Titolo della tesi: Which is the cause of NMJs damage in distrophic mdx mice?
Duchenne muscular dystrophy (DMD) is characterized by the absence of dystrophin, a cytoskeletal protein that connects several protein complexes, including those maintaining the proper organization of the neuromuscular junction (NMJ). The absent of dystrophin leads to muscle membrane instability, followed by muscle inflammation, infiltration of pro-inflammatory macrophages and fibrosis. Inflammation causes a substantial part of muscle damage in DMD, and anti- inflammatory treatments can slow the course of the disease, in humans and mice. In the disease, there is a complex crosstalk between cells of the immune system, in particular macrophages and fibroblasts. Pro-inflammatory macrophages (MΦs), required to clear debris of dead cells, are constitutively present in dystrophic muscles and contribute to muscle damage through the release of nitric oxide (NO). Pro-wound healing MΦs are less harmful, as they reduce NO release by pro-inflammatory MΦs and promote muscle regeneration. In the context of DMD, the main drawback in pushing MΦs towards an anti-inflammatory phenotype is that they release TGFβ1, promoting fibroblast proliferation and muscle fibrosis. Indeed, in DMD patients, abundance of anti-inflammatory MΦs correlates with enhanced fibrosis and poor prognosis. Both MΦ phenotype and fibroblast proliferation critically depend on the activity of the Ca2+-activated K+ channel KCa3.1 (KCNN4), which therefore could represent a key factor in determining muscle damage in DMD.
Dystrophin is also essential to the proper organization of the neuromuscular junction express. Denervation or partial overlap of pre- and post-synaptic structures are observed, and the endplate acquires a fragmented structure in DMD patients and in all animal models of the disease. Functional changes are also observed, with reduced postsynaptic response to acetylcholine (ACh) and a compensatory increase in quantal content. The damage to the NMJ can impair neuromuscular transmission and contribute to muscle weakness, exacerbating disease progression.Since NMJ fragmentation is also observed in acutely injured or ageing mouse muscles, it is still debated whether the damage to the NMJ is due to the absence of dystrophin or to cycles of damage and regeneration of muscle fibers. In either case, in the dystrophic muscle, ongoing reinnervation of newly formed muscle fibers is required to maintain muscle function.
In my PhD project, I focused my attention on these topics, analysing the impairment of NMJ in DMD to address if the NMJ fragmentation, reported in DMD patients and mdx mice, could be due to the absence of dystrophin or as a second consequences of muscular damage.