Daniele Bano

Cleavage of the plasma membrane Na+/Ca2+ exchanger in excitotoxicity

In brain ischemia, gating of postsynaptic glutamate receptors and other membrane channels triggers intracellular Ca2+ overload and cell death. In excitotoxic settings, the initial Ca2+ influx through glutamate receptors is followed by a second uncontrolled Ca2+ increase that leads to neuronal demise. Here we report that the major plasma membrane Ca2+ extruding system, the Na+/Ca2+ exchanger (NCX), is cleaved during brain ischemia and in neurons undergoing excitotoxicity. Inhibition of Ca2+-activated proteases (calpains) by overexpressing their endogenous inhibitor protein, calpastatin or the expression of an NCX isoform not cleaved by calpains, prevented Ca2+ overload and rescued neurons from excitotoxic death. Conversely, down-regulation of NCX by siRNA compromised neuronal Ca2+ handling, transforming the Ca2+ transient elicited by non-excitotoxic glutamate concentrations into a lethal Ca2+overload. Thus, proteolytic inactivation of NCX-driven neuronal Ca2+ extrusion is responsible for the delayed excitotoxic Ca2+ deregulation and neuronal death.

Ca2+ signals and death programmes in neurons

Cell death programmes are generally defined by biochemical/genetic routines that are linked to their execution and by the appearance of more or less typical morphological features. However, in pathological settings death signals may engage complex and interacting lethal pathways, some of which are common to different cells, whereas others are linked to a specific tissue and differentiation pattern. In neurons, death programmes can be spatially and temporally segregated. Most importantly physiological Ca2+ signals are essential for cell function and survival. On the other hand, Ca2+ overload or perturbations of intracellular Ca2+ compartmentalization can activate or enhance mechanisms leading to cell death. An imbalance between Ca2+ influx and efflux from cells is the initial signal leading to Ca2+ overload and death of ischaemic neurons or cardiomyocytes. Alterations of intracellular Ca2+ storage can integrate with death signals that do not initially require Ca2+, to promote processing of cellular components and death by apoptosis or necrosis. Finally, Ca2+ can directly activate catabolic enzymes such as proteases, phospholipases and nucleases that directly cause cell demise and tissue damage.

p73 regulates maintenance of neural stem cell

Research highlights ► TAp73 is expressed in neural stem cells and its expression increases following their differentiation. ► Neural stem cells from p73 null mice have a reduced proliferative potential. ► p73-deficient neural stem cells show reduced expression of members of the Sox-2 and Notch gene families. ► Neurogenic areas are reduced in the brains of embryonic and adult p73−/− mice.

p73: A Multifunctional Protein in Neurobiology

Abstractp73, a transcription factor of the p53 family, plays a key role in many biological processes including neuronal development. Indeed, mice deficient for both TAp73 and ΔNp73 isoforms display neuronal pathologies, including hydrocephalus and hippocampal dysgenesis, with defects in the CA1-CA3 pyramidal cell layers and the dentate gyrus. TAp73 expression increases in parallel with neuronal differentiation and its ectopic expression induces neurite outgrowth and expression of neuronal markers in neuroblastoma cell lines and neural stem cells, suggesting that it has a pro-differentiation role. In contrast, ΔNp73 shows a survival function in mature cortical neurons as selective ΔNp73 null mice have reduced cortical thickness. Recent evidence has also suggested that p73 isoforms are deregulated in neurodegenerative pathologies such as Alzheimer’s disease, with abnormal tau phosphorylation. Thus, in addition to its increasingly accepted contribution to tumorigenesis, the p73 subfamily also plays a role in neuronal development and neurodegeneration.

Cannabinoid receptor 2 deficiency results in reduced neuroinflammation in an Alzheimer's disease mouse model

Several studies have indicated that the cannabinoid receptor 2 (CB2) plays an important role in neuroinflammation associated with Alzheimers disease (AD) progression. The present study examined the role of CB2 in microglia activation in vitro as well as characterizing the neuroinflammatory process in a transgenic mouse model of AD (APP/PS1 mice). We demonstrate that microglia harvested from CB2(-/-) mice were less responsive to pro-inflammatory stimuli than CB2(+/+) microglia, based on the cell surface expression of ICAM and CD40 and the release of chemokines and cytokines CCL2, IL-6, and TNFα. Transgenic APP/PS1 mice lacking CB2 showed reduced percentages of microglia and infiltrating macrophages. Furthermore, they showed lowered expression levels of pro-inflammatory chemokines and cytokines in the brain, as well as diminished concentrations of soluble Aβ 40/42. The reduction in neuroinflammation did not affect spatial learning and memory in APP/PS1*CB2(-/-) mice. These data suggest a role for the CB2 in Alzheimers disease-associated neuroinflammation, independent of influencing Aβ-mediated pathology and cognitive impairment.

Ryanodine receptor-2 upregulation and nicotine-mediated plasticity.

