Dendritic spine
A dendritic spine is a small membrane protrusion from a neuron's dendrite that typically receives input from a single axon at the synapse. Dendritic spines serve as a storage site for synaptic strength and help transmit electrical signals to the neuron's cell body. Most spines have a bulbous head, and a thin neck that connects the head of the spine to the shaft of the dendrite. The dendrites of a single neuron can contain hundreds to thousands of spines. In addition to spines providing an anatomical substrate for memory storage and synaptic transmission, they may also serve to increase the number of possible contacts between neurons. It has also been suggested that changes in the activity of neurons have a positive effect on spine morphology.
Structure
Dendritic spines are small with spine head volumes ranging 0.01 μm3 to 0.8 μm3. Spines with strong synaptic contacts typically have a large spine head, which connects to the dendrite via a membranous neck. The most notable classes of spine shape are "thin", "stubby", "mushroom", and "bifurcated". Electron microscopy studies have shown that there is a continuum of shapes between these categories. The variable spine shape and volume is thought to be correlated with the strength and maturity of each spine-synapse.Distribution
Dendritic spines usually receive excitatory input from axons, although sometimes both inhibitory and excitatory connections are made onto the same spine head. Excitatory axon proximity to dendritic spines is not sufficient to predict the presence of a synapse, as demonstrated by the Lichtman lab in 2015.Spines are found on the dendrites of most principal neurons in the brain, including the pyramidal neurons of the neocortex, the medium spiny neurons of the striatum, and the Purkinje cells of the cerebellum. Dendritic spines occur at a density of up to 5 spines/1 μm stretch of dendrite. Hippocampal and cortical pyramidal neurons may receive tens of thousands of mostly excitatory inputs from other neurons onto their equally numerous spines, whereas the number of spines on Purkinje neuron dendrites is an order of magnitude larger.
Cytoskeleton and organelles
The cytoskeleton of dendritic spines is particularly important in their synaptic plasticity; without a dynamic cytoskeleton, spines would be unable to rapidly change their volumes or shapes in responses to stimuli. These changes in shape might affect the electrical properties of the spine. The cytoskeleton of dendritic spines is primarily made of filamentous actin. tubulin Monomers and microtubule-associated proteins are present, and organized microtubules are present. Because spines have a cytoskeleton of primarily actin, this allows them to be highly dynamic in shape and size. The actin cytoskeleton directly determines the morphology of the spine, and actin regulators, small GTPases such as Rac, RhoA, and CDC42, rapidly modify this cytoskeleton. Overactive Rac1 results in consistently smaller dendritic spines.In addition to their electrophysiological activity and their receptor-mediated activity, spines appear to be vesicularly active and may even translate proteins. Stacked discs of the smooth endoplasmic reticulum have been identified in dendritic spines. Formation of this "spine apparatus" depends on the protein synaptopodin and is believed to play an important role in calcium handling. "Smooth" vesicles have also been identified in spines, supporting the vesicular activity in dendritic spines. The presence of polyribosomes in spines also suggests protein translational activity in the spine itself, not just in the dendrite.
Morphogenesis
The morphogenesis of dendritic spines is critical to the induction of long-term potentiation. The morphology of the spine depends on the states of actin, either in globular or filamentous forms. The role of Rho family of GTPases and its effects in the stability of actin and spine motility has important implications for memory. If the dendritic spine is the basic unit of information storage, then the spine's ability to extend and retract spontaneously must be constrained. If not, information may be lost. Rho family of GTPases makes significant contributions to the process that stimulates actin polymerization, which in turn increases the size and shape of the spine. Large spines are more stable than smaller ones and may be resistant to modification by additional synaptic activity. Because changes in the shape and size of dendritic spines are correlated with the strength of excitatory synaptic connections and heavily depend on remodeling of its underlying actin cytoskeleton, the specific mechanisms of actin regulation, and therefore the Rho family of GTPases, are integral to the formation, maturation, and plasticity of dendritic spines and to learning and memory.RhoA pathway
One of the major Rho GTPases involved in spine morphogenesis is RhoA, a protein that also modulates the regulation and timing of cell division. In the context of activity in neurons, RhoA is activated in the following manner: once calcium has entered a cell through NMDA receptors, it binds to calmodulin and activates CaMKII, which leads to the activation of RhoA. The activation of the RhoA protein will activate ROCK, a RhoA kinase, which leads to the stimulation of LIM kinase, which in turn inhibits the protein cofilin. Cofilin's function is to reorganize the actin cytoskeleton of a cell; namely, it depolymerizes actin segments and thus inhibits the growth of growth cones and the repair of axons.A study conducted by Murakoshi et al. in 2011 implicated the Rho GTPases RhoA and Cdc42 in dendritic spine morphogenesis. Both GTPases were quickly activated in single dendritic spines of pyramidal neurons in the CA1 region of the rat hippocampus during structural plasticity brought on by long-term potentiation stimuli. Concurrent RhoA and Cdc42 activation led to a transient increase in spine growth of up to 300% for five minutes, which decayed into a smaller but sustained growth for thirty minutes. The activation of RhoA diffused around the vicinity of the spine undergoing stimulation, and it was determined that RhoA is necessary for the transient phase and most likely the sustained phase as well of spine growth.
