Expression of ezrin radixin moesin proteins in the adult subventricular zone and the rostral migratory stream

Radixin: Roles in the Nervous System and Beyond

BACKGROUND

Radixin is an ERM family protein that includes radixin, moesin, and ezrin. The importance of ERM family proteins has been attracting more attention, and studies on the roles of ERM in biological function and the pathogenesis of some diseases are accumulating. In particular, we have found that radixin is the most dramatically changed ERM protein in elevated glucose-treated Schwann cells.

METHOD

We systemically review the literature on ERM, radixin in focus, and update the roles of radixin in regulating cell morphology, interaction, and cell signaling pathways. The potential of radixin as a therapeutic target in neurodegenerative diseases and cancer was also discussed.

RESULTS

Radixin research has focused on its cell functions, activation, and pathogenic roles in some diseases. Radixin and other ERM proteins maintain cell shape, growth, and motility. In the nervous system, radixin has been shown to prevent neurodegeneration and axonal growth. The activation of radixin is through phosphorylation of its conserved threonine residues. Radixin functions in cell signaling pathways by binding to membrane proteins and relaying the cell signals into the cells. Deficiency of radixin has been involved in the pathogenic process of diseases in the central nervous system and diabetic peripheral nerve injury. Moreover, radixin also plays a role in cell growth and drug resistance in multiple cancers. The trials of therapeutic potential through radixin modulation have been accumulating. However, the exact mechanisms underlying the roles of radixin are far from clarification.

CONCLUSIONS

Radixin plays various roles in cells and is involved in developing neurodegenerative diseases and many types of cancers. Therefore, radixin may be considered a potential target for developing therapeutic strategies for its related diseases. Further elucidation of the function and the cell signaling pathways that are linked to radixin may open the avenue to finding novel therapeutic strategies for diseases in the nervous system and other body systems.

An implantable human stem cell-derived tissue-engineered rostral migratory stream for directed neuronal replacement

The rostral migratory stream (RMS) facilitates neuroblast migration from the subventricular zone to the olfactory bulb throughout adulthood. Brain lesions attract neuroblast migration out of the RMS, but resultant regeneration is insufficient. Increasing neuroblast migration into lesions has improved recovery in rodent studies. We previously developed techniques for fabricating an astrocyte-based Tissue-Engineered RMS (TE-RMS) intended to redirect endogenous neuroblasts into distal brain lesions for sustained neuronal replacement. Here, we demonstrate that astrocyte-like-cells can be derived from adult human gingiva mesenchymal stem cells and used for TE-RMS fabrication. We report that key proteins enriched in the RMS are enriched in TE-RMSs. Furthermore, the human TE-RMS facilitates directed migration of immature neurons in vitro. Finally, human TE-RMSs implanted in athymic rat brains redirect migration of neuroblasts out of the endogenous RMS. By emulating the brain’s most efficient means for directing neuroblast migration, the TE-RMS offers a promising new approach to neuroregenerative medicine.

O’Donnell et al. describe their Tissue-Engineered Rostral Migratory Stream (TE-RMS) comprised of human astrocyte-like cells that can be derived from adult gingival stem cells within one week, which reorganizes into bundles of bidirectional, longitudinally-aligned astrocytes to emulate the endogenous RMS. Establishing immature neuronal migration in vitro and in vivo, their study demonstrates surgical feasibility and proof-of-concept evidence for this nascent technology.

Neuroinflammation Mediated by Glia Maturation Factor Exacerbates Neuronal Injury in an in vitro Model of Traumatic Brain Injury

