Secreted Versus Membrane-anchored Collagenases: RELATIVE ROLES IN FIBROBLAST-DEPENDENT COLLAGENOLYSIS AND INVASION*

Fibroblasts degrade type I collagen, the major extracellular protein found in mammals, during events ranging from bulk tissue resorption to invasion through the three-dimensional extracellular matrix. Current evidence suggests that type I collagenolysis is mediated by secreted as well as membrane-anchored members of the matrix metalloproteinase (MMP) gene family. However, the roles played by these multiple and possibly redundant, degradative systems during fibroblast-mediated matrix remodeling is undefined. Herein, we use fibroblasts isolated from Mmp13^−/−, Mmp8^−/−, Mmp2^−/−, Mmp9^−/−, Mmp14^−/− and Mmp16^−/− mice to define the functional roles for secreted and membrane-anchored collagenases during collagen-resorptive versus collagen-invasive events. In the presence of a functional plasminogen activator-plasminogen axis, secreted collagenases arm cells with a redundant collagenolytic potential that allows fibroblasts harboring single deficiencies for either MMP-13, MMP-8, MMP-2, or MMP-9 to continue to degrade collagen comparably to wild-type fibroblasts. Likewise, Mmp14^−/− or Mmp16^−/− fibroblasts retain near-normal collagenolytic activity in the presence of plasminogen via the mobilization of secreted collagenases, but only Mmp14 (MT1-MMP) plays a required role in the collagenolytic processes that support fibroblast invasive activity. Furthermore, by artificially tethering a secreted collagenase to the surface of Mmp14^−/− fibroblasts, we demonstrate that localized pericellular collagenolytic activity differentiates the collagen-invasive phenotype from bulk collagen degradation. Hence, whereas secreted collagenases arm fibroblasts with potent matrix-resorptive activity, only MT1-MMP confers the focal collagenolytic activity necessary for supporting the tissue-invasive phenotype.In the postnatal state, fibroblasts are normally embedded in a self-generated three-dimensional connective tissue matrix composed largely of type I collagen, the major extracellular protein found in mammals (1–3). Type I collagen not only acts as a structural scaffolding for the associated mesenchymal cell populations but also regulates gene expression and cell function through its interactions with collagen binding integrins and discoidin receptors (2, 4). Consistent with the central role that type I collagen plays in defining the structure and function of the extracellular matrix, the triple-helical molecule is resistant to almost all forms of proteolytic attack and can display a decades-long half-life in vivo (4–6). Nonetheless, fibroblasts actively remodel type I collagen during wound healing, inflammation, or neoplastic states (2, 7–13).To date type I collagenolytic activity is largely confined to a small subset of fewer than 10 proteases belonging to either the cysteine proteinase or matrix metalloproteinase (MMP)² gene families (4, 14–18). As all collagenases are synthesized as inactive zymogens, complex proteolytic cascades involving serine, cysteine, metallo, and aspartyl proteinases have also been linked to collagen turnover by virtue of their ability to mediate the processing of the pro-collagenases to their active forms (13, 15, 19). After activation, each collagenase can then cleave native collagen within its triple-helical domain, thus precipitating the unwinding or “melting” of the resulting collagen fragments at physiologic temperatures (4, 15). In turn, the denatured products (termed gelatin) are susceptible to further proteolysis by a broader class of “gelatinases” (4, 15). Collagen fragments are then either internalized after binding to specific receptors on the cell surface or degraded to smaller peptides with potent biological activity (20–24).Previous studies by our group as well as others have identified MMPs as the primary effectors of fibroblast-mediated collagenolysis (20, 25, 26). Interestingly, adult mouse fibroblasts express at least six MMPs that can potentially degrade type I collagen, raising the possibility of multiple compensatory networks that are designed to preserve collagenolytic activity (25). Four of these collagenases belong to the family of secreted MMPs, i.e. MMP-13, MMP-8, MMP-2, and MMP-9, whereas the other two enzymes are members of the membrane-type MMP subgroup, i.e. MMP-14 (MT1-MMP) and MMP-16 (MT3-MMP) (13, 27–29). From a functional perspective, the specific roles that can be assigned to secreted versus membrane-anchored collagenases remain undefined. As such, fibroblasts were isolated from either wild-type mice or mice harboring loss-of-function deletions in each of the major secreted and membrane-anchored collagenolytic genes, and the ability of the cells to degrade type I collagen was assessed. Herein, we demonstrate that fibroblasts mobilize either secreted or membrane-anchored MMPs to effectively degrade type I collagen in qualitatively and quantitatively distinct fashions. However, under conditions where fibroblasts use either secreted and membrane-anchored MMPs to exert quantitatively equivalent collagenolytic activity, only MT1-MMP plays a required role in supporting a collagen-invasive phenotype. These data establish a new paradigm wherein secreted collagenases are functionally limited to bulk collagenolytic processes, whereas MT1-MMP uniquely arms the fibroblast with a focalized degradative activity that mediates subjacent collagenolysis as well as invasion.     

