Immediate early gene c-fos regulates the synthesis of phospholipids but not of gangliosides

From seconds to days: Neural plasticity viewed through a lipid lens

Many adult neurons are dynamically remodeled across timescales ranging from the rapid addition and removal of specific synaptic connections, to largescale structural plasticity events that reconfigure circuits over hours, days, and months. Membrane lipids, including brain-enriched sphingolipids, play crucial roles in these processes. In this review, we summarize progress at the intersection of neuronal activity, lipids, and structural remodeling. We highlight how brain activity modulates lipid metabolism to enable adaptive structural plasticity, and showcase glia as key players in membrane remodeling. These studies reveal that lipids act as critical signaling molecules that instruct the dynamic architecture of the brain.

Circadian Regulation and Clock-Controlled Mechanisms of Glycerophospholipid Metabolism from Neuronal Cells and Tissues to Fibroblasts

Along evolution, living organisms developed a precise timekeeping system, circadian clocks, to adapt life to the 24-h light/dark cycle and temporally regulate physiology and behavior. The transcriptional molecular circadian clock and metabolic/redox oscillator conforming these clocks are present in organs, tissues, and even in individual cells, where they exert circadian control over cellular metabolism. Disruption of the molecular clock may cause metabolic disorders and higher cancer risk. The synthesis and degradation of glycerophospholipids (GPLs) is one of the most highly regulated metabolisms across the 24-h cycle in terms of total lipid content and enzyme expression and activity in the nervous system and individual cells. Lipids play a plethora of roles (membrane biogenesis, energy sourcing, signaling, and the regulation of protein-chromatin interaction, among others), making control of their metabolism a vital checkpoint in the cellular organization of physiology. An increasing body of evidence clearly demonstrates an orchestrated and sequential series of events occurring in GPL metabolism across the 24-h day in diverse retinal cell layers, immortalized fibroblasts, and glioma cells. Moreover, the clock gene Per1 and other circadian-related genes are tightly involved in the regulation of GPL synthesis in quiescent cells. However, under proliferation, the metabolic oscillator continues to control GPL metabolism of brain cancer cells even after molecular circadian clock disruption, reflecting the crucial role of the temporal metabolism organization in cell preservation. The aim of this review is to examine the control exerted by circadian clocks over GPL metabolism, their synthesizing enzyme expression and activities in normal and tumorous cells of the nervous system and in immortalized fibroblasts.

Impairing activation of phospholipid synthesis by c-Fos interferes with glioblastoma cell proliferation

Glioblastoma multiforme is the most aggressive type of tumor of the CNS with an overall survival rate of approximately one year. Since this rate has not changed significantly over the last 20 years, the development of new therapeutic strategies for the treatment of these tumors is peremptory. The over-expression of the proto-oncogene c-Fos has been observed in several CNS tumors including glioblastoma multiforme and is usually associated with a poor prognosis. Besides its genomic activity as an AP-1 transcription factor, this protein can also activate phospholipid synthesis by a direct interaction with key enzymes of their metabolic pathways. Given that the amino-terminal portion of c-Fos (c-Fos-NA: amino acids 1-138) associates to but does not activate phospholipid synthesizing enzymes, we evaluated if c-Fos-NA or some shorter derivatives are capable of acting as dominant-negative peptides of the activating capacity of c-Fos. The over-expression or the exogenous administration of c-Fos-NA to cultured T98G cells hampers the interaction between c-Fos and PI4K2A, an enzyme activated by c-Fos. Moreover, it was observed a decrease in tumor cell proliferation rates in vitro and a reduction in tumor growth in vivo when a U87-MG-generated xenograft on nude mice is intratumorally treated with recombinant c-Fos-NA. Importantly, a smaller peptide of 92 amino acids derived from c-Fos-NA retains the capacity to interfere with tumor proliferation in vitro and in vivo. Taken together, these results support the use of the N-terminal portion of c-Fos, or shorter derivatives as a novel therapeutic strategy for the treatment of glioblastoma multiforme.

The moonlighting protein c-Fos activates lipid synthesis in neurons, an activity that is critical for cellular differentiation and cortical development

Differentiation of neuronal cells is crucial for the development and function of the nervous system. This process involves high rates of membrane expansion, during which the synthesis of membrane lipids must be tightly regulated. In this work, using a variety of molecular and biochemical assays and approaches, including immunofluorescence microscopy and FRET analyses, we demonstrate that the proto-oncogene c-Fos (c-Fos) activates cytoplasmic lipid synthesis in the central nervous system and thereby supports neuronal differentiation. Specifically, in hippocampal primary cultures, blocking c-Fos expression or its activity impairs neuronal differentiation. When examining its subcellular localization, we found that c-Fos co-localizes with endoplasmic reticulum markers and strongly interacts with lipid-synthesizing enzymes, whose activities were markedly increased in vitro in the presence of recombinant c-Fos. Of note, the expression of c-Fos dominant-negative variants capable of blocking its lipid synthesis-activating activity impaired neuronal differentiation. Moreover, using an in utero electroporation model, we observed that neurons with blocked c-Fos expression or lacking its AP-1-independent activity fail to initiate cortical development. These results highlight the importance of c-Fos-mediated activation of lipid synthesis for proper nervous system development.

