[Show abstract][Hide abstract] ABSTRACT: Vascular endothelial growth factor (VEGF) is a regulator of vascularization in development and is a key growth factor in tissue repair. In disease, VEGF contributes to vascularization of solid tumors and arthritic joints. This study examines the role of the mRNA-binding protein AUF1/heterogeneous nuclear ribonucleoprotein D (AUF1) in VEGF gene expression. We show that overexpression of AUF1 in mouse macrophage-like RAW-264.7 cells suppresses endogenous VEGF protein levels. To study 3' untranslated region (UTR)-mediated regulation, we introduced the 3' UTR of VEGF mRNA into a luciferase reporter gene. Coexpression of AUF1 represses VEGF-3' UTR reporter expression in RAW-264.7 cells and in mouse bone marrow-derived macrophages. The C-terminus of AUF1 contains arginine-glycine-glycine (RGG) repeat motifs that are dimethylated. Deletion of the RGG domain of AUF1 eliminated the repressive effects of AUF1. Surprisingly, expression of an AUF1-RGG peptide reduced endogenous VEGF protein levels and repressed VEGF-3' UTR reporter activity in RAW-264.7 cells. These findings demonstrate that AUF1 regulates VEGF expression, and this study identifies an RGG peptide that suppresses VEGF gene expression.
Molecular biology of the cell 02/2012; 23(8):1414-22. DOI:10.1091/mbc.E11-06-0545 · 4.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In Sjögren's syndrome, keratoconjunctivitis sicca (dry eye) is associated with infiltration of lacrimal glands by leukocytes and consequent losses of tear-fluid production and the integrity of the ocular surface. We investigated the effect of blockade of the lymphotoxin-beta receptor (LTBR) pathway on lacrimal-gland pathology in the NOD mouse model of Sjögren's syndrome.
Male NOD mice were treated for up to ten weeks with an antagonist, LTBR-Ig, or control mouse antibody MOPC-21. Extra-orbital lacrimal glands were analyzed by immunohistochemistry for high endothelial venules (HEV), by Affymetrix gene-array analysis and real-time PCR for differential gene expression, and by ELISA for CXCL13 protein. Leukocytes from lacrimal glands were analyzed by flow-cytometry. Tear-fluid secretion-rates were measured and the integrity of the ocular surface was scored using slit-lamp microscopy and fluorescein isothiocyanate (FITC) staining. The chemokine CXCL13 was measured by ELISA in sera from Sjögren's syndrome patients (n = 27) and healthy controls (n = 30). Statistical analysis was by the two-tailed, unpaired T-test, or the Mann-Whitney-test for ocular integrity scores.
LTBR blockade for eight weeks reduced B-cell accumulation (approximately 5-fold), eliminated HEV in lacrimal glands, and reduced the entry rate of lymphocytes into lacrimal glands. Affymetrix-chip analysis revealed numerous changes in mRNA expression due to LTBR blockade, including reduction of homeostatic chemokine expression. The reduction of CXCL13, CCL21, CCL19 mRNA and the HEV-associated gene GLYCAM-1 was confirmed by PCR analysis. CXCL13 protein increased with disease progression in lacrimal-gland homogenates, but after LTBR blockade for 8 weeks, CXCL13 was reduced approximately 6-fold to 8.4 pg/mg (+/- 2.7) from 51 pg/mg (+/-5.3) in lacrimal glands of 16 week old control mice. Mice given LTBR blockade exhibited an approximately two-fold greater tear-fluid secretion than control mice (P = 0.001), and had a significantly improved ocular surface integrity score (P = 0.005). The mean CXCL13 concentration in sera from Sjögren's patients (n = 27) was 170 pg/ml, compared to 92.0 pg/ml for sera from (n = 30) healthy controls (P = 0.01).
Blockade of LTBR pathways may have therapeutic potential for treatment of Sjögren's syndrome.
