NGF steers microglia toward a neuroprotective phenotype

The Impact of Obesity on Microglial Function: Immune, Metabolic and Endocrine Perspectives

Increased life expectancy in combination with modern life style and high prevalence of obesity are important risk factors for development of neurodegenerative diseases. Neuroinflammation is a feature of neurodegenerative diseases, and microglia, the innate immune cells of the brain, are central players in it. The present review discusses the effects of obesity, chronic peripheral inflammation and obesity-associated metabolic and endocrine perturbations, including insulin resistance, dyslipidemia and increased glucocorticoid levels, on microglial function.

Assessing Microglial Dynamics by Live Imaging

Microglia are highly dynamic in the brain in terms of their ability to migrate, proliferate, and phagocytose over the course of an individual's life. Real-time imaging is a useful tool to examine how microglial behavior is regulated and how it affects the surrounding environment. However, microglia are sensitive to environmental stimuli, so they possibly change their state during live imaging in vivo, mainly due to surgical damage, and in vitro due to various effects associated with culture conditions. Therefore, it is difficult to perform live imaging without compromising the properties of the microglia under physiological conditions. To overcome this barrier, various experimental conditions have been developed; recently, it has become possible to perform live imaging of so-called surveillant microglia in vivo, ex vivo, and in vitro, although there are various limitations. Now, we can choose in vivo, ex vivo, or in vitro live imaging systems according to the research objective. In this review, we discuss the advantages and disadvantages of each experimental system and outline the physiological significance and molecular mechanisms of microglial behavior that have been elucidated by live imaging.

Snapshot of microglial physiological functions

Microglia as a defensive arm of the nervous system emerged early in evolution. The surveilling microglia with motile and ramified processes are the main phenotype in the healthy CNS; the surveilling microglial patrol neuronal somata, dendrites, dendritic spines and axons. Increasing evidence suggests that microglia play fundamental roles in development, maturation and ageing of the brain, as well as contribute to a variety of physiological brain processes including sleep and circadian rhythm. Physiological state of microglia is tightly regulated by brain microenvironment and controlled by a sophisticated system of receptors and signalling cascades including ionotropic and metabotropic purinoceptors, pattern-recognition receptors, and receptors for chemokines and cytokines. Microglia also utilise ion channels and transporters in regulating ionic homeostasis and various aspects of microglial function. The major ion transporters expressed by microglia include Na⁺/H⁺ exchanger 1 and Na⁺/Ca²⁺ exchangers, which are involved in regulation of pH_i and Ca²⁺ homeostasis during microglial physiological responses. Microglial cells control development, maturation and plasticity of neuronal ensembles through controlled physiological phagocytosis of synapses or synaptic fragments - processes known as synaptic pruning and trogocytosis. This special issue on "Physiological roles of microglia" is an assembly of papers written by the leading experts in this research field. We start this special issue with this snapshot of microglial physiological functions as a prelude to the indepth discussion of microglia in physiological processes in the nervous system.

Involvement of Bradykinin Receptor 2 in Nerve Growth Factor Neuroprotective Activity

Neurotrophin nerve growth factor (NGF) has been demonstrated to upregulate the gene expression of bradykinin receptor 2 (B2R) on sensory neurons, thus facilitating nociceptive signals. The aim of the present study is to investigate the involvement of B2R in the NGF mechanism of action in nonsensory neurons in vitro by using rat mixed cortical primary cultures (CNs) and mouse hippocampal slices, and in vivo in Alzheimer’s disease (AD) transgenic mice (5xFAD) chronically treated with NGF. A significant NGF-mediated upregulation of B2R was demonstrated by microarray, Western blot, and immunofluorescence analysis in CNs, indicating microglial cells as the target of this modulation. The B2R involvement in the NGF mechanism of action was also demonstrated by using a selective B2R antagonist which was able to reverse the neuroprotective effect of NGF in CNs, as revealed by viability assay, and the NGF-induced long-term potentiation (LTP) in hippocampal slices. To confirm in vitro observations, B2R upregulation was observed in 5xFAD mouse brain following chronic intranasal NGF treatment. This study demonstrates for the first time that B2R is a key element in the neuroprotective activity and synaptic plasticity mediated by NGF in brain cells.