Expression of estrogen receptor α variants and c-Fos in rat mammary gland and tumors

Breast cancer is currently the leading cause of cancer death among women worldwide. AP-1 (c-Fos/c-Jun) is associated with proliferation and survival, while cytoplasmic c-Fos activates phospholipid synthesis in cells induced to differentiate or grow. Estrogen receptor α 46 (ERα46) is a splice variant of full-length ERα66 and it is known that it has an inhibitory role in cancer cell growth. We investigated c-Fos localization, its relationship to AP-1, the non genomic pathway of phospho-Tyr537-ERα66, as well as ERα46 and ERα66 isoforms in rat mammary gland development and carcinogenic transformation, and in mammary tumors. Female rats were injected: a) saline solution (Control mammary gland, CMG) or b) N-Nitroso-N-methyl urea (NMU), and samples were taken at 60, 90, 120 and 150 days of life. In addition, we analyzed hormone-dependent (HD) and independent (HI) tumors in ovariectomized rats, and intact tumors (IT) in non-ovariectomized ones. Our results show that, in CMG, nuclear c-Fos and proliferation decreased with age, AP-1 content was low, and nuclear ERα46/ERα66 ratio was higher than 1. In NMU, nuclear c-Fos and proliferation increased with carcinogenic transformation, AP-1 content was high, and nuclear ERα46/ERα66 was below 1. As tumor grade increased, proliferation, nuclear c-Fos and AP-1 expression were negatively associated to nuclear ERα46/ERα66 in IT. In HD, nuclear ERα46/ERα66, nuclear c-Fos expression, AP-1 levels and proliferation were lower than in HI, whose growth is estrogen-independent. Phospho-Tyr537-ERα66 content and ERK1/2 activation were associated with AP-1 levels and cell proliferation. Collectively, our findings support the notion that variant detection and ERα46/ERα66 ratio could shed light on the role of ERα isoforms in mammary gland transformation and the behavior of ERα positive mammary tumors.

Lipid Metabolism in Neurons: A Brief Story of a Novel c-Fos-Dependent Mechanism for the Regulation of Their Synthesis

The mechanisms that coordinately regulate lipid synthesis in the nervous system together with the high rates of membrane biogenesis needed to support cell growth are largely unknown as are their subcellular site of synthesis. c-Fos, a well-known AP-1 transcription factor, has emerged as a unique protein with the capacity to associate to specific enzymes of the pathway of synthesis of phospholipids at the endoplasmic reticulum and activate their synthesis to accompany genomic decisions of growth. Herein, we discuss this effect of c-Fos in the context of neuronal differentiation and also with respect to pathologies of the nervous system such as the development and growth of tumors. We also provide insights into the sub-cellular sites where this regulation occurs at the endoplasmic reticulum membranes and the molecular mechanism by which c-Fos exerts this activity.

Mechanistic insights into the nongenomic regulation of phospholipid synthesizing enzymes

Lipid synthesis is a complex process regulated at multiple levels. Here, we will discuss nongenomic regulatory mechanisms, particularly the activation and/or recruitment of key enzymes to membranes. The phospholipid synthesis enzymes Lipin and CTP:phosphocholine cytidylyltransferase are taken as examples of these mechanisms that are mediated by posttranslational modifications or by an intrinsic property of the enzyme that senses lipid composition. In addition, special emphasis will be put on another relevant non genomic lipid synthesis regulation mechanism that is dependent on c-Fos, a protein that has deserved less attention so far. This latter regulatory mechanism is emerging as an important determinant for processes that require high rates of lipid synthesis such as those of growth and proliferation.

Old players with a newly defined function: Fra-1 and c-Fos support growth of human malignant breast tumors by activating membrane biogenesis at the cytoplasm

