Structural insights and activating mutations in diverse pathologies define mechanisms of deregulation for phospholipase C gamma enzymes

Intron retention in PI-PLC γ1 mRNA as a key mechanism affecting MMP expression in human primary fibroblast-like synovial cells

The intron retention (IR) is a phenomenon utilized by cells to allow diverse fates at the same mRNA, leading to a different pattern of synthesis of the same protein. In this study, we analyzed the modulation of phosphoinositide-specific phospholipase C (PI-PLC) enzymes by Harpagophytum procumbens extract (HPE) in synoviocytes from joins of osteoarthritis (OA) patients. In some samples, the PI-PLC γ1 isoform mature mRNA showed the IR and, in these synoviocytes, the HPE treatment increased the phenomenon. Moreover, we highlighted that as a consequence of IR, a lower amount of PI-PLC γ1 was produced. The decrease of PI-PLC γ1 was associated with the decrease of metalloprotease-3 (MMP-3), and MMP-13, and ADAMTS-5 after HPE treatment. The altered expression of MMPs is a hallmark of the onset and progression of OA, thus substances able to decrease their expression are very desirable. The interesting outcomes of this study are that 35% of analyzed synovial tissues showed the IR phenomenon in the PI-PLC γ1 mRNA and that the HPE treatment increased this phenomenon. For the first time, we found that the decrease of PI-PLC γ1 protein in synoviocytes interferes with MMP production, thus affecting the pathways involved in the MMP expression. This finding was validated by the silencing of PI-PLC γ1 in synoviocytes where the IR phenomenon was not present. Our results shed new light on the biochemical mechanisms involved in the degrading enzyme production in the joint of OA patients, suggesting a new therapeutic target and highlighting the importance of personalized medicine.

The hypermorphic PLCγ2 S707Y variant dysregulates microglial cell function - Insight into PLCγ2 activation in brain health and disease, and opportunities for therapeutic modulation

Phospholipase C-gamma 2 (PLCγ2) is highly expressed in hematopoietic and immune cells, where it is a key signalling node enabling diverse cellular functions. Within the periphery, gain-of-function (GOF) PLCγ2 variants, such as the strongly hypermorphic S707Y, cause severe immune dysregulation. The milder hypermorphic mutation PLCγ2 P522R increases longevity and confers protection in central nervous system (CNS) neurodegenerative disorders, implicating PLCγ2 as a novel therapeutic target for treating these CNS indications. Currently, nothing is known about what consequences strong PLCγ2 GOF has on CNS functionality, and more precisely on the specific biological functions of microglia. Using the PLCγ2 S707Y variant as a model of chronic activation we investigated the functional consequences of strong PLCγ2 GOF on human microglia. PLCγ2 S707Y expressing human inducible pluripotent stem cells (hiPSC)-derived microglia exhibited hypermorphic enzymatic activity under both basal and stimulated conditions, compared to PLCγ2 wild type. Despite the increase in PLCγ2 enzymatic activity, the PLCγ2 S707Y hiPSC-derived microglia display diminished functionality for key microglial processes including phagocytosis and cytokine secretion upon inflammatory challenge. RNA sequencing revealed a downregulation of genes related to innate immunity and response, providing molecular support for the phenotype observed. Our data suggests that chronic activation of PLCγ2 elicits a detrimental phenotype that is contributing to unfavourable CNS functions, and informs on the therapeutic window for targeting PLCγ2 in the CNS. Drug candidates targeting PLCγ2 will need to precisely mimic the effects of the PLCγ2 P522R variant on microglial function, but not those of the PLCγ2 S707Y variant.

De novo variants in PLCG1 are associated with hearing impairment, ocular pathology, and cardiac defects

Phospholipase C isozymes (PLCs) hydrolyze phosphatidylinositol 4,5-bisphosphate into inositol 1,4,5-trisphosphate and diacylglycerol, important signaling molecules involved in many cellular processes. PLCG1 encodes the PLCγ1 isozyme that is broadly expressed. Hyperactive somatic mutations of PLCG1 are observed in multiple cancers, but only one germline variant has been reported. Here we describe three unrelated individuals with de novo heterozygous missense variants in PLCG1 (p.Asp1019Gly, p.His380Arg, and p.Asp1165Gly) who exhibit variable phenotypes including hearing loss, ocular pathology and cardiac septal defects. To model these variants in vivo, we generated the analogous variants in the Drosophila ortholog, small wing (sl). We created a null allele slT2A and assessed the expression pattern. sl is broadly expressed, including in wing discs, eye discs, and a subset of neurons and glia. Loss of sl causes wing size reductions, ectopic wing veins and supernumerary photoreceptors. We document that mutant flies exhibit a reduced lifespan and age-dependent locomotor defects. Expressing wild-type sl in slT2A mutant rescues the loss-of-function phenotypes whereas expressing the variants causes lethality. Ubiquitous overexpression of the variants also reduces viability, suggesting that the variants are toxic. Ectopic expression of an established hyperactive PLCG1 variant (p.Asp1165His) in the wing pouch causes severe wing phenotypes, resembling those observed with overexpression of the p.Asp1019Gly or p.Asp1165Gly variants, further arguing that these two are gain-of-function variants. However, the wing phenotypes associated with p.His380Arg overexpression are mild. Our data suggest that the PLCG1 de novo heterozygous missense variants are pathogenic and contribute to the features observed in the probands.

