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The extracellular matrix (ECM) molecules chondroitin-6-sulfate proteoglycan (CS-PG) and cytotactin/tenascin (CT), present on subpopulations of astroglia or their precursors during development, can inhibit neurite outgrowth in vitro. However, it is not known whether these molecules are expressed within the mature CNS following injury, where they could contribute to regenerative failure. Thus, the expression of various ECM molecules that affect axon growth was examined in areas of reactive gliosis caused by implanting a piece of nitrocellulose into the cortex of neonatal and adult animals. The expression of these molecules was compared to the amount of neurite outgrowth that occurred in vitro when the damaged CNS tissue from animals of various ages was removed intact and used as a substrate in explant culture. The results demonstrate that the growth-promoting molecules laminin, collagen type IV, and fibronectin were present around the implant in all experimental groups. In comparison, CS-PG and CT were present within and around the area of the lesion only in adult animals. In vivo, these molecules were colocalized with intensely glial fibrillary acidic protein (GFAP)-positive astrocytes in and immediately adjacent to the scar, but not with other equally intensely GFAP-positive astrocytes in the cortex away from the site of injury. CT and CS-PG were present in gray matter areas of the cortex that had been directly damaged during the implant procedure and in the corpus callosum when lesioned during implantation. In vitro, the glial tissue removed from the lesion site of neonatal animals supported neurite outgrowth, while scars removed from adult animals did not. The inability of the adult glial scar tissue to support neurite outgrowth was best correlated with the expression of CS-PG and CT, suggesting that these molecules may be involved in limiting the growth of regenerating axons in the CNS after injury.

Spinal cord and brain injuries lead to complex cellular and molecular interactions within the central nervous system in an attempt to repair the initial tissue damage. Many studies have illustrated the importance of the glial cell response to injury, and the influences of inflammation and wound healing processes on the overall morbidity and permanent disability that result. The abortive attempts of neuronal regeneration after spinal cord injury are influenced by inflammatory cell activation, reactive astrogliosis and the production of both growth promoting and inhibitory extracellular molecules. Despite the historical perspective that the glial scar was a mechanical barrier to regeneration, inhibitory molecules in the forming scar and methods to overcome them have suggested molecular modification strategies to allow neuronal growth and functional regeneration. Unlike myelin associated inhibitory molecules, which remain at largely static levels before and after central nervous system trauma, inhibitory extracellular matrix molecules are dramatically upregulated during the inflammatory stages after injury providing a window of opportunity for the delivery of candidate therapeutic interventions. While high dose methylprednisolone steroid therapy alone has not proved to be the solution to this difficult clinical problem, other strategies for modulating inflammation and changing the make up of inhibitory molecules in the extracellular matrix are providing robust evidence that rehabilitation after spinal cord and brain injury has the potential to significantly change the outcome for what was once thought to be permanent disability.