A shared characteristic of tumor cells is their exacerbated growth. Consequently, tumor cells demand high rates of phospholipid synthesis required for membrane biogenesis to support their growth. c-Fos, in addition to its AP-1 transcription factor activity, is the only protein known up to date that is capable of activating lipid synthesis in normal and brain tumor tissue. For this latter activity, c-Fos associates to the endoplasmic reticulum (ER) through its N-terminal domain and activates phospholipid synthesis, an event that requires it Basic Domain (BD) (aa 139–159). Fra-1, another member of the FOS family of proteins, is over-expressed in human breast cancer cells and its BD is highly homologous to that of c-Fos with two conservative substitutions in its basic amino acids. Consequently, herein we examined if Fra-1 and/or c-Fos participate in growth of breast cancer cells by activating phospholipid synthesis as found previously for c-Fos in brain tumors. We found both Fra-1 and c-Fos over-expressed in >95% of human ductal breast carcinoma biopsies examined contrasting with the very low or undetectable levels in normal tissue. Furthermore, both proteins associate to the ER and activate phospholipid synthesis in cultured MCF7 and MDA-MB231 breast cancer cells and in human breast cancer samples. Stripping tumor membranes of Fra-1 and c-Fos prior to assaying their lipid synthesis capacity in vitro results in non-activated lipid synthesis levels that are restored to their initial activated state by addition of Fra-1 and/or c-Fos to the assays. In MDA-MB231 cells primed to proliferate, blocking Fra-1 and c-Fos with neutralizing antibodies blocks lipid-synthesis activation and cells do not proliferate. Taken together, these results disclose the cytoplasmic activity of Fra-1 and c-Fos as potential targets for controlling growth of breast carcinomas by decreasing the rate of membrane biogenesis required for growth.

Controlling cytoplasmic c-Fos controls tumor growth in the peripheral and central nervous system

Some 20 years ago c-Fos was identified as a member of the AP-1 family of inducible transcription factors (Angel and Karin in Biochim Biophys Acta 1072:129-157, 1991). More recently, an additional activity was described for this protein: it associates to the endoplasmic reticulum and activates the biosynthesis of phospholipids (Bussolino et al. in FASEB J 15:556-558, 2001), (Gil et al. in Mol Biol Cell 15:1881-1894, 2004), the quantitatively most important components of cellular membranes. This latter activity of c-Fos determines the rate of membrane genesis and consequently of growth in differentiating PC12 cells (Gil et al. in Mol Biol Cell 15:1881-1894, 2004). In addition, it has been shown that c-Fos is over-expressed both in PNS and CNS tumors (Silvestre et al. in PLoS One 5(3):e9544, 2010). Herein, it is shown that c-Fos-activated phospholipid synthesis is required to support membrane genesis during the exacerbated growth characteristic of brain tumor cells. Specifically blocking c-Fos-activated phospholipid synthesis significantly reduces proliferation of tumor cells in culture. Blocking c-Fos expression also prevents tumor progression in mice intra-cranially xeno-grafted human brain tumor cells. In NPcis mice, an animal model of the human disease Neurofibromatosis Type I (Cichowski and Jacks in Cell 104:593-604, 2001), animals spontaneously develop tumors of the PNS and the CNS, provided they express c-Fos (Silvestre et al. in PLoS One 5(3):e9544, 2010). Treatment of PNS tumors with an antisense oligonucleotide that specifically blocks c-Fos expression also blocks tumor growth in vivo. These results disclose cytoplasmic c-Fos as a new target for effectively controlling brain tumor growth.

The kinase c-Src and the phosphatase TC45 coordinately regulate c-Fos tyrosine phosphorylation and c-Fos phospholipid synthesis activation capacity

Our previous work showed that in T98G cells, a human glioblastoma multiforme-derived cell line, the association of c-Fos to the endoplasmic reticulum (ER) and consequently, the capacity of c-Fos to activate phospholipid synthesis, is regulated by the phosphorylation state of tyrosine (tyr) residues #10 and #30 of c-Fos. The small amount of c-Fos present in quiescent cells is tyr-phosphorylated, is dissociated from the ER membranes and does not activate phospholipid synthesis. However, on induction of the cell to re-enter growth, c-Fos expression is rapidly induced, it is found dephosphorylated, associated to ER membranes and activating phospholipid synthesis (Portal et al., 2007). Herein, using in vivo and in vitro experimental strategies, we show that the kinase c-Src is capable of phosphorylating tyr residues of c-Fos whereas the phosphatase TC45 T-cell protein-tyr phosphatase (TC-PTP) dephosphorylates them, thus enabling c-Fos/ER association and activation of phospholipid synthesis. Results also suggest that the regulation of the phosphorylation/dephosphorylation cycle of c-Fos occurs at the TC-PTP level: induction of cells to re-enter growth promotes the translocation of TC45 from a nuclear to a cytoplasmic location concomitant with its activation. Activated TC45 in its turn promotes dephosphorylation of pre-formed c-Fos, enabling cells to rapidly activate phospholipid synthesis to respond to its growth demands.