Deletion of the Chloride Transporter Slc26a7 Causes Distal Renal Tubular Acidosis and Impairs Gastric Acid Secretion

SLC26A7 (human)/Slc26a7 (mouse) is a recently identified chloride-base exchanger and/or chloride transporter that is expressed on the basolateral membrane of acid-secreting cells in the renal outer medullary collecting duct (OMCD) and in gastric parietal cells. Here, we show that mice with genetic deletion of Slc26a7 expression develop distal renal tubular acidosis, as manifested by metabolic acidosis and alkaline urine pH. In the kidney, basolateral Cl⁻/HCO₃⁻ exchange activity in acid-secreting intercalated cells in the OMCD was significantly decreased in hypertonic medium (a normal milieu for the medulla) but was reduced only mildly in isotonic medium. Changing from a hypertonic to isotonic medium (relative hypotonicity) decreased the membrane abundance of Slc26a7 in kidney cells in vivo and in vitro. In the stomach, stimulated acid secretion was significantly impaired in isolated gastric mucosa and in the intact organ. We propose that SLC26A7 dysfunction should be investigated as a potential cause of unexplained distal renal tubular acidosis or decreased gastric acid secretion in humans.The collecting duct segment of the distal kidney nephron plays a major role in systemic acid base homeostasis by acid secretion and bicarbonate absorption. The acid secretion occurs via H⁺-ATPase and H-K-ATPase into the lumen and bicarbonate is absorbed via basolateral Cl⁻/HCO₃⁻ exchangers (1–4). The tubules, which are located within the outer medullary region of the kidney collecting duct (OMCD),² have the highest rate of acid secretion among the distal tubule segments and are therefore essential to the maintenance of acid base balance (2).The gastric parietal cell is the site of generation of acid and bicarbonate through the action of cytosolic carbonic anhydrase II (5, 6). The intracellular acid is secreted into the lumen via gastric H-K-ATPase, which works in conjunction with a chloride channel and a K⁺ recycling pathway (7–10). The intracellular bicarbonate is transported to the blood via basolateral Cl⁻/HCO₃⁻ exchangers (11–14).SLC26 (human)/Slc26 (mouse) isoforms are members of a conserved family of anion transporters that display tissue-specific patterns of expression in epithelial cells (15–24). Several SLC26 members can function as chloride/bicarbonate exchangers. These include SLC26A3 (DRA), SLC26A4 (pendrin), SLC26A6 (PAT1 or CFEX), SLC26A7, and SLC26A9 (25–31). SLC26A7 and SLC26A9 can also function as chloride channels (32–34).SLC26A7/Slc26a7 is predominantly expressed in the kidney and stomach (28, 29). In the kidney, Slc26a7 co-localizes with AE1, a well-known Cl⁻/HCO₃⁻ exchanger, on the basolateral membrane of (acid-secreting) A-intercalated cells in OMCD cells (29, 35, 36) (supplemental Fig. 1). In the stomach, Slc26a7 co-localizes with AE2, a major Cl⁻/HCO₃⁻ exchanger, on the basolateral membrane of acid secreting parietal cells (28). To address the physiological function of Slc26a7 in the intact mouse, we have generated Slc26a7 ko mice. We report here that Slc26a7 ko mice exhibit distal renal tubular acidosis and impaired gastric acidification in the absence of morphological abnormalities in kidney or stomach.     

Adipose Triglyceride Lipase Is Implicated in Fuel- and Non-fuel-stimulated Insulin Secretion

