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Ephrins keep dendritic spines in shape © 2003 Nature Publishing Group http://www.nature.com/natureneuroscience
Scott M. Thompson The EphB ephrin receptors are involved in synapse formation. New work now implicates the EphA family, as glial ephrin3A activates the EphA4 receptor on dendritic spines to stabilize spine shape. For a century or more, static pictures of Golgi-stained cells in fixed tissue1 have dominated our impressions of dendritic spines, the postsynaptic partner in many excitatory synapses. Only within the last five years have we been liberated from the Golgi stain and come to appreciate how dynamic the structure of a spine really is. Time-lapse imaging shows that adult spines can change shape by as much as 30% of their length or width within a few seconds to minutes1. Furthermore, high-frequency stimulation induces NMDA receptor–dependent protrusion of new dendritic processes in the adult hippocampus1 as well as spine bifurcation, also within minutes. These data suggest that changes in spine number or shape may contribute to synaptic plasticity, perhaps by providing a lasting structural substrate for newly encoded memories. Our understanding of the molecular mechanisms by which the shape and density of dendritic spines are regulated remains incomplete. It is becoming more apparent, however, that the glial cells that ensheath many synapses2 (Fig. 1a) exert an important modulatory influence in this process3. In this issue, an elegant and technically outstanding examination of ephrin signaling by Murai et al.4 sheds new light on how neuron–glia signaling regulates synaptic structure. Ephrins are extracellular signaling molecules that activate Eph receptor tyrosine kinases. Although promiscuity abounds, the EphA family of receptors are generally activated by the ephrin-A ligands, which are glycosyl-phosphatidylinositol-anchored to the cell surface, whereas the EphB receptors are activated by the transmembrane ephrinB ligands5. Activation of Eph receptors causes their homodimerization and autophosphorylation. The author is in the Department of Physiology, 655 W. Baltimore Street, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA. e-mail:
[email protected]
EphB receptors have already been implicated in synapse formation, but the role of EphA receptors has been less clear. EphB receptor–dependent signaling is important in early development for proper axonal pathfinding and for establishment of topographic projections 6. Later on, activation of EphB receptors triggers the transformation of filopodia into spines and the clustering of NMDA receptors7, an early stage in the formation of functional excitatory synapses. Manipulation of EphB signaling produces changes in NMDA receptor–mediated synaptic excitation and in the maturation of spines7. Murai et al.4 have now begun to reveal the role of EphA receptors. They report that the EphA receptor EphA4 is highly expressed throughout the hippocampus, primarily in spines adjacent to presynaptic nerve terminals in both early postnatal animals and adults. They also show that numerous EphA-favoring ligands are expressed in the hippocampus, with ephrin-A3 being the most highly expressed A ligand. Surprisingly, they report that ephrin-A3 is expressed exclusively on glial fibrillary acidic protein (GFAP)-immunoreactive astrocytic processes surrounding synapses. This conclusion was stunningly confirmed with single-cell RT-PCR on cells that had been laser microdissected from hippocampal tissue: ephrin-A3 mRNA was present only in astrocytes and not in CA1 pyramidal neurons. What are the functional consequences of EphA receptor activation? Using phosphotyrosine-specific antibodies, Murai et al. 4 found that the EphA4 receptors were activated heavily early in postnatal life, but relatively little in adults. They conclude that there is not much constitutive activation of EphA4 in the adult, despite the high level of EphA4 and ephrin-A3 expression. When the authors acutely increased the level of EphA receptor activation in cultured hippocampal slices, however, they were able to detect not only rapid EphA4 tyrosine phos-
nature neuroscience • volume 6 no 2 • february 2003
phorylation, but also a significant global reduction in the length and number of spines in fluorescently labeled CA1 cell dendrites. This decrease occurred within only 45 minutes. In contrast, when they decreased activation of endogenous EphA4 receptors, then long, disorganized and irregularly shaped spines were observed. Similarly, spines in the hippocampus of EphA4 receptor knockout mice were abnormally long and disorganized. Taken together, these results indicate that signaling between glial ephrin-A3 and neuronal EphA4 receptors is an important determinant of spine morphology, even in the adult hippocampus. Not all effects of EphB receptors on synaptic transmission depend on the tyrosine kinase activity of the receptor. EphB receptor–mediated potentiation of NMDA receptor–mediated synaptic Ca 2+ influx 8 , as well as EphB receptor–mediated synaptic maturation7, are blocked by mutations in the kinase domain of the receptor. The deficits in synaptic plasticity observed in EphB2 receptor knockout mice, in contrast, can be rescued by kinase-inactive EphB2 constructs9,10. Murai et al. 4 provide evidence that receptor tyrosine kinase activity is required for regulation of spine shape by EphA4. When CA1 cells in hippocampal slice cultures were transfected with a kinase-inactive EphA4 receptor construct, abnormal and disorganized spines were observed. This result is essentially identical to that seen after knocking out or acutely decreasing the activation of EphA4 receptors. The authors conclude that the overexpression of the kinaseinactive mutant receptor acts as a dominant-negative receptor, preventing the endogenous ephrin-A3 ligand from properly regulating spine shape. The work of Murai et al. 4 leaves us with the following model for the regulation of spine shape (Fig. 1b). In the adult hippocampus, spine shape is held relatively constant by the spatially restricted, contact-mediated interaction between ephrin-A3, expressed on the surface of astrocytes, and EphA4 receptors, expressed in the plasma membrane of spines. The normal rapid microscopic changes in spine shape3, or ‘morphing’, result in low-level activation of EphA4 and trigger the retraction of the spine away from the astrocyte. Is there any evidence that this process occurs in the brain? Dunaevsky et al.11 have measured the ongoing motility of 103
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Fig. 1. Ephrin signaling between astrocytes and neurons regulates dendritic spine shape. (a) Electron micrograph of a spine (red) and its apposed presynaptic terminal (yellow) surrounded by astrocytic processes (blue). Scale bar, 0.5 µm. Adapted from ref. 2 (also see http://synapses.bu.edu/anatomy/astro3d/ 3d_essay.stm). (b) Model of astrocyte–spine interactions. At rest (middle), spine shape is maintained by transient contact and repulsion between the ephrin-A3 expressing astrocytes (blue) and EphA4-expressing spines (red). Decreased contact between the spine and the astrocyte (top), for example as a result of astrocyte shrinkage, decreases the activation of EphA4 by ephrin-A3, leading to spine growth. Increased contact (bottom), for example as a result of gliosis or astrocytic swelling, triggers spine retraction.
