Galectin-1 in regenerating motoneurons

Peripheral nerve-derived fibroblasts promote neurite outgrowth in adult dorsal root ganglion neurons more effectively than skin-derived fibroblasts

NEW FINDINGS

What is the central question of this study? Although fibroblasts are involved in the regenerative process associated with peripheral nerve injury, detailed information regarding their characteristics is largely lacking. What is the main finding and its importance? Nerve-derived fibroblasts have a greater neurite-promoting effect than skin-derived fibroblasts, and epineurium-derived fibroblasts can promote neurite outgrowth more effectively than parenchyma-derived fibroblasts. The epineurium-derived fibroblasts and parenchyma-derived fibroblasts have distinctly different molecular profiles, including genes of soluble factors to promote axonal growth. Fibroblasts are molecularly and functionally different depending on their localization in nerve tissue, and epineurium-derived fibroblasts might be involved in axon regeneration after peripheral nerve injury more than previously thought.

ABSTRACT

Although fibroblasts (Fb) are components of a peripheral nerve involved in the regenerative process associated with peripheral nerve injury, detailed information regarding their characteristics is largely lacking. The objective of the present study was to investigate the capacity of Fb derived from peripheral nerves to stimulate the outgrowth of neurites from adult dorsal root ganglion neurons and to clarify their molecular characteristics. Fibroblasts were prepared from the epineurium and parenchyma of rat sciatic nerves and skin. The Fb derived from epineurium showed the greatest effect on neurite outgrowth, followed by the Fb derived from parenchyma, indicating that Fb derived from nerves promote neurite outgrowth more effectively than skin-derived Fb. Although both soluble and cell-surface factors contributed evenly to the neurite-promoting effect of nerve-derived Fb, in crush and transection injury models, Fb were not closely associated with regenerating axons, indicating that only soluble factors from Fb are available to regenerating axons. A transcriptome analysis revealed that the molecular profiles of these Fb were distinctly different and that the gene expression profiles of soluble factors that promote axonal growth are unique to each Fb. These findings indicate that Fb are molecularly and functionally different depending on their localization in nerve tissue and that Fb derived from epineurium might be involved more than was previously thought in axon regeneration after peripheral nerve injury.

Axonal Regeneration Mediated by a Novel Axonal Guidance Pair, Galectin-1 and Secernin-1

Galectin-1 (Gal-1), a member of the Galectin family, is expressed in various tissues and responsible for multiple biological activities. Previous studies reported that extracellular Gal-1 participated in axonal growth and repair, and Gal-1 knockout mice exhibited memory impairment. However, no study has demonstrated the direct contribution of intracellular Gal-1 upregulation in neurons to promoting axonal regeneration in the brain and recovering memory function. In the present study, we found that axonal growth is promoted by overexpression of Gal-1 via adeno-associated virus serotype 9 delivery in primary cultured hippocampal neurons. Moreover, Gal-1 was expressed on the membranes of growth cones in hippocampal neurons and interacted with a novel axonal guidance molecule, Secernin-1, which was secreted from prefrontal cortex (PFC) neurons. Gal-1-overexpression-driven axonal growth was enhanced when recombinant (extracellular) Secernin-1 was treated to the axonal site in a neuron device chamber. Direct binding of extracellular Secernin-1 with Gal-1 was detected through immunoprecipitation and immunocytochemistry, demonstrating that Gal-1 possibly works as an axonal guidance receptor for Secernin-1 in hippocampal neurons. In the PFC, the expression of Gal-1 in axonal shafts and terminals of hippocampal neurons was decreased in the 5XFAD mouse model of Alzheimer's disease (AD). Overexpression of Gal-1 in hippocampal neurons recovered memory deficits and induced axonal regeneration toward the PFC in 5XFAD mice. This study suggests that the enhanced interaction of Secernin-1 and Gal-1 can be harnessed as a therapeutic strategy for long-distance and direction-specific axonal regeneration in AD.

Animal Galectins and Plant Lectins as Tools for Studies in Neurosciences

Lectins are proteins or glycoproteins of non-immunological origin capable of reversibly and specifically binding to glycoconjugates. They exist in free form or associated with cells and are widely distributed in nature, being found in plants, microorganisms, and animals. Due to their characteristics and mainly due to the possibility of reversible binding to glycoconjugates, lectins have stood out as important tools in research involving Neurobiology. These proteins have the ability to modulate molecular targets in the central nervous system (CNS) which may be involved with neuroplasticity, neurobehavioral effects, and neuroprotection. The present report integrates existing information on the activity of animal and plant lectins in different areas of Neuroscience, presenting perspectives to direct new research on lectin function in the CNS, providing alternatives for understanding neurological diseases such as mental disorders, neurodegenerative, and neuro-oncological diseases, and for the development of new drugs, diagnoses and therapies in the field of Neuroscience.

