Xylem-borne cytokinins: still in search of a role?

Leaf expansion and xylem cytokinin concentration ([X-CK]) decrease in response to nitrogen (N) deprivation. Debate continues over cause, effect, and correlation. Supporting studies provide, at best, correlative evidence that [X-CK] controls leaf growth in response to N-deprivation, while dissenting studies indicate that leaf growth responses to N can be independent of changes in X-CK supply to leaves. A model is proposed to evaluate the physiological significance to leaf growth of changes in plant and environment N concentrations, and plant CK concentrations.     

Plants and animals: a different taste for microbes?

Plants and animals can recognize potential pathogens by detecting pathogen-associated molecular patterns (PAMPs). Significant advances over the past few years have begun to unveil the molecular basis of PAMP perception by pattern recognition receptors (PRRs). Although these discoveries highlight common recognition strategies among higher eukaryotes, they also show differences with respect to the nature of the receptors involved and the exact molecular patterns recognized. This suggests a convergent evolution of microbe sensing by the innate immune systems of these various organisms.     

Mitochondria and aging: a role for the permeability transition?

When mitochondria are subjected to oxidative stress and relatively high [Ca2+], they undergo a "permeability transition" in which the inner membrane becomes freely permeable to low-molecular-weight solutes. This phenomenon reflects reversible deformation of the adenine nucleotide translocase, the loss of its native gating properties and the stabilization of the deformed state by cyclophilin-D. The permeability transition may be a factor in cell dysfunction associated with aging. This can manifest in a number of ways ranging, in the most severe, from impaired energy transduction and compromised viability to more subtle influences on the propagation of Ca2+ signals. This article critically examines data relevant to this issue.     

A pivotal role for beta-aminoisobutyric acid and oxidative stress in dihydropyrimidine dehydrogenase deficiency?

Dihydropyrimidine dehydrogenase (DPD) constitutes the first step of the pyrimidine degradation pathway in which the pyrimidine bases uracil and thymine are catabolised to beta-alanine and beta-aminoisobutyric acid (beta-AIB), respectively. The mean concentration of beta-AIB was approximately 5- to 8-fold lower in urine of patients with a DPD deficiency, when compared to age-matched controls. Comparable levels of 8-hydroxydeoxyguanosine (8-OHdG) were present in urine from controls and DPD patients at the age <2 year. In contrast, slightly elevated levels of 8-OHdG were detected in urine from DPD patients with an age >2 year, suggesting the presence of increased oxidative stress.     

Skin aging: a role for telomerase and telomere dynamics?

Skin is a complex tissue composed of two very different compartments -- the continuously renewing epidermis made up mostly by keratinocytes and the underlying matrix-rich dermis with the resting fibroblasts as its major cellular components. Both compartments are tightly interconnected and a paracrine mutual interaction is essential for epidermal growth, differentiation, and tissue homeostasis. Skin aging is commonly viewed as wrinkle formation, hair greying, and impaired wound healing. Nevertheless, the epidermis as the outermost shield needs to remain intact in order to guarantee an inside-out and outside-in barrier function throughout life time of a human being. Furthermore, the epidermis is one of the few regenerative tissues that express telomerase, the ribonucleoprotein complex that can counteract telomere erosion, one of the presently mostly favoured potential mechanisms causing cellular aging. This raises the question whether in the epidermis telomerase is able to counteract telomere erosion and thereby to prevents a telomere-dependent aging process and consequently which part of the skin is responsible for the most obvious changes associated with skin aging.     

Trypanosomiasis and trypanotolerance in cattle: a role for congopain?

A Trypanosoma congolense cysteine protease (congopain) elicits a high IgG response in trypanotolerant but not in trypanosusceptible cattle during primary infections. As discussed here by Edith Authié, this observation suggests that congopain, like other parasite cysteine proteases, may play a role in pathogenicity and that more efficient immune responses to congopain may contribute to trypanotolerance.     

Cortical maps: a role for astrocytes?

