Opposite Enzyme Effect on Wild-type and Mutant Tau Raises Concerns over Popular Alzheimer’s Model

A key to untangling abnormal structures believed to be responsible for the demise of neurons in neurodegenerative diseases may lie in the interaction between two proteins, tau and Pin1.

Pin1 has a diametrically opposite impact on tau levels and tauopathy phenotype depending on whether the tau is wild-type or the P301L mutant.

Alzheimer’s disease (AD) and frontotemporal dementia with parkinsonism-17 (FTDP-17) are two disorders characterized by dementia that share a common abnormality, a brain accumulation of tau proteins in rigid tangles. In healthy neurons, tau binds tubulin with the help of the Pin1 enzyme and promotes microtubule stabilization and assembly. In conditions of tauopathy, hyperphosphorylated tau aggregates in tangles and is incapable of binding microtubules, causing dysfunction and neural loss. Pin1 is important in tauopathy since it can untangle tau by promoting the removal of phosphates. Furthermore, the loss of Pin1 promotes a tanglelike pathology, suggesting Pin levels are important in this neurodegenerative phenotype.

The laboratory of Kun Ping Lu, HMS professor of medicine, sought to determine whether elevating Pin1 levels could prevent neuronal degeneration associated with Alzheimer’s and FTDP-17.

In the May Journal of Clinical Investigation, they report the surprising discovery that Pin1 has diametrically opposite effects on tauopathy in two different experimental mouse models of Alzheimer’s, raising concern over the appropriateness of their use and suggesting the existence of multiple mechanisms for tauopathy generation.

The P301L tau mutation is found in FTDP-17, a relatively rare neurodegenerative disease. It happens to be the preferred Alzheimer’s model, because of the rapid tauopathy generated when it is introduced into mice. However, while tau levels are elevated in AD patients, no tau mutants have been found.

Since loss of Pin1 activity is a major factor contributing to the development of Alzheimer’s, Lu’s lab sought to determine whether Pin1 overexpression could inhibit tau-related neurodegeneration. As expected, Pin1 prevented tau hyperphosphorylation, tau accumulation in tangle complexes, and neural degeneration in wild-type tau transgenic mice.

The investigators were surprised not to get the same results in mice expressing P301L tau. Not only did Pin1 not prevent tauopathy in P301L tau mice, but the condition became worse. When the researchers evaluated Pin1 inhibition in mice expressing wild-type and mutant tau, they again found opposite effects. Pin1 inhibition caused neurodegeneration in transgenic mice expressing wild-type tau, as previously found; however, neurodegeneration was inhibited in the tau mutant mice.

Lu and his colleagues determined that Pin1 controls the stability and accumulation of the tau protein. When Pin1 is present in neuronal cells, tau is degraded, and when Pin1 is removed, tau proteins become stabilized. However, the opposite occurred in neuronal cells expressing the tau P301L mutant; increased tau levels in cells overexpressing Pin exacerbated the tauopathy.

Currently, there is no effective therapy for Alzheimer’s. The use of “overexpression of Pin1 to prevent neurodegeneration is quite exciting,” said Lu. “We don’t know why the tau mutant caused such an aggressive change, but it raises serious questions about the models for AD that are used.” While the P301L tau mutation doesn’t represent Alzheimer’s, the conclusions made from studies with these mice are useful for FTDP-17.

—Kafi Meadows

RNA Splicing Secret Reveals Link Between Nutrients and Protein Translation

Splicing gives mRNAs an edge when it comes to translation. A message stitched together from multiple exons produces more proteins than its seamless counterpart. A study in the April 18 issue of Cell may explain why.

Working in the lab of HMS professor of cell biology John Blenis, postdoctoral researcher Xiaoju Max Ma unearthed surprising connections between several proteins, exposing the mechanistic missing link between splicing and translational control. Ironically, he made the discovery while excavating a different pathway.

Ma was probing SKAR, a protein downstream of the nutrient-sensing hub called mammalian target of rapamycin (mTOR), when he ran into the field of splicing. In a previous study, the Blenis lab showed that SKAR serves as a docking site for the activated form of the kinase S6K1, which is phosphorylated by mTOR. Ma found that SKAR binds to mRNAs as a result of splicing. He further mapped the location of the SKAR-binding site to the exon junction complex (EJC), which is deposited on the mRNA during splicing.
“We didn’t expect EJCs to be involved,” Ma said. “This study provides the first evidence that splicing-mediated translational control is connected with nutrient sensing and cellular metabolism.”

