Lysosomes degrade macromolecules and recycle metabolites as well as being involved in diverse cellular processes such as plasma membrane repair, immune responses, nutrient sensing and signaling, and cell death. Impairment of lysosome function contributes to the pathogenesis of many diseases including lysosomal storage diseases, neurodegenerative disorders and cancer. However, systematic dissection of lysosome homeostasis, including biogenesis, dynamics and integrity maintenance, at an organismal level has not been achieved and the role of lysosomes in physiology and pathology remains poorly understood.
I. Lysosomes are highly dynamic in C. elegans
We observed a variety of dynamic changes of lysosomes that appear to associate with larval development, stress conditions and adult aging in C. elegans . These intriguing observations provide us unique opportunities to study lysosome dynamics and functions in a multicellular organism. We aim to identify signals/cellular processes that trigger/involve lysosomal changes, dissect underlying signaling pathways and reveal regulatory mechanisms and physiological significance. To do this, we have developed research tools to monitor and analyze lysosome dynamics and activity in live C. elegans and have identified more than 10 genes involved in controlling lysosome dynamics and functions. We are in the progress of analyzing their function.
II. Lysosomes maintain cell homeostasis essential for embryonic and larval development.
i) The lysosomal lysine/arginine transporter LAAT-1 maintains lysosome function and amino acid homeostasis for normal embryogenesis
We identified LAAT-1 (lysosomal amino acid transporter 1) from a genetic screen for regulators of apoptotic cell clearance. Loss of LAAT-1 causes accumulation of lysine and arginine in enlarged lysosomes that are defective in cargo degradation. We demonstrate that LAAT-1 (and its human counterpart, PQLC2) exports lysine and arginine from lysosomal lumen to maintain the availability of these two essential amino acids in the cytosol for normal embryogenesis. Our work also provides a molecular explanation for how cysteamine alleviates cystinosis, a lysosomal storage disease caused by dysfunction of lysosomal cystine transporter (Liu et al., Science 2012). The LAAT-1 study reveals the critical role of lysosomes in embryogenesis and addresses a long-standing issue regarding the therapeutic treatment of cystinosis. It proves that C. elegans can serve as an effective research model to reveal physiological regulation and function of lysosomes.
ii) RNST-2 degrades ribosomal RNA delivered by autophagy to maintain embryogenesis and larval development
Ribosome degradation through the autophagy-lysosome pathway is crucial for cell survival during nutrient starvation, but whether it occurs under normal growth conditions and contributes to animal physiology remains unaddressed. We identified RNST-2, a C. elegans T2 family endoribonuclease, as the key enzyme that degrades ribosomal RNA in lysosomes. We found that loss of rnst-2 causes accumulation of rRNA and ribosomal proteins in enlarged lysosomes and both phenotypes are suppressed by blocking autophagy, indicating that RNST-2 mediates autophagic degradation of ribosomal RNA in lysosomes. rnst-2(lf) mutants are defective in embryonic and larval development and are short-lived. Remarkably, simultaneous loss of RNST-2 and de novo synthesis of pyrimidine nucleotides leads to complete embryonic lethality, which is suppressed by supplements of uridine or cytidine. Our study reveals an essential role of autophagy-dependent degradation of ribosomal RNA in maintaining nucleotide homeostasis during animal development (Liu et al., eLife 2018).
The LAAT-1 and RNST-2 work reveals essential roles of lysosomes in animal development. Our ongoing projects aim to understand how lysosome homeostasis is regulated and contribute to C. elegans development.
III．Maintence of lysosomal membrane integrity
We have identified scav-3/LIMP-2's important function in maintaining lysosomal membrane integrity (Li et al., JCB 2016). scav-3 encodes a lysosomal membrane protein homologous to human LIMP-2. We found that SCAV-3 is a key regulator of lysosome integrity. Loss of SCAV-3 causes rupture of lysosome membranes and significantly shortens the lifespan of C. elegans. Both phenotypes were suppressed by reinforced expression of LMP-1 or LMP-2 (the C. elegans LAMPs), indicating that maintenance of lysosome integrity is an important factor in lifespan regulation. Remarkably, inhibition of insulin/IGF-1 signaling efficiently suppresses lysosome damage and extends the lifespan in scav-3(lf) animals, indicating that the insulin/IGF-1 pathway, which regulates dauer formation, stress resistance and longevity, is also involved in modulating lysosome membrane integrity. This work reveals that SCAV-3 is essential for preserving lysosome membrane integrity and that modulation of lysosome integrity by the insulin/IGF-1 signaling pathway affects longevity.
The lysosomal lumen contains more than 60 different soluble acid hydrolases that act in coordination to degrade all types of macromolecule including proteins, nucleic acids, carbohydrates, lipids and cellular debris. Thus, lysosomes have long been considered as “suicide bags”. The damage of lysosomal membrane causes leakage of the luminal content into the cytosol, leading to membrane trafficking defects, abnormal energy metabolism and even cell death. To further understand how lysosome membrane integrity is maintained and contribute to animal physiology, we developed cell biological tools to monitor lysosomal damage in live C. elegans and performed modified genetic screens to search for genes and cellular pathways that modulate lysosome membrane integrity. On the other hand, we are developing genetic and cell biological tools to investigate how worms respond to and resolve lysosomal damage.