[Show abstract][Hide abstract] ABSTRACT: The immunoglobulin (Ig) superfamily consists of many critical immune regulators, including the B7 family ligands and receptors. In this study, we identify a novel and structurally distinct Ig superfamily inhibitory ligand, whose extracellular domain bears homology to the B7 family ligand PD-L1. This molecule is designated V-domain Ig suppressor of T cell activation (VISTA). VISTA is primarily expressed on hematopoietic cells, and VISTA expression is highly regulated on myeloid antigen-presenting cells (APCs) and T cells. A soluble VISTA-Ig fusion protein or VISTA expression on APCs inhibits T cell proliferation and cytokine production in vitro. A VISTA-specific monoclonal antibody interferes with VISTA-induced suppression of T cell responses by VISTA-expressing APCs in vitro. Furthermore, anti-VISTA treatment exacerbates the development of the T cell-mediated autoimmune disease experimental autoimmune encephalomyelitis in mice. Finally, VISTA overexpression on tumor cells interferes with protective antitumor immunity in vivo in mice. These findings show that VISTA, a novel immunoregulatory molecule, has functional activities that are nonredundant with other Ig superfamily members and may play a role in the development of autoimmunity and immune surveillance in cancer.
Journal of Experimental Medicine 03/2011; 208(3):577-92. DOI:10.1084/jem.20100619 · 12.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Given the importance of IgG Fc receptors in immune regulation, we hypothesized that Fcg receptor type III (FcgRIII, CD16) plays an important role in atherogenesis. We therefore analysed the formation of arterial lesions in LDL receptor-deficient (LDLR(-/-)) and FcgRIII(-/-)xLDLR(-/-) double knockout mice at three different points up to 24 weeks of exposure to a high-fat diet.
Analysis of Oil Red-O-stained sections revealed no difference in lesion formation between strains after 6 weeks of a high-fat diet, and a modest decrease after 14 weeks in double knockouts relative to LDLR(-/-) controls. After 24 weeks, lesion formation was decreased in the aortic root (30%) and innominate artery (50%) in FcgRIII double knockouts relative to LDLR(-/-) controls. Analysis of peripheral CD4+ T-cells by intracellular flow cytometry from double knockouts after 24 weeks of a high-fat diet revealed statistically significant increases in the percentages of cells producing interferon-gamma, interleukin (IL)-10, and IL-4 relative to controls, differences that were also observed by analyses of whole aortas for cytokine mRNA levels. As determined by flow cytometry, FcgRIII deficiency resulted in an expansion of CD4+ cells and an increase in the CD4 to CD8 ratio. Analysis of plasma anti-oxidized LDL (OxLDL) antibodies by chemiluminescent assay revealed that IgG1 and IgG2c titers to OxLDL were increased in FcgRIII (-/-)xLDLR(-/-) double knockouts relative to LDLR(-/-) controls, while total IgG levels were similar.
These results reveal altered immunity in FcgRIII(-/-)xLDLR(-/-) mice and a reduction in lesion formation associated with increased production of IL-10 by an expansion of CD4+ T-cells. The reduction in lesion formation was manifest well after evidence of an immune response to OxLDL, suggesting that FcgRIII contributes to lesion progression in murine atherosclerosis.
Cardiovascular Research 09/2009; 85(1):224-31. DOI:10.1093/cvr/cvp300 · 5.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The lymphotoxin-beta receptor (LTbetaR) pathway is important in the development and maintenance of lymphoid structures. Blocking this pathway has proven beneficial in murine models of autoimmune diseases such as diabetes and rheumatoid arthritis. The aim of this study was to determine the effects of LTbetaR pathway blockade on Sjögren syndrome (SS)-like salivary gland disease in non-obese diabetic (NOD) mice.
The course of SS-like disease was followed in NOD mice that were given lymphotoxin-beta receptor-immunoglobulin fusion protein (LTbetaR-Ig) starting at 9 weeks of age. Treatment was given as a single weekly dose for 3, 7, or 10 weeks. Age-matched NOD mice treated with mouse monoclonal IgG1, or not treated at all, were used as controls. The severity of inflammation, cellular composition, and lymphoid neogenesis in the submandibular glands were determined by immunohistochemistry. Mandibular lymph nodes were also studied. Saliva flow rates were measured, and saliva was analyzed by a multiplex cytokine assay. The salivary glands were analyzed for CXCL13, CCL19, and CCL21 gene expression by quantitative polymerase chain reaction.
