Hemispheric Cortical, Cerebellar and Caudate Atrophy Associated to Cognitive Impairment in Metropolitan Mexico City Young Adults Exposed to Fine Particulate Matter Air Pollution

Exposures to fine particulate matter PM2.5 are associated with Alzheimer’s, Parkinson’s (AD, PD) and TDP-43 pathology in young Metropolitan Mexico City (MMC) residents. High-resolution structural T1-weighted brain MRI and/or Montreal Cognitive Assessment (MoCA) data were examined in 302 volunteers age 32.7 ± 6.0 years old. We used multivariate linear regressions to examine cortical surface area and thickness, subcortical and cerebellar volumes and MoCA in ≤30 vs. ≥31 years old. MMC residents were exposed to PM2.5 ~ 30.9 µg/m3. Robust hemispheric differences in frontal and temporal lobes, caudate and cerebellar gray and white matter and strong associations between MoCA total and index scores and caudate bilateral volumes, frontotemporal and cerebellar volumetric changes were documented. MoCA LIS scores are affected early and low pollution controls ≥ 31 years old have higher MoCA vs. MMC counterparts (p ≤ 0.0001). Residency in MMC is associated with cognitive impairment and overlapping targeted patterns of brain atrophy described for AD, PD and Fronto-Temporal Dementia (FTD). MMC children and young adult longitudinal studies are urgently needed to define brain development impact, cognitive impairment and brain atrophy related to air pollution. Identification of early AD, PD and FTD biomarkers and reductions on PM2.5 emissions, including poorly regulated heavy-duty diesel vehicles, should be prioritized to protect 21.8 million highly exposed MMC urbanites.

We have reported fluid and crystallized cognition deficits in MMC children cohorts age 6.9 ± 0.67 years old and 10.7 ± 2.7 years old, versus low-pollution matched children, along MRI prefrontal white matter hyperintense lesions, white matter volume differences in bitemporal and right parietal and no changes in the subcortical regions, including the hippocampus, caudate, putamen, globus pallidus and amygdala [13,14].We have also examined 134 consecutive MMC forensic autopsies in subjects ≤ 30 years, average age 20.03 ± 6.38 years (range 11 months to 30 years) and documented neuropathological hallmarks of Alzheimer's disease (AD) in 99% of autopsies, and in a simultaneous clinical study of 150 seemingly healthy young urbanites age 21.6 ± 3.5 y, we reported cognitive impairment in 66% of them using the Montreal Cognitive Assessment instrument (MoCA) [15]. In a MoCA study of 517 urbanites, average age 21.6 ± 5.8 years old exposed to PM 2.5 in various Mexican polluted cities, 55% of the population was cognitively impaired with scores ≤ 25 (normal [26][27][28][29][30] [16]. In sharp contrast, MMC residents ≥ 31 years old had MoCA scores on average 20.4 ± 3.4 vs. low pollution controls 25.2 ± 2.4 (p < 0.0001) [17]. The information from these Mexican cognitive studies suggested that brain volumetric changes were significant beyond age 31, and since COVID restricted our capacity to have brain MRI volunteers in a low pollution city, we decided to compare ≥31 years vs. ≤30 years old subjects while we wait for the opportunity to screen the controls.
Brain structural MRI allows for the noninvasive assessment of cortical and subcortical morphology. Hemispheric and subcortical regional specificity of targeted brain structures affected by air pollutant exposure is at the core of our interest in MMC highly exposed populations, knowing that in forensic samples with no extra-neural pathology, 99.5% of individuals 40 and younger have Alzheimer's disease (AD) hallmarks, 23% have Parkinson's disease (PD) and 18% TDP-43 pathology. In this study, we focused on MMC healthy middle-class college educated volunteers and used brain cortical thickness, surface area, subcortical volumes and MoCA results to compare individuals ≤30 versus ≥31 years old with a lifetime residency in one of the most polluted cities in North America. We hypothesized that lifetime PM 2.5 exposures would be associated with progressive regional brain changes and worse cognitive performance. We report significant differences in gray, white matter and CSF volumes, regional cortical thickness, cortical surface area, caudate and cerebellar volumes between MMC residents younger than 30 years versus older than 31 years and provide cognitive correlates taking into account formal education years, sex and age.

