2.5. Vitamins in Amyotrophic Lateral Sclerosis
Amyotrophic lateral sclerosis (ALS) is a fatal form of motor neuron disease (MND) characterized by progressive degeneration of motor neurons in the brain and spinal cord. The loss of motor neurons leads to the deterioration of whole-body muscle mass
[58][59][60]. Similar to MS, very few studies have been performed regarding the role of vitamins in ALS. However, as in MS, there is a strong correlation between ALS and VitD supplementation. The active form of VitD is reduced in ALS patients and animal models of ALS
[61][62]. VitD was shown to be protective in motor neurons in vitro, and plasma levels of VitD were directly correlated to the severity of the disease in ALS patients
[63]. Genetic studies have shown that VitD is linked to ALS pathology through the regulation of various immune components such as toll-like receptors, major histocompatibility complex (MHC) class II molecules, poly (ADP-ribose) polymerase 1 (PARP1), and heme oxygenase-1 (HO-1)
[61]. In addition, VitD influences ALS pathology through cell-signaling mechanisms, including Wnt/β-catenin, mitogen-activated protein kinase (MAPK), glutamate, prostaglandins, reactive oxygen species (ROS), matrix metalloproteinases, and nitric oxidase synthase
[61]. In transgenic mouse models of ALS, VitD supplementation reduced symptoms of muscle weakness and improved motor functional capacity but did not prevent the final disease outcome
[64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][34][35][36][37][38][39][40][82][83][84][85][86][87][88][89][90][91][92][93][94][95][96][97][98][99][100][101][102][103][104][105][106][107][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][108][109][110][111][112][113][114][115][116][117][118][119][120][121][122][123][124][125][126][127][128][129][130][131][132][133][134][58][59][60][62][63][135][136][137]. In contrast, a recent study of ALS patients found no reduction in VitD levels, and no benefit for VitD supplementation for improving the prognosis of this disease
[138]. Therefore, future studies are needed to determine the role of VitD in ALS pathophysiology.
2.6. Vitamins in Prion Disease
Prion disease (PRD), also known as transmissible spongiform encephalopathies, is a rare progressive neurodegenerative disorder caused by abnormal prion protein accumulation, which leads to subsequent brain damage
[139]. Structural differences occur between two forms of a prion protein
[140]. PrPC is the normal prion protein, while the misfolded PrPSc (scrapie) is the pathogenic form
[141]. The difference between the two versions of the prion protein is in the secondary structure
[142][143]. Secondary structure α-helix motifs within the normal protein are converted to β-sheet secondary structures, which leads to protein misfolding and toxicity. The molecular mechanisms for this conversion are unclear
[144]. Vitamins prevent the transformation of the normal form of the prion protein towards the pathogenic form
[145]. Various micronutrients, including copper (Cu) and iron (Fe), are also involved in the vitamin-meditated maintenance of the normal form of the prion protein
[146]. In addition, it is thought that oxidative stress and inflammation lead to the formation of the pathogenic form of the prion protein. Therefore, compounds or vitamins with antioxidative and anti-inflammatory characteristics should be beneficial in decreasing the risk of PRD
[145].
Cobalamin (Cbl, Vitamin B12) deficiency plays a major role in the disturbance of the connection between the CNS and the peripheral nervous system (PNS)
[147][148]. Cbl deficiency mainly affects glial cells, myelin sheaths, and the interstitium of the nervous system
[149][150]. In the rat CNS, Cbl deficiency causes a reduction in the epidermal growth factor (EGF) and an enhancement of the activity of tumor necrosis factor-α (TNF-α), which leads to myelin damage and glial activation in both the CNS and PNS
[151]. Interestingly, Cbl inhibits the nuclear localization of the NF-κB pathway, which is responsible for the upregulation of cytokines such as TNF-α and affects the conversion of the normal form of PrPC to the diseased form
[152]. Another vitamin relevant to PRD is VitD2, which effectively crossed the BBB and suppressed PrPc oligomerization, a required step before PrPSc formation
[153].
2.7. Vitamins in Age-Related Macular Degeneration
Not only influential in PD, AD, and HD, vitamins also exert their therapeutic response against age-related macular degeneration (AMD). Oral supplementation and modifications in diet show significant protection against AMD
[154]. The Age-Related Eye Disease Study 2 (AREDS2) research group suggested that lutein/zeaxanthin plays a very vital role in the protection against AMD
[155]. In AMD, the cone cell abnormalities caused by oxidative stress were significantly improved by VitD supplementation
[156]. A clinical study on the Korean population exhibits that the degeneration of AMD progresses due to a lower level of VitD
[157]. In the pathogenesis of AMD, the VitD metabolism plays a very novel role as suggested by a system-biology-based analysis. Therefore, we can say that vitamins are also very effective against AMD. More study will be needed to confirm the applicability and suitability of vitamins in AMD.
3. Conclusions and Future Prospective
Oxidative stress and neuroinflammation are the two major factors involved in the progression of neurodegenerative diseases. As such, compounds having antioxidative and anti-inflammatory properties should provide significant neuroprotection. In recent years, accumulating evidence has suggested a beneficial role for vitamins in protecting CNS diseases, as both WSV and FSV can protect neurons from death
[158][159]. PD, AD, HD, MS, ALS, and PRD are the major neurodegenerative diseases found in humans. Animal models for these diseases have been utilized to prove the therapeutic efficacy of vitamins. However, the utilization of vitamins in therapies has been delayed due to studies which suggest no correlation between vitamin levels and neurodegenerative diseases. Despite this, several clinical trials support a potential role for vitamins in future therapies of neurodegenerative diseases. Hormesis is an important factor and should be taken into consideration while designing the dose of vitamins. Hormesis differentiates between the beneficial and toxic activity of vitamins at a particular dose. Multivitamin supplementation shows therapeutic solid potential for neurodegenerative diseases as compared to a single vitamin-based option. Multiple signaling pathways are involved in the multivitamin approach that can boost the antioxidative response manifold.
Here, we discussed both beneficial and controversial studies involving the role of vitamins in neurodegenerative diseases. Future studies involving different drug-dose paradigms, diverse animal models, and various geographical locations are necessary to determine precisely the potential roles for vitamins in therapies to treat neurodegenerative diseases. Moreover, human clinical trials are desirable to prove the efficacy and the protective role of vitamins in neurodegenerative diseases.