Significance of serum VIP and PACAP in multiple sclerosis: an exploratory case-control study

Untangling the mess of CGRP levels as a migraine biomarker: an in-depth literature review and analysis of our experimental experience

BACKGROUND

Calcitonin gene-related peptide (CGRP) is the most promising candidate to become the first migraine biomarker. However, literature shows clashing results and suggests a methodological source for such discrepancies. We aimed to investigate some of these methodological factors to evaluate the actual role of CGRP as biomarker.

METHODS

Previous to the experimental part, we performed a literature review of articles measuring CGRP in migraine patients. Using our 399 bio-bank sera samples, we performed a series of experiments to test the validity of different ELISA kits employed, time of sample processing, long-term storage, sampling in rest or after moderate exercise. Analysis of in-house data was performed to analyse average levels of the peptide and the effect of sex and age.

RESULTS

Literature review shows the high variability in terms of study design, determination methods, results and conclusions obtained by studies including CGRP determinations in migraine patients. CGRP measurements depends on the method and specific kit employed, also on the isoform detected, showing completely different ranges of concentrations. Alpha-CGRP and beta-CGRP had median with IQR levels of 37.5 (28.2-54.4) and 4.6 (2.4-6.4)pg/mL, respectively. CGRP content is preserved in serum within the 24 first hours when samples are stored at 4°C after clotting and immediate centrifugation. Storages at -80°C of more than 6 months result in a decrease in CGRP levels. Moderate exercise prior to blood extraction does not modulate the concentration of the peptide. Age positively correlates with beta-CGRP content and men have higher alpha-CGRP levels than women.

CONCLUSIONS

We present valuable information for CGRP measurements in serum. ELISA kit suitability should be tested prior to the experiments. Alpha and beta-CGRP levels should be analysed separately as they can show different behaviours even within the same condition. Samples can be processed in a 24-h window if they have been kept in 4°C and should not be stored for more than 6 months at -80°C before assayed. Patients do not need to rest before the blood extraction unless they have performed a high-endurance exercise. For comparative studies, sex and age should be accounted for as these parameters can impact CGRP concentrations.

Potential Crosstalk between the PACAP/VIP Neuropeptide System and Endoplasmic Reticulum Stress-Relevance to Multiple Sclerosis Pathophysiology

Multiple sclerosis (MS) is an immune-mediated disorder characterized by focal demyelination and chronic inflammation of the central nervous system (CNS). Although the exact etiology is unclear, mounting evidence indicates that endoplasmic reticulum (ER) stress represents a key event in disease pathogenesis. Pituitary adenylate cyclase-activating peptide (PACAP) and vasoactive intestinal peptide (VIP) are two structurally related neuropeptides that are abundant in the CNS and are known to exert neuroprotective and immune modulatory roles. Activation of this endogenous neuropeptide system may interfere with ER stress processes to promote glial cell survival and myelin self-repair. However, the potential crosstalk between the PACAP/VIP system and ER stress remains elusive. In this review, we aim to discuss how these peptides ameliorate ER stress in the CNS, with a focus on MS pathology. Our goal is to emphasize the importance of this potential interaction to aid in the identification of novel therapeutic targets for the treatment of MS and other demyelinating disorders.

Role of endocrine PACAP in age-related diseases

Pituitary adenylate cyclase activating polypeptide (PACAP) is a conserved neuropeptide, which confers diverse anti-aging endocrine and paracrine/autocrine effects, including anti-apoptotic, anti-inflammatory and antioxidant action. The results of the in vivo and in vitro experiments show that increasing emphasis is being placed on the diagnostic/prognostic biomarker potential of this neuropeptide in a wide array of age-related diseases. After the initial findings regarding the presence and alteration of PACAP in different body fluids in physiological processes, an increasing number of studies have focused on the changes of its levels in various pathological conditions associated with advanced aging. Until 2016 - when the results of previous human studies were reviewed - a vast majority of the studies had dealt with age-related neurological diseases, like cerebrovascular and neurodegenerative diseases, multiple sclerosis, as well as some other common diseases in elderly such as migraine, traumatic brain injury and post-traumatic stress disorder, chronic hepatitis and nephrotic syndrome. The aim of this review is to summarize the old and the new results and highlight those 'classical' and emerging clinical fields in which PACAP may become subject to further investigation as a diagnostic and/or prognostic biomarker in age-related diseases.

Distribution of PACAP and PAC1 Receptor in the Human Eye

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide with widespread distribution and diverse biological functions. Several studies show that PACAP has strong cytoprotective effects mediated mostly through its specific PAC1 receptor (PAC1-R) and it plays important roles in several pathological conditions. Its distribution and altered expression are known in various human tissues, but there is no descriptive data about PACAP and its receptors in the human eyebulb. Since PACAP38 is the dominant form of the naturally occurring PACAP, our aim was to investigate the distribution of PACAP38-like immunoreactivity in the human eye and to describe the presence of PAC1-R. Semiquantitative evaluation was performed after routine histology and immunohistochemical labeling on human eye sections. Our results showed high level of immunopositivity in the corneal epithelium and endothelium. Within the vascular layer, the iris and the ciliary body had strong immunopositivity for both PACAP and PAC1-R. Several layers of the retina showed immunoreactivity for PACAP and PAC1-R, but the ganglion cell layer had a special pattern in the immunolabeling. Labeling was observed in the neuropil within the optic nerve in both cases and glial cells displayed immunoreactivity for PAC1-R. In summary, our study indicates the widespread occurrence of PACAP and its specific receptor in the human eye, implying that the results from in vitro and animal studies have translational value and most probably are also present in the human eye.