Introduction: Over the last three years, we have demonstrated the complex role of melatonin, a hormone produces mainly in the brain, in the development of scoliosis and in particular by reporting for the first time that cells from AIS patients cannot respond to melatonin, which contrasted with similar cells isolated from healthy subjects. We have determined that this phenomenon is caused by chemical modifications affecting the activity of Gi proteins, a group of small proteins normally associated with both melatonin receptors. Interestingly, previous studies showed that melatonin deficiency could also induce a scoliosis suggesting that the asymmetrical growth of the spine in humans and in melatonin deficient animals could be caused by a common downstream effector regulated by melatonin. This study was then designed to determine and characterise the early biochemical, cellular and molecular changes underlying the formation of spinal deformities in growing pinealectomized chicken and in bipedal C57Bl/6 mice, a naturally melatonin deficient strain of mice.
Methods: For this study, 145 newly hatched chickens (Mountain Hubbard) were purchased at a local hatchery and divided into three distinct groups. First group, pinealectomized (n=100), underwent complete removal of the pineal gland. The second group, sham (n=20), underwent superficial cranial incision without the ablation of the pineal gland. The third group, control (n=25), the chickens did not undergo any surgical procedure. All surgeries were performed by the same surgeon between day three and five after hatching. At days 14, 21 and 28 chicken underwent radiographic examination with a DEXA bone densitometer (PIXImus II, Lunar Corp., Madison, WI). Each digital image was evaluated for the presence of scoliosis and the degree of curvature was measured. Cobb angle threshold value of 10° and higher was retained as a significant scoliotic condition. Blood samples (1 to 2 ml) were taken from a peripheral wing vein of each chicken (from 6 am to 9 am) at the age of 14, 21 and 28 days. Sera were collected by centrifugation and immediately stored at −80°C until assayed. Serum melatonin concentrations were determined using an ELISA method (IBL, Hamburg, Germany). At day 28, chicken were euthanised and tissues were collected to extract mRNA for expression analysis or proteins for subsequent detection. C57Bl/6 mice (n=50) were purchased from Charles-Rivers and bipedal mice were generated by removing the forelimbs and tail after weaning (three weeks old) according to a protocol approved by our institutional animal health care committee. Sera of AIS patients and matched healthy controls were also analysed to determine the levels of circulating P factor using an ELISA assay.
Results: Our results demonstrated a more dynamic variation of circulating melatonin level only in pinealectomised chicken developing a scoliosis, which allowed us to separate scoliotic chicken in two distinct groups. In the first group, the animals showed a biphasic response with a strong decrease of melatonin level between days 14 to 21, followed by a rapid recovery to almost reach the normal values at day 28. In the second group, pinealectomised chickens showed a linear decrease of circulating melatonin over the three-week period while, non-scoliotic pinealectomised chicken showed non-significant variations in melatonin concentration with values close to those obtained with the shams. At the molecular level, expression analysis demonstrated higher expression of a gene encoding a protein that has been termed P factor only in paraspinal muscles of pinealectomized chicken developing a scoliosis. Accumulation of P factor was also confirmed at the protein level by Western blot analysis. Bipedal C57Bl/6 mice, which are naturally melatonin deficient, developed also scoliotic deformities in a proportion of 45% over a two-month period. Interestingly, we observed that genetically modified mice devoid of P factor (n=60) or one of its receptor (n=40) in the same genomic background (C57Bl/6) cannot develop a scoliosis in the same conditions. Moreover, P factor circulating levels in scoliotic patients showed a 2–4 fold increase when compared to healthy matched individuals.
Conclusions: These results showed for the first time a more dynamic variation in circulating melatonin levels among pinealectomised chicken, which was unsuspected by previous studies. Interestingly, a transient decrease of circulating melatonin level was sufficient to induce scoliotic deformities during the first two weeks even if melatonin concentration was subsequently recovered a week later. This may explain why melatonin injection in pinealectomised chicken is not always efficient in preventing scoliosis. Taken together, these observations further suggest that a melatonin decrease below a certain threshold during a specific postnatal window may be sufficient to trigger a scoliosis and reconcile the data concerning AIS patients showing in most of the studies no significant variation when analysed at late stages. The study of early molecular changes in animal models also led us to identify a novel factor, which appears essential to initiate scoliosis through a specific signalling action. The clinical relevance of the P factor in AIS and related spinal syndromes is further strengthened by the detection of high levels of P factor only in scoliotic patients and could pave the way for the development of innovative diagnosis tools as well as the first pharmacological treatments to prevent scoliosis deformities in children.
Research project supported by La Fondation Yves Cotrel de l’Institut de France
