PIP2 depletion and altered endocytosis caused by expression of Alzheimer's disease‐protective variant PLCγ2 R522

In vitro studies have demonstrated that the P522R variant induces a slight increase in cytosolic calcium, both basally and upon stimulation, indicating mild hypermorphic activity.

“In vitro studies have demonstrated that the P522R variant induces a slight increase in cytosolic calcium, both basally and upon stimulation, indicating mild hypermorphic activity. 10 , 11 , 12”

The P522R-associated calcium changes are associated with improved survival, enhanced phagocytic/endocytic activity, and increased acute inflammatory response.

“Collectively, these changes are associated with improved survival, enhanced phagocytic/endocytic activity, and increased acute inflammatory response. 11 , 12”

In vivo studies of the P522R variant have been limited to wild-type (non-AD) P522R knock-in mice, which reported a trend toward increased expression of Cst7, Tyrobp, Clec7a, and Ccl3 by quantitative polymerase chain reaction of total brain mRNA.

“Despite this recent progress, studies of the P522R variant in vivo have thus far remained limited to the examination of wild‐type (non‐AD) P522R knock‐in (KI) mice, which reported a trend toward increased expression of Cst7, Tyrobp, Clec7a, and Ccl3 by quantitative polymerase chain reaction of total brain mRNA. 11 , 12”

The P522R mutation increases phagocytosis of Aβ.

“For example, studies showed that the P522R mutation reduced phagocytosis of Escherichia coli but increased phagocytosis of Aβ [14, 15].”

The discrepancy was hypothesized to be due to the size of the cargo, with studies showing lower phagocytosis of larger particles such as zymosan and E. coli, and more phagocytosis of smaller-sized dextrans.

“This discrepancy was hypothesized to be due to the size of the cargo, with studies showing lower phagocytosis of larger particles such as zymosan and E. coli, and more phagocytosis of smaller-sized dextrans [14, 15].”

Previous studies have demonstrated that the PLCγ2-P522R variant increases the phagocytosis of Aβ 1-42 in both mouse microglia and human iPSC models.

“previous studies have demonstrated that the PLCγ2-P522R variant also increases the phagocytosis of Aβ 1–42 in both mouse microglia and human iPSC models [17, 53, 54].”

In human microglia, PLCG2P522R increases enzymatic activity.

“In human microglia, PLCG2P522R has been shown to increase enzymatic activity 30”

In human microglia, PLCG2P522R increases amyloid beta (Aβ) uptake.

“increased amyloid beta (Aβ) uptake, 27 , 30”

In human microglia, PLCG2P522R decreases Escherichia coli uptake.

“decreased Escherichia coli uptake, 30”

In human microglia, PLCG2P522R decreases zymosan uptake.

“decreased zymosan uptake, 27 , 30”

The PLCγ2 P522R protective variant is a functional hypermorph.

“The phospholipase C-γ−2 (PLCγ2) P522R protective variant is a functional hypermorph (12–16), while it is predicted that M28L is a loss-of-function variant (17).”

In microglia, PLCγ2 activity is downstream of TREM2 and Fc receptors, regulating inflammatory processes, migration, and endocytosis.

“In microglia PLCγ2 activity has been documented to be downstream of TREM2 (19) and Fc receptors (15), regulating inflammatory processes, migration, and endocytosis (13, 16, 25, 33, 34).”

The role of PLCG2 has not yet been comprehensively explored.

“However, the role of PLCG2 has not yet been comprehensively explored (Tsai et al., 2021; Maguire et al., 2021).”

The P522R mutation reduces phagocytosis of Escherichia coli.

“For example, studies showed that the P522R mutation reduced phagocytosis of Escherichia coli but increased phagocytosis of Aβ [14, 15].”

The literature addressing the effect of the P522R mutation on phagocytosis is inconsistent, showing both increased and decreased phagocytosis.

“The literature addressing the effect of the P522R mutation on phagocytosis is inconsistent, showing both increased and decreased phagocytosis [14–16].”

A hypermorphic variant in PLCG2, rs72824905, is protective against AD risk.

“a hypermorphic variant in PLCG2, rs72824905, is protective against AD risk [9–11].”

PLCγ2 acts downstream of TREM2 signalling, modulating microglial functions such as phagocytosis, lipid metabolism, survival and cytokine release.

“Recent studies have identified PLCγ2 downstream of TREM2 signalling, modulating different microglial functions such as phagocytosis, lipid metabolism, survival and cytokine release [14, 15].”

The P522R variant is a mild hypermorph that potentiates the enzymatic activity of PLCγ2.

“Recent studies have suggested that the P522R variant is a mild hypermorph [15–17], serving to potentiate the enzymatic activity of the protein”

PLCG2 P522R has previously been reported to influence selective cargo uptake, suggesting a shift towards endocytic pathways.

“PLCγ2P522R has previously been reported to influence selective cargo uptake, suggesting a shift towards endocytic pathways [15].”

Other studies suggest that P522R impairs phagocytic clearance while enhancing endocytic clearance.

“Conversely however, other studies have suggested that in fact phagocytic clearance is impaired, while endocytic clearance is enhanced [15]”

The PLCγ2P522R variant may positively impact directly on disease-relevant pathology clearance.

“suggesting that the variant may positively impact directly on disease relevant pathology clearance, as previously suggested [15]”

The presence of the PLCγ2P522R variant in microglia enhances Ca2+ release in response to physiological stimuli.

“Recent reports have demonstrated that the presence of the PLCγ2P522R variant in microglia enhances Ca2+ release in response to physiological stimuli [15].”

The PLCγ2-R522 variant is hyperfunctional and alters microglial endocytic and phagocytic activity, including modestly increased β-amyloid uptake in human iPSC-derived microglia.

“We previously demonstrated that the PLCγ2‐R522 variant is hyperfunctional and alters microglial endocytic and phagocytic activity, including modestly increased β‐amyloid uptake in human iPSC‐derived microglia (Maguire et al. 2021).”

PLCγ2R522 is hyperfunctional, enhancing intracellular Ca2+ signaling and modulating microglial endocytosis and phagocytosis.

“We and others have shown that PLCγ2R522 is hyperfunctional, enhancing intracellular Ca2+ signaling and modulating microglial endocytosis and phagocytosis (Maguire et al. 2021; Magno et al. 2019).”

Several studies have suggested that the protective role of the P522R PLCγ2 variant is linked to functional effects on microglia.

“Several studies have suggested that the protective role of the P522R PLCγ2 variant is linked to functional effects on microglia (Maguire et al., 2021;Takalo et al., 2020;Tsai et al., 2023).”

Microglia and macrophages from PLCγ2 P522R knock-in mice have enhanced calcium responses following stimulation.

“With respect to PLC activity, some of these studies have demonstrated that microglia and macrophages from PLCγ2 P522R knock-in (KI) mice have enhanced calcium responses following stimulation (Maguire et al., 2021); similarly, an enhanced IP1 production has been observed in growing bone marrow-derived macrophages (BMDMs) from the KI mice (Takalo et al., 2020).”

Altered amyloid clearance is unlikely to be the dominant mechanism of protection in the App NL-G-F model.

“the largely unchanged total amyloid burden observed here suggests that altered amyloid clearance is unlikely to be the dominant mechanism of protection in the App NL‐G‐F model.”