Lifespan trajectory of myelin integrity and maximum motor speed

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263

Several theories link processes of development and aging in humans. In neuroscience, one model posits for instance that healthy agerelated brain degeneration mirrors development, with the areas of the brain thought to develop later also degenerating earlier. However, intrinsic evidence for such a link between healthy aging and development in brain structure remains elusive. Here, we show that a data-driven analysis of brain structural variation across 484 healthy participants (8-85 y) reveals a largely-but not only-transmodal network whose lifespan pattern of age-related change intrinsically supports this model of mirroring development and aging. We further demonstrate that this network of brain regions, which develops relatively late during adolescence and shows accelerated degeneration in old age compared with the rest of the brain, characterizes areas of heightened vulnerability to unhealthy developmental and aging processes, as exemplified by schizophrenia and Alzheimer's disease, respectively. Specifically, this network, while derived solely from healthy subjects, spatially recapitulates the pattern of brain abnormalities observed in both schizophrenia and Alzheimer's disease. This network is further associated in our large-scale healthy population with intellectual ability and episodic memory, whose impairment contributes to key symptoms of schizophrenia and Alzheimer's disease. Taken together, our results suggest that the common spatial pattern of abnormalities observed in these two disorders, which emerge at opposite ends of the life spectrum, might be influenced by the timing of their separate and distinct pathological processes in disrupting healthy cerebral development and aging, respectively.M any phylogenetic or ontogenetic models attempt to relate development and aging at genetic, molecular, or cognitive systems levels (1-4). In neuroscience, one of the most popular hypotheses in this respect postulates that the process of healthy age-related brain decline mirrors developmental maturation. This concept was first introduced in 1881 as a "loi de régression" (Ribot's law) when Théodule Ribot, a French philosopher, observed that the destruction of memories progresses in reverse order to that of their formation: from the unstable to the stable, from the newly formed memories to older "sensory, instinctive" memories (5). More generally, this hypothesis postulates that the sequence of events associated with brain decline should present itself in reverse order to the series of events related to brain development, with brain regions thought to develop relatively late-at both ontogenetic and phylogenetic levels-also degenerating relatively early (2, 6, 7).One way of tracking this hypothesized mirroring pattern of development and aging in the human brain is to use the information provided at a macroscopic level by structural MRI in large-scale, lifespan human populations. Although structural MRI does not distinguish between the various cellular mechanisms underpinning development and aging processes [e.g., de...

Section: Discussionmentioning

confidence: 90%

A common brain network links development, aging, and vulnerability to disease

Douaud

Groves

Tamnes³

et al. 2014

300

263

“…In contrast, we found that pronounced increases in the density of myelinated axons in the human neocortex continue after adolescence and into the third decade, providing further evidence of extraordinary prolonged neocortical maturation. MRI studies also indicate that growth in the volume of the cortical white matter persists well beyond puberty (25)(26)(27)(28)39). Additionally, in humans, neocortical dendritic development and synaptogenesis exhibit heterogeneity across the processing hierarchy, with the greatest delay in maturation characterizing the prefrontal cortex (40)(41)(42)(43)(44).…”

