NGF steers microglia toward a neuroprotective phenotype

Microglia are the resident immune cells of the Central Nervous System (CNS). Beside classic in ammatory activities shared with macrophages, microglia actively participate in activity-dependent plasticity and learning processes [1] [2], as sculpting the neuronal circuitry during development [3] [4]. Microglia have been shown to be key players in the pathogenesis and progression of many neurodegenerative disorders and they are responsible for brain homeostasis and monitor the brain environment with their ever-moving processes [5] [6]. However, their role, either promoting or preventing pathology, is debated. On one hand, excessive activation of microglia leads to oxidative stress, neuroinammation, and eventually neuronal death [7]. On the other hand, microglial activation might be harnessed to carry out protective activities in the brain, such as phagocytosis of aggregates, synaptic pruning and formation, and the maintenance of healthy neuronal circuits [8]. Therefore, it is important to identify and modulate selectively the neuroprotective activities of microglia. The idea of microglia cells as the natural scavengers of the brain becomes especially interesting when dealing with diseases with the loss of proteostasis such as Alzheimer's disease. In the search of neuroprotective agents against neurodegeneration, neurotrophins have been historically considered as potential therapeutic candidates but usually with actions targeted to natural neuronal population. In this thesis I tested the hypothesis that microglia represent a new target cell for Nerve Growth Factor (NGF) in the brain. So far sparse experiments in the literature suggest this insight. In the literature microglia cells are known to be a source of neurotrophins [9] [10][11], most notably the Brain Derived Neurotrophic Factor (BDNF) which has been shown to promote synapse formation [1] and NGF [12] [13]. However, the extent of the modulation NGF might exert on physiological microglial functions and how this effect might come into play in neurodegenerative disorders has not been in- vestigated yet. Indeed, the main cellular targets of the neurotrophin Nerve Growth Factor (NGF) [14] in the central nervous system are considered to be the cholinergic neurons of the basal forebrain (BFCNs) [15], while its sources are mainly cortical and hippocampal neurons [16]. Consistently, interference with NGF signaling (trkA-NGF signalling) in the adult brain leads to de cits of the cholinergic system that has been related to the mechanisms driving neurodegeneration, as in the AD11 transgenic mouse model [17] [18]. The expression of anti-NGF antibodies selectively neutralizing mature NGF in the adult brain determines a progressive comprehensive neurodegeneration with neuroinflammation as the earliest observed change, at a presymptomatic phase [19] [20]. A similar progressive neurodegeneration is observed in trans- genic mice expressing a neutralizing anti TrkA antibody in the adult brain [21]. Changes in NGF homeostasis in the brain, with particular regard to the ratio of NGF to proNGF levels, have also been linked to Alzheimer's disease [22]. However the overall neurodegenerative picture induced by anti- NGF or anti-TrkA antibodies in those transgenic models is much broader than what one would expect on the basis of an action of the antibodies on the BFCNs exclusively. Moreover, the loss of NGF-TrkA signaling in the CNS, obtained by conditionally deleting NGF or TrkA genes in CNS cells derived from nestin-positive cells, has proven not to be sufficient in inducing severe cognitive impairments or neurodegen- eration in mice [23]. Altogether, this body of results has motivated our search for non neuronal targets of NGF in the adult brain. Microglia was a strong candidate, because (1) previous work had suggested that NGF could modulate some aspects of microglial cells in culture [12] and (2) transcriptomic studies in the AD11 mouse model expressing anti-NGF had shown that neuroinammation is the earliest phenotypic alteration, already at a presymptomatic phase (1 month of age; [19] [20]). In this thesis I show that microglia cells are true target of NGF both in vivo and in vitro and that the activity carried out by this neurotrophin on these myeloid cells might result neuroprotective in the context of Alzheimer's Disease.