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Current
Pharmaceutical Design
ISSN: 1381-6128
Current Pharmaceutical Design
Volume 12, Number 14, 2006
Contents
G Protein-Coupled Receptor Drug Targets
Executive Editors: P.M. Sexton and A. Christopoulos
Editorial Pp. 1681-1682
Mechanisms of ERK1/2 Regulation by Seven Transmembrane Domain
Receptors Pp. 1683-1702
T.D. Werry, A. Christopoulos and P.M. Sexton
[Abstract]
G Proteins in Drug Screening: From Analysis of Receptor-G
Protein Specificity to Manipulation of GPCR-Mediated Signalling
Pathways Pp. 1703-1715
E. Kostenis
[Abstract]
Integrative Functional Assays, Chemical Genomics and
High Throughput Screening: Harnessing Signal Transduction
Pathways to a Common HTS Readout Pp. 1717-1729
E.S. Burstein, F. Piu, J-N. Ma, J.T. Weissman,
E.A. Currier, N.R. Nash, D.M. Weiner, T.A. Spalding, H.H.
Schiffer, A.L.D. Tredici and M.R. Brann
[Abstract]
Glucagon and Glucagon-Like Peptide Receptors as Drug Targets
Pp. 1731-1750
J.L. Estall and D.J. Drucker
[Abstract]
Cannabinoid Receptors as Therapeutic Targets Pp.
1751-1769
S. Pavlopoulos, G.A. Thakur, S.P. Nikas, and A. Makriyannis
[Abstract]
A Role for Information Collection, Management, and
Integration in Structure-Function Studies of G-Protein Coupled
Receptors Pp. 1771-1783
L. Shi and J.A. Javitch
[Abstract]
Screening the Receptorome Yields Validated Molecular
Targets for Drug Discovery Pp. 1785-1795
B.L. Roth and W.K. Kroeze
[Abstract]
Co-Ordinated Covalent Modification of G-Protein Coupled
Receptors Pp. 1797-1808
I. Torrecilla and A.B. Tobin
[Abstract]
Abstracts
[Back
to top]
Editorial
G Protein-Coupled Receptor Drug Targets
G protein-coupled receptors (GPCRs) represent the largest
known single gene family in the human genome. This superfamily
of cell-surface proteins also constitutes the major target
for approximately half the medicines on the market today.
This issue of Current Pharmaceutical Design focuses on different
aspects of the biology and the study of these receptors, with
a broad emphasis on how these studies are influencing the
modern drug discovery process.
In her article, Evi Kostenis [2] presents an overview of functional
assay systems for GPCRs that take advantage of the best-known
signal transduction system associated with these receptors,
namely, that of the G proteins themselves. The ability to
translate ligand dependent and independent modulation of GPCRs
into a measurable functional response in a manner that is
as portable across GPCR systems as possible remains an important
priority, and challenge, to GPCR-based drug discovery. The
article describes how the manipulation of G protein alpha
subunits, in particular, has been used to address this issue
with respect to high throughput screening of GPCRs.
Ethan Burstein and colleagues [3] focus on the emergence of
chemical genomics, which combines genomic information, combinatorial
chemistry and functional high throughput screening to accelerate
drug discovery. In particular, the utility of functional assay
platforms that allow a common screening approach against the
widest possible array of genomic targets are discussed. The
role of GPCR constitutive activity, intracellular signalling
promiscuity, and G protein independent signaling are discussed
within the context of general strategies for creating homogeneous
assay platforms.
Jen Estall and Daniel Drucker [4] focus on a specific subset
of GPCRs, those targeted by glucagon and glucagon-like peptides,
and discuss the potential of these receptors as drug targets.
These receptors play essential roles in energy intake, absorption
and disposal, and are currently hot targets with respect to
diabetes, the regulation of food intake and related metabolic
conditions both in the central nervous system and the periphery.
The current state of knowledge with respect to selective agonists
and antagonists, as well as specific aspects of the regulation
of this system, are discussed.
In their article, Spiro Pavlopoulos and colleagues [5] focus
on another specific GPCR family that has also emerged as a
very promising therapeutic target in recent years, namely
the cannabinoid CB1
and CB2
receptors. Although there has been anectodal evidence for
medicinal properties of cannabis and cannabis-related compounds
for centuries, it is only in the modern molecular era that
significant attempts have been made to delineate the biology
of the receptors for these compounds and to design compounds
that maintain therapeutic efficacy while minimizing and/or
eliminating unwanted psychoactive effects. The discovery of
endogenous cannabinoid compounds has greatly facilitated this
process, and the article focuses on essential pharmacophoric
elements that can be used to lead to selective signalling
at cannabinoid receptors.
