Branching Off: New Insight Into Lysosomes as Tubular Organelles

Snapshots of mitochondrial fission imaged by cryo-scanning transmission electron tomography

Mitochondria undergo constant remodeling via fission, fusion, extension and degradation. Fission, in particular, depends on the accumulation of mitochondrial fission factor (MFF) and subsequent recruitment of the dynamin-related protein DRP1 (also known as DNM1L). We used cryo-scanning transmission electron tomography (cryo-STET) to investigate mitochondrial morphologies in MFF mutant (MFF-/-) mouse embryonic fibroblast (MEF) cells in ATP-depleting conditions that normally induce fission. The capability of cryo-STET to image through the cytoplasmic volume to a depth of 1 µm facilitated visualization of intact mitochondria and their surroundings. We imaged changes in mitochondrial morphology and cristae structure, as well as contacts with the endoplasmic reticulum (ER), degradative organelles and the cytoskeleton at stalled fission sites. We found disruption of the outer mitochondrial membrane at contact sites with the ER and degradative organelles at sites of mitophagy. We identified fission sites where the inner mitochondrial membrane is already separated while the outer membrane is still continuous. Although MFF is a general fission factor, these observations demonstrate that mitochondrial fission can proceed to the final stage in its absence. The use of cryo-STET allays concerns about the loss of structures due to sample thinning required for tomography using cryo-transmission electron microscopy.

Increased expression of the small lysosomal gene SVIP in the Drosophila gut suppresses pathophysiological features associated with a high-fat diet

Lysosomes are digestive organelles that are crucial for nutrient sensing and metabolism. Lysosome impairment is linked to a broad spectrum of metabolic disorders, underscoring their importance to human health. Thus, lysosomes are an attractive target for metabolic disease therapies. In previous work, we discovered a novel class of tubular lysosomes that are morphologically and functionally distinct from traditionally described vesicular lysosomes. Tubular lysosomes are present in multiple tissues, are broadly conserved from invertebrates to mammals, are more proficient at degrading autophagic cargo than vesicular lysosomes, and delay signs of tissue aging when induced ectopically. Thus, triggering tubular lysosome formation presents one mechanism to increase lysosome activity and, notably, overproduction of the small lysosomal protein, SVIP, is a robust genetic strategy for triggering lysosomal tubulation on demand. In this study, we examine whether SVIP overexpression in the fly gut can suppress pathophysiological phenotypes associated with an obesogenic high-fat diet. Indeed, our results indicate that increasing SVIP expression in the fly gut reduces lipid accumulation, suppresses body mass increase, and improves survival in flies fed a high-fat diet. Collectively, these data hint that increasing lysosomal activity through induction of tubular lysosomal networks, could be one strategy to combat obesity-related pathologies.

Benzo-pyrrolidinyl substituted silicon phthalocyanines: A novel two-photon lysosomal nanoprobe for in vitro photodynamic therapy

Lysosomes are pivotal in diverse physiological phenomena, encompassing autophagy, apoptosis, and cellular senescence. The demand for precise tumors treatment has led to the development of specific lysosome-targeting probes capable of elucidating lysosomal dynamics and facilitating targeted cell death. In this research, we report the synthesis and characterization of a novel benzopyrrolidinyl-substituted silicon phthalocyanine (Py-SiPc), designed for selective lysosome labeling and Fluorescence imaging-guided in vitro photodynamic therapy. Furthermore, we encapsulated Py-SiPc within a biocompatible nanocarrier, dipalmitoylphosphatidylethanolamine-polyethylene glycol 2000 (DSPE), to create water-soluble nanoparticles (DSPE@Py-SiPc). These nanoparticles exhibit exceptional lysosome labeling capabilities, as evidenced by bioimaging techniques. Upon exposure to laser irradiation, DSPE@Py-SiPc efficiently induces the production of reactive oxygen species, impairing lysosomal function and triggering lysosomal-mediated cell death. The DSPE@Py-SiPc system emerges as a promising photosensitizer.

Human SVIP is sufficient to stimulate tubular lysosomes and extend healthspan in well-fed Caenorhabditis elegans

Small VCP Interacting Protein (SVIP) is essential for maintaining a unique form of tubular lysosomes (TLs) in Drosophila . Although Caenorhabditis elegans do not have an annotated SVIP ortholog, expression of Drosophila SVIP in the C. elegans intestine induces TLs constitutively, increases autophagic activity, and extends healthspan. Here, we find that expression of the human ortholog of SVIP in the C. elegans gut causes similar physiological and phenotypic effects as Drosophila SVIP , albeit some effects were less pronounced. These results demonstrate that human SVIP can induce functional TLs in C. elegans but may be a weaker allele compared to Drosophila SVIP .