Traumatic brain injury (TBI) is the primary cause of death and disability affecting over 10 million people in the industrialized world. TBI causes a wide spectrum of secondary molecular and cellular complications in the brain. However, the pathological events are still not yet fully understood. Previously, we have shown that the glia maturation factor (GMF) is a mediator of neuroinflammation in neurodegenerative diseases. To identify the potential molecular pathways accompanying TBI, we used an in vitro cell culture model of TBI. A standardized injury was induced by scalpel cut through a mixed primary cell culture of astrocytes, microglia and neurons obtained from both wild type (WT) and GMF-deficient (GMF-KO) mice. Cell culture medium and whole-cell lysates were collected at 24, 48, and 72 h after the scalpel cuts injury and probed for oxidative stress using immunofluorescence analysis. Results showed that oxidative stress markers such as glutathione and glutathione peroxidase were significantly reduced, while release of cytosolic enzyme lactate dehydrogenase along with nitric oxide and prostaglandin E2 were significantly increased in injured WT cells compared with injured GMF-KO cells. In addition, injured WT cells showed increased levels of oxidation product 4-hydroxynonenal and 8-oxo-2'-deoxyguanosine compared with injured GMF-KO cells. Further, we found that injured WT cells showed a significantly increased expression of glial fibrillary acidic protein, ionized calcium binding adaptor molecule 1, and phosphorylated ezrin/radixin/moesin proteins, and reduced microtubule associated protein expression compared with injured GMF-KO cells after injury. Collectively, our results demonstrate that GMF exacerbates the oxidative stress-mediated neuroinflammation that could be brought about by TBI-induced astroglial activation.

Podoplanin Gene Disruption in Mice Promotes in vivo Neural Progenitor Cells Proliferation, Selectively Impairs Dentate Gyrus Synaptic Depression and Induces Anxiety-Like Behaviors

Podoplanin (Pdpn), a brain-tumor-related glycoprotein identified in humans and animals, is endogenously expressed in several organs critical for life support such as kidney, lung, heart and brain. In the brain, Pdpn has been identified in proliferative nestin-positive adult neural progenitor cells and in neurons of the neurogenic hippocampal dentate gyrus (DG), a structure associated to anxiety, critical for learning and memory functions and severely damaged in people with Alzheimer's Disease (AD). The in vivo role of Pdpn in adult neurogenesis and anxiety-like behavior remained however unexplored. Using mice with disrupted Pdpn gene as a model organism and applying combined behavioral, molecular biological and electrophysiological assays, we here show that the absence of Pdpn selectively impairs long-term synaptic depression in the neurogenic DG without affecting the CA3-Schaffer's collateral-CA1 synapses. Pdpn deletion also enhanced the proliferative capacity of DG neural progenitor cells and diminished survival of differentiated neuronal cells in vitro. In addition, mice with podoplanin gene disruption showed increased anxiety-like behaviors in experimentally validated behavioral tests as compared to wild type littermate controls. Together, these findings broaden our knowledge on the molecular mechanisms influencing hippocampal synaptic plasticity and neurogenesis in vivo and reveal Pdpn as a novel molecular target for future studies addressing general anxiety disorder and synaptic depression-related memory dysfunctions.

Perspectives for Ezrin and Radixin in Astrocytes: Kinases, Functions and Pathology

Astrocytes are increasingly perceived as active partners in physiological brain function and behaviour. The structural correlations of the glia–synaptic interaction are the peripheral astrocyte processes (PAPs), where ezrin and radixin, the two astrocytic members of the ezrin-radixin-moesin (ERM) family of proteins are preferentially localised. While the molecular mechanisms of ERM (in)activation appear universal, at least in mammalian cells, and have been studied in great detail, the actual ezrin and radixin kinases, phosphatases and binding partners appear cell type specific and may be multiplexed within a cell. In astrocytes, ezrin is involved in process motility, which can be stimulated by the neurotransmitter glutamate, through activation of the glial metabotropic glutamate receptors (mGluRs) 3 or 5. However, it has remained open how this mGluR stimulus is transduced to ezrin activation. Knowing upstream signals of ezrin activation, ezrin kinase(s), and membrane-bound binding partners of ezrin in astrocytes might open new approaches to the glial role in brain function. Ezrin has also been implicated in invasive behaviour of astrocytomas, and glial activation. Here, we review data pertaining to potential molecular interaction partners of ezrin in astrocytes, with a focus on PKC and GRK2, and in gliomas and other diseases, to stimulate further research on their potential roles in glia-synaptic physiology and pathology.

