Sublytic C5b-9-stimulated Schwann cell survival through PI 3-kinase-mediated phosphorylation of BAD

Emerging role of complement in COVID-19 and other respiratory virus diseases

The complement system, a key component of innate immunity, provides the first line of defense against bacterial infection; however, the COVID-19 pandemic has revealed that it may also engender severe complications in the context of viral respiratory disease. Here, we review the mechanisms of complement activation and regulation and explore their roles in both protecting against infection and exacerbating disease. We discuss emerging evidence related to complement-targeted therapeutics in COVID-19 and compare the role of the complement in other respiratory viral diseases like influenza and respiratory syncytial virus. We review recent mechanistic studies and animal models that can be used for further investigation. Novel knockout studies are proposed to better understand the nuances of the activation of the complement system in respiratory viral diseases.

Complement terminal pathway inhibition reduces peritoneal injuries in a rat peritonitis model

Peritonitis and the resulting peritoneal injuries are common problems that prevent long-term peritoneal dialysis (PD) therapy in patients with end-stage kidney diseases. Previously, we have analyzed the relationship between the complement system and progression of peritoneal injuries associated with PD, particularly focusing on the early activation pathways and effects of the anaphylatoxins. We here utilized a novel mAb 2H2 that blocks assembly of the membrane attack complex (MAC) to investigate roles of the complement terminal pathway in PD-associated peritoneal injury. We intraperitoneally injected mAb 2H2 anti-C5b-7 (2.5 or 5 mg/rat) once or twice over the five-day course of the experiment to investigate the effects of inhibiting formation of MAC in a fungal rat peritonitis model caused by repeated intraperitoneal administration of zymosan after methylglyoxal pretreatment (Zy/MGO model). Rats were sacrificed on day 5 and macroscopic changes in both parietal and visceral peritoneum evaluated. Peritoneal thickness, the abundance of fibrinogen and complement C3 and MAC deposition in tissue and accumulation of inflammatory cells were pathologically assessed. The results showed that mAb 2H2, but not isotype control mAb, reduced peritoneal thickness and accumulation of inflammatory cells in a dose and frequency-dependent manner in the Zy/MGO model. These effects were accompanied by decreased C3, MAC, and fibrinogen deposition in peritoneum. In conclusion, in the rat Zy/MGO model, complement terminal pathway activation and MAC formation substantially contributed to development of peritoneal injuries, suggesting that MAC-targeted therapies might be effective in preventing development of peritoneal injuries in humans.

Cd59 and inflammation regulate Schwann cell development

Efficient neurotransmission is essential for organism survival and is enhanced by myelination. However, the genes that regulate myelin and myelinating glial cell development have not been fully characterized. Data from our lab and others demonstrates that cd59, which encodes for a small GPI-anchored glycoprotein, is highly expressed in developing zebrafish, rodent, and human oligodendrocytes (OLs) and Schwann cells (SCs), and that patients with CD59 dysfunction develop neurological dysfunction during early childhood. Yet, the function of Cd59 in the developing nervous system is currently undefined. In this study, we demonstrate that cd59 is expressed in a subset of developing SCs. Using cd59 mutant zebrafish, we show that developing SCs proliferate excessively and nerves may have reduced myelin volume, altered myelin ultrastructure, and perturbed node of Ranvier assembly. Finally, we demonstrate that complement activity is elevated in cd59 mutants and that inhibiting inflammation restores SC proliferation, myelin volume, and nodes of Ranvier to wildtype levels. Together, this work identifies Cd59 and developmental inflammation as key players in myelinating glial cell development, highlighting the collaboration between glia and the innate immune system to ensure normal neural development.

Complement as a powerful "influencer" in the brain during development, adulthood and neurological disorders

The complement system was long considered as only a powerful effector arm of the immune system that, while critically protective, could lead to inflammation and cell death if overactivated, even in the central nervous system (CNS). However, in the past decade it has been recognized as playing critical roles in key physiological processes in the CNS, including neurogenesis and synaptic remodeling in the developing and adult brain. Inherent in these processes are the interactions with cells in the brain, and the cascade of interactions and functional consequences that ensue. As a result, investigations of therapeutic approaches for both suppressing excessive complement driven neurotoxicity and aberrant sculpting of neuronal circuits, require broad (and deep) knowledge of the functional activities of multiple components of this highly evolved and regulated system to avoid unintended negative consequences in the clinic. Advances in basic science are beginning to provide a roadmap for translation to therapeutics, with both small molecule and biologics. Here, we present examples of the critical roles of proper complement function in the development and sculpting of the nervous system, and in enabling rapid protection from infection and clearance of dying cells. Microglia are highlighted as important command centers that integrate signals from the complement system and other innate sensors that are programed to provide support and protection, but that direct detrimental responses to aberrant activation and/or regulation of the system. Finally, we present promising research areas that may lead to effective and precision strategies for complement targeted interventions to promote neurological health.

