Apolipoprotein E4, Gender, Body Mass Index, Inflammation, Insulin Resistance, and Air Pollution Interactions: Recipe for Alzheimer’s Disease Development in Mexico City Young Females

Abstract

Given the epidemiological trends of increasing Alzheimer’s disease (AD) and growing evidence that exposure and lifestyle factors contribute to AD risk and pathogenesis, attention should be paid to variables such as air pollution, in order to reduce rates of cognitive decline and dementia. Exposure to fine particulate matter (PM_2.5) and ozone (O₃) above the US EPA standards is associated with AD risk. Mexico City children experienced pre- and postnatal high exposures to PM_2.5, O₃, combustion-derived iron-rich nanoparticles, metals, polycyclic aromatic hydrocarbons, and endotoxins. Exposures are associated with early brain gene imbalance in oxidative stress, inflammation, innate and adaptive immune responses, along with epigenetic changes, accumulation of misfolded proteins, cognitive deficits, and brain structural and metabolic changes. The Apolipoprotein E (APOE) 4 allele, the most prevalent genetic risk for AD, plays a key role in the response to air pollution in young girls. APOE 4 heterozygous females with >75% to <94% BMI percentiles are at the highest risk of severe cognitive deficits (1.5–2 SD from average IQ). This review focused on the relationships between gender, BMI, systemic and neural inflammation, insulin resistance, hyperleptinemia, dyslipidemia, vascular risk factors, and central nervous system involvement in APOE4 urbanites exposed to PM_2.5 and magnetite combustion-derived iron-rich nanoparticles that can reach the brain. APOE4 young female heterozygous carriers constitute a high-risk group for a fatal disease: AD. Multidisciplinary intervention strategies could be critical for prevention or amelioration of cognitive deficits and long-term AD progression in young individuals at high risk.

INTRODUCTION

Increasing evidence suggests that Alzheimer’s disease (AD) pathogenesis includes strong interactions between a harmful environment and systemic and neural inflammatory, immunological, and metabolic responses impacting the brain from the early stages of AD development. Detrimental effects of exposures to complex mixtures of air pollutants are seen in urban children and young adults including progressive cognitive deficits together with structural, volumetric, and neuropathological hallmarks of neurodegeneration. In the developing brain, from fetus through adolescence and young adulthood, harmful exposures can lead to dysregulation of genes and result in oxidative stress, neuroinflammation, DNA damage, and NFκB signaling, deficiencies in neuroprotective mechanisms, and activation of neurodegenerative cascades. Genes that confer risk for early or late onset AD pose additional burdens that strongly influence occurrence of neurodegeneration.

The apolipoprotein E (APOE) ε4 allele is the most prevalent genetic risk factor for AD [1], and poses a stronger risk in women. In Mexico City and the US, ε3 is the most common allele, while the prevalence of ε4 allele in Mexico varies from 8.4% in Guadalajara, to 11.5% in Nayarit and Durango, to 28% among Huichol Indians [2]. In our experience (LCG personal data), 18–20% of Mexico City residents carry an allele 4, compared to about 13% of a predominantly white Caucasian population in the Framingham Heart Study [3, 4].

However, since AD is an aging-associated disease, co-factors, particularly those that accelerate various aspects of aging most likely interact with the APOE ε4 allele to impact key factors favoring neurodegeneration, likely at an earlier age than in people who lack an APOE ε4 allele [5]. Systemic inflammation, metabolic imbalance, and neuroinflammation are recognized factors contributing to neurodegeneration, and the very same effects occur in response to chronic exposure to urban air pollution [5-20]. Based on a 9- year cohort follow-up study of 95,690 individuals, 65 years of age and older, Jung et al. estimated that AD risk increases by 211% per 10.91 ppb increase in ozone (O₃), and by 138% per 4.34 μg/m³ increase in environmental particulate matter <2.5 μm (PM_2.5) [21]. Their findings suggest long-term exposure to levels of O₃ and PM_2.5 that exceed the current US EPA standards increases risk of AD [21].

In this review, we present published data concerning cognitive, behavioral, and cerebral neuroimaging effects of air pollutant exposure in children, and posit a hypothesis that positive feedback loops set the stage for progressive neurodegeneration. However, in Mexico City, the etiopathogenic factors are likely to be multifaceted such that, besides chronic exposures to complex mixtures of air pollutants, lifestyle and socioeconomic factors also cause stresses that drive systemic inflammation, insulin resistance, hyperleptinemia, neuroinflammation, neurodegeneration, structural/metabolic brain changes, and epigenetic modifications. APOE ε4 genotype is at the center-piece of our hypothesis.

Our discussion will focus on evidence that the effects of APOE ε4 allele “cooperate” with chronic air pollutant exposures and socioeconomic/lifestyle factors to promote obesity, diabetes, systemic inflammation, insulin resistance, and cognitive impairment, leading to AD-type neurodegeneration at high rates and at early ages (Fig. 1).

