Lecture Summaries | Learning and Memory: Activity-Controlled Gene Expression in the Nervous System | Biology

Lecture summaries.
WEEK #	TOPICS	LECTURE SUMMARIES
1	Introduction	Getting to know each other Overview and discussion of syllabus General course policies, administrative details How to efficiently search for literature (PubMed, EndNote, isi web of knowledge, MIT Libraries Vera Multisearch) Considerations in reading and analyzing primary scientific literature "Introduction to Brain Function: From Behavior to Molecule" (lecture by instructor)
2	Cellular correlates of memory and memory erasure: Long-term potentiation (LTP) and long-term depression (LTD) of synaptic strength	Memory reflects the storage of information. The discovery that synapses can undergo long-lasting changes in the strength of their transmission has fueled speculations that the brain potentially stores memory within synapses. Electrophysiological recordings from neurons have demonstrated that repeated high-frequency stimulation of neurons can lead to a long-lasting enhancement of synaptic transmission between neuronal cells. This phenomenon was named long-term potentiation (LTP) and was found to last for hours, days and even weeks. Hence, long-lasting potentiation of synaptic strength has been postulated to represent a cellular correlate of memory. Likewise, the opposite phenomenon has been observed. Low-frequency stimulation was found to lead to a long-lasting decrease in synaptic strength, named long-term depression (LTD). LTD may represent a cellular correlate of memory erasure. Together, the existence of LTP and LTD has inspired thousands of studies attempting to unveil the secrets of memory. This week, we will discuss LTP, and in future weeks we will consider both LTP and LTD. Bliss and Lomo demonstrated that synapses can undergo long-term potentiation, and Dunwiddie and Lych showed that Ca²⁺ ions play a crucial role for the induction of LTP.
3	The NMDA receptor, a gatekeeper for synaptic plasticity	The NMDA (N-methyl-D-aspartate) receptor is one of a number of neuronal transmembrane receptors that respond to the neurotransmitter glutamate. This receptor can by distinguished pharmacologically from other glutamate receptors by its response to the small chemical NMDA. Upon binding of glutamate a conformational change within the NMDA receptor (NMDAR) takes place and an ion-conducting pore opens up. However, a magnesium ion is located in the pore of the channel and will block conductance unless membrane depolarization relieves this block. Only when the neuron is depolarized from prior stimulation will the magnesium ion leave the channel pore and make way for the flux of cations — among them calcium that triggers intracellular signaling. Because the channel will allow ion flux only when glutamate is present and the neuron is already depolarized, the NMDAR acts as a molecular coincidence detector. This simple mechanism ensures that conductivity through the NMDA receptor occurs only when a cell that has just been excited receives a second stimulus. The NMDAR has been implicated in a variety of plasticity mechanisms at the synapse, such as LTP, LTD, and also synaptic maturation and neuronal survival. Early studies used pharmacological approaches to discriminate among various glutamate receptors. Today we know that the NMDA receptor is composed of multiple molecularly distinct subunits and that complicated rules govern its composition, signaling properties and delivery to postsynaptic sites. Collingridge and colleagues used a pharmacological approach to demonstrate that N-methyl-D-aspartate has profound effects on synaptic transmission. Years later, Barria and Malinow demonstrated that trafficking of the NMDAR to synapses depends on its subunit composition.
4	Making silent synapses talk: Activation of the NMDA receptor recruits AMPA-type glutamate receptors to synaptic sites	Besides the NMDA receptor, the majority of glutamate-gated ion channels in neuronal surface membranes are AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic-acid)-type glutamate receptors. In most synapses, NMDA- and AMPA-type glutamate receptors coexist, suggesting that fully operational postsynaptic sites need both these types of receptors. However, some glutamatergic synapses probably do not contain both of these types of ion-conducting glutamate receptors. While those synapses have detectable amounts of NMDARs they seemed to lack functional AMPARs, raising the question whether synaptic transmission is actually happening at those sites. One theory suggests that during synaptogenesis NMDARs are incorporated into synapses early on. Only upon their activation (which requires coincident membrane depolarization and presynaptic glutamate release), AMPARs get recruited to young synapses, rendering them fully functional. This model proposes coincident pre- and postsynaptic activity as a key regulator of synapse stabilization and maturation. In this way, inappropriate synaptic connections that do not get used do not become operational and are not stabilized. Two classic studies provided evidence for such a process and have proven to be highly influential concerning the way we think about synapse maturation.
