Phenotypic knockout of nerve growth factor in adult transgenic mice reveals severe deficits in basal forebrain cholinergic neurons, cell death in the spleen, and skeletal muscle dystrophy

The dependence of basal forebrain cholinergic neurons on NGF: The case in Alzheimer pathology

This chapter discusses the dependency of basal forebrain cholinergic neurons (BFCNs) on endogenous nerve growth factor (NGF) for the structural and physiologic maintenance of the neuronal cell somata, axonal projections, and terminal synapses. It covers the discovery of NGF and the occurrence of a CNS neurotrophin family and their cognate receptors and their signaling mechanisms. It concludes with a description of the NGF metabolic pathway and its dysregulation in Alzheimer disease (AD) and Down syndrome pathology, explaining the progressive atrophy of BFCNs, which starts at preclinical stages and is reflected in body fluid biomarkers.

From Lab Bench to Hope: Emerging Gene Therapies in Clinical Trials for Alzheimer's Disease

Alzheimer's disease is a progressive neurodegenerative disorder that affects memory and cognitive abilities, affecting millions of people around the world. Current treatments focus on the management of symptoms, as no effective therapy has been approved to modify the underlying disease process. Gene therapy is a promising approach that can offer disease-modifying treatment for AD, targeting various aspects of the pathophysiology of the disease. This review presents a comprehensive overview of the current state of gene therapy research for AD, with a specific focus on clinical trials and preclinical studies that have used nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), apolipoprotein E2 (APOE2), and human telomerase reverse transcriptase (hTERT) as therapeutic gene therapy approaches. These gene targets have shown potential to alleviate the neuropathology of AD in animal studies and have demonstrated feasibility and safety in non-human primates. Despite the failure of the NGF gene therapy approach in clinical trials, we have reviewed and highlighted the reported findings and evaluations from the trials. Furthermore, the review included the conclusions of postmortem brain tissue analysis of AD patients who received NGF gene therapy. The goal is to learn from the failed trials and improve the approach in the future. Although gene therapy shows promise, it faces several challenges and limitations, including optimizing gene delivery methods, enhancing safety and efficacy profiles, and determining long-term results. This review contributes to the growing body of literature on innovative treatments for AD and highlights the need for more research and development to advance gene therapy as a viable treatment option for AD.

Neurotization of decellularized muscle graft increases de novo type I slow muscle fiber formation and large fiber size frequency

Volumetric muscle loss (VML) injuries are the result of extreme trauma from battlefield injuries, tumor ablations, and other physical traumas such as car crash injuries. The abrupt loss of muscle restricts the tissue's remaining regenerative capacity, leading to loss of satellite cells, peripheral nerve connections, and aberrant fibrosis. Prior research from our lab demonstrated that decellularized muscle matrix (DMM) supported regeneration of de novo fibers within the graft. The goal of this study was to determine whether DMM treated with a peripheral nerve using neurotization surgeries would enhance muscle regeneration and innervation. Forty-eight male Sprague Dawley rats were randomized and received a 1.5×1 cm defect treated with no treatment empty defect (ED), DMM, or autograft with a direct peroneal (antagonist) neurotization or tibial via end to side graft (agonist) neurotization. DMM grafts treated with neurotization utilizing either peroneal or tibial nerve axons increased fast twitch fibers within the grafted area compared to untreated DMM or ED. Additionally, the frequency distribution of myofiber size shifted toward a healthier morphology in the tibial nerve axon neurotized DMM compared to the uninjured medial head. Lastly, Nanostring gene results showed DMM treated with a neurotization shifted expression towards a more regenerative phenotype with some myogenic markers returning to sham levels. These data indicate that injured muscle treated with DMM and neurotization becomes pro-regenerative and can contribute to the functionalization of DMM. STATEMENT OF SIGNIFICANCE: Extremity soft tissue trauma like volumetric muscle loss (VML) can result in permanent loss of skeletal muscle mass, denervation, and ischemia posing a significant clinical challenge. VML injuries disrupt normal tissue architecture in addition to intramuscular axons which are critical elements in muscle function and regeneration. The overall objective of this study was to enhance axon growth into a VML injury treated with decellularized muscle matrix (DMM) using neurotization. DMM is an acellular biomaterial capable of regenerating skeletal muscle; however, without bona fide neuromuscular connections, functional gains are small. This study demonstrates that introducing motor axons into an acellular regenerative material using neurotization enhanced muscle regeneration and promoted slow twitch fiber formation.

Delivery of Neuroregenerative Proteins to the Brain for Treatments of Neurodegenerative Brain Diseases

Neurodegenerative brain diseases such as Alzheimer's disease (AD), multiple sclerosis (MS), and Parkinson's disease (PD) are difficult to treat. Unfortunately, many therapeutic agents for neurodegenerative disease only halt the progression of these diseases and do not reverse neuronal damage. There is a demand for finding solutions to reverse neuronal damage in the central nervous system (CNS) of patients with neurodegenerative brain diseases. Therefore, the purpose of this review is to discuss the potential for therapeutic agents like specific neurotrophic and growth factors in promoting CNS neuroregeneration in brain diseases. We discuss how BDNF, NGF, IGF-1, and LIF could potentially be used for the treatment of brain diseases. The molecule's different mechanisms of action in stimulating neuroregeneration and methods to analyze their efficacy are described. Methods that can be utilized to deliver these proteins to the brain are also discussed.

METTL14 mediates nerve growth factor-stimulated testosterone synthesis in porcine theca cells†

Ovarian theca cells produce testosterone, which acts as a vital precursor substance for synthesizing estrogens during follicular development. Nerve growth factor (NGF) has been shown to participate in reproductive physiology, specifically to follicular development and ovulation. There is currently no available data on the impact of NGF on testosterone synthesis in porcine theca cells. Furthermore, m6A modification is the most common internal modification in eukaryotic mRNAs that are closely associated with female gametogenesis, follicle development, ovulation, and other related processes. It is also uncertain whether the three main enzymes associated with m6A, such as Writers, Erasers, and Readers, play a role in this process. The present study, with an in vitro culture model, investigated the effect of NGF on testosterone synthesis in porcine theca cells and the role of Writers-METTL14 in this process. It was found that NGF activates the PI3K/AKT signaling pathway through METTL14, which regulates testosterone synthesis in porcine theca cells. This study will help to further elucidate the mechanisms by which NGF regulates follicular development and provide new therapeutic targets for ovary-related diseases in female animals. Summary Sentence  The present study investigated the effect of NGF on testosterone synthesis in porcine theca cells. It was found that NGF activates the PI3K/AKT signaling pathway through METTL14, which regulates testosterone synthesis in porcine theca cells.

Targeting Molecular Mechanisms of Obesity- and Type 2 Diabetes Mellitus-Induced Skeletal Muscle Atrophy with Nerve Growth Factor

Skeletal muscle plays a critical role in metabolic diseases, such as obesity and type 2 diabetes mellitus (T2DM). Muscle atrophy, characterized by a decrease in muscle mass and function, occurs due to an imbalance between the rates of muscle protein synthesis and degradation. This study aimed to investigate the molecular mechanisms that lead to muscle atrophy in obese and T2DM mouse models. Additionally, the effect of nerve growth factor (NGF) on the protein synthesis and degradation pathways was examined. Male mice were divided into three groups: a control group that was fed a standard chow diet, and two experimental groups that were fed a Western diet. After 8 weeks, the diabetic group was injected with streptozotocin to induce T2DM. Each group was then further divided into NGF-treated or non-treated control group. In the gastrocnemius muscles of the Western diet group, increased expressions of myostatin, autophagy markers, and ubiquitin ligases were observed. Skeletal muscle tissue morphology indicated signs of muscle atrophy in both obese and diabetic mice. The NGF-treated group showed a prominent decrease in the protein levels of myostatin and autophagy markers. Furthermore, the NGF-treated group showed an increased Cyclin D1 level. Western diet-induced obesity and T2DM may be linked to muscle atrophy through upregulation of myostatin and subsequent increase in the ubiquitin and autophagy systems. Moreover, NGF treatment may improve muscle protein synthesis and cell cycling.