Fig. 1.

Conceptual framework of how air pollution interacts with apolipoprotein E (APOE) ε4 genotype, female gender, and obesity to drive metabolic syndrome (MetS), cognitive decline and Alzheimer’s disease (AD). In the setting of chronic high-level exposures to PM_2.5 and significantly high concentrations of iron-rich combustion-derived magnetite nanoparticles, present in Mexico City’s polluted air, individuals carrying an APOE ε4 allele, particularly girls, are predisposed to develop insulin resistance and metabolic dysfunction which drive obesity. Having a single risk factor (APOE ε4, increased BMI, or female sex) increases the likelihood of developing mild insulin resistance, Type 2 diabetes mellitus (T2DM), and mild cognitive impairment (MCI). Dual risk factors are more precarious and promote moderate insulin resistance, MetS, and cognitive decline. Finally, having three risk factors is the most serious as that state predisposes children to developing obesity, severe systemic insulin resistance (IR), MetS, and brain IR, and ultimately dementia due to AD.

AIR POLLUTANT EXPOSURES, COGNITIVE AND BEHAVIORAL EFFECTS IN CHILDREN

Millions of children around the world regularly breathe air that has unsafe, toxic levels of air pollutants. Chronic exposure to polluted air has been linked to premature deaths, particularly in developing nations [22]. Although everyone is potentially harmed, genetic factors, age, gender, nutritional status, epigenetic factors, cognitive reserve, socioeconomic status, health status, occupation, and lifestyles pertaining to physical activity and modes of transportation, can increase “susceptibility” to the adverse effects of chronic air pollutant exposure. What makes this type of research difficult to generate clear conclusive messages is that risk is modified by a host of environmental factors including characteristics of air pollutants, e.g., the nature and levels of toxic gases, the sources and sizes of organic and inorganic components of PM, meteorological conditions, and ambient temperature [23-26]. The abundant presence of Fe-rich magnetite nanoparticles in the brains of Mexico City young and old residents, the location of these particles in key organelles such as mitochondria, their association with transition metals, and their ubiquitous presence in airborne PM warrants the need to examine these powerful producers of damaging reactive oxygen species as part of the pollution scenario [27].

Air pollution-associated cognitive, behavioral, and cerebral imaging effects in the pediatric population are broad-ranging in nature and severity, and include: 1) behavioral problems; 2) deficits in sustained attention; 3) higher indices of hyperactivity; 4) decreased verbal IQ index; 5) reduced growth in cognitive development; 6) lower scores on metacognition assessments; and 7) reduced functional integration and segregation in key brain networks such as the default mode network and stimulus-driven mental operations [28-36]. In addition, significant reductions in placental brain-derived neurotrophic factor (BDNF) and synapsin 1, which are key mediators of normal neurodevelopmental trajectories, have been linked to childhood neurobehavioral deficits following maternal chronic air pollutant exposures during pregnancy [37].

We strongly suggest that air pollution is a causative agent because in contrast to children exposed to low concentration of PM and other pollutants, children residing in Mexico City where the ambient fine PM (PM_2.5), the magnetite combustion-derived nanoparticles, and ozone concentrations are significant, exhibit pronounced cognitive deficits with metabolic, structural, and volumetric changes in the brain [6, 10, 12, 15, 16]. Furthermore, more recent findings revealed that the APOE ε4 genotype is a major factor contributing to worse adverse effects associated with air pollution in Mexico City [18]. These observations lend strong support to the concept that APOE ε4 genotype interacts with molecular and biochemical adverse effects of childhood environmental exposures, including air pollution, to enhance deleterious effects of metabolic dysfunction that already has been shown to cause or propagate neurodegenerative diseases, including AD.

APOE ε4 AND CHILDREN’S NEURAL RESPONSES

The fact APOE ε4 allele is the strongest known genetic risk factor for late and early onset AD, and it is strongly associated with oxidative stress, mitochondrial dysfunction, endoplasmic reticulum stress, immune dysfunction, dysregulated lipid and glucose metabolism, and impairments in white matter (WM) integrity is relevant to highly exposed air pollution pediatric populations since we are able to identify precisely these associations in children [1, 38-50]. Its direct link to AD pathology pertains in part to the earlier and higher levels of amyloid-β (Aβ)_1-42 accumulation in brains of individuals with APOE ε4 allele. Mechanistically, APOE ε4 influences Aβ clearance and aggregation, and it also impacts AD pathology in Aβ-independent ways [1, 51-54]. Besides the APOE ε4 genotype, age and female sex are established risk factors for AD [1, 51, 55-57].