© 2003 Nature Publishing Group http://www.nature.com/natureneuroscience
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receptor activation will be the next critical task. It will be of considerable interest to determine whether the number of NMDA or AMPA receptors at the spine head changes as the result of EphA receptor activation. And if spine shape is correlated with synaptic efficacy, then another question arises: does spine retraction weaken the synapse because of the shrinkage of the spine head or strengthen the synapse because of the shortening of the spine neck? Either way, these new results 4 will keep synaptic neurobiologists in shape by stimulating more work. 1. Nimchinsky, E.A., Sabatini, B.L. & Svoboda, K. Annu. Rev. Physiol. 64, 313–353 (2002). 2. Ventura, R. & Harris, K.M. J. Neurosci. 19, 6897–6906 (1999).
spines in the dendrites of adult cerebellar Purkinje cells and have correlated that motility with the amount of contact that the spines have with surrounding cellular processes, including glia, as determined post-hoc with electron microscopy. They found that more cell contact with spines is generally correlated with less motility, a result consistent with the work of Murai et al.4. If astrocyte–spine contacts were to increase, for example as a result of cell swelling or gliosis, then spine retraction is the predicted outcome. Gliosis is a common consequence of many forms of brain injury. In fact, many human neuropathologies characterized by gliosis, such as chronic human epilepsy 12, are associated with a decreased size and density of spines. In contrast, if astrocyte–spine contacts were to decrease, then one would expect spine growth. It is interesting to note that periods of elevated estrogen levels over 104
the estrus cycle are correlated with both shrinkage of hippocampal astrocytes13 and concomitant increases in spine number and size14. Like all good science, these exciting results invite us to seek the answers to even more questions. What receptors and/or ligands do the presynaptic nerve terminals express? Are they also affected by changes in Eph receptor activation? What are the downstream signaling pathways engaged upon EphA receptor activation? Previous studies have established links between Eph receptors and several small GTPases—molecules known to modulate the actin cytoskeleton and spines. EphB receptors, for example, increase cdc42 activity in less than five minutes as the result of interactions with associated guanine nucleotide exchange factors15. Determining the functional consequences of the changes in spine shape produced by different levels of EphA4
3. Ullian, E.M., Sapperstein, S.K., Christopherson, K.S. & Barres, B.A. Science 291, 657–661 (2001). 4. Murai, K.K., Nguyn, L.N., Irie, F., Yamaguchi, Y. & Pasquale, E.B. Nat. Neurosci. 6, 153–160 (2003). 5. Flanagan, J.G. & Vanderhaeghen, P. Annu. Rev. Neurosci. 21, 309–345 (1998). 6. Wilkinson, D.G. Nat. Rev. Neurosci. 2, 155–164 (2001). 7. Ethell, I.M. et al. Neuron 31, 1001–1013 (2001). 8. Dalva, M.B. et al. Cell 103, 945–956 (2000). 9. Grunwald, I.C. et al. Neuron 32, 1027–1040 (2001). 10. Henderson, J.T. et al. Neuron 32, 1041–1056 (2001). 11. Dunaevsky, A., Blazeski, R., Yuste, R. & Mason, C. Nat. Neurosci. 4, 685–686 (2001). 12. Scheibel, M.E., Crandall, P.H. & Scheibel, A.B. Epilepsia 9, 89–102 (1974). 13. Klintsova, A., Levy, W.B. & Desmond, N.L. Brain Res. 690, 269–274 (1995). 14. Woolley, C.S. & McEwen, B.S. J. Neurosci. 12, 2549–2554 (1992). 15. Irie, F. & Yamaguchi, Y. Nat. Neurosci. 5, 1117–1118 (2002).
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