Edoardo Moretto

Impairments in axonal transport have been implicated in the pathogenesis of tauopathies, including frontotemporal dementia and Alzheimer’s disease, yet the underlying mechanisms and reversibility of these deficits are largely unknown. In particular, the impacts of tau mutations, phosphorylation and aggregation on axonal transport in vivo remain controversial. By using two-photon imaging of axonal transport of BDNF granules in the mouse cortex, we reveal that deficits in axonal transport arise in vivo at early stages of tau pathology, preceding tangle formation and neuronal death. Mechanistically, these impairments are caused by the enlargement of tau envelopes on microtubules, which act as functional barriers for transport. Crucially, these deficits are reversed by inhibiting MAPK p38α. Together, our work demonstrates that tau pathology causes reversible deficits in axonal transport in vivo, posing the basis for pharmacological interventions to restore the physiological flux of axonal organelles and cargoes in tauopathies.

Protocadherins are key regulators of neurodevelopment and synaptic function, acting not only as adhesion molecules but also as synaptic hubs for intracellular signaling. Here, we uncover a novel activity-dependent signaling pathway for Pcdh9 , a protocadherin linked to Autism Spectrum Disorder and Major Depressive Disorder. By combining biochemical and immunohistochemistry approaches on neuronal cultures, we show that neuronal activity triggers Matrix Metalloproteases (MMP)-dependent cleavage of PCDH9, generating a C-terminal fragment (CTF) that translocates to the nucleus. PCDH9 CTF overexpression promotes dendritic growth, increases spine density, and concomitantly strengthens excitatory synaptic transmission. These findings identify PCDH9 CTF as a novel activity-dependent signaling molecule that links synaptic activity to structural remodeling and functional modulation, suggesting a new mechanism by which synaptic activity shapes neuronal properties.

Social isolation and perceived loneliness are increasingly recognized as serious public health concerns, with extensive evidence linking them to adverse mental and physical health outcomes. Defined, respectively, as the objective lack of social interactions and the subjective feeling of insufficient connection, both conditions are present across various age groups and are associated with elevated risks of cognitive decline and psychiatric disorders. Epidemiological studies have also identified a strong association between chronic social isolation and the development of metabolic syndrome (MetS) and cardiovascular diseases (CVD), potentially mediated by dysregulated stress responses, immune function, and endocrine signaling. Animal models of social deprivation have proven instrumental in elucidating the biological underpinnings of these effects, revealing disruptions in neurotransmitter systems and in the hypothalamic–pituitary–adrenal (HPA) axis, with important downstream metabolic alterations. This review explores the molecular and cellular mechanisms linking social isolation to MetS and CVD, with a focus on oxidative stress, inflammation, mitochondrial dysfunction, and impaired autophagy. A deeper understanding of these pathways is essential to guide the development of targeted interventions and to reduce the long-term health burden associated with social disconnection.

Protocadherins, a family of adhesion molecules with a crucial role in cell–cell interactions, have emerged as key players in neurodevelopmental and psychiatric disorders. In particular, growing evidence links genetic alterations in the protocadherin 9 ( PCDH9 ) gene with autism spectrum disorder and major depressive disorder. Furthermore, Pcdh9 deletion induces neuronal defects in the mouse somatosensory cortex, accompanied by sensorimotor and memory impairment. However, the synaptic and molecular mechanisms of PCDH9 in the brain remain largely unknown, particularly concerning its impact on brain pathology. To address this question, we conducted a comprehensive investigation of PCDH9’s role in the male mouse hippocampus at the ultrastructural, biochemical, transcriptomic, electrophysiological, and network levels. We show that PCDH9 mainly localizes at glutamatergic synapses and its expression peaks in the first week after birth, a crucial time window for synaptogenesis. Strikingly, Pcdh9 KO neurons exhibit oversized presynaptic terminal and postsynaptic density in the CA1. Synapse overgrowth is sustained by the widespread upregulation of synaptic genes, as revealed by single-nucleus RNA-seq (snRNA-seq), and the dysregulation of key drivers of synapse morphogenesis, including the SHANK2/CORTACTIN pathway. At the functional level, these structural and transcriptional abnormalities result in increased excitatory postsynaptic currents (mEPSC) and reduced network activity in the CA1 of Pcdh9 KO mice. In conclusion, our work uncovers Pcdh9 ’s pivotal role in shaping the morphology and function of CA1 excitatory synapses, thereby modulating glutamatergic transmission within hippocampal circuits.

