LAMP2A as a therapeutic target in Parkinson disease

Abnormal aggregation of SNCA/α-synuclein plays a crucial role in Parkinson disease (PD) pathogenesis. SNCA levels determine its toxicity, and its accumulation, even to a small extent, may be a risk factor for neurodegeneration. One of the main pathways for SNCA degradation is chaperone-mediated autophagy (CMA), a selective form of autophagy, while aberrant SNCA may act as a CMA inhibitor. In the current punctum we summarize our recent data showing that induction of CMA, via overexpression of the protein controlling its rate-limiting step, the lysosomal receptor LAMP2A, effectively decreases SNCA levels and ameliorates SNCA-induced neurodegeneration, both in neuronal cell culture systems and in the rat brain. Such findings suggest that modulation of LAMP2A and, consequently, CMA, represents a viable therapeutic target for PD and other synucleinopathies where SNCA accumulation and aggregation plays a fundamental role.     

Dietary lipids and aging compromise chaperone-mediated autophagy by similar mechanisms

Chaperone-mediated autophagy (CMA) is a selective form of autophagy whose distinctive feature is the fact that substrate proteins are translocated directly from the cytosol across the lysosomal membrane for degradation inside lysosomes. CMA substrates are cytosolic proteins bearing a pentapeptide motif in their sequence that, when recognized by the cytosolic chaperone HSPA8/HSC70, targets them to the surface of the lysosomes. Once there, substrate proteins bind to the lysosome-associated membrane protein type 2 isoform A (LAMP2A), inducing assembly of this receptor protein into a higher molecular weight protein complex that is used by the substrate proteins to reach the lysosomal lumen. CMA is constitutively active in most cells but it is maximally activated under conditions of stress.     

Discovery of a novel type of autophagy targeting RNA

Regulated degradation of cellular components by lysosomes is essential to maintain biological homeostasis. In mammals, three forms of autophagy, macroautophagy, microautophagy and chaperone-mediated autophagy (CMA), have been identified. Here, we showed a novel type of autophagy, in which RNA is taken up directly into lysosomes for degradation. This pathway, which we term “RNautophagy,” is ATP-dependent, and unlike CMA, is independent of HSPA8/Hsc70. LAMP2C, a lysosomal membrane protein, serves as a receptor for this pathway. The cytosolic tail of LAMP2C specifically binds to almost all total RNA derived from mouse brain. The cytosolic sequence of LAMP2C and its affinity for RNA are evolutionarily conserved from nematodes to humans. Our findings shed light on the mechanisms underlying RNA homeostasis in higher eukaryotes.     

Dietary lipids and aging compromise chaperone-mediated autophagy by similar mechanisms

Chaperone-mediated autophagy (CMA) is a selective form of autophagy whose distinctive feature is the fact that substrate proteins are translocated directly from the cytosol across the lysosomal membrane for degradation inside lysosomes. CMA substrates are cytosolic proteins bearing a pentapeptide motif in their sequence that, when recognized by the cytosolic chaperone HSPA8/HSC70, targets them to the surface of the lysosomes. Once there, substrate proteins bind to the lysosome-associated membrane protein type 2 isoform A (LAMP2A), inducing assembly of this receptor protein into a higher molecular weight protein complex that is used by the substrate proteins to reach the lysosomal lumen. CMA is constitutively active in most cells but it is maximally activated under conditions of stress.     

Revisiting LAMP1 as a marker for degradative autophagy-lysosomal organelles in the nervous system