Expression of novel opsins and intrinsic light responses in the mammalian retinal ganglion cell line RGC-5. Presence of OPN5 in the rat retina

The vertebrate retina is known to contain three classes of photoreceptor cells: cones and rods responsible for vision, and intrinsically photoresponsive retinal ganglion cells (RGCs) involved in diverse non-visual functions such as photic entrainment of daily rhythms and pupillary light responses. In this paper we investigated the potential intrinsic photoresponsiveness of the rat RGC line, RGC-5, by testing for the presence of visual and non-visual opsins and assessing expression of the immediate-early gene protein c-Fos and changes in intracellular Ca²⁺mobilization in response to brief light pulses. Cultured RGC-5 cells express a number of photopigment mRNAs such as retinal G protein coupled receptor (RGR), encephalopsin/panopsin (Opn3), neuropsin (Opn5) and cone opsin (Opn1mw) but not melanopsin (Opn4) or rhodopsin. Opn5 immunoreactivity was observed in RGC-5 cells and in the inner retina of rat, mainly localized in the ganglion cell layer (GCL). Furthermore, white light pulses of different intensities and durations elicited changes both in intracellular Ca²⁺ levels and in the induction of c-Fos protein in RGC-5 cell cultures. The results demonstrate that RGC-5 cells expressing diverse putative functional photopigments display intrinsic photosensitivity which accounts for the photic induction of c-Fos protein and changes in intracellular Ca²⁺ mobilization. The presence of Opn5 in the GCL of the rat retina suggests the existence of a novel type of photoreceptor cell.

c-Fos activates and physically interacts with specific enzymes of the pathway of synthesis of polyphosphoinositides

c-Fos increases the overall synthesis of polyphosphoinositides by an AP-1–independent mechanism involving activation of CDP-diacyl­glycerol synthase and phosphatidylinositol (PtdIns) 4-kinase II α but not of PtdIns synthase or PtdIns 4-kinase II β. Coimmunoprecipitation and FRET experiments show that c-Fos physically associates only with the enzymes it activates.

The oncoprotein c-Fos is a well-recognized AP-1 transcription factor. In addition, this protein associates with the endoplasmic reticulum and activates the synthesis of phospholipids. However, the mechanism by which c-Fos stimulates the synthesis of phospholipids in general and the specific lipid pathways activated are unknown. Here we show that induction of quiescent cells to reenter growth promotes an increase in the labeling of polyphosphoinositides that depends on the expression of c-Fos. We also investigated whether stimulation by c-Fos of the synthesis of phosphatidylinositol and its phosphorylated derivatives depends on the activation of enzymes of the phosphatidylinositolphosphate biosynthetic pathway. We found that c-Fos activates CDP-diacylglycerol synthase and phosphatidylinositol (PtdIns) 4-kinase II α in vitro, whereas no activation of phosphatidylinositol synthase or of PtdIns 4-kinase II β was observed. Both coimmunoprecipitation and fluorescence resonance energy transfer experiments consistently showed a physical interaction between the N-terminal domain of c-Fos and the enzymes it activates.

Growth of peripheral and central nervous system tumors is supported by cytoplasmic c-Fos in humans and mice

BACKGROUND

We have previously shown that the transcription factor c-Fos is also capable of associating to endoplasmic reticulum membranes (ER) and activating phospholipid synthesis. Herein we examined phospholipid synthesis status in brain tumors from human patients and from NPcis mice, an animal model of the human disease Neurofibromatosis Type 1 (NF1).

PRINCIPAL FINDINGS

In human samples, c-Fos expression was at the limit of detection in non-pathological specimens, but was abundantly expressed associated to ER membranes in tumor cells. This was also observed in CNS of adult tumor-bearing NPcis mice but not in NPcis fos(-/-) KO mice. A glioblastoma multiforme and a malignant PNS tumor from a NF1 patient (MPNST) showed a 2- and 4- fold c-Fos-dependent phospholipid synthesis activation, respectively. MPNST samples also showed increased cell proliferation rates and abundant c-Fos expression.

CONCLUSIONS

Results highlight a role of cytoplasmic c-Fos as an activator of phospholipid synthesis in events demanding high rates of membrane biogenesis as occurs for the exacerbated growth of tumors cells. They also disclose this protein as a potential target for controlling tumor growth in the nervous system.

Content of endoplasmic reticulum and Golgi complex membranes positively correlates with the proliferative status of brain cells

Although the molecular and cellular basis of particular events that lead to the biogenesis of membranes in eukaryotic cells has been described in detail, understanding of the intrinsic complexity of the pleiotropic response by which a cell adjusts the overall activity of its endomembrane system to accomplish these requirements is limited. Here we carried out an immunocytochemical and biochemical examination of the content and quality of the endoplasmic reticulum (ER) and Golgi apparatus membranes in two in vivo situations characterized by a phase of active cell proliferation followed by a phase of declination in proliferation (rat brain tissue at early and late developmental stages) or by permanent active proliferation (gliomas and their most malignant manifestation, glioblastomas multiforme). It was found that, in highly proliferative phases of brain development (early embryo brain cells), the content of ER and Golgi apparatus membranes, measured as total lipid phosphorous content, is higher than in adult brain cells. In addition, the concentration of protein markers of ER and Golgi is also higher in early embryo brain cells and in human glioblastoma multiforme cells than in adult rat brain or in nonpathological human brain cells. Results suggest that the amount of endomembranes and the concentration of constituent functional proteins diminish as cells decline in their proliferative activity.

