Blockade of chondroitin sulfate proteoglycans-induced axonal growth inhibition by LOTUS

Nogo Receptor Antagonist LOTUS Promotes Neurite Outgrowth through Its Interaction with Teneurin-4

Neurite outgrowth is a crucial process for organizing neuronal circuits in neuronal development and regeneration after injury. Regenerative failure in the adult mammalian central nervous system (CNS) is attributed to axonal growth inhibitors such as the Nogo protein that commonly binds to Nogo receptor-1 (NgR1). We previously reported that lateral olfactory tract usher substance (LOTUS) functions as an endogenous antagonist for NgR1 in forming neuronal circuits in the developing brain and improving axonal regeneration in the adult injured CNS. However, another molecular and cellular function of LOTUS remains unknown. In this study, we found that cultured retinal explant neurons extend their neurites on the LOTUS-coating substrate. This action was also observed in cultured retinal explant neurons derived from Ngr1-deficient mouse embryos, indicating that the promoting action of LOTUS on neurite outgrowth may be mediated by unidentified LOTUS-binding protein(s). We therefore screened the binding partner(s) of LOTUS by using a liquid chromatography-tandem mass spectrometry (LC-MS/MS). LC-MS/MS analysis and pull-down assay showed that LOTUS interacts with Teneurin-4 (Ten-4), a cell adhesion molecule. RNAi knockdown of Ten-4 inhibited neurite outgrowth on the LOTUS substrate in retinoic acid (RA)-treated Neuro2A cells. Furthermore, a soluble form of Ten-4 attenuates the promoting action on neurite outgrowth in cultured retinal explant neurons on the LOTUS substrate. These results suggest that LOTUS promotes neurite outgrowth by interacting with Ten-4. Our findings may provide a new molecular mechanism of LOTUS to contribute to neuronal circuit formation in development and to enhance axonal regeneration after CNS injury.

Unveiling the modulation of Nogo receptor in neuroregeneration and plasticity: Novel aspects and future horizon in a new frontier

Neurodegenerative diseases (NDs) such as Alzheimer's, Parkinson's, Multiple Sclerosis, Hereditary Spastic Paraplegia, and Amyotrophic Lateral Sclerosis have emerged as the most dreaded diseases due to a lack of precise diagnostic tools and efficient therapies. Despite the fact that the contributing factors of NDs are still unidentified, mounting evidence indicates the possibility that genetic and cellular changes may lead to the significant production of abnormally misfolded proteins. These misfolded proteins lead to damaging effects thereby causing neurodegeneration. The association between Neurite outgrowth factor (Nogo) with neurological diseases and other peripheral diseases is coming into play. Three isoforms of Nogo have been identified Nogo-A, Nogo-B and Nogo-C. Among these, Nogo-A is mainly responsible for neurological diseases as it is localized in the CNS (Central Nervous System), whereas Nogo-B and Nogo-C are responsible for other diseases such as colitis, lung, intestinal injury, etc. Nogo-A, a membrane protein, had first been described as a CNS-specific inhibitor of axonal regeneration. Several recent studies have revealed the role of Nogo-A proteins and their receptors in modulating neurite outgrowth, branching, and precursor migration during nervous system development. It may also modulate or affect the inhibition of growth during the developmental processes of the CNS. Information about the effects of other ligands of Nogo protein on the CNS are yet to be discovered however several pieces of evidence have suggested that it may also influence the neuronal maturation of CNS and targeting Nogo-A could prove to be beneficial in several neurodegenerative diseases.

