Writer: Elena Lynott
Editor: Helen Chow
The neuronal mechanisms of long term potentiation (LTP) and long term depression (LTD) underlie basic memory formation and learning by strengthening synaptic connections between neurons in LTP, and weakening these connections in LTD. While LTP works to establish new neuronal connections critical to memory formation, LTD is also crucial for creating new memories; this process facilitates the removal of old memories by weakening synapses to essentially clear space in the brain for new learning to occur. Despite LTD’s synaptic reduction being necessary for memory formation, LTD can also contribute to intense synaptic degradation which is associated with neurodegenerative disorders and intellectual disabilities.
As a multifaceted process, LTD plays a large role in synaptic plasticity which can be both beneficial and harmful to functioning brains. The specific processes involved in LTD are still widely unknown and actively being researched, however the foundation of LTD revolves around the low intensity depolarization of NMDA channel receptors in the neuronal membrane, which allows for the entry of a low concentration of calcium ions into the cell. This light stimulation and calcium depolarization causes the expulsion of AMPA channel receptors from the same cell membrane, and subsequent synaptic shrinkage (Figure 1). Thus, synaptic plasticity is dependent on the function of NMDA receptors in neuron membranes.
New research has recently come to light regarding the underlying mechanisms involved in LTD and associated synaptic shrinkage. By running several sequencing tests with gene knockout techniques as well as microscopy and image analysis of dendrites, this research ultimately concluded that an RNA helicase, DDX6, plays important roles in gene and protein silencing, and therefore is required for NMDA channel-dependent dendritic spine shrinkage [1]. Furthermore, this finding is important because different mutations in this key RNA helicase can inhibit or overregulate dendritic spine shrinkage, causing imperative complications. This study delves specifically into dendritic spine shrinkage, which plays a huge role in the refinement, control, and impairment of brain circuits [2]. These findings add to the depth of knowledge known about synaptic plasticity procedures which could build our current understanding of neurological diseases and disorders related to synapse shrinkage.
In normally functioning brain circuits, a sequence of protein associations work together to repress gene expression beginning at level of mRNA [3]. MicroRNA (miRNA), which regulates the process of translation by degrading target genes [3] are needed for NMDA channel regulations regarding synaptic plasticity. MicroRNAs begin the formation of spine shrinkage-facilitating protein complexes. In order to repress gene expression encoding for synaptic growth, thus allowing for dendritic spine shrinkage, miRNA binds with proteins called argonauts and LimK proteins [1]. This nexus, called the RNA induced silencing complex, or RISC, also requires the recently recognized DDX6 RNA helicase to properly function and promote dendritic spine shrinkage [1].
By immunostaining proteins with purified antibodies from rat embryonic cortical neuronal cultures, it was found that low levels of NMDA channel stimulation indicative to that of LTD actually increases the binding of DDX6 to LimK1, a LimK protein that selectively silences genes when inhibited [1], proving that this RISC complex regulates spine shrinkage with LTD. DDX6 and argonauts actually begin their silencing activities when binding to LimK1; LimK1 polymerizes the actin filament in dendrites which promotes dendritic spine growth, so LimK1 silencing causes actin depolymerisation and consequent spine shrinkage [1]. In addition testing with immunocytochemistry used to localize specific proteins in a cell, it was found that argonaut proteins and DDX6 were localized in dendritic spines after low NMDA stimulation [1], further drawing connections between spine morphology and RISC workings.
In order to hone in on the relevance of DDX6 in these synaptic processes, gene knockout methods were practiced on DDX6. Gene knockout inactivates specific genes, inhibiting their function, so here DDX6 was knocked out using short hairpin RNA (shRNA) [1]. In response to this test, it was found that lack of DDX6 stopped the binding of argonaut proteins to LimK1, so RISC gene silencing and dendritic spine shrinkage could not occur [1]. Therefore, DDX6 helicase activity is required for NMDA-dependent silencing and synaptic modifications. These shRNA tests actually also showed that no functioning DDX6 caused a small increase in dendritic spine size [1], promoting the hypothesis that DDX6 allows for dendritic spine shrinkage because when DDX6 was removed, dendrite spine size changed in a growing manner, contrasting the shrinkage seen with DDX6.
In this RISC induced gene silencing, NMDA channels first enable LTD to occur, and hence dendritic spine shrinkage due to the synaptic weakening associated with LTD occurs also. As earlier stated, although it seems counterintuitive, LTD needs to occur for memory formation to occur. Therefore, any changes in this complicated, yet vital system would be detrimental to the development of new memories and learning abilities. While DDX6 is emerging as an important part of RISC function due to its role in NMDA-induced dendritic spine shrinkage, mutations are also beginning to be identified which disrupts this system. One such mutation, the R386E mutation in DDX6, inhibits DDX6 binding and association with other proteins in RISC [1]. This mutation is damaging to neural processes as synaptic plasticity relates to so much of basic and higher order brain functions. Neurodevelopmental DDX6 Syndrome is linked to intellectual disabilities and developmental delays, and it is caused by DDX6 helicase mutations [4]. These disabilities have not yet been extensively studied, so currently there is not too much research surrounding DDX6 syndromes. However, what has been found is that DDX6 mutations that cause Neurodevelopmental DDX6 Syndrome exhibit disabilities in their neuronal development as displayed by a thin corpus callosum in individuals with the mutation [5].
Hopefully, future research will be able to give us better insight into how DDX6 specifically regulates neuronal disorders. In terms of LTP and LTD in general, there is also a lack of extensive research. Despite the fact that dysregulation of LTD has been found to have a large role in the development of neurodegenerative diseases, like Alzheimer’s disease and dementia, drugs approved for neurological treatment focus on regulating the actual neurotransmitters, mainly acetylcholine and glutamate, rather than their receptors and synapses [6]. While there is still much to be discovered about how brain networks operate, the recognition of RNA helicase DDX6 is a large step in fostering our understanding of dendritic plasticity, what molecular mechanisms are responsible for synaptic connectivity, and how these systems promote or inhibit the ways in which our brains function.
References
[1]: An essential role for the RNA helicase DDX6 in NMDA receptor-dependent gene silencing and dendritic spine shrinkage | Scientific Reports (nature.com)
[5]: Rare De Novo Missense Variants in RNA Helicase DDX6 Cause Intellectual Disability and Dysmorphic Features and Lead to P-Body Defects and RNA Dysregulation - PMC (nih.gov)