Reduced lipolysis in hormone-sensitive lipase-deficient mice is associated with impaired glucose-stimulated insulin secretion (GSIS), suggesting that endogenous β-cell lipid stores provide signaling molecules for insulin release. Measurements of lipolysis and triglyceride (TG) lipase activity in islets from HSL^−/− mice indicated the presence of other TG lipase(s) in the β-cell. Using real time-quantitative PCR, adipose triglyceride lipase (ATGL) was found to be the most abundant TG lipase in rat islets and INS832/13 cells. To assess its role in insulin secretion, ATGL expression was decreased in INS832/13 cells (ATGL-knockdown (KD)) by small hairpin RNA. ATGL-KD increased the esterification of free fatty acid (FFA) into TG. ATGL-KD cells showed decreased glucose- or Gln + Leu-induced insulin release, as well as reduced response to KCl or palmitate at high, but not low, glucose. The K_ATP-independent/amplification pathway of GSIS was considerably reduced in ATGL-KD cells. ATGL^−/− mice were hypoinsulinemic and hypoglycemic and showed decreased plasma TG and FFAs. A hyperglycemic clamp revealed increased insulin sensitivity and decreased GSIS and arginine-induced insulin secretion in ATGL^−/− mice. Accordingly, isolated islets from ATGL^−/− mice showed reduced insulin secretion in response to glucose, glucose + palmitate, and KCl. Islet TG content and FFA esterification into TG were increased by 2-fold in ATGL^−/− islets, but glucose usage and oxidation were unaltered. The results demonstrate the importance of ATGL and intracellular lipid signaling for fuel- and non-fuel-induced insulin secretion.Free fatty acids (FFA)⁵ and other lipid molecules are important for proper glucose-stimulated insulin secretion (GSIS) by β-cells. Thus, deprivation of fatty acids (FA) in vivo (1) diminishes GSIS, whereas a short term exposure to FFA enhances it (1–3). In contrast, a sustained provision of FA, particularly in the presence of high glucose in vitro, is detrimental to β-cells in that it reduces insulin gene expression (4) and secretion (5) and induces β-cell apoptosis (6). The FA supply to the β-cells can be from exogenous sources, such as plasma FFAs and lipoproteins, or endogenous sources, such as intracellular triglyceride (TG) stores. Studies from our laboratory (7–10) and others (11, 12) support the concept that the hydrolysis of endogenous TG plays an important role in fuel-induced insulin secretion because TG depletion with leptin (13) or inhibition of TG lipolysis by lipase inhibitors such as 3,5-dimethylpyrazole (7) or orlistat (11, 12) markedly curtail GSIS in rat islets. Furthermore, mice with β-cell-specific knock-out of hormone-sensitive lipase (HSL), which hydrolyzes both TG and diacylglycerol (DAG), show defective first phase GSIS in vivo and in vitro (14).Lipolysis is an integral part of an essential metabolic pathway, the TG/FFA cycle, in which FFA esterification onto a glycerol backbone leading to the synthesis of TG is followed by its hydrolysis with the release of the FFA that can then be re-esterified. Intracellular TG/FFA cycling is known to occur in adipose tissue of rats and humans (15, 16) and also in liver and skeletal muscle (17). It is generally described as a “futile cycle” as it leads to the net hydrolysis of ATP with the generation of heat (18). However, several studies have shown that this cycle has important functions in the cell. For instance, in brown adipose tissue, it contributes to overall thermogenesis (17, 19). In islets from the normoglycemic, hyperinsulinemic, obese Zucker fatty rat, increased GSIS is associated with increased glucose-stimulated lipolysis and FA esterification, indicating enhanced TG/FFA cycling (10). Stimulation of lipolysis by glucose has also been observed in isolated islets from normal rats (12) and HSL^−/− mice (8) indicating the presence of glucose-responsive TG/FFA cycling in pancreatic β-cells.The identity of the key lipases involved in the TG/FFA cycle in pancreatic islets is uncertain. HSL is expressed in islets (20), is up-regulated by long term treatment with elevated glucose (21), and is associated with insulin secretory granules (22). In addition, our earlier results suggested that elevated HSL expression correlates with augmented TG/FFA cycling in islets of Zucker fatty rats (10). However, it appears that other lipases may contribute to lipolysis and the regulation of GSIS in islet tissue. Thus, results from studies using HSL^−/− mice showed unaltered GSIS (8, 23), except in fasted male mice (8, 9) in which lipolysis was decreased but not abolished. Furthermore, HSL^−/− mice show residual TG lipase activity (8) indicating the presence of other TG lipases.Recently, adipocyte triglyceride lipase (ATGL; also known as Desnutrin, TTS-2, iPLA₂-ζ, and PNPLA2) (24–26) was found to account for most if not all of the residual lipolysis in HSL^−/− mice (26, 27). Two homologues of ATGL, Adiponutrin and GS2, have been described in adipocytes (24). All three enzymes contain a patatin-like domain with broad lipid acyl-hydrolase activity. However, it is not known if adiponutrin and GS2 are actually TG hydrolases. An additional lipase, TG hydrolase or carboxylesterase-3, has been identified in rat adipose tissue (28, 29). Although the hydrolysis of TG is catalyzed by all these lipases, HSL can hydrolyze both TG and DAG, the latter being a better substrate (30).In this study, we observed that besides HSL, ATGL (31), adiponutrin, and GS2 are expressed in rat islets and INS832/13 cells, with ATGL being the most abundant. We then focused on the role of ATGL in fuel-stimulated insulin secretion in two models, INS832/13 β-cells in which ATGL expression was reduced by RNA interference-knockdown (ATGL-KD) and ATGL^−/− mice.     

