Maximum reproductive lifespan correlates with CD33rSIGLEC gene number: Implications for NADPH oxidase-derived reactive oxygen species in aging

Therapeutic potential to target sialylation and SIGLECs in neurodegenerative and psychiatric diseases

Sialic acids, commonly found as the terminal carbohydrate on the glycocalyx of mammalian cells, are pivotal checkpoint inhibitors of the innate immune system, particularly within the central nervous system (CNS). Sialic acid-binding immunoglobulin-like lectins (SIGLECs) expressed on microglia are key players in maintaining microglial homeostasis by recognizing intact sialylation. The finely balanced sialic acid-SIGLEC system ensures the prevention of excessive and detrimental immune responses in the CNS. However, loss of sialylation and SIGLEC receptor dysfunctions contribute to several chronic CNS diseases. Genetic variants of SIGLEC3/CD33, SIGLEC11, and SIGLEC14 have been associated with neurodegenerative diseases such as Alzheimer's disease, while sialyltransferase ST8SIA2 and SIGLEC4/MAG have been linked to psychiatric diseases such as schizophrenia, bipolar disorders, and autism spectrum disorders. Consequently, immune-modulatory functions of polysialic acids and SIGLEC binding antibodies have been exploited experimentally in animal models of Alzheimer's disease and inflammation-induced CNS tissue damage, including retinal damage. While the potential of these therapeutic approaches is evident, only a few therapies to target either sialylation or SIGLEC receptors have been tested in patient clinical trials. Here, we provide an overview of the critical role played by the sialic acid-SIGLEC axis in shaping microglial activation and function within the context of neurodegeneration and synaptopathies and discuss the current landscape of therapies that target sialylation or SIGLECs.

Neuroimmunomodulatory properties of polysialic acid

Polymeric sialic acid (polysialic acid, polySia) is a remarkable posttranslational modification of only few select proteins. The major, and most prominent polySia protein carrier is the neural cell adhesion molecule NCAM. Here, the key functions of polySia are to regulate interactions of NCAM and to balance cellular interactions in brain development and plasticity. During recent years, however, increasing evidence points towards a role of polySia in the modulation of immune responses. These immunomodulatory functions can be mediated by polySia on proteins other than NCAM, presented either on the cell surface or released into the extracellular space. This perspective review summarizes our current knowledge and addresses major open questions on polySia and polySia receptors in modulating innate immune responses in the brain.

Discovery, classification, evolution and diversity of Siglecs

Immunoglobulin (Ig) superfamily proteins play diverse roles in vertebrates, including regulation of cellular responses by sensing endogenous or exogenous ligands. Siglecs are a family of glycan-recognizing proteins belonging to the Ig superfamily (i.e., I-type lectins). Siglecs are expressed on various leukocyte types and are involved in diverse aspects of immunity, including the regulation of inflammatory responses, leukocyte proliferation, host-microbe interaction, and cancer immunity. Sialoadhesin/Siglec-1, CD22/Siglec-2, and myelin-associated glycoprotein/Siglec-4 were among the first to be characterized as members of the Siglec family, and along with Siglec-15, they are relatively well-conserved among tetrapods. Conversely, CD33/Siglec-3-related Siglecs (CD33rSiglecs, so named as they show high sequence similarity with CD33/Siglec-3) are encoded in a gene cluster with many interspecies variations and even intraspecies variations within some lineages such as humans. The rapid evolution of CD33rSiglecs expressed on leukocytes involved in innate immunity likely reflects the selective pressure by pathogens that interact and possibly exploit these Siglecs. Human Siglecs have several additional unique and/or polymorphic properties as compared with closely related great apes, changes possibly related to the loss of the sialic acid Neu5Gc, another distinctly human event in sialobiology. Multiple changes in human CD33rSiglecs compared to great apes include many examples of human-specific expression in non-immune cells, coinciding with human-specific diseases involving such cell types. Some Siglec gene polymorphisms have dual consequences-beneficial in a situation but detrimental in another. The association of human Siglec gene polymorphisms with several infectious and non-infectious diseases likely reflects the ongoing competition between the host and microbial pathogens.

Planetary Metronome as a Regulator of Lifespan and Aging Rate: The Metronomic Hypothesis

A metronomic mechanism for the duration control of ontogenetic cycle periods of an animal is proposed. The components of the proposed metronomic system include the ventricular system of the brain, planet Earth as a generator of metronomic signals, and temporal DNA (tDNA) as a substrate that is epigenetically marked to measure elapsed time of ontogenesis. The metronomic system generates repetitive signals in the form of hydrodynamic disturbances in the cerebrospinal fluid (CSF). The metronomic effect arises due to the superposition of two processes - the near-wall unidirectional flow of CSF and oscillations in the movement of the planet. Hydrodynamic impacts of the metronome are transformed into nerve impulses that initiate epigenetic modification of tDNA in neurons, changing the content of factors expressed by this DNA for innervated targets of the body. The duration of ontogenetic cycle periods, including duration of the adult life, depends on the rate of addition of epigenetic marks to tDNA. This rate depends mainly on the frequency of the metronomic signals used by each particular species. But epigenetic modifications can also be influenced by factors that modulate metabolism and the rate of chromatin modifications, such as a calorie-restricted diet.

