White matter pathways as both a target and mediator of health behaviors

Indirect Effects of Racial Discrimination on Health Outcomes Through Prefrontal Cortical White Matter Integrity

BACKGROUND

Racial discrimination is consistently associated with adverse health outcomes and has been linked to structural decrements in brain white matter. However, it is unclear whether discrimination-related neuroplastic changes could indirectly affect health outcomes. Our goal was to evaluate indirect associations of racial discrimination on health outcomes through white matter microstructure in a sample of trauma-exposed Black women.

METHODS

A trauma study in an urban hospital setting recruited 79 Black women who received a history and physical examination to assess medical disorders (compiled into a summed total of disorder types). Participants reported on experiences of racial discrimination and underwent diffusion tensor imaging; fractional anisotropy values were extracted from white matter pathways previously linked to racial discrimination (corpus callosum, including the body and genu; anterior cingulum bundle; and superior longitudinal fasciculus) and entered into mediational models.

RESULTS

Indirect effects of racial discrimination on medical disorders through left anterior cingulum bundle fractional anisotropy were significant (β = 0.07, SE = 0.04, 95% CI [0.003, 0.14]) after accounting for trauma and economic disadvantage. Indirect effects of racial discrimination on medical disorders through corpus callosum genu fractional anisotropy were also significant (β = 0.08, SE = 0.04, 95% CI [0.01, 0.16]).

CONCLUSIONS

Racial discrimination may increase risk for medical disorders via neuroplastic effects on microstructural integrity of stress-sensitive prefrontal white matter tracts. Racial discrimination-related changes in these tracts may affect health behaviors, which, in turn, influence vulnerability for medical disorders. These data highlight the connections between racial discrimination, prefrontal white matter connections, and incidence of medical disorders in Black Americans.

Acculturation in Context and Brain Health in Older Latino Adults: A Diffusion Tensor Imaging Study

BACKGROUND

Latinos are at higher risk of developing mild cognitive impairment (MCI) and Alzheimer's disease than non-Latino Whites. Acculturation factors may influence this risk, yet there are few studies that have examined associations of acculturation, particularly in the context of socioenvironmental and familial factors, and brain health in older Latinos.

OBJECTIVE

To examine potential associations between acculturation in context and brain health in older Latinos.

METHODS

Using three previously established composites of acculturation-in-context, (acculturation-related: nativity status, language preference, acculturation scores; contextually-related socioenvironmental: perceived discrimination, loneliness/social isolation, social network size; and familism), and diffusion-tensor imaging (DTI), associations with white matter structural integrity were examined in 92 Latino adults without dementia participating in one of three epidemiological studies of aging. Linear regression models were used to test associations with DTI-derived metrics (fractional anisotropy, FA; trace) as separate outcomes and acculturation composite scores as individual predictors, while adjusting for age, sex, education, scanner, and white matter hyperintensities (voxelwise and total volumes normalized by intracranial volume).

RESULTS

Higher scores on the socioenvironmental composite were associated with lower FA in two clusters of left-hemisphere connections. Cluster 1 was dominated by both short association pathways connecting frontal regions and projection pathways connecting frontal regions with the thalamus. Cluster 2 was dominated by long association pathways connecting parietal, frontal, and temporal regions.

CONCLUSIONS

This study of older Latino adults demonstrated an association between reduced brain white matter integrity and contextually related socioenvironmental experiences known to increase risk of MCI and Alzheimer's disease.

Altered Levels of Plasma Inflammatory Cytokines and White Matter Integrity in Bipolar Disorder Patients With Suicide Attempts

OBJECTIVE

Bipolar disorder (BD) has a higher lifetime rate of suicide attempts (SA) than other psychiatric disorders. Furthermore, BD patients with SA (BD + S) are prone to a worse quality of life. However, the pathophysiology of BD + S is poorly understood. To further reveal the potential mechanisms of BD + S, abnormalities in peripheral plasma inflammatory cytokines and brain white matter (WM) in BD + S, as well as the correlation between them are investigated.

