Zaman Mirzadeh

Neural Stem Cells Confer Unique Pinwheel Architecture to the Ventricular Surface in Neurogenic Regions of the Adult Brain

Neural stem cells (NSCs, B1 cells) are retained in the walls of the adult lateral ventricles but, unlike embryonic NSCs, are displaced from the ventricular zone (VZ) into the subventricular zone (SVZ) by ependymal cells. Apical and basal compartments, which in embryonic NSCs play essential roles in self-renewal and differentiation, are not evident in adult NSCs. Here we show that SVZ B1 cells in adult mice extend a minute apical ending to directly contact the ventricle and a long basal process ending on blood vessels. A closer look at the ventricular surface reveals a striking pinwheel organization specific to regions of adult neurogenesis. The pinwheels core contains the apical endings of B1 cells and in its periphery two types of ependymal cells: multiciliated (E1) and a type (E2) characterized by only two cilia and extraordinarily complex basal bodies. These results reveal that adult NSCs retain fundamental epithelial properties, including apical and basal compartmentalization, significantly reshaping our understanding of this adult neurogenic niche.

Functional outcome after language mapping for glioma resection.

BACKGROUND Language sites in the cortex of the brain vary among patients. Language mapping while the patient is awake is an intraoperative technique designed to minimize language deficits associated with brain-tumor resection. METHODS To study language function after brain-tumor resection with language mapping, we examined 250 consecutive patients with gliomas. Positive language sites (i.e., language regions in the cortex of the brain, 1 cm by 1 cm, which were temporarily inactivated by means of a bipolar electrode) were identified and categorized into cortical language maps. The tumors were resected up to 1 cm from the cortical areas where intraoperative stimulation produced a disturbance in language. Our resection strategy did not require identification of the stimulation-induced language sites within the field of exposure. RESULTS A total of 145 of the 250 patients (58.0%) had at least one site with an intraoperative stimulation-induced speech arrest, 82 patients had anomia, and 23 patients had alexia. Overall, 3094 of 3281 cortical sites (94.3%) were not associated with stimulation-induced language deficits. A total of 159 patients (63.6%) had intact speech preoperatively. One week after surgery, baseline language function remained in 194 patients (77.6%), it worsened in 21 patients (8.4%), and 35 patients (14.0%) had new speech deficits. However, 6 months after surgery, only 4 of 243 surviving patients (1.6%) had a persistent language deficit. Cortical maps generated with intraoperative language data also showed surprising variability in language localization within the dominant hemisphere. CONCLUSIONS Craniotomies tailored to limit cortical exposure, even without localization of positive language sites, permit most gliomas to be aggressively resected without language deficits. The composite language maps generated in our study suggest that our current models of human language organization insufficiently account for observed language function.

Corridors of migrating neurons in the human brain and their decline during infancy

The subventricular zone of many adult non-human mammals generates large numbers of new neurons destined for the olfactory bulb. Along the walls of the lateral ventricles, immature neuronal progeny migrate in tangentially oriented chains that coalesce into a rostral migratory stream (RMS) connecting the subventricular zone to the olfactory bulb. The adult human subventricular zone, in contrast, contains a hypocellular gap layer separating the ependymal lining from a periventricular ribbon of astrocytes. Some of these subventricular zone astrocytes can function as neural stem cells in vitro, but their function in vivo remains controversial. An initial report found few subventricular zone proliferating cells and rare migrating immature neurons in the RMS of adult humans. In contrast, a subsequent study indicated robust proliferation and migration in the human subventricular zone and RMS. Here we find that the infant human subventricular zone and RMS contain an extensive corridor of migrating immature neurons before 18 months of age but, contrary to previous reports, this germinal activity subsides in older children and is nearly extinct by adulthood. Surprisingly, during this limited window of neurogenesis, not all new neurons in the human subventricular zone are destined for the olfactory bulb—we describe a major migratory pathway that targets the prefrontal cortex in humans. Together, these findings reveal robust streams of tangentially migrating immature neurons in human early postnatal subventricular zone and cortex. These pathways represent potential targets of neurological injuries affecting neonates.

