Age-related gene expression signatures from limb skeletal muscles and the diaphragm in mice and rats reveal common and species-specific changes.

BACKGROUND: As a result of aging, skeletal muscle undergoes atrophy and a decrease in function. This age-related skeletal muscle weakness is known as "sarcopenia". Sarcopenia is part of the frailty observed in humans. In order to discover treatments for sarcopenia, it is necessary to determine appropriate preclinical models and the genes and signaling pathways that change with age in these models. METHODS AND RESULTS: To understand the changes in gene expression that occur as a result of aging in skeletal muscles, we generated a multi-time-point gene expression signature throughout the lifespan of mice and rats, as these are the most commonly used species in preclinical research and intervention testing. Gastrocnemius, tibialis anterior, soleus, and diaphragm muscles from male and female C57Bl/6J mice and male Sprague Dawley rats were analyzed at ages 6, 12, 18, 21, 24, and 27 months, plus an additional 9-month group was used for rats. More age-related genes were identified in rat skeletal muscles compared with mice; this was consistent with the finding that rat muscles undergo more robust age-related decline in mass. In both species, pathways associated with innate immunity and inflammation linearly increased with age. Pathways linked with extracellular matrix remodeling were also universally downregulated. Interestingly, late downregulated pathways were exclusively found in the rat limb muscles and these were linked to metabolism and mitochondrial respiration; this was not seen in the mouse. CONCLUSIONS: This extensive, side-by-side transcriptomic profiling shows that the skeletal muscle in rats is impacted more by aging compared with mice, and the pattern of decline in the rat may be more representative of the human. The observed changes point to potential therapeutic interventions to avoid age-related decline in skeletal muscle function.

Species-specific effects of neuregulin-1ß (cimaglermin alfa) on glucose handling in animal models and humans with heart failure.

Neuregulin-1ß is a member of the neuregulin family of growth factors and is critically important for normal development and functioning of the heart and brain. A recombinant version of neuregulin-1ß, cimaglermin alfa (also known as glial growth factor 2 or GGF2) is being investigated as a possible therapy for heart failure. Previous studies suggest that neuregulin-1ß stimulation of skeletal muscle increases glucose uptake and, specifically, sufficient doses of cimaglermin alfa acutely produce hypoglycemia in pigs. Since acute hypoglycemia could be a safety concern, blood glucose changes in the above pig study were further investigated. In addition, basal glucose and glucose disposal were investigated in mice. Finally, as part of standard clinical chemistry profiling in a single ascending-dose human safety study, blood glucose levels were evaluated in patients with heart failure after cimaglermin alfa treatment. A single intravenous injection of cimaglermin alfa at doses of 0.8mg/kg and 2.6mg/kg in mice resulted in a transient reduction of blood glucose concentrations of approximately 20% and 34%, respectively, at 2h after the treatment compared to pre-treatment levels. Similar results were observed in diabetic mice. Treatment with cimaglermin alfa also increased blood glucose disposal following oral challenge in mice. However, no significant alterations in blood glucose concentrations were found in human heart failure patients at 0.5 and 2h after treatment with cimaglermin alfa over an equivalent human dose range, based on body surface area. Taken together, these data indicate strong species differences in blood glucose handling after cimaglermin alfa treatment, and particularly do not indicate that this phenomenon should affect human subjects.

Recovery of sensorimotor function following sciatic nerve injury across multiple rat strains.

BACKGROUND: Peripheral nerve injury (PNI) can result in neurodegenerative changes leading to motor, sensory and autonomic dysfunction. Injury to the rat sciatic nerve is used to model pathophysiologic processes following PNI and assess the efficacy of therapeutic interventions. Frequently, temporal changes in the sciatic functional index (SFI), a measure of sensorimotor integration are measured in rats to assess functional recovery following sciatic nerve injury. However, multiple rat strains and behavioral endpoints have been employed to investigate pathophysiology of PNI and impact of therapeutic intervention on recovery, raising the possibility that rat strain may influence the outcome of such studies. NEW METHOD: The temporal course of recovery from sham, sciatic nerve crush or transection injury was assessed using SFI determined by two methods (footprint and DigiGait), and proprioceptive hind limb placement (a measure of proprioceptive integrity) of the sciatic nerve innervation, in male Sprague Dawley, Lewis, Fischer, Wistar and Long Evans rats. RESULTS: The SFI profile, as assessed by both inked footprint analysis and DigiGait, following sciatic nerve injury was remarkably conserved across strains. Dramatic strain-related differences were observed in the latency to place the crush- or transection-injured hind limb following proprioceptive hind limb stimulation. COMPARISON WITH EXISTING METHOD: The novelty of this study is the parallel comparison of multiple strains using existing and novel tests. CONCLUSION: These results suggest that some sensorimotor function tests may be sensitive to the choice of strain, as evidenced by the differences between SFI and proprioceptive function outcomes.

Characterization of behavioral and neuromuscular junction phenotypes in a novel allelic series of SMA mouse models.

A number of mouse models for spinal muscular atrophy (SMA) have been genetically engineered to recapitulate the severity of human SMA by using a targeted null mutation at the mouse Smn1 locus coupled with the transgenic addition of varying copy numbers of human SMN2 genes. Although this approach has been useful in modeling severe SMA and very mild SMA, a mouse model of the intermediate form of the disease would provide an additional research tool amenable for drug discovery. In addition, many of the previously engineered SMA strains are multi-allelic by design, containing a combination of transgenes and targeted mutations in the homozygous state, making further genetic manipulation difficult. A new genetic engineering approach was developed whereby variable numbers of SMN2 sequences were incorporated directly into the murine Smn1 locus. Using combinations of these alleles, we generated an allelic series of SMA mouse strains harboring no, one, two, three, four, five, six or eight copies of SMN2. We report here the characterization of SMA mutants in this series that displayed a range in disease severity from embryonic lethal to viable with mild neuromuscular deficits.