*Synaptic Dysfunction in Neurodegenerative Diseases: A Comprehensive Review* describes a key molecular or cellular mechanism implicated in neurodegenerative disease. This page provides a detailed overview of the pathway components, signaling cascades, and their relevance to conditions such as Alzheimer's disease, Parkinson's disease, and related disorders.
Synaptic Dysfunction in Neurodegenerative Diseases: A Comprehensive Review [@aggleton2010]
--- [@sze2000]
The loss of synaptic integrity is increasingly recognized as a central, early pathological event that precedes overt neuronal death and correlates strongly with the onset of cognitive decline in several major neurodegenerative disorders. In Alzheimer disease (AD), quantitative morphometric analyses demonstrated that the density of synaptic contacts declines decades before clinical symptoms become manifest, and the magnitude of this loss predicts the severity of dementia more reliably than the burden of amyloid plaques or neurofibrillary tangles (Terry et al., 1991; Masliah et al., 1994)【1,2】. Likewise, in Parkinson disease (PD) the earliest neurochemical alterations involve the dopaminergic terminal in the striatum, where reductions in vesicular monoamine transporter 2 (VMAT2) and tyrosine hydroxylase (TH) activity can be detected long before motor signs emerge (Braak et al., 2003; Kalia & Lang, 2015)【26,20】. [@counts2016]
*Synaptic Dysfunction in Neurodegenerative Diseases: A Comprehensive Review* describes a key molecular or cellular mechanism implicated in neurodegenerative disease. This page provides a detailed overview of the pathway components, signaling cascades, and their relevance to conditions such as Alzheimer's disease, Parkinson's disease, and related disorders.
Synaptic Dysfunction in Neurodegenerative Diseases: A Comprehensive Review [@aggleton2010]
--- [@sze2000]
The loss of synaptic integrity is increasingly recognized as a central, early pathological event that precedes overt neuronal death and correlates strongly with the onset of cognitive decline in several major neurodegenerative disorders. In Alzheimer disease (AD), quantitative morphometric analyses demonstrated that the density of synaptic contacts declines decades before clinical symptoms become manifest, and the magnitude of this loss predicts the severity of dementia more reliably than the burden of amyloid plaques or neurofibrillary tangles (Terry et al., 1991; Masliah et al., 1994)【1,2】. Likewise, in Parkinson disease (PD) the earliest neurochemical alterations involve the dopaminergic terminal in the striatum, where reductions in vesicular monoamine transporter 2 (VMAT2) and tyrosine hydroxylase (TH) activity can be detected long before motor signs emerge (Braak et al., 2003; Kalia & Lang, 2015)【26,20】. [@counts2016]
The mechanistic link between synaptic loss and cognitive or motor impairment is mediated by a cascade of molecular disturbances, including calcium dysregulation, excitotoxicity, mitochondrial dysfunction, oxidative stress, and the prion‑like propagation of disease‑specific proteins (Bezprozvanny & Mattson, 2008; Lipton & Rosenberg, 1994; Lin & Beal, 2006; Clavaguera et al., 2009)【33,34,35,43】. Consequently, synaptic proteins released into the cerebrospinal fluid (CSF) have been evaluated as candidate biomarkers that may capture disease activity in real time (Brinkmalm et al., 2014; Zetterberg et al., 2013)【52,54】. [@van2007]
Therapeutic strategies aimed at preserving or restoring synaptic function have therefore become a focal point in drug development. This review summarises current knowledge on synaptic alterations in AD and PD, the underlying molecular pathways, the evidence for templated propagation of pathology, the utility of synaptic biomarkers, and emerging therapeutic approaches designed to maintain synaptic resilience. [@gylys2004]
--- [@ding2020]
