Gum infection may play a central role in the development of Alzheimer's disease (AD), new research suggests.
A multinational team of investigators carried out a series of experiments to examine the potential impact of Porphyromonas gingivalis, the major pathogen of chronic periodontitis, on AD development. They studied mouse models as well as examined the cerebrospinal fluid (CSF) and postmortem brain tissue of individuals with AD.
The presence of oral P gingivalis infection in mice resulted in brain infiltration by the bacteria that was accompanied by increased production of Aβ1-42, a component of amyloid plaques implicated in AD.
Gingipains were also found to damage tau, a protein necessary for normal neuronal function and whose disruption is likewise implicated in AD.
Moreover, DNA from P gingivalis was found in the CSF of living AD patients and in postmortem studies of AD patients.
Small molecule gingipain inhibitors administered to mice reduced the bacterial load of P gingivalis in the brain, blocked Aβ1-42 production, reduced neuroinflammation, and rescued hippocampal neurons in mice.
"Our findings indicate that there is a significantly increased load of gingipains, cysteine protease virulence factors secreted by P gingivalis, in the brain of AD patients, when compared to nondemented controls," said lead author Stephen Dominy, MD, chief scientific officer of Cortexyme, the company that funded the study.
"We demonstrate in the paper that these pathological effects can be blocked by gingipain inhibitors in wild-type mice, providing a new strategy for Alzheimer's treatment by specifically targeting P gingivalis and gingipains instead of Aβ or other downstream pathways," he said.
The study was published online January 23 in Science Advances.
Evidence of Causation?
AD patients "exhibit neuroinflammation consistent with infection," and "infectious agents have been found in the brain and postulated to be involved with AD," the authors write.
Until now, however, there has been no "robust evidence of causation."
"Chronic periodontitis (CP) and infection with Porphyromonas gingivalis — a keystone pathogen in the development of CP — have been identified as significant risk factors for developing Aβ plaques, dementia, and AD," they state.
P gingivalis is an "asaccharolytic Gram-negative anaerobic bacteria that produces major virulence factors known as gingipains, which are cysteine proteases consisting of lysine-gingipain (Kgp), arginine-gingipain A (RgpA), and arginine-gingipain B (RgpB)," they explain.
"The link between AD and bacterial infection has been suggested before and, additionally, emerging evidence has suggested that Aβ1-42 is an antimicrobial peptide," Dominy said.
"We investigated this particular pathogen because multiple prior studies, including epidemiology and mechanistic studies, had been published suggesting a link between Porphyromonas gingivalis and AD," he continued.
"Furthermore, a new paper by Ilievski et al independently demonstrated that oral infection of wild-type mice with P gingivalis resulted in P gingivalis infection of the brain and an increase in Aβ1-42 and tau pathology, along with brain inflammation and neurodegeneration," he added.
The researchers approached the issues from a number of angles in human beings and in mouse models.
Using tissue microarrays of brain tissue cores from the middle temporal gyrus (MTG), the researchers compared the brains of individuals with AD and neurologically normal individuals.
Postmortem Brain Data
They began by examining postmortem brain samples of close to 100 individuals with and without AD, testing the tissue samples for gingipains.
Of the AD samples, 96% were positive for RgpB and 91% for Kgp, vs 52% and 39%, respectively, in the brains of nondemented patients without controls.
Similarly, there was a "highly significant" correlation between RgpB load as well as Kgp load and tau load. Further assays found that tau is fragmented by gingipains.
When the researchers tested for ubiquitin, a small protein tag that marks damaged proteins for degradation by proteasomes and accumulates in tau tangles and Aβ plaques, they likewise found significant correlation between RgpB load and ubiquitin load, as well as between Kgp load and ubiquitin load.
RgpB and Kgp staining were observed in 39% and 52% of nondemented samples, respectively, "revealing a continuum of gingipain and AD pathology already present in the controls," the authors comment.
They note that these findings are "consistent with the concept of preclinical AD."
The researchers describe CSF as a "window" into brain infection, "providing insight into the neuropathogenesis of infectious agents."
They therefore analyzed CSF and saliva samples from 10 patients diagnosed with probable AD who had mild to moderate cognitive impairment.
Investigators detected and quantified copies of the hmuY gene by qPCFR in the CSF and saliva in 7 of the 10 clinically diagnosed AD patients, providing "additional evidence for P gingivalis infection in the brain of AD patients."
When the researchers compared brain samples from individuals with other non-AD neurological diseases, such as Parkinson's disease, they found no significant differences in gingipain load compared with controls.
"In summary, both RgpB and Kgp antigens in brain independently demonstrated a significant correlation with AD diagnosis, tau load, and ubiquitin load," the authors write.
They point out that in AD, the hippocampus is one of the first areas to be damaged. Analysis of brain tissue revealed that RgpB co-localized with neurons, astrocytes, and pathology in the hippocampus of patients with AD.