Treatment with LTbetaR-Ig prevented the increase in size and number of focal infiltrates normally observed in this SS-like disease. Compared with the controls, the submandibular glands of LTbetaR-Ig-treated mice had fewer and smaller T- and B-cell zones and fewer high endothelial venules per given salivary gland area. Follicular dendritic cell networks were lost in LTbetaR-Ig-treated mice. CCL19 expression was also dramatically inhibited in the salivary gland infiltrates. Draining lymph nodes showed more gradual changes after LTbetaR-Ig treatment. Saliva flow was partially restored in mice treated with 10 LTbetaR-Ig weekly injections, and the saliva cytokine profile of these mice resembled that of mice in the pre-disease state.
Our findings show that blocking the LTbetaR pathway results in ablation of the lymphoid organization in the NOD salivary glands and thus an improvement in salivary gland function.
[Show abstract][Hide abstract] ABSTRACT: Stat5 proteins are critical signaling molecules activated by many cytokines. Within the immune system, Stat5 plays important roles related to the development of thymocytes and proliferation of T cells. Stat5 has been implicated in malignant transformation, and moreover, the activated tyrosine phosphorylated form of Stat5 is frequently observed in human lymphomas. We previously demonstrated the oncogenic potential of Stat5, with thymic lymphoblastic lymphomas developing in a significant proportion of transgenic (TG) mice overexpressing Stat5a or Stat5b in lymphocytes. In addition, immunization or expression of a T-cell receptor (TCR) transgene augmented the rate of tumor formation. Here, we investigate the mechanism of Stat5-mediated lymphomagenesis by exploring the contributions of major histocompatibility complex (MHC)/TCR and pre-TCR signals. We present data demonstrating that Stat5b TG mice unexpectedly develop CD8(+) lymphoma even in the absence of either pre-TCR signaling or normal thymic selection. Indeed, acceleration of Stat5b transgene-mediated lymphoma occurred on TCRalpha(-/-) and pre-TCRalpha(-/-) backgrounds. In light of these data, we propose a model in which alterations in T-cell development at the double-negative/double-positive (DN/DP) stages cooperate with cytokine-mediated pathways in immature thymocytes to give rise to lymphoblastic T-cell lymphomas in Stat5b TG mice.
[Show abstract][Hide abstract] ABSTRACT: Receptor activator of nuclear factor-kappaB ligand (RANKL) promotes osteoclast differentiation from monocyte precursors by inducing a cohort of genes, including tartrate-resistant acid phosphatase (TRAP) and matrix metalloproteinase-9 (MMP-9). A family of synthetic triterpenoids with antiinflammatory and pro-apoptotic properties was described to modulate differentiation in monocytic cell lineages. We therefore investigated the ability of the potent and bioavailable synthetic triterpenoid TP-222 to inhibit RANKL-induced osteoclast formation and MMP-9 expression from monocytic precursor cells.
Osteoclast formation was assayed by staining for TRAP-positive multinucleated cells. MMP-9 expression was measured by quantitative RT-PCR, Western blot, immunohistochemistry, and gel zymography. In vivo effects of TP-222 were assessed by daily intraperitoneal injection of 4-week-old mice for 7 days followed by measurement of osteoclast number and MMP-9 expression at the cartilage/bone junction of the epiphyseal growth plate.
RANKL promoted and TP-222 (300 nM) inhibited osteoclast formation in cultures of RAW264.7 cells or bone marrow-derived monocytes. RANKL also induced MMP-9 expression in RAW264.7 cells and this was reduced by concurrent or subsequent addition of TP-222. TP-222 treatment significantly reduced the mean number of osteoclasts present at the cartilage/bone interface compared to vehicle-injected control mice. Morphometric analyses of tissue sections showed that TP-222 treatment reduced the amount of immunoreactive MMP-9 present in both mononucleated pre-osteoclasts and osteoclasts.
Our data demonstrate that TP-222 inhibits osteoclast formation and MMP-9 expression in vitro and in vivo, and suggest that triterpenoids may be useful compounds for modulating bone resorption diseases.
