TIE2 is a receptor-like tyrosine kinase expressed almost exclusively in endothelial cells and early hemopoietic cells and required for the normal development of vascular structures during embryogenesis. We report the identification of a secreted ligand for TIE2, termed Angiopoietin-1, using a novel expression cloning technique that involves intracellular trapping and detection of the ligand in COS cells. The structure of Angiopoietin-1 differs from that of known angiogenic factors or other ligands for receptor tyrosine kinases. Although Angiopoietin-1 binds and induces the tyrosine phosphorylation of TIE2, it does not directly promote the growth of cultured endothelial cells. However, its expression in close proximity with developing blood vessels implicates Angiopoietin-1 in endothelial developmental processes.

We report the identification of ligands for Tyro 3 (alternatively called Sky, rse, brt, or tif) and Axl (alternatively, Ark or UFO), members of a previously orphan family of receptor-like tyrosine kinases. These ligands correspond to protein S, a protease regulator that is a potent anticoagulant, and Gas6, a protein related to protein S but lacking any known function. Our results are reminiscent of recent findings that the procoagulant thrombin, a protease that drives clot formation by cleaving fibrinogen to form fibrin, also binds and activates intracellular signaling via a G protein-coupled cell surface receptor. Proteases and protease regulators that also activate specific cell surface receptors may serve to integrate coagulation with associated cellular responses required for tissue repair and growth, as well as to coordinate protease cascades and associated cellular responses in other systems, such as those involved in growth and remodeling of the nervous system.

Formation of th neuromuscular junction depends upon reciprocal inductive interactions between the developing nerve and muscle, resulting in the precise juxtaposition of a differentiated nerve terminal with a highly specialized patch on the muscle membrane, termed the motor endplate. Agrin is a nerve-derived factor that can induced molecular reorganizations at the motor endplate, but the mechanism of action of agrin remains poorly understood. MuSK is a receptor tyrosine kinase localized to the motor endplate, seemingly well positioned to receive a key nerve-derived signal. Mice lacking either agrin or MuSK have recently been generated and exhibit similarly profound defects in their neuromuscular junctions. Here we demonstrate that agrin acts via a receptor complex that includes MuSK as well as a myotube-specific accessory component.

Mammalian cells constantly monitor and respond to a myriad of extracellular signals, often by using cell surface receptors. Two important classes of cell surface receptors include the receptor tyrosine kinases, which recognize peptide growth factors such as insulin, and the integrins, which most often mediate binding to components of the extracellular matrix. We report that the collagens serve as ligands for the previously orphan family of discoidin domain-containing receptor-like tyrosine kinases. The unexpected realization that an extracellular matrix molecule can directly serve as a ligand for receptor tyrosine kinases provides an example of ligands shared by integrins and receptor tyrosine kinases, and this finding seems likely to change prevailing views about the mechanisms by which cells perceive and respond to the extracellular matrix.

Near-infrared spectroscopy (NIRS) is a well-known method used to measure muscle oxygenation and hemodynamics in vivo. The application of arterial occlusions allows for the assessment of muscle oxygen consumption (mVo(2)) using NIRS. The aim of this study was to measure skeletal muscle mitochondrial capacity using blood volume-corrected NIRS signals that represent oxygenated hemoglobin/myoglobin (O(2)Hb) and deoxygenated hemoglobin/myoglobin (HHb). We also assessed the reliability and reproducibility of NIRS measurements of resting oxygen consumption and mitochondrial capacity. Twenty-four subjects, including four with chronic spinal cord injury, were tested using either the vastus lateralis or gastrocnemius muscles. Ten healthy, able-bodied subjects were tested on two occasions within a period of 7 days to assess the reliability and reproducibility. NIRS signals were corrected for blood volume changes using three different methods. Resting oxygen consumption had a mean coefficient of variation (CV) of 2.4% (range 1-32%). The recovery of oxygen consumption (mVo(2)) after electrical stimulation at 4 Hz was fit to an exponential curve, which represents mitochondrial capacity. The time constant for the recovery of mVo(2) was reproducible with a mean CV of 10% (range 1-22%) only when correcting for blood volume changes. We also examined the effects of adipose tissue thickness on measurements of mVo(2). We found the mVo(2) measurements using absolute units to be influenced by adipose tissue thickness (ATT), and this relationship was removed when an ischemic calibration was performed, supporting its use to compare mVo(2) between individuals of varying ATT. In conclusion, in vivo oxidative capacity can be assessed using blood volume-corrected NIRS signals with a high degree of reliability and reproducibility.