Age-associated reduction of nuclear shape dynamics in excitatory neurons of the visual cortex.

Neurons decline in their functionality over time, and age-related neuronal alterations are associated with phenotypes of neurodegenerative diseases. In nonneural tissues, an infolded nuclear shape has been proposed as a hallmark of aged cells and neurons with infolded nuclei have also been reported to be associated with neuronal activity. Here, we performed time-lapse imaging in the visual cortex of Nex-Cre;SUN1-GFP mice. Nuclear infolding was observed within 10 min of stimulation in young nuclei, while the aged nuclei were already infolded pre-stimulation and showed reduced dynamics of the morphology. In young nuclei, the depletion of the stimuli restored the nucleus to a spherical shape and reduced the dynamic behavior, suggesting that nuclear infolding is a reversible process. We also found the aged nucleus to be stiffer than the young one, further relating to the age-associated loss of nuclear shape dynamics. We reveal temporal changes in the nuclear shape upon external stimulation and observe that these morphological dynamics decrease with age.

Inhomogeneous mechanotransduction defines the spatial pattern of apoptosis-induced compensatory proliferation.

The number of cells in tissues is controlled by cell division and cell death, and its misregulation could lead to pathological conditions such as cancer. To maintain the cell numbers, a cell-elimination process called apoptosis also stimulates the proliferation of neighboring cells. This mechanism, apoptosis-induced compensatory proliferation, was originally described more than 40 years ago. Although only a limited number of the neighboring cells need to divide to compensate for the apoptotic cell loss, the mechanisms that select cells to divide have remained elusive. Here, we found that spatial inhomogeneity in Yes-associated protein (YAP)-mediated mechanotransduction in neighboring tissues determines the inhomogeneity of compensatory proliferation in Madin-Darby canine kidney (MDCK) cells. Such inhomogeneity arises from the non-uniform distribution of nuclear size and the non-uniform pattern of mechanical force applied to neighboring cells. Our findings from a mechanical perspective provide additional insight into how tissues precisely maintain homeostasis.

Interfacial friction and substrate deformation mediate long-range signal propagation in tissues.

Tissue layers can generally slide at the interface, accompanied by the dissipation due to friction. Nevertheless, it remains elusive how force could propagate in a tissue with such interfacial friction. Here, we elaborate the force dynamics in a prototypical multilayer system in which an epithelial monolayer was cultivated upon an elastic substrate in contact with a hard surface, and discover a novel mechanism of pronounced force propagation over a long distance due to interfacial dynamics between substrate layers. We derived an analytical model for the dynamics of the elastic substrate under the shear stress provided by the cell layer at the surface boundary and the friction at bottom. The model reveals that sliding between substrate layers leads to an expanding stretch regime from a shear regime of substrate deformation in time and space. The regime boundary propagating diffusively with a speed depending on the stiffness, thickness, and slipperiness of the substrate, is a robust nature of a deformed elastic sheet with interfacial friction. These results shed new light on force propagation in tissues and our model could serve as a basis for studies of such propagation in a more complex tissue environment.

Author Correction: Role of extrinsic mechanical force in the development of the RA-I tactile mechanoreceptor.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Role of extrinsic mechanical force in the development of the RA-I tactile mechanoreceptor.

Rapidly adapting type I (RA-I) mechanoreceptors play an important role in sensing the low-frequency vibration aspects of touch. The structure of the RA-I mechanoreceptor is extremely complex regardless of its small size, limiting our understanding of its mechanotransduction. As a result of the emergence of bioengineering, we previously proposed an in vitro bioengineering approach for RA-I receptors to overcome this limitation. Currently, the in vitro bioengineering approach for the RA-I receptor is not realizable given the lack of knowledge of its morphogenesis. This paper demonstrates our first attempt to interpret the cellular morphogenesis of the RA-I receptor. We found indications of extrinsic mechanical force nearby the RA-I receptor in the developing fingertip. Using a mechanical compression device, the axon of dorsal root ganglion (DRG) neurons buckled in vitro into a profile that resembled the morphology of the RA-I receptor. This work encourages further implementation of this bioengineering approach in tactile receptor-related research.

