Fragile X mental retardation, the most common form of inherited mental impairment, is caused by mutations in the fmr-1 gene, coding for fragile X mental retardation protein (FMRP). A body of circumstantial evidence has suggested that neuronal FMRP plays a role in protein translation, particularly local dendritic translation in response to synaptic activity. These findings open the possibility that FMRP could play a critical role in the synaptic plasticity believed to underlie learning and memory, functions that are severely affected in Alzheimer disease.
In an article published 17 November 2004 in the early online edition of PNAS, William Greenough's team at the University of Illinois at Urbana-Champaign, demonstrate the importance of FMRP by showing what happens when the protein is absent. Greenough and colleagues found that mice with the fmr-1 gene knocked out have deficits in dendritic protein translation, specifically in their ability to upregulate protein synthesis in response to neurotransmitter activation of postsynaptic receptors.
This year has seen the publication of several studies that add detail to the role of FMRP in postsynaptic membranes. Two research groups confirmed that FMRP participates in complexes with polyribosomes (Khandjian et al., 2004; Stefani et al., 2004), and other researchers demonstrated that FMRP is rapidly transported into dendrites in response to KCl depolarization (Antar et al., 2004). In their study, Greenough, first authors Ivan Jeanne Weiler and Chad Spangler, and colleagues studied the dendritic protein translation machinery in tissue from two-week-old fmr-1 KO mice, which have a phenotype reminiscent of human fragile X syndrome, including immature dendritic spine morphology.
Weiler and colleagues worked with preparations called "synaptoneurosomes." These are derived from tissue that is homogenized and passed through successively smaller filters, leaving behind subcellular particles containing neuronal processes, including intact synapses. Pools of synaptoneurosomes can be stimulated with receptor agonists or KCl to simulate synaptic transmission.
In agreement with earlier studies, Weiler and colleagues found that wild-type synaptoneurosomes from visual cortex responded to K+ depolarization with rapid (within two minutes) assembly and incorporation of mRNA into polyribosomes (P-mRNA). By contrast, synaptoneurosomes from the knockout mice lacked this ability. Similarly, agonist stimulation of metabotropic glutamate receptors (mGluRs) induced rapid increases in P-mRNA in the wild-type, but not the fmr-1 KO mice.
Supporting the notion that FMRP is critical to local dendritic, activity-dependent protein translation, the authors found that FMRP-negative preparations were deficient in protein synthesis. Five minutes after mGluR stimulation, synaptoneurosomes from wild-type mice showed a burst of translational activity, as indicated by incorporation of radioactively labeled methionine. In contrast, synaptoneurosomes from the KO mice were unable to respond with increased protein synthesis in this time frame.
To address the concern that mGlu receptors might be downregulated in the KO mice, the authors bypassed the receptors and directly stimulated protein kinase C, which lies downstream of mGluR in the pathway that promotes rapid assembly of polyribosomes. This also failed to activate the dendritic translation machinery in the fmr-1 KO mice. Finally, Weiler and colleagues used electron microscopy to examine layer IV visual cortical tissue, finding that a significantly lower proportion of synapses in the fmr-1 KO mice had polyribosome assemblies in their vicinities.
Speculation has given FMRP several possible roles in regulation of protein translation, including the transport of mRNA and a role in the actual translation process. The authors suggest a model whereby FMRP binds certain mRNAs as early as in the nucleus, before the mRNA complex is transported to the dendrites. There, FMRP remains bound to the mRNA until signaled in some way by synaptic activity. Then the mRNA, according to this model, is released for rapid translation at polyribosomes near synapses.
What proteins might be impacted by the absence of FMRP? One group is neurotransmitters, speculate the authors. Any diminution in proteins critical for receptor proliferation in the postsynaptic membrane might have effects on synaptic plasticity, and hence, one might extrapolate, on processes critical for learning and memory.—Hakon Heimer