Protocadherins are cell adhesion molecules with crucial role in cell-cell contacts, whose mutations or altered expression have been implicated in multiple brain disorders. In particular, growing evidence links genetic alterations in Protocadherin 9 ( PCDH9 ) gene with Autism Spectrum Disorder (ASD) and Major Depression Disorder (MDD). Furthermore, Pcdh9 deletion induces neuronal defects in the mouse somatosensory cortex, accompanied by sensorimotor and memory impairment. However, the synaptic and molecular mechanisms underlying Pcdh9 physiological function and its involvement in brain pathology remain largely unknown. To this aim, we conducted a comprehensive investigation of PCDH9 role in the mouse hippocampus at the ultrastructural, biochemical, transcriptomic, electrophysiological and network level. We show that PCDH9 mainly localizes at glutamatergic synapses and its expression peaks in the first week after birth, a crucial time window for synaptogenesis. Strikingly, Pcdh9 KO neurons exhibit oversized presynaptic terminal and postsynaptic density (PSD) in the CA1. Synapse overgrowth is sustained by the broad up-regulation of synaptic genes and the dysregulation of key drivers of synapse morphogenesis, as revealed by single-nucleus RNAseq. Synaptic and transcriptional defects are accompanied by increased EPSC frequency and disturbances in the hippocampal network activity of Pcdh9 KO mice. In conclusion, our work indicates that Pcdh9 regulates the morphology and function of excitatory synapses in the CA1, thereby affecting glutamatergic transmission in hippocampal circuitries.

Background: The spatiotemporal spread of pathological tau observed across anatomically connected brain areas in Alzheimer’s disease suggests that tau pathology propagates trans‐synaptically. The underlying endocytic mechanisms of tau occurring at the post‐synapse remains poorly characterised. This study aims to investigate the mechanism/s controlling neuronal endocytosis of tau at the synapse, and understand how this mechanism is altered in tauopathies, with the goal of identifying targets to prevent pathological tau spread.Method: To achieve this ambitious target, human tau undergoing synaptic endocytosis will be monitored in living neurons with confocal microscopy in specifically designed unidirectional microfluidic devices. Incorporating this novel model, targeted screening using pharmacological and genetic approaches will identify which endocytic mechanisms are exploited by pathological tau to enter neurons. Finally, targets of interest will be further investigated through co‐localisation studies to human tau and post‐synaptic markers using super‐resolution microscopy.Result: Preliminary experiments showed that by infecting an isolated population of neurons with the P301S‐tau mutant, tau‐positive puncta and aggregates are detected in synaptically connected neurons, suggestive of trans‐synaptic tau propagation. In addition, we demonstrated that tau transfer is dependent on neuronal stimulation, supporting the notion that tau spreads across synapses in an activity‐dependent manner.Conclusion: Further work is required to optimise this experimental set up for investigating the exact mechanisms underlying tau endocytosis at the post‐synaptic level. Elucidating the specific pathways involved in tau uptake at the synapse will aid the identification of novel targets to prevent pathogenic tau spread and tauopathy progression.

