Current Drug Targets-CNS & Neurological Disorders - Volume 4, Issue 5, 2005

Scientific research in cognition and cognitive pharmacology is entering an exciting era. Not only are we beginning through intensive investigations around the world, to better understand the synaptic and molecular mechanisms underlying cognition and cognitive disorders, but several therapeutic targets are also being examined for developing cognitive therapeutic agents and many more are starting to emerge. The excitement is best illustrated by the review articles in this hot topic theme issue of Current Drug Targets - CNS and Neurological Disorders. These articles cover important aspects of cognition therapeutics and therapeutic targets, including lowdensity lipoprotein receptor-related protein (LRP; by Harris-White and Frautschy), the cAMP responsive element binding protein (CREB; by Sheena A. Josselyn and Peter V. Nguyen), memantine for the glutamate hyperactivity (by Horst J. Koch, Gökhan Uyanik and David Fischer-Barnicol), the cannabinoid system (by Franjo Grotenhermen), the testosterone system (by E. Hogervorst and S.D. Moffat), the protein kinase C substrates (by Miao-Kun Sun and Daniel L. Alkon), cholesterol and the ApoE (by Daniela Fenili and JoAnne McLaurin), and Aβ removal/reduction (by Edith G. McGeer and Patrick L. McGeer). LRP is highly expressed in the pyramidal cells and plays an important role in synaptic transmission. LRP itself and its many ligands, including ApoE, α2-macroglobulin, the transforming growth factor-β , and the insulin-like growth factor, are important therapeutic targets in cognition and cognition disorders. Intracellular A accumulation leads to impaired neuronal metabolism and synaptic dysfunctions and is probably responsible for triggering the onset of cognitive dysfunction in Alzheimer's disease (AD). Therefore, blocking Aβ uptake and intracellular accumulation, including LRP-mediated neuronal uptake of ApoE-bound Aβ , has obvious therapeutic values in anti dementia therapy. The cAMP responsive element binding protein (CREB) represents another target in developing novel treatment of memory disorders. Cognition is impaired by many more pathogenic cascades, which serve as potential therapeutic targets. Glutamate hyperactivity, for instance, is involved in AD, vascular dementia, and other neurodegenerative disorders. Early findings also define an involvement of cholinergic deficits in AD. These studies have led to some antidementia drugs, such as cholinesterase inhibitors and memantine. Although these drugs are used clinically to treat AD patients, whether they can change over a long period, the rate of the disease progression is an entirely different issue. The cannabinoid system plays an important role in signal transduction. Its receptor agonists reduce Aβ neurotoxicity and thus possess an anti-AD action. The cannabinoid receptor1 antagonists, on the other hand, can improve memory in rats. Further studies are needed to evaluate potentials of the cannabinoid receptor agonists and antagonists in cognitive pharmacology. Effects of testosterone on cognition and cognitive disorders include a reduction in both Aβ formation and tau hyperphosphorylation, suggesting that the male hormone has a protective action against AD and cognitive decline. But further studies are required to define its long-term benefit. Protein kinase C activators have been found to produce several promising effects on AD and cognition, including an enhancement of spatial cognition, a reduction of Aβ formation and accumulation. Elevated cholesterol and the apolipoprotein E ε4 allele are important risk factors for AD. Immunization with Aβ , inhibitors of A formation, inhibitors of tau hyper-phosphorylation, cholesterol-lowering agents, β-sheet breaker peptides, and various miscellaneous agents and strategies presented in these review articles are currently the leading strategies in developing cognitive therapeutics. We hope that our efforts will be rewarded with new developments and facilitated emergence of therapeutic agents that are able to slow down or block the progression of cognitive disorders.

