Exploring the Syndecan-Mediated Cellular Internalization of the SARS-CoV-2 Omicron Variant

SARS-CoV-2 variants evolve to rely more on heparan sulfate (HS) for viral attachment and subsequent infection. In our earlier work, we demonstrated that the Delta variant’s spike protein binds more strongly to HS compared to WT SARS-CoV-2, leading to enhanced cell internalization via syndecans (SDCs), a family of transmembrane HS proteoglycans (HSPGs) facilitating the cellular entry of the original strain. Using our previously established ACE2- or SDC-overexpressing cellular models, we now compare the ACE2- and SDC-dependent cellular uptake of heat-inactivated WT SARS-CoV-2 with the Delta and Omicron variants. Internalization studies with inactivated virus particles showed that ACE2 overexpression could not compensate for the loss of HS in Omicron’s internalization, suggesting that this variant primarily uses HSPGs to enter cells. Although SDCs increased the internalization of all three viruses, subtle differences could be detected between their SDC isoform preferences. The Delta variant particularly benefitted from SDC1, 2, and 4 overexpression for cellular entry, while SDC4 had the most prominent effect on Omicron internalization. The SDC4 knockdown (KD) in Calu-3 cells reduced the cellular uptake of all three viruses, but the inhibition was the most pronounced for Omicron. The polyanionic heparin also hindered the cellular internalization of all three viruses with a dominant inhibitory effect on Omicron. Omicron’s predominant HSPG affinity, combined with its preference for the universally expressed SDC4, might account for its efficient transmission yet reduced pathogenicity

Syndecan-3 as a Novel Biomarker in Alzheimer's Disease

Early diagnosis of Alzheimer's disease (AD) is of paramount importance in preserving the patient's mental and physical health in a fairly manageable condition for a longer period. Reliable AD detection requires novel biomarkers indicating central nervous system (CNS) degeneration in the periphery. Members of the syndecan family of transmembrane proteoglycans are emerging new targets in inflammatory and neurodegenerative disorders. Reviewing the growing scientific evidence on the involvement of syndecans in the pathomechanism of AD, we analyzed the expression of the neuronal syndecan, syndecan-3 (SDC3), in experimental models of neurodegeneration. Initial in vitro studies showed that prolonged treatment of tumor necrosis factor-alpha (TNF-α) increases SDC3 expression in model neuronal and brain microvascular endothelial cell lines. In vivo studies revealed elevated concentrations of TNF-α in the blood and brain of APPSWE-Tau transgenic mice, along with increased SDC3 concentration in the brain and the liver. Primary brain endothelial cells and peripheral blood monocytes isolated from APPSWE-Tau mice exhibited increased SDC3 expression than wild-type controls. SDC3 expression of blood-derived monocytes showed a positive correlation with amyloid plaque load in the brain, demonstrating that SDC3 on monocytes is a good indicator of amyloid pathology in the brain. Given the well-established role of blood tests, the SDC3 expression of monocytes could serve as a novel biomarker for early AD detection

Mutual Inhibition of Antithrombin III and SARS-CoV-2 Cellular Attachment to Syndecans: Implications for COVID-19 Treatment and Vaccination

Antithrombin III (ATIII) is a potent endogenous anticoagulant that binds to heparan sulfate proteoglycans (HSPGs) on endothelial cells' surfaces. Among these HSPGs, syndecans (SDCs) are crucial as transmembrane receptors bridging extracellular ligands with intracellular signaling pathways. Specifically, syndecan-4 (SDC4) has been identified as a key receptor on endothelial cells for transmitting the signaling effects of ATIII. Meanwhile, SDCs have been implicated in facilitating the cellular internalization of SARS-CoV-2. Given the complex interactions between ATIII and SDC4, our study analyzed the impact of ATIII on the virus entry into host cells. While ATIII binds to all SDC isoforms, it shows the strongest affinity for SDC4. SDCs' heparan sulfate chains primarily influence ATIII's SDC attachment, although other parts might also play a role in ATIII's dominant affinity toward SDC4. ATIII significantly reduces SARS-CoV-2's cellular entry into cell lines expressing SDCs, suggesting a competitive inhibition mechanism at the SDC binding sites, particularly SDC4. Conversely, the virus or its spike protein decreases the availability of SDCs on the cell surface, reducing ATIII's cellular attachment and hence contributing to a procoagulant environment characteristic of COVID-19

