Melanocortin‐1 receptor structure and functional regulation

The melanogenic actions of the melanocortins are mediated by the melanocortin‐1 receptor (MC1R). MC1R is a member of the G‐protein‐coupled receptors (GPCR) superfamily expressed in cutaneous and hair follicle melanocytes. Activation of MC1R by adrenocorticotrophin or α‐melanocyte stimulating hormone is positively coupled to the cAMP signaling pathway and leads to a stimulation of melanogenesis and a switch from the synthesis of pheomelanins to the production of eumelanic pigments. The functional behavior of the MC1R agrees with emerging concepts in GPCR signaling including dimerization, coupling to more than one signaling pathway and a high agonist‐independent constitutive activity accounting for inverse agonism phenomena. In addition, MC1R displays unique properties such as an unusually high number of natural variants often associated with clearly visible phenotypes and the occurrence of endogenous peptide antagonists. Therefore MC1R is an ideal model to study GPCR function. Here we review our current knowledge of MC1R structure and function, with emphasis on information gathered from the analysis of natural variants. We also discuss recent data on the regulation of MC1R function by paracrine and endocrine factors and by external stimuli such as ultraviolet light.     

Phosphatidylinositol inhibits microtubule assembly by binding to microtubule-associated protein 2 at a single, specific, high-affinity site.

The effects of various anionic phospholipids on the in vitro assembly of MAP2/tubulin microtubules has been examined. We show that the potency to inhibit is related to the polarity of the phospholipids and that this is consistent with a mode of action involving the sequencing of microtubule-associated proteins (MAPs) by nonspecific electrostatic interactions. The inhibitory potency of phosphatidylinositol (PI) is, however, considerably larger than predicted by this model. The effects of PI on MAP2/tubulin microtubule assembly have therefore been examined in greater detail by preparing phosphatidylcholine (PC) liposomes doped with increasing amounts of PI. We show that when the PI is sufficiently dispersed by dilution with PC, it inhibits microtubule assembly by binding to MAP2 with an apparent stoichiometry, after correction for the bilamellar nature of the liposomes, of 1:1 mol.mol-1 PI:MAP2. Furthermore, we show that the Kd of this interaction is in the submicromolar range.     

The precursor of interleukin-1 alpha is phosphorylated at residue serine 90

Mononuclear phagocytes release interleukin-1 (IL-1), a 17-kDa polypeptide with diverse biological activities. IL-1 is synthesized as a precursor (31 kDa) which lacks a signal sequence or hydrophobic domains that could facilitate transmembrane translocation. Possible postsynthetic modifications of IL-1 that might account for its cellular transport were examined. We found that lipopolysaccharide stimulated, but not unstimulated, murine macrophages incorporated 32PO4 into the IL-1 alpha precursor (31 kDa) predominantly at residue serine 90. Released IL-1 alpha (17 kDa) is not phosphorylated in agreement with peptide sequence data that the site of 32P incorporation is in the amino-terminal one-third of the precursor. Approximately 10% of the phosphorylated IL-1 alpha precursor is membrane bound and associated with a fraction enriched in lysosomal vesicles. Together these data suggest mechanisms by which the postsynthetic proteolysis of the IL-1 alpha precursor may be modified and cellular transport of IL-1 alpha is accomplished.     

The immunological network at the site of tumor rejection

The tumor mass irrespective of its type or location in the body has long been shrouded in mystery and even today we still have only a tentative handle on its secrets. Attempts to manipulate either the tumor cells per se or host-derived leukocytes have, on the whole, not been successful or at best questionable. The ability of the host to respond immunologically to TSTA is well documented, yet again attempts to manipulate this response have been disappointing. One of the problems has been a lack of knowledge concerning the tumor mass and its constituents, such as the intratumor leukocytes, and the significance of their presence to the biological properties of the neoplasm [8,9,80]. The purpose in studying the immunological network is, in part, to try to assign a function to these cells on the premise that lymphoid elements and macrophages have a potential role to play in recognition of TSTA. The advantage of adoptive immunotherapy model systems is that tumor rejection can be achieved under controlled conditions and this allows an analysis of the immunological network and its individual circuits. At the same time, valuable information on the mechanisms of action during adoptive immunotherapy and how best to improve therapeutic protocols is acquired.     

THE INDIRECT EFFECT OF PHOTOPERIOD ON TRACHEID DIAMETER IN PINUS RESINOSA

Larson , Philip R. (Lake States Forest Experiment Station, Rhinelander, Wis.) The indirect effect of photoperiod on tracheid diameter in Pinus resinosa. Amer. Jour. Bot. 49 (2) : 132–137. Illus. 1962.—The influence of photoperiod on tracheid diameter was studied by independently exposing the buds and needles to different photoperiods. Long days promoted needle elongation and large-diameter tracheid production, whereas short days brought about cessation of needle elongation and the transition to narrow-diameter tracheids. The response to photoperiod varied with the stage of plant development. During active extension growth of the shoot, the bud exerted a decisive regulatory influence on tracheid diameter, but the needles became the principal source of the stimulus contributing to tracheid diameter when extension growth terminated. It is suggested that the effect of photoperiod on tracheid diameter is indirect and associated with auxin production and distribution of the terminal meristems.