Study Population, Inclusion and Exclusion Criteria
This 2018-2020 protocol was conducted according to the Revised Helsinki Declaration of 2000 and approved by the review boards and ethics committees at the Universidad del Valle de Mexico (16 March 2016) and the University of Montana (IRB# 206 R-09 and 185-20). Written consent was obtained from the adult volunteers and from parents of minors, who also signed a verbal consent. This study was aimed to middle class volunteers and was advertised by word of mouth among families involved in previous clinical studies and through social networking. The participating 304 right-handed volunteers included: (1) thirty-four MMC volunteers with brain MRI/MoCA, (2) MMC only MoCA 233 volunteers, and (3) control, low pollution city (Hermosillo) MoCA 35 volunteers. Subjects completed a baseline examination and were considered clinically healthy. Briefly, inclusion criteria involved each volunteer being able to validly and safely complete the clinical visits and MoCA and/or brain MRI studies, negative smoking history and environmental tobacco/nicotine exposure, lifelong residency in MMC or the control city, residency Toxics 2022, 10, 156 3 of 22 within a diameter of 5 km of the closest monitoring station from their residence, and unremarkable clinical histories, including the absence of a history of hospitalizations for respiratory illnesses, ear-nose-throat (ENT) and oral symptomatology and/or surgery, head trauma, systemic or respiratory viral diseases, cardiovascular diseases, diabetes, metabolic syndrome, obesity, and family histories of Alzheimer's, Parkinson's, frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). We specifically excluded subjects with active participation in team sports, head trauma, who had occupational exposures to toxic substances and who had a history of drug abuse, including alcohol and marijuana. Sporadic, social intake of alcohol was recorded in 12% of participant adults. All included subjects were taking no medications, slept in bedrooms with no carpeting and had open windows for ventilation, had kitchens separated from living and sleeping areas and used gas for cooking. We examined cross-sectional regional and hemispheric specific differences in brain morphometry and cognition in Metropolitan Mexico City ≤30 years old (average 22 ± 3.3 years) (n: 20) versus MMC volunteers ≥ 31 years old (average age 42.7 ± 9.3 years) (n: 14) and included MoCA only data from MMC and Hermosillo controls for MoCA scores comparison.

Brain MRI Acquisition and Processing
The 3D MRI for all subjects was acquired on a 1.5 Tesla 5T Signa Excite HD MR (General Electric Milwaukee WI, USA) with an 8 Channel Brain Array. High-resolution T1 weighted anatomical images were acquired, as well as T2 weighted images using a 2D multi-slice dual fast spin echo sequence and fluid attenuated inversion recovery images. All sequences covered the entire surface of the brain. Atlas moderated probabilistic tissue segmentations of white matter, gray matter and cerebrospinal fluid were computed in automatic fashion including skull stripping, intensity inhomogeneity correction and intensity calibration. Next, subcortical structures and 90 cortical parcellations (Neuromorphometrics Inc., Somerville, MA, USA) were determined via a multi-atlas multi-modality segmentation process. The parcellation for each subject was then combined with that subject's tissue classification to obtain white matter, gray matter and cerebral-spinal fluid volumes for each region. The corresponding volumetric measurements for ROIs and parcellations were automatically computed. Cortical surface reconstruction and subcortical segmentation were accomplished via FreeSurfer (version 5.3) including total gray and white matter and subcortical volumes, cortical thickness and cortical surface area estimates for cortical regions via the Desikan-Killiany Atlas [18,19]. At the Neuro Image Research and Analysis Lab, University of Carolina at Chapel Hill, T1-weighted structural images underwent quality control (QC) across five categories, both prior to and after post-processing to gauge intensity inhomogeneity, white matter underestimation, pial overestimation, and magnetic susceptibility artefact [19]. Only images passing QC for all categories were included in this study. In summary, measurements on structural MRI data included (i) whole brain tissue volumes of white matter and gray matter, CSF, (ii) cortical regions and quantification of WM/GM/CSF, (iii) measurements of all major sub-cortical structures, including the hippocampus, amygdala, caudate, putamen, globus pallidus and lateral ventricles, and (iv) cortical thickness for whole brain and lobar parcellations.