Contribution of syndecans to the cellular entry of SARS-CoV-2

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel emerging pathogen causing an unprecedented pandemic in 21st century medicine. Due to the significant health and economic burden of the current SARS-CoV-2 outbreak, there is a huge unmet medical need for novel interventions effectively blocking SARS-CoV-2 infection. Unknown details of SARS-CoV-2 cellular biology hamper the development of potent and highly specific SARS-CoV-2 therapeutics. Angiotensin-converting enzyme-2 (ACE2) has been reported to be the primary receptor for SARS-CoV-2 cellular entry. However, emerging scientific evidence suggests the involvement of additional membrane proteins, such as heparan sulfate proteoglycans, in SARS-CoV-2 internalization. Here, we report that syndecans, the evolutionarily conserved family of transmembrane proteoglycans, facilitate the cellular entry of SARS-CoV-2. Among syndecans, the lung abundant syndecan-4 was the most efficient in mediating SARS-CoV-2 uptake. The S1 subunit of the SARS-CoV-2 spike protein plays a dominant role in the virus’s interactions with syndecans. Besides the polyanionic heparan sulfate chains, other parts of the syndecan ectodomain, such as the cell-binding domain, also contribute to the interaction with SARS-CoV-2. During virus internalization, syndecans colocalize with ACE2, suggesting a jointly shared internalization pathway. Both ACE2 and syndecan inhibitors exhibited significant efficacy in reducing the cellular entry of SARS-CoV-2, thus supporting the complex nature of internalization. Data obtained on syndecan specific in vitro assays present syndecans as novel cellular targets of SARS-CoV-2 and offer molecularly precise yet simple strategies to overcome the complex nature of SARS-CoV-2 infection

Syndecan-3 as a Novel Biomarker in Alzheimer's Disease

Early diagnosis of Alzheimer's disease (AD) is of paramount importance in preserving the patient's mental and physical health in a fairly manageable condition for a longer period. Reliable AD detection requires novel biomarkers indicating central nervous system (CNS) degeneration in the periphery. Members of the syndecan family of transmembrane proteoglycans are emerging new targets in inflammatory and neurodegenerative disorders. Reviewing the growing scientific evidence on the involvement of syndecans in the pathomechanism of AD, we analyzed the expression of the neuronal syndecan, syndecan-3 (SDC3), in experimental models of neurodegeneration. Initial in vitro studies showed that prolonged treatment of tumor necrosis factor-alpha (TNF-α) increases SDC3 expression in model neuronal and brain microvascular endothelial cell lines. In vivo studies revealed elevated concentrations of TNF-α in the blood and brain of APPSWE-Tau transgenic mice, along with increased SDC3 concentration in the brain and the liver. Primary brain endothelial cells and peripheral blood monocytes isolated from APPSWE-Tau mice exhibited increased SDC3 expression than wild-type controls. SDC3 expression of blood-derived monocytes showed a positive correlation with amyloid plaque load in the brain, demonstrating that SDC3 on monocytes is a good indicator of amyloid pathology in the brain. Given the well-established role of blood tests, the SDC3 expression of monocytes could serve as a novel biomarker for early AD detection

The Nuclear Localization Signal of NF-κB p50 Enters the Cells via Syndecan-Mediated Endocytosis and Inhibits NF-κB Activity

