LRRK2 as a modulator of lysosomal calcium homeostasis with downstream effects on autophagy

Alterations in autophagy are thought to underlie various neurodegenerative diseases including Parkinson disease (PD). Previous studies have indicated that the PD gene leucine rich repeat kinase 2 (LRRK2) is involved in this process, but its mechanism of action has remained unknown. Our recent work describes how LRRK2 acts through calcium-mediated events originating from acidic stores to regulate autophagy and cell survival, which may give rise to novel therapeutic strategies.     

Cryo-EM structure of the heptameric calcium homeostasis modulator 1 channel

Calcium homeostasis modulator 1 (CALHM1) is a voltage- and Ca²⁺-gated ATP channel that plays an important role in neuronal signaling. However, as the previously reported CALHM structures are all in the ATP-conducting state, the gating mechanism of ATP permeation is still elusive. Here, we report cryo-EM reconstructions of two Danio rerio CALHM1 heptamers with ordered or flexible long C-terminal helices at resolutions of 3.2 Å and 2.9 Å, respectively, and one D. rerio CALHM1 octamer with flexible long C-terminal helices at a resolution of 3.5 Å. Structural analysis shows that the heptameric CALHM1s are in an ATP-nonconducting state with a central pore diameter of approximately 6.6 Å. Compared with those inside the octameric CALHM1, the N-helix inside the heptameric CALHM1 is in the “down” position to avoid steric clashing with the adjacent TM1 helix. Molecular dynamics simulations show that as the N-helix moves from the “down” position to the “up” position, the pore size of ATP molecule permeation increases significantly. Our results provide important information for elucidating the mechanism of ATP molecule permeation in the CALHM1 channel.     

Elevated ceramide is downstream of altered calcium homeostasis in low serum-induced apoptosis

Two immortalized cell lines, sup (+)and sup (), derived from mutagenized Syrian hamster embryo cells,were used to study the relationship and temporal order between calciumand ceramide signals during apoptosis. The early preneoplastic cells,termed sup (+), suppress tumorigenicity when hybridized with tumorcells, whereas later-stage sup () cells do not. In reduced serumconditions, sup (+) cells cease proliferating and undergo apoptosis; incontrast, sup () cells continue slow growth and undergo necrosis. Insup (+) cells, decreased endoplasmic reticulum (ER) calcium occurs 4 h after low serum treatment and precedes apoptosis. Significant elevations in ceramide are observed 16 h after reduced serumtreatment of sup (+) cells but are not found in sup () cells.Inhibiting ER calcium depletion in low serum-treated sup (+) cells bytreating with high levels of calcium prevents both ceramide generation and apoptosis. Conversely, inducing ER calcium depletion in sup ()cells by treating with low serum plus thapsigargin results in elevatedceramide levels and apoptosis. Furthermore, C₆-ceramide treatment induced apoptosis of sup () cells in low serum, a condition that does not normally cause apoptosis. C₆-ceramidetreatment did not induce apoptosis in either sup (+) or sup () cellsin 10% serum but did cause G₂/M arrest. These studies showthat ceramide production is downstream of ER calcium release.

     

The expression of damage-regulated autophagy modulator 2 (DRAM2) contributes to autophagy induction

Autophagy is a membrane trafficking process involved in intracellular degradation and recycling in eukaryotic cells. DRAM2 (damage-regulated autophagy modulator 2) is a homologue of DRAM that regulates p53-mediated cell death. As its name implies, DRAM expression induces autophagy in a p53-dependent manner; however, the role of DRAM2 in autophagy is not clear. In this study, we report that DRAM2 expression contributes to autophagy induction. Overexpression of DRAM2 induces cytoplasmic GFP-LC3 punctuates, and increases the level of endogenous LC3-II. Moreover, the silencing of endogenous DRAM2 interferes with starvation-induced autophagy. Thus, we propose that DRAM2 as well as DRAM are involved in autophagy.     