Chondroitin sulfate proteoglycans (CSPGs) present a barrier to axon regeneration. However, no specific receptor for the inhibitory effect of CSPGs has been identified. We showed that a transmembrane protein tyrosine phosphatase, PTPσ, binds with high affinity to neural CSPGs. Binding involves the chondroitin sulfate chains and a specific site on the first immunoglobulin-like domain of PTPσ. In culture, PTPσ −/− neurons show reduced inhibition by CSPG. A PTPσ fusion protein probe can detect cognate ligands that are up-regulated specifically at neural lesion sites. After spinal cord injury, PTPσ gene disruption enhanced the ability of axons to penetrate regions containing CSPG. These results indicate that PTPσ can act as a receptor for CSPGs and may provide new therapeutic approaches to neural regeneration.Recovery after central nervous system (CNS) injury is minimal, leading to substantial current interest in potential strategies to overcome this challenge (1-5). Chondroitin sulfate proteoglycans (CSPGs) show dramatic up-regulation after neural injury, within the extracellular matrix of scar tissue and in the perineuronal net within more-distant targets of the severed axons (6,7). The inhibitory nature of CSPGs is reflected not only in the formation of dystrophic axonal retraction bulbs that fail to regenerate through the lesion (8), but also in the limited capacity for collateral sprouting of spared fibers (8,9). This inhibition can be relieved by chondroitinase ABC digestion of the chondroitin sulfate (CS) side chains, which can promote regeneration and sprouting and restore lost function (10)(11)(12)(13)(14). It has been known for nearly two decades that sulfated proteoglycans are major contributors to the repulsive nature of the glial scar (15); however, the precise inhibitory mechanism remains poorly understood. Because the identification of specific neuronal receptors for CSPGs has been lacking, relatively nonspecific mechanisms brought about by arrays of negatively charged sulfate (16) or the occlusion of substrate adhesion molecules (17) have been suggested.Transmembrane protein tyrosine phosphatases (PTPs) form a large and diverse molecular family and have a structure typical of transmembrane cell-surface receptors (18,19). In previous work, we and others have found that PTPσ and other PTPs in the leukocyte antigen-related (LAR) subfamily can act as receptors for heparan sulfate proteoglycans (HSPGs) (20)(21)(22), and these PTPs are involved in axon guidance and synapse formation during development (18- ‡To whom correspondence should be addressed. flanagan@hms.harvard.edu. * These authors contributed equally to this work. † Present address: Motor Neuron Center, Columbia University, New York, NY 10032, USA. (Fig. 1A). Using a cell-free system with recombinant fusion proteins of the PTPσ extra-cellular domain with an immunoglobulin Fc tag (PTPσ-Fc) and neurocan with an alkaline phosphatase tag (Ncn-AP), a binding interaction was indeed identified (P < 0.001) (Fig. 1B). Genuine biological ligand-re...

The central serotonin (5-HT) neurotransmitter system is an important modulator of diverse physiological processes and behaviors; however, the transcriptional mechanisms controlling its development are largely unknown. The Pet-1 ETS factor is a precise marker of developing and adult 5-HT neurons and is expressed shortly before 5-HT appears in the hindbrain. Here we show that in mice lacking Pet-1, the majority of 5-HT neurons fail to differentiate. Remaining ones show deficient expression of genes required for 5-HT synthesis, uptake, and storage. Significantly, defective development of the 5-HT system is followed by heightened anxiety-like and aggressive behavior in adults. These findings indicate that Pet-1 is a critical determinant of 5-HT neuron identity and implicate a Pet-1-dependent program in serotonergic modulation of behavior.

It is widely accepted that the adult mammalian central nervous system (CNS) is unable to regenerate axons. In addition to physical or molecular barriers presented by glial scarring at the lesion site, it has been suggested that the normal myelinated CNS environment contains potent growth inhibitors or lacks growth-promoting molecules. Here we investigate whether adult CNS white matter can support long-distance regeneration of adult axons in the absence of glial scarring, by using a microtransplantation technique that minimizes scarring to inject minute volumes of dissociated adult rat dorsal root ganglia directly into adult rat CNS pathways. This atraumatic injection procedure allowed considerable numbers of regenerating adult axons immediate access to the host glial terrain, where we found that they rapidly extended for long distances in white matter, eventually invading grey matter. Abortive regeneration correlated precisely with increased levels of proteoglycans within the extracellular matrix at the transplant interface, whereas successfully regenerating transplants were associated with minimal upregulation of these molecules. Our results demonstrate, to our knowledge for the first time, that reactive glial extracellular matrix at the lesion site is directly associated with failure of axon regrowth in vivo, and that adult myelinated white matter tracts beyond the glial scar can be highly permissive for regeneration.

No abstract

Astrocytes react to CNS injury by building a dense wall of filamentous processes around the lesion. Stromal cells quickly take up residence in the lesion core and synthesize connective tissue elements that contribute to fibrosis. Oligodendrocyte precursor cells proliferate within the lesion and help to entrap dystrophic axon tips. Here we review evidence that this aggregate scar acts as the major barrier to regeneration of axons after injury. We also consider several exciting new interventions that allow axons to regenerate beyond the glial scar, and discuss the implications of this work for the future of regeneration biology.