The impact of trophic and immunomodulatory factors on oligodendrocyte maturation: Potential treatments for encephalopathy of prematurity

Encephalopathy of prematurity (EoP) is a major cause of morbidity in preterm neonates, causing neurodevelopmental adversities that can lead to lifelong impairments. Preterm birth-related insults, such as cerebral oxygen fluctuations and perinatal inflammation, are believed to negatively impact brain development, leading to a range of brain abnormalities. Diffuse white matter injury is a major hallmark of EoP and characterized by widespread hypomyelination, the result of disturbances in oligodendrocyte lineage development. At present, there are no treatment options available, despite the enormous burden of EoP on patients, their families, and society. Over the years, research in the field of neonatal brain injury and other white matter pathologies has led to the identification of several promising trophic factors and cytokines that contribute to the survival and maturation of oligodendrocytes, and/or dampening neuroinflammation. In this review, we discuss the current literature on selected factors and their therapeutic potential to combat EoP, covering a wide range of in vitro, preclinical and clinical studies. Furthermore, we offer a future perspective on the translatability of these factors into clinical practice.

Protective Functions of Reactive Astrocytes Following Central Nervous System Insult

Astrocytes play important roles in numerous central nervous system disorders including autoimmune inflammatory, hypoxic, and degenerative diseases such as Multiple Sclerosis, ischemic stroke, and Alzheimer’s disease. Depending on the spatial and temporal context, activated astrocytes may contribute to the pathogenesis, progression, and recovery of disease. Recent progress in the dissection of transcriptional responses to varying forms of central nervous system insult has shed light on the mechanisms that govern the complexity of reactive astrocyte functions. While a large body of research focuses on the pathogenic effects of reactive astrocytes, little is known about how they limit inflammation and contribute to tissue regeneration. However, these protective astrocyte pathways might be of relevance for the understanding of the underlying pathology in disease and may lead to novel targeted approaches to treat autoimmune inflammatory and degenerative disorders of the central nervous system. In this review article, we have revisited the emerging concept of protective astrocyte functions and discuss their role in the recovery from inflammatory and ischemic disease as well as their role in degenerative disorders. Focusing on soluble astrocyte derived mediators, we aggregate the existing knowledge on astrocyte functions in the maintenance of homeostasis as well as their reparative and tissue-protective function after acute lesions and in neurodegenerative disorders. Finally, we give an outlook of how these mediators may guide future therapeutic strategies to tackle yet untreatable disorders of the central nervous system.

The delicate balance between neurotoxicity and neuroprotection in the context of HIV-1 infection

Human immunodeficiency virus type-1 (HIV-1) causes a spectrum of neurological impairments, termed HIV-associated neurocognitive disorder (HAND), following the infiltration of infected cells into the brain. Even though the implementation of antiretroviral therapy reduced the systemic viral load, the prevalence of HAND remains unchanged and infected patients develop persisting neurological disturbances affecting their quality of life. As a result, HAND have gained importance in basic and clinical researches, warranting the need of developing new adjunctive treatments. Nonetheless, a better understanding of the molecular and cellular mechanisms remains necessary. Several studies consolidated their efforts into elucidating the neurotoxic signaling leading to HAND including the deleterious actions of HIV-1 viral proteins and inflammatory mediators. However, the scope of these studies is not sufficient to address all the complexity related to HAND development. Fewer studies focused on an altered neuroprotective capacity of the brain to respond to HIV-1 infection. Neurotrophic factors are endogenous polyproteins involved in neuronal survival, synaptic plasticity, and neurogenesis. Any defects in the processing or production of these crucial factors might compose a risk factor rendering the brain more vulnerable to neuronal damages. Due to their essential roles, they have been investigated for their diverse interplays with HIV-1 infection. In this review, we present a complete description of the neurotrophic factors involved in HAND. We discuss emerging concepts for their therapeutic applications and summarize the complex mechanisms that down-regulate their production in favor of a neurotoxic environment. For certain factors, we finally address opposing roles that rather lead to increased inflammation.

