Versican V2 and the central inhibitory domain of Nogo-A inhibit neurite growth via p75NTR/NgR-independent pathways that converge at RhoA

Mitochondrial Miro GTPases coordinate mitochondrial and peroxisomal dynamics

Mitochondria and peroxisomes are highly dynamic, multifunctional organelles. Both perform key roles for cellular physiology and homoeostasis by mediating bioenergetics, biosynthesis, and/or signalling. To support cellular function, they must be properly distributed, of proper size, and be able to interact with other organelles. Accumulating evidence suggests that the small atypical GTPase Miro provides a central signalling node to coordinate mitochondrial as well as peroxisomal dynamics. In this review, I summarize our current understanding of Miro-dependent functions and molecular mechanisms underlying the proper distribution, size and function of mitochondria and peroxisomes.

Modulatory properties of extracellular matrix glycosaminoglycans and proteoglycans on neural stem cells behavior: Highlights on regenerative potential and bioactivity

Despite the poor regenerative capacity of the adult central nervous system (CNS) in mammals, two distinct regions, subventricular zone (SVZ) and the subgranular zone (SGZ), continue to generate new functional neurons throughout life which integrate into the pre-existing neuronal circuitry. This process is not fixed but highly modulated, revealing many intrinsic and extrinsic mechanisms by which this performance can be optimized for a given environment. The capacity for self-renewal, proliferation, migration, and multi-lineage potency of neural stem cells (NSCs) underlines the necessity of controlling stem cell fate. In this context, the native and local microenvironment plays a critical role, and the application of this highly organized architecture in the CNS has been considered as a fundamental concept in the generation of new effective therapeutic strategies in tissue engineering approaches. The brain extracellular matrix (ECM) is composed of biomacromolecules, including glycosaminoglycans, proteoglycans, and glycoproteins that provide various biological actions through biophysical and biochemical signaling pathways. Herein, we review predominantly the structure and function of the mentioned ECM composition and their regulatory impact on multiple and diversity of biological functions, including neural regeneration, survival, migration, differentiation, and final destiny of NSCs.

RhoA-ROCK Signaling as a Therapeutic Target in Traumatic Brain Injury

Traumatic brain injury (TBI) is a leading cause of death and disability worldwide. TBIs, which range in severity from mild to severe, occur when a traumatic event, such as a fall, a traffic accident, or a blow, causes the brain to move rapidly within the skull, resulting in damage. Long-term consequences of TBI can include motor and cognitive deficits and emotional disturbances that result in a reduced quality of life and work productivity. Recovery from TBI can be challenging due to a lack of effective treatment options for repairing TBI-induced neural damage and alleviating functional impairments. Central nervous system (CNS) injury and disease are known to induce the activation of the small GTPase RhoA and its downstream effector Rho kinase (ROCK). Activation of this signaling pathway promotes cell death and the retraction and loss of neural processes and synapses, which mediate information flow and storage in the brain. Thus, inhibiting RhoA-ROCK signaling has emerged as a promising approach for treating CNS disorders. In this review, we discuss targeting the RhoA-ROCK pathway as a therapeutic strategy for treating TBI and summarize the recent advances in the development of RhoA-ROCK inhibitors.

Deacetylation of Miro1 by HDAC6 blocks mitochondrial transport and mediates axon growth inhibition

Inhibition of histone deacetylase 6 (HDAC6) was shown to support axon growth on the nonpermissive substrates myelin-associated glycoprotein (MAG) and chondroitin sulfate proteoglycans (CSPGs). Though HDAC6 deacetylates α-tubulin, we find that another HDAC6 substrate contributes to this axon growth failure. HDAC6 is known to impact transport of mitochondria, and we show that mitochondria accumulate in distal axons after HDAC6 inhibition. Miro and Milton proteins link mitochondria to motor proteins for axon transport. Exposing neurons to MAG and CSPGs decreases acetylation of Miro1 on Lysine 105 (K105) and decreases axonal mitochondrial transport. HDAC6 inhibition increases acetylated Miro1 in axons, and acetyl-mimetic Miro1 K105Q prevents CSPG-dependent decreases in mitochondrial transport and axon growth. MAG- and CSPG-dependent deacetylation of Miro1 requires RhoA/ROCK activation and downstream intracellular Ca²⁺ increase, and Miro1 K105Q prevents the decrease in axonal mitochondria seen with activated RhoA and elevated Ca²⁺ These data point to HDAC6-dependent deacetylation of Miro1 as a mediator of axon growth inhibition through decreased mitochondrial transport.

Overexpression of Nogo receptor 3 (NgR3) correlates with poor prognosis and contributes to the migration of epithelial cells of nasopharyngeal carcinoma patients

Lymph node metastasis (N classification) is one of the most important prognostic factors of nasopharyngeal carcinoma (NPC), and nerve involvement is associated with the transition of the N category in NPC patients. Although the nervous system has been reported to participate in many types of cancer progression, its functions in NPC progression remains unknown. Through analysis of gene profiling data, we demonstrate an enrichment of genes associated with neuronal development and differentiation in NPC tissues and cell lines. Among these genes, Nogo receptor 3 (NgR3), which was originally identified in the nervous system and plays a role in nerve development and regeneration, was inappropriately overexpressed in NPC cells and tissues. Immunohistochemical analysis demonstrated that the overexpression of NgR3 was correlated with poor prognosis in NPC patients. Overexpression of NgR3 promoted, and knocking down NgR3 inhibited, NPC cell migration and invasion in vitro and metastasis in vivo. The ability of NgR3 to promote cell migration was triggered by the downregulation of E-cadherin and enhanced cytoskeletal rearrangement and cell polarity, which were correlated with the activation of focal adhesion kinase (FAK). Collectively, NgR3 is a novel indicator of poor outcomes in NPC patients and plays an important role in driving the progression of NPC. These results suggest a potential link between the nervous system and NPC progression.

KEY MESSAGES

Genes involved in the neuronal biological process are enriched in nasopharyngeal carcinoma. Overexpression of NgR3 correlates with poor prognosis of nasopharyngeal carcinoma. NgR3 promotes NPC cell migration by downregulating E-cadherin. NgR3 promotes NPC cell polarity and enhances the formation of NPC cell pseudopodia by activating FAK/Src pathway.

