Sunlight Incidence,Vitamin D Deficiency,and Alzheimer's Disease

Data selection

Solar incidence and AD

Experimental studies involving VD and AD are rare, usually restricted to a group of patients at a given location. ^{14,17 –22} The average concentration of VD is not known in many countries, making difficult an epidemiological analysis in large scale. In the absence of large-scale available data about serum VD concentrations in patients with AD, we investigated the relationship between AD death rate and the average annual solar incidence of 172 countries for which data were available (Supplementary Table S1). Solar incidence ranges from 2.12 kWh/m²/day in Iceland to 6.37 kWh/m2/day in Niger. The distribution midpoint (4.25 kWh/m²/day) was used to classify the countries into two groups: those with the average solar incidence above the midpoint (high-incidence group, containing 129 countries) and those with the average solar incidence below the midpoint (low-incidence group, containing 43 countries). According to Figure 1a, countries with high solar incidence have significantly lower AD death rate (4.47 deaths/100,000 habitants in average) than countries with low solar incidence (12.34 deaths/100,000 habitants in average).

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FIG. 1.

AD death rates in countries with high and low sunlight incidence. Distribution of AD death rates per 100,000 habitants in countries with average annual solar incidence greater than 4.25 kWh/m²/day (high) and equal or lower than 4.25 kWh/m²/day (low): (a) all 172 countries present at World Life Expectancy repository (129 countries in high group and 43 countries in low group); (b) the 90 countries presenting life expectancy equal or greater than 75 years (55 countries in high group and 35 countries in low group). Boxes represent the IQR, the line inside it represents the median, and the bottom and top lines of the box are the first and the third quartiles, respectively. Whiskers limits are the lowest and the highest observation within 1.5 of IQR from the lower and upper quartiles. *P < .05 (Wilcoxon–Mann–Whitney test) when comparing to respective low group. Statistics and graphs were generated using R language. IQR, interquartile range; AD, Alzheimer's disease.

Since AD is prevalent in elderly people, we evaluated the life expectancy of the 172 countries analyzed (Supplementary Fig. S1) to evaluate if AD death rate could be biased by life expectancy. In fact, countries with high solar incidence have lower life expectancy than countries with low solar incidence (Supplementary Fig. S1a) and all evaluated countries with life expectancy below 75 years have AD death rate less than 10 deaths per 100,000 habitants (Supplementary Fig. S1b). We then analyzed the subgroup of countries with life expectancy equal or greater than 75 years (90 countries in total). Figure 1b shows that low solar incidence countries have significantly greater AD deaths rate when compared with high solar incidence countries, even when low life expectancy countries are excluded from the analysis.

Although the data presented here do not confirm the relationship between VD deficiency and AD, it can be interpreted as an indicative of this relationship, since solar incidence affects directly the synthesis of VD3. ^{23 –27} Hypovitaminosis D usually reflects a reduced sun exposure. This condition must be treated with an adequate exposure to solar radiation instead of exogenous supplements. ²⁷ According to Krzywanski et al., sun exposure proves to be more efficient in VD acquisition than oral supplementation. Proximity to the equator line is a strong stimulus for VD synthesis since efficiency in VD production increases in high ultraviolet B radiation. ²⁵ Supplementation with VD2 (obtained by foods) is described to be less efficient than supplementation with VD3 (obtained by ultraviolet radiation). ¹³ In addition, the relationship between VD receptor (VDR) and VD concentration may be influenced by sun exposure but not by dietary intake of VD. ²⁶

Proteins associated with VD deficiency and AD

Several proteins are known to be involved with AD pathophysiological processes (Fig. 2a). Other proteins are described as having neuroprotective effect on AD (Fig. 2b), improving the prognosis and/or reducing the damage caused by the disease. Some of the proteins involved with AD are directly or indirectly regulated by VD (Table 1). Among the proteins known to be involved in AD progression, one of the most important is APP. APP (also known as Aβ protein) is a transmembrane protein that is cleaved to form Ap oligomers (amyloid peptides). These fragments form the protein base of the amyloid plaques found in AD patients. ²⁸ The biological functions of this protein have been investigated for many years and believed that APP is involved in synaptic and cellular adhesion, axonal degeneration and inhibition, intracellular signaling, apoptosis, among others. In addition, a body of evidence suggests a trophic function of APP in neurons and synapses. ²⁹ APP fragments are intimately involved with events that lead to progressive neurodegeneration in AD. Toxicity of Ap monomers leads to calcium homeostasis disturbance and changes in brain-derived neurotrophic factor (BDNF) levels (Fig. 2a), affecting the well functioning of neurons and their synapses. ^30,31 Low levels of BDNF are found in Alzheimer's patients. ³²

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FIG. 2.

