Ionotropic Glutamate Receptor Technologies

307 Pages·2016·9.988 MB·English

by  Gabriela K. Popescu (eds.)

Neuromethods 106
Gabriela K. Popescu Editor
Ionotropic
Glutamate
Receptor
Technologies
N
EUROMETHODS
Series Editor
Wolfgang Walz
University of Saskatchewan
Saskatoon, SK, Canada
For further volumes:
h ttp://www.springer.com/series/7657

Ionotropic Glutamate
Receptor Technologies
Edited by
Gabriela K. Popescu
Department of Biochemistry, University at Buffalo, Buffalo, NY, USA
Editor
Gabriela K. P opescu
Department of Biochemistry
University at Buffalo
Buffalo, NY , U SA
ISSN 0893-2336 ISSN 1940-6045 (electronic)
Neuromethods
ISBN 978-1-4939-2811-8 ISBN 978-1-4939-2812-5 (eBook)
DOI 10.1007/978-1-4939-2812-5
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v

Foreword
Ionotropic glutamate receptors (iGluRs) are glutamate-gated ion channels that mediate fast
excitatory neurotransmission in the central nervous system (CNS). They mediate the nor-
mal development and function of the CNS and play critical roles in numerous neurologic
and psychiatric disorders. The already high and increasing burden of neurodegenerative
diseases across the world has prompted national interest to better understand the structure
and function of the nervous system. This ambitious desideratum will require more advanced
knowledge of the structure, function, and biological activities of iGluRs.
The fi eld of ligand-gated ion channels has its roots in the pharmacologic investigations
of excitable tissues in the second part of the nineteenth century. The observations that
drugs and poisons (atropine, nicotine, morphine, etc.) have dramatic effects on the electri-
cal properties of tissues and organs (heartbeat, muscle twitch, etc.) were most parsimoni-
ously explained by postulating the existence of chemically receptive proteins that form
transmembrane pores. Since then, the evolution of theories and concepts in the fi eld of
ligand-activated channels has followed closely the development of technologies to apply
and withdraw ligands (perfusion techniques), to record and interpret electrical signals
(electrophysiology), and to access increasingly diverse biological preparations (tissue cul-
ture, molecular biology). Many of these concepts and theories were developed in the fi rst
part of the twentieth century, in advance of the fi rst direct observation of a single-channel
current and before the cells of the mammalian central nervous system were amenable to
direct experimental investigation. In fact, these concepts evolved primarily from observa-
tions of the more easily accessible nerve-muscle synapse; because the ion channel respon-
sible for the endplate current is the muscle acetylcholine receptor, to this day, in many
textbooks and reviews, the term ligand-gated ion channel often refers to this founding
receptor and its family of pentameric ligand-gated ion channels, despite the growing diver-
sity of proteins in this class.
After the Second World War, two simultaneous technologic advances propelled the
glutamate-gated ion channels to the experimentally accessible range. First, increased
knowledge on how to prepare and maintain explants of central nervous tissue made it
progressively feasible to investigate their molecular and cellular properties. In parallel,
methods to record and measure electrical currents across biological membranes—fi rst
with sharp intracellular electrodes and then with patch-clamp microelectrodes—made it
possible to observe and record the electrical properties of much smaller cells in the man-
ner already used for the neuromuscular junction. With what was at the time the state of
the art in electrophysiology, it was repeatedly demonstrated that glutamate and related
amino acids excited central neurons. In fact, kainate, a glutamate analogue, had been a
well-known neurotoxin and was used by neuroanatomists to trace neuronal circuits by
staining degenerating axons. Still, up until the early 1980s, the pervasive thought was
that glutamate is too important a metabolite, and its concentration in brain too high, for
it to possibly be a neurotransmitter, and many scientists speculated that its excitatory
effects had no physiologic meaning. More than two decades of careful pharmacologic
vii
viii Foreword
and neurophysiologic investigations were necessary to slowly bring into unanimous
acceptance that in the CNS excitatory postsynaptic currents are largely the result of
iGluR activations.
The molecular biology revolution of the late twentieth century allowed for the fi rst
time the preparation and thus the functional investigation of proteins of defi ned molecular
identity. It also exposed an unanticipated multiplicity of ligand-gated channels, which are
organized not only as the prototypical acetylcholine receptor as pentamers but also as tri-,
tetra-, or hexameric proteins. Furthermore, it became clear that ligand-gated channels are
not only expressed at synapses and on the surface of cells but also embedded in virtually
every biological membrane. For iGluRs, as well, this has been a particularly exciting era. It
was demonstrated during this time that functional iGluRs assemble as homo- or hetero-
tetramers of homologous subunits, and the subsequent decoding of several genomes has
established that mammalian iGluRs assemble from a family of 18 homologous subunits.
These cluster further into three classes or types, corresponding to the already established
pharmacologically based nomenclature as AMPA, kainate, and NMDA receptors.
The current volume compiles methods that have afforded important conceptual
