Anatomy of the complete mouse eye vasculature explored by light-sheet fluorescence microscopy exposes subvascular-specific remodeling in development and pathology

Eye development and function rely on precise establishment, regression and maintenance of its many sub-vasculatures. These crucial vascular properties have been extensively investigated in eye development and disease utilizing genetic and experimental mouse models. However, due to technical limitations, individual studies have often restricted their focus to one specific sub-vasculature. Here, we apply a workflow that allows for visualization of complete vasculatures of mouse eyes of various developmental stages. Through tissue depigmentation, immunostaining, clearing and light-sheet fluorescence microscopy (LSFM) entire vasculatures of the retina, vitreous (hyaloids) and uvea were simultaneously imaged at high resolution. In silico dissection provided detailed information on their 3D architecture and interconnections. By this method we describe successive remodeling of the postnatal iris vasculature, involving sprouting and pruning, following its disconnection from the embryonic feeding hyaloid vasculature. In addition, we demonstrate examples of conventional and LSFM-mediated analysis of choroidal neovascularization after laser-induced wounding, showing added value of the presented workflow in analysis of modelled eye disease. These advancements in visualization and analysis of the respective eye vasculatures in development and complex eye disease open for novel observations of their functional interplay at a whole-organ level.

Directionality of developing skeletal muscles is set by mechanical forces

Formation of oriented myofibrils is a key event in musculoskeletal development. However, the mechanisms that drive myocyte orientation and fusion to control muscle directionality in adults remain enigmatic. Here, we demonstrate that the developing skeleton instructs the directional outgrowth of skeletal muscle and other soft tissues during limb and facial morphogenesis in zebrafish and mouse. Time-lapse live imaging reveals that during early craniofacial development, myoblasts condense into round clusters corresponding to future muscle groups. These clusters undergo oriented stretch and alignment during embryonic growth. Genetic perturbation of cartilage patterning or size disrupts the directionality and number of myofibrils in vivo. Laser ablation of musculoskeletal attachment points reveals tension imposed by cartilage expansion on the forming myofibers. Application of continuous tension using artificial attachment points, or stretchable membrane substrates, is sufficient to drive polarization of myocyte populations in vitro. Overall, this work outlines a biomechanical guidance mechanism that is potentially useful for engineering functional skeletal muscle.

ARG1-expressing microglia show a distinct molecular signature and modulate postnatal development and function of the mouse brain

Molecular diversity of microglia, the resident immune cells in the CNS, is reported. Whether microglial subsets characterized by the expression of specific proteins constitute subtypes with distinct functions has not been fully elucidated. Here we describe a microglial subtype expressing the enzyme arginase-1 (ARG1; that is, ARG1⁺ microglia) that is found predominantly in the basal forebrain and ventral striatum during early postnatal mouse development. ARG1⁺ microglia are enriched in phagocytic inclusions and exhibit a distinct molecular signature, including upregulation of genes such as Apoe, Clec7a, Igf1, Lgals3 and Mgl2, compared to ARG1^- microglia. Microglial-specific knockdown of Arg1 results in deficient cholinergic innervation and impaired dendritic spine maturation in the hippocampus where cholinergic neurons project, which in turn results in impaired long-term potentiation and cognitive behavioral deficiencies in female mice. Our results expand on microglia diversity and provide insights into microglia subtype-specific functions.

Spatiotemporal characterization of cellular tau pathology in the human locus coeruleus-pericoerulear complex by three-dimensional imaging

Tau pathology of the noradrenergic locus coeruleus (LC) is a hallmark of several age-related neurodegenerative disorders, including Alzheimer's disease. However, a comprehensive neuropathological examination of the LC is difficult due to its small size and rod-like shape. To investigate the LC cytoarchitecture and tau cytoskeletal pathology in relation to possible propagation patterns of disease-associated tau in an unprecedented large-scale three-dimensional view, we utilized volume immunostaining and optical clearing technology combined with light sheet fluorescence microscopy. We examined AT8⁺ pathological tau in the LC/pericoerulear region of 20 brains from Braak neurofibrillary tangle (NFT) stage 0-6. We demonstrate an intriguing morphological complexity and heterogeneity of AT8⁺ cellular structures in the LC, representing various intracellular stages of NFT maturation and their diverse transition forms. We describe novel morphologies of neuronal tau pathology such as AT8⁺ cells with fine filamentous somatic protrusions or with disintegrating soma. We show that gradual dendritic atrophy is the first morphological sign of the degeneration of tangle-bearing neurons, even preceding axonal lesions. Interestingly, irrespective of the Braak NFT stage, tau pathology is more advanced in the dorsal LC that preferentially projects to vulnerable forebrain regions in Alzheimer's disease, like the hippocampus or neocortical areas, compared to the ventral LC projecting to the cerebellum and medulla. Moreover, already in the precortical Braak 0 stage, 3D analysis reveals clustering tendency and dendro-dendritic close appositions of AT8⁺ LC neurons, AT8⁺ long axons of NFT-bearing cells that join the ascending dorsal noradrenergic bundle after leaving the LC, as well as AT8⁺ processes of NFT-bearing LC neurons that target the 4th ventricle wall. Our study suggests that the unique cytoarchitecture, comprised of a densely packed and dendritically extensively interconnected neuronal network with long projections, makes the human LC to be an ideal anatomical template for early accumulation and trans-neuronal spreading of hyperphosphorylated tau.