Complement Inhibition Targeted to Injury Specific Neoepitopes Attenuates Atherogenesis in Mice

Rationale: Previous studies have indicated an important role for complement in atherosclerosis, a lipid-driven chronic inflammatory disease associated to oxidative stress in the vessel wall. However, it remains unclear how complement is activated in the process of atherogenesis. An accepted general model for complement activation in the context of ischemia reperfusion injury is that ischemia induces the exposure of neoepitopes that are recognized by natural self-reactive IgM antibodies, and that in turn activate complement. Objective: We investigated whether a similar phenomenon may be involved in the pathogenesis of atherosclerosis, and whether interfering with this activation event, together with inhibition of subsequent amplification of the cascade at the C3 activation step, can provide protection against atherogenesis. Methods and Results: We utilized C2scFv-Crry, a novel construct consisting of a single chain antibody (scFv) linked to Crry, a complement inhibitor that functions at C3 activation. The scFv moiety was derived from C2 IgM mAb that specifically recognizes phospholipid neoepitopes known to be expressed after ischemia. C2scFv-Crry targeted to the atherosclerotic plaque of Apoe -/- mice, demonstrating expression of the C2 neoepitope. C2scFv-Crry administered twice per week significantly attenuated atherosclerotic plaque in the aorta and aortic root of Apoe -/- mice fed with a high-fat diet (HFD) for either 2 or 4 months, and treatment reduced C3 deposition and membrane attack complex formation as compared to vehicle treated mice. C2scFv-Crry also inhibited the uptake of oxidized low-density-lipoprotein (oxLDL) by peritoneal macrophages, which has been shown to play a role in pathogenesis, and C2scFv-Crry-treated mice had decreased lipid content in the lesion with reduced oxLDL levels in serum compared to vehicle-treated mice. Furthermore, C2scFv-Crry reduced the deposition of endogenous total IgM in the plaque, although it did not alter serum IgM levels, further indicating a role for natural IgM in initiating complement activation. Conclusion: Neoepitope targeted complement inhibitors represent a novel therapeutic approach for atherosclerosis.

Complement C6 deficiency exacerbates pathophysiology after spinal cord injury

Historically, the membrane attack complex, composed of complement components C5b-9, has been connected to lytic cell death and implicated in secondary injury after a CNS insult. However, studies to date have utilized either non-littermate control rat models, or mouse models that lack significant C5b-9 activity. To investigate what role C5b-9 plays in spinal cord injury and recovery, we generated littermate PVG C6 wildtype and deficient rats and tested functional and histological recovery after moderate contusion injury using the Infinite Horizon Impactor. We compare the effect of C6 deficiency on recovery of locomotor function and histological injury parameters in PVG rats under two conditions: (1) animals maintained as separate C6 WT and C6-D homozygous colonies; and (2) establishment of a heterozygous colony to generate C6 WT and C6-D littermate controls. The results suggest that maintenance of separate homozygous colonies is inadequate for testing the effect of C6 deficiency on locomotor and histological recovery after SCI, and highlight the importance of using littermate controls in studies involving genetic manipulation of the complement cascade.

The role of complement membrane attack complex in dry and wet AMD - From hypothesis to clinical trials

Data from human dry and wet age-related macular degeneration (AMD) eyes support the hypothesis that constant 'tickover' of the alternative complement pathway results in chronic deposition of the complement membrane attack complex (MAC) on the choriocapillaris and the retinal pigment epithelium (RPE). Sub-lytic levels of MAC lead to cell signaling associated with tissue remodeling and the production of cytokines and inflammatory molecules. Lytic levels of MAC lead to cell death. CD59 is a naturally occurring inhibitor of the assembly of MAC. CD59 may thus be therapeutically efficacious against the pathophysiology of dry and wet AMD. The first gene therapy clinical trial for geographic atrophy - the advanced form of dry AMD has recently completed recruitment. This trial is studying the safety and tolerability of expressing CD59 from an adeno-associated virus (AAV) vector injected once into the vitreous. A second clinical trial assessing the efficacy of CD59 in wet AMD patients is also under way. Herein, the evidence for the role of MAC in the pathophysiology of dry as well as wet AMD and the scientific rationale underlying the use of AAV- delivered CD59 for the treatment of dry and wet AMD is discussed.