Loss of TMEM16A Causes a Defect in Epithelial Ca2+-dependent Chloride Transport

Molecular identification of the Ca²⁺-dependent chloride channel TMEM16A (ANO1) provided a fundamental step in understanding Ca²⁺-dependent Cl⁻ secretion in epithelia. TMEM16A is an intrinsic constituent of Ca²⁺-dependent Cl⁻ channels in cultured epithelia and may control salivary output, but its physiological role in native epithelial tissues remains largely obscure. Here, we demonstrate that Cl⁻ secretion in native epithelia activated by Ca²⁺-dependent agonists is missing in mice lacking expression of TMEM16A. Ca²⁺-dependent Cl⁻ transport was missing or largely reduced in isolated tracheal and colonic epithelia, as well as hepatocytes and acinar cells from pancreatic and submandibular glands of TMEM16A^−/− animals. Measurement of particle transport on the surface of tracheas ex vivo indicated largely reduced mucociliary clearance in TMEM16A^−/− mice. These results clearly demonstrate the broad physiological role of TMEM16A^−/− for Ca²⁺-dependent Cl⁻ secretion and provide the basis for novel treatments in cystic fibrosis, infectious diarrhea, and Sjöegren syndrome.Electrolyte secretion in epithelial tissues is based on the major second messenger pathways cAMP and Ca²⁺, which activate the cystic fibrosis transmembrane conductance regulator (CFTR)² Cl⁻ channels and Ca²⁺-dependent Cl⁻ channels, respectively (1–3). CFTR conducts Cl⁻ in epithelial cells of airways, intestine, and the ducts of pancreas and sweat gland, while Ca²⁺-dependent Cl⁻ channels secrete Cl⁻ in pancreatic acini and salivary and sweat glands (4–6). Controversy exists as to the contribution of these channels to Cl⁻ secretion in submucosal glands of airways and the relevance for cystic fibrosis (7–9). While cAMP-dependent Cl⁻ secretion by CFTR is well examined, detailed analysis of epithelial Ca²⁺-dependent Cl⁻ secretion is hampered by the lack of a molecular counterpart. Although bestrophins may form Ca²⁺-dependent Cl⁻ channels and facilitate Ca²⁺-dependent Cl⁻ secretion in epithelial tissues (10, 11), they are unlikely to form secretory Cl⁻ channels in the apical cell membrane, because Ca²⁺-dependent Cl⁻ secretion is still present in epithelia of mice lacking expression of bestrophin (12). Bestrophins may rather have an intracellular function by facilitating receptor mediated Ca²⁺ signaling and activation of membrane localized channels (13). With the discovery that TMEM16A produces Ca²⁺-activated Cl⁻ currents with biophysical and pharmacological properties close to those in native epithelial tissues, these proteins are now very likely candidates for endogenous Ca²⁺-dependent Cl⁻ channels (14–17). In cultured airway epithelial cells, small interfering RNA knockdown of endogenous TMEM16A largely reduced calcium-dependent chloride secretion (16). However, apart from preliminary studies of airways and salivary glands, the physiological significance of TMEM16A in native epithelia, particularly in glands, is unclear (14, 17).     

Increased Protein Stability of FGF1 Can Compensate for Its Reduced Affinity for Heparin

Human FGF1 (fibroblast growth factor 1) is a powerful signaling molecule with a short half-life in vivo and a denaturation temperature close to physiological. Binding to heparin increases the stability of FGF1 and is believed to be important in the formation of FGF1·fibroblast growth factor receptor (FGFR) active complex. In order to reveal the function of heparin in FGF1·FGFR complex formation and signaling, we constructed several FGF1 variants with reduced affinity for heparin and with diverse stability. We determined their biophysical properties and biological activities as well as their ability to translocate across cellular membranes. Our study showed that increased thermodynamic stability of FGF1 nicely compensates for decreased binding of heparin in FGFR activation, induction of DNA synthesis, and cell proliferation. By stepwise introduction of stabilizing mutations into the K118E (K132E) FGF1 variant that shows reduced affinity for heparin and is inactive in stimulation of DNA synthesis, we were able to restore the full mitogenic activity of this mutant. Our results indicate that the main role of heparin in FGF-induced signaling is to protect this naturally unstable protein against heat and/or proteolytic degradation and that heparin is not essential for a direct FGF1-FGFR interaction and receptor activation.FGF1 (fibroblast growth factor 1) belongs to a family of polypeptide growth factors comprising in humans 22 structurally related proteins (1, 2). The signaling induced by the growth factor leads to a wide range of cellular responses during development as well as in adult life, such as growth regulation, differentiation, survival, stress response, migration, and proliferation of different cell types (3). The biological activity of FGF1 is exerted through binding to four high affinity cell surface receptors (FGFR1–4), resulting in receptor dimerization and transphosphorylation in its tyrosine kinase domain (4, 5). The activated FGFR³ induces cellular response by initiating several signaling cascades, including mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase/Akt, and phospholipase C-γ (PLC-γ) pathways (6).In addition to FGFRs, FGF1 binds to heparan sulfates (HS) associated with proteoglycans at the cell surface and in the extracellular matrix (7). Among the physiological sugars, the highest affinity for FGF1 is shown by heparin, a widely used linear, highly sulfated polysaccharide composed of 2-O-sulfated iduronic acid and 6-O-sulfated, N-sulfated glucosamine units (8).Despite many years of research, there is still controversy regarding the molecular role of heparin/HS in FGF1- and FGF2-induced signaling. Thus, the question of whether or not the linkage of two molecules of the growth factor by heparin/HS is an absolute prerequisite for induction of FGFR dimerization is still open. Numerous studies have concluded that the presence of heparin/HS is obligatory for FGF signaling. It is widely believed that heparin/HS is directly involved in receptor dimerization and is critical for mitogenic response stimulated by the growth factor (4, 6, 8–10).On the other hand, several authors working on FGF1 and FGF2 have suggested that there is no mandatory requirement for heparin for the assembly and activation of the FGF·FGFR complex. They imply that heparin only plays a role in association of two molecules of the growth factor and therefore facilitates their binding to FGFR (11). It has been reported that FGF1 and FGF2 can interact with the FGFR and trigger phosphorylation of p42/44 MAPK and activation of other signaling pathways even in the absence of HS (12–16).The accepted role of heparin/HS in FGF1 signaling is to prevent the degradation of the growth factor (17). The interaction with heparin or HS protects FGF1 against heat, acidic pH, and proteases (18, 19). HS also seems to regulate the activity of different FGFs by creating their local reservoir and generating a concentration gradient of the growth factor (6, 17).The binding of FGF1 to heparin/HS is mediated by specific residues forming a positively charged patch on the protein surface (20, 21). The major contribution is made by Lys¹¹⁸ (Lys¹³² in the full-length numbering system), which was identified by Harper and Lobb (22), and Lys¹¹² and Arg¹²² (23, 24). Additional residues of FGF1 involved in the interaction with heparin are the positively charged Lys¹¹³, Arg¹¹⁹, and Lys¹²⁸ and the polar Asn¹⁸, Asn¹¹⁴, and Gln¹²⁷ (20, 21). Site-directed mutagenesis and other studies have revealed the importance of Lys¹¹⁸ not only in heparin binding but also for the biological function of FGF1 (22, 25, 26). It was shown that the K118E (K132E) mutant is inactive in stimulation of DNA synthesis, although its affinity for FGFR and the ability to activate signaling cascades is not reduced (27, 28). Despite extensive research, the reason for the lack of mitogenic potential of K118E FGF1 is still not clear.In this paper, we verified the function of heparin in FGF1·FGFR complex formation and signaling by constructing several FGF1 mutants with reduced affinity for heparin. To recover the stability of these variants, which could no longer be stabilized by heparin, we supplemented them stepwise with stabilizing mutations (29). We analyzed thoroughly their biological activity and their ability to translocate across cellular membranes (30–34). Interestingly, the full mitogenic activity of the K118E FGF1 variant was restored by the introduced stabilizing mutations.Our results indicate that the main role of heparin in FGF-induced signaling is to protect this naturally unstable protein against heat denaturation and proteolytic degradation and that the increased stability of the growth factor can compensate for reduced heparin binding.     