Section: Discussionmentioning

confidence: 99%

Prolonged myelination in human neocortical evolution

Miller

Duka

Stimpson

et al. 2012

559

454

Nerve myelination facilitates saltatory action potential conduction and exhibits spatiotemporal variation during development associated with the acquisition of behavioral and cognitive maturity. Although human cognitive development is unique, it is not known whether the ontogenetic progression of myelination in the human neocortex is evolutionarily exceptional. In this study, we quantified myelinated axon fiber length density and the expression of myelinrelated proteins throughout postnatal life in the somatosensory (areas 3b/3a/1/2), motor (area 4), frontopolar (prefrontal area 10), and visual (areas 17/18) neocortex of chimpanzees (N = 20) and humans (N = 33). Our examination revealed that neocortical myelination is developmentally protracted in humans compared with chimpanzees. In chimpanzees, the density of myelinated axons increased steadily until adult-like levels were achieved at approximately the time of sexual maturity. In contrast, humans displayed slower myelination during childhood, characterized by a delayed period of maturation that extended beyond late adolescence. This comparative research contributes evidence crucial to understanding the evolution of human cognition and behavior, which arises from the unfolding of nervous system development within the context of an enriched cultural environment. Perturbations of normal developmental processes and the decreased expression of myelin-related molecules have been related to psychiatric disorders such as schizophrenia. Thus, these species differences suggest that the human-specific shift in the timing of cortical maturation during adolescence may have implications for vulnerability to certain psychiatric disorders.C omparative studies suggest that human neurobiological development is unique. For example, humans differ from other primates in extending a rapid, fetal-like brain mass growth rate into the first postnatal year, thereby achieving relatively large adult brain size (1). Gene expression patterns related to postnatal development of the prefrontal cortex are delayed in humans compared with chimpanzees and macaque monkeys (2). In addition, synapse maturation (3, 4) and axon myelination (5, 6) occur during later life history stages in humans compared with macaques. Furthermore, recent volumetric data obtained by using in vivo MRI demonstrates that human neural development and aging differ from those of our close nonhuman primate relatives (7-9). Together, these observations indicate that a marked delay in the developmental schedule of human neocortex may play an important role in the growth of connections that contribute to our species-specific cognitive abilities by providing greater opportunities for social learning to influence the establishment of circuits.

“…Scientifically, it was chosen because it consistently shows significant schizophrenia-related WM deficits (33,44,45). The corpus callosum was also chosen because it is composed of commissural fibers that facilitate long-interhemispheric signal transmission and shows strong phenotypic and genetic associations with processing speed in humans and primates (46)(47)(48)(49). In addition, the corpus callosum provides the feasibility needed for testing the specific hypothesis.…”

Section: Methodsmentioning

confidence: 99%

Diffusion-weighted imaging uncovers likely sources of processing-speed deficits in schizophrenia

Kochunov

Rowland

Fieremans

et al. 2016

Self Cite

Schizophrenia, a devastating psychiatric illness with onset in the late teens to early 20s, is thought to involve disrupted brain connectivity. Functional and structural disconnections of cortical networks may underlie various cognitive deficits, including a substantial reduction in the speed of information processing in schizophrenia patients compared with controls. Myelinated white matter supports the speed of electrical signal transmission in the brain. To examine possible neuroanatomical sources of cognitive deficits, we used a comprehensive diffusion-weighted imaging (DWI) protocol and characterized the white matter diffusion signals using diffusion kurtosis imaging (DKI) and permeability-diffusivity imaging (PDI) in patients (n = 74), their nonill siblings (n = 41), and healthy controls (n = 113). Diffusion parameters that showed significant patient-control differences also explained the patient-control differences in processing speed. This association was also found for the nonill siblings of the patients. The association was specific to processing-speed abnormality but not specific to working memory abnormality or psychiatric symptoms. Our findings show that advanced diffusion MRI in white matter may capture microstructural connectivity patterns and mechanisms that govern the association between a core neurocognitive measureprocessing speed-and neurobiological deficits in schizophrenia that are detectable with in vivo brain scans. These non-Gaussian diffusion white matter metrics are promising surrogate imaging markers for modeling cognitive deficits and perhaps, guiding treatment development in schizophrenia.diffusion-weighted imaging | schizophrenia | processing speed | cognitive deficits | endophenotypes A lthough pharmaceutical interventions alleviate clinical symptoms, such as delusions and hallucinations, for some patients, schizophrenia remains a debilitating illness, often times leading to long-term disabilities and severe cognitive deficits (1). Discovering the underlying neurobiology behind these core cognitive deficits in schizophrenia patients may present a viable strategy to identify biomarkers for supporting pathophysiology and comprehensive treatment research. Two decades of research have implicated impaired brain connectivity as a source of functional disability in schizophrenia (1, 2). Delayed information processing is one of the most robust cognitive deficits (3, 4) and may contribute to other cognitive impairments in working memory and executive function (4-6).Myelinated axons in the brain's white matter (WM) support its functionality by propagating electric signal transmissions through saltatory conductance (7,8). Reports of WM abnormalities in patients with schizophrenia are common and include reduced fractional anisotropy (FA) of water diffusion measured by diffusion tensor imaging (DTI) MRI (9-11) as well as reduced axonal myelin levels and glial cell density in postmortem brain studies (12)(13)(14). Identifying the key WM microstructural properties that explain the patient-control...