Parkinson disease (PD) is the second most common neurodegenerative condition. Insulin-like growth factor-1 (IGF-1), a neurotrophic factor, plays an essential role in neuronal survival and brain functions. IGF-1 resistance, characterized by the increase of circulating IGF-1 with impaired IGF-1 function, plays a role in disease progression, cognitive impairment, and pathology of idiopathic PD [1
]. Thus, changes of plasma IGF-1 have been under clinical investigation for monitoring IGF-1 function, in order to predict the prognosis and treatment responses in PD [5
]. However, the measurable IGF-1 in plasma is largely inactive. The majority of plasma IGF-1 is bound to IGF binding protein (IGFBP)s, in which more than 75% is IGFBP-3 [6
]. Binding to IGFBP-3 prevents IGF-1 from activating IGF-1 receptors and from being metabolized [6
]. Thus, IGFBP-3 regulates IGF-1 function in both stimulatory and inhibitory manners, namely, autocrine regulation of IGF-1 [6
]. Only a small amount of unbound IGF-1 (or free IGF-1) in plasma is bioactive, and the window of opportunity for directly detecting unbound IGF-1 is small, because free IGF-1 is either rapidly metabolized or internalized after interacting with IGF receptors. Therefore, plasma IGF-1 concentration does not represent the function of IGF-1. Nonetheless, the IGF-1 concentration in plasma is still frequently evaluated for indicating IGF-1 function [5
]. As an alternative, the ratio of IGF-1/IGFBP-3 has been used for indicating ‘free’ bioactive IGF-1. Yet, the measurement includes a large amount of free IGFBP-3 [8
]; thus, is still not a reliable representation of bioavailable IGF-1.
A better representation of the bioavailability of IGF-1 is instead feasible by measuring the levels of cyclic glycine-proline (cGP), a metabolite from ‘free’, bioactive IGF-1 [8
]. The cGP is formed from the N-terminal tripeptide of IGF-1 [7
] after being cleaved [10
] by an acid enzyme [13
]. The N-terminal of IGF-1 is a primary binding site for IGFBP-3 [12
], and cGP retains the same affinity for interacting with IGFBP-3 in a concentration-dependent manner [15
]. The function of cGP is mediated through competition with the binding of IGF-1 to IGFBP-3 [15
], in which the higher ratio of cGP/IGF-1 associates with a greater free IGF-1 and better IGF-1 function [15
]. Administration of cGP protects the brain from ischemic injury in rats [15
] by improving IGF-1 function [15
]. A structure analogue of cGP also protects dopamine neurons from 6-hydroxydopamine-induced injury and improves long-term functional recovery in a rat model of PD [17
High consumption of berry-fruits has been reported to be associated with a lower risk of PD [19
]. In an open-label study [20
], PD patients display a lower Hospital-associated Anxiety and Depression Scale (HADS) score after the supplementation with blackcurrant anthocyanin (BCA, 35% anthocyanins). However, the authors have disassociated the improved neuropsychological outcome with the supplementation; thus, the mechanism underlying the beneficial effect of BCA remains unclear. It has been reported that the effects of anthocyanin on preventing the apoptosis and cardiac dysfunction of diabetic rats are mediated by activating IGF-1 receptors and signaling pathways [21
]. In addition, purple wheat is high in anthocyanins, and the effects of purple wheat on prolonging the life span of Caenorhabditis elegans are associated with the IGF-1 signaling pathway [22
]. We, therefore, sought to examine the possibility of BCA intake on altering IGF-1 function by examining CSF and plasma levels of cGP, IGF-1 and IGFBP-3.
As a neuropeptide, cGP is a natural nutrient of the BCA. The supplementation of the BCA led to an increase of CSF cGP in PD patients, suggesting oral availability and effective brain uptake of cGP. The cGP concentrations were correlated between the CSF and plasma, suggesting the plasma cGP may be the potential source. Given the well-characterized function and mechanisms of cGP in brain protection, the supplementation of the BCA may be further developed for treating neurological conditions through a larger clinical trial.