Cerebrospinal Fluid Galectin-1 Levels Discriminate Patients with Parkinsonism from Controls

Parkinson’s disease (PD) is the second most common neurodegenerative disorder in elderly people. Currently, the diagnosis of PD is based on neurological examination, neuroimaging, and the response to dopaminergic medication. The diagnosis can be challenging, especially at early disease stages, when the symptoms of patients with atypical parkinsonism (APD) may strongly overlap. Therefore, reliable biomarkers that are able to identify patients with PD are much needed. Here, we aimed to identify and validate new biomarkers for PD in cerebrospinal fluid (CSF). We performed a profiling experiment using mass spectrometry (MS) of CSF from ten PD patients and ten matched non-neurological controls. We selected one protein, galectin-1 (Gal-1), which was differentially expressed in PD vs. controls, and quantified its concentrations in CSF by enzyme-linked immunosorbent assay (ELISA) in three new cohorts of 37 PD patients, 21 APD patients, and 44 controls. CSF levels of Gal-1 were lower in PD in both the discovery and validation experiments and discriminated PD from controls with moderate–high accuracy levels (ELISA: area under the curve = 0.7). Similar levels of Gal-1 were found in PD and APD. Gal-1 levels were correlated to age in all groups and correlated in the PD patients to CSF levels of total tau, phosphorylated tau, neurofilament light chain (NFL), and the mini-mental state examination (MMSE) score. We conclude that MS profiling of proteins may be a useful tool to identify novel biomarkers of neurological diseases and that CSF Gal-1 levels may discriminate PD from non-neurological controls.

Progranulin functions as a cathepsin D chaperone to stimulate axonal outgrowth in vivo

Loss of function mutations in progranulin (GRN) cause frontotemporal dementia, but how GRN haploinsufficiency causes neuronal dysfunction remains unclear. We previously showed that GRN is neurotrophic in vitro. Here, we used an in vivo axonal outgrowth system and observed a delayed recovery in GRN-/- mice after facial nerve injury. This deficit was rescued by reintroduction of human GRN and relied on its C-terminus and on neuronal GRN production. Transcriptome analysis of the facial motor nucleus post injury identified cathepsin D (CTSD) as the most upregulated gene. In aged GRN-/- cortices, CTSD was also upregulated, but the relative CTSD activity was reduced and improved upon exogenous GRN addition. Moreover, GRN and its C-terminal granulin domain granulinE (GrnE) both stimulated the proteolytic activity of CTSD in vitro. Pull-down experiments confirmed a direct interaction between GRN and CTSD. This interaction was also observed with GrnE and stabilized the CTSD enzyme at different temperatures. Investigating the importance of this interaction for axonal regeneration in vivo we found that, although individually tolerated, a combined reduction of GRN and CTSD synergistically reduced axonal outgrowth. Our data links the neurotrophic effect of GRN and GrnE with a lysosomal chaperone function on CTSD to maintain its proteolytic capacity.

A novel microglial subset plays a key role in myelinogenesis in developing brain

Microglia are resident macrophages of the central nervous system that contribute to homeostasis and neuroinflammation. Although known to play an important role in brain development, their exact function has not been fully described. Here, we show that in contrast to healthy adult and inflammation‐activated cells, neonatal microglia show a unique myelinogenic and neurogenic phenotype. A CD11c⁺ microglial subset that predominates in primary myelinating areas of the developing brain expresses genes for neuronal and glial survival, migration, and differentiation. These cells are the major source of insulin‐like growth factor 1, and its selective depletion from CD11c⁺ microglia leads to impairment of primary myelination. CD11c‐targeted toxin regimens induced a selective transcriptional response in neonates, distinct from adult microglia. CD11c⁺ microglia are also found in clusters of repopulating microglia after experimental ablation and in neuroinflammation in adult mice, but despite some similarities, they do not recapitulate neonatal microglial characteristics. We therefore identify a unique phenotype of neonatal microglia that deliver signals necessary for myelination and neurogenesis.

mRNA Transcriptomics of Galectins Unveils Heterogeneous Organization in Mouse and Human Brain

Background: Galectins, a family of non-classically secreted, β-galactoside binding proteins is involved in several brain disorders; however, no systematic knowledge on the normal neuroanatomical distribution and functions of galectins exits. Hence, the major purpose of this study was to understand spatial distribution and predict functions of galectins in brain and also compare the degree of conservation vs. divergence between mouse and human species. The latter objective was required to determine the relevance and appropriateness of studying galectins in mouse brain which may ultimately enable us to extrapolate the findings to human brain physiology and pathologies.

