Rostral migratory stream neuroblasts turn and change directions in stereotypic patterns

Unveiling the fate and potential neuroprotective role of neural stem/progenitor cells in multiple sclerosis

Chronic pathological conditions often induce persistent systemic inflammation, contributing to neuroinflammatory diseases like Multiple Sclerosis (MS). MS is known for its autoimmune-mediated damage to myelin, axonal injury, and neuronal loss which drive disability accumulation and disease progression, often manifesting as cognitive impairments. Understanding the involvement of neural stem cells (NSCs) and neural progenitor cells (NPCs) in the remediation of MS through adult neurogenesis (ANG) and gliogenesis-the generation of new neurons and glial cells, respectively is of great importance. Hence, these phenomena, respectively, termed ANG and gliogenesis, involve significant structural and functional changes in neural networks. Thus, the proper integration of these newly generated cells into existing circuits is not only key to understanding the CNS's development but also its remodeling in adulthood and recovery from diseases such as MS. Understanding how MS influences the fate of NSCs/NPCs and their possible neuroprotective role, provides insights into potential therapeutic interventions to alleviate the impact of MS on cognitive function and disease progression. This review explores MS, its pathogenesis, clinical manifestations, and its association with ANG and gliogenesis. It highlights the impact of altered NSCs and NPCs' fate during MS and delves into the potential benefits of its modifications. It also evaluates treatment regimens that influence the fate of NSCS/NPCs to counteract the pathology subsequently.

Ventricular-subventricular zone stem cell niche adaptations in a mouse model of post-infectious hydrocephalus

Congenital post-infectious hydrocephalus (PIH) is a condition characterized by enlargement of the ventricular system, consequently imposing a burden on the associated stem cell niche, the ventricular-subventricular zone (V-SVZ). To investigate how the V-SVZ adapts in PIH, we developed a mouse model of influenza virus-induced PIH based on direct intracerebroventricular injection of mouse-adapted influenza virus at two distinct time points: embryonic day 16 (E16), when stem cells line the ventricle, and postnatal day 4 (P4), when an ependymal monolayer covers the ventricle surface and stem cells retain only a thin ventricle-contacting process. Global hydrocephalus with associated regions of astrogliosis along the lateral ventricle was found in 82% of the mice infected at P4. Increased ependymogenesis was observed at gliotic borders and throughout areas exhibiting intact ependyma based on tracking of newly divided cells. Additionally, in areas of intact ependyma, stem cell numbers were reduced; however, we found no significant reduction in new neurons reaching the olfactory bulb following onset of ventriculomegaly. At P4, injection of only the non-infectious viral component neuraminidase resulted in limited, region-specific ventriculomegaly due to absence of cell-to-cell transmission. In contrast, at E16 intracerebroventricular injection of influenza virus resulted in death at birth due to hypoxia and multiorgan hemorrhage, suggesting an age-dependent advantage in neonates, while the viral component neuraminidase resulted in minimal, or no, ventriculomegaly. In summary, we tracked acute adaptations of the V-SVZ stem cell niche following onset of ventriculomegaly and describe developmental changes that help mitigate the severity of congenital PIH.

In vivo imaging in transgenic songbirds reveals superdiffusive neuron migration in the adult brain

Neuron migration is a key phase of neurogenesis, critical for the assembly and function of neuronal circuits. In songbirds, this process continues throughout life, but how these newborn neurons disperse through the adult brain is unclear. We address this question using in vivo two-photon imaging in transgenic zebra finches that express GFP in young neurons and other cell types. In juvenile and adult birds, migratory cells are present at a high density, travel in all directions, and make frequent course changes. Notably, these dynamic migration patterns are well fit by a superdiffusive model. Simulations reveal that these superdiffusive dynamics are sufficient to disperse new neurons throughout the song nucleus HVC. These results suggest that superdiffusive migration may underlie the formation and maintenance of nuclear brain structures in the postnatal brain and indicate that transgenic songbirds are a useful resource for future studies into the mechanisms of adult neurogenesis.