The NG2-glia is a potential target to maintain the integrity of neurovascular unit after acute ischemic stroke

The neurovascular unit (NVU) plays a critical role in health and disease. In the current review, we discuss the critical role of a class of neural/glial antigen 2 (NG2)-expressing glial cells (NG2-glia) in regulating NVU after acute ischemic stroke (AIS). We first introduce the role of NG2-glia in the formation of NVU during development as well as aging-induced damage to NVU and accompanying NG2-glia change. We then discuss the reciprocal interactions between NG2-glia and the other component cells of NVU, emphasizing the factors that could influence NG2-glia. Damage to the NVU integrity is the pathological basis of edema and hemorrhagic transformation, the most dreaded complication after AIS. The role of NG2-glia in AIS-induced NVU damage and the effect of NG2-glia transplantation on AIS-induced NVU damage are summarized. We next discuss the role of NG2-glia and the effect of NG2-glia transplantation in oligodendrogenesis and white matter repair as well as angiogenesis which is associated with the outcome of the patients after AIS. Finally, we review the current strategies to promote NG2-glia proliferation and differentiation and propose to use the dental pulp stem cells (DPSC)-derived exosome as a promising strategy to reduce AIS-induced injury and promote repair through maintaining the integrity of NVU by regulating endogenous NG2-glia proliferation and differentiation.

Brain-derived neurotrophic factor (BDNF) induces antagonistic action to Nogo signaling by the upregulation of lateral olfactory tract usher substance (LOTUS) expression

Neurons in the central nervous system (CNS) have limited capacity for axonal regeneration after trauma and neurological disorders due to an endogenous nonpermissive environment for axon regrowth in the CNS. Lateral olfactory tract usher substance (LOTUS) contributes to axonal tract formation in the developing brain and axonal regeneration in the adult brain as an endogenous Nogo receptor-1 (NgR1) antagonist. However, how LOTUS expression is regulated remains unclarified. This study examined molecular mechanism of regulation in LOTUS expression and found that brain-derived neurotrophic factor (BDNF) increased LOTUS expression in cultured hippocampal neurons. Exogenous application of BDNF increased LOTUS expression at both mRNA and protein levels in a dose-dependent manner. We also found that pharmacological inhibition with K252a and gene knockdown by siRNA of tropomyosin-related kinase B (TrkB), BDNF receptor suppressed BDNF-induced increase in LOTUS expression. Further pharmacological analysis of the TrkB signaling pathway revealed that BDNF increased LOTUS expression through mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) cascades, but not phospholipase C-γ (PLCγ) cascade. Additionally, treatment with c-AMP response element binding protein (CREB) inhibitor partially suppressed BDNF-induced LOTUS expression. Finally, neurite outgrowth assay in cultured hippocampal neurons revealed that BDNF treatment-induced antagonism for NgR1 by up-regulating LOTUS expression. These findings suggest that BDNF may acts as a positive regulator of LOTUS expression through the TrkB signaling, thereby inducing an antagonistic action for NgR1 function by up-regulating LOTUS expression. Also, BDNF may synergistically affect axon regrowth through the upregulation of LOTUS expression.

LOTUS suppresses amyloid β-induced dendritic spine elimination through the blockade of amyloid β binding to PirB

BACKGROUND

Alzheimer's disease (AD) is the most common neurodegenerative disease worldwide but has no effective treatment. Amyloid beta (Aβ) protein, a primary risk factor for AD, accumulates and aggregates in the brain of patients with AD. Paired immunoglobulin-like receptor B (PirB) has been identified as a receptor of Aβ and Aβ-PirB molecular interactions that cause synapse elimination and synaptic dysfunction. PirB deletion has been shown to suppress Aβ-induced synaptic dysfunction and behavioral deficits in AD model mice, implying that PirB mediates Aβ-induced AD pathology. Therefore, inhibiting the Aβ-PirB molecular interaction could be a successful approach for combating AD pathology. We previously showed that lateral olfactory tract usher substance (LOTUS) is an endogenous antagonist of type1 Nogo receptor and PirB and that LOTUS overexpression promotes neuronal regeneration following damage to the central nervous system, including spinal cord injury and ischemic stroke. Therefore, in this study, we investigated whether LOTUS inhibits Aβ-PirB interaction and Aβ-induced dendritic spine elimination.

METHODS

The inhibitory role of LOTUS against Aβ-PirB (or leukocyte immunoglobulin-like receptor subfamily B member 2: LilrB2) binding was assessed using a ligand-receptor binding assay in Cos7 cells overexpressing PirB and/or LOTUS. We assessed whether LOTUS inhibits Aβ-induced intracellular alterations and synaptotoxicity using immunoblots and spine imaging in a primary cultured hippocampal neuron.

