Discoveries

Modulating the Formation of Stress-Related Memories

July 18, 2017

Synaptic plasticity – the ability of neurons to increase or decrease the strength of signal transmission in response to changes in synaptic activity – is a key process in learning, cognition, and memory. Considerable data suggest that noradrenergic neurons that project their axons to the cortex and hippocampus play a role in the development and retention of memories associated with fear or stress. They do this by promoting long-term potentiation, a form of synaptic plasticity characterized by a sustained increase in signal transmission in response to a stimulus, which is often glutamate released by excitatory neurons. Now, Basic Sciences investigators Jeff Conn and Craig Lindsley, along with their laboratories, use novel selective allosteric modulators of group II metabotropic glutamate receptors (mGlu2 and mGlu3) to show that mGlu3 in hippocampal astrocytes suppresses the development of long-term potentiation in response to noradrenergic stimulation. They show that noradrenaline released by noradrenergic neurons increases cAMP levels in both neurons and neighboring astrocytes. In neurons, the cAMP promotes development of long-term potentiation in response to a glutamate stimulus. However, in the astrocytes, glutamate also activates mGlu3, which increases cAMP levels even more. Breakdown of the excess cAMP yields high concentrations of adenosine that travels to the nearby excitatory glutamatergic nerve terminals, suppressing their release of glutamate. In the absence of glutamate, long-term potentiation does not develop, even if noradrenergic signaling occurs. These findings may have important implications for the prevention and/or treatment of post-traumatic stress disorder in which stress- or fear-related memories play a critical role.  The work is published in the journal Neuropsychopharmacology [A. G. Walker, et al., (2017) Neuropsychopharmacology, published online June 30, 2017, DOI:10.1038/nnp.2017.136].

Image reproduced by permission from Macmillan Publishers Ltd, from A. G. Walker, et al., (2017) Neuropsychopharmacology, published online June 30, 2017, DOI:10.1038/nnp.2017.136]. Copyright 2017 

Discovering the Secrets of a Cancer Drug Resistance Protein

April 27, 2017

The enzyme γ-secretase is primarily known for the role it places in processing amyloid precursor protein (APP). A product of this process is the generation of small peptides that aggregate in the brain to form the neurotoxic amyloid deposits found in the brains of Alzheimer’s disease patients. Attempts to prevent or treat Alzheimer’s disease by inhibiting the action of γ-secretase have resulted in high toxicity due to the fact that the enzyme also processes the Notch-1 protein, a cell membrane receptor critical to the regulation of cell-to-cell interactions. This led, Vanderbilt Basic Sciences investigator Chuck Sanders and his laboratory to carry out structural studies of the Notch-1- transmembrane domain (Notch-TMD), the region of the protein that is cleaved by γ-secretase. Their goal was to identify differences between the Notch-TMD and C99 (the comparable domain of the APP) that could guide the development of selective inhibitors of APP processing. The investigators used solution NMR (nuclear magnetic resonance) and computational approaches to construct a model of the Notch-TMD. Their model revealed that the Notch-TMD has a straight intramembrane α-helical domain that adjusts its degree of tilt to compensate for variations in membrane thickness. This distinguishes it from the transmembrane region of C99, which is bent and adds or subtracts amino acids at its N-terminus to accommodate membrane thickness variability. The latter mechanism, which alters the position of the residues within the membrane leads to changes in the site of γ-secretase cleavage  of C99 that, in turn, alters the proportion of highly amyloidogenic peptides produced. Such variability in cleavage site is not expected with the tilt adjustment mechanism of the Notch-TMD. A more therapeutically relevant difference between the two proteins is the existence of a cholesterol binding site in C99 that was not found in the Notch-TMD. This site has been exploited to discover inhibitors of γ-secretase cleavage that block APP but not Notch-TMD processing and potentially pave the way for the development of therapies for Alzheimer’s disease that have acceptable levels of toxicity. The work is published in the journal Science Advances [C. L. Deatherage, et al., (2017), Sci. Adv., 3, 31602794].

Figure reproduced under the Creative Commons Attribution-NonCommercial 4.0 International License from C. L. Deatherage, et al., (2017), Sci. Adv., 3, 31602794.

Par3 Promotes Mammary Cell Survival by an Unexpected Mechanism

March 31, 2017

​Epithelial cells form the lining of cavities and the surfaces of organs and blood vessels in animals. A distinctive feature of epithelial cells is their polarization, as indicated by the segregation of cell cortex proteins into functionally distinct regions. In particular, epithelia have apical and basolateral domains separated by tight junctions that attach the cells to one another. Par3 is a large scaffold protein that is localized to the lateral membrane close to the tight junction. Although many Par3 binding partners are known, its function is not fully understood. Recently, Vanderbilt Basic Sciences investigator Ian Macara reported that Par3 is necessary for the survival of mammary epithelial cells. Now, he and his postdoctoral fellow Syed Mukhtar Ahmed, show that Par3 serves as a receptor for the exocyst, an eight subunit protein complex that tethers exocytotic vesicles to the lateral plasma membrane. Macara and Ahmed began their journey by trying to explain why knockdown of Par3 expression using RNAi techniques caused mammary epithelial cells to undergo death by apoptosis. They soon discovered that the activity of an anti-apoptotic protein kinase, AKT, was reduced in Par3 deficient cells. This then led them to discover that the cells also lacked normal levels of PIP3, a membrane lipid required for activation of AKT. PIP3 is generated by the enzyme PI 3-kinase, which is one of many proteins carried to the membrane by the exocyst as part of the E-cadherin:β-catenin complex. Further studies demonstrated that Par3 binds directly to one of the exocyst subunits, and the researchers were able to identify the specific region of Par3 required for this interaction. Expression of this small piece of the Par3 protein in Par3 deficient cells reversed many of the effects of Par3 deficiency and protected the cells from apoptosis. These findings provide important new insight into Par3 function and reveal how the exocyst docks at the tight junction region of the plasma membrane. They also provide important new information about mammary cell function and survival. The work is published in the journal Nature Communications [S. M. Ahmed & I. G. Macara., (2017) Nat. Commun., 8, 14867].

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