Chronic pharmacological blockade of the Na+ /Ca2+ exchanger modulates the growth and development of the Purkinje cell dendritic arbor in mouse cerebellar slice cultures

Nuclear localization of NCX: Role in Ca2+ handling and pathophysiological implications

Numerous lines of evidence indicate that nuclear calcium concentration ([Ca²⁺]_n) may be controlled independently from cytosolic events by a local machinery. In particular, the perinuclear space between the inner nuclear membrane (INM) and the outer nuclear membrane (ONM) of the nuclear envelope (NE) likely serves as an intracellular store for Ca²⁺ ions. Since ONM is contiguous with the endoplasmic reticulum (ER), the perinuclear space is adjacent to the lumen of ER thus allowing a direct exchange of ions and factors between the two organelles. Moreover, INM and ONM are fused at the nuclear pore complex (NPC), which provides the only direct passageway between the nucleoplasm and cytoplasm. However, due to the presence of ion channels, exchangers and transporters, it has been generally accepted that nuclear ion fluxes may occur across ONM and INM. Within the INM, the Na⁺/Ca²⁺ exchanger (NCX) isoform 1 seems to play an important role in handling Ca²⁺ through the different nuclear compartments. Particularly, nuclear NCX preferentially allows local Ca²⁺ flowing from nucleoplasm into NE lumen thanks to the Na⁺ gradient created by the juxtaposed Na⁺/K⁺-ATPase. Such transfer reduces abnormal elevation of [Ca²⁺]_n within the nucleoplasm thus modulating specific transductional pathways and providing a protective mechanism against cell death. Despite very few studies on this issue, here we discuss those making major contribution to the field, also addressing the pathophysiological implication of nuclear NCX malfunction.

Distribution of and steroid hormone effects on calbindin-D9k in the immature rat brain

Calbindin-D_9k (CaBP-9k), one of the major calcium-binding and calcium-buffering proteins, is important in the physiological functioning of organs. The neuroanatomical localization of CaBP-9k in the rodent brain has not been reported; thus, this study investigated the neuroanatomical distribution of CaBP-9k and the regulation of CaBP-9k expression on steroid hormones in the immature rat brain. To confirm the influence of steroid hormones on CaBP-9k expression, immature female rats were injected for 5 days with estrogen (E2), progesterone (P4), dexamethasone (DEX), and their antagonists (ICI 182, 780 and RU 486). The localization and expression of the CaBP-9k protein in brain regions were identified by immunofluorescence and western blot assays, respectively. We observed that CaBP-9k expression was especially strong in hypothalamus, cerebellum, and brain stem. In addition, CaBP-9k was colocalized with mature-, GABAergic, dopaminergic, and oxytocinergic neurons. We also observed that the CaBP-9k protein level was significantly increased by P4 and reversed by antagonist RU 486 treatment in immature rat brain. In summary, CaBP-9k positive cells have a wide distribution in the immature rat brain, and CaBP-9k expression is regulated by P4. We suggest that CaBP-9k expression regulated by steroid hormone may serve as an important regulator of cytosolic calcium concentration in the brain.

Selective inhibition of mitochondrial sodium-calcium exchanger protects striatal neurons from α-synuclein plus rotenone induced toxicity

Progressive accumulation of α-synuclein (α-syn) and exposure to environmental toxins are risk factors that may both concur to Parkinson’s disease (PD) pathogenesis. Electrophysiological recordings of field postsynaptic potentials (fEPSPs) and Ca²⁺ measures in striatal brain slices and differentiated SH-SY5Y cells showed that co-application of α-syn and the neurotoxic pesticide rotenone (Rot) induced Ca²⁺ dysregulation and alteration of both synaptic transmission and cell function. Interestingly, the presence of the mitochondrial NCX inhibitor CGP-37157 prevented these alterations. The specific involvement of the mitochondrial NCX was confirmed by the inability of the plasma membrane inhibitor SN-6 to counteract such phenomenon. Of note, using a siRNA approach, we found that NCX1 was the isoform specifically involved. These findings suggested that NCX1, operating on the mitochondrial membrane, may have a critical role in the maintenance of ionic Ca²⁺ homeostasis in PD and that its inhibition most likely exerts a protective effect in the toxicity induced by α-syn and Rot.