Results: In order to fill this crucial gap in our understanding of brain galectins, we analyzed the in situ hybridization and microarray data of adult mouse and human brain respectively, from the Allen Brain Atlas, to resolve each galectin-subtype’s spatial distribution across brain distinct cytoarchitecture. Next, transcription factors (TFs) that may regulate galectins were identified using TRANSFAC software and the list obtained was further curated to sort TFs on their confirmed transcript expression in the adult brain. Galectin-TF cluster analysis, gene-ontology annotations and co-expression networks were then extrapolated to predict distinct functional relevance of each galectin in the neuronal processes. Data shows that galectins have highly heterogeneous expression within and across brain sub-structures and are predicted to be the crucial targets of brain enriched TFs. Lgals9 had maximal spatial distribution across mouse brain with inferred predominant roles in neurogenesis while LGALS1 was ubiquitously expressed in human. Limbic region associated with learning, memory and emotions and substantia nigra associated with motor movements showed strikingly high expression of LGALS1 and LGALS8 in human vs. mouse brain. The overall expression profile of galectin-8 was most preserved across both these species, however, galectin-9 showed maximal preservation only in the cerebral cortex.

Conclusion: It is for the first time that a comprehensive description of galectins’ mRNA expression profile in brain is presented. Results suggests that spatial transcriptome changes in galectins may contribute to differential brain functions and evolution across species that highlights galectins as novel signatures of brain heterogeneity and functions, which if disturbed, can promote several brain disorders.

Electrical Stimulation to Enhance Axon Regeneration After Peripheral Nerve Injuries in Animal Models and Humans

Injured peripheral nerves regenerate their lost axons but functional recovery in humans is frequently disappointing. This is so particularly when injuries require regeneration over long distances and/or over long time periods. Fat replacement of chronically denervated muscles, a commonly accepted explanation, does not account for poor functional recovery. Rather, the basis for the poor nerve regeneration is the transient expression of growth-associated genes that accounts for declining regenerative capacity of neurons and the regenerative support of Schwann cells over time. Brief low-frequency electrical stimulation accelerates motor and sensory axon outgrowth across injury sites that, even after delayed surgical repair of injured nerves in animal models and patients, enhances nerve regeneration and target reinnervation. The stimulation elevates neuronal cyclic adenosine monophosphate and, in turn, the expression of neurotrophic factors and other growth-associated genes, including cytoskeletal proteins. Electrical stimulation of denervated muscles immediately after nerve transection and surgical repair also accelerates muscle reinnervation but, at this time, how the daily requirement of long-duration electrical pulses can be delivered to muscles remains a practical issue prior to translation to patients. Finally, the technique of inserting autologous nerve grafts that bridge between a donor nerve and an adjacent recipient denervated nerve stump significantly improves nerve regeneration after delayed nerve repair, the donor nerves sustaining the capacity of the denervated Schwann cells to support nerve regeneration. These reviewed methods to promote nerve regeneration and, in turn, to enhance functional recovery after nerve injury and surgical repair are sufficiently promising for early translation to the clinic.

Galectin-1 deficiency improves axonal swelling of motor neurones in SOD1(G93A) transgenic mice

AIMS

Galectin-1, a member of the β-galactoside-binding lectin family, accumulates in neurofilamentous lesions in the spinal cords of both sporadic and familial amyotrophic lateral sclerosis (ALS) patients with a superoxide dismutase 1 gene (SOD1) mutation (A4V). The aim of this study was to evaluate the roles of endogenous galectin-1 in the pathogenesis of ALS.

METHODS

Expression of galectin-1 in the spinal cord of mutant SOD1 transgenic (SOD1(G93A) ) mice was examined by pathological analysis, real-time RT-PCR and Western blotting. The effects of galectin-1 deficiency were evaluated by cross-breeding SOD1(G93A) mice with galectin-1 null (Lgals1(-/-) ) mice.

RESULTS

Before ALS-like symptoms developed in SOD1(G93A) /Lgals1(+/+) mice, strong galectin-1 immunoreactivity was observed in swollen motor axons and colocalized with aggregated neurofilaments. Electron microscopic observations revealed that the diameters of swollen motor axons in the spinal cord were significantly smaller in SOD1(G93A) /Lgals1(-/-) mice, and there was less accumulation of vacuoles compared with SOD1(G93A) /Lgals1(+/+) mice. In symptomatic SOD1(G93A) /Lgals1(+/+) mice, astrocytes surrounding motor axons expressed a high level of galectin-1.

CONCLUSIONS

Galectin-1 accumulates in neurofilamentous lesions in SOD1(G93A) mice, as previously reported in humans with ALS. Galectin-1 accumulation in motor axons occurs before the development of ALS-like symptoms and is associated with early processes of axonal degeneration in SOD1(G93A) mice. In contrast, galectin-1 expressed in astrocytes may be involved in axonal degeneration during symptom presentation.

What makes a RAG regeneration associated?

