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NGF, the principal neurotrophic factor for basal forebrain cholinergic neurons (BFCNs), has been correlated to Alzheimer's disease (AD) because of the selective vulnerability of BFCNs in AD. These correlative links do not substantiate a comprehensive cause-effect mechanism connecting NGF deficit to overall AD neurodegeneration. A demonstration that neutralizing NGF activity could have consequences beyond a direct interference with the cholinergic system came from studies in the AD11 mouse model, in which the expression of a highly specific anti-NGF antibody determines a neurodegeneration that encompasses several features of human AD. Because the transgenic antibody binds to mature NGF much more strongly than to proNGF and prevents binding of mature NGF to the tropomyosin-related kinase A (TrkA) receptor and to p75 neurotrophin receptor (p75NTR), we postulated that neurodegeneration in AD11 mice is provoked by an imbalance of proNGF/NGF signaling and, consequently, of TrkA/p75NTR signaling. To test this hypothesis, in this study we characterize the phenotype of two lines of transgenic mice, one in which TrkA signaling is inhibited by neutralizing anti-TrkA antibodies and a second one in which anti-NGF mice were crossed to p75NTR(exonIII(-/-)) mice to abrogate p75NTR signaling. TrkA neutralization determines a strong cholinergic deficit and the appearance of beta-amyloid peptide (Abeta) but no tau-related pathology. In contrast, abrogating p75NTR signaling determines a full rescue of the cholinergic and Abeta phenotype of anti-NGF mice, but tau hyperphosphorylation is exacerbated. Thus, we demonstrate that inhibiting TrkA signaling activates Abeta accumulation and that different streams of AD neurodegeneration are related in complex ways to TrkA versus p75NTR signaling.

Nerve growth factor (NGF) deficits are linked to Alzheimer's Disease (AD), due to the role of NGF on basal forebrain cholinergic neurons (BFCN). We have further established that a disequilibrium in NGF signaling and/or processing from its precursor proNGF is also directly and causally related to the aberrant activation of an amyloidogenic route to neurodegeneration. The therapeutic potential of using human NGF to provide a long-lasting cholinergic trophic support, thereby preventing or slowing cognitive decline in AD patients, has therefore a strong rationale. However, a simple and practical means of delivering NGF to the brain in a safe and long-term manner, limiting the undesired adverse effects of NGF in activating nociceptive responses, has represented a significant challenge. For this reason, pilot clinical studies have been performed so far with invasive approaches requiring neurosurgery. We obtained a proof of principle, in neurodegeneration animal models, of an alternative, non-invasive delivery of NGF through an intranasal route, which facilitates access of NGF to the central nervous system (CNS), while minimizing the biodistribution of NGF to compartments where it activates undesired effects, such as pain. The ideal NGF product for a non invasive NGF-based therapy would be a recombinant NGF that, while exhibiting an identical biological activity to that of human NGF, can be traced, against the endogenous NGF, in order to optimize the therapeutical dose range and meet the required therapeutic window. We describe an engineered mutein of hNGF, hNGF-61, that is selectively recognized, against endogenous NGF, by a specific antibody. hNGF-61 mutein has an identical potency and bioactivity profile as hNGF, in vitro and in vivo. Moreover, hNGF-61 and hNGF are equally effective in rescuing the behavioral and neurodegenerative phenotype in adult and aged AD11 anti-NGF mice. Finally, we demonstrated that intranasally delivered hNGF-61 is significantly more effective than ocularly applied hNGF-61, to determine phenotypic rescue in AD11 mice. The development of hNGF-61 towards clinical applications in AD patients is under way.

We previously showed that anti-nerve growth factor (NGF) antibodies expressed in transgenic mice (AD11) elicit a progressive neurodegeneration, comprising the triad of Alzheimer's disease (AD) hallmarks: cholinergic loss, tau hyperphosphorylation, and amyloid-beta peptide formation. However, since anti-NGF antibodies are produced both in the brain and in peripheral tissues of AD11 mice, the contribution of peripheral neutralization of NGF to the onset of brain neurodegeneration was still unexplored. To address this question, we characterized a line of transgenic mice (AD10) in which anti-NGF antibodies are obligatorily produced only in lymphocytes, being initially found in blood. In AD10 mice, peripheral NGF neutralization elicits shrinkage of superior cervical ganglia (immunosympathectomy) and, as a consequence of this, peripheral anti-NGF antibodies cross the blood brain barrier (BBB) and reach the brain, generating an NGF-dependent neurodegeneration, largely superimposable to that observed in AD11 mice. This demonstrates that peripherally originated anti-NGF antibodies can generate a neurodegeneration in the central nervous system of an animal model. Consistently, peripherally-delivered NGF is effective in preventing the onset of the central cholinergic deficit. These findings could have a direct relevance for some human sporadic AD cases, highlighting the role of the BBB disruption and suggesting a causally relevant role of circulating antibodies in AD pathology.