METHODS

We tested the levels of TNF-α, IL-1β, and IL-6 in peripheral plasma and collected the diffusion tensor imaging (DTI) data from 14 BD + S, 24 BD patients without SA (BD-S), and 26 healthy controls (HCs). The three groups were matched by age and gender. The levels of TNF-α, IL-1β, and IL-6 were detected by Luminex multifactor detection technology, and the fractional anisotropy (FA) values were employed to depict the alterations of WM. Partial correlation analyses were conducted to detect correlations between levels of TNF-α, IL-1β, and IL-6 and changes of WM, and the relationships between severity of clinical symptoms, including scores of HAMD-17 and YMRS, and cytokine levels or FA values in all groups.

RESULTS

For plasma inflammatory cytokines, there was no significant difference in their levels except for IL-6 among the three groups. Post-hoc analyses revealed that increased IL-6 level was only detected in BD + S (p < 0.05, Bonferroni correction). For DTI, BD + S showed specifically decreased FA in the bilateral middle cerebellar peduncle and the left superior corona radiata compared to BD-S and HCs (p < 0.05, Bonferroni correction). Additionally, both BD + S and BD-S groups revealed decreased FA in the bilateral body and genu of corpus callosum (CC) compared to HCs (p < 0.05, Bonferroni correction). No significant correlation between plasma inflammatory cytokines and WM integrity was found. In the BD + S group, we found negative correlation between the scores of YMRS and FA values of the left middle cerebellar peduncle (r = -0.74, p = 0.035).

CONCLUSION

The inflammation and impaired WM integrity may provide a scientific basis to understand the potential mechanisms of BD + S.

Associations between Neighborhood SES and Functional Brain Network Development

Higher socioeconomic status (SES) in childhood is associated with stronger cognitive abilities, higher academic achievement, and lower incidence of mental illness later in development. While prior work has mapped the associations between neighborhood SES and brain structure, little is known about the relationship between SES and intrinsic neural dynamics. Here, we capitalize upon a large cross-sectional community-based sample (Philadelphia Neurodevelopmental Cohort, ages 8-22 years, n = 1012) to examine associations between age, SES, and functional brain network topology. We characterize this topology using a local measure of network segregation known as the clustering coefficient and find that it accounts for a greater degree of SES-associated variance than mesoscale segregation captured by modularity. High-SES youth displayed stronger positive associations between age and clustering than low-SES youth, and this effect was most pronounced for regions in the limbic, somatomotor, and ventral attention systems. The moderating effect of SES on positive associations between age and clustering was strongest for connections of intermediate length and was consistent with a stronger negative relationship between age and local connectivity in these regions in low-SES youth. Our findings suggest that, in late childhood and adolescence, neighborhood SES is associated with variation in the development of functional network structure in the human brain.

Dissecting the Fornix in Basic Memory Processes and Neuropsychiatric Disease: A Review

