Autism Spectrum Disorders (ASD) are complicated neurodevelopmental conditions that result in impairments of social communication amongst other psychological and behavioral symptoms. It is classified as a heterogeneous condition because epigenetics and environmental factors both play major roles in the idiosyncrasy of symptom and clinical presentation (Masi, DeMayo, Glozier, & Guastella, 2017). Growing literature finds that a subset of children with ASD shows abnormal neuron growth trajectories (Kaushik & Zarbalis, 2016). The human brain contains billions of neurons that, after production, must migrate to the cortex. After neural migration, neurons undergo morphological changes during prenatal and postnatal periods to fit their function, such as elongation of axons or the branching of dendrites (Gilbert & Man, 2017). Disruptions of any mechanism during these processes would cause abnormalities in brain development and function. The loss of connectivity of neurons, especially those in higher-order areas of the cerebral cortex, leads to neurodevelopmental disorders including ASD (Gilbert & Man, 2017). Furthermore, the cerebral cortex and corpus callosum are susceptible to physiological changes as neuron growth trajectories alters, which may add on to the severity and presentation of ASD symptoms. Abnormal neuron migration and developmental neurogenesis are thus both important biomarkers of ASD and should be targets for interventions during treatment.
Overview of Neuron Migration
The cerebral cortex, the outermost layer of the brain, consists mostly of two types of neurons: excitatory (glutaminergic) and inhibitory (GABAergic) neurons. The two types of neurons are born in different areas of the brain. Excitatory neurons, which constitute the majority of the cerebral cortex, are born either in the ventricular or subventricular zone while the inhibitory neurons are born in the ventral part of the telencephalon, known as the subpallium (Reiner, Karzbrun, Kshirsagar, & Kaibuchi, 2016). Within the cortex are also glial cells, which are non-neuron cells that provide structure and facilitate the growth of neurons. Radial glial cells (RGCs) are involved in neurogenesis and neuronal migration, as in the developing brain, they guide migration and ultimately provide a scaffold for the proper pattern of synaptic connectivity (Pan, Wu, & Yuan, 2019). Approximately 75%-90% of human neurons use glial-guided migration (Gilbert & Man, 2017). Inhibitory neurons, on the other hand, first participate in tangential migration across the plane of the glial fiber system before migrating along radial glial cells to their proper positions (Reiner et al., 2016).
Figure 1. Representation of neural migration pathways of inhibitory (left) and excitatory (right) neurons. Inhibitory neurons migrate tangentially from the subpallium to the cortex, while excitatory neurons migrate radially. (Reiner et al., 2016)
Neuronal migration deficits represent an important pathology of autistic brains. Subjects with ASD and rodent ASD models show deficiencies in radial migration of both excitatory neurons and tangential migration of inhibitory neurons within the cerebral cortex (Pan et al., 2019). Animal studies corroborate that neurons which undergo abnormal neural migration may result in behavioral consequences depending on the region of the brain the neurons incorrectly migrate to. In adult rats, migratory deficits of neurons to the thalamus, the sensory "gatekeeper" of the brain, were induced and subtle behavioral consequences were observed, namely deficits in behavioral habituation (Vomund, De Souza Silva, Huston, & Korth, 2017). Unfortunately, it is still unclear as to how abnormal neuronal migration affects the circuity of neural networks. One hypothesis is that misplaced neurons detrimentally affect neural network function due to the lack of appropriate innervation at the location (Pan et al., 2019). Neurons in the wrong location will not fire properly as fibers travel and innervate, or excite, neurons in their designated cell layer. Consequently, if neurons are not in their proper locations, brain regions lose their input and/or output connectivity. Additionally, misplaced neurons in brains with ASD are observed to have delayed maturation which could possibly explain the later adolescent and overall development in children with autism (Jerzy Wegiel et al., 2010). Deficits in neuronal migration thus fits well with the overall pathology as seen in ASD patients (Reiner et al., 2016).
Abnormal Neurogenesis During Early Development
Dysregulation of neurogenesis, the process of generating new neurons by neuronal progenitor cells (NPCs), is another key mechanism that underlies neurodevelopmental disorders like ASD. Proper developmental neurogenesis depends upon the regulation of symmetric divisions of RGCs, which give rise to more RGCs, and asymmetric divisions that generate NPCs (Kaushik & Zarbalis, 2016). It can also be influenced by environmental factors such as pollution and toxins in the mother's body during pregnancy. NPCs then give rise to all of the glial and neuronal cells in the brain and spinal cord. Research supports that a subset of ASD cases show an increase in neurons in the prefrontal cortex during the prenatal neurodevelopment and the early childhood period (Gilbert & Man, 2017). A drastic change between the number of neurons after prenatal development is indicative of changes in developmental neurogenesis (Kaushik & Zarbalis, 2016). The process of neurogenesis itself consists of multiple steps, including NPC proliferation, differentiation, migration, and maturation (Fan & Pang, 2017). It is hypothesized that an increased rate of proliferation in NPCs during the embryonic stage is likely linked to aberrant neurogenesis. Studies showed that ASD-derived NPCs displayed faster proliferation rates than NPCs in controlled subjects, which is consistent with early brain overgrowth seen in ASD patients (Marchetto et al., 2017).
