Balancing repair and tolerance of DNA damage caused by alkylating agents

Alkylating agents constitute a major class of frontline chemotherapeutic drugs that inflict cytotoxic DNA damage as their main mode of action, in addition to collateral mutagenic damage. Numerous cellular pathways, including direct DNA damage reversal, base excision repair (BER) and mismatch repair (MMR), respond to alkylation damage to defend against alkylation-induced cell death or mutation. However, maintaining a proper balance of activity both within and between these pathways is crucial for a favourable response of an organism to alkylating agents. Furthermore, the response of an individual to alkylating agents can vary considerably from tissue to tissue and from person to person, pointing to genetic and epigenetic mechanisms that modulate alkylating agent toxicity

Regeneration of Reinnervated Rat Soleus Muscle Is Accompanied by Fiber Transition Toward a Faster Phenotype

The functional recovery of skeletal muscles after peripheral nerve transection and microsurgical repair is generally incomplete. Several reinnervation abnormalities have been described even after nerve reconstruction surgery. Less is known, however, about the regenerative capacity of reinnervated muscles. Previously, we detected remarkable morphological and motor endplate alterations after inducing muscle necrosis and subsequent regeneration in the reinnervated rat soleus muscle. In the present study, we comparatively analyzed the morphometric properties of different fiber populations, as well as the expression pattern of myosin heavy chain isoforms at both immunohistochemical and mRNA levels in reinnervated versus reinnervated-regenerated muscles. A dramatic slow-to-fast fiber type transition was found in reinnervated soleus, and a further change toward the fast phenotype was observed in reinnervated-regenerated muscles. These findings suggest that the (fast) pattern of reinnervation plays a dominant role in the specification of fiber phenotype during regeneration, which can contribute to the long-lasting functional impairment of the reinnervated muscle. Moreover, because the fast II fibers (and selectively, a certain population of the fast IIB fibers) showed better recovery than did the slow type I fibers, the faster phenotype of the reinnervated-regenerated muscle seems to be actively maintained by selective yet undefined cues. (J Histochem Cytochem 56:111–123, 2008

Myosin heavy chain isoform transitions in canine skeletal muscles during postnatal growth

To gain a better understanding of the normal characteristics of developing canine muscles, myosin heavy chain (MHC) isoform expression was analysed in the axial and limb skeletal muscles of 18 young dogs whose ages ranged from the late prenatal stage to 6 months. We compared the results of immunohistochemistry using ten monoclonal antibodies, specific to different MHC isoforms, and enzyme-histochemical reactions, which demonstrate the activity of myofibrillar ATPase, succinate dehydrogenase (SDH) and α-glycerophosphate dehydrogenase (α-GPDH). In the skeletal muscles of fetuses and neonatal dogs the developmental isoforms MHC-emb and MHC-neo were prevalent. In all muscles the primary fibres, located centrally in each muscle fascicle, strongly expressed the slow isoform MHC-I. The adult fast isoform MHC-IIa was first noted in some of the secondary fibres on fetal day 55. During the first 10 days after birth, the expression of MHC-emb declined, as did that of MHC-neo during the second and third weeks. Correspondingly, the expression of MHC-IIa, and later, of MHC-I increased in the secondary fibres. Between the sixth week and second month the expression of MHC-IIx became prominent. The slow rhomboideus muscle exhibited an early expression of the slow isoform in the secondary fibres. Our results indicate that the timing of muscle maturation depends on its activity immediately following birth. The fastest developing muscle was the diaphragm, followed by the fast muscles. A pronounced changeover from developmental to adult isoforms was noted at 4–6 weeks of age, which coincides with the increased physical activity of puppies

E-C Coupling and Contractile Characteristics of Mechanically Skinned Single Fibres from Young Rats During Rapid Growth and Maturation

The postnatal growth of rats involves a developmental phase (0 to ∼3 weeks), a rapid growth phase (∼3 to ∼10 weeks), and a slower maturation phase (∼10 weeks+). In this study, we investigated the age-related changes in excitation–contraction (E–C) coupling characteristics of mammalian skeletal muscle, during rapid growth (4– 10 weeks) and maturation (10–21 weeks) phases, using single, mechanically skinned fibres from rat extensor digitorum longus (EDL) muscle. Fibres from rats aged 4 and 8 weeks produced lower maximum T-system depolarizationinduced force responses and fewer T-system depolarizationinduced force responses to 75% run-down than those produced by fibres from rats aged 10 weeks and older. The sensitivity of the contractile apparatus to Ca2+ in fibres from 4-week rats was significantly higher than that in fibres from 10-week rats; however, the maximum Ca2+-activated force per skinned fibre cross-sectional area (specific force) developed by fibres from 4-week rats was on average ∼44% lower than the values obtained for all the other age groups. In agreement with the age difference in specific force, the MHC content of EDL muscles from 4-week rats was ∼29% lower than that of 10-week rats. Thus, mechanically skinned fibres from rats undergoing rapid growth are less responsive to T-system depolarization and maximal Ca2+ activation than fibres from rats at the later stage of maturation or adult rats. These results suggest that during the rapid growth phase in rats, the structure and function of elements involved in E–C coupling in fast-twitch skeletal muscle continue to undergo significant changes