The triggering receptor expressed on myeloid cells 2 (TREM2) is a microglial innate immune receptor associated with a lethal form of early, progressive dementia, Nasu-Hakola disease, and with an increased risk of Alzheimer's disease. Microglial defects in phagocytosis of toxic aggregates or apoptotic membranes were proposed to be at the origin of the pathological processes in the presence of Trem2 inactivating mutations. Here, we show that TREM2 is essential for microglia-mediated synaptic refinement during the early stages of brain development. The absence of Trem2 resulted in impaired synapse elimination, accompanied by enhanced excitatory neurotransmission and reduced long-range functional connectivity. Trem2 mice displayed repetitive behavior and altered sociability. TREM2 protein levels were also negatively correlated with the severity of symptoms in humans affected by autism. These data unveil the role of TREM2 in neuronal circuit sculpting and provide the evidence for the receptor's involvement in neurodevelopmental diseases.

Neuronal circuit assembly requires the fine balance between synapse formation and elimination. Microglia, through the elimination of supernumerary synapses, have an established role in this process. While the microglial receptor TREM 2 and the soluble complement proteins C1q and C3 are recognized as key players, the neuronal molecular components that specify synapses to be eliminated are still undefined. Here, we show that exposed phosphatidylserine ( PS ) represents a neuronal “eat‐me” signal involved in microglial‐mediated pruning. In hippocampal neuron and microglia co‐cultures, synapse elimination can be partially prevented by blocking accessibility of exposed PS using Annexin V or through microglial loss of TREM 2. In vivo , PS exposure at both hippocampal and retinogeniculate synapses and engulfment of PS ‐labeled material by microglia occurs during established developmental periods of microglial‐mediated synapse elimination. Mice deficient in C1q, which fail to properly refine retinogeniculate connections, have elevated presynaptic PS exposure and reduced PS engulfment by microglia. These data provide mechanistic insight into microglial‐mediated synapse pruning and identify a novel role of developmentally regulated neuronal PS exposure that is common among developing brain structures.

Neuronal circuits assembly requires the fine equilibrium between synapse formation and elimination. Microglia, through the elimination of supernumerary synapses, have an established role in this process. While the microglial receptor TREM2 and the soluble complement proteins C1q and C3 are recognized key players in this process, the neuronal molecular components that tag synapses to be eliminated are still undefined. Here we show that exposed phosphatidylserine (PS) represents a neuronal 'eat-me' signal enabling microglial-mediated synapse pruning. In hippocampal neuron and microglia co-cultures, synapse elimination can be prevented by blocking accessibility of exposed PS using Annexin V or through microglial loss of TREM2. In vivo, exposed PS is detectable at both hippocampal and retinogeniculate synapses, where exposure coincides with the onset of synapse elimination and increased PS engulfment by microglia. Mice deficient in C1q, which fail to properly refine retinogeniculate connections, display elevated exposed PS and reduced PS engulfment by microglia. These data provide mechanistic insight into microglial-mediated synapse pruning and identify a novel role of developmentally regulated PS exposure that is common among developing brain structures.

Bone marrow Mesenchymal Stem Cells (BM-MSCs), due to their strong protective and anti-inflammatory abilities, have been widely investigated in the context of several diseases for their possible therapeutic role, based on the release of a highly proactive secretome composed of soluble factors and Extracellular Vesicles (EVs). BM-MSC-EVs, in particular, convey many of the beneficial features of parental cells, including direct and indirect β-amyloid degrading-activities, immunoregulatory and neurotrophic abilities. Therefore, EVs represent an extremely attractive tool for therapeutic purposes in neurodegenerative diseases, including Alzheimer’s disease (AD). We examined the therapeutic potential of BM-MSC-EVs injected intracerebrally into the neocortex of APPswe/PS1dE9 AD mice at 3 and 5 months of age, a time window in which the cognitive behavioral phenotype is not yet detectable or has just started to appear. We demonstrate that BM-MSC-EVs are effective at reducing the Aβ plaque burden and the amount of dystrophic neurites in both the cortex and hippocampus. The presence of Neprilysin on BM-MSC-EVs, opens the possibility of a direct β-amyloid degrading action. Our results indicate a potential role for BM-MSC-EVs already in the early stages of AD, suggesting the possibility of intervening before overt clinical manifestations.