Regenerative failure remains a significant barrier for functional recovery after central nervous system (CNS) injury. As such, understanding the physiological processes that regulate axon regeneration is a central focus of regenerative medicine. Studying the gene transcription responses to axon injury of regeneration competent neurons, such as those of the peripheral nervous system (PNS), has provided insight into the genes associated with regeneration. Though several individual “regeneration-associated genes” (RAGs) have been identified from these studies, the response to injury likely regulates the expression of functionally coordinated and complementary gene groups. For instance, successful regeneration would require the induction of genes that drive the intrinsic growth capacity of neurons, while simultaneously downregulating the genes that convey environmental inhibitory cues. Thus, this view emphasizes the transcriptional regulation of gene “programs” that contribute to the overall goal of axonal regeneration. Here, we review the known RAGs, focusing on how their transcriptional regulation can reveal the underlying gene programs that drive a regenerative phenotype. Finally, we will discuss paradigms under which we can determine whether these genes are injury-associated, or indeed necessary for regeneration.

Galectin-1-secreting neural stem cells elicit long-term neuroprotection against ischemic brain injury

Galectin-1 (gal-1), a special lectin with high affinity to β-galactosides, is implicated in protection against ischemic brain injury. The present study investigated transplantation of gal-1-secreting neural stem cell (s-NSC) into ischemic brains and identified the mechanisms underlying protection. To accomplish this goal, secretory gal-1 was stably overexpressed in NE-4C neural stem cells. Transient cerebral ischemia was induced in mice by middle cerebral artery occlusion for 60 minutes and s-NSCs were injected into the striatum and cortex within 2 hours post-ischemia. Brain infarct volume and neurological performance were assessed up to 28 days post-ischemia. s-NSC transplantation reduced infarct volume, improved sensorimotor and cognitive functions, and provided more robust neuroprotection than non-engineered NSCs or gal-1-overexpressing (but non-secreting) NSCs. White matter injury was also ameliorated in s-NSC-treated stroke mice. Gal-1 modulated microglial function in vitro, by attenuating secretion of pro-inflammatory cytokines (TNF-α and nitric oxide) in response to LPS stimulation and enhancing production of anti-inflammatory cytokines (IL-10 and TGF-β). Gal-1 also shifted microglia/macrophage polarization toward the beneficial M2 phenotype in vivo by reducing CD16 expression and increasing CD206 expression. In sum, s-NSC transplantation confers robust neuroprotection against cerebral ischemia, probably by alleviating white matter injury and modulating microglial/macrophage function.

Galectin-1 in injured rat spinal cord: implications for macrophage phagocytosis and neural repair

Galectin (Gal)-1 is a small carbohydrate-binding protein and immune modulatory cytokine that is synthesized locally at the site of peripheral nerve injury. In this environment, Gal1 can promote regeneration of injured peripheral axons, in part by modifying the function of macrophages recruited to the site of injury. Unlike in injured peripheral nerves, macrophages do not promote axon regeneration in the injured central nervous system (CNS), perhaps because Gal1 levels are not regulated appropriately. Because the dynamics and cellular localization of endogenous Gal1 have not been rigorously characterized after CNS injury, we examined the spatio-temporal distribution of Gal1 in rat spinal cords subjected to a standardized contusion injury. Whereas Gal1 was not expressed in uninjured spinal cord, it was significantly upregulated after SCI, especially within the lesion core. Gal1 was expressed in ~40% of lesion-localized macrophages at 3-28 days post-injury (dpi), and in ~45% of astrocytes in the lesion border at 7-28 dpi. Most lesion-localized Gal1+ macrophages did not express the phagocytosis marker ED1, and Gal1+ cells contained less phagocytosed lipids. These data suggest that time- and location-dependent regulation of Gal1 by macrophages (and astrocytes) could be important for modulating phagocytosis, inflammation/gliosis, and axon growth after SCI.

Glycan-dependent binding of galectin-1 to neuropilin-1 promotes axonal regeneration after spinal cord injury

Following spinal cord injury (SCI), semaphorin 3A (Sema3A) prevents axonal regeneration through binding to the neuropilin-1 (NRP-1)/PlexinA4 receptor complex. Here, we show that galectin-1 (Gal-1), an endogenous glycan-binding protein, selectively bound to the NRP-1/PlexinA4 receptor complex in injured neurons through a glycan-dependent mechanism, interrupts the Sema3A pathway and contributes to axonal regeneration and locomotor recovery after SCI. Although both Gal-1 and its monomeric variant contribute to de-activation of microglia, only high concentrations of wild-type Gal-1 (which co-exists in a monomer-dimer equilibrium) bind to the NRP-1/PlexinA4 receptor complex and promote axonal regeneration. Our results show that Gal-1, mainly in its dimeric form, promotes functional recovery of spinal lesions by interfering with inhibitory signals triggered by Sema3A binding to NRP-1/PlexinA4 complex, supporting the use of this lectin for the treatment of SCI patients.

