Imaging the Development of Cerebral Cortical Radial Units: Implications for Cortical Minicolumn Assembly and Autism

Lay Summary

Are Neuronal Defects in the Cerebral Cortex Linked to Autism?

Investigators will use two-photon cellular imaging during development in mice with a genetic form of autism to see whether neurons fail to migrate and connect properly in the cerebral cortex.   Co-funding for this study will be provided by Autism Speaks.

During development, neurons produced at a similar time from progenitor cells migrate and occupy similar positions in the cerebral cortex.  They are layered horizontally, and then connect vertically into columns. In this way, neurons that transmit excitatory messages connect with those that transmit inhibitory messages, forming local communication networks.  In children with autism, these neural columns are more narrow and dense than in normally developing children.  The researchers hypothesize that these abnormalities are linked to autism, possibly due to an imbalance between excitatory and inhibitory neurons, or to impairments in their connections. They will explore this hypothesis in mice with Fragile X syndrome (a genetically caused form of autism).  Using two-photon cellular imaging, they will follow migrating neurons into the cerebral cortex, and map their connections using electrophysiology recordings.

Significance:  This study may reveal how abnormal cortical excitation or inhibition is linked to autism and lead to improved treatment.

Abstract

Imaging the Development of Cerebral Cortical Radial Units: Implications for Cortical Minicolumn Assembly and Autism

Autism, or autism spectrum disorder (ASD), is a developmental disorder of the nervous system. Despite its prevalence, the key neural defects that cause autistic behaviors, such as deficits in social interactions and verbal and non-verbal communications, to emerge during the first few years of life are not clear. The cerebral cortex plays a central role in nearly all high-order brain functions, including language and social interaction. Therefore, defects in the cerebral cortex development and function likely contribute centrally to the pathophysiology of autism.

The cerebral cortex is a highly organized brain structure. During development, cortical neurons born at a similar time migrate and occupy similar positions in the cortex, resulting in a horizontally layered arrangement of cortical neurons. Moreover, cortical neurons are functionally assembled into vertical columns—cortical minicolumns, in which ~80-120 excitatory pyramidal neurons and inhibitory interneurons connect into a local network. Cortical minicolumns are the basic information processing units in the cortex. Recent studies showed that the cortical minicolumn structure is abnormal in size and density in autistic brains compared to normal controls; however, the cause of this cerebral cortex defect is not clear.

Cortical minicolumns likely originate from radial units in the developing cortex, which is formed by asymmetric cell division of cortical progenitor cells, i.e., radial glial cells, and radial migration of postmitotic neurons. The proposed work will employ powerful imaging techniques together with electrophysiology and mouse genetics approaches to investigate the anatomical and functional development of cortical radial units of both cortical excitatory and inhibitory neurons. Furthermore, we will explore the link between maldevelopment of cortical radial units and autism. Our central hypothesis is that defects in the development of excitatory neuron and inhibitory neuron radial units are responsible for the abnormalities in cortical minicolumns observed in autistic patients. The specific aims are the following:

Aim 1: To visualize the migration, the morphogenesis, and the assembly of excitatory and inhibitory neuron radial units in the developing mouse cortex. We will inject retroviruses expressing fluorescence protein (e.g. EGFP) into the lateral ventricle of developing mouse embryos in utero to label individual dividing radial glial cells and their neuronal progeny. The migration, the morphogenesis, and the assembly of fluorescence labeled radial units will be monitored with a custom-built two photon laser scanning microscope (TPLSM).

Aim 2: To map the synaptic connectivity formation and maturation among neurons in excitatory and inhibitory neuron radial units in the developing mouse cortex. We will combine optical probing technique (e.g., glutamate photo-uncaging) with electrophysiology to examine the synapse formation and maturation among neurons in cortical radial units at different developmental stages using our custom-built TPLSM.

Aim 3: To examine the developmental defects of excitatory pyramidal neuron and inhibitory interneuron radial units in fragile X mental retardation 1 (FRM1) gene knockout mice. Fragile X syndrome due to mutations in the FRM1 gene is the most common known single gene cause of autism. Therefore, the FRM1 knockout mouse offers an excellent model to explore the link between development defects of cortical radial units and autism. We will examine the anatomical and the functional development of cortical radial units of both excitatory and inhibitory neurons in the FRM1 knockout mice.

Our proposed study integrates state-of-the-art imaging techniques with electrophysiology and mouse genetics approaches. It will not only provide new insight into the anatomical and functional organization of the mammalian cerebral cortex, but will also reveal the potential link between the maldevelopment of the cerebral cortex and the pathophysiology of autism. Furthermore, the methods established here will allow us to investigate the function of other candidate genes implicated in autism and to define the genetic basis of autism.