Lei Shi and Jonathan Javitch [6] turn their attention to the
area of informatics as applied to the study of GPCRs. Specifically,
they highlight the key role for informatic approaches utilizing
sequence information, mutagenesis data and the vast published
literature to facilitate studies of GPCRs in the current climate
characterized by a dearth of atomic resolution models. General
approaches for integrating the vast amounts of information
already available in databases and the literature, and the
application to structure-funciton studies of GPCRs, are presented.
Bryan Roth and Wesley Kroeze [7] draw attention to the wealth
of therapeutic targets potentially available in the “receptorome”,
i.e., that portion of the human genome that encodes all receptors.
By using a series of illuminating case studies, they demonstrate
how massively parallel screening of the receptorome is a powerful
drug discovery platform that can validate drug targets and
yield novel information not only on mechanisms of drug action,
but also on previously poorly understood mechanisms underlying
drug side-effects. In addition, an overview of useful GPCR-related
databases is also provided.
Finally, Ignacio Torrecilla and Andrew Tobin [8] highlight
the importance of delineating mechanisms that regulate GPCRs
beyond the acute effects mediated by drugs that either activate
or inhibit these receptors. In particular, their article focuses
on the dynamic mechanisms that underlie covalent modifications
of these receptors mediated by the processes of phosphorylation,
palmitoylation and ubiquitination; it is envisaged that these
pathways can provide alternative targets to the GPCRs themselves
in the drug discovery process.
We are extremely grateful to all our contributors for their
efforts and patience during the preparation of this issue,
and believe that it will serve as a timely overview of much
of the state of play in
GPCR-based drug discovery.
References
[1] Werry TD, Christopoulos A, Sexton PM. Mechanisms of ERK1/2
Regulation by Seven-Transmembrane-Domain Receptors. Curr Pham
Design 2006; 12(14): 1683-1702.
[2] Kostenis E. G Proteins in Drug Screening: From Analysis
of Receptor-G Protein Specificity to Manipulation of GPCR-Mediated
Signalling Pathways. Curr Pham Design 2006; 12(14): 1703-1715.
[3] Burstein ES, Piu F, Ma J-N, Weissman JT, Currier EA, Nash
NR, Weiner DM, Spalding TA, Schiffer HH, Tredici ALD, Brann
MR. Integrative Functional Assays, Chemical Genomics and High
Throughput Screening: Harnessing Signal Transduction Pathways
to a Common HTS Readout. Curr Pham Design 2006; 12(14): 1717-1729.
[4] Estall JL, Drucker DJ. Glucagon and Glucagon-Like Peptide
Receptors as Drug Targets. Curr Pham Design 2006; 12(14):
1731-1750.
[5] Pavlopoulos S, Thakur GA, Nikas SP, Makriyannis A. Cannabinoid
Receptors as Therapeutic Targets. Curr Pham Design 2006; 12(14):
1751-1769.
[6] Shi L, Javitch JA. A Role for Information Collection,
Management, and Integration in Structure-Function Studies
of G-Protein Coupled Receptors. Curr Pham Design 2006; 12(14):
1771-1783.
[7] Roth BL, Kroeze WK. Screening the Receptorome Yields Validated
Molecular Targets for Drug Discovery. Curr Pham Design 2006;
12(14): 1785-1795.
[8] Torrecilla I, Tobin AB. Co-Ordinated Covalent Modification
of G-Protein Coupled Receptors. Curr Pham Design 2006; 12(14):
1797-1808.
Patrick M. Sexton
Arthur Christopoulos
Executive Guest Editors
Howard Florey Institute
University of Melbourne
Parkville, Melbourne
Australia
[Back to top]
Mechanisms of ERK1/2 Regulation by Seven
Transmembrane Domain Receptors
T.D. Werry, A. Christopoulos and P.M. Sexton
Control of cell growth and differentiation
has long been a focus of intense research interest, particularly
in the context of cancer therapeutics. The evolutionarily-conserved
extracellular signal-regulated kinases 1 and 2 (ERK1/2) are
serine-threonine kinases that respond to a wide range of mitogens
and growth factors to initiate changes in cellular proliferation
and differentiation, and are the most important members of
the mitogen-activated protein kinase (MAPK) family in terms
of seven transmembrane-domain receptor (7TMR)-mediated regulation
of mitogenic processes. Regulation of the ERK1/2 signaling
cascade by 7TMRs is highly complex and cell type-specific.