Evolution of Human-Specific Alleles Protecting Cognitive Function of Grandmothers

The myelomonocytic receptor CD33 (Siglec-3) inhibits innate immune reactivity by extracellular V-set domain recognition of sialic acid (Sia)-containing "self-associated molecular patterns" (SAMPs). We earlier showed that V-set domain-deficient CD33-variant allele, protective against late-onset Alzheimer's Disease (LOAD), is derived and specific to the hominin lineage. We now report multiple hominin-specific CD33 V-set domain mutations. Due to hominin-specific, fixed loss-of-function mutation in the CMAH gene, humans lack N-glycolylneuraminic acid (Neu5Gc), the preferred Sia-ligand of ancestral CD33. Mutational analysis and molecular dynamics (MD)-simulations indicate that fixed change in amino acid 21 of hominin V-set domain and conformational changes related to His45 corrected for Neu5Gc-loss by switching to N-acetylneuraminic acid (Neu5Ac)-recognition. We show that human-specific pathogens Neisseria gonorrhoeae and Group B Streptococcus selectively bind human CD33 (huCD33) as part of immune-evasive molecular mimicry of host SAMPs and that this binding is significantly impacted by amino acid 21 modification. In addition to LOAD-protective CD33 alleles, humans harbor derived, population-universal, cognition-protective variants at several other loci. Interestingly, 11 of 13 SNPs in these human genes (including CD33) are not shared by genomes of archaic hominins: Neanderthals and Denisovans. We present a plausible evolutionary scenario to compile, correlate, and comprehend existing knowledge about huCD33-evolution and suggest that grandmothering emerged in humans.

CD33 is downregulated by influenza virus H1N1pdm09 and induces ROS and the TNF-α, IL-1β, and IL-6 cytokines in human mononuclear cells

The influenza A virus (IAV) H1N1pdm09 induces exacerbated inflammation, contributing to disease complications. Inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α), favor an inflammatory response that aids viral replication and survival. A pathway by which spontaneous TNF-α production occurs involves either the reduction of Siglec-3 (CD33) levels or the absence of its ligand, sialic acid. Influenza virus uses sialic acid to enter cells by reducing their expression; however, the role of CD33 in IAV H1N1pdm09 stimulation and its relationship with inflammation have not yet been studied. To evaluate the role of CD33 in proinflammatory cytokine production in IAV H1N1pdm09 stimulation, peripheral blood mononuclear cells from healthy subjects were incubated with IAV H1N1pdm09. We observed that the infection caused an increase in the mRNA expression of proinflammatory cytokines such as TNF-α, interleukin (IL)-1β, and IL-6 and a significant reduction in CD33 expression by monocytes at an early stage of infection. Additionally, suppressor of cytokine signaling 3 (SOCS-3) mRNA expression was upregulated at 6 h, and reactive oxygen species (ROS) production increased at 1.5 h. Moreover, a significant reduction in CD33 expression on the cell surface of monocytes from influenza patients or of IAV H1N1pdm09-stimulated monocytes incubated in vitro was observed by flow cytometry. The results suggest that the decrease in CD33 and increase of SOCS-3 expression induced by IAV H1N1pdm09 triggered TNF-α secretion and ROS production, suggesting an additional way to exacerbate inflammation during viral infection.

Siglecs Modulate Activities of Immune Cells Through Positive and Negative Regulation of ROS Generation

Reactive oxygen species (ROS) are a group of oxygen-containing highly-reactive molecules produced from oxidative metabolic processes or in response to intracellular signals like cytokines and external stimuli like pathogen attack. They regulate a range of physiological processes and are involved in innate immune responses against infectious agents. Deregulation of ROS contributes to a plethora of disease conditions. Sialic acids are carbohydrates, present on cell surfaces or soluble proteins. Sialic acid-binding immunoglobulin-like lectins (Siglecs) recognize and bind to sialic acids. These are widely expressed on various types of immune cells. Siglecs modulate immune activation and can promote or inhibit ROS generation under different contexts. Siglecs promote ROS-dependent cell death in neutrophils and eosinophils while limiting oxidative stress associated with chronic obstructive pulmonary disease (COPD), sickle cell disease (SCD), coronavirus disease-2019 (COVID-19), etc. This review distinguishes itself in summarizing the current understanding of the role of Siglecs in moderating ROS production and their distinct effect on different immune cells; that ultimately determine the cellular response and the disease outcome. This is an important field of investigation having scope for both expansion and medical importance.