advances in the iGluR fi eld within the past decade. Among these recent developments per-
haps the most spectacular are several atomic structures for functional AMPA and NMDA
receptors, and parts of kainate receptors. Along with these new structural benchmarks,
modern means of identifying and sorting intramolecular motions seek to associate confor-
mational changes with state lifetimes and ultimately with functional output. Thus, a com-
mon goal is to organize observed structural changes into a coherent chronologic sequence
that narrates the molecular trajectories that produce function. Zooming out from the
atomic to the molecular and cellular levels, methodologic advances described in this volume
expose mechanisms that control receptor assembly, oligomerization, expression, and traf-
fi cking, and provide approaches to identify or count molecular assemblies expressed on cells
or at specifi c synapses. Lastly, the volume would have been incomplete without giving a
modern account of classic electrophysiologic approaches that evaluate receptor function
following mutagenesis, pharmacologic treatment, and a variety of stimulation protocols,
whether for recombinant or native receptors.
Taken together the chapters in this volume outline the contemporary landscape of
iGluR technologies. They highlight exciting advances in the fi eld in a manner designed to
facilitate additional investigations along these newly forged tracks. Necessarily, they also
illustrate that progress has been uneven across the three classes of iGluRs, most likely due
to the specifi c experimental challenges associated with each receptor type. By assisting new
and established investigators to adopt these technologies, the present volume may expedite
the development of the next generation of approaches and techniques to produce a com-
prehensive understanding of how iGluRs work to fulfi ll their essential biologic functions in
the CNS.
Buffalo, NY, USA Gabriela K. Popescu
Prefa ce
Glutamate is the principal excitatory neurotransmitter in the brain and spinal cord and its
rapid action at more than 90 % of central synapses occurs through membrane receptors of
the ionotropic glutamate receptor (iGluR) family. Since their molecular cloning in the early
1990s, the number of PubMed indexed articles focusing on these receptors’ structure,
function, and role in health and disease has exploded in the 1990s and has held steady dur-
ing the past 15 years at ~2,500 publications per year, with no sign of a decline.
This large number of publications refl ects the constant and substantial advances in our
collective understanding of these receptors but also the development of new technologies
that allow scientists to address gaps in knowledge in this area. Increasingly, scientifi c jour-
nals that report primary research have moved to enforcing page limits for the articles they
are willing to review and publish. This fact has resulted in deliberate abbreviation of the
Methods section, usually accomplished by extensive referencing of previous literature or by
relegating a major part of this section to Supplementary Material. This practice has made it
cumbersome to follow the technical procedures and quite diffi cult to implement these in a
lab with no previous experience with the particular technique. The chapters in this volume
of N euromethods describe techniques, methods, and approaches that are either specifi c to
iGluRs or have advanced the fi eld signifi cantly in recent years. They are intended as detailed
practical guides that will facilitate the implementation of these technologies in new or
established laboratories.
Despite the critical roles of iGluRs in health and disease, much remains unknown about
the operation, modulation, and the biological functions of iGluRs. The development,
maintenance, and experience-dependent plasticity of excitatory CNS synapses depend criti-
cally on the activity of iGluRs; and iGluRs participate in fundamental aspects of develop-
ment and behavior including learning and memory, information processing, and cognition.
In addition, iGluRs mediate glutamate neurotoxicity, a key component of pathology in a
number of neurodegenerative conditions. Newly delineated atomic models of functional
iGluRs have galvanized the fi eld with new information that had been previously diffi cult to
obtain and have formulated new questions in iGluR research. In addition, recent national
initiatives into the structure and function of the brain are sure to increase the demand for
accessible techniques to evaluate the structure, function, and physiologic contributions of
iGluRs. Helping investigators to implement successfully iGluR-specifi c methods will accel-
erate the pace of discovery in this important scientifi c area.
This volume compiles practical guides, organized as chapters, to technologies that are
used currently to investigate iGluR structure, function, and physiology. Chapters focus on
a particular approach that has been proven successful in revealing fundamental aspects of
iGluRs’ involvement with health and disease. The fi rst section includes methods that can
help illuminate the assembly, traffi cking, molecular composition, and subcellular location of
iGluRs. The second section describes approaches used to understand the atomic organiza-
tion of iGluRs and the intramolecular motions associated with function. The last section
provides techniques to monitor receptor activity in real time, whether from single m olecules
or receptor populations, and approaches to assemble a storyboard of conformational
ix