Stanniocalcin 2 is a negative modulator of store-operated calcium entry

The regulation of cellular Ca(2+) homeostasis is essential for innumerable physiological and pathological processes. Stanniocalcin 1, a secreted glycoprotein hormone originally described in fish, is a well-established endocrine regulator of gill Ca(2+) uptake during hypercalcemia. While there are two mammalian Stanniocalcin homologs (STC1 and STC2), their precise molecular functions remain unknown. Notably, STC2 is a prosurvival component of the unfolded protein response. Here, we demonstrate a cell-intrinsic role for STC2 in the regulation of store-operated Ca(2+) entry (SOCE). Fibroblasts cultured from Stc2 knockout mice accumulate higher levels of cytosolic Ca(2+) following endoplasmic reticulum (ER) Ca(2+) store depletion, specifically due to an increase in extracellular Ca(2+) influx through store-operated Ca(2+) channels (SOC). The knockdown of STC2 expression in a hippocampal cell line also potentiates SOCE, and the overexpression of STC2 attenuates SOCE. Moreover, STC2 interacts with the ER Ca(2+) sensor STIM1, which activates SOCs following ER store depletion. These results define a novel molecular function for STC2 as a negative modulator of SOCE and provide the first direct evidence for the regulation of Ca(2+) homeostasis by mammalian STC2. Furthermore, our findings implicate the modulation of SOCE through STC2 expression as one of the prosurvival measures of the unfolded protein response.     

Partial restoration of mutant enzyme homeostasis in three distinct lysosomal storage disease cell lines by altering calcium homeostasis

A lysosomal storage disease (LSD) results from deficient lysosomal enzyme activity, thus the substrate of the mutant enzyme accumulates in the lysosome, leading to pathology. In many but not all LSDs, the clinically most important mutations compromise the cellular folding of the enzyme, subjecting it to endoplasmic reticulum–associated degradation instead of proper folding and lysosomal trafficking. A small molecule that restores partial mutant enzyme folding, trafficking, and activity would be highly desirable, particularly if one molecule could ameliorate multiple distinct LSDs by virtue of its mechanism of action. Inhibition of L-type Ca²⁺ channels, using either diltiazem or verapamil—both US Food and Drug Administration–approved hypertension drugs—partially restores N370S and L444P glucocerebrosidase homeostasis in Gaucher patient–derived fibroblasts; the latter mutation is associated with refractory neuropathic disease. Diltiazem structure-activity studies suggest that it is its Ca²⁺ channel blocker activity that enhances the capacity of the endoplasmic reticulum to fold misfolding-prone proteins, likely by modest up-regulation of a subset of molecular chaperones, including BiP and Hsp40. Importantly, diltiazem and verapamil also partially restore mutant enzyme homeostasis in two other distinct LSDs involving enzymes essential for glycoprotein and heparan sulfate degradation, namely α-mannosidosis and type IIIA mucopolysaccharidosis, respectively. Manipulation of calcium homeostasis may represent a general strategy to restore protein homeostasis in multiple LSDs. However, further efforts are required to demonstrate clinical utility and safety.     

Osteoactivin acts as downstream mediator of BMP-2 effects on osteoblast function

Our laboratory previously showed that osteoactivin (OA) is a novel, osteoblast-related glycoprotein that plays a role in osteoblast differentiation and function. The purpose of this study was to examine the regulation of OA expression by BMP-2 and the role OA plays as a downstream mediator of BMP-2 effects in osteoblast function. Using primary osteoblast cultures, we tested different doses of BMP-2 on the regulation of OA expression during osteoblast development. To test whether Smad-1 signaling is responsible for BMP-2 regulation of OA expression, osteoblast cultures were transfected with Smad1 siRNA, treated with 50 ng/ml of BMP-2 and analyzed by Western blot. BMP-2 treatment increased OA mRNA and protein expression in a dose-dependent manner and this upregulation was blocked in Smad1 siRNA transfected cultures. We next examined whether the role of OA as a downstream mediator of BMP-2 effects on osteoblast differentiation and matrix mineralization. Osteoblast cultures were transfected with OA antisense oligonucleotides and treated with 50 ng/ml of BMP-2. Cultures transfected with OA antisense oligonucleotides and treated with BMP-2 showed a reduction of OA expression associated with a significant reduction in early and late differentiation markers induced by BMP-2. Therefore, OA acts, at least in part, as a downstream mediator of BMP-2 effects on osteoblast differentiation and matrix mineralization. Our findings suggest that BMP-2 regulates OA expression through the Smad1 signaling pathway. Our data also emphasize that OA protein acts as a downstream mediator of BMP-2 effects on osteoblast differentiation and function.     

siRNA screening of the kinome identifies ULK1 as a multidomain modulator of autophagy