Intravital Imaging of the Murine Subventricular Zone with Three Photon Microscopy

The mouse subventricular zone (SVZ) produces neurons throughout life. It is useful for mechanism discovery and is relevant for regeneration. However, the SVZ is deep, significantly restricting live imaging since current methods do not extend beyond a few hundred microns. We developed and adapted three-photon microscopy (3PM) for non-invasive deep brain imaging in live mice, but its utility in imaging the SVZ niche was unknown. Here, with fluorescent dyes and genetic labeling, we show successful 3PM imaging in the whole SVZ, extending to a maximum depth of 1.5 mm ventral to the dura mater. 3PM imaging distinguished multiple SVZ cell types in postnatal and juvenile mice. We also detected fine processes on neural stem cells interacting with the vasculature. Previous live imaging removed overlying cortical tissue or lowered lenses into the brain, which could cause inflammation and alter neurogenesis. We found that neither astrocytes nor microglia become activated in the SVZ, suggesting 3PM does not induce major damage in the niche. Thus, we show for the first time 3PM imaging of the SVZ in live mice. This strategy could be useful for intravital visualization of cell dynamics, molecular, and pathological perturbation and regenerative events.

The Celsr3-Kif2a axis directs neuronal migration in the postnatal brain

The tangential migration of immature neurons in the postnatal brain involves consecutive migration cycles and depends on constant remodeling of the cell cytoskeleton, particularly in the leading process (LP). Despite the identification of several proteins with permissive and empowering functions, the mechanisms that specify the direction of migration remain largely unknown. Here, we report that planar cell polarity protein Celsr3 orients neuroblasts migration from the subventricular zone (SVZ) to olfactory bulb (OB). In Celsr3-forebrain conditional knockout mice, neuroblasts loose directionality and few can reach the OB. Celsr3-deficient neuroblasts exhibit aberrant branching of LP, de novo LP formation, and decreased growth rate of microtubules (MT). Mechanistically, we show that Celsr3 interacts physically with Kif2a, a MT depolymerizing protein and that conditional inactivation of Kif2a in the forebrain recapitulates the Celsr3 knockout phenotype. Our findings provide evidence that Celsr3 and Kif2a cooperatively specify the directionality of neuroblasts tangential migration in the postnatal brain.

Directional Persistence of Cell Migration in Schizophrenia Patient-Derived Olfactory Cells

Cell migration is critical for brain development and linked to several neurodevelopmental disorders, including schizophrenia. We have shown previously that cell migration is dysregulated in olfactory neural stem cells from people with schizophrenia. Although they moved faster than control cells on plastic substrates, patient cells were insensitive to regulation by extracellular matrix proteins, which increase the speeds of control cells. As well as speed, cell migration is also described by directional persistence, the straightness of movement. The aim of this study was to determine whether directional persistence is dysregulated in schizophrenia patient cells and whether it is modified on extracellular matrix proteins. Directional persistence in patient-derived and control-derived olfactory cells was quantified from automated live-cell imaging of migrating cells. On plastic substrates, patient cells were more persistent than control cells, with straighter trajectories and smaller turn angles. On most extracellular matrix proteins, persistence increased in patient and control cells in a concentration-dependent manner, but patient cells remained more persistent. Patient cells therefore have a subtle but complex phenotype in migration speed and persistence on most extracellular matrix protein substrates compared to control cells. If present in the developing brain, this could lead to altered brain development in schizophrenia.

Enhanced NRF2 expression mitigates the decline in neural stem cell function during aging

Although it is known that aging affects neural stem progenitor cell (NSPC) biology in fundamental ways, the underlying dynamics of this process are not fully understood. Our previous work identified a specific critical period (CP) of decline in NSPC activity and function during middle age (13–15 months), and revealed the reduced expression of the redox‐sensitive transcription factor, NRF2, as a key mediator of this process. Here, we investigated whether augmenting NRF2 expression could potentially mitigate the NSPC decline across the identified CP. NRF2 expression in subventricular zone (SVZ) NSPCs was upregulated via GFP tagged recombinant adeno‐associated viral vectors (AAV‐NRF2‐eGFP), and its cellular and behavioral effects compared to animals that received control vectors (AAV‐eGFP). The vectors were administered into the SVZs of aging rats, at time points either before or after the CP. Results indicate that animals that had received AAV‐NRF2‐eGFP, prior to the CP (11 months of age), exhibited substantially improved behavioral function (fine olfactory discrimination and motor tasks) in comparison to those receiving control viruses. Further analysis revealed that NSPC proliferation, self‐renewal, neurogenesis, and migration to the olfactory bulb had significantly increased upon NRF2 upregulation. On the other hand, increasing NRF2 after the CP (at 20 months of age) produced no notable changes in NSPC activity at either cellular or behavioral levels. These results, for the first time, indicate NRF2 pathway modulation as a means to support NSPC function with age and highlight a critical time‐dependency for activating NRF2 to enhance NSPC function.