Macropinocytosis in Different Cell Types: Similarities and Differences

Macropinocytosis is a unique pathway of endocytosis characterised by the nonspecific internalisation of large amounts of extracellular fluid, solutes and membrane in large endocytic vesicles known as macropinosomes. Macropinocytosis is important in a range of physiological processes, including antigen presentation, nutrient sensing, recycling of plasma proteins, migration and signalling. It has become apparent in recent years from the study of specialised cells that there are multiple pathways of macropinocytosis utilised by different cell types, and some of these pathways are triggered by different stimuli. Understanding the physiological function of macropinocytosis requires knowledge of the regulation and fate of the macropinocytosis pathways in a range of cell types. Here, we compare the mechanisms of macropinocytosis in different primary and immortalised cells, identify the gaps in knowledge in the field and discuss the potential approaches to analyse the function of macropinocytosis in vivo.

Therapeutic Effects of Simultaneous Delivery of Nerve Growth Factor mRNA and Protein via Exosomes on Cerebral Ischemia

Stroke is the leading neurological cause of death and disability all over the world, with few effective drugs. Nerve growth factor (NGF) is well known for its multifaceted neuroprotective functions post-ischemia. However, the lack of an efficient approach to systemically deliver bioactive NGF into ischemic region hinders its clinical application. In this study, we engineered the exosomes with RVG peptide on the surface for neuron targeting and loaded NGF into exosomes simultaneously, with the resultant exosomes denoted as NGF@Exo^RVG. By systemic administration of NGF@Exo^RVG, NGF was efficiently delivered into ischemic cortex, with a burst release of encapsulated NGF protein and de novo NGF protein translated from the delivered mRNA. Moreover, NGF@Exo^RVG was found to be highly stable for preservation and function efficiently for a long time in vivo. Functional study revealed that the delivered NGF reduced inflammation by reshaping microglia polarization, promoted cell survival, and increased the population of doublecortin-positive cells, a marker of neuroblast. The results of our study suggest the potential therapeutic effects of NGF@Exo^RVG for stroke. Moreover, the strategy proposed in our study may shed light on the clinical application of other neurotrophic factors for central nervous system diseases.

Unveiling functional motions based on point mutations in biased signaling systems: A normal mode study on nerve growth factor bound to TrkA

Many receptors elicit signal transduction by activating multiple intracellular pathways. This transduction can be triggered by a non-specific ligand, which simultaneously activates all the signaling pathways of the receptors. However, the binding of one biased ligand preferentially trigger one pathway over another, in a process called biased signaling. The identification the functional motions related to each of these distinct pathways has a direct impact on the development of new effective and specific drugs. We show here how to detect specific functional motions by considering the case of the NGF/TrkA-Ig2 complex. NGF-mediated TrkA receptor activation is dependent on specific structural motions that trigger the neuronal growth, development, and survival of neurons in nervous system. The R221W mutation in the ngf gene impairs nociceptive signaling. We discuss how the large-scale structural effects of this mutation lead to the suppression of collective motions necessary to induce TrkA activation of nociceptive signaling. Our results suggest that subtle changes in the NGF interaction network due to the point mutation are sufficient to inhibit the motions of TrkA receptors putatively linked to nociception. The methodological approach presented in this article, based jointly on the normal mode analysis and the experimentally observed functional alterations due to point mutations provides an essential tool to reveal the structural changes and motions linked to the disease, which in turn could be necessary for a drug design study.