Autophagy is a vital response to nutrient starvation. Here, we screened a kinase-specific siRNA library using an autophagy assay in human embryonic kidney 293 cells that measures lipidation of the marker protein GFP-LC3 following amino acid starvation. This screen identified ULK1 in addition to other novel candidates that could be confirmed with multiple siRNAs. Knockdown of ULK1, but not the related kinase ULK2, inhibited the autophagic response. Also, ULK1 knockdown inhibited rapamycin-induced autophagy consistent with a role downstream of mTOR. Overexpression of ULK1 inhibited autophagy and this inhibition was independent of its kinase activity. Deletion of the PDZ domain-binding Val-Tyr-Ala motif at the ULK1 C terminus generated a more potent dominant-negative protein. Further deletions revealed that the minimal ULK1 dominant-negative region could be mapped to residues 1-351. Full-length ULK1 localized to cytoplasmic structures, some of which were GFP-LC3-positive, and this localization required the conserved C-terminal domain. In contrast, ULK1-(1-351) was diffuse in the cytoplasm. These experiments reveal at least two domains in ULK1 which likely function via unique sets of effectors to regulate autophagy.     

Mutations in LRRK2 as a cause of Parkinson's disease

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common known cause of late-onset Parkinson's disease (PD). Clinical and pathological studies have demonstrated that in the majority of cases LRRK2 mutations lead to PD with classical clinical and pathological features. However, in some patients the pathological features can be distinct and/or more extensive than typically seen in PD. Collectively, these findings provide important clues into the mechanisms by which LRRK2 mutations can lead to demise of dopaminergic neurons. The understanding of LRRK2 protein function and its gene regulation and the consequences of mutations are still at their infancy, but scientific findings are progressing at a rapid pace. Although more detailed information on LRRK2 is still needed in the quest for therapeutic intervention that could halt or slow the progression of disease, here we summarize the current information on the biological and pathological properties of LRRK2.     

Somatostatin as a modulator of vagal effects on heart rhythm]

In 11 experiments on anesthetised cats burst stimulation of peripheral cut end of right vagus nerve leads to synchronization of cardiac and vagus rhythms. Alterations of burst sequence frequency within definite limits has been synchronously reproduced by heart thus creating managed bradycardia possibility. Somatostatin (10(-8)-10(-9) M intravenously) decreases heart rate and inhibits total vagus chronotropic effect. Vagolytic effect of somatostatin caused a decrease of tonic component of the vagus chronotropic effect. On the other hand, somatostatin augmented the extent of the vagal synchronizing influences and caused enlargement of the ranges of managed bradycardia. The observed results testify to participation of the peptidergic mechanisms in genesis of vagal managed bradycardia.     

Characterization of TAE684 as a potent LRRK2 kinase inhibitor

Leucine-rich repeat kinase 2 (LRRK2) is linked to Parkinson's disease and may represent an attractive therapeutic target. Here we report a 2,4-dianilino-5-chloro-pyrimidine, TAE684, a previously reported inhibitor of anaplastic lymphoma kinase (ALK), is also a potent inhibitor of LRRK2 kinase activity (IC(50) of 7.8nM against wild-type LRRK2, 6.1nM against the G2019S mutant). TAE684 substantially inhibits Ser910 and Ser935 phosphorylation of both wild-type LRRK2 and G2019S mutant at a concentration of 0.1-0.3μM in cells and in mouse spleen and kidney, but not in brain, following oral doses of 10mg/kg.     

Platinum drugs and neurotoxicity: effects on intracellular calcium homeostasis

[Pt(O,O′-acac)(γ-acac)(DMS)] (PtAcacDMS) is a new platinum compound showing low reactivity with nucleobases and specific reactivity with sulfur ligands intracellularly. It induces apoptosis in breast cancer cells, but appears to be less neurotoxic to the developing cerebellum than cisplatin (cisPt). The aim of this study was to assess the neurotoxicity of platinum compounds on calcium homeostasis in the dentate gyrus and Cornu Ammonis regions of the hippocampal formation during rat postnatal development. Two intracellular calcium homeostasis systems were taken for measurement, calbindin, a calcium buffer protein, and a plasma membrane calcium ATPase (PMCA1). The platinum compounds showed different effects on these markers in the two areas. One day after injection (PD11), cisPt decreased calbindin immunoreactivity and PMCA1 labeling in both regions; at PD17, the downregulation of PMCA1 persisted. Instead, PtAcacDMS produced varying effects on calbindin immunoreactivity in the two regions at PD11 and PD17; but in all cases, the changes incurred in calbindin immunoreactivity were counterbalanced by changes produced in PMCA1 expression. In conclusion, PtAcacDMS seems to affect calcium homeostasis in the central nervous system differently than cisPt. Both the platinum compounds act early to alter the calbindin buffering system. However, the most important difference between cisPt and PtAcacDMS is that, in vivo, the latter acts early to stimulate calcium efflux from nerve cells as reflected by its effect on PMCA1. The rapid onset of an activated calcium pump appears to be essential to cope with the excessive intracellular calcium concentration stemming from the downregulation of calbindin which could damage neuron function and morphology.     

