Post-transcriptional regulation and mRNA Transport

Molecular motors actively transport many types of cargo along the cytoskeleton in a wide range of organisms. One class of cargo is localized mRNAs, which are transported by myosin on actin filaments or by kinesin and dynein on microtubules. How the cargo is kept at its final intracellular destination and whether the motors are recycled after completion of transport are poorly understood. A new RNA anchoring assay in living Drosophila blastoderm embryos has been used to show that apical anchoring of mRNA after completion of dynein transport does not depend on actin or on continuous active transport by the motor. Instead, apical anchoring of RNA requires microtubules and involves dynein as a static anchor that remains with the cargo at its final destination. This study proposes a general principle that could also apply to other dynein cargo and to some other molecular motors, whereby cargo transport and anchoring reside in the same molecule (Delanoue, 2005).

This study has used a specific RNA anchoring assay to distinguish between the four main models that could explain how apical wg and pair-rule mRNA (runt, and fushi tarazu) are retained in the apical cytoplasm after their transport by dynein. The models that have been proposed could also apply to other molecular motors and their various cargos. (1) The dynein motor could release the RNA cargo at its final destination, allowing the RNA to bind to an actin-dependent static anchor and the motor to participate in further transport. (2) The anchor could be MT associated rather than actin based. (3) RNA could be retained in the apical cytoplasm by continuous active transport without anchoring. (4) The motor itself could retain the cargo and turn into a static anchor when it reaches the final destination (Delanoue, 2005).

At the outset of this study, it was anticipated that cargo anchoring via actin was the most likely possibility given that actin is thought to be involved in anchoring of many other RNAs. It was also thought that after a motor completes a transport cycle, it releases the cargo and is available for transport of new cargo. However, in general, there has not been very good direct evidence showing that such a model is correct because of the lack of an assay that could discriminate between the transport and anchoring steps. In this study, two specific assays were used: one for transport and another for anchoring. Both anchoring and transport were assayed at the same time in the same embryo using two distinct RNAs. These specific assays have allowed a test and refutation of the prevailing actin anchoring model at least in the case of runt, fushi tarazu and wg apical mRNA localization in the Drosophila blastoderm embryo. Against expectations, the results show that the fourth model is correct, namely that wg and pair-rule RNA are anchored by a dynein-dependent mechanism so that the motor molecules are maintained to the site of anchoring with the cargo. The data shows that the requirement for dynein to anchor the apical RNA is independent of the ATPase activity of the motor and its transport cofactors Egl and BicD, all of which are required for the active transport of the RNA. These observations are best explained by a model in which the dynein motor involved in apical transport of RNA does not release the cargo and acts as a static anchor at the final destination (Delanoue, 2005).

It is interesting to consider how a dynamic motor such as dynein could turn into a static anchor after completion of cargo transport. Dynein is a large multicomplex motor that is difficult to work with in vitro. Nevertheless, many of the subunits of dynein are defined and the force-generating protein, Dhc, is thought to contain physically distinct ATPase and MT binding domains. It is therefore easy to imagine how the motor could change to a static anchor by remaining attached to MTs via the MT binding domain and losing its ATPase force-generating capacity. Indeed, ATPase-independent MT binding has been observed with dynein under in vitro conditions. While it is difficult to compare in vitro studies with the current studies in vivo, the latter are likely to show much more complex and varied interactions with proteins in the cell. Indeed, anchoring may also involve interactions with additional components not present in vitro, such as MT-associated proteins (MAPs), which could stabilize the binding of dynein to the apical MTs or could physically obstruct the motor movement. Another possibility could be anchoring through association with ribosomes, but this can be ruled out in the case of wg and pair-rule RNA, since RNAs lacking a coding region can be transported and anchored correctly. Alternative hypotheses, which cannot be ruled out, include a change of conformation or modifications of the structure of the dynein-dynactin complex. While the data demonstrate conclusively a new RNA-anchoring function for dynein, they do not allow distinguishing between the various hypotheses of how this anchoring occurs at the molecular level, nor test definitively whether Dynactin is required for anchoring. p50/dynamitin is present with the anchored RNA, and overexpression of p50/dynamitin and a Glued/p150 allele cause a partial inhibition of RNA localization with no obvious effects on anchoring. These results suggest, but do not demonstrate conclusively, that Dynactin is not required for anchoring. Furthermore, while it is shown that the ATPase activity of the motor is not required for anchoring, this observation does not test whether dynactin is required in addition to dynein for anchoring (Delanoue, 2005).

