Growth promoting and inhibitory effects of glial cells in the mammalian nervous system

Extracellular environment contribution to astrogliosis-lessons learned from a tissue engineered 3D model of the glial scar

Glial scars are widely seen as a (bio)mechanical barrier to central nervous system regeneration. Due to the lack of a screening platform, which could allow in-vitro testing of several variables simultaneously, up to now no comprehensive study has addressed and clarified how different lesion microenvironment properties affect astrogliosis. Using astrocytes cultured in alginate gels and meningeal fibroblast conditioned medium, we have built a simple and reproducible 3D culture system of astrogliosis mimicking many features of the glial scar. Cells in this 3D culture model behave similarly to scar astrocytes, showing changes in gene expression (e.g., GFAP) and increased extra-cellular matrix production (chondroitin 4 sulfate and collagen), inhibiting neuronal outgrowth. This behavior being influenced by the hydrogel network properties. Astrocytic reactivity was found to be dependent on RhoA activity, and targeting RhoA using shRNA-mediated lentivirus reduced astrocytic reactivity. Further, we have shown that chemical inhibition of RhoA with ibuprofen or indirectly targeting RhoA by the induction of extracellular matrix composition modification with chondroitinase ABC, can diminish astrogliosis. Besides presenting the extracellular matrix as a key modulator of astrogliosis, this simple, controlled and reproducible 3D culture system constitutes a good scar-like system and offers great potential in future neurodegenerative mechanism studies, as well as in drug screenings envisaging the development of new therapeutic approaches to minimize the effects of the glial scar in the context of central nervous system disease.

Development of a stereotaxic device for low impact implantation of neural constructs or pieces of neural tissues into the mammalian brain

Implanting pieces of tissue or scaffolding material into the mammalian central nervous system (CNS) is wrought with difficulties surrounding the size of tools needed to conduct such implants and the ability to maintain the orientation and integrity of the constructs during and after their transplantation. Here, novel technology has been developed that allows for the implantation of neural constructs or intact pieces of neural tissue into the CNS with low trauma. By “laying out” (instead of forcibly expelling) the implantable material from a thin walled glass capillary, this technology has the potential to enhance neural transplantation procedures by reducing trauma to the host brain during implantation and allowing for the implantation of engineered/dissected tissues or constructs in such a way that their orientation and integrity are maintained in the host. Such technology may be useful for treating various CNS disorders which require the reestablishment of point-to-point contacts (e.g., Parkinson's disease) across the adult CNS, an environment which is not normally permissive to axonal growth.

Cellular and molecular mediators of neuroinflammation in the pathogenesis of Parkinson's disease

Neuroinflammation is a host-defense mechanism associated with restoration of normal structure and function of the brain and neutralization of an insult. Increasing neuropathological and biochemical evidence from the brains of individuals with Parkinson's disease (PD) provides strong evidence for activation of neuroinflammatory pathways. Microglia, the resident innate immune cells, may play a major role in the inflammatory process of the diseased brain of patients with PD. Although microglia forms the first line of defense for the neural parenchyma, uncontrolled activation of microglia may directly affect neurons by releasing various molecular mediators such as inflammatory cytokines (tumor necrosis factor-α, interleukin [IL]-6, and IL-1β), nitric oxide, prostaglandin E2, and reactive oxygen and nitrogen species. Moreover, recent studies have reported that activated microglia phagocytose not only damaged cell debris but also intact neighboring cells. This phenomenon further supports their active participation in self-enduring neuronal damage cycles. As the relationship between PD and neuroinflammation is being studied, there is a realization that both cellular and molecular mediators are most likely assisting pathological processes leading to disease progression. Here, we discuss mediators of neuroinflammation, which are known activators released from damaged parenchyma of the brain and result in neuronal degeneration in patients with PD.

