Spine apparatus

The spine apparatus (SA) is a specialized form of endoplasmic reticulum (ER) that is found in a subpopulation of dendritic spines in central neurons. It was discovered by E. G. Gray in 1959 when he applied electron microscopy to fixed cortical tissue. [Gray EG (1959) Electron microscopy of synaptic contacts on dendrite spines of the cerebral cortex. Nature 1959 Jun 6;183(4675):1592-3 [http://www.ncbi.nlm.nih.gov/pubmed/13666826] ] The SA consists of a series of stacked discs that are thought to be connected to each other and to the dendritic system of ER-tubules. [Cooney JR, Hurlburt JL, Selig DK, Harris KM, Fiala JC (2002) Endosomal compartments serve multiple hippocampal dendritic spines from a widespread rather than a local store of recycling membrane. J Neurosci. 22(6):2215-24. [http://www.ncbi.nlm.nih.gov/pubmed/11896161] ] . The actin binding protein synaptopodin (which has originally been described in podocytes of the kidney) is an essential component of the SA.Deller T, Merten T, Roth SU, Mundel P, Frotscher M. (2000) Actin-associated protein synaptopodin in the rat hippocampal formation: localization in the spine neck and close association with the spine apparatus of principal neurons. [http://www.ncbi.nlm.nih.gov/pubmed/10701442 J Comp Neurol. 418(2):164-81.] ] Mice that lack the gene for synaptopodin do not form a spine apparatus.. The SA is believed to play a critical role in learning and memory.

Morphology

In CA1 pyramidal cells of the hippocampus, a spine apparatus is found in about 20% of all dendritic spines. The SA is mostly found in large mushroom-shaped spines which are thought to carry strong synapses. Not all spiny cells form a SA: Purkinje cells of the cerebellum, for example, have many dendritic spines, but no spine apparatus.

Function

Dendritic spines in cortical neurons contain mainly ryanodine receptors, ion channels that are capable of releasing calcium from the endoplasmic reticulum. Therefore, it has long been speculated that the spine apparatus might be involved in calcium signaling inside the spine. A knockout mouse lacking synaptopodin provided the first evidence for an involvement of the spine apparatus in synaptic plasticity.Deller T, Korte M, Chabanis S, Drakew A, Schwegler H, Stefani GG, Zuniga A, Schwarz K, Bonhoeffer T, Zeller R, Frotscher M, Mundel P. (2003) Synaptopodin-deficient mice lack a spine apparatus and show deficits in synaptic plasticity. [http://www.ncbi.nlm.nih.gov/pubmed/12928494 PNAS 100(18):10494-9] ] These mice, which had no spine apparatus, had impaired long-term potentiation in the hippocampus and also deficits in spatial learning. However a mechanistic link between the SA and synaptic plasticity has not yet been established.

References

External links

[http://synapses.clm.utexas.edu/anatomy/sa/sa.stm Synapse Web: Electron microscopy of the spine apparatus]