Front. Neurol., 24 June 2022Sec. Autonomic Neuroscience
https://doi.org/10.3389/fneur.2022.900048 View all 5 Articles
Downregulation of Neurofilament Light Chain Expression in Human Neuronal-Glial Cell Co-Cultures by a Microbiome-Derived Lipopolysaccharide-Induced miRNA-30b-5p
Aileen I. Pogue1, Vivian R. Jaber2, Nathan M. Sharfman2, Yuhai Zhao2,3 and Walter J. Lukiw1,2,4,5*
- 1Alchem Biotech Research, Toronto, ON, Canada
- 2LSU Neuroscience Center, Louisiana State University Health Science Center, New Orleans, LA, United States
- 3Department of Cell Biology and Anatomy, Louisiana State University Health Science Center, New Orleans, LA, United States
- 4Department of Ophthalmology, Louisiana State University Health Science Center, New Orleans, LA, United States
- 5Department Neurology, Louisiana State University Health Science Center, New Orleans, LA, United States
Microbiome-derived Gram-negative bacterial lipopolysaccharide (LPS) has been shown by multiple laboratories to reside within Alzheimer's disease (AD)-affected neocortical and hippocampal neurons. LPS and other pro-inflammatory stressors strongly induce a defined set of NF-kB (p50/p65)-sensitive human microRNAs, including a brain-enriched Homo sapien microRNA-30b-5p (hsa-miRNA-30b-5p; miRNA-30b). Here we provide evidence that this neuropathology-associated miRNA, known to be upregulated in AD brain and LPS-stressed human neuronal-glial (HNG) cells in primary culture targets the neurofilament light (NF-L) chain mRNA 3'-untranslated region (3'-UTR), which is conducive to the post-transcriptional downregulation of NF-L expression observed within both AD and LPS-treated HNG cells. A deficiency of NF-L is associated with consequent atrophy of the neuronal cytoskeleton and the disruption of synaptic organization. Interestingly, miRNA-30b has previously been shown to be highly expressed in amyloid-beta (Aβ) peptide-treated animal and cell models, and Aβ peptides promote LPS entry into neurons. Increased miRNA-30b expression induces neuronal injury, neuron loss, neuronal inflammation, impairment of synaptic transmission, and synaptic failure in neurodegenerative disease and transgenic murine models. This gut microbiota-derived LPS-NF-kB-miRNA-30b-NF-L pathological signaling network: (i)underscores a positive pathological link between the LPS of gastrointestinal (GI)-tract microbes and the inflammatory neuropathology, disordered cytoskeleton, and disrupted synaptic signaling of the AD brain and stressed brain cells; and (ii) is the first example of a microbiome-derived neurotoxic glycolipid having significant detrimental miRNA-30b-mediated actions on the expression of NF-L, an abundant neuron-specific filament protein known to be important in the maintenance of neuronal cell shape, axonal caliber, and synaptic homeostasis.
The gastrointestinal (GI) tract of Homo sapienscontains a complex, dynamic, and highly interactive community of microorganisms collectively known as the GI-tract microbiome possessing a staggering complexity and diversity. Composed of about ~1015microorganisms from many thousands of different microbial species, the vast majority of human GI-tract microbes are composed of anaerobic or facultative anaerobic bacteria with aerobic bacteria, fungi, protozoa, Archaebacteria (an ancient intermediate microbial group between the prokaryotes and eukaryotes), viruses, and other microorganisms making up the remainder (1–3). Increasing research evidence has demonstrated that the composition of the GI-tract microbiome can significantly affect normal physiological homeostasis and contribute to the pathogenesis of diseases ranging from various types of inflammatory bowel disease to cancer to neurodegenerative disorders such as Alzheimer's disease [AD; (3–7)]. Gut microbiota can interact with the central nervous system (CNS) through the microbiota-gut-brain axis and through interactions mediated by metabolic and hormonal signaling, neural stimulation, and microbial secretions that both enhance and disrupt neurophysiology and neurological health. Deleterious microbial secretions are composed of neurotoxins, such as microbial amyloids, small bacterial RNAs, and endotoxins, such as fragilysin and lipopolysaccharide (LPS) that together represent some of the most pro-inflammatory and neurotoxic substances known (6–12). Together, complex mixtures of GI-tract-derived neurotoxins damage both colonic epithelial and neurovascular barriers, in part by inducing cleavage of the zonula adherens protein E-cadherin and other cell-cell adhesion molecules, thereby disrupting cell-cell adhesion, and enabling the translocation of these potent neurotoxins across aged or damaged plasma membranes, and into the systemic circulation, into CNS and PNS compartments and across the plasma membrane of brain cells (3–12). One major class of microbiome-derived neurotoxin is the Gram-negative bacteria-derived lipoprotein glycoconjugate lipopolysaccharide (LPS) that has been reported by several independent research groups to reside within the brain cells and CNS tissues of aged patients affected with AD and in AD murine models (10–15). Many different variations of LPS are derived from different human microbiome-resident Gram-negative bacteria; for example, species such as the anaerobic bacterium Bacteroides fragilisare capable of secreting particularly pro-inflammatory and neurotoxic forms of LPS, such as BF-LPS, which penetrate physiological barriers, including brain cell plasma membranes (8–18). Importantly, Aβ peptides, one neuropathological hallmark for AD, have recently been shown to further support the translocation of LPS into neurons, probably via transient channel formation through the neuronal plasma membrane (6–12).
This “Perspectives” paper ties together several recent observations linking increased LPS and LPS-induced NF-kB signaling with increases in a pathogenic human CNS-enriched NF-kB-sensitive microRNA-30b. We provide the first evidence that increased miRNA-30b is capable of targeting the 3'-UTR of the neuron-specific neurofilament light (NF-L) chain messenger RNA (mRNA), thus linking this action to the decreased expression of NF-L, a cytoskeletal element known to be downregulated within CNS neurons in AD affected brain, in stressed HNG cells in primary culture and in transgenic murine models of AD (14–21). In doing so, NF-L depletion disrupts normal neuronal cell shape, cytoarchitecture, and synaptic organization. This is the first example of a microbiome-initiated pathogenic pathway linking LPS, an abundant microbial glycolipid neurotoxin, with the miRNA-mediated downregulation of an essential neuron-specific cytoskeletal component normally required to maintain the cytoarchitecture and signaling functions of the neuron.
Neurofilament Light Chain Protein and AD
The neuron-specific neurofilament light (NF-L) chain protein of the neurofilament (NF) triplet bundle consisting of NF-L, neurofilament medium, and heavy chains (NF-M, NF-H): (i) is normally the most abundant neurofilamentous structural element in neurons; (ii) is a key scaffolding component of the axoskeleton of healthy neurons, to which other neuronal cytoskeletal proteins attach; and (iii)interacts directly with multiple synaptic-phosphoproteins to support and coordinate neuronal cell shape, cytoarchitecture, neurotransmission, synaptogenesis, and inter-neuronal synaptic signaling (19–24).