BACKGROUND:Latent TGFβ binding protein-2 (LTBP2) is a fibrillin 1 binding component of the microfibril. LTBP2 is the only LTBP protein that does not bind any isoforms of TGFβ, although it may interfere with the function of other LTBPs or interact with other signaling pathways. RESULTS:Here, we investigate mice lacking Ltbp2 (Ltbp2-/- ) and identify multiple phenotypes that impact bodyweight and fat mass, and affect bone and skin development. The alterations in skin and bone development are particularly noteworthy since the strength of these tissues is differentially affected by loss of Ltbp2. Interestingly, some tissues that express high levels of Ltbp2, such as the aorta and lung, do not have a developmental or homeostatic phenotype. CONCLUSIONS:Analysis of these mice show that LTBP2 has complex effects on development through direct effects on the extracellular matrix (ECM) or on signaling pathways that are known to regulate the ECM.

Abdominal aortic aneurysms (AAAs) represent a multifactorial, proteolytic disorder involving disintegration of the matrix structure within the AAA wall. Intrinsic deficiency of adult vascular cells to regenerate and repair the wall elastic matrix, which contributes to vessel stretch and recoil, is a major clinical challenge to therapeutic reversal of AAA growth. In this study, we investigate the involvement of epidermal growth factor receptor-mitogen activated protein kinase (EGFR-MAPK) pathway in the activation of aneurysmal smooth muscle cells (SMCs) by neutrophil elastase, and how EGFR can be targeted for elastic matrix regeneration. We have demonstrated that neutrophil elastase activates EGFR and downregulates expression level of key elastin homeostasis genes (elastin, crosslinking enzyme-lysyl oxidase, and fibulin4) between a dose range of 1-10 mu g/mL (p < 0.05). It also incites downstream proteolytic outcomes by upregulating p-extracellular signal-regulated kinase (ERK)1/2 (p < 0.0001) and matrix metalloprotease 2 (MMP2) at a protein level, which is significantly downregulated upon EGFR-specific inhibition by tyrosine kinase inhibitor AG1478 (p-ERK1/2 and MMP2 [p < 0.05]). Moreover, we have shown that EGFR inhibition suppresses collagen amounts in aneurysmal SMCs (p < 0.05) and promotes robust formation of elastic fibers by enhancing its deposition in the extracellular space. Hence, the EGFR-MAPK pathway in aneurysmal cells can be targeted to provide therapeutic effects toward stimulating vascular matrix regeneration. Impact statementProteolytic disorders such as aortal expansions, called abdominal aortic aneurysms (AAAs), are characterized by naturally irreversible enzymatic breakdown and loss of elastic fibers, a problem that has not yet been surmounted by existing tissue engineering approaches. In this work, we show, for the first time, how epidermal growth factor receptor (EGFR) inhibition provides downstream benefits in elastic fiber assembly and deposition in aneurysmal smooth muscle cell cultures. This work can open future possibilities for development of EGFR-targeted drug-based therapies not only for vessel wall repair in AAAs but also other proteolytically compromised elastic tissues.

Arterial stiffening is a significant predictor of cardiovascular disease development and mortality. In elastic arteries, stiffening refers to the loss and fragmentation of elastic fibers, with a progressive increase in collagen fibers. Type VIII collagen (Col-8) is highly expressed developmentally, and then once again dramatically upregulated in aged and diseased vessels characterized by arterial stiffening. Yet its biophysical impact on the vessel wall remains unknown. The purpose of this study was to test the hypothesis that Col-8 functions as a matrix scaffold to maintain vessel integrity during extracellular matrix (ECM) development. These changes are predicted to persist into the adult vasculature, and we have tested this in our investigation. Through our in vivo and in vitro studies, we have determined a novel interaction between Col-8 and elastin. Mice deficient in Col-8 (Col8-/-) had reduced baseline blood pressure and increased arterial compliance, indicating an enhanced Windkessel effect in conducting arteries. Differences in both the ECM composition and VSMC activity resulted in Col8-/- carotid arteries that displayed increased crosslinked elastin and functional distensibility, but enhanced catecholamine-induced VSMC contractility. In vitro studies revealed that the absence of Col-8 dramatically increased tropoelastin mRNA and elastic fiber deposition in the ECM, which was decreased with exogenous Col-8 treatment. These findings suggest a causative role for Col-8 in reducing mRNA levels of tropoelastin and the presence of elastic fibers in the matrix. Moreover, we also found that Col-8 and elastin have opposing effects on VSMC phenotype, the former promoting a synthetic phenotype, whereas the latter confers quiescence. These studies further our understanding of Col-8 function and open a promising new area of investigation related to elastin biology.