And SKAR links the EJC directly to translation initiation—the rate-limiting step in the process of protein synthesis—since fully activated S6K1 phosphorylates proteins associated with the mRNA’s cap-binding complex, which homes to the ribosome. Ma used short interfering RNAs to confirm the connection. He knocked down SKAR, S6K1, and an EJC core component, one at a time. The spliced mRNAs in the cells failed to produce proteins efficiently.

Given adequate nutrients, SKAR, S6K1, and the EJC combine forces to activate the initiation of a pioneer round of translation. During this process, a ribosome (with help from helicases activated by S6K1) unwinds secondary structures in the mRNA. This efficient unwinding, which is unique to spliced mRNAs, may have a lasting impact on the following rounds of translation, which account for overall protein production.

It is the pioneer round of translation that sets spliced mRNAs apart. Following this special round, a spliced mRNA—stripped of its adornments—resembles any other mRNA and employs common steady-state translation machinery.

“Given the importance of the pioneer round of translation, Max has uncovered an important link between metabolic processes and the rate of protein synthesis,” Blenis said. “This research has implications for aberrant growth regulation, which underlies human diseases ranging from cancer to diabetes.”

In addition to pinpointing SKAR’s residence, Ma expanded the list of potential S6K1 targets associated with an mRNA’s cap-binding complex. The new list includes some surprises.

“The next step is to validate them,” Blenis said. “We know that mTOR and splicing regulate translation initiation through the EJC, but this trio may also control other processes.”

—Alyssa Kneller

Combo Treatment Releases Brake on Antitumor Immune Responses

It has been a long-held dream in immunology to create a cancer vaccine—but the problem with malignant tumors is that they are notoriously good at evading the immune system. One way they are known to do this is by increasing activity of a specialized group of T cells, known as T regulatory cells (Tregs), which suppress immune responses to self-antigens and reduce inflammation.

An HMS team has now shown that when Treg activity is downregulated in mice receiving local tumor treatment, the mice can be cured of a metastatic cancer—apparently via disinhibition of an antitumor immune response. The work is reported in the April 8 Proceedings of the National Academy of Sciences.

The research team, led by Michael Hamblin, HMS associate professor of dermatology at Massachusetts General Hospital, used photodynamic therapy (PDT) to treat mice with a highly metastatic form of cancer. The treatment involves systemic administration of a photosensitive drug that becomes activated when the tumor is exposed to light of an appropriate wavelength. The activation of the drug causes production of reactive oxygen species, which induce apoptosis and necrosis in cancer cells and destroy the tumor blood supply. “People are often treated with PDT for cancer—not normally for metastatic cancer—but in cancer you don’t always know whether it’s metastasized,” said Hamblin.

Over the years there had been scattered reports that PDT not only destroys tumors locally but also evokes antitumor immune responses that target and attack distant metastases. But this effect has been difficult to reproduce. “One of the theories of why people do not always see this is because of Tregs,” explained Hamblin.

The HMS team showed that if PDT is combined with a low dose of the chemotherapy agent cyclophosphamide (CY), it can effectively cure metastatic cancer in mice. Furthermore, when T cells were taken from the spleens of cured mice and mixed with the cancer cells, they specifically killed cells from the target tumor in vitro. “Sort of like a vaccine,” said Hamblin, “[the treatment] kills the tumor in such a way that you also vaccinate the host against it at the same time.”

The team subsequently performed a series of in vitro experiments, which confirmed that the mechanism by which the PDT-CY treatment worked was via inhibition of cancer-induced Treg activation. “No one had really looked into the role of Tregs in PDT-induced immunity. So what this paper shows is that they are important. And if they are present, they can actually diminish the effectiveness of treatment,” explained Pawel Mroz, research fellow in dermatology and co-author on the paper.

“This could easily be clinically applied,” said Hamblin. “People are treated with PDT for cancer everyday, but often the cancer comes back. So people getting PDT for cancer could also get a regimen with low dose cyclophosphamide; it isn’t going to be that much more of a burden.”

—Yvonna Reekie

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