c-Fos activates glucosylceramide synthase and glycolipid synthesis in PC12 cells

It has been demonstrated that c-Fos has, in addition to its well recognized AP-1 transcription factor activity, the capacity to associate to the endoplasmic reticulum and activate key enzymes involved in the synthesis of phospholipids required for membrane biogenesis during cell growth and neurite formation. Because membrane genesis requires the coordinated supply of all its integral membrane components, the question emerges as to whether c-Fos also activates the synthesis of glycolipids, another ubiquitous membrane component. We show that c-Fos activates the metabolic labeling of glycolipids in differentiating PC12 cells. Specifically, c-Fos activates the enzyme glucosylceramide synthase (GlcCerS), the product of which, GlcCer, is the first glycosylated intermediate in the pathway of synthesis of glycolipids. By contrast, the activities of GlcCer galactosyltransferase 1 and lactosylceramide sialyltransferase 1 are essentially unaffected by c-Fos. Co-immunoprecipitation experiments in cells co-transfected with c-Fos and a V5-tagged version of GlcCerS evidenced that both proteins participate in a physical association. c-Fos expression is tightly regulated by specific environmental cues. This strict regulation assures that lipid metabolism activation will occur as a response to cell requirements thus pointing to c-Fos as an important regulator of key membrane metabolisms in membrane biogenesis-demanding processes.

N-Terminal c-Fos tyrosine phosphorylation regulates c-Fos/ER association and c-Fos-dependent phospholipid synthesis activation

c-Fos dephosphorylated on tyrosine (c-Fos), a component of the activator protein-1 (AP-1) family of transcription factors, is expressed at very low levels in resting cells. However, its expression is rapidly upregulated in cells undergoing G(0) to S phase transition leading to AP-1-dependent gene transcription responses. In addition, cytoplasmic c-Fos associates to the endoplasmic reticulum (ER) membranes and activates phospholipid synthesis during cell growth and differentiation. Herein, it is shown that in T98G cells, c-Fos/ER association and consequently phospholipid synthesis activation is regulated by the phosphorylated state of c-Fos tyrosine (tyr) residues. The small amount of c-Fos present in quiescent T98G cells is tyr-phosphorylated and not ER-membrane bound. In growing cells, it is dephosphorylated, associated to ER membranes and promotes phospholipid synthesis activation. Impairing tyr-dephosphorylation abrogates phospholipid synthesis activation and reduces proliferation rates to those of quiescent cells. Substitution of tyr residues 10, 30, 106 and 337 evidence tyr 10 and 30 as relevant for this regulatory phenomenon. It is concluded that phosphorylation of tyr residues 10 and 30 of c-Fos regulate the rate of synthesis of phospholipids by regulating c-Fos/ER association.

Rhythms of glycerophospholipid synthesis in retinal inner nuclear layer cells

The present study demonstrates that the biosynthesis of phospholipids in the inner nuclear layer cells of the chicken retina displays daily rhythms under constant illumination conditions. The vertebrate retina contains circadian oscillators and photoreceptors (PRCs) that temporally regulate its own physiology and synchronize the whole organism to the daily environmental changes. We have previously reported that chicken photoreceptors and retinal ganglion cells (RGCs) present significant daily variations in their phospholipid biosynthesis under constant illumination conditions. Herein, we demonstrate that cell preparations highly enriched in inner nuclear layer cells also exhibit a circadian-regulated phospholipid labeling after the in vivo administration of [(32)P]phosphate or [(3)H]glycerol both in animals maintained under constant darkness or light for at least 48h. In constant darkness, there was a significant incorporation of both precursors into phospholipids with the highest levels of labeling around midday and dusk. In constant light, the labeling of (32)P-phospholipids was also significantly higher during the day and early night whereas the incorporation of [(3)H]glycerol into phospholipids, that indicates de novo biosynthesis, was greater during the day but probably reflecting a higher precursor availability at those phases. We also measured the in vitro activity of phosphatidate phosphohydrolase and diacylglycerol lipase in preparations obtained from the dark condition. The two enzymes exhibited the highest activity levels late in the day. When we assessed the in vitro incorporation of [(14)C]oleate into different lysophospholipids from samples collected at different phases in constant darkness, reaction catalyzed by lysophospholipid acyltransferases II, labeling showed a complex pattern of daily activity. Taken together, these results demonstrate that the biosynthesis of phospholipids in cells of the chicken retinal inner nuclear layer exhibits a daily rhythmicity under constant illumination conditions, which is controlled by a circadian clock.

