br As DAPK is dependent on
As DAPK1 is dependent on Ca2+/CaM for its activity, its role in induction of autophagy is Ca2+-dependent and will be described in Section 5.1. LRRK1 and 2 were however shown to regulate autophagy in numerous studies where the role of Ca2+ was not investigated, and these mechanisms will be described here below. As LRRK2 has an important role in the etiology of PD, special attention will be devoted to the latter protein.
Ca2+ ions are ideally suited to act as intracellular messenger and are for that reason ubiquitously exploited as such . In Mammalia, Ca2+ signals regulate a plethora of cellular functions including fertilization, proliferation, gene activation, metabolism, UNC 0646 and muscle contraction [99,100]. Importantly, Ca2+ signaling also plays a controlling role in cellular life and death decisions [, , , ]. The versatility of Ca2+ in independently regulating many different processes, is explained by the properties of the Ca2+ signals in terms of amplitude and frequency, as well as by the subcellular location of those Ca2+ signals. Of special importance in this context are the so-called Ca2+ microdomains that allow a very local regulation of cellular processes. The existence and the regulation of such Ca2+ microdomains play a crucial role in the processes of programmed cell death or apoptosis and are likely of great importance also for the regulation of autophagy [, , ]. As autophagy is an equally important cellular process, which moreover exist in a reciprocal relation with apoptosis [34,108,109], it could be expected to be regulated by Ca2+, and many studies have probed whether and how Ca2+ impacted autophagy (reviewed in [, , , , , , , ]). The obtained results have however delivered a surprisingly complicated message concerning the role of Ca2+ in autophagy. While it is clear that Ca2+ can regulate autophagy, studies have proposed either an inhibitory or a stimulatory function. Moreover, the origin of the Ca2+ signals acting on the autophagy process can be very diverse as Ca2+ can as well enter the cell from the extracellular space as from the endoplasmic reticulum and Ca2+ handling by the mitochondria and the lysosomes can both also participate in the process [107,118]. This complex behavior is due to the fact that the autophagy process consists of a series of independent, consecutive steps (see Section 1) while cellular Ca2+ homeostasis is under control of multiple Ca2+-handling proteins, the so-called Ca2+-signaling toolkit . All cellular compartments participate in the regulation of the cellular Ca2+ homeostasis and consequently many tens of Ca2+ channels, Ca2+ pumps, Ca2+ exchangers and Ca2+-binding proteins are expressed in each cell type. It is meanwhile very well appreciated that the basal Ca2+ concentration in the cell is maintained by the coordinated action of these various proteins, and that they are instrumental for creating the complex spatio-temporal Ca2+ signals that regulate cell function. Moreover, abnormal changes in their expression levels or activity lead to pathological situations . It is outside the scope of this review to discuss all members of the Ca2+-signaling toolkit involved in the modulation of autophagy and we will focus here only on those that are necessary for the topic of this review (see Section 5), i.e. CaM, the inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) and the two-pore channel (TPC) 2. CaM is a ubiquitously expressed protein of 148 a.a. containing 4 Ca2+-binding sites, 2 in its N-terminal domain and 2 in its C-terminal domain. Their affinity for Ca2+ ranges between 5 × 10−7 and 5 × 10−6 M . It is therefore ideally suited to sense Ca2+ concentrations in the physiological range and thus to serve as intracellular Ca2+ sensor. In their Ca2+-bound conformation, both the N-terminal and the C-terminal domains expose a hydrophobic cleft that can engage other proteins, leading to changes in their activity . The CaM target database http://calcium.uhnres.utoronto.ca/ctdb/  contains over 350 entries, including DAPK1 [51,52], DAPK2  and the IP3R, which contains several CaM-binding sites [, , , , , , , , , ].