Proteins associated with VD and AD. Proteins related to AD are shown according to their neurotoxic (a) or neuroprotective (b) role in AD. APP is directly involved with AD neurotoxicity. APP is involved in reducing BDNF levels, disrupts PTEN signaling and calcium homeostasis. The ɛ4 APO E allele contributes to APP deposition and neurofibrillary tangle formation. APP and RAGE exert a positive feedback on their own expression. RAGE is indirectly involved with neurofibrillary tangle formation and stimulates BACE-1 and neuroinflammation, which are indirectly related to AD. RAGE is activated by ROS and also increases oxidative stress. RyR and mTORC are involved with neurotoxicity in AD. The increased calcium influx sensitizes RyR. VD is able to suppress RAGE and APP expression and is indirectly associated with antioxidant synthesis calcium influx. Klotho is able to reduce ROS and inflammation, to reduce calcium influx, and to have a beneficial role in synapses and myelination. VD and VDR stimulate Klotho synthesis and VDR suppresses APP. VD enhances PTEN expression, PTEN inhibits mTORC and decreases TAU aggregates, playing a neuroprotective role in AD. Elements in the figure are not showed in real scale. ROS, reactive oxygen species; VD, vitamin D; APP, amyloid beta precursor protein; VDR, vitamin D receptor; RyR, ryanodine receptor; PTEN, phosphatase and tensin homologue; RAGE, receptor for advanced glycation end products; BACE-1, β-site amyloid protein cleavage enzyme 1; BDNF, brain-derived neurotrophic factor; APOE, apolipoprotein E.

Dysregulation in amyloid metabolism, such as APP clearance decrease and/or APP overproduction, is described as the most important molecular event leading to AD pathology and symptoms. An important risk factor for AD is the ɛ4 allele of apolipoprotein E (Apo E). ³³ The ɛ4 allele contributes to amyloid protein deposition. Currently, two models seek to explain the role of Apo E in APP accumulation. The first consists in endocytosis of soluble APP and Apo E associated with a lipid particle followed by APP aggregation in amyloid fibrils, which are released into the extracellular environment. The second justifies increased APP production through cellular cholesterol increase. The cholesterol deposition in cellular membranes contributes to Aβ protein synthesis. ³⁴ The ɛ4 allele of Apo E is associated with neurofibrillary tangles, amyloid proteins, phosphorylated TAU protein, and accelerated memory decline. ³⁵

Experimental data showed that VD deficiency contributes to APP accumulation, reducing the protection against cognitive decline, contributing to poor performance in tasks related to memory and learning, and suppressing the hippocampal synaptic plasticity. Human studies showed that individuals with high concentrations of VD have low amounts of APP, regardless of gender, age, family history, and concentrations of Apo E. Treatment with VD can stimulate Aβ peptide phagocytosis, suppress APP gene transcription, and Aβ oligomer synthesis, controlling the inflammatory process and increasing the VDR expression. ³⁰

In addition to the amyloid precursor protein, the ryanodine receptor (RyR) is also associated with AD progression (Fig. 2a). RyR is a family (RyR1, RyR2, and RyR3) of ion channels known to be responsible for calcium release from intracellular reserves during striated muscle contraction. RyR might be regulated by several proteins, ions, as well as redox modifications. RyR3 is the brain isoform expressed in the hippocampus, thalamus, Purkinje cells, and striatum, and is related to synaptic transmission and plasticity. ³⁶ RyR 1 and RyR 2 are also found in the brain, although they are not predominantly present in the nervous system. ³⁷ The increased calcium influx through L-type calcium channels sensitizes RyR, which is associated with memory loss and age-related cognition decline. Antioxidants prevent a decline in cognition, long-term depolarization, and memory loss by preventing the RyR sensitization. In this way, VD is important for the indirect synthesis of antioxidant enzymes and thus in AD prevention. ^{1,3,38 –41} In addition, VD decreases calcium influx through L-type calcium channels, preventing RyR sensitization. Aβ oligomers are responsible for increasing intracellular calcium concentration, contributing to memory loss by inducing long-term depolarization and by activating RyR. Dantrolene sodium, a drug used in AD treatment, inhibits RyR by reducing the intracellular calcium. Klotho is also involved with intracellular calcium uptake reduction, preventing the forgetfulness of AD. ⁴²