Recent advances in our knowledge of this effector pathway
have revealed that its regulation is at least partly independent
of traditional G protein-mediated actions arising from the
stimulation of 7TMRs. This review summarizes the current position
of our knowledge of ERK1/2 regulation, and illustrates the
wealth of potential targets available for the development
of new strategies for the treatment of proliferative and other
ERK-related disorders.
[Back to top]
G Proteins in Drug Screening: From Analysis
of Receptor-G Protein Specificity to Manipulation of GPCR-Mediated
Signalling Pathways
E. Kostenis
Seven transmembrane G protein coupled receptors (7TM GPCRs)
represent one of the largest gene familes in the human genome.
Because of the size of the GPCR family, their proven history
of being valuable targets for small molecule drug design,
the fact that the absolute number of GPCRs that are targets
for current medicines represents only a small fraction of
the total encoded by the human genome, and that ligands for
GPCRs do not have to enter the cell to exert their function,
it is very likely that GPCRs will remain major targets for
the pharmaceutical industry in the foreseeable future. Despite
recent evidence indicating that GPCRs can provide information
to cells, that does not require activation of G proteins (“signaling
at zero G”), most of the GPCRs known to date function
via interaction with and activation of heterotrimeric
(αβγ
) G proteins. Thus, assay systems translating ligand
modulation of GPCRs into G protein-dependent intracellular
responses are a key component of both basic research and the
drug discovery process. This article will review the current
knowledge and recent progress in understanding molecular aspects
of specific receptor-G protein recognition. It will also highlight
how the knowledge generated by such studies can be transformed
into assay systems for GPCR drug discovery.
[Back to top]
Integrative Functional Assays, Chemical Genomics and
High Throughput Screening: Harnessing Signal Transduction
Pathways to a Common HTS Readout
E.S. Burstein, F. Piu, J-N. Ma, J.T. Weissman,
E.A. Currier, N.R. Nash, D.M. Weiner, T.A. Spalding, H.H.
Schiffer, A.L.D. Tredici and M.R. Brann
Chemical genomics is a drug discovery strategy that relies
heavily on high-throughput screening (HTS) and therefore benefits
from functional assay platforms that allow HTS against all
relevant genomic targets. Receptor Selection and Amplification
Technology (R-SAT™) is a cell-based, high-throughput
functional assay where the receptor stimulus is translated
into a measurable cellular response through an extensive signaling
cascade occurring over several days. The large biological
and chronological separation of stimulus from response provides
numerous opportunities for enabling assays and increasing
assay sensitivity. Here we review strategies for building
homogeneous assay platforms across large gene families by
redirecting and/or amplifying signal transduction pathways.
[Back to top]
Glucagon and Glucagon-Like Peptide Receptors as Drug
Targets
J.L. Estall and D.J. Drucker
Glucagon and the glucagon-like peptides are derived from a
common proglucagon precursor, and regulate energy homeostasis
through interaction with a family of distinct G protein coupled
receptors. Three proglucagon-derived peptides, glucagon, GLP-1,
and GLP-2, play important roles in energy intake, absorption,
and disposal, as elucidated through studies utilizing peptide
antagonists and receptor knockout mice. The essential role
of glucagon in the control of hepatic glucose production,
taken together with data from studies employing glucagon antagonists,
glucagon receptor antisense oligonucleotides, and glucagon
receptor knockout mice, suggest that reducing glucagon action
may be a useful strategy for the treatment of type 2 diabetes.
GLP-1 secreted from gut endocrine cells controls glucose homeostasis
through glucose-dependent enhancement of β
cell function and reduction of glucagon secretion and gastric
emptying. GLP-1 administration is also associated with reduction
of food intake, prevention of weight gain, and expansion of
β cell
mass through stimulation of β
cell proliferation, and prevention of apoptosis. GLP-1R agonists,
as well as enzyme inhibi-tors that prevent GLP-1 degradation,
are in late stage clinical trials for the treatment of type
2 diabetes. Exenatide (Exendin-4) has been approved for the
treatment of type 2 diabetes in the United States in April
2005. GLP-2 promotes energy absorption, inhibits gastric acid
secretion and gut motility, and preserves mucosal epithelial
integrity through enhancement of crypt cell proliferation
and reduction of epithelial apoptosis. A GLP-2R agonist is
being evaluated in clinical trials for the treatment of inflammatory
bowel disease and short bowel syndrome. Taken together, the
separate receptors for glucagon, GLP-1, and GLP-2 represent
important targets for developing novel therapeutic agents
for the treatment of disorders of energy homeostasis.