This review will focus primarily on the role of the low density lipoprotein receptor-related protein (LRP-1) in neuronal synapse formation and function in Alzheimer's Disease (AD). We review the role that its ligands may have in cognition or AD: apolipoprotein E (ApoE), α2-macroglobulin, Transforming Growth Factor-Beta (TGFβ), Tissue Plasminogen Activator (tPA), insulin growth factor binding protein-3 (IGFBP-3), which all bind LRP-1 and apolipoprotein J (ApoJ), which is a ligand for LRP-2. After reviewing its role as a signaling receptor, we discuss the connection between LRP and the NMDA glutamate receptor via the post synaptic density 95 (PSD-95) neuronal scaffold protein and the implications it may have for memory and cognition. Finally, we discuss the evidence supporting a role for LRP in AD. Although the evidence for LRP as a genetic risk factor is weak, many of its ligands impose genetic risk, and have been implicated in AD pathogenic cascades. We discuss the role of LRP in amyloid precursor protein (APP) processing and production of beta-amyloid (Aβ). We identify LRP ligands that accelerate aggregation of toxic Aβ species. LRP mediates crucial pathways in AD pathogenesis such as Aβ clearance, Aβ uptake, intraneuronal Aβ accumulation and A associated neuron death. Interestingly, the TGFβ-V receptor is LRP-1. Data show that one critical ligand TGFβ2, associated with neurodegeneration in amyloid diseases, induces LRP expression in PC12 cells. Data from rodent infusion models demonstrate the impact of TGFβ2in modifying Aβ induced Long Term Potentiation (LTP) responses, presynaptic proteins, lipid peroxidation, gliosis and staining for neuronal nuclei. The evidence supports a complex and significant role of LRP in cognition and AD.

In neurons, appropriate long-term adaptive responses to changes in the environment require the conversion of extracellular stimuli into discrete intracellular signals. Many of these signals involve the regulation of gene expression. The cAMP responsive element binding protein (CREB) is a nuclear transcription factor that modulates transcription of genes containing cAMP responsive elements (CRE sites) in their promoters. CREB is a key part of many intracellular signaling events that critically regulate many neural functions. Numerous studies on invertebrates and vertebrates demonstrate that CREB is critical for long-term memory. Here, we review the key features of CREB-dependent transcription and critically evaluate the data examining the roles of CREB in different forms of plasticity, including longterm memory in mammals. Because learning and memory have been linked to specific types of synaptic plasticity in several species, we also review studies on the role of CREB in long-term facilitation in Aplysia and in hippocampal longterm potentiation (LTP). Several human cognitive disorders have been linked to alterations of CREB-regulated gene expression. Therefore, we explore the possibility of targeting CREB function in developing novel treatment strategies. Finally, we highlight areas of research on CREB that are ripe for further advancement.

Memantine has been clinically used in the treatment of organic disorders in Germany for over ten years and has now been approved in Europe and also in the US for moderate to severe Alzheimer's disease. The rationale for this indication is strongly related to the physiological and pathological role of glutamate in neurotransmission. Glutamate is an agonist of NMDA, kainate and AMPA (ionotropic) receptors, where its influence on NMDA receptors plays an important role with regard to neuronal plasticity effecting memory and learning. Excessive levels of glutamate result in neurotoxicity, in part by overactivation of NMDA receptors. Memantine acts as an uncompetitive antagonist of NMDA receptors and therefore compensates for this overactivation. Furthermore, memantine is a neuroprotective agent in various animal models based on both neurodegenerative and vascular processes, as it ameliorates cognitive and memory deficits. Memantine was effective and safe in several clinical studies, particularly in Alzheimer's disease. The compound is completely absorbed after oral intake and undergoes little metabolism. Having a low probability for drug-drug interactions, memantine, in principle, is suited for elderly patients exposed to multiple therapeutic therapies.

