Homeostatic plasticity is thought to play an important role in maintaining the stability of neuronal circuits. During one form of homeostatic plasticity, referred to as synaptic scaling, activity blockade leads to a compensatory increase in synaptic transmission by stimulating in dendrites the local translation and synaptic insertion of the AMPA receptor subunit GluR1. We have previously shown that all-trans retinoic acid (RA) mediates activity blockadeinduced synaptic scaling by activating dendritic GluR1 synthesis and that this process requires RAR␣, a member of the nuclear RA receptor family. This result raised the question of where RAR␣ is localized in dendrites and whether its localization is regulated by RA and/or activity blockade. Here, we show that activity blockade or RA treatment in neurons enhances the concentration of RAR␣ in the dendritic RNA granules and activates local GluR1 synthesis in these RNA granules. Importantly, the same RNA granules that contain RAR␣ also exhibit an accumulation of GluR1 protein but with a much slower time course than that of RAR␣, suggesting that the former regulates the latter. Taken together, our results provide a direct link between dendritically localized RAR␣ and local GluR1 synthesis in RNA granules during RA-mediated synaptic signaling in homeostatic synaptic plasticity.FMRP ͉ GluR1 ͉ local protein translation ͉ synaptic scaling H omeostatic synaptic plasticity plays important roles in maintaining the stability of neural networks through the adjustment of global synaptic strength (1, 2). A particular form of homeostatic plasticity, referred to as synaptic scaling, modifies all synapses onto a neuron concurrently in a multiplicative fashion, with greater synaptic adjustment at stronger synapses than at weaker ones, allowing global changes of synaptic activity while preserving relative synaptic strength between individual synapses (3, 4). One well characterized form of synaptic scaling is the increase in synaptic strength induced by chronic blockade of neuronal activity with tetradotoxin (TTX) and the NMDA receptor antagonist APV. This form of synaptic scaling, manifested by an increase in synaptic glutamate receptor response, is mediated by the local synthesis and synaptic insertion of homomeric GluR1 receptors (5-7).We recently found that all-trans retinoic acid (RA), the active form of retinoids in the brain, mediates activity blockade-induced synaptic scaling (8). We demonstrated that activity blockadewhich induces homeostatic plasticity-strongly stimulates neuronal RA synthesis, which in turn activates dendritic GluR1 synthesis. We thus described a function of RA as a synaptic signal that operates during homeostatic plasticity to up-regulate synaptic strength by increasing the size of the postsynaptic glutamate receptor response. Additionally, we suggested that RAR␣ may be critically involved in this form of RA signaling because RAR␣ is present in dendrites, and knocking down RAR␣ blocks synaptic scaling. These results were surprising, given our previous under...

Excitatory synapses in the CNS are formed on both dendritic spines and shafts. Recent studies show that the density of shaft synapses may be independently regulated by behavioral learning and the induction of synaptic plasticity, suggesting that distinct mechanisms are involved in regulating these two types of synapses. Although the molecular mechanisms underlying spinogenesis and spine synapse formation are being delineated, those regulating shaft synapses are still unknown. Here, we show that postsynaptic ephrinB3 expression promotes the formation of glutamatergic synapses specifically on the shafts, not on spines. Reducing or increasing postsynaptic ephrinB3 expression selectively decreases or increases shaft synapse density, respectively. In the ephrinB3 knock-out mouse, although spine synapses are normal, shaft synapse formation is reduced in the hippocampus. Overexpression of glutamate receptor-interacting protein 1 (GRIP1) rescues ephrinB3 knockdown phenotype by restoring shaft synapse density. GRIP1 knockdown prevents the increase in shaft synapse density induced by ephrinB3 overexpression. Together, our results reveal a novel mechanism for independent modulation of shaft synapses through ephrinB3 reverse signaling.

Subunit 7a of mouse cytochrome c oxidase (Cox) displays a contractile muscle-specific isoform, Cox7a1, that is the major cardiac form. To gain insight into the role of this isoform, we have produced a new knockout mouse line that lacks Cox7a1. We show that homozygous and heterozygous Cox7a1 knockout mice, although viable, have reduced Cox activity and develop a dilated cardiomyopathy at 6 weeks of age. Surprisingly, the cardiomyopathy improves and stabilizes by 6 months of age. Cox7a1 knockout mice incorporate more of the “liver-type” isoform Cox7a2 into the cardiac Cox holoenzyme and, also surprisingly, have higher tissue ATP levels.

We have previously shown that male Npc1 heterozygous mice (Npc1+/−), as compared to homozygous wild-type mice (Npc1+/+), both maintained on the —lean BALB/cJ genetic background, become obese on a high fat but not on a low fat diet. We have now extended this result for female heterozygous mice. When fed high-fat diet, the Npc1+/− white adipose weight is also increased in females, therefore following the same trend as males. Bile transporters which had previously been found to be altered in Npc1−/− mice on a high fat diet, showed related, but small, changes in mRNA levels but large changes in protein expression. We have addressed the possible role of caveolae in these differences. It has long been known that caveolin 1 is increased in the liver (sex not specified) of Npc1+/− (compared to Npc1+/+ and Npc1−/−) mice and in heterozygous cultured skin fibroblasts of NPC1 carriers. We now find that caveolin 1 is increased in male, but not female liver and female, but not male adipose tissue. The caveolin 1 increase was not accompanied by changes in another caveolar protein, polymerase1 and transcript release factor (Ptrf). The numbers of caveolae in female adipose cells could not be correlated with levels of caveolae. Thus, we conclude that Npc1 affects female as well as male obesity and bile transporters but that effects on caveolin 1 are not discernible.

Anti-PM-2 is a monoclonal antibody that has been developed against the ECM of embryo/larvae of the starfish Pisaster ochraceus. Immunofluorescent staining shows that the PM-2 epitope is present in the cortical granules of unfertilized eggs and is released into the perivitelline space on fertilization. At the blastula stage, staining is very faint and limited to the blastocoel and a few granules within the cells. Strong staining appears in the embryonic/larval body cavity shortly after gastrulation and continues to increase in both the embryonic/larval body cavity and lumen of the gut at least until the bipinnaria stage. The presence of PM-2 in the Golgi apparatus, its susceptibility to enzymes that attack carbohydrates, and inhibition of PM-2 synthesis by tunicamycin, a drug that inhibits the linkage of carbohydrate moieties to protein backbone chains, suggest that the PM-2 epitope is or contains carbohydrate. Western blots of the whole embryo homogenates show bands at molecular weights of 130, 122, 100, 70, and 50 kDa. As embryos grow, two other high molecular weight (greater than 200 kDa) bands also appear. This suggests that the epitope is present on a series of molecules and that some of the lower MW molecules are precursors of the higher MW ones. A single 24-h exposure to the antibody just posthatching appears to inhibit normal mesenchymal migration at the gastrula stage, and if development of these treated embryos/larvae is allowed to continue to the bipinnaria stage, the embryos are stunted and have a smaller oral hood and esophagus. Long-term exposure results in stunted animals with distorted shapes. Such animals develop a very small embryonic/larval body cavity or none at all and differentiation of the larval GI tract fails to occur. The results suggest that molecules exhibiting the PM-2 epitope are necessary for the proper formation of the blastocoel, for mesenchyme cell movement and for proper development of the larvae GI tract.