Current Drug Targets Volume 2, Number 3, 2001
Contents
The NMDA Receptor
NMDA
Receptor Pharmacology: Perspectives from Molecular Biology Pp. 215-231
Subunit
Characterization of NMDA Receptors Pp. 233-239
Neuroprotection by NMDA Receptor Antagonists in a Variety
of Neuropathologies Pp. 241-271
NMDA Receptors and
Learning and Memory Processes Pp. 273-283
Ifenprodil,
a Novel NMDA Receptor Antagonist: Site and Mechanism of Action Pp. 285-298
Keith Williams
Convergence of PKC-Dependent Kinase
Signal Cascades on NMDA Receptors Pp. 299-312
J.F. MacDonald, S.A. Kotecha, W.-Y. Lu and M.F. Jackson
Structure-Function Relationships of
the NMDA Receptor Antagonist Conantokin Peptides Pp. 313-322
Mary Prorok and Francis J. Castellino
Activators and Inhibitors of the Ion
Channel of the NMDA Receptor Pp. 323-329
Rebecca C. Klein and Francis J. Castellino
The
Use of Antibody Engineering to Create Novel Drugs that Target N-methyl-D-Aspartate Receptors Pp. 331-345
[Back to top] NMDA
Receptor Pharmacology: Perspectives from Molecular Biology
The NMDA receptor is an
important target for drug development, with agents from many different classes
acting on this receptor. While the severe side effects associated with complete
NMDA receptor blockade have limited clinical usefulness of most antagonists,
the understanding of the multiple forms of NMDA receptors provides an
opportunity for development of subtype specific agents with potentially fewer
side effects. Different NMDA receptor subtypes are assembled from combinations
of NR1 and NR2 subunits with each subunit conveying distinct properties. The
NR1 subunit is the glycine binding subunit and exists as 8 splice variants of a
single gene. The glutamate binding subunit is the NR2 subunit, which is
generated as the product of four distinct genes, and provides most of the
structural basis for heterogeneity in NMDA receptors. Pharmacological
heterogeneity results from differences in the structure of ligand binding
regions, as well as structural differences between subtypes in a modulatory
region called the LIVBP-like domain. This region in NR1 and NR2B controls the
action of NR2B-selective drugs like ifenprodil, while this domain in receptors
containing the NR2A subunit controls the action of NR2A-selective drugs such as
zinc. This suggests that NMDA receptor subtype selective drugs can be created,
and further understanding of subtype specific mechanisms ultimately may allow
successful use of NMDA receptor antagonists as therapeutic agents.
[Back to top] Subunit
Characterization of NMDA Receptors
NMDA receptors are a subclass
of excitatory, ionotropic L-glutamate neurotransmitter receptors. They
are.heteromeric, integral membrane proteins being formed by the assembly of the
obligatory NR1 subunit together with modulatory NR2 subunits of which four different
types, NR2A - NR2D, have been described. This results in a heterogenous
population of receptor proteins with distinct pharmacological and biophysical
properties thus yielding potential for the development of NMDA receptor
subtype-selective therapeutic agents. Anti-NMDA receptor subunit antibodies
have been generated and used in immunoprecipitation or immunoaffinity
purification studies to determine the in vivo subunit complements of NMDA
receptors. This article summarizes knowledge on the subunit compositions of
NMDA receptors based on these approaches together with the current status of
NMDA receptor subunit stoichiometry and hence quaternary structure of native
NMDA receptors.
[Back to top]
Neuroprotection by NMDA Receptor Antagonists in a Variety
of Neuropathologies
Because of adverse reactions,
early efforts to introduce high affinity competitive or use-dependent NMDA
receptor antagonists into patients suffering from stroke, head trauma or
epilepsy met with failure. Later it was discovered that both low affinity
use-dependent NMDA receptor antagonists and compounds with selective affinity
for the NR2B receptor subunit met the criteria for safe administration into
patients. Furthermore, these low affinity antagonists exhibit significant
mechanistic differences from their higher affinity counterparts. Success of the
latter is attested to the ability of the following low affinity compounds to be
marketed: 1) Cough suppressant – dextromethorphan (available for decades); 2)
Parkinson’s disease - amantadine, memantine and budipine; 3) Dementia –
memantine; and 4) Epilepsy - felbamate. Moreover, Phase III clinical trials are
ongoing with remacemide for epilepsy and Huntington’s disease and head trauma
for HU-211. A host of compounds are or were under evaluation for the possible
treatment of stroke, head trauma, hyperalgesia and various neurodegenerative
disorders. Despite the fact that other drugs with associated NMDA receptor mechanisms
have reached clinical status, this review focuses only on those competitive and
use-dependent NMDA receptor antagonists that reached clinical trails. The
ensuing discussions link the in vivo pharmacological investigations that led to
the success/mistakes/ failures for eventual testing of promising compounds in
the clinic.
[Back to
top] NMDA Receptors and
Learning and Memory Processes
In the first part of this
review studies are considered in which pre- or post-training peripheral or
intracerebroventricular administrations of competitive or noncompetitive
N-methyl-D-aspartate (NMDA) receptor antagonists were carried out in a variety
of animal species tested in different experimental conditions, in order to
investigate the effects of these drugs on acquisition and memory processes. In
particular, post-training treatments, which are known to affect memory
consolidation, ruling out the possible “aspecific effects” linked to the pre-training
administrations, show that the NMDA receptor antagonists impair memory in
animals tested in various tasks. Memory impairments are also evident when the
NMDA antagonists (in particular AP5) are injected into different brain
structures, including amygdala and hippocampus.
