MTOR and proximal spinal muscular atrophy: In rodents, studies on Spinal Muscular Atrophy (SMA) led to the identification of CTRP3 (C1QTNF3, C1q/Tumor Necrosis Factor-related protein 3), which is secreted by muscle and promotes protein synthesis locally in motor neurons, via the activation of mTOR (Figure 4A).