Astrocytes are a multifunctional cell type in the nervous system that can influence neurons and synapses in numerous ways. Astrocytes have been suggested to play important roles in synapse formation during development, as well as in multiple forms of synaptic plasticity in the developing and adult brain. Astrocytes respond to nearby neural activity with elevations in cytosolic calcium concentration, and in sensory cortex these calcium responses have been shown to be topographically aligned to neuronal sensory maps. Here, we review recent evidence for astrocyte interactions with neural circuits, with particular emphasis on how these interactions may shape the development, arrangement and plasticity of cortical sensory maps.     

Hybridization and speciation in angiosperms: a role for pollinator shifts?

The majority of convincingly documented cases of hybridization in angiosperms has involved genetic introgression between the parental species or formation of a hybrid species with increased ploidy; however, homoploid (diploid) hybridization may be just as common. Recent studies, including one in BMC Evolutionary Biology, show that pollinator shifts can play a role in both mechanisms of hybrid speciation.     

Angiogenesis and rhodopsin-like receptors: a role for N-terminal acidic residues?

Numerous rhodopsin-like G-protein coupling receptors induce or inhibit angiogenesis. The active human receptors include several chemokine receptors, apelin APJ receptor, neuropeptide Y Y2 receptor, Duffy antigen, and herpes virus-8 receptor. A common and striking feature of these receptors is the large fraction (up to 42%) of residues with anionic sidechains (Asp, Glu, and benzene anions Tyr, Trp, and Phe) in the N-terminal extracellular domain. These residues (which are frequently clustered) can assist the binding of ligand peptides, but should also support interactions that help tubular arraying of cells, e.g., via cationic bridges and/or hydrogen bonding with cell-connecting receptors such as integrins, or with proteins of the extracellular matrix.     

Erythropoietin and the skin: a role for epidermal oxygen sensing?

Erythropoietin (EPO), long appreciated as the chief endocrine regulator of red blood cell formation, is now recognized to exert many additional functions outside the bone marrow. Thus, the quest is on to define the full range of EPO functions in the physiology and pathology of non‐hematopoietic tissues. Two recent studies in man and mice have highlighted the importance of the mammalian skin as one peripheral tissue with a previously unsuspected role in EPO biology; both, as a target and as a source of EPO. In addition, the skin has been proposed to function as an oxygen sensor. The present hypothesis essay critically reviews the currently available evidence for this and provides a unifying theoretical scenario for intracutaneous EPO functions and for a potential role of the skin in the control of EPO production. Mainly, we propose that the skin itself directly contributes to the up‐regulation of EPO plasma levels in response to hypoxia.     

Neuropeptide interactions and REM sleep: a role for Urotensin II?

Urotensin II (UII) is a peptide with structural similarity to the somatostatin family with potent vasoconstrictor activity. UII receptor is expressed broadly in the periphery, and most notably in the heart and microvessels. In the brain, the UII receptor can be detected in the spinal cord and in cholinergic nuclei in the brainstem known to be involved in REM sleep regulation. Recent data suggest that, in addition to their vasoactive properties, UII receptor ligands may have excitatory activity on a selective group of neurons that modulate REM sleep. This review focuses on the implications of these findings for the neurobiology of REM sleep regulation and discusses the possible impact of UII and other neuropeptides on the balance of the alternation between sleep states.     

Photoentrainment in mammals: a role for cryptochrome?

There is growing evidence in support of the hypothesis that, in mammals, photoreceptive tasks are segregated into those associated with creating a detailed visual image of the environment and those involved in the photic regulation of temporal biology. The hypothesis that this segregation extends to the use of different photoreceptors remains unproven, but published reports from several mammalian species that circadian photoentrainment survives a degree of retinal degeneration sufficient to induce visual blindness suggest that this may be so. This has lead to speculation that mammals might employ a dedicated 'circadian photoreceptor' distinct from the rod and cone cells of the visual system. The location and nature of this putative circadian photoreceptor has become a matter of conjecture. The latest candidates to be put forward as potential circadian photopigments are the mammalian cryptochrome proteins (CRY1 and 2), putative vitamin-B2 based photopigments. To date, published experimental evidence falls short of a definitive demonstration that these proteins form the basis of circadian photoreception in mammals. Consequently, this review aims to assess their suitability for this task in light of what we know regarding the biology of the cyrptochromes and the nature of mammalian photoentrainment.