A covariotide model explains apparent phylogenetic structure of oxygenic photosynthetic lineages

The aims of the work were (1) to develop statistical tests to identify whether substitution takes place under a covariotide model in sequences used for phylogenetic inference and (2) to determine the influence of covariotide substitution on phylogenetic trees inferred for photosynthetic and other organisms. (Covariotide and covarion models are ones in which sites that are variable in some parts of the underlying tree are invariable in others and vice versa.) Two tests were developed. The first was a contingency test, and the second was an inequality test comparing the expected number of variable sites in two groups with the observed number. Application of these tests to 16S rDNA and tufA sequences from a range of nonphotosynthetic prokaryotes and oxygenic photosynthetic prokaryotes and eukaryotes suggests the occurrence of a covariotide mechanism. The degree of support for partitioning of taxa in reconstructed trees involving these organisms was determined in the presence or absence of sites showing particular substitution patterns. This analysis showed that the support for splits between (1) photosynthetic eukaryotes and prokaryotes and (2) photosynthetic and nonphotosynthetic organisms could be accounted for by patterns arising from covariotide substitution. We show that the additional problem of compositional bias in sequence data needs to be considered in the context of patterns of covariotide/covarion substitution. We argue that while covariotide or covarion substitution may give rise to phylogenetically informative patterns in sequence data, this may not always be so.      

Myelin Deficits Produced by Early Postnatal Exposure to Inorganic Lead or Triethyltin Are Persistent

Abstract: Long-Evans rat pups were exposed to either inorganic lead (400 mg Pb as lead acetate/kg body weight/day) or triethyltin sulfate (1.0 mg/kg body weight/day), by gastric intubation, from 2 days through 29 days of age. The rats were then weaned and placed on standard lab chow ad libitum. At 30 days of age, leadtreated rats exhibited statistically significant decreases in body and brain weights (22% and 17%, respectively), and the concentration of forebrain myelin was significantly reduced, by 21% relative to the 4.9 mg myelin protein/g brain in control rats. Although these animals recovered from the body weight deficits after several months, the deficits in brain weight and myelin concentration were still present at 120 days of age. This suggests that the lead-induced myelin deficits were permanent. Lead levels in brain, which were maximal at 30 days of age when the treatment was terminated, decreased more slowly than in other organs and were still 30% of maximal levels at 120 days of age. Triethyltin-treated animals also had significantly decreased body and brain weights (20% and 11%, respectively) at 30 days of age, and an even more severe reduction in forebrain myelin concentration (33%). These animals also regained a normal body weight by 120 days of age, but again the deficits in brain weight and myelin concentration persisted. Tin levels in brain and other organs had decreased to control levels by 60 days of age. Animals malnourished by maternal deprivation to match the body weights of the treated animals had myelin deficits that were less severe than those in the treated animals at 30 days of age (approximately 11% less than controls); however, these myelin deficits also persisted throughout the subsequent 90-day recovery period examined. The apparent lack of recovery from CNS myelin deficits produced by neonatal exposure to different heavy metals or to malnutrition reemphasizes the vulnerability of the developing nervous system to a wide range of metabolic insults.     

Different patterns of arachidonate metabolism in autologous human blood monocytes and alveolar macrophages

As peripheral blood monocytes (PBM) differentiate into tissue macrophages, they undergo a variety of functional changes. One such difference which has been described is an enhanced metabolism of arachidonic acid (AA) via the 5-lipoxygenase (5-LO) pathway in alveolar macrophages (AM) as compared to PBM. In order to elucidate a possible mechanism for this difference, we compared the metabolism of endogenously released AA mobilized by agonists and of exogenously supplied fatty acid in adherent autologous PBM and AM obtained from six normal subjects. Exogenous AA was metabolized to larger amounts of both cyclooxygenase (CO) and 5-LO products by PBM as compared with AM. Although the two cell types released similar amounts of endogenous AA in response to ionophore A23187, marked differences in the pattern of its metabolism were observed. In PBM, a large proportion of released AA remained unmetabolized, and that which was metabolized was converted predominantly to CO products. In contrast, arachidonate released by AM was efficiently metabolized, predominantly via the 5-LO pathway. Similar results were obtained when cells were stimulated with the particulate zymosan, with PBM synthesizing mainly CO and AM, mainly 5-LO eicosanoids. In addition, culture of PBM for up to 5 days in an aerobic environment did not alter their response to A23187 stimulation. These results suggest that the lesser 5-LO metabolism by PBM than AM is not explained by lesser phospholipase or 5-LO activities, but rather a compartmentalization of the endogenous AA deacylated by phospholipase and the 5-LO enzyme in the PBM. The acquisition of the capacity to metabolize endogenous AA to large quantities of 5-LO products as mononuclear phagocytes differentiate in the lung may equip them with the ability to mount an inflammatory response in the alveolar space.     