Slc26a9 Is Inhibited by the R-region of the Cystic Fibrosis Transmembrane Conductance Regulator via the STAS Domain

SLC26 proteins function as anion exchangers, channels, and sensors. Previous cellular studies have shown that Slc26a3 and Slc26a6 interact with the R-region of the cystic fibrosis transmembrane conductance regulator (CFTR), (R)CFTR, via the Slc26-STAS (sulfate transporter anti-sigma) domain, resulting in mutual transport activation. We recently showed that Slc26a9 has both nCl⁻-HCO₃⁻ exchanger and Cl⁻ channel function. In this study, we show that the purified STAS domain of Slc26a9 (a9STAS) binds purified (R)CFTR. When Slc26a9 and (R)CFTR fragments are co-expressed in Xenopus oocytes, both Slc26a9-mediated nCl⁻-HCO₃⁻ exchange and Cl⁻ currents are almost fully inhibited. Deletion of the Slc26a9 STAS domain (a9-ΔSTAS) virtually eliminated the Cl⁻ currents with only a modest affect on nCl⁻-HCO₃⁻ exchange activity. Co-expression of a9-ΔSTAS and the (R)CFTR fragment did not alter the residual a9-ΔSTAS function. Replacing the Slc26a9 STAS domain with the Slc26a6 STAS domain (a6-a9-a6) does not change Slc26a9 function and is no longer inhibited by (R)CFTR. These data indicate that the Slc26a9-STAS domain, like other Slc26-STAS domains, binds CFTR in the R-region. However, unlike previously reported data, this binding interaction inhibits Slc26a9 ion transport activity. These results imply that Slc26-STAS domains may all interact with (R)CFTR but that the physiological outcome is specific to differing Slc26 proteins, allowing for dynamic and acute fine tuning of ion transport for various epithelia.Slc26 genes and proteins have attracted the attention of physiologists and geneticists. Why? Slc26a1 (Sat-1) was characterized as a Na⁺-independent SO₄²⁻ transporter (1). Given the transport characteristics of the founding member of the gene family, Slc26 proteins were assumed to be sulfate transporters. Disease phenotypes, clone characterization, and family additions demonstrate that the Slc26 proteins are anion transporters or channels (2–4). These proteins have varied tissue expression patterns. At one extreme, Slc26a5 in mammals is found in the hair cells of the inner ear (5), whereas Slc26a2 (DTDST) is virtually ubiquitous in epithelial tissues (2).Several Slc26 proteins are found in the epithelia of the lung, intestine, stomach, pancreas, and kidney, usually in apical membranes. Interestingly these are also tissues and membranes in which the cystic fibrosis transmembrane conductance regulator (CFTR)⁵ has been found functionally or by immunohistochemistry. Ko and co-workers (6–8) examined the distribution of Slc26a3 and Slc26a6 in HCO₃⁻ secretory epithelia, and asked if an interaction might occur between these Slc26 proteins and CFTR. In particular, these studies indicate that in expression systems, there is a reciprocal-stimulatory interaction of the STAS (sulfate transporter anti-sigma) domains of Slc26a3 and Slc26a6 with the regulatory region (R-region) of CFTR. These investigators hypothesized that this stimulatory interaction could account for the differences in pancreatic insufficiency and sufficiency observed in cystic fibrosis patients. Nevertheless, knock-out Slc26a6 mouse studies reveal more complicated cell and tissue physiology (see “Discussion”).Slc26a9 has been reported to be a Cl⁻-HCO₃⁻ exchanger (9, 10) or a large Cl⁻ conductance (3, 11, 12). Loriol and co-workers (12) indicated that SLC26A9 has a Cl⁻ conductance that may be stimulated by HCO₃⁻. Two other groups have indicated that the Cl⁻ conductance is not affected by the presence of HCO₃⁻ (10, 11). We have recently demonstrated that Slc26a9 functions as both an electrogenic nCl⁻-HCO₃⁻ exchanger and a Cl⁻ channel (10). Dorwart and colleagues (11) found that WNK kinases inhibited the SLC26A9 Cl⁻ conductance but that this effect was independent of kinase activity. One group has a preliminary report indicating that WNK3 decreased Cl⁻ uptake, whereas WNK4 increased Cl⁻ uptake via Slc26a9 expressed in Xenopus oocytes (13).Slc26a9 and CFTR are also co-expressed in several tissues. Slc26a9 protein has been localized to epithelia of the stomach and lung (9, 10, 14), although mRNA is also detectable in brain, heart, kidney, small intestine, thymus, and ovary (10). The R-region of CFTR was previously shown to increase the activity of Slc26a3 and Slc26a6 by interaction with STAS domains (6, 15, 16). Because Slc26a9 displays several different modes of ion transport, we asked if the R-region of CFTR would also increase the activity of Slc26a9. Our results indicate that the R-region of CFTR does interact with the STAS domain of Slc26a9. However, in the case of Slc26a9 this apparently similar interaction results in inhibition of Slc26a9 ion transport.     

