Niche expansion and temperature sensitivity of tropical African montane forests

Aim Climate and land‐use change will have a dramatic impact on future ecosystems through alterations to species ranges and community composition. When forming conservation strategies, correlative species distribution models are often created to assess risks for individual species. These models are based on the assumption of climatic equilibrium, such that the modern range is representative of the full range of conditions under which species could thrive. However, the palaeo‐ecological record illustrates examples of disequilibrium in species today, and recent studies suggest that many species could occur in much broader climatic settings than previously thought. Montane ecosystems are thought to be at disproportionate risk due to temperature sensitivity and restricted geographical ranges. However, in the Afrotropics the palaeo‐ecological record shows that montane forest taxa expanded into the lowlands numerous times, suggesting a possible tolerance to warm temperatures. Location Africa. Methods We integrate palaeo‐ecological and palaeo‐climatic data in order to compare climate conditions in which species are currently found with those in the past. We use species distribution models to construct potential modern ranges for Afromontane species based on modern distributions and distributions in the palaeo‐ecological record in order to evaluate the equilibrium of species ranges. Results We show that many Afromontane trees have occupied warmer climates in the past, which suggests that the current low‐elevation boundaries are not set by climate. Interestingly, the species with the largest disequilibrium between palaeo‐ and modern distributions are those whose modern distributions show the least temperature sensitivity. Mapping of species potential ranges based on modern and palaeo‐ distributions clearly shows that suitable climate conditions exist today in the lowlands for less temperature‐sensitive species. Main conclusions These results imply that the current range of these forest trees does not necessarily inform risk from climatic change, and that human land use may be the major pressure for many species in the future.     

Functional reconstitution of the alpha 2-adrenergic receptor with guanine nucleotide regulatory proteins in phospholipid vesicles

We describe the successful reconstitution of functional interactions between an inhibitory adenylate cyclase-coupled receptor and various nucleotide-binding regulatory proteins in phospholipid vesicles. The receptor is the alpha 2-adrenergic receptor (alpha 2AR) which has been partially purified (approximately 500-5000-fold) from human platelet membranes. The nucleotide-binding regulatory proteins include purified preparations of human erythrocyte Ni and Ns, bovine retinal transducin and the recently discovered bovine brain No. Addition of the physiologic ligand, epinephrine, to vesicles containing the alpha 2AR and Ni results in stimulation of the GTPase activity in Ni. This stimulation of GTPase activity by epinephrine is prevented in the presence of the alpha-adrenergic antagonist, phentolamine, which indicates that a functional reconstitution of the alpha 2AR and Ni has been established. The maximum turnover number for the alpha 2AR-mediated epinephrine-stimulated GTPase activity in Ni is similar to the maximal turnover numbers obtained for the beta-adrenergic receptor-mediated isoproterenol-stimulated GTPase activity in Ns and the rhodopsin-mediated light-stimulated GTPase activity in transducin (0.5-1.5 mol of Pi released per min per mol of nucleotide regulatory protein). Functional similarities between the alpha 2AR and rhodopsin are observed in their interactions with the various nucleotide-binding regulatory proteins. Thus, both of these receptor proteins are capable of promoting the maximal activation of Ni and No while being much less effective in promoting the activation of Ns. However, there are differences between the alpha 2AR and rhodopsin in their interactions with transducin. Specifically, while rhodopsin will maximally activate transducin, the alpha 2AR is much less effective in promoting this activation (i.e. approximately 20% as effective as rhodopsin). Overall, these results suggest the following specificities of interaction: for rhodopsin, transducin approximately equal to Ni approximately equal to No much greater than Ns; while for alpha 2AR, Ni approximately equal to No greater than transducin greater than or equal to Ns.     

Synaptosomal Sialyltransferase Glycosylates Surface Proteins that Are Inaccessible to the Action of Membrane-Bound Sialidase

Sialyltransferase has been characterized in P2 pellets derived from animals of increasing age. The enzyme was found to be associated with the plasma membrane and to be developmentally regulated at times coincident with cell migration and fibre outgrowth. This regulation appeared to be due, in part, to an endogenous competitive inhibitor in the P2 pellet but not in the synaptosome. Optimal transfer of [14C]N-acetylneuraminic acid to endogenous synaptosomal acceptors was achieved only in the absence of detergent. Furthermore, the transferred sialic acid was found to be inaccessible to the action of membrane-bound sialidase. The significance of these findings is discussed.     