Constitutively activating mutations in GNAQ (Gαq) or GNA11 (Gα11) are the oncogenic drivers of uveal (eye) melanoma (UM), occurring in over 90% of tumors. We show that constitutively active Gαq in UM cells can be targeted by the cyclic depsipeptide FR900359 (FR). FR inhibits GDP/GTP exchange allosterically to trap constitutively active Gαq in inactive GDP-bound Gαβγ heterotrimers, and allosteric inhibition of other Gα subunits can be achieved by introduction of an FR binding site. In UM cells driven by constitutively active Gαq, FR inhibits second messenger signaling, arrests proliferation, and reinstates melanocytic differentiation. At higher doses, FR also induces apoptosis. FR has no effect on Gαq/11-wild-type UM cells. FR promotes UM cell differentiation by reactivating polycomb repressive complex 2 (PRC2)-mediated gene silencing, antagonized by a heretofore unrecognized effector system of constitutively active Gαq in UM. Constitutively active Gαq and PRC2 therefore provide important therapeutic targets for UM. Further, the development of FR analogs specific for other Gα subunit subtypes may provide novel therapeutic approaches for diseases driven by constitutively active Gα subunits or multiple G protein-coupled receptors where targeting a single receptor is ineffective. This abstract is also being presented as Poster A10. Citation Format: Michael D. Onken, Carol M. Makepeace, Kevin M. Kaltenbronn, Stanley M. Kanai, Tyson D. Todd, Shiqi Wang, Thomas J. Broekelmann, Prabakar Kumar Rao, John A. Cooper, Kendall J. Blumer. Pharmacologic targeting of Gq reveals new pathways in uveal melanoma [abstract]. In: Proceedings of the AACR Special Conference on Melanoma: From Biology to Target; 2019 Jan 15-18; Houston, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(19 Suppl):Abstract nr PR09.

Abstract Dramatic cardiomegaly arising from gain-of-function (GoF) mutations in the ATP-sensitive potassium (KATP) channels genes, ABCC9 and KCNJ8, is a characteristic feature of Cantú syndrome (CS). How potassium channel over-activity results in cardiac hypertrophy, as well as the long-term consequences of cardiovascular remodeling in CS, is unknown. Using genome-edited mouse models of CS, we therefore sought to dissect the pathophysiological mechanisms linking KATP channel GoF to cardiac remodeling. We demonstrate that chronic reduction of systemic vascular resistance in CS is accompanied by elevated renin–angiotensin signaling, which drives cardiac enlargement and blood volume expansion. Cardiac enlargement in CS results in elevation of basal cardiac output, which is preserved in aging. However, the cardiac remodeling includes altered gene expression patterns that are associated with pathological hypertrophy and are accompanied by decreased exercise tolerance, suggestive of reduced cardiac reserve. Our results identify a high-output cardiac hypertrophy phenotype in CS which is etiologically and mechanistically distinct from other myocardial hypertrophies, and which exhibits key features of high-output heart failure (HOHF). We propose that CS is a genetically-defined HOHF disorder and that decreased vascular smooth muscle excitability is a novel mechanism for HOHF pathogenesis.

Microfibril-associated glycoprotein-1 (MAGP-1) is a component of vertebrate extracellular matrix (ECM) microfibrils that, together with the fibrillins, contributes to microfibril function. Many of the phenotypes associated with MAGP-1 gene inactivation are consistent with dysregulation of the transforming growth factor β (TGFβ)/bone morphogenetic protein (BMP) signaling system. We have previously shown that full-length MAGP-1 binds active TGFβ-1 and some BMPs. The work presented here further defines the growth factor-binding domain of MAGP-1. Using recombinant domains and synthetic peptides, along with surface plasmon resonance analysis to measure the kinetics of the MAGP-1-TGFβ-1 interaction, we localized the TGFβ- and BMP-binding site in MAGP-1 to a 19-amino acid-long, highly acidic sequence near the N terminus. This domain was specific for binding active, but not latent, TGFβ-1. Growth factor activity experiments revealed that TGFβ-1 retains signaling activity when complexed with MAGP-1. Furthermore, when bound to fibrillin, MAGP-1 retained the ability to interact with TGFβ-1, and active TGFβ-1 did not bind fibrillin in the absence of MAGP-1. The absence of MAGP was sufficient to raise the amount of total TGFβ stored in the ECM of cultured cells, suggesting that the MAGPs compete with the TGFβ large latent complex for binding to microfibrils. Together, these results indicate that MAGP-1 plays an active role in TGFβ signaling in the ECM.