Multiple Genomic Events Altering Hominin SIGLEC Biology and Innate Immunity Predated the Common Ancestor of Humans and Archaic Hominins

Human-specific pseudogenization of the CMAH gene eliminated the mammalian sialic acid (Sia) Neu5Gc (generating an excess of its precursor Neu5Ac), thus changing ubiquitous cell surface “self-associated molecular patterns” that modulate innate immunity via engagement of CD33-related-Siglec receptors. The Alu-fusion-mediated loss-of-function of CMAH fixed ∼2–3 Ma, possibly contributing to the origins of the genus Homo. The mutation likely altered human self-associated molecular patterns, triggering multiple events, including emergence of human-adapted pathogens with strong preference for Neu5Ac recognition and/or presenting Neu5Ac-containing molecular mimics of human glycans, which can suppress immune responses via CD33-related-Siglec engagement. Human-specific alterations reported in some gene-encoding Sia-sensing proteins suggested a “hotspot” in hominin evolution. The availability of more hominid genomes including those of two extinct hominins now allows full reanalysis and evolutionary timing. Functional changes occur in 8/13 members of the human genomic cluster encoding CD33-related Siglecs, all predating the human common ancestor. Comparisons with great ape genomes indicate that these changes are unique to hominins. We found no evidence for strong selection after the Human–Neanderthal/Denisovan common ancestor, and these extinct hominin genomes include almost all major changes found in humans, indicating that these changes in hominin sialobiology predate the Neanderthal–human divergence ∼0.6 Ma. Multiple changes in this genomic cluster may also explain human-specific expression of CD33rSiglecs in unexpected locations such as amnion, placental trophoblast, pancreatic islets, ovarian fibroblasts, microglia, Natural Killer(NK) cells, and epithelia. Taken together, our data suggest that innate immune interactions with pathogens markedly altered hominin Siglec biology between 0.6 and 2 Ma, potentially affecting human evolution.

Human-specific microglial Siglec-11 transcript variant has the potential to affect polysialic acid-mediated brain functions at a distance

CD33-related Siglecs are often found on innate immune cells and modulate their reactivity by recognition of sialic acid-based "self-associated molecular patterns" and signaling via intracellular tyrosine-based cytosolic motifs. Previous studies have shown that Siglec-11 specifically binds to the brain-enriched polysialic acid (polySia/PSA) and that its microglial expression in the brain is unique to humans. Furthermore, human microglial Siglec-11 exists as an alternate splice form missing the exon encoding the last (fifth) Ig-like C2-set domain of the extracellular portion of the protein, but little is known about the functional consequences of this variation. Here, we report that the recombinant soluble human microglial form of Siglec-11 (hSiglec-11(4D)-Fc) binds endogenous and immobilized polySia better than the tissue macrophage form (hSiglec-11(5D)-Fc) or the chimpanzee form (cSiglec-11(5D)-Fc). The Siglec-11 protein is also prone to aggregation, potentially influencing its ligand-binding ability. Additionally, Siglec-11 protein can be secreted in both intact and proteolytically cleaved forms. The microglial splice variant has reduced proteolytic release and enhanced incorporation into exosomes, a process that appears to be regulated by palmitoylation of cysteines in the cytosolic tail. Taken together, these data demonstrate that human brain specific microglial hSiglec-11(4D) has different molecular properties and can be released on exosomes and/or as proteolytic products, with the potential to affect polySia-mediated brain functions at a distance.

Bilirubin Nanomedicines for the Treatment of Reactive Oxygen Species (ROS)-Mediated Diseases

Reactive oxygen species (ROS) are chemically reactive species that are produced in cellular aerobic metabolism. They mainly include superoxide anion, hydrogen peroxide, hydroxyl radicals, singlet oxygen, ozone, and nitric oxide and are implicated in many physiological and pathological processes. Bilirubin, a cardinal pigment in the bile, has been increasingly investigated to treat cancer, diabetes, ischemia-reperfusion injury, asthma, and inflammatory bowel diseases (IBD). Indeed, bilirubin has been shown to eliminate ROS production, so it is now considered as a promising therapeutic agent for ROS-mediated diseases and can be used for the development of antioxidative nanomedicines. This review summarizes the current knowledge of the physiological mechanisms of ROS production and its role in pathological changes and focuses on discussing the antioxidative effects of bilirubin and its application in the experimental studies of nanomedicines. Previous studies have shown that bilirubin was mainly used as a responsive molecule in the microenvironment of ROS overproduction in neoplastic tissues for the development of anticancer nanodrugs; however, it could also exert powerful ROS scavenging activity in chronic inflammation and ischemia-reperfusion injury. Therefore, bilirubin, as an inartificial ROS scavenger, is expected to be used for the development of nanomedicines against more diseases due to the universality of ROS involvement in human pathological conditions.