RESULTS

We found that LOTUS inhibits the binding of Aβ to PirB overexpressed in Cos7 cells. In addition, we found that Aβ-induced dephosphorylation of cofilin and Aβ-induced decrease in post-synaptic density-95 expression were suppressed in cultured hippocampal neurons from LOTUS-overexpressing transgenic (LOTUS-tg) mice compared with that in wild-type mice. Moreover, primary cultured hippocampal neurons from LOTUS-tg mice improved the Aβ-induced decrease in dendritic spine density. Finally, we studied whether human LOTUS protein inhibits Aβ binding to LilrB2, a human homolog of PirB, and found that human LOTUS inhibited the binding of Aβ to LilrB2 in a similar manner.

CONCLUSIONS

This study implied that LOTUS improved Aβ-induced synapse elimination by suppressing Aβ-PirB interaction in rodents and inhibited Aβ-LilrB2 interaction in humans. Our findings revealed that LOTUS may be a promising therapeutic agent in counteracting Aβ-induced AD pathologies.

Axonal Regeneration Through Autologous Grafts: Does the Axonal Load Influence Regeneration?

INTRODUCTION

Two-stage free functional muscle transfers for long-standing facial palsy can yield unpredictable results. Earlier studies have demonstrated incomplete regeneration across neurorrhaphies in native nerve and higher donor axonal counts correlating with improved outcomes but axonal count in nerve grafts have not been as thoroughly reviewed. To investigate the impact of varying axonal counts in autologous grafts on functional outcomes of repair.

MATERIALS AND METHODS

Animals were allocated into three groups: Direct Nerve Repair (DNR, n = 50), Small Nerve Graft (SNG, n = 50), and Large Nerve Graft (LNG, n = 50). All grafts were inset into the Posterior Auricular Nerve with ear movement recovery (EMR) monitored as functional outcome. At various postoperative weeks (POWs), excised specimens were imaged with electron microscopy. Axonal counts were measured proximal to, distal (DAC) to, and within grafts. Total Success Ratio (TSR) was calculated.

RESULTS

In DNR, DAC was significantly lower than proximal axonal counts at all POWs, with maximum TSR of 80%. TSR for LNG and SNG were significantly lower at all POWs when compared to DNR, with maximums of 56% and 38%, respectively. LNG had a significantly larger DAC than SNG at POW12 and beyond. A direct relationship was present between DAC and EMR for all values.

CONCLUSIONS

Higher native axonal count of autologous nerve grafts resulted in higher percentage of regeneration across neurorrhaphies.

The Implication of Reticulons (RTNs) in Neurodegenerative Diseases: From Molecular Mechanisms to Potential Diagnostic and Therapeutic Approaches

Reticulons (RTNs) are crucial regulatory factors in the central nervous system (CNS) as well as immune system and play pleiotropic functions. In CNS, RTNs are transmembrane proteins mediating neuroanatomical plasticity and functional recovery after central nervous system injury or diseases. Moreover, RTNs, particularly RTN4 and RTN3, are involved in neurodegeneration and neuroinflammation processes. The crucial role of RTNs in the development of several neurodegenerative diseases, including Alzheimer's disease (AD), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), or other neurological conditions such as brain injury or spinal cord injury, has attracted scientific interest. Reticulons, particularly RTN-4A (Nogo-A), could provide both an understanding of early pathogenesis of neurodegenerative disorders and be potential therapeutic targets which may offer effective treatment or inhibit disease progression. This review focuses on the molecular mechanisms and functions of RTNs and their potential usefulness in clinical practice as a diagnostic tool or therapeutic strategy.