TCR Pathway Mutations in Mature T Cell Lymphomas

Mature T cell lymphomas are heterogeneous neoplasms that are aggressive and resistant to treatment. Many of these cancers retain immunological properties of their cell of origin. They express cytokines, cytotoxic enzymes, and cell surface ligands normally induced by TCR signaling in untransformed T cells. Until recently, their molecular mechanisms were unclear. Recently, high-dimensional studies have transformed our understanding of their cellular and genetic characteristics. Somatic mutations in the TCR signaling pathway drive lymphomagenesis by disrupting autoinhibitory domains, increasing affinity to ligands, and/or inducing TCR-independent signaling. Collectively, most of these mutations augment signaling pathways downstream of the TCR. Emerging data suggest that these mutations not only drive proliferation but also determine lymphoma immunophenotypes. For example, RHOA mutations are sufficient to induce disease-relevant CD4+ T follicular helper cell phenotypes. In this review, we describe how mutations in the TCR signaling pathway elucidate lymphoma pathophysiology but also provide insights into broader T cell biology.

Pleckstrin Homology [PH] domain, structure, mechanism, and contribution to human disease

The pleckstrin homology [PH] domain is a structural fold found in more than 250 proteins making it the 11th most common domain in the human proteome. 25% of family members have more than one PH domain and some PH domains are split by one, or several other, protein domains although still folding to give functioning PH domains. We review mechanisms of PH domain activity, the role PH domain mutation plays in human disease including cancer, hyperproliferation, neurodegeneration, inflammation, and infection, and discuss pharmacotherapeutic approaches to regulate PH domain activity for the treatment of human disease. Almost half PH domain family members bind phosphatidylinositols [PIs] that attach the host protein to cell membranes where they interact with other membrane proteins to give signaling complexes or cytoskeleton scaffold platforms. A PH domain in its native state may fold over other protein domains thereby preventing substrate access to a catalytic site or binding with other proteins. The resulting autoinhibition can be released by PI binding to the PH domain, or by protein phosphorylation thus providing fine tuning of the cellular control of PH domain protein activity. For many years the PH domain was thought to be undruggable until high-resolution structures of human PH domains allowed structure-based design of novel inhibitors that selectively bind the PH domain. Allosteric inhibitors of the Akt1 PH domain have already been tested in cancer patients and for proteus syndrome, with several other PH domain inhibitors in preclinical development for treatment of other human diseases.

Activation Mechanisms and Diverse Functions of Mammalian Phospholipase C

Phospholipase C (PLC) plays pivotal roles in regulating various cellular functions by metabolizing phosphatidylinositol 4,5-bisphosphate in the plasma membrane. This process generates two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol, which respectively regulate the intracellular Ca²⁺ levels and protein kinase C activation. In mammals, six classes of typical PLC have been identified and classified based on their structure and activation mechanisms. They all share X and Y domains, which are responsible for enzymatic activity, as well as subtype-specific domains. Furthermore, in addition to typical PLC, atypical PLC with unique structures solely harboring an X domain has been recently discovered. Collectively, seven classes and 16 isozymes of mammalian PLC are known to date. Dysregulation of PLC activity has been implicated in several pathophysiological conditions, including cancer, cardiovascular diseases, and neurological disorders. Therefore, identification of new drug targets that can selectively modulate PLC activity is important. The present review focuses on the structures, activation mechanisms, and physiological functions of mammalian PLC.