Complement in the fundamental processes of the cell

Once regarded solely as an activator of innate immunity, it is now clear that the complement system acts in an assortment of cells and tissues, with immunity only one facet of a diverse array of functions under the influence of the complement proteins. Throughout development, complement activity has now been demonstrated from early sperm-egg interactions in fertilisation, to regulation of epiboly and organogenesis, and later in refinement of cerebral synapses. Complement has also been shown to regulate homeostasis of adult tissues, controlling cell processes such as migration, survival, repair, and regeneration. Given the continuing emergence of such novel actions of complement, the existing research likely represents only a fraction of the myriad of functions of this complex family of proteins. This review is focussed on outlining the current knowledge of complement family members in the regulation of cell processes in non-immune systems. It is hoped this will spur research directed towards revealing more about the role of complement in these fundamental cell processes.

Complement Membrane Attack and Tumorigenesis: A SYSTEMS BIOLOGY APPROACH

Tumor development driven by inflammation is now an established phenomenon, but the role that complement plays remains uncertain. Recent evidence has suggested that various components of the complement (C) cascade may influence tumor development in disparate ways; however, little attention has been paid to that of the membrane attack complex (MAC). This is despite abundant evidence documenting the effects of this complex on cell behavior, including cell activation, protection from/induction of apoptosis, release of inflammatory cytokines, growth factors, and ECM components and regulators, and the triggering of the NLRP3 inflammasome. Here we present a novel approach to this issue by using global gene expression studies in conjunction with a systems biology analysis. Using network analysis of MAC-responsive expression changes, we demonstrate a cluster of co-regulated genes known to have impact in the extracellular space and on the supporting stroma and with well characterized tumor-promoting roles. Network analysis highlighted the central role for EGF receptor activation in mediating the observed responses to MAC exposure. Overall, the study sheds light on the mechanisms by which sublytic MAC causes tumor cell responses and exposes a gene expression signature that implicates MAC as a driver of tumor progression. These findings have implications for understanding of the roles of complement and the MAC in tumor development and progression, which in turn will inform future therapeutic strategies in cancer.

The membrane attack complex as an inflammatory trigger

The final common pathway of all routes of complement activation involves the non-enzymatic assembly of a complex comprising newly formed C5b with the plasma proteins C6, C7, C8 and C9. When assembly occurs on a target cell membrane the forming complex inserts into and through the bilayer to create a pore, the membrane attack complex (MAC). On some targets, pore formation causes rapid lytic destruction; however, most nucleated cell targets resist lysis through a combination of ion pumps, membrane regulators and active recovery processes. Cells survive but not without consequence. The MAC pore causes ion fluxes and directly or indirectly impacts several important signalling pathways that in turn activate a diverse series of events in the cell, many of which are highly pro-inflammatory. Although this non-lytic, pro-inflammatory role of MAC has been recognised for thirty years, no consensus signalling pathway has emerged. Recent work, summarised here, has implicated specific signalling routes and, in some cells, inflammasome involvement, opening the door to novel approaches to therapy in complement-driven pathologies.

Complement system in pathogenesis of AMD: dual player in degeneration and protection of retinal tissue

Age-related macular degeneration (AMD) is the most common cause of blindness among the elderly, especially in Western countries. Although the prevalence, risk factors, and clinical course of the disease are well described, its pathogenesis is not entirely elucidated. AMD is associated with a variety of biochemical abnormalities, including complement components deposition in the retinal pigment epithelium-Bruch's membrane-choriocapillaris complex. Although the complement system (CS) is increasingly recognized as mediating important roles in retinal biology, its particular role in AMD pathogenesis has not been precisely defined. Unrestricted activation of the CS following injury may directly damage retinal tissue and recruit immune cells to the vicinity of active complement cascades, therefore detrimentally causing bystander damage to surrounding cells and tissues. On the other hand, recent evidence supports the notion that an active complement pathway is a necessity for the normal maintenance of the neurosensory retina. In this scenario, complement activation appears to have beneficial effect as it promotes cell survival and tissue remodeling by facilitating the rapid removal of dying cells and resulting cellular debris, thus demonstrating anti-inflammatory and neuroprotective activities. In this review, we discuss both the beneficial and detrimental roles of CS in degenerative retina, focusing on the diverse aspects of CS functions that may promote or inhibit macular disease.