Heterozygous missense mutations in lysyl oxidase (LOX) are associated with thoracic aortic aneurysms and dissections. To assess how LOX mutations modify protein function and lead to aortic disease, we studied the factors that influence the onset and progression of vascular aneurysms in mice bearing a Lox mutation (p.M292R) linked to aortic dilation in humans. We show that mice heterozygous for the M292R mutation did not develop aneurysmal disease unless challenged with increased hemodynamic stress. Vessel dilation was confined to the ascending aorta, although in both ascending and descending aortae, changes in vessel wall structure, smooth muscle cell number, and inflammatory cell recruitment differed between WT and mutant animals. Studies with isolated cells revealed that M292R-mutant LOX is retained in the endoplasmic reticulum and ultimately cleared through an autophagy/proteasome pathway. Because the mutant protein does not transit to the Golgi, where copper incorporation occurs, the protein is never catalytically active. These studies show that the M292R mutation results in LOX loss of function due to a secretion defect that predisposes the ascending aorta in mice (and by extension humans with similar mutations) to arterial dilation when exposed to risk factors that impart stress to the arterial wall.

Fibulin‐4 (FBLN4) is an extracellular matrix protein essential for elastic fiber assembly in large conduit arteries. While its exact role in the process remains unknown, it is hypothesized that, through binding lysyl oxidase (LOX), FBLN4 facilitates elastin crosslinking and mature elastic fiber formation. To assess whether FBLN4 and LOX interact in the process of elastic fiber formation in vivo, we took a genetic approach where we bred recently characterized mice carrying a disease‐causing mutation in Fbln4 (Fbln4E57K) with LOX insufficient (Lox+/−) mice and examined the consequences of LOX insufficiency on the cardiovascular system of Fbln4E57K mice. By 3–4 months of age Fbln4E57K;Lox+/− mice were similar in size to Fbln4E57K mice and littermates. Ascending aortic aneurysms, which were incompletely penetrant and confined to the ascending aorta in Fbln4E57K mice were significantly worsened by LOX insufficiency as they were fully penetrant and extended from the aortic root through the aortic arch in Fbln4E57K;Lox+/− mice. Additionally, arterial tortuosity and elastic fiber fragmentation of large conduit arteries were exacerbated by LOX insufficiency. Interestingly, muscular arteries remained intact. Fbln4E57K;Lox+/− mice had significant cardiac hypertrophy and widening of the pulse pressure that was due to lower diastolic blood compared to Fbln4E57K mice. The cardiac hypertrophy and decrease in diastolic blood pressure are likely a consequence of worsened aortic regurgitation as large artery stiffness was not exacerbated by LOX insufficiency in Fbln4E57K mice. In summary, LOX insufficiency exacerbated the vascular phenotype seen in mice with mutant FBLN4 suggesting that there is a functional interaction between the two molecules. Interestingly, elastic fiber formation was unaffected in muscular arteries raising the possibility that the process of elastic fiber assembly, previously thought to be the same in all elastic tissues, may differ between vascular beds.Support or Funding InformationNIH, NHLBI ‐ 1K08HL135400This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Constitutively activating mutations in the G-protein alpha subunits GNAQ(Gαq) and GNA11(Gα11) act as oncogenic drivers in over 90% of uveal (eye) melanoma (UM) tumors. We show that constitutively active Gαq and Gα11 can be targeted in UM cells by the cyclic depsipeptide FR900359 (FR). FR inhibits GDP/GTP guanine nucleotide exchange allosterically to trap constitutively active Gαq/11 in inactive GDP-bound Gαβγ heterotrimers. FR inhibits second messenger signaling, arrests proliferation and reinstates melanocytic differentiation in UM cells driven by constitutively active Gαq or Gα11. At higher doses, FR also induces apoptosis. The re-differentiation and anti-proliferative effects of FR are not seen in UM cells that lack mutations in Gαq or Gα11. FR promotes UM cell differentiation by reactivating polycomb repressive complex 2 (PRC2)-mediated gene silencing, and this re-differentiation can be blocked with an EZH2 inhibitor. The effector system regulating PRC2 downstream of constitutively active Gαq/11 in UM is currently under investigation. Preliminary data from human primary tumor samples suggest that targeting constitutively active Gαq/11 with FR could provide an important therapeutic approach for UM.Citation Format: Michael D. Onken, Carol M. Makepeace, Kevin M. Kaltenbronn, Stanley M. Kanai, Tyson D. Todd, Shiqi Wang, Thomas J. Broekelmann, Prabakar Kumar Rao, John A. Cooper, Kendall J. Blumer. Targeting nucleotide exchange to inhibit Gq/11 driver mutations in uveal melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1764.