Calculation of Accumulated PM 2.5 Exposure
We obtained a detailed residency and exposure history from each volunteer. The collected data consisted of historical 24-h average PM 2.5 mass concentrations sampled at the closest monitoring station to their residence. The PM 2.5 data were collected from the air quality-monitoring network of Mexico City's Secretariat of the Environment under a six-day schedule following the recommendations of the US EPA for PM samplers [20]. Given that systematic PM 2.5 measurements were not available before 2004, fine particle concentrations PM 2.5 for the period 1989 to 2004 were estimated from PM 10 measurements performed in the same monitoring sites for that period. The associated PM 2.5 levels were then estimated from the average of the slopes of the linear regression analysis of the 24-h . Upon entrance to the study, the geolocation of the subject residency, school and workplace were mapped out and the cumulative exposure of the PM 2.5 was calculated and integrated into the database. The cumulative exposure was obtained by summing up the annual average PM 2.5 mass concentration in excess to the US EPA annual NAAQS from the year of the test (2019) and backwards up to 1989. We applied the current primary US EPA annual standard for PM 2.5 stands for an annual mean of 12.0 µg/m 3 averaged over three years.

Neurocognitive Performance
The Spanish version of MoCA was used in this study (Certification MXCRECE191274-01). The MMC 34 brain MRI volunteers plus 233 MMC subjects and 35 low pollution controls underwent MoCA testing. MoCA assesses global cognitive function and contains 10 subtests [21][22][23][24][25][26]. MoCA scores were converted into six index scores based on the combinations used by Julayanont et al. and Petersen et al. [24,26].

Covariates
The following covariates were included in the main analysis: age, sex, years of formal education, BMI, MoCA scores, cognition indexes, and socioeconomic status (SES). For subcortical volume analysis, intracranial volume (ICV) was a covariate.

Study City and Air Quality
The MMC area covers~7585 km 2 and is located on an elevated basin 2240 m above sea level surrounded by mountain ridges on three sides. MMC has a population of~21.8 million people. Emissions from~five million vehicles, over 50,000 industries and LP gas, industrial and household solvents, and vapors of oil derived liquid fuels combine with high solar radiation and poor ventilation to produce a severe air pollution problem with a strong oxidizing capacity [27][28][29][30]. MMC residents have been exposed to high levels of primary fine and ultrafine particles as well as secondary air pollutants including secondary organic aerosols and ozone concentrations at levels above United States National Air Ambient Quality Standards (NAAQS) all year round during the last two decades [20, [27][28][29][30]. High levels of black carbon (BC), polycyclic aromatic hydrocarbons (PAHs), semi-volatile organic compounds from incomplete combustion of carbonaceous fuels such as gasoline and diesel, as well as metals from brake and tire wear have been historically found in the PM 2.5 fraction of MMC [31][32][33][34][35]. As a result, MMC residents, including children and pregnant women, are exposed to outdoor elevated NP concentrations rich in PAHs and metals [36,37]. Commuting in any of the urban transport modes available in the urban area is also associated with high NPs exposures [38]. Traveling in the MMC subway system results in high PM 2.5 exposures between 34 and 93 µg m −3 ; equivalent PAHs concentrations ranging from 19 to 41 ng m −3 ; and NPs up to 50,300 ± 10,600 (# cm −3 ) with an average size of 38.5 ± 15.9 nm, and elevated concentrations of Fe, Cu, Ni, Cr and Mn [38,39]. Metals present in the underground subway are the result of friction, brake wear, and sparking from rail grinding. Ozone has been also above the US EPA standards, with higher concentrations in Southwest MMC [28,33].
Hermosillo, the selected control city, is located in the southern extreme of the Sonora Desert in northwestern Mexico. It has a population of nearly 813,000 people and an urban area of around 168 km 2 (centroid radius of~7.4 km). The climate is dry and characterized by arid to semiarid conditions with good wind ventilation conditions. The urban area has diverse emission sources of air pollutants, including traffic, domestic combustion and industry, and is surrounded by areas with strong agricultural activities. Most streets are unpaved and, as a result, the re-suspension of dusts with dominant particulate matter ≥2.5 µm constitutes the main air pollution problem [40,41]. Several heavy metals have been identified in dust, and lead seems to be the most important [42]. The anthropogenic lead content in the dust is greater than the geogenic one, suggesting that since leadedgasoline has not been used in Mexico in the last 30 years, this signature shows a Pb-legacy. Lead chromate (crocoite) from yellow paint in the inhalable fraction of dusts has also been reported [43]. However, the Pb PM concentration does not exceed the respective NAAQS standard (0.15 µg/m 3 of lead in total suspended particles (TSP) in a 3-month average) [42]. The control city has concentrations of PM 2.5 , SO 2 , NO 2 , CO, SO 2 and O 3 below the respective US EPA NAAQS short-and long-term exposures [44].