Muscle Injury Induces Postoperative Cognitive Dysfunction

Postoperative cognitive dysfunction (POCD) is a major complication affecting patients of any age undergoing surgery. This syndrome impacts everyday life up to months after hospital discharge, and its pathophysiology still remains unclear. Translational research focusing on POCD is based on a wide variety of rodent models, such as the murine tibial fracture, whose severity can limit mouse locomotion and proper behavioral assessment. Besides, influence of skeletal muscle injury, a lesion encountered in a wide range of surgeries, has not been explored in POCD occurrence. We propose a physical model of muscle injury in CX3CR1^GFP/+ mice (displaying green fluorescent microglial cells) to study POCD, with morphological, behavioral and molecular approaches. We highlighted: alteration of short- and long-term memory after muscle regeneration, wide microglial reactivity in the brain, including hippocampus area, 24 hours after muscle injury, and an alteration of central brain derived neurotrophic factor (BDNF) and nerve growth factor (NGF) balance, 28 days after muscle injury. Our results suggest for the first time that muscle injury can have early as well as late impacts on the brain. Our CX3CR1^GFP/+ model can also facilitate microglial investigation, more specifically their pivotal role in neuroinflammation and synaptic plasticity, in the pathophysiology of POCD.

Nerve growth factor activates autophagy in Schwann cells to enhance myelin debris clearance and to expedite nerve regeneration

Rationale: Autophagy in Schwann cells (SCs) is crucial for myelin debris degradation and clearance following peripheral nerve injury (PNI). Nerve growth factor (NGF) plays an important role in reconstructing peripheral nerve fibers and promoting axonal regeneration. However, it remains unclear if NGF effect in enhancing nerve regeneration is mediated through autophagic clearance of myelin debris in SCs.

Methods: In vivo, free NGF solution plus with/without pharmacological inhibitors were administered to a rat sciatic nerve crush injury model. In vitro, the primary Schwann cells (SCs) and its cell line were cultured in normal medium containing NGF, their capable of swallowing or clearing degenerated myelin was evaluated through supplement of homogenized myelin fractions.

Results: Administration of exogenous NGF could activate autophagy in dedifferentiated SCs, accelerate myelin debris clearance and phagocytosis, as well as promote axon and myelin regeneration at early stage of PNI. These NGF effects were effectively blocked by autophagy inhibitors. In addition, inhibition of the p75 kD neurotrophin receptor (p75^NTR) signal or inactivation of the AMP-activated protein kinase (AMPK) also inhibited the NGF effect as well.

Conclusions: NGF effect on promoting early nerve regeneration is closely associated with its accelerating autophagic clearance of myelin debris in SCs, which probably regulated by the p75^NTR/AMPK/mTOR axis. Our studies thus provide strong support that NGF may serve as a powerful pharmacological therapy for peripheral nerve injuries.

The NGFR100W Mutation Specifically Impairs Nociception without Affecting Cognitive Performance in a Mouse Model of Hereditary Sensory and Autonomic Neuropathy Type V

Nerve growth factor (NGF) is a key mediator of nociception, acting during the development and differentiation of dorsal root ganglion (DRG) neurons, and on adult DRG neuron sensitization to painful stimuli. NGF also has central actions in the brain, where it regulates the phenotypic maintenance of cholinergic neurons. The physiological function of NGF as a pain mediator is altered in patients with Hereditary Sensory and Autonomic Neuropathy type V (HSAN V), caused by the 661C>T transition in the Ngf gene, resulting in the R100W missense mutation in mature NGF. Homozygous HSAN V patients present with congenital pain insensitivity, but are cognitively normal. This led us to hypothesize that the R100W mutation may differentially affect the central and peripheral actions of NGF. To test this hypothesis and provide a mechanistic basis to the HSAN V phenotype, we generated transgenic mice harboring the human 661C>T mutation in the Ngf gene and studied both males and females. We demonstrate that heterozygous NGF^R100W/wt mice display impaired nociception. DRG neurons of NGF^R100W/wt mice are morphologically normal, with no alteration in the different DRG subpopulations, whereas skin innervation is reduced. The NGF^R100W protein has reduced capability to activate pain-specific signaling, paralleling its reduced ability to induce mechanical allodynia. Surprisingly, however, NGF^R100W/wt mice, unlike heterozygous mNGF^+/- mice, show no learning or memory deficits, despite a reduction in secretion and brain levels of NGF. The results exclude haploinsufficiency of NGF as a mechanistic cause for heterozygous HSAN V mice and demonstrate a specific effect of the R100W mutation on nociception.SIGNIFICANCE STATEMENT The R100W mutation in nerve growth factor (NGF) causes Hereditary Sensory and Autonomic Neuropathy type V, a rare disease characterized by impaired nociception, even in apparently clinically silent heterozygotes. For the first time, we generated and characterized heterozygous knock-in mice carrying the human R100W-mutated allele (NGF^R100W/wt). Mutant mice have normal nociceptor populations, which, however, display decreased activation of pain transduction pathways. NGF^R100W interferes with peripheral and central NGF bioavailability, but this does not impact on CNS function, as demonstrated by normal learning and memory, in contrast with heterozygous NGF knock-out mice. Thus, a point mutation allows neurotrophic and pronociceptive functions of NGF to be split, with interesting implications for the treatment of chronic pain.