[Back to top]
Cannabinoid Receptors as Therapeutic Targets
S. Pavlopoulos, G.A. Thakur, S.P. Nikas, and A. Makriyannis
The cannabinoid receptors CB1 and CB2 are family A, G-protein
Coupled Receptors that mediate the effects of cannabinoids,
a class of compounds that are so named because the first members
were isolates of the cannabis plant. In recent history, there
has been much anecdotal evidence that the potent and diverse
physiological responses produced by these compounds can be
turned to therapeutic benefit for a wide variety of maladies.
The remarkable abundance of cannabinoid receptors and the
discovery of several endogenous ligands along with enzyme
and transporter proteins for which they are substrates, suggests
that an endogenous cannabinoid neuromodulatory system is an
important mediator of biological function. For these reasons
CB1 and CB2 receptors are attractive targets for the design
of therapeutic ligands. The action of these receptors, however,
may also be modulated by manipulating the enzymes and membrane
transporters that regulate the endogenous ligands. Despite
the range of physiological processes and activities that are
mediated by cannabinoid receptors, it is clear that it is
possible to produce ligands that result in differential responses.
In this paper, we review the pharmacophoric elements that
lead to these differential responses and in order to discuss
them in context we present an overview of structural aspects
governing cannabinoid receptor function, the cannabinergic
system and its physiological functions.
[Back to top]
A Role for Information Collection, Management, and
Integration in Structure-Function Studies of G-Protein Coupled
Receptors
L. Shi and J.A. Javitch
Elucidation of protein function is greatly facilitated
by the availability of an atomic resolution structure or a
reliable molecular model. The difficulty of obtaining atomic
resolution structures of membrane proteins in general, and
of G-protein coupled receptors (GPCRs) in particular, has
made the information available from sequence analysis, mutagenesis,
and the literature on related GPCRs exceptionally important.
Here, we review previous studies of GPCR structure-function
from the perspectives of sequence analysis, management of
mutagenesis and ligand binding data, and literature data mining.
The knowledge derived from these information resources not
only constitutes the prerequisites for reliable molecular
modeling, but also can provide other insights into GPCR functions.
Finally, we review approaches for information integration
and applying knowledge discovery techniques to structure-function
studies of GPCRs, including molecular modeling itself.
[Back to top]
Screening the Receptorome Yields Validated Molecular
Targets for Drug Discovery
B.L. Roth and W.K. Kroeze
With the recently completed sequencing and annotation of
the human genome, it has become clear that a significant portion
of the genome encodes signal-transducing molecules including
receptors, protein kinases, ion channels, transporters and
coupling proteins. This review focuses on membrane-localized
receptors, which represent the largest single group of signal-transducing
molecules. Indeed, one can estimate that nearly 10% of the
human genome encodes membrane-localized receptors (e.g. G-protein
coupled receptors, ligand-gated ion channels and transporters).
We have defined that portion of the human genome that encodes
‘receptors’ the receptorome. In this
article, we will demonstrate how the massively parallel screening
of the receptorome provides a facile and under-utilized screening
platform for drug discovery. Using case studies, we will show
how receptorome-based screening elucidates the mechanisms
responsible for serious side-effects of both approved and
investigational medications. Additionally, we will provide
evidence that receptorome-based screening provides insights
into novel therapeutic indications of approved medications
and serves to validate targets for therapeutic drug discovery.
[Back to top]
Co-Ordinated Covalent Modification of G-Protein Coupled
Receptors
I. Torrecilla and A.B. Tobin
The G-protein coupled receptor (GPCR) gene family represents
one of the largest families in the mammalian genome. The flexibility
of signalling and widespread tissue distribution of these
receptors has allowed GPCRs to be employed in the physiological
regulation of nearly all biological functions. This, coupled
with the fact that it is possible to chemically produce highly
specific ligands to these receptors have made GPCRs attractive
targets for pharmacological intervention in a wide variety
of disease states. When targeting GPCRs in therapeutic drug
design it is traditional, and eminently sensible, to focus
on ligands that will provide agonism, antagonism or allosteric
modulation. However, as more is understood of the mechanisms
that regulate GPCRs, and in particular the dynamic covalent
modifications that might endow tissue specific functions,
then these regulatory processes may provide alternative targets
for GPCR drug discovery. In this review we consider three
of the covalent modifications which are considered to regulate
the function of GPCRs namely; receptor phosphorylation, palmitoylation
and ubiquitination. In particular, we will describe the mechanisms
of modification, the functional consequences and the relationship
between these three covalent modification events.
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