Notch activation in the mouse mammary luminal lineage leads to ductal hyperplasia and altered partitioning of luminal cell subtypes

Hyperactivated Notch signalling has been implicated in breast cancer, but how elevated levels of Notch signalling contribute to mammary dysplasia and tumorigenesis is not fully understood. In this study, we express an activated form of Notch1 in the mouse mammary luminal lineage and analyse the consequences for tumour formation and the transcriptomic landscape in the luminal lineage. Simultaneous conditional activation of a Notch1 intracellular domain (Notch1 ICD) and EGFP in the luminal lineage was achieved by removal of a stop cassette by CRE-recombinase expression from the whey acidic protein (WAP) promoter. Mice in which Notch1 ICD was activated in the luminal lineage (WAP-CRE;R26-N1ICD mice) exhibit ductal hyperplasia after lactation with an increase in branching frequency and in the number of side-branch ends in the ductal tree. A subset of the mice developed mammary tumours and the majority of the tumour cells expressed EGFP (as a proxy for Notch1 ICD), indicating that the tumours originate from the Notch1 ICD-expressing cells. Single-cell transcriptome analysis of the EGFP-positive mammary cells identified six subtypes of luminal cells. The same six subtypes were found in control mice (WAP-CRE;R26-tdTomato mice expressing the tdTomato reporter from WAP-CRE-mediated activation), but the proportion of cells in the various subtypes differed between the WAP-CRE;R26-N1ICD and control WAP-CRE;R26-tdTomato mice. In conclusion, we show that Notch1 ICD expression in the luminal lineage produces a ductal hyperplasia and branching phenotype accompanied by altered luminal cell subtype partitioning.

The T-type Ca2+ Channel Cav3.2 Regulates Differentiation of Neural Progenitor Cells during Cortical Development via Caspase-3

Here we report that the low-voltage-dependent T-type calcium (Ca²⁺) channel Ca_v3.2, encoded by the CACNA1H gene, regulates neuronal differentiation during early embryonic brain development through activating caspase-3. At the onset of neuronal differentiation, neural progenitor cells exhibited spontaneous Ca²⁺ activity. This activity strongly correlated with the upregulation of CACNA1H mRNA. Cells exhibiting robust spontaneous Ca²⁺ signaling had increased caspase-3 activity unrelated to apoptosis. Inhibition of Ca_v3.2 by drugs or viral CACNA1H knock down resulted in decreased caspase-3 activity followed by suppressed neurogenesis. In contrast, when CACNA1H was overexpressed, increased neurogenesis was detected. Cortical slices from Cacna1h knockout mice showed decreased spontaneous Ca²⁺ activity, a significantly lower protein level of cleaved caspase-3, and microanatomical abnormalities in the subventricular/ventricular and cortical plate zones when compared to their respective embryonic controls. In summary, we demonstrate a novel relationship between Ca_v3.2 and caspase-3 signaling that affects neurogenesis in the developing brain.

Cdk5 phosphorylation of ErbB4 is required for tangential migration of cortical interneurons

Interneuron dysfunction in humans is often associated with neurological and psychiatric disorders, such as epilepsy, schizophrenia, and autism. Some of these disorders are believed to emerge during brain formation, at the time of interneuron specification, migration, and synapse formation. Here, using a mouse model and a host of histological and molecular biological techniques, we report that the signaling molecule cyclin-dependent kinase 5 (Cdk5), and its activator p35, control the tangential migration of interneurons toward and within the cerebral cortex by modulating the critical neurodevelopmental signaling pathway, ErbB4/phosphatidylinositol 3-kinase, that has been repeatedly linked to schizophrenia. This finding identifies Cdk5 as a crucial signaling factor in cortical interneuron development in mammals.