Reduced Purkinje cell size is compatible with near normal morphology and function of the cerebellar cortex in a mouse model of spinocerebellar ataxia

Spinocerebellar ataxia type 14 (SCA14) is a dominantly inherited neurodegenerative disease caused by diverse mutations in the Protein Kinase C gamma (PKCγ) gene which is one of the crucial signaling molecules of Purkinje cells. We have previously created a mouse model of SCA14 by transgenic expression of a mutated PKCγ gene causing SCA14 with a mutation in the catalytic domain. Purkinje cells from the mutated mice have a strong reduction of their dendritic tree in organotypic slice cultures typical for increased PKC activity. There was no overt degeneration of Purkinje cells in vivo and the cerebellum appeared morphologically normal with the exception of lobule 7 where abnormal Purkinje cells were present. Besides from mild motor deficits the mice have no major phenotype. We have now done a more extensive study of cerebellar morphology in these mice and show by rapid Golgi staining that there is a marked reduction of Purkinje cell dendritic tree size throughout the cerebellum. Despite this reduction in dendritic tree size, climbing fiber innervation of Purkinje cells as visualized by immunostaining for the vesicular glutamate transporter 2 (vGlut2) appeared normal in most parts of the cerebellum. The same was true for the expression of the activity and plasticity markers pS6, c-Fos and Arc. These finding suggest that the cerebellar cortex in the transgenic mice is functioning fairly normal and that the reduction of dendritic tree size and the increased PKC activity can be compensated in most Purkinje cells. Around cerebellar lobule 7 there was high transgene expression from the L7 promotor and Purkinje cells showed abnormal morphologies. Climbing fiber innervation as well as the expression of the activity and plasticity markers was strongly disturbed in this area. Our results show that there is substantial potential for functional compensation in the cerebellar cortex. In lobule 7, an area with high transgene expression, compensation failed resulting in Purkinje cell degeneration and dysfunction.

Na+/Ca2+ exchanger 1 on nuclear envelope controls PTEN/Akt pathway via nucleoplasmic Ca2+ regulation during neuronal differentiation

Nuclear envelope (NE) is a Ca²⁺-storing organelle controlling neuronal differentiation through nuclear Ca²⁺ concentrations ([Ca²⁺]_n). However, how [Ca²⁺]_n regulates this important function remains unknown. Here, we investigated the role of the nuclear form of the Na⁺/Ca²⁺ exchanger 1(nuNCX1) during the different stages of neuronal differentiation and the involvement of PTEN/PI3'K/Akt pathway. In neuronal cells, nuNCX1 was detected on the inner membrane of the NE where protein expression and activity of the exchanger increased during NGF-induced differentiation. nuNCX1 activation by Na⁺-free perfusion induced a time-dependent activation of nuclear-resident PI3K/Akt pathway in isolated nuclei. To discriminate the contribution of nuNCX1 from those of plasma membrane NCX, we generated a chimeric protein composed of the fluorophore EYFP, the exchanger inhibitory peptide, and the nuclear localization signal, named XIP-NLS. Fura-2 measurements on single nuclei and patch-clamp experiments in whole-cell configuration showed that XIP-NLS selectively inhibited nuNCX1. Once it reached the nuclear compartment, XIP-NLS increased the nucleoplasmic Ca²⁺ peak elicited by ATP and reduced Akt phosphorylation, GAP-43 and MAP-2 expression through nuclear-resident PTEN induction. Furthermore, in accordance with the prevention of the neuronal phenotype, XIP-NLS significantly reduced TTX-sensitive Na⁺ currents and membrane potential during neuronal differentiation. The selective inhibition of nuNCX1 by XIP-NLS increased the percentage of β III tubulin-positive immature neurons in mature cultures of MAP-2-positive cortical neurons, thus unraveling a new function for nuNCX1 in regulating neuronal differentiation through [Ca²⁺]_n-dependent PTEN/PI3K/Akt pathway.