EndoN treatment allows neuroblasts to leave the rostral migratory stream and migrate towards a lesion within the prefrontal cortex of rats

The binding properties of neural cell adhesion molecule are modulated by a polysialic acid moiety. This plays an important role in the migration of adult born neuroblasts from their area of origin, the subventricular zone, towards the olfactory bulb. Polysialisation increases the migration speed of the cells and helps to prevent the neuroblasts from leaving their migration route, the rostral migratory stream. Here, we evaluated the potential of intraventricular application of endoneuraminidase-N, an enzyme that specifically cleaves polysialic acid from neural cell adhesion molecule, in a rat model for structural prefrontal cortex damage. As expected, endoneuraminidase-N caused the rostral migratory stream to become wider, with a less uniform cellular orientation. Furthermore, endoneuraminidase-N treatment caused the neuroblasts to leave the rostral migratory stream and migrate towards the lesioned tissue. Despite the neuroblasts not being differentiated into neurons after a survival time of three weeks, this technique provides a solid animal model for future work on the migration and differentiation of relocated neuroblasts and might provide a basis for a future endogenous stem cell-based therapy for structural brain damage. The experiments were approved by the local animal care committee (522-27-11/02-00, 115; Senatorin für Wissenschaft, Gesundheit und Verbraucherschutz, Bremen, Germany) on February 10, 2016.

PICS: a platform for planar imaging of curved surfaces of brain and other tissue

Optical imaging of wholemount tissue samples provides greater understanding of structure-function relationships as the architecture of these specimens is generally well preserved. However, difficulties arise when attempting to stitch together images of multiple regions of larger, oddly shaped specimens. These difficulties include (1) maintaining consistent signal-to-noise ratios when the overlying sample surface is uneven, (2) ensuring sample viability when live samples are required, and (3) stabilizing the specimen in a fixed position in a flowing medium without distorting the tissue sample. To address these problems, we designed a simple and cost-efficient device that can be 3D-printed and machined. The design for the device, named the Platform for Planar Imaging of Curved Surfaces (PICS), consists of a sample holder, or "cap" with gaps for fluid flow and a depression for securing the sample in a fixed position without glue or pins, a basket with two arms that move along an external radius to rotate the sample around a central axis, and a customizable platform designed to fit on a commercially available temperature control system for slice electrophysiology. We tested the system using wholemounts of the murine subventricular zone (SVZ), which has a high degree of curvature, to assess sample viability and image quality through cell movement for over an hour for each sample. Using the PICS system, tissues remained viable throughout the imaging sessions, there were no noticeable decreases in the image SNR across an imaging plane, and there was no noticeable displacement of the specimen due to fluid flow.

In Vivo Electroporation and Time-Lapse Imaging of the Rostral Migratory Stream in Developing Rodent Brain

Interneurons in the olfactory bulb are generated from neuronal precursor cells migrating from the anterior subventricular zone (SVZa) throughout the embryonic and postnatal life of mammals. This article describes basic methods for in vivo electroporation to label SVZa cells of both embryonic and postnatal rats. In addition, it describes three methods for tracing SVZa progenitors and following their migration pathway and differentiation, including immunohistochemistry, time-lapse live imaging in slice culture, and time-lapse imaging following transplantation in slice culture. These methods may be applied to all strains of rats and mice, including reporter mice. They may also be combined with methods such as BrdU labeling, tamoxifen injection, and electrophysiology, allowing one to observe proliferation or control gene expression at specific times and for specific neuronal functions. With time-lapse live imaging, details of labeled cells can be studied, including morphology, motility pattern, differentiation, and crosstalk between cells. © 2019 by John Wiley & Sons, Inc.