LRRK2 kinase activity is dependent on LRRK2 GTP binding capacity but independent of LRRK2 GTP binding

Leucine rich repeat kinase 2 (LRRK2) is a Parkinson's disease (PD) gene that encodes a large multidomain protein including both a GTPase and a kinase domain. GTPases often regulate kinases within signal transduction cascades, where GTPases act as molecular switches cycling between a GTP bound "on" state and a GDP bound "off" state. It has been proposed that LRRK2 kinase activity may be increased upon GTP binding at the LRRK2 Ras of complex proteins (ROC) GTPase domain. Here we extensively test this hypothesis by measuring LRRK2 phosphorylation activity under influence of GDP, GTP or non-hydrolyzable GTP analogues GTPγS or GMPPCP. We show that autophosphorylation and lrrktide phosphorylation activity of recombinant LRRK2 protein is unaltered by guanine nucleotides, when co-incubated with LRRK2 during phosphorylation reactions. Also phosphorylation activity of LRRK2 is unchanged when the LRRK2 guanine nucleotide binding pocket is previously saturated with various nucleotides, in contrast to the greatly reduced activity measured for the guanine nucleotide binding site mutant T1348N. Interestingly, when nucleotides were incubated with cell lysates prior to purification of LRRK2, kinase activity was slightly enhanced by GTPγS or GMPPCP compared to GDP, pointing to an upstream guanine nucleotide binding protein that may activate LRRK2 in a GTP-dependent manner. Using metabolic labeling, we also found that cellular phosphorylation of LRRK2 was not significantly modulated by nucleotides, although labeling is significantly reduced by guanine nucleotide binding site mutants. We conclude that while kinase activity of LRRK2 requires an intact ROC-GTPase domain, it is independent of GDP or GTP binding to ROC.     

Bcl-2-regulated calcium signals as common mediators of both apoptosis and autophagy

The calcium ion, a major intracellular second messenger, is a known mediator of apoptosis and is regulated by the antiapoptotic protein Bcl-2. A paper by H?yer-Hasen et al. (2007) in the current issue of Molecular Cell indicates that calcium also mediates the induction of macroautophagy in a Bcl-2 regulated fashion and identifies a signaling pathway through which calcium exerts its action. These intriguing findings provoke speculation as to how a cell decides to undergo either apoptosis or macroautophagy in response to calcium signals.     

Functional analysis of picornavirus 2B proteins: effects on calcium homeostasis and intracellular protein trafficking

The family Picornaviridae consists of a large group of plus-strand RNA viruses that share a similar genome organization. The nomenclature of the picornavirus proteins is based on their position in the viral RNA genome but does not necessarily imply a conserved function of proteins of different genera. The enterovirus 2B protein is a small hydrophobic protein that, upon individual expression, is localized to the endoplasmic reticulum (ER) and the Golgi complex, reduces ER and Golgi complex Ca(2+) levels, most likely by forming transmembrane pores, and inhibits protein trafficking through the Golgi complex. At present, little is known about the function of the other picornavirus 2B proteins. Here we show that rhinovirus 2B, which is phylogenetically closely related to enterovirus 2B, shows a similar subcellular localization and function to those of enterovirus 2B. In contrast, 2B proteins of hepatitis A virus, foot-and-mouth disease virus, and encephalomyocarditis virus, all of which are more distantly related to enteroviruses, show a different localization and have little, if any, effects on Ca(2+) homeostasis and intracellular protein trafficking. Our data suggest that the 2B proteins of enterovirus and rhinovirus share the same function in virus replication, while the other picornavirus 2B proteins support the viral life cycle in a different manner. Moreover, we show that an enterovirus 2B protein that is retained in the ER is unable to modify Ca(2+) homeostasis and inhibit protein trafficking, demonstrating the importance of Golgi complex localization for its functioning.     