Background: The fornix is the primary axonal tract of the hippocampus, connecting it to modulatory subcortical structures. This review reveals that fornix damage causes cognitive deficits that closely mirror those resulting from hippocampal lesions. Methods: We reviewed the literature on the fornix, spanning non-human animal lesion research, clinical case studies of human patients with fornix damage, as well as diffusion-weighted imaging (DWI) work that evaluates fornix microstructure in vivo. Results: The fornix is essential for memory formation because it serves as the conduit for theta rhythms and acetylcholine, as well as providing mnemonic representations to deep brain structures that guide motivated behavior, such as when and where to eat. In rodents and non-human primates, fornix lesions lead to deficits in conditioning, reversal learning, and navigation. In humans, damage to the fornix manifests as anterograde amnesia. DWI research reveals that the fornix plays a key role in mild cognitive impairment and Alzheimer's Disease, and can potentially predict conversion from the former to the latter. Emerging DWI findings link perturbations in this structure to schizophrenia, mood disorders, and eating disorders. Cutting-edge research has investigated how deep brain stimulation of the fornix can potentially attenuate memory loss, control epileptic seizures, and even improve mood. Conclusions: The fornix is essential to a fully functioning memory system and is implicated in nearly all neurological functions that rely on the hippocampus. Future research needs to use optimized DWI methods to study the fornix in vivo, which we discuss, given the difficult nature of fornix reconstruction. Impact Statement The fornix is a white matter tract that connects the hippocampus to several subcortical brain regions and is pivotal for episodic memory functioning. Functionally, the fornix transmits essential neurotransmitters, as well as theta rhythms, to the hippocampus. In addition, it is the conduit by which memories guide decisions. The fornix is biomedically important because lesions to this tract result in irreversible anterograde amnesia. Research using in vivo imaging methods has linked fornix pathology to cognitive aging, mild cognitive impairment, psychosis, epilepsy, and, importantly, Alzheimer's Disease.

Evidence for genetic correlation between human cerebral white matter microstructure and inflammation

White matter microstructure is affected by immune system activity via the actions of circulating pro-inflammatory cytokines. Although white matter microstructure and inflammatory measures are significantly heritable, it is unclear if overlapping genetic factors influence these traits in humans. We conducted genetic correlation analyses of these traits using randomly ascertained extended pedigrees from the Genetics of Brain Structure and Function Study (N = 1862, 59% females, ages 18-97 years; 42 ± 15.7). White matter microstructure was assessed using fractional anisotropy (FA) calculated from diffusion tensor imaging (DTI). Circulating levels (pg/mL) of pro-inflammatory cytokines (IL-6, IL-8, and TNFα) phenotypically associated with white matter microstructure were quantified from blood serum. All traits were significantly heritable (h² ranging from 0.41 to 0.66 for DTI measures and from 0.18 to 0.30 for inflammatory markers). Phenotypically, higher levels of circulating inflammatory markers were associated with lower FA values across the brain (r = -.03 to r = -.17). There were significant negative genetic correlations between most DTI measures and IL-8 and TNFα, although effects for TNFα were no longer significant when covarying for body mass index. Genetic correlations between DTI measures and IL-6 were not significant. Understanding the genetic correlation between specific inflammatory markers and DTI measures may help researchers focus questions related to inflammatory processes and brain structure.

Potential Circadian Rhythms in Oligodendrocytes? Working Together Through Time

Oligodendrocytes (OL) are the only myelinating cells of the central nervous system thus interferences, either environmental or genetic, with their maturation or function have devastating consequences. Albeit so far neglected, one of the less appreciated, nevertheless possible, regulators of OL maturation and function is the circadian cycle. Yet, disruptions in these rhythms are unfortunately becoming a common "disorder" in the today's world. The temporal patterning of behaviour and physiology is controlled by a circadian timing system based in the anterior hypothalamus. At the molecular level, circadian rhythms are generated by a transcriptional/translational feedback system that regulates transcription and has a major impact on cellular function(s). Fundamental cellular properties/functions in most cell types vary with the daily circadian cycle: OL are unlikely an exception! To be clear, the presence of circadian oscillators or the cell-specific function(s) of the circadian clock in OL has yet to be defined. Furthermore, we wish to entertain the idea of links between the "thin" evidence on OL intrinsic circadian rhythms and their interjection(s) at different stages of lineage progression as well as in supporting/regulating OL crucial function: myelination. Individuals with intellectual and developmental syndromes as well as neurodegenerative diseases present with a disrupted sleep/wake cycle; hence, we raise the possibility that these disturbances in timing can contribute to the loss of white matter observed in these disorders. Preclinical and clinical work in this area is needed for a better understanding of how circadian rhythms influence OL maturation and function(s), to aid the development of new therapeutic strategies and standards of care for these patients.