Neonatal rats were purchased from Charles River Laboratories, and NRVMs were isolated using previously published techniques (45 (link)). NRVMs were serum starved for 6 hours before 18-hour treatment, unless otherwise specified. The following reagents were used: activin A (0 to 100 ng/ml: 338 AC, R&D Systems), GDF11 (100 ng/ml: 120–11, PeproTech), GDF8 (100 ng/ml: 788G8, R&D Systems), CDD866 (0 to 100 μg/ml; Novartis), MG132 (10 μM; Sigma-Aldrich), and A01 (10 μM; Sigma-Aldrich). Proteasome activity was measured using the 20S Proteasome Activity Assay (MilliporeSigma).