Background: Tau is a protein abundantly expressed in neurons where it modulates the stability of axonal microtubules, thus contributing to the regulation of axonal transport of several organelles. Tau aggregates in a group of neurodegenerative diseases named tauopathies, which include frontotemporal dementia (FTD) and Alzheimer’s disease. Recent in vitro work has uncovered the existence of tau “islands”, a microtubule‐bound multimeric state of tau, which is distinct from pathological aggregates. However, whether these structures exist in intact neurons remains unclear.Method: We investigated both cultured mouse neurons and mice in vivo, using immunoistochemistry, live imaging and two photon microscopy.Results: we found that human tau showed regions of higher density along axons, reminiscent of tau islands. FTD‐linked mutations, known to increase pathological phosphorylation and aggregation of tau, induces larger islands, an effect that is reversed by inhibition of p38 MAPK, known to phosphorylate tau at multiple sites. Functionally, axonal transport of BDNF‐containing secretory granules is affected by FTD‐linked mutant tau as assessed both in vitro and in vivo by using a new assay based on two‐photon microscopy. Interestingly, this impairment occurred very early on, before overt tau aggregation. Inhibition of p38 MAPK was able to partially rescue the defects in axonal transport both in vitro and in vivo.Conclusion: Our data suggests that tau island size regulates axonal transport, an effect dependent on tau phosphorylation. Inefficient organelles transport may have severe consequences on the activity and plasticity of neuronal circuits. The evidence that reducing tau phosphorylation by inhibiting p38 MAPK potentiated axonal transport points towards inhibition of p38 MAPK as a promising therapeutic strategy in tauopathies.

Intracellular trafficking of AMPA receptors is a tightly regulated process which involves several adaptor proteins, and is crucial for the activity of excitatory synapses both in basal conditions and during synaptic plasticity. We found that, in rat hippocampal neurons, an intracellular pool of the tetraspanin TSPAN5 promotes exocytosis of AMPA receptors without affecting their internalisation. TSPAN5 mediates this function by interacting with the adaptor protein complex AP4 and Stargazin and possibly using recycling endosomes as a delivery route. This work highlights TSPAN5 as a new adaptor regulating AMPA receptor trafficking.

Pathological tau aggregates propagate across functionally connected neuronal networks in human neurodegenerative pathologies, such as Alzheimer’s disease. However, the mechanism underlying this process is poorly understood. Several studies have showed that tau release is dependent on neuronal activity and that pathological tau is found in the extracellular space in free form, as well as in the lumen of extracellular vesicles. We recently showed that metabotropic glutamate receptor activity and the SNAP25 integrity modulate the release of pathological tau from human and mouse synaptosomes. Here, we have leveraged botulinum neurotoxins (BoNTs), which impair neurotransmitter release by cleaving specific synaptic SNARE proteins, to dissect molecular mechanisms related to tau release at synapses. In particular, we have tested the effect of botulinum neurotoxin A (BoNT/A) on the synaptic release of tau in primary mouse neurons. Hippocampal neurons were grown in microfluidic chambers and transduced with lentiviruses expressing human tau (hTau). We found that neuronal stimulation significantly increases the release of mutant hTau, whereas wild-type hTau is unaffected. Importantly, BoNT/A blocks mutant hTau release, indicating that this process is modulated by SNAP25 in intact neurons. These results suggest that BoNTs are potent tools to study the spreading of pathological proteins in neurodegenerative diseases and will play a central role in identifying novel molecular targets for the development of therapeutic interventions to treat tauopathies.

Several neurodegenerative diseases are characterized by the accumulation of aggregated misfolded proteins. These pathological agents have been suggested to propagate in the brain via mechanisms similar to that observed for the prion protein, where a misfolded variant is transferred from an affected brain region to a healthy one, thereby inducing the misfolding and/or aggregation of correctly folded copies. This process has been characterized for several proteins, such as α-synuclein, tau, amyloid beta (Aβ) and less extensively for huntingtin and TDP-43. α-synuclein, tau, TDP-43 and huntingtin are intracellular proteins, and their aggregates are located in the cytosol or nucleus of neurons. They have been shown to spread between cells and this event occurs, at least partially, via secretion of these protein aggregates in the extracellular space followed by re-uptake. Conversely, Aβ aggregates are found mainly extracellularly, and their spreading occurs in the extracellular space between brain regions. Due to the inherent nature of their spreading modalities, these proteins are exposed to components of the extracellular matrix (ECM), including glycans, proteases and core matrix proteins. These ECM components can interact with or process pathological misfolded proteins, potentially changing their properties and thus regulating their spreading capabilities. Here, we present an overview of the documented roles of ECM components in the spreading of pathological protein aggregates in neurodegenerative diseases with the objective of identifying the current gaps in knowledge and stimulating further research in the field. This could potentially lead to the identification of druggable targets to slow down the spreading and/or progression of these pathologies.