Leonard Dode

The Secretory Pathway Ca2+/Mn2+-ATPase 2 Is a Golgi-localized Pump with High Affinity for Ca2+ Ions

Accumulation of Ca2+ into the Golgi apparatus is mediated by sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs) and by secretory pathway Ca2+-ATPases (SPCAs). Mammals and birds express in addition to the housekeeping SPCA1 (human gene name ATP2C1, cytogenetic position 3q22.1) a homologous SPCA2 isoform (human gene name ATP2C2, cytogenetic position 16q24.1). We show here that both genes present an identical exon/intron layout. We confirmed that hSPCA2 has the ability to transport Ca2+, demonstrated its Mn2+-transporting activity, showed its Ca2+- and Mn2+-dependent phosphoprotein intermediate formation, and documented the insensitivity of these functional activities to thapsigargin inhibition. The mRNA encoding hSPCA2 showed a limited tissue expression pattern mainly confined to the gastrointestinal and respiratory tract, prostate, thyroid, salivary, and mammary glands. Immunocytochemical localization in human colon sections presented a typical apical juxtanuclear Golgi-like staining. The expression in COS-1 cells allowed the direct demonstration of 45Ca2+ (K0.5 = 0.27 μm) or 54Mn2+ transport into an A23187-releasable compartment.

The SERCA3-type of organellar Ca2+pumps

Of all the SERCA pumps, SERCA3 was the latest to be described and the least well known. Its primary structure deviates more than usual from the other members of the SERCA family. It is not known whether its remarkably low affinity for Ca2+ (K0.5 > 1μM) observed upon expression in the COS cell system occurs also in its normal cellular context. SERCA3 is particularly expressed at high levels in different types of blood cells and related cells like platelets, lymphocytes, mast cells and arterial endothelial cells. It is also found in cerebellar Purkinje neurons. The physiological significance of this expression pattern remains unknown.

Distribution and isoform diversity of the organellar Ca2+ pumps in the brain

The gene family of organellar-type Ca2+ transport ATPases consists of three members. SERCA1 is expressed exclusively in fast skeletal muscle; SERCA2 is ubiquitously expressed, whereas SERCA3 is considered to be mainly expressed in cells of the hematopoietic lineage and in some epithelial cells. In the brain, the organellar-type Ca2+ transport ATPases are almost exclusively transcribed from the SERCA2 gene. Four different SERCA2 mRNAs have been described (classes 1-4). However, unlike in nonneuronal cells, which express the class 1, 2, and 3 splice variants, the main SERCA2 mRNA in the brain is the class 4 messenger. Similar to classes 2 and 3, the class 4 codes for the ubiquitously expressed SERCA2b protein. Recently, we have reported the distribution of the SERCA isoforms in the brain (Baba-Aissa et al., 1996a,b). SERCA2b was present in most neurons of all investigated brain regions. The highest levels were found in the Purkinje neurons of the cerebellum and in the pyramidal cells of the hippocampus. Interestingly, SERCA3 and SERCA2a are coexpressed along with SERCA2b in the Purkinje neurons, but are weakly expressed in the other brain regions if present at all. Since these three protein isoforms have a different affinity for Ca2+, their possible roles in relation to Ca2+ stores in neurons are discussed.

Purkinje neurons express the SERCA3 isoform of the organellar type Ca2+-transport ATPase

We report the distribution of the sarco(endo)plasmic reticulum Ca2+ ATPase 3 (SERCA3) isoform in the rat brain. Compared to SERCA2 isoform, which is found in all brain regions, SERCA3 is specifically expressed in the Purkinje neurons. This conclusion is based on immunochemical observations using SERCA3- and SERCA2b-specific antibodies, in-situ hybridization using SERCA3-specific oligonucleotide probes and single-cell reverse transcription-polymerase chain reaction (RT-PCR). Immunocytochemistry clearly revealed the expression of SERCA3 in the cell body and in the dentritic processes of the Purkinje neurons. Single-cell ratio RT-PCR showed that Purkinje neurons expressed 3-fold lower levels of SERCA3 mRNA compared to SERCA2 mRNA. SERCA3 expression is very low or absent in the rat cerebrum and brainstem. It is known that the SERCA3 Ca2+ pump has an approximately 5-fold lower affinity for Ca2+ when expressed in COS cells as compared to other SERCA members [15]. If this property is also valid in a neuronal context, the expression of the SERCA3 Ca(2+)-pump isoform could have important functional implications for the regulation of the cytosolic Ca2+ concentration in Purkinje neurons.

DISEASES INVOLVING THE GOLGI CALCIUM PUMP

Secretory-pathway Ca2(+)-transport ATPases (SPCA) provide the Golgi apparatus with Ca2+ and Mn2+ needed for the normal functioning of this organelle. Loss of one functional copy of the human SPCA1 gene (ATP2C1) causes Hailey-Hailey disease, a rare skin disorder characterized by recurrent blisters and erosions in the flexural areas. Here, we will review the properties and functional role of the SPCAs. The relationship between Hailey-Hailey disease and its defective gene (ATP2C1) will be adressed as well.