Molecules involved in the crosstalk between immune- and peripheral nerve Schwann cells

Schwann cells are the myelinating glial cells of the peripheral nervous system and establish myelin sheaths on large caliber axons in order to accelerate their electrical signal propagation. Apart from this well described function, these cells revealed to exhibit a high degree of differentiation plasticity as they were shown to re- and dedifferentiate upon injury and disease as well as to actively participate in regenerative- and inflammatory processes. This review focuses on the crosstalk between glial- and immune cells observed in many peripheral nerve pathologies and summarizes functional evidences of molecules, regulators and factors involved in this process. We summarize data on Schwann cell's role presenting antigens, on interactions with the complement system, on Schwann cell surface molecules/receptors and on secreted factors involved in immune cell interactions or para-/autocrine signaling events, thus strengthening the view for a broader (patho) physiological role of this cell lineage.

Sublytic complement protects prostate cancer cells from tumour necrosis factor-α-induced cell death

Inflammation is a critical component of tumour progression. Although complement and tumour necrosis factor (TNF)-α potentially exert significant anti-tumour effects, both mediators may also promote tumour progression. It has been demonstrated that sublytic complement confers resistance on tumour cells not only against lytic complement, but also other danger molecules such as perforin. In low concentrations, TNF promotes survival of malignant cells rather than exerting cytotoxic activity. In this study, we tested if sublytic complement is able to interfere with TNF-mediated tumour cell killing. Our results demonstrate that either subcytotoxic concentrations of TNF or sublytic complement rescue prostate carcinoma cells (DU145) from TNF-α-mediated cell death. Upon pretreatment with low-dose TNF-α, but not upon pre-exposure to sublytic complement, TNF resistance was associated with the down-regulation of TNF receptor 1 (TNF-R1) expression. Complement-induced protection against TNF-mediated apoptosis accompanied the induction of anti-apoptotic proteins [B cell leukaemia/lymphoma (Bcl)-2 and Bcl-xL] at an early stage followed by inhibition of the TNF-induced decrease in the amount of Bcl-2 and Bcl-xL. Cell protection also accompanied the inhibition of caspase-8 activation, poly (ADP-ribose) polymerase (PARP)-1 cleavage and the activation of nuclear factor (NF)-κB. Our data extend our current view on the induction of tumour cell resistance against cytotoxic mediators supporting the role of the tumour microenvironment in mediating protection against the anti-cancer immune response.

Cancer and the complement cascade

Despite significant research on the role of inflammation and immunosurveillance in the immunologic microenvironment of tumors, little attention has been given to the oncogenic capabilities of the complement cascade. The recent finding that complement may contribute to tumor growth suggests an insidious relationship between complement and cancer, especially in light of evidence that complement facilitates cellular proliferation and regeneration. We address the hypothesis that complement proteins promote carcinogenesis and suggest mechanisms by which complement can drive the fundamental features of cancer. Evidence shows that this diverse family of innate immune proteins facilitates dysregulation of mitogenic signaling pathways, sustained cellular proliferation, angiogenesis, insensitivity to apoptosis, invasion and migration, and escape from immunosurveillance. Given that the traditionally held functions for the complement system include innate immunity and cancer defense, our review suggests a new way of thinking about the role of complement proteins in neoplasia.

Implication of complement system and its regulators in Alzheimer's disease

Alzheimer’s disease (AD) is an age-related neurodegenerative disease that affects approximately 24 million people worldwide. A number of different risk factors have been implicated in AD, however, neuritic (amyloid) plaques are considered as one of the defining risk factors and pathological hallmarks of the disease. Complement proteins are integral components of amyloid plaques and cerebral vascular amyloid in Alzheimer brains. They can be found at the earliest stages of amyloid deposition and their activation coincides with the clinical expression of Alzheimer's dementia. This review emphasizes on the dual key roles of complement system and complement regulators (CRegs) in disease pathology and progression. The particular focus of this review is on currently evolving strategies for design of complement inhibitors that might aid therapy by restoring the fine balance between activated components of complement system, thus improving the cognitive performance of patients. This review discusses these issues with a view to inspiring the development of new agents that could be useful for the treatment of AD.