Microfibril-associated glycoproteins 1 and 2 (MAGP-1, MAGP-2) are protein components of extracellular matrix microfibrils. These proteins interact with fibrillin, the core component of microfibrils, and impart unique biological properties that influence microfibril function in vertebrates. MAGPs bind active forms of TGFβ and BMPs and are capable of modulating Notch signaling. Mutations in MAGP-1 or MAGP-2 have been linked to thoracic aneurysms and metabolic disease in humans. MAGP-2 has also been shown to be an important biomarker in several human cancers. Mice lacking MAGP-1 or MAGP-2 have defects in multiple organ systems, which reflects the widespread distribution of microfibrils in vertebrate tissues. This review summarizes our current understanding of the function of the MAGPs and their relationship to human disease.

Elastin and collagen levels in tissues are frequently difficult to measure because of each protein's limited solubility. This chapter provides detailed methodology for the determination of elastin, collagen, and total protein levels in a single tissue sample. All three assays start with an acid hydrolysate of the tissue, which breaks the tissue-associated proteins down to their component amino acids. Marker amino acids unique to each protein (desmosine for elastin and hydroxyproline for collagen) are then quantified. Total protein content, useful as a denominator for data normalization, can also be measured from a portion of the hydrolysate using an assay for free amino groups. These measurements are performed using convenient 96-well assay plates and require only a plate reader to determine absorbance.

A plant-derived compound may treat patients with diseases caused by Gα mutations.

Increased vascular stiffness correlates with a higher risk of cardiovascular complications in aging adults. Elastin (ELN) insufficiency, as observed in patients with Williams-Beuren syndrome or with familial supravalvular aortic stenosis, also increases vascular stiffness and leads to arterial narrowing. We used Eln+/- mice to test the hypothesis that pathologically increased vascular stiffness with concomitant arterial narrowing leads to decreased blood flow to end organs such as the brain. We also hypothesized that drugs that remodel arteries and increase lumen diameter would improve flow. To test these hypotheses, we compared carotid blood flow using ultrasound and cerebral blood flow using MRI-based arterial spin labeling in wild-type (WT) and Eln+/- mice. We then studied how minoxidil, an ATP-sensitive K+ channel opener and vasodilator, affects vessel mechanics, blood flow, and gene expression. Both carotid and cerebral blood flows were lower in Eln+/- mice than in WT mice. Treatment of Eln+/- mice with minoxidil lowered blood pressure and reduced functional arterial stiffness to WT levels. Minoxidil also improved arterial diameter and restored carotid and cerebral blood flows in Eln+/- mice. The beneficial effects persisted for weeks after drug removal. RNA-Seq analysis revealed differential expression of 127 extracellular matrix-related genes among the treatment groups. These results indicate that ELN insufficiency impairs end-organ perfusion, which may contribute to the increased cardiovascular risk. Minoxidil, despite lowering blood pressure, improves end-organ perfusion. Changes in matrix gene expression and persistence of treatment effects after drug withdrawal suggest arterial remodeling. Such remodeling may benefit patients with genetic or age-dependent ELN insufficiency. NEW & NOTEWORTHY Our work with a model of chronic vascular stiffness, the elastin ( Eln)+/- mouse, shows reduced brain perfusion as measured by carotid ultrasound and MRI arterial spin labeling. Vessel caliber, functional stiffness, and blood flow improved with minoxidil. The ATP-sensitive K+ channel opener increased Eln gene expression and altered 126 other matrix-associated genes.