Neurosteroids as regulators of neuroinflammation

Neuroinflammation is a physiological protective response in the context of infection and injury. However, neuroinflammation, especially if chronic, may also drive neurodegeneration. Neurodegenerative diseases, such as multiple sclerosis (MS), Alzheimer's disease (AD), Parkinson's disease (PD) and traumatic brain injury (TBI), display inflammatory activation of microglia and astrocytes. Intriguingly, the central nervous system (CNS) is a highly steroidogenic environment synthesizing steroids de novo, as well as metabolizing steroids deriving from the circulation. Neurosteroid synthesis can be substantially affected by neuroinflammation, while, in turn, several steroids, such as 17β-estradiol, dehydroepiandrosterone (DHEA) and allopregnanolone, can regulate neuroinflammatory responses. Here, we review the role of neurosteroids in neuroinflammation in the context of MS, AD, PD and TBI and describe underlying molecular mechanisms. Moreover, we introduce the concept that synthetic neurosteroid analogues could be potentially utilized for the treatment of neurodegenerative diseases in the future.

Microglia-neuron crosstalk: Signaling mechanism and control of synaptic transmission

The continuous crosstalk between microglia and neurons is required for microglia housekeeping functions and contributes to brain homeostasis. Through these exchanges, microglia take part in crucial brain functions, including development and plasticity. The alteration of neuron-microglia communication contributes to brain disease states with consequences, ranging from synaptic function to neuronal survival. This review focuses on the signaling pathways responsible for neuron-microglia crosstalk, highlighting their physiological roles and their alteration or specific involvement in disease. In particular, we discuss studies, establishing how these signaling allow microglial cells to control relevant physiological functions during brain development, including synaptic formation and circuit refinement. In addition, we highlight how microglia and neurons interact functionally to regulate highly dynamical synaptic functions. Microglia are able to release several signaling molecules involved in the regulation of synaptic activity and plasticity. On the other side, molecules of neuronal origin control microglial processes motility in an activity-dependent manner. Indeed, the continuous crosstalk between microglia and neurons is required for the sensing and housekeeping functions of microglia and contributes to the maintenance of brain homeostasis and, particularly, to the sculpting of neuronal connections during development. These interactions lay on the delicate edge between physiological processes and homeostasis alteration in pathology and are themselves altered during neuroinflammation. The full description of these processes could be fundamental for understanding brain functioning in health and disease.

Effects of Neurotrophic Factors in Glial Cells in the Central Nervous System: Expression and Properties in Neurodegeneration and Injury

Astrocytes, oligodendrocytes, and microglia are abundant cell types found in the central nervous system and have been shown to play crucial roles in regulating both normal and disease states. An increasing amount of evidence points to the critical importance of glia in mediating neurodegeneration in Alzheimer’s and Parkinson’s diseases (AD, PD), and in ischemic stroke, where microglia are involved in initial tissue clearance, and astrocytes in the subsequent formation of a glial scar. The importance of these cells for neuronal survival has previously been studied in co-culture experiments and the search for neurotrophic factors (NTFs) initiated after finding that the addition of conditioned media from astrocyte cultures could support the survival of primary neurons in vitro. This led to the discovery of the potent dopamine neurotrophic factor, glial cell line-derived neurotrophic factor (GDNF). In this review, we focus on the relationship between glia and NTFs including neurotrophins, GDNF-family ligands, CNTF family, and CDNF/MANF-family proteins. We describe their expression in astrocytes, oligodendrocytes and their precursors (NG2-positive cells, OPCs), and microglia during development and in the adult brain. Furthermore, we review existing data on the glial phenotypes of NTF knockout mice and follow NTF expression patterns and their effects on glia in disease models such as AD, PD, stroke, and retinal degeneration.