5	AMPA receptor endocytosis as a key mechanism for regulating synaptic strength	Mature synapses are composed of a presynaptic terminal and a postsynaptic site. Both parts contain highly specialized molecular machinery to accomplish the complex task of synaptic transmission. The presynaptic terminal releases a diffusible neurotransmitter (glutamate in excitatory synapses of the mammalian central nervous system) in a highly regulated and quantal manner. The postsynaptic specialization of excitatory synapses consists of a characteristically dense submembrane protein meshwork (the postsynaptic density) and is often located in morphologically distinct protrusions named dendritic spines. Many mechanisms regulate synaptic strength, both on the pre- and the postsynaptic sides. One key mechanism is the regulated surface expression of AMPA-type ionotropic glutamate receptors in postsynaptic spine heads. These receptors constantly cycle between the surface and intracellular compartments, thereby enabling the synapse to react rapidly in cases of synaptic plasticity. Two crucial studies helped establish that the endocytosis and reinsertion of AMPA receptors from the spine surface membrane is a pivotal mechanism neurons use to fine-tune synaptic strength.
6	Metabotropic glutamate receptors (mGluRs) are potent mediators of long-term synaptic depression (LTD)	Cells have developed several mechanisms to receive and convey signals from their environment. Ligand-gated ion channels, such as the kainate, AMPA- and NMDA-type glutamate receptors translate presynaptic activity into changes of membrane potential of the postsynaptic cell. Binding of glutamate to these receptors directly leads to ion influx through a pore within the transmembrane receptor. A different mechanism of signal transduction from the cell surface encompasses metabotropic as opposesd to ionotropic receptors. Metabotropic receptors are different with respect to their membrane topology and do not form an ion-conductive pore. Rather, they associate with trimeric G-proteins on their cytosolic sides and convey signals through agonist-induced association and dissociation with these key signaling proteins. Various isoforms of metabotropic glutamate receptors (mGluRs) exist and are expressed both pre- and postsynaptically. The activation of mGluRs can lead to long-lasting synaptic depression. Accordingly, chemical stimulation of these receptors may induce mGluR-LTD. The mechanisms of how such synaptic weakening is achieved – either pre- or postsynaptically — have remained largely elusive until two studies showed evidence that mGluR activation affects the localization and surface expression of postsynaptic AMPA receptors.
7	Synaptic scaling as a mechanism to globally tune synapse strength	Neurons may store information by strengthening or weakening some synapses and not others. However, potentiation of synaptic strength is possible only within a limited range and will eventually reach a ceiling. Prolonged elevated network activity could theoretically lead to a plateau where all synapses are operating at maximum strength and reach the ceiling of potentiation. As a consequence formerly expressed differences in synaptic strength among individual synapses would be lost in such a scenario. To preserve the relative strengths of synapses within the same neuron, the cell may have mechanisms to elevate or dampen global synapse strength according to overall network activity. Indeed, early studies like the Turrigiano et al paper showed that neurons react to decreased network activity by up-regulating postsynaptic surface receptors globally, a process called synaptic scaling. Synaptic scaling is thought to be a bidirectional homeostatic form of synaptic plasticity, adjusting the overall strength of all synapses to meet the current network requirements. The molecular mechanisms that operate to express synaptic scaling are not fully understood. However, Shepherd and colleagues showed that the activity-regulated protein Arc/Arg3.1 mediates synaptic scaling of AMPA receptors.