Evaluation method regarding the effect of psychotropic drugs on driving performance: A literature review.

Although automobile driving is necessary for many people, including patients with mental disorders, the influence of psychotropic drugs on driving performance remains unclear and requires scientific verification. Therefore, the objective of this study was to conduct a review of the literature in order to aid the development of a valid evaluation method regarding the influence of medication on driving performance. We conducted a literature search using two sets of terms on PubMed. One set was related to psychotropic drugs, and the other to driving tests. We excluded reviews and case studies and added literature found on other sites. A total of 121 relevant reports were found. The experiments were roughly divided into on-the-road tests (ORT) and driving simulators (DS). Although highway driving tests in ORT are most often used to evaluate driving performance, DS are becoming increasingly common because of their safety and low cost. The validity of evaluation methods for alcohol should be verified; however, we found that there were few validated tests, especially for DS. The scenarios and measurement indices of each DS were different, which makes it difficult to compare the results of DS studies directly. No evaluation indices, except for SD of lateral position, were sufficiently validated. Although highway ORT are the gold standard, DS were shown to have an increasing role in evaluating driving performance. The reliability of DS needs to be established, as does their validation with alcohol in order to accumulate more high-quality evidence.

Differentiating cells mechanically limit the interkinetic nuclear migration of progenitor cells to secure apical cytogenesis.

Many proliferative epithelia are pseudostratified because of cell cycle-dependent interkinetic nuclear migration (IKNM, basal during G1 and apical during G2). Although most epithelia, including early embryonic neuroepithelia (≤100 µm thick), undergo IKNM over the entire apicobasal extent, more apicobasally elongated (300 µm) neural progenitor cells (radial glial cells) in the mid-embryonic mouse cerebral wall move their nuclei only within its apical (100 µm) compartment, leaving the remaining basal region nucleus-free (fiber-like). How this IKNM range [i.e. the thickness of a pseudostratified ventricular zone (VZ)] is determined remains unknown. Here, we report external fencing of IKNM and the VZ by differentiating cells. When a tight stack of multipolar cells immediately basal to the VZ was 'drilled' via acute neuron-directed expression of diphtheria toxin, IKNM of apicobasally connected progenitor cells continued further towards the basal region of the cell (200 µm). The unfencing-induced basally overshot nuclei stay in S phase for too long and do not move apically, suggesting that external limitation of IKNM is necessary for progenitors to undergo normal cytogenetic behaviors. Thus, physical collaboration between progenitors and differentiating cells, including neurons, underlies brain development.

Elasticity-based boosting of neuroepithelial nucleokinesis via indirect energy transfer from mother to daughter.

Neural progenitor cells (NPCs), which are apicobasally elongated and densely packed in the developing brain, systematically move their nuclei/somata in a cell cycle-dependent manner, called interkinetic nuclear migration (IKNM): apical during G2 and basal during G1. Although intracellular molecular mechanisms of individual IKNM have been explored, how heterogeneous IKNMs are collectively coordinated is unknown. Our quantitative cell-biological and in silico analyses revealed that tissue elasticity mechanically assists an initial step of basalward IKNM. When the soma of an M-phase progenitor cell rounds up using actomyosin within the subapical space, a microzone within 10 µm from the surface, which is compressed and elastic because of the apical surface's contractility, laterally pushes the densely neighboring processes of non-M-phase cells. The pressed processes then recoil centripetally and basally to propel the nuclei/somata of the progenitor's daughter cells. Thus, indirect neighbor-assisted transfer of mechanical energy from mother to daughter helps efficient brain development.

Radial Glial Fibers Promote Neuronal Migration and Functional Recovery after Neonatal Brain Injury.