The Journal of Rheumatology 06/2007; 34(5):1058-68. · 3.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The lymphotoxin axis is important for the maintenance of several specialized lymphoid microenvironments in secondary lymphoid tissue. Lymphoid-tissue architecture is highly plastic and requires continual homeostatic signaling to maintain its basal functional state. The cellularity of lymph nodes in adult mice was reduced by systemic blockade of lymphotoxin-beta receptor (LTbeta R) signaling with a soluble decoy receptor both in resting and reactive settings. This reduction in cellularity resulted from greatly impaired lymphocyte entry into lymph nodes due to decreased levels of peripheral lymph node addressing (PNAd) and MAdCAM on high endothelial venules (HEV). LTbeta R signaling was required to maintain normal levels of RNA expression of MAdCAM, and also of PNAd by regulating the expression of key enzymes and scaffold proteins required for its assembly. Thus, the homeostatic maintenance of functional HEV status in adult mice relies largely on LTbeta R signaling.
[Show abstract][Hide abstract] ABSTRACT: A lymphotoxin-beta (LTbeta) receptor-Ig fusion protein (LTbetaR-Ig) was used to evaluate the importance of the lymphotoxin/LIGHT axis in the development and perpetuation of arthritis. Prophylactic treatment with the inhibitor protein LTbetaR-Ig blocked the induction of collagen-induced arthritis in mice and adjuvant arthritis in Lewis rats. Treatment of mice with established collagen-induced arthritis reduced the severity of arthritic symptoms and joint tissue damage. However, in a passive model of anti-collagen Ab-triggered arthritis, joint inflammation was not affected by LTbetaR-Ig treatment precluding LT/LIGHT involvement in the very terminal immune complex/complement/FcR-mediated effector phase. Collagen-II and Mycobacterium-specific T cell responses were not impaired, yet there was evidence that the overall response to the mycobacterium was blunted. Serum titers of anti-collagen-II Abs were reduced especially during the late phase of disease. Treatment with LTbetaR-Ig ablated follicular dendritic cell networks in the draining lymph nodes, suggesting that impaired class switching and affinity maturation may have led to a decreased level of pathological autoantibodies. These data are consistent with a model in which the LT/LIGHT axis controls microenvironments in the draining lymph nodes. These environments are critical in shaping the adjuvant-driven initiating events that impact the subsequent quality of the anti-collagen response in the later phases. Consequently, blockade of the LT/LIGHT axis may represent a novel approach to the treatment of autoimmune diseases such as rheumatoid arthritis that involve both T cell and Ab components.
The Journal of Immunology 08/2003; 171(1):115-26. DOI:10.4049/jimmunol.171.1.115 · 4.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The therapeutic potential of placental growth factor (PlGF) and its receptor Flt1 in angiogenesis is poorly understood. Here, we report that PlGF stimulated angiogenesis and collateral growth in ischemic heart and limb with at least a comparable efficiency to vascular endothelial growth factor (VEGF). An antibody against Flt1 suppressed neovascularization in tumors and ischemic retina, and angiogenesis and inflammatory joint destruction in autoimmune arthritis. Anti-Flt1 also reduced atherosclerotic plaque growth and vulnerability, but the atheroprotective effect was not attributable to reduced plaque neovascularization. Inhibition of VEGF receptor Flk1 did not affect arthritis or atherosclerosis, indicating that inhibition of Flk1-driven angiogenesis alone was not sufficient to halt disease progression. The anti-inflammatory effects of anti-Flt1 were attributable to reduced mobilization of bone marrow-derived myeloid progenitors into the peripheral blood; impaired infiltration of Flt1-expressing leukocytes in inflamed tissues; and defective activation of myeloid cells. Thus, PlGF and Flt1 constitute potential candidates for therapeutic modulation of angiogenesis and inflammation.
Nature Medicine 09/2002; 8(8):831-40. DOI:10.1038/nm731 · 27.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The zinc-finger protein tristetraprolin (TTP) has been demonstrated to regulate tumor necrosis factor alpha (TNFalpha) messenger RNA (mRNA) instability in murine macrophages. We sought to develop a model system to characterize the effects of human TTP (hTTP) on TNFalpha 3'-untranslated region (3'-UTR)-mediated expression. We also generated a specific polyclonal antibody against hTTP that enabled the examination of the subcellular distribution of hTTP and its RNA binding in vivo.