Adult neurogenesis is a multistep process regulated by several extrinsic factors, including neurotrophins. Among them, little is known about the role of nerve growth factor (NGF) in the neurogenic niches of the mouse. Here we analyzed the biology of adult neural stem cells (NSCs) from the subventricular zone (SVZ) of AD11 anti-NGF transgenic mice, in which the expression of the recombinant antibody aD11 leads to a chronic postnatal neutralization of endogenous NGF. We showed that AD11-NSCs proliferate 10-fold less, with respect to their control counterparts, and display a significant impairment in their ability to differentiate into β-tubulin positive neurons. We found a considerable reduction in the number of SVZ progenitors and neuroblasts also in vivo, which correlates with a lower number of newborn neurons in the olfactory bulbs of AD11 mice and a severe deficit in the ability of these mice to discriminate between different odors. We also demonstrated that, in AD11 mice, the morphology of both SVZ-resident and neurosphere-derived astrocytes is significantly altered. We were able to reproduce the AD11 phenotype in vitro, by acutely treating wild type NSCs with the anti-NGF antibody, further demonstrating that both the proliferation and the differentiation defects are due to the NGF deprivation. Consistently, the proliferative impairment of AD11 progenitors, as well as the atrophic morphology of AD11 astrocytes, can be partly rescued in vitro and in vivo by exogenous NGF addition. Altogether, our results demonstrate a causal link between NGF signaling and proper proliferation and differentiation of neural stem cells from the SVZ. Adult neurogenesis is a multistep process regulated by several extrinsic factors, including neurotrophins. Among them, little is known about the role of nerve growth factor (NGF) in the neurogenic niches of the mouse. Here we analyzed the biology of adult neural stem cells (NSCs) from the subventricular zone (SVZ) of AD11 anti-NGF transgenic mice, in which the expression of the recombinant antibody aD11 leads to a chronic postnatal neutralization of endogenous NGF. We showed that AD11-NSCs proliferate 10-fold less, with respect to their control counterparts, and display a significant impairment in their ability to differentiate into β-tubulin positive neurons. We found a considerable reduction in the number of SVZ progenitors and neuroblasts also in vivo, which correlates with a lower number of newborn neurons in the olfactory bulbs of AD11 mice and a severe deficit in the ability of these mice to discriminate between different odors. We also demonstrated that, in AD11 mice, the morphology of both SVZ-resident and neurosphere-derived astrocytes is significantly altered. We were able to reproduce the AD11 phenotype in vitro, by acutely treating wild type NSCs with the anti-NGF antibody, further demonstrating that both the proliferation and the differentiation defects are due to the NGF deprivation. Consistently, the proliferative impairment of AD11 progenitors, as well as the atrophic morphology of AD11 astrocytes, can be partly rescued in vitro and in vivo by exogenous NGF addition. Altogether, our results demonstrate a causal link between NGF signaling and proper proliferation and differentiation of neural stem cells from the SVZ.

Highlights ► We hypothesize that neurodegeneration in anti-NGF is due to NGF/proNGF imbalance. ► We propose that the proNGF co- receptor sortilin contributes to the neurodegeneration. ► We analyzed anti-NGF mice crossed to sortilin knockout mice. ► Sortilin loss partially protected AD10 anti-NGF mice from neurodegeneration., Sortilin is a member of the family of vacuolar protein sorting 10 protein domain receptors which has emerged as a co-receptor in cell death and neurodegeneration processes mediated by proneurotrophins. Here we tested the possibility that sortilin deficiency interferes with behavioral and neuropathological endpoints in a chronic Nerve Growth factor (NGF)-deprivation model of Alzheimer’s disease (AD), the AD10 anti-NGF mouse. AD10 mice show cholinergic deficit, increased APP processing and tau hyper-phosphorylation, resulting in behavioral deficits in learning and memory paradigms assessed by novel object recognition and Morris water maze tests. Sort1−/− mice were crossed with AD10 anti-NGF mice and the neurodegenerative phenotype was studied. We found that the loss of sortilin partially protected AD10 anti-NGF mice from neurodegeneration. A protective effect was observed on non-spatial memory as assessed by novel object recognition, and histopathologically at the level of Aβ and BFCNs, while the phosphotau increase was unaltered by knocking out sortilin. We suggest that sortilin might be involved in different aspects of neurodegeneration in a complex way, supporting the view that sortilin functions in the CNS are broader than being a co-receptor in proneurotrophin and neurotrophin signaling.

Nerve growth factor (NGF) was discovered because of its neurotrophic actions on sympathetic and sensory neurons in the developing chicken embryo. NGF was subsequently found to influence and regulate the function of many neuronal and non neuronal cells in adult organisms. Little is known, however, about the possible actions of NGF during early embryonic stages. However, mRNAs encoding for NGF and its receptors TrkA and p75 NTR are expressed at very early stages of avian embryo development, before the nervous system is formed. The question, therefore, arises as to what might be the functions of NGF in early chicken embryo development, before its well-established actions on the developing sympathetic and sensory neurons. To investigate possible roles of NGF in the earliest stages of development, stage HH 11–12 chicken embryos were injected with an anti-NGF antibody (mAb αD11) that binds mature NGF with high affinity. Treatment with anti-NGF, but not with a control antibody, led to a dose-dependent inversion of the direction of axial rotation. This effect of altered rotation after anti NGF injection was associated with an increased cell death in somites. Concurrently, a microarray mRNA expression analysis revealed that NGF neutralization affects the expression of genes linked to the regulation of development or cell proliferation. These results reveal a role for NGF in early chicken embryo development and, in particular, in the regulation of somite survival and axial rotation, a crucial developmental process linked to left–right asymmetry specification.