Mechanisms of lysosomal tubulation and sorting driven by LRRK2

Lysosomes are dynamic cellular structures that adaptively remodel their membrane in response to stimuli, including membrane damage. Lysosomal dysfunction plays a central role in the pathobiology of Parkinson's disease (PD). Gain-of-function mutations in Leucine-rich repeat kinase 2 (LRRK2) cause familial PD and genetic variations in its locus increase the risk of developing the sporadic form of the disease. We previously uncovered a process we term LYTL (LYsosomal Tubulation/sorting driven by LRRK2), wherein membrane-damaged lysosomes generate tubules sorted into mobile vesicles. Subsequently, these vesicles interact with healthy lysosomes. LYTL is orchestrated by LRRK2 kinase activity, via the recruitment and phosphorylation of a subset of RAB GTPases. Here, we summarize the current understanding of LYTL and its regulation, as well as the unknown aspects of this process.

DAF-16/FOXO and HLH-30/TFEB comprise a cooperative regulatory axis controlling tubular lysosome induction in C. elegans

Although life expectancy has increased, longer lifespans do not always align with prolonged healthspans and, as a result, the occurrence of age-related degenerative diseases continues to increase. Thus, biomedical research has been shifting focus to strategies that enhance both lifespan and healthspan concurrently. Two major transcription factors that have been heavily studied in the context of aging and longevity are DAF-16/FOXO and HLH-30/TFEB; however, how these two factors coordinate to promote longevity is still not fully understood. In this study, we reveal a new facet of their cooperation that supports healthier aging in C. elegans. Namely, we demonstrate that the combinatorial effect of daf-16 and hlh-30 is required to trigger robust lysosomal tubulation, which contributes to systemic health benefits in late age by enhancing cross-tissue proteostasis mechanisms. Remarkably, this change in lysosomal morphology can be artificially induced via overexpression of SVIP, a previously characterized tubular lysosome stimulator, even when one of the key transcription factors, DAF-16, is absent. This adds to growing evidence that SVIP could be utilized to employ tubular lysosome activity in adverse conditions or disease states. Mechanistically, intestinal overexpression of SVIP leads to nuclear accumulation of HLH-30 in gut and non-gut tissues and triggers global gene expression changes that promotes systemic health benefits. Collectively, our work reveals a new cellular process that is under the control of DAF-16 and HLH-30 and provides further insight into how these two transcription factors may be exerting their pro-health effects.

ASO-mediated knockdown or kinase inhibition of G2019S-Lrrk2 modulates lysosomal tubule-associated antigen presentation in macrophages

Genetic variation around the LRRK2 gene affects risk for both familial and sporadic Parkinson's disease (PD). LRRK2 levels have become an appealing target for potential PD therapeutics with LRRK2 antisense oligonucleotides (ASOs) now moving toward clinical trials. However, LRRK2 has been suggested to play a fundamental role in peripheral immunity, and it is currently unknown if targeting increased LRRK2 levels in peripheral immune cells will be beneficial or deleterious. Here it was observed that G2019S macrophages exhibited increased stimulation-dependent lysosomal tubule formation (LTF) and MHC-II trafficking from the perinuclear lysosome to the plasma membrane in an mTOR-dependent manner with concomitant increases in pro-inflammatory cytokine release. Both ASO-mediated knockdown of mutant Lrrk2 and LRRK2 kinase inhibition ameliorated this phenotype and decreased these immune responses in control cells. Given the critical role of antigen presentation, lysosomal function, and cytokine release in macrophages, it is likely LRRK2-targeting therapies with systemic activity may have therapeutic value with regard to mutant LRRK2, but deleterious effects on the peripheral immune system, such as altered pathogen control in these cells, should be considered when reducing levels of non-mutant LRRK2.

SPIN-4/Spinster supports sperm activation in C. elegans via sphingosine-1-phosphate transport

Spermiogenesis, a sperm-activation step, is crucial for the transformation of immotile spermatids into motile sperm. Though membrane transport of ions and molecules across the sperm plasma membrane has been implicated in this process, the full repertoire of transporters involved, and their respective substrates, is unclear. Here, we report that the major facilitator superfamily transporter SPIN-4/Spinster governs efficient spermiogenesis and fertility in the hermaphrodite nematode Caenorhabditis elegans. Unlike other C. elegans Spinster paralogs, SPIN-4 is germline-expressed. Moreover, SPIN-4 expression is gamete-specific; it is strongly expressed in developing sperm, where it localizes to the plasma membrane, but it is absent from oocytes. Consistent with these expression data, we demonstrate that knocking out spin-4 impairs sperm development, leading to the formation of non-motile sperm that lack pseudopodia. Consequently, hermaphrodites homozygous for the spin-4(knu1099) knockout allele show extensive sperm wasting and reduced self-progeny. We observe similar defects when we genetically inhibit production of sphingosine-1-phosphate, a lipid molecule that stimulates cell motility when exported extracellularly by Spinster homologs in other contexts. Remarkably, extracellular supplementation with sphingosine-1-phosphate rescues sperm activation and motility in the absence of SPIN-4, suggesting that Spinster-dependent efflux of sphingosine-1-phosphate plays a key role in sperm mobilization. These findings identify a new signaling mechanism in C. elegans spermiogenesis entailing Spinster and sphingosine-1-phosphate.