Statistical Analysis
We first calculated the sample mean and sample standard deviation of the total MoCA score in each group including Non-MRI-MMC age ≤ 30 years, Non-MRI-MMC age ≥ 31 years, MRI-MMC age ≤ 30 years, MRI-MMC age ≥ 31 years, Hermosillo age ≤ 30 years, and Hermosillo age ≥ 31 years and included Cognitive Domain scores and index scores (Supplemental Tables S1 and S2). Next, we performed multiple linear regressions to test the significance of mean difference of various group scores after adjusting age, gender, BMI and education years. We consider these pairs of groups: (i) Non-MRI-MMC age ≤ 30 years and Hermosillo age≤ 30 years, (ii) Non-MRI-MMC age ≥ 31 years and Hermosillo age ≥ 31 years, (iii) MRI-MMC age ≤ 30 years and Hermosillo age ≤ 30 years, and (iv) MRI-MMC age ≥ 31 years and Hermosillo age ≥ 31 years. The adjusted p-values are reported in Supplemental Tables S1 and S2. Between-group differences in 34 righthanded subjects (≤30 y vs. ≥31 y) were assessed using multivariable linear regressions of the pooled means of regional cortical thickness (mm), regional and total cortical surface area (mm 2 ), regional subcortical volume on the age-group, and intracranial volume (ICV; mm 3 ) as predictors. P values were corrected for false discovery rate (FDR). We also performed multivariate linear regression of MoCA index scores on regional cerebral volumes and cortical thickness, age and intracranial volume, and calculated p-values for the significance of the linear association between various MoCA index scores and regional cerebral volumes and cortical thickness. We used MS-Excel and statistical software "R" to perform these analyses.

Air Pollution
Twenty-two million MMC residents are exposed to PM 2.5 above the annual US EPA NAAQS. Figures 1 and 2 show the backward trend of the annual mean concentrations averaged over three years of PM 2.5 24-h data and the respective cumulative PM 2.5 in excess of the annual US EPA NAAQS for Southwest and East MMC sectors in accordance with the volunteers' age, starting in 2019. Volunteers of age ≤ 30 years old living in either one of the MMC sectors targeted have been exposed to an equivalent average of nearly two times the reference USEPA standard, while volunteers age ≥ 30 years old have been exposed to annual concentrations between 35 and 45 µg/m 3 , three to four times the level allowed in the US. Taking into consideration that the estimated backward trend of cumulative PM 2.5 mass concentrations for Southwest and East MMC was represented by a straight line, the rates of PM 2.5 accumulation as people aged would be 28.36 µg/m 3 /year and 33.4 µg/m 3 /year, respectively. Remarkably, North and West MMC sectors register higher cumulative PM 2.5 levels compared to SW and East MMC, thus displacements of a person within the urban area would not have reduced the level of exposure.
To show the contrast of PM 2.5 air quality between MMC and Hermosillo, the low pollution control city, we plotted the time series of the maxima PM 2.5 daily averages registered for the period 2019-2020 ( Figure 3). In sharp contrast, Hermosillo has PM 2.5 concentrations below the USEPA standard versus MMC with unhealthy PM 2.5 concentrations regardless of pre-COVID-19 and COVID-19 times. oxics 2021, 9, x FOR PEER REVIEW 6 higher cumulative PM2.5 levels compared to SW and East MMC, thus displacement person within the urban area would not have reduced the level of exposure.