Complement and the central nervous system: emerging roles in development, protection and regeneration

As expanding research reveals the novel ability of complement proteins to promote proliferation and regeneration of tissues throughout the body, the concept of the complement cascade as an innate immune effector has changed rapidly. In particular, its interactions with the central nervous system have provided a wealth of information regarding the ability of complement proteins to mediate neurogenesis, synaptogenesis, cell migration, neuroprotection, proliferation and regeneration. At numerous phases of the neuronal and glial cell cycle, complement proteins exert direct or indirect influence over their behavior and fate. Neuronal stem cells differentiate and migrate in response to complement, and it prevents injury and death in adult cells in response to toxic agents. Furthermore, complement proteins promote survival via anti-apoptotic actions, and can facilitate clearance and regeneration of injured tissues in various models of CNS disease. In summary, we highlight the protean abilities of complement proteins in the central nervous system, underscoring an exciting avenue of research that has yielded greater understanding of complement's role in central nervous system health and disease.

Anti-GD(2) with an FC point mutation reduces complement fixation and decreases antibody-induced allodynia

Monoclonal antibodies against GD(2) ganglioside, such as ch14.18, the human-mouse chimeric antibody, have been shown to be effective for the treatment of neuroblastoma. However, treatment is associated with generalized, relatively opiate-resistant pain. We investigated if a point mutation in ch14.18 antibody (hu14.18K332A) to limit complement-dependent cytotoxicity (CDC) would ameliorate the pain behavior, while preserving antibody-dependent cellular cytotoxicity (ADCC). In vitro, CDC and ADCC were measured using europium-TDA assay. In vivo, allodynia was evaluated by measuring thresholds to von Frey filaments applied to the hindpaws after injection of either ch14.18 or hu14.18K332 into wild type rats or rats with deficient complement factor 6. Other rats were pretreated with complement factor C5a receptor antagonist and tested following ch14.18 injection. The mutation reduces the antibody's ability to activate complement, while maintaining its ADCC capabilities. Injection of hu14.18K322 (1 or 3mg/kg) produced faster resolving allodynia than that engendered by ch14.18 (1mg/kg). Injection of ch14.18 (1mg/kg) into rats with C6 complement deficiency further reduced antibody-induced allodynia, while pre-treatment with complement factor C5a receptor antagonist completely abolished ch14.18-induced allodynia. These findings showed that mutant hu14.18 K322 elicited less allodynia than ch14.18 and that ch14.18-elicited allodynia is due to activation of the complement cascade: in part, to formation of membrane attack complex, but more importantly to release of complement factor C5a. Development of immunotherapeutic agents with decreased complement-dependent lysis while maintaining cellular cytotoxicity may offer treatment options with reduced adverse side effects, thereby allowing dose escalation of therapeutic antibodies.

Innate immunity in the nervous system

The complement (C) system plays a central role in innate immunity and bridges innate and adaptive immune responses. A fine balance of C activation and regulation mediates the elimination of invading pathogens and the protection of the host from excessive C deposition on healthy tissues. If this delicate balance is disrupted, the C system may cause injury and contribute to the pathogenesis of various diseases, including neurodegenerative disorders and neuropathies. Here we review evidence indicating that C factors and regulators are locally synthesized in the nervous system and we discuss the evidence supporting the protective or detrimental role of C activation in health, injury, and disease of the nerve.

Fingolimod and related compounds in a spontaneous autoimmune polyneuropathy

We investigated potential therapeutic effects of sphingosine-1-phosphate (S1P) receptor modulators FTY720 (fingolimod) and selective S1P1 agonist SEW2871 on a spontaneous autoimmune polyneuropathy (SAP) when given orally at 7 mo (anticipated disease onset) for 4 weeks. Clinical severity, electrophysiologic and histological findings were ameliorated in mice treated with 1 mg/kg of FTY720. Subsequent studies showed that SEW2871 was also effective in halting the progression of SAP, which was accompanied by decreased proliferative and cytokine responses to myelin protein zero (P0), and an increase in regulatory T cells. We conclude that S1P receptor modulators may play a therapeutic role in autoimmune neuropathies.