G-CSF drives autoinflammation in APLAID

Missense mutations in PLCG2 can cause autoinflammation with phospholipase C gamma 2-associated antibody deficiency and immune dysregulation (APLAID). Here, we generated a mouse model carrying an APLAID mutation (p.Ser707Tyr) and found that inflammatory infiltrates in the skin and lungs were only partially ameliorated by removing inflammasome function via the deletion of caspase-1. Also, deleting interleukin-6 or tumor necrosis factor did not fully prevent APLAID mutant mice from autoinflammation. Overall, these findings are in accordance with the poor response individuals with APLAID have to treatments that block interleukin-1, JAK1/2 or tumor necrosis factor. Cytokine analysis revealed increased granulocyte colony-stimulating factor (G-CSF) levels as the most distinct feature in mice and individuals with APLAID. Remarkably, treatment with a G-CSF antibody completely reversed established disease in APLAID mice. Furthermore, excessive myelopoiesis was normalized and lymphocyte numbers rebounded. APLAID mice were also fully rescued by bone marrow transplantation from healthy donors, associated with reduced G-CSF production, predominantly from non-hematopoietic cells. In summary, we identify APLAID as a G-CSF-driven autoinflammatory disease, for which targeted therapy is feasible.

Study of NAD-interacting proteins highlights the extent of NAD regulatory roles in the cell and its potential as a therapeutic target

Nicotinamide adenine dinucleotide (NAD) levels are essential for the normal physiology of the cell and are strictly regulated to prevent pathological conditions. NAD functions as a coenzyme in redox reactions, as a substrate of regulatory proteins, and as a mediator of protein-protein interactions. The main objectives of this study were to identify the NAD-binding and NAD-interacting proteins, and to uncover novel proteins and functions that could be regulated by this metabolite. It was considered if cancer-associated proteins were potential therapeutic targets. Using multiple experimental databases, we defined datasets of proteins that directly interact with NAD - the NAD-binding proteins (NADBPs) dataset - and of proteins that interact with NADBPs - the NAD-protein-protein interactions (NAD-PPIs) dataset. Pathway enrichment analysis revealed that NADBPs participate in several metabolic pathways, while NAD-PPIs are mostly involved in signalling pathways. These include disease-related pathways, namely, three major neurodegenerative disorders: Alzheimer's disease, Huntington's disease, and Parkinson's disease. Then, the complete human proteome was further analysed to select potential NADBPs. TRPC3 and isoforms of diacylglycerol (DAG) kinases, which are involved in calcium signalling, were identified as new NADBPs. Potential therapeutic targets that interact with NAD were identified, that have regulatory and signalling functions in cancer and neurodegenerative diseases.

A frequent PLCγ1 mutation in adult T-cell leukemia/lymphoma determines functional properties of the malignant cells

BACKGROUND

Development of adult T-cell leukemia/lymphoma (ATL) involves human T-cell leukemia virus type 1 (HTLV-1) infection and accumulation of somatic mutations. The most frequently mutated gene in ATL (36 % of cases) is phospholipase C gamma1 (PLCG1). PLCG1 is also frequently mutated in other T-cell lymphomas. However, the functional consequences of the PLCG1 mutations in cancer cells have not been characterized.

METHODS

We compared the activity of the wild-type PLCγ1 with that of a mutant carrying a hot-spot mutation of PLCγ1 (S345F) observed in ATL, both in cells and in cell-free assays. To analyse the impact of the mutation on cellular properties, we quantified cellular proliferation, aggregation, chemotaxis and apoptosis by live cell-imaging in an S345F+ ATL-derived cell line (KK1) and a KK1 cell line in which we reverted the mutation to the wild-type sequence using CRISPR/Cas9 and homology-directed repair.

FINDINGS

The PLCγ1 S345F mutation results in an increase of basal PLC activity in vitro and in different cell types. This higher basal activity is further enhanced by upstream signalling. Reversion of the S345F mutation in the KK1 cell line resulted in reduction of the PLC activity, lower rates of proliferation and aggregation, and a marked reduction in chemotaxis towards CCL22. The PLCγ1-pathway inhibitors ibrutinib and ritonavir reduced both the PLC activity and the tested functions of KK1 cells.

INTERPRETATION

Consistent with observations from clinical studies, our data provide direct evidence that activated variants of the PLCγ1 enzyme contribute to the properties of the malignant T-cell clone in ATL.

FUNDING

MRC (UK) Project Grant (P028160).

PLCγ2 impacts microglia-related effectors revealing variants and pathways important in Alzheimer’s disease

Alzheimer’s disease (AD) is an irreversible neurodegenerative disease mainly characterized by memory loss and cognitive decline. The etiology of AD is complex and remains incompletely understood. In recent years, genome-wide association studies (GWAS) have increasingly highlighted the central role of microglia in AD pathology. As a trans-membrane receptor specifically present on the microglia in the central nervous system, phosphatidylinositol-specific phospholipase C gamma 2 (PLCγ2) plays an important role in neuroinflammation. GWAS data and corresponding pathological research have explored the effects of PLCG2 variants on amyloid burden and tau pathologies that underline AD. The link between PLCγ2 and other AD-related effectors in human and mouse microglia has also been established, placing PLCγ2 downstream of the triggering receptor expressed on myeloid cells 2 (TREM2), toll-like receptor 4 (TLR4), Bruton’s tyrosine kinase (BTK), and colony-stimulating factor 1 receptor (CSF1R). Because the research on PLCγ2’s role in AD is still in its early stages, few articles have been published, therefore in this paper, we integrate the relevant research published to date, review the structural features, expression patterns, and related pathways of PLCγ2, and summarize the recent studies on important PLCG2 variants related to AD. Furthermore, the possibility and challenge of using PLCγ2 to develop therapeutic drugs for AD are also discussed.