Transfection of reporter gene constructs were used to functionally characterize the role of hTTP in regulating TNFalpha expression in a 3'-UTR-dependent manner. An immunoprecipitation reverse transcription-polymerase chain reaction technique, immunoblotting, immunocytochemistry, and sucrose density fractionation were used to identify and localize hTTP.
We found that hTTP interacted with human TNFalpha mRNA in the cytoplasm. The presence of the TNFalpha 3'-UTR was sufficient to confer binding by TTP in vivo. This interaction resulted in reduced luciferase reporter gene activity in a TNFalpha 3'-UTR adenine-uridine-rich element (ARE)-dependent manner. Immunoblotting and immunocytochemistry indicated that endogenous and transfected hTTP localized to the cytoplasm. Results of sucrose density fractionation studies were consistent with a polysomal location of hTTP. In rheumatoid synovium, hTTP expression was restricted to cells in the synovial lining layers.
Through the development of an antiserum specific for hTTP, we have been able to demonstrate that hTTP binds specifically to the TNFalpha 3'-UTR and reduces reporter gene expression in an ARE-specific manner. These studies establish that hTTP is likely to function in a similar, if not identical manner, in the posttranscriptional regulation of TNFalpha. Understanding the posttranscriptional regulation of TNFalpha biosynthesis is important for the development of novel treatment strategies in rheumatoid arthritis.
[Show abstract][Hide abstract] ABSTRACT: Vascular endothelial growth factor (VEGF) is abundant in synovium and synovial fluids, where it probably contributes to vascular permeability and angiogenesis in arthritic joints. To investigate the probable sources of VEGF in synovium, we compared the ability of several cytokines (TGF-beta, platelet-derived growth factor (PDGF), IL-1, tumour necrosis factor (TNF), basic fibroblast growth factor (bFGF) that are associated with arthritis and angiogenesis, to stimulate secretion of VEGF protein by human synovial fibroblasts. TGF-beta was the strongest inducer of VEGF secretion; six times more VEGF was secreted when cells were stimulated by TGF-beta than when stimulated by PDGF or IL-1 for 24 h. TNF-alpha and bFGF did not stimulate any secretion of VEGF. The stimulatory effects of TGF-beta and IL-1 on VEGF secretion were additive. Hypoxic culture alone also stimulated VEGF secretion, but more importantly, hypoxic culture conditions doubled the rate of VEGF secretion stimulated by the cytokines TGF-beta and IL-1. When dermal and synovial fibroblasts were stimulated identically by hypoxia and cytokines (TGF-beta and IL-1), synovial fibroblasts secreted four times more VEGF than did dermal fibroblasts. Thus in rheumatoid arthritis, the capacity of synovial fibroblasts in the hypoxic environment to secrete large amounts of VEGF in response to cytokines such as TGF-beta probably contributes significantly to angiogenesis in the synovium.
[Show abstract][Hide abstract] ABSTRACT: Collagen-induced arthritis (CIA) is an animal model for the human autoimmune disease rheumatoid arthritis (RA). CIA can be induced in several species including primates by immunization with heterologous type-II collagen (CII). Polyclonal antibodies are formed upon immunization with CII that exhibit a broad range of epitope specificities (some that cross-react with hose CII); however, only antibodies directed against certain specific epitopes on CII are arthritogenic. Recently, the importance of cognate interactions between T-cells and B-cells to the induction of CIA was demonstrated by administration of monoclonal antibodies against a T-cell surface protein, gp39. Blocking the interaction of T-cell gp39, with its receptor/ligand on the surface of B-cells (CD40), completely blocked induction of CIA in mice. A concomitant reduction in the level of anti-CII IgG produced in anti-gp39-treated animals was observed, demonstrating the crucial importance of T-cell:B-cell interactions via gp39:CD-40 binding to the primary immune response to CII in vivo and therefore to the induction of CIA. Other features of CIA are important in elucidating the condition and this article will deal with some important issues.
