Reactive Astrocytosis in a Mouse Model of Chronic Polyamine Catabolism Activation

Abstract

Background: In the brain, polyamines are mainly synthesized in neurons, but preferentially accumulated in astrocytes, and are proposed to be involved in neurodegenerative/neuroinflammatory disorders and neuron injury. A transgenic mouse overexpressing spermine oxidase (SMOX, which specifically oxidizes spermine) in the neocortex neurons (Dach-SMOX mouse) was proved to be a model of increased susceptibility to excitotoxic injury. Methods: To investigate possible alterations in synapse functioning in Dach-SMOX mouse, both cerebrocortical nerve terminals (synaptosomes) and astrocytic processes (gliosomes) were analysed by assessing polyamine levels, ezrin and vimentin content, glutamate AMPA receptor activation, calcium influx, and catalase activity. Results: The main findings are as follows: (i) the presence of functional calcium-permeable AMPA receptors in synaptosomes from both control and Dach-SMOX mice, and in gliosomes from Dach-SMOX mice only; (ii) reduced content of spermine in gliosomes from Dach-SMOX mice; and (iii) down-regulation and up-regulation of catalase activity in synaptosomes and gliosomes, respectively, from Dach-SMOX mice. Conclusions: Chronic activation of SMOX in neurons leads to major changes in the astrocyte processes including reduced spermine levels, increased calcium influx through calcium-permeable AMPA receptors, and stimulation of catalase activity. Astrocytosis and the astrocyte process alterations, depending on chronic activation of polyamine catabolism, result in synapse dysregulation and neuronal suffering.

Critical Role of Astrocytic Polyamine and GABA Metabolism in Epileptogenesis

“…Noteworthily, animals over-expressing SPM oxidase (SMOX) ( Cervelli et al, 2013 ) developed epileptic seizures and oxidative stress ( Leonetti et al, 2020 ). Reportedly, over-expression of SMOX caused robustly increased activity most probably associated with the release of Glu from Bergman gliosomes ( Cervetto et al, 2015 , 2016 ) and release of SPM ( Cervetto et al, 2021 ). We may raise the possibility that clinical seizures and Snyder-Robinson syndrome, which is the only known genetic disorder is associated with the polyamine metabolic pathway.…”

Transgenic Mouse Overexpressing Spermine Oxidase in Cerebrocortical Neurons: Astrocyte Dysfunction and Susceptibility to Epileptic Seizures

“…Reactive astrogliosis in the cerebral cortex of Dach-SMOX mice is indicated by an increased number of astrocytes and morphological cellular changes consisting in hypertrophy and wide ramification [12,36]. Consistent with the presence of reactive astrocytes and a neuroinflammation condition [69], a relative abundance of astrocyte processes versus nerve terminals was observed in Dach-SMOX mice, with the increase in GFAP-positive particles, and a reduction of synaptophysin-positive particles [36], together with increased levels of the astroglial markers ezrin and vimentin [43]. Indeed, vimentin is a potential marker for reactive astrocytes [67,68] of relevance in the control of the function of astrocytes and astrocyte processes in reactive astrocytosis [70].…”

“…The results obtained from the study on SMOX overexpressing mice are of relevance also in the scenario of the increasingly recognized importance of astrocytes in brain function [ 39 , 40 , 41 ], which is now shifting from a neurocentric to a neuro-astrocentric view [ 42 ]. In fact, chronic overexpression of SMOX in cortical neurons of Dach-SMOX mice severely affects their astrocyte morphology and function, and heavily influences cerebrocortical synapse functioning [ 43 ]. The findings support roles for endogenous PAs in maintaining neuron–astrocyte cross-talk and in neuroprotection [ 44 ], and that an imbalance of PA synthesis and flux can alter the neuron–glial communication in the brain [ 16 , 44 , 45 ].…”

“…It can be hypothesized that PAs are among these trophic factors produced in neurons, then released and accumulated in glial cells [ 16 , 44 , 45 ]. In turn, reactive astrocytes may have detrimental effects on neurons, contributing to the dysregulation of synapse functioning [ 43 ]. The processes of reactive astrocytes in Dach-SMOX mice undergo the following modifications that might impact on glutamatergic synapse function: reduced Spm content, expression of AMPA GluA2-lacking receptors linked to Ca 2+ entry and activation of glutamate release, increased xc − transporter function and increased glutamate in-out transport, reduced expression of EAAT1 and EAAT2.…”

“…However, in-depth analyses at the synaptic level by assessing morphology and function of purified preparations of nerve terminals, synaptosomes, and of astrocyte processes, gliosomes, prepared from the cerebral cortex of adult SMOX-overexpressing and control mice, revealed some differences. A lower content of Spm was found in the astrocyte processes of Dach-SMOX mice as compared to controls, while no difference was found in the PA content in the nerve terminals [ 43 ]. This finding is in some ways puzzling, because in Dach-SMOX mice, SMOX is specifically overexpressed in neuronal cells.…”