New insights into the effects of APP gene dose on synapse in Down syndrome

Synaptic dysfunction: Alzheimer’s disease (AD) is a prevalent form of dementia, affecting over 35 million people worldwide (Tzioras et al., 2023). A synapse serves as the connection point between neurons, facilitating the transmission of information from one neuron to another. Dynamic alterations in synapses, known as synaptic plasticity, play a pivotal role in cognitive processes such as learning and memory. Synaptic loss has been identified as a key contributor to cognitive decline in AD patients. Studies have shown that the soluble forms of amyloid-beta (Aβ) and tau proteins are toxic to synapses, leading to cognitive impairment in animal models (Spires-Jones and Hyman, 2014). Additionally, the formation of oligomers of tau and Aβ can spread pathology through synaptic connections in the brain, emphasizing the vital role of synapses in disease progression. Despite the significance of synapses in AD, effective treatments that prevent or slow synaptic loss are currently lacking. A deeper understanding of pathological changes in synapses could provide crucial biomarkers for early diagnosis and effective treatment. Down syndrome (DS), or trisomy 21, is a genetic disorder that results from an extra copy of chromosome 21 or part of it. It is the most common genetic cause of intellectual and developmental disabilities and is linked to early-onset AD. Individuals with DS typically exhibit AD-like brain pathology (DS-AD) by the age of 40, including amyloid plaques and neurofibrillary tangles, which progress to dementia by age 60 (Chen et al., 2021). However, there is limited research on synaptic loss in DS-AD. Given the similarities between DS-AD and AD pathology, it is reasonable to speculate that individuals with DS-AD may also experience similar synaptic changes observed in AD. Amyloid precursor protein (APP) dose-dependent synaptic protein reduction in DS: Neurotransmitter-containing synaptic vesicles release their contents into the synaptic cleft, initiating synaptic transmission between neurons. Specific receptors on the postsynaptic neuron then bind to these neurotransmitters. The fusion of synaptic vesicles with the plasma membrane requires the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, composed of the vesicular SNARE protein synaptobrevin and the target membrane SNARE proteins syntaxin-1 and synaptosomal-associated protein-25 (SNAP-25) (Sudhof, 2012). Changes in these core proteins involved in synaptic vesicle fusion can lead to synaptic loss and neurological dysfunction. Limited knowledge exists regarding the levels of synaptic proteins in the DS brain and the distinctions between DS and DS-AD. While numerous reports have investigated the levels of synaptic proteins in AD and mild cognitive impairment, they have uncovered inconsistent reductions in various proteins located both pre- and post-synaptically. Given the comparable underlying pathologies of AD and DS-AD, it would be expected that the DS-AD brain would demonstrate comparable protein alterations. Our research revealed decreased levels of several synaptic proteins in the frontal cortex of individuals with AD and DS-AD, including SNARE proteins syntaxin 1A and SNAP25, synaptic vesicle proteins synaptophysin and synapsin 1, as well as postsynaptic density protein PSD95. Notably, reductions in SNARE complex levels were correlated with changes in other synaptic proteins (Chen et al., 2023a), indicating that the loss of these essential proteins necessary for neurotransmitter transmission may signify synaptic dysfunction or loss in DS-AD. Changes in this set of proteins in the brains of DS-AD subjects are consistent with sporadic AD cases. However, no changes in synaptic proteins or SNARE complexes were observed in DS patients without dementia (Chen et al., 2023a), suggesting that molecular and cellular events that lead to synaptic dysfunction and defects may jointly contribute to the potential pathogenesis of DS-AD and AD comorbidities. Consistently, we observed a notable age-dependent decline in the levels of SNARE proteins syntaxin 1A and SNAP25 in the Dp16 DS mouse model. Notably, findings from both the rare case of partial trisomy 21 (PT-DS) (Doran et al., 2017) and Dp16 mice with App gene normalization (Dp16: App++−) indicate that an increased dosage of the APP gene is crucial for synaptic dysfunction or loss (Figure 1).Figure 1: Increased APP gene dose induces the reduction of synaptic