Most of neuron migration and neurogenesis occurs prenatally, but the process does not finish until about 5 months after birth, which could be linked to regressive autism. Even though abnormalities in these processes are not direct causes of ASD, they can serve as biomarkers. These processes also occur at the beginning of brain development. Thus, it is absolutely vital that these changes are detected early on to ensure that the correct treatment can be provided promptly.
Genetics Although the exact cause of the dysregulation is unknown, animal studies have given insight to the genes that might be responsible for regulating neural progenitor proliferation. One gene in particular, the WDFY3 gene, has shown profound effects on both neural progenitor proliferation and neuronal migration in mice studies (Kaushik & Zarbalis, 2016). The known function of the WDFY3 geneis its involvement in linking specific proteins in a process known as selective macroautophagy, or the degradation of cellular contents by lysosomes to recycle macromolecular contents (Orosco et al., 2014). Studies show that mice with the WDFY3 geneknocked out showed an increase in proliferative neural progenitor cell divisions, which in turn produces more radial glial cells. Results support the hypothesis that mutant mice with the altered WDFY3 gene will produce more Pax6+ radial glial cells in the ventricular zone compared to the control (Orosco et al., 2014). The population of progenitors was shown to increase, and thus mice displayed an enlarged cerebral cortex (Kaushik & Zarbalis, 2016). This suggests that the WDFY3 gene is vital for the normal regulation of cell cycle and cellular division. The loss of the gene ultimately results in pathological changes that are characteristic of ASD (Orosco et al., 2014). Currently, the WDFY3 gene, amongst many others, is identified as an intellectual disability, developmental delay, and autism risk gene (Napoli et al., 2018).
Figure 2. Measure of Pax6+ radial glial cell population area shows significant increase in mutant WDFY3 gene mice compared to the wild type. (Orosco et al., 2014)
In ASD patients, abnormal neural migration and developmental neurogenesis can result in changes in neural density and volume, or it may lead to aberrant growth trajectories in certain regions of the brain (Reiner et al., 2016). Magnetic Resonance Imaging (MRI) has shown various forms of brain malformations of altered brain volume, typically that of the cortex, cerebellum and limbic system (Pan et al., 2019).
The most coherent finding of structural brain changes is an overgrowth of the cerebral cortex in early childhood as a defining feature in a subset of children with ASD (Kaushik & Zarbalis, 2016). Accelerated cortex volume is observed for children around 2-4 years of age, which is also the current earliest age of diagnosis, while older ASD patients show a decreased volume or no significant difference compared to the control (Ha, Sohn, Kim, Sim, & Cheon, 2015). As the most developed part of the brain, the cerebral cortex integrates many neural circuits and is in charge of many higher-order functions such as language and emotional processing. Abnormalities in regions of the cerebral cortex like the prefrontal lobe, Broca’s area, or Wernicke’s area may then be related to defects in social language processing and attention as seen in ASD cases (Ha et al., 2015). Developments of these specific regions are especially critical at young ages. According to psychologist Lev Vygotsky’s theory of Zone of Proximal Development, social interaction is one of the most important part of a child learner’s psychological development. Any higher mental function must go through an external social stage, such as the teaching of or guidance of a skill, before it becomes internalized as a mental function (Shabani, Khatib, & Ebadi, 2010). Thus, structural anomalies and changes in neural circuits in the cortex during early childhood are extremely detrimental to the child’s social and communication skills.
Figure 3. Areas of the prefrontal cortex that are affected by cerebral cortex overgrowth. Broca’s area is involved in speech production, and Wernicke’s area is involved in language comprehension. Both areas are important in ASD social deficits.