Achieving CNS axon regeneration by manipulating convergent neuro-immune signaling

After central nervous system (CNS) trauma, axons have a low capacity for regeneration. Regeneration failure is associated with a muted regenerative response of the neuron itself, combined with a growth-inhibitory and cytotoxic post-injury environment. After spinal cord injury (SCI), resident and infiltrating immune cells (especially microglia/macrophages) contribute significantly to the growth-refractory milieu near the lesion. By targeting both the regenerative potential of the axon and the cytotoxic phenotype of microglia/macrophages, we may be able to improve CNS repair after SCI. In this review, we discuss molecules shown to impact CNS repair by affecting both immune cells and neurons. Specifically, we provide examples of pattern recognition receptors, integrins, cytokines/chemokines, nuclear receptors and galectins that could improve CNS repair. In many cases, signaling by these molecules is complex and may have contradictory effects on recovery depending on the cell types involved or the model studied. Despite this caveat, deciphering convergent signaling pathways on immune cells (which affect axon growth indirectly) and neurons (direct effects on axon growth) could improve repair and recovery after SCI. Future studies must continue to consider how regenerative therapies targeting neurons impact other cells in the pathological CNS. By identifying molecules that simultaneously improve axon regenerative capacity and drive the protective, growth-promoting phenotype of immune cells, we may discover SCI therapies that act synergistically to improve CNS repair and functional recovery.

Downstream effector molecules in successful peripheral nerve regeneration

The robust axon regeneration that occurs following peripheral nerve injury is driven by transcriptional activation of the regeneration program and by the expression of a wide range of downstream effector molecules from neuropeptides and neurotrophic factors to adhesion molecules and cytoskeletal adaptor proteins. These regeneration-associated effector molecules regulate the actin-tubulin machinery of growth-cones, integrate intracellular signalling and stimulatory and inhibitory signals from the local environment and translate them into axon elongation. In addition to the neuronally derived molecules, an important transcriptional component is found in locally activated Schwann cells and macrophages, which release a number of cytokines, growth factors and neurotrophins that support neuronal survival and axonal regeneration and that might provide directional guidance cues towards appropriate peripheral targets. This review aims to provide a comprehensive up-to-date account of the transcriptional regulation and functional role of these effector molecules and of the information that they can give us with regard to the organisation of the regeneration program.

Transplantation of human neural stem/progenitor cells overexpressing galectin-1 improves functional recovery from focal brain ischemia in the Mongolian gerbil

Transplantation of human neural stem/progenitor cells (hNSPCs) is a promising method to regenerate tissue from damage and recover function in various neurological diseases including brain ischemia. Galectin-1(Gal1) is a lectin that is expressed in damaged brain areas after ischemia. Here, we characterized the detailed Gal1 expression pattern in an animal model of brain ischemia. After brain ischemia, Gal1 was expressed in reactive astrocytes within and around the infarcted region, and its expression diminished over time. Previously, we showed that infusion of human Gal1 protein (hGal1) resulted in functional recovery after brain ischemia but failed to reduce the volume of the ischemic region. This prompted us to examine whether the combination of hNSPCs-transplantation and stable delivery of hGal1 around the ischemic region could reduce the ischemic volume and promote better functional recovery after brain ischemia. In this study, we transplanted hNSPCs that stably overexpressed hGal1 (hGal1-hNSPCs) in a model of unilateral focal brain ischemia using Mongolian gerbils. Indeed, we found that transplantation of hGal1-hNSPCs both reduced the ischemic volume and improved deficits in motor function after brain ischemia to a greater extent than the transplantation of hNSPCs alone. This study provides evidence for a potential application of hGal1 with hNSPCs-transplantation in the treatment of brain ischemia.

Wallerian degeneration: gaining perspective on inflammatory events after peripheral nerve injury

In this review, we first provide a brief historical perspective, discussing how peripheral nerve injury (PNI) may have caused World War I. We then consider the initiation, progression, and resolution of the cellular inflammatory response after PNI, before comparing the PNI inflammatory response with that induced by spinal cord injury (SCI).In contrast with central nervous system (CNS) axons, those in the periphery have the remarkable ability to regenerate after injury. Nevertheless, peripheral nervous system (PNS) axon regrowth is hampered by nerve gaps created by injury. In addition, the growth-supportive milieu of PNS axons is not sustained over time, precluding long-distance regeneration. Therefore, studying PNI could be instructive for both improving PNS regeneration and recovery after CNS injury. In addition to requiring a robust regenerative response from the injured neuron itself, successful axon regeneration is dependent on the coordinated efforts of non-neuronal cells which release extracellular matrix molecules, cytokines, and growth factors that support axon regrowth. The inflammatory response is initiated by axonal disintegration in the distal nerve stump: this causes blood-nerve barrier permeabilization and activates nearby Schwann cells and resident macrophages via receptors sensitive to tissue damage. Denervated Schwann cells respond to injury by shedding myelin, proliferating, phagocytosing debris, and releasing cytokines that recruit blood-borne monocytes/macrophages. Macrophages take over the bulk of phagocytosis within days of PNI, before exiting the nerve by the circulation once remyelination has occurred. The efficacy of the PNS inflammatory response (although transient) stands in stark contrast with that of the CNS, where the response of nearby cells is associated with inhibitory scar formation, quiescence, and degeneration/apoptosis. Rather than efficiently removing debris before resolving the inflammatory response as in other tissues, macrophages infiltrating the CNS exacerbate cell death and damage by releasing toxic pro-inflammatory mediators over an extended period of time. Future research will help determine how to manipulate PNS and CNS inflammatory responses in order to improve tissue repair and functional recovery.

