Neurotrophic growth factors stimulate glycosaminoglycan synthesis in identified retinal cell populations in vitro

Glycosaminoglycans (GAG) are known to participate in central nervous system processes such as development, cell migration, and neurite outgrowth, but little is known with respect to their regulation through soluble neurotrophic factors. In the present study, we have addressed this issue using cell culture models of three distinct cell populations derived from young rat retinas, namely, purified M uller glia, pigmented epithelium, and neurons respectively. Cultures were maintained in chemically defined media in the presence or absence of either basic fibroblast or epidermal growth factor. In control glial and epithelial cultures, hyaluronic acid dominated the soluble GAG pool, with lesser contributions from dermatan sulfate, chondroitin sulfate, and heparan sulfate (in decreasing order). Retinal neuronal GAG were almost exclusively chondroitin sulfate (approximately 90%). Treatment of glial and epithelial cultures with either factor led to dose-dependent increases in especially hyaluronic acid synthesis (a maximum 6-fold increase relative to control levels), with smaller but consistent changes in chondroitin sulfate. Similar treatment of retinal neurons did not lead to any changes in GAG synthesis. These data indicate that glia and pigment epithelia are the principal sources of GAG components in retina at least in vitro, and that endogenous neurotrophic growth factors can greatly modify GAG synthesis in these two retinal cell populations. Such data suggest that a delicate balance may exist between growth factor availability and glycoconjugate metabolism in vivo, participating in normal or pathological states of the retina.      

Properties of monoclonal antibodies specific for determinants of a protein antigen, myoglobin

Monoclonal hybridoma antibodies specific for the protein antigen sperm whale myoglobin were produced using hyperimmune spleen cells from mice with the genetic trait of high responsiveness to myoglobin. Antibodies from the several clones tested were found to produce linear Scatchard plots, as predicted for homogeneous antibodies, and to possess high affinities for the immunogen (KA congruent to 10(9) M-1). None of the monoclonal antibodies tested reacted with either fragment (1-55) or fragment (132-153) of sperm whale myoglobin. Competitive binding assays using human and horse myoglobins suggested that several of these monoclonal antibodies, which can readily distinguish these myoglobins, recognize different antigenic determinants on the myoglobin molecule. Studies using additional myoglobin sequence variants as competitors should be able to more closely define these antigenic determinants.     

M wave potentiation during and after muscle activity

The M wave (muscle compound action potential) has been shown to enlarge between successive 3-s voluntary contractions of the human thenar and extensor digitorum brevis (EDB) muscles. The changes, which affected both the amplitude and the area of the M wave, were more obvious in the thenar than in the EDB muscles. In the thenar muscles the mean amplitude was already significantly enlarged after the first voluntary contraction and close to the maximal value by the third (mean maximal increase 23.6 +/- 12.6% of control). The increase in mean M wave area was more gradual, reaching a maximum of 29.3 +/- 14.1% at 100 s. After the voluntary thenar contractions ceased, the amplitude of the M wave subsided more rapidly than the area and had regained the control value within 50 s. The magnitude and time course of the increase in EDB M wave area (maximum change 25.9 +/- 15.2%) were similar to those of the thenar muscles; however, the subsequent decline was slower. The amplitude of the EDB M wave showed the least change, and the maximum increase (11.4 +/- 9.6%) occurred early in the postcontraction period. In both muscles the changes in M wave amplitude and area were significantly different from the control values.     

Kinetic Basis for the Conjugation of Auxin by a GH3 Family Indole-acetic Acid-Amido Synthetase

The GH3 family of acyl-acid-amido synthetases catalyze the ATP-dependent formation of amino acid conjugates to modulate levels of active plant hormones, including auxins and jasmonates. Initial biochemical studies of various GH3s show that these enzymes group into three families based on sequence relationships and acyl-acid substrate preference (I, jasmonate-conjugating; II, auxin- and salicylic acid-conjugating; III, benzoate-conjugating); however, little is known about the kinetic and chemical mechanisms of these enzymes. Here we use GH3-8 from Oryza sativa (rice; OsGH3-8), which functions as an indole-acetic acid (IAA)-amido synthetase, for detailed mechanistic studies. Steady-state kinetic analysis shows that the OsGH3-8 requires either Mg²⁺ or Mn²⁺ for maximal activity and is specific for aspartate but accepts asparagine as a substrate with a 45-fold decrease in catalytic efficiency and accepts other auxin analogs, including phenyl-acetic acid, indole butyric acid, and naphthalene-acetic acid, as acyl-acid substrates with 1.4–9-fold reductions in k_cat/K_m relative to IAA. Initial velocity and product inhibition studies indicate that the enzyme uses a Bi Uni Uni Bi Ping Pong reaction sequence. In the first half-reaction, ATP binds first followed by IAA. Next, formation of an adenylated IAA intermediate results in release of pyrophosphate. The second half-reaction begins with binding of aspartate, which reacts with the adenylated intermediate to release IAA-Asp and AMP. Formation of a catalytically competent adenylated-IAA reaction intermediate was confirmed by mass spectrometry. These mechanistic studies provide insight on the reaction catalyzed by the GH3 family of enzymes to modulate plant hormone action.