Modulation of the Mesenchymal Stem Cell Secretome Using Computer-Controlled Bioreactors: Impact on Neuronal Cell Proliferation, Survival and Differentiation

In recent years it has been shown that the therapeutic benefits of human mesenchymal stem/stromal cells (hMSCs) in the Central Nervous System (CNS) are mainly attributed to their secretome. The implementation of computer-controlled suspension bioreactors has shown to be a viable route for the expansion of these cells to large numbers. As hMSCs actively respond to their culture environment, there is the hypothesis that one can modulate its secretome through their use. Herein, we present data indicating that the use of computer-controlled suspension bioreactors enhanced the neuroregulatory profile of hMSCs secretome. Indeed, higher levels of in vitro neuronal differentiation and NOTCH1 expression in human neural progenitor cells (hNPCs) were observed when these cells were incubated with the secretome of dynamically cultured hMSCs. A similar trend was also observed in the hippocampal dentate gyrus (DG) of rat brains where, upon injection, an enhanced neuronal and astrocytic survival and differentiation, was observed. Proteomic analysis also revealed that the dynamic culturing of hMSCs increased the secretion of several neuroregulatory molecules and miRNAs present in hMSCs secretome. In summary, the appropriate use of dynamic culture conditions can represent an important asset for the development of future neuro-regenerative strategies involving the use of hMSCs secretome.

Radixin knockdown by RNA interference suppresses human glioblastoma cell growth in vitro and in vivo

Radixin, a member of the ERM (ezrin-radixin-moesin) family, plays important roles in cell motility, invasion and tumor progression. It is expressed in a variety of normal and neoplastic cells, including many types of epithelial and lymphoid examples. However, its function in glioblastomas remains elusive. Thus, in this study, radixin gene expression was first examined in the glioblastoma cells, then suppressed with a lentivirus-mediated short-hairpin RNA (shRNA) method.We found that there were high levels of radixin expression in glioblastoma U251cells. Radixin shRNA caused down-regulation of radixin gene expression and when radixin-silenced cells were implanted into nude mice, tumor growth was significantly inhibited as compared to blank control cells or non- sense shRNA cells. In addition, microvessel density in the tumors was significantly reduced. Thrombospondin-1 (TSP-1) and E-cadherin were up-regulated in radixin- suppressed glioblastoma U251 cells. In contrast, MMP9 was down-regulated. Taken together, our findings suggest that radixin is involved in GBM cell migration and invasion, and implicate TSP-1, E-cadherin and MMP9 as metastasis-inducing factors.

Molecular approaches for manipulating astrocytic signaling in vivo

Astrocytes are the predominant glial type in the central nervous system and play important roles in assisting neuronal function and network activity. Astrocytes exhibit complex signaling systems that are essential for their normal function and the homeostasis of the neural network. Altered signaling in astrocytes is closely associated with neurological and psychiatric diseases, suggesting tremendous therapeutic potential of these cells. To further understand astrocyte function in health and disease, it is important to study astrocytic signaling in vivo. In this review, we discuss molecular tools that enable the selective manipulation of astrocytic signaling, including the tools to selectively activate and inactivate astrocyte signaling in vivo. Lastly, we highlight a few tools in development that present strong potential for advancing our understanding of the role of astrocytes in physiology, behavior, and pathology.

Ezrin mediates neuritogenesis via down-regulation of RhoA activity in cultured cortical neurons

Neuronal morphogenesis is implicated in neuronal function and development with rearrangement of cytoskeletal organization. Ezrin, a member of Ezrin/Radixin/Moesin (ERM) proteins links between membrane proteins and actin cytoskeleton, and contributes to maintenance of cellular function and morphology. In cultured hippocampal neurons, suppression of both radixin and moesin showed deficits in growth cone morphology and neurite extensions. Down-regulation of ezrin using siRNA caused impairment of netrin-1-induced axon outgrowth in cultured cortical neurons. However, roles of ezrin in the neuronal morphogenesis of the cultured neurons have been poorly understood. In this report, we performed detailed studies on the roles of ezrin in the cultured cortical neurons prepared from the ezrin knockdown (Vil2^kd/kd) mice embryo that showed a very small amount of ezrin expression compared with the wild-type (Vil2^+/+) neurons. Ezrin was mainly expressed in cell body in the cultured cortical neurons. We demonstrated that the cultured cortical neurons prepared from the Vil2^kd/kd mice embryo exhibited impairment of neuritogenesis. Moreover, we observed increased RhoA activity and phosphorylation of myosin light chain 2 (MLC2), as a downstream effector of RhoA in the Vil2^kd/kd neurons. In addition, inhibition of Rho kinase and myosin II rescued the impairment of neuritogenesis in the Vil2^kd/kd neurons. These data altogether suggest a novel role of ezrin in the neuritogenesis of the cultured cortical neurons through down-regulation of RhoA activity.