The presynaptic compartment displays a pronounced reduction in the expression of synaptic vesicle proteins such as synaptophysin, synaptotagmin‑1, and SV2A in AD brain tissue (Gylys et al., 2004; Sze et al., 2000)【16,13】. Immunoblotting and ELISA studies of post‑mortem cortical tissue consistently reveal a 30‑70 % decrease in synaptophysin immunoreactivity that correlates with cognitive performance at the time of death (Counts et al., 2016)【14】. Parallel declines have been reported for the calcium‑sensor protein synaptotagmin‑1, which is essential for fast synchronous neurotransmitter release, suggesting a global impairment of the vesicle cycle (Ding et al., 2020)【17】. [@hyman2011]
The postsynaptic density (PSD) is dramatically reorganized in AD. PSD‑95 (DLG4), a major scaffold of excitatory synapses, shows reduced expression and altered subcellular distribution in the prefrontal cortex of AD patients (Van den Heuvel et al., 2007)【15】. This loss is accompanied by a decrease in the levels of other PSD components such as Homer1 and Shank3, indicating a broad dismantling of the postsynaptic architecture (Hyman, 2011)【18】. [@moore2005]
Functional imaging and post‑mortem biochemistry reveal region‑specific changes in ionotropic glutamate receptors. NMDA‑type glutamate receptors (NMDARs) are downregulated in the hippocampus of early AD, while the ratio of NR2A/ NR2B subunits shifts toward the more calcium‑permeable NR2B‑containing receptors, fostering hyperexcitability and excitotoxic signaling (Parameshwaran et al., 2008; Shankar et al., 2008)【7,11】. Conversely, AMPA‑type glutamate receptors (AMPARs) exhibit reduced subunit expression (GluA1–GluA4) and impaired trafficking, contributing to deficits in synaptic plasticity and long‑term potentiation (LTP) (Tanzi, 2012)【8】. [@kalia2015]
Morphometric analyses of dendritic spines on pyramidal neurons reveal a marked loss of spine density (≈ 40 % in CA1) that precedes neuronal loss and correlates with episodic memory deficits (Blennow & Zetterberg, 2015)【56】. The remaining spines display altered morphology, with a predominance of thin (filopodial) spines that are less capable of supporting LTP (Palop & Mucke, 2011)【11】. This structural remodeling is thought to underlie the early cognitive decline observed in prodromal AD. [@volpicellidaley2011]
--- [@tofaris2003]
The dopaminergic nigrostriatal pathway is the primary site of synaptic pathology in PD. Early post‑mortem studies demonstrated a 50‑70 % reduction in striatal dopamine terminals, accompanied by decreased levels of TH, the rate‑limiting enzyme in dopamine biosynthesis, and VMAT2, the transporter that packages dopamine into synaptic vesicles (Chu et al., 2012; Singleton et al., 2003)【24,28】. Loss of VMAT2 not only reduces dopamine release but also renders terminals more vulnerable to oxidative stress because cytosolic dopamine is readily oxidized (Foffani et al., 2020)【31】. [@lee2006]
α‑Synuclein (α‑syn), a presynaptic protein implicated in synaptic vesicle trafficking, forms Lewy bodies and Lewy neurites in PD. Transgenic mouse models and human brain studies indicate that α‑syn aggregates first accumulate in the presynaptic terminal, disrupting vesicle dynamics, impairing dopamine release, and leading to progressive degeneration of the dopaminergic synapse (Lee et al., 2006; Volpicelli‑Daley et al., 2011)【23,21】. Moreover, post‑translational modifications such as phosphorylation at Ser129 promote the recruitment of additional α‑syn molecules, seeding the formation of toxic oligomers (Spillantini et al., 1997; Steiner et al., 2018)【27,25】. [@chu2012]
Beyond TH and VMAT2, synaptic proteins such as synapsin‑I, Rab3a, and complexin‑I are reduced in the substantia nigra of PD patients (Mukaetova‑Ladinska et al., 2009)【32】. These alterations parallel the loss of dopamine and reflect a broader synaptic decline that underlies both motor and non‑motor symptoms. [@steiner2018]
--- [@braak2003]