proteins and retromer complex subunits in DS.Increased dosage of the APP gene results in elevated levels of both APP and its processing products, including Aβ species. In DS-AD, but not in DS without a dementia diagnosis, a reduction of synaptic proteins dependent on APP occurs, along with decreased levels of the SNARE complex. DS-AD shows reductions in synaptic proteins in both the presynaptic and postsynaptic compartments, which is similar to AD. However, in both DS-AD and DS, there is an APP-dependent reduction in retromer subunits. The increased levels of Aβ42 and Aβ40 arising from increased APP gene dosage in DS decrease retromer subunit levels and function, in turn leading to increased Aβ production. Thus, APP gene dosage-induced changes in both synaptic proteins and the retromer complex can synergize with other affected pathways to drive synaptic dysfunction at different stages in DS. Created with BioRender.com. AD: Alzheimer’s disease; APP: amyloid precursor protein; Aβ: amyloid-beta; DS: Down syndrome; SNARE: soluble N-ethylmaleimide-sensitive factor attachment protein receptor.Synaptic retromer dysfunction in DS: The presynaptic terminal contains not only synaptic vesicles, but also a variety of components involved in the exocytosis and endocytosis processes. One critical component is the retromer protein complex, which is responsible for endosomal protein sorting. This complex recognizes specific transmembrane proteins and facilitates their transport to the trans-Golgi network or recycling back to the plasma membrane. Studies have highlighted the potential involvement of retromer complex in neurodegenerative diseases, including AD in which VPS26 and VPS35 subunits were reduced. Dysfunction of the retromer complex results in increased production of Aβ species. Genetic studies also linked AD to several retromer-associated proteins such as SNX1, SNX3, Rab7a, and SORL1/SORLA (Brodin and Shupliakov, 2018). In individuals with DS, the pathogenic mechanisms may be similar to AD due to the increased expression of APP and Aβ production. Additionally, the upregulation of microRNA miR-155 encoded by chromosome 21 has been found to decrease the expression of SNX27, resulting in synaptic dysfunction (Wang et al., 2013). Moreover, upregulation of hippocampal SNX27 in DS mice can improve synaptic and cognitive deficits. Therefore, understanding the molecular basis of retromer component reduction in DS may lead to new strategies for addressing synaptic loss. We observed significant reductions in the retromer core subunits VPS26A, VPS26B, and VPS29 in DS and DS-AD, but not in PT-DS (Chen et al., 2023b). The downregulation of these subunits was also observed in the brains of 16-month-old Dp16 mice and was dependent on APP gene dosage. Additionally, GluA1 displayed the same changes as the retromer subunits, reflecting alterations in retromer functions (Figure 1). To identify the cellular location of the reduction of retromer subunits, we isolated synaptosomes from the cortex of 20-month-old male 2N and Dp16 mice. We found significant enrichment of each retromer core subunit in synaptosomes. Notably, the reduction of retromer subunits observed throughout the homogenate was also observed in synaptosomes rather than the cytoplasmic fraction. These results suggest that synapses are the primary cellular location of retromer activity. Importantly, the administration of a novel gamma-secretase modulator, designed to specifically target the γ-site cleavages of the γ-secretase complex, resulted in a reduction of Aβ42 and Aβ40 levels, along with an increase in the levels of nontoxic Aβ38 and Aβ37. This treatment successfully restored the expression of VPS26A, VPS26B, VPS29, and GluA1, which were found to be diminished in Dp16 mice (Chen et al., 2023b). Limitations and perspective: Synaptic changes in DS follow a hierarchical pattern, with synaptic function affected in the early stages of childhood and young adulthood, followed by AD-related synaptic and neuronal loss in later stages. The latter is characterized by a reduction in synaptic markers. It is worth highlighting that individuals with DS, particularly children and young adults, experience a general decline in cognitive abilities, which can be attributed to specific developmental changes occurring in the DS brain. These changes encompass deficits in neurogenesis, neuronal maturation, and synaptogenesis. Our research findings strongly indicate a significant correlation between modifications in