The insulin-like growth factor 1 receptor is essential for axonal regeneration in adult central nervous system neurons

Axonal regeneration is an essential condition to re-establish functional neuronal connections in the injured adult central nervous system (CNS), but efficient regrowth of severed axons has proven to be very difficult to achieve. Although significant progress has been made in identifying the intrinsic and extrinsic mechanisms involved, many aspects remain unresolved. Axonal development in embryonic CNS (hippocampus) requires the obligate activation of the insulin-like growth factor 1 receptor (IGF-1R). Based on known similarities between axonal growth in fetal compared to mature CNS, we decided to examine the expression of the IGF-1R, using an antibody to the βgc subunit or a polyclonal anti-peptide antibody directed to the IGF-R (C20), in an in vitro model of adult CNS axonal regeneration, namely retinal ganglion cells (RGC) derived from adult rat retinas. Expression of both βgc and the β subunit recognized by C20 antibody were low in freshly isolated adult RGC, but increased significantly after 4 days in vitro. As in embryonic axons, βgc was localised to distal regions and leading growth cones in RGC. IGF-1R-βgc co-localised with activated p85 involved in the phosphatidylinositol-3 kinase (PI3K) signaling pathway, upon stimulation with IGF-1. Blocking experiments using either an antibody which neutralises IGF-1R activation, shRNA designed against the IGF-1R sequence, or the PI3K pathway inhibitor LY294002, all significantly reduced axon regeneration from adult RGC in vitro (∼40% RGC possessed axons in controls vs 2-8% in the different blocking studies). Finally, co-transfection of RGC with shRNA to silence IGF-1R together with a vector containing a constitutively active form of downstream PI3K (p110), fully restored axonal outgrowth in vitro. Hence these data demonstrate that axonal regeneration in adult CNS neurons requires re-expression and activation of IGF-1R, and targeting this system may offer new therapeutic approaches to enhancing axonal regeneration following trauma.

Role of the lesion scar in the response to damage and repair of the central nervous system

Traumatic damage to the central nervous system (CNS) destroys the blood–brain barrier (BBB) and provokes the invasion of hematogenous cells into the neural tissue. Invading leukocytes, macrophages and lymphocytes secrete various cytokines that induce an inflammatory reaction in the injured CNS and result in local neural degeneration, formation of a cystic cavity and activation of glial cells around the lesion site. As a consequence of these processes, two types of scarring tissue are formed in the lesion site. One is a glial scar that consists in reactive astrocytes, reactive microglia and glial precursor cells. The other is a fibrotic scar formed by fibroblasts, which have invaded the lesion site from adjacent meningeal and perivascular cells. At the interface, the reactive astrocytes and the fibroblasts interact to form an organized tissue, the glia limitans. The astrocytic reaction has a protective role by reconstituting the BBB, preventing neuronal degeneration and limiting the spread of damage. While much attention has been paid to the inhibitory effects of the astrocytic component of the scars on axon regeneration, this review will cover a number of recent studies in which manipulations of the fibroblastic component of the scar by reagents, such as blockers of collagen synthesis have been found to be beneficial for axon regeneration. To what extent these changes in the fibroblasts act via subsequent downstream actions on the astrocytes remains for future investigation.

Time course Analysis of Gene expression patterns in ZebrafIsh Eye during Optic Nerve Regeneration

It is well-established that neurons in the adult mammalian central nervous system (CNS) are terminally differentiated and, if injured, will be unable to regenerate their connections. In contrast to mammals, zebrafish and other teleosts display a robust neuroregenerative response. Following optic nerve crush (ONX), retinal ganglion cells (RGC) regrow their axons to synapse with topographically correct targets in the optic tectum, such that vision is restored in ~21 days. What accounts for these differences between teleostean and mammalian responses to neural injury is not fully understood. A time course analysis of global gene expression patterns in the zebrafish eye after ONX can help to elucidate cellular and molecular mechanisms that contribute to a successful neuroregeneration. To define different phases of regeneration after ONX, alpha tubulin 1 ( tuba1) and growth-associated protein 43 ( gap43), markers previously shown to correspond to morphophological events, were measured by real time quantitative PCR (qPCR). Microarray analysis was then performed at defined intervals (6 hours, 1, 4, 12, and 21 days) post-ONX and compared to SHAM. Results show that optic nerve damage induces multiple, phase-related transcriptional programs, with the maximum number of genes changed and highest fold-change occurring at 4 days. Several functional groups affected by optic nerve regeneration, including cell adhesion, apoptosis, cell cycle, energy metabolism, ion channel activity, and calcium signaling, were identified. Utilizing the whole eye allowed us to identify signaling contributions from the vitreous, immune and glial cells as well as the neural cells of the retina. Comparisons between our dataset and transcriptional profiles from other models of regeneration in zebrafish retina, heart and fin revealed a subset of commonly regulated transcripts, indicating shared mechanisms in different regenerating tissues. Knowledge of gene expression patterns in all components of the eye in a model of successful regeneration provides an entry point for functional analyses, and will help in devising hypotheses for testing normal and toxic regulatory factors.