Cdc42 pathway
Cdc42 has been implicated in many different functions including dendritic growth, branching, and branch stability. Calcium influx into the cell through NMDA receptors binds to calmodulin and activates the Ca2+/calmodulin-dependent protein kinases II. In turn, CaMKII is activated and this activates Cdc42, after which no feedback signaling occurs upstream to calcium and CaMKII. If tagged with monomeric-enhanced green fluorescent protein, one can see that the activation of Cdc42 is limited to just the stimulated spine of a dendrite. This is because the molecule is continuously activated during plasticity and immediately inactivates after diffusing out of the spine. Despite its compartmentalized activity, Cdc42 is still mobile out of the stimulated spine, just like RhoA. Cdc42 activates PAK, which is a protein kinase that specifically phosphorylates and, therefore, inactivates ADF/cofilin. Inactivation of cofilin leads to increased actin polymerization and expansion of the spine's volume. Activation of Cdc42 is required for this increase in spinal volume to be sustained.Observed changes in structural plasticity
Murakoshi, Wang, and Yasuda examined the effects of Rho GTPase activation on the structural plasticity of single dendritic spines elucidating differences between the transient and sustained phases.Transient changes in structural plasticity
Applying a low-frequency train of two-photon glutamate uncaging in a single dendritic spine can elicit rapid activation of both RhoA and Cdc42. During the next two minutes, the volume of the stimulated spine can expand to 300 percent of its original size. However, this change in spine morphology is only temporary; the volume of the spine decreases after five minutes. Administration of C3 transferase, a Rho inhibitor, or glycyl-H1152, a Rock inhibitor, inhibits the transient expansion of the spine, indicating that activation of the Rho-Rock pathway is required in some way for this process.Sustained changes in structural plasticity
After the transient changes described above take place, the spine's volume decreases until it is elevated by 70 to 80 percent of the original volume. This sustained change in structural plasticity will last about thirty minutes. Once again, administration of C3 transferase and Glycyl-H1152 suppressed this growth, suggesting that the Rho-Rock pathway is necessary for more persistent increases in spinal volume. In addition, administration of the Cdc42 binding domain of Wasp or inhibitor targeting Pak1 activation-3 decreases this sustained growth in volume, demonstrating that the Cdc42-Pak pathway is needed for this growth in spinal volume as well. This is important because sustained changes in structural plasticity may provide a mechanism for the encoding, maintenance, and retrieval of memories. The observations made may suggest that Rho GTPases are necessary for these processes.Physiology
Receptor activity
Dendritic spines express glutamate receptors on their surface. The TrkB receptor for BDNF is also expressed on the spine surface, and is believed to play a role in spine survival. The tip of the spine contains an electron-dense region referred to as the "postsynaptic density". The PSD directly apposes the active zone of its synapsing axon and comprises ~10% of the spine's membrane surface area; neurotransmitters released from the active zone bind receptors in the postsynaptic density of the spine. Half of the synapsing axons and dendritic spines are physically tethered by calcium-dependent cadherin, which forms cell-to-cell adherent junctions between two neurons.Glutamate receptors are localized to the postsynaptic density, and are anchored by cytoskeletal elements to the membrane. They are positioned directly above their signalling machinery, which is typically tethered to the underside of the plasma membrane, allowing signals transmitted by the GluRs into the cytosol to be further propagated by their nearby signalling elements to activate signal transduction cascades. The localization of signalling elements to their GluRs is particularly important in ensuring signal cascade activation, as GluRs would be unable to affect particular downstream effects without nearby signallers.
Signalling from GluRs is mediated by the presence of an abundance of proteins, especially kinases, that are localized to the postsynaptic density. These include calcium-dependent calmodulin, CaMKII, PKC, PKA, Protein Phosphatase-1, and Fyn tyrosine kinase. Certain signallers, such as CaMKII, are upregulated in response to activity.
Spines are particularly advantageous to neurons by compartmentalizing biochemical signals. This can help to encode changes in the state of an individual synapse without necessarily affecting the state of other synapses of the same neuron. The length and width of the spine neck has a large effect on the degree of compartmentalization, with thin spines being the most biochemically isolated spines.