APOE ε4 is a key modulator of children’s responses to cumulative air pollution exposures, leading to early detrimental effects on cognition [18]. In a study by Dean et al. [58], brain magnetic resonance imaging of 162 healthy, 2- to 25-month-old infants and toddlers showed that the APOE ε4 carriers had reduced WM myelin water fraction and gray matter volume measurements compared with APOE ε4-negative subjects. Altered WM myelin water fraction measurements were observed in the precuneus, middle and posterior cingulate, lateral temporal, and medial occipitotemporal regions, which are targeted in AD [58]. Independent of air pollution or other known chronic exposure effects, Heise et al. [46] used diffusion tensor imaging to show that APOE ε4 modulates brain WM structure long before clinical and neurophysiological manifestations of impending AD emerge in healthy younger (aged 20–35 years) and older (aged 50–78 years) adults.

Through the use of neuroimaging and assessments of neurofunction, Chang et al. [59] also demonstrated striking effects of APOE ε4 in young children. Those with ε2/ε4 genotype had the smallest hippocampi, while those with ε4/ε4 genotype had the lowest measurements of hippocampal fractional anisotropy and age-dependent thinning of the entorhinal cortex, and those with ε3/ε4 genotype had the largest cortical area measurements in the medial orbitofrontal region [59]. Importantly, the APOE ε4 effects on brain structure were predictive of low scores on executive function and working memory tasks [59]. Similarly, using combined approaches to assess effects of APOE genotype on brain function and structure, Filippini et al. [60] found that brain functional over-activity of young APOE ε4 carriers was disproportionately reduced with advancing age, before the onset of measurable memory impairment. While researchers generally agree that APOE genotype has intrinsic effects on differentiation of functional networks in the brain [61-63] and impact cerebrovascular reactivity in young subjects, whether genotype is sufficient and causal remains controversial. For example, contrary to the above studies, Dell’Acqua et al. [52] found no associations between microstructural properties in WM and the APOE ε4 or APOE ε2 alleles in young healthy adolescents. They concluded that neural effects of these variants are not evident in 14-year-olds, and may only develop later in life. While their observations appear to contradict results from other studies, in fact they may reinforce our proposed interactive roles of APOE ε4 genotype with injurious early life exposures such as air pollution since the Dell’Acqua et al. study was conducted with subjects who resided in low pollution environments. This concept should be considered in future studies that evaluate pediatric populations from multiple residential environments that differ with respect to levels of air pollution exposure [59].

APOE ε4 AND SYSTEMIC CHANGES

APOE influences lipid serum levels in children and adults [64, 65]. Correspondingly in a study of 400 women from Southwest Mexico, APOE ε4 carrier status was found to be correlated with high levels of low-density lipoprotein cholesterol [66]. Furthermore, the APOE ε4 allele is an independent risk factor for Type 2 diabetes mellitus (T2DM), cerebrovascular disease, and dyslipidemia across ethnic groups [66-72]. In a study of 300 Mexican children age 9.05 ± 0.80 years, 46% of children were either overweight or obese, and waist circumference, a surrogate marker of visceral obesity and insulin resistance, was significantly correlated with APOE ε4 allele carrier status [71]. In essence, besides its role in Aβ clearance, APOE β4 carrier status promotes dyslipidemia, phospholipid dysregulation, and insulin resistance, which increase the risk for AD [50, 53, 54, 73].

APOE ε4 AND VASCULAR RISK FACTORS

Vascular pathology is extremely common in neurodegenerative diseases [74-82], yet it is not known whether this is a cause or consequence of the underlying disease mechanisms. However, given the strong association between APOE ε4 and atherosclerotic cardiovascular disease, it is likely that APOE ε4 carrier status contributes to vascular degeneration and dysfunction in AD and vascular dementia [74]. This concept is supported by the finding that harboring one or two APOE ε4 alleles elevates the risk of vascular dementia [78]. The lower risk for vascular dementia compared with AD [78] is explainable by the other adverse effects that APOE ε4 has on Aβ_1-42, glucose, and lipid metabolism, and rates of T2DM and metabolic syndrome (MetS). It is widely accepted that diabetes (controlled and uncontrolled) and MetS along with vascular risk factors, smoking, and excess body weight/obesity negatively impact multiple cognitive domains [81] and increase the risks for mild cognitive impairment and AD.

The confusion regarding the role of vascular factors in dementia concerns the nature of vascular-mediated injury. It is important to distinguish effects of large infarcts resulting from large and medium size vascular occlusions from extensive small vessel disease, which is quite prominent in WM. While large infarcts alone are not sufficient to cause dementia and instead lead to focal deficits, less well delineated types of injury falling under the category of leukoaraiosis, occur in many neurodegenerative diseases, including AD. In the Nun-Study [83], although brain infarcts alone were found to have little effect on cognitive status, their presence did increase the risk of dementia associated with AD pathology. A second pertinent finding in that study was that APOE ε4 carrier status significantly influenced development of AD but not vascular dementia. However, since the Nun study [83] included only women, it was not possible to assess effects of sex. Moreover, as discussed earlier, it is likely that the APOE ε4 genotype’s effects are heterogeneous [76] and mediated through various pathways including metabolism, insulin responsiveness, and Aβ clearance.