Cellular composition and cytoarchitecture of the adult human subventricular zone: A niche of neural stem cells

The lateral wall of the lateral ventricle in the human brain contains neural stem cells throughout adult life. We conducted a cytoarchitectural and ultrastructural study in complete postmortem brains (n = 7) and in postmortem (n = 42) and intraoperative tissue (n = 43) samples of the lateral walls of the human lateral ventricles. With varying thickness and cell densities, four layers were observed throughout the lateral ventricular wall: a monolayer of ependymal cells (Layer I), a hypocellular gap (Layer II), a ribbon of cells (Layer III) composed of astrocytes, and a transitional zone (Layer IV) into the brain parenchyma. Unlike rodents and nonhuman primates, adult human glial fibrillary acidic protein (GFAP)+ subventricular zone (SVZ) astrocytes are separated from the ependyma by the hypocellular gap. Some astrocytes as well as a few GFAP‐cells in Layer II in the SVZ of the anterior horn and the body of the lateral ventricle appear to proliferate based on proliferating cell nuclear antigen (PCNA) and Ki67 staining. However, compared to rodents, the adult human SVZ appears to be devoid of chain migration or large numbers of newly formed young neurons. It was only in the anterior SVZ that we found examples of elongated Tuj1+ cells with migratory morphology. We provide ultrastructural criteria to identify the different cells types in the human SVZ including three distinct types of astrocytes and a group of displaced ependymal cells between Layers II and III. Ultrastructural analysis of this layer revealed a remarkable network of astrocytic and ependymal processes. This work provides a basic description of the organization of the adult human SVZ. J. Comp. Neurol. 494:415–434, 2006.

Coupling between hydrodynamic forces and planar cell polarity orients mammalian motile cilia

In mammals, motile cilia cover many organs, such as fallopian tubes, respiratory tracts and brain ventricles. The development and function of these organs critically depend on efficient directional fluid flow ensured by the alignment of ciliary beating. To identify the mechanisms involved in this process, we analysed motile cilia of mouse brain ventricles, using biophysical and molecular approaches. Our results highlight an original orientation mechanism for ependymal cilia whereby basal bodies first dock apically with random orientations, and then reorient in a common direction through a coupling between hydrodynamic forces and the planar cell polarity (PCP) protein Vangl2, within a limited time-frame. This identifies a direct link between external hydrodynamic cues and intracellular PCP signalling. Our findings extend known PCP mechanisms by integrating hydrodynamic forces as long-range polarity signals, argue for a possible sensory role of ependymal cilia, and will be of interest for the study of fluid flow-mediated morphogenesis.

β‐Catenin Signaling Promotes Proliferation of Progenitor Cells in the Adult Mouse Subventricular Zone

The subventricular zone (SVZ) is the largest germinal zone in the mature rodent brain, and it continuously produces young neurons that migrate to the olfactory bulb. Neural stem cells in this region generate migratory neuroblasts via highly proliferative transit‐amplifying cells. The Wnt/β‐catenin signaling pathway partially regulates the proliferation and neuronal differentiation of neural progenitor cells in the embryonic brain. Here, we studied the role of β‐catenin signaling in the adult mouse SVZ. β‐Catenin‐dependent expression of a destabilized form of green fluorescent protein was detected in progenitor cells in the adult SVZ of Axin2‐d2EGFP reporter mice. Retrovirus‐mediated expression of a stabilized β‐catenin promoted the proliferation of Mash1+ cells and inhibited their differentiation into neuroblasts. Conversely, the expression of Dkk1, an inhibitor of Wnt signaling, reduced the proliferation of Mash1+ cells. In addition, an inhibitor of GSK3β promoted the proliferation of Mash1+ cells and increased the number of new neurons in the olfactory bulb 14 days later. These results suggest that β‐catenin signaling plays a role in the proliferation of progenitor cells in the SVZ of the adult mouse brain.