Lipid Metabolism During Early Stages of Wallerian Degeneration in the Rat Sciatic Nerve

We examined changes in biosynthetic capacity of sciatic nerve during the early stages of Wallerian degeneration, utilizing a model that permits exclusion of nonresident cells from degenerating nerve. Sciatic nerve segments were placed in either 5-microns pore (allowing infiltration of nonresident cells) or 0.22-microns pore (excluding nonresident cells) Millipore diffusion chambers and then implanted in the peritoneal cavity of the same 32-34-day-old rat. At times up to 7 days postsurgery, nerve segments from the chambers, as well as control segments from the contralateral sciatic nerve, were removed and their capacity to incorporate radioactive precursors into lipids and proteins assayed in vitro. In nerve segments from both the 0.22- and 5-microns pore chambers, incorporation of [14C]acetate into total lipids was decreased relative to control by 50% at 12 h postsurgery and by 85% at day 3. This decreased incorporation of [14C]acetate reflects primarily decreased de novo synthesis of cholesterol and of fatty acyl residues incorporated into glycerolipids and sphingolipids. There was a preferentially decreased synthesis of cerebrosides and cholesterol (components enriched in myelin) relative to other lipids, while cholesterol esters and free fatty acids (products of membrane degradation) accounted for a greater proportion of the greatly reduced levels of total lipid label. In contrast to [14C]acetate, incorporation of [3H]glycerol into lipids was increased up to fourfold, relative to control, 1 day after nerve transection.(ABSTRACT TRUNCATED AT 250 WORDS)     

Turnover of Axonally Transported Phospholipids in Nerve Endings of Retinal Ganglion Cells

We have investigated the metabolic turnover of axonally transported phospholipids in myelinated axons (optic tract) and nerve endings (superior colliculus) of retinal ganglion cells. One week following intraocular injection of [2-3H]glycerol, turnover rates for individual phospholipid classes in the retina (which contains a number of other cell types in addition to the ganglion cells) were all very similar to each other, with apparent half-lives of approximately 7 days. Apparent half-lives of labeled phospholipids in superior colliculus (presumably primarily in retinal ganglion cell nerve endings) were 10 days for both choline and inositol phosphoglycerides and 13 days for both serine and diacylethanolamine phosphoglycerides. Subcellular fractionation data obtained from superior colliculus at various times after injection suggested that apparent turnover rates determined for nerve ending phospholipids probably were not significantly affected by transfer of axonally transported 3H lipids into myelin. Apparent half-lives for phospholipids in optic tract were somewhat longer than in superior colliculus, ranging from 11 to 18 days. The slower turnover rates in optic tract may, in part, reflect the transfer of some axonal lipids to the more metabolically stable pool of lipids in the myelin ensheathing the retinal ganglion cell axons. In both optic tract and superior colliculus, apparent half-lives for axonally transported phospholipids labeled with [32P]phosphate were only slightly longer than for [2-3H]glycerol, while those for [14C]choline and [3H]acetate were markedly longer, indicating differing degrees of metabolic conservation or reutilization of these precursors relative to glycerol.     

Synthesis of antithrombin III and alpha-1-antitrypsin by the perfused rat liver.

Livers isolated from control or turpentine-injected rats were perfused for 3 h with human red cells suspended in Krebs-Henseleit solution containing bovine serum albumin, dextran, glucose, heparin, cortisol, insulin, a mixture of 20 amino acids and [3H]leucine. Changes in the concentrations of antithrombin III and alpha-1-antitrypsin were evaluated by rocket immunoelectrophoresis using specific antisera, and incorporation of the 3H radioactivity into the total protein, albumin, antitrhombin III and alpha-1-antitrypsin in the perfusate was measured. The results indicate that both antithrombin III and alpha-1-antitrypsin are synthesized in the liver. Local inflammation induced in the liver donors moderately stimulated the synthesis of alpha-1-antitrypsin but it affected only marginally that of antithrombin III.     