Whatever the molecular basis for the dynein anchoring function that was uncovered, it seems likely that the described anchoring does not involve a single dynein molecule anchoring a single RNA molecule. Instead, the RNA cargo is likely to consist of particles containing many RNA molecules and probably many motor complexes. The cargo is thus likely to remain strongly attached to at least some motor molecules throughout transport and anchoring. However, it is not yet known what the linkers between the RNA and motors are (Delanoue, 2005).

Little is also known about the mechanism of anchoring of other dynein cargos, although the mechanism of transport of RNA by dynein could be very similar to other cargos such as lipid droplets. Dynein is also required for nuclear positioning and tethering in many systems, so its role as a static anchor may be widespread. Furthermore, some kinesin-like proteins are also thought to interact with static cell components, and recent in vitro studies show that myosin VI can switch from a motor to an anchor under tension. This process has been proposed to stabilize actin cytoskeletal structures and link protein complexes to actin structures. It is therefore proposed that myosins, kinesins, and dynein may all be able to switch under certain circumstances from dynamic motors to static anchors and that the observations of this study may represent a general principle for anchoring of some cargos following transport to their final cytoplasmic destination (Delanoue, 2005).

Fragile X mental retardation protein regulates trans-synaptic signaling in Drosophila

Fragile X syndrome (FXS), the most common inherited determinant of intellectual disability and autism spectrum disorders, is caused by loss of the fragile X mental retardation 1 (FMR1) gene product (FMRP), an mRNA-binding translational repressor. A number of conserved FMRP targets have been identified in the well-characterized Drosophila FXS disease model, but FMRP is highly pleiotropic in function and the full spectrum of FMRP targets has yet to be revealed. In this study, screens for upregulated neural proteins in Drosophila fmr1 (dfmr1) null mutants reveal strong elevation of two synaptic heparan sulfate proteoglycans (HSPGs): GPI-anchored glypican Dally-like protein (Dlp) and transmembrane Syndecan (Sdc). Earlier work has shown that Dlp and Sdc act as co-receptors regulating extracellular ligands upstream of intracellular signal transduction in multiple trans-synaptic pathways that drive synaptogenesis. Consistently, dfmr1 null synapses exhibit altered WNT signaling, with changes in both Wingless (Wg) ligand abundance and downstream Frizzled-2 (Fz2) receptor C-terminal nuclear import. Similarly, a parallel anterograde signaling ligand, Jelly belly (Jeb), and downstream ERK phosphorylation (dpERK) are depressed at dfmr1 null synapses. In contrast, the retrograde BMP ligand Glass bottom boat (Gbb) and downstream signaling via phosphorylation of the transcription factor MAD (pMAD) seem not to be affected. To determine whether HSPG upregulation is causative for synaptogenic defects, HSPGs were genetically reduced to control levels in the dfmr1 null background. HSPG correction restored both (1) Wg and Jeb trans-synaptic signaling, and (2) synaptic architecture and transmission strength back to wild-type levels. Taken together, these data suggest that FMRP negatively regulates HSPG co-receptors controlling trans-synaptic signaling during synaptogenesis, and that loss of this regulation causes synaptic structure and function defects characterizing the FXS disease state (Friedman, 2013).