Expression of Nogo receptor 1 in the regeneration process of the mouse olfactory epithelium

Nogo receptor 1 (NgR1) is the most important Nogo-A receptor. By its interaction with myelin-associated inhibitory proteins, NgR1 inhibits the regeneration of axons and is extensively expressed in the central nervous system. However, the expression of NgR1 in regenerable neurons, such as olfactory neurons, and its expression in the regeneration progress of olfactory neurons have not been reported. In this study, we demonstrated that NgR1 was expressed in the cell bodies of certain mature olfactory receptor neurons (ORNs) but was not expressed in immature ORNs in the olfactory epithelium (OE) of normal adult mice. On day 21 after OE injury, NgR1 was expressed not only in the cell bodies of mature ORNs but also in the cell bodies of glial fibrillary acidic protein (GFAP)-positive cells in the top and submucosal layers of the OE. On day 48 after model establishment, NgR1 expression decreased in the cell bodies of the GFAP-positive cells. On day 56 after model establishment, no NgR1 expression was found in the cell bodies of the GFAP-positive cells, and NgR1 was again expressed only in the mature ORNs. Our results demonstrated that NgR1 expression is upregulated in the OE after injury, which suggests that NgR1 might be involved in the regeneration of the OE.

Houshiheisan and its components promote axon regeneration after ischemic brain injury

Houshiheisan, a classic prescription in traditional Chinese medicine, contains Flos Chrysanthemi, Radix Saposhnikoviae, Ramulus Cinnamomi, Rhizoma Chuanxiong, Radix et Rhizoma Asari, Radix Platycodonis, Rhizoma Atractylodis macrocephalae, Poria, Rhizoma Zingiberis, Radix Angelicae sinensis, Radix et Rhizoma Ginseng, Radix Scutellariae and Concha Ostreae. According to traditional Chinese medicine theory, Flos Chrysanthemi, Radix Saposhnikoviae, Ramulus Cinnamomi, Rhizoma Chuanxiong, Radix et Rhizoma Asari and Radix Platycodonis are wind-dispelling drugs; Rhizoma Atractylodis macrocephalae, Poria, Rhizoma Zingiberis, Radix Angelicae sinensis and Radix et Rhizoma Ginseng are deficiency-nourishing drugs. A large number of randomized controlled trials have shown that Houshiheisan is effective in treating stroke, but its mechanism of action is unknown. Axonal remodeling is an important mechanism in neural protection and regeneration. Therefore, this study explored the effect and mechanism of action of Houshiheisan on the repair of axons after cerebral ischemia. Rat models of focal cerebral ischemia were established by ligating the right middle cerebral artery. At 6 hours after model establishment, rats were intragastrically administered 10.5 g/kg Houshiheisan or 7.7 g/kg wind-dispelling drug or 2.59 g/kg deficiency-nourishing drug. These medicines were intragastrically administered as above every 24 hours for 7 consecutive days. Houshiheisan, and its wind-dispelling and deficiency-nourishing components reduced the neurological deficit score and ameliorated axon and neuron lesions after cerebral ischemia. Furthermore, Houshiheisan, and its wind-dispelling and deficiency-nourishing components decreased the expression of proteins that inhibit axonal remodeling: amyloid precursor protein, neurite outgrowth inhibitor protein A (Nogo-A), Rho family small GTPase A (RhoA) and Rho-associated kinase 2 (Rock2), and increased the expression of growth associated protein-43, microtubule-associated protein-2, netrin-1, Ras-related C3 botulinum toxin substrate 1 (Rac1) and cell division cycle 42 (Cdc42). The effect of Houshiheisan was stronger than wind-dispelling drugs or deficiency-nourishing drugs alone. In conclusion, Houshiheisan, and wind-dispelling and deficiency-nourishing drugs promote the repair of axons and nerve regeneration after cerebral ischemia through Nogo-A/RhoA/Rock2 and Netrin-1/Rac1/Cdc42 signaling pathways. These effects are strongest with Houshiheisan.

Sphingosine-1-Phosphate and the S1P3 Receptor Initiate Neuronal Retraction via RhoA/ROCK Associated with CRMP2 Phosphorylation

The bioactive lipid sphingosine-1-phosphate (S1P) is an important regulator in the nervous system. Here, we explored the role of S1P and its receptors in vitro and in preclinical models of peripheral nerve regeneration. Adult sensory neurons and motor neuron-like cells were exposed to S1P in an in vitro assay, and virtually all neurons responded with a rapid retraction of neurites and growth cone collapse which were associated with RhoA and ROCK activation. The S1P1 receptor agonist SEW2871 neither activated RhoA or neurite retraction, nor was S1P-induced neurite retraction mitigated in S1P1-deficient neurons. Depletion of S1P3 receptors however resulted in a dramatic inhibition of S1P-induced neurite retraction and was on the contrary associated with a significant elongation of neuronal processes in response to S1P. Opposing responses to S1P could be observed in the same neuron population, where S1P could activate S1P1 receptors to stimulate elongation or S1P3 receptors and retraction. S1P was, for the first time in sensory neurons, linked to the phosphorylation of collapsin response-mediated protein-2 (CRMP2), which was inhibited by ROCK inhibition. The improved sensory recovery after crush injury further supported the relevance of a critical role for S1P and receptors in fine-tuning axonal outgrowth in peripheral neurons.

Chondroitin Sulfate Impairs Neural Stem Cell Migration Through ROCK Activation

Brain injuries such as trauma and stroke lead to glial scar formation by reactive astrocytes which produce and secret axonal outgrowth inhibitors. Chondroitin sulfate proteoglycans (CSPG) constitute a well-known class of extracellular matrix molecules produced at the glial scar and cause growth cone collapse. The CSPG glycosaminoglycan side chains composed of chondroitin sulfate (CS) are responsible for its inhibitory activity on neurite outgrowth and are dependent on RhoA activation. Here, we hypothesize that CSPG also impairs neural stem cell migration inhibiting their penetration into an injury site. We show that DCX+ neuroblasts do not penetrate a CSPG-rich injured area probably due to Nogo receptor activation and RhoA/ROCK signaling pathway as we demonstrate in vitro with neural stem cells cultured as neurospheres and pull-down for RhoA. Furthermore, CS-impaired cell migration in vitro induced the formation of large mature adhesions and altered cell protrusion dynamics. ROCK inhibition restored migration in vitro as well as decreased adhesion size.

