Distinct developmental principles underlie the formation of ipsilateral and contralateral whisker-related axonal patterns of layer 2/3 neurons in the barrel cortex

Characterization of the Inner and Outer Fiber Layers in the Developing Cerebral Cortex of Gyrencephalic Ferrets

Changes in the cerebral cortex of mammals during evolution have been of great interest. Ferrets, monkeys, and humans have more developed cerebral cortices compared with mice. Although the features of progenitors in the developing cortices of these animals have been intensively investigated, those of the fiber layers are still largely elusive. By taking the advantage of our in utero electroporation technique for ferrets, here we systematically investigated the cellular origins and projection patterns of axonal fibers in the developing ferret cortex. We found that ferrets have 2 fiber layers in the developing cerebral cortex, as is the case in monkeys and humans. Axonal fibers in the inner fiber layer projected contralaterally and subcortically, whereas those in the outer fiber layer sent axons to neighboring cortical areas. Furthermore, we performed similar experiments using mice and found unexpected similarities between ferrets and mice. Our results shed light on the cellular origins, the projection patterns, the developmental processes, and the evolution of fiber layers in mammalian brains.

Development of the whisker-to-barrel cortex system

This review provides an overview on the development of the rodent whisker-to-barrel cortex system from late embryonic stage to the end of the first postnatal month. During this period the system shows a remarkable transition from a mostly genetic-molecular driven generation of crude connectivity, providing the template for activity-dependent structural and functional maturation and plasticity, to the manifestation of a complex behavioral repertoire including social interactions. Spontaneous and sensory-evoked activity is present in neonatal barrel cortex and control the generation of the cortical architecture. Half a century after its first description by Woolsey and van der Loos the whisker-to-barrel cortex system with its unique and clear topographic organization still offers the exceptional opportunity to study sensory processing and complex behavior.

Molecular investigations of development and diseases of the brain of higher mammals using the ferret

The brains of higher mammals such as primates and carnivores contain well-developed unique brain structures. Uncovering the physiological functions, developmental mechanisms and evolution of these brain structures would greatly facilitate our understanding of the human brain and its diseases. Although the anatomical and electrophysiological features of these brain structures have been intensively investigated, our knowledge about their molecular bases is still limited. To overcome this limitation, genetic techniques for the brains of carnivores and primates have been established, and molecules whose expression patterns correspond to these brain structures were identified recently. To investigate the functional roles of these molecules, rapid and efficient genetic manipulation methods for higher mammals have been explored. In this review, recent advances in molecular investigations of the brains of higher mammals are discussed, mainly focusing on ferrets (Mustela putorius furo).

CRISPR/Cas9-mediated gene knockout in the mouse brain using in utero electroporation

The CRISPR/Cas9 system has recently been adapted for generating knockout mice to investigate physiological functions and pathological mechanisms. Here, we report a highly efficient procedure for brain-specific disruption of genes of interest in vivo. We constructed pX330 plasmids expressing humanized Cas9 and single-guide RNAs (sgRNAs) against the Satb2 gene, which encodes an AT-rich DNA-binding transcription factor and is responsible for callosal axon projections in the developing mouse brain. We first confirmed that these constructs efficiently induced double-strand breaks (DSBs) in target sites of exogenous plasmids both in vitro and in vivo. We then found that the introduction of pX330-Satb2 into the developing mouse brain using in utero electroporation led to a dramatic reduction of Satb2 expression in the transfected cerebral cortex, suggesting DSBs had occurred in the Satb2 gene with high efficiency. Furthermore, we found that Cas9-mediated targeting of the Satb2 gene induced abnormalities in axonal projection patterns, which is consistent with the phenotypes previously observed in Satb2 mutant mice. Introduction of pX330-NeuN using our procedure also resulted in the efficient disruption of the NeuN gene. Thus, our procedure combining the CRISPR/Cas9 system and in utero electroporation is an effective and rapid approach to achieve brain-specific gene knockout in vivo.

Contralateral targeting of the corpus callosum in normal and pathological brain function

The corpus callosum connects the two cortical hemispheres of the mammalian brain and is susceptible to structural defects during development, which often result in significant neuropsychological dysfunction. To date, such individuals have been studied primarily with regards to the integrity of the callosal tract at the midline. However, the mechanisms regulating the contralateral targeting of the corpus callosum, after midline crossing has occurred, are less well understood. Recent evidence suggests that defects in contralateral targeting can occur in isolation from midline-tract malformations, and may have significant functional implications. We propose that contralateral targeting is a crucially important and relatively under-investigated event in callosal development, and that defects in this process may constitute an undiagnosed phenotype in several neurological disorders.

Classic Cadherins Mediate Selective Intracortical Circuit Formation in the Mouse Neocortex

Understanding the molecular mechanisms underlying the formation of selective intracortical circuitry is one of the important questions in neuroscience research. "Barrel nets" are recently identified intracortical axonal trajectories derived from layer 2/3 neurons in layer 4 of the primary somatosensory (barrel) cortex. Axons of layer 2/3 neurons are preferentially distributed in the septal regions of layer 4 of the barrel cortex, where they show whisker-related patterns. Because cadherins have been viewed as potential candidates that mediate the formation of selective neuronal circuits, here we examined the role of cadherins in the formation of barrel nets. We disrupted the function of cadherins by expressing dominant-negative cadherin (dn-cadherin) using in utero electroporation and found that barrel nets were severely disrupted. Confocal microscopic analysis revealed that expression of dn-cadherin reduced the density of axons in septal regions in layer 4 of the barrel cortex. We also found that cadherins were important for the formation, rather than the maintenance, of barrel nets. Our results uncover an important role of cadherins in the formation of local intracortical circuitry in the neocortex.

Molecular investigations of the brain of higher mammals using gyrencephalic carnivore ferrets

The brains of mammals such as carnivores and primates contain developed structures not found in the brains of mice. Uncovering the physiological importance, developmental mechanisms and evolution of these structures using carnivores and primates would greatly contribute to our understanding of the human brain and its diseases. Although the anatomical and physiological properties of the brains of carnivores and primates have been intensively examined, molecular investigations are still limited. Recently, genetic techniques that can be applied to carnivores and primates have been explored, and molecules whose expression patterns correspond to these structures were reported. Furthermore, to investigate the functional importance of these molecules, rapid and efficient genetic manipulation methods were established by applying electroporation to gyrencephalic carnivore ferrets. In this article, I review recent advances in molecular investigations of the brains of carnivores and primates, mainly focusing on ferrets (Mustela putorius furo).

The development of suckling behavior of neonatal mice is regulated by birth

Background

Although the function of the sensory system rapidly develops soon after birth in newborn pups, little is known about the mechanisms triggering this functional development of the sensory system.

Results

Here we show that the birth of pups plays an active role in the functional development of the sensory system. We first optimized the experimental procedure for suckling behavior using neonatal mouse pups. Using this procedure, we found that preterm birth selectively accelerated the development of suckling behavior in neonatal pups, but not that of motor performance, suggesting that the birth of pups regulates the functional development of the sensory system soon after birth.

Conclusions

Taken together with our recent findings that birth itself regulates the initiation of sensory map formation in the somatosensory and visual systems, these results support the idea that the birth of pups actively regulates the anatomical and functional development of the sensory system.