Objectives-To investigate the role of matrix metalloproteinase (MMP)-13/collagenase-3 in osteoarthritis (OA).Methods-Surgically-induced OA in knees of MMP-13 knock out (KO) and wild type (WT) mice was compared. Femoral and tibial cartilage aggrecan loss (0-3), erosion (0-7) and chondrocyte hypertrophy (0-1), as well as osteophyte size (0-3) and maturity (0-3) were histologically scored. Serial sections were stained for collagen type X and the MMP-generated aggrecan neo-epitope DIPEN.Results-Following surgery, aggrecan loss and cartilage erosion were more severe in the tibia than femur (p<0.01) and tibial cartilage erosion increased with time (p<0.05) in WT mice. Cartilaginous osteophytes were present at 4 weeks and underwent ossification, with size and maturity increasing by 8 weeks (p<0.01). There was no difference between genotypes in aggrecan loss or cartilage erosion at 4 weeks. Tibial cartilage erosion in KO mice was less than WT at 8 weeks (p<0.02). Cartilaginous osteophytes were larger in KO at 4 weeks (p<0.01), but by 8 weeks osteophyte maturity and size were no different from WT. Articular chondrocyte hypertrophy with positive type X collagen and DIPEN staining occurred in both WT and KO joints.Conclusions-These studies have confirmed that structural cartilage damage in mouse experimental OA is dependent on MMP-13 activity. Chondrocyte hypertrophy is not regulated by MMP-13 activity in this model and does not in itself lead to cartilage erosion. MMP-13 deficiency can inhibit cartilage erosion in the presence of aggrecan depletion, supporting the potential for therapeutic intervention in established OA with MMP-13 inhibitors.Progressive erosion of articular cartilage is a significant determinant of prognosis and the need for joint replacement surgery in osteoarthritis (OA). Proteolysis of the principal NIH Public Access Author ManuscriptArthritis Rheum. Author manuscript; available in PMC 2010 December 1. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript cartilage extracellular matrix constituents, aggrecan and the type II/IX/XI collagen network, directly causes erosion as well as predisposing the tissue to mechanical disruption even with loading at physiological levels. Aggrecan proteolysis and loss precedes and may be prerequisite for subsequent collagenolysis (1). A distintegrin and metalloproteinase with thrombospondin repeat (ADAMTS) enzymes are responsible for pathological aggrecanolysis (2,3). ADAMTS-5 is the predominant arthritis-associated enzyme in mice, since animals deficient in ADAMTS-5 activity are protected from cartilage erosion in OA and inflammatory arthritis (4-6). Ablating the ADAMTS cleavage site in the interglobular domain of aggrecan also blocks cartilage structural damage, confirming that the effect in ADAMTS-5-deficient mice is due to inhibition of aggrecanolysis (7).The above studies demonstrate that inhibiting the initiation of aggrecan loss can prevent subsequent structural cartilage damage/erosion in arthritis. Clinically, it is likely that e...

Erk1/Erk2 MAP kinases are key regulators of cell behaviour and their activation is generally associated with tyrosine kinase signalling. However, TGF-beta stimulation also activates Erk MAP kinases through an undefined mechanism, albeit to a much lower level than receptor tyrosine kinase stimulation. We report that upon TGF-beta stimulation, the activated TGF-beta type I receptor (TbetaRI) recruits and directly phosphorylates ShcA proteins on tyrosine and serine. This dual phosphorylation results from an intrinsic TbetaRI tyrosine kinase activity that complements its well-defined serine-threonine kinase function. TGF-beta-induced ShcA phosphorylation induces ShcA association with Grb2 and Sos, thereby initiating the well-characterised pathway linking receptor tyrosine kinases with Erk MAP kinases. We also found that TbetaRI is tyrosine phosphorylated in response to TGF-beta. Thus, TbetaRI, like the TGF-beta type II receptor, is a dual-specificity kinase. Recruitment of tyrosine kinase signalling pathways may account for aspects of TGF-beta biology that are independent of Smad signalling.

The ion selectivity of pumps and channels is central to their ability to perform a multitude of functions. Here we investigate the mechanism of the extraordinary selectivity of the human voltage gated proton channel1, hHV1. This selectivity is essential to its ability to regulate reactive oxygen species production by leukocytes2–4, histamine secretion by basophils5, sperm capacitation6, and airway pH7. The most selective ion channel known, HV1 shows no detectable permeability to other ions1. Opposing classes of selectivity mechanisms postulate that (a) a titratable amino acid residue in the permeation pathway imparts proton selectivity1, 8–11, or (b) water molecules “frozen” in a narrow pore conduct protons while excluding other ions12. Here we identify Aspartate112 as a crucial component of the selectivity filter of hHV1. When a neutral amino acid replaced Asp112, the mutant channel lost proton specificity and became anion selective or did not conduct. Only the glutamate mutant remained proton specific. Mutation of the nearby Asp185 did not impair proton selectivity, suggesting that Asp112 plays a unique role. Although histidine shuttles protons in other proteins, when histidine or lysine replaced Asp112, the mutant channel was still anion permeable. Evidently, the proton specificity of hHV1 requires an acidic group at the selectivity filter.