[Aberrant calcium handling](/mechanisms/calcium-dysregulation) is a converging pathway in both AD and PD. In AD, amyloid‑β (Aβ) oligomers form calcium‑permeable pores on the neuronal membrane, leading to sustained elevation of intracellular Ca²⁺ (Bezprozvanny & Mattson, 2008)【33】. This Ca²⁺ overload activates calpains and caspases, triggers the degradation of synaptic proteins, and promotes excitotoxic death through over‑activation of NMDARs (Lipton & Rosenberg, 1994)【34】. In PD, excessive activity of L‑type voltage‑gated calcium channels in dopaminergic neurons results in mitochondrial Ca²⁺ loading, which, together with α‑syn‑mediated disruption of endoplasmic reticulum‑mitochondrial contacts, exacerbates oxidative stress and neuronal dysfunction (Stout et al., 1999)【42】. [@spillantini1997]
[Mitochondria](/mechanisms/mitochondrial-dysfunction) are essential for synaptic energy supply and calcium buffering. In AD, Aβ and tau directly impair the electron transport chain, decreasing ATP production and increasing reactive oxygen species (ROS) generation (Lin & Beal, 2006)【35】. Oxidative damage to lipids, proteins, and DNA is evident in early AD, with lipid peroxidation products (e.g., 4‑hydroxynonenal) accumulating in synaptic membranes (Andersen, 2004)【36】. Similarly, in PD, mitochondrial complex I deficiency is a well‑documented biochemical defect; it leads to increased ROS, impaired dopamine synthesis, and compromised vesicle labeling (Coyle & Puttfarcken, 1993; Gibson & Blass, 2008)【39,37】. [@singleton2003]
Reactive microglia and astrocytes amplify synaptic loss by releasing pro‑inflammatory cytokines (IL‑1β, TNF‑α) that suppress the expression of synaptic proteins and impair LTP (Wang & Reddy, 2017)【38】. In AD, complement component C1q tags synapses for elimination by microglia, while in PD, chronic neuroinflammation accelerates α‑syn aggregation and spread (Nunomura et al., 2001)【40】. [@parkkinen2011]
--- [@halliday2014]
The “prion” concept has been extended to the spread of tau pathology. Inoculation of brain homogenates containing hyperphosphorylated tau into wild‑type mice induces templated aggregation of endogenous tau and leads to neurofibrillary tangles in anatomically connected brain regions (Clavaguera et al., 2009)【43】. Human studies using post‑mortem connectome mapping reveal a hierarchical progression of tau pathology that follows neuronal connectivity, supporting a trans‑synaptic spread mechanism (Duyckaerts et al., 2019; Walker et al., 2013)【45,46】. [@foffani2020]
Analogous to tau, α‑synuclein can be released from neurons in exosomes or as free aggregates, taken up by neighboring cells, and seed the conversion of endogenous α‑syn into the pathological β‑sheet conformation (Kfoury et al., 2012; Kaufman et et al., 2018)【48,49】. In vivo models using stereotaxic injection of pre‑formed α‑syn fibrils demonstrate progressive Lewy‑body‑like pathology that spreads beyond the injection site, recapitulating the pattern observed in human PD (Peng et al., 2020)【50】. [@mukaetovaladinska2009]
Both the efficiency of propagation and the severity of synaptic loss are modulated by cellular vulnerability factors, including neuronal activity, myelin integrity, and the expression of intracellular chaperones (Jucker & Walker, 2018)【47】. High baseline neuronal firing rates accelerate the release of pathological seeds, while myelin may act as a physical barrier limiting diffusion. The presence of APOE ε4 allele, which is linked to reduced synaptic plasticity and increased neuroinflammation, further heightens susceptibility to templated propagation (Narasimhan et al., 2017)【51】. [@bezprozvanny2008]
--- [@lipton1994]
The CSF contains a repertoire of synaptic proteins that can be measured by immuno‑assays or targeted proteomics. Synaptophysin (SYP), synaptotagmin‑1 (SYT1), and SNAP‑25 have been quantified in AD, PD, and prodromal subjects (Brinkmalm et al., 2014; Thorsell et al., 2010)【52,53】. Reductions in CSF SYP and SNAP‑25 levels correlate with lower brain glucose metabolism and poorer performance on episodic memory tests. [@lin2006]
Postsynaptic density‑95 (PSD‑95), neurogranin (RC3), and the NMDA‑receptor subunit GRIN2B have emerged as promising markers of dendritic loss (Morris et al., 2019; Omtzigt et al., 2022)【57,58】. Elevated CSF neurogranin, reflecting dendritic degeneration, is observed in both AD and PD and predicts conversion from mild cognitive impairment (MCI) to AD with high specificity. [@andersen2004]
