NeuroLeman Network

Carnosine is a natural dipeptide widely distributed in mammalian tissues and exists at particularly high concentrations in skeletal and cardiac muscles and brain. A growing body of evidence shows that carnosine is involved in many cellular defense mechanisms against oxidative stress, including inhibition of amyloid-β (Aβ) aggregation, modulation of nitric oxide (NO) metabolism, and scavenging both reactive nitrogen and oxygen species. Different types of cells are involved in the innate immune response, with macrophage cells representing those primarily activated, especially under different diseases characterized by oxidative stress and systemic inflammation such as depression and cardiovascular disorders. Microglia, the tissue-resident macrophages of the brain, are emerging as a central player in regulating key pathways in central nervous system inflammation; with specific regard to Alzheimer’s disease (AD) these cells exert a dual role: on one hand promoting the clearance of Aβ via phagocytosis, on the other hand increasing neuroinflammation through the secretion of inflammatory mediators and free radicals. The activity of carnosine was tested in an in vitro model of macrophage activation (M1) (RAW 264.7 cells stimulated with LPS + IFN-γ) and in a well-validated model of Aβ-induced neuroinflammation (BV-2 microglia treated with Aβ oligomers). An ample set of techniques/assays including MTT assay, trypan blue exclusion test, high performance liquid chromatography, high-throughput real-time PCR, western blot, atomic force microscopy, microchip electrophoresis coupled to laser-induced fluorescence, and ELISA aimed to evaluate the antioxidant and anti-inflammatory activities of carnosine was employed. In our experimental model of macrophage activation (M1), therapeutic concentrations of carnosine exerted the following effects: 1) an increased degradation rate of NO into its non-toxic end-products nitrite and nitrate; 2) the amelioration of the macrophage energy state, by restoring nucleoside triphosphates and counterbalancing the changes in ATP/ADP, NAD+/NADH and NADP+/NADPH ratio obtained by LPS + IFN-γ induction; 3) a reduced expression of pro-oxidant enzymes (NADPH oxidase, Cyclooxygenase-2) and of the lipid peroxidation product malondialdehyde; 4) the rescue of antioxidant enzymes expression (Glutathione peroxidase 1, Superoxide dismutase 2, Catalase); 5) an increased synthesis of transforming growth factor-β1 (TGF-β1) combined with the negative modulation of interleukines 1β and 6 (IL-1β and IL-6), and 6) the induction of nuclear factor erythroid-derived 2-like 2 (Nrf2) and heme oxygenase-1 (HO-1). In our experimental model of Aβ-induced neuroinflammation, carnosine: 1) prevented cell death in BV-2 cells challenged with Aβ oligomers; 2) lowered oxidative stress by decreasing the expression of inducible nitric oxide synthase and NADPH oxidase, and the concentrations of nitric oxide and superoxide anion; 3) decreased the secretion of pro-inflammatory cytokines such as IL-1β simultaneously rescuing IL-10 levels and increasing the expression and the release of TGF-β1; 4) prevented Aβ-induced neurodegeneration in primary mixed neuronal cultures challenged with Aβ oligomers and these neuroprotective effects was completely abolished by SB431542, a selective inhibitor of type-1 TGF-β receptor. Overall, our data suggest a novel multimodal mechanism of action of carnosine underlying its protective effects in macrophages and microglia and the therapeutic potential of this dipeptide in counteracting pro-oxidant and pro-inflammatory phenomena observed in different disorders characterized by elevated levels of oxidative stress and inflammation such as depression, cardiovascular disorders, and Alzheimer’s disease.

The amyloid cascade hypothesis for Alzheimer disease ((Hardy and Selkoe, 2002; Hardy and Higgins, 1992; Selkoe, 1991), updated in (Karran et al., 2011) provides a linear model for the pathogenesis of AD with Aβ accumulation upstream and Tau pathology, inflammation, synaptic dysfunction, neuronal loss and dementia downstream, all interlinked, initiated and driven by Aβ42 peptides or oligomers. The genetic mutations causing familial Alzheimer disease seem to support this model. The nagging problem remains however that the postulated causal, and especially the ’driving’ role of abnormal Aβ aggregation or Aβ oligomer formation could not be convincingly demonstrated until now. Indeed, many questions (e.g. what causes Aβ toxicity, what is the relation between Aβ and Tau pathology, what causes neuronal death, why is amyloid deposition not correlated with dementia etc…) were already raised when the amyloid hypothesis was conceived 25 years ago. These questions remain in essence unanswered. It seems that the old paradigm is not tenable: the amyloid cascade is too linear, too neurocentric, and does not take into account the long time lag between the biochemical phase i.e. the appearance of amyloid plaques and neuronal tangles and the ultimate clinical phase, i.e. the manifestation of dementia. The pathways linking these two phases must be complex and tortuous. We have called this the cellular phase of AD (De Strooper and Karran, 2016) to suggest that a long period of action and reaction involving neurons, neuronal circuitry but also microglia, astroglia, oligodendrocytes, and the vasculature underlies the disease. In fact it is this long disease process that should be studied in the coming years. While microglia are part of this process, they should not be considered as the only component of the cellular phase. We expect that further clinical investigations and novel tools will allow to diagnose the effects of the cellular changes in the brain and provide clinical signs for this so called preclinical or prodromal AD. Furthermore the better understanding of this phase will lead to completely novel drug targets and treatments and will lead to an era where patients will receive an appropriate therapy according to their clinical stage. In this view anti-amyloid therapy is probably only effective and useful in the very early stage of the disease and AD does no longer equal to dementia. We will discuss in our talk how single cell technology and transplantation of human iPS cells into mouse brain allow to start to map in a systematic way the cellular phase of Alzheimer’s Disease.

A long-term treatment with antidepressants reduces the risk to develop AD and different second-generation antidepressants such as selective serotonin reuptake inhibitors (SSRIs) are currently studied for their neuroprotective properties in AD. An impairment of neurotrophic factors signaling seems to be a common pathophysiological event in depression and AD. In particular a deficit of transforming growth factor-β1 (TGF-β1) and increased oxidative stress have been found both in depression and AD. In the present work the SSRI fluoxetine and the new multimodal antidepressant vortioxetine were tested for their ability to prevent memory deficits and depressive-like phenotype in a non-transgenic mouse model of AD (i.c.v. Aβ1-42 injection) by rescue of TGF-β1 signaling. The same drugs were also tested for their ability to modulate the expression of pro-oxidant genes as well as of genes related to the antioxidant machinery.