Mechanisms of Plasticity

Outline Objective Mechanism Control of gene expression Kinase cascades and Phosphorylation/dephosphorylation General Learning/LTP Synaptic strengthening Control of gene expression Transcription factors Epigenetics

Definition and learning objective Neuroplasticity The brain’s ability to reorganize itself by forming new neural connections throughout life and adjust their activities in response to the environment Take home: Many environmental and context dependent cues can modulate how our brain functions throughout life both adaptively and maladaptively

Activity of neurons regulates gene expression and neuroplasticity Inputs Outputs Neurotransmitters Neuroprotection Stressors Circadian entrainment Light Growth and survival Growth Factors Synaptic plasticity CREB Neurons act by integrating a variety of input from many sources. Depolarization of neurons and elevation of cAMP strongly stimulates the phosphorylation of CREB at Ser133 in order to activate CREB-dependent transcription

Converging pathways onto CREB 1st signal 2nd messengers Amplification Protein Kinases Multiple signals able to regulate CREB dependent transcription, through the activation of second messenger cascades

G alpha S is stimulatory, and will increase cyclic amp levels which will increase PKA . G alpha I is inhibitory and will decrease cyclic AMP levels, and G alpha Q acts on phospholipase C which will cleave membrane bound PIP2 to activate diacyl glycerol and inosititol triphosphate (IP3) transduction pathways ultimately increasing calcium levels and PKC. There is also the G alpha 12/13 subunit that regulates cell processes through the use of guanine nucleotide exchange factors, but we wont talk about this pathway in this class. Paolo Sassone-Corsi Cold Spring Harb Perspect Biol 2012;4:a011148 ©2012 by Cold Spring Harbor Laboratory Press

All protein kinases are regulated in a similar way. 2nd msg cAMP cGMP DAG Ca2+/Cam 1st msg Growth factors

Mechanism: Phosphorylation Phosphorylation events are regulated by virtually every type of external and internal stimulus. As such protein phosphorylation can be viewed as the major molecular currency of intracellular regulation and is one of the central mechanism underlying diverse types of neural plasticity. A phosphorylation or dephosphorylation event usually results in a functional change in the the substrate, through changing its cellular location, ability to associate with other proteins, its enzymatic activity… etc.

This is just to highlight the complexity of how these signaling pathways actually are in real life. Note Interplay between kinase cascades. Cross talk, synergy, antagonism, common target: CREB Allyn & Bacon 2004

Long Term Potentiation The persistent strengthening of synapses based on recent patterns of behavior Ok, now getting to the meat and potatoes of this lecture and the molecular mechanisms underlying plasticity: long term potentiation.

Long Term Potentiation Summary of the early phase of LTP.

Plasticity during Learning -Stronger synapses PKA phosphorylation of AMPA receptors allows them to stay open longer when glutamate binds. Prolonged exposure activates NMDA receptors, CamKII, NOS, and leads to more glutamate release as well as the transcription of new AMPA receptors.

Early – local and increases NO Late Kinases

This is a representation of the post synaptic responses in order to show the enhanced efficacy of synaptic connection after high frequency depolarization of pre and post synaptic neurons

Post translational regulation of neuronal activity

Mechanism: Phosphorylation

In the case of a Signal B that increases Ca, then the cacliu calmodulin dependent phosphatase, calcineurin is activated which sill dephosphorylate DARPP32. In this case, then Protein phosphatase-1 is not inhibited and the sodium potassium pump will be able to be dephosphylated into its active state and reset the membrane potential. What regulates whether the pump is active? The balance of signal A and signal B.

Mechanisms of DARPP-32 regulation Dopamine higher- phosphorylates threonine 24 – phosphorylates darp32 which then inhibits PP1 - inhibits dephosphorylation events. Glutamate higher – favors phosphorylates on threonin76 and this does not inhibit PP1 or dephosphorylation events

Regulation of plasticity at a transcriptional level

Gene Expression: Central Dogma Genes encode proteins and proteins dictate cellular function. DNA contains the genetic code or blue print, that is the instructions for making the different proteins in a cell. These instructions are transcribed into RNA, and then mRNA is synthesized which is the instructions for making the proteins at the ribosomes

DNA Theoretically the same in all cells of an individual Reality is that somatic mutations occur frequently -Adaptive vs maladaptive (cancer) vs silent changes Genome wide high throughput sequencing Variation between individuals GWAS Inheritance Patterns Dominance vs. recessivity

Transcription Factors Trans acting – separate from the DNA strand. Cis acting – attached to the DNA. Trans-acting factors bind to cis activng factors but not to the RNA polymerase complex. Together this complex is a positive regulator of gene expression.

How to measure gene regulation? Direct method: One method of measuring gene regulation is by Chromatic immunoprecipitation, which is a way to measure the amount of a transcription factor bound to a particular DNA sequence. Can also be used to detect which tf’s are bound to that DNA sequence if combined with mass spectrometry.

How to measure gene regulation? Indirect way : measure the expression of target genes . Microarray: measures differential gene expression levels between two samples. mouse brains – the right is the WT the right is the KO.

Epigenetics: Global gene expression regulation -acetylation -methylation

Transcriptional enhancers recruit histone acetyl transferases (HATs) Transcriptional repressors recruit HDACs and DNMTs

Epigenetic changes are influenced by signaling from the membrane This slide serves to illustrate how signals from the membrane can influence kinases like CaMK which are then able to phosphorylate epigenetic repressors and they are then unable to associate with DNA

Specific histone marks are associated with changes in gene expression Maintains repressive H3K27me3 mark Multipotent stem cell Bivalent genes: Poised for activation but silent Differentiation Background for the paper: Bivalent epigenetic marks primes key developmental genes for later activation. Maintains permissive the H3K4me3 mark

The transcription factor SOX2 promotes hippocampal neurogenesis through inhibiting repressive epigenetic histone marks SOX2 limits the deposition of the repressive epigenetic histone mark H3K27Me3, specifically at the promoter regions of the proneural genes, NGN2 and NeuroD1. Loss of SOX2 expression impairs the expression of NGN2 and NeuroD1 in vitro.

Loss of SOX2 impairs hippocampal neurogenesis in vivo Loss of SOX2 expression impairs the expression of NGN2 and NeuroD1 in vivo. This study describes a novel role for SOX2 in neuronal differentiation, by modulating the epigenetic programs that allow for adult neurogenesis to proceed.

Summary Learning objectives Mechanism of plasticity Kinase cascades and Phosphorylation/dephosphorylation Long Term Potentiation Synaptic strengthening Control of gene expression Transcription factors Epigenetics