Enhancement of neural regeneration as a therapeutic strategy for Alzheimer's disease (Review)

Alzheimer's disease (AD), the most common cause of dementia worldwide, has gradually become a global health concern for society and individuals with the process of global ageing. Although extensive research has been carried out on AD, the etiology and pathological mechanism of the disease are still unclear, and there is no specific drug to cure or delay AD progression. The exploration of enhancing nerve regeneration in AD has gradually attracted increasing attention. In the current review, the existing therapeutic strategies were summarized to induce nerve regeneration which can increase the number of neurons, and improve the survival of neurons, the plasticity of synapses and synaptic activity. The strategies include increasing neurotrophic expression (such as brain-derived neurotrophic factor and nerve growth factor), inhibiting acetylcholinesterase (such as donepezil, tacrine, rivastigmine and galanthamine), elevating histone deacetylase levels (such as RGFP-966, Tasquinimod, CM-414 and 44B), stimulating the brain by physiotherapy (such as near-infrared light, repetitive transcranial magnetic stimulation, and transcranial direct current stimulation) and transplanting exogenous neural stem cells. However, further evaluations need to be performed to determine the optimal treatment. The present study reviews recent interventions for enhancing adult neurogenesis and attempts to elucidate their mechanisms of action, which may provide a theoretical basis for inducing nerve regeneration to fight against AD.

Aripiprazole combined with nerve growth factor improves cognitive function in mice with schizophrenia model

The pathogenesis and treatment of cognitive dysfunction in patients with schizophrenia (SCZ) remains a challenge. Exploring new effective treatment strategies is relevant for the improvement of cognitive function. Aripiprazole (ARI) is an atypical antipsychotic that improves some cognitive functions. Nerve growth factor (NGF) has been shown to improve cognitive function in certain neurological impairments and partial neurological deficits, but its mechanism of action in cognitive dysfunction in SCZ is unclear. In this study, we established schizophrenia mouse model with dizocilpine (MK-801); treated mice with ARI alone or in combination with NGF; assessed spontaneous activity and cognitive function using open field test and Morris water maze test; and measured brain-derived neurotrophic factor (BDNF) protein and mRNA expression levels using immunohistochemistry and molecular biology assays. The results showed that ARI alone or in combination with NGF can improve increased spontaneous activity and spatial learning memory deficits in model mice by elevating BDNF expression levels in prefrontal cortex (PFC) and hippocampus (HIP). The results suggest that ARI combined with NGF can improve cognitive function in SCZ, which provides new ideas and directions for the clinical treatment of cognitive dysfunction in SCZ.

New perspectives on the basal forebrain cholinergic system in Alzheimer's disease

The basal forebrain cholinergic system (BFCS) has long been implicated in age-related cognitive changes and the pathophysiology of Alzheimer's disease (AD). Limitations of cholinergic interventions helped to inspire a shift away from BFCS in AD research. A resurgence in interest in the BFCS following methodological and analytical advances has resulted in a call for the BFCS to be examined in novel frameworks. We outline the basic structure and function of the BFCS, its role in supporting cognitive and affective function, and its vulnerability to aging and AD. We consider the BFCS in the context of the amyloid hypothesis and evolving concepts in AD research: resilience and resistance to pathology, selective neuronal vulnerability, trans-synaptic pathology spread and sleep health. We highlight 1) the potential role of the BFCS in cognitive resilience, 2) recent work refining understanding about the selective vulnerability of BFCS to AD, 3) BFCS connectivity that suggests it is related to tau spreading and neurodegeneration and 4) the gap between BFCS involvement in AD and sleep-wake cycles.

Small-Molecule Cdc25A Inhibitors Protect Neuronal Cells from Death Evoked by NGF Deprivation and 6-Hydroxydopamine

Alzheimer's disease (AD) and Parkinson's disease (PD) are the two most common neurodegenerative diseases that are presently incurable. There have been reports of aberrant activation of cell cycle pathways in neurodegenerative diseases. Previously, we have found that Cdc25A is activated in models of neurodegenerative diseases, including AD and PD. In the present study, we have synthesized a small library of molecules targeting Cdc25A and tested their neuroprotective potential in cellular models of neurodegeneration. The Buchwald reaction and amide coupling were crucial steps in synthesizing the Cdc25A-targeting molecules. Several of these small-molecule inhibitors significantly prevented neuronal cell death induced by nerve growth factor (NGF) deprivation as well as 6-hydroxydopamine (6-OHDA) treatment. Lack of NGF signaling leads to neuron death during development and has been associated with AD pathogenesis. The NGF receptor TrkA has been reported to be downregulated at the early stages of AD, and its reduction is linked to cognitive failure. 6-OHDA, a PD mimic, is a highly oxidizable dopamine analogue that can be taken up by the dopamine transporters in catecholaminergic neurons and can induce cell death by reactive oxygen species (ROS) generation. Some of our newly synthesized molecules inhibit Cdc25A phosphatase activity, block loss of mitochondrial activity, and inhibit caspase-3 activation caused by NGF deprivation and 6-OHDA. Hence, it may be proposed that Cdc25A inhibition could be a therapeutic possibility for neurodegenerative diseases and these Cdc25A inhibitors could be effective treatments for AD and PD.

Reduced levels of NGF shift astrocytes toward a neurotoxic phenotype

Nerve growth factor (NGF) is critical for neuronal physiology during development and adulthood. Despite the well-recognized effect of NGF on neurons, less is known about whether NGF can actually affect other cell types in the central nervous system (CNS). In this work, we show that astrocytes are susceptible to changes in ambient levels of NGF. First, we observe that interfering with NGF signaling in vivo via the constitutive expression of an antiNGF antibody induces astrocytic atrophy. A similar asthenic phenotype is encountered in an uncleavable proNGF transgenic mouse model (TgproNGF#72), effectively increasing the brain proNGF levels. To examine whether this effect on astrocytes is cell-autonomous, we cultured wild-type primary astrocytes in the presence of antiNGF antibodies, uncovering that a short incubation period is sufficient to potently and rapidly trigger calcium oscillations. Acute induction of calcium oscillations by antiNGF antibodies is followed by progressive morphological changes similar to those observed in antiNGF AD11 mice. Conversely, incubation with mature NGF has no effect on either calcium activity nor on astrocytic morphology. At longer timescales, transcriptomic analysis revealed that NGF-deprived astrocytes acquire a proinflammatory profile. In particular, antiNGF-treated astrocytes show upregulation of neurotoxic transcripts and downregulation of neuroprotective mRNAs. Consistent with that data, culturing wild-type neurons in the presence of NGF-deprived astrocytes leads to neuronal cell death. Finally, we report that in both awake and anesthetized mice, astrocytes in layer I of the motor cortex respond with an increase in calcium activity to acute NGF inhibition using either NGF-neutralizing antibodies or a TrkA-Fc NGF scavenger. Moreover, in vivo calcium imaging in the cortex of the 5xFAD neurodegeneration mouse model shows an increased level of spontaneous calcium activity in astrocytes, which is significantly reduced after acute administration of NGF. In conclusion, we unveil a novel neurotoxic mechanism driven by astrocytes, triggered by their sensing and reacting to changes in the levels of ambient NGF.

Dynamic Responsive Inguinal Scaffold Activates Myogenic Growth Factors Finalizing the Regeneration of the Herniated Groin

BACKGROUND

Postoperative chronic pain caused by fixation and/or fibrotic incorporation of hernia meshes are the main concerns in inguinal herniorrhaphy. As inguinal hernia is a degenerative disease, logically the treatment should aim at stopping degeneration and activating regeneration. Unfortunately, in conventional prosthetic herniorrhaphy no relationship exists between pathogenesis and treatment. To overcome these incongruences, a 3D dynamic responsive multilamellar scaffold has been developed for fixation-free inguinal hernia repair. Made of polypropylene like conventional flat meshes, the dynamic behavior of the scaffold allows for the regeneration of all typical inguinal components: connective tissue, vessels, nerves, and myocytes. This investigation aims to demonstrate that, moving in tune with the groin, the 3D scaffold attracts myogenic growth factors activating the development of mature myocytes and, thus, re-establishing the herniated inguinal barrier.