Characterization of the membrane interactions of phospholipase Cγ reveals key features of the active enzyme

PLCγ enzymes are autoinhibited in resting cells and form key components of intracellular signaling that are also linked to disease development. Insights into physiological and aberrant activation of PLCγ require understanding of an active, membrane-bound form, which can hydrolyze inositol-lipid substrates. Here, we demonstrate that PLCγ1 cannot bind membranes unless the autoinhibition is disrupted. Through extensive molecular dynamics simulations and experimental evidence, we characterize membrane binding by the catalytic core domains and reveal previously unknown sites of lipid interaction. The identified sites act in synergy, overlap with autoinhibitory interfaces, and are shown to be critical for the phospholipase activity in cells. This work provides direct evidence that PLCγ1 is inhibited through obstruction of its membrane-binding surfaces by the regulatory region and that activation must shift PLCγ1 to a conformation competent for membrane binding. Knowledge of the critical sites of membrane interaction extends the mechanistic framework for activation, dysregulation, and therapeutic intervention.

Dynamics of allosteric regulation of the phospholipase C-γ isozymes upon recruitment to membranes

Numerous receptor tyrosine kinases and immune receptors activate phospholipase C-γ (PLC-γ) isozymes at membranes to control diverse cellular processes including phagocytosis, migration, proliferation, and differentiation. The molecular details of this process are not well understood. Using hydrogen-deuterium exchange mass spectrometry, we show that PLC-γ1 is relatively inert to lipid vesicles that contain its substrate, phosphatidylinositol 4,5-bisphosphate (PIP₂), unless first bound to the kinase domain of the fibroblast growth factor receptor (FGFR1). Exchange occurs throughout PLC-γ1 and is exaggerated in PLC-γ1 containing an oncogenic substitution (D1165H) that allosterically activates the lipase. These data support a model whereby initial complex formation shifts the conformational equilibrium of PLC-γ1 to favor activation. This receptor-induced priming of PLC-γ1 also explains the capacity of a kinase-inactive fragment of FGFR1 to modestly enhance the lipase activity of PLC-γ1 operating on lipid vesicles but not a soluble analog of PIP₂ and highlights potential cooperativity between receptor engagement and membrane proximity. Priming is expected to be greatly enhanced for receptors embedded in membranes and nearly universal for the myriad of receptors and co-receptors that bind the PLC-γ isozymes.

A kinetic model of phospholipase C-γ1 linking structure-based insights to dynamics of enzyme autoinhibition and activation

Phospholipase C-γ1 (PLC-γ1) is a receptor-proximal enzyme that promotes signal transduction through PKC in mammalian cells. Because of the complexity of PLC-γ1 regulation, a two-state (inactive/active) model does not account for the intricacy of activation and inactivation steps at the plasma membrane. Here, we introduce a structure-based kinetic model of PLC-γ1, considering interactions of its regulatory Src homology 2 (SH2) domains and perturbation of those dynamics upon phosphorylation of Tyr783, a hallmark of activation. For PLC-γ1 phosphorylation to dramatically enhance enzyme activation as observed, we found that high intramolecular affinity of the C-terminal SH2 (cSH2) domain-pTyr783 interaction is critical, but this affinity need not outcompete the autoinhibitory interaction of the cSH2 domain. Under conditions for which steady-state PLC-γ1 activity is sensitive to the rate of Tyr783 phosphorylation, maintenance of the active state is surprisingly insensitive to the phosphorylation rate, since pTyr783 is well protected by the cSH2 domain while the enzyme is active. In contrast, maintenance of enzyme activity is sensitive to the rate of PLC-γ1 membrane (re)binding. Accordingly, we found that hypothetical PLC-γ1 mutations that either weaken autoinhibition or strengthen membrane binding influence the activation kinetics differently, which could inform the characterization of oncogenic variants. Finally, we used this newly informed kinetic scheme to refine a spatial model of PLC/PKC polarization during chemotaxis. The refined model showed improved stability of the polarized pattern while corroborating previous qualitative predictions. As demonstrated here for PLC-γ1, this approach may be adapted to model the dynamics of other receptor- and membrane-proximal enzymes.