Since the discovery of an endogenous cannabinoid system, research into the pharmacology and therapeutic potential of cannabinoids has steadily increased. Two subtypes of G-protein coupled cannabinoid receptors, CB1 and CB2, have been cloned and several putative endogenous ligands (endocannabinoids) have been detected during the past 15 years. The main endocannabinoids are arachidonoyl ethanolamide (anandamide) and 2-arachidonoyl glycerol (2-AG), derivatives of arachidonic acid, that are produced "on demand" by cleavage of membrane lipid precursors. Besides phytocannabinoids of the cannabis plant, modulators of the cannabinoid system comprise synthetic agonists and antagonists at the CB receptors and inhibitors of endocannabinoid degradation. Cannabinoid receptors are distributed in the central nervous system and many peripheral tissues, including immune system, reproductive and gastrointestinal tracts, sympathetic ganglia, endocrine glands, arteries, lung and heart. There is evidence for some non-receptor dependent mechanisms of cannabinoids and for endocannabinoid effects mediated by vanilloid receptors. Properties of CB receptor agonists that are of therapeutic interest include analgesia, muscle relaxation, immunosuppression, anti-inflammation, antiallergic effects, improvement of mood, stimulation of appetite, antiemesis, lowering of intraocular pressure, bronchodilation, neuroprotection and antineoplastic effects. The current main focus of clinical research is their efficacy in chronic pain and neurological disorders. CB receptor antagonists are under investigation for medical use in obesity and nicotine addiction. Additional potential was proposed for the treatment of alcohol and heroine dependency, schizophrenia, conditions with lowered blood pressure, Parkinson's disease and memory impairment in Alzheimer's disease.

Low testosterone (T) levels may predispose to Alzheimer disease (AD), but it is unclear whether this is a comorbid effect due to cachexia, subclinical hyperthyroidism or other co-morbidity. The biological plausibility for potential protective effects of T on brain functions is substantial. In addition, higher levels of gonadotropins found in older cases with AD suggest that low levels of T are not due to brain degeneration and that the hypothalamic-pituitary-gonadal (HPG) axis is still intact. Men genetically at risk for AD were also already found to have lower levels of T. However, despite having lower levels of T, women do not show accelerated cognitive decline with age when compared to men. In addition, castration has not necessarily shown a decline in cognitive functions; some studies even found improvement of memory recall. Age may be an important factor when assessing optimal levels of T and several studies suggest that free or bioavailable T may be a better marker than total T levels when investigating associations of androgen activity with cognitive function. Small-scale T intervention trials in elderly men with and without dementia suggest that some cognitive deficits may be reversed, at least in part, by short term T supplementation. Age and prior hypogonadism may play an important role in therapy success and these factors should be investigated in more detail in future large scale randomized controlled studies. For elderly women, T treatment does not seem to have additional benefits over estrogen treatment for postmenopausal complaints and cognitive decline and may increase cardiovascular disease.

PKC plays an important role in many types of learning and memory. Evidence has been provided that PKC activation and translocation are induced in associative learning tasks. PKC inhibition, on the other hand, impairs learning and memory, consistent with the observations that transgenic animal models with a particular PKC isoform deficit exhibit impaired capacity in cognition. The dramatic impact of PKC pharmacology on learning and memory is further emphasized by a regulatory role of PKC isozymes in amyloid production and accumulation. Recent study reveals that PKC activation greatly reduces neurotoxic amyloid production and accumulation. PKC activators, therefore, may have important therapeutic values in the treatment of dementia, especially when fine-tuning of selective isoform activity can be effectively achieved pharmacologically, with further development of isozymes-specific agents. The success of antidementia therapy with agents that act on PKC signaling cascades depends on whether such agents at their effective doses would significantly disrupt or interfere with other vital functions that rely on a narrow range of PKC activities. org

Alzheimer's disease (AD) is a debilitating disease that affects many people. In order to reduce the number of people diagnosed with this disease, drug strategies need to be implemented that target early steps in disease pathogenesis. Elevated cholesterol levels and presence of the apolipoprotein E 4 allele increase AD risk. How these two factors may contribute to AD pathogenesis and some therapeutic strategies for alleviating AD risk will be discussed.

The amyloid cascade hypothesis postulates that accumulation of beta-amyloid (Abeta) plays a key role in the development of Alzheimer's disease (AD). Accordingly, much effort has gone into reducing the amyloid burden, especially in transgenic mice expressing mutations in human amyloid precursor protein. Such mice develop amyloid plaques but not neurofibrillary tangles. Immunization with Abeta and other inflammatory stimuli, inhibitors of Abeta formation, cholesterol lowering agents, beta-sheet breaker peptides, antioxidants and various miscellaneous agents have been found to reduce the more soluble Abeta in such transgenic mice. Whether they would affect the more consolidated, cross-linked Abeta of AD and, if they did, whether that would really prove an effective treatment for the disease remains for future research to determine.

