Ruolo della Lipocalina-2 nella fisiopatologia ossea e muscolare

Lcn2 is an iron-binding adipokine that is involved in many pato-physiological processes, such as kidney metabolism, iron homeostasis, innate immunity. In our laboratory, Lcn2 has been demostrated to be a mechano-responsive gene, that is incrased in bone following mechanical unloading, which is the leading cause of disuse osteoporosis. In this thesis, first we demonstrate that Lcn2 has a dual function in bone, since while its overexpression in bone and bone cells causes inflammation and correlates to low bone mass, removing it also gives rise to an osteopenic phenotype, due to impaired energy metabolism that eventually causes osteoblast dysfunction. Lcn2-ko mice also present with polyuria and glycosuria, which may make the phenotype of this mouse even more complex. Lcn2 is also increased in femurs of the MDX mouse model of Duchenne Muscular Dystrophy (DMD), Given this, and since muscle is the closest collaborator of bone, we decided to investigate whether Lcn2 had any role in muscle phisiopathology. We first noticed that serum LCN2 was increased following high-intensity exercise (Gran Sasso Vertical Run) in healthy athletes, and the same was true for markers of muscle damage Creatine Kinase (CK) and Myoglobin. Lcn2 also correlated with Dikkopf (DKK)-1, a known determinant of low bone mass, which was also increased following this high-intensity run. Studying Lcn2-ko mice, we realised that muscle function was normal (grip strength meter). However, the size of muscle fibers was smaller in Lcn2-ko mice, and we gathered some transcriptional indication that myogenesis is activated at a higher level at 3 months of age, which is reverted at 12 months. Furthermore, Lcn2-ko mice never had detectable interleukin (IL)-6 in their serum, which was the case in WT mice, strengthening the idea that Lcn2 is a determinant of inflammation. Therefore, removing Lcn2 is not detrimental for muscle overall, and causes few changes in muscle physiology. Since Lcn2 is increased in MDX mouse femurs, and DMD is characterised by inflammation and mechanical unloading, we investigated whether removing Lcn2 would improve bone and muscle phenotype of MDX mice. We found increased Lcn2 serum levels in MDX mice at 1, 3, 6 and 12 months of age. Consistently, Lcn2 mRNA was higher in MDX versus WT diaphragm, quadriceps, soleus and extensor digitorum longus muscles. Immunohistochemistry confirmed this and also showed that Lcn2 is mainly expressed by mononuclear cells in diaphragm and quadriceps, although positive muscle fibers were also observed. Lcn2 also correlated with the inflammatory cytokines IL1B and TNFA. Based on these results, we ablated Lcn2 in MDX by crossbreeding them with Lcn2-ko mice (double mutant/dMUT) and analyzed their bone and muscle phenotype. MicroCT analyses showed higher trabecular Bone volume/Tissue volume (BV/TV)% and number in dMUT mice vs MDX, likely due to reduced bone resorption, evaluated by serum carboxy-terminal of crosslinked collagen. Similar results were found at 6-month-old-mice, although also osteoblast number was increased in dMUT, compared to MDX. Intriguingly, 3-month-old dMUT mice had higher muscle strength, evaluated by grip strength meter. This was consistent with the fact that dMUT had increased intact muscle fibres and strongly reduced serum CK. Similar results were found in 6-month-old mice. To strengthen these findings, we inhibited Lcn2 by treating 2-month-old MDX mice with a Lcn2-blocking monoclonal antibody, which increased Tb.BV/TV, and reduced osteoclast surface/bone surface compared to MDX mice treated with irrelevant IgG. On the muscle side, grip force was increased, and diaphragm fibrosis was reduced by Lcn2-mAb. Increased BV/TV was also observed when treating 2-week-old MDX to mimic a preventive treatment. In conclusion, these results point at Lcn2 as a determinant of musculoskeletal health in physiopathological conditions, and that this protein is a candidate target in DMD-induced bone and muscle wasting.