Deficiency in parvalbumin, but not in calbindin D-28k upregulates mitochondrial volume and decreases smooth endoplasmic reticulum surface selectively in a peripheral, subplasmalemmal region in the soma of Purkinje cells

Abstract

Abstract-The Ca 2؉ -binding proteins parvalbumin (PV) and calbindin D-28k (CB) are key players in the intracellular Ca 2؉ -buffering in specific cells including neurons and have profound effectson spatiotemporal aspects of Ca 2؉ transients. The previously observed increase in mitochondrial volume density in fast-twitch muscle of PV؊/؊ mice is viewed as a specific compensation mechanism to maintain Ca 2؉ homeostasis. Since cerebellar Purkinje cells (PC) are characterized by high expression levels of the Ca 2؉ buffers PV and CB, the question was raised, whether homeostatic mechanisms are induced in PC lacking these buffers. Mitochondrial volume density, i.e. relative mitochondrial mass was increased by 40% in the soma of PV؊/؊ PC. Upregulation of mitochondrial volume density was not homogenous throughout the soma, but was selectively restricted to a peripheral region of 1.5 m width underneath the plasma membrane. Accompanied was a decreased surface of subplasmalemmal smooth endoplasmic reticulum (sPL-sER) in a shell of 0.5 m thickness underneath the plasma membrane. These alterations were specific for the absence of the "slow-onset" buffer PV, since in CB؊/؊ mice neither changes in peripheral mitochondria nor in sPL-sER were observed. This implicates that the morphological alterations are aimed to specifically substitute the function of the slow buffer PV. We propose a novel concept that homeostatic mechanisms of components involved in Ca 2؉ homeostasis do not always occur at the level of similar or closely related molecules. Rather the cell attempts to restore spatiotemporal aspects of Ca 2؉ signals prevailing in the undisturbed (wildtype) situation by subtly fine tuning existing components involved in the regulation of Ca 2؉ fluxes.Key words: calcium-binding, buffers, EF-hand, homeostasis, morphology.Ca 2ϩ ions are such ubiquitous second messengers that meaningful information must be contained in the subtle spatiotemporal aspects of Ca 2ϩ transients. A complex machinery of Ca 2ϩ entry and release systems, mobile and immobile Ca 2ϩ buffers, transient Ca 2ϩ -storage devices and Ca 2ϩ -extrusion systems governs the shape and spreading of intracellular Ca 2ϩ transients (Berridge et al., 2003). Affinities, kinetics of binding and release of Ca 2ϩ ions, the relative mobility and the geometrical distribution of all components, that is, the interplay between these systems finally shapes the spatiotemporal aspects of a Ca 2ϩ signal. Cerebellar Purkinje cells (PC) are characterized by extensive Ca 2ϩ signaling in somata, dendrites and spines elicited by either climbing fiber or parallel fiber stimulation. Following depolarization-evoked rises in the intracellular Ca 2ϩ concentration ([Ca 2ϩ ] i ) in the PC somata, endoplasmic reticulum (ER) and plasma membrane Ca 2ϩ pumps and the Na ϩ -Ca 2ϩ exchanger contribute to PC [Ca 2ϩ ] i clearance (Fierro et al., 1998). Since these systems only accounted for approximately 60% of total Ca 2ϩ clearing, mitochondria were additionally postulated to play a role. These organelles have...

Activation of phenotypically distinct neuronal subpopulations in the anterior subdivision of the rat basolateral amygdala following acute and repeated stress

“…Prior experiments in cerebellar Purkinje cells indicated that deficits in CB and PV resulted in increased neuronal spine length and volume in these cells (Vecellio et al, 2000;Chen et al, 2006). Although we did not find any changes in the actual number of PV ϩ or CB ϩ neurons in response to RRS, it interesting to note that these two populations displayed the most profound deficits in c-Fos expression.…”

Cytosolic Ca2+ Buffers

“…The most striking alterations in the absence of PVare the morphological changes observed in the soma. The volume of mitochondria, Ca 2þ sequestrating organelles that also serve as transient Ca 2þ stores (Billups and Forsythe 2002;Murchison and Griffith 2000), is increased by about 40% selectively in a narrow compartment underneath the plasma membrane (Chen et al 2006). Concomitantly, the subplasmalemmal smooth ER compartment is decreased (Fig.…”

“…One argument is the generality of the mechanism and a second one the reciprocality as exemplified for the relationship between PV content and mitochondrial volume. In the absence of PV in PV 2/2 mice, an up-regulation of mitochondria occurs not only in PV-ergic Purkinje cells (Chen et al 2006) or cerebellar stellate and basket cells (B Schwaller, unpubl. ), but is also seen in PV 2/2 fast-twitch muscle fibers (Chen et al 2001).…”

IGF signaling between blastema and wound epidermis is required for fin regeneration

“…Protein samples were separated by SDS-PAGE (12.5%) and transferred onto nitrocellulose membrane using semi-dry blotting apparatus (Bio-Rad). Membranes were stained with Ponceau S to check for even loading of the gels and then further processed as described before (Chen et al, 2006) using total-Akt antibody (1:1000, Cell Signaling Technology) and phospho-Akt antibody (1:1000, Ser 473, Cell Signaling Technology).…”