Fluid Biomarkers for Synaptic Dysfunction and Loss

Abstract

Synapses are the site for brain communication where information is transmitted between neurons and stored for memory formation. Synaptic degeneration is a global and early pathogenic event in neurodegenerative disorders with reduced levels of pre- and postsynaptic proteins being recognized as a core feature of Alzheimer's disease (AD) pathophysiology. Together with AD, other neurodegenerative and neurodevelopmental disorders show altered synaptic homeostasis as an important pathogenic event, and due to that, they are commonly referred to as synaptopathies. The exact mechanisms of synapse dysfunction in the different diseases are not well understood and their study would help understanding the pathogenic role of synaptic degeneration, as well as differences and commonalities among them and highlight candidate synaptic biomarkers for specific disorders. The assessment of synaptic proteins in cerebrospinal fluid (CSF), which can reflect synaptic dysfunction in patients with cognitive disorders, is a keen area of interest. Substantial research efforts are now directed toward the investigation of CSF synaptic pathology to improve the diagnosis of neurodegenerative disorders at an early stage as well as to monitor clinical progression. In this review, we will first summarize the pathological events that lead to synapse loss and then discuss the available data on established (eg, neurogranin, SNAP-25, synaptotagmin-1, GAP-43, and α-syn) and emerging (eg, synaptic vesicle glycoprotein 2A and neuronal pentraxins) CSF biomarkers for synapse dysfunction, while highlighting possible utilities, disease specificity, and technical challenges for their detection.

Keywords: Alzheimer’s disease; Synaptic biomarkers; cerebrospinal fluid; proteomics; synaptopathies.

Figure 1.

Synaptic and neuronal biomarkers location. The picture is a schematic representation of the most studied synaptic biomarkers described in this review. As it can be noticed, most of the candidate biomarkers are localized presynaptically, with the exception of neurogranin and neuronal pentraxins (NPTX), which has also been described to be present presynaptically. Many proteins are involved in synaptic vesicle assembly and neurotransmitters release, like synaptotagmin-1 (syt 1), synaptophysin, SNAP-25, and SV2A. α-Synuclein (α-syn) can be found as a soluble form in the cytoplasm, but also associating with membrane lipids as, for instance, with synaptic vesicles and mitochondria. GAP-43 shows high density in the presynaptic terminal, where depending on its phosphorylation status, participates in neuronal growth modulating actin or in synaptic plasticity modulating synaptic vesicle trafficking. Together with actin filaments and microtubules, neurofilaments are cytoskeletal elements of the neurons, providing mechanical strength and stability. Tau protein, mainly expressed in axons, binds to tubulin and induce its polymerization into microtubules, which support axon outgrowth and elongation. α-syn indicates synuclein; DCV, dense-core vesicles; GAP-43, growth-associated protein 43; LTP, long-term potentiation; NPTX, neuronal pentraxin; SNAP-25, synaptosomal-associated protein 25. Figure made with www.biorender.com.

Figure 2.

Proteomic approaches in synaptic biomarkers discovery and validation. Proteomic studies can start with large explorative investigations in brain tissue, which might lead to the discovery of new candidate biomarkers. However, these studies can be seen as starting points, and they have no clinical utilities. Thus, investigations in CSF are needed to be able to translate the biomarker discovery into a tool of clinical use. Once the biomarker has been validated in CSF, further investigations can be carried in blood, a biofluid with higher accessibility and cheaper to use. On the other hand blood is further away from the brain and the targeted protein level might be susceptible to peripheral contribution, resulting in lower biomarker specificity and confounding results. A possible approach to overcome this problem is the use of plasma-derived neuronal exosomes. These investigations can be carried out with a targeted or non-targeted approach. In the diagram, pros and cons of both approaches are highlighted. ELISA indicates enzyme-linked immunosorbent assay; IP, immunoprecipitation; LC-MS, liquid chromatography-mass spectrometry; PRM, parallel reaction monitoring; SIMOA, single molecule array; WB, Western blot. Figure made with www.biorender.com.