The aspects of degenerative vascular pathology that are least understood in relation to AD and probably many other forms of neurodegeneration, concern the mechanisms and consequences of microvascular disease. This subject has proven to be stubbornly difficult to tease out due to the presence of concomitant pathology in WM. Brun and Englund were first to recognize WM atrophy and degeneration in AD [84]. Subsequently, it was found that WM pathology and vascular degeneration occurred early in the course of, including in its preclinical stages [85]. Moreover, AD-associated WM microvascular pathology resembles the histopathologic changes associated with T2DM and hypertension. Is WM microvascular pathology always the result of metabolic dysfunction as occurs throughout the body in diabetes and hypertension? What is the role of the APOE ε4 allele? Does it cause injury or does it fail to protect? This discussion is important because it is well established that aging is the most important risk factor for AD, yet vascular risk factors and reduced WM integrity are most prevalent in older adults, and APOE ε4 carrier status exacerbates microvascular disease related alterations in WM microstructure [86].

Microvascular disease contributes to brain pathology in three major ways: 1) it restricts flow due to narrowed lumens; 2) vasoreactivity is poor due to fibrosis of the walls; and 3) poor integrity leads to leakage of blood products and other potentially toxic substances from the peripheral circulation. Cerebral vasoreactivity is critical for maintaining continuous perfusion. Reduced vasoreactivity, which marks microvascular injury, is significantly associated with APOE ε4 carrier status [82]. With regard to the loss of vascular integrity, evidence suggests that a glial-lymphatic (glymphatic) system distributed in perivascular spaces has an important role in clearing Aβ from the brain [80]. Using convective flow generated by arterial pulsations, glymphatic clearance may be responsible for draining approximately 60% of Aβ from the brain to cervical lymph nodes [87]. Early in AD, reduced cerebral blood flow and local gray matter blood volume correlate with increased vascular leakage, and potentially links neurovascular damage to impairments in cerebral blood flow, function of neurovascular unit, and integrity of the blood-brain barrier [88]. Mechanistically, dysfunction of the neurovascular unit is mediated by diseased/damaged astrocytes and pericytes during the course of AD development [89-91]. Early damage to the neurovascular unit is a key feature of Mexico City residents [17, 19]. Tight junction and neural antibodies are present in serum and cerebrospinal fluid (CSF) of Mexico City children. Cryptic ‘self’ tight junction antigens can trigger an autoimmune response potentially contributing to the neuroinflammatory status. Major findings in healthy Mexico City resident dogs (age 3.17 ± 0.74 years), and children and teens (age: 12.64 ± 4.2 years) versus clean air controls, included leaking capillaries and small arterioles, significant amounts of lipofuscin in pericytes, smooth muscle and endothelial cells, thick cerebrovascular basement membranes with small deposits of amyloid, patchy absence of the perivascular glial sheet, enlarged Virchow-Robin spaces and nanosize particles (20–48 nm) in endothelial cells, basement membranes, mitochondria, axons, and dendrites. Tight junctions, a key component of the neurovascular unit, were abnormal in Mexico City versus control dogs (χ² < 0.0001), and WM perivascular damage was significantly worse in Mexico City dogs (p = 0.002) [19]. Thus, the microvascular component is strongly associated with air pollution environmental exposures in young urbanites and undoubtedly contributes to brain pathology.

THE IMPACT OF POLLUTED AIR ON A DEVELOPING APOE ε4 BRAIN: THE MEXICO CITY FEMALE SCENARIO. DO ALL ROADS LEAD TO ALZHEIMER’S DISEASE IN YOUNG GIRLS?

The daily challenges faced by most children living in Mexico City are multifaceted and dire. They can be categorized as: environmental (severe air pollution), genetic (APOE ε4 allele), socioeconomic (poverty, high urban stress), nutrition and lifestyle-based (high sugar intake, poor unbalanced diets, and lack of exercise), maternal-child health based (maternal obesity and diabetes), and social (low safety, poor schools, violence, addictions, adolescent pregnancies, very low rate of criminal convictions) [92-99].

Over the last two decades, life expectancy in Mexico has stagnated due to the high rates of homicides among men [95], T2DM and its complications, and childhood and adolescent obesity and MetS [93,100]. Canudas-Ramos and coworkers [95] stated that the number of years Mexicans lived with perceived vulnerability has increased by 30.5 million person-years over the last 10 years. This situation has a serious detrimental health impact over children and adults alike.

Mexican children and young adolescents are affected by obesity, MetS, and vitamin deficiencies. Children ages 10.6 ± 2.7 years with exogenous obesity and body mass indices (BMI) greater than 2.0 standard deviations from the mean, showed high prevalence rates (37.5% to 54.5%) of MetS [100, 101]. The lack of exposure to UV light, the nutrition deficiencies, and the reluctance of parents of allowing children to play outdoors due to the violence, are translated into vitamin D deficiencies among 87% of clinically healthy children residing in Mexico City (serum 25-hydroxyvitamin D below 30 ng/mL) [14].