Radial glia (RG) are embryonic neural stem cells (NSCs) that produce neuroblasts and provide fibers that act as a scaffold for neuroblast migration during embryonic development. Although they normally disappear soon after birth, here we found that RG fibers can persist in injured neonatal mouse brains and act as a scaffold for postnatal ventricular-subventricular zone (V-SVZ)-derived neuroblasts that migrate to the lesion site. This injury-induced maintenance of RG fibers has a limited time window during post-natal development and promotes directional saltatory movement of neuroblasts via N-cadherin-mediated cell-cell contacts that promote RhoA activation. Transplanting an N-cadherin-containing scaffold into injured neonatal brains likewise promotes migration and maturation of V-SVZ-derived neuroblasts, leading to functional improvements in impaired gait behaviors. Together these results suggest that RG fibers enable postnatal V-SVZ-derived neuroblasts to migrate toward sites of injury, thereby enhancing neuronal regeneration and functional recovery from neonatal brain injuries.

Differences in the Mechanical Properties of the Developing Cerebral Cortical Proliferative Zone between Mice and Ferrets at both the Tissue and Single-Cell Levels.

Cell-producing events in developing tissues are mechanically dynamic throughout the cell cycle. In many epithelial systems, cells are apicobasally tall, with nuclei and somata that adopt different apicobasal positions because nuclei and somata move in a cell cycle-dependent manner. This movement is apical during G2 phase and basal during G1 phase, whereas mitosis occurs at the apical surface. These movements are collectively referred to as interkinetic nuclear migration, and such epithelia are called "pseudostratified." The embryonic mammalian cerebral cortical neuroepithelium is a good model for highly pseudostratified epithelia, and we previously found differences between mice and ferrets in both horizontal cellular density (greater in ferrets) and nuclear/somal movements (slower during G2 and faster during G1 in ferrets). These differences suggest that neuroepithelial cells alter their nucleokinetic behavior in response to physical factors that they encounter, which may form the basis for evolutionary transitions toward more abundant brain-cell production from mice to ferrets and primates. To address how mouse and ferret neuroepithelia may differ physically in a quantitative manner, we used atomic force microscopy to determine that the vertical stiffness of their apical surface is greater in ferrets (Young's modulus = 1700 Pa) than in mice (1400 Pa). We systematically analyzed factors underlying the apical-surface stiffness through experiments to pharmacologically inhibit actomyosin or microtubules and to examine recoiling behaviors of the apical surface upon laser ablation and also through electron microscopy to observe adherens junction. We found that although both actomyosin and microtubules are partly responsible for the apical-surface stiffness, the mouse

Two-Photon Imaging of DiO-Labelled Meissner Corpuscle in Living Mouse's Fingertip.

Meissner corpuscles are the fast adapting type I (FA-I) mechanoreceptor that locates at the dermal papillae of skin. The Meissner corpuscle is well known for its complex structure, consisting of spiral axons, lamellar cells, and a collagen capsule. Fluorescent microscopy has become a convenient method for observing the Meissner corpuscle and its inner structure. This method requires preparing samples with fingertip cross-sections and performing antibody staining before observation. Various kinds of microscopy can be used for observation, such as confocal microscopy, transmission electron microscopy (TEM), or scanning electron microscopy (SEM). Although the anatomical shape, distribution, and components of Meissner corpuscle are recognized, they have been mostly determined from observations of fixed tissues. Therefore, knowledge of mechanical transduction is limited by the lack of in vivo experiments and individual differences among samples. In this study, we propose a novel less invasive imaging method that incorporates a staining technique with lipophilic carbocyanine [Formula: see text] and two-photon microscopy. This combination allows us to repetitively observe the Meissner corpuscle in a living mouse.

Ferret-mouse differences in interkinetic nuclear migration and cellular densification in the neocortical ventricular zone.