Transcribing the path to neurological recovery-From early signals through transcription factors to downstream effectors of successful regeneration

The peripheral nervous system is known to regenerate comparatively well and this ability is mirrored in the de novo expression or upregulation of a wide variety of molecules involved in axonal outgrowth starting with transcription factors, but also including growth-stimulating substances, guidance and cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions. Recent studies using pharmacological agents, and global as well as neuron-selective gene inactivation techniques have shed light on those endogenous molecules that play a non-redundant role in mediating regenerative axonal outgrowth in vivo. The aim of the current review is to sketch the sequence of molecular events from early sensors of injury to transcription factors to downstream effectors that cooperate in successful regeneration and functional recovery.

A role for galectin-1 in the immune response to peripheral nerve injury

Galectin-1 (Gal1) is a multi-functional protein that has key roles in organismal growth and survival. In the adult nervous system, Gal1 promotes axonal regeneration following peripheral nerve injury. Although the mechanism by which Gal1 promotes regeneration is unclear, previous reports suggested that Gal1 acts indirectly by activating macrophages. An appropriate response of macrophages is crucial for repair of injured nerves: these immune cells remove obstructive axon and myelin debris in the distal nerve. Here we establish a role for Gal1 in the accumulation of immune cells following peripheral axotomy. We used immunohistochemistry to visualize macrophages (F4/80) in wild-type (Lgals1(+/+)) and knockout (Lgals1(-/-)) mouse sciatic nerves following injury and/or manipulation of Gal1 levels. Density of F4/80 immunoreactivity, which peaks around 3 days post-injury, was decreased in Lgals1(+/+) nerves injected with Gal1 antibody. The typical injury-induced peak of macrophage/microglial density was delayed in the sciatic nerves and fifth lumbar dorsal root ganglia of Lgals1(-/-) mice relative to control mice. Injection of oxidized Gal1 into uninjured sciatic nerve promoted the accumulation of macrophages in Lgals1(+/+) nerves. Finally, we used transplants of sciatic nerve to uncover a compensatory mechanism in Lgals1(-/-) mice that allows for macrophage accumulation (albeit delayed and diminished) following axotomy. We conclude that Gal1 is necessary to direct the typical accumulation of macrophages in the injured peripheral nerve, and that Gal1 is sufficient to promote macrophage accumulation in the uninjured nerve of wild-type mice.

Galectin-1 expression in innervated and denervated skeletal muscle

Galectin-1 is a soluble carbohydrate-binding protein with a particularly high expression in skeletal muscle. Galectin-1 has been implicated in skeletal muscle development and in adult muscle regeneration, but also in the degeneration of neuronal processes and/or in peripheral nerve regeneration. Exogenously supplied oxidized galectin-1, which lacks carbohydrate-binding properties, has been shown to promote neurite outgrowth after sciatic nerve sectioning. In this study, we compared the expression of galectin-1 mRNA and immunoreactivity in innervated and denervated mouse and rat hind-limb and hemidiaphragm muscles. The results show that galectin-1 mRNA expression and immunoreactivity are up-regulated following denervation. The galectin-1 mRNA is expressed in the extrasynaptic and perisynaptic regions of the muscle, and its immunoreactivity can be detected in both regions by Western blot analysis. The results are compatible with a role for galectin-1 in facilitating reinnervation of denervated skeletal muscle.

Lack of galectin-1 results in defects in myoblast fusion and muscle regeneration

Galectin-1 has been implicated in the development of skeletal muscle, being maximally expressed at the time of myofiber formation. Furthermore, in the presence of exogenous galectin-1, mononuclear myoblasts show increased fusion in vitro. In the current study, we have used the galectin-1 null mouse to elucidate the role of galectin-1 in skeletal muscle development and regeneration. Myoblasts derived from the galectin-1 mutant showed a reduced ability to fuse in vitro. In galectin-1 null mutants, there was evidence of a delay in muscle fiber development at the neonatal stage and muscle fiber diameter was reduced when compared with wild-type at the adult stage. Muscle regeneration was also compromised in the galectin-1 mutant with the process being delayed and a reduced fiber size being maintained. These results, therefore, show a definitive role for galectin-1 in fusion of myoblasts both in vitro, in vivo, and in regeneration after recovery from induced injury.