The Conformational State of the BTK Substrate PLCγ Contributes to Ibrutinib Resistance

Mutations in PLCγ, a substrate of the tyrosine kinase BTK, are often found in patients who develop resistance to the BTK inhibitor Ibrutinib. However, the mechanisms by which these PLCγ mutations cause Ibrutinib resistance are unclear. Under normal signaling conditions, BTK mediated phosphorylation of Y783 within the PLCγ cSH2-linker promotes the intramolecular association of this site with the adjacent cSH2 domain resulting in active PLCγ. Thus, the cSH2-linker region in the center of the regulatory gamma specific array (γSA) of PLCγ is a key feature controlling PLCγ activity. Even in the unphosphorylated state this linker exists in a conformational equilibrium between free and bound to the cSH2 domain. The position of this equilibrium is optimized within the properly regulated PLCγ enzyme but may be altered in the context of mutations. We therefore assessed the conformational status of four resistance associated mutations within the PLCγ γSA and find that they each alter the conformational equilibrium of the γSA leading to a shift toward active PLCγ. Interestingly, two distinct modes of mutation induced activation are revealed by this panel of Ibrutinib resistance mutations. These findings, along with the recently determined structure of fully autoinhibited PLCγ, provide new insight into the nature of the conformational change that occurs within the γSA regulatory region to affect PLCγ activation. Improving our mechanistic understanding of how B cell signaling escapes Ibrutinib treatment via mutations in PLCγ will aid in the development of strategies to counter drug resistance.

In vitro reconstitution reveals cooperative mechanisms of adapter protein-mediated activation of phospholipase C-γ1 in T cells

Activation of T cells upon engagement of the T cell antigen receptor rapidly leads to a number of phosphorylation and plasma membrane recruitment events. For example, translocation of phospholipase-Cγ1 (PLC−γ1) to the plasma membrane and its association with the transmembrane adapter protein LAT and two other adapter proteins, Gads and SLP-76, are critical events in the early T cell activation process. We have previously characterized the formation of a tetrameric LAT-Gads-SLP-76-PLC−γ1 complex by reconstitution in vitro and have also characterized the thermodynamics of tetramer formation. In the current study, we define how PLC−γ1 recruitment to liposomes, which serve as a plasma membrane surrogate, and PLC−γ1 activation are regulated both independently and additively by recruitment of PLC−γ1 to phosphorylated LAT, by formation of the LAT-Gads-SLP-76-PLC−γ1 tetramer, and by tyrosine phosphorylation of PLC−γ1. The recently solved structure of PLC−γ1 indicates that, in the resting state, several PLC−γ1 domains inhibit its enzymatic activity and contact with the plasma membrane. We propose the multiple cooperative steps that we observed likely lead to conformational alterations in the regulatory domains of PLC−γ1, enabling contact with its membrane substrate, disinhibition of PLC−γ1 enzymatic activity, and production of the phosphoinositide cleavage products necessary for T cell activation.

Conformational switches that control the TEC kinase – PLCγ signaling axis

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TEC kinases and PLCγ transition between autoinhibited state and active conformation.

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PLCγ structures reveal both autoinhibited form and active form of gamma specific array (γSA); the four regulatory domains unique to the PLCγ isozymes.

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Domain dynamics likely control activation mechanism.

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PLCγ phosphorylation triggers conformational switch.

Cell surface receptors such as the T-cell receptor (TCR) and B-cell receptor (BCR) engage with external stimuli to transmit information into the cell and initiate a cascade of signaling events that lead to gene expression that drives the immune response. At the heart of controlling T- and B-cell cell signaling, phospholipase Cγ hydrolyzes membrane associated PIP₂, leading to generation of the second messengers IP₃ and DAG. These small molecules trigger mobilization of intracellular Ca²⁺ and promote transcription factor transport into the nucleus launching the adaptive immune response. The TEC family kinases are responsible for phosphorylating and activating PLCγ, and our group aims to understand mechanisms that regulate immune cell signal transduction by focusing on this kinase/phospholipase axis in T-cells and B-cells. Here, we review the current molecular level understanding of how the TEC kinases (ITK and BTK) and PLCγ1/2 are autoinhibited prior to activation of cell surface receptors, how TEC kinases are activated to specifically recognize the PLCγ substrate, and how conformational changes induced by phosphorylation trigger PLCγ activation.

The rapidly evolving role of cryo-EM in drug design

Since the early 2010s, cryo-electron microscopy (cryo-EM) has evolved to a mainstream structural biology method in what has been dubbed the "resolution revolution". Pharma companies also began to use cryo-EM in drug discovery, evidenced by a growing number of industry publications. Hitherto limited in resolution, throughput and attainable molecular weight, cryo-EM is rapidly overcoming its main limitations for more widespread use through a new wave of technological advances. This review discusses how cryo-EM has already impacted drug discovery, and how the state-of-the-art is poised to further revolutionize its application to previously intractable proteins as well as new use cases.