Nogo receptor antagonist LOTUS exerts suppression on axonal growth-inhibiting receptor PIR-B

Damaged axons in the adult mammalian central nervous system have a restricted regenerative capacity mainly because of Nogo protein, which is a major myelin-associated axonal growth inhibitor with binding to both receptors of Nogo receptor-1 (NgR1) and paired immunoglobulin-like receptor (PIR)-B. Lateral olfactory tract usher substance (LOTUS) exerts complete suppression of NgR1-mediated axonal growth inhibition by antagonizing NgR1. However, the regulation of PIR-B functions in neurons remains unknown. In this study, protein-protein interactions analyses found that LOTUS binds to PIR-B and abolishes Nogo-binding to PIR-B completely. Reverse transcription-polymerase chain reaction and immunocytochemistry revealed that PIR-B is expressed in dorsal root ganglions (DRGs) from wild-type and Ngr1-deficient mice (male and female). In these DRG neurons, Nogo induced growth cone collapse and neurite outgrowth inhibition, but treatment with the soluble form of LOTUS completely suppressed them. Moreover, Nogo-induced growth cone collapse and neurite outgrowth inhibition in Ngr1-deficient DRG neurons were neutralized by PIR-B function-blocking antibodies, indicating that these Nogo-induced phenomena were mediated by PIR-B. Our data show that LOTUS negatively regulates a PIR-B function. LOTUS thus exerts an antagonistic action on both receptors of NgR1 and PIR-B. This may lead to an improvement in the defective regeneration of axons following injury.

LOTUS as an endogenous protein converting the adult central nervous system environment from nonpermissive to permissive for axonal regrowth after brain injury

Central nervous system (CNS) injury, such as spinal cord injury (SCI), results in severe sensory and motor deficits due to the poor regenerative capacity of the adult CNS primarily caused by a damaged CNS environment containing a large amount of axonal growth inhibitors, such as Nogo receptor-1 (NgR1), which inhibits axonal regrowth strongly after SCI, and its five ligands. Lateral olfactory tract usher substance (LOTUS), identified in the developing brain, completely antagonizes NgR1 function, promoting neuronal regeneration and functional recovery after SCI. Therefore, we hypothesized that LOTUS might be a useful natural agent for the clinical treatment of SCI in order to increase functional recovery by converting the CNS environment from nonpermissive to permissive for neuronal regeneration. Currently, we are attempting to administer LOTUS after SCI by protein injection or gene transfection. In this report, I discuss the probability of clinical application of LOTUS for future therapy of brain injury.

Axonal regeneration and functional recovery driven by endogenous Nogo receptor antagonist LOTUS in a rat model of unilateral pyramidotomy

The adult mammalian central nervous system (CNS) rarely recovers from injury. Myelin fragments contain axonal growth inhibitors that limit axonal regeneration, thus playing a major role in determining neural recovery. Nogo receptor-1 (NgR1) and its ligands are among the inhibitors that limit axonal regeneration. It has been previously shown that the endogenous protein, lateral olfactory tract usher substance (LOTUS), antagonizes NgR1-mediated signaling and accelerates neuronal plasticity after spinal cord injury and cerebral ischemia in mice. However, it remained unclear whether LOTUS-mediated reorganization of descending motor pathways in the adult brain is physiologically functional and contributes to functional recovery. Here, we generated LOTUS-overexpressing transgenic (LOTUS-Tg) rats to investigate the role of LOTUS in neuronal function after damage. After unilateral pyramidotomy, motor function in LOTUS-Tg rats recovered significantly compared to that in wild-type animals. In a retrograde tracing study, labeled axons spanning from the impaired side of the cervical spinal cord to the unlesioned hemisphere of the red nucleus and sensorimotor cortex were increased in LOTUS-Tg rats. Anterograde tracing from the unlesioned cortex also revealed enhanced ipsilateral connectivity to the impaired side of the cervical spinal cord in LOTUS-Tg rats. Moreover, electrophysiological analysis showed that contralesional cortex stimulation significantly increased ipsilateral forelimb movement in LOTUS-Tg rats, which was consistent with the histological findings. According to these data, LOTUS overexpression accelerates ipsilateral projection from the unlesioned cortex and promotes functional recovery after unilateral pyramidotomy. LOTUS could be a future therapeutic option for CNS injury.

