15 Jan 2020

How do neurons die in Alzheimer’s disease? Perhaps by necroptosis, a form of programmed cell death triggered by inflammation. In the December Acta Neuropathologica, researchers led by Dietmar Thal, Bart De Strooper, and Sriram Balusu at KU Leuven, Belgium, reported finding necrosomes, complexes of activated necroptotic proteins, inside granulovacuolar bodies in the Alzheimer’s brain. These bodies consist of large cytoplasmic vacuoles that contain dense granular material. Such vacuoles are common in AD. They develop early on in hippocampal pyramidal neurons and eventually appear in connected brain areas. Necrosomes inside these vacuoles correlated with tau pathology and neuronal loss, but not with amyloid plaques, the authors report, implicating granulovacuolar degeneration (GVD) and necroptosis in neuronal death in AD.

Activated necrosomes hide out in granulovacuolar bodies in AD brain.

The complexes associate with tau toxicity and neuron loss.

Causal relationships? TBD.

“The demonstration of necrosomes in GVD in AD brains is interesting and new, and opens up lines of research and potentially new therapeutic targets,” Steve Finkbeiner at the University of California, San Francisco, wrote to Alzforum (full comment below). Shijun Zhang at Virginia Commonwealth University in Richmond said the paper makes a valuable contribution. “The analyses are solid and convincing … the study adds further evidence to support the possible involvement of necroptosis as one of the mechanisms underlying neuronal degeneration in AD,” he wrote (full comment below).

Junying Yuan and colleagues at Harvard Medical School first described necroptosis 15 years ago (Jun 2005 news). Like apoptosis, the process is activated by tumor necrosis factor receptors, but it has different downstream mediators. First the TNF receptor phosphorylates receptor-interacting serine/threonine protein kinase 1 (RIPK1), switching it on. RIPK1 then binds and activates RIPK3, forming the necrosome. When this complex recruits mixed-lineage kinase-like pseudokinase (MLKL), which oligomerizes and punches holes in the cell membrane, the cell dies.

Tau and Necroptosis. Necroptotic proteins (green) occur together with phosphorylated tau (red) in some neurons (arrow; overlay looks yellow), but not in others (arrowhead). Nuclei are blue. [Courtesy of Koper et al., Acta Neuropathologica.]

Necroptosis previously has been linked to AD. Salvatore Oddo and colleagues at Arizona State University, Tempe, reported elevated levels of necroptotic proteins in AD brain, which correlated with tau tangles, brain atrophy, and cognitive decline (Jul 2017 news).

De Strooper and colleagues became interested in necroptosis when they found it in human neurons they had transplanted into the brains of APPPS1 mice (Feb 2017 news). To learn if necroptosis figures in AD pathology, they turned to postmortem brain.

First author Marta Koper immunostained sections of brain tissue from 23 people who had had AD, from 24 who had pathologically confirmed preclinical AD, and from 16 healthy controls. Antibodies detected phosphorylated, activated versions of RIPK1, RIPK3, and MLKL in all of the AD brains, in 22 of the preclinical AD brains, and in two control samples. Activated necrosomes were only inside granulovacuolar bodies in neurons.

These vacuoles are bound by a double membrane and are believed to represent malfunctioning late-stage autophagosomes (Funk et al., 2011). They were first described in AD brain in 1911, and granulovacuolar degeneration is considered a characteristic of the disease (Kahn et al., 1985; Okamoto et al., 1991; Kurdi et al., 2016).

Koper and colleagues found necroptotic GVD bodies (GVDn+) scattered throughout all regions of AD brain they examined: hippocampus, entorhinal cortex, hypothalamus, amygdala, temporal cortex, and frontal cortex. They were most numerous in the hippocampus; sparse in frontal cortex.

Because granulovacuolar degeneration begins in the hippocampus, the authors homed in on this region. They found no spatial relationship between GVDn+ and plaques, but a close relationship with tangles. About 40 percent of GVDn+ neurons contained phosphorylated tau (see image above) and in a regression analysis, the extent of GVD correlated better with the Braak neurofibrillary tangle stage than with the amount of plaque or clinical status.

Regions with GVDn+ tended to have fewer neurons, indicating degeneration. In the hippocampus, the proportion of GVDn+ neurons was none in control brain, 10 percent in preclinical brain, and 42 percent in AD. Neuronal density tracked in tandem, reaching 135 cells per square millimeter in control hippocampus, 109 in preclinical AD, and 81 in AD hippocampus. Granulovacuolar degeneration spreads to the frontal cortex late in disease. In this region, the authors counted no difference between preclinical and control, but in AD, 2 percent of frontal cortex neurons harbored necrosome-positive granulovacuoles and there were 99 neurons per square millimeter, compared with 113 in control cortex.

Do GVDn+ and neurofibrillary tangles underlie neuronal death? How do these two pathologies relate to each other? Tangles could induce granulovacuolar bodies. GVD occurs in mouse models of tauopathy, and seeding of tau tangles triggers GVD in cultured neurons and mouse models (Lewis et al., 2001; Köhler et al., 2014; Wiersma et al., 2019). However, the relationship could be indirect, as well. “There could be two parallel pathways that cause tau phosphorylation and RIPK1 phosphorylation,” De Strooper wrote to Alzforum.

Another puzzle is why necrosomes appear only inside GVD vacuoles. Perhaps these vacuoles sequester the necrosomes, preventing them from reaching the cell membrane and in that way prolong the life of the neuron. “We need to develop cellular and animal models to investigate causal relationships and molecular mechanisms,” noted De Strooper, who plans to investigate the effect of GVDn+ in chimeric mice. Finkbeiner suggested following single neurons that contain necrosome-positive vacuoles to see whether the pathology shortens or lengthens their lifespans.

Beyond GVD bodies, cells may have other ways of fending off necroptosis. A recent paper characterized two families with autosomal-dominant mutations in RIPK1. Even though the mutations rendered the protein constitutively active, predictably upping inflammation and cell death in blood, fibroblasts from carriers had low RIPK1 expression and resisted necroptosis (Tao et al., 2019, and related comments). “The compensation … may point to potential mechanisms for modulating cell-death speed, possibly in neurodegenerative disorders as well,” Thal suggested.

Does necroptosis play a role in glial cells? It’s unclear. The authors saw activated necroptotic proteins only in neurons, but Oddo, and later Veronique Miron’s group at the University of Edinburgh, did report active necrosome components in microglia (Jun 2019 news). Microglia might interact with necroptotic neurons, Miron noted. In the present study, microglia appear with necroptosing neurons in preclinical AD tissue. This would position the microglia to respond to damage wrought by necroptosis, which could, in turn, modify their behavior, she wrote (full comment below).

If necroptosis pushes vulnerable neurons toward destruction in AD, inhibiting this process could stave off disease progression. RIPK1 inhibitors are being developed. Partners Denali/Sanofi are testing DNL747 in AD and ALS, and DNL758 in auto-immune disease, and Rigel Pharmaceuticals started Phase 1 for its RIPK1 inhibitor R552 (see PRNewswire). GlaxoSmithKline ran eight Phase 1 and 2 trials of GSK2982772 in various inflammatory disorders, but removed this RIPK1 inhibitor from its clinical pipeline.—Madolyn Bowman Rogers