Galectins in the tumor endothelium: opportunities for combined cancer therapy

Galectins are emerging as a family of proteins that play an important role in several steps of tumorigenesis. Evidence is accumulating that galectins are expressed by the tumor endothelium, where they contribute to different steps of tumor progression such as immune escape and metastasis. Recent studies have identified an important role for galectins in tumor angiogenesis. Moreover, it has been shown that galectins in the endothelium can be targeted for therapeutic applications. This opens a window of opportunity for the development of tumor-type independent treatment strategies. This review focuses on the expression of galectins in the tumor endothelium, their contribution to tumor progression, and their application in tumor-type independent cancer therapy.

Identification of a lectin causing the degeneration of neuronal processes using engineered embryonic stem cells

Unlike the mechanisms involved in the death of neuronal cell bodies, those causing the elimination of processes are not well understood owing to the lack of suitable experimental systems. As the neurotrophin receptor p75(NTR) is known to restrict the growth of neuronal processes, we engineered mouse embryonic stem (ES) cells to express an Ngfr (p75(NTR)) cDNA under the control of the Mapt locus (the gene encoding tau), which begins to be active when ES cell-derived progenitors start elongating processes. This caused a progressive, synchronous degeneration of all processes, and a prospective proteomic analysis showed increased levels of the sugar-binding protein galectin-1 in the p75(NTR)-engineered cells. Function-blocking galectin-1 antibodies prevented the degeneration of processes, and recombinant galectin-1 caused the processes of wild-type neurons to degenerate first, followed by the cell bodies. In vivo, the application of a glutamate receptor agonist, a maneuver known to upregulate p75(NTR), led to an increase in the amount of galectin-1 and to the degeneration of neurons and their processes in a galectin-1-dependent fashion. Section of the sciatic nerve also rapidly upregulated levels of p75(NTR) and galectin-1 in terminal Schwann cells, and the elimination of nerve endings was delayed at the neuromuscular junction of mice lacking Lgals1 (the gene encoding galectin-1). These results indicate that galectin-1 actively participates in the elimination of neuronal processes after lesion, and that engineered ES cells are a useful tool for studying relevant aspects of neuronal degeneration that have been hitherto difficult to analyze.

The making of successful axonal regeneration: Genes, molecules and signal transduction pathways

Unlike its central counterpart, the peripheral nervous system is well known for its comparatively good potential for regeneration following nerve fiber injury. This ability is mirrored by the de novo expression or upregulation of a wide variety of molecules including transcription factors, growth-stimulating substances, cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions, that promote neurite outgrowth in cultured neurons. However, their role in vivo is less known. Recent studies using neutralizing antibodies, gene inactivation and overexpression techniques have started to shed light on those endogenous molecules that play a key role in axonal outgrowth and the process of successful functional repair in the injured nervous system. The aim of the current review is to provide a summary on this rapidly growing field and the experimental techniques used to define the specific effects of candidate signaling molecules on axonal regeneration in vivo.

Galectin-1 is essential in tumor angiogenesis and is a target for antiangiogenesis therapy

We describe that galectin-1 (gal-1) is a receptor for the angiogenesis inhibitor anginex, and that the protein is crucial for tumor angiogenesis. gal-1 is overexpressed in endothelial cells of different human tumors. Expression knockdown in cultured endothelial cells inhibits cell proliferation and migration. The importance of gal-1 in angiogenesis is illustrated in the zebrafish model, where expression knockdown results in impaired vascular guidance and growth of dysfunctional vessels. The role of gal-1 in tumor angiogenesis is demonstrated in gal-1-null mice, in which tumor growth is markedly impaired because of insufficient tumor angiogenesis. Furthermore, tumor growth in gal-1-null mice no longer responds to antiangiogenesis treatment by anginex. Thus, gal-1 regulates tumor angiogenesis and is a target for angiostatic cancer therapy.

Galectin-1: a small protein with major functions

Galectins are a family of carbohydrate-binding proteins with an affinity for beta-galactosides. Galectin-1 (Gal-1) is differentially expressed by various normal and pathological tissues and appears to be functionally polyvalent, with a wide range of biological activity. The intracellular and extracellular activity of Gal-1 has been described. Evidence points to Gal-1 and its ligands as one of the master regulators of such immune responses as T-cell homeostasis and survival, T-cell immune disorders, inflammation and allergies as well as host-pathogen interactions. Gal-1 expression or overexpression in tumors and/or the tissue surrounding them must be considered as a sign of the malignant tumor progression that is often related to the long-range dissemination of tumoral cells (metastasis), to their dissemination into the surrounding normal tissue, and to tumor immune-escape. Gal-1 in its oxidized form plays a number of important roles in the regeneration of the central nervous system after injury. The targeted overexpression (or delivery) of Gal-1 should be considered as a method of choice for the treatment of some kinds of inflammation-related diseases, neurodegenerative pathologies and muscular dystrophies. In contrast, the targeted inhibition of Gal-1 expression is what should be developed for therapeutic applications against cancer progression. Gal-1 is thus a promising molecular target for the development of new and original therapeutic tools.