Spatiotemporal expression patterns of chondroitin sulfate proteoglycan mRNAs in the developing rat brain

Chondroitin sulfate proteoglycans (CSPGs) are pluripotent components of the extracellular matrix in the brain. Although previous studies have examined the developmental change in certain CSPGs in the whole brain, no known systematic studies have been carried out on the temporal or spatial expression of CSPGs. Here, we used quantitative real-time PCR to examine the CSPG mRNAs expression in the postnatal developing rat brain starting from postnatal day 5-42, mainly focusing on the parietal cortex, hippocampus, and corpus callosum. Results were further verified by immunohistochemistry. Our results showed that aggrecan, brevican, phosphacan, and NG2 generally showed upregulation across developmental stages in all three regions. Neurocan showed a rapid increase until postnatal day 10 in all three regions. Versican, however, showed a sharp decrease until postnatal day 10. Cross-region analysis showed higher expression of most CSPG members in the corpus callosum during later stages of development. Further immunohistochemistry staining confirmed our results by showing prominent CSPGs protein expression in the corpus callosum. In summary, our study reported specific temporal-expression and spatial-expression patterns of the CSPGs species. These results are consistent with the known roles of these members in neurodevelopment. The current study provided clues for the development of CSPGs as potential treatment targets in neurodevelopmental disorders.

Keratan sulfate (KS)-proteoglycans and neuronal regulation in health and disease: the importance of KS-glycodynamics and interactive capability with neuroregulatory ligands

Compared to the other classes of glycosaminoglycans (GAGs), that is, chondroitin/dermatan sulfate, heparin/heparan sulfate and hyaluronan, keratan sulfate (KS), have the least known of its interactive properties. In the human body, the cornea and the brain are the two most abundant tissue sources of KS. Embryonic KS is synthesized as a linear poly-N-acetyllactosamine chain of d-galactose-GlcNAc repeat disaccharides which become progressively sulfated with development, sulfation of GlcNAc is more predominant than galactose. KS contains multi-sulfated high-charge density, monosulfated and non-sulfated poly-N-acetyllactosamine regions and thus is a heterogeneous molecule in terms of chain length and charge distribution. A recent proteomics study on corneal KS demonstrated its interactivity with members of the Slit-Robbo and Ephrin-Ephrin receptor families and proteins which regulate Rho GTPase signaling and actin polymerization/depolymerization in neural development and differentiation. KS decorates a number of peripheral nervous system/CNS proteoglycan (PG) core proteins. The astrocyte KS-PG abakan defines functional margins of the brain and is up-regulated following trauma. The chondroitin sulfate/KS PG aggrecan forms perineuronal nets which are dynamic neuroprotective structures with anti-oxidant properties and roles in neural differentiation, development and synaptic plasticity. Brain phosphacan a chondroitin sulfate, KS, HNK-1 PG have roles in neural development and repair. The intracellular microtubule and synaptic vesicle KS-PGs MAP1B and SV2 have roles in metabolite transport, storage, and export of neurotransmitters and cytoskeletal assembly. MAP1B has binding sites for tubulin and actin through which it promotes cytoskeletal development in growth cones and is highly expressed during neurite extension. The interactive capability of KS with neuroregulatory ligands indicate varied roles for KS-PGs in development and regenerative neural processes.

White-matter repair: Interaction between oligodendrocytes and the neurovascular unit

There are currently no adequate treatments for white-matter injury, which often follows central nervous system maladies and their accompanying neurodegenerative processes. Indeed, the white matter is compromised by the deterioration of the blood–brain barrier and the demyelination of neuronal axons. Key repairs to the white matter are mediated by oligodendrocyte lineage cells after damaging events. Oligodendrocytes are supported by other cells in the neurovascular unit and these cells collaborate in processes such as angiogenesis, neurogenesis, and oligodendrogenesis. Understanding the various interactions between these cells and oligodendrocytes will be imperative for developing reparative therapies for impaired white matter. This minireview will discuss how oligodendrocytes and oligodendrocyte lineage cells mend damage to the white matter and restore brain function ensuing neural injury.