Schwann Cell Expressed Nogo-B Modulates Axonal Branching of Adult Sensory Neurons Through the Nogo-B Receptor NgBR

In contrast to the central nervous system (CNS) nerve fibers do regenerate in the peripheral nervous system (PNS) although in a clinically unsatisfying manner. A major problem is excessive sprouting of regenerating axons which results in aberrant reinnervation of target tissue and impaired functional recovery. In the CNS, the reticulon protein Nogo-A has been identified as a prominent oligodendrocyte expressed inhibitor of long-distance growth of regenerating axons. We show here that the related isoform Nogo-B is abundantly expressed in Schwann cells in the PNS. Other than Nogo-A in oligodendrocytes, Nogo-B does not localize to the myelin sheath but is detected in the ER and the plasma membrane of Schwann cells. Adult sensory neurons that are cultured on nogo-a/b deficient Schwann cells form significantly fewer axonal branches vs. those on wildtype Schwann cells, while their maximal axonal extension is unaffected. We demonstrate that this effect of Nogo-B on neuronal morphology is restricted to undifferentiated Schwann cells and is mediated by direct physical contact between these two cell types. Moreover, we show that blocking the Nogo-B specific receptor NgBR, which we find expressed on sensory neurons and to interact with Schwann cell expressed Nogo-B, produces the same branching phenotype as observed after deletion of Nogo-B. These data provide evidence for a novel function of the nogo gene that is implemented by the Nogo-B isoform. The remarkably specific effects of Nogo-B/NgBR on axonal branching, while leaving axonal extension unaffected, are of potential clinical relevance in the context of excessive axonal sprouting after peripheral nerve injury.

Heterodimerization of p45-p75 modulates p75 signaling: structural basis and mechanism of action

The formation of a p45-p75 heterodimer overrides p75’s inhibition of nerve regeneration by stopping p75 homodimers from forming and creating a complex with the Nogo receptor.

The p75 neurotrophin receptor, a member of the tumor necrosis factor receptor superfamily, is required as a co-receptor for the Nogo receptor (NgR) to mediate the activity of myelin-associated inhibitors such as Nogo, MAG, and OMgp. p45/NRH2/PLAIDD is a p75 homologue and contains a death domain (DD). Here we report that p45 markedly interferes with the function of p75 as a co-receptor for NgR. P45 forms heterodimers with p75 and thereby blocks RhoA activation and inhibition of neurite outgrowth induced by myelin-associated inhibitors. p45 binds p75 through both its transmembrane (TM) domain and DD. To understand the underlying mechanisms, we have determined the three-dimensional NMR solution structure of the intracellular domain of p45 and characterized its interaction with p75. We have identified the residues involved in such interaction by NMR and co-immunoprecipitation. The DD of p45 binds the DD of p75 by electrostatic interactions. In addition, previous reports suggested that Cys257 in the p75 TM domain is required for signaling. We found that the interaction of the cysteine 58 of p45 with the cysteine 257 of p75 within the TM domain is necessary for p45–p75 heterodimerization. These results suggest a mechanism involving both the TM domain and the DD of p45 to regulate p75-mediated signaling.

Injuries to the brain and spinal cord often result in paralysis due to the fact that the injured nerves cannot regrow to reach their normal targets and carry out their functions. At the injury sites, there are proteins released from the damaged myelin that bind the Nogo receptor (NgR) on the nerve and inhibit its regeneration. The NgR needs to form a complex with the p75 neurotrophin receptor in order to mediate this inhibitory signal. Here we found that p45, a homologue of p75, can also bind to p75 and block its inhibitory activity when overexpressed. To perform its function, p75 needs to dimerize through both its transmembrane and intracellular domains, facilitating the recruitment of several proteins. Our structural and functional studies show that p45 binds specifically to conserved regions in the p75 transmembrane and the intracellular domain and that this blocks p75 dimerization along with its downstream signaling. Thus, this study demonstrates that altering the oligomerization of p75 is a good strategy to override p75's inhibitory effects on nerve regeneration, and it opens the door for the design of specific p75 inhibitors for therapeutic applications.

Nogo receptor homolog NgR2 expressed in sensory DRG neurons controls epidermal innervation by interaction with Versican

Primary sensory afferents of the dorsal root ganglion (DRG) that innervate the skin detect a wide range of stimuli, such as touch, temperature, pain, and itch. Different functional classes of nociceptors project their axons to distinct target zones within the developing skin, but the molecular mechanisms that regulate target innervation are less clear. Here we report that the Nogo66 receptor homolog NgR2 is essential for proper cutaneous innervation. NgR2(-/-) mice display increased density of nonpeptidergic nociceptors in the footpad and exhibit enhanced sensitivity to mechanical force and innocuous cold temperatures. These sensory deficits are not associated with any abnormality in morphology or density of DRG neurons. However, deletion of NgR2 renders nociceptive nonpeptidergic sensory neurons insensitive to the outgrowth repulsive activity of skin-derived Versican. Biochemical evidence shows that NgR2 specifically interacts with the G3 domain of Versican. The data suggest that Versican/NgR2 signaling at the dermo-epidermal junction acts in vivo as a local suppressor of axonal plasticity to control proper density of epidermal sensory fiber innervation. Our findings not only reveal the existence of a novel and unsuspected mechanism regulating epidermal target innervation, but also provide the first evidence for a physiological role of NgR2 in the peripheral nervous system.

Rho-independent stimulation of axon outgrowth and activation of the ERK and Akt signaling pathways by C3 transferase in sensory neurons

Peripheral nerve injury triggers the activation of RhoA in spinal motor and peripheral sensory neurons. RhoA activates a number of effector proteins including the Rho-associated kinase, ROCK, which targets the cytoskeleton and leads to inhibition of neurite outgrowth. Blockade of the Rho/ROCK pathway by pharmacological means improves axon regeneration after experimental injury. C3_bot transferase, an exoenzyme produced by Clostridium botulinum, inactivates RhoA by ADP-ribosylation. It has been successfully applied in experimental CNS lesions to facilitate axon regeneration. Up to now it was not investigated thoroughly whether C3_bot exerts positive effects on peripheral axon regeneration as well. In the present study, recombinant membrane permeable C3_bot produced a small, but significant, axon outgrowth effect on peripheral sensory neurons dissociated from adult dorsal root ganglia (DRG) of the rat. Neuronal overexpression of C3, however, did not enhance axonal growth. Moreover, transfection of plasmids encoding dominant negative RhoA or RhoA specific shRNAs failed to increase axonal growth. Furthermore, we show that the C3_bot mutant, C3_E174Q, which lacks RhoA inhibitory activity, still stimulates axonal growth. When analyzing possible signaling mechanisms we found that extracellular signal-regulated kinase (ERK) and Akt are activated by C3_bot and ERK is induced by the C3_E174Q mutant. Upregulation of kinase activities by C3_bot occurs significantly faster than inactivation of RhoA indicating a RhoA-independent pathway of action by C3_bot. The induction of ERK signaling by C3_bot was detected in embryonic hippocampal neurons, too. Taken together, although RhoA plays a central role for inhibition of axon outgrowth by myelin-derived inhibitors, it does not interfere with axonal growth of sensory neurons on a permissive substrate in vitro. C3_bot blocks neuronal RhoA activity, but its positive effects on axon elongation and branching appear to be mediated by Rho independent mechanisms involving activation of axon growth promoting ERK and Akt kinases.

