We have identified the relationship between molecular pathology (estimated with [¹⁸F]AV- 1451 PET) and synaptic density (estimated with [¹¹C]UCB-J PET), in patients with the primary tauopathies of Progressive Supranuclear Palsy and Corticobasal Degeneration (inferred in vivo from amyloid-negative corticobasal syndrome). There are three principal results: (i) regions with higher synaptic density have higher pathology, (ii) within regions, synaptic density becomes less dependent on [¹⁸F]AV-1451 binding as disease severity increases, and (iii) between regions, increased cortical [¹⁸F]AV-1451 binding is associated with reduced subcortical synaptic density. We interpret these three findings in the context of connectivity-based susceptibility to tauopathy, synaptotoxic effects of tauopathy, and cortico- subcortical diaschisis, respectively.

The effect of 4R hyperphosphorylated tau pathology such as PSP and CBD ^22,39, on synaptic function and density is complex. It involves both direct and indirect pathways of injury with changes in cellular physiology preceding the loss of neurons. Through direct pathways, pathological tau interferes with dendritic morphology, synaptic protein expression, the number of NMDA (N-methyl-D-Aspartate) and AMPA (α-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid) receptors on the pre-synaptic membrane, mitochondrial function, synaptic vesicle numbers, and ultimately synaptic loss (for a review of animal studies illustrating various direct tau-induced synaptic abnormalities see ⁴⁰). Tau also directly affects the axon cytoskeleton and trafficking, as well as the soma ⁴¹. Indirectly, hyperphosphorylated tau adversely affects the functioning of the neuronal support network, including glia cells and astrocytes ^42,43. These events are affected by the stage and severity of the disease process, and in relation to regional differences in connectivity which we discuss next (concepts schematically illustrated in Figure 1).

We identified a positive relationship between the binding of [¹¹C]UCB-J and [¹⁸F]AV-1451 such that areas of the brain with higher synaptic density develop higher pathology. This accords with preclinical and clinical models of tauopathy in which the strength of local network connectivity facilitates the transneuronal spread of tau pathology ^10,24,44,45.

However, the relationship between tau accumulation and synaptic density changes with disease progression, at least as inferred from the cross-sectional moderation by disease severity (Figure 2B). With increasing scores on the PSP rating scale, synaptic density becomes less dependent on local tau accumulation. In other words, in areas with relatively low tau accumulation synaptic density is minimally affected, whereas in areas with higher tau accumulation there is reduction of synaptic density as the disease progresses and this preferentially occurs in synapse rich areas. As the disease progresses, other pathological processes may contribute to synaptic loss, such as inflammation, another predictor of prognosis and mediator of synaptic loss ⁴⁶. There is therefore not a simple linear relationship between tau accumulation and synaptic density in moderate and advanced disease. This observation accords with human post-mortem and animal studies. In post mortem studies of the tauopathy Alzheimer’s disease, there is a biphasic synaptic protein response during disease progression, with increases in synaptophysin/syntaxin/SNAP-25 in early Braak stages and synaptic loss observed only when the disease has progressed to the neocortex ¹⁶. In the P301L transgenic mouse model of PSP-like tauopathy, there is a differential loss of synapses, as well as synaptic proteins, depending on disease stage¹⁵.

To understand the biphasic relationship between tau accumulation and synaptic density, one must consider other key players in synaptotoxicity in tauopathies, such as neuroinflammation ⁴⁷. Recent in vivo studies have confirmed the regional co-localisation of inflammation and [¹⁸F]AV-1451 binding in PSP ⁴⁸, in line with previous in vivo ^49,50, and post mortem⁵¹ reports of the tight interplay between neuroinflammation and tau accumulation in tauopathies. There is growing evidence that these two pathological processes affect synaptic function both independently and synergistically ^40,42.