With regard to obesity and alterations in metabolism, normal weight healthy children living in Mexico City have significant elevations in serum leptin and the powerful vasoconstrictor, endothelin-1. Leptin levels are strongly associated with the cumulative levels of PM_2.5 exposure. Therefore, chronic exposures to high environmental levels of PM_2.5 are associated with basal hyperleptinemia, altered appetite-regulating peptides, and increases in endothelin-1 (contributing to brain hypoperfusion) in clinically healthy Mexico City children [14, 102]. These findings may account for the increased rates of hypertension and early development of atherosclerosis among adolescents residing in Mexico City [103], and increased rates of liver injury manifested by elevated serum aminotransferase levels with MetS and obesity in adults who grew up and remained living in Mexico City [104].

Genes by environment

The impact of environmental exposures on susceptibility of Mexican populations to obesity, T2DM, and MetS is governed by genetic backgrounds that dictate susceptibility to disease and proneness to inflammatory responses. For example, functional polymorphisms of inflammatory-response genes such as peroxisome proliferator-activated receptor-γ2, are associated with insulin resistance and T2DM [105-107]. Native American ancestry is also a strong risk factor for diabetes among overweight and obese Mexican-American women [108], indicating that interactions between BMI and Native American ancestry modulate insulin resistance disease proneness. These interactive effects extend further and become very complex when considering interactions among genetic ancestry, environment, obesity, and behavioral patterns of food intake in Mexican mestizo children and adolescents, particularly girls [109, 110]. Conceivably, the complexity of these interactive responses could be the result of epigenetic modifications of brain function. For example, generations of maternal dietary imbalances that fail to meet nutritional needs of the fetus can cause intrauterine growth restriction, preterm delivery, low birth weight, excessive impairments in postnatal growth, and metabolic functions [108].

Maternal-fetal/child health and inflammation

The chronic inflammatory state associated with urban polluted environments renders placental barriers permeable to environmental insults [111, 112] that can significantly impact fetal development, including via epigenetic changes. In this regard, Tsamou and coworkers [112] measured expression levels of 6 candidate microRNAs (miRs) in 210 paired placental-neonate blood samples. Trimester-specific PM_2.5 exposures were estimated based on home address. Multiple regression analysis showed that placental expression of miR-21, miR-146a, and miR-222 was inversely associated with PM_2.5 exposure in the 2nd trimester, while placental expression of miR-20a and miR-21 was positively associated with 1st trimester exposures. The tumor suppressor phosphatase and tensin homolog (PTEN) is a common target of the affected miRs, and placental PTEN expression was strongly and positively associated with 3rd trimester PM_2.5 exposure [112]. PTEN is a multifunctional protein that is deregulated in many types of cancer; its silencing by mutation promotes tumorigenesis [113-116] and PTEN suppression could enhance axonal growth and functional recovery in adult central nervous system after injury [117]. However, while low levels of PTEN promote carcinogenesis and axonal growth, high levels activate pro-apoptosis and cellular degeneration pathways [118]. In addition, insulin and IGF-1 regulate PTEN phosphorylation [119] and a fat diet induces insulin resistance through TNFα and positive modulation of PTEN [120]. In fact, one of the common genetic variants of PTEN is associated with peripheral insulin resistance and T2DM [121]. Insulin resistance and T2DM have significant adverse effects on placenta and fetus [122]. Thus, the marked placental trimester specific changes in the tumor suppressor phosphatase and tensin homolog associated with PM2.5 certainly deserve investigation in different environmental scenarios.

Systemic inflammation and brain development

The PM_2.5 and toxic metabolic exposures that occur during pregnancy introduce one of the first hits to the developing brain. A second hit is introduced by return of the mothers with neonates to the urban home environment with continuous high levels of air pollution. The attendant chronic inflammation produced in the respiratory tract of infants leads to chronic systemic inflammation. Circulating inflammatory mediators can reach the brain, establishing a link between inflammatory responses to PM_2.5 and neuroinflammation [5-8, 10-12, 13, 123-125]. Consequences include increased formation of reactive oxygen species [126]. Another source of injury and neuroinflammation is direct inhalation of Ultrafine PM (UFPM), magnetite nanoparticles, PM–associated lipopolysaccharides (PM-LPS), and metal uptake via olfactory neurons which extend to corticolimbic pathways in the brain, i.e., the most vulnerable targets of AD. UFPM and magnetite nanoparticles can also traffic to the central nervous system via cranial nerves such as the trigeminal and vagus, the systemic circulation, and macrophage-like cells originating in the lungs [5, 8, 27]. Neuroinflammation is initiated by innate immune responses, activated via interactions between circulating cytokines and constitutively expressed cytokine receptors on brain endothelial cells, and propagated by local adaptive immune responses [127]. Innate immunity if mediated by monocytes that produce and secrete TNFα, interleukin-1β (IL-1β), and IL-6, which recruit and stimulate other immune cells. Fine and UFPM exposures may trigger neuroinflammatory cascades leading to endothelial cell activation and disruption of the neurovascular unit. We speculate that attendant alterations in the innate immune response to PM_2.5 and UFPM could trigger production of autoantibodies to actin and occludin/zonulin [17, 19] that attack cell junctions and neural proteins, worsen neuroinflammation, and set the stage for later onset of neurodegeneration. Without early intervention, such as avoidance of PM_2.5 and UFPM exposures [5, 13, 128], the developing brains are rendered highly susceptible to progressive neurodegeneration [128-130].