In a second part of this review
some recent studies are considered showing the existence of: a)
cholinergic-glutamatergic interactions; b) interactions between NMDA receptors
and opioid system, and c) interactions between NMDA receptor antagonists
(MK-801) and cocaine, in the modulation of memory processes of laboratory
animals.
The results of some studies
showing the involvement of glutamatergic mechanisms in Alzheimer’s disease are
finally reported, and the therapeutic efficacy of glutamatergic drugs in the
treatment of this disease is considered.
[Back to top] Ifenprodil,
a Novel NMDA Receptor Antagonist: Site and Mechanism of Action
Keith Williams
[Back to top] Convergence
of PKC-Dependent Kinase Signal Cascades on NMDA Receptors
Synaptic
plasticity, or long-term potentiation (LTP), of excitatory synapses in the
hippocampus contributes to learning and the establishment of spatial memories.
In the CA1 region, induction of LTP enhances the function of postsynaptic a-amino-3-hydroxy-5-methyl-4-isoxazole
propionate receptors (AMPARs) because of the Ca2+-calmodulin
kinase II (CaMKII)-dependent phosphorylation of this subtype of glutamate
receptor. Entry of Ca2+, via N-methyl-D-aspartate
receptors (NMDARs), during strong synaptic stimulation provides the stimulus to
trigger phosphorylation of AMPARs. However, this induction also requires
activation of a protein kinase C (PKC)-dependent tyrosine kinase signal cascade
and a concomitant upregulation of NMDARs. This review focuses upon NMDARs as
potential targets of PKC and/or of the PKC-dependent tyrosine kinase cascade.
PKC, acting via the CAKb/Src tyrosine kinase cascade, enhances NMDAR activation and may increase
the number of receptors expressed in synapses. In contrast, direct
phosphorylation of NMDARs by PKC increases the sensitivity of NMDA channel
inactivation to intracellular Ca2+. In CA1 neurons, PKC provides a
point of convergence of control of NMDARs and synaptic plasticity for a wide
variety of G-protein coupled and growth factor receptors.
[Back to top] Structure-Function
Relationships of the NMDA Receptor Antagonist Conantokin Peptides
The three
members of the conantokin peptide family identified to date are
conantokin(con)-G, -T and -R. Their defining attributes include a high relative
content of g-carboxyglutamic
acid (Gla), N-terminal sequence identity, as well as considerable overall sequence
homology, and antagonism of the N-methyl-D-aspartate receptor (NMDAR). As
promising templates for the design of neuroprotective agents, a thorough
evaluation of structure-function relationships in these peptides will be
invaluable in aiding rational drug modeling. To this end, a comprehensive
assessment of the contributions of individual residues to conantokin structure
and function is required. The current review summarizes recent efforts in this
area, and also includes the effects of peptide length, as well as
structural-stabilization and –destabilization on the structural and inhibitory
profiles of an extensive panel of conantokin derivatives.
[Back to top] Activators
and Inhibitors of the Ion Channel of the NMDA Receptor
The
involvement of the glutamate-glycine activated ion channels of the NMDA
receptor in various neurophysiological processes has made this ion channel the
focus of intense research. The excessive release of glutamate in a variety of
neuronal hypoxic conditions implicates the NMDA receptor in a number of
neuropatholological states, such as stroke, chronic pain, Parkinson's disease,
Alzheimer's disease, ALS, and epilepsy, among others, thus making this receptor
a prime drug target candidate. A variety of agents are known to be effective in
opening and closing of the ion channels of this receptor, among the latter
group of agents is the peptidic conantokins. Through the use of
electrophysiological measurements with a number of cell types containing
natural and recombinant subunits of the NMDA receptor, much knowledge is
evolving regarding the mechanism of action of activators and inhibitors of the
NMDA receptor ion channels. In addition, structure-function studies of the
conantokins in these systems have been revealing in terms of their
complimentary sites on the NMDA receptor. These relationships serve as the main
focus of this review.
[Back
to top] The Use of Antibody Engineering to Create Novel Drugs that
Target N-methyl-D-Aspartate
Receptors
According
to a recent World Health Organization survey, there are over four hundred
million people worldwide suffering from mental and neurological disorders;
schizophrenia affects some forty-five million people, and unipolar major
depression ranked fifth in major causes of disability and death. Clearly it is
of the utmost importance to develop new, effective, and safe neuro-pharmaceuticals
with this increasing “global burden of disease”. To this end, we have developed
a strategy of generating monoclonal antibodies that act as modulators of the
cell-surface central nervous system receptor-ion channel complexes.
In this review we will focus on the
generation and characterization of a monoclonal antibody that acts as a partial
agonist to the N-methyl-D-aspartate receptor. The creation of peptide mimetics,
derived from this monoclonal antibody, that may be useful as cognitive enhancers
and protect neurons hypoxic and ischemic insults caused by stroke, will also be
discussed.