synaptic proteins and the onset of dementia associated with AD. It is noteworthy that the frontal cortex is comparatively less affected and experiences later involvement in comparison to other brain regions like the entorhinal cortex. It is necessary to evaluate the alterations in synaptic proteins within other vulnerable brain regions. Nevertheless, further investigations are imperative to ascertain whether alterations in synaptic proteins are present in children and young adults with DS, and if so, to elucidate the role of developmental deficits in these changes. Furthermore, the reported functional imbalance between synaptic excitation and inhibition, as well as the morphological enlargement of spines, may contribute to the cognitive dysfunction observed in children and young adults with DS. The precise links between alterations in synaptic proteins and changes in electrophysiological features and even behavioral outcomes need to be defined, although the specific roles of individual synaptic proteins have not yet been fully elucidated. Biomarkers have been utilized to define the disease status of AD, including AD in DS. It would be valuable to investigate whether changes in brain levels of individual synaptic proteins can be reflected in their corresponding levels in body fluids. Further research is needed to evaluate the efficacy of utilizing biomarkers associated with synaptic proteins, thereby enhancing our understanding of their potential as diagnostic tools. Contrarily, there was no observed correlation between reductions in retromer complex proteins and the diagnosis of dementia in individuals with DS. This finding stands in contrast to the results obtained for synaptic proteins above, which exhibited reduced levels in DS-AS but not in DS or PT-DS. This difference suggests that while the increased dosage of the APP gene affected synaptic proteins and retromer subunits, the clinical significance of these changes differed. Increased Rab5 activity is a characteristic feature observed in both AD and DS, and it has been demonstrated to accelerate the internalization of APP into the endolysosomal system where increased processing of APP results in the formation of C-terminal fragments and Aβ species, leading to elevated levels of endosomal Aβ (Grbovic et al., 2003). The retromer complex, which is responsible for recycling and retrieving proteins from endosomes to the plasma membrane, is involved in this process. The accumulation of Aβ within endosomes at the synapse can have detrimental effects on the stability of retromer subunits, consequently leading to the loss of GluA1 and synaptic dysfunction. In line with this, dysfunction of the retromer complex impairs the trafficking of APP from endosomes and promotes increased cleavage of APP by β-secretase, thereby enhancing the production of Aβ, particularly Aβ42 and Aβ40. These elevated levels of Aβ species further contribute to the reduction of retromer levels, creating a positive feedback loop that links retromer dysfunction to increased Aβ production. The different patterns of changes in synaptic proteins and retromer complex subunits in DS-AD and DS suggest that retromer changes may start from the very early stages, while synaptic protein changes occur predominantly in the later stages. In preclinical studies of AD mouse models, gamma-secretase modulators have been extensively tested for their ability to reduce plaques and soluble Aβ, making them a promising therapeutic approach for AD. Gamma-secretase modulator treatment has been shown to prevent the reduction of the retromer complex subunits in Dp16 mice. Further study is needed to determine whether it can reverse the loss of synaptic proteins. Both synaptic proteins and the retromer complex play a critical role in synaptic transmission, as evidenced by the changes observed in SNARE and GluA1 receptor levels in DS and DS mouse models. While the effects of increased APP gene dosage on other synaptic pathways in DS are still not completely clear, further research is necessary to investigate how these downstream changes synergistically contribute to the synaptic microstructural changes, the synaptic transmission deficits, and subsequent cognitive dysfunction across various stages. Nonetheless, these findings reinforce the critical role of increased APP gene dosage in disrupting synaptic function, which aligns with the requirement of APP gene dosage in the development of AD in DS. C-Editors: Zhao M, Liu WJ, Qiu Y; T-Editor: Jia Y