Postnatal Deletion of Numb/Numblike Reveals Repair and Remodeling Capacity in the Subventricular Neurogenic Niche

Neural stem cells are retained in the postnatal subventricular zone (SVZ), a specialized neurogenic niche with unique cytoarchitecture and cell-cell contacts. Although the SVZ stem cells continuously regenerate, how they and the niche respond to local changes is unclear. Here we generated nestin-creER(tm) transgenic mice with inducible Cre recombinase in the SVZ and removed Numb/Numblike, key regulators of embryonic neurogenesis from postnatal SVZ progenitors and ependymal cells. This resulted in severe damage to brain lateral ventricle integrity and identified roles for Numb/Numblike in regulating ependymal wall integrity and SVZ neuroblast survival. Surprisingly, the ventricular damage was eventually repaired: SVZ reconstitution and ventricular wall remodeling were mediated by progenitors that escaped Numb deletion. Our results show a self-repair mechanism in the mammalian brain and may have implications for both niche plasticity in other areas of stem cell biology and the therapeutic use of neural stem cells in neurodegenerative diseases.

Combinations of genetic mutations in the adult neural stem cell compartment determine brain tumour phenotypes

It has been suggested that intrinsic brain tumours originate from a neural stem/progenitor cell population in the subventricular zone of the post‐natal brain. However, the influence of the initial genetic mutation on the phenotype as well as the contribution of mature astrocytes to the formation of brain tumours is still not understood. We deleted Rb/p53, Rb/p53/PTEN or PTEN/p53 in adult subventricular stem cells; in ectopically neurografted stem cells; in mature parenchymal astrocytes and in transplanted astrocytes. We found that only stem cells, but not astrocytes, gave rise to brain tumours, independent of their location. This suggests a cell autonomous mechanism that enables stem cells to generate brain tumours, whereas mature astrocytes do not form brain tumours in adults. Recombination of PTEN/p53 gave rise to gliomas whereas deletion of Rb/p53 or Rb/p53/PTEN generated primitive neuroectodermal tumours (PNET), indicating an important role of an initial Rb loss in driving the PNET phenotype. Our study underlines an important role of stem cells and the relevance of initial genetic mutations in the pathogenesis and phenotype of brain tumours.

Cilia Organize Ependymal Planar Polarity

Multiciliated epithelial cells, called ependymal cells, line the ventricles in the adult brain. Most ependymal cells are born prenatally and are derived from radial glia. Ependymal cells have a remarkable planar polarization that determines orientation of ciliary beating and propulsion of CSF. Disruption of ependymal ciliary beating, by injury or disease, results in aberrant CSF circulation and hydrocephalus, a common disorder of the CNS. Very little is known about the mechanisms guiding ependymal planar polarity and whether this organization is acquired during ependymal cell development or is already present in radial glia. Here we show that basal bodies in ependymal cells in the lateral ventricle walls of adult mice are polarized in two ways: (1) rotational; angle of individual basal bodies with respect to their long axis and (2) translational; the position of basal bodies on the apical surface of the cell. Conditional ablation of motile cilia disrupted rotational orientation, but translational polarity was largely preserved. In contrast, translational polarity was dramatically affected when radial glial primary cilia were ablated earlier in development. Remarkably, radial glia in the embryo have a translational polarity that predicts the orientation of mature ependymal cells. These results suggest that ependymal planar cell polarity is a multistep process initially organized by primary cilia in radial glia and then refined by motile cilia in ependymal cells.

The Subventricular Zone En-face: Wholemount Staining and Ependymal Flow

The walls of the lateral ventricles contain the largest germinal region in the adult mammalian brain. The subventricular zone (SVZ) in these walls is an extensively studied model system for understanding the behavior of neural stem cells and the regulation of adult neurogenesis. Traditionally, these studies have relied on classical sectioning techniques for histological analysis. Here we present an alternative approach, the wholemount technique, which provides a comprehensive, en-face view of this germinal region. Compared to sections, wholemounts preserve the complete cytoarchitecture and cellular relationships within the SVZ. This approach has recently revealed that the adult neural stem cells, or type B1 cells, are part of a mixed neuroepithelium with differentiated ependymal cells lining the lateral ventricles. In addition, this approach has been used to study the planar polarization of ependymal cells and the cerebrospinal fluid flow they generate in the ventricle. With recent evidence that adult neural stem cells are a heterogeneous population that is regionally specified, the wholemount approach will likely be an essential tool for understanding the organization and parcellation of this stem cell niche.