Prosaposin maintains adult neural stem cells in a state associated with deep quiescence.

In most vertebrates, adult neural stem cells (NSCs) continuously give rise to neurons in discrete brain regions. A critical process for maintaining NSC pools over long periods of time in the adult brain is NSC quiescence, a reversible and tightly regulated state of cell-cycle arrest. Recently, lysosomes were identified to regulate the NSC quiescence-proliferation balance. However, it remains controversial whether lysosomal activity promotes NSC proliferation or quiescence, and a finer influence of lysosomal activity on NSC quiescence duration or depth remains unexplored. Using RNA sequencing and pharmacological manipulations, we show that lysosomes are necessary for NSC quiescence maintenance. In addition, we reveal that expression of psap, encoding the lysosomal regulator Prosaposin, is enriched in quiescent NSCs (qNSCs) that reside upstream in the NSC lineage and display a deep/long quiescence phase in the adult zebrafish telencephalon. We show that shRNA-mediated psap knockdown increases the proportion of activated NSCs (aNSCs) as well as NSCs that reside in shallower quiescence states (signed by ascl1a and deltaA expression). Collectively, our results identify the lysosomal protein Psap as a (direct or indirect) quiescence regulator and unfold the interplay between lysosomal function and NSC quiescence heterogeneities.

Meta-analysis of the association of prosaposin polymorphisms rs4747203 and rs885828 with risk of Parkinson's disease.

BACKGROUND: Previous research has established a connection between polymorphisms rs4747203 and rs885828 in the prosaposin (PSAP) gene and an increased risk of Parkinson's disease (PD). However, other studies have found no significant difference in risk compared to the general population. METHODS: To evaluate the current evidence linking rs4747203 and rs885828 to PD risk, we conducted a comprehensive search of PubMed, the Web of Science, Embase, and the Cochrane Library for relevant studies up until May 2023. In addition, we analyzed data from the publicly available "PD Variant Browser". We performed a meta-analysis using Stata 17.0 to synthesize the findings from the selected studies. RESULTS: Our meta-analysis, which included data from six published studies and the public database, revealed no significant association between PD risk and either rs4747203 [OR (95% CI) = 0.99 (0.93-1.05), I2 = 90.3%, P = 0.635] or rs885828 [OR (95% CI) = 1.01 (0.95-1.07), I2 = 90.7%, P = 0.773]. These results remained consistent when examining subgroups of individuals within or outside of Asia. CONCLUSION: The available evidence does not support an association between the genotype at rs4747203 or rs885828 and the risk of PD.

Prosaposin variants in sporadic, familial, and early-onset Parkinson's disease: a Taiwanese case-control study and meta-analysis.

Polymorphisms in the PSAP gene, which encodes prosaposin and is involved in the lysosomal function, yielded conflicting results regarding the association with Parkinson's disease (PD). Therefore, this study aims to investigate the role of PSAP in familial PD (FPD), early onset PD (EOPD) with age at onset before 50 years old, and sporadic PD (SPD) among Taiwanese population, and summarize relevant studies via meta-analysis. By sequencing exon 1 to 14 in 183 FPD and 219 EOPD, two novel exonic variants were found in EOPD, including p.A146E (c.437C > A) on exon 5 and p.Y248C (c.743A > G) on exon 7. Furthermore, four previously reported intronic variants (rs142614739/rs74733861), rs749823, rs4747203 and rs885828) in intron 11 and 12 were analyzed in 485 SPD and 712 in-hospital controls, in addition to the aforementioned FPD and EOPD groups. The adjusted odd ratios (ORs) by age and sex, only rs142614739 was significantly associated with higher risk of EOPD (OR = 1.85, 95% CI = 1.33-2.58). The risk effect was further confirmed by the meta-analysis of the association between rs142614739 and the risk of PD in both common effect (OR = 1.29, 95% CI = 1.11-1.50) and random effect (OR = 1.29, 95% CI = 1.11-1.50). Our findings suggest that the PSAP rs142614739 variant is associated with the risk of EOPD. Further functional studies are warranted to elucidate the biochemical mechanisms.

Prosaposin maintains lipid homeostasis in dopamine neurons and counteracts experimental parkinsonism in rodents.

Prosaposin (PSAP) modulates glycosphingolipid metabolism and variants have been linked to Parkinson's disease (PD). Here, we find altered PSAP levels in the plasma, CSF and post-mortem brain of PD patients. Altered plasma and CSF PSAP levels correlate with PD-related motor impairments. Dopaminergic PSAP-deficient (cPSAPDAT) mice display hypolocomotion and depression/anxiety-like symptoms with mildly impaired dopaminergic neurotransmission, while serotonergic PSAP-deficient (cPSAPSERT) mice behave normally. Spatial lipidomics revealed an accumulation of highly unsaturated and shortened lipids and reduction of sphingolipids throughout the brains of cPSAPDAT mice. The overexpression of α-synuclein via AAV lead to more severe dopaminergic degeneration and higher p-Ser129 α-synuclein levels in cPSAPDAT mice compared to WT mice. Overexpression of PSAP via AAV and encapsulated cell biodelivery protected against 6-OHDA and α-synuclein toxicity in wild-type rodents. Thus, these findings suggest PSAP may maintain dopaminergic lipid homeostasis, which is dysregulated in PD, and counteract experimental parkinsonism.

Genetic ablation of Saposin-D in Krabbe disease eliminates psychosine accumulation but does not significantly improve demyelination.

Krabbe disease is an inherited demyelinating disease caused by a genetic deficiency of the lysosomal enzyme galactosylceramide (GalCer) ß-galactosidase (GALC). The Twitcher (Twi) mouse is a naturally occurring, genetically and enzymatically authentic mouse model that mimics infantile-onset Krabbe disease. The major substrate for GALC is the myelin lipid GalCer. However, the pathogenesis of Krabbe disease has long been explained by the accumulation of psychosine, a lyso-derivative of GalCer. Two metabolic pathways have been proposed for the accumulation of psychosine: a synthetic pathway in which galactose is transferred to sphingosine and a degradation pathway in which GalCer is deacylated by acid ceramidase (ACDase). Saposin-D (Sap-D) is essential for the degradation of ceramide by ACDase in lysosome. In this study, we generated Twi mice with a Sap-D deficiency (Twi/Sap-D KO), which are genetically deficient in both GALC and Sap-D and found that very little psychosine accumulated in the CNS or PNS of the mouse. As expected, demyelination with the infiltration of multinucleated macrophages (globoid cells) characteristic of Krabbe disease was milder in Twi/Sap-D KO mice than in Twi mice both in the CNS and PNS during the early disease stage. However, at the later disease stage, qualitatively and quantitatively comparable demyelination occurred in Twi/Sap-D KO mice, particularly in the PNS, and the lifespans of Twi/Sap-D KO mice were even shorter than that of Twi mice. Bone marrow-derived macrophages from both Twi and Twi/Sap-D KO mice produced significant amounts of TNF-α upon exposure to GalCer and were transformed into globoid cells. These results indicate that psychosine in Krabbe disease is mainly produced via the deacylation of GalCer by ACDase. The demyelination observed in Twi/Sap-D KO mice may be mediated by a psychosine-independent, Sap-D-dependent mechanism. GalCer-induced activation of Sap-D-deficient macrophages/microglia may play an important role in the neuroinflammation and demyelination in Twi/Sap-D KO mice.

A zebrafish model of combined saposin deficiency identifies acid sphingomyelinase as a potential therapeutic target.

Sphingolipidoses are a subcