Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Columnar Organization of Mammalian Neocortex

 

Nature 486, 118121 (07 June 2012)

Clonally related visual cortical neurons show similar stimulus feature selectivity

Division of Neurobiology, Department of Molecular and Cell Biology, Helen Wills Neuroscience Institute, University of California, Berkeley, California 94720, USA

Ye Li, Hui Lu, Pei-lin Cheng & Yang Dan

Howard Hughes Medical Institute, University of California, Berkeley, California 94720, USA

Ye Li, Hui Lu & Yang Dan

Department of Neurobiology & Behavior, State University of New York at Stony Brook, Stony Brook, New York 11794, USA

Shaoyu Ge

Developmental Biology Program, Memorial Sloan-Kettering Cancer Centre, 1275 York Avenue, New York, New York 10065, USA

Huatai Xu & Song-Hai Shi

 

A fundamental feature of the mammalian neocortex is its columnar organization. In the visual cortex, functional columns consisting of neurons with similar orientation preferences have been characterized extensively, but how these columns are constructed during development remains unclea. The radial unit hypothesis posits that the ontogenetic columns formed by clonally relatred neurons migrating along the same radial glial fibre during corticogenesis provide the basis for functional columns in adult neocortex. However, a direct correspondence between the ontogenetic and functional columns has not been demonstrated. Here we show that, despite the lack of a discernible orientation map in mouse visual cortex, sister neurons in the same radial clone exhibit similar orientation preferences. Using a retroviral vector encoding green fluorescent protein to label radial clones of excitatory neurons, and in vivo two-photon calcium imaging to measure neuronal response properties, we found that sister neurons preferred similar orientations whereas nearby non-sister neurons showed no such relationship. Interestingly, disruption of gap junction coupling by viral expression of a dominant-negative mutant of Cx26 (also known as Gjb2) or by daily administration of a gap junction blocker, carbenoxolone, during the first postnatal week greatly diminished the functional similarity between sister neurons, suggesting that the maturation of ontogenetic into functional columns requires intercellular communication through gap junctions. Together with the recent finding of preferential excitatory connections among sister neurons, our results support the radial unit hypothesis and unify the ontogenetic and functional columns in the visual cortex.

Intracortical excitatory connections are highly non-random, organizing the neurons into fine-scale subnetworks. A recent study showed that sister neurons in the same radial clone are much more connected to each other than to nearby non-sister neurons, suggesting that the radial clones may provide a basis for subnetwork organization. The high connectivity between sister cells should also contribute to their functional similarity as observed in our study. However, inputs from the sister cells alone are likely to be insufficient to determine stimulus selectivity, as each neuron receives inputs from ~1,000 other neurons, whereas each radial clone only consists of tens of neurons. Other factors, such as common inputs to the sister cells, may also have important roles. In mouse V1, layer 2/3 neurons with similar orientation tuning are shown to be preferentially interconnected. A significant fraction of these neurons may be sister cells, exhibiting similar orientation tuning and preferential connectivity.

Although the columnar structure has long been thought to be a fundamental organizational principle of the neocortex, the existence of a basic processing unit has remained controversial. Although the anatomical minicolumns observed in adult cortex are believed to arise from ontogenetic columns, the relationship between the functional columns and mini/ontogenetic columns remained speculative. Our results demonstrate a direct correspondence between them in V1, at least in superficial layers where neurons are most orientation selective. Contrary to the notion of random organization, our study shows that orientation tuning is organized in columns even in rodent visual cortex. The fine spatial scale of ontogenetic columns may also explain the extraordinary precision of the orientation map in cat visual cortex. The interspecies difference in macroscopic cortical organization may be due to differences in the horizontal connections between ontogenetic columns, which can lead to either a smoothly varying map or apparent salt-and-pepper organization. Thus, our results support the view that the ontogenetic columns, rather than the macroscopic functional columns, constitute the basic units of cortical processing.

 

 

 

Nature 486, 113117 (07 June 2012)

Preferential electrical coupling regulates neocortical lineage-dependent microcircuit assembly

Institute of Neurobiology, Institutes of Brain Science and State Key Laboratory of Medical Neurobiology, Fudan University, 138 Yixueyuan Road, Shanghai 200032, China

Yong-Chun Yu, Yinghui Fu, Xing-Hua Yao & Jian Ma

Developmental Biology Program, Memorial Sloan-Kettering Cancer Centre, 1275 York Avenue, New York, New York 10065, USA

Shuijin He, She Chen, Keith N. Brown, Kate P. Gao & Song-Hai Shi

Neuroscience Graduate Program, Weill Cornell Medical College, 1230 York Avenue, New York, New York 10065, USA

Keith N. Brown, Kate P. Gao & Song-Hai Shi

National Centre for Microscopy and Imaging Research and Department of Neurosciences, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0608, USA

Gina E. Sosinsky

Department of Biomedical Informatics, Comprehensive Cancer Center Biomedical Informatics Shared Resource, The Ohio State University, 333 West 10th Avenue, Columbus, Ohio 43210, USA

Kun Huang

 

Radial glial cells are the primary neural progenitor cells in the developing neocortex. Consecutive asymmetric divisions of individual radial glial progenitor cells produce a number of sister excitatory neurons that migrate along the elongated radial glial fibre, resulting in the formation of ontogenetic columns. Moreover, sister excitatory neurons in ontogenetic columns preferentially develop specific chemical synapses with each other rather than with nearby non-siblings. Although these findings provide crucial insight into the emergence of functional columns in the neocortex, little is known about the basis of this lineage-dependent assembly of excitatory neuron microcircuits at single-cell resolution. Here we show that transient electrical coupling between radially aligned sister excitatory neurons regulates the subsequent formation of specific chemical synapses in the neocortex. Multiple-electrode whole-cell recordings showed that sister excitatory neurons preferentially form strong electrical coupling with each other rather than with adjacent non-sister excitatory neurons during early postnatal stages. This preferential coupling allows selective electrical communication between sister excitatory neurons, promoting their action potential generation and synchronous firing. Interestingly, although this electrical communication largely disappears before the appearance of chemical synapses, blockade of the electrical communication impairs the subsequent formation of specific chemical synapses between sister excitatory neurons in ontogenetic columns. These results suggest a strong link between lineage-dependent transient electrical coupling and the assembly of precise excitatory neuron microcircuits in the neocortex.

Sister excitatory neurons in individual ontogenetic columns preferentially develop specific chemical synapses with each other rather than with adjacent non-sister excitatory neurons. Given the almost complete overlap of the dendritic fields of neighbouring excitatory neurons, it is unclear how this lineage-dependent assembly of precise columnar microcircuits is controlled at the individual cell level. Some studies have suggested that gap-junction-mediated neuronal communication is involved in the formation of local connectivity in the developing neocortex, even though direct evidence of electrically coupled neocortical neurons at early developmental stages is lacking. In this study, we set out to investigate whether gap-junction-mediated electrical coupling exists between sister excitatory neurons in ontogenetic columns and, if so, whether this coupling regulates the preferential formation of chemical synapses between sister excitatory neurons.

Gap junctions are composed of two membrane-docked hexameric hemi-channels that consist of connexin proteins from two adjacent cells. There are ~20 genes encoding connexins in rodents, and the corresponding protein symbols are denoted as CX plus the calculated molecular mass of the protein. Of these proteins, CX26 and CX43 have been shown to be abundantly expressed in the developing neocortex at embryonic and neonatal stages. Consistent with this, we found that developing neurons in the neonatal neocortex expressed CX26. Moreover, CX26-positive puncta were present at the dendrodendritic and dendrosomatic contacts of radially aligned sister excitatory neurons that were labelled by in utero intraventricular injection of low-titre enhanced green fluorescence protein (eGFP)-expressing retrovirus at embryonic day 12 to 13 (E1213), indicating the existence of gap junctions between sister excitatory neurons in ontogenetic columns.

Gap junctions mediate intercellular adhesion and the exchange of small molecules (typically less than 1kDa), including low-molecular-mass dyes and ions that can be detected experimentally. Previous dye injection experiments have suggested the presence of gap junctions between progenitor cells in the embryonic neocortex and between neurons in the neonatal neocortex. Although these studies have provided important insight, the accuracy of dye coupling in revealing the presence of gap junctions has been debated. To circumvent this issue and to quantitatively examine gap junction channel activity, we performed whole-cell patch-clamp recording experiments to study gap junctions between sister excitatory neurons in ontogenetic columns.

Transient electrical coupling occurs before the establishment of mature patterns of synaptic connectivity in many developing nervous systems, and in some cases, this electrical coupling is crucial for the development of chemical synapses. Previously, dye coupling was observed in the developing neocortex. In addition, it has been suggested that the formation of electrically coupled neuronal domains might help to guide the emergence of chemically transmitting neuronal circuits. However, electrically coupled neuronal pairs have not been reported in the neocortex at early developmental stages. By performing dual and quadruple whole-cell recording experiments, which allow the detection of gap-junction-mediated electrical coupling with high sensitivity and spatial precision, we demonstrated the electrical coupling of excitatory neurons in the early postnatal neocortex. Moreover, we revealed that neocortical excitatory neurons show a high preference for forming strong electrical coupling with their sister excitatory neurons but not with nearby non-sister excitatory neurons.

Furthermore, we found that strong electrical coupling between sister excitatory neurons in ontogenetic columns promotes their action potential generation and synchronous firing. Although previous studies have suggested that electrical coupling is crucial for robust synchronous activity in the neonatal neocortex, the precise function of electrical coupling has been elusive. Our results show that electrical transmission between sister excitatory neurons in ontogenetic columns is required for the development of precise chemical synapses between these neurons. These findings provide clear evidence of the role of gap junctions in regulating precise neuronal circuit assembly in the neocortex.