METHODS

Biopsy samples excised from the 3D scaffold at different postoperative stages were stained with H&E and Azan-Mallory; immunohistochemistry for NGF and NGFR p75 was performed to verify the degree of involvement of muscular growth factors in the neomyogenesis.

RESULTS

Histological evidence of progressive muscle development and immunohistochemical proof of NFG and NFGRp75 contribution in neomyogenesis within the 3D scaffold was documented and statistically validated.

CONCLUSION

The investigation appears to confirm that a 3D polypropylene scaffold designed to confer dynamic responsivity, unlike the fibrotic scar plate of static meshes, attracts myogenic growth factors turning the biological response into tissue regeneration. Newly developed muscles allow the scaffold to restore the integrity of the inguinal barrier.

The relationships between neuroglial and neuronal changes in Alzheimer's disease, and the related controversies II:

Since the original description of Alzheimer´s disease (AD), research into this condition has mainly focused on assessing the alterations to neurons associated with dementia, and those to the circuits in which they are involved. In most of the studies on human brains and in many models of AD, the glial cells accompanying these neurons undergo concomitant alterations that aggravate the course of neurodegeneration. As a result, these changes to neuroglial cells are now included in all the "pathogenic cascades" described in AD. Accordingly, astrogliosis and microgliosis, the main components of neuroinflammation, have been integrated into all the pathogenic theories of this disease, as discussed in this part of the two-part monograph that follows an accompanying article on gliopathogenesis and glioprotection. This initial reflection verified the implication of alterations to the neuroglia in AD, suggesting that these cells may also represent therapeutic targets to prevent neurodegeneration. In this second part of the monograph, we will analyze the possibilities of acting on glial cells to prevent or treat the neurodegeneration that is the hallmark of AD and other pathologies. Evidence of the potential of different pharmacological, non-pharmacological, cell and gene therapies (widely treated) to prevent or treat this disease is now forthcoming, in most cases as adjuncts to other therapies. A comprehensive AD multimodal therapy is proposed in which neuronal and neuroglial pharmacological treatments are jointly considered, as well as the use of new cell and gene therapies and non-pharmacological therapies that tend to slow down the progress of dementia.

Targeting the Cation-Chloride Co-Transporter NKCC1 to Re-Establish GABAergic Inhibition and an Appropriate Excitatory/Inhibitory Balance in Selective Neuronal Circuits: A Novel Approach for the Treatment of Alzheimer's Disease

GABA, the main inhibitory neurotransmitter in the adult brain, depolarizes and excites immature neurons because of an initially higher intracellular chloride concentration [Cl-]i due to the delayed expression of the chloride exporter KCC2 at birth. Depolarization-induced calcium rise via NMDA receptors and voltage-dependent calcium channels is instrumental in shaping neuronal circuits and in controlling the excitatory (E)/inhibitory (I) balance in selective brain areas. An E/I imbalance accounts for cognitive impairment observed in several neuropsychiatric disorders. The aim of this review is to summarize recent data on the mechanisms by which alterations of GABAergic signaling alter the E/I balance in cortical and hippocampal neurons in Alzheimer's disease (AD) and the role of cation-chloride co-transporters in this process. In particular, we discuss the NGF and AD relationship and how mice engineered to express recombinant neutralizing anti-NGF antibodies (AD11 mice), which develop a neurodegenerative pathology reminiscent of that observed in AD patients, exhibit a depolarizing action of GABA due to KCC2 impairment. Treating AD and other forms of dementia with bumetanide, a selective KCC2 antagonist, contributes to re-establishing a proper E/I balance in selective brain areas, leading to amelioration of AD symptoms and the slowing down of disease progression.

A Microglial Function for the Nerve Growth Factor: Predictions of the Unpredictable

Microglia are the only immune cell population present in the brain parenchyma. Their vantage position in the central nervous system (CNS) enables these myeloid cells to perform the most disparate of tasks: from the classical immune functions of fighting infections and surveilling the extracellular space for pathogens and damage, to sculpting the neuronal circuitry by pruning unnecessary synapses and assisting neurons in spine formation, aiding in the maintenance of brain homeostasis. The neurotrophin field has always been dominated by the neurocentric view that the primary target of these molecules must be neurons: this holds true even for the Nerve Growth Factor (NGF), which owes its popularity in the neuroscience community to its trophic and tropic activity towards sensory and sympathetic neurons in the peripheral nervous system, and cholinergic neurons in the CNS. The increasing evidence that microglia are an integral part of neuronal computation calls for a closer look as to whether these glial cells are capable of responding directly to NGF. In this review, we will first outline evidence in support of a role for NGF as a molecule mediating neuroimmune communication. Then, we will illustrate some of those non-immune features that have made microglial cells one of the hottest topics of this last decade. In conclusion, we will discuss evidence in support of a microglial function for NGF.

Getting Into the Brain: The Intranasal Approach to Enhance the Delivery of Nerve Growth Factor and Its Painless Derivative in Alzheimer’s Disease and Down Syndrome

The neurotrophin Nerve Growth Factor (NGF) holds a great potential as a therapeutic candidate for the treatment of neurological diseases. However, its safe and effective delivery to the brain is limited by the fact that NGF needs to be selectively targeted to the brain, to avoid severe side effects such as pain and to bypass the blood brain barrier. In this perspective, we will summarize the different approaches that have been used, or are currently applied, to deliver NGF to the brain, during preclinical and clinical trials to develop NGF as a therapeutic drug for Alzheimer’s disease. We will focus on the intranasal delivery of NGF, an approach that is used to deliver proteins to the brain in a non-invasive, safe, and effective manner minimizing systemic exposure. We will also describe the main experimental facts related to the effective intranasal delivery of a mutant form of NGF [painless NGF, human nerve growth factor painless (hNGFp)] in mouse models of Alzheimer’s disease and compare it to other ways to deliver NGF to the brain. We will also report new data on the application of intranasal delivery of hNGFp in Down Syndrome mouse model. These new data extend the therapeutic potential of hNGFp for the treatment of the dementia that is progressively associated to Down Syndrome. In conclusion, we will show how this approach can be a promising strategy and a potential solution for other unmet medical needs of safely and effectively delivering this neuroprotective neurotrophin to the brain.

hiPSC-Derived Schwann Cells Influence Myogenic Differentiation in Neuromuscular Cocultures

Motoneurons, skeletal muscle fibers, and Schwann cells form synapses, termed neuromuscular junctions (NMJs). These control voluntary body movement and are affected in numerous neuromuscular diseases. Therefore, a variety of NMJ in vitro models have been explored to enable mechanistic and pharmacological studies. So far, selective integration of Schwann cells in these models has been hampered, due to technical limitations. Here we present robust protocols for derivation of Schwann cells from human induced pluripotent stem cells (hiPSC) and their coculture with hiPSC-derived motoneurons and C2C12 muscle cells. Upon differentiation with tuned BMP signaling, Schwann cells expressed marker proteins, S100b, Gap43, vimentin, and myelin protein zero. Furthermore, they displayed typical spindle-shaped morphologies with long processes, which often aligned with motoneuron axons. Inclusion of Schwann cells in coculture experiments with hiPSC-derived motoneurons and C2C12 myoblasts enhanced myotube growth and affected size and number of acetylcholine receptor plaques on myotubes. Altogether, these data argue for the availability of a consistent differentiation protocol for Schwann cells and their amenability for functional integration into neuromuscular in vitro models, fostering future studies of neuromuscular mechanisms and disease.