Nitric oxide synthases (NOSs) are classified functionally, based on whether calmodulin binding is Ca 2؉ -dependent (cNOS) or Ca 2؉ -independent (iNOS). This key dichotomy has not been defined at the molecular level. Here we show that cNOS isoforms contain a unique polypeptide insert in their FMN binding domains which is not shared with iNOS or other related flavoproteins. Previously identified autoinhibitory domains in calmodulin-regulated enzymes raise the possibility that the polypeptide insert is the autoinhibitory domain of cNOSs. Consistent with this possibility, three-dimensional molecular modeling suggested that the insert originates from a site immediately adjacent to the calmodulin binding sequence. Synthetic peptides derived from the 45-amino acid insert of endothelial NOS were found to potently inhibit binding of calmodulin and activation of cNOS isoforms. This inhibition was associated with peptide binding to NOS, rather than free calmodulin, and inhibition could be reversed by increasing calmodulin concentration. In contrast, insert-derived peptides did not interfere with the arginine site of cNOS, as assessed from [ 3 H]N G -nitro-L-arginine binding, nor did they potently effect iNOS activity. Limited proteolysis studies showed that calmodulin's ability to gate electron flow through cNOSs is associated with displacement of the insert polypeptide; this is the first specific calmodulin-induced change in NOS conformation to be identified. Together, our findings strongly suggest that the insert is an autoinhibitory control element, docking with a site on cNOSs which impedes calmodulin binding and enzymatic activation. The autoinhibitory control element molecularly defines cNOSs and offers a unique target for developing novel NOS activators and inhibitors.Nitric oxide is a ubiquitous cell-signaling molecule, with protean roles in physiology and pathophysiology (1-3). Encoded by distinct genes, mammalian NO synthases (NOSs) 1 comprise a family of three calmodulin-dependent biopterohemoflavoproteins that are functionally distinguished by their modes of regulation (4). The two constitutively expressed isoforms of NOS (cNOSs), first identified in neuronal cells (nNOS) and endothelial cells (eNOS), remain dormant until calcium/calmodulin (Ca 2ϩ /CaM) binding is actuated by transient elevations in intracellular Ca 2ϩ . This Ca 2ϩ -dependent mode of regulation provides pulses of NO for moment-to-moment modulation of vascular tone and neurosignaling. In contrast, activity of the immunostimulant-induced isoform of NOS (iNOS) is Ca 2ϩ -independent, providing continuous high output NO generation for host defense. A remarkably high affinity for CaM, even at basally low levels of intracellular calcium, is responsible for the Ca 2ϩ independence of iNOS (5). Whether a given NOS isoform binds CaM in a Ca 2ϩ -dependent or -independent manner has been assumed to be a property solely of the amino acid sequence specified by a 20 -25-amino acid CaM binding site. However, this restrictive view is challenged by findings that ...

The reactive oxygen species (ROS)-producing enzyme NADPH oxidase (NOX) was first identified in the membrane of phagocytic cells. For many years, its only known role was in immune defense, where its ROS production leads to the destruction of pathogens by the immune cells. NOX from phagocytes catalyzes, via one-electron trans-membrane transfer to molecular oxygen, the production of the superoxide anion. Over the years, six human homologs of the catalytic subunit of the phagocyte NADPH oxidase were found: NOX1, NOX3, NOX4, NOX5, DUOX1, and DUOX2. Together with the NOX2/gp91phox component present in the phagocyte NADPH oxidase assembly itself, the homologs are now referred to as the NOX family of NADPH oxidases. NOX are complex multidomain proteins with varying requirements for assembly with combinations of other proteins for activity. The recent structural insights acquired on both prokaryotic and eukaryotic NOX open new perspectives for the understanding of the molecular mechanisms inherent to NOX regulation and ROS production (superoxide or hydrogen peroxide). This new structural information will certainly inform new investigations of human disease. As specialized ROS producers, NOX enzymes participate in numerous crucial physiological processes, including host defense, the post-translational processing of proteins, cellular signaling, regulation of gene expression, and cell differentiation. These diversities of physiological context will be discussed in this review. We also discuss NOX misregulation, which can contribute to a wide range of severe pathologies, such as atherosclerosis, hypertension, diabetic nephropathy, lung fibrosis, cancer, or neurodegenerative diseases, giving this family of membrane proteins a strong therapeutic interest.