FXS is widely considered a disease state arising from synaptic dysfunction, with pre- and postsynaptic defects well characterized in the Drosophila disease model. There has been much work documenting FXS phenotypes in humans as well as in animal models, but there has been less progress on mechanistic underpinnings. This study focuses on the extracellular synaptomatrix in FXS owing to identification of pharmacological and genetic interactions between FMRP and secreted MMPs, a mechanism that is conserved in mammals. Other studies have also highlighted the importance of the synaptomatrix in synaptogenesis, particularly the roles of membrane-anchored HSPGs as co-receptors regulating trans-synaptic signaling. Importantly, it has been shown that FMRP binds HSPG mRNAs, thereby presumably repressing translation. Based on these multiple lines of evidence, this study hypothesized that the FMRP-MMP-HSPG intersection provides a coordinate mechanism for the pre- and postsynaptic defects characterizing the FXS disease state, with trans-synaptic signaling orchestrating synapse maturation across the synaptic cleft (Friedman, 2013).

In testing this hypothesis, a dramatic upregulation of GPI-anchored glypican Dlp and transmembrane Sdc HSPGs was discovered at dfmr1 null NMJ synapses. Indeed, these are among the largest synaptic molecular changes reported in the Drosophila FXS disease model. Importantly, HSPGs have been shown to play key roles in synaptic development. For example, the mammalian HSPG Agrin has long been known to regulate acetylcholine receptors, interconnected with a glycan network modulating trans-synaptic signaling. In Drosophila, Dlp, Sdc and Perlecan HSPGs mediate axon guidance, synapse formation and trans-synaptic signaling. Previous work on dlp mutants reports elevated neurotransmission, paradoxically similar to the Dlp overexpression phenotype shown in this study. However, the previous study does not show Dlp overexpression electrophysiological data, although it does show increased active zone areas consistent with strengthened neurotransmission. The same study reports that Dlp overexpression decreases bouton number on muscle 6/7, which differs from finding in this study of increased bouton number on muscle 4. Because HSPG co-receptors regulate trans-synaptic signaling, dfmr1 mutants were tested for changes in three established pathways at the Drosophila NMJ. Strong alterations in both Wg and Jeb pathways were found, with anterograde signaling being downregulated in both cases. In contrast, no change was found in the retrograde BMP Gbb pathway, suggesting that FMRP plays specific roles in modulating anterograde trans-synaptic signaling during synaptogenesis (Friedman, 2013).

The defect in Jeb signaling seems to be simple to understand, with decreased synaptomatrix ligand abundance coupled to decreased dpERK nuclear localization. However, there is no known link to HSPG co-receptor regulation. It has been shown earlier that Jeb signaling is regulated by another synaptomatrix glycan mechanism, providing a clear precedent for this level of regulation. In contrast, the Wnt pathway exhibits an inverse relationship between Wg ligand abundance (elevated) and Fz2-C nuclear signaling (reduced). This apparent contradiction is explained by the dual activity of the Dlp co-receptor, which stabilizes extracellular Wg to retain it at the membrane, but also competes with the Fz2 receptor. This ‘exchange-factor mechanism’ is competitively dependent on the ratio of Dlp co-receptor to Fz2 receptor, with a higher ratio causing more Wg to be competed away from Fz2. Indeed, it has been demonstrated that the same elevated Wg surface retention couples to decreased downstream Fz2-C signaling in an independent HSPG regulative mechanism at the Drosophila NMJ. This study suggests that in the dfmr1 null synapse, highly elevated Dlp traps Wg, thereby preventing it from binding Fz2 to initiate signaling (Friedman, 2013).

