Editorial for the Special Issue: Microglia-Neuron interactions in health and disease - novel perspectives for translational research

CX3CL1 Pathway as a Molecular Target for Treatment Strategies in Alzheimer's Disease

Alzheimer's disease (AD) is a scourge for patients, caregivers and healthcare professionals due to the progressive character of the disease and the lack of effective treatments. AD is considered a proteinopathy, which means that aetiological and clinical features of AD have been linked to the deposition of amyloid β (Aβ) and hyperphosphorylated tau protein aggregates throughout the brain, with Aβ and hyperphosphorylated tau representing classical AD hallmarks. However, some other putative mechanisms underlying the pathogenesis of the disease have been proposed, including inflammation in the brain, microglia activation, impaired hippocampus neurogenesis and alterations in the production and release of neurotrophic factors. Among all, microglia activation and chronic inflammation in the brain gained some attention, with researchers worldwide wondering whether it is possible to prevent and stop, respectively, the onset and progression of the disease by modulating microglia phenotypes. The following key points have been established so far: (i) Aβ deposition in brain parenchyma represents repeated stimulus determining chronic activation of microglia; (ii) chronic activation and priming of microglia make these cells lose neuroprotective functions and favour damage and loss of neurons; (iii) quiescent status of microglia at baseline prevents chronic activation and priming, meaning that the more microglia are quiescent, the less they become neurotoxic. Many molecules are known to modulate the quiescent baseline state of microglia, attracting huge interest among scientists as to whether these molecules could be used as valuable targets in AD treatment. The downside of the coin came early with the observation that quiescent microglia do not display phagocytic ability, being unable to clear Aβ deposits since phagocytosis is crucial for Aβ clearance efficacy. A possible solution for this issue could be found in the modulation of microglia status at baseline, which could help maintain both neuroprotective features and phagocytic ability at the same time. Among the molecules known to influence the baseline status of microglia, C-X3-chemokine Ligand 1 (CX3CL1), also known as Fractalkine (FKN), is one of the most investigated. FKN and its microglial receptor CX3CR1 are crucial players in the interplay between neurons and microglia, modulating the operation of some neural circuits and the efficacy and persistence of immune response against injury. In addition, CX3CL1 regulates synaptic pruning and plasticity in the developmental age and in adulthood, when it strongly impacts the hippocampus neurogenesis of the adult. CX3CL1 has an effect on Aβ clearance and tau phosphorylation, as well as in microglia activation and priming. For all the above, CX3CL1/CX3CR1 signalling has been widely studied in relation to AD pathogenesis, and its biochemical pathway could hide molecular targets for novel treatment strategies in AD. This review summarizes the possible role of CX3CL1 in AD pathogenesis and its use as a potential target for AD treatment.

Neuroinflammatory Responses and Parkinson' Disease: Pathogenic Mechanisms and Therapeutic Targets

Parkinson's disease (PD) is the second most common age-related neurodegenerative disorders of the central nervous system, which mainly impairs the motor system. However, the pathogenic mechanisms are still unclear. Gene-environment complex interaction leads to selective dopaminergic neuron death in PD. Growing evidences supports that neuroinflammatory responses are involved in the pathogenesis of PD. This review critically discusses current studies on the inflammatory response of the pathological process of PD. The mechanisms and strategies of modifying inflammatory responses would be potential treatments for neurodegenerative diseases. Graphical abstract Activated microglia canpromote the damage ofdopaminergic neurons, which inturn aggravates the activation ofmicroglia in the process of PD. Atthe same time, microglia canactivate astrocytes throughproliferation and secretion ofinflammatory factors. The role ofastrocytes on the loss ofdopaminergic neurons is stillcontroversial in PD. (Nonsteroidalanti-inflammatory drugs,NSAIDs. adiposed-derived stemcells, ADSCs.nicotinamideadenine dinucleotide phosphate,NADPH. signal transducers andactivators of transcription,STAT.DJ-1,Aliases forPARK7.mesencephalic astrocytederivedneurotrophic factor,MANF.Ciliary neurotrophicfactor,CNTF.glial cell linederivedneurotrophic factor,GDNF.Wnt Family Member1,Wnt1). Graphical abstract Mitochondrial dysfunction causes neuroinflammation throughDAMPs and a series of factors such as oxidative stress andinflammatory bodies in PD. (Damage-associated molecular patterns,DAMPs. reactive oxygen species, ROS). Graphical abstract Various mechanismsparticipate in NLRP3 activation,causing microglia activation inPD. ( -synuclein, -syn.) TolllikeReceptor 2, TLR2. Toll-likeReceptor 4, TLR4. TumorNecrosis Factor, TNF.Apoptosisassociated speck like proteincontaining a CARD, ASC).