TGFβ-Neurotrophin Interactions in Heart, Retina, and Brain

Ischemic insults to the heart and brain, i.e., myocardial and cerebral infarction, respectively, are amongst the leading causes of death worldwide. While there are therapeutic options to allow reperfusion of ischemic myocardial and brain tissue by reopening obstructed vessels, mitigating primary tissue damage, post-infarction inflammation and tissue remodeling can lead to secondary tissue damage. Similarly, ischemia in retinal tissue is the driving force in the progression of neovascular eye diseases such as diabetic retinopathy (DR) and age-related macular degeneration (AMD), which eventually lead to functional blindness, if left untreated. Intriguingly, the easily observable retinal blood vessels can be used as a window to the heart and brain to allow judgement of microvascular damages in diseases such as diabetes or hypertension. The complex neuronal and endocrine interactions between heart, retina and brain have also been appreciated in myocardial infarction, ischemic stroke, and retinal diseases. To describe the intimate relationship between the individual tissues, we use the terms heart-brain and brain-retina axis in this review and focus on the role of transforming growth factor β (TGFβ) and neurotrophins in regulation of these axes under physiologic and pathologic conditions. Moreover, we particularly discuss their roles in inflammation and repair following ischemic/neovascular insults. As there is evidence that TGFβ signaling has the potential to regulate expression of neurotrophins, it is tempting to speculate, and is discussed here, that cross-talk between TGFβ and neurotrophin signaling protects cells from harmful and/or damaging events in the heart, retina, and brain.

Intranasal delivery of BDNF rescues memory deficits in AD11 mice and reduces brain microgliosis

A decrease in brain-derived neurotrophic factor (BDNF), a neurotrophin essential for synaptic function, plasticity and neuronal survival, is evident early in the progression of Alzheimer's disease (AD), being apparent in subjects with mild cognitive impairment or mild AD, and both proBDNF and mature BDNF levels are positively correlated with cognitive measures. BDNF delivery is, therefore, considered of great interest as a potentially useful therapeutic strategy to contrast AD. Invasive BDNF administration has indeed been recently used in animal models of AD with promising results in rescuing memory deficits, synaptic density and cell loss. Here, we tested whether non-invasive intranasal administration of different BDNF concentrations after the onset of cognitive and anatomical deficits (6 months of age) could rescue neuropathological and memory deficits in AD11 mice, a model of NGF deprivation-induced neurodegeneration. In addition to AD hallmarks, we investigated BDNF effects on microglia presence in the brain of AD11 mice, since alterations in microglia activation have been associated with ageing-related cognitive decline and with the progression of neurodegenerative diseases, including AD. We found that intranasal delivery of 42 pmol BDNF (1 μM), but not PBS, was sufficient to completely rescue performance of AD11 mice both in the object recognition test and in the object context test. No further improvement was obtained with 420 pmol (10 μM) BDNF dose. The strong improvement in memory performance in BDNF-treated mice was not accompanied by an amelioration of AD-like pathology, Aβ burden, tau hyperphosphorylation and cholinergic deficit, but there was a dramatic decrease of CD11b immunoreactive brain microglia. These results reinforce the potential therapeutic uses of BDNF in AD and the non-invasive intranasal route as an effective delivery strategy of BDNF to the brain. They also strengthen the connection between neuroinflammation and neurodegenerative dementia and suggest microglia as a possible mediator of BDNF therapeutic actions in the brain.

Nerve Growth Factor Neutralization Promotes Oligodendrogenesis by Increasing miR-219a-5p Levels

In the brain, the neurotrophin Nerve growth factor (NGF) regulates not only neuronal survival and differentiation, but also glial and microglial functions and neuroinflammation. NGF is known to regulate oligodendrogenesis, reducing myelination in the central nervous system (CNS). In this study, we found that NGF controls oligodendrogenesis by modulating the levels of miR-219a-5p, a well-known positive regulator of oligodendrocyte differentiation. We exploited an NGF-deprivation mouse model, the AD11 mice, in which the postnatal expression of an anti-NGF antibody leads to NGF neutralization and progressive neurodegeneration. Notably, we found that these mice also display increased myelination. A microRNA profiling of AD11 brain samples and qRT-PCR analyses revealed that NGF deprivation leads to an increase of miR-219a-5p levels in hippocampus and cortex and a corresponding down-regulation of its predicted targets. Neurospheres isolated from the hippocampus of AD11 mice give rise to more oligodendrocytes and this process is dependent on miR-219a-5p, as shown by decoy-mediated inhibition of this microRNA. Moreover, treatment of AD11 neurospheres with NGF inhibits miR-219a-5p up-regulation and, consequently, oligodendrocyte differentiation, while anti-NGF treatment of wild type (WT) oligodendrocyte progenitors increases miR-219a-5p expression and the number of mature cells. Overall, this study indicates that NGF inhibits oligodendrogenesis and myelination by down-regulating miR-219a-5p levels, suggesting a novel molecular circuitry that can be exploited for the discovery of new effectors for remyelination in human demyelinating diseases, such as Multiple Sclerosis.

Genetic Therapies for Alzheimer's Disease: A Scoping Review

Effective, disease modifying therapies for Alzheimer's disease (AD) remain a quandary, following a panoply of expensive failures in human clinical trials. Given the stagnation in therapeutics, alternative approaches are needed. Recent successes of genetic therapies in other neurodegenerative diseases may highlight the way forward. This scoping review explores suggested targets of genetic therapy in AD, with a focus on vector-based approaches in pre-clinical and clinical trials. Putative targets of genetic therapies tested in pre-clinical trials include amyloid pathway intermediates and enzymes modulation, tau protein downregulation, APOE4 downregulation and APOE2 upregulation, neurotrophin expression (nerve growth factor (NGF) and brain-derived neurotrophic factor), and inflammatory cytokine alteration, among several other approaches. There have been three completed human clinical trials for genetic therapy in AD patients, all of which upregulated NGF in AD patients, showing some mixed evidence of benefit. Several impediments remain to be surpassed before genetic therapies can be successfully applied to AD, including the challenge of delivering monogenic genetic therapies for complex polygenic disorders, risks in the dominant delivery method (intracranial injection), stability of genetic therapies in vivo, poor translatability of pre-clinical AD models, and the expense of genetic therapy production. Genetic therapies represent an exciting opportunity within the world of AD therapeutics, but clinical applications likely remain a long term, rather than short term, possibility.

The evolution of nerve growth factor inhibition in clinical medicine

Nerve growth factor (NGF) is a neurotrophin that activates nociceptive neurons to transmit pain signals from the peripheral to the central nervous system and that exerts its effects on neurons by signalling through tyrosine kinase receptors. Antibodies that inhibit the function of NGF and small molecule inhibitors of NGF receptors have been developed and tested in clinical studies to evaluate the efficacy of NGF inhibition as a form of analgesia in chronic pain states including osteoarthritis and chronic low back pain. Clinical studies in individuals with painful knee and hip osteoarthritis have revealed that NGF inhibitors substantially reduce joint pain and improve function compared with NSAIDs for a duration of up to 8 weeks. However, the higher tested doses of NGF inhibitors also increased the risk of rapidly progressive osteoarthritis in a small percentage of those treated. This Review recaps the biology of NGF and the studies that have been performed to evaluate the efficacy of NGF inhibition for chronic musculoskeletal pain states. The adverse events associated with NGF inhibition and the current state of knowledge about the mechanisms involved in rapidly progressive osteoarthritis are also discussed and future studies proposed to improve understanding of this rare but serious adverse event.

ProNGF/p75NTR Axis Drives Fiber Type Specification by Inducing the Fast-Glycolytic Phenotype in Mouse Skeletal Muscle Cells

Despite its undisputable role in the homeostatic regulation of the nervous system, the nerve growth factor (NGF) also governs the relevant cellular processes in other tissues and organs. In this study, we aimed at assessing the expression and the putative involvement of NGF signaling in skeletal muscle physiology. To reach this objective, we employed satellite cell-derived myoblasts as an in vitro culture model. In vivo experiments were performed on Tibialis anterior from wild-type mice and an mdx mouse model of Duchenne muscular dystrophy. Targets of interest were mainly assessed by means of morphological, Western blot and qRT-PCR analysis. The results show that proNGF is involved in myogenic differentiation. Importantly, the proNGF/p75NTR pathway orchestrates a slow-to-fast fiber type transition by counteracting the expression of slow myosin heavy chain and that of oxidative markers. Concurrently, proNGF/p75NTR activation facilitates the induction of fast myosin heavy chain and of fast/glycolytic markers. Furthermore, we also provided evidence that the oxidative metabolism is impaired in mdx mice, and that these alterations are paralleled by a prominent buildup of proNGF and p75NTR. These findings underline that the proNGF/p75NTR pathway may play a crucial role in fiber type determination and suggest its prospective modulation as an innovative therapeutic approach to counteract muscle disorders.