Response gene to complement 32 is required for C5b-9 induced cell cycle activation in endothelial cells

Proliferation of vascular endothelial cells (EC) and smooth muscle cells (SMC) is a critical event in angiogenesis and atherosclerosis. We previously showed that the C5b-9 assembly during complement activation induces cell cycle in human aortic EC (AEC) and SMC. C5b-9 can induce the expression of Response Gene to Complement (RGC)-32 and over expression of this gene leads to cell cycle activation. Therefore, the present study was carried out to test the requirement of endogenous RGC-32 for the cell cycle activation induced by C5b-9 by knocking-down its expression using siRNA. We identified two RGC-32 siRNAs that can markedly reduce the expression of RGC-32 mRNA in AEC. RGC-32 silencing in these cells abolished DNA synthesis induced by C5b-9 and serum growth factors, indicating the requirement of RGC-32 activity for S-phase entry. RGC-32 siRNA knockdown also significantly reduced the C5b-9 induced CDC2 activation and Akt phosphorylation. CDC2 does not play a role in G1/S transition in HeLa cells stably overexpressing RGC-32. RGC-32 was found to physically associate with Akt and was phosphorylated by Akt in vitro. Mutation of RGC-32 protein at Ser 45 and Ser 47 prevented Akt mediated phosphorylation. In addition, RGC-32 was found to regulate the release of growth factors from AEC. All these data together suggest that cell cycle induction by C5b-9 in AEC is RGC-32 dependent and this is in part through regulation of Akt and growth factor release.

Complement activation in the Parkinson's disease substantia nigra: an immunocytochemical study

BACKGROUND

Inflammatory processes are increased in the Parkinson's disease (PD) brain. The long-term use of nonsteroidal anti-inflammatory drugs has been associated, in retrospective studies, with decreased risk for PD, suggesting that inflammation may contribute to development of this disorder. The objective of this study was to determine the extent of complement activation, a major inflammatory mechanism, in PD.

METHODS

Substantia nigra specimens from young normal subjects (n = 11-13), aged normal subjects (n = 24-28), and subjects with PD (n = 19-20), Alzheimer's disease (AD; n = 12-13), and dementia with Lewy bodies (DLB; n = 9) were stained for iC3b and C9, representing early- and late-stage complement activation, respectively. Numbers of iC3b+, C9+, and total melanized neurons in each section were counted in a blinded fashion. Nonparametric analyses were used to evaluate differences between groups and to evaluate correlations between complement staining, numbers of melanized neurons, and the duration of PD.

RESULTS

Lewy bodies in both PD and DLB specimens stained for iC3b and C9. Staining was also prominent on melanized neurons. The percentage of iC3b+ neurons was significantly increased in PD vs. aged normal and AD specimens, and in young normal vs. aged normal specimens. C9 immunoreactivity was significantly increased in PD vs. AD specimens, but unlike iC3b, the increased C9 staining in PD and young normal specimens did not achieve statistical significance vs. aged normal specimens. iC3b and C9 staining in PD specimens was not correlated with the numbers of remaining melanized neurons, nor with the duration of PD.

CONCLUSION

Complement activation occurs on Lewy bodies and melanized neurons in the PD substantia nigra. Early complement activation (iC3b) is increased on melanized neurons in PD vs. aged normal specimens, and late-stage complement activation (C9) also tends to increase. This latter finding suggests that complement activation may contribute to loss of dopaminergic neurons in some individuals with PD. Complement activation on melanized neurons appears to decrease with normal aging, suggesting a possible neuroprotective role for this process in the normal substantia nigra.

C5b-9-induced endothelial cell proliferation and migration are dependent on Akt inactivation of forkhead transcription factor FOXO1

