Normal vision can compensate for the loss of the circadian clock

Circadian clocks are thought to be essential for timing the daily activity of animals, and consequently increase fitness. This view was recently challenged for clock-less fruit flies and mice that exhibited astonishingly normal activity rhythms under outdoor conditions. Compensatory mechanisms appear to enable even clock mutants to live a normal life in nature. Here, we show that gradual daily increases/decreases of light in the laboratory suffice to provoke normally timed sharp morning (M) and evening (E) activity peaks in clock-less flies. We also show that the compound eyes, but not Cryptochrome (CRY), mediate the precise timing of M and E peaks under natural-like conditions, as CRY-less flies do and eyeless flies do not show these sharp peaks independently of a functional clock. Nevertheless, the circadian clock appears critical for anticipating dusk, as well as for inhibiting sharp activity peaks during midnight. Clock-less flies only increase E activity after dusk and not before the beginning of dusk, and respond strongly to twilight exposure in the middle of the night. Furthermore, the circadian clock responds to natural-like light cycles, by slightly broadening Timeless (TIM) abundance in the clock neurons, and this effect is mediated by CRY.     

Protocadherin PAPC is expressed in the CNC and can compensate for the loss of PCNS

Protocadherins represent the biggest subgroup within the cadherin superfamily of transmembrane glycoproteins. In contrast to classical type I cadherins, protocadherins in general exhibit only moderate adhesive activity. During embryogenesis, they are involved in cell signaling and regulate diverse morphogenetic processes, including morphogenetic movements during gastrulation and neural crest migration. The two protocadherins paraxial protocadherin (PAPC) and axial protocadherin (AXPC) are indispensable for proper gastrulation movements in Xenopus and zebrafish. The closest relative PCNS instead, is required for neural crest and somite formation. Here, we show that cranial neural crest (CNC) cells in addition to PCNS express PAPC, but not AXPC. Overexpression of PAPC resulted in comparable migration defects as knockdown of PCNS. Moreover, reconstitution experiments revealed that PAPC is able to replace PCNS in CNC cells, indicating that both protocadherins can regulate CNC migration. genesis 52:120–126. © 2013 Wiley Periodicals, Inc.     

Neuroligin-1 loss is associated with reduced tenacity of excitatory synapses

Neuroligins (Nlgns) are postsynaptic, integral membrane cell adhesion molecules that play important roles in the formation, validation, and maturation of synapses in the mammalian central nervous system. Given their prominent roles in the life cycle of synapses, it might be expected that the loss of neuroligin family members would affect the stability of synaptic organization, and ultimately, affect the tenacity and persistence of individual synaptic junctions. Here we examined whether and to what extent the loss of Nlgn-1 affects the dynamics of several key synaptic molecules and the constancy of their contents at individual synapses over time. Fluorescently tagged versions of the postsynaptic scaffold molecule PSD-95, the AMPA-type glutamate receptor subunit GluA2 and the presynaptic vesicle molecule SV2A were expressed in primary cortical cultures from Nlgn-1 KO mice and wild-type (WT) littermates, and live imaging was used to follow the constancy of their contents at individual synapses over periods of 8-12 hours. We found that the loss of Nlgn-1 was associated with larger fluctuations in the synaptic contents of these molecules and a poorer preservation of their contents at individual synapses. Furthermore, rates of synaptic turnover were somewhat greater in neurons from Nlgn-1 knockout mice. Finally, the increased GluA2 redistribution rates observed in neurons from Nlgn-1 knockout mice were negated by suppressing spontaneous network activity. These findings suggest that the loss of Nlgn-1 is associated with some use-dependent destabilization of excitatory synapse organization, and indicate that in the absence of Nlgn-1, the tenacity of excitatory synapses might be somewhat impaired.     

