Cellular Prion Protein Promotes Regeneration of Adult Muscle Tissue

It is now well established that the conversion of the cellular prion protein, PrP^C, into its anomalous conformer, PrP^Sc, is central to the onset of prion disease. However, both the mechanism of prion-related neurodegeneration and the physiologic role of PrP^C are still unknown. The use of animal and cell models has suggested a number of putative functions for the protein, including cell signaling, adhesion, proliferation, and differentiation. Given that skeletal muscles express significant amounts of PrP^C and have been related to PrP^C pathophysiology, in the present study, we used skeletal muscles to analyze whether the protein plays a role in adult morphogenesis. We employed an in vivo paradigm that allowed us to compare the regeneration of acutely damaged hind-limb tibialis anterior muscles of mice expressing, or not expressing, PrP^C. Using morphometric and biochemical parameters, we provide compelling evidence that the absence of PrP^C significantly slows the regeneration process compared to wild-type muscles by attenuating the stress-activated p38 pathway, and the consequent exit from the cell cycle, of myogenic precursor cells. Demonstrating the specificity of this finding, restoring PrP^C expression completely rescued the muscle phenotype evidenced in the absence of PrP^C.The cellular prion protein (PrP^C) is a glycoprotein, prominently expressed in the mammalian central nervous system (CNS) and lymphoreticular system, that is anchored to the cell external surface through a glycolipidic moiety. The bad reputation acquired by PrP^C originates from the notion that an aberrant conformer of it (PrP^Sc) is the major component of the prion, the unconventional infectious particle that causes fatal neurodegenerative disorders, i.e., transmissible spongiform encephalopathies (TSE) or prion diseases (56). A wealth of evidence has suggested that the function of PrP^C is beneficial to the cell, but currently, our detailed comprehension of its physiology remains poor. In this respect, the availability of knockout (KO) paradigms for PrP^C has provided less crucial information than expected. Subtle phenotypes, e.g., mild neuropathologic, cognitive, and behavioral deficits, have been described in PrP-KO mice (17, 50), but these animals generally live a normal life span without displaying obvious developmental defects (8, 42). Importantly, the same holds true when the expression of PrP^C is postnatally abrogated (40). The extensive search for PrP^C''s raison d''être has ascribed to the protein a plethora of functions (for updated reviews, see references 1 and 35); among these, roles in cell adhesion, migration, and differentiation have been proposed whereby PrP^C could act by modulating different cell-signaling pathways (63). In this framework, a variety of neuronal proteins have been hypothesized to interact with PrP^C (reviewed in references 1 and 11), for example, cell adhesion molecules or extracellular matrix proteins, which could explain the capacity of PrP^C to mediate the neuritogenesis and neuronal differentiation observed in several cell model systems (13, 22, 23, 27, 36, 59, 64).Although neurons are generally regarded as the model of choice for unraveling the function of PrP^C, the expression of the protein in several other organs suggests that PrP^C has a conserved role in different tissues. Thus, important insight into PrP^C function may also be provided by the analysis of extraneural tissues. One such tissue is skeletal muscle, which has been shown to express PrP^C at significant levels (43, 46) and has been found to upregulate PrP^C levels under stress conditions (71). On the other hand, ablation of the PrP gene has been shown to directly affect skeletal muscles, for example, by enhancing oxidative damage (30) or by diminishing tolerance for physical exercise (51). Skeletal muscles have also been associated with prion pathology, as evidenced by the accumulation of PrP^Sc (or PrP^Sc-like forms) in the muscles of TSE-affected humans and animals (2, 3, 6, 21, 53, 67) and by transgenic-mouse models of some inherited TSEs (16). In addition, overexpression of wild-type (WT) PrP^C (25, 68), or expression of TSE-associated mutants of the protein (16, 66), generates myopathic traits in transgenic mice.In light of these notions, and because intact muscle tissues are more amenable to in vivo manipulations than neural tissue, we set out to analyze the potential role of PrP^C in tissue morphogenesis (38, 41, 46) using an in vivo skeletal-muscle paradigm from two congenic mouse lines expressing (WT) or not expressing (PrP-KO) PrP^C. Importantly, to verify that the PrP-KO muscle phenotype was specifically dependent on the absence of PrP^C, we used PrP-KO mice reconstituted with a PrP transgene (PrP-Tg). The applied protocol consisted of first characterizing the degeneration of the hind-limb tibialis anterior (TA) muscle and then evaluating the myogenic process from the response to inflammation to the full recovery of the muscle. By combining acute insult with adult age, this strategy also had the potential to bypass possible compensatory mechanisms that might mask PrP-KO phenotypes during embryogenesis and/or in adulthood under normal conditions (65).In this study, we provide evidence that, compared to animals expressing PrP^C (WT and PrP-Tg), recovery from damage of adult skeletal muscles was significantly slower in PrP-KO mice. Analysis of the different stages of muscle regeneration allowed us to conclude that PrP^C is one of the factors that govern the early phases of this process, in which the proliferation and differentiation of myogenic precursor cells take place.