Cornelia Dorner-Ciossek

Insensitivity to Aβ42-lowering nonsteroidal anti-inflammatory drugs and γ-secretase inhibitors is common among aggressive presenilin-1 mutations

Aβ42-lowering nonsteroidal anti-inflammatory drugs (NSAIDs) constitute the founding members of a new class of γ-secretase modulators that avoid side effects of pan-γ-secretase inhibitors on NOTCH processing and function, holding promise as potential disease-modifying agents for Alzheimer disease (AD). These modulators are active in cell-free γ-secretase assays indicating that they directly target the γ-secretase complex. Additional support for this hypothesis was provided by the observation that certain mutations in presenilin-1 (PS1) associated with early-onset familial AD (FAD) change the cellular drug response to Aβ42-lowering NSAIDs. Of particular interest is the PS1-ΔExon9 mutation, which provokes a pathogenic increase in the Aβ42/Aβ40 ratio and dramatically reduces the cellular response to the Aβ42-lowering NSAID sulindac sulfide. This FAD PS1 mutant is unusual as a splice-site mutation results in deletion of amino acids Thr291–Ser319 including the endoproteolytic cleavage site of PS1, and an additional amino acid exchange (S290C) at the exon 8/10 splice junction. By genetic dissection of the PS1-ΔExon9 mutation, we now demonstrate that a synergistic effect of the S290C mutation and the lack of endoproteolytic cleavage is sufficient to elevate the Aβ42/Aβ40 ratio and that the attenuated response to sulindac sulfide results partially from the deficiency in endoproteolysis. Importantly, a wider screen revealed that a diminished response to Aβ42-lowering NSAIDs is common among aggressive FAD PS1 mutations. Surprisingly, these mutations were also partially unresponsive to γ-secretase inhibitors of different structural classes. This was confirmed in a mouse model with transgenic expression of the PS1-L166P mutation, in which the potent γ-secretase inhibitor LY-411575 failed to reduce brain levels of soluble Aβ42. In summary, these findings highlight the importance of genetic background in drug discovery efforts aimed at γ-secretase, suggesting that certain AD mouse models harboring aggressive PS mutations may not be informative in assessing in vivo effects of γ-secretase modulators and inhibitors.

Inhibition of acetylcholinesterase and phosphodiesterase-9A has differential effects on hippocampal early and late LTP.

Donepezil is the current standard symptomatic treatment of mild-to-moderate Alzheimers disease (AD) patients. It aims to compensate for the deficit in cholinergic neurotransmission by blocking acetylcholinesterase (AChE) and thus increases the concentration of extracellular acetylcholine. However, experience from clinical practice demonstrated that AChE inhibitors only have moderate treatment effects. As a potential new approach for memory enhancement, inhibition of specific phosphodiesterases (PDEs) has gained attention. Among those are PDE9A inhibitors which increase the levels of the second messenger cyclic guanosine monophosphate (cGMP) intracellularly. In order to gain more insight into the potential impact of extracellularly acting AChEs and intracellularly acting PDE9A inhibitors on synaptic plasticity, we analyzed the effects of the AChE inhibitor donepezil and the PDE9A inhibitor BAY 73-6691 on long-term potentiation (LTP) in rat hippocampal slices, a widely accepted cellular experimental model of memory formation. Generally, LTP can be differentiated into an early and a late form, being protein-synthesis independent and protein-synthesis dependent, respectively. Donepezil was found to increase early LTP, but did not affect late LTP. In contrast, BAY 73-6691 demonstrated enhancing effects on both early and late LTP and even transformed early into late LTP. Furthermore, it was shown that this transformation into late LTP was dependent on the NO-cGMP-PKG pathway. In conclusion, this study demonstrates that BAY 73-6691 exhibits a stronger effect in enhancing and prolonging LTP than donepezil suggesting that PDE9 inhibition might be more efficacious in enhancing learning and memory.

PDE9A inhibition rescues amyloid beta-induced deficits in synaptic plasticity and cognition.