An in vitro model of the inhibition of axon growth in the lesion scar formed after central nervous system injury

After central nervous system (CNS) injury, meningeal fibroblasts migrate in the lesion center to form a fibrotic scar which is surrounded by end feet of reactive astrocytes. The fibrotic scar expresses various axonal growth-inhibitory molecules and creates a major impediment for axonal regeneration. We developed an in vitro model of the scar using coculture of cerebral astrocytes and meningeal fibroblasts by adding transforming growth factor-beta1 (TGF-beta1), a potent fibrogenic factor. Addition of TGF-beta1 to this coculture resulted in enhanced proliferation of fibroblasts and the formation of cell clusters which consisted of fibroblasts inside and surrounded by astrocytes. The cell cluster in culture densely accumulated the extracellular matrix molecules and axonal growth-inhibitory molecules similar to the fibrotic scar, and remarkably inhibited the neurite outgrowth of cerebellar neurons. Therefore, this culture system can be available to analyze the inhibitory property in the lesion site of CNS.

Nano neurology and the four P's of central nervous system regeneration: preserve, permit, promote, plasticity

True nanomaterials are delivered as a specific structure, or combination of structures, designed to deliver the therapeutic intact, directly to the site, requiring a much lower dose. These materials use very specific and deliberate molecular structures that can interact with neurons or protein structures inside the cells. Until recently, functional recovery of the central nervous system (CNS) was an unattainable goal and nanotechnology was an invisible science. A well-planned treatment spaced over time will produce functional return in the CNS. The four P's of CNS regeneration is a new framework for approaching CNS injury and evidence shows that nanotechnology is currently being used for stroke rehabilitation and, in several clinical trials, the treatment of scar formation blockade in the spinal cord. The four components are preserve, permit, promote, and plasticity.

Semaphorins in axon regeneration: developmental guidance molecules gone wrong?

Semaphorins are developmental axon guidance cues that continue to be expressed during adulthood and are regulated by neural injury. During the formation of the nervous system, repulsive semaphorins guide axons to their targets by restricting and channelling their growth. They affect the growth cone cytoskeleton through interactions with receptor complexes that are linked to a complicated intracellular signal transduction network. Following injury, regenerating axons stop growing when they reach the border of the glial-fibrotic scar, in part because they encounter a potent molecular barrier that inhibits growth cone extension. A number of secreted semaphorins are expressed in the glial-fibrotic scar and at least one transmembrane semaphorin is upregulated in oligodendrocytes surrounding the lesion site. Semaphorin receptors, and many of the signal transduction components required for semaphorin signalling, are present in injured central nervous system neurons. Here, we review evidence that supports a critical role for semaphorin signalling in axon regeneration, and highlight a number of challenges that lie ahead with respect to advancing our understanding of semaphorin function in the normal and injured adult nervous system.

Apoptosis is required during early stages of tail regeneration in Xenopus laevis

The Xenopus tadpole is able to regenerate its tail, including skin, muscle, notochord, spinal cord and neurons and blood vessels. This process requires rapid tissue growth and morphogenesis. Here we show that a focus of apoptotic cells appears in the regeneration bud within 12 h of amputation. Surprisingly, when caspase-3 activity is specifically inhibited, regeneration is abolished. This is true of tails both before and after the refractory period. Programmed cell death is only required during the first 24 h after amputation, as later inhibition has no effect on regeneration. Inhibition of caspase-dependent apoptosis results in a failure to induce proliferation in the growth zone, a mispatterning of axons in the regenerate, and the appearance of ectopic otoliths in the neural tube, in the context of otherwise normal continued development of the larva. Larvae amputated during the refractory stage exhibit a much broader domain of caspase-3-positive cells, suggesting a window for the amount of apoptosis that is compatible with normal regeneration. These data reveal novel roles for apoptosis in development and indicate that a degree of apoptosis is an early and obligate component of normal tail regeneration, suggesting the possibility of the existence of endogenous inhibitory cells that must be destroyed by programmed cell death for regeneration to occur.