It is well established that cationic peptides can enter cells following attachment to polyanionic membrane components. We report that the basic nuclear localization signal (NLS) of the NF-κB p50 subunit is internalized via lipid raft-dependent endocytosis mediated by heparan sulfate proteoglycans and exerts significant NF-κB inhibitory activities both in vitro and in vivo. In vitro uptake experiments revealed that the p50 NLS peptide (CYVQRKRQKLMP) enters the cytoplasm and accumulates in the nucleus at 37 °C. Depleting cellular ATP pools or decreasing temperature to 4 °C abolished peptide internalization, confirming the active, energy-dependent endocytic uptake. Co-incubation with heparan sulfate or replacing the peptide’s basic residues with glycines markedly reduced the intracellular entry of the p50 NLS, referring to the role of polyanionic cell-surface proteoglycans in internalization. Furthermore, treatment with methyl-β-cyclodextrin greatly inhibited the peptide’s membrane translocation. Overexpression of the isoforms of the syndecan family of transmembrane proteoglycans, especially syndecan-4, increased the cellular internalization of the NLS, suggesting syndecans’ involvement in the peptide’s cellular uptake. In vitro , p50 NLS reduced NF-κB activity in TNF-α-induced L929 fibroblasts and LPS-stimulated RAW 264.7 macrophages. TNF-α-induced ICAM-1 expression of HMEC-1 human endothelial cells could also be inhibited by the peptide. Fifteen minutes after its intraperitoneal injection, the peptide rapidly entered the cells of the pancreas, an organ with marked syndecan-4 expression. In an acute pancreatitis model, an inflammatory disorder triggered by the activation of stress-responsive transcription factors like NF-κB, administration of the p50 NLS peptide reduced the severity of pancreatic inflammation by blocking NF-κB transcription activity and ameliorating the examined laboratory and histological markers of pancreatitis

Syndecan-4 Mediates the Cellular Entry of Adeno-Associated Virus 9

Due to their low pathogenicity, immunogenicity, and long-term gene expression, adeno-associated virus (AAV) vectors emerged as safe and efficient gene delivery tools, over-coming setbacks experienced with other viral gene delivery systems in early gene therapy trials. Among AAVs, AAV9 can translocate through the blood-brain barrier (BBB), making it a promising gene delivery tool for transducing the central nervous system (CNS) via systemic administration. Recent reports on the shortcomings of AAV9-mediated gene delivery into the CNS require reviewing the molecular base of AAV9 cellular biology. A more detailed understanding of AAV9’s cellular entry would eradicate current hurdles and enable more efficient AAV9-based gene therapy approaches. Syndecans, the transmembrane family of heparan-sulfate proteoglycans, facilitate the cellular uptake of various viruses and drug delivery systems. Utilizing human cell lines and syndecan-specific cellular assays, we assessed the involvement of syndecans in AAV9’s cellular entry. The ubiquitously expressed isoform, syndecan-4 proved its superiority in facilitating AAV9 internalization among syndecans. Introducing syndecan-4 into poorly transducible cell lines enabled robust AAV9-dependent gene transduction, while its knockdown reduced AAV9’s cellular entry. Attachment of AAV9 to syndecan-4 is mediated not just by the polyanionic heparan-sulfate chains but also by the cell-binding domain of the extracellular syndecan-4 core protein. Co-immunoprecipitation assays and affinity proteomics also confirmed the role of syndecan-4 in the cellular entry of AAV9. Overall, our findings highlight the universally expressed syndecan-4 as a significant contributor to the cellular internalization of AAV9 and provide a molecular-based, rational explanation for the low gene delivery potential of AAV9 into the CNS

The Nuclear Localization Signal of NF-κB p50 Enters the Cells via Syndecan-Mediated Endocytosis and Inhibits NF-κB Activity