Regulation of TrkB cell surface expression-a mechanism for modulation of neuronal responsiveness to brain-derived neurotrophic factor

Neurotrophin signaling via receptor tyrosine kinases is essential for the development and function of the nervous system in vertebrates. TrkB activation and signaling show substantial differences to other receptor tyrosine kinases of the Trk family that mediate the responses to nerve growth factor and neurotrophin-3. Growing evidence suggests that TrkB cell surface expression is highly regulated and determines the sensitivity of neurons to brain-derived neurotrophic factor (BDNF). This translocation of TrkB depends on co-factors and modulators of cAMP levels, N-glycosylation, and receptor transactivation. This process can occur in very short time periods and the resulting rapid modulation of target cell sensitivity to BDNF could represent a mechanism for fine-tuning of synaptic plasticity and communication in complex neuronal networks. This review focuses on those modulatory mechanisms in neurons that regulate responsiveness to BDNF via control of TrkB surface expression.

Nerve growth factor in metabolic complications and Alzheimer's disease: Physiology and therapeutic potential

As the population ages, obesity and metabolic complications as well as neurological disorders are becoming more prevalent, with huge economic burdens on both societies and families. New therapeutics are urgently needed. Nerve growth factor (NGF), first discovered in 1950s, is a neurotrophic factor involved in regulating cell proliferation, growth, survival, and apoptosis in both central and peripheral nervous systems. NGF and its precursor, proNGF, bind to TrkA and p75 receptors and initiate protein phosphorylation cascades, resulting in changes of cellular functions, and are associated with obesity, diabetes and its complications, and Alzheimer's disease. In this article, we summarize changes in NGF levels in metabolic and neuronal disorders, the signal transduction initiated by NGF and proNGF, the physiological and pathophysiological relevance, and therapeutic potential in treating chronic metabolic diseases and cognitive decline.

Mechanisms of altered skeletal muscle action potentials in the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) patients suffer from progressive and debilitating motor dysfunction for which only palliative treatment is currently available. Previously, we discovered reduced skeletal muscle Cl^- channel (ClC-1) and inwardly rectifying K⁺ channel (Kir) currents in R6/2 HD transgenic mice. To further investigate the role of ClC-1 and Kir currents in HD skeletal muscle pathology, we measured the effect of reduced ClC-1 and Kir currents on action potential (AP) repetitive firing in R6/2 mice using a two-electrode current clamp. We found that R6/2 APs had a significantly lower peak amplitude, depolarized maximum repolarization, and prolonged decay time compared with wild type (WT). Of these differences, only the maximum repolarization was accounted for by the reduction in ClC-1 and Kir currents, indicating the presence of additional ion channel defects. We found that both K_V1.5 and K_V3.4 mRNA levels were significantly reduced in R6/2 skeletal muscle compared with WT, which explains the prolonged decay time of R6/2 APs. Overall, we found that APs in WT and R6/2 muscle significantly and progressively change during activity to maintain peak amplitude despite buildup of Na⁺ channel inactivation. Even with this resilience, the persistently reduced peak amplitude of R6/2 APs is expected to result in earlier fatigue and may help explain the motor impersistence experienced by HD patients. This work lays the foundation to link electrical changes to force generation defects in R6/2 HD mice and to examine the regulatory events controlling APs in WT muscle.

Neurotrophins BDNF and NT4/5 accelerate dental pulp stem cell migration

Neurotrophic factors play important roles in neuron survival, growth and differentiation. In the present research, the expression of multiple neurotrophins and their effects on cell migration were studied in the dental pulp stem cells (DPSCs). Human DPSCs from five patients were cultured. Expression of neurotrophins and their receptors were evaluated by PCR, immunofluorescent staining and ELISA. Scratch assay was performed in the presence or absence of neurotrophic factors. Level of phosphorylated-ERK was evaluated with Western blotting. Neurotrophins were expressed at various levels in the DPSCs. Treatment of 100 ng/ml BDNF or NT4/5 accelerated wound healing in scratch assay and elevated the expression of phosphorylated–ERK. The work indicated that neurotrophins promoted human DPSCs migration in vitro.

Muscle Progenitors Derived from Extraocular Muscles Express Higher Levels of Neurotrophins and their Receptors than other Cranial and Limb Muscles

Extraocular muscles (EOMs) show resistance to muscle dystrophies and sarcopenia. It has been recently demonstrated that they are endowed with different types of myogenic cells, all of which present an outstanding regenerative potential. Neurotrophins are important modulators of myogenic regeneration and act promoting myoblast proliferation, enhancing myogenic fusion rates and protecting myotubes from inflammatory stimuli. Here, we adapted the pre-plate cell isolation technique to obtain myogenic progenitors from the rat EOMs, and quantified their in vitro expression of neurotrophins and their receptors by RT–qPCR and immunohistochemistry, respectively. The results were compared with the expression on progenitors isolated from buccinator, tongue and limb muscles. Our quantitative analysis of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF) and neurotrophin-3 (NT-3) transcripts showed, for the first time, that EOMs-derived cells express more of these factors and that they expressed TrkA, but not TrkB and TrkC receptors. On the contrary, the immunofluorescence analysis demonstrated high expression of p75^NTR on all myogenic progenitors, with the EOMs-derived cells showing higher expression. Taken together, these results suggest that the intrinsic trophic differences between EOMs-derived myogenic progenitors and their counterparts from other muscles could explain why those cells show higher proliferative and fusion rates, as well as better regenerative properties.

Focused ultrasound delivery of a selective TrkA agonist rescues cholinergic function in a mouse model of Alzheimer's disease

The degeneration of cholinergic neurons is a prominent feature of Alzheimer's disease (AD). In animal models of injury and aging, nerve growth factor (NGF) enhances cholinergic cell survival and function, contributing to improved memory. In the presence of AD pathology, however, NGF-related therapeutics have yet to fulfill their regenerative potential. We propose that stimulating the TrkA receptor, without p75^NTR activation, is key for therapeutic efficacy. Supporting this hypothesis, the selective TrkA agonist D3 rescued neurotrophin signaling in TgCRND8 mice, whereas NGF, interacting with both TrkA and p75^NTR, did not. D3, delivered intravenously and noninvasively to the basal forebrain using MRI-guided focused ultrasound (MRIgFUS)-mediated blood-brain barrier (BBB) permeability activated TrkA-related signaling cascades and enhanced cholinergic neurotransmission. Recent clinical trials support the safety and feasibility of MRIgFUS BBB modulation in AD patients. Neuroprotective agents targeting TrkA, combined with MRIgFUS BBB modulation, represent a promising strategy to counter neurodegeneration in AD.

NGF protects neuroblastoma cells against β-amyloid-induced apoptosis via the Nrf2/HO-1 pathway

As one of the main neurotrophic factors, nerve growth factor (NGF) participates in various processes related to viability, plasticity, and neuronal growth. NGF is known to protect against cell death and toxicity triggered by β-amyloid (Aβ), but the underlying mechanism remains unclear. Here, we investigated this process in SKNSH neuroblastoma, in which NGF reduced cell death induced by Aβ25-35. Furthermore, NGF suppressed the production of reactive oxygen species (ROS) and promoted antioxidant function via Aβ25-35. Additionally, we demonstrated that NGF impaired the activation of the JNK/c-Jun signaling pathway and significantly increased Nrf2 nuclear translocation and HO-1 expression. Nrf2 elimination abolished the protective effect of NGF-1 on Aβ25-35-induced ROS generation, apoptosis, and activation of the JNK/c-Jun pathway. The results of our study indicate that NGF protects neuroblastoma against injury triggered by Aβ25-35 via suppression of ROS-JNK/c-Jun pathway stimulation through the Nrf2/HO-1 pathway.