An altered FtsA can compensate for the loss of essential cell division protein FtsN in Escherichia coli

FtsN is the last known essential protein component to be recruited to the Escherichia coli divisome, and has several special properties. Here we report the isolation of suppressor mutants of ftsA that allow viability in the absence of ftsN. Cells producing the FtsA suppressors exhibited a mild cell division deficiency in the absence of FtsN, and no obvious phenotype in its presence. Remarkably, these altered FtsA proteins also could partially suppress a deletion of ftsK or zipA, were less toxic than wild-type FtsA when in excess, and conferred resistance to excess MinC, indicating that they share some properties with the previously isolated FtsA* suppressor mutant, and bypass the need for ftsN by increasing the integrity of the Z ring. TolA, which normally requires FtsN for its recruitment to the divisome, localized proficiently in the suppressed ftsN null strain, strongly suggesting that FtsN does not recruit the Tol-Pal complex directly. Therefore, despite its classification as a core divisome component, FtsN has no unique essential function but instead promotes overall Z ring integrity. The results strongly suggest that FtsA is conformationally flexible, and this flexibility is a key modulator of divisome function at all stages.     

Immunological responses and actin dynamics in macrophages are controlled by N-cofilin but are independent from ADF

Dynamic changes in the actin cytoskeleton are essential for immune cell function and a number of immune deficiencies have been linked to mutations, which disturb the actin cytoskeleton. In macrophages and dendritic cells, actin remodelling is critical for motility, phagocytosis and antigen presentation, however the actin binding proteins, which control antigen presentation have been poorly characterized. Here we dissect the specific roles of the family of ADF/cofilin F-actin depolymerizing factors in macrophages and in local immune responses. Macrophage migration, cell polarization and antigen presentation to T-cells require n-cofilin mediated F-actin remodelling. Using a conditional mouse model, we show that n-cofilin also controls MHC class II-dependent antigen presentation. Other cellular processes such as phagocytosis and antigen processing were found to be independent of n-cofilin. Our data identify n-cofilin as a novel regulator of antigen presentation, while ADF on the other hand is dispensable for macrophage motility and antigen presentation.     

Differences in the cytoskeleton in inhibitory and excitatory synapses

The cytoskeleton of afferent chemical synapses, with various ultrastructure of contact zones, was examined in the Mauthner cells of the goldfish. The synapses with combined active zones and desmosome-like specialized contacts possessed a well developed cytoskeleton consisting of filaments and microtubules oriented towards the synaptic apposition. Regular arrays of synaptic vesicles oriented in the same direction were observed beyond and near the active zones. The cytoskeleton of the synapses lacking desmosome-like formations was diffusely organized throughout the boutons. The distribution of vesicles in the vicinity of active zones was also not ordered. The role of cytoskeleton in organization of the two morphologically distinct synapses is discussed. A special function of cytoskeleton as an intermediary between synaptoplasm and membrane is regarded as a necessary basis for plasticity of excitatory rather than inhibitory synapses.     

Receptor trafficking and the plasticity of excitatory synapses

Newly discovered features of the trafficking of AMPA receptors to and from the postsynaptic membrane of excitatory synapses are now bringing the mechanisms of synaptic plasticity into focus. Recent advances, including the existence of slots, anchors, transport factors and pathways for activity-dependent control, have elucidated the role of the individual AMPA receptor subunits and their binding partners. The latest views describe how subunit type dictates the assembly of heteromeric receptors, and how these heteromers interact with the receptor trafficking machinery and synaptic anchorage factors. Moreover, phosphorylation may play an important role in receptor transport and synaptic turnover.     

Neuroenergetics and the kinetic design of excitatory synapses

Why is the characteristic timescale of neural information processing in the millisecond range, corresponding to a 'clock speed' of about 1 kHz, whereas the clock speed of modern computers is about 3 GHz? Here we investigate how the brain's energy supply limits the maximum rate at which the brain can compute, and how the molecular components of excitatory synapses have evolved properties that are matched to the information processing they perform.     