Microfibril-associated glycoprotein-1 (MAGP1) is an extracellular matrix protein that interacts with fibrillin and is involved in regulating the bioavailability of signaling molecules such as TGFβ. Mice with germline MAGP1 deficiency (Mfap2−/−) develop increased adiposity, hyperglycemia, insulin resistance, bone marrow adipose tissue expansion, reduced cancellous bone mass, cortical bone thinning and bone fragility. The goal of this study was to assess whether the Mfap2−/− bone phenotypes were due to loss of MAGP1 locally or secondary to a change in whole body physiology (metabolic dysfunction). To do this, mice with conditional deletion of MAGP1 in the limb skeleton were generated by crossing MAGP1-flox mice (Mfap2lox/lox) with Prx1-Cre mice. Mfap2Prx−/− mice did not show any changes in peripheral adiposity, hyperglycemia or insulin sensitivity, but did have increased bone length and cancellous bone loss that was comparable to the germline Mfap2−/− knockout. Unlike the germline knockout, marrow adiposity, cortical bone thickness and bone strength in Mfap2Prx−/− mice were normal. These findings implicate systemic metabolic dysfunction in the development of bone fragility in germline Mfap2−/− mice. An unexpected finding of this study was the detection of MAGP1 protein in the Mfap2Prx−/− hematopoietic bone marrow, despite the absence of MAGP1 protein in osseous bone matrix and absent Mfap2 transcript expression at both sites. This suggests MAGP1 from a secondary site may accumulate in the bone marrow, but not be incorporated into the bone matrix, during times of regional MAGP1 depletion.

Deficiency or insufficiency of Fibulin‐4 (FBLN4) in mice leads to abnormal elastic fiber formation in large vessels with consequent aneurysm formation and arterial tortuosity, while FBLN4 mutations in humans lead to autosomal recessive cutis laxa type 1B (ARCL 1B), a multisystem disorder characterized by inelastic skin, arterial tortuosity, aortic aneurysms, and pulmonary emphysema and thought to be the result of aberrant elastic fiber formation. We sought to determine the consequences of a disease‐causing mutation in FBLN4 (E57K) on the cardiovascular system and vascular elastic fibers in a mouse model of ARCL 1B. Fbln4 E57K/E57K mice were hypertensive and developed arterial elongation, tortuosity and ascending aortic aneurysms. Smooth muscle cell (SMC) organization within the wall of conduit arteries was abnormal and elastic fibers were fragmented and had a spongy, moth‐eaten appearance. Surprisingly, elastin and SMCs in the mesenteric, saphenous, and renal arteries of homozygous mutant mice were normal and seemingly unaffected by the E57K mutation. Furthermore, elastin and collagen content of large and small arteries were unchanged in Fbln4 E57K/E57K mice. While the E57K mutation did not affect Fbln4 mRNA expression, FBLN4 protein levels were reduced in Fbln4 E57K/E57K ascending aorta, but unchanged in mesenteric arteries. These results suggest a differential role for FBLN4 in elastic fiber assembly between elastic and muscular arteries. Future studies investigating tissue‐specific elastic fiber assembly may lead to novel therapeutic interventions for ARCL 1B and other disorders of elastic fiber assembly. Support or Funding Information CMH is a Scholar of the Child Health Research Center at Washington University School of Medicine, National Institutes of Health (NIH) K12‐HD076224. She also received support from the NIH training grant 2T32HD043010‐11 and the Mallinckrodt Foundation Physician‐Scientist Training Program Fellow support. This work was funded by NIH grants R01‐HL53325 and R01‐HL105314 to RPM.

Homozygous or compound heterozygous mutations in fibulin-4 (FBLN4) lead to autosomal recessive cutis laxa type 1B (ARCL1B), a multisystem disorder characterized by significant cardiovascular abnormalities, including abnormal elastin assembly, arterial tortuosity, and aortic aneurysms. We sought to determine the consequences of a human disease-causing mutation in FBLN4 (E57K) on the cardiovascular system and vascular elastic fibers in a mouse model of ARCL1B. Fbln4(E57K/E57K) mice were hypertensive and developed arterial elongation, tortuosity, and ascending aortic aneurysms. Smooth muscle cell organization within the arterial wall of large conducting vessels was abnormal, and elastic fibers were fragmented and had a moth-eaten appearance. In contrast, vessel wall structure and elastic fiber integrity were normal in resistance/muscular arteries (renal, mesenteric, and saphenous). Elastin cross-linking and total elastin content were unchanged in large or small arteries, whereas elastic fiber architecture was abnormal in large vessels. While the E57K mutation did not affect Fbln4 mRNA levels, FBLN4 protein was lower in the ascending aorta of mutant animals compared to wild-type arteries but equivalent in mesenteric arteries. We found a differential role of FBLN4 in elastic fiber assembly, where it functions mainly in large conduit arteries. These results suggest that elastin assembly has different requirements depending on vessel type. Normal levels of elastin cross-links in mutant tissue call into question FBLN4' s suggested role in mediating lysyl oxidase-elastin interactions. Future studies investigating tissuespecific elastic fiber assembly may lead to novel therapeutic interventions for ARCL1B and other disorders of elastic fiber assembly.