NgR1 and NgR3 are receptors for chondroitin sulfate proteoglycans

In the adult mammalian CNS, chondroitin sulfate proteoglycans (CSPGs) and myelin–associated inhibitors (MAIs) stabilize neuronal structure and restrict compensatory sprouting following injury. The Nogo receptor family members NgR1 and NgR2 bind to MAIs and have been implicated in neuronal inhibition. Here we show that NgR1 and NgR3 bind with high–affinity to the glycosaminoglycan moiety of proteoglycans and participate in CSPG inhibition in cultured neurons. Nogo receptor triple mutants (NgR123^−/−), but not single mutants, show enhanced axonal regeneration following retro–orbital optic nerve crush injury. The combined loss of NgR1 and NgR3 (NgR13^−/−), but not NgR1 and NgR2 (NgR12^−/−), is sufficient to mimic the NgR123^−/− regeneration phenotype. Regeneration in NgR13^−/− mice is further enhanced by simultaneous ablation of RPTPσ, a known CSPG receptor. Collectively, these results identify NgR1 and NgR3 as novel CSPG receptors, demonstrate functional redundancy among CSPG receptors, and provide unexpected evidence for shared mechanisms of MAI and CSPG inhibition.

Chondroitin sulfate proteoglycans down-regulate spine formation in cortical neurons by targeting tropomyosin-related kinase B (TrkB) protein

Chondroitin sulfate proteoglycans (CSPGs) are components of the extracellular matrix that inhibit axonal sprouting and experience-dependent plasticity. Although protein-tyrosine phosphatase σ (PTPσ) has been proven to be a receptor for CSPGs, its downstream signaling has remained a mystery. Here, we show that CSPGs target and dephosphorylate tropomyosin-related kinase B, the receptor of brain-derived neurotrophic factor (BDNF), via PTPσ in embryonic cortical neurons in vitro. Whereas BDNF promoted dendritic spine formation in embryonic cortical neurons, CSPGs abolished the effects of BDNF and eliminated existing dendritic spines when BDNF was present. The latter effect was dependent on the p75 receptor, presumably because BDNF binding to the p75 receptor elicits elimination of dendritic spines. These results suggest that the inhibitory activity of CSPGs on dendritic spine formation operates through the targeting of neurotrophins at the receptor level.

Minimal essential length of Clostridium botulinum C3 peptides to enhance neuronal regenerative growth and connectivity in a non-enzymatic mode

C3 ADP-ribosyltransferase is a valuable tool to study Rho-dependent cellular processes. In the current study we investigated the impact of enzyme-deficient peptides derived from Clostridium botulinum C3 transferase in the context of neuronal process elongation and branching, synaptic connectivity, and putative beneficial effects on functional outcome following traumatic injury to the CNS. By screening a range of peptidic fragments, we identified three short peptides from C3bot that promoted axon and dendrite outgrowth in cultivated hippocampal neurons. Furthermore, one of these fragments, a 26-amino acid peptide covering the residues 156-181 enhanced synaptic connectivity in primary hippocampal culture. This peptide was also effective to foster axon outgrowth and re-innervation in organotypical brain slice culture. To evaluate the potential of the 26mer to foster repair mechanisms after CNS injury we applied this peptide to mice subjected to spinal cord injury by either compression impact or hemisection. A single local administration at the site of the lesion improved locomotor recovery. In addition, histological analysis revealed an increased serotonergic input to lumbar motoneurons in treated compared with control mice. Pull-down assays showed that lesion-induced up-regulation of RhoA activity within the spinal cord was largely blocked by C3bot peptides despite the lack of enzymatic activity.

The challenges of long-distance axon regeneration in the injured CNS

Injury to the central nervous system (CNS) that results in long-tract axonal damage typically leads to permanent functional deficits in areas innervated at, and below, the level of the lesion. The initial ischemia, inflammation, and neurodegeneration are followed by a progressive generation of scar tissue, dieback of transected axons, and demyelination, creating an area inhibitory to regrowth and recovery. Two ways to combat this inhibition is to therapeutically target the extrinsic and intrinsic properties of the axon-scar environment. Scar tissue within and surrounding the lesion site can be broken down using an enzyme known as chondroitinase. Negative regulators of adult neuronal growth, such as Nogo, can be neutralized with antibodies. Both therapies greatly improve functional recovery in animal models. Alternatively, modifying the intrinsic growth properties of CNS neurons through gene therapy or pharmacotherapy has also shown promising axonal regeneration after injury. Despite these promising therapies, the main challenge of long-distance axon regeneration still remains; achieving a level of functional and organized connectivity below the level of the lesion that mimics the intact CNS.

Role of engineered nanocarriers for axon regeneration and guidance: current status and future trends

There are approximately 1.5 million people who experience traumatic injuries to the brain and 265,000 who experience traumatic injuries to the spinal cord each year in the United States. Currently, there are few effective treatments for central nervous system (CNS) injuries because the CNS is refractory to axonal regeneration and relatively inaccessible to many pharmacological treatments. Smart, remotely tunable, multifunctional micro- and nanocarriers hold promise for delivering treatments to the CNS and targeting specific neurons to enhance axon regeneration and synaptogenesis. Furthermore, assessing the efficacy of treatments could be enhanced by biocompatible nanovectors designed for imaging in vivo. Recent developments in nanoengineering offer promising alternatives for designing biocompatible micro- and nanovectors, including magnetic nanostructures, carbon nanotubes, and quantum dot-based systems for controlled release of therapeutic and diagnostic agents to targeted CNS cells. This review highlights recent achievements in the development of smart nanostructures to overcome the existing challenges for treating CNS injuries.

Combinatorial therapy stimulates long-distance regeneration, target reinnervation, and partial recovery of vision after optic nerve injury in mice

The optic nerve has been widely studied for insights into mechanisms that suppress or promote axon regeneration after central nervous system injury. Following optic nerve damage in adult mammals, retinal ganglion cells (RGCs) normally fail to regenerate their axons, resulting in blindness in patients who suffer from neurodegenerative diseases such as glaucoma or who have sustained traumatic injury to the optic nerve. Over the past several decades, many groups have investigated the basis of regenerative failure in the hope of developing strategies to stimulate the regrowth of axons and restore visual function. New findings show that a combination of therapies that act synergistically to activate RGCs' intrinsic growth state enables these cells to regenerate their axons the full length of the optic nerve, across the optic chiasm, and into the brain, where they establish synapses in appropriate target zones and restore limited visual responses. These treatments involve the induction of a limited inflammatory response in the eye to increase levels of oncomodulin and other growth factors; elevation of intracellular cAMP; and deletion of the pten gene in RGCs. Although these methods cannot be applied in the clinic, they point to strategies that might be.