A Western blot (WB) analysis revealed that Kgp was also present in the cerebral cortex, another area of the brain affected in AD.
Analysis of DNA isolated from the same tissue used for the previous analyses identified P gingivalis 16S rRNA and hmuY genes in the cerebral cortex of AD samples and in nondemented Kgp-positive control brains.
Gingipain-induced aggregation did not take place when gingipains were pretreated with iodoacetamide, an irreversible cysteine protease inhibitor, "indicating that the proteolytic activity of the gingipains was responsible for the morphological changes," the authors suggest.
The researchers developed a "library" of potent and selective reversible and irreversible small-molecule gingipain inhibitors, including COR286 and COR271, which are irreversible, and COR119, a covalent Kgp inhibitors, which is reversible.
COR286 and COR271 were both found effective in blocking P gingivalis-induced cell death in a concentration-dependent manner.
Oral Health Underemphasized
Interestingly, two broad-spectrum antibiotics (moxifloxacin and doxycycline), even at concentrations that reduce bacterial survival in vitro, did not provide the same level of cellular protection.
To determine if oral exposure to P gingivalis would result in brain infiltration and induction of the "stereotypical" AD marker Aβ1-42, the researchers conducted a series of studies in mouse models.
Mice were orally infected every other day with P gingivalis over a 6-week period. Some were infected with Kgp knockout, while others with RgpA RgpB double knockout P gingivalis. After 6 weeks of oral infection,
P gingivalis was found to have invaded the brain of all eight mice, but colonization was decreased by gingipain knockout strains or treatment with COR119.
Moreover, mouse brain Aβ1-42 increased significantly after oral infection, compared with mock-infected or COR119-treated mice.
Aβ1-42 was found to have antibacterial effects against P gingivalis, with the proportion of dead and dying P gingivalis bacterium significantly increasing after incubation with Aβ1-42.
Oral administration of the Kgp inhibitor COR271 was found to treat P gingivalis brain infection and prevent loss of hippocampal GAD67 neurons in vivo at 5 weeks, based on histological analysis of the mouse brains.
While moxifloxacin was also beneficial in prevention the increase in brain colonization between day 35 and day 70, it was not effective in reducing the load before the 5-week baseline.
Treatment with COR388, a "highly potent" selective irreversible small-molecule inhibitor of Kgp and an analog CPR271 had dose-dependent effects on brain gingivalis infection, Aβ1-42 levels, and tumor necrosis factor–α levels.
"Certainly, good oral health and reducing the P gingivalis load in the mouth, especially at an early age, should be taken more seriously than it currently is, based on our data," said Dominy.
"However, we would like to emphasize that we don't believe that targeting P gingivalis in the mouth will help with an established P gingivalis brain infection," he noted.
"That is why we have created highly potent, brain penetrant, anti-gingipain small-molecules to target P gingivalis and gingipains in the brain."
Commenting on the study for Medscape Medical News, Sim Singhrao, MPhil, PhD, senior research fellow, School of Dentistry, University of Central Lancashire, Preston, England, who was not involved with the study, said that the researchers "have made a stronger case for pathogen-related dementia."
She suggested that geriatricians and psychiatrists should be "open to the idea that gum disease [and] poor oral hygiene play a role in the onset of dementia and they should change attitudes towards accepting pathogen-related dementia."
Moreover, "scientists need to develop tests that can better inform clinicians about their patient's risk for gum disease and dementia developing," she urged.
"In the meantime, this report is alarming and everyone can do more to modify their oral hygiene habits with the help of dental professionals," she emphasized.
Also commenting on the study for Medscape Medical News, Angela Kamer, DDS, associate professor, New York University College of Dentistry, who was not involved with the study, said it implies that, "although some of these findings replicate previous studies supporting the role of this pathogen in AD, the uniqueness of this study is that a novel drug targeting P gingivalis interrupts these processes in the brain."
She also said it's very important to note that "although these studies are important and [have] advanced our knowledge, the experimental studies are on animal models."
Future research should focus on whether treatments targeting periodontal disease and its associated bacteria might affect AD risk in human beings, she said.
Dominy noted that the gingipain inhibitor COR388 recently completed phase 1 clinical trials in healthy older adults and in AD patients and was shown to be well tolerated.
Based on these results, Cortexyme is "rapidly moving" COR388 into a phase 2/3 efficacy trial in AD patients later in 2019.
"If COR388 proves effective in AD modification, then prevention trials can begin in the near future," he said.
Two of the authors were supported by grants from the National Science Center and NIH/NIDCR. The study was funded by Cortexyme Inc. Dominy is a cofounder and employee of Cortexyme and also owns Cortexyme stock. The other authors' disclosures are listed on the original paper. Singhrao and Kamer have disclosed no relevant financial relationships.
Sci Adv. Published online January 23, 2019. Full text
Medscape Medical News © 2019
Cite this: Gum Disease Strongly Implicated in Alzheimer's - Medscape - Feb 01, 2019.