Lysosomes serve as the degradation hubs for macroautophagic/autophagic and endocytic components, thus maintaining cellular homeostasis essential for neuronal survival and function. LAMP1 (lysosomal associated membrane protein 1) and LAMP2 are distributed among autophagic and endolysosomal organelles. Despite widespread distribution, LAMP1 is routinely used as a lysosome marker and LAMP1-positive organelles are often referred to as lysosomal compartments. By applying immuno-electron microscopy (iTEM) and confocal imaging combined with Airyscan microscopy, we expand on the limited literature to provide a comprehensive and quantitative analysis of LAMP1 distribution in various autophagic and endolysosomal organelles in neurons. Our study demonstrates that a significant portion of LAMP1-labeled organelles lack major lysosomal hydrolases. BSA-gold pulse-chase assay further shows heterogeneous degradative capacities of LAMP1-labled organelles. In addition, LAMP1 intensity is not a sensitive readout to assess lysosomal deficits in familial amyotrophic lateral sclerosis-linked motor neurons in vivo. Our study thus calls for caution when interpreting LAMP1-labeled organelles in the nervous system where LAMP1 intensity, trafficking, and distribution do not necessarily represent degradative lysosomes or autolysosomes under physiological and pathological conditions.

Abbreviations: ALS: amyotrophic lateral sclerosis; BSA: bovine serum albumin; DRG: dorsal root ganglion; IGF2R/CI-M6PR: insulin like growth factor 2 receptor; iTEM: immuno-transmission electron microscopy; LAMP1/2: lysosomal associated membrane protein 1/2; P80: postnatal day 80; sMNs: spinal motor neurons     

LAMP2A overexpression in breast tumors promotes cancer cell survival via chaperone-mediated autophagy

Lysosome-associated membrane protein type 2A (LAMP2A) is a key protein in the chaperone-mediated autophagy (CMA) pathway. LAMP2A helps in lysosomal uptake of modified and oxidatively damaged proteins directly into the lumen of lysosomes for degradation and protein turnover. Elevated expression of LAMP2A was observed in breast tumor tissues of all patients under investigation, suggesting a survival mechanism via CMA and LAMP2A. Reduced expression of the CMA substrates, GAPDH and PKM, was observed in most of the breast tumor tissues when compared with the normal adjacent tissues. Reactive oxygen species (ROS) mediated oxidative stress damages regulatory cellular components such as DNA, proteins and/or lipids. Protein carbonyl content (PCC) is widely used as a measure of total protein oxidation in cells. Ectopic expression of LAMP2A reduces PCC and thereby promotes cell survival during oxidative stress. Furthermore, inhibition of LAMP2A stimulates accumulation of GAPDH, AKT1 phosphorylation, generation of ROS, and induction of cellular apoptosis in breast cancer cells. Doxorubicin, which is a chemotherapeutic drug, often becomes ineffective against tumor cells with time due to chemotherapeutic resistance. Breast cancer cells deficient of LAMP2A demonstrate increased sensitivity to the drug. Thus, inhibiting CMA activity in breast tumor cells can be exploited as a potential therapeutic application in the treatment of breast cancer.     

LAMP2A overexpression in breast tumors promotes cancer cell survival via chaperone-mediated autophagy

Lysosome-associated membrane protein type 2A (LAMP2A) is a key protein in the chaperone-mediated autophagy (CMA) pathway. LAMP2A helps in lysosomal uptake of modified and oxidatively damaged proteins directly into the lumen of lysosomes for degradation and protein turnover. Elevated expression of LAMP2A was observed in breast tumor tissues of all patients under investigation, suggesting a survival mechanism via CMA and LAMP2A. Reduced expression of the CMA substrates, GAPDH and PKM, was observed in most of the breast tumor tissues when compared with the normal adjacent tissues. Reactive oxygen species (ROS) mediated oxidative stress damages regulatory cellular components such as DNA, proteins and/or lipids. Protein carbonyl content (PCC) is widely used as a measure of total protein oxidation in cells. Ectopic expression of LAMP2A reduces PCC and thereby promotes cell survival during oxidative stress. Furthermore, inhibition of LAMP2A stimulates accumulation of GAPDH, AKT1 phosphorylation, generation of ROS, and induction of cellular apoptosis in breast cancer cells. Doxorubicin, which is a chemotherapeutic drug, often becomes ineffective against tumor cells with time due to chemotherapeutic resistance. Breast cancer cells deficient of LAMP2A demonstrate increased sensitivity to the drug. Thus, inhibiting CMA activity in breast tumor cells can be exploited as a potential therapeutic application in the treatment of breast cancer.     