Nerve Growth Factor modulates LPS - induced microglial glycolysis and inflammatory responses

Microglia, the parenchymal immune cells of the central nervous system, orchestrate neuroinflammation in response to infection or damage, and promote tissue repair. However, aberrant microglial responses are integral to neurodegenerative diseases and critically contribute to disease progression. Thus, it is important to elucidate how microglia - mediated neuroinflammation is regulated by endogenous factors. Here, we explored the effect of Nerve Growth Factor (NGF), an abundant neurotrophin, on microglial inflammatory responses. NGF, via its high affinity receptor TrkA, downregulated LPS - induced production of pro-inflammatory cytokines and NO in primary mouse microglia and inhibited TLR4 - mediated activation of the NF-κB and JNK pathways. Furthermore, NGF attenuated the LPS - enhanced glycolytic activity in microglia, as suggested by reduced glucose uptake and decreased expression of the glycolytic enzymes Pfkβ3 and Ldhα. Consistently, 2DG - mediated glycolysis inhibition strongly downregulated LPS - induced cytokine production in microglial cells. Our findings demonstrate that NGF attenuates pro-inflammatory responses in microglia and may thereby contribute to regulation of microglia - mediated neuroinflammation.

Chronic electrical stimulation reduces hyperalgesia and associated spinal changes induced by peripheral nerve injury

OBJECTIVES

We aimed to investigate if different protocols of electrical stimulation following nerve injury might improve neuropathic pain outcomes and modify associated plastic changes at the spinal cord level.

MATERIALS AND METHODS

Adult rats were subjected to sciatic nerve transection and repair, and distributed in four groups: untreated (SNTR, n = 12), repeated acute electrical stimulation (rAES, 50 Hz, one hour, n = 12), chronic electrical stimulation (CES, 50 Hz, one hour, n = 12), and increasing-frequency chronic electrical stimulation (iCES, one hour, n = 12) delivered during two weeks following the lesion. The threshold of nociceptive withdrawal to mechanical stimuli was evaluated by means of a Von Frey algesimeter during three weeks postlesion. Spinal cord samples were processed by immunohistochemistry for labeling glial cells, adrenergic receptors, K⁺ -Cl^- cotransporter 2 (KCC2) and GABA.

RESULTS

Acute electrical stimulation (50 Hz, one hour) delivered at 3, 7, and 14 days induced an immediate increase of mechanical pain threshold that disappeared after a few days. Chronic electrical stimulation given daily reduced mechanical hyperalgesia until the end of follow-up, being more sustained with the iCES than with constant 50 Hz stimulation (CES). Chronic stimulation protocols restored the expression of β2 adrenergic receptor and of KCC2 in the dorsal horn, which were significantly reduced by nerve injury. These treatments decreased also the activation of microglia and astrocytes in the dorsal horn.

CONCLUSION

Daily electrical stimulation, especially if frequency-patterned, was effective in ameliorating hyperalgesia after nerve injury, and partially preventing the proinflammatory and hyperalgesic changes in the dorsal horn associated to neuropathic pain.