Involvement of metabotropic glutamate receptor 5 signaling in activity-related proliferation of adult hippocampal neural stem cells

Adult hippocampal neural stem cells can be activated by hippocampal neural activities. When focal cerebral ischemia, known as middle cerebral artery occlusion (MCAO), occurs, neural stem cells are activated to promote their proliferation. However, the mechanism by which these cells are activated is still unclear. Here, we indicate the involvement of metabotropic glutamate receptor 5 (mGluR5) signaling in neural stem cells in their activity-related proliferation after MCAO. We found mGluR5 molecules on neural stem cells by using calcium imaging. We detected the activation of neural stem cells by adding the mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine. On a hippocampal slice, the activation of neural stem cells to promote their proliferation was initiated by theta-burst electrical stimulation at the perforant pathway, and this activation was significantly blocked by an mGluR5 antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP). In addition to this, the injection of the blood-brain barrier-permeable mGluR5 agonist 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide into live mice promoted the proliferation of neural stem cells. Moreover, in vivo theta-burst electrical stimulation induced proliferation of neural stem cells. A chronic field recording study showed that the activity of the hippocampal formation was elevated after MCAO. Finally, we observed that the mGluR5 antagonist MPEP significantly blocked the stimulated proliferation of neural stem cells induced by MCAO, by blocking mGluR5 signaling. Our results suggest that glutamates released by the elevated neural activities after MCAO may trigger mGluR5 signaling in neural stem cells to promote their proliferation.

Differences of migratory behavior between direct progeny of apical progenitors and basal progenitors in the developing cerebral cortex

Cerebral cortical neurons are known to be produced from both apical progenitors in the ventricular zone (VZ) and basal (intermediate) progenitors in the subventricular zone (SVZ). On the other hand, we have shown that many SVZ cells assume multipolar morphology and show a characteristic movement termed "multipolar migration." The relationship between multipolar cells and basal progenitors in the SVZ has yet to be investigated. Herein, we followed postmitotic cells generated in the VZ and found that they stayed for more than 10 h in the VZ after becoming postmitotic and then accumulated in the lower part of the SVZ (multipolar cell accumulation zone: MAZ) as multipolar cells (slowly exiting population: SEP), whereas basal progenitors rapidly migrated into the SVZ or intermediate zone (IZ) (rapidly exiting population: REP) with somal translocation morphology. Although REP reached the SVZ/IZ earlier than the SEP, REP stayed within in the SVZ/IZ, whereas SEP moved steadily and entered the CP prior to the REP. We also observed SEP to eventually differentiate into pyramidal neurons in layers II/III. This study provides in vivo evidence of differences in migration modes between postmitotic cells generated from apical progenitors and basal progenitors.

Neuronal migration in cortical development

Cortical formation in the developing brain is a highly complicated process involving neuronal production (through symmetric or asymmetric cell division) interaction of radial glia with neuronal migration, and multiple modes of neuronal migration. It has been convincingly demonstrated by numerous studies that radial glial cells are neural stem cells. However, the processes by which neurons arise from radial glia and migrate to their final destinations in vivo are not yet fully understood. Recent studies using time-lapse imaging of neuronal migration are giving investigators an increasingly more detailed understanding of the mitotic behavior of radial glia and the migrating behavior of their daughter cells. In this review, we describe recent progress in elucidating neuronal migration in brain formation and how neuronal migration is disturbed by mutations in genes that control this process.

Layer-specific Production of Nitric Oxide during Cortical Circuit Formation in Postnatal Mouse Brain

In the developing cerebral cortex, neuronal nitric oxide synthase (nNOS) is expressed abundantly, but temporarily. During the early postnatal stage, cortical neurons located in the multi-layered structure of the cortical plate start forming well-organized cortical circuits, but little is known about the molecular machinery for layer-specific circuit formation. To address the involvement of nitric oxide (NO), we utilized a new NO indicator (DAR-4M) and developed a protocol for the real-time imaging of NO produced in fresh cortical slices upon N-methyl-D-aspartic acid stimulation. At postnatal day 0 (P0), NO production was restricted to the deep layers (layers V and VI) of the somatosensory cortex where transient synapses are formed. At P10, the production of NO was expanded to layer IV where large numbers of thalamocortical axons form synapses. The pattern of NO production could correspond to active sites for synaptic formation. This study is the first clear demonstration of NO production in the postnatal mouse neocortex. The findings presented may reflect a function of NO in relation to the layer-specific development of neural circuits in the neocortex.