Neuronal Activity at Synapse Resolution:Reporters and Effectors for Synaptic Neuroscience

The development of methods for the activity-dependent tagging of neurons enabled a new way to tackle the problem of engram identification at the cellular level, giving rise to groundbreaking findings in the field of memory studies. However, the resolution of activity-dependent tagging remains limited to the whole-cell level. Notably, events taking place at the synapse level play a critical role in the establishment of new memories, and strong experimental evidence shows that learning and synaptic plasticity are tightly linked. Here, we provide a comprehensive review of the currently available techniques that enable to identify and track the neuronal activity with synaptic spatial resolution. We also present recent technologies that allow to selectively interfere with specific subsets of synapses. Lastly, we discuss how these technologies can be applied to the study of learning and memory

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

Abstract 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

A continuous model of physiological prion aggregation suggests a role for Orb2 in gating long-term synaptic information.

The regulation of mRNA translation at the level of the synapse is believed to be fundamental in memory and learning at the cellular level. The family of cytoplasmic polyadenylation element binding (CPEB) proteins emerged as an important RNA-binding protein family during development and in adult neurons. Drosophila Orb2 (homologue of mouse CPEB3 protein and of the neural isoform of Aplysia CPEB) has been found to be involved in the translation of plasticity-dependent mRNAs and has been associated with long-term memory. Orb2 protein presents two main isoforms, Orb2A and Orb2B, which form an activity-induced amyloid-like functional aggregate, thought to be the translation-inducing state of the RNA-binding protein. Here we present a first two-states continuous differential model for Orb2A-Orb2B aggregation. This model provides new working hypotheses for studying the role of prion-like CPEB proteins in long-term synaptic plasticity. Moreover, this model can be used as a first step to integrate translation- and protein aggregation-dependent phenomena in synaptic facilitation rules

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

Molecular Dynamics Simulations of the NGF-TrkA Domain 5 Complex and Comparison with Biological Data

AbstractThe nerve growth factor (NGF) is an important pharmacological target for Alzheimer's and other neurodegenerative diseases. Its action derives partly from its binding to the tyrosine kinase A receptor (TrkA). Here we study energetics and dynamics of the NGF-TrkA complex by carrying out multinanosecond molecular dynamics simulations, accompanied by electrostatic calculations based on the Poisson-Boltzmann equation. Our calculations, which are based on the x-ray structure of the complex, suggest that some of the mutations affecting dramatically the affinity of the complex involve residues that form highly favorable, direct or water-mediated hydrogen bond interactions at the ligand-receptor interface and, in some cases, that also critically participate to the large-scale motions of the complex. Furthermore, our calculations offer a rationale for the small effect on binding affinity observed upon specific mutations involving large changes in electrostatics (i.e., the charged-to-neutral mutations). Finally, these calculations, used along with the mutagenesis data, provide a basis for designing new peptides that mimic NGF in TrkA binding function

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

Understanding pain perception through genetic painlessness diseases: The role of NGF and proNGF.

Nerve growth factor (NGF), by binding to TrkA and p75(NTR) receptors, regulates the survival and differentiation of sensory neurons during development and mediates pain transmission and perception during adulthood, by acting at different levels of the nervous system. Key to understanding the role of NGF as a pain mediator is the finding that mutations (namely, R121W, V232fs and R221W) in the NGF gene cause painlessness disease Hereditary Sensory and Autonomic Neuropathy type V (HSAN V). Here we shall review the consequences of these NGF mutations, each of which results in specific clinical signs: R221W determines congenital pain insensitivity with no overt cognitive disabilities, whereas V232fs and R121W also result in intellectual disability, thus showing similarities to HSAN IV, which is caused by mutations in TrkA, rather than to HSAN V. Comparing the cellular, biochemical and clinical findings of these mutations could help in better understanding not only the possible mechanisms underlying HSAN V, but also mechanisms of NGF signalling and roles. These mutations alter the balance between NGF and proNGF in favour of an accumulation of the latter, suggesting a possible role of proNGF as a molecule with an analgesic role. Furthermore, the neurotrophic and pronociceptive functions of NGF are split by the R221W mutation, making NGF variants based on this mutation interesting for designing therapeutic applications for many diseases. This review emphasizes the possibility of using the mutations involved in "painlessness" clinical disorders as an innovative approach to identify new proteins and pathways involved in pain transmission and perception.Outstanding questions: Why do homozygous HSAN V die postnatally? What is the cause of this early postnatal lethality?Is the development of a mouse or a human feeling less pain affecting higher cognitive and perceptual functions?What is the consequence of the HSAN V mutation on the development of joints and bones? Are the multiple fractures observed in HSAN V patients due exclusively to the carelessness consequent to not feeling pain, or also to an intrinsic frailty of their bones?Are heterodimers of NGF(WT) and NGF(R221W) in the heterozygote state formed? And if so, what are the properties of these heterodimeric proteins?How is the processing of proNGF(R221W) to NGF(R221W) affected by the mutation

An Optimized Procedure for the Site-Directed Labeling of NGF and proNGF for Imaging Purposes

Neurotrophins are growth factors of fundamental importance for the development, survival and maintenance of different neuronal and non-neuronal populations. Over the years, the use of labeled neurotrophins has helped in the study of their biological functions, leading to a better understanding of the processes that regulate their transport, traffic, and signaling. However, the diverse and heterogeneous neurotrophin labeling strategies adopted so far have often led to poorly reproducible protocols and sometimes conflicting conclusions. Here we present a robust, reliable, and fast method to obtain homogeneous preparations of fluorescent proNGF and NGF with 1:1 labeling stoichiometry. This strategy is well suited for several applications, ranging from advanced imaging techniques such as single particle tracking, to analyses that require large amounts of neurotrophins such as in vivo monitoring of protein biodistribution. As a proof of the quality of the labeled NGF and proNGF preparations, we provide a quantitative analysis of their colocalization with proteins involved in the signaling endosome function and sorting. This new analysis allowed demonstrating that proNGF localizes at a sub-population of endosomes not completely overlapped to the one hosting NGF

Blocking the NGF-TrkA Interaction Rescues the Developmental Loss of LTP in the Rat Visual Cortex Role of the Cholinergic System

AbstractAlthough nerve growth factor (NGF) is a crucial factor in the activity-dependent development and plasticity of visual cortex, its role in synaptic efficacy changes is largely undefined. We demonstrate that the maintenance phase of long-term potentiation (LTP) is blocked by local application of exogenous NGF in rat visual cortex at an early stage of postnatal development. Long-term depression (LTD) and bidirectional plasticity are unaffected. At later postnatal ages, blockade of either endogenous NGF by immunoadhesin (TrkA-IgG) or TrkA receptors by monoclonal antibody rescues LTP. Muscarinic receptor activation/inhibition suggests that LTP dependence on NGF is mediated by the cholinergic system. These results indicate that NGF regulates synaptic strength in well-characterized cortical circuitries