8	Midterm assignment – Neurite outgrowth: constructing elaborate neuronal circuits	During development, the nervous system needs to accomplish two major tasks. After having found their spatial destiny within the brain, they first need to grow an arbor of neurites, both axonal and dendritic, and then they have to establish the correct connections to form functional circuits. BDNF is a small secreted molecule that was originally identified as a survival factor for neurons. However, it soon became clear that BDNF also positively regulates dendritic outgrowth, as demonstrated in the visual system of ferrets, using particle-mediated gene transfer technology in combination with organotypic slice culture and imaging. Another small secreted molecule, CPG15, was demonstrated to increase axon arborization, using the visual system of the Xenopus laevis tadpoles as a model system and employing a combination of imaging and electrophysiological techniques.
9	Synaptogenesistd	The formation of functional synaptic contacts is central to neuronal communication and the establishment of network activity. The neuromuscular junction, the site where nerve terminals end and synapse onto muscle fibers, has served as a model system because of its many similarities to synapses in the central nervous system (CNS). The sequence of events that leads to the molecular construction of brain synapses has remained more elusive, mainly because of the overwhelmingly complex CNS anatomy. Unlike at the neuromuscular junction pre- and postsynaptic sides cannot be easily distinguished from each other and are not as readily accessible to molecular manipulation. Two studies have pioneered the elucidation of central synapse formation, on the pre- and postsynaptic sides, respectively.
10	Field trip to the Eli and Edithe Broad Institute and the Nedivi Lab	We will visit the Broad Institute and the Nedivi lab in the Department of Brain and Cognitive Sciences to view cutting-edge technology and its application to neuroscience. The Broad Institute harbors the RNAi consortium, a platform that aims to provide an array of viruses expressing small hairpin RNAs directed against all known mouse genes. Researchers at the Broad Institute will show how they use large-scale robotics and a viral knockdown library to screen for genes that regulate neurite outgrowth in young cultured cortical neurons. Next, we will visit the Nedivi lab to see a two-photon microscope used for deep tissue imaging. The Nedivi lab uses this imaging technique to chronically observe interneurons in the visual cortex of the mouse. Employing various techniques, researchers in the lab can construct three-dimensional images of living neurons and can reconstruct their complex arborization in its entirety. They also can identify structural changes that occur in response to altered network activity, e.g. through manipulation of sensory input to the visual system. We will witness image acquisition from live animals and learn how these images are compiled to generate information about 3D-structural changes in the brain.
11	Activity-induced synapse turnover	Long after the concept of activity-induced circuit strengthening was established, it remained unclear whether changes in neuron physiology were accompanied or even caused by concomitant morphological changes. Engert and colleagues used chemical perfusion on a subcellular level in combination with tissue imaging to demonstrate that morphological changes such as the activity-induced growth of new spines correlate with altered network physiology in the hippocampus. A later study enhanced our understanding of activity-induced synapse elimination and elucidated the molecular mechanisms at work when synapses get destabilized and are eventually lost.
12	Activity-regulated gene expression in neurons	Neurons are postmitotic cells that have lost the ability to proliferate. Neurons nevertheless respond to treatment with growth hormones by inducing transcription factors that in dividing cells trigger cell growth, mitosis and proliferation. A milestone in our understanding of activity-regulated gene expression in neurons was the discovery that rather than growth factors the bona fide stimulator of neuronal transcription was membrane depolarization and Ca²⁺-influx — hallmarks of synaptic electrical activity. These results prompted researchers in the early 1990s to screen for genes that respond to electrical activity. To date, many activity-regulated genes have been discovered. Their specific roles in synaptic plasticity and brain function are being investigated.
13	Final assignment and closing remarks	Oral presentation (details see page 2): Students will give a 10-15 minute Power Point presentation describing a research article. The article must be directly relevant to the course. Papers should be chosen by week 12 and need to be pre-approved by the instructor. The group will provide feedback. We will summarize the course material and make concluding remarks.