Heparan Sulfate Proteoglycans (HSPGs) Serve as the Mediator Between Monomeric Tau and Its Subsequent Intracellular ERK1/2 Pathway Activation

Abstract

The conversion of soluble tau protein to insoluble, hyperphosphorylated neurofibrillary tangles (NFTs) is a major hallmark leading to neuronal death observed in neurodegenerative tauopathies. Unlike NFTs, the involvement of monomeric tau in the progression of tau pathology has been less investigated. Using live-cell confocal microscopy and flow cytometry, we demonstrate that soluble 0N4R monomers were rapidly endocytosed by SH-SY5Y and C6 glioma cells via actin-dependent macropinocytosis. Further, cellular endocytosis of monomeric tau has been demonstrated to be HSPG-dependent, as shown in C6 glial cells with genetic knockouts of xylosyltransferase-1—a key enzyme in HSPG synthesis—with a reduced level of tau uptake. Tau internalization subsequently triggers ERK1/2 activation and therefore, the upregulation of IL-6 and IL-1β . The role of ERK1/2 in regulating the levels of pro-inflammatory gene transcripts was confirmed by inhibiting the MEK-ERK1/2 signaling pathway, which led to the attenuated IL-6 and IL-1β expressions but not that of TNF-α . Moreover, as a key regulator of tau internalization, LRP1 (low-density lipoprotein receptor-related protein 1) levels were downregulated in response to monomeric tau added to C6 cells, while it was upregulated in HSPG-deficient cells, suggesting that the involvement of LRP1 in tau uptake depends on the presence of HSPGs on the cell surface. The subsequent LRP1 knockdown experiment we performed shows that LRP1 deficiency leads to an attenuated propensity for tau uptake and further elevated IL-6 gene expression. Collectively, our data suggest that tau has multiple extracellular binding partners that mediate its internalization through distinct mechanisms. Additionally, this study demonstrates the important role of both HSPGs and LRP1 in regulating cellular immune responses to tau protein monomers, providing a novel target for alleviating the neuroinflammatory environment before the formation of neurofibrillary tangles. Supplementary Information The online version contains supplementary material available at 10.1007/s12031-021-01943-2.

An Association of CSF Apolipoprotein E glycosylation and amyloid-beta 42 in individuals who carry the APOE4 allele

“…The presence of negatively charge sialic acids on apoE repels the negatively charge heparin, thus reducing its binding. HSPGs not only promote Aβ aggregation, but also promote glia cell in ammation response and the propagation of toxic tau species [40]. They further veri ed that apoE3ch produced the lowest levels of Aβ 42 toxic oligomers, then further veri ed apoE3ch had the lowest binding a nity to heparin among the other three isoforms (E2 < E3 < E4) [42].…”

“…Heparan sulfate proteoglycans (HSPGs) recognize apoE's LDLR binding region and affect Aβ cellular uptake [40]. Interestingly, a new apoE isoform stemming from a unique mutation in the APOE3 gene and resulting in the replacement of Arg 136 with Ser 136 , dubbed Christchurch ApoE3 (APOE3ch), was shown to have a 98% reduced a nity for HSPGs and 60% reduced a nity for LDLR, indicating that ApoE3ch has a LDLR binding a nity between apoE2 and apoE3 [41].…”

Regulation of autophagy, lipid metabolism, and neurodegenerative pathology by heparan sulfate proteoglycans

“…Compromising heparan sulfate biosynthesis in mature neurons or ectopic expression of an enzyme that degrades heparan sulfate suppress the accumulation of amyloid in mouse models of AD ( Jendresen et al, 2015 ; Liu et al, 2016 ). Later, it was discovered that HSPGs internalize Tau, affect Tau aggregation and release of misfolded Tau to neighboring cells, providing a mechanism of intercellular spread mediated by this protein ( Holmes et al, 2013 ; Mah et al, 2021 ; Huynh et al, 2022 ; Song et al, 2022 ). HSPG-mediated uptake of monomeric Tau also activates Erk signaling, promoting pro-inflammatory processes ( Song et al, 2022 ), another potential aspect of this heparan sulfate-mediated process.…”

“…Later, it was discovered that HSPGs internalize Tau, affect Tau aggregation and release of misfolded Tau to neighboring cells, providing a mechanism of intercellular spread mediated by this protein ( Holmes et al, 2013 ; Mah et al, 2021 ; Huynh et al, 2022 ; Song et al, 2022 ). HSPG-mediated uptake of monomeric Tau also activates Erk signaling, promoting pro-inflammatory processes ( Song et al, 2022 ), another potential aspect of this heparan sulfate-mediated process. More recently HSPGs were found to regulate autophagy ( Ning et al, 2015 ; Reynolds-Peterson et al, 2017 ; Reynolds-Peterson et al, 2020 ), and affect lipid accumulation ( Yamashita et al, 2018 ), two processes that are also disrupted in AD.…”

The Role of Extracellular Matrix in Human Neurodegenerative Diseases

“…Similarly to CSPGs, HSPGs not only appear in the close neighborhood of β-amyloid plaques, but also co-localize with hyperphosphorilated tau. Actually, HSPGs regulate cellular immune responses to tau protein monomers, which creates a neuroinflammatory environment for tangle formation [ 179 ]. The early accumulation of HSPGs, as well as the related molecular mechanisms in disease development, made HSPGs primary and key players in the unifying hypothesis of Alzheimer’s disease [ 180 , 181 ].…”