Circadian rhythm disruption and retinal dysfunction:a bidirectional link in Alzheimer’s disease?

Dysfunction in circadian rhythms is a common occurrence in patients with Alzheimer’s disease.A predominant function of the retina is circadian synchronization,carrying information to the brain through the retinohypothalamic tract,which projects to the suprachiasmatic nucleus.Notably,Alzheimer’s disease hallmarks,including amyloid-β,are present in the retinas of Alzheimer’s disease patients,followed/associated by structural and functional disturbances.However,the mechanistic link between circadian dysfunction and the pathological changes affecting the retina in Alzheimer’s disease is not fully understood,although some studies point to the possibility that retinal dysfunction could be considered an early pathological process that directly modulates the circadian rhythm.

Aquaporin 5 in Alzheimer’s disease:a link between oral and brain pathology?

The involvement of aquaporins(AQPs)in the development of diseases has been widely described(Azad et al.,2021).AQP5 has been described in astrocytes changing after traumatic brain injuries(Chai et al.,2013),but the precise role of AQP5 in Alzheimer’s disease(AD)pathology is yet to be understood.We have recently reported that AQP5 expression changes during the development of AD(Antequera et al.,2022).The AQP5 expression in salivary glands is decreased in 6-month-old APP/PS1 mice and AD patients.This decrease in AQP5 expression could be involved in the mechanism of salivary gland dysfunction described in a previous study(Antequera et al.,2021).Now,we propose a new indirect role of AQP5 in the connection between infection-induced oral dysbiosis and AD(Sureda et al.,2020).Here,we suggest that the proinflammatory response induced by oral pathogen infection results in the downregulation of AQP5 contributing to the salivary gland secretory dysfunction.All these alterations destabilize the peripheral immune-inflammatory balance and exacerbate neuroinflammation and neurodegeneration leading to AD pathology.

Implication of salivary lactoferrin and periodontal-mediated infections in Alzheimer's disease

Lactoferrin is an antimicrobial prote in characterized by the exertion of many protective functions,including antibacterial,antifungal,antiviral,and antiparasitic properties,as well as anti-inflammatory and immunomodulatory activities(Kruzel et al.,2017).Lactoferrin is one of the major proteins present in exocrine secretions,including saliva,and is therefore associated with host defense against oral pathogens and control of the oral microbiome.In recent years,it has become clear that alterations in the oral microbiome may contribute to opportunistic pathogen infections in the brains of Alzheimer’s disease(AD)patients and thus participate in or contribute to the development of this neurodegenerative disease(Sureda et al.,2020).Pathogenic oral microbes can affect neurological processes by entering brain tissue through various pathways and directly damaging the central nervous system.In the central nervous system,oral microbes may trigger an immune response that increases amyloidβ(Aβ)production and may even trigger the Aβcascade to promote the onset of AD,as we discuss in our previous study supporting the“infectious hypothesis”in AD(González-Sánchez et al.,2020).

Cerebrospinal fluid neurogranin as a new player in prion disease diagnosis and prognosis

Neurogranin (Ng) and its role as Alzheimer’s disease (AD) biomarker: Ng is a calmodulin-binding protein mainly expressed in cerebral structures such as the cortex,hippocampus and striatum.It is mainly located in the dendritic processes,particularly in post-synaptic compartments,but also in the cytosolic compartment,being likely involved in the regulation of the intracellular calcium-calmodulin signaling pathway (Represa et al.,1990).In the last decade,a plethora of studies have demonstrated that cerebrospinal fluid (CSF) Ng is increased in AD patients and in individuals with an ADlike CSF profile (Kester et al.,2015a).This increase seems to be disease-specific because other neurodegenerative conditions including frontotemporal dementia,Lewy body dementia,Parkinson’s disease,progressive supranuclear palsy,multiple system atrophy or Huntington’s disease,present CSF Ng concentrations similar to controls (Wellington et al.,2016).Ng levels in CSF appear to be elevated in mild cognitive impairment (MCI)-affected individuals who progress to AD and are highly related to memory and cognitive function (Kester et al.,2015a;Tarawneh et al.,2016),which indicates that this protein may serve as an early AD biomarker with diagnostic utility in pre-dementia disease stages,and with prognostic utility to predict cognitive decline and MCI-to-AD conversion.

Neuronal circuitry reconstruction after stem cell therapy in damaged brain

Transplantation of neuronal precursors derived from human pluripotent stem cells is a promising therapy for the treatment of neurological disorders associated with neuronal loss,such us neurodegenerative diseases,brain trauma and stroke.The functional integration of grafted neurons differentiated from stem cells into the host injured neuronal circuitry has been a major challenge in cell therapy strategies for brain repair(Palma-Tortosa et al.,2021).Even though other cell types or mechanisms may provide modest clinical improvements,neuronal replacement and reconstruction of the damaged area are crucial for an optimal and long-term recovery.