Neuroprotective effects of cGP have been demonstrated in a rat model with ischemic brain injury [16
]. The treatment effects of cGP are mediated by improving IGF-1function by increasing bioavailable IGF-1 [15
]. Administration of a structure cGP analogue, cyclic Glycine-2ally-Proline (NNZ 2591), after the onset of motor deficits, improves long-term motor function in a rat model of PD [18
] and normalizes neuroplasticity in a rat model with acute memory impairment [23
]. The effectiveness of cGP and cGP analogue in brain protection and functional recovery proves that cGP is a neurotrophic agent. Supplementation of the BCA, which has such neurotrophic nutrient, increased CSF cGP concentration in PD patients. The data provide the first clinical evidence for oral availability and effective central uptake of cGP following the intake of foods.
cGP is a small and lipophilic molecule (192 d) and is able to cross the blood-brain barrier (BBB) in vivo [16
]. Approximately 52% of plasma (endogenous) cGP found in the CSF before the supplementation, which increased to 71% after the supplementation. Given the ability of cGP crossing the BBB, the significant correlation between CSF and plasma concentration of cGP may suggest that the plasma cGP could be a likely source for the increase of CSF cGP, even though the supplementation did not change plasma concentration of cGP. We did not see a correlation between cGP and IGF-1 in the CSF, which could not exclude the possibility that a small part of CSF cGP forms from IGF-1 in the central nerve system (CNS), as the enzyme that cleaves IGF-1 also occurs in the CNS [14
In contrast to cGP, the CSF concentration of IGF-1 was about 1% of that in the plasma, suggesting that CSF IGF-1 was largely independent of circulating IGF-1. IGF-1 is a larger molecule (7600 d) than cGP, with limited ability to cross the BBB [24
]. Using post-mortem human brain tissue, we have recently reported that the BBB function is not compromised in most of the brain regions of idiopathic PD cases that show the absence of dementia-related pathology [25
]. It is possible that the demand for trophic supports of degenerating brains promoted cGP transfer from plasma to CSF. Indeed, the direct administration of cGP to CSF protects brains from ischemic injury [15
]. Thus, further increase of CSF cGP after supplementation of anthocyanin may potentially lead to the improvement of IGF-1 function in PD brains. Activating IGF-1 receptors and signalling pathways has been suggested to be the mechanism underlying the treatment effects of anthocyanin that prevents apoptosis and cardiac dysfunction in diabetic rats [21
]. However, clinical benefits from BCA supplementation were not conclusive, as the changes of CSF cGP were not significantly correlated with the HADS scores (p
= 0.18, R = 0.39, data not shown). Large clinical trials are essential to confirm the efficacy of BCA supplementation in clinical outcome of PD.
The majority of plasma IGF-1 is inactive due to the binding to IGFBPs [6
]. Nonetheless, the increase in plasma IGF-1 has been used for indicating IGF-1 resistances in PD patients [1
]. However, such changes of IGF-1 in plasma are not always observed [26
]. The increased plasma IGF-1 in PD may be an ineffective endocrine response to produce more circulating IGF-1. The limited central uptake of IGF-1 could be a contributing factor to IGF-1 resistance in PD patients [1
]. Circulating IGF-1 may decline when the condition of PD deteriorates to the stage with the complication of cognitive impairment [5
Apart from increasing IGF-1 production, namely, endocrine regulation, the IGF-1 function is also regulated by improving autocrine/paracrine regulations, particularly under the situation that IGF-1 production is insufficient [8
]. The deficiency of IGF-1 could be the result of impaired autocrine regulation trough interacting with IGFBPs [8
]. We have reported the role of cGP in regulating IGF-1 bioavailability by competing with IGF-1 binding to IGFBP-3 [15
] under both physiological [9
] and pathological conditions [8
]. The competitive binding between IGF-1 and cGP is concentration-dependent, resulting in more cGP and more active IGF-1. Our observations from other clinical and experimental studies show that the increase of cGP and/or cGP/IGF-1 ratio is associated with weight changes in obese women [8
], post-natal development [9
], and spontaneous recovery in stroke patients [30
]. Thus, the changes of plasma cGP and/or cGP/IGF-1 ratio may provide an additional indication of IGF-1 function. Autocrine regulation of IGF-1 may present in the CNS, which could be different from that in plasma [6
]. Given limited access to CSF samples, a larger clinical trial is essential for determining whether plasma cGP and cGP/IGF-1 ratio would provide a reliable indication for IGF-1 function in PD and other neurological conditions with intact BBB.
Even though the results were significant, this pilot study has a clear limitation due to small sample size. The interpretation of the results should be cautious until confirmation is received from large clinical trials. Total of 6 out of 7 patients had positively responded to the supplementation by showing increases of cGP in the CSF, suggesting the change is sensitive. One patient did not respond to the treatment and had the highest score of PDQ-39. With 10 patients in each group, we also detected a significant decrease of plasma cGP in hypertensive women and increase of cGP/IGF-1 ratio in obese women [8
]. The longitudinal design used in current trial may eliminate some clinical variations and improve the sensitivity of the changes in plasma cGP and cGP/IGF-1 ratio [8
]. The clinical research for evaluating cGP and cGP/IGF-1 ratio for the IGF-1 function is still in its infancy. If it confirmed through large trials, the changes of plasma cGP would help to individualize BCA intervention.