A computational in silico approach to predict high-risk coding and non-coding SNPs of human PLCG1 gene

PLCG1 gene is responsible for many T-cell lymphoma subtypes, including peripheral T-cell lymphoma (PTCL), angioimmunoblastic T-cell lymphoma (AITL), cutaneous T-cell lymphoma (CTCL), adult T-cell leukemia/lymphoma along with other diseases. Missense mutations of this gene have already been found in patients of CTCL and AITL. The non-synonymous single nucleotide polymorphisms (nsSNPs) can alter the protein structure as well as its functions. In this study, probable deleterious and disease-related nsSNPs in PLCG1 were identified using SIFT, PROVEAN, PolyPhen-2, PhD-SNP, Pmut, and SNPS&GO tools. Further, their effect on protein stability was checked along with conservation and solvent accessibility analysis by I-mutant 2.0, MUpro, Consurf, and Netsurf 2.0 server. Some SNPs were finalized for structural analysis with PyMol and BIOVIA discovery studio visualizer. Out of the 16 nsSNPs which were found to be deleterious, ten nsSNPs had an effect on protein stability, and six mutations (L411P, R355C, G493D, R1158H, A401V and L455F) were predicted to be highly conserved. Among the six highly conserved mutations, four nsSNPs (R355C, A401V, L411P and L455F) were part of the catalytic domain. L411P, L455F and G493D made significant structural change in the protein structure. Two mutations-Y210C and R1158H had post-translational modification. In the 5' and 3' untranslated region, three SNPs, rs139043247, rs543804707, and rs62621919 showed possible miRNA target sites and DNA binding sites. This in silico analysis has provided a structured dataset of PLCG1 gene for further in vivo researches. With the limitation of computational study, it can still prove to be an asset for the identification and treatment of multiple diseases associated with the target gene.

Mechanism of activation and the rewired network: New drug design concepts

Precision oncology benefits from effective early phase drug discovery decisions. Recently, drugging inactive protein conformations has shown impressive successes, raising the cardinal questions of which targets can profit and what are the principles of the active/inactive protein pharmacology. Cancer driver mutations have been established to mimic the protein activation mechanism. We suggest that the decision whether to target an inactive (or active) conformation should largely rest on the protein mechanism of activation. We next discuss the recent identification of double (multiple) same-allele driver mutations and their impact on cell proliferation and suggest that like single driver mutations, double drivers also mimic the mechanism of activation. We further suggest that the structural perturbations of double (multiple) in cis mutations may reveal new surfaces/pockets for drug design. Finally, we underscore the preeminent role of the cellular network which is deregulated in cancer. Our structure-based review and outlook updates the traditional Mechanism of Action, informs decisions, and calls attention to the intrinsic activation mechanism of the target protein and the rewired tumor-specific network, ushering innovative considerations in precision medicine.

Emerging roles of PLCγ1 in endothelial biology

Phospholipase C γ1 (PLCγ1) is a member of the PLC family that functions as signal transducer by hydrolyzing membrane lipid to generate second messengers. The unique protein structure of PLCγ1 confers a critical role as a direct effector of VEGFR2 and signaling mediated by other receptor tyrosine kinases. The distinct vascular phenotypes in PLCγ1-deficient animal models and the gain-of-function mutations of PLCγ1 found in human endothelial cancers point to a major physiological role of PLCγ1 in the endothelial system. In this review, we discuss aspects of physiological and molecular function centering around PLCγ1 in the context of endothelial cells and provide a perspective for future investigation.

Recruitment of phospholipase Cγ1 to the non-structural membrane protein pK15 of Kaposi Sarcoma-associated herpesvirus promotes its Src-dependent phosphorylation

