Synaptosome as a tool in Alzheimer’s disease research

Highlights

• Synaptosomes are biochemically isolated pinched off and resealed synaptic terminals.

• Synaptosomes constitute “halfway house” between neurochemistry and electrophysiology.

• Synaptosomes serve as ex vivo model for studying synaptic dysfunction in AD.

• Synaptosomes are key for location-specific analysis of pathogenic species in AD.

• Data points to dysfunctions in membrane composition, oxidative damage and cell signalling.

Abstract

Synapse dysfunction is an integral feature of Alzheimer’s disease (AD) pathophysiology. In fact, prodromal manifestation of structural and functional deficits in synapses much prior to appearance of overt pathological hallmarks of the disease indicates that AD might be considered as a degenerative disorder of the synapses. Several research instruments and techniques have allowed us to study synaptic function and plasticity and their alterations in pathological conditions, such as AD. One such tool is the biochemically isolated preparations of detached and resealed synaptic terminals, the “synaptosomes”. Because of the preservation of many of the physiological processes such as metabolic and enzymatic activities, synaptosomes have proved to be an indispensable ex vivo model system to study synapse physiology both when isolated from fresh or cryopreserved tissues, and from animal or human post-mortem tissues. This model system has been tremendously successful in the case of post-mortem tissues because of their accessibility relative to acute brain slices or cultures. The current review details the use of synaptosomes in AD research and its potential as a valuable tool in furthering our understanding of the pathogenesis and in devising and testing of therapeutic strategies for the disease.

Introduction

The major pathway of inter-neuronal communication is formed by what are known as chemical synapses, which are characterized by employment of a broad range of “neurotransmitters” and “neuropeptides” for propagation of signals in lieu of absence of any direct physical contact points between neurons. Structurally, a typical chemical synapse consists of an axon terminus or the presynapse, synaptic cleft and the postsynaptic density or the PSD (Harris and Littleton, 2015). The pre- and postsynaptic entities are uniquely distinguishable by visible densities along their corresponding cell membranes. Hence the presynapse is highly enriched in synaptic vesicles which store neurotransmitters, and mitochondria which are a bioenergetic source. Arrival of an action potential at the presynapse triggers release of neurotransmitters from the vesicles into the synaptic cleft in an intricate exocytosis-dependent and calcium-regulated process. The PSD has a dense collection of clustered receptors on its cytoplasmic surface that bind to the neurotransmitters present in the synaptic cleft and initiate the postsynaptic responses. Of note, communication through chemical synapses is plastic with dynamic short and long-term alterations in their signalling capacities, strengths and nature of downstream effector responses. It is not surprising then that chemical transmission in the nervous system is under a potent regulation mediated by complex and interlinked protein-driven molecular mechanisms, making them arguably one of the most complex molecular functional system known to biologists.

Alzheimer’s disease (AD) is among the primary causes of dementia in the aged population worldwide. Synapses are known to be a primary and early target in AD pathogenesis (DeKosky and Scheff, 1990, LaFerla and Oddo, 2005, Selkoe, 2002). In fact, prodromal loss of spines and structural alternations in synapse structure and functions are observed long before the onset of overt pathological hallmarks (such as amyloid plaques and neurofibrillary tangles) of the disease (Hsia et al., 1999, Terry et al., 1991). Importantly, loss of synapse structure and density (rather than plaque deposition, tangle accumulation or neuronal loss) is a strong correlate of cognitive, behavioural and memory deficits observed along the course of the disease (DeKosky and Scheff, 1990, Scheff et al., 1991, Terry et al., 1991). Not surprisingly then, AD is often attributed primarily as a degenerative disorder of the synapses (Selkoe, 2002). Several observations support this hypothesis; first, proteins and their fragments central to AD pathology; amyloid precursor protein (APP), amyloid beta peptides (Aβ) and pathogenic tau species are all localized in synaptic compartments (Fein et al., 2008, Henkins et al., 2012, Tai et al., 2014) and are secreted upon induction of synaptic activity (Cirrito et al., 2005, Cirrito et al., 2008, Sokolow et al., 2015). Second, connectivity, but not proximity, regulates propagation of the toxic species, indicating a trans-synaptic mechanism (Ahmed et al., 2014, Dujardin et al., 2014, Liu et al., 2012). Third, current therapeutic strategies against AD mainly focus upon preserving synaptic functions (Tan, 2014). It should be noted here that contemporary anti-AD drugs and therapies offer only symptomatic relief and currently no ameliorative therapy has proved to elucidate any disease-retarding or modifying properties. This is mainly due to the complex and varying influences of a plethora of wide-ranged genetic and environmental factors on the pathogenesis of AD (Crous-Bou et al., 2017, Ferreira et al., 2020). In this regard, a comprehensive understanding of the disease pathology, identification of specific biomarkers and therapeutic targets, and by extension, successful assembly of a suitable disease modifying strategy necessitates a complete picture of the fundamental role of synaptic deficits in the progression of AD and the molecular mechanisms and players involved.

Several novel instruments and techniques have been developed (and continue to be developed) specifically to study both the electrical and biochemical aspects of synaptic physiology, as well as their alterations in neuropathological states, particularly neurodegenerative conditions like AD. None of them have been more indispensable than the ex vivo neurobiochemical preparations of “synaptosomes”, subcellular brain fractions that are enriched in synaptic terminals. The present review attempts to summarize the knowledge leap achieved by synaptosomal research in AD. First, a brief outline of the biochemical technique for isolation of synaptosomes and their research utilities is provided; followed by a thorough analysis of the major findings of the studies that utilized synaptosomes as an ex vivo model in an effort to contribute to and further our understanding of the complex mechanisms of synaptic dysfunction that is central to AD pathology.