim & Reid, 1987). It is an endocrine hormone that signals to the brain and acts as a satiety factor; its presence in plasma is about 10 times higher than that of CGRP (Hay, 2017). Research on AMY deposition in brain neurons has been discussed for a role in Alzheimer’s disease (Mietlicki-Baase et al., 2017) and AMY has been found to alter viability of human brain pericytes (Schultz, Byman, Fex & Wennstrom, 2017).
Previously, we reported that AMY is expressed in feline TG neurons, and that AMY relax in vitro and in vivo cerebral vessels (Edvinsson, Goadsby & Uddman, 2001). A detailed report showed AMY expression in a few small to medium sized TG neurons which co-express CGRP and in C-fibres (Edvinsson, Grell & Warfvinge, 2020).
Receptors for CGRP and AMY are related and share components, CLR/RAMP1 and CTR/RAMP1, respectively. Given the close relationship between AMY and CGRP, and since release of the peptides may act on either or both receptors, it is understandable that experiments with AMY aimed to trigger migraine and to block migraine with mAbs towards AMY are on the way. However, early result from a trial on migraine patients did not reveal any prophylactic effect of the mAbs towards AMY (Ashina, 2019).
AM is most often considered an endothelial peptide and was first isolated in 1993 (Kitamura et al., 1993). AM is generally expressed and participates in a variety of physiological functions including vasodilation, bronchodilatation, growth and hormone regulation (Ferrero et al., 2018). Furthermore, AM is involved in pathophysiological processes such as hypertension, retinopathy and tumour genesis (Ferrero, Larrayoz, Gil-Bea, Martinez & Ramirez, 2018). In mammals, endothelial AM-ir is present in low concentrations in the vascular endothelium (Satoh et al., 1996). AM is found in neurons and glial cells (Serrano et al., 2000). We have reported the presence of AM-ir in the thin cytoplasm of the glial cells, SGCs and cells enveloping neuronal processes, probably myelinating cells. In addition, AM-ir has been noted to occur in cranial vascular endothelium (Edvinsson, Grell & Warfvinge, 2020). Its molecular relation to CGRP has resulted in suggestion that AM may have a role in migraine pathophysiology (Juhl, Petersen, Larsen, Jansen-Olesen & Olesen, 2006). However, intravenous AM infusion did not cause migraine-like attacks in man (Ashina, Hansen, BO & Olesen, 2017). This view is in line with our demonstration that there is no AM in TG neurons but presence in glial cells and vascular endothelium (Edvinsson, Grell & Warfvinge, 2020).