Peroxisome Ca2+ homeostasis in animal and plant cells.

Peroxisomes are organelles enclosed by a single membrane, with a high matrix protein concentration that includes at least 50 different enzymes involved in several pathways; among these, the best known are fatty acid -oxidation, scavenging of hydrogen peroxide (functions that in animal cells are partially shared by mitochondria), and the synthesis of etherphospholipids [52, 87]. In particular, peroxisomes are endowed with several enzymes capable of disposing of oxygen peroxide (generated in large amount by oxidative reactions), e.g., catalase, glutathione peroxidase and peroxiredoxin V (for an exhaustive review on peroxisomal biochemistry see [88]). Specialized peroxisomal functions are found in some cells, e.g., plants and fungi, such as fatty acid degradation and synthesis of phytohormones [21] (see also Chapter 14, 16 this book). These organelles are numerous and dispersed throughout the entire cytosol and can rapidly adapt to cellular demand by increasing their size or numbers; the increase in the number of peroxisomes is mainly triggered by lipids, through the activation of a ligand-dependent transcription factor (Peroxisome Proliferator Activated Receptor, PPAR), mostly mediated by a fission process similar to that described for mitochondria [52]. The peroxisomal membrane is impermeable to high MW molecules (> 1000 Da) and several specific carriers are expressed in the organelles for taking up different metabolites ([87] and see Chapters 2 and 10 of this book). Recent reports have demonstrated the existence of peroxisomal membrane transporters (from yeast to mammalian cells) [68, 78, 85], thus suggesting a strictly regulated activity of peroxisomal pathways, acting in concert with cytosolic metabolism. Apart from their metabolic functions, peroxisomes have emerged as organelles important in determining cell fate, contributing to regulate cell differentiation as well as embryo development and morphogenesis [79]. Indeed, the lipids synthesized by peroxisomes can be targeted to the nucleus by specific binding proteins, where they activate lipid-dependent transcription factors (including PPARs) that in turn regulate the expression of several genes involved in differentiation and development [77]. The cellular importance of peroxisomes has been recently reinforced by the fact that impairment of peroxisomal activity or biogenesis is linked to different genetic disorders in humans, most of which are associated with severe neurological symptoms, as in the Zellweger syndrome spectrum [74]. Given that any enzymatic activity is highly sensitive to the ionic composition of the environment where they operate, it is surprising that information on the luminal ion content of peroxisomes, as well as the permeability characteristics of their membrane to ions, is scarce and contradictory. For example, the luminal pH of peroxisomes has been reported to be indistinguishable, more alkaline or more acidic than that of the cytoplasm. In particular, using a targeted GFP based pH indicator, Drago et al and Jankowski et al concluded that no appreciable pH gradient exists across the membrane of mammalian cell lines [15, 27], while Dansen et al. (using another genetically encoded pH probe) reported that the peroxisomal pH in human fibroblasts is slightly alkaline. A similar conclusion was reached by Waterham et al. and van Roermund et al. in yeast [13, 83, 90]. On the contrary, also in yeast, Lasorsa et al. found that the peroxisomal pH is slightly acidic due to the operation of specific adenine nucleotide transporters [31]. The reason for these discrepancies and the mechanisms for the possible generation of the pH gradient remain unsolved. To the best of our knowledge, no direct measurement of intra-peroxisomal monovalent cation or anion concentration has yet been performed, although, based on indirect evidence, Drago et al. [15] concluded that the Na+ concentration is indistinguishable between peroxisomes and cytosol. Although it is an issue potentially of crucial importance in the understanding possible regulatory signals for their metabolic activity, very little and contrasting information is available (see below) about the Ca2+ concentration in the peroxisome lumen, [Ca2+]p, and peroxisomal Ca2+ handling by the organelles. In this chapter we will focus on the mechanism regulating peroxisomal Ca2+ homeostasis in mammalian and plant cells. We will also discuss recent data concerning the possible functional role of the changes in intra-peroxisomal Ca2+ level occurring in living cells. As to this latter aspect, we will discuss only data obtained in plants, as no information is presently available concerning the functional role of intra-peroxisomal Ca2+ in mammalian cells.