Dysregulation of the Wg nuclear import pathway (FNI) provides a plausible mechanism to explain synapse development defects underlying the FXS disease state, with established roles in activity-dependent modulation of synaptic morphogenesis and neurotransmission. FXS has long been associated with defects in activity-dependent architectural modulation, including postsynaptic spine formation, synapse pruning and functional plasticity. Although it is surely not the only player, aberrant Wg signaling could play a part in these deficiencies. Importantly, it has been shown that the FNI pathway is involved in shuttling large RNA granules out of the postsynaptic nucleus, providing a potential intersection with the FMRP RNA transport mechanism. However, the Wg FNI pathway is not the only Wnt signaling at the Drosophila NMJ, with other outputs including the canonical, divergent canonical and planar cell polarity pathways, which could be dysregulated in dfmr1 nulls. For example, a divergent canonical retrograde pathway proceeds through GSK3β (Shaggy) to alter microtubule assembly, and the FXS disease state is linked to dysregulated GSK3β and microtubule stability misregulation via Drosophila Futsch/mammalian MAP1B. Moreover, it has been shown that the secreted HSPG Perlecan (Drosophila Trol) regulates bidirectional Wnt signaling to affect Drosophila NMJ structure and/or function, via anterograde FNI and retrograde divergent canonical pathways. It is also important to note that previous studies show that a reduction in the FNI pathway, due to decreased Fz2-C trafficking to the nucleus, leads to decreased NMJ bouton number. Future work is needed to fully understand connections between FMRP, HSPGs, the multiple Wnt signaling pathways and the established defects in the synaptic microtubule cytoskeleton in the FXS disease state (Friedman, 2013).

Adding to the complications of FXS trans-synaptic signaling regulation, it was shown that two trans-synaptic signaling pathways are suppressed in parallel: the Wg and Jeb pathways. Possibly even more promising for clinical relevance, it has been established that the Jeb signaling functions as a repressor of neurotransmission strength at the Drosophila NMJ, with jeb and alk mutants presenting increased evoked synaptic transmission. Consistently, loss of FMRP leads to increased EJC amplitudes, which could be due, at least partially, to misregulated Jeb-Alk signaling. Importantly, it has been shown that dfmr1 null neurotransmission defects are due to a combination of pre- and postsynaptic changes, and that there is a non-cell-autonomous requirement for FMRP in the regulation of functional changes in the synaptic vesicle (SV) cycle underlying neurotransmission strength. Additionally, jeb and alk mutants exhibit synaptic structural changes consistent with this FMRP interaction, including a larger NMJ area and synaptic bouton maturation defects, which are markedly similar to the structural overelaboration phenotypes of the FXS disease state. These data together suggest that altered Jeb-Alk trans-synaptic signaling plays a role in the synaptic dysfunction characterizing the dfmr1 null. The study proposes that Wg and Jeb signaling defects likely interact, in synergistic and/or antagonistic ways, to influence the combined pre- and postsynaptic alterations characterizing the FXS disease state (Friedman, 2013).

Although trans-synaptic signaling pathways, and in particular both Wnt and Jeb-Alk pathways, have been proposed to be involved in the manifestation of a number of neurological disorders, this study provides the first evidence that aberrant trans-synaptic signaling is causally involved in an FXS disease model. The study proposes a mechanism in which FMRP acts to regulate trans-synaptic ligands by depressing expression of membrane-anchored HSPG co-receptors. HSPG overexpression alone is sufficient to cause both synaptic structure and function defects characterizing the FXS disease state. Increasing HSPG abundance in the postsynaptic cell is enough to increase the number of presynaptic branches and synaptic boutons, as well as elevate neurotransmission. Correlation with these well-established dfmr1 null synaptic phenotypes suggests that HSPG elevation could be a causal mechanism. Conclusively, reversing HSPG overexpression in the dfmr1 null is sufficient to correct Wnt and Jeb signaling, and to restore normal synaptic structure and function. Because there is no dosage compensation, HSPG heterozygosity offsets the elevation caused by loss of dfmr1. Correcting both Dlp and Sdc HSPGs in the dfmr1 background restores Wg and Jeb signaling to control levels. Correcting Dlp levels by itself restores synaptic architecture, but both Dlp and Sdc have to be corrected to restore normal neurotransmission in dfmr1 null synapses. Taken together, these results from the Drosophila FXS disease model provide exciting new insights into the mechanisms of synaptic phenotypes caused by the loss of FMRP, and promising avenues for new therapeutic treatment strategies (Friedman, 2013).

wingless continued: Biological Overview | Evolutionary Homologs | Transcriptional regulation |Targets of Activity | Protein Interactions | Developmental Biology | Effects of Mutation | References

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