Potent proinflammatory markers appear to be significantly elevated in the brains of black patients with Alzheimer’s disease, compared to the brains of white patients, while cytokines with a potentially neuroprotective role are decreased, a Food and Drug Administration researcher reported Jan. 11 at an agency grand rounds presentation
These differences may be driven by the NLRP3 gene, which, in the presence of neuronal insults like beta amyloid aggregates, can direct microglia to pump out a stew of inflammatory cytokines and chemokines, Dr. Ferguson said.
“This proposed pathway may help explain why black patients are twice as likely as white ones to develop AD, said Dr. Ferguson, acting director of FDA Division of Neurotoxicology. “Once NLRP3 is activated, it leads to chronically increased levels of inflammatory cytokines. Once they are chronically increased, it could lead to increased synaptic dysfunction, cognitive impairment, and cell death.”
Dr. Ferguson and her colleague, Vijayalakshmi Varma, PhD, FDA research biologist, obtained brain tissue samples from 12 black patients with AD and 12 white ones. She did not have baseline severity staging for the cohort, but said that the patients were a mean of 81 years old, and had confirmed AD pathology at the time of death.
The researchers examined neurodegenerative proteins and cytokine levels in the BA21 area of the brain. Located in the temporal lobe, BA21 is important in language and auditory processing. It generally exhibits atrophy and the characteristic AD lesions of beta amyloid plaques and tau tangles early in the disease.
In a previously published report of this neuropathological cohort, the team examined a number of markers of neurodegeneration including S100B, the soluble form of the receptor for advanced glycation end-products (sRAGE); glial cell-derived neurotrophic factor (GDNF); and amyloid beta (AB) 40 and AB42 (J Alzheimers Dis. 2017:59; 57-66).
RAGE helps mediate the transport of AB through the cell membrane and the blood brain barrier. S100B is a protein that has been implicated in the formation of tau tangles; other studies suggest that it interacts with RAGE to promote tau phosphorylation.
Compared to levels in the brains of white patients, brains of black patients contained 17% more S100B and 121% more AB42. The AB42/40 ratio was increased by almost 500% over that seen in the white sample.
The new, unpublished data used this same neuropathological cohort, but examined 40 known neuroinflammatory markers. Compared to the white sample, the black sample showed:
IL-1B, thought to increase amyloid precursor protein and promote tau phosphorylation, increased by 109% MIG, an attractant for activated T-cells, increased by 37%
TRAIL, a ligand that induces apoptosis. increased by 50%
“S 100 B is an astrocytic calcium binding protein,” Dr. Ferguson said. “We know that there are increased brain and cerebrospinal levels of S100 be in mild-moderate Alzheimer’s. S100 B can trigger neuroinflammatory signaling pathways. S100 B is also a potential biomarker of blood-brain barrier permeability. We see increased serum levels in such things as unmedicated schizophrenia and depression as well as brain injury.”
However, Dr. Ferguson said, “There were a couple of cytokines significantly that were decreased in African-American sample, including IL-8. Decreased IL-8 brain levels have been described in those with Alzheimer’s and recent studies have indicated that IL-8 may have a protective role in Alzheimer’s pathogenesis.”
IL-3, which activates mature neutrophils and macrophages, was also significantly decreased.
The researchers also saw some gender differences. Women had about 20% less CCL25 and CCL26 than men, and 32% less CxCL1 (fracktalkine). CCL26 (Eotaxin-3) was decreased significantly (19%) in women. Fractalkine is essential for microglial cell migration. CCL25 and CCL26 are also mobilizers of immune cells.
Dr. Ferguson postulated that at least some of this neuroinflammatory profile could be related to the formation of inflammasomes – multi-protein oligomeric structures formed in microglia in the brain by the influence of the NLRP3 gene.
“There is a lot of evidence that inflammasomes are involved in AD pathogenesis,” she said. Messenger RNA for the NLRP3 inflammasome is up-regulated in blood from AD patients. Alzheimer’s transgenic mice without the gene show decreased hippocampal and cortical AB40 and 42, increased microglial phagocytosis, and better memory. There is also some evidence that the NLRP3 inflammasome releases minute protein particles called apoptosis-associated (ASC) specks. Some researchers think these particles help seed amyloid throughout the brain. Based on these findings, Dr. Ferguson postulated a potential pathway for the increased prevalence and severity of AD among blacks.
Proinflammatory cytokines are released in response to rising AB levels. These promote the formation of the NLRP3 inflammasome within activated microglia. The inflammasome releases more chemokines and cytokines, leading to a chronic proinflammatory state that may actually promote amyloid seeding. This leads to synaptic dysfunction, cognitive dysfunction, and neuronal death
Dr. Ferguson said work on the samples will continue.
“We also have CSF and hippocampal tissue and intend to look at similar endpoints in those,” she said. “If as a result of this, we can find gene variants associated with ethnicity and Alzheimer’s, we may be able to establish genetic profiles to identify those at high risk. If we can do that, we may be able to intervene early before the person starts showing cognitive deficits and slow the progression of the disease – even develop precision medications for disease intervention.”
This article was updated 1/11/18.