Function of ezrin-radixin-moesin proteins in migration of subventricular zone-derived neuroblasts following traumatic brain injury

Throughout life, newly generated neuroblasts from the subventricular zone migrate toward the olfactory bulb through the rostral migratory stream. Upon brain injury, these migrating neuroblasts change their route and begin to migrate toward injured regions, which is one of the regenerative responses after brain damage. This injury-induced migration is triggered by stromal cell-derived factor 1 (SDF1) released from microglia near the damaged site; however, it is still unclear how these cells transduce SDF1 signals and change their direction. In this study, we found that SDF1 promotes the phosphorylation of ezrin-radixin-moesin (ERM) proteins, which are key molecules in organizing cell membrane and linking signals from the extracellular environment to the intracellular actin cytoskeleton. Blockade of ERM activation by overexpressing dominant-negative ERM (DN-ERM) efficiently perturbed the migration of neuroblasts. Considering that DN-ERM-expressing neuroblasts failed to maintain proper migratory cell morphology, it appears that ERM-dependent regulation of cell shape is required for the efficient migration of neuroblasts. These results suggest that ERM activation is an important step in the directional migration of neuroblasts in response to SDF1-CXCR4 signaling following brain injury.

Radixin inhibition decreases adult neural progenitor cell migration and proliferation in vitro and in vivo

Neuronal progenitors capable of long distance migration are produced throughout life in the subventricular zone (SVZ). Migration from the SVZ is carried out along a well-defined pathway called the rostral migratory stream (RMS). Our recent finding of the specific expression of the cytoskeleton linker protein radixin in neuroblasts suggests a functional role for radixin in RMS migration. The ezrin-radixin-moesin (ERM) family of proteins is capable of regulating migration through interaction with the actin cytoskeleton and transmembrane proteins. The ERM proteins are differentially expressed in the RMS with radixin and moesin localized to neuroblasts, and ezrin expression confined to astrocytes of the glial tubes. Here, we inhibited radixin function using the quinocarmycin analog DX52-1 which resulted in reduced neuroblast migration in vitro, while glial migration remained unaltered. Furthermore, the morphology of neuroblasts was distorted resulting in a rounded shape with no or short polysialylated neural cell adhesion molecule positive processes. Intracerebroventricular infusion of the radixin inhibitor resulted in accumulation of neuroblasts in the anterior SVZ. Neuroblast chains were short and intermittently interrupted in the SVZ and considerably disorganized in the RMS. Moreover, we studied the proliferation activity in the RMS after radixin inhibition, since concentrated radixin expression has been demonstrated in the cleavage furrow of dividing cells, which indicates a role of radixin in cell division. Radixin inhibition decreased neuroblast proliferation, whereas the proliferation of other cells in the RMS was not affected. Our results demonstrate a significant role for radixin in neuroblast proliferation and migration.

Radixin expression in microglia after cortical stroke lesion

Stroke induces extensive tissue remodeling, resulting in the activation of several cell types in the brain as well as recruitment of blood-borne leucocytes. Radixin is part of a cytoskeleton linker protein family with the ability to connect transmembrane proteins to the actin cytoskeleton, promoting cell functions involving a dynamic cytoskeleton such as morphological changes, cell division and migration which are common events of different cell types after stroke. In the healthy adult brain radixin is expressed in Olig2(+) cells throughout the brain and in neural progenitor cells in the subventricular zone. In the current study, we detected a 2.5 fold increase in the number of radixin positive cells in the peri-infarct cortex two weeks after the induction of cortical stroke by photothrombosis. Similarly, the number of Olig2(+) cells increased in the peri-infarct area after stroke; however, the number of radixin(+)/Olig2(+) cells was unchanged. Neural progenitor cells maintained radixin expression on their route to the infarct. More surprising however, was the expression of radixin in activated microglia in the peri-infarct cortex. Seventy percent of Iba1(+) cells expressed radixin after stroke, a population which was not present in the control brain. Furthermore, activation of radixin was predominantly detected in the peri-infarct region of oligodendrocyte progenitors and microglia. The specific location of radixin(+) cells in the peri-infarct region and in microglia suggests a role for radixin in microglial activation after stroke.

