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 Edward George Gray in 1959 when he applied electron microscopy to fixed cortical tissue. 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. The actin binding protein synaptopodin (which was originally described in podocytes of the kidney) is an essential component of the SA. 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, but the exact function of the spine apparatus is still considered relatively enigmatic. 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 Edward George Gray in 1959 when he applied electron microscopy to fixed cortical tissue. 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. The actin binding protein synaptopodin (which was originally described in podocytes of the kidney) is an essential component of the SA. 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, but the exact function of the spine apparatus is still considered relatively enigmatic. The spine apparatus consists of membranous saccules and tubules along with wispy filamentous material and is usually associated with a large mushroom-shaped dendritic spine. The wispy filamentous material is a cytoskeletal network which is responsible for the maintenance and alteration of spine shape and controls effectiveness of axospinous synapses. The morphology of the spine apparatus is highly indicative of and similar to the morphology and structure of the smooth surfaced endoplasmic reticulum of the dendrite. Consisting of continuous parallel flattened cisternae, the spine apparatus has a large surface area which is beneficial for its function. Benefits of the large surface area of the spine apparatus include increased electronic properties of the spine and contribution to longitudinal resistance of the cytoplasm. The spine apparatus occupies a large portion of the volume of the spine stalk, which allows it to contribute significantly to the longitudinal resistance of the cytoplasm. Therefore, the spine apparatus can have a direct effect on the membrane potential of the spine plasma membrane by variation in position and volume. The spine apparatus structure allows for dynamic changes in the surface area of the spine plasma membrane. For example, calcium-dependent mechanisms, similar to ones associated with cell shape and maintenance, have been linked to dynamic changes of spine plasma membrane surface area. These calcium-dependent mechanisms have a direct correlation with dynamic changes in dendritic spines, and hence spine plasma membrane surface area. The connection to the smooth endoplasmic reticulum also suggests a potential pathway for the transfer of proteins between the spine and dendrite. Also, the similarity between the smooth endoplasmic reticulum and the spine apparatus suggests that the spine apparatus could function as a reservoir for calcium ions. The structures of spine apparatuses differ depending on their elaboration and position in the spine. Spines can be classified into four different categories: thin, stubby, mushroom, and branched. The shape of the spine apparatus differs depending upon which spine type it is associated with. For example, when the spine apparatus is associated with thin spines, the morphology is simple and consists of a very basic tubular form; however, when the spine apparatus is associated with mushroom shaped dendritic spines, the morphology is a complex laminar arrangement of saccules in the spine head and stalk. For some time, the function of the spine apparatus has been considered enigmatic. Recent evidence, however, suggests the spine apparatus may possess several distinct functions. After elucidating the structure of the spine apparatus, Spacek and Harris noted a continuation of the smooth endoplasmic reticulum into the spine apparatus, where it then takes on a lamellar structure. This observation suggests the SA might play a role in vesicular transport, although a specific mechanism is not yet clear. Furthermore, Pierce et al. proposed that the spine apparatus may be involved in post-translational protein processing, similar to that observed in the Golgi apparatus, and function in the post-translational processing of GluR1 and GluR2 subunits, which are locally translated in dentritic spines, of AMPA receptors. The spine apparatus has also been shown to be involved in the post-translational processing and spatial delivery of NMDA receptors, which also function as glutamate receptors and play a significant role in controlling synaptic plasticity. Considering immunostaining studies have identified NMDARs and AMPARs in the spine apparatus, it has been proposed that the spine apparatus may be critical to the localization of AMPARs and NMDARs to synapses during LTP formation. The appearance of molecular markers for satellite secretory pathways provides further evidence that the spine apparatus plays a role in local integral membrane protein translocation and processing. More specifically, the protein translocation site marker (Sec61α) and the Golgi cisternae markers (giantin and α-mannosidase II) have been observed in the spine apparatus. Synaptic activity triggers Ca2+ influx into dendritic spines via NMDA receptors and voltage-dependent calcium channels. Free Ca2+ ions are rapidly removed from the cytoplasm through Na+/Ca2+ exchangers in the plasma membrane and by sarco/endoplasmic reticulum Ca2+ ATPases (SERCA pumps) that mediate Ca2+ uptake into the smooth endoplasmic reticulum (sER). The spine apparatus, as a sub-compartment of the sER, has a large surface area and is thought to act as an efficient calcium buffer inside the spine (Figure 2).