Postsynaptic organization and regulation of excitatory synapses

Dynamic regulation of synaptic efficacy is one of the mechanisms thought to underlie learning and memory. Many of the observed changes in efficacy, such as long-term potentiation and long-term depression, result from the functional alteration of excitatory neurotransmission mediated by postsynaptic glutamate receptors. These changes may result from the modulation of the receptors themselves and from regulation of protein networks associated with glutamate receptors. Understanding the interactions in this synaptic complex will yield invaluable insight into the molecular basis of synaptic function. This review focuses on the molecular organization of excitatory synapses and the processes involved in the dynamic regulation of glutamate receptors.     

Changing partners in an obligate symbiosis: a facultative endosymbiont can compensate for loss of the essential endosymbiont Buchnera in an aphid

Almost all aphids harbour an endosymbiotic bacterium, Buchnera aphidicola, in bacteriocytes. Buchnera synthesizes essential nutrients and supports growth and reproduction of the host. Over the long history of endosymbiosis, many essential genes have been lost from the Buchnera genome, resulting in drastic genome reduction and the inability to live outside the host cells. In turn, when deprived of Buchnera, the host aphid suffers retarded growth and sterility. Buchnera and the host aphid are often referred to as highly integrated almost inseparable mutualistic partners. However, we discovered that, even after complete elimination of Buchnera, infection with a facultative endosymbiotic gamma-proteobacterium called pea aphid secondary symbiont (PASS) enabled survival and reproduction of the pea aphid. In the Buchnera-free aphid, PASS infected the cytoplasms of bacteriocytes that normally harbour Buchnera, establishing a novel endosymbiotic system. These results indicate that PASS can compensate for the essential role of Buchnera by physiologically and cytologically taking over the symbiotic niche. By contrast, PASS negatively affected the growth and reproduction of normal host aphids by suppressing the essential symbiont Buchnera. These findings illuminate complex symbiont-symbiont and host-symbiont interactions in an endosymbiotic system, and suggest a possible evolutionary route to novel obligate endosymbiosis by way of facultative endosymbiotic associations.     

Nonhomogeneous excitatory synapses of a crab stomach muscle

Neuromuscular synapses of pyloric muscle P1 in the blue crab Callinectes sapidus were examined using electrophysiological and electron microscopic methods. The muscle is innervated by a single excitatory axon of the stomatogastric ganglion. Excitatory postsynaptic potentials show striking facilitation at very low frequencies of stimulation, indicating very slow decay of the facilitation process after a single nerve impulse. Quantal content of transmitter release at a low frequency of stimulation averaged 1.5. Evidence was obtained that not all synapses on a muscle fiber are equivalent. This was particularly evident at the morphological level in serially sectioned nerve terminals. On each nerve terminal examined, a wide range of synapse sizes was found. Synaptic contact areas ranged from less than 0.5 micron2 to almost 10 micron2; the latter value is large compared with those obtained for other crustacean neuromuscular synapses. Most of the smaller synapses lacked the presynaptic dense bodies which are putative release sites for the transmitter substance. The larger synapses all had presynaptic dense bodies, and some showed evidence of splitting apart into smaller subunits. It is postulated that about half the morphologically identified synapses are relatively inactive.     

hSMUG1 can functionally compensate for Ung1 in the yeast Saccharomyces cerevisiae

There are at least four distinct families of enzymes that recognize and remove uracil from DNA. Family-3 (SMUG1) enzymes have recently been identified and have a preference for uracil in single-stranded DNA when assayed in vitro. Here we investigate the in vivo function of SMUG1 using the yeast Saccharomyces cerevisiae as a model system. These organisms lack a SMUG1 homologue and use a single enzyme, Ung1 to carry out uracil-repair. When a wild-type strain is treated with antifolate agents to induce uracil misincorporation into DNA, S-phase arrest and cellular toxicity occurs. The arrest is characteristic of checkpoint activation due to single-strand breaks caused by continuous uracil removal and self-defeating DNA repair. When uracil-DNA glycosylase is deleted (deltaung1), cells continue through S-phase and arrest at G(2)/M, presumably due to the effects of stable uracil misincorporation in DNA. Pulsed field gel electrophoresis (PFGE) demonstrates that cells are able to complete DNA replication with uracil-substituted DNA and do not experience the extensive strand breakage attributed to uracil-DNA glycosylase-mediated repair. As a result, these cells experience early protection from antifolate-induced cytotoxicity. When either UNG1 or SMUG1 functions are reintroduced back into the null strain and then subjected to antifolate treatment, the cells revert back to the wild-type phenotype as shown by a restored sensitivity to drug and S-phase arrest. The arrest is accompanied by the accumulation of replication intermediates as determined by PFGE. Collectively, these data indicate that SMUG1 can act as a functional homolog of the family-1 uracil-DNA glycosylase enzymes.     