Southwest MMC
Age at the year of the test (years)  To show the contrast of PM2.5 air quality between MMC and Hermosillo, the pollution control city, we plotted the time series of the maxima PM2.5 daily ave registered for the period 2019-2020 ( Figure 3). In sharp contrast, Hermosillo has concentrations below the USEPA standard versus MMC with unhealthy concentrations regardless of pre-COVID-19 and COVID-19 times.  To show the contrast of PM2.5 air quality between MMC and Hermosillo, the pollution control city, we plotted the time series of the maxima PM2.5 daily aver registered for the period 2019-2020 ( Figure 3). In sharp contrast, Hermosillo has concentrations below the USEPA standard versus MMC with unhealthy concentrations regardless of pre-COVID-19 and COVID-19 times.

Study Population and Demographics
The participating 302 healthy volunteers included: (1)

Study Population and Demographics
The participating 302 healthy volunteers included: (1)

Total Gray and White Matter Volumes and CSF, Cortical Thickness, Cortical Surface Area, and Intracranial Volume ICV
We found significant differences in total gray matter, CSF volumes, cortical thickness and surface area in supratentorial regions and volume changes in subcortical regions, including cerebellar gray and white matter and bilateral caudate volumes ( Table 2, Figures 4-6). Indeed, gray matter was significantly decreased in ≥31 years old volunteers p < 0.0001 vs. ≤30 y, with robust decreases in cortical thickness predominantly in the left hemispheric brain regions, including the temporal middle gyrus, cingulate and frontal superior, middle and inferior lobes (Figure 4, Table 2).  Significant changes in surface areas were also documented ( Figure 5). Significant changes in surface areas were also documented ( Figure 5).  Significant changes in surface areas were also documented ( Figure 5).   Figure 5. Visualization of the statistical significance of surface area measurements in cortical regions (left). On the right, the inflated surface. Non-significant regions are in white, significant regions (following correction for multiple comparison) are shown in blue (p < 0.05) to red (p < 0.001).

Subcortical Volume
There was a significant decrease in right cerebellar gray and white matter and left cerebellar gray matter, and a predominant right smaller caudate and lesser left caudate involvement ( Figure 6). No differences were documented for the hippocampus, putamen, pallidum, amygdala and nucleus accumbens ( Table 2). CSF was significantly increased in ≥31 years old volunteers (p = 0.0008).  Table 1, Supplemental Tables S1 and S2).

MoCA Total Score, Index Scores, Cortical Thickness and Subcortical Volumes
We analyzed the correlation between MoCA total scores, index scores and the different cortical and subcortical volumes, cortical thickness and surface areas in MMC/MRI ≤ 30 versus ≥31 years old subjects (Table 3).

Subcortical Volume
There was a significant decrease in right cerebellar gray and white matter and left cerebellar gray matter, and a predominant right smaller caudate and lesser left caudate involvement ( Figure 6). No differences were documented for the hippocampus, putamen, pallidum, amygdala and nucleus accumbens ( Table 2). CSF was significantly increased in ≥31 years old volunteers (p = 0.0008).  Table 1, Supplemental Tables S1 and S2).

