Biology subjects

North, K. N.

Publications and source records attributed to North, K. N..

4 recordsLinked to original sources

bioRxiv2021

Loss of α-actinin-3 confers protection from eccentric contraction damage in fast-twitch EDL muscles from aged mdx dystrophic mice by reducing pathological fibre branching

The common null polymorphism (R577X) in the ACTN3 gene is present in over 1.5 billion people worldwide and results in the absence of the protein -actinin-3 from the Z-discs of fast-twitch skeletal muscle fibres. We have previously reported that this polymorphism is a modifier of dystrophin deficient Duchenne Muscular Dystrophy. To investigate the mechanism underlying this we use a double knockout (dk)Actn3KO/mdx (dKO) mouse model which lacks both dystrophin and sarcomere -actinin-3. We used dKO mice and mdx dystrophic mice at 12 months (aged) to investigate the correlation between morphological changes to the fast-twitch dKO EDL and the reduction in force deficit produced by an in vitro eccentric contraction protocol. In the aged dKO mouse we found a marked reduction in fibre branching complexity that correlated with protection from eccentric contraction induced force deficit. Complex branches in the aged dKO EDL fibres (28%) were substantially reduced compared to aged mdx EDL fibres (68%) and this correlates with a graded force loss over three eccentric contractions for dKO muscles ([~]35% after first contraction, [~]66% overall) compared to an abrupt drop in mdx upon the first eccentric contraction ([~]73% after first contraction, [~]89% after three contractions). In dKO protection from eccentric contraction damage was linked with a doubling of SERCA1 pump density the EDL. We propose that the increased oxidative metabolism of fast-twitch glycolytic fibres characteristic of the null polymorphism (R577X) and increase in SR Ca2+ pump proteins reduces muscle fibre branching and decreases susceptibility to eccentric injury in the dystrophinopathies.

genetics↗

bioRxiv2021

Lifespan analysis of dystrophic mdx fast-twitch muscle morphology and its impact on contractile function

Duchenne muscular dystrophy is caused by the absence of the protein dystrophin from skeletal muscle and is characterized by progressive cycles of necrosis/regeneration. Using the dystrophin deficient mdx mouse model we studied the morphological and contractile chronology of dystrophic skeletal muscle pathology in fast twitch EDL muscles from animals 4-22 months of age containing 100% regenerated muscle fibers. Catastrophically, the older age groups lost [~]80% of their maximum force after one eccentric contraction of 20% strain, with the greatest loss [~]93% recorded in senescent 22 month old mdx mice. In old age groups there was minimal force recovery [~]24% after 120 minutes, correlated with a dramatic increase in the number and complexity of branched fibers. This data supports our two-stage model where a "tipping point" is reached when branched fibers rupture irrevocably on eccentric contraction. These findings have important implications for pre-clinical drug studies and genetic rescue strategies.

physiology↗

bioRxiv2020

ACTN3 genotype influences skeletal muscle mass regulation and response to dexamethasone

Homozygosity for the common ACTN3 null polymorphism (ACTN3 577X) results in -actinin-3 deficiency in ~20% of humans worldwide and is linked to reduced sprint and power performance in both elite athletes and the general population. -Actinin-3 deficiency is also associated with reduced muscle mass and strength, increased risk of sarcopenia in the elderly, and altered response to muscle wasting induced by denervation and immobilisation. ACTN3 genotype is also a disease modifier for Duchenne muscular dystrophy (DMD), with -actinin-3 deficiency associated with slower disease progression. Here we show that -actinin-3 plays a key role in the regulation of protein synthesis and breakdown signalling in skeletal muscle, and its influence on muscle mass begins during early postnatal muscle development. Actn3 genotype also influences the skeletal muscle response to the glucocorticoid dexamethasone. Following acute dexamethasone exposure, transcriptomic analyses by RT-qPCR and RNA-sequencing show reduced atrophy signalling (Mstn, Tmem100, mRas, Fbxo32, Trim63) and anti-inflammatory response in -actinin-3 deficient mice compared to wild-type. -Actinin-3 deficiency also protects against muscle wasting following prolonged daily treatment with dexamethasone in female, but not male mice. In combination, these data suggest that ACTN3 R577X is a pharmacogenetic variant influencing the anti-inflammatory and muscle wasting response to glucocorticoid therapy.

genetics↗

bioRxiv2020

Loss of α-actinin-3 during human evolution provides superior cold resilience and muscle heat generation

The fast skeletal muscle protein -actinin-3 is absent in 1.5 billion people worldwide due to homozygosity for a nonsense polymorphism in the ACTN3 gene (R577X) 1. The prevalence of the 577X allele increased as modern humans moved to colder climates, suggesting a link between -actinin-3 deficiency and improved cold tolerance 1,2. Here, we show that humans lacking -actinin-3 (XX) are superior in maintaining core body temperature during cold-water immersion due to changes in skeletal muscle thermogenesis. Muscles of XX individuals displayed a shift towards more slow-twitch isoforms of myosin heavy chain (MyHC) and sarcoplasmic reticulum (SR) proteins, accompanied by altered neuronal muscle activation resulting in increased tone rather than overt shivering 3,4. Experiments on Actn3 knockout mice showed no alterations in brown adipose tissue (BAT) properties that could explain the improved cold tolerance in XX individuals. Thus, this study provides a clear mechanism for the positive selection of the ACTN3 X-allele in cold climates and supports a key thermogenic role of skeletal muscle during cold exposure in humans.

genetics↗