Neuroinflammation and neurodegeneration

Both systemic and neural inflammation propagate cascades of neurodegeneration by driving pathogenic innate and adaptive immune responses in the brain [131-138]. Chronic exposures to PM_2.5 and toxic metabolites can produce these effects [139] because they readily cross blood-brain and blood-placental-fetal barriers. Besides vasculocentric pathology, neuroinflammation increases reactive oxygen species which triggers aberrant signaling that mediates hyperphosphorylation of tau and activation of ubiquitin-proteasome pathways associated with neurodegeneration, including AD [140-141]. One of our most disturbing observations was that major AD-associated neuropathologic abnormalities including tau hyperphosphorylation and Aβ_1-42 diffuse plaques in the frontal lobe, and deposition of aggregated, hyperphosphorylated α-synuclein in olfactory nerves and key brainstem nuclei develop very early and are present in brains of young Mexico City residents [5, 9, 7, 19, 142]. Likewise, changes in major neural proteins are detectable in cerebrospinal fluid samples read as normal [20]. In 129 CSF samples from children, ages 11.9 ± 4.8 years, the mean levels of Aβ_1-42 and BDNF were significantly lower in Mexico City versus clean air control subjects (p = 0.005 and p = 0.02, respectively) [20]. Cumulative exposures to PM_2.5 up to 5 μg/m³ increased CSF levels of total cellular prion protein (TPrP), whereas higher exposure levels decreased CSF TPrP. The clinical, neurobehavioral, and neuropathological correlates of PM_2.5 exposure-related alterations in CSF Aβ_1-42, BDNF, and TPrP among young Mexico City urbanites include impairments in cognitive processing, deficits in odor identification, downregulation of frontal cellular prion protein PrP^C, and histopathological lesions of AD and Parkinson’s disease [20].

Excessive vulnerability of girls in Mexico City

Based on human and experimental data, we have argued that chronic exposures to ambient air pollutants drives inflammatory cascades that disrupt fetal and brain development and function. These effects could potentially adversely affect long-term brain function and activate major cascades of neurodegeneration. Given the in utero exposures mediated by blood-placental-fetus transfer of PM and other air pollutants, there is also potential for transgeneration transmission of innate and adaptive aberrant neuroinflammatory responses. A disturbing aspect of this scenario is that girls residing in Mexico City are significantly more vulnerable to these air pollutant responses, and as discussed earlier, this increased vulnerability is linked to APOE ε4 carrier status together with systemic insulin resistance mediated in part by overweight body habitus [18].

In the cohort study of 105 Mexico City children (12.32 ± 5.4 years), APOE ε3/ε4 (n: 36) genotype was associated with deficits in attention, short-term memory, and Verbal, Performance and Full Scale IQ assessments relative to APOE ε3/ε3 (n = 69). In addition, fasting blood glucose was significantly higher in APOE ε4 carriers. Further dissection of the data divulged additional co-factors mediating these effects. Among them were BMI and female sex. BMIs were higher in APOE ε3/ε4 females than in APOE ε3/ε4 males and APOE ε3/ε3 boys and girls, and girls with BMIs between the 75th and 94th percentiles had the largest deficits in Total IQ, Performance IQ, Digit Span, Picture Arrangement, Block Design, and Object Assembly, i.e., they were at high risk for severe cognitive impairment. In addition, female sex was the main variable accounting for inter-group differences in serum insulin, HOMA-IR and leptin [18]. Altogether, the findings support our thesis that Mexico City APOE ε4 young girls are at significantly increased risk for developing neurodegeneration, particularly AD.

