TALLER N° 3 Igualdad de genero

Previous studies have shown that young (2-5 week old) mdx skeletal muscle contains significantly less taurine relative to non-dystrophic controls, and that taurine content appears to increase with stabilisation of the disease and muscle repair (McIntosh et al., 1998a, McIntosh et al., 1998b). Specifically, McIntosh et al. (1998) examined taurine content in the TA and DIA of young (<3 week old), adolescent (3-6 week old) and adult (> 6 week old) mdx mice, finding that lower taurine was characteristic of active degeneration within mdx skeletal muscle, but increased with repair until ultimately reaching a greater level in adult mdx mice compared to controls. The use of similar time points and the same muscle (TA) in the current study allows direct comparison with those results. This study confirmed that taurine is lower in mdx mice during acute degeneration (day 28 & 35) and increases with repair (day 45), until reaching a greater concentration than controls (day 70). The same general pattern was also observed in another hind limb muscle (GAST), with lower taurine at days 28 and 35, yet higher by day 45. The novel finding in the present study was that despite substantial muscle damage and degeneration, taurine supplementation was able to increase hind limb skeletal muscle taurine content to above that of control measures at all time points within the TA, and at most points within GAST. This is significant, as taurine content of skeletal muscle has been shown to be a marker of muscle repair, correlating closely with regeneration and myogenic cell proliferation (McIntosh et al., 1998b).

It should be noted, however, that taurine supplementation appears to become less effective with long-term treatment, as by day 70, in both the TA and GAST, taurine supplemented mice appear to be unable to maintain a steady increase in taurine muscle content, with smaller differences in TA than at earlier time points, while the GAST actually showed significantly less taurine content than controls, despite supplementation. In fact, taurine content of GAST was lower than that of untreated mdx mice, although this did not reach statistical significance. This result was unexpected, as while it is assumed that there is a ceiling for intramuscular taurine content, and that eventually despite increasing supplementation the accumulation of taurine in skeletal muscle would plateau, it was not hypothesised that taurine treatment would become ineffective with chronic treatment. This result is unlikely to be due to alterations in TauT expression, as there was no significant difference found between MDX and TAU groups at day 28 or

106

day 70, and any difference from CON animals appears consistent at both day 28 and day 70. However, it is possible that the activity of the transporter is downregulated with chronic supplementation, or that taurine efflux is increased. Another possible explanation is that taurine supplementation is not ineffective, but rather taurine content becomes normalised within the mdx muscle, negating the need to further increase intracellular stores despite the increased availability of taurine. Unfortunately, there are very few examples in the literature of long-term taurine treatment (Di Leo et al., 2002, Yu et al., 2007), with none of the available investigations examining a time course of taurine treatment to determine if there are any changes associated with the duration of supplementation. Pierno et al. (1998) has used chronic (2-3 months) taurine treatment in aged rats, finding that taurine treatment significantly increased muscle taurine content to above non-supplemented aged controls. Yu et al. (2008) showed similar results within the retina of diabetic rats, with 3 months of chronic taurine treatment significantly increasing taurine content back to control values. This, however, is not different from results obtained in the current study, as the decrease in taurine content is not significantly different between the two dystrophic groups, MDX and TAU, but rather between CON and TAU. It is unknown if the observed trend at day 70 for lower taurine content in supplemented mdx mice associated with long-term taurine treatment would have continued with increasing age. To the author’s knowledge, there is no study that has examined taurine content of skeletal muscle after any longer than 3 months of treatment, and as such the likelihood of this phenomenon of decreased taurine accumulation in muscle with chronic treatment cannot be evaluated at present.

Taurine content of the DIA was also examined in this study, as while this muscle displays a relatively mild dystrophic phenotype at the time points examined, it does undergo significant degeneration and fibrosis at approximately 6 months of age (Louboutin et al., 1993), which could possibly be attenuated if taurine treatment is effective early in the lifespan. In addition, previous investigations have shown that taurine content is significantly lowered in the mdx DIA, even at a young age (McIntosh et al., 1998a, McIntosh et al., 1998b). Results from the current study support that MDX DIA taurine content is significantly lower compared to CON muscle, and that supplementation with taurine significantly increases taurine content in the DIA muscle of the mdx mouse. As taurine has been shown to be essential for normal skeletal muscle function, and is able to combat many of the key pathological features of dystrophy, such

107

as impaired Ca2+ handling, increased oxidative stress and chronic inflammation, it is possible that this increase in endogenous taurine associated with supplementation in the

mdx DIA, may decrease pathology later in life.