Prof. Peter Sonderegger, Department of Biochemistry, University of Zürich

Abstract

Sarcopenia is a particular type of skeletal muscle atrophy that affects almost 25% of the individuals aged 65 years and increases to 30 – 50% in those aged 80 years. It plays a major role in the loss of muscular strength, decreased metabolism, and gradual reduction of bone density. Therefore, sarcopenia is a major cause for frailty and functional impairment that progressively affects pepople at old age.

The etiology of sarcopenia is still not clear. The long list of suspects includes motoneuron loss and motor-unit remodeling, decreased capacity of motoneurons to innervate regenerating muscle fibers, instability of the neuromuscular junction (NMJ), physical inactivity, mitochondrial dysfunction, oxidative damage, impaired excitation-contraction coupling, decline in satellite cell activation and proliferation, as well as nutritional, hormonal, metabolic and immunological deficits associated with advanced age.

We recently discovered that proteolytic cleavage of agrin by the neuronal serine protease neurotrypsin results in NMJ degeneration and, in turn, loss of muscle fibers, resulting in a sarcopenia-like muscle atrophy. Besides muscle fiber loss, characteristic alterations of neurotrypsin-induced sarcopenia included an increased heterogeneity of fiber thickness, centralized nuclei, fiber-type grouping, and fiber-type redistribution towards an increased proportion of type-I fibers. Thus, overexpression of the agrin-cleaving protease neurotrypsin in motoneurons installed the full sarcopenia phenotype in young-adult mice.

However, our studies of neurotrypsin-deficient mice and of mice co-expressing cleavage-resistant agrin together with overexpressed neurotrypsin indicated that age-dependent sarcopenia develops in the absence of neurotrypsin and is not prevented by cleavage-resistant agrin. Thus, the results of our recent studies define neurotrypsin- and age-dependent as the common final outcome of two etiologically distinct entities.

The present project is aimed at answering the question whether pathologically elevated activity of neurotrypsin resulting in premature senescence of the NMJ represents a pathogenic mechanism of a naturally occurring form of sarcopenia-like muscle atrophy. The proposed studies should provide insight that allows answering the question whether excessive activation of the zymogenic pro-form of neurotrypsin may represent a plausible pathogenic mechanism for precocious sarcopenia-like muscle atrophy. In preliminary studies, we identified proprotein convertase 5 (PC5), a member of the subtilisin/kexin family of proprotein convertases, as the most likely in vivo activator of neurotrypsin. Proposed experiments shall work out the molecular and cellular details of the neurotrypsin-activating machinery of PC5 and its regulatory mechanisms during development, adult life, and at old age. 

Specific aims of our planned projects include:

- The expression and purification of PC5 for in vitro activation studies of neurotrypsin

- The generation of highly specific antibodies against the neurotrypsin-activating proprotein convertase PC5

- The investigation of the activity-dependence of the neuronal exocytosis of PC5

- Studies on the role of agrin as an enhancer of neurotrypsin activation

The results are expected to shed light on a potential pathogenic mechanism of precocious sarcopenia-like muscle atrophy and, possibly, will open new ways for therapeutic interventions.