MoCA Total Score, Index Scores, Cortical Thickness and Subcortical Volumes
We analyzed the correlation between MoCA total scores, index scores and the different cortical and subcortical volumes, cortical thickness and surface areas in MMC/MRI ≤ 30 vs. ≥31 years old subjects (Table 3).
Remarkably, MoCA total scores were significantly associated with right and left caudate decreased volume and left orbital gyrus' surface area. Orientation was associated with the right temporal superior transverse gyrus, in spite orientation scores were marginally significant between groups (Supplemental Table S2). Executive Index Scores (EIS) were strongly associated with decreased volume on the left orbital lateral sulcus, left frontal inferior triangular gyrus and right cerebellar white matter (Table 3). Language Index Scores (LIS) were associated with the right temporal superior transverse gyrus, left gyrus and cingulate middle anterior and posterior sulcus, and left orbital gyrus volume. The Visuospatial index (VIS) was associated with the left orbital lateral sulcus, and the Attention Index Score (AIS) was associated with the right temporal superior transverse gyrus volume. The Summary Score was strongly associated to decreased volume in the orbital left gyrus (p = 0.0009). Table 3. The adjusted p values (adjusted for age and intracranial volume) for the significance of the linear association between MoCA Index scores and regional cerebral volume.
Strikingly, the poorer cognitive performance in MMC residents was associated with caudate and left orbital gyrus atrophy in keeping with orbital frontal cortex connections with regions processing visual, spatial, emotional information and social cognition, PFCcaudate brain wiring, and caudate functions affecting learning, memory, reward and motivation [69][70][71][72][73][74]. The current findings are not unexpected, i.e., the cerebellar atrophy is significant and could have a clinical counterpart in gait and equilibrium abnormalities described by our group in young MMC urbanites [75], cognitive deficits [16,57], the association between hearing loss and decreased brainstem and cerebellar volumes in AD cases [76], and gait and cognitive abnormalities associated with regional cerebellar atrophy in elderly fallers [77]. Cerebellar atrophy is particularly intriguing in view of our published cerebellar higher concentrations of magnetite (estimated from their saturation remanent magnetisation (SIRM) values) in MMC young forensic cases [2] and the recent description of regional cerebellar hypermetabolism in AD [78].
MoCA index scores are valid measures in MCI, and Kim et al., [90] have shown MoCA-OIS (Orientation) and MoCA total score distinguished between MCI and dementia groups in both AD and vascular cognitive impairment (VCI). In MMC ≥ 31 years old, the combined score of delay recall, EIS, VIS and LIS reached values below the cutoff score. Indeed, LIS scores were strongly associated with decreased volumes in the right temporal superior transverse, left cingulate middle posterior and anterior gyrus and sulcus and the left orbital gyrus.
In the background of 21.8 million MMC residents with sustained exposures to PM 2.5 above USEPA annual standards, Falcon et al.'s work [55] is indeed an obligated reference. Falcon and coworkers [55] documented significant changes in cerebral volumes for Barcelona's cognitively intact residents average age 58.6 years old with 80% family history of AD and~50% APOE4 allele carriers. Specifically, exposures to NO 2 were associated with decreased gray matter (GM) in the precuneus and greater WM volume in the splenium of the corpus callosum and inferior longitudinal fasciculus, while PM 2.5 exposures above the USEPA annual standard (16.3 µg/m 3 range 8.4-24.2) were associated with greater GM in the cerebellum and WM in the splenium of the corpus callosum, the superior longitudinal fasciculus, and cingulate gyrus. It is crucial to emphasize that Falcon's study was done in cognitively intact individuals with significant AD genetic risk, while ours was done in subjects with average MoCA scores in the range of MCI, no genetic risk factors and lifetime exposures to PM 2.5 concentrations significantly higher than in Barcelona.
The key issue in Falcon et al.'s work is the impact of specific air pollutants upon GM and WM volumes of targeted brain areas in a highly genetically vulnerable population even before cognitive changes take place. In sharp contrast, in MMC residents, the historically documented early neuropathological development of AD, PD and TDP-43 alterations is critical to the hemispheric and subcortical volume changes recorded for an adult middleaged population that already has cognitive deficits.
The bilateral caudate atrophy (R > L) and their relationship to MoCA total scores deserves a comment. The caudate gray matter volume association with the degree of social intelligence (SI) as measured by the Guilford-Sullivan test was explored by Votinov and coworkers [98]. Their findings of the Guilford-Sullivan test positively correlating with the FC between seeds in the right caudate head and two clusters within the right superior temporal gyrus and bilateral precuneus are remarkable since both regions are part of the theory of mind (ToM) network. Social intelligence deficits [99,100], intense apathy [64] and poorer cognitive function are all related to caudate function and disruption of associative and limbic pathways from/to the PFC and are described in PD patients [62,63]. Interestingly, global measures of striatum such as total striatum, nucleus accumbens, caudate nuclei, and putamen may not be significantly different between PD patients and controls, although inward surface displacement of caudal-motor striatum (the region first and most dopamine depleted in PD) distinguished PD patients from controls in the work of Khan et al. [101]. The integrity of white matter cortico-striatal connections in caudal-motor and adjacent striatal sub-regions (i.e., executive and temporal striatum) was reduced for PD patients relative to controls [101]. Interestingly, loneliness and human emotions are related to changes to the ventral striatum and cerebellum [57,102], while in adolescents, negative associations were found between left hemisphere caudate volume and scores on 'total wellbeing' [103].
The issue of the significant caudate involvement in MMC residents is critical for many reasons: (1) We have described neuropathological early PD in 23% of forensic autopsies in MMC individuals ≤40 years old and brainstem, olfactory bulb and neuroenteric system α-synuclein starting in children and progressing as the individuals grow older in Mexico City [3,91,92,104,105]; (2) In the same forensic study, we have documented extensive hyperphosphorylated tau in substantia nigrae in adolescents [3]; and (3) The literature is very clear regarding the association between structural brain changes in rapid eye movement sleep disorders (RBD) and PD, and in fact RBD is considered a prodromal state of PD [58]. RBD individuals in the Holtbernd et al., study [58] showed increased volume of the right caudate nucleus compared to controls, and higher cerebellar volume compared with both PD subjects and controls.