Axon regeneration in peripheral nerves is enhanced by proteoglycan degradation

Regeneration of axons in the peripheral nervous system is enhanced by the removal of glycosaminoglycan side chains (GAGs) of chondroitin sulfate proteoglycans. However, some axons regenerate poorly despite such treatment, suggesting the existence of additional inhibitors. We compared the effects of enzymatic removal of GAGs from chondroitin sulfate proteoglycans versus two other proteoglycan species, heparan sulfate and keratan sulfate proteoglycans, on the regeneration of peripheral axons. Common fibular (CF) nerves of thy-1-YFP-H mice were cut and repaired using short segments of CF nerves harvested from wild-type littermates and pre-treated with a GAG-degrading enzyme for 1 h prior to nerve repair. Axonal regeneration was assayed by measuring the lengths of profiles of YFP+ axons in optical sections of the grafted nerves 1 week later. Except for grafts treated with keratanase, more and longer axon profiles were encountered in enzyme-treated grafts than in control grafts. Heparinase III treatments induced the greatest number of axons to enter into the graft. The proportions of axon profiles longer than 1000 microm were greater in grafts treated with chondroitinase ABC or heparinase I, but not with either keratanase or heparinase III. More regenerative sprouts were observed after treatment with heparinase I than any other enzymes. Treatment with a mixture of all four enzymes resulted in an enhancement of axon regeneration which was greater than that observed after treatment with any of the enzymes individually. The effects of chondroitinase ABC and heparinase III were correlated with specific GAG degradation. We believe that enzymatic removal of GAGs is especially effective in promoting the ability of regenerating axons to select their pathway in the distal stump (or nerve graft) and, in the case of chondroitinase ABC or heparinase I, it may also promote growth within that pathway.

Regulation of neuronal and glial galectin-1 expression by peripheral and central axotomy of rat primary afferent neurons

Galectin-1 (Gal1) is an endogenously-expressed protein important for the embryonic development of the full complement of primary sensory neurons and their synaptic connections in the spinal cord. Gal1 also promotes axonal regeneration following peripheral nerve injury, but the regulation of Gal1 by axotomy in primary afferent neurons has not yet been examined. Here, we show by immunohistochemistry and in situ hybridization that Gal1 expression is differentially regulated by peripheral nerve injury and by dorsal rhizotomy. Following peripheral nerve injury, the proportion of Gal1-positive DRG neurons was increased. An increase in the proportion of large-diameter DRG neurons immunopositive for Gal1 was paralleled by an increase in the depth of immunoreactivity in the dorsal horn, where Gal1-positive terminals are normally restricted to laminae I and II. Dorsal rhizotomy did not affect the proportions of neurons containing Gal1 mRNA or protein, but did deplete the ipsilateral dorsal horn of Gal1 immunoreactivity, indicating that it is transported centrally by dorsal root axons. Dorsal rhizotomy also resulted in an increase in Gal1 mRNA the nerve peripheral to the PNS-CNS interface (likely within Schwann cells and/or macrophages), and to a lesser extent within deafferented spinal cord regions undergoing Wallerian degeneration. This latter increase was notable in the dorsal columns and along the prior trajectories of myelinated afferents into the deeper dorsal horn. These results show that neuronal and glial expressions of Gal1 are tightly correlated with regenerative success. Thus, the differential expression pattern of Gal1 following peripheral axotomy and dorsal rhizotomy suggests that endogenous Gal1 may be a factor important to the regenerative response of injured axons.

Molecular mechanisms in successful peripheral regeneration

Peripheral nerve injury is normally followed by a robust regenerative response. Here we describe the early changes associated with injury from the initial rise in intracellular calcium and the subsequent activation of transcription factors and cytokines leading to an inflammatory reaction, and the expression of growth factors, cytokines, neuropeptides, and other secreted molecules involved in cell-to-cell communication promoting regeneration and neurite outgrowth. The aim of this review is to summarize the molecular mechanisms that play a part in executing successful regeneration.

New roles for galectins in brain tumors--from prognostic markers to therapeutic targets

Despite advances in diagnosis and treatment, brain tumors continue to be the leading cause of cancer-related death in patients under 35 years of age, demonstrating the need for better prognostic and therapeutic targets. Galectins, a family of mammalian carbohydrate binding proteins, are involved in many processes important for tumor survival and dissemination, including proliferation, apoptosis, transcriptional regulation, intracellular signaling, cell adhesion, and cell migration. Several galectins are expressed in human brain, with many galectins demonstrating altered expression during tumor progression. Thus, galectins and the functions regulated by this family of proteins are potential targets for the diagnosis and treatment of brain cancer. This review highlights the roles of galectins in cancer and specifically, the developing field of galectins in brain cancer.