The cyclic nucleotide cGMP is an important intracellular messenger for synaptic plasticity and memory function in rodents. Therefore, inhibition of cGMP degrading phosphodiesterases, like PDE9A, has gained interest as potential target for treatment of cognition deficits in indications like Alzheimers disease (AD). In fact, PDE9A inhibition results in increased hippocampal long-term potentiation and exhibits procognitive effects in rodents. To date, however, no evidence has been published linking PDE9A inhibition to the pathologic hallmarks of AD such as amyloid beta (Aβ) deposition. Therefore, we investigated the role of PDE9A inhibition in an AD relevant context by testing its effects on Aβ-related deficits in synaptic plasticity and cognition. The PDE9A inhibitor BAY 73-6691 was found to restore long-term potentiation impaired by Aβ42 oligomers. Furthermore, we demonstrated that BAY 73-6691 enhanced cGMP levels in the hippocampus of APP transgenic tg2576 mice and improved memory performance of these mice. Altogether, our results support the hypothesis that inhibition of PDE9A could be a beneficial approach for the treatment of memory impairment in AD patients.

Detecting spatial memory deficits beyond blindness in tg2576 Alzheimer mice

The retinal degeneration Pde6b(rd1) (rd) mutation can be a major pitfall in behavioral studies using tg2576 mice bred on a B6:SJL genetic background, 1 of the most widely used models of Alzheimers disease. After a pilot study in wild type mice, performance of 8- and 16-month-old tg2576 mice were assessed in several behavioral tasks with the challenge of selecting 1 or more task(s) showing robust memory deficits on this genetic background. Water maze acquisition was impossible in rd homozygotes, whereas Y-maze alternation, object recognition, and olfactory discrimination were unaffected by both the transgene and the rd mutation. Spatial memory retention of 8- and 16-month-old tg2576 mice, however, was dramatically affected independently of the rd mutation when mice had to recognize a spatial configuration of objects or to perform the Barnes maze. Thus, the latter tasks appear extremely useful to evaluate spatial memory deficits and to test cognitive therapies in tg2576 mice and other mouse models bred on a background susceptible to visual impairment.

Touchscreen tasks in mice to demonstrate differences between hippocampal and striatal functions

In mammals, hippocampal and striatal regions are engaged in separable cognitive processes usually assessed through species-specific paradigms. To reconcile cognitive testing among species, translational advantages of the touchscreen-based automated method have been recently promoted. However, it remains undetermined whether similar neural substrates would be involved in such behavioral tasks both in humans and rodents. To address this question, the effects of hippocampal or dorso-striatal fiber-sparing lesions were first assessed in mice through a battery of tasks (experiment A) comprising the acquisition of two touchscreen paradigms, the Paired Associates Learning (dPAL) and Visuo-Motor Conditional Learning (VMCL) tasks, and a more classical T-maze alternation task. Additionally, we sought to determine whether post-acquisition hippocampal lesions would alter memory retrieval in the dPAL task (experiment B). Pre-training lesions of dorsal striatum caused major impairments in all paradigms. In contrast, pre-training hippocampal lesions disrupted the performance of animals trained in the T-maze assay, but spared the acquisition in touchscreen tasks. Nonetheless, post-training hippocampal lesions severely impacted the recall of the previously learned dPAL task. Altogether, our data show that, after having demonstrated their potential in genetically modified mice, touchscreens also reveal perfectly adapted to taxing functional implications of brain structures in mice by means of lesion approaches. Unlike its human counterpart requiring an intact hippocampus, the acquisition of the dPAL task requires the integrity of the dorsal striatum in mice. The hippocampus only later intervenes, when acquired information needs to be retrieved. Touchscreen assays may therefore be suited to study striatal- or hippocampal-dependent forms of learnings in mice.

Prolonged stability by cyclization: Macrocyclic phosphino dipeptide isostere inhibitors of β-secretase (BACE1)

Cyclization of recently reported linear phosphino dipeptide isostere inhibitors of BACE1 via side chain olefin metathesis yielded macrocyclic BACE1 inhibitors. The most potent compound II-P1 (IC(50) of 47nM) and the corresponding linear analog I were tested for serum stability. The approach led to three times prolonged half life serum stability of 44min for the macrocyclic inhibitor II-P1 compared to the linear compound I.

Optimization of touchscreen-based behavioral paradigms in mice: implications for building a battery of tasks taxing learning and memory functions.

Although many clinical pathological states are now detectable using imaging and biochemical analyses, neuropsychological tests are often considered as valuable complementary approaches to confirm diagnosis, especially for disorders like Alzheimer’s or Parkinson’s disease, and schizophrenia. The touchscreen-based automated test battery, which was introduced two decades ago in humans to assess cognitive functions, has recently been successfully back-translated in monkeys and rodents. We focused on optimizing the protocol of three distinct behavioral paradigms in mice: two variants of the Paired Associates Learning (PAL) and the Visuo-Motor Conditional Learning (VMCL) tasks. Acquisition of these tasks was assessed in naive versus pre-trained mice. In naive mice, we managed to define testing conditions allowing significant improvements of learning performances over time in the three aforementioned tasks. In pre-trained mice, we observed differential acquisition rates after specific task combinations. Particularly, we identified that animals previously trained in the VMCL paradigm subsequently poorly learned the sPAL rule. Together with previous findings, these data confirm the feasibility of using such behavioral assays to evaluate the power of different models of cognitive dysfunction in mice. They also highlight the risk of interactions between tasks when rodents are run through a battery of different cognitive touchscreen paradigms.