In sharp contrast, PD patients had decreased volumes in the basal ganglia, midbrain, pedunculopontine nuclei, and cerebellum [58]. Remarkably, Holtbernd et al., commented upon the "co-occurrence of neurodegeneration and compensatory mechanisms that fail with emerging PD pathology" [58]. In our MMC middle-aged adults, caudate atrophy is already present and pRBD [94] is documented in 32.7% of MMC residents with PTSD, raising the issue of PTSD and neurodegeneration association in highly exposed air pollution residents [106][107][108][109] and the strong possibility of establishing a link between RBD documented by polysomnography and caudate atrophy in future MMC studies.
It is clear for MMC residents that a convergent pathophysiological hallmark of PD is affecting the striatum, as described in PD patients by Li et al. [65]. In their study of 84 PD patients versus 70 matched healthy controls, the modulation of early caudate atrophy over other brain structures showed that GM atrophy progressively expands from basal ganglia to the angular gyrus, temporal areas, and eventually spreads through the subcortical-cortical networks as PD progresses. Remarkably, Li et al. [65] identified a shared caudate-associated degeneration network including the basal ganglia, thalamus, cerebellum, sensorimotor cortex, and cortical association areas with the PD progressive factors. The authors emphasized the importance of the early caudate atrophy and potential clinical applications in the development of early predictors of PD onset and progress [65]. The striking caudate atrophy (and cerebellar involvement) in our MMC subjects in the presence of autonomic data in children and young adults (personal communication with Nora Vacaseydel MD) obligate us to consider such finding as an early future predictor of PD onset in highly exposed air pollution individuals [110][111][112]. The involvement of somatosensory cortices (in PD [65]) and decreased functional connectivity in the Default Mode Network (DMN) in AD as discussed by Weisman et al., and Becci and Giacomussi's work [113,114] add significant knowledge to this discussion. Weismann et al. [113] commented that cognitive domains modulate cortical somatosensory processing and when attention and processing speed abilities are considered in AD cases, differences in gamma-frequency somatosensory response amplitude and gating become obvious and are accompanied by statistical suppression effects, thus early documentation of somatosensory processing is critical for identifying individuals at high risk of proteinopathies.
MMC middle-age adults are showing an MRI overlap in cortical and subcortical structures involved in AD, PD, LBD and FTD associated to cognitive decline and our deep concern relates to three main issues: (1) The need to identify subjects at risk as early as possible, and to predict the progression of the neurodegenerative processes at a stage when we can still effectively intervene; (2) The extensive hemispheric cortical and subcortical involvement in young urbanites is likely already impacting key processes such as social intelligence, the high risk for eating disorders, attention-deficit/hyperactivity disorder, conduct and sleep disorders, bullying behaviors, and depression [98][99][100][115][116][117]; and (3) The study of the progression of the structural brain changes and its association with clinical variables, including cognition deficits, should be of great concern for researchers studying highly exposed air pollution, children, teens and young adults.
The paradigm of neurodegenerative disease being associated with aging certainly does not apply in the setting of high air pollution scenarios, and thus we are obligated to develop guidance on the use of cognitive testing, neuroimaging and diagnosing potential early clinical neurodegenerative manifestations such as abnormal gait, BAEPS, PTSD and sleep disorders in young urbanites.
The current preclinical criteria aimed at elderly populations that is being used in the current literature [118][119][120][121][122][123] is certainly not ideal. We must agree with Dubois et al., [124] that the NIA-AA [119] that relies on biomarkers has limitations, and that those limitations certainly apply to young cohorts with no comorbidities, but with cognition deficits associated with extensive MRI changes. Specifically, we know that 55% of the MMC population (average age 21.6 ± 5.8 years, with 13.7 ± 1.3 formal education years) is already cognitively impaired, but we are very aware it will be extremely difficult to apply the ATN biomarker classification system (amyloid beta [A], pathologic tau [T], and neurodegeneration [N]) for predicting conversion from mild cognitive impairment (MCI) to dementia [118][119][120]. Moreover, of deep concern is the fact that in Aβ-positive elderly [121], 16% of variance in cross-sectional cognitive impairment was accounted for by Aβ, 46-47% by tau, and 25-29% by atrophy, and the Aβ-tau-atrophy pathway accounted for 50% to 56% of variance in longitudinal cognitive decline [121].
Furthermore, to complicate matters, current AD progression studies [122] support pivotal roles for regional amyloid beta (Aβ) and tau deposition, the identification of genetic contributions and comorbidities such as vascular disease that we do not have in children and young adults.
We are in the position of going to the ATX(N) system [123] applicable to children and young adults and incorporating novel biomarkers such as neuroimmune dysregulation and blood-brain barrier alterations that are already documented in children [2,3,13,14].
The study has several strengths, including the selection of clinically healthy collegeeducated individuals, with no family history of neurodegenerative diseases, no exposure to harmful drugs, including alcohol, or head injury of any etiology and with lifelong residency in MMC. Moreover, we performed the neuropathological studies of the 203 forensic MMC autopsies in a cohort with similar backgrounds as our MRI cases, which allowed us to establish associations with AD, PD and TDP-43 pathology discussed in the forensic cases [2,3]. Limitations should be noted. It is a small sample in a cross-sectional design limiting causal inference, and due to the COVID epidemic we were unable to include the planned controls for which we had the MoCA studies. However, our results remained robust in terms of the extensive hemispheric cortical thickness and surface volume involvement and the caudate and cerebellar atrophy in a population for which we had documented high concentrations of magnetite in the cerebellum, PD neuropathology in 23% of consecutive 203 MMC autopsies, gait and equilibrium abnormalities and sleep disorders [2][3][4]75,94].