Migration and proliferation of aortic endothelial cells (AEC) are critical processes involved in angiogenesis, atherosclerosis, and postangioplasty restenosis. Activation of complement and assembly of the C5b-9 complement complex have been implicated in the pre-lesional stage of atherogenesis and progression of the atherosclerotic lesion. We have shown that C5b-9 induces proliferation and activates phosphatidylinositol 3-kinase (PI3K), but it is unknown whether this can lead to activation of Akt in AEC, a major downstream target of PI3K, or if C5b-9 can induce the migration of AEC, a critical step in angiogenesis. In this study, we show that C5b-9 induces AEC proliferation and migration and also activates the PI3K/Akt pathway. C5b-9 activates Akt as shown by in vitro kinase assay and phosphorylation of Ser-473. C5b-9-induced cell cycle activation was inhibited by pretreatment with LY294002 (PI3K inhibitor), SH-5 (Akt inhibitor), or transfection with Akt siRNA. These data suggests that the PI3K/Akt pathway is required for C5b-9-induced cell cycle activation. FOXO1, a member of forkhead transcription factor family, was phosphorylated at Ser-256 and inactivated after C5b-9 stimulation as shown by a decrease in DNA binding and cytoplasmic relocalization. Cytoplasmic relocalization was significantly reduced after pretreatment with LY294002, SH-5, or transfection with Akt siRNA. Silencing FOXO1 expression using siRNA stimulated AEC proliferation and regulated angiogenic factor release. Our data indicate that C5b-9 regulation of the cell cycle activation in AEC through Akt pathway is dependent on inactivation of FOXO1.

JNK1 activation mediates C5b-9-induced P0 mRNA instability and P0 gene expression in Schwann cells

The protein zero (P0) glycoprotein is an important component of compact peripheral nerve myelin produced by the glial cells of the mammalian peripheral nervous system. P0 mRNA expression is reduced following exposure of Schwann cells to sublytic C5b-9, the terminal activation complex of the complement cascade. Sublytic complement treatment decreased P0 mRNA by 81% within 6 h and required C5b-9 assembly. C5b-9 induced a threefold increase in both JNK1 activity and c-jun mRNA within 20 and 30 min, respectively, compared with cells treated with either human serum depleted of complement component C7 (C7dHS) or medium alone. Sublytic C5b-9 stimulation, in the presence of the transcription inhibitor Actinomycin D, decreased P0 mRNA expression by 52%, indicating that mRNA was selectively destabilized. This effect was prevented by pretreatment with L-JNK inhibitor 1 (L-JNKI1). To study a potential inhibition of P0 gene transcription, we transfected Schwann cells with a P0 promoter-firefly luciferase construct. Sublytic C5b-9 stimulation of the transfected cells decreased luciferase activity by 82% at 6 h, and this effect was prevented by pretreatment with L-JNKI1 inhibitor. Our results indicate that the ability of C5b-9 in vitro to affect P0 gene expression is mediated via JNK1 activation that leads to enhanced mRNA decay and transcriptional repression of P0.

Intrinsic Resistance of Hepatocytes to Complement-Mediated Injury

When activated on or in the vicinity of cells, complement usually causes loss of function and sometimes cell death. Yet the liver, which produces large amounts of complement proteins, clears activators of complement and activated complexes from portal blood without obvious injury or impaired function. We asked whether and to what extent hepatocytes resist injury and loss of function mediated by exposure to complement. Using cells isolated from porcine livers as a model system, we found that, in contrast to endothelial cells, hepatocytes profoundly resist complement-mediated lysis and exhibit normal synthetic and conjugative functions when complement is activated on their surface. The resistance of hepatocytes to complement-mediated injury was not a function of cell surface control of the complement cascade but rather an intrinsic resistance of the cells dependent on the PI3K/Akt pathway. The resistance of hepatocytes to complement might be exploited in developing approaches to the treatment of hepatic failure or more broadly to the treatment of complement-mediated disease.

The terminal complement complex inhibits apoptosis in vascular smooth muscle cells by activating an autocrine IGF-1 loop

Two counteracting processes determine accumulation of human vascular smooth muscle cells (SMCs) in atherosclerotic lesions: cell proliferation and apoptosis. SMCs synthesize insulin-like growth factor-1 (IGF-1), which potently inhibits apoptosis. The terminal complement complex C5b-9 interacts with SMCs in early human atherogenesis. In this study, we investigated whether C5b-9 may activate the IGF-1 system in SMCs, resulting in the inhibition of SMC apoptosis. C5b-9 generation on SMCs in vitro markedly reduced CD95-mediated apoptosis as assessed by flowcytometric analysis of annexin V binding and in caspase 3 assays. C5b-9 induced both significant IGF-1 release and up-regulation of IGF-1 binding sites in SMCs. Immunoneutralization of IGF-1 with a monoclonal IGF-1 antibody abolished the antiapoptotic effects of C5b-9. We conclude that C5b-9 inhibits apoptosis in SMCs by inducing an autocrine IGF-1 loop. This mechanism may contribute to the accumulation of SMCs in early human atherosclerotic lesions.