Age-related synaptic dysfunction in Tg2576 mice starts as a failure in early long-term potentiation which develops into a full abolishment of late long-term potentiation

Tg2576 mice are widely used to study amyloid‐dependent synaptic dysfunction related to Alzheimers disease. However, conflicting data have been reported for these mice with regard to basal transmission as well as the in vitro correlate of memory, long‐term potentiation (LTP). Some studies show clear impairments, whereas others report no deficiency. The present study uses hippocampal slices from 3‐, 10‐, and 15‐month‐old wild‐type (WT) and Tg2576 mice to evaluate synaptic function in each group, including experiments to investigate basal synaptic transmission, short‐ and long‐term plasticity by inducing paired‐pulse facilitation, and both early and late LTP. We show that synaptic function remains intact in hippocampal slices from Tg2576 mice at 3 months of age. However, both early and late LTP decline progressively during aging in these mice. This deterioration of synaptic plasticity starts affecting early LTP, ultimately leading to the abolishment of both forms of LTP in 15‐month‐old animals. In comparison, WT littermates display normal synaptic parameters during aging. Additional pharmacological investigation into the involvement of NMDA receptors and L‐type voltage‐gated calcium channels in LTP suggests a distinct mechanism of induction among age groups, demonstrating that both early and late LTP are differentially affected by these channels in Tg2576 mice during aging.

Improving synaptic plasticity and cognitive function in rodents by the novel phosphodiesterase 9A inhibitor bi 409306

AD features were replicated in human apoE4 target replacement mice (in contrast to human apoE3 replacement mice) thus providing a pharmacological model for assessing therapeutic modalities in apoE4 AD. Results: In a lead selection two ABCA1 agonist with favorable brain penetration were compared for their efficacy assessing AD phenotype characteristics in the hippocampus showing superior effects by CogpepB. CogpepB doseresponse was assessed by administering 20 and 60mg/kg/48 hour for 6 weeks assessing Novel Object Recognition, NOR). A highly statistically significant improvement in NOR compared to placebo was found by the lower dose with no additional benefit by the higher dose. In a 3study 20mg/kg/48h CogpepB was administered for 6 weeks to apoE4 and apoE3 mice and compared to placebo assessing Morris water maze test derived cognition which showed highly significant improvement in the apoE4 active group to that of the apoE3 mice. Interestingly, CogpepB treatment had no effect on apoE3 mice suggesting that the treatment effect is limited to apoE4 mice, potentially by correction of apoE4’s conformational restrictions with regard to ABCA1 interaction and lipid transfer from cells to extra-cellular apoE4. This mechanism of action notion was supported by direct demonstration of increased apoE4 lipidation by CogpepB. Conclusions: Taken together the findings suggest that an ABCA1 agonist, CogpepB, designed from the lipid binding segment of apoE is able to correct the apoE4 lipidation insufficiency thereby normalizing apoE4 phenotype and cognition. CogpepB therefore represents a novel therapeutic approach to apoE4 AD.

Role of PDE9 in Cognition

Inhibition of phosphodiesterases (PDEs) has been demonstrated to enhance performance of animals in various cognition tasks and accordingly PDE inhibitors have been proposed as new approach for treatment of cognitive dysfunction (Reneerkens et al. Psychopharmacology 202:419-443, 2009; Schmidt Curr Top Med Chem 10(2):222-230, 2010). One of the eleven PDE isoforms, showing expression in cognition relevant brain regions across species, is PDE9, which hydrolyzes cGMP only. Furthermore, it is well established that the nitric oxide (NO)/cGMP pathway and NMDA receptor signaling has a crucial function in synaptic plasticity and cognitive function. In this chapter, we will provide an overview on PDE9, its expression and function in the brain, and hence, its relevance for synaptic plasticity and cognitive performance. Moreover, the recent advances of PDE9 inhibition as potential therapeutic approach for treatment of cognitive dysfunction in CNS disorders will be discussed.