It is well established that cationic peptides can enter cells following attachment to polyanionic membrane components. We report that the basic nuclear localization signal (NLS) of the NF-κB p50 subunit is internalized via lipid raft-dependent endocytosis mediated by heparan sulfate proteoglycans and exerts significant NF-κB inhibitory activities both in vitro and in vivo. In vitro uptake experiments revealed that the p50 NLS peptide (CYVQRKRQKLMP) enters the cytoplasm and accumulates in the nucleus at 37 °C. Depleting cellular ATP pools or decreasing temperature to 4 °C abolished peptide internalization, confirming the active, energy-dependent endocytic uptake. Co-incubation with heparan sulfate or replacing the peptide’s basic residues with glycines markedly reduced the intracellular entry of the p50 NLS, referring to the role of polyanionic cell-surface proteoglycans in internalization. Furthermore, treatment with methyl-β-cyclodextrin greatly inhibited the peptide’s membrane translocation. Overexpression of the isoforms of the syndecan family of transmembrane proteoglycans, especially syndecan-4, increased the cellular internalization of the NLS, suggesting syndecans’ involvement in the peptide’s cellular uptake. In vitro , p50 NLS reduced NF-κB activity in TNF-α-induced L929 fibroblasts and LPS-stimulated RAW 264.7 macrophages. TNF-α-induced ICAM-1 expression of HMEC-1 human endothelial cells could also be inhibited by the peptide. Fifteen minutes after its intraperitoneal injection, the peptide rapidly entered the cells of the pancreas, an organ with marked syndecan-4 expression. In an acute pancreatitis model, an inflammatory disorder triggered by the activation of stress-responsive transcription factors like NF-κB, administration of the p50 NLS peptide reduced the severity of pancreatic inflammation by blocking NF-κB transcription activity and ameliorating the examined laboratory and histological markers of pancreatitis

Biodistribution and Cellular Internalization of Inactivated SARS-CoV-2 in Wild-Type Mice

Despite the growing list of identified SARS-CoV-2 receptors, the human angiotensin-converting enzyme 2 (ACE2) is still viewed as the main cell entry receptor mediating SARS-CoV-2 internalization. It has been reported that wild-type mice, like other rodent species of the Muridae family, cannot be infected with SARS-CoV-2 due to differences in their ACE2 receptors. On the other hand, the consensus heparin-binding motif of SARS-CoV-2's spike protein, PRRAR, enables the attachment to rodent heparan sulfate proteoglycans (HSPGs), including syndecans, a transmembrane HSPG family with a well-established role in clathrin- and caveolin-independent endocytosis. As mammalian syndecans possess a relatively conserved structure, we analyzed the cellular uptake of inactivated SARS-CoV-2 particles in in vitro and in vivo mice models. Cellular studies revealed efficient uptake into murine cell lines with established syndecan-4 expression. After intravenous administration, inactivated SARS-CoV-2 was taken up by several organs in vivo and could also be detected in the brain. Internalized by various tissues, inactivated SARS-CoV-2 raised tissue TNF-alpha levels, especially in the heart, reflecting the onset of inflammation. Our studies on in vitro and in vivo mice models thus shed light on unknown details of SARS-CoV-2 internalization and help broaden the understanding of the molecular interactions of SARS-CoV-2

Contribution of syndecans to cellular uptake and fibrillation of alpha-synuclein and tau

Scientific evidence suggests that alpha-synuclein and tau have prion-like properties and that prionlike spreading and seeding of misfolded protein aggregates constitutes a central mechanism for neurodegeneration. Heparan sulfate proteoglycans (HSPGs) in the plasma membrane support this process by attaching misfolded protein fibrils. Despite of intense studies, contribution of specific HSPGs to seeding and spreading of alpha-synuclein and tau has not been explored yet. Here we report that members of the syndecan family of HSPGs mediate cellular uptake of alpha-synuclein and tau fibrils via a lipid-raft dependent and clathrin-independent endocytic route. Among syndecans, the neuron predominant syndecan-3 exhibits the highest affinity for both alpha-synuclein and tau. Syndecan-mediated internalization of alpha-synuclein and tau depends heavily on conformation as uptake via syndecans start to dominate once fibrils are formed. Overexpression of syndecans, on the other hand, reduces cellular uptake of monomeric alpha-synuclein and tau, yet exerts a fibril forming effect on both proteins. Data obtained from syndecan overexpressing cellular models presents syndecans, especially the neuron predominant syndecan-3, as important mediators of seeding and spreading of alpha-synuclein and tau and reveal how syndecans contribute to fundamental molecular events of a-synuclein and tau pathology