Several reports indicated that SMA mice that received treatment before disease onset exhibited a satisfactory recovery of SMN levels [
17,
20,
22,
27,
28,
66], an improvement in SMA symptoms [
17,
20,
22,
27,
28,
66], and rescue of the SMA-like phenotype [
28,
66]. Based on these results, SMA mice received combined treatment for 1 month beginning on PND 1. The initial results revealed improved SMN levels in the brains and spinal cords of the mice that received combined treatment relative to those of the mice that received
L-AA alone (Figure
6A-D). In addition, combined treatment improved the motor performance of the adult mice with late-onset SMA (Figure
5C), most probably because of a protective effect on motor neurons (Figure
6E-H) and an improvement in muscle pathology (Figure
7) due to the recovery of SMN levels. These findings are consistent with the fact that SMN is required for normal development and that early treatment resulting in sufficient SMN levels improves the prognosis in SMA models. While early treatment has been shown to be essential to mitigating disease severity in mice with early-onset SMA, whether early drug intervention is also necessary for mice with late-onset SMA has not been established. Hua
et al. reported that early (E15) ASO injection in mice with late-onset SMA [
26] dramatically reduced the disease severity and improved motor function, indicating that early treatment is also beneficial in late-onset SMA. However, most cases of type II and III SMA are not diagnosed in the earlier stages of the disease, making it necessary to evaluate the efficacy of later interventions in the late-onset SMA model. In this study, we did not evaluate whether combined treatment administered to mice with late-onset SMA near or after the onset of symptoms could achieve ideal therapeutic effects. However, there has been some evidence that other drugs such as SV that correct the
SMN2 alternative splicing may be therapeutic even when administered later in the disease progression. Some reports have indicated that type II SMA patients that receive VPA for 6 months at disease onset showed significant increases in muscle strength and function [
67,
68]. Additionally, some type II and III SMA patients who received salbutamol treatment for 6 months at disease onset also showed an increase of muscle strength [
69]. These findings therefore support the possibility that combined
L-AA and SV treatment applied at later stages of late-onset SMA may be beneficial and present a promising avenue for further study. However, the efficacy of later interventions with combined treatment should be further investigated. SV treatment (20 mg/kg once daily) alone caused substantially reduced weight gain and mortality before PND 6 in mice (Figure
3). Decreasing the SV dosage to 15 mg/kg prevented lethality but still resulted in a reduced growth rate in juvenile mice (Additional file
5). By contrast, the mice that received combined treatment displayed normal growth rates with no obvious hepatic or renal damage (Figure
3 and Table
1) and reduced vanadium accumulation in the liver and kidneys on PND 30 (Table
2) that decreased to very low levels in the adult mice after drug therapy ended. The effects of SV on the liver or kidneys are of particular concern because of their involvement in the excretory mechanism [
43,
44]. In addition, a report indicated that the daily tolerance of vanadium ranges from 10 to 60 μg/day in humans [
70]. However, the average basal and normative vanadium requirement has been difficult to ascertain. Data acquired from deprivation studies in animals indicated that 2 to 25 ng/day vanadium often induced no significant clinical effects, and many diets supply approximately 15 to 30 μg of vanadium daily, suggesting that dietary intake of vanadium of approximately 10 μg/day is safe [
71]. Moreover, the addition of
L-AA dramatically reduces SV-induced toxicity
in vitro and
in vivo (Figures
1 and
3).
L-AA has very low toxicity, and the minimum dietary requirement in humans is generally 40 to 100 mg/day, however, concentrations of up to 100-fold higher have been shown to be within a safe range [
72]. Therefore, combined treatment provides a novel and useful strategy for SMA therapy in the near future.