Nicotine, the major psychoactive component of cigarette smoke, modulates neuronal activity to produce Ca2+‐dependent changes in gene transcription. However, the downstream targets that underlie the long‐term effects of nicotine on neuronal function, and hence behaviour, remain to be elucidated. Here, we demonstrate that nicotine administration to mice upregulates levels of the type 2 ryanodine receptor (RyR2), a Ca2+‐release channel present on the endoplasmic reticulum, in a number of brain areas associated with cognition and addiction, notably the cortex and ventral midbrain. Nicotine‐mediated RyR2 upregulation was driven by CREB, and caused a long‐lasting reinforcement of Ca2+ signalling via the process of Ca2+‐induced Ca2+ release. RyR2 upregulation was itself required for long‐term phosphorylation of CREB in a positive‐feedback signalling loop. We further demonstrate that inhibition of RyR‐activation in vivo abolishes sensitization to nicotine‐induced habituated locomotion, a well‐characterised model for onset of drug dependence. Our findings, therefore, indicate that gene‐dependent reprogramming of Ca2+ signalling is involved in nicotine‐induced behavioural changes.

The enemy at the gates. Ca2+ entry through TRPM7 channels and anoxic neuronal death.

In brain ischemia, gating of postsynaptic glutamate receptors is thought to initiate Ca2+ overload leading to excitotoxic neuronal death. In this issue, Aarts and colleagues describe a novel mechanism, whereby gating of TRPM7, a Ca2+-permeable nonselective cation channel, mediates Ca2+ overload and demise of anoxic neurons.

Loss of apoptosis-inducing factor critically affects MIA40 function

Mitochondrial apoptosis-inducing factor (AIF) influences the oxidative phosphorylation (OXPHOS) system and can be recruited as a mediator of cell death. Pathogenic mutations in the AIFM1 gene cause severe human diseases. Clinical manifestations include inherited peripheral neuropathies, prenatal cerebral abnormalities and progressive mitochondrial encephalomyopathies. In humans, rodents and invertebrates, AIF deficiency results in loss of respiratory complexes and, therefore, impaired OXPHOS. The molecular mechanisms underlying AIF-induced mitochondrial dysfunction remain elusive. Here we show that AIF physically interacts with the oxidoreductase CHCHD4/MIA40. In patient-derived fibroblasts as well as in tissues and glia cells from Harlequin (Hq) mutant mice, AIF deficiency correlates with decreased MIA40 protein levels, without affecting mRNA transcription. Importantly, MIA40 overexpression counteracts loss of respiratory subunits in Hq cells. Together, our findings suggest that MIA40 reduction contributes to the effects of AIF deficiency on OXPHOS, as it may impact on the correct assembly and maintenance of the respiratory subunits. This may be relevant for the development of new therapeutic approaches for AIF-related mitochondrial disorders.

Alteration of the nuclear pore complex in Ca(2+)-mediated cell death.

Cell death requires coordinated intracellular signalling before disassembly of cell architecture by degradative enzymes. Although the death signalling cascades that involve the mitochondria, the ER and the plasma membrane have been extensively characterized, only a handful of studies have examined the functional and structural alterations of the nuclear pore complex (NPC) during neuronal death. Here, we show that during excitotoxic neuronal degeneration calpains redistributed across the nuclear envelope and mediated the degradation of NPC components causing altered permeability of the nuclear membrane. In primary dissociated neurons, simultaneous recording of cytosolic [Ca2+] and localization of fluorescent proteins showed that the onset of Ca2+ overload signalled a progressive increase in the diffusion of small reporter molecules across the nuclear envelope. Later, calpain-mediated changes in nuclear pore permeability allowed accumulation of large proteins in the nucleus. Further, in a model of excitotoxic neuronal degeneration in Caenorhabditis elegans, we found similar nuclear changes and redistribution of fluorescent probes across the nuclear membrane in dying neurons. Our findings strongly suggest that increased leakiness of the nuclear barrier affects nucleocytoplasmic transport, alters the localization of proteins across the nuclear envelope and it is likely to be involved in Ca2+-dependent cell death, including ischemic neuronal demise.

Ageing, neuronal connectivity and brain disorders: an unsolved ripple effect.

Cognitive decline associated with ageing and age-related disorders emerges as one of the greatest health challenges in the next decades. To date, the molecular mechanisms underlying the onset of neuronal physiological changes in the central nervous system remain unclear. Functional MRI and PET studies have indicated the decline in working memory performance in older adults. Similarly, age-related disorders, such as Alzheimer’s disease, are associated with changes in the prefontral cortex and related neural circuitry, which underlines the decline of integrative function between different brain regions. This is mainly attributed to the loss of synaptic connectivity, which is a feature commonly observed in neurodegenerative disorders. In humans, the morphological and functional changes in neurons, such as reduction of spine numbers and synaptic dysfunction, precede the first signs of cognitive decline and likely contribute to pathology progression. Thus, a new scenario emerges in which apparently unrelated diseases present common features, such as the remodelling of neuronal circuitries promoted by ageing. For many years, ageing was considered a process of slow deterioration triggered by accidental environmental factors. Conversely, it is now evident that ageing is a biological process tightly controlled by evolutionary highly conserved signalling pathways. Importantly, genetic mutations that enhance longevity significantly delay the loss of synaptic connectivity and, therefore, the onset of age-related brain disorders. Accordingly, tweaking ageing might be an attractive approach to prevent cognitive decline caused by age-related synaptic dysfunction.