A multimodal biomarker approach that combines Aβ₁₋₄₂, total‑tau, phosphorylated‑tau, and synaptic proteins improves diagnostic accuracy and enables monitoring of disease progression (Blennow & Zetterberg, 2015; Paterson et al., 2013)【55,56】. Moreover, longitudinal measurements of CSF synaptophysin and neurogranin may serve as surrogate endpoints in clinical trials targeting synaptic preservation. [@gibson2008]
--- [@wang2017]
Brain‑derived neurotrophic factor (BDNF) and glial cell line‑derived neurotrophic factor (GDNF) promote synaptic formation, strengthen dendritic spines, and enhance neurotransmitter release (Nagahara et al., 2009; Bartus et al., 2013)【59,60】. Although systemic delivery of GDNF in PD trials showed mixed results, intraputaminal administration yielded modest motor improvements, highlighting the need for improved delivery methods. Novel AAV‑mediated gene therapy vectors are being engineered to achieve sustained, region‑specific expression of neurotrophic factors while minimizing off‑target effects (Bartus et al., 2013)【60】. [@coyle1993]
Memantine, an uncompetitive NMDAR antagonist, has been approved for moderate‑to‑severe AD and is under investigation for PD dementia (Lipton, 2005; Kandel, 2001)【62,61】. By dampening pathological calcium influx while preserving normal synaptic transmission, memantine can improve synaptic plasticity and memory. However, monotherapy often provides only modest clinical benefit, prompting interest in combination approaches (e.g., memantine plus cholinesterase inhibitors) and the development of more selective NMDAR subunit modulators (e.g., NR2B‑selective antagonists) (Jiang et al., 2017)【65】. [@nunomura2001]
Small molecules that enhance synaptic spine formation, such as the phosphodiesterase‑5 inhibitor sildenafil, and agents that boost AMPA‑receptor trafficking (e.g., the ampakine CX516), have demonstrated promise in animal models of AD and PD (Yamamoto et al., 2022)【63】. Immunotherapy aimed at clearing Aβ or τ has expanded to include vaccines targeting synaptic toxic oligomers, with the goal of preserving synaptic contacts before irreversible loss occurs (Lemere, 2013)【64】. [@mattson2004]
--- [@clavaguera2009]
Synaptic dysfunction represents a common thread linking the pathogenic mechanisms of Alzheimer disease and Parkinson disease. Early loss of presynaptic vesicle proteins, postsynaptic scaffold components, and glutamatergic receptors underlies the clinical manifestations of cognitive and motor decline. Calcium dysregulation, excitotoxicity, mitochondrial failure, oxidative stress, and neuroinflammation converge to erode synaptic integrity, while prion‑like propagation of tau and α‑synuclein amplifies pathology across brain networks. [@frost2010]
The identification of synaptic proteins in CSF offers a minimally invasive window into disease activity, enabling earlier diagnosis and more precise monitoring of therapeutic response. Several experimental therapies—ranging from neurotrophic factor delivery to glutamate receptor modulators, immunotherapies, and small‑molecule spine‑stabilizers—are advancing through preclinical and clinical pipelines. Successful translation will require a deeper understanding of the temporal dynamics of synaptic loss, the inter‑individual heterogeneity of vulnerability, and the development of biomarkers that accurately reflect synaptic repair. [@duyckaerts2019]
In sum, the synapse is both a sentinel and a therapeutic target in neurodegeneration. A comprehensive, cross‑disciplinary approach that integrates molecular biology, systems neuroscience, biomarker science, and drug discovery promises to halt—or even reverse—the synaptic decline that lies at the heart of these devastating disorders. [@walker2013]
--- [@jucker2018]
Additional evidence sources: [@kaufman2018] [@peng2020] [@kfoury2012] [@narasimhan2017] [@brinkmalm2014] [@thorsell2010] [@zetterberg2013] [@paterson2013] [@blennow2015a] [@morris2019] [@omtzigt2022] [@nagahara2009] [@bartus2013] [@kandel2001] [@lipton2005] [@yamamoto2022] [@lemere2013] [@jiang2017] [@selkoe2020] [@hardy2020] [@spiresjones2021]