No abstract

Glioblastoma multiforme (GBM) is the most aggressive form of glioma, with life expectancy of around 2 years after diagnosis, due to recidivism and to the blood-brain barrier (BBB) limiting the amount of drugs which reach the residual malignant cells, thus contributing to the failure of chemotherapies. To bypass the obstacles imposed by the BBB, we investigated the use of nanotechnologies combined with radiotherapy, as a potential therapeutic strategy for GBM. We used poly(lactic-co-glycolic acid) (PLGA) nanoparticles (PNP) conjugated to chlorotoxin (CTX), a peptide reported to bind selectively to glioma cells. Silver nanoparticles were entrapped inside the functionalized nanoparticles (Ag-PNP-CTX), to allow detection and quantification of the cellular uptake by confocal microscopy, both in vitro and in vivo. In vitro experiments performed with different human glioblastoma cell lines showed higher cytoplasmic uptake of Ag-PNP-CTX, with respect to nonfunctionalized nanoparticles. In vivo experiments showed that Ag-NP-CTX efficiently targets the tumor, but are scarcely effective in crossing the blood brain barrier in the healthy brain, where dispersed metastatic cells are present. We show here that single whole brain X-ray irradiation, performed 20 h before nanoparticle injection, enhances the expression of the CTX targets, MMP-2 and ClC-3, and, through BBB permeabilization, potently increases the amount of internalized Ag-PNP-CTX even in dispersed cells, and generated an efficient antitumor synergistic effect able to inhibit in vivo tumor growth. Notably, the application of Ag-PNP-CTX to irradiated tumor cells decreases the extracellular activity of MMP-2. By targeting dispersed GBM cells and reducing MMP-2 activity, the combined use of CTX-nanovectors with radiotherapy may represent a promising therapeutic approach toward GBM.

Substantial data indicate that amyloid-β (Aβ), the major component of senile plaques, plays a central role in Alzheimer’s Disease and indeed the assembly of naturally occurring amyloid peptides into cytotoxic aggregates is linked to the disease pathogenesis. Although Aβ42 is a highly aggregating form of Aβ, the co-occurrence of shorter Aβ peptides might affect the aggregation potential of the Aβ pool. In this study we aimed to assess whether the structural behavior of human Aβ42 peptide inside the brain is influenced by the concomitant presence of N-terminal fragments produced by the proteolytic activity of glial cells. We show that the occurrence of the human C-terminal truncated 1–24 Aβ fragment impairs Aβ42 clearance through blood brain barrier and promotes the formation of Aβ42 aggregates even in the healthy brain. By showing that Aβ1-24 has seeding properties for aggregate formation in intracranially injected wild type mice, our study provide the proof-of-concept that peptides produced upon Aβ42 cleavage by activated glial cells may cause phenotypic defects even in the absence of genetic mutations associated with Alzheimer’s Disease, possibly contributing to the development of the sporadic form of the pathology.

The capacity to guarantee the proper excitatory/inhibitory balance is one of the most critical steps during early development responsible for the correct brain organization, function, and plasticity. GABAergic neurons guide this process leading to the right structural organization, brain circuitry, and neuronal firing. Here, we identified the ataxia telangiectasia mutated (ATM), a serine/threonine protein kinase linked to DNA damage response, as crucial in regulating neurotransmission. We found that reduced levels of ATM in the hippocampal neuronal cultures produce an excitatory/inhibitory unbalance toward inhibition as indicated by the higher frequency of miniature inhibitory postsynaptic current events and an increased number of GABAergic synapses. In vivo, the increased inhibition still persists and, even if a higher excitation is also present, a reduced neuronal excitability is found as indicated by the lower action potential frequency generated in response to high-current intensity stimuli. Finally, we found an elevated extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation in heterozygous hippocampi associated with lower expression levels of the ERK1/2 phosphatase PP1. Given that the neurodegenerative condition associated with genetic mutations in the Atm gene, ataxia telangiectasia, presents a variable phenotype with impairment in cognition, our molecular findings provide a logical frame for a more clear comprehension of cognitive defects in the pathology, opening to novel therapeutic strategies.