mTOR-related synaptic pathology causes autism spectrum disorder-associated functional hyperconnectivity

Postmortem studies have revealed increased density of excitatory synapses in the brains of individuals with autism spectrum disorder (ASD), with a putative link to aberrant mTOR-dependent synaptic pruning. ASD is also characterized by atypical macroscale functional connectivity as measured with resting-state fMRI (rsfMRI). These observations raise the question of whether excess of synapses causes aberrant functional connectivity in ASD. Using rsfMRI, electrophysiology and in silico modelling in Tsc2 haploinsufficient mice, we show that mTOR-dependent increased spine density is associated with ASD -like stereotypies and cortico-striatal hyperconnectivity. These deficits are completely rescued by pharmacological inhibition of mTOR. Notably, we further demonstrate that children with idiopathic ASD exhibit analogous cortical-striatal hyperconnectivity, and document that this connectivity fingerprint is enriched for ASD-dysregulated genes interacting with mTOR or Tsc2. Finally, we show that the identified transcriptomic signature is predominantly expressed in a subset of children with autism, thereby defining a segregable autism subtype. Our findings causally link mTOR-related synaptic pathology to large-scale network aberrations, revealing a unifying multi-scale framework that mechanistically reconciles developmental synaptopathy and functional hyperconnectivity in autism.

SIRT1 Regulates Tau Expression and Tau Synaptic Pathology

Background: Amyloid plaques and neurofibrillary tangles are two pathological hallmarks of Alzheimer’s disease (AD). However, synaptic deficits occur much earlier and correlate stronger with cognitive decline than amyloid plaques and neurofibrillary tangles. Mislocalization of tau is an early hallmark of neurodegeneration and precedes aggregations. Sirtuin type 1 (SIRT1) is a deacetylase which acts on proteins including transcriptional factors and associates closely with AD. Objective: The present study investigated the association between SIRT1 and tau expression in cells and in mice brains. Methods: Western blot was performed to detected tau, SIRT1, C/EBPα, and GAPDH protein levels. Immunological fluorescence assay was used to assess tau localization in primary cortical neuronal cells. Golgi staining was performed to evaluated dendritic spine morphology in mice brains. Results: In the present study, we found that SIRT1 negatively regulates expression of tau at the transcriptional level through transcriptional factor C/EBPα. Inhibition of the activity of SIRT1 limits the distribution of tau to the neurites. In the meantime, the alteration of dendritic spine morphology is also observed in the brains of SIRT1+/– mice. Conclusion: SIRT1 may be a potential drug target for early intervention in AD.

Synaptic Alterations in a Transgenic Model of Tuberous Sclerosis Complex: Relevance to Autism Spectrum Disorders

Tuberous sclerosis complex (TSC) is a rare, multi-system genetic disease with serious neurological and mental symptoms, including autism. Mutations in the TSC1/TSC2 genes lead to the overactivation of mTOR signalling, which is also linked to nonsyndromic autism. Our aim was to analyse synaptic pathology in a transgenic model of TSC: two-month-old male B6;129S4-Tsc2tm1Djk/J mice with Tsc2 haploinsufficiency. Significant brain-region-dependent alterations in the expression of several synaptic proteins were identified. The most prominent changes were observed in the immunoreactivity of presynaptic VAMP1/2 (ca. 50% increase) and phospho-synapsin-1 (Ser62/67) (ca. 80% increase). Transmission electron microscopy demonstrated serious ultrastructural abnormalities in synapses such as a blurred structure of synaptic density and a significantly increased number of synaptic vesicles. The impairment of synaptic mitochondrial ultrastructure was represented by excessive elongation, swelling, and blurred crista contours. Polyribosomes in the cytoplasm and swollen Golgi apparatus suggest possible impairment of protein metabolism. Moreover, the delamination of myelin and the presence of vacuolar structures in the cell nucleus were observed. We also report that Tsc2+/− mice displayed increased brain weights and sizes. The behavioural analysis demonstrated the impairment of memory function, as established in the novel object recognition test. To summarise, our data indicate serious synaptic impairment in the brains of male Tsc2+/− mice.

Synaptic tau: A pathological or physiological phenomenon?

In this review, we discuss the synaptic aspects of Tau pathology occurring during Alzheimer’s disease (AD) and how this may relate to memory impairment, a major hallmark of AD. Whilst the clinical diagnosis of AD patients is a loss of working memory and long-term declarative memory, the histological diagnosis is the presence of neurofibrillary tangles of hyperphosphorylated Tau and Amyloid-beta plaques. Tau pathology spreads through synaptically connected neurons to impair synaptic function preceding the formation of neurofibrillary tangles, synaptic loss, axonal retraction and cell death. Alongside synaptic pathology, recent data suggest that Tau has physiological roles in the pre- or post- synaptic compartments. Thus, we have seen a shift in the research focus from Tau as a microtubule-stabilising protein in axons, to Tau as a synaptic protein with roles in accelerating spine formation, dendritic elongation, and in synaptic plasticity coordinating memory pathways. We collate here the myriad of emerging interactions and physiological roles of synaptic Tau, and discuss the current evidence that synaptic Tau contributes to pathology in AD.