Heterodimerization of the Sialidase NEU1 with the Chaperone Protective Protein/Cathepsin A Prevents Its Premature Oligomerization

Lysosomal neuraminidase-1 (NEU1) forms a multienzyme complex with β-galactosidase and protective protein/cathepsin A (PPCA). Because of its association with PPCA, which acts as a molecular chaperone, NEU1 is transported to the lysosomal compartment, catalytically activated, and stabilized. However, the mode(s) of association between these two proteins both en route to the lysosome and in the multienzyme complex has remained elusive. Here, we have analyzed the hydrodynamic properties of PPCA, NEU1, and a complex of the two proteins and identified multiple binding sites on both proteins. One of these sites on NEU1 that is involved in binding to PPCA can also bind to other NEU1 molecules, albeit with lower affinity. Therefore, in the absence of PPCA, as in the lysosomal storage disease galactosialidosis, NEU1 self-associates into chain-like oligomers. Binding of PPCA can reverse self-association of NEU1 by causing the disassembly of NEU1-oligomers and the formation of a PPCA-NEU1 heterodimeric complex. The identification of binding sites between the two proteins allowed us to create innovative structural models of the NEU1 oligomer and the PPCA-NEU1 heterodimeric complex. The proposed mechanism of interaction between NEU1 and its accessory protein PPCA provides a rationale for the secondary deficiency of NEU1 in galactosialidosis.Mammalian neuraminidases have been classified as lysosomal (NEU1),⁴ cytosolic (NEU2), plasma membrane (NEU3), and mitochondria/lysosomal (NEU4) based on their subcellular distributions, pH optimum, kinetic properties, responses to ions and detergents, and substrate specificities (1–3). Of the four sialidases, only NEU1 is ubiquitously expressed at different levels in various tissues and cell types (4–7). The importance of these proteins in normal cellular physiology is illustrated by the numerous metabolic processes that they control, including cell proliferation and differentiation, cell adhesion, membrane fusion and fluidity, immunocyte function, and receptor modification (8–21).NEU1 initiates the intralysosomal hydrolysis of sialo-oligosaccharides, -glycolipids, and -glycoproteins by removing their terminal sialic acid residues. In human and murine tissues, NEU1 forms a complex with at least two other proteins, β-galactosidase and the protective protein/cathepsin A (PPCA) (22). By virtue of their association with PPCA, NEU1 and β-galactosidase acquire their active and stable conformation in lysosomes. However, PPCA appears to function as a crucial chaperone/transport protein for NEU1. Because NEU1 is poorly mannose 6-phosphorylated, it depends on PPCA for correct compartmentalization and catalytic activation in lysosomes (23–25). Only a small amount of PPCA and β-galactosidase activities is found in the NEU1-PPCA-β-galactosidase complex, which instead contains all of the NEU1 catalytic activity (24–27). By understanding how and when NEU1 and PPCA interact, how they regulate each other in different cell types, and what determinants control their association, we may gain important insight into their significance in physiologic and pathologic conditions.The absence of NEU1 is associated with two neurodegenerative diseases that involve glycoprotein metabolism; sialidosis, which is caused by structural lesions in the lysosomal NEU1 locus (28), and galactosialidosis (GS), a combined deficiency of NEU1 and β-galactosidase which is caused by the absence of PPCA (22). Patients with sialidosis and those with GS have similar clinical and biochemical features, and both diseases are characterized by multiple phenotypes that are classified according to the age of onset and severity of the symptoms.Previously, we generated two animal models of primary or secondary NEU1 deficiency, Neu1^−/− mice and Ppca^−/− mice. Both mouse models have a profound loss of Neu1 activity in multiple tissues and develop clinical, biochemical, and pathologic manifestations resembling those seen in patients with severe sialidosis and GS (29–31). Neu1^−/− mice are phenotypically similar but not identical to Ppca^−/− mice and, like children with the disease, exhibit a time-dependent splenomegaly associated with extramedullary hematopoiesis (30, 31). We found that the cause of these phenotypic abnormalities is the gradual loss of retention of hematopoietic progenitors within the bone niche due to exacerbated lysosomal exocytosis of bone marrow cells. The latter process is negatively regulated by NEU1 activity (31).The mode of interaction between PPCA and NEU1 and the mechanism of catalytic activation are not well understood. Here we present biochemical, analytical, and structural analyses of NEU1, PPCA, and the PPCA-NEU1 complex by using purified baculovirus (BV)-expressed wild-type and mutagenized recombinant enzymes and synthetic peptides.     