c-Jun interacts with phospholipids and c-Fos at the interface

We describe c-Jun, a widely studied transcription factor that participates in cell proliferation, differentiation, and tumorigenesis, as amphitropic. We show that c-Jun forms stable monolayers and interacts favorably, although in a nonselective manner, with phospholipids at the interface. The surface activity of c-Jun, together with that of c-Fos, its common partner in AP-1 transcription heterodimers, drives interfacial complex formation. We show that AP-1 is very stable at the air-water interface and suggest that AP-1 may not be substantially formed in solution as a stable equimolar association of both proteins.

c-Fos and phosphatidylinositol-4,5-bisphosphate reciprocally reorganize in mixed monolayers

The transcription factor c-Fos has surface thermodynamic properties that allow it to differentially interact with phospholipids, especially PIP2. It regulates phospholipid metabolism both in vivo and in vitro, and modulates degradation of phospholipid monolayers by phospholipases in a way that depends on the membrane intermolecular packing (i.e., surface lateral pressure). With the aim to understand details of the interactions of c-Fos at the membrane level, we studied the surface packing, dipole potential, compressibility and topography of mixed films of the protein with PIP2. We show that c-Fos changes the packing of liquid-expanded PIP2 monolayers, in a different manner with respect to its effect on the similarly liquid-expanded dilauroylphosphatidylcholine monolayers. The changes at the local molecular level are transduced to long-range inhomogeneities of the surface, detected by Brewster angle (BAM) and epifluorescence microscopy (EFM). Our results highlight the capacity of c-Fos to alter the packing and dipole potential of the lipid-protein interface. This involves variations of the surface in-plane elasticity and lateral segregation of phase domains. These dynamic, reversible alterations of surface organization provide a basis by which c-Fos may transduce molecular information at the membrane level.

Retinal ganglion cells are autonomous circadian oscillators synthesizing N-acetylserotonin during the day

Retinal ganglion cells send visual and circadian information to the brain regarding the environmental light-dark cycles. We investigated the capability of retinal ganglion cells of synthesizing melatonin, a highly reliable circadian marker that regulates retinal physiology, as well as the capacity of these cells to function as autonomous circadian oscillators. Chick retinal ganglion cells presented higher levels of melatonin assessed by radioimmunoassay during both the subjective day in constant darkness and the light phase of a light-dark cycle. Similar changes were observed in mRNA levels and activity of arylalkylamine N-acetyltransferase, a key enzyme in melatonin biosynthesis, with the highest levels of both parameters during the subjective day. These daily variations were preceded by the elevation of cyclic-AMP content, the second messenger involved in the regulation of melatonin biosynthesis. Moreover, cultures of immunopurified retinal ganglion cells at embryonic day 8 synchronized by medium exchange synthesized a [3H]melatonin-like indole from [3H]tryptophan. This [3H]indole was rapidly released to the culture medium and exhibited a daily variation, with levels peaking 8 h after synchronization, which declined a few hours later. Cultures of embryonic retinal ganglion cells also showed self-sustained daily rhythms in arylalkylamine N-acetyltransferase mRNA expression during at least three cycles with a period near 24 h. These rhythms were also observed after the application of glutamate. The results demonstrate that chick retinal ganglion cells may function as autonomous circadian oscillators synthesizing a melatonin-like indole during the day.

Synthesis of retinal ganglion cell phospholipids is under control of an endogenous circadian clock: Daily variations in phospholipid-synthesizing enzyme activities

Retinal ganglion cells (RGCs) are major components of the vertebrate circadian system. They send information to the brain, synchronizing the entire organism to the light-dark cycles. We recently reported that chicken RGCs display daily variations in the biosynthesis of glycerophospholipids in constant darkness (DD). It was unclear whether this rhythmicity was driven by this population itself or by other retinal cells. Here we show that RGCs present circadian oscillations in the labeling of [32P]phospholipids both in vivo in constant light (LL) and in cultures of immunopurified embryonic cells. In vivo, there was greater [32P]orthophosphate incorporation into total phospholipids during the subjective day. Phosphatidylinositol (PI) was the most 32P-labeled lipid at all times examined, displaying maximal levels during the subjective day and dusk. In addition, a significant daily variation was found in the activity of distinct enzymes of the pathway of phospholipid biosynthesis and degradation, such as lysophospholipid acyltransferases (AT II), phosphatidate phosphohydrolase (PAP), and diacylglycerol lipase (DGL) in cell preparations obtained in DD, exhibiting differential but coordinated temporal profiles. Furthermore, cultures of immunopurified RGCs synchronized by medium exchange displayed a circadian fluctuation in the phospholipid labeling. The results demonstrate that chicken RGCs contain circadian oscillators capable of generating metabolic oscillations in the biosynthesis of phospholipids autonomously.