The transmembrane protein Klotho is present in cerebrospinal fluid and is known to be expressed in the brain choroid plexus, parathyroid glands, and kidney tubular cells. ⁴³ Klotho is associated with antioxidant synthesis, reasoning, memory, and cognition. ^31,44 In addition, Klotho increases the expression of peroxiredoxins (Prx-2, Prx-3) and thioredoxin reductase 1, which act together to reduce ROS. ⁴² Klotho gene transcription is stimulated by VD through interaction with VDR and retinoid X receptor (RXR) (Fig. 2b). Klotho deficiency is related to aging, early death, weight loss, infertility, calcified arteries, atherosclerosis, emphysema, skin atrophy, and osteoporosis. ⁴⁵ The protein is found in low concentrations in women when compared with man, elderly people, VD deficients, and Alzheimer's patients. ⁴³ Klotho synthesis decreases with aging and overexpression is associated with a longer life. Studies have associated Klotho deficiency with increased lipid peroxidation and hippocampal DNA damage in mice, leading to cognitive decline, aging, and cerebral pathology. ⁴³

Klotho plays an important role in brain myelination and synapses at the hippocampus and frontal cortex. ^43,46 Another suggested role of Klotho is a protective effect on inflammation by suppressing TNF-α expression and attenuation of NF-κB activation and IkB phosphorylation. ³¹ Experiments have reported that Klotho deficiency is involved in cyclooxygenase 2 (COX-2) overexpression. ⁴⁷ COX may be involved in neurodegenerative mechanisms, and administration of nonsteroidal anti-inflammatory drugs might reduce AD risk. In fact, COX2 is involved with APP deposition and is found to be increased in areas related to memory in AD patients. ⁴⁸ The importance of Klotho in synapses, antioxidant synthesis, protective effect on inflammation, and protection of neuronal myelin may point that protein as central in AD.

Mammalian target of rapamycin complex 1 (mTORC1) is a complex regulated by insulin pathway, formed by the proteins LST8, KOG1, TCO89, TOR1, PRAS40, and DEPTOR. It is known that Klotho indirectly regulates the activity of mTORC1 by inhibiting the insulin signaling pathway. ^42,49 mTORC1 controls cell growth through signaling pathways that respond to nutritional integration signals, such as availability of amino acids. mTORC1 is also involved in cell life, environmental stress response, and autophagy. ⁴⁹ Increase in mTORC1 signaling pathways is associated with aging, AD, cancer, and type 2 diabetes. The exact mechanisms by which the mTORC1 complex influences AD are not understood. However, it is known that VD inhibits mTORC1 and increases the expression of phosphatase and tensin homolog (PTEN) transcript 4, which also inhibits mTORC activity (Fig. 2a). ⁴²

PTEN is an intracellular protein present in low concentrations in AD patients. PTEN is important for cell survival and growth, participating in several cell-signaling pathways. PTEN has a lipid phosphatase activity that modulates cell cycle progression, inhibits cell migration, and integrin-mediated cell spreading. PTEN plays a key role in AKT-mTOR signaling pathway, controlling the integration process of newly developed neurons during adult neurogenesis. ⁵⁰ Researches provide evidences that Aβ oligomers disrupt PTEN signaling, which might lead to loss of synapses. ⁵¹ VD enhances PTEN expression through the VD receptor and thereby prevents AD progression (Fig. 2b). ^42,52 PTEN can be inhibited in VD deficiency, impairing cellular functions and neuronal survival, playing a neuroprotective role in AD. PTEN has been described to affect TAU phosphorylation, aggregation, and association with microtubules. Significant PTEN loss in AD patient brains has been correlated with increased phospho-tau at Ser-214 concentration. ⁵³

The receptor for advanced glycation end products (RAGE) is also associated with AD progression. RAGE is a member of cell surface immunoglobulin families, described as a signal transduction receptor for nonenzymatic glycation end products and is considered a proinflammatory receptor. ⁵⁴ Advanced glycation end products (AGEs), generated by hyperglycemia and/or oxidative stress, can mediate inflammatory cell activation through RAGE. It can result in hypoxia, ischemia, and arterial lesion. ⁵⁵ There is an increasing body of evidence of AGE involvement in neurodegenerative processes, including AD. RAGE may be involved in Ap monomer aggregation. Experiments suggested that RAGE can bind to Aβ42 protein and regulates its transport across the blood/brain barrier. ⁵⁶ This receptor is involved in amyloid protein production and accumulation, neurofibrillary tangle formation, synaptic transmission deficit, and neurodegeneration. RAGE contributes to Aβ protein synthesis by increasing inflammatory response and oxidative stress. ⁵⁷

