Trends in Neurosciences
ViewpointActin and the agile spine: how and why do dendritic spines dance?
Section snippets
Defining spines
For the purposes of this article, a dendritic spine is defined as a postsynaptic structure apposed to a presynaptic terminal. Spiney protrusions that lack presynaptic contacts have been observed on cerebellar Purkinje cells, for example, in the case of specific mouse mutants (see Ref. 21 for review); however, Purkinje cells might be an unusual exception, as spines that lack nerve terminals have not been described so far in other types of neuron. It remains unclear how similar such ‘unattached’
Spines have an actin-based cytoskeleton
Although spines vary widely in size and shape, they all contain a network of narrow filaments and a characteristic organelle, the postsynaptic density (PSD), which contains clusters of neurotransmitter receptors and associated proteins, as well as cytoskeletal elements28, 29. It has long been assumed that actin provides the main structural basis for cytoskeletal organization in dendritic spines, as spines mostly lack microtubules and intermediate filaments3, 30. Recently, it was confirmed
Spine actin filaments: stable or dynamic?
Although actin is associated with cell motility, it also has crucial roles in the formation of stable cellular structures, such as hair-cell stereocilia, brush-border microvilli and striated muscle. Thus, cells use either dynamic or stable configurations of actin filaments to fulfil their needs for motility versus structural integrity. Actin exists either as polymerized filaments (F-actin) or unpolymerized globular subunits (G-actin). The balance between these states is ATP dependent and
The function of spines
Multiple perspectives on the function of dendritic spines have received attention3, 4. First, spines expand the connective opportunities for a dendrite, which effectively widens the cylinder of 3D space occupied by a given dendrite, while still enabling tight packing of synapses. Second, it has been proposed that spines isolate the synapse from the dendritic shaft electrically. Although perhaps true to some degree, modeling studies indicate that alterations in spine shape have only a minor
Beyond the barrier: roles for motility
None of the roles so far suggested for spines requires that they change shape rapidly to perform their duties. Indeed, the word ‘spine’ itself suggests a certain rigidity. The views of spine function proposed to date imply a rather static existence for the spine. Although such functions could be modified gradually via changes in spine morphology, this does not explain why a dendritic spine requires motility second by second. Given that the primary function of a spine is to transduce synaptic
A flexible future
Dendritic spines are tiny structures about 1 μm3 in volume. Their small size and lack of biochemical accessibility has limited understanding of their regulation and function. In 1982, Francis Crick conjectured insightfully that dendritic spines ‘twitch’ – that they possess a rapid actin-based motility – and that this rapid twitching might underlie short-term information storage at synapses68. His article challenged investigators to probe the nature and function of the dendritic spine
Acknowledgements
The author thanks A. Hipolito and P. Mas for help with figure preparation, and members of the Halpain laboratory, K. Harris, A. Matus and V. Fowler, for helpful discussions. The author’s research is supported by grants from the National Institutes of Health.
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