Kaposi Sarcoma-associated herpesvirus (KSHV) causes three human malignancies, Kaposi Sarcoma (KS), Primary Effusion Lymphoma (PEL) and the plasma cell variant of multicentric Castleman’s Disease (MCD), as well as an inflammatory cytokine syndrome (KICS). Its non-structural membrane protein, pK15, is among a limited set of viral proteins expressed in KSHV-infected KS tumor cells. Following its phosphorylation by Src family tyrosine kinases, pK15 recruits phospholipase C gamma 1 (PLCγ1) to activate downstream signaling cascades such as the MEK/ERK, NFkB and PI3K pathway, and thereby contributes to the increased proliferation and migration as well as the spindle cell morphology of KSHV-infected endothelial cells. Here, we show that a phosphorylated Y⁴⁸¹EEVL motif in pK15 preferentially binds into the PLCγ1 C-terminal SH2 domain (cSH2), which is involved in conformational changes occurring during the activation of PLCγ1 by receptor tyrosine kinases. We determined the crystal structure of a pK15 12mer peptide containing the phosphorylated pK15 Y⁴⁸¹EEVL motif in complex with a shortened PLCγ1 tandem SH2 (tSH2) domain. This structure demonstrates that the pK15 peptide binds to the PLCγ1 cSH2 domain in a position that is normally occupied by the linker region connecting the PLCγ1 cSH2 and SH3 domains. We also show that longer pK15 peptides containing the phosphorylated pK15 Y⁴⁸¹EEVL motif can increase the Src-mediated phosphorylation of the PLCγ1 tSH2 region in vitro. This pK15-induced increase in Src-mediated phosphorylation of PLCγ1 can be inhibited with the small pK15-derived peptide which occupies the PLCγ1 cSH2 domain. Our findings thus suggest that pK15 may act as a scaffold protein to promote PLCγ1 activation in a manner similar to the cellular scaffold protein SLP-76, which has been shown to promote PLCγ1 activation in the context of T-cell receptor signaling. Reminiscent of its positional homologue in Epstein-Barr Virus, LMP2A, pK15 may therefore mimic aspects of antigen-receptor signaling. Our findings also suggest that it may be possible to inhibit the recruitment and activation of PLCγ1 pharmacologically.

Kaposi’s Sarcoma-Associated Herpesvirus (KSHV) causes three human malignancies (Kaposi Sarcoma, Primary Effusion Lymphoma, Multicentric Castleman’s Disease) and an inflammatory condition, KICS. One of its non-structural membrane proteins, pK15, is expressed in tumor cells and has previously been shown to contribute to its ability to reactivate from latency and to its pathogenetic properties in endothelial cells by recruiting the cellular signaling enzyme phospholipase Cγ1 (PLCγ1). Here we investigate the interaction of pK15 with PLCγ1, report the structure of a PLCγ1 domain in complex with a pK15 peptide and show that pK15 primes PLCγ1 for phosphorylation by the cellular kinase Src. We also show that the pK15-dependent activation of PLCγ1 can be inhibited with a small peptide. Our findings therefore identify the pK15-PLCγ1 interaction as a putative druggable target and provide the basis for the development of small molecule inhibitors that could perhaps serve to inhibit KSHV replication and pathogenesis.

TREM2/PLCγ2 signalling in immune cells: function, structural insight, and potential therapeutic modulation

The central role of the resident innate immune cells of the brain (microglia) in neurodegeneration has become clear over the past few years largely through genome-wide association studies (GWAS), and has rapidly become an active area of research. However, a mechanistic understanding (gene to function) has lagged behind. That is now beginning to change, as exemplified by a number of recent exciting and important reports that provide insight into the function of two key gene products – TREM2 (Triggering Receptor Expressed On Myeloid Cells 2) and PLCγ2 (Phospholipase C gamma2) – in microglia, and their role in neurodegenerative disorders. In this review we explore and discuss these recent advances and the opportunities that they may provide for the development of new therapies.

A novel somatic PLCG2 variant associated with resistance to BTK and SYK inhibition in chronic lymphocytic leukemia

The treatment of chronic lymphocytic leukemia (CLL) has been transformed by the use of targeted small molecules inhibiting components of the B cell receptor (BCR) signaling pathway (Haematologica, 103, 2018 and e204; Curr Hematol Malig Rep, 14, 2019, 302). Chief among these is ibrutinib, an irreversible inhibitor of Bruton tyrosine kinase (BTK), which produces deep, durable responses in CLL with good tolerability (Haematologica, 103, 2018 and e204). Though prolonged exposure to the drug can exert selective pressure on CLL cells and allow for the emergence of drug-resistant clones, primary ibrutinib treatment failure is rare (Expert Rev Hematol, 11 and 2018, 185; N Engl J Med, 370, 2014 and 2352; N Engl J Med, 373, 2015 and 25, 2425; Blood, 128, 2016 and 2199). Activating mutations in the gene PLCG2, which encodes a downstream target of BTK, appear to enable constitutive BCR signaling and have been associated with ibrutinib resistance (Int J Cancer, 146 and 2020, 85; J Clin Oncol, 35, 2017 and 1437; Blood, 126, 2015 and 61). In recent years, novel investigational agents have targeted other components of the BCR pathway. Among these is entospletinib, an orally bioavailable, selective inhibitor of splenic tyrosine kinase (SYK) (Blood, 126, 2015 and 1744), which lies upstream of the enzyme phospholipase C-gamma-2 (PLCG2). Here, we describe a patient who was found to harbor a novel somatic variant of PLCG2 and experienced a lack of treatment response to both ibrutinib and entospletinib.