Visualization of Rostral Migratory Stream in the Developing Rat Brain by In Vivo Electroporation

Interneurons in the olfactory bulb (OB) are generated from neuronal precursor cells migrating from anterior subventricular zone (SVZa) not only in the developing embryo but also throughout the postnatal life of mammals. In the present study, we established an in vivo electroporation assay to label SVZa cells of rat both at embryonic and postnatal ages, and traced SVZa progenitors and followed their migration pathway and differentiation. We found that labeled cells displayed high motility. Interestingly, the postnatal cells migrated faster than the embryonic cells after applying this assay at different ages of brain development. Furthermore, based on brain slice culture and time-lapse imaging, we analyzed the detail migratory properties of these labeled precursor neurons. Finally, tissue transplantation experiments revealed that cells already migrated in subependymal zone of OB were transplanted back into rostral migratory stream (RMS), and these cells could still migrate out tangentially along RMS to OB. Taken together, these findings provide an in vivo labeling assay to follow and trace migrating cells in the RMS, their maturation and integration into OB neuron network, and unrecognized phenomena that postnatal SVZa progenitor cells with higher motility than embryonic cells, and their migration was affected by extrinsic environments.

Redirection of neuroblast migration from the rostral migratory stream into a lesion in the prefrontal cortex of adult rats

Clinical treatment of structural brain damage today is largely limited to symptomatic approaches and the avoidance of secondary injury. However, neuronal precursor cells are constantly produced within specified regions of the mammalian brain throughout life. Here we evaluate the potential of the known chemoattractive properties of the glycoprotein laminin on neuroblasts to relocate the cells into damaged brain areas. Injection of a thin laminin tract, leading from the rostral migratory stream to an excitotoxic lesion within the medial prefrontal cortex of rats, enabled neuroblasts to migrate away from their physiological route towards the olfactory bulb into the lesion site. Once they reached the damaged tissue, they migrated further in a non-uniform orientation within the lesion. Furthermore, our data indicate that the process of diverted migration is still active 6 weeks after the treatment and that at least some of the neuroblasts are capable of maturing into adult neurons.

Magnetic resonance imaging of odorant activity-dependent migration of neural precursor cells and olfactory bulb growth

Neural progenitors or neuroblasts are produced by precursor cells in the subventricular zone (SVZ) and migrate along the rostral migratory stream (RMS) to the olfactory bulbs (OB) throughout life. In the OB, these adult born neurons either die or replace existing olfactory interneurons, playing a critical role in the stabilization of OB circuitry. Although several aspects of the addition of new neurons into the OB have been studied, it is unclear whether long-distance activity from the OB can regulate the influx of migrating neuroblasts along the RMS. In this study, iron oxide-assisted MRI was used to track the migration of neuroblasts in combination with reversible naris occlusion to manipulate odorant-induced activity. It was found that decreasing olfactory activity led to a decrease in the rate of neuroblast migration along the RMS. Removal of the naris occlusion led to an increase in migratory rate back to control levels, indicating that olfactory activity has regulatory function on neuroblast migration in the RMS. Blocking odorant activity also led to an arrest in OB growth and re-opening the block led to a rapid re-growth returning the bulb size to control levels. Furthermore, pharmacogenetic elimination of the neuroblasts demonstrated that they were required for re-growth of the bulb following sensory deprivation. Together, these results show that sensory activity, neural migration and OB growth are tightly coupled in an interdependent manner.

Loss of galectin-3 decreases the number of immune cells in the subventricular zone and restores proliferation in a viral model of multiple sclerosis

Multiple sclerosis (MS) frequently starts near the lateral ventricles, which are lined by subventricular zone (SVZ) progenitor cells that can migrate to lesions and contribute to repair. Because MS-induced inflammation may decrease SVZ proliferation and thus limit repair, we studied the role of galectin-3 (Gal-3), a proinflammatory protein. Gal-3 expression was increased in periventricular regions of human MS in post-mortem brain samples and was also upregulated in periventricular regions in a murine MS model, Theiler's murine encephalomyelitis virus (TMEV) infection. Whereas TMEV increased SVZ chemokine (CCL2, CCL5, CCL, and CXCL10) expression in wild type (WT) mice, this was inhibited in Gal-3(-/-) mice. Though numerous CD45+ immune cells entered the SVZ of WT mice after TMEV infection, their numbers were significantly diminished in Gal-3(-/-) mice. TMEV also reduced neuroblast and proliferative SVZ cell numbers in WT mice but this was restored in Gal-3(-/-) mice and was correlated with increased numbers of doublecortin+ neuroblasts in the corpus callosum. In summary, our data showed that loss of Gal-3 blocked chemokine increases after TMEV, reduced immune cell migration into the SVZ, reestablished SVZ proliferation and increased the number of progenitors in the corpus callosum. These results suggest Gal-3 plays a central role in modulating the SVZ neurogenic niche's response to this model of MS.