Rho/ROCK pathway and neural regeneration: a potential therapeutic target for central nervous system and optic nerve damage

Rho-associated kinase (ROCK) is a serine/threonine kinase and one of the major downstream effectors of the small GTPase RhoA. The Rho/ROCK pathway is closely related to the pathogenesis of several central nervous system (CNS) disorders, and involved in many aspects of neuronal functions including neurite outgrowth and retraction. In the adult CNS, the damaged neuron regeneration is very difficult due to the presence of myelin-associated axon growth inhibitors such as Nogo, myelin-associated glycoprotein (MAG) and oligodendrocyte-myelin glycoprotein (Omgp), etc. The effects of these axon growth inhibitors are reversed by blocking the Rho/ROCK pathway in vitro, and the inhibition of Rho/ROCK pathway can promote axon regeneration and functional recovery in the injured CNS in vivo. In addition, the therapeutic effects of the Rho/ROCK inhibitors have also been demonstrated in some animal models and the Rho/ROCK pathway becomes an attractive target for the development of drugs for treating CNS disorders. In this review, we summarized on the effect of the Rho and the downstream factor ROCK in neural regeneration, and the potential therapeutic effect of Rho/ROCK inhibitors in the survival and axonal regeneration of retinal ganglion cells was also discussed.

Rho GTPases as regulators of morphological neuroplasticity

GTPases function as intracellular, bimolecular switches by adopting different conformational states in response to binding GDP or GTP. Their activation is mediated through cell-surface receptors. Rho GTPases act on several downstream effectors involved in cellular morphogenesis, cell polarity, migration and cell division. In neurons, Rho GTPases regulate various features of dendritic and axonal outgrowth during development and regeneration mainly through their effects on the cytoskeleton. This review summarizes the main functions of Rho, Rac and Cdc42 GTPases as key regulators of morphological neuroplasticity under normal and pathological conditions.

C3 peptide enhances recovery from spinal cord injury by improved regenerative growth of descending fiber tracts

Functional recovery and regeneration of corticospinal tract (CST) fibers following spinal cord injury by compression or dorsal hemisection in mice was monitored after application of the enzyme-deficient Clostridium botulinum C3-protein-derived 29-amino-acid fragment C3bot154-182. This peptide significantly improved locomotor restoration in both injury models as assessed by the open-field Basso Mouse Scale for locomotion test and Rotarod treadmill experiments. These data were supported by tracing studies showing an enhanced regenerative growth of CST fibers in treated animals as visualized by anterograde tracing. Additionally, C3bot154-182 stimulated regenerative growth of raphespinal fibers and improved serotonergic input to lumbar α-motoneurons. These in vivo data were confirmed by in vitro data, showing an enhanced axon outgrowth of α-motoneurons and hippocampal neurons cultivated on normal or growth-inhibitory substrates after application of C3bot154-182. The observed effects were probably caused by a non-enzymatic downregulation of active RhoA by the C3 peptide as indicated by pull-down experiments. By contrast, C3bot154-182 did not induce neurite outgrowth in primary cultures of dorsal root ganglion cells. In conclusion, C3bot154-182 represents a novel, promising tool to foster axonal protection and/or repair, as well as functional recovery after traumatic CNS injury.

Neurites regrowth of cortical neurons by GSK3beta inhibition independently of Nogo receptor 1

Lesioned axons do not regenerate in the adult mammalian CNS, owing to the over-expression of inhibitory molecules such as myelin-derived proteins or chondroitin sulphate proteoglycans. In order to overcome axon inhibition, strategies based on extrinsic and intrinsic treatments have been developed. For myelin-associated inhibition, blockage with NEP1-40, receptor bodies or IN-1 antibodies has been used. In addition, endogenous blockage of cell signalling mechanisms induced by myelin-associated proteins is a potential tool for overcoming axon inhibitory signals. We examined the participation of glycogen synthase kinase 3beta (GSK3beta) and extracellular-related kinase (ERK) 1/2 in axon regeneration failure in lesioned cortical neurons. We also investigated whether pharmacological blockage of GSK3beta and ERK1/2 activities facilitates regeneration after myelin-directed inhibition in two models: (i) cerebellar granule cells and (ii) lesioned entorhino-hippocampal pathway in slice cultures, and whether the regenerative effects are mediated by Nogo Receptor 1 (NgR1). We demonstrate that, in contrast to ERK1/2 inhibition, the pharmacological treatment of GSK3beta inhibition strongly facilitated regrowth of cerebellar granule neurons over myelin independently of NgR1. Finally, these regenerative effects were corroborated in the lesioned entorhino-hippocampal pathway in NgR1-/- mutant mice. These results provide new findings for the development of new assays and strategies to enhance axon regeneration in injured cortical connections.

Protein microarray analysis identifies cyclic nucleotide phosphodiesterase as an interactor of Nogo-A

Nogo-A, a neurite outgrowth inhibitor, is expressed exclusively on oligodendrocytes and neurons in the CNS. The central domain of Amino-Nogo spanning amino acids 567-748 in the human Nogo-A designated NIG, mediates persistent inhibition of axonal outgrowth and induces growth cone collapse by signaling through an as yet unidentified NIG receptor. We identified 82 NIG-interacting proteins by screening a high-density human protein microarray composed of 5000 proteins with a recombinant NIG protein as a probe. Following an intensive database search, we selected 12 neuron/oligodendrocyte-associated NIG interactors. Among them, we verified the molecular interaction of NIG with 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNP), a cell type-specific marker of oligodendrocytes, by immunoprecipitation and cell imaging analysis. Although CNP located chiefly in the cytoplasm of oligodendrocytes might not serve as a cell-surface NIG receptor, it could act as a conformational stabilizer for the intrinsically unstructured large segment of Amino-Nogo.

Overcoming amino-Nogo-induced inhibition of cell spreading and neurite outgrowth by 12-O-tetradecanoylphorbol-13-acetate-type tumor promoters

The N-terminal domain of NogoA, called amino-Nogo, inhibits axonal outgrowth and cell spreading via a largely unknown mechanism. In the present study, we show that amino-Nogo decreases Rac1 activity and inhibits fibroblast spreading. 12-O-Tetradecanoylphorbol-13-acetate-type tumor promoters, such as phorbol 12-myristate 13-acetate (PMA) and teleocidin, increase Rac1 activity and overcome the amino-Nogo-induced inhibition of cell spreading. The stimulating effect of tumor promoters on cell spreading requires activation of protein kinase D and the subsequent activation of Akt1. Furthermore, we identified Akt1 as a new signaling component of the amino-Nogo pathway. Akt1 phosphorylation is decreased by amino-Nogo. Activation of Akt1 with a cell-permeable peptide, TAT-TCL1, blocks the amino-Nogo inhibition. Finally, we provide evidence that these signaling pathways operate in neurons in addition to fibroblasts. Our results suggest that activation of protein kinase D and Akt1 are approaches to promote axonal regeneration after injury.