Entering the lysosome through a transient gate by chaperone-mediated autophagy

A subset of cytosolic proteins can be selectively degraded in lysosomes through chaperone-mediated autophagy. The lysosomal-membrane protein type 2A (LAMP-2A) acts as the receptor for the substrates of chaperone-mediated autophagy (CMA), which should undergo unfolding before crossing the lysosomal membrane and reaching the lumen for degradation. Translocation of substrates is assisted by chaperones on both sides of the membrane, but the actual steps involved in this process and the characteristics of the translocation complex were, for the most part, unknown. We have now found that rather than a stable translocon at the lysosomal membrane, CMA substrates bind to monomers of LAMP-2A driving the organization of this protein into a high molecular weight multimeric complex that mediates translocation. Assembly and disassembly of LAMP-2A into and from this complex is dynamic and it is regulated by hsc70 and hsp90, the two lysosomal chaperones related to CMA. This work thus unveils a unique mechanism of protein translocation across the lysosomal membrane, which involves only transient discontinuity of the membrane. The possible advantages of this transitory lysosomal translocon are discussed in light of the unique properties of the lysosomal compartment.

Addendum to: Bandyopadhyay U, Kaushik S, Vartikovski L, Cuervo AM. Dynamic organization of the receptor for chaperone-mediated autophagy at the lysosomal membrane. Mol Cell Biol 2008; 28:5747-63; DOI: 10.1128/MCB.02070-07.     

Hispolon promotes MDM2 downregulation through chaperone-mediated autophagy

Amplification and overexpression of murine double minute (MDM2) has been observed in several human cancers. Some chemotherapeutic agents cause MDM2 ubiquitination and degradation in a proteasome-dependent system. In addition to the proteasome system, chaperone-mediated autophagy (CMA) is a lysosomal pathway for selective misfolded protein degradation. Molecular chaperone heat shock cognate 70 protein (Hsc70) recognizes the misfolded proteins, which are then delivered to lysosome-associated membrane protein type 2A (LAMP2A) for lysosomal degradation. Our previous study reported that hispolon was able to induce cell apoptosis and downregulate MDM2 expression. In this study, our results showed that the proteasome inhibitor, MG132, could not inhibit hispolon-induced MDM2 downregulation. In contrast, both inhibition of lysosomes with NH₄Cl and inhibition of LAMP2A using siRNA partially attenuated hispolon-induced MDM2 downregulation. To determine whether Hsc70 recognizes MDM2 on amino acids 135-141, SMP14 antibody was used to compete with Hsc70 for interaction with MDM2. After Hsc70 knockdown, SMP14 antibody immunoprecipitated increased MDM2. We also found that hispolon induced increased association of Hsp70, Hsc70, Hsp90 and LAMP2A with MDM2. This association was inhibited in cells pretreated with geldanamycin (GA), an Hsp90 inhibitor. GA also attenuated hispolon-induced MDM2 downregulation. Meanwhile, inhibition of Hsc70 using siRNA attenuated hispolon-induced MDM2 downregulation. Our study provides the first example of the ability of hispolon to mediate MDM2 downregulation in lysosomes through the CMA pathway.     

CMA restricted to mammals and birds: myth or reality?

Chaperone-mediated autophagy (CMA) is a major pathway of lysosomal proteolysis essential for the control of intermediary metabolism. So far, the absence of any identifiable LAMP2A – a necessary and limiting protein for CMA – outside of the tetrapod clade, led to the paradigm that this cellular function was (presumably) restricted to mammals and birds. However, after we identified expressed sequences displaying high sequence homology with the mammalian LAMP2A in several fish species, our findings challenge that view and suggest that CMA likely appeared much earlier during evolution than initially thought. Hence, our results do not only shed an entirely new light on the evolution of CMA, but also bring new perspectives on the possible use of complementary genetic models, such as zebrafish or medaka for studying CMA function from a comparative angle/view.     