Innovative Therapy for Alzheimer's Disease-With Focus on Biodelivery of NGF

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder associated with abnormal protein modification, inflammation and memory impairment. Aggregated amyloid beta (Aβ) and phosphorylated tau proteins are medical diagnostic features. Loss of memory in AD has been associated with central cholinergic dysfunction in basal forebrain, from where the cholinergic circuitry projects to cerebral cortex and hippocampus. Various reports link AD progression with declining activity of cholinergic neurons in basal forebrain. The neurotrophic molecule, nerve growth factor (NGF), plays a major role in the maintenance of cholinergic neurons integrity and function, both during development and adulthood. Numerous studies have also shown that NGF contributes to the survival and regeneration of neurons during aging and in age-related diseases such as AD. Changes in neurotrophic signaling pathways are involved in the aging process and contribute to cholinergic and cognitive decline as observed in AD. Further, gradual dysregulation of neurotrophic factors like NGF and brain derived neurotrophic factor (BDNF) have been reported during AD development thus intensifying further research in targeting these factors as disease modifying therapies against AD. Today, there is no cure available for AD and the effects of the symptomatic treatment like cholinesterase inhibitors (ChEIs) and memantine are transient and moderate. Although many AD treatment studies are being carried out, there has not been any breakthrough and new therapies are thus highly needed. Long-term effective therapy for alleviating cognitive impairment is a major unmet need. Discussion and summarizing the new advancements of using NGF as a potential therapeutic implication in AD are important. In summary, the intent of this review is describing available experimental and clinical data related to AD therapy, priming to gain additional facts associated with the importance of NGF for AD treatment, and encapsulated cell biodelivery (ECB) as an efficient tool for NGF delivery.

Trophic Factors in Inflammation and Regeneration: The Role of MANF and CDNF

Regeneration is an important process in multicellular organisms, responsible for homeostatic renewal and repair of different organs after injury. Immune cell activation is observed at early stages of the regenerative response and its regulation is essential for regenerative success. Thus, immune regulators play central roles in optimizing regenerative responses. Neurotrophic factors (NTFs) are secreted molecules, defined by their ability to support neuronal cell types. However, emerging evidence suggests that they can also play important functions in the regulation of immune cell activation and tissue repair. Here we discuss the literature supporting a role of NTFs in the regulation of inflammation and regeneration. We will focus, in particular, in the emerging roles of mesencephalic astrocyte-derived neurotrophic factor (MANF) and cerebral dopamine neurotrophic factor (CDNF) in the regulation of immune cell function and in the central role that immune modulation plays in their biological activity in vivo. Finally, we will discuss the potential use of these factors to optimize regenerative success in vivo, both within and beyond the nervous system.

Painless Nerve Growth Factor: A TrkA biased agonist mediating a broad neuroprotection via its actions on microglia cells

Nerve Growth Factor (NGF) is a therapeutic candidate for Alzheimer's disease, based on its well known actions on basal forebrain cholinergic neurons. However, because of its pro-nociceptive activity, in current clinical trials NGF has to be administered intraparenchymally into the brain by neurosurgery via cell or gene therapy approaches. To prevent the NGF pain-inducing collateral effects, thus avoiding the necessity for local brain injection, we developed painless NGF (hNGFp), based on the human genetic disease Hereditary Sensory and Autonomic Neuropathy type V (HSAN V). hNGFp has similar neurotrophic activity as wild type human NGF, but its pain sensitizing activity is tenfold lower. Pharmacologically, hNGFp is a biased receptor agonist of NGF TrkA receptor. The results of recent studies shed new light on the neuroprotective mechanism by hNGFp and are highly relevant for the planning of NGF-based clinical trials. The intraparenchymal delivery of hNGFp, as used in clinical trials, was simulated in the 5xFAD mouse model and found to be inefficacious in reducing Aβ plaque load. On the contrary, the same dose of hNGFp administered intranasally, which was rather widely biodistributed in the brain and did not induce pain sensitization, blocked APP processing into amyloid and restored synaptic plasticity and memory in this aggressive neurodegeneration model. This potent and broad neuroprotection by hNGFp was found to be mediated by hNGFp actions on glial cells. hNGFp increases inflammatory proteins such as the soluble TNFα receptor II and the chemokine CXCL12. Independent work has shown that NGF has a potent anti-inflammatory action on microglia and steers them towards a neuroprotective phenotype. These studies demonstrate that microglia cells are a new target cell of NGF in the brain and have therapeutic significance: i) they establish that the neuroprotective actions of hNGFp relies on a widespread exposure of the brain, ii) they identify a new anti-neurodegenerative pathway, linking hNGFp to inflammatory chemokines and cytokines via microglia, a common target for new therapeutic opportunities for neurodegenerative diseases, iii) they extend the neuroprotective potential of hNGFp beyond its classical cholinergic target, thereby widening the range of neurological diseases for which this neurotrophic factor might be used therapeutically, iv) they help interpreting the results of current NGF clinical trials in AD and the design of future trials with this new potent therapeutic candidate.

