Effects of pharmacologic sclerostin inhibition or testosterone administration on soleus muscle atrophy in rodents after spinal cord injury

Abstract

Sclerostin is a circulating osteocyte-derived glycoprotein that negatively regulates Wnt-signaling after binding the LRP5/LRP6 co-receptors. Pharmacologic sclerostin inhibition produces bone anabolic effects after spinal cord injury (SCI), however, the effects of sclerostin-antibody (Scl-Ab) on muscle morphology remain unknown. In comparison, androgen administration produces bone antiresorptive effects after SCI and some, but not all, studies have reported that testosterone treatment ameliorates skeletal muscle atrophy in this context. Our purposes were to determine whether Scl-Ab prevents hindlimb muscle loss after SCI and compare the effects of Scl-Ab to testosterone enanthate (TE), an agent with known myotrophic effects. Male Sprague-Dawley rats aged 5 months received: (A) SHAM surgery (T8 laminectomy), (B) moderate-severe contusion SCI, (C) SCI+TE (7.0 mg/wk, im), or (D) SCI+Scl-Ab (25 mg/kg, twice weekly, sc). Twenty-one days post-injury, SCI animals exhibited a 31% lower soleus mass in comparison to SHAM, accompanied by >50% lower soleus muscle fiber cross-sectional area (fCSA) (p<0.01 for all fiber types). Scl-Ab did not prevent soleus atrophy, consistent with the relatively low circulating sclerostin concentrations and with the 91–99% lower LRP5/LRP6 gene expressions in soleus versus tibia (p<0.001), a tissue with known anabolic responsiveness to Scl-Ab. In comparison, TE partially prevented soleus atrophy and increased levator ani/bulbocavernosus (LABC) mass by 30–40% (p<0.001 vs all groups). The differing myotrophic responsiveness coincided with a 3-fold higher androgen receptor gene expression in LABC versus soleus (p<0.01). This study provides the first direct evidence that Scl-Ab does not prevent soleus muscle atrophy in rodents after SCI and suggests that variable myotrophic responses in rodent muscles after androgen administration are influenced by androgen receptor expression.

Locomotor training with adjuvant testosterone preserves cancellous bone and promotes muscle plasticity in male rats after severe spinal cord injury

“…Our laboratory has developed a rodent severe contusion SCI model, that displays several characteristics of the severe incomplete SCI population, including the complete inability to support the hindlimbs in stance or to perform voluntary over‐ground stepping for at least 3 months postinjury, extensive cancellous bone deficits at the distal femur and proximal tibia (Otzel, Conover, et al, ), and ~50% lower circulating testosterone than uninjured controls for upwards of 2 months postinjury (Beggs et al, ; Otzel, Conover, et al, ; Yarrow, Conover, et al, ). Using this model, we have reported a >50% reduction in soleus muscle fiber cross‐sectional area (fCSA) occurs within 21 days of SCI, although we did not observe definitive signs of the hallmark slow‐oxidative to fast‐glycolytic fiber‐type transition at this relatively early postinjury time point (Phillips et al, ). Furthermore, we have reported that testosterone enanthate (TE) drug treatment attenuated cancellous bone loss and maintained mass of the sublesional androgen‐sensitive non–weight‐bearing levator ani/bulbocavernosus (LABC) muscle complex in both young (Yarrow, Conover, et al, ) and skeletally mature rodents after severe SCI (Phillips et al, ; Yarrow et al, ), with supraphysiologic TE producing the most robust effects.…”

“…Using this model, we have reported a >50% reduction in soleus muscle fiber cross‐sectional area (fCSA) occurs within 21 days of SCI, although we did not observe definitive signs of the hallmark slow‐oxidative to fast‐glycolytic fiber‐type transition at this relatively early postinjury time point (Phillips et al, ). Furthermore, we have reported that testosterone enanthate (TE) drug treatment attenuated cancellous bone loss and maintained mass of the sublesional androgen‐sensitive non–weight‐bearing levator ani/bulbocavernosus (LABC) muscle complex in both young (Yarrow, Conover, et al, ) and skeletally mature rodents after severe SCI (Phillips et al, ; Yarrow et al, ), with supraphysiologic TE producing the most robust effects. However, in our previous studies, supraphysiologic TE did not prevent the soleus fCSA atrophy after severe SCI, perhaps because the soleus exhibits ~70% lower androgen receptor expression than the androgen‐sensitive LABC (Phillips et al, ) or because the TE treatment duration (21 days) was not sufficient to observe muscular improvements.…”

“…Bone outcomes were evaluated at the distal femur or proximal tibia because these skeletal sites display dramatic bone loss and are the most prone to fracture in persons with SCI (Cirnigliaro et al, ). Muscle outcomes were evaluated in the soleus because it is mostly composed of type I fibers that exhibit significant atrophy (Phillips et al, ) and a phenotypic slow‐oxidative to fast‐glycolytic fiber‐type transition in response to disuse and because the rat soleus is recruited during locomotion and standing (Jayaraman et al, ; Liu et al, , ; Stevens et al, ). The LABC was assessed because it is an androgen‐sensitive sublesional non–weight‐bearing muscle (Yarrow, Conover, et al, ).…”

“…Furthermore, little to no detectable bone loss occurs in rodents in response to mild or moderate contusion SCI (Voor et al, ), likely because occasional voluntary hindlimb stepping is preserved in these models. In comparison, only a few studies have examined musculoskeletal adaptations occurring after severe contusion SCI (Lin et al, ; Otzel, Conover, et al, ; Phillips et al, ; Voor et al, ; Ye et al, ), a model that is indicative of the severely impaired incomplete SCI population and that is not typically associated with recovery of hindlimb bone or muscle parameters or the ability to perform hindlimb stepping (Otzel, Conover, et al, ). Our laboratory has developed a rodent severe contusion SCI model, that displays several characteristics of the severe incomplete SCI population, including the complete inability to support the hindlimbs in stance or to perform voluntary over‐ground stepping for at least 3 months postinjury, extensive cancellous bone deficits at the distal femur and proximal tibia (Otzel, Conover, et al, ), and ~50% lower circulating testosterone than uninjured controls for upwards of 2 months postinjury (Beggs et al, ; Otzel, Conover, et al, ; Yarrow, Conover, et al, ).…”

“…Furthermore, we have reported that testosterone enanthate (TE) drug treatment attenuated cancellous bone loss and maintained mass of the sublesional androgen‐sensitive non–weight‐bearing levator ani/bulbocavernosus (LABC) muscle complex in both young (Yarrow, Conover, et al, ) and skeletally mature rodents after severe SCI (Phillips et al, ; Yarrow et al, ), with supraphysiologic TE producing the most robust effects. However, in our previous studies, supraphysiologic TE did not prevent the soleus fCSA atrophy after severe SCI, perhaps because the soleus exhibits ~70% lower androgen receptor expression than the androgen‐sensitive LABC (Phillips et al, ) or because the TE treatment duration (21 days) was not sufficient to observe muscular improvements. Furthermore, we are unaware of any study that has examined the bone or muscle responses to TE treatment when administered adjuvant to bodyweight‐supported TM training.…”