Axonal transport of glycerophospholipids following intracerebral injection of glycerol into substantia nigra or lateral geniculate body

[2-³H]Glycerol was injected into one substantia nigra of adult rats. Incorporation of radioactivity into lipids at the injection site was maximal by 2 hr, after which it declined. Rapidly transported³H-labeled lipids were just beginning to accumulate in the primary projection site, the ipsilateral corpus striatum by 2 hr, as evidenced by 20-fold higher levels of lipid radioactivity in the projection site relative to control regions. However, the bulk of labeled lipid arrived between 6 hr and 3 days postinjection, suggesting either a prolonged period of release of rapidly transported lipids from the nerve cell bodies or a slow rate of transport for the later arriving lipids. Colchicine applied locally to the fibers of this tract blocked the axonal transport of lipids to the striatum almost completely. Choline and ethanolamine phosphoglycerides were the major transported lipids, accounting for approximately 60% and 25%, respectively, of the total. Similar results were obtained in studies of [2-³H]glycerol-labeled lipids synthesized in the lateral geniculate body and transported to the visual cortex. The rapid axonal transport of lipids labeled with [³²P]phosphate (injected simultaneously with [2-³H]glycerol) could also be demonstrated in both tracts. However, in contrast to [2-³H]glycerol, considerable amounts of³²P soluble label were present in the projection sites, and colchicine only partially blocked the accumulation of³²P-labeled lipid. These results demonstrate the relative utility of [2-³H]glycerol as a lipid precursor for examination of axonal transport in intrabrain tracts. Characteristics of lipid axonal transport in these two intrabrain tracts are similar to each other and are also similar to those previously described for retinal ganglion cells, indicating a common requirement for the axonal transport of these membrane constituents to axons and nerve endings in widely divergent CNS tracts.Presented in part at the 11th meeting of the American Society for Neurochemistry, Houston, Texas, March 1980.     

Axonal transport of phospholipids in rat visual system.

Abstract— Seventeen day old rats were injected intraocularly with a phospholipid precursor, [³²P]phosphate, and a glycoprotein precursor, [³H]fucose. Animals were killed between 1 h and 21 days later, and structures of the visual pathway (retina, optic nerve, optic tract, lateral geniculate body, and superior colliculus) were dissected. Radioactivity in phospholipids ([³²P] in solvent-extracted material) and in glycoproteins ([³H] in solvent-extracted residue) was determined. Incorporation of [³H]fucose into retinal glycoproteins peaked at 6–8 h. Labelled glycoproteins were present in superior colliculus by 2h after injection, indicating a rapid rate of transport; maximal labelling was at 8–10 h after injection. Incorporation of [³²P]phosphate into retinal phospholipids peaked at 1 day after injection. Phospholipids were also rapidly transported since label was present in the superior colliculus by 3 h after injection: however, maximal labelling did not occur until 5–6 days. These results indicate that newly synthesized phospholipids enter a preexisting pool, part of which is later committed to transport at a rapid rate. Transported phospholipids were catabolized at the nerve endings with a maximum half-life of several days; there was minimal recycling of precursor label. Lipids were fractionated by thin-layer chromatography, and radioactivity in individual phospholipid classes determined. Choline and ethanolamine phosphoglycerides were the major transported phospholipids, together accounting for approx 85% of the total transported lipid radioactivity. At early time points, the ratio of radioactivity in choline phosphoglycerides to that in ethanolamine phosphoglycerides increased in structures progressively removed from the site of synthesis (retina) but by 2 days approached a constant value. In each structure, choline phosphoglyceride-ethanolamine phosphoglyceride radioactivity ratios decreased with time, rapidly at first, but plateaued by 2 days. These results indicate that choline phosphoglycerides are committed to transport sooner than ethanolamine phosphoglycerides. Some experiments were also conducted using [2-³H]glycerol as a phospholipid precursor. Results concerning incorporation of this precursor into individual phospholipid classes and their subsequent axonal transport were comparable to those obtained using [³²P]phosphate, with the following exceptions: (a) incorporation of [2-³H]glycerol into retinal phospholipids was relatively rapid (near-maximal levels at 1 h after injection) although transport to the superior colliculus showed an extended time course very similar to [³²P]-labelled lipids; (b) [2-³H]glycerol was somewhat less efficient than [³²P]phosphate in labelling lipids committed to transport relative to labelling those which remained in the retina; and (c) [2-³H]glycerol did not label plasmalogens.