Therapeutic effects of remediating autophagy failure in a mouse model of Alzheimer disease by enhancing lysosomal proteolysis

The extensive autophagic-lysosomal pathology in Alzheimer disease (AD) brain has revealed a major defect: in the proteolytic clearance of autophagy substrates. Autophagy failure contributes on several levels to AD pathogenesis and has become an important therapeutic target for AD and other neurodegenerative diseases. We recently observed broad therapeutic effects of stimulating autophagic-lysosomal proteolysis in the TgCRND8 mouse model of AD that exhibits defective proteolytic clearance of autophagic substrates, robust intralysosomal amyloid-β peptide (Aβ) accumulation, extracellular β-amyloid deposition and cognitive deficits. By genetically deleting the lysosomal cysteine protease inhibitor, cystatin B (CstB), to selectively restore depressed cathepsin activities, we substantially cleared Aβ, ubiquitinated proteins and other autophagic substrates from autolysosomes/lysosomes and rescued autophagic-lysosomal pathology, as well as reduced total Aβ40/42 levels and extracellular amyloid deposition, highlighting the underappreciated importance of the lysosomal system for Aβ clearance. Most importantly, lysosomal remediation prevented the marked learning and memory deficits in TgCRND8 mice. Our findings underscore the pathogenic significance of autophagic-lysosomal dysfunction in AD and demonstrate the value of reversing this dysfunction as an innovative therapeautic strategy for AD.     

Therapeutic effects of remediating autophagy failure in a mouse model of Alzheimer disease by enhancing lysosomal proteolysis

The extensive autophagic-lysosomal pathology in Alzheimer disease (AD) brain has revealed a major defect

in the proteolytic clearance of autophagy substrates. Autophagy failure contributes on several levels to AD pathogenesis and has become an important therapeutic target for AD and other neurodegenerative diseases. We recently observed broad therapeutic effects of stimulating autophagic-lysosomal proteolysis in the TgCRND8 mouse model of AD that exhibits defective proteolytic clearance of autophagic substrates, robust intralysosomal amyloid-β peptide (Aβ) accumulation, extracellular β-amyloid deposition and cognitive deficits. By genetically deleting the lysosomal cysteine protease inhibitor, cystatin B (CstB), to selectively restore depressed cathepsin activities, we substantially cleared Aβ, ubiquitinated proteins and other autophagic substrates from autolysosomes/lysosomes and rescued autophagic-lysosomal pathology, as well as reduced total Aβ40/42 levels and extracellular amyloid deposition, highlighting the underappreciated importance of the lysosomal system for Aβ clearance. Most importantly, lysosomal remediation prevented the marked learning and memory deficits in TgCRND8 mice. Our findings underscore the pathogenic significance of autophagic-lysosomal dysfunction in AD and demonstrate the value of reversing this dysfunction as an innovative therapeautic strategy for AD.     

Evidence of lysosomal membrane permeabilization in mucopolysaccharidosis type I: Rupture of calcium and proton homeostasis

Mucopolysaccharidosis type I (MPS I) is caused by a deficiency of α‐iduronidase (IDUA), which leads to intralysosomal accumulation of glysosaminoglycans. Patients with MPS I present a wide range of clinical manifestations, but the mechanisms by which these alterations occur are still not fully understood. Genotype–phenotype correlations have not been well established for MPS I; hence, it is likely that secondary and tertiary alterations in cellular metabolism and signaling may contribute to the physiopathology of the disease. The aim of this study was to analyze Ca²⁺ and H⁺ homeostasis, lysosomal leakage of cysteine proteases, and apoptosis in a murine model of MPS I. After exposition to specific drugs, cells from Idua?/? mice were shown to release more Ca²⁺ from the lysosomes and endoplasmic reticulum than Idua+/+ control mice, suggesting a higher intraorganelle store of this ion. A lower content of H⁺ in the lysosomes and in the cytosol was found in cells from Idua?/? mice, suggesting an alteration of pH homeostasis. In addition, Idua?/? cells presented a higher activity of cysteine proteases in the cytosol and an increased rate of apoptotic cells when compared to the control group, indicating that lysosomal membrane permeabilization might occur in this model. Altogether, our results suggest that secondary alterations—as changes in Ca²⁺ and H⁺ homeostasis and lysosomal membrane permeabilization—may contribute for cellular damage and death in the physiopathology of MPS I. J. Cell. Physiol. 223: 335–342, 2010. © 2010 Wiley‐Liss, Inc.     

Increased LRRK2 kinase activity alters neuronal autophagy by disrupting the axonal transport of autophagosomes

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