Modulation of Cytosolic Phospholipase A2 as a Potential Therapeutic Strategy for Alzheimer's Disease

Background

Alzheimer's disease (AD) is a neurodegenerative disorder lacking any curative treatment up to now. Indeed, actual medication given to the patients alleviates only symptoms. The cytosolic phospholipase A2 (cPLA2-IVA) appears as a pivotal player situated at the center of pathological pathways leading to AD and its inhibition could be a promising therapeutic approach.

Objective

A cPLA2-IVA inhibiting peptide was identified in the present work, aiming to develop an original therapeutic strategy.

Methods

We targeted the cPLA2-IVA using the phage display technology. The hit peptide PLP25 was first validated in vitro (arachidonic acid dosage [AA], cPLA2-IVA cellular translocation) before being tested in vivo. We evaluated spatial memory using the Barnes maze, amyloid deposits by MRI and immunohistochemistry (IHC), and other important biomarkers such as the cPLA2-IVA itself, the NMDA receptor, AβPP and tau by IHC after i.v. injection in APP/PS1 mice.

Results

Showing a high affinity for the C2 domain of this enzyme, the peptide PLP25 exhibited an inhibitory effect on cPLA2-IVA activity by blocking its binding to its substrate, resulting in a decreased release of AA. Coupled to a vector peptide (LRPep2) in order to optimize brain access, we showed an improvement of cognitive abilities of APP/PS1 mice, which also exhibited a decreased number of amyloid plaques, a restored expression of cPLA2-IVA, and a favorable effect on NMDA receptor expression and tau protein phosphorylation.

Conclusions

cPLA2-IVA inhibition through PLP25 peptide could be a promising therapeutic strategy for AD.

Re-Evaluating the Relevance of the Oxygen-Glucose Deprivation Model in Ischemic Stroke: The Example of Cdk Inhibition

Previous research has shown that cyclin-dependent kinases (Cdks) that play physiological roles in cell cycle regulation become activated in post-mitotic neurons after ischemic stroke, resulting in apoptotic neuronal death. In this article, we report our results using the widely used oxygen-glucose deprivation (OGD) in vitro model of ischemic stroke on primary mouse cortical neurons to investigate whether Cdk7, as part of the Cdk-activating kinase (CAK) complex that activates cell cycle Cdks, might be a regulator of ischemic neuronal death and may potentially constitute a therapeutic target for neuroprotection. We found no evidence of neuroprotection with either pharmacological or genetic invalidation of Cdk7. Despite the well-established idea that apoptosis contributes to cell death in the ischemic penumbra, we also found no evidence of apoptosis in the OGD model. This could explain the absence of neuroprotection following Cdk7 invalidation in this model. Neurons exposed to OGD seem predisposed to die in an NMDA receptor-dependent manner that could not be prevented further downstream. Given the direct exposure of neurons to anoxia or severe hypoxia, it is questionable how relevant OGD is for modeling the ischemic penumbra. Due to remaining uncertainties about cell death after OGD, caution is warranted when using this in vitro model to identify new stroke therapies.

Proliferation of hippocampal progenitors relies on p27-dependent regulation of Cdk6 kinase activity

Neural stem cells give rise to granule dentate neurons throughout life in the hippocampus. Upon activation, these stem cells generate fast proliferating progenitors that complete several rounds of divisions before differentiating into neurons. Although the mechanisms regulating the activation of stem cells have been intensively studied, little attention has been given so far to the intrinsic machinery allowing the expansion of the progenitor pool. The cell cycle protein Cdk6 positively regulates the proliferation of hippocampal progenitors, but the mechanism involved remains elusive. Whereas Cdk6 functions primarily as a cell cycle kinase, it can also act as transcriptional regulator in cancer cells and hematopoietic stem cells. Using mouse genetics, we show here that the function of Cdk6 in hippocampal neurogenesis relies specifically on its kinase activity. The present study also reveals a specific regulatory mechanism for Cdk6 in hippocampal progenitors. In contrast to the classical model of the cell cycle, we observe that the Cip/Kip family member p27, rather than the Ink4 family, negatively regulates Cdk6 in the adult hippocampus. Altogether, our data uncover a unique, cell type-specific regulatory mechanism controlling the expansion of hippocampal progenitors, where Cdk6 kinase activity is modulated by p27.

Genetic and pharmacological inhibition of Cdk1 provides neuroprotection towards ischemic neuronal death

Cell cycle proteins are mainly expressed by dividing cells. However, it is well established that these molecules play additional non-canonical activities in several cell death contexts. Increasing evidence shows expression of cell cycle regulating proteins in post-mitotic cells, including mature neurons, following neuronal insult. Several cyclin-dependent kinases (Cdks) have already been shown to mediate ischemic neuronal death but Cdk1, a major cell cycle G2/M regulator, has not been investigated in this context. We therefore examined the role of Cdk1 in neuronal cell death following cerebral ischemia, using both in vitro and in vivo genetic and pharmacological approaches. Exposure of primary cortical neurons cultures to 4 h of oxygen–glucose deprivation (OGD) resulted in neuronal cell death and induced Cdk1 expression. Neurons from Cdk1-cKO mice showed partial resistance to OGD-induced neuronal cell death. Addition of R-roscovitine to the culture medium conferred neuroprotection against OGD-induced neuronal death. Transient 1-h occlusion of the cerebral artery (MCAO) also leads to Cdk1 expression and activation. Cdk1-cKO mice displayed partial resistance to transient 1-h MCAO. Moreover, systemic delivery of R-roscovitine was neuroprotective following transient 1-h MCAO. This study demonstrates that promising neuroprotective therapies can be considered through inhibition of the cell cycle machinery and particularly through pharmacological inhibition of Cdk1.