神经元内蛋白分区化合成和降解

佚名

ating local translation of CAMKII mRNA at synapses in response to signals generated by synaptic activation.

Whereas the evidence to date indicates that translation of CAMKII is regulated via NMDA receptor activation, translation of other mRNAs may be regulated in other ways. Local synthesis of FMRP appears to be regulated by mGluR activation; for example, treatment of synaptoneurosomes with agonists for metabotropic glutamate receptors causes a rapid increase in the amount of FMRP detectable by Western blot (Weiler et al., 1997). The translation of other dendritic mRNAs appears to be insensitive to neurotransmitter activation. For example, using another measure of translation (association of mRNAs with polysomes), Bagni et al. (2000) confirmed that glutamate application or depolarization recruited CAMKII mRNA to polyribosomes, but did not recruit mRNAs for InsP3R1 or Arc.

Another recent study reports, however, that Arc mRNA translation in synaptosomes is strikingly upregulated by exogenous recombinant reelin acting through integrin receptors (Dong et al., 2003). This induction was blocked by echistatin (which blocks integrin receptors) and by rapamycin, implying that translation is regulated through the rapamycin-sensitive kinase mammalian target of rapamycin (mTOR, also known as FRAP kinase and RAFT-1). Previous studies had shown that several putative components of this translational signaling pathway, including mTOR, 4E-BP1, 4E-BP-2, and eIF-4E, are present in dendrites (Tang et al., 1998), and that rapamycin blocks several forms of protein synthesis-dependent synaptic plasticity (Steward and Schuman, 2001; Tang et al., 1998). It remains to be seen how integrin receptor-mediated signals and signals generated by neurotransmitters are integrated by the translational machinery at synapses.

Adding still further to the complexity is the evidence that several dendritically localized mRNAs have internal ribosome entry sites (IRESs), including CAMKII, Arc, dendrin, MAP2, and RC3, and that these mRNAs can be translated in a cap-dependent or cap-independent fashion (Pinkstaff et al., 2001). Interestingly, studies of dicistronic constructs with two different reporters revealed that IRES-mediated translation was relatively more efficient in dendrites (Pinkstaff et al., 2001), and studies using bicistronic constructs in Aplysia neurons have revealed that egg-laying hormone, which triggers a bout of intense activity, causes a switch from cap-dependent to cap-independent translation (Dyer et al., 2003). It remains to be seen whether IRES-mediated translation is regulated by synaptic activation in mammalian neurons or by other signals impinging on dendrites.

The Fragile X/BC1 Connection
Still another mechanism for controlling translation at synapses involves Fragile X mental retardation protein (FMRP) and a pol-3 RNA transcript called BC1. Fragile X mental retardation syndrome is caused by a mutation in the gene encoding FMRP (usually an expanded trinucleotide repeat that is hypermethylated, inhibiting gene transcription). FMRP is an RNA binding protein, and EM immunocytochemical studies revealed that FMR protein is concentrated around SPRCs (Feng et al., 1997). On this basis, Feng et al. proposed that FMRP might be involved in targeting mRNAs to dendrites or regulating their translation. Studies of FMRP knockout mice revealed that there were no gross abnormalities in the dendritic localization of representative dendritic mRNAs (MAP2, CAMKII, and Arc), but this study did not exclude the possibility of subtle deficits in mRNA targeting (Steward et al., 1998).

Subsequent studies sought to define the mRNAs bound by FMRP. One identified a “G quartet” domain that appears to be one motif that mediates binding of mRNA to FMRP, but none of the principle “dendritic” mRNAs (Table 1) have this domain (Darnell et al., 2001). Another study identified a different set of mRNAs that interact with FMRP (Miyashiro et al., 2003), some of which are localized in proximal dendrites, but again none of the mRNAs that are abundant in dendrites turned up.

A new twist to the story has come from a very recent study that indicates that FMRP acts as a repressor of translation of several of the principle dendritic mRNAs including CAMKII and Arc, as well as β-actin and FXR2 (a protein related to FMRP) (Zalfa et al., 2003). This study also showed that FMRP interacts with the regulated mRNAs via a noncoding pol-3 transcript called BC1, which has previously been shown to be localized in dendrites (Tiedge et al., 1991). Interestingly, BC1 contains sequences that are predicted to base pair with sequences in MAP1B, CAMKII, and Arc mRNAs. These results suggest that BC1 may link particular mRNAs to FMRP, leading to repression of translation. This ties in nicely with other work implicating BC1 as a regulator of translation initiation of dendritic mRNAs (Wang et al., 2002). These findings have led to the interesting idea that the loss of FMRP in Fragile X mental retardation syndrome could lead to a dysregulation of mRNA translation at the synapse, disrupting synaptic function (Zalfa et al., 2003).

