19 Jan 2019

A paper in the January 15 Cell Reports suggests that tauopathies cause defects in nucleocytoplasmic transport. Scientists led by Rick Livesey, University College London, report that mutated tau gives rise to microtubules that prod neuronal nuclei, like fingers poking a balloon. Normally plump and round, the nuclei then morph into wrinkly organelles that leak proteins and RNA. Dissolving microtubules restores nuclear morphology and integrity. The results support the idea that nucleocytoplasmic transport takes a hit in tauopathies, and that these diseases have something in common with ALS, progeria, and other disorders that disrupt the nuclear membrane.

Neurons derived from FTD patients have wrinkled nuclei.

Microtubules protrude into the nuclear membrane, causing it to fold.

Deformed nuclei leak, but breaking up the microtubules restores proper shape and transport.

“This paper makes a strong claim that tau is having a major effect on nuclear architecture,” said Bess Frost, University of Texas Health, San Antonio, who was not involved in the study. Frost’s lab recently reported invaginations of the nuclear membrane that disrupt RNA processing in a fly model of tauopathy (Dec 2017 conference news; Cornelison et al., 2018). She has also documented nuclear invaginations in postmortem human Alzheimer's disease brain tissue (Frost, 2016). She found actin filaments in these invaginations, and given Livesey’s data, thinks the filaments likely contain microtubules. “This gives us a great understanding of the physical pressures on the nucleus caused by tau,” she told Alzforum. Such results are further evidence for a gain-of-function effect for mislocalized tau, she added.

Turned In. In neurons derived from healthy controls, microtubules (colored lines) skirt the nucleus (left). In neurons derived from FTD patients who carry tau mutations (middle, right), the fibers push the nuclear membrane inward, distorting its shape. [Courtesy of Paonessa et al., 2019.]

In neurons from patients with tau-related dementias, including FTD and Alzheimer’s disease, previous studies have reported tau wandering from axons into cell bodies and dendrites (Thies and Mandelkow, 2007; Fu et al., 2016). Livesey wondered if this affected microtubule dynamics in these compartments, since tau stabilizes these filaments. As luck would have it, colleagues of his who study cancer and Hutchinson-Gilford progeria syndrome, a disease of premature aging, had noticed microtubules in the cell body that deformed the nucleus and disrupted nucleocytoplasmic transport (Broers et al., 2006; Kelley et al., 2011). Could the same thing happen in tauopathies?

To find out, first author Francesco Paonessa and colleagues derived neurons from pluripotent stem cells grown from fibroblasts donated by FTD patients. They examined neurons carrying P301L and IVS10+16 tau mutations. The latter increases the amount of tau carrying exon 10, which encodes the second microtubule-binding repeat, leading to a higher ratio of four-repeat to three-repeat tau. The researchers detected mutant tau in cell bodies and dendrites, then Paonessa used real-time microscopy to study nuclei dynamics. In control cells, microtubules sideswiped and glanced off nuclei, which preserved their smooth, oval shape. In the FTD neurons, microtubules poked into the neuronal membrane, causing whole folds to cave in (see image below).

Turned In. In neurons derived from healthy controls, microtubules (colored lines) skirt the nucleus (left). In neurons derived from FTD patients who carry tau mutations (middle, right), the fibers push the nuclear membrane inward, distorting its shape. [Courtesy of Paonessa et al., 2019.]

Three-dimensional super-resolution imaging offered an even closer view. It confirmed that large swaths of the neuronal membrane pushed toward the center of the nucleus, sometimes crossing all the way to the other side. Tau appeared inside these invaginations lining the outer membrane of the nucleus but not penetrating through it. “The nuclei looked like raisins,” Livesey said (see image below). “It was quite dramatic.”

Would these observations hold up in vivo? Paonessa and colleagues examined postmortem brain tissue from eight FTD patients aged 48 to 71 who had carried the IVS10+16 mutation. They compared samples to those from eight age-matched, non-demented controls. Neuronal nuclear invaginations appeared in both sets of samples, but they were larger and more numerous in the FTD tissue. This suggests that people develop folds in neuronal nuclei with age, but that tauopathies worsen them, Livesey said.

Marred surface. The normally oval nucleus in control cells (left) takes on a jagged, disheveled appearance in cells that express mutated tau (middle, right). [Courtesy of Paonessa et al., 2019.]

Interestingly, these folds almost always contained nuclear pore complexes, the channels responsible for passing components into and out of the nucleus. To find out if a folded membrane disrupted nucleocytoplasmic transport, Paonessa and colleagues attached a nuclear localization sequence to red florescent protein and a nuclear exclusion sequence to green florescent protein. While control cells maintained a stark contrast between both, RFP turned up in the cytoplasm of FTD cells and GFP turned up in the nucleus, suggesting their nuclear pores leaked in both directions. However, if the researchers depolymerized microtubules using nocodazole, the nuclei bounced back to a healthy, rounded shape and nucleocytoplasmic transport returned to normal. The results suggest that this condition is acute and reversible at this stage, said Livesey.

These results complement findings from the groups of Bradley Hyman, Massachusetts General Hospital, Boston, and Jeff Rothstein, Johns Hopkins University, Baltimore. They reported that tau interacts directly with nuclear pore complexes and interferes with nucleocytoplasmic transport (Sep 2018 news). Because Hyman and Rothstein studied cells from Alzheimer’s patients, which contain hyperphosphorylated wild-type tau, these observations may extend to all forms of tau, not just mutated ones, Livesey said. It may be that this happens in any dementia where tau ends up in the cell body, he told Alzforum.

“The findings converge on this idea that changes in tau biology lead to changes in nuclear membrane function in dementias,” Livesey said. Dysfunctional nucleocytoplasmic transport has emerged as a feature of ALS and FTD, which lie on opposite ends of a disease spectrum. The work counters the notion that tauopathies, which are protein-aggregation diseases, are separate from amyotrophic lateral sclerosis, which is thought of as a disease of nucleocytoplasmic transport and RNA biology, he told Alzforum. “This indicates that there may be some degree of overlap in pathogenesis when it comes to nuclear membrane function,” Livesey said. That overlap might prove insightful for studying pathogenesis and developing therapeutics, he told Alzforum.

“The approaches and interpretation are intriguing,” Hyman wrote to Alzforum. “The data are quite clear that tau impacts the nuclear membrane. That’s very complementary and largely in concert to the conclusions we drew from our own studies with Jeff Rothstein.”

Livesey noted that he found no evidence of tau aggregation in this study. Neither did Frost in her studies. “It seems to me that this effect of tau on nuclear architecture is independent of tau aggregation, which is novel and exciting,” Frost said.

It is unclear why nuclei become leaky when the membrane folds in on itself. Livesey said it is also unclear at what stage of disease this occurs, whether it is toxic, or if and when it becomes irreversible. He is interested in exploring whether these invaginations lead to downstream changes in chromatin.—Gwyneth Dickey Zakaib