c-Fos activated phospholipid synthesis is required for neurite elongation in differentiating PC12 cells

We have previously shown that c-Fos activates phospholipid synthesis through a mechanism independent of its genomic AP-1 activity. Herein, using PC12 cells induced to differentiate by nerve growth factor, the genomic effect of c-Fos in initiating neurite outgrowth is shown as distinct from its nongenomic effect of activating phospholipid synthesis and sustaining neurite elongation. Blocking c-Fos expression inhibited differentiation, phospholipid synthesis activation, and neuritogenesis. In cells primed to grow, blocking c-Fos expression determined neurite retraction. However, transfected cells expressing c-Fos or c-Fos deletion mutants with capacity to activate phospholipid synthesis sustain neurite outgrowth and elongation in the absence of nerve growth factor. Results disclose a dual function of c-Fos: it first releases the genomic program for differentiation and then associates to the endoplasmic reticulum and activates phospholipid synthesis. Because phospholipids are key membrane components, we hypothesize this latter phenomenon as crucial to support membrane genesis demands required for cell growth and neurite elongation.

The metabolism of phospholipids oscillates rhythmically in cultures of fibroblasts and is regulated by the clock protein PERIOD 1

The mammalian circadian timing system is composed of countless cell oscillators distributed throughout the body and central pacemakers regulating temporal physiology and behavior. Peripheral clocks display circadian rhythms in gene expression both in vivo and in culture. We examined the biosynthesis of phospholipids as well as the expression of the clock gene period 1 (Per1) and its potential involvement in the regulation of the phospholipid metabolism in cultured quiescent NIH 3T3 cells synchronized by a 2 h serum shock. A 30 min pulse of radiolabeled precursor was given at phases ranging from 0.5 to 62 h after serum treatment. We observed a daily rhythm in the phospholipid labeling that persisted at least for two cycles, with levels significantly decreasing 29 and 58 h after treatment. Per1 expression exhibited a rapid and transient induction and a daily rhythmicity in antiphase to the lipid labeling. After Per1 expression knockdown, the rhythm of phospholipid labeling was lost. Furthermore, in cultures of CLOCK mutant fibroblasts--cells with a clock mechanism impairment--PER1 was equally expressed at all times examined and the phospholipid labeling did not oscillate. The results demonstrate that the biosynthesis of phospholipids oscillates daily in cultured fibroblasts by an endogenous clock mechanism involving Per1 expression.

Circadian phototransduction and the regulation of biological rhythms

The vertebrate circadian system that controls most biological rhythms is composed of multiple oscillators with varied hierarchies and complex levels of organization and interaction. The retina plays a key role in the regulation of daily rhythms and light is the main synchronizer of the circadian system. To date, the identity of photoreceptors/photopigments responsible for the entrainment of biological rhythms is still uncertain; however, it is known that phototransduction must occur in the eye because light entrainment is lost with eye removal. The retina is also rhythmic in physiological and metabolic activities as well as in gene expression. Retinal oscillators may act like clocks to induce changes in the visual system according to the phase of the day by predicting environmental changes. These oscillatory and photoreceptive capacities are likely to converge all together on selected retinal cells. The aim of this overview is to present the current knowledge of retinal physiology in relation to the circadian timing system.

Immediate early gene expression within the visual system: light and circadian regulation in the retina and the suprachiasmatic nucleus

Immediate early genes are a family of genes that share the characteristic of having their expression rapidly and transiently induced upon stimulation of neuronal and non-neuronal cells. In this review, first a short description of the IEGs is given, then it is discussed the stimulus-induced and circadian-induced variations in the expression of IEGs in the visual system, mainly in the retina and the suprachiasmatic nucleus. The possible physiological consequences of these variations in IEG expression are also considered. Finally, we refer to two aspects of our recent studies and those of other laboratories involving light-driven IEG expression. The first is the finding that in the chick retina, the expression of c-fos is differentially modulated in the different cell types and that c-fos regulates the synthesis of the quantitatively most important lipids of all cells, the phospholipids, by a non-genomic mechanism. The second is the occurrence of differential waves of IEG expression in the mammalian suprachiasmatic nucleus regarding light induction or spontaneous oscillations.