As is evident, the story regarding translational regulation is rapidly evolving, and the final answer is likely to be complex. Translation of certain mRNAs appears to be regulated by signals generated by particular neurotransmitter receptors as well as other signals, and the translation of different mRNAs appears to be controlled in different ways. It remains to be seen whether different control mechanisms exist at different types of synapses.

Protein Synthesis in Axons
The idea for local protein synthesis in axons has been controversial until recently (for a review, see Giuditta et al., 2002). It is now well accepted, however, that mRNAs and translational machinery are present in the neurites of invertebrates, which have the characteristics of both axons and dendrites (van Minnen and Syed, 2001). There is abundant evidence that local synthesis in invertebrate neurites is critical for several different forms of activity-dependent synaptic plasticity (Martin et al., 1997; Sutton et al., 2001). It is not yet clear whether the critical protein synthesis-dependent events are on the presynaptic or postsynaptic side, or whether different mechanisms operate for different types of synaptic modification.

In adult vertebrate axons, polyribosomes are present beneath synapses on axon initial segments (Steward and Ribak, 1986), but are generally not detectable in distal axons. mRNAs encoding the neuropeptide neurotransmitters oxytocin, vasopressin, and prodynorphin are present in axon terminals of the hypothalamo-hypophyseal tract (Mohr et al., 1991), and mRNAs for the olfactory marker protein and various odorant receptors are present in the axon terminals of olfactory neurons that terminate in the olfactory bulb (Ressler et al., 1994; Vassar et al., 1994). The significance of the localization of mRNAs in these axons is not clear, because ribosomes have not been detected in these axon terminals.

Protein Synthesis in Growing Axons and Growth Cones
Local protein synthesis appears to be especially important in growing axons, especially within growth cones. There is evidence, for example, that local synthesis of β-actin is important for cytoskeletal remodeling at the leading edge of the growth cone and within filopodia (Bassell et al., 1998). β-actin mRNA is localized in growth cones, and neurotrophin treatments that stimulate axon growth in culture also enhance the localization of β-actin mRNA in growth cones (Zhang et al., 1999). Localization of β-actin mRNA appears to be mediated by a cis-acting sequence (termed the zip code) that is recognized by a protein called the zip code binding protein (ZBP). Disruption of the interaction between ZBP and β-actin mRNA disrupted mRNA localization, reduced β-actin protein levels within growth cones, and impaired growth cone motility (Zhang et al., 2001).

In contrast to the situation in adult organisms, where polyribosomes are not detected in axons except in initial segments, polyribosomes are abundant in growth cones of at least some growing axons in vitro (Bassell et al., 1998) and in vivo (Tennyson, 1970). It is interesting that the significance of the latter observation is only now becoming apparent.

Recent evidence also implicates local protein synthesis within growth cones in growth cone turning in response to guidance cues. Growth cones extend and retract filopodia and ruffled membranes, and the net direction of extension is determined by where extension/retraction occurs. Turning is caused by extension on one side and collapse on the other, and is triggered by local attractive and repulsive cues in the environment. Studies of chemotropic responses of growth cones of Xenopus retinal ganglion cells have demonstrated that growth cone collapse and turning away in response to sema3A are blocked by protein synthesis inhibitors, even in growth cones that had been separated from their cell bodies (Campbell and Holt, 2001). Protein synthesis inhibitors also blocked the attractive turning response normally seen when retinal growth cones from young embryos (stage 24) were exposed to a gradient of netrin-1 as well as the repulsive turning induced by netrin-1 when neurons were grown on laminin. Together, these results indicated that local protein synthesis within the growth cone is essential for both repulsive and attractive guidance mechanisms, regardless of the exact extracellular stimulus that induces the collapse or turning response. Moreover, exposure to sema3A or netrin-1 triggered a burst of protein synthesis within growth cones, as evidenced by a rapid phosphorylation of the elongation factor eIF-4E, and increased incorporation of labeled amino acids.

One paradox is that protein synthesis is important for both collapse

上一页 [1] [2] [3] [4] [5] [6] 下一页