A Novel Function of Heparan Sulfate in the Regulation of Cell-Cell Fusion

Despite the important contribution of cell-cell fusion in the development and physiology of eukaryotes, little is known about the mechanisms that regulate this process. Our study shows that glycosaminoglycans and more specifically heparan sulfate (HS) expressed on the cell surface and extracellular matrix may act as negative regulator of cell-cell fusion. Using herpes simplex virus type-1 as a tool to enhance cell-cell fusion, we demonstrate that the absence of HS expression on the cell surface results in a significant increase in cell-cell fusion. An identical phenomenon was observed when other viruses or polyethylene glycol was used as fusion enhancer. Cells deficient in HS biosynthesis showed increased activity of two Rho GTPases, RhoA and Cdc42, both of which showed a correlation between increased activity and increased cell-cell fusion. This could serve as a possible explanation as to why HS-deficient cells showed significantly enhanced cell-cell fusion and suggests that HS could regulate fusion via fine tuning of RhoA and Cdc42 activities.Cell-cell fusion is an important physiological process widespread in organisms ranging from yeast to humans (1). It is critical for several biological phenomena including fertilization, placenta formation, skeletal muscle and bone development, tumorigenesis, immune response, and stem cell differentiation (1–9). Defects in cell-cell fusion can lead to serious diseases, such as myotonic dystrophy, centronuclear myopathy, preeclampsia, and osteopetrosis (10–13). Defects in sperm-egg fusion are a major cause of infertility (5). Cell-cell fusion has also been utilized for therapeutic applications, including the generation of monoclonal antibody-producing hybridomas (14) as well as new agents for cancer immunotherapy (15–17).Because of its critical nature, many studies have looked at the mechanism by which cell-cell fusion occurs. Although it can occur in a variety of different biological processes, many of the fusion events share common characteristics (8). For example, tetraspanin proteins function in gamete-, myoblast-, macrophage-, and virus-mediated fusion events (18–21). Although many mediators of cell-cell fusion are known, little is known about the fine-tuning mechanisms that may regulate the membrane fusion process.Viruses have been a useful tool for studying cell-cell fusion since the discovery that they could induce the fusion of somatic cells in vitro (22). Enveloped viruses, like herpes simplex virus type-1 (HSV-1),² use transmembrane viral proteins to mediate fusion with the host cell during entry and spread (23–25). For HSV-1, fusion occurs after the virus has attached to host cells by binding to heparan sulfate (HS) using glycoproteins gB and gC (26). Fusion of the virus envelope with the plasma membrane requires that an additional glycoprotein, gD, binds to one of its receptors, a process that also requires HSV-1 gB, gH, and gL (27–29). During HSV-1-mediated cell-cell fusion, gB, gD, gH, and gL are expressed on the surface of infected cells, allowing them to bind and fuse with surrounding uninfected cells, forming syncytia.Heparan sulfate proteoglycans are ubiquitously expressed cell surface molecules composed of a protein core, commonly syndecan, covalently attached to one or more HS glycosaminoglycan (GAG) side chains via a linker region (30). HS polysaccharide chains are composed of alternating hexuronic acid and d-glucosamine units (30, 31). HS chains undergo extensive modifications during their biosynthesis, including sulfation and epimerization, resulting in a variety of structurally diverse HS chains (30, 32–33). This diversity allows HS to interact with an array of functionally unrelated proteins and participate in various processes, such as the regulation of embryonic development, angiogenesis, blood coagulation, growth factor/cytokine interactions, cell adhesion, and lipid metabolism (30).Much remains to be learned about the cell-cell fusion mechanism and regulation of this phenomenon. The purpose of our study was to examine the effect of HS on cell-cell fusion and how it may function in the fusion mechanism. Using HSV-1 as a tool, we discovered that the absence of HS from the cell surface significantly enhanced the ability of cells to fuse with each other. This effect was also seen independently of HSV-1 in cells that neither expressed HSV-1 glycoproteins nor their receptors. This suggests a novel role for HS as a negative regulator and a fine-tuner of cell-cell fusion events.     