Role of oligodendrocyte-neurovascular unit in white matter repair

White matter injury caused by acute or chronic neuropathologies is a major characteristic of many CNS diseases, and an effective treatment is still out of our reach. White matter damage is associated with the collapse of the axon-myelin complex and with blood-brain barrier (BBB) breakdown, which results in disruption of white matter function. While white matter damage cannot completely resolve spontaneously, some compensative responses may occur after the injury. Oligodendrocyte lineage cells perform critical functions in repairing damaged white matter. In this mini-review, we will focus on the reparative actions of the oligodendrocytes, and highlight the important role of oligodendrocyte lineage cells in brain recovery after injury.

LOTUS Inhibits Neuronal Apoptosis and Promotes Tract Regeneration in Contusive Spinal Cord Injury Model Mice

Nogo receptor-1 (NgR1) signaling is involved in the limitation of axonal regeneration following spinal cord injury (SCI) through collapsing the growth cone and inhibiting neurite outgrowth. Lateral olfactory tract usher substance (LOTUS), a NgR antagonist, suppresses these pathological conditions. A previous report demonstrated the positive effects of LOTUS expression on motor function through raphespinal tract regeneration using pan-neuronally LOTUS-overexpressing transgenic mice. However, this report used a hemi-section model, which does not represent the majority of clinical SCI cases, and lacked a detailed histological analysis of other descending tracts. To determine the true therapeutic effects of LOTUS, we used a more clinically relevant contusive SCI model in female transgenic mice. Definitive tracing analyses revealed that LOTUS promoted the extensive regeneration of the reticulospinal tract across the lesion site and suppressed axonal dieback of corticospinal tract (CST). A significant increase in raphespinal tract fibers was seen from the subacute to the chronic phase after the injury, strongly suggesting that LOTUS promoted translesional elongation of this tract. Furthermore, histological analyses revealed that LOTUS had a neuroprotective effect on the injured spinal cord through suppressing cellular apoptosis during the acute phase. These neuroprotective and regenerative effects contributed to significant motor functional recovery and restoration of the motor evoked potential (MEP). Therefore, LOTUS application could prove beneficial in the treatment of SCI by promoting axonal regeneration of some descending fibers, reducing axonal dieback of CST fibers and encouraging motor function recovery.

The soluble form of LOTUS inhibits Nogo receptor type 1-mediated signaling induced by B lymphocyte stimulator and chondroitin sulfate proteoglycans

There are global efforts in developing therapeutic strategies for central nervous system (CNS) injuries using multimodal approaches. Nogo receptor type 1 (NgR1) has been known as a primary molecule limiting neuronal regeneration in the adult CNS. We identified lateral olfactory tract usher substance (LOTUS) as an endogenous NgR1 antagonist. Membrane-bound LOTUS interacts with NgR1 and inhibits its function by blocking its ligand binding. Five molecules including Nogo, myelin-associated glycoprotein (MAG), oligodendrocyte myelin glycoprotein (OMgp), B lymphocyte stimulator (BLyS) and chondroitin sulfate proteoglycans (CSPGs) have been identified as NgR1 ligands. These ligands bind to NgR1 and activate NgR1 signaling, leading to axon growth inhibition such as growth cone collapse and neurite outgrowth inhibition. We have recently reported that the soluble form of LOTUS (s-LOTUS) also suppressed NgR1-mediated signaling induced by myelin axonal inhibitors (MAIs) including Nogo, MAG and OMgp by binding with both NgR1 and its co-receptor p75 neurotrophin receptor (p75^NTR). Though s-LOTUS has been reported to suppress MAIs, whether s-LOTUS also suppresses NgR1 signaling induced by BLyS and CSPGs remains to be elucidated. Here, we show that s-LOTUS inhibits NgR1-mediated signaling induced by BLyS and CSPGs. Although treatment with s-LOTUS did not suppress BLyS-NgR1 interaction, s-LOTUS inhibited growth cone collapse and neurite outgrowth inhibition induced by BLyS and CSPGs in chick dorsal root ganglion (DRG) neurons. Furthermore, s-LOTUS compensated for the suppressive function of endogenous LOTUS in NgR1-mediated signaling in olfactory bulb neurons of lotus-knockout mice. These findings suggest that s-LOTUS is a potent therapeutic agent for neuronal regeneration in the CNS injuries.