Proteinopathies is a catch-all term for neurodegenerative diseases characterised by specific post translational modifications, involving the disease-related aggregation and neuronal accumulation of otherwise unstructured proteins, leading to neurodegeneration and eventual neuron loss. The term proteinopathies looks at the similarities between the molecular pathogenesis of neurodegenerative diseases such as AD, Parkinson’s disease (PD) and Huntington’s disease: the respective proteins underlying these diseases are tau/amyloid-b, a-synuclein, huntingtin and mutated forms of tau (in familial cases). Several other diseases have also been classified as proteinopathies (Golde et al., 2013).
A unifying characteristic of proteinopathies that continues to attract major research attention is the prion hypothesis: the mechanism by which aggregated and misfolded forms of causative proteins can be transmitted between synaptically-connected neurons. Through this strategy, abnormal forms of each
protein are gradually distributed throughout the brain, leading to synaptic dysfunction and neuron death (Frost and Diamond, 2010; Golde et al., 2013). 1.6.1 Neuronal internalisation and transmission of tau pathology: the transneuronal propagation and the selective vulnerability
hypotheses
Although mounting evidence suggests that tau dysfunction causes neurodegeneration, the mechanistic basis for this association is not fully understood. Two main mechanisms have been proposed, referred to as the trans- neuronal propagation and the selective vulnerability spread hypotheses.
Transneuronal propagation
Against the backdrop that tau pathology in AD is predicted to occur in a defined, stereotypical mechanism (Braak and Braak, 1991), many scientists became interested in understanding how this may occur in experimental models. The hypothesis that misfolded tau may propagate in a transcellular manner, similar to the prion protein, was formulated and prompted a series of investigations into the cellular internalisation, secretion and propagation of dysfunctional tau. In line with this, many studies have reported that exogenously added tau protein can be internalised by both neuronal and non-neuronal cells (Clavaguera et al., 2009; de Calignon et al., 2012; Lasagna-Reeves et al., 2012a). Whilst in the cell, the internalised protein interacts with endogenous tau and propagate intracellular toxicity (Lasagna-Reeves et al., 2012; Michel et al., 2014; Usenovic et al., 2015). The mechanism of aggregation can therefore be referred to as template misfolding since the internalised protein, acting as a “seed” or “template,” induces the aggregation or misfolding of endogenous tau (Stancu et al., 2015). Some of the internalised tau (perhaps in complex with some endogenous tau) is re-secreted into the extracellular environment from where propagation continues to neighbouring cells. Remarkably, all the pre-NFT (monomers, oligomers, filaments) tau species have been found to be internalised when supplied exogenously to cells and can be secreted to initiate the transmission process (Guo and Lee, 2011; Jackson et al., 2016; Lasagna-Reeves et al., 2012a; Michel et al., 2014; Usenovic et al., 2015). Moreover, these observations have been consistently made by modelling tauopathies in a range of model systems, including physiologically active whole organisms (Clavaguera et al., 2009;
Papanikolopoulou and Skoulakis, 2011), neuronal and neuron-like tissues (Michel et al., 2014; Usenovic et al., 2015; Wauters et al., 2016), ex vivo brain tissues (Fá et al., 2016) and post-mortem brains (Lasagna-Reeves et al., 2012a). The minimal tau peptide sequence required for this propagation behavioural is thought to be a 31-residue peptide that includes the hexapeptide motif in R3 (Stöhr et al., 2017). Neuronal accumulation and propagation of dysfunctional tau causes toxicity by impairing neurotransmission through neurite retraction and soma loss (Stancu et al., 2015; Usenovic et al., 2015), damaging electrical communication (Fá et al., 2016; Lasagna-Reeves et al., 2012a), and triggering memory and cognitive changes (Fá et al., 2016; Stancu et al., 2015).