Classical benzodiazepines (BZs) are the most widely prescribed drugs acting on the central nervous system (CNS). They exert their therapeutic effects via binding to the BZ-site of GABAA receptors, and allosterically modulating the chloride flux through the ion channel complex. Given the multiple actions of classical BZs, the serious limitations to their usefulness have directed much research into development of novel ligands for the BZ-site with retained therapeutic effectiveness and minimal side effects. From the studies of CNS-active chemical constituents of medicinal herbs, some members of the family of flavonoids were demonstrated to have moderate binding affinities for the BZ-site. In vivo studies revealed that these compounds were mostly partial agonists of GABAA receptors, and only a few flavonoids were shown to possess antagonistic activities. At effective anxiolytic doses, the actions of partial agonistic flavonoids were often not accompanied by sedative and myorelaxant side effects. Based on structure-activity relationship (SAR) studies, incorporation of electronegative groups to the C6 and C3' on the flavone backbone was found to yield significant increases in the binding affinities for the BZ-site. It was also shown that 2'-hydroxyl was a critical moiety on flavonoids with regard to BZ-site binding. These have guided the identification of several synthetic flavonoids with high BZ-site binding affinity and in vivo activity, and further quantitative SAR studies resulted in the development of several pharmacophore models. This review attempts to summarize these findings, which has led to the establishment of flavonoids as potential therapeutics for GABAA receptor-mediated disorders.

In transthyretin (TTR) amyloidosis TTR variants deposit as amyloid fibrils giving origin, in most cases, to peripheral polyneuropathy, cardiomyopathy, carpal tunnel syndrome and/or amyloid deposition in the eye. More than eighty TTR variants are known, most of them being pathogenic. The mechanism of TTR fibril formation is still not completely elucidated. However it is widely accepted that the amino acid substitutions in the TTR variants contribute to a destabilizing effect on the TTR tetramer molecule, which in particular conditions dissociate into non native monomeric intermediates that aggregate and polymerize in amyloid fibrils that further elongate. Since this is a multi-step process there is the possibility to impair TTR amyloid fibril formation at different stages of the process namely by tetramer stabilization, inhibition of fibril formation or fibril disruption. Till now the only efficient therapy available is liver transplant when performed in an early phase of the onset of the disease symptoms. Since this is a very invasive therapy alternatives are desirable. In that sense, several compounds have been proposed to impair amyloid formation or disruption. Based on the proposed mechanism for TTR amyloid fibril formation we discuss the action of some of the proposed TTR stabilizers such as derivatives of some NSAIDs (diflunisal, diclofenac, flufenamic acid, and derivatives) and the action of amyloid disrupters such as 4'-iodo-4'-deoxydoxorubicin (I-DOX) and tetracyclines. Among all these compounds, TTR stabilizers seem to be the most interesting since they would impair very early the process of amyloid formation and could also have a prophylactic effect.

In susceptible individuals, stressors can increase the risk of onset of depression and recent brain imaging studies have shown morphometric alterations in the limbic system of patients affected by depression. The volume loss observed in the hippocampus of depressed individuals suggests a possible involvement of structural neuronal plasticity in the pathogenesis of depression. Stressful conditions in animals can result in impaired structural neuronal plasticity in the hippocampus, characterised by retraction of apical dendrites and decreased neurogenesis. The intrinsic dynamic instability of the cytoskeletal microtubular system is essential for neuronal remodelling and plasticity. We have recently shown that both acute and chronic stress decrease microtubular dynamics in the rat hippocampus. Other authors have demonstrated that proteins functionally involved in the regulation of microtubule dynamics can be altered by stress in the rodent hippocampus. Furthermore, the existence of a link between stress-induced microtubular changes and depression is further strengthened by evidence showing that both acute and chronic treatment with antidepressant drugs can affect the expression of microtubular proteins. The present review will introduce a growing body of evidence suggesting that stress-induced alterations in neuronal plasticity might be considered the final result of activation and/or inhibition of molecular cascades regulating the dynamics of the microtubular system. In addition, the prospect of targeting microtubules as a pharmacotherapeutic approach to treat mood disorders will be discussed.