Parkinsonian toxin-induced oxidative stress inhibits basal autophagy in astrocytes via NQO2/quinone oxidoreductase 2: Implications for neuroprotection

Oxidative stress (OS) stimulates autophagy in different cellular systems, but it remains controversial if this rule can be generalized. We have analyzed the effect of chronic OS induced by the parkinsonian toxin paraquat (PQ) on autophagy in astrocytoma cells and primary astrocytes, which represent the first cellular target of neurotoxins in the brain. PQ decreased the basal levels of LC3-II and LC3-positive vesicles, and its colocalization with lysosomal markers, both in the absence and presence of chloroquine. This was paralleled by increased number and size of SQSTM1/p62 aggregates. Downregulation of autophagy was also observed in cells chronically exposed to hydrogen peroxide or nonlethal concentrations of PQ, and it was associated with a reduced astrocyte capability to protect dopaminergic cells from OS in co-cultures. Surprisingly, PQ treatment led to inhibition of MTOR, activation of MAPK8/JNK1 and MAPK1/ERK2-MAPK3/ERK1 and upregulation of BECN1/Beclin 1 expression, all signals typically correlating with induction of autophagy. Reduction of OS by NMDPEF, a specific NQO2 inhibitor, but not by N-acetylcysteine, abrogated the inhibitory effect of PQ and restored autophagic flux. Activation of NQO2 by PQ or menadione and genetic manipulation of its expression confirmed the role of this enzyme in the inhibitory action of PQ on autophagy. PQ did not induce NFE2L2/NRF2, but when it was co-administered with NMDPEF NFE2L2 activity was enhanced in a SQSTM1-independent fashion. Thus, a prolonged OS in astrocytes inhibits LC3 lipidation and impairs autophagosome formation and autophagic flux, in spite of concomitant activation of several pro-autophagic signals. These findings outline an unanticipated neuroprotective role of astrocyte autophagy and identify in NQO2 a novel pharmacological target for its positive modulation.     

Stabilization of MORC2 by estrogen and antiestrogens through GPER1- PRKACA-CMA pathway contributes to estrogen-induced proliferation and endocrine resistance of breast cancer cells

ABSTRACT

Aberrant activation of estrogen signaling through three ESR (estrogen receptor) subtypes, termed ESR1/ERα, ESR2/ERβ, and GPER1 (G protein-coupled estrogen receptor 1), is implicated in breast cancer pathogenesis and progression. Antiestrogens tamoxifen (TAM) and fulvestrant (FUL) are effective for treatment of ESR1-positive breast tumors, but development of resistance represents a major clinical challenge. However, the molecular mechanisms behind these events remain largely unknown. Here, we report that 17β-estradiol (E2), TAM, and FUL stabilize MORC2 (MORC family CW-type zinc finger 2), an emerging oncoprotein in human cancer, in a GPER1-dependent manner. Mechanistically, GPER1 activates PRKACA (protein kinase cAMP-activated catalytic subunit alpha), which in turn phosphorylates MORC2 at threonine 582 (T582). Phosphorylated MORC2 decreases its interaction with HSPA8 (heat shock protein family A [Hsp70] member 8) and LAMP2A (lysosomal associated membrane protein 2A), two core components of the chaperone-mediated autophagy (CMA) machinery, thus protecting MORC2 from lysosomal degradation by CMA. Functionally, knockdown of MORC2 attenuates E2-induced cell proliferation and enhances cellular sensitivity to TAM and FUL. Moreover, introduction of wild-type MORC2, but not its phosphorylation-lacking mutant (T582A), in MORC2-depleted cells restores resistance to antiestrogens. Clinically, the phosphorylation levels of MORC2 at T582 are elevated in breast tumors from patients undergoing recurrence after TAM treatment. Together, these findings delineate a phosphorylation-dependent mechanism for MORC2 stabilization in response to estrogen and antiestrogens via blocking CMA-mediated lysosomal degradation and uncover a dual role for MORC2 in both estrogen-induced proliferation and resistance to antiestrogen therapies of breast cancer cells.