Developmental abnormalities may also lead to a loss in the integrity of brain connectivity, which is essential for higher-order social, emotional, and communicative abilities in ASD subjects (Jarek Wegiel et al., 2018). The corpus collosum is a bundle of fibers that connects the left and right hemispheres of the brain, facilitating in the integration of information between the two. Imaging reveals that the corpus collosum is where interhemispheric connections transfer cognitive, motor, and sensory information. Specifically, MRI findings indicate that there is a reduction of corpus collosum thickness amongst certain ASD subjects (Jarek Wegiel et al., 2017). In certain segments of the corpus collosum, less axons were found. Furthermore, axons with smaller diameters were more prevalent than axons of average diameters in ASD patients. The thickness of myelin sheathes, which insulates neurons and allows electrical impulses to travel faster, was also measured, yet no significant difference was found (Jarek Wegiel et al., 2018). These differences may be traced to abnormalities in either the mechanism controlling the amount of neuron connections between the hemispheres, or the mechanism that directly controls the structure of the neuron, including axon diameter, area, and myelin thickness.
Figure 4. The corpus collosum of divided into five segments and compared between ASD patients and the control group.
Figure 5. Differences between axon diameter distribution of ASD subjects and control. Across all 5 segments of the corpus collosum, subjects with ASD had a higher distribution of axons with smaller diameter compared to the control group.
Corpus collosum agenesis (when an organ fails to develop properly during the embryonic stage) and hypoplasia (the underdevelopment or incomplete development of an organ) both lead to a reduced number of axons, and therefore a loss of connectivity between the right and left hemispheres of the brain compared to the average brain (Jarek Wegiel et al., 2018). Even though the causal relationship between corpus collosum agenesis and ASD remains unclear as not all patients with ASD display corpus collosum agenesis or hypoplasia and vice versa, similar pathologies between the two reveal that ASD may be a developmental disorder that arises from atypical connectivity of the brain (Paul, Corsello, Kennedy, & Adolphs, 2014).
Cerebellum Symptoms of ASD involve deficits in higher-order cognitive function. The cerebellum’s role in motor movements and balance has been explored in great detail over the past decades, but recent attention has been focused on its involvement in cognitive function, such as emotion regulation. Most studies therefore focus on assessing the size, density, and number of neurons in the cerebellum. Consistent with findings of other brain regions, studies show ASD subjects have reduced Purkinje cells and reduced cerebellar volume (Rogers et al., 2013). Purkinje cells are cells found in the cerebellum: they have long extensive dendrites which make them morphologically unique. Purkinje cells need to receive thousands of inputs from other neurons of the brain, yet they are the only output cells in the cerebellum. This may be why they are characterized by their numerous dendrites.
Figure 6. Image of a Purkinje cell and its extensive dendrites
Purkinje cells are inhibitory; thus, reduced numbers of Purkinje cells due to abnormalities during development may lead to excess firing in the cerebellum (Rogers et al., 2013). The other leading theory of the effects of reduced cerebellar volume is the lack of Purkinje cells or dysregulation of the neural circuitry may lead to a disconnection with the rest of the brain. Studies show that the cerebellum projects to regions such as the prefrontal cortex. If the cerebellum-prefrontal cortex circuit is disturbed or altered, it may adversely impact cognitive functions. One loop is known as the cerebello-thalamo-cortical loop. This loop has been investigated for its role in motor functions, and the organization of the loop also encompasses brain regions that are primarily involved in cognitive processing (Wang, Kloth, & Badura, 2014). The cerebellum also projects to the ventral tegmental area, which contains cells activated by the neurotransmitter dopamine. Dopamine is a hormone that has been found to affect a wide range of functions such as balance and mood. These dopaminergic cells also ultimately terminate in the prefrontal cortex, so disruptions of this circuit will also have effects on the prefrontal cortex and influence cognitive functions. Mice studies show that cerebellar damage significantly affects the release of dopamine in the prefrontal cortex, and abnormal dopamine functions are often observed in ASD subjects (Rogers et al., 2013).
Figure 7. Neural circuitries from the cerebellum to the prefrontal cortex. Pathways highlighted in red are glutaminergic pathways (require neurotransmitter glutamate) and pathways highlighted in green are dopaminergic (require dopamine).
Even though it is unclear whether cerebellar neuropathology is the cause of ASD related symptoms, its effects on the prefrontal cortex suggests that the cerebellum is not only involved in motor movements and balance, and that the complicated existing neural circuitries indicate that more research is required.
ASD is a complex neurodevelopmental disorder, and it is becoming more prevalent as diagnosis methods become better and broader. Understanding how abnormalities in neural migration and neurogenesis in the developing brain can lead to structural changes of different brain regions might help us understand the pathology of ASD and how to treat it. Despite the heterogeneity and complexities that genetics and environmental causes bring, further insight into the relationship between abnormal neural migration or neural growth and ASD can serve as important diagnostic biomarkers and allow earlier detection, intervention, and treatment for ASD subjects.
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