Mammalian alpha 1-adrenergic receptor. Purification and characterization of the native receptor ligand binding subunit

alpha 1-Adrenergic receptors from a cultured smooth muscle cell line (DDT1 MF-2) have been solubilized with digitonin and purified to apparent homogeneity by sequential chromatography on a biospecific affinity support (Sepharose-A55453 (4-amino-6,7-dimethoxy-2-[4-[5-(4-amino-3-phenyl) pentanoyl]-1-piperazinyl]-quinazoline), an alpha 1 receptor-selective antagonist), a wheat germ agglutinin-agarose gel, and a high performance steric exclusion liquid chromatography column. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography of iodinated purified receptor preparations reveals a peptide with an apparent Mr = 80,000 that co-migrates with the peptide labeled by the specific alpha 1-adrenergic receptor photoaffinity probe 4-amino-6,7-dimethoxy-2-[4-[5-(4-azido-3-[125I]iodophenyl)pentanoyl] -1-piperazinyl] quinazoline. The specific activity (approximately 13,600 pmol of ligand binding/mg of protein) of purified receptor preparations is consistent with that expected for a pure peptide of Mr = 80,000 containing a single ligand binding site. Overall yields approximate 14% of initial crude particulate binding. The purified receptor preparations bind agonist and antagonist ligands with appropriate alpha 1-adrenergic specificity, stereoselectivity, and affinity. Peptide maps of the pure alpha 1-adrenergic receptor and the pure human platelet alpha 2-adrenergic receptor (Regan, J.W., Nakata, H., DeMarinis, R.M., Caron, M.G., and Lefkowitz, R.J. (1986) J. Biol. Chem. 261, 3894-3900) using several different proteases suggest that these two receptors show little if any structural homology.     

Classification of chemical agents as to their ability to induce long-or short-patch DNA repair in human cells

33 chemical agentsand UV- and γ-irradiation were tested for their comparative ability to induce long-patch or short-patch repair using the 5-bromodeoxyuridine photolysis assay. For 11 chemical agents repair was long-patch in nature as determined by calculated patch size and response of xeroderma pigmentosum cells relative to normal human cells. Typical patch sizes as measured by this assay were about 90 nucleotides for UV repair, a range of 30 to 70 nucleotides for a variety of known and suspected UV-mimetic chemicals, and 3–4 nucleotides for γ-radiation. Alkaylating agens previously shown to induce short-patch repair were shown also to induce long-patch repair.     

DNA replication in human cells treated with methyl methanesulfonate

Methyl methanesulfonate (MMS) affects the production of DNA in human cells by reducing the rate of DNA synthesis and by causing the DNA to be synthesized in smaller than normal segments. DNA profiles from alkaline sucrose gradients from cells treated with MMS for 1 h and pulse-labeled at 2.5 h after treatment show more slow-sedimenting DNA than profiles from untreated cells or treated cells pulsed at 0.5 or 4 h after the 1-h treatment. Upon incubation of the pulse-labeled DNA there is an increase in the amount of fast-sedimenting DNA in each sample, indicating repair of the lesions.The amount of DNA synthesized is also reduced 2.5 h after a 1-h treatment but is at near normal levels at 0.5 and 4 h. The reduction in the size of the DNA segments synthesized and the reduction in the rate of DNA synthesis probably reflect the formation and repair of lesions in the parental DNA.     

Response of continuous cultures to stimuli in glucose feed rate and dilution rate

The response of continuous cultures of yeast was investigated following step disturbances in glucose feed rate and dilution rate. The responses of the culture to the stimuli were oscillatory. The oscillatory responses were explained in terms of cell synchrony which was induced by the step change. An understanding of continuous cultures to stimuli was made possible with an appreciation of the inherently oscillatory events occurring in the single cell cycle between one mitosis and the next. Step changes in glucose feed rate and dilution rate induced a partial synchrony, which enabled the inherently oscillatory behavior of the individual cells to be made observable in the culture as a whole.     

Evidence for Excision of Ultraviolet-Induced Pyrimidine Dimers from the DNA of Human Cells In Vitro

Within 12-24 hr after human cells were irradiated with ultraviolet light, approximately 50% of the ultraviolet-induced pyrimidine dimers were lost from the DNA. Pyrimidine dimers were found in the TCA-soluble fraction of ultraviolet-irradiated cells at 24 hr. Excess thymidine, caffeine, or hydroxyurea had no effect on the loss of pyrimidine dimers from the DNA of ultraviolet-irradiated cells.     

Stable expression of recombinant human alpha 2-adrenoceptor subtypes in two mammalian cell lines: characterization with [3H]rauwolscine binding, inhibition of adenylate cyclase and RNase protection assay.

Cloning of the genes encoding distinct subtypes of human alpha 2-adrenergic receptors (alpha 2-AR) allows the separate recombinant expression of each individual subtype in heterologous systems. We report here the transfection, selection and preliminary pharmacological characterization of two mammalian cell lines, adherent Shionogi S115 mouse mammary tumour cells and human B-lymphoblastoid IBW4 cells growing in suspension, expressing the human alpha 2-AR subtypes alpha 2-C4 and alpha 2-C10 at densities of approx. 2 x 10(5) receptors/cell. Transfection of the subtype genes was verified using a specific RNase protection assay. Pharmacological characterization was carried out with [3H]rauwolscine binding, which was inhibited by oxymetazoline and prazosin in a subtype-selective manner. The sensitivity of (-)-noradrenaline binding to the GTP-analogue 5'-guanylylimidodiphosphate suggested that the receptors are coupled to G-proteins. This was verified in S115 cells by efficient inhibition of forskolin-stimulated cAMP production by the alpha 2-AR agonists, (-)-noradrenaline and clonidine. These cell lines thus appear to be suitable for pharmacological studies on receptor function and ligand binding.