Curcumin's neuroprotective efficacy in Drosophila model of idiopathic Parkinson's disease is phase specific: implication of its therapeutic effectiveness

Abstract

Selective degeneration of dopaminergic neurons in the substantia nigra underlies the basic motor impairments of Parkinson's disease (PD). Curcumin has been used for centuries in traditional medicines in India. Our aim is to understand the efficacy of genotropic drug curcumin as a neuroprotective agent in PD. Analysis of different developmental stages in model organisms revealed that they are characterized by different patterns of gene expression which is similar to that of developmental stages of human. Genotropic drugs would be effective only during those life cycle stages for which their target molecules are available. Hence there exists a possibility that targets of genotropic compounds such as curcumin may not be present in all life stages. However, no reports are available in PD models illustrating the efficacy of curcumin in later phases of adult life. This is important because this is the period during which late-onset disorders such as idiopathic PD set in. To understand this paradigm, we tested the protective efficacy of curcumin in different growth stages (early, late health stage, and transition phase) in adult Drosophila flies. Results showed that it can rescue the motor defects during early stages of life but is ineffective at later phases. This observation was substantiated with the finding that curcumin treatment could replenish depleted brain dopamine levels in the PD model only during early stages of life cycle, clearly suggesting its limitation as a therapeutic agent in late-onset neurodegenerative disorders such as PD.

FIG. 1.

Survival proportions of Oregon K male flies on regular culture medium. Flies were grown on normal culture medium, and survival was recorded until all of the flies died. Health span extends from day 4/5 to 30 days; the transition phase is 31–60 days of adult span, and the senescent span is 61–120 days. Maximum life span is 121 days and median life span is 95 days.

FIG. 2.

Concentration- and time-dependent mortality of Drosophila melanogaster (Oregon K) exposed to paraquat (PQ). Adult male flies (4–5 days) were exposed to six different concentratins of PQ (2.5, 5, 10, 15, 20, 40 mM). PQ exposure induced concentration-dependent lethality. Comparison of survival curves reveals that the response difference among different tested concentrations was significant (log-rank [Mantel–Cox test, p<0.0001]).

FIG. 3.

Assessing curcumin toxicity. Effect of increasing concentrations of curcumin (25 μM to 50 mM) for 10 days on survival of adult male flies (4–5 days). Feeding of 2.5 mM or higher concentration affected viability, whereas concentrations lower than that exhibited no observable toxicity. Data were collected every 24 hr for each group.

FIG. 4.

Curcumin (C) protects against paraquat (PQ)-induced mobility defects in 4- to 5-day-old flies under co-treatment and pre-treatment regimes as tested in a negative geotaxis assay. The distance a male fly climbs in 12 sec after 24 hr (A) and 48 hr (B) of exposure to 10 mM PQ or 10 mM PQ with different concentrations of curcumin was assayed. Feeding of curcumin alone showed no effect on speed of the fly, whereas ingestion of PQ alone adversely affected mobility. PQ and C co-feeding resulted in significant improvement in mobility performance. In the pre-treatment regime, curcumin protects against PQ-induced mobility defects after 24 hr (C) and 48 hr (D) in a negative geotaxis assay, suggesting that the rescue effect of curcumin is not due to antagonistic interaction with PQ (one-way analysis of variance [ANOVA] followed by Newman–Keuls multiple comparison test showed that the protective efficacy of curcumin in the co-treatment regime is highly significant compared to the PQ-treated group). (***) p<0.0001.

FIG. 5.

Curcumin (C) protects against paraquat (PQ)-induced mobility defects (in co-treatment regime) in 30-day-old flies after 24 hr (A) and 48 hr (B) exposure to PQ (***) p<0.0001 compared to the PQ-treated group.

FIG. 6.

Curcumin's failure to rescue paraquat (PQ)-mediated mobility defects during the transition phase. Curcumin (C) fails to protect against PQ-induced mobility defects in 55-day-old flies (transition phase) in the co-treatment regime after 24 hr (A) and 48 hr (B) of exposure and in the pre-treatment regime after 24 hr (C) and 48 hr (D) exposure to PQ. NS, lack of significance between the PQ-treated group and rescue groups.

FIG. 7.

Quantification of brain dopamine level with high-performance liquid chromatography (HPLC). Chromatogram of standard dopamine showing a retention time between 3.196 and 3.442 min (A) and chromatogram for Drosophila head tissue extract, showing a peak during the observed time window for standard dopamine (B) (peak for dopamine is pointed with an arrow in both the panels). Peaks can be seen (at 5 min and 7.5 min) much away from retention time window observed for standard dopamine that amount to artifact.

FIG. 8.

Quantification of dopamine levels in head tissue. Relative dopamine levels in 5-day (health span) and 55-day (transition phase) fly brains exposed to 10 mM paraquat (PQ) for 24 hr. (***) p<0.0001 compared to control and curcumin alone fed. NS, lack of significance between toxin-treated group and rescue group, i.e., PQ+curcumin).