The relationship between tauopathy and synaptic density is even more intriguing when considering the change in synaptic density in one region as a function of pathology in another. There are strong correlations between [¹¹C]UCB-J binding within the basal ganglia and brain stem and [¹⁸F]AV-1451 binding in most cortical areas, with the association between synaptic density in the caudate, putamen, and substantia nigra, and cortical tau remaining significant at p<0.05. The reverse association, between subcortical [¹⁸F]AV-1451 and cortical [¹¹C]UCB-J binding is also observed (Supplementary Figure 3) but is dismissed here as uninterpretable in view of subcortical off-target binding of [¹⁸F]AV-1451. The significant negative correlation between cortical [¹⁸F]AV-1451 binding and synaptic density in the basal ganglia could be a reflection of severe disease in the basal ganglia and accumulating pathology in the neocortex. In other words, synapses are severely affected in the basal ganglia as one of the earliest sites of pathology, with pathology spreading and accumulating in synapse-rich areas of the brain, for example the neocortex. A second explanation is that loss of descending cortico-striatal axons due to cortical pathology, may cause diaschisis, affecting subcortical synaptic density even further. Previous analysis of diffusion tensor imaging in patients with PSP/CBD have revealed extensive white matter abnormalities (within the main association fibres) beyond the degree of cortical atrophy ^52,53 resulting in loss of cortical afferents onto subcortical structures. A third, though not mutually exclusive, explanation is the weakening of cortical-subcortical functional connectivity resulting from dysfunctional synapses rather than synaptic loss ²⁴.

Although at a regional level there is a positive correlation between [¹¹C]UCB-J and [¹⁸F]AV- 1451 BP_ND, we are not directly measuring either synaptic function or the synaptotoxic tau oligomers. This caveat must be borne in mind when interpreting PET data. It is the preclinical models that have shown that oligomers of tau are toxic to synaptic function, even in the absence of tau polymers/fibrils ^11,12. By the time tau aggregates are established, oligomers of tau are expected cortically, and perhaps interfering with cortical function and the integrity of descending axons.

There are other limitations to our study. First, the low affinity of [¹⁸F]AV-1451 for PSP and CBD 4R tau. Even where this radioligand recapitulates the distribution of post-mortem neuropathology in PSP and CBD, and binds PSP 4R tau, the affinity is very much lower than for 3R tau in Alzheimer’s disease ^21,39. Second, there is well-established off-target binding of [¹⁸F]AV-1451, particularly within subcortical structures where monoamine oxidase is present. Off-target binding is most prominent in the basal ganglia which we excluded before running our statistical analyses. We included these regions in the detailed descriptive correlation matrices in Supplementary Figure 3 for completeness sake, noting the strong negative correlations between cortical [¹⁸F]AV-1451 BP_ND and subcortical [¹¹C]UCB-J BP_ND. Third, we note that in PET studies of neurodegeneration with atrophy, grey matter volume loss can affect the interpretation of PET signals. However, synaptic loss in PSP and CBD occurs even in areas of the brain without discernible atrophy on MRI ^7,8. Nonetheless, we used a stringent partial volume correction method (GTM) to minimise the effect of atrophy on our ligand cross-correlations. Of note, our data without partial volume correction yield similar results in all the main analyses (Supplementary Figure 1 and 2). Lastly, the cross-sectional design of this study limits the interpretation of the dynamic relationship between tau accumulation and synaptic loss. Although we include patients at various stages of their illness, a longitudinal design is necessary to test the dynamic relationship we propose, and the mediation of synaptic loss by progressive tauopathy.

In conclusion, we demonstrate a widespread positive association between [¹⁸F]AV-1451 and [¹¹C]UCB-J binding in patients with symptomatic PSP and amyloid-negative corticobasal syndromes. Individual variability in this association correlates with disease severity. The complex relationship between tau accumulation and synaptic density in vivo may explain changes in cognitive and motor physiology. We hope that these insights will inform the design of new clinical trials to arrest PSP and CBD.