Nano neuro knitting: peptide nanofiber scaffold for brain repair and axon regeneration with functional return of vision

Nanotechnology is often associated with materials fabrication, microelectronics, and microfluidics. Until now, the use of nanotechnology and molecular self assembly in biomedicine to repair injured brain structures has not been explored. To achieve axonal regeneration after injury in the CNS, several formidable barriers must be overcome, such as scar tissue formation after tissue injury, gaps in nervous tissue formed during phagocytosis of dying cells after injury, and the failure of many adult neurons to initiate axonal extension. Using the mammalian visual system as a model, we report that a designed self-assembling peptide nanofiber scaffold creates a permissive environment for axons not only to regenerate through the site of an acute injury but also to knit the brain tissue together. In experiments using a severed optic tract in the hamster, we show that regenerated axons reconnect to target tissues with sufficient density to promote functional return of vision, as evidenced by visually elicited orienting behavior. The peptide nanofiber scaffold not only represents a previously undiscovered nanobiomedical technology for tissue repair and restoration but also raises the possibility of effective treatment of CNS and other tissue or organ trauma.

Cellular pathology of Parkinson?s disease: astrocytes, microglia and inflammation

Parkinson's disease (PD) is a frequent neurological disorder of the basal ganglia, which is characterized by the progressive loss of dopaminergic neurons mainly in the substantia nigra pars compacta (SNpc). Inflammatory processes have been shown to be associated with the pathogenesis of PD. Activated microglia, as well as to a lesser extent reactive astrocytes, are found in the area associated with cell loss, possibly contributing to the inflammatory process by the release of pro-inflammatory prostaglandins or cytokines. Further deleterious factors released by activated microglia or astrocytes are reactive oxygen species. On the other hand, they may mediate neuroprotective properties by the release of trophic factors or the uptake of glutamate. In this review, we will discuss the different aspects of activated glial cells and potential mechanisms that mediate or protect against cell loss in PD.

Molecules involved in reactive sprouting in the hippocampus

Denervation of the hippocampus triggers reactive responses in neurons and glial cells in their affected strata in a temporally ordered fashion. Many of these responses have been studied extensively, focusing on the one hand on glial initiation and clearing responses during the degeneration phase and, on the other, on transneuronal reorganization and the newly adjusted physiological balance. We used the entorhinal cortex lesion (ECL) as a model system to study the cues that underlie the layer-specific sprouting response. This lesion destroys the perforant path, which is a massive excitatory projection to the dentate gyrus and hippocampus proper. In the deafferented zones of the hippocampus, sprouting of the remaining unlesioned fibers occurs, which replaces the lost afferences of the perforant path. We focus on candidate molecules which govern the layer-specific sprouting of the remaining axons and, in particular, on membrane-bound cues. The fact that layer-specific sprouting occurs even in the adult central nervous system (CNS) provides a valuable model for understanding the mechanisms of reactive neuronal growth and reorganization in the adult CNS. Isolation and analysis of the molecules involved in these mechanisms are important steps in understanding the potential and limitations of regeneration in the CNS.

Coexistence of neuropeptides and their possible relation to neuritic regeneration in primary cultures of magnocellular neurons isolated from adult rat supraoptic nuclei

The coexistence of vasopressin (VP), oxytocin (OXY), galanin (GAL) and cholecystokinin (CCK) and the synthesis of GAL and CCK during neuritic regeneration was investigated in cultured magnocellular neurons, isolated from adult rat supraoptic nuclei. Double-labelling immunofluorescence was performed after 7 days of culture using primary antibodies for VP, OXY, GAL and CCK (paired in all possible combinations) and secondary antibodies labelled with either fluorescein or rhodamine. Confocal laser scanning microscopy revealed the coexistence of the mentioned peptides in all possible combinations, an unexpected result considering that the only combinations observed in tissue sections are VP-GAL and OXY-CCK. Freshly dispersed cells were devoid of any neuritic processes and showed a very poor immunocytochemical staining reaction for GAL and CCK. In contrast, neurons cultured for 7, 12 and 21 days showed many neurites and a strong immunoreactivity for GAL and CCK indicative of an increased synthesis of both peptides in the regenerating neurons. This increased synthetic activity is consistent with transient upregulation of these peptides observed in situ after hypophysectomy by other authors. The results suggest that the upregulation of GAL and CCK is functionally related to the neuronal regeneration processes observed during culture and that the 'uncommon' coexistences as well as the prolonged sythesis of GAL and CCK may be due to the lack of environmental inputs, which normally regulate the expression and up- and downregulation of these peptides in vivo.