Increased arterial stiffness is a common characteristic of humans with Williams-Beuren syndrome and mouse models of elastin insufficiency. Arterial stiffness is associated with multiple negative cardiovascular outcomes, including myocardial infarction, stroke, and sudden death. Therefore, identifying therapeutic interventions that improve arterial stiffness in response to changes in elastin levels is of vital importance. The goal of this study was to determine the effect of chronic pharmacologic therapy with different classes of antihypertensive medications on arterial stiffness in elastin insufficiency. Elastin-insufficient mice 4-6 wk of age and wild-type littermates were subcutaneously implanted with osmotic micropumps delivering a continuous dose of one of the following: vehicle, losartan, nicardipine, or propranolol for 8 wk. At the end of treatment period, arterial blood pressure and large artery compliance and remodeling were assessed. Our results show that losartan and nicardipine treatment lowered blood pressure and pulse pressure in elastin-insufficient mice. Elastin and collagen content of abdominal aortas as well as ascending aorta and carotid artery biomechanics were not affected by any of the drug treatments in either genotype. By reducing pulse pressure and shifting the working pressure range of an artery to a more compliant region of the pressure-diameter curve, antihypertensive medications may mitigate the consequences of arterial stiffness, an effect that is drug class independent. These data emphasize the importance of early recognition and long-term management of hypertension in Williams-Beuren syndrome and elastin insufficiency.

The v3 integrin stimulates the resorptive capacity of the differentiated osteoclast (OC) by organizing its cytoskeleton via the tyrosine kinase, Syk. Thus, Syk-deficient OCs fails to spread or form actin rings, in vitro and in vivo. The Syk family of tyrosine kinases consists of Syk itself and Zap70 which are expressed by different cell types. Because of their structural similarity, and its compensatory properties in other cells, we asked if Zap70 can substitute for absence of Syk in OCs. While expression of Syk, as expected, normalizes the cytoskeletal abnormalities of Syk-/- OCs, Zap70 fails do so. In keeping with this observation, Syk, but not Zap70, rescues v3 integrin-induced SLP76 phosphorylation in Syk-/- OCs. Furthermore the kinase sequence of Syk partially rescues the Syk-/- phenotype but full normalization also requires its SH2 domains. Surprisingly, expression of Zap70 inhibits WT OC spreading, actin ring formation and bone resorptive activity, but not differentiation. In keeping with arrested cytoskeletal organization, Zap70 blocks integrin-activated endogenous Syk and Vav3, SLP76 phosphorylation. Such inhibition requires Zap70 kinase activity, as it is abolished by mutation of the Zap70 kinase domain. Thus, while the kinase domain of Syk is uniquely required for OC function that of Zap70 inhibits it. J. Cell. Biochem. 114: 1871-1878, 2013. (c) 2013 Wiley Periodicals, Inc.

Type 1 phosphotidylinosotol-4 phosphate 5 kinase gamma (PIP5KI gamma) is central to generation of phosphotidylinosotol (4,5)P-2 (PI(4,5)P-2). PIP5KI gamma also participates in cytoskeletal organization by delivering talin to integrins, thereby enhancing their ligand binding capacity. As the cytoskeleton is pivotal to osteoclast function, we hypothesized that absence of PIP5KI gamma would compromise their resorptive capacity. Absence of the kinase diminishes PI(4,5) abundance and desensitizes pre-cursors to RANK ligand-stimulated differentiation. Thus, PIP5KI gamma(-/-) osteoclasts are reduced in number in vitro and confirm physiological relevance in vivo. Despite reduced numbers, PIP5KI gamma(-/-) osteoclasts surprisingly have normal cytoskeletons and effectively resorb bone. PIP5KI gamma overexpression, which increases PI(4,5)P-2, also delays osteoclast differentiation and reduces cell number but in contrast to cells lacking the kinase, its excess disrupts the cytoskeleton. The cytoskeleton-disruptive effects of excess PIP5KI gamma reflect its kinase activity and are independent of talin recognition. The combined arrested differentiation and disorganized cytoskeleton of PIP5KI gamma-transduced osteoclasts compromises bone resorption. Thus, optimal PIP5KI gamma and PI(4,5)P-2 expression, by osteoclasts, are essential for skeletal homeostasis.