Aβ oligomers activate RAGE when ROS are generated (Fig. 2a). RAGE activation can result in neuroinflammation, neurodegeneration, and memory loss. It is also known that RAGE increases oxidative stress by NADPH oxidase activation. ⁴² RAGE is involved in TAU hyperphosphorylation, a phenomenon observed in AD. ^{57 –59} TAU phosphorylation allows proteins to bind together and form tangled threads. ⁶⁰ Treatment with hesperidin can suppress oxidative stress and inflammation through RAGE inhibition, conferring neuroprotection. ⁶¹

The G8S2 polymorphism of the RAGE receptor encoding gene is also associated with AD and this polymorphism increases AD risk. G8S2 polymorphism may contribute to Aβ protein production and its accumulation in the brain since the single nucleotide polymorphism (SNP) is located in an exon that may be involved in signaling change responsible for accelerating amyloid protein processing. In addition, RAGE activation stimulates β-site amyloid protein cleavage enzyme 1 expression, which is important for Aβ protein production (Fig. 2a). The amyloid protein and RAGE can exert a positive feedback on their own production and expression. The G8S2 polymorphism may also be responsible for increasing Aβ protein transport in the brain. ⁶²

Deletion of RAGE gene has been described to decrease intracellular Aβ levels. This deletion decreases mitochondrial dysfunction in cortical neurons in culture. ⁶² RAGE inhibition is also involved with the reduction of inflammatory cell migration and proliferation. ⁵⁵ VD treatment was able to reduce RAGE expression in cell line experiments and decrease RAGE expression in diabetic rats. ^{63 –66}

Finally, VD receptor is associated with a neuroprotective effect in AD, improving the prognosis (Fig. 2b). VDR is the only high-affinity receptor to VD and it is the main mediator for VD actions. The genomic and nongenomic actions of VD are mediated by nuclear and membrane receptors, respectively, and the VDR is identical in both locations. It has been proposed that VDR may use alternative mechanisms to interact with genomic DNA. These alternative mechanisms can explain some of the specific cellular actions of VDR, as well the repressive functions on gene transcription. ^17,67 In the human brain, VDR is expressed mainly in the hypothalamus, thalamus, hippocampus, dentate gyrus, substantia nigra, and cortex, both in neurons and astrocytes. VD has several functions in the brain, such as cell proliferation, differentiation, and apoptosis, and it can contribute to a neuroprotective effect. ^14,17,64,68 Experiments with mice showed that they could not survive without VD receptors. ⁶⁹

Polymorphisms in VD receptor have been described as risk factors for AD by impairing learning and memory. These polymorphisms deregulate calcium and redox signaling pathways and increase calcium influx. ⁴² VDR polymorphisms may also decrease VD affinity to the receptor, affecting neurotrophin expression, which may lead to aging and neuronal death. ¹⁷ In fact, an increase in VDR polymorphisms was found in Alzheimer's brains and a decrease in VDR mRNA levels was reported in hippocampus affected by AD. ¹ Polymorphisms in the VDR gene promoter 5′ region affect mRNA expression, whereas in the 3′ untranslated region affect mRNA stability. The “A” allele of the Apal VDR polymorphism has been suggested as a risk allele to AD. Another risk allele is the “T” allele, of the Taql polymorphism. ²² Another VDR polymorphism associated with AD corresponds to the CDX2 allele polymorphism (SNP CDX2), which results in decreased VD receptor activity and increased Aβ protein expression. ⁷⁰ These polymorphisms may be associated with changes in translation efficiency and mRNA stability. ²²

In vitro experiments showed that VDR overexpression or VD treatment suppresses APP transcription in neuroblastoma cells. ⁷⁰ In addition, amyloid protein can trigger neurodegeneration and cytotoxicity by suppressing the VDR expression. ⁷¹ VDR interacts with SMAD 3 protein, which is involved in Aβ protein processing through TGF-β signaling. Finally, VDR inhibition may result in neurotoxic effects, cognitive and behavioral deficits, premature aging, decreased life expectancy, and calcium homeostasis dysregulation. ¹⁷