ProNGF Is a Cell-Type-Specific Mitogen for Adult Hippocampal and for Induced Neural Stem Cells

The role of proNGF, the precursor of nerve growth factor (NGF), in the biology of adult neural stem cells (aNSCs) is still unclear. Here, we analyzed adult hippocampal neurogenesis in AD11 transgenic mice, in which the constitutive expression of anti-NGF antibody leads to an imbalance of proNGF over mature NGF. We found increased proliferation of progenitors but a reduced neurogenesis in the AD11 dentate gyrus (DG)-hippocampus (HP). Also in vitro, AD11 hippocampal neural stem cells (NSCs) proliferated more, but were unable to differentiate into morphologically mature neurons. By treating wild-type hippocampal progenitors with the uncleavable form of proNGF (proNGF-KR), we demonstrated that proNGF acts as mitogen on aNSCs at low concentration. The mitogenic effect of proNGF was specifically addressed to the radial glia-like (RGL) stem cells through the induction of cyclin D1 expression. These cells express high levels of p75^NTR , as demonstrated by immunofluorescence analyses performed ex vivo on RGL cells isolated from freshly dissociated HP-DG or selected in vitro from NSCs by leukemia inhibitory factor. Clonogenic assay performed in the absence of mitogens showed that RGLs respond to proNGF-KR by reactivating their proliferation and thus leading to neurospheres formation. The mitogenic effect of proNGF was further exploited in the expansion of mouse-induced neural stem cells (iNSCs). Chronic exposure of iNSCs to proNGF-KR increased their proliferation. Altogether, we demonstrated that proNGF acts as mitogen on hippocampal and iNSCs. Stem Cells 2019;37:1223-1237.

Early life stress disrupts intestinal homeostasis via NGF-TrkA signaling

Early childhood is a critical period for development, and early life stress may increase the risk of gastrointestinal diseases including irritable bowel syndrome (IBS). In rodents, neonatal maternal separation (NMS) induces bowel dysfunctions that resemble IBS. However, the underlying mechanisms remain unclear. Here we show that NMS induces expansion of intestinal stem cells (ISCs) and their differentiation toward secretory lineages including enterochromaffin (EC) and Paneth cells, leading to EC hyperplasia, increased serotonin production, and visceral hyperalgesia. This is reversed by inhibition of nerve growth factor (NGF)-mediated tropomyosin receptor kinase A (TrkA) signalling, and treatment with NGF recapitulates the intestinal phenotype of NMS mice in vivo and in mouse intestinal organoids in vitro. Mechanistically, NGF transactivates Wnt/β-catenin signalling. NGF and serotonin are positively correlated in the sera of diarrhea-predominant IBS patients. Together, our findings provide mechanistic insights into early life stress-induced intestinal changes that may translate into treatments for gastrointestinal diseases.

Regulation of cholinergic basal forebrain development, connectivity, and function by neurotrophin receptors

Cholinergic basal forebrain (cBF) neurons are defined by their expression of the p75 neurotrophin receptor (p75NTR) and tropomyosin-related kinase (Trk) neurotrophin receptors in addition to cholinergic markers. It is known that the neurotrophins, particularly nerve growth factor (NGF), mediate cholinergic neuronal development and maintenance. However, the role of neurotrophin signalling in regulating adult cBF function is less clear, although in dementia, trophic signalling is reduced and p75NTR mediates neurodegeneration of cBF neurons. Here we review the current understanding of how cBF neurons are regulated by neurotrophins which activate p75NTR and TrkA, B or C to influence the critical role that these neurons play in normal cortical function, particularly higher order cognition. Specifically, we describe the current evidence that neurotrophins regulate the development of basal forebrain neurons and their role in maintaining and modifying mature basal forebrain synaptic and cortical microcircuit connectivity. Understanding the role neurotrophin signalling plays in regulating the precision of cholinergic connectivity will contribute to the understanding of normal cognitive processes and will likely provide additional ideas for designing improved therapies for the treatment of neurological disease in which cholinergic dysfunction has been demonstrated.

Nerve growth factor metabolic dysfunction contributes to sevoflurane-induced cholinergic degeneration and cognitive impairments

General anesthesia with sevoflurane is associated with an increased incidence of postoperative cognitive dysfunction. Previous studies have shown that sevoflurane anesthesia can affect the integrity and function of basal forebrain cholinergic neurons (BFCNs) which are essential for learning and memory. However, the underlying mechanisms remain largely unknown. Here, we demonstrated that exposure to 2.5% sevoflurane induced significant loss of BFCNs and caused impairments of the spatial and the fear memory. Further, sevoflurane exposure significantly reduced the level of nerve growth factor (NGF), an important factor for the survival and phenotype maintenance of BFCNs, by disrupting its synthesis pathways in the brain. More importantly, NGF administration not only prevented the loss of BFCNs but also ameliorated the cognitive impairments in sevoflurane-treated mice. Our findings indicate that NGF metabolic dysfunction contributes to sevoflurane-associated BFCNs degeneration and subsequent cognitive deficits.

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.

Targeting Alzheimer's disease with gene and cell therapies

Alzheimer's disease (AD) causes dementia in both young and old people affecting more than 40 million people worldwide. The two neuropathological hallmarks of the disease, amyloid beta (Aβ) plaques and neurofibrillary tangles consisting of protein tau are considered the major contributors to the disease. However, a more complete picture reveals significant neurodegeneration and decreased cell survival, neuroinflammation, changes in protein and energy homeostasis and alterations in lipid and cholesterol metabolism. In addition, gene and cell therapies for severe neurodegenerative disorders have recently improved technically in terms of safety and efficiency and have translated to the clinic showing encouraging results. Here, we review broadly current data within the field for potential targets that could modify AD through gene and cell therapy strategies. We envision that not only Aβ will be targeted in a disease-modifying treatment strategy but rather that a combination of treatments, possibly at different intervention times may prove beneficial in curing this devastating disease. These include decreased tau pathology, neuronal growth factors to support neurons and modulation of neuroinflammation for an appropriate immune response. Furthermore, cell based therapies may represent potential strategies in the future.

From the Cover: Evaluation of the Effects of Tanezumab, a Monoclonal Antibody Against Nerve Growth Factor, on the Sympathetic Nervous System in Adult Cynomolgus Monkeys (Macaca fascicularis): A Stereologic, Histomorphologic, and Cardiofunctional Assessment

Tanezumab, a humanized monoclonal antibody against nerve growth factor is in development for treatment of chronic pain. Three nonclinical studies assessed effects of clinically relevant and supratherapeutic doses of tanezumab on the sympathetic nervous system (SNS) of adult nonhuman primates. Study 1 evaluated potential effects of subcutaneous (SC) tanezumab (1.2 mg/kg every 8 weeks [Q8W]) on SNS in cynomolgus monkeys for 3 or 6 months and reversibility or persistence of any effects through a nondosing/recovery period. Study 2 evaluated whether neuronal cell death occurs shortly after a single SC tanezumab injection (1.2 mg/kg). Assessments for these two studies included evaluations of superior cervical and cervicothoracic ganglia for neuronal cell death and morphology. Study 3 evaluated effects of SC tanezumab (1.2 mg/kg Q8W and 30 mg/kg/week) over 6 months on sympathetic control of cardiovascular function. Tanezumab exposure was associated with stereologic changes in sympathetic ganglia, including smaller ganglion volume, and smaller average neuron size/area beginning at 2 weeks and reaching maximal levels by 1 month with no further progression through 6 months. These changes were not associated with clinical signs, completely reversed upon tanezumab withdrawal, and were not considered adverse. Tanezumab had no adverse effects on sympathetic control of cardiovascular function. These data support the conclusion that tanezumab administration for up to 6 months has no adverse effects on SNS morphology or function and does not cause neuronal cell death in adult nonhuman primates.