Being a young female in Mexico City is associated with negative IQ effects, particularly on Performance IQ. There is an extensive literature examining gender and neurocognitive function [143-151], but the literature is scarce in identifying complex air pollution exposures as a risk factor for gender related cognition deficits in children [152]. Factors at play to explain sex differences in selective attention for example, include underlying sex differences in core abilities, including spatial or verbal cognition [153], while others include variables such as exercise regime, duration, and intensity playing a role in hippocampal structural plasticity and in adult hippocampal neurogenesis [149]. Gender differences in volume and surface area are observed across time during brain development, suggesting a few cerebral regions exhibit cortical developmental changes as a function of gender [154]. The Gender Similarities Hypothesis states that males and females are quite similar on the majority-but not all-psychological variables [155]. Therefore, we expect equal cognitive abilities for males and females. This is interesting when one considers data on the dementia risk in both sexes, given that we see clear sex-specific differences in the modulation of redox proteins and WM and mitochondria proteomes of females [156]. Laws and colleagues reported that men outperform women at the same stage of AD in episodic memory, visuospatial tasks, language, and semantic abilities [157]. Thus, the gender differences are carried out at later times in life.

The issue is important because we see the significant contrast between APOE ε4 versus ε3 Mexico City children in the reduction of the right frontal WM NAA/Cr ratio in association with decreased attention, short-term memory, and significantly lower scores in Verbal and Full Scale IQ [15, 16]. Moreover, we have reported APOE modulated the group effects between WISC-R and left frontal and parietal WM, and hippocampus metabolites.

Very relevant for this review, hyperphosphorylated tau, Aβ_1-42 diffuse plaques, and neuroinflammation, are already present in Mexico City children regardless of APOE status but are significantly more severe in APOE ε4 children [5]. Equally relevant, APOE4 impacted negatively both children and parents sharing the allele 4 [16].

Reynolds and co-workers suggested APOE could represent a variability gene for depressive symptoms and spatial reasoning, but without an impact on BMI [158]. Ihle et al. [159] did not report APOE ε4–related cognitive effects in children and young adults, while Heise and co-workers [46] found in APOE ε4 carriers versus non-carriers, a reduction of fractional anisotropy and increase in mean diffusivity using diffusion tensor imaging in healthy 20–35 and 50-78-year-old adults. Interestingly, Heise and coworkers [46] found no significant interactions between APOE ε4 and age, suggesting that differences in WM structure do not show significant differential changes with age.

Both, the early reduction of fractional anisotropy and the increase in mean diffusivity values in APOE ε4 young individuals is a key observation for our Mexico City cohorts, given that WM metabolic and structural alterations impact cognition [46, 78, 160-171]. We strongly support that the interaction between air pollution and APOE alleles has to be taken into account at all ages [15, 16, 172]. Torres-Pérez and colleagues’ [173] work on APOE ε4 as a risk factor for cardiovascular disease is a key reference for this review. They analyzed 4,408 men and their data showed a gene dose-dependent association between APOE ε4 and increased risk for MetS [173]. This association was primarily from the overweight individuals. The same group evaluated 3,908 healthy young individuals from the Coronary Artery Risk Development in Young Adults cohort followed-up for 25 years, and APOE ε4 was significantly associated with increased risk of developing MetS. The critical interplay between APOE ε4 and the longitudinal development of fatness towards the onset of MetS occurred throughout the study, a key observation for Mexico City APOE ε4 girls. Torres-Pérez and colleagues concluded APOE ε4 increases MetS risk in a dose-dependent manner [173].

The work of Torres-Pérez [173] substantiates our pediatric findings: female gender is the main variable accounting for the difference in insulin, HOMA-IR, and leptin, while APOE ε4 is critical for the fasting glucose levels.

We are dealing with an ill-fated interplay of gender, APOE ε4, and metabolic abnormalities in young girls residing in a highly-polluted environment.

A key study by Jiménez-Pavón et al. [152] in European adolescents shows leptin as the only risk factor for insulin resistance in male adolescents, while in females, leptin, vitamin D, and fitness were the key independent risk factors [152]. Thus, gender is very important for insulin resistance in adolescents and relationships between obesity/adiposity and vitamin D reservoirs along with expression of insulin receptors and glucose transport likely play key roles in insulin resistance [152].

The issue is important because we have published evidence that systemic inflammation and immunodysregulation are early findings in exposed Mexico City children [8, 123] and since overweight and obesity causes low grade systemic inflammation-associated with dysfunctional adipose tissue [174, 175], consideration for a synergistic effect with air pollution inflammation is justified [176].

Lean Mexico City children exhibit hyperleptinemia and food reward hormone dysregulation versus controls, and PM_2.5 cumulative exposures are strongly positively associated with leptin [14].

The trajectory of seemingly healthy Mexico City children suggests that hyperleptinemia and food reward hormone dysregulation can be detected in the early pubertal stage (11.1 ± 3.2 years), before these children start putting on weight and before APOE ε4 heterozygous females show severe cognitive deficits (1.5-2SD from average IQ) at age 12.32 ± 5.4 years [18]. More worrisome, at age 11.9 ± 4.8 years, their CSF values for Aβ_1-42 and BDNF concentrations are already significantly lower versus controls [20]. Lavigne and coworkers [177] have a relevant paper showing strong associations between air pollution variables and cord blood leptin levels adjusted for birth weight z-score. Their sample included 1,257 mother-infant pairs from the Maternal-Infant Research on Environmental Chemicals Study, conducted in Canada between 2008 and 2011 [177]. We agree with Lavigne that prenatal exposure is very relevant to the potential development of childhood obesity.