Acute and chronic synaptic pathology in multiple sclerosis gray matter

Objectives: To investigate the extent of synaptic loss, and the contribution of gray matter (GM) inflammation and demyelination to synaptic loss, in multiple sclerosis (MS) brain tissue. Methods: This study was performed on two different post-mortem series of MS and control brains, including deep GM and cortical GM. MS brain samples had been specifically selected for the presence of active demyelinating GM lesions. Over 1,000,000 individual synapses were identified and counted using confocal microscopy, and further characterized as glutamatergic/GABAergic. Synaptic counts were also correlated with neuronal/axonal loss. Results: Important synaptic loss was observed in active demyelinating GM lesions (−58.9%), while in chronic inactive GM lesions, synaptic density was only mildly reduced compared to adjacent non-lesional gray matter (NLGM) (−12.6%). Synaptic loss equally affected glutamatergic and GABAergic synapses. Diffuse synaptic loss was observed in MS NLGM compared to control GM (−21.2% overall). Conclusion: This study provides evidence, in MS brain tissue, of acute synaptic damage/loss during active GM inflammatory demyelination and of synaptic reorganization in chronically demyelinated GM, affecting equally glutamatergic and GABAergic synapses. Furthermore, this study provides a strong indication of widespread synaptic loss in MS NLGM also independently from focal GM demyelination.

The Effects of CSF Neurogranin and APOE ε4 on Cognition and Neuropathology in Mild Cognitive Impairment and Alzheimer’s Disease

Cerebrospinal fluid (CSF) measurements of neurogranin (Ng) have emerged as a promising biomarker for cognitive decline in mild cognitive impairment (MCI) and Alzheimer’s disease (AD). The apolipoprotein E ε4 (APOE ε4) allele is by far the most consistent genetic risk factor for AD. However, it is not known whether the pathophysiological roles of Ng in MCI or AD are related to APOEε4. We stratified 250 participants from the Alzheimer’s Disease Neuroimaging Initiative (ADNI) database into cognitively normal (CN) ε4 negative (CN ε4−), CN ε4 positive (CN ε4+), MCI ε4 negative (MCI ε4−), MCI ε4 positive (MCI ε4+), AD ε4 negative (AD ε4−), and AD ε4 positive (AD ε4+). CSF Ng levels were significantly increased in APOE ε4 carriers compared to APOE ε4 non-carriers with MCI. In addition, CSF Ng identified MCI ε4+ versus CN ε4−, but not MCI ε4− versus CN ε4−. Similarly, CSF Ng negatively correlated with Mini-Mental State Examination (MMSE) scores at baseline in the MCI ε4+ group. Our findings support the use of CSF Ng as a biomarker of synaptic pathology for AD. We propose that the roles of CSF Ng in the pathophysiology of MCI may be related to APOE ε4.

Arc rescues defective synaptic plasticity and cognitive dysfunction in sepsis-associated encephalopathy

Sepsis-associated encephalopathy (SAE) is a major and frequent complication in patients with sepsis resulting in delirium and premature death. Sepsis survivors commonly suffer from long-term cognitive impairment causing immense burden on patients, caregivers, and economic health systems. The underlying pathophysiology of SAE is largely unresolved, thus treatment options are missing. We report that experimental polymicrobial sepsis in mice induces synaptic pathology in the central nervous system underlying defective long-term potentiation and cognitive dysfunction. Analysis of differentially expressed genes revealed severely affected downregulation of genes related to neuronal and synaptic signaling in the brain, e.g. of the activity-regulated cytoskeleton-associated protein (Arc), of the transcription-regulatory EGR family, and of the dual-specificity phosphatase 6 (Dusp6). On the protein level, ARC expression and mitogen-activated protein (MAP) kinase signaling in the brain was disturbed during SAE. For targeted rescue of dysregulated synaptic signaling and plasticity, we overexpressed ARC in the hippocampus by bilateral in-vivo stereotactic microinjection of an adeno-associated virus containing a neuron-specific plasmid of the Arc transgene. Hereby, defective synaptic plasticity and signaling in the hippocampus were restored and memory function improved. Accordingly, synaptic plasticity, neuronal spine pathology, and memory dysfunction also improved when post-septic mice were subjected to enriched environment demonstrating the potential for activity-induced recovery of long-term cognitive dysfunction. Together, we identified synaptic pathology of neurocognitive dysfunction after severe systemic infection and provide a proof-of-concept approach to interfere with SAE pathomechanisms leading to cognitive improvement.