Development of pain therapies targeting nerve growth factor signal transduction and the strategies used to resolve safety issues

Therapeutic agents commonly used in the management of chronic pain have limited effectiveness and may be associated with issues of dependence and tolerability. Thus, a large unmet medical need exists for the development of safe and effective therapeutics for treatment of chronic pain. A novel approach includes identification of intracellular signals involved in the pain transduction pathway, such as nerve growth factor (NGF). Monoclonal antibodies targeting NGF, such as tanezumab, fulranumab and fasinumab, have been investigated for the treatment of chronic pain conditions. Due to unexpected joint adverse events in clinical studies and concerns about sympathetic nervous system toxicity in animals, these agents were placed on 2 separate partial clinical holds, which were subsequently lifted after rigorous evaluations were conducted to understand how inhibition of NGF impacts safety. To share learnings regarding the rigorous evaluation of clinical and nonclinical safety data which contributed to the removal of these partial clinical holds, this article reviews the rationale for developing agents that target NGF as potential treatments for chronic pain, describes nonclinical and clinical studies of these agents, and describes strategies used to evaluate whether inhibition of NGF has negative effects on joint or sympathetic nervous system safety.

Hypofunctional TrkA Accounts for the Absence of Pain Sensitization in the African Naked Mole-Rat

The naked mole-rat is a subterranean rodent lacking several pain behaviors found in humans, rats, and mice. For example, nerve growth factor (NGF), an important mediator of pain sensitization, fails to produce thermal hyperalgesia in naked mole-rats. The sensitization of capsaicin-sensitive TRPV1 ion channels is necessary for NGF-induced hyperalgesia, but naked mole-rats have fully functional TRPV1 channels. We show that exposing isolated naked mole-rat nociceptors to NGF does not sensitize TRPV1. However, the naked mole-rat NGF receptor TrkA displays a reduced ability to engage signal transduction pathways that sensitize TRPV1. Between one- and three-amino-acid substitutions in the kinase domain of the naked mole-rat TrkA are sufficient to render the receptor hypofunctional, and this is associated with the absence of heat hyperalgesia. Our data suggest that evolution has selected for a TrkA variant that abolishes a robust nociceptive behavior in this species but is still compatible with species fitness.

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TRPV1 ion channels in naked mole-rat nociceptors are not sensitized by NGF

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Naked mole-rat TRPV1 channels are sensitized by NGF in mouse nociceptors

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NGF activation of naked mole-rat TrkA receptors does not sensitize TRPV1

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One to three amino acids in the naked mole-rat TrkA receptors may render it hypofunctional

The fate of the brain cholinergic neurons in neurodegenerative diseases

The aims of this review are: 1) to describe which cholinergic neurons are affected in brain neurodegenerative diseases leading to dementia; 2) to discuss the possible causes of the degeneration of the cholinergic neurons, 3) to summarize the functional consequences of the cholinergic deficit. The brain cholinergic system is basically constituted by three populations of phenotypically similar neurons forming a series of basal forebrain nuclei, the midpontine nuclei and a large population of striatal interneurons. In Alzheimer's disease there is an extensive loss of forebrain cholinergic neurons accompanied by a reduction of the cholinergic fiber network of the cortical mantel and hippocampus. The midpontine cholinergic nuclei are spared. The same situation occurs in the corticobasal syndrome and dementia following alcohol abuse and traumatic brain injury. Conversely, in Parkinson's disease, the midpontine nuclei degenerate, together with the dopaminergic nuclei, reducing the cholinergic input to thalamus and forebrain whereas the forebrain cholinergic neurons are spared. In Parkinson's disease with dementia, Lewis Body Dementia and Parkinsonian syndromes both groups of forebrain and midpontine cholinergic nuclei degenerate. In Huntington's disease a dysfunction of the striatal cholinergic interneurons without cell loss takes place. The formation and accumulation of misfolded proteins such as β-amyloid oligomers and plaques, tau protein tangles and α-synuclein clumps, and aggregated mutated huntingtin play a crucial role in the neuronal degeneration by direct cellular toxicity of the misfolded proteins and through the toxic compounds resulting from an extensive inflammatory reaction. Evidences indicate that β-amyloid disrupts NGF metabolism causing the degeneration of the cholinergic neurons which depend on NGF for their survival, namely the forebrain cholinergic neurons, sparing the midpontine and striatal neurons which express no specific NGF receptors. It is feasible that the latter cholinergic neurons may be damaged by direct toxicity of tau, α-synuclein and inflammations products through mechanisms not fully understood. Attention and learning and memory impairment are the functional consequences of the forebrain cholinergic neuron dysfunction, whereas the loss of midpontine cholinergic neurons results primarily in motor and sleep disturbances.

The chemokine CXCL12 mediates the anti-amyloidogenic action of painless human nerve growth factor

Nerve growth factor is a therapeutic candidate for Alzheimer’s disease. Due to its pain-inducing activity, in current clinical trials nerve growth factor is delivered locally into the brain by neurosurgery, but data on the efficacy of local nerve growth factor delivery in decreasing amyloid-β deposition are not available. To reduce the nerve growth factor pain-inducing side effects, thus avoiding the need for local brain injection, we developed human painless nerve growth factor (hNGFp), inspired by the human genetic disease hereditary sensory and autonomic neuropathy type V. hNGFp has identical neurotrophic potency as wild-type human nerve growth factor, but a 10-fold lower pain sensitizing activity. In this study we first mimicked, in the 5xFAD mouse model, the intraparenchymal delivery of hNGFp used in clinical trials and found it to be ineffective in decreasing amyloid-β plaque load. On the contrary, the same dose of hNGFp delivered intranasally, which was widely biodistributed in the brain and did not induce pain, showed a potent anti-amyloidogenic action and rescued synaptic plasticity and memory deficits. We found that hNGFp acts on glial cells, modulating inflammatory proteins such as the soluble TNFα receptor II and the chemokine CXCL12. We further established that the rescuing effect by hNGFp is mediated by CXCL12, as pharmacological inhibition of CXCL12 receptor CXCR4 occludes most of hNGFp effects. These findings have significant therapeutic implications: (i) we established that a widespread exposure of the brain is required for nerve growth factor to fully exert its neuroprotective actions; and (ii) we have identified a new anti-neurodegenerative pathway as a broad target for new therapeutic opportunities for neurodegenerative diseases.

Impaired NGF/TrkA Signaling Causes Early AD-Linked Presynaptic Dysfunction in Cholinergic Primary Neurons

Alterations in NGF/TrkA signaling have been suggested to underlie the selective degeneration of the cholinergic basal forebrain neurons occurring in vivo in AD (Counts and Mufson, 2005; Mufson et al., 2008; Niewiadomska et al., 2011) and significant reduction of cognitive decline along with an improvement of cholinergic hypofunction have been found in phase I clinical trial in humans affected from mild AD following therapeutic NGF gene therapy (Tuszynski et al., 2005, 2015). Here, we show that the chronic (10–12 D.I.V.) in vitro treatment with NGF (100 ng/ml) under conditions of low supplementation (0.2%) with the culturing serum-substitute B27 selectively enriches the basal forebrain cholinergic neurons (+36.36%) at the expense of other non-cholinergic, mainly GABAergic (−38.45%) and glutamatergic (−56.25%), populations. By taking advantage of this newly-developed septo-hippocampal neuronal cultures, our biochemical and electrophysiological investigations demonstrate that the early failure in excitatory neurotransmission following NGF withdrawal is paralleled by concomitant and progressive loss in selected presynaptic and vesicles trafficking proteins including synapsin I, SNAP-25 and α-synuclein. This rapid presynaptic dysfunction: (i) precedes the commitment to cell death and is reversible in a time-dependent manner, being suppressed by de novo external administration of NGF within 6 hr from its initial withdrawal; (ii) is specific because it is not accompanied by contextual changes in expression levels of non-synaptic proteins from other subcellular compartments; (ii) is not secondary to axonal degeneration because it is insensible to pharmacological treatment with known microtubule-stabilizing drug such paclitaxel; (iv) involves TrkA-dependent mechanisms because the effects of NGF reapplication are blocked by acute exposure to specific and cell-permeable inhibitor of its high-affinity receptor. Taken together, this study may have important clinical implications in the field of AD neurodegeneration because it: (i) provides new insights on the earliest molecular mechanisms underlying the loss of synaptic/trafficking proteins and, then, of synapes integrity which occurs in vulnerable basal forebrain population at preclinical stages of neuropathology; (ii) offers prime presynaptic-based molecular target to extend the therapeutic time-window of NGF action in the strategy of improving its neuroprotective in vivo intervention in affected patients.