Receptors for myelin inhibitors: Structures and therapeutic opportunities

Many studies have indicated that the inability of adult mammalian central nervous system (CNS) to regenerate after injury is partly due to the existence of growth-inhibitory molecules associated with CNS myelin. Studies over the years have led to the identification of multiple myelin-associated inhibitors, among which Nogo, myelin-associated glycoprotein (MAG) and oligodendrocyte-myelin glycoprotein (Omgp) represent potentially major contributors to CNS axon regeneration failure. Here we review in vitro and in vivo investigations into these inhibitory ligands and their functional mechanisms, focusing particularly on the neuronal receptors that mediate the inhibitory signals from these myelin molecules. A better understanding of the receptors for myelin-associated inhibitors could provide opportunities to decipher the mechanism of restriction in CNS regeneration, and lead to the development of potential therapeutic targets in neurodegenerative diseases and neurological injury. We will discuss the structures of the receptors and therapeutic opportunities that might arise based on this information.

Optimisation of siRNA-mediated RhoA silencing in neuronal cultures

In investigating the consequences of gene silencing in axon growth disinhibition strategies in cultured retinal ganglion cells (RGC), we conducted experiments designed to silence RhoA signalling in PC12 and primary adult rat retinal cell cultures (containing RGC) by siRNA-mediated RhoA mRNA knockdown. We demonstrate wide differences in the levels of RhoA mRNA knockdown, dose-dependent cell toxicity, and induction of endogenous inflammatory cytokine and interferon responses to siRNA therapy. Toxicity effects observed with RhoA-siRNA was significantly reduced with "Stealth" chemical modification of the sequence, promoting approximately 50% and 70% knockdown of RhoA mRNA and protein in retinal cells, respectively, while promoting significant disinhibited RGC neurite outgrowth in the presence of inhibitory CNS myelin. Our results highlight differential responsiveness of cell lines compared to primary cultured cells, and demonstrate the efficacy of the "Stealth" modification to reduce siRNA-induced interferon responses, thereby increasing target cell viability and reducing off-target effects of the delivered nucleic acids.

Effect of the oligodendrocyte myelin glycoprotein (OMgp) on the expansion and neuronal differentiation of rat neural stem cells

The oligodendrocyte myelin glycoprotein (OMgp) inhibits axon regeneration after injury in the adult mammalian central nervous system. However its function during brain development remains largely unknown. The present study aims to analyze a possible role for OMgp during neurogenesis. We showed that neural stem cells (NSC) extracted from the whole mesencephalon of rat embryos (E14) and cultured as free floating neurospheres expressed both OMgp and its receptor Nogo-R1. An over-expression of OMgp affected NSC expansion by reducing cell proliferation, but did not affect their differentiation into neurons. These findings indicate a new role for OMgp during brain development as a possible regulator of neurogenesis. Moreover, they suggest a possible implication for OMG gene in the etiology of neurofibromatosis type 1 forms characterized by a deletion of the NF1 gene locus containing OMG.

Inhibitory activity of myelin-associated glycoprotein on sensory neurons is largely independent of NgR1 and NgR2 and resides within Ig-Like domains 4 and 5

Myelin-associated glycoprotein (MAG) is a sialic acid binding Ig-like lectin (Siglec) which has been characterized as potent myelin-derived inhibitor of neurite outgrowth. Two members of the Nogo-receptor (NgR) family, NgR1 and NgR2, have been identified as neuronal binding proteins of MAG. In addition, gangliosides have been proposed to bind to and confer the inhibitory activity of MAG on neurons. In this study, we investigated the individual contribution of NgRs and gangliosides to MAG-mediated inhibition of sensory neurons derived from dorsal root ganglia (DRG) of ngr1, ngr2 or ngr1/ngr2 deletion mutants. We found no disinhibition of neurite growth in the absence of either NgR1 or NgR2. Sensory neurons deficient for both NgR proteins displayed only a moderate reduction of MAG-mediated inhibition of neurite growth. If treated with Vibrio cholerae neuraminidase (VCN), inhibition by MAG is further attenuated but still not annulled. Thus, disrupting all known protein and ganglioside receptors for MAG in sensory neurons does not fully abolish its inhibitory activity pointing to the existence of as yet unidentified receptors for MAG. Moreover, by employing a variety of protein mutants, we identified the Ig-like domains 4 or 5 of MAG as necessary and sufficient for growth arrest, whereas abolishing MAG's ability to bind to sialic acid did not interfere with its inhibitory activity. These findings provide new insights into the inhibitory function of MAG and suggest similarities but also major differences in MAG inhibition between sensory and central nervous system (CNS) neurons.

Loss of function genetic screens reveal MTGR1 as an intracellular repressor of beta1 integrin-dependent neurite outgrowth

Integrins are transmembrane receptors that promote neurite growth and guidance. To identify regulators of integrin-dependent neurite outgrowth, here we used two loss of function genetic screens in SH-SY5Y neuroblastoma cells. First, we screened a genome-wide retroviral library of genetic suppressor elements (GSEs). Among the many genes identified in the GSE screen, we isolated the hematopoetic transcriptional factor MTGR1 (myeloid translocation gene-related protein-1). Treatment of SH-SY5Y cells with MTGR1 siRNA enhanced neurite outgrowth and concurrently increased expression of GAP-43, a protein linked to neurite outgrowth. Second, we transduced SH-SY5Y with a genome-wide GFP-labeled lentiviral siRNA library, which expressed 40,000 independent siRNAs targeting 8500 human genes. From this screen we isolated GFI1 (growth factor independence-1), which, like MTGR1, is a member of the myeloid translocation gene on 8q22 (MTG8)/ETO protein complex of nuclear repressor proteins. These results reveal novel contributions of MTGR1 and GFI1 to the regulation of neurite outgrowth and identify novel repressors of integrin-dependent neurite outgrowth.