Copper deficient rat heart can compensate for doxorubicin-induced oxidant stress

The hypothesis that copper (Cu) alters drug metabolizing enzymes and functions as an antioxidant nutrient in doxorubicin cardiotoxicity was tested. Male Sprague-Dawley rats were fed Cu adequate (⁺Cu; 5 mg Cu/kg of diet), marginally Cu deficient (MCu; 1.2 mg Cu/kg of diet), or severely Cu deficient (⁻Cu; 0.5 mg Cu/kg of diet) diets for 6 wk. Doxorubicin (1, 2, or 4 mg/kg body wt) or saline were administered intraperitoneally 1 time/wk for 4 wk. Compared to control hearts, Cu, Zn superoxide dismutase activity was decreased by 9% in MCu rats and by 21–40% in⁻Cu rats. Glutathione peroxidase activity was elevated 5–15% in⁻Cu rats. Doxorubicin administration increased heart Cu, Zn superoxide dismutase activity in⁺Cu and⁻Cu rats 18 h after the last of 4 injections, but not 18 h after 1 injection. There was no synergism between doxorubicin and Cu deficiency on lipid peroxidation, plasma creatine phosphokinase, cardiac hypertrophy, electrocardiographic abnormalities, or morphological changes. Heart glutathione S-transferase activity was decreased by Cu deficiency, and like Cu, Zn superoxide dismutase activity, returned to normal in⁻Cu rats given doxorubicin. Thus, the Cu deficient rat heart may be able to compensate for doxorubicin-induced oxidant stress by increasing the activity of Cu,Zn superoxide dismutase and glutathione S-transferase.     

Intracellular Ca can compensate for the lack of NADPH oxidase-derived ROS in endothelial cells

The aim of the present study is to determine the role of intracellular Ca²⁺ in VEGF signaling. We demonstrate that reduction in Ca²⁺ by chelating compound BAPTA-AM or by IP₃-endoplasmic reticulum blocker 2-APB selectively inhibited VEGF-induced activation of c-Src-PI3K-Akt but not ERK1/2 in human coronary artery endothelial cells (HCAEC). We also show that the selective inhibitory effects of NADPH oxidase knockdown on VEGF-mediated activation of c-Src-PI3K-Akt signaling and cell proliferation in HCAEC can be reversed by increase in intracellular Ca²⁺. These results suggest an essential role for Ca²⁺ in redox-dependent selective activation of c-Src-PI3K-Akt and endothelial cell proliferation.     

Mitochondrial-derived vesicles compensate for loss of LC3-mediated mitophagy

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Novel protein kinase A-dependent long-term depression of excitatory synapses

Dopamine neurons of the ventral tegmental area (VTA) are critically involved in processing novel and rewarding information, and mediate the addictive properties of many drugs of abuse. Excitatory synapses on these neurons, like those in other brain regions, exhibit long-term depression (LTD). Amphetamine or dopamine block LTD at VTA synapses, indicating that both pathological and local physiological stimuli regulate LTD. Here we show that in common with other forms of LTD, VTA LTD results from a selective decrease in AMPA receptor function accompanied by a decrease in cell surface AMPA receptors. However, unlike the case for any previously described form of LTD, activation of cyclic AMP-dependent protein kinase (PKA) is necessary and sufficient to trigger LTD at synapses on VTA dopamine neurons.