Neurofibrillary tangles and amyloid plaques constitute the hallmark brain lesions of Alzheimer’s disease (AD) patients. Tangles are composed of fibrillar aggregates of the microtubule-associated protein tau, and plaques comprise fibrillar forms of a proteolytic cleavage product, amyloid-β (Aβ). Although plaques and tangles are the end-stage lesions in AD, small oligomers of Aβ and tau are now receiving increased attention as they are shown to correlate best with neurotoxicity. One key question of debate, however, is which of these pathologies appears first and hence is upstream in the pathocascade. Studies suggest that there is an intense crosstalk between the two molecules and, based on work in animal models, there is increasing evidence that Aβ, at least in part, exerts its toxicity via tau, with the Src kinase Fyn playing a crucial role in this process. In other experimental paradigms, Aβ and tau have been found to exert both separate and synergistic modes of toxicity. The challenge, however, is to integrate these different scenarios into a coherent picture. Furthermore, the ability of therapeutic interventions targeting just one of these molecules, to successfully neutralize the toxicity of the other, needs to be ascertained to improve current therapeutic strategies, such as immunotherapy, for the treatment of AD. Although this article is not intended to provide a comprehensive review of the currently pursued therapeutic strategies, we will discuss what has been achieved by immunotherapy and, in particular, how the inherent limitations of this approach can possibly be overcome by novel strategies that involve single-chain antibodies.

The microtubule-associated protein tau has a critical role in Alzheimer disease and related tauopathies. There is accumulating evidence that tau aggregates spread and replicate in a prion-like manner, with the uptake of pathological tau seeds causing misfolding and aggregation of monomeric tau in recipient cells. Here we focused on small extracellular vesicles enriched for exosomes that were isolated from the brains of tau transgenic rTg4510 and control mice. We found that these extracellular vesicles contained tau, although the levels were significantly higher in transgenic mice that have a pronounced tau pathology. Tau in the vesicles was differentially phosphorylated, although to a lower degree than in the brain cells from which they were derived. Several phospho-epitopes (AT8, AT100, and AT180) thought to be critical for tau pathology were undetected in extracellular vesicles. Despite this, when assayed with FRET tau biosensor cells, extracellular vesicles derived from transgenic mice were capable of seeding tau aggregation in a threshold-dependent manner. We also observed that the dye used to label extracellular vesicle membranes was still present during nucleation and formation of tau inclusions, suggesting either a role for membranes in the seeding or in the process of degradation. Together, we clearly demonstrate that extracellular vesicles can transmit tau pathology. This indicates a role for extracellular vesicles in the transmission and spreading of tau pathology. The characteristics of tau in extracellular vesicles and the seeding threshold we identified may explain why tau pathology develops very slowly in neurodegenerative diseases such as Alzheimer disease.

In Alzheimer disease and related disorders, the microtubule-associated protein tau aggregates and forms cytoplasmic lesions that impair neuronal physiology at many levels. In addition to affecting the host neuron, tau aggregates also spread to neighboring, recipient cells where the misfolded tau aggregates, in a manner similar to prions, actively corrupt the proper folding of soluble tau, and thereby impair cellular functions. One vehicle for the transmission of tau aggregates are secretory nanovesicles known as exosomes. Here, we established a simple model of a neuronal circuit using a microfluidics culture system in which hippocampal neurons A and B were seeded into chambers 1 and 2, respectively, extending axons via microgrooves in both directions and thereby interconnecting. This system served to establish two models to track exosome spreading. In the first model, we labeled the exosomal membrane by coupling tetraspanin CD9 with either a green or red fluorescent tag. This allowed us to reveal that interconnected neurons exchange exosomes only when their axons extend in close proximity. In the second model, we added exosomes isolated from the brains of tau transgenic rTg4510 mice (i.e. exogenous, neuron A-derived) to neurons in chamber 1 (neuron B) interconnected with neuron C in chamber 2. This allowed us to demonstrate that a substantial fraction of the exogenous exosomes were internalized by neuron B and passed then on to neuron C. This transportation from neuron B to C was achieved by a mechanism that is consistent with the hijacking of secretory endosomes by the exogenous exosomes, as revealed by confocal, super-resolution and electron microscopy. Together, these findings suggest that fusion events involving the endogenous endosomal secretory machinery increase the pathogenic potential and the radius of action of pathogenic cargoes carried by exogenous exosomes.

Male development in mammals is normally initiated by the Y-linked gene Sry, which activates expression of Sox9, leading to a cascade of gene activity required for testis formation. Although defects in this genetic cascade lead to human disorders of sex development (DSD), only a dozen DSD genes have been identified, and causes of 46,XX DSD (XX maleness) other than SRY translocation are almost completely unknown. Here, we show that transgenic expression of Sox10, a close relative of Sox9, in gonads of XX mice resulted in development of testes and male physiology. The degree of sex reversal correlated with levels of Sox10 expression in different transgenic lines. Sox10 was expressed at low levels in primordial gonads of both sexes during normal mouse development, becoming male-specific during testis differentiation. SOX10 protein was able to activate transcriptional targets of SOX9, explaining at a mechanistic level its ability to direct male development. Because over-expression of SOX10 alone is able to mimic the XX DSD phenotypes associated with duplication of human chromosome 22q13, and given that human SOX10 maps to 22q13.1, our results functionally implicate SOX10 in the etiology of these DSDs.

The microtubule-associated protein tau has a critical role in Alzheimer’s disease and other tauopathies. A proposed pathomechanism in the progression of tauopathies is the trans-synaptic spreading of tau seeds, with a role for exosomes which are secretory nanovesicles generated by late endosomes. Our previous work demonstrated that brain-derived exosomes isolated from tau transgenic rTg4510 mice encapsulate tau seeds with the ability to induce tau aggregation in recipient cells. We had also shown that exosomes can hijack the endosomal pathway to spread through interconnected neurons. Here, we reveal how tau seeds contained within internalized exosomes exploit mechanisms of lysosomal degradation to escape the endosome and induce tau aggregation in the cytosol of HEK293T-derived ‘tau biosensor cells’. We found that the majority of the exosome-containing endosomes fused with lysosomes to form endolysosomes. Exosomes induced their permeabilization, irrespective of the presence of tau seeds, or whether the exosomal preparations originated from mouse brains or HEK293T cells. We also found that permeabilization is a conserved mechanism, operating in both non-neuronal tau biosensor cells and primary neurons. However, permeabilization of endolysosomes only occurred in a small fraction of cells, which supports the notion that permeabilization occurs by a thresholded mechanism. Interestingly, tau aggregation was only induced in cells that exhibited permeabilization, presenting this as an escape route of exosomal tau seeds into the cytosol. Overexpression of RAB7, which is required for the formation of endolysosomes, strongly increased tau aggregation. Conversely, inhibition of lysosomal function with alkalinizing agents, or by knocking-down RAB7, decreased tau aggregation. Together, we conclude that the enzymatic activities of lysosomes permeabilize exosomal and endosomal membranes, thereby facilitating access of exosomal tau seeds to cytosolic tau to induce its aggregation. Our data underscore the importance of endosomal membrane integrity in mechanisms of cellular invasion by misfolded proteins that are resistant to lysosomal degradation.