Light exposure activates retina ganglion cell lysophosphatidic acid acyl transferase and phosphatidic acid phosphatase by a c-fos-dependent mechanism

We previously reported that the biosynthesis of phospholipids in the avian retina is altered by light stimulation, increasing significantly in ganglion cells in light and in photoreceptor cells in dark. In the present work, we have determined that light significantly increases the incorporation of [3H]glycerol into retina ganglion cell glycerophospholipids in vivo by a Fos-dependent mechanism because an oligonucleotide antisense to c-fos mRNA substantially blocked the light-dark differences. We also studied in vitro the enzyme activities of phosphatidate phosphohydrolase (PAPase), lysophosphatidate acyl transferase (AT II), and phosphatidylserine synthase from retinas of chickens exposed to light or dark. Higher PAPase I and AT II activities were found in incubations of retinal ganglion cells from animals exposed to light; no increase was observed in preparations obtained from light-exposed animals treated with the c-fos antisense oligonucleotide. No light-dark differences were found in phosphatidylserine synthase activity. These findings support the idea that a coordinated photic regulation of PAPase I and AT II is taking place in retina ganglion cells. This constitutes a reasonable mechanism to obtain an overall increased synthesis of glycerophospholipids in stimulated cells that is mediated by the expression of Fos-like proteins.

A simple method to obtain retinal cell preparations highly enriched in specific cell types. Suitability for lipid metabolism studies

The neural retina is a highly complex tissue composed of excitatory and inhibitory neurons and of glial cells. The biosynthesis of lipids that occurs in the retina may be distinctly regulated in one neuronal type of cells with respect to another. To study the cell-type-specific aspects of lipid metabolism, a method for the separation of different retinal cell populations is needed. Herein, we describe a very simple procedure to isolate preparations highly enriched in specific retinal cell types that are suitable for in vitro biochemical assays. The method consists of selectively obtaining photoreceptors (PRC) and retina ganglion cells (RGC) from lyophilized chicken retinas using Scotch tape to assess, then, the in vitro incorporation of labeled precursors into phospholipid moieties. When their metabolic capability was assayed, it was found that these cell preparations maintain their enzyme activities intact to incorporate (32)P-phosphate into phospholipids in vitro at a similar rate as observed in fresh tissue after 1 h incubation. The highest proportion of labeling was observed in phosphatidylethanolamine (PE), followed by phosphoinositides (PIPs), phosphatidylcholine (PC) and phosphatidic acid (PA). Phosphatidate-phosphohydrolase (PAPase), a key enzyme of glycerolipid metabolism, exhibits similar levels of activity when assessed in fresh or frozen cell preparations, indicating that the lyophilization procedure does not significantly affect this activity. It is concluded that different cell populations obtained by the experimental procedure described herein, are useful to study the cellular metabolism and its regulation.

Light affects c-fos expression and phospholipid synthesis in both retinal ganglion cells and photoreceptor cells in an opposite way for each cell type

Retina photoreceptor and ganglion cells isolated from chicks that in vivo were exposed to light have a different phospholipid labeling capacity than those from chicks in the dark. In the light exposed animals, the phospholipid labeling in the ganglion cells is higher (Delta% 45, p<0.005) than in those maintained in the dark, whereas in the photoreceptor cells, the opposite occurs, that is, the phospholipid labeling is higher in the dark than in light. The light-dark differences for phospholipid labeling correlate with the expression of c-fos: when c-fos expression increases (both in mRNA and in c-Fos protein content), phospholipid labeling increases concomitantly. That is, in ganglion cells, c-fos expression and the phospholipid synthesis is higher in light with respect to dark, whereas in photoreceptor cells, c-fos expression and phospholipid synthesis is higher in dark with respect to light. Moreover, when an oligonucleotide antisense to c-fos is administered intraocularly prior to separating the animals into light and dark, no differences in c-fos expression and, consequently, no differences in phospholipid synthesis are found between animals in light and dark. Taken together, these results point to a novel mechanism by which rapid genomic responses to cell stimulation are converted to longer lasting changes in the cell components.

Light exposure stimulates the activity of ganglioside glycosyltransferases of retina ganglion cells

In chicks submitted to light stimulation, the synthesis of gangliosides of the retina ganglion cell increases with respect to chicks maintained in the dark. In an attempt to elucidate if the activation of glycosyltransferases participates in the establishment of these light-dark differences detected in vivo, we examined the activity of a key ganglioside glycosyltransferase, the GalNAc-T (N-acetylgalactosaminyltransferase) that converts GM3 to GM2, in the retina ganglion cells isolated from light and dark exposed chicks. We found that GalNAc-T and other glycosyltransferases are active in these ganglion cell preparations; the kinetic parameters for GalNAc-T were similar to those previously reported for chick retina. The other glycosyltransferase activities assayed were the galactosyltransferase (Gal-T2) that converts GM2 to GM1 and the N-acetylneuraminyltransferase (Sialyl-T1) that converts lactosylceramide to GM3. The three glycosyltransferase activities were higher in the ganglion cell preparations obtained from chicks exposed to light compared to those maintained in the dark. For the GalNAc-T activity, the differences disappear when the cell preparations are sonicated or if the assays are carried out in the presence of detergents or if the end product of the reaction is added to the incubates. The results indicate that the activation of the glycosyltransferases is part of the phenomenon required for cells to achieve the precise rate of synthesis of gangliosides needed in vivo.