KCa3.1 modulates neuroblast migration along the rostral migratory stream (RMS) in vivo

From the subventricular zone (SVZ), neuronal precursor cells (NPCs), called neuroblasts, migrate through the rostral migratory stream (RMS) to become interneurons in the olfactory bulb (OB). Ion channels regulate neuronal migration during development, yet their role in migration through the adult RMS is unknown. To address this question, we utilized Nestin-CreER(T2)/R26R-YFP mice to fluorescently label neuroblasts in the adult. Patch-clamp recordings from neuroblasts reveal K(+) currents that are sensitive to intracellular Ca(2+) levels and blocked by clotrimazole and TRAM-34, inhibitors of intermediate conductance Ca(2+)-activated K(+) (KCa3.1) channels. Immunolabeling and electrophysiology show KCa3.1 expression restricted to neuroblasts in the SVZ and RMS, but absent in OB neurons. Time-lapse confocal microscopy in situ showed inhibiting KCa3.1 prolonged the stationary phase of neuroblasts' saltatory migration, reducing migration speed by over 50%. Both migration and KCa3.1 currents could also be inhibited by blocking Ca(2+) influx via transient receptor potential (TRP) channels, which, together with positive immunostaining for transient receptor potential canonical 1 (TRPC1), suggest that TRP channels are an important Ca(2+) source modulating KCa3.1 activity. Finally, injecting TRAM-34 into Nestin-CreER(T2)/R26R-YFP mice significantly reduced the number of neuroblasts that reached the OB, suggesting an important role for KCa3.1 in vivo. These studies describe a previously unrecognized protein in migration of adult NPCs.

Ependymal ciliary dysfunction and reactive astrocytosis in a reorganized subventricular zone after stroke

Subventricular zone (SVZ) astrocytes and ependymal cells are both derived from radial glia and may have similar gliotic reactions after stroke. Diminishing SVZ neurogenesis worsens outcomes in mice, yet the effects of stroke on SVZ astrocytes and ependymal cells are poorly understood. We used mouse experimental stroke to determine if SVZ astrocytes and ependymal cells assume similar phenotypes and if stroke impacts their functions. Using lateral ventricular wall whole mount preparations, we show that stroke caused SVZ reactive astrocytosis, disrupting the neuroblast migratory scaffold. Also, SVZ vascular density and neural proliferation increased but apoptosis did not. In contrast to other reports, ependymal denudation and cell division was never observed. Remarkably, however, ependymal cells assumed features of reactive astrocytes post stroke, robustly expressing de novo glial fibrillary acidic protein, enlargening and extending long processes. Unexpectedly, stroke disrupted motile cilia planar cell polarity in ependymal cells. This suggested ciliary function was affected and indeed ventricular surface flow was slower and more turbulent post stroke. Together, these results demonstrate that in response to stroke there is significant SVZ reorganization with implications for both pathophysiology and therapeutic strategies.

Subventricular zone cell migration: lessons from quantitative two-photon microscopy

Neuroblasts born in the adult subventricular zone (SVZ) migrate long distances in the rostral migratory stream (RMS) to the olfactory bulbs where they integrate into circuitry as functional interneurons. As very little was known about the dynamic parameters of SVZ neuroblast migration, we used two-photon time-lapse microscopy to analyze migration in acute slices. This involved analyzing 3D stacks of images over time and uncovered several novel aspects of SVZ migration: chains remain stable, cells can be immotile for extensive periods, morphology does not necessarily correlate with motility, neuroblasts exhibit local exploratory motility, dorsoventral migration occurs throughout the striatal SVZ, and neuroblasts turn at distinctive angles. We investigated these novel findings in the SVZ and RMS from the population to the single cell level. In this review we also discuss some technical considerations when setting up a two-photon microscope imaging system. Throughout the review we identify several unsolved questions about SVZ neuroblast migration that might be addressed with current or emerging techniques.