Emerging cues mediating astroglia lineage restriction of progenitor cells in the injured/diseased adult CNS

Other than specific neurogenic regions, the adult central nervous system (CNS) is not conducive for neuronal regeneration and neurogenesis, particularly at sites of injury or neurodegeneration. Engraftment of neural stem/progenitor cells into non-neurogenic regions or sites of injury/disease invariably results mainly in astroglia differentiation. The reasons for such a lineage restriction have not been well defined. Recent findings have brought to light some underlying novel mechanistic basis for this preferential differentiation into astroglia. The more oxidized state of pathological brain tissue leads to upregulation of the protein deacetylase sirtuin 1 (Sirt1). Sirt1 appears to stabilize a co-repressor complex of Hairy/enhancer of split (Hes)1, thereby suppressing expression of the proneuronal transcription factor Mash1, and directs progenitor cell differentiation towards the glia lineage. Sirt1 upregulated by CNS inflammation may also inhibit neuronal differentiation. Myelin-associated inhibitors such as Nogo, acting through the Nogo-66 receptor (NgR), also appear to promote neural stem/progenitor cell differentiation into astrocytes. Understanding the molecular basis of glia lineage restriction of neural progenitors in the injured or diseased CNS would provide handles to improving the success of stem cell-based transplantation therapy.

Aggrecan components differentially modulate nerve growth factor-responsive and neurotrophin-3-responsive dorsal root ganglion neurite growth

Aggrecan is one of the major chondroitin sulfate proteoglycans (CSPGs) expressed in the central nervous system. The signaling pathways activated downstream of cell interaction with aggrecan and with CSPGs in general and the importance of chondroitin sulfate-glycosaminoglycan side chains in their inhibition are unclear. Therefore, to analyze the effect of different components of aggrecan in inhibiting neurite growth, neurite outgrowth was quantified in an in vitro model in which chick dorsal root ganglion (DRG) explants were grown on substrates containing aggrecan bound to hyaluronan and link protein as a macromolecular aggregate, aggrecan monomers, hyaluronan, or ChABC-treated aggrecan. Aggrecan aggregate, aggrecan monomer, and hyaluronan inhibited neurite outgrowth from nerve growth factor (NGF)- and neurotrophin-3 (NT3)-responsive DRG neurons. Aggrecan inhibition was dependent on its chondroitin sulfate-glycosaminoglycans, as ChABC digestion alleviated neurite inhibition because of aggrecan. Growth cones displayed full or partial collapse on aggrecan aggregate, hyaluronan, and ChABC-treated aggrecan. Inhibition of Rho kinase (ROCK) with Y27632 increased neurite growth on some but not all of the aggrecan components tested. With NGF in the culture medium, Y27632 increased neurite outgrowth on aggrecan aggregate, monomers, and ChABC-treated aggrecan, but not on hyaluronan. The ROCK inhibitor also increased NT3-responsive outgrowth on aggrecan aggregate and hyaluronan, but not on ChABC-treated aggrecan. This study showed that the matrix proteoglycan aggrecan and its components have multiple effects on neurite outgrowth and that some of these effects involve the Rho/ROCK pathway.

Reinduction of ErbB2 in astrocytes promotes radial glial progenitor identity in adult cerebral cortex

Radial glial cells play a critical role in the construction of mammalian brain by functioning as a source of new neurons and by providing a scaffold for radial migration of new neurons to their target locations. Radial glia transform into astrocytes at the end of embryonic development. Strategies to promote functional recovery in the injured adult brain depend on the generation of new neurons and the appropriate guidance of these neurons to where they are needed, two critical functions of radial glia. Thus, the competence to regain radial glial identity in the adult brain is of significance for the ability to promote functional repair via neurogenesis and targeted neuronal migration in the mature brain. Here we show that the in vivo induction of the tyrosine kinase receptor, ErbB2, in mature astrocytes enables a subset of them to regain radial glial identity in the mature cerebral cortex. These new radial glial progenitors are capable of giving rise to new neurons and can support neuronal migration. These studies indicate that ErbB2 signaling critically modulates the functional state of radial glia, and induction of ErbB2 in distinct adult astrocytes can promote radial glial identity in the mature cerebral cortex.

Inhibition of oligodendrocyte precursor cell differentiation by myelin-associated proteins

Object

Promoting repair of central nervous system (CNS) white matter represents an important approach to easing the course of a number of tragic neurological diseases. For this purpose, strategies are currently being evaluated for transplanting cells capable of generating new oligodendrocytes into areas of demyelination and/or enhancing the potential of endogenous stem/precursor cells to give rise to new oligodendrocytes. Emerging evidence, however, indicates that increasing the presence of cells capable of forming new myelin sheaths is not sufficient to promote repair because of unknown inhibitors that accumulate in lesions as a consequence of myelin degeneration and impair the generation of new oligodendrocytes. The aim of the present study was to characterize the nature of the inhibitory molecules present in myelin.

Methods

Differentiation of primary rat oligodendrocyte precursor cells (OPCs) in the presence of CNS and peripheral nervous system myelin was assessed by immunocytochemical methods. The authors further characterized the nature of the inhibitors by submitting myelin membrane preparations to biochemical precipitation and digestion. Finally, OPCs were grown on purified Nogo-A, oligodendrocyte myelin glycoprotein, and myelin-associated glycoprotein, the most prominent inhibitors of axon regeneration.

Results

Myelin membrane preparations induced a differentiation block in OPCs that was associated with down-regulation of expression of the transcription factor Nkx2.2. The inhibitory activity in myelin was restricted to the CNS and was predominantly associated with white matter. Furthermore, the results demonstrate that myelin proteins that are distinct from the most prominent inhibitors of axon outgrowth are specific inhibitors of OPC differentiation.

Conclusions

The inhibitory effect of unknown myelin-associated proteins should be considered in future treatment strategies aimed at enhancing CNS repair.

Disorganized microtubules underlie the formation of retraction bulbs and the failure of axonal regeneration

Axons in the CNS do not regrow after injury, whereas lesioned axons in the peripheral nervous system (PNS) regenerate. Lesioned CNS axons form characteristic swellings at their tips known as retraction bulbs, which are the nongrowing counterparts of growth cones. Although much progress has been made in identifying intracellular and molecular mechanisms that regulate growth cone locomotion and axonal elongation, a comprehensive understanding of how retraction bulbs form and why they are unable to grow is still elusive. Here we report the analysis of the morphological and intracellular responses of injured axons in the CNS compared with those in the PNS. We show that retraction bulbs of injured CNS axons increase in size over time, whereas growth cones of injured PNS axons remain constant. Retraction bulbs contain a disorganized microtubule network, whereas growth cones possess the typical bundling of microtubules. Using in vivo imaging, we find that pharmacological disruption of microtubules in growth cones transforms them into retraction bulb-like structures whose growth is inhibited. Correspondingly, microtubule destabilization of sensory neurons in cell culture induces retraction bulb formation. Conversely, microtubule stabilization prevents the formation of retraction bulbs and decreases axonal degeneration in vivo. Finally, microtubule stabilization enhances the growth capacity of CNS neurons cultured on myelin. Thus, the stability and organization of microtubules define the fate of lesioned axonal stumps to become either advancing growth cones or nongrowing retraction bulbs. Our data pinpoint microtubules as a key regulatory target for axonal regeneration.