A Functional P2X7 Splice Variant with an Alternative Transmembrane Domain 1 Escapes Gene Inactivation in P2X7 Knock-out Mice

The ATP-activated P2X7 receptor channel is involved in immune function and inflammatory pain and represents an important drug target. Here we describe a new P2X7 splice variant (P2X7(k)), containing an alternative intracellular N terminus and first transmembrane domain encoded by a novel exon 1 in the rodent P2rx7 gene. Whole cell patch clamp recordings of the rat isoform expressed in HEK293 cells revealed an 8-fold higher sensitivity to the agonist Bz-ATP and much slower deactivation kinetics when compared with the P2X7(a) receptor. Permeability measurements in Xenopus oocytes show a high permeability for N-methyl-d-glucamine immediately upon activation, suggesting that the P2X7(k) channel is constitutively dilated upon opening. The rates of agonist-induced dye uptake and membrane blebbing in HEK cells were also increased. PCR analyses and biochemical analysis by SDS-PAGE and BN-PAGE indicate that the P2X7(k) variant escapes gene deletion in one of the available P2X7^−/− mice strains and is strongly expressed in the spleen. Taken together, we describe a novel P2X7 isoform with distinct functional properties that contributes to the diversity of P2X7 receptor signaling. Its presence in one of the P2X7^−/− strains has important implications for our understanding of the role of this receptor in health and disease.P2X receptors (P2XRs)³ are ATP-gated cation channels. They consist of three subunits (1, 2) each containing two transmembrane domains (TMDs) linked by an extracellular ligand-binding domain (3). The P2X7 receptor is distinguished from other P2X receptors by its long intracellular C terminus, a low ATP sensitivity (EC₅₀: 100 μm to 1 mm), and its ability to induce “cell permeabilization,” meaning that upon prolonged ATP application the opening of a permeation pathway for large molecules is induced. This process eventually leads to apoptosis, requires the C terminus (3–6), and may be mediated by interaction with pannexin hemichannels (7). In addition, “pore dilation,” which allows the passage of the large cation NMDG, is observed if extracellular sodium is replaced by NMDG (8), a property also displayed by the P2X2 (9) and P2X4 (10) receptors. This pore dilation is assumed to represent an intrinsic property of these P2X receptors (11, 12), although it can be influenced by interaction with intracellular proteins (13). However, both processes are still poorly understood.P2X7 receptors are found on cells of the hematopoietic lineage, in epithelia, and endothelia. Several studies report its expression and/or function in neurons, although its presence here is under debate (14, 15). So far, nine splice variants (P2X7(b) through P2X7(j)) have been described, only one of which was shown to be, at least partially, functional (16, 17). In addition, numerous single nucleotide polymorphisms have been identified in the human P2X7 receptor. Some of these have been found to cause either gain or loss of function and have been associated with chronic lymphocytic leukemia, bone fracture risk, and impaired immune functions (18–20). Recent genetic studies indicate an association between the Gln-460 → Arg polymorphism and familial depressive disorders (21).Two P2X7-deficient mouse lines have been described. In the mouse line generated by Glaxo, the P2rx7 gene was knocked out by insertion of a lacZ transgene into exon 1 (22). In the mouse line generated by Pfizer (23) a neomycin cassette was inserted into exon 13, replacing a region that encodes Cys-506–Pro-532 of the intracellular C terminus of the receptor. The Pfizer P2X7 KO mice demonstrated the involvement of this receptor in bone formation (24), cytokine production, and inflammation (23, 25) while the Glaxo^−/− mice established its role in inflammatory and neuropathic pain (26). All these findings and multiple subsequent studies based on these mouse models defined the P2X7R as a promising target for the development of innovative therapeutic strategies, and selective P2X7 inhibitors are already in clinical trials for the treatment of rheumatoid arthritis (27).Here, we describe a novel P2X7 isoform with an alternative N terminus and TMD 1. Compared with the originally identified P2X7(a) variant, it has increased agonist sensitivity and a higher propensity to form NMDG-permeable pores and permit dye uptake. Due to a novel alternative exon 1 and translation start, this splice variant escapes inactivation in the Glaxo P2X7^−/− mice and thus could account for possible inconsistencies between results obtained with different P2X7^−/− mouse lines (28).