Obesity is increased in low socioeconomic status minority populations, and the problem is of serious concern in Mexicans and Mexican-Americans for whom socioeconomic disadvantage, race/ethnic disparities and genetics play a key role in overweight and obesity status [84, 178-181].

Mexico City children have two serious problems: 1) They are unable to play outdoors or to travel to places where they can safely exercise due to the violence on the streets, playgrounds, and the public transport system, and 2) their intake of beverages with high fructose is encouraged by the media and allowed by the school system.

Mexico City has witnessed a steady increase in violent crime in the last decade along with a negative trust of authorities [182-183]. Homicides, kidnappings, armed robberies, car thefts, and various forms of residential/street crime are everyday concerns [184]. Muggah and Vilalta [182] discussed the strong relationships between high-crime areas and underdevelopment, income, and family disruption. Kristiansson et al. [94] hypothesized that the effects of poverty and associated air pollution-related stress on impaired cognitive skills are mediated by inflammatory cytokines. All these factors are negatively impacting children and teens, unable to have age-related activities outside the home. Also of serious health negative impact, Mexico is the world’s biggest per capita consumer of soft drinks and the change from cane sugar to high fructose corn syrup will aggravate obesity, chronic metabolic disease, cognitive decline, and the risk of AD [185-187]. Making the problem more complicated,>85% of Mexico City children and teens have 25-hydroxyvitamin D serum concentrations below <30 ng/mL [14], and since vitamin D synthesis is greatly influenced by outdoor pollution, latitude, altitude, darker skin pigmentation, and deficient nutritional intake [14, 188, 189], the problem is likely not to be solved for a while.

THE RECIPE FOR ALZHEIMER’S DISEASE DEVELOPMENT AND WHAT ABOUT PREVENTION?

A growing theme for prevention of neurodegenerative diseases is the characterization of individuals’ APOE genotype to identify at-risk participants [173]. Currently, our group is making progress in identifying APOE ε4 girls with serious cognitive deficits. The work of Sampedro et al. [190] is pertinent to our discussion: the effects of APOE ε4 on brain metabolism and structure are modified by gender. Their work demonstrates healthy elderly female APOE ε4 carriers had lower CSF Aβ_1-42 and higher CSF p-tau181p values compared with non-carriers and showed greater hypometabolism and atrophy than male carriers [190].

The dreadful picture in Mexico City APOE ε4 girls could signal their future trajectory toward the development of progressively worse cognitive impairment and AD.

This is complicated by underprovided Mexico City public schools, deficient in the development of executive function skills with a resultant lack of cognitive reserves [11], deficits in Performance and Total IQ tests in APOE ε4 children with a resultant negative impact on a projected 2.16 million APOE ε4 pediatric carriers out of the 24 million residents in the Mexico City Metropolitan Area including 16 delegaciones in Mexico City and 28 municipios in the adjacent Mexico State.

We would like to be optimistic in this issue and assume we have roughly a 50-year window of opportunity between the time urban children might be experiencing the detrimental effects we are describing and when they might undergo neurologic diagnostic assessment, allowing for targeted preventive intervention of vitamin D deficiency and insufficiency, access to indoor spaces for exercise away from air pollutants, public school curricula improvement to build executive function skills and increase their cognitive reserves, free school healthy lunches, and good pediatric care including mental health services. Early interventions should be integrated in health and educational agendas targeting Mexico City children, especially girls. The need for interventions aimed at breaking the cycle of poverty, poor food security, high unemployment, economic inequality, low maternal education, poor quality of the home environment epidemic violence, feminicides, impunity, corruption, addictions, air pollution, and their negative health consequences becomes heightened [95, 191, 192].

CONCLUSIONS AND FUTURE PERSPECTIVES

Chronic exposures to ambient air pollutants drives inflammatory cascades that disrupt fetal and brain development and function. Epidemiological human data and experimental animal studies support our thesis that Mexico City young populations with chronic exposures to high, above standard concentrations of PM_2.5 and ozone, together with other significant insulin-metabolic derangements including obesity, T2DM, and MetS, female sex, and APOE ε4 carrier status, are at significantly increased risk for developing neurodegeneration, particularly AD. Factors such as the high exposures to endotoxins, metals, polycyclic aromatic hydrocarbons, and massive presence of high-temperature, combustion-derived iron-rich magnetite in brains of young and old Mexico City residents alike ought to be included in this unfortunate equation.

Identifying early determinants of gender-specific risk trajectories would greatly facilitate multidisciplinary prevention efforts for potentially modifying disease course at early stages. What are we waiting for?