Calcineurin-Dependent Homeostatic Response of C. elegans Muscle Cells upon Prolonged Activation of Acetylcholine Receptors

Pharmacological adaptation is a common phenomenon observed during prolonged drug exposure and often leads to drug resistance. Understanding the cellular events involved in adaptation could provide new strategies to circumvent this resistance issue. We used the nematode Caenorhabditis elegans to analyze the adaptation to levamisole, an ionotropic acetylcholine receptor agonist, used for decades to treat nematode parasitic infections. Genetic screens in C. elegans identified "adapting mutants" that initially paralyze upon exposure to levamisole as the wild type (WT), but recover locomotion after a few hours whereas WT remain paralyzed. Here, we show that levamisole induces a sustained increase in cytosolic calcium concentration in the muscle cells of adapting mutants, lasting several hours and preceding a decrease in levamisole-sensitive acetylcholine receptors (L-AChR) at the muscle plasma membrane. This decrease correlated with a drop in calcium concentration, a relaxation of the animal's body and a resumption of locomotion. The decrease in calcium and L-AChR content depends on calcineurin activation in muscle cells. We also showed that levamisole adaptation triggers homeostatic mechanisms in muscle cells including mitochondria remodeling, lysosomal tubulation and an increase in autophagic activity. Levamisole adaptation thus provides a new experimental paradigm for studying how cells cope with calcium stress.

Totally tubular: ASO-mediated knock-down of G2019S -Lrrk2 modulates lysosomal tubule-associated antigen presentation in macrophages

Genetic variation around the LRRK2 gene affects risk of both familial and sporadic Parkinson's disease (PD). LRRK2 levels have become an appealing target for potential PD-therapeutics with LRRK2 antisense oligonucleotides (ASOs) now in clinical trials. However, LRRK2 has been suggested to play a fundamental role in peripheral immunity, and it is currently unknown if targeting increased LRRK2 levels in peripheral immune cells will be beneficial or deleterious. Furthermore, the precise role of LRRK2 in immune cells is currently unknown, although it has been suggested that LRRK2-mediated lysosomal function may be crucial to immune responses. Here, it was observed that G2019S macrophages exhibited increased stimulation-dependent lysosomal tubule formation (LTF) and MHC-II trafficking from the perinuclear lysosome to the plasma membrane in an mTOR dependent manner with concomitant increases in pro-inflammatory cytokine release. Both ASO-mediated knock down of mutant Lrrk 2 and LRRK2 kinase inhibition ameliorated this phenotype and decreased these immune responses in control cells. Given the critical role of antigen presentation, lysosomal function, and cytokine release in macrophages, it is likely LRRK2-targetting therapies may have therapeutic value with regards to mutant LRRK2 but deleterious effects on the peripheral immune system, such as altered pathogen control and infection resolution.

Morpholinyl silicon phthalocyanine nanoparticles with lysosome cell death and two-photon imaging functions for in vitro photodynamic therapy of cancer cells

The lysosome is an important target for realizing antitumor therapy. Lysosomal cell death exerts significant therapeutic effects on apoptosis and drug-resistance. The development of lysosome-targeting nanoparticles to obtain efficient cancer treatment is challenging. In this article, nanoparticles composed of DSPE@M-SiPc and possessing bright two-photon fluorescence, lysosome targeting ability, and photodynamic therapy multifunctionalities are prepared by encapsulating morpholinyl-substituted silicon phthalocyanine (M-SiPc) with 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(poly(ethylene glycol))-2000] (DSPE). Two photon fluorescence bioimaging showed that M-SiPc and DSPE@M-SiPc mainly locate in lysosomes after cellular internalization. Upon irradiation, DSPE@M-SiPc effectively generates reactive oxygen species and damages the function of lysosome, subsequently leading to lysosomal cell death. DSPE@M-SiPc is a promising photosensitizer for cancer treatment.

The functional importance of VCP to maintaining cellular protein homeostasis

The AAA-ATPase (ATPases associated with diverse cellular activities) valosin-containing protein (VCP), is essential for many cellular pathways including but not limited to endoplasmic reticulum-associated degradation (ERAD), DNA damage responses, and cell cycle regulation. VCP primarily identifies ubiquitylated proteins in these pathways and mediates their unfolding and degradation by the 26S proteasome. This review summarizes recent research on VCP that has uncovered surprising new ways that this ATPase is regulated, new aspects of recognition of substrates and novel pathways and substrates that utilize its activity.