The thick outer subventricular zone (OSVZ) is characteristic of the development of human neocortex. How this region originates from the ventricular zone (VZ) is largely unknown. Recently, we showed that over-proliferation-induced acute nuclear densification and thickening of the VZ in neocortical walls of mice, which lack an OSVZ, causes reactive delamination of undifferentiated progenitors and invasion by these cells of basal areas outside the VZ. In this study, we sought to determine how VZ cells behave in non-rodent animals that have an OSVZ. A comparison of mid-embryonic mice and ferrets revealed: (1) the VZ is thicker and more pseudostratified in ferrets. (2) The soma and nuclei of VZ cells were horizontally and apicobasally denser in ferrets. (3) Individual endfeet were also denser on the apical (ventricular) surface in ferrets. (4) In ferrets, apicalward nucleokinesis was less directional, whereas basalward nucleokinesis was more directional; consequently, the nuclear density in the periventricular space (within 16 µm of the apical surface) was smaller in ferrets than in mice, despite the nuclear densification seen basally in ferrets. These results suggest that species-specific differences in nucleokinesis strategies may have evolved in close association with the magnitudes and patterns of nuclear stratification in the VZ.

Neurogenin2-d4Venus and Gadd45g-d4Venus transgenic mice: visualizing mitotic and migratory behaviors of cells committed to the neuronal lineage in the developing mammalian brain.

To achieve highly sensitive and comprehensive assessment of the morphology and dynamics of cells committed to the neuronal lineage in mammalian brain primordia, we generated two transgenic mouse lines expressing a destabilized (d4) Venus controlled by regulatory elements of the Neurogenin2 (Neurog2) or Gadd45g gene. In mid-embryonic neocortical walls, expression of Neurog2-d4Venus mostly overlapped with that of Neurog2 protein, with a slightly (1 h) delayed onset. Although Neurog2-d4Venus and Gadd45g-d4Venus mice exhibited very similar labeling patterns in the ventricular zone (VZ), in Gadd45g-d4Venus mice cells could be visualized in more basal areas containing fully differentiated neurons, where Neurog2-d4Venus fluorescence was absent. Time-lapse monitoring revealed that most d4Venus(+) cells in the VZ had processes extending to the apical surface; many of these cells eventually retracted their apical process and migrated basally to the subventricular zone, where neurons, as well as the intermediate neurogenic progenitors that undergo terminal neuron-producing division, could be live-monitored by d4Venus fluorescence. Some d4Venus(+) VZ cells instead underwent nuclear migration to the apical surface, where they divided to generate two d4Venus(+) daughter cells, suggesting that the symmetric terminal division that gives rise to neuron pairs at the apical surface can be reliably live-monitored. Similar lineage-committed cells were observed in other developing neural regions including retina, spinal cord, and cerebellum, as well as in regions of the peripheral nervous system such as dorsal root ganglia. These mouse lines will be useful for elucidating the cellular and molecular mechanisms underlying development of the mammalian nervous system.

Lhx1 in the proximal region of the optic vesicle permits neural retina development in the chicken.

How the eye forms has been one of the fundamental issues in developmental biology. The retinal anlage first appears as the optic vesicle (OV) evaginating from the forebrain. Subsequently, its distal portion invaginates to form the two-walled optic cup, which develops into the outer pigmented and inner neurosensory layers of the retina. Recent work has shown that this optic-cup morphogenesis proceeds as a self-organizing activity without any extrinsic molecules. However, intrinsic factors that regulate this process have not been elucidated. Here we show that a LIM-homeobox gene, Lhx1, normally expressed in the proximal region of the nascent OV, induces a second neurosensory retina formation from the outer pigmented retina when overexpressed in the chicken OV. Lhx2, another LIM-homeobox gene supposed to be involved in early OV formation, could not substitute this function of Lhx1, while Lhx5, closely related to Lhx1, could replace it. Conversely, knockdown of Lhx1 expression by RNA interference resulted in the formation of a small or pigmented vesicle. These results suggest that the proximal region demarcated by Lhx1 expression permits OV development, eventually dividing the two retinal domains.