Phospholipase C families: Common themes and versatility in physiology and pathology

Phosphoinositide-specific phospholipase Cs (PLCs) are expressed in all mammalian cells and play critical roles in signal transduction. To obtain a comprehensive understanding of these enzymes in physiology and pathology, a detailed structural, biochemical, cell biological and genetic information is required. In this review, we cover all these aspects to summarize current knowledge of the entire superfamily. The families of PLCs have expanded from 13 enzymes to 16 with the identification of the atypical PLCs in the human genome. Recent structural insights highlight the common themes that cover not only the substrate catalysis but also the mechanisms of activation. This involves the release of autoinhibitory interactions that, in the absence of stimulation, maintain classical PLC enzymes in their inactive forms. Studies of individual PLCs provide a rich repertoire of PLC function in different physiologies. Furthermore, the genetic studies discovered numerous mutated and rare variants of PLC enzymes and their link to human disease development, greatly expanding our understanding of their roles in diverse pathologies. Notably, substantial evidence now supports involvement of different PLC isoforms in the development of specific cancer types, immune disorders and neurodegeneration. These advances will stimulate the generation of new drugs that target PLC enzymes, and will therefore open new possibilities for treatment of a number of diseases where current therapies remain ineffective.

Severe Autoinflammatory Manifestations and Antibody Deficiency Due to Novel Hypermorphic PLCG2 Mutations

Autoinflammatory diseases (AIDs) were first described as clinical disorders characterized by recurrent episodes of seemingly unprovoked sterile inflammation. In the past few years, the identification of novel AIDs expanded their phenotypes toward more complex clinical pictures associating vasculopathy, autoimmunity, or immunodeficiency. Herein, we describe two unrelated patients suffering since the neonatal period from a complex disease mainly characterized by severe sterile inflammation, recurrent bacterial infections, and marked humoral immunodeficiency. Whole-exome sequencing detected a novel, de novo heterozygous PLCG2 variant in each patient (p.Ala708Pro and p.Leu845_Leu848del). A clear enhanced PLCγ2 activity for both variants was demonstrated by both ex vivo calcium responses of the patient's B cells to IgM stimulation and in vitro assessment of PLC activity. These data supported the autoinflammation and PLCγ2-associated antibody deficiency and immune dysregulation (APLAID) diagnosis in both patients. Immunological evaluation revealed a severe decrease of immunoglobulins and B cells, especially class-switched memory B cells, with normal T and NK cell counts. Analysis of bone marrow of one patient revealed a reduced immature B cell fraction compared with controls. Additional investigations showed that both PLCG2 variants activate the NLRP3-inflammasome through the alternative pathway instead of the canonical pathway. Collectively, the evidences here shown expand APLAID diversity toward more severe phenotypes than previously reported including dominantly inherited agammaglobulinemia, add novel data about its genetic basis, and implicate the alternative NLRP3-inflammasome activation pathway in the basis of sterile inflammation.

Phosphatidylinositol(4,5)bisphosphate: diverse functions at the plasma membrane

Phosphatidylinositol(4,5) bisphosphate (PI(4,5)P2) has become a major focus in biochemistry, cell biology and physiology owing to its diverse functions at the plasma membrane. As a result, the functions of PI(4,5)P2 can be explored in two separate and distinct roles - as a substrate for phospholipase C (PLC) and phosphoinositide 3-kinase (PI3K) and as a primary messenger, each having unique properties. Thus PI(4,5)P2 makes contributions in both signal transduction and cellular processes including actin cytoskeleton dynamics, membrane dynamics and ion channel regulation. Signalling through plasma membrane G-protein coupled receptors (GPCRs), receptor tyrosine kinases (RTKs) and immune receptors all use PI(4,5)P2 as a substrate to make second messengers. Activation of PI3K generates PI(3,4,5)P3 (phosphatidylinositol(3,4,5)trisphosphate), a lipid that recruits a plethora of proteins with pleckstrin homology (PH) domains to the plasma membrane to regulate multiple aspects of cellular function. In contrast, PLC activation results in the hydrolysis of PI(4,5)P2 to generate the second messengers, diacylglycerol (DAG), an activator of protein kinase C and inositol(1,4,5)trisphosphate (IP3/I(1,4,5)P3) which facilitates an increase in intracellular Ca2+. Decreases in PI(4,5)P2 by PLC also impact on functions that are dependent on the intact lipid and therefore endocytosis, actin dynamics and ion channel regulation are subject to control. Spatial organisation of PI(4,5)P2 in nanodomains at the membrane allows for these multiple processes to occur concurrently.