Clinical Immunology and Immunopathology 11/1994; 73(1):11-8. DOI:10.1006/clin.1994.1164
[Show abstract][Hide abstract] ABSTRACT: Vascular permeability factor (VPF, also known as vascular endothelial growth factor or VEGF), is a potent microvascular permeability enhancing cytokine and a selective mitogen for endothelial cells. It has been implicated in tumor angiogenesis and ascites fluid accumulation. Since development of the destructive synovial pannus in rheumatoid arthritis (RA) is associated with changes in vascular permeability (synovial fluid accumulation), synovial cell hyperplasia, and angiogenesis, we examined synovial fluids (SFs) and joint tissue for the expression and local accumulation of VPF/VEGF. VPF/VEGF was detected in all of 21 synovial fluids examined and when measured by an immunofluorimetric assay, ranged from 6.9 to 180.5 pM. These levels are biologically significant, since < 1 pM VPF/VEGF can elicit responses from its target cells, endothelial cells. Levels of VPF/VEGF were highest in rheumatoid arthritis fluids (n = 10), with a mean value (+/- SEM) of 59.1 +/- 18.0 pM, vs. 21.4 +/- 2.3 pM for 11 SFs from patients with other forms of arthritis (p = 0.042). In situ hybridization studies that were performed on joint tissues from patients with active RA revealed that synovial lining macrophages strongly expressed VPF/VEGF mRNA, and that microvascular endothelial cells of nearby blood vessels strongly expressed mRNA for the VPF/VEGF receptors, flt-1 and KDR. Immunohistochemistry performed on inflamed rheumatoid synovial tissue revealed that the VPF/VEGF peptide was localized to macrophages within inflamed synovium, as well as to microvascular endothelium, its putative target in the tissue. Together, these findings indicate that VPF/VEGF may have an important role in the pathogenesis of RA.
Journal of Experimental Medicine 07/1994; 180(1):341-6. · 12.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Studies have implicated tumour necrosis factor-alpha (TNF-alpha) in type-II collagen (CII)-induced arthritis (CIA), a well established animal model of human rheumatoid arthritis. Precisely how TNF is involved in CIA is not yet clear. In this study the effects of TNF on CIA were examined, independent of its potential effects on the immune response, by performing peri-articular injection of TNF in combination with passive immunization of rats. A sub-arthritic dose (5 mg) of affinity-purified anti-CII IgG, which alone was insufficient to induce spontaneous clinical arthritis, was used throughout the study. Obvious clinical arthritis that persisted for several days was rapidly induced by injections of 100 ng TNF into hindpaws of rats that were passively immunized shortly before the TNF injection. Injections of TNF in non-immunized control rats did not induce clinical arthritis, nor did buffer-only injections in passively immunized controls. The clinical arthritic response was a local phenomenon, limited only to the TNF-injected hindpaws. No swelling was observed in the opposite, buffer-injected hindpaws, indicating the effects of TNF were not systemic. Depletion of peripheral blood phagocytes with anti-rat neutrophil antiserum before passive immunization completely abolished the ability of TNF to induce clinical arthritis, identifying phagocytic cells as the essential target cells in evoking this arthritic response. A role for complement activation was also demonstrated in this model through the use of a soluble recombinant version of CD35, the cell surface complement receptor type-1 (sCR1, BRL55730), which significantly reduced TNF-induced arthritis in phagocyte-replete rats.
[Show abstract][Hide abstract] ABSTRACT: Annexin-1 (also called lipocortin-1 or p35), a putative substrate of the epidermal growth factor/receptor kinase, protein kinase C, and transglutaminase, was immunolocalized in embryonic, neonatal, adult, and diseased human epidermis. In embryonic skin intense annexin-1 immunoreactivity was found in the periderm at 54 d estimated gestational age (EGA). Later (EGA = 91-143 d), annexin-1 immunoreactivity was restricted to basal keratinocytes. In neonatal skin, basal cells were often more heavily stained than were suprabasal keratinocytes, which were also stained. Only basal keratinocytes stained in adult plantar skin, but in thin skin annexin-1 was present in the basal, suprabasal, and sometimes even in the granular layers of the epidermis. Often, annexin-1 appeared concentrated around the perimeter of cells, especially tonofilament/desmosome-rich keratinocytes of the spinous-cell layer. At high magnification, annexin-1 appeared associated with distinct structures and was very granular in appearance in the intensely stained ductal keratinocytes of eccrine sweat glands, cells that are very highly enriched in keratin tonofilaments. This striking distribution in certain keratinocytes enriched in tonofilaments suggests a role for annexin-1 in cytoskeletal functions.