Intrathecal treatment with anti-Nogo-A antibody improves functional recovery in adult rats after stroke

Stroke often results in devastating neurological disabilities with no specific treatment available to improve functional recovery. Neurite growth inhibitory proteins such as Nogo-A play a critical role in impeding regain of function after stroke. We have reported that treatment with anti-Nogo-A antibody using the intracerebroventricular route resulted in improvement of function and neuroplasticity in adult or aged rats after stroke. This present study tested a more clinically accessible route for applying anti-Nogo-A antibodies, the intrathecal route. Anti-Nogo-A or control antibody was administered intrathecally at lower lumbar levels 1 week after middle cerebral artery occlusion in adult rats. Our results show that anti-Nogo-A antibody delivered by this intrathecal route for 2 weeks penetrated into brain parenchyma and bound to myelin-enriched structures such as the corpus callosum and striatal white matter. Animals receiving anti-Nogo-A antibody treatment significantly improved recovery of function on the skilled forelimb reaching task as compared to stroke only and stroke/control antibody animals. These findings show that anti-Nogo-A antibody delivered through the intrathecal route is as effective in restoring lost functions after stroke as the intracerebroventricular route. This is of great importance for the future application of anti-Nogo-A immunotherapy for ischemic stroke treatment.

The pivotal role of RhoA GTPase in the molecular signaling of axon growth inhibition after CNS injury and targeted therapeutic strategies

The dogma that the adult central nervous system (CNS) is nonpermissive to axonal regeneration is beginning to fall in the face of increased understanding of the molecular and cellular biology of axon outgrowth. It is now appreciated that axon growth is regulated by a combination of extracellular factors related to the milieu of the developing or adult CNS and the presence of injury, and intracellular factors related to the "growth state" of the developing or regenerating neuron. Several critical points of convergence within the developing or regenerating neuron for mediating intracellular cell signaling effects on the growth cone cytoskeleton have been identified, and their modulation has produced marked increases in axon outgrowth within the "nonpermissive" milieu of the adult injured CNS. One such critical convergence point is the small GTPase RhoA, which integrates signaling events produced by both myelin-associated inhibitors (e.g., NogoA) and astroglial-derived inhibitors (chondroitin sulfate proteoglycans) and regulates the activity of downstream effectors that modulate cytoskeletal dynamics within the growth cone mediating axon outgrowth or retraction. Inhibition of RhoA has been associated with increased outgrowth on nonpermissive substrates in vitro and increased axon regeneration in vivo. We are developing lentiviral vectors that modulate RhoA activity, allowing more long-term expression than is possible with current approaches. These vectors may be useful in regenerative strategies for spinal cord injury, brain injury, and neurodegenerative diseases including Parkinson's disease, Alzheimer's disease, and Huntington's disease.

Rewiring the injured CNS: lessons from the optic nerve

The optic nerve offers a number of advantages for investigating mechanisms that govern axon regeneration in the CNS. Although mature retinal ganglion cells (RGCs) normally show no ability to regenerate injured axons through the optic nerve, this situation can be partially reversed by inducing an inflammatory response in the eye. The secretion of a previously unknown growth factor, oncomodulin, along with co-factors, causes RGCs to undergo dramatic changes in gene expression and regenerate lengthy axons into the highly myelinated optic nerve. By themselves, strategies that counteract inhibitory signals associated with myelin and the glial scar are insufficient to promote extensive regeneration in this system. However, combinatorial treatments that activate neurons' intrinsic growth state and overcome inhibitory signals result in dramatic axon regeneration in vivo. Because of the ease of introducing trophic factors, soluble receptors, drugs, or viruses expressing any gene or small interfering RNA of interest into RGCs, this system is ideal for identifying intracellular signaling pathways, transcriptional cascades, and ligand-receptor interactions that enable axon regeneration to occur in the CNS.

Involvement of the myelin-associated inhibitor Nogo-A in early cortical development and neuronal maturation

Nogo-A is a myelin-associated protein expressed by neurons and myelinating mature oligodendrocytes in the central nervous system. Although most research has focused on the participation of Nogo-A in the prevention of axonal regeneration and plasticity in the adult, little attention has been paid to the putative functions of Nogo-A during embryonic development. Here we examined the general pattern and cell-specific distribution of Nogo-A in the prenatal mouse telencephalon. In addition, we studied the development of the major axon tracts and radial and tangential migration in Nogo-A/B/C knockout mice. The pattern of Nogo-A showed distinct distribution in radial glia and postmitotic neurons, in which it is particularly enriched in developing axons. Similarly, Nogo-A was enriched at the leading process of tangentially migrating interneurons but not detectable in radial migrating neurons. Although a low level of Nogo-A appears to be on the surface of many cortical neurons, most proteins have intracellular localization. In Nogo-deficient background, neurons displayed early polarization and increased branching in vitro, probably reflecting a cell-intrinsic role of Nogo proteins in branching reduction, and early tangential migration was delayed. On the basis of these observations, we propose that Nogo proteins, particularly Nogo-A, are involved in multiple processes during cortical development.

Competition and cooperation between tenascin-R, lecticans and contactin 1 regulate neurite growth and morphology

The extracellular matrix molecule tenascin-R (TN-R) and the proteoglycans of the lectican family show an overlapping distribution in the developing brain, have been implicated in similar cellular processes and form a complex network of interactions. Previously, we have demonstrated that TN-R induces microprocesses along neurites and enlarged growth cones of tectal cells by interacting with the cell adhesion molecule contactin 1. Here, we describe competition and cooperation between TN-R, lecticans and contactin 1, and their functional consequences for tectal cells. Aggrecan, brevican and neurocan inhibit the effects of TN-R on microprocess formation and growth cone size. This blocking effect is due to competition of lecticans with binding of TN-R to its neuronal receptor contactin 1, as shown by a sandwich-binding assay. Interaction of aggrecan with TN-R fibronectin type III domains 4-A is necessary for its inhibitory effect on both microprocess formation and TN-R binding to contactin 1. However, the chondroitin sulfate chains are not involved. Time-lapse video microscopy showed that aggrecan has no acute effect on motility and morphology of microprocesses and growth cones but induces long-term neurite retraction after pre-treatment with TN-R. In contrast to the competition described above, TN-R cooperates with brevican and neurocan to induce attachment of tectal cells and neurite outgrowth, probably by forming a bridge between the lectican substrate and contactin 1 as the neuronal receptor. Our findings suggest that a complex network of protein-protein interactions within the brain extracellular matrix, as shown here for TN-R and lecticans, is important for the fine-regulation of developmental processes such as microprocess formation along the neurite and neurite outgrowth.