EGF-induced expansion of migratory cells in the rostral migratory stream

The presence of neural stem cells in the adult brain is currently widely accepted and efforts are made to harness the regenerative potential of these cells. The dentate gyrus of the hippocampal formation, and the subventricular zone (SVZ) of the anterior lateral ventricles, are considered the main loci of adult neurogenesis. The rostral migratory stream (RMS) is the structure funneling SVZ progenitor cells through the forebrain to their final destination in the olfactory bulb. Moreover, extensive proliferation occurs in the RMS. Some evidence suggest the presence of stem cells in the RMS, but these cells are few and possibly of limited differentiation potential. We have recently demonstrated the specific expression of the cytoskeleton linker protein radixin in neuroblasts in the RMS and in oligodendrocyte progenitors throughout the brain. These cell populations are greatly altered after intracerebroventricular infusion of epidermal growth factor (EGF). In the current study we investigate the effect of EGF infusion on the rat RMS. We describe a specific increase of radixin⁺/Olig2⁺ cells in the RMS. Negative for NG2 and CNPase, these radixin⁺/Olig2⁺ cells are distinct from typical oligodendrocyte progenitors. The expanded Olig2⁺ population responds rapidly to EGF and proliferates after only 24 hours along the entire RMS, suggesting local activation by EGF throughout the RMS rather than migration from the SVZ. In addition, the radixin⁺/Olig2⁺ progenitors assemble in chains in vivo and migrate in chains in explant cultures, suggesting that they possess migratory properties within the RMS. In summary, these results provide insight into the adaptive capacity of the RMS and point to an additional stem cell source for future brain repair strategies.

F-spondin/spon1b expression patterns in developing and adult zebrafish

F-spondin, an extracellular matrix protein, is an important player in embryonic morphogenesis and CNS development, but its presence and role later in life remains largely unknown. We generated a transgenic zebrafish in which GFP is expressed under the control of the F-spondin (spon1b) promoter, and used it in combination with complementary techniques to undertake a detailed characterization of the expression patterns of F-spondin in developing and adult brain and periphery. We found that F-spondin is often associated with structures forming long neuronal tracts, including retinal ganglion cells, the olfactory bulb, the habenula, and the nucleus of the medial longitudinal fasciculus (nMLF). F-spondin expression coincides with zones of adult neurogenesis and is abundant in CSF-contacting secretory neurons, especially those in the hypothalamus. Use of this new transgenic model also revealed F-spondin expression patterns in the peripheral CNS, notably in enteric neurons, and in peripheral tissues involved in active patterning or proliferation in adults, including the endoskeleton of zebrafish fins and the continuously regenerating pharyngeal teeth. Moreover, patterning of the regenerating caudal fin following fin amputation in adult zebrafish was associated with F-spondin expression in the blastema, a proliferative region critical for tissue reconstitution. Together, these findings suggest major roles for F-spondin in the CNS and periphery of the developing and adult vertebrate.

Adult neurogenesis and neurite outgrowth are impaired in LRRK2 G2019S mice

The generation and maturation of adult neural stem/progenitor cells are impaired in many neurodegenerative diseases, among them is Parkinson's disease (PD). In mammals, including humans, adult neurogenesis is a lifelong feature of cellular brain plasticity in the hippocampal dentate gyrus (DG) and in the subventricular zone (SVZ)/olfactory bulb system. Hyposmia, depression, and anxiety are early non-motor symptoms in PD. There are parallels between brain regions associated with non-motor symptoms in PD and neurogenic regions. In autosomal dominant PD, mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are frequent. LRRK2 homologs in non-vertebrate systems play an important role in chemotaxis, cell polarity, and neurite arborization. We investigated adult neurogenesis and the neurite development of new neurons in the DG and SVZ/olfactory bulb system in bacterial artificial chromosome (BAC) human Lrrk2 G2019S transgenic mice. We report that mutant human Lrrk2 is highly expressed in the hippocampus in the DG and the SVZ of adult Lrrk2 G2019S mice. Proliferation of newly generated cells is significantly decreased and survival of newly generated neurons in the DG and olfactory bulb is also severely impaired. In addition, after stereotactic injection of a GFP retrovirus, newly generated neurons in the DG of Lrrk2 G2019S mice exhibited reduced dendritic arborization and fewer spines. This loss in mature, developed spines might point towards a decrease in synaptic connectivity. Interestingly, physical activity partially reverses the decrease in neuroblasts observed in Lrrk2 G2010S mice. These data further support a role for Lrrk2 in neuronal morphogenesis and provide new insights into the role of Lrrk2 in adult neurogenesis.