Tau propagation is achieved through release and uptake between neighbouring neurons, for which several pathways have been reported (Fig. 1.9). In tau secretion via exocytosis for example, cytosolic tau is encapsulated in endosomal vesicles which undergo endocytic processing and later fuse with the plasma membrane to release tau in exosomes (Wang et al., 2017; Saman, et al., 2012) or ectosomes (Dujardin, et al., 2014) to the extracellular space. The secretion of tau in exosomes has been reported in cellular tauopathy models and in CSF of AD patients (Saman, et al., 2012). In the M1C human neuroblastoma cell line, exosome secreted 0N4R tau is phosphorylated at specific sites (Saman, et al., 2012). Differential release of tau isoforms (0N4R>1N4R/1N3R) has also been reported in the same cell line (Kim et al., 2010). In human AD CSF, exosome- associated tau is aggregated and phosphorylated at threonine 181, which is a diagnostic marker of AD (Saman, et al., 2012). Other mechanisms of tau secretion include tunneling nanotubular release between connected neurons (Tardivel et al., 2016; Abounit et al., 2016), and the extracellular release of tau in the membrane- free form (Chai et al., 2012). Tau in the extracellular space can be taken up by receiving neurons through several routes, with endocytosis being the commonest. Endocytosis has been reported for various forms of tau (monomers, oligomers, and fibrils), using cell lines overexpressing specific forms of tau or physiological levels of tau. (Frost et al., 2009; Guo and Lee, 2011; Michel et al., 2013; Wu et al., 2013). Furthermore, tau in exosomes can be directly transferred between neurons (Wang et al., 2017). Another mechanism is heparan sulphate-mediated macropinocytosis (Holmes et al., 2013).
Selective vulnerability model
A critique of the transneuronal propagation model argues that the physical internalisation, release and subsequent propagation proposed may be unrealistic because: (i) the model is incapable of explaining cell autonomy in degeneration and death; (ii) the Braak staging (Braak and Braak, 1991) used as a premise does not support the proposed transmission hypothesis; and (iii) the model does not address the patient heterogeneity and the brain regional vulnerability observed in proteinopathic protein accumulation. On this basis, an alternative model, referred to as the “selective vulnerability” hypothesis has been proposed. This model suggests that neuronal vulnerability to toxic insults (e.g., from misfolded tau) is selectively dependent on its viability: neurons stressed from the extracellular build-up of tau (or due to any other reason) would be easier targets compared to healthy ones. Healthy cells may become susceptible over time as the disease progresses and/or the levels of the stressor increase (Walsh and Selkoe, 2016). There are various mechanisms of transmission reported for the trans-neuronal propagation (Fig. 1.9), whilst Fig. 1.10 describes the counter selective vulnerability model.
Tau seeds 1
Fibrils
Ectosomes
Donor neuron Receiving
neuron Vesicular bodies Macropinocytosis 2 3 4 Nanotubules Exosomes 5 Extracellular Space Free Tau 6 Through Membrane Endosome 7 Receptor- mediated
Figure 1.9. Mechanisms of misfolded tau transmission in the transneuronal propagation model for extracellular tau.
Misfolded or abnormal tau seeds (could be aggregated or not) accumulate in the neuron (1). Some of these are secreted through various routes, such as encapsulation and subsequent release by ectocytosis (2) or exocytosis (3). Nanotubular transfer between connected neuron is another possibility (4). Uptake of secreted tau can occur by naked-form passage through the plasma membrane of the receiving neuron (6), endocytosis (7), or macropinocytosis (5).
Figure 1.10. The selective vulnerability model of tau toxicity.
(A) The transneuronal spread model proposes physical transfer of misfolded tau between synaptically connected neurons, and thus brain regions, without accounting for cell autonomy. (B) The selective vulnerability model suggests that neurons of compromised viability will be first affected by possible insult from misfolded tau. Vulnerability can be transferred trans-synaptically through diffusible metabolic factors, instead of protein internalisation and release (Walsh and Selkoe, 2016).