ProNGF Drives Localized and Cell Selective Parvalbumin Interneuron and Perineuronal Net Depletion in the Dentate Gyrus of Transgenic Mice

ProNGF, the precursor of mature Nerve Growth Factor (NGF), is the most abundant NGF form in the brain and increases markedly in the cortex in Alzheimer's Disease (AD), relative to mature NGF. A large body of evidence shows that the actions of ProNGF and mature NGF are often conflicting, depending on the receptors expressed in target cells. TgproNGF#3 mice, expressing furin-cleavage resistant proNGF in CNS neurons, directly reveal consequences of increased proNGF levels on brain homeostasis. Their phenotype clearly indicates that proNGF can be a driver of neurodegeneration, including severe learning and memory behavioral deficits, cholinergic deficits, and diffuse immunoreactivity for A-beta and A-beta-oligomers. In aged TgproNGF#3 mice spontaneous epileptic-like events are detected in entorhinal cortex-hippocampal slices, suggesting occurrence of excitatory/inhibitory (E/I) imbalance. In this paper, we investigate the molecular events linking increased proNGF levels to the epileptiform activity detected in hippocampal slices. The occurrence of spontaneous epileptiform discharges in the hippocampal network in TgproNGF#3 mice suggests an impaired inhibitory interneuron homeostasis. In the present study, we detect the onset of hippocampal epileptiform events at 1-month of age. Later, we observe a regional- and cellular-selective Parvalbumin interneuron and perineuronal net (PNN) depletion in the dentate gyrus (DG), but not in other hippocampal regions of TgproNGF#3 mice. These results demonstrate that, in the hippocampus, the DG is selectively vulnerable to altered proNGF/NGF signaling. Parvalbumin interneuron depletion is also observed in the amygdala, a region strongly connected to the hippocampus and likewise receiving cholinergic afferences. Transcriptome analysis of TgproNGF#3 hippocampus reveals a proNGF signature with broad down-regulation of transcription. The most affected mRNAs modulated at early times belong to synaptic transmission and plasticity and extracellular matrix (ECM) gene families. Moreover, alterations in the expression of selected BDNF splice variants were observed. Our results provide further mechanistic insights into the vicious negative cycle linking proNGF and neurodegeneration, confirming the regulation of E/I homeostasis as a crucial mediating mechanism.

Neurotrophin receptor agonists and antagonists as therapeutic agents: An evolving paradigm

Neurodegenerative disorders are prevalent, complex and devastating conditions, with very limited treatment options currently available. While they manifest in many forms, there are commonalities that link them together. In this review, we will focus on neurotrophins - a family of related factors involved in neuronal development and maintenance. Neurodegenerative diseases often present with a neurotrophin imbalance, in which there may be decreases in trophic signaling through Trk receptors for example, and/or increases in pro-apoptotic activity through p75. Clinical trials with neurotrophins have continuously failed due to their poor pharmacological properties as well as the unavoidable activation of p75. Thus, there is a need for drugs without such setbacks. Small molecule neurotrophin mimetics are favorable options since they can selectively activate Trks or inactivate p75. In this review, we will initially present a brief outline of how these molecules are synthesized and their mechanisms of action; followed by an update in the current state of neurotrophins and small molecules in major neurodegenerative diseases. Although there has been significant progress in the development of potential therapeutics, more studies are needed to establish clear mechanisms of action and target specificity in order to transition from animal models to the assessment of safety and use in humans.

NGF controls APP cleavage by downregulating APP phosphorylation at Thr668: relevance for Alzheimer's disease

NGF has been implicated in forebrain neuroprotection from amyloidogenesis and Alzheimer's disease (AD). However, the underlying molecular mechanisms are still poorly understood. Here, we investigated the role of NGF signalling in the metabolism of amyloid precursor protein (APP) in forebrain neurons using primary cultures of septal neurons and acute septo-hippocampal brain slices. In this study, we show that NGF controls the basal level of APP phosphorylation at Thr668 (T668) by downregulating the activity of the Ser/Thr kinase JNK(p54) through the Tyr kinase signalling adaptor SH2-containing sequence C (ShcC). We also found that the specific NGF receptor, Tyr kinase A (TrkA), which is known to bind to APP, fails to interact with the fraction of APP molecules phosphorylated at T668 (APP(pT668) ). Accordingly, the amount of TrkA bound to APP is significantly reduced in the hippocampus of ShcC KO mice and of patients with AD in which elevated APP(pT668) levels are detected. NGF promotes TrkA binding to APP and APP trafficking to the Golgi, where APP-BACE interaction is hindered, finally resulting in reduced generation of sAPPβ, CTFβ and amyloid-beta (1-42). These results demonstrate that NGF signalling directly controls basal APP phosphorylation, subcellular localization and BACE cleavage, and pave the way for novel approaches specifically targeting ShcC signalling and/or the APP-TrkA interaction in AD therapy.

Nerve growth factor enhances the CRE-dependent transcriptional activity activated by nobiletin in PC12 cells

Prevention and treatment of Alzheimer disease are urgent problems for elderly people in developed countries. We previously reported that nobiletin, a poly-methoxylated flavone from the citrus peel, improved the symptoms in various types of animal models of memory loss and activated the cAMP responsive element (CRE)-dependent transcription in PC12 cells. Nobiletin activated the cAMP/PKA/MEK/Erk/MAPK signaling pathway without using the TrkA signaling activated by nerve growth factor (NGF). Here, we examined the effect of combination of nobiletin and NGF on the CRE-dependent transcription in PC12 cells. Although NGF alone had little effect on the CRE-dependent transcription, NGF markedly enhanced the CRE-dependent transcription induced by nobiletin. The NGF-induced enhancement was neutralized by a TrkA antagonist, K252a. This effect of NGF was effective on the early signaling event elicited by nobiletin. These results suggested that there was crosstalk between NGF and nobiletin signaling in activating the CRE-dependent transcription in PC12 cells.

Impairment of the nerve growth factor pathway driving amyloid accumulation in cholinergic neurons: the incipit of the Alzheimer's disease story?

The current idea behind brain pathology is that disease is initiated by mild disturbances of common physiological processes. Overtime, the disruption of the neuronal homeostasis will determine irreversible degeneration and neuronal apoptosis. This could be also true in the case of nerve growth factor (NGF) alterations in sporadic Alzheimer's disease (AD), an age-related pathology characterized by cholinergic loss, amyloid plaques and neurofibrillary tangles. In fact, the pathway activated by NGF, a key neurotrophin for the metabolism of basal forebrain cholinergic neurons (BFCN), is one of the first homeostatic systems affected in prodromal AD. NGF signaling dysfunctions have been thought for decades to occur in AD late stages, as a mere consequence of amyloid-driven disruption of the retrograde axonal transport of neurotrophins to BFCN. Nowadays, a wealth of knowledge is potentially opening a new scenario: NGF signaling impairment occurs at the onset of AD and correlates better than amyloid load with cognitive decline. The recent acceleration in the characterization of anatomical, functional and molecular profiles of early AD is aimed at maximizing the efficacy of existing treatments and setting novel therapies. Accordingly, the elucidation of the molecular events underlying APP metabolism regulation by the NGF pathway in the septo-hippocampal system is crucial for the identification of new target molecules to slow and eventually halt mild cognitive impairment (MCI) and its progression toward AD.