Conclusions
Metropolitan Mexico City children and young adults historically have neuropathological hallmarks of AD, PD and TDP-43 pathology. The current study found MMC residents exposed to sustained, yearlong PM2.5 concentrations averaging 30.9 µg/m 3 per year (well above the USEPA annual standard 12 µg/m 3 ) showed robust hemispheric differences in frontal and temporal lobes, caudate and cerebellar gray and white matter when compared to ≤30 vs. ≥31 years old residents. Significant associations between MoCA total scores and caudate bilateral volumes and specific reductions in frontal and temporal cortical thickness and cerebellar white matter are remarkable in a healthy population with no comorbidities. Residency in Metropolitan Mexico City is associated with multi-domain cognitive impairment and an overlap of targeted patterns of structural brain atrophy as seen in AD, PD and FTD.
Prospective follow-up studies of MMC children and young adults are urgently needed to determine the impact on brain development and cognitive impairment and brain atrophy risk across pediatric and young adulthood ages.
PM2.5 emissions controls, including regulating heavy diesel vehicles, should be prioritized. We need an early biomarker identification of neurodevelopmental and neurodegenerative effects of air pollution in young MMC residents. This is a serious health crisis, and we desperately need support.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/toxics10040156/s1, Table S1: Summary of MoCA cognition scores in subjects ≥ 31 y and ≤30 y in Metropolitan Mexico City MRI and non-MRI subjects and Control City Hermosillo. Ad-justed p-value refers to the p-values after removing the linear effects of age, gender, BMI, and education years. and Table S2 Informed Consent Statement: Informed consent was obtained from all subjects involved in the study, written consent was obtained from the adult volunteers and from parents of minors, who also signed a verbal consent.