How to reprogram microglia toward beneficial functions

Microglia, brain cells of nonneural origin, orchestrate the inflammatory response to diverse insults, including hypoxia/ischemia or maternal/fetal infection in the perinatal brain. Experimental studies have demonstrated the capacity of microglia to recognize pathogens or damaged cells activating a cytotoxic response that can exacerbate brain damage. However, microglia display an enormous plasticity in their responses to injury and may also promote resolution stages of inflammation and tissue regeneration. Despite the critical role of microglia in brain pathologies, the cellular mechanisms that govern the diverse phenotypes of microglia are just beginning to be defined. Here we review emerging strategies to drive microglia toward beneficial functions, selectively reporting the studies which provide insights into molecular mechanisms underlying the phenotypic switch. A variety of approaches have been proposed which rely on microglia treatment with pharmacological agents, cytokines, lipid messengers, or microRNAs, as well on nutritional approaches or therapies with immunomodulatory cells. Analysis of the molecular mechanisms relevant for microglia reprogramming toward pro‐regenerative functions points to a central role of energy metabolism in shaping microglial functions. Manipulation of metabolic pathways may thus provide new therapeutic opportunities to prevent the deleterious effects of inflammatory microglia and to control excessive inflammation in brain disorders.

Microglia/Astrocytes-Glioblastoma Crosstalk: Crucial Molecular Mechanisms and Microenvironmental Factors

In recent years, the functions of glial cells, namely, astrocytes and microglia, have gained prominence in several diseases of the central nervous system, especially in glioblastoma (GB), the most malignant primary brain tumor that leads to poor clinical outcomes. Studies showed that microglial cells or astrocytes play a critical role in promoting GB growth. Based on the recent findings, the complex network of the interaction between microglial/astrocytes cells and GB may constitute a potential therapeutic target to overcome tumor malignancy. In the present review, we summarize the most important mechanisms and functions of the molecular factors involved in the microglia or astrocytes-GB interactions, which is particularly the alterations that occur in the cell's extracellular matrix and the cytoskeleton. We overview the cytokines, chemokines, neurotrophic, morphogenic, metabolic factors, and non-coding RNAs actions crucial to these interactions. We have also discussed the most recent studies regarding the mechanisms of transportation and communication between microglial/astrocytes - GB cells, namely through the ABC transporters or by extracellular vesicles. Lastly, we highlight the therapeutic challenges and improvements regarding the crosstalk between these glial cells and GB.

NGF steers microglia toward a neuroprotective phenotype

Microglia are the sentinels of the brain but a clear understanding of the factors that modulate their activation in physiological and pathological conditions is still lacking. Here we demonstrate that Nerve Growth Factor (NGF) acts on microglia by steering them toward a neuroprotective and anti-inflammatory phenotype. We show that microglial cells express functional NGF receptors in vitro and ex vivo. Our transcriptomic analysis reveals how, in primary microglia, NGF treatment leads to a modulation of motility, phagocytosis and degradation pathways. At the functional level, NGF induces an increase in membrane dynamics and macropinocytosis and, in vivo, it activates an outward rectifying current that appears to modulate glutamatergic neurotransmission in nearby neurons. Since microglia are supposed to be a major player in Aβ peptide clearance in the brain, we tested the effects of NGF on its phagocytosis. NGF was shown to promote TrkA-mediated engulfment of Aβ by microglia, and to enhance its degradation. Additionally, the proinflammatory activation induced by Aβ treatment is counteracted by the concomitant administration of NGF. Moreover, by acting specifically on microglia, NGF protects neurons from the Aβ-induced loss of dendritic spines and inhibition of long term potentiation. Finally, in an ex-vivo setup of acute brain slices, we observed a similar increase in Aβ engulfment by microglial cells under the influence of NGF. Our work substantiates a role for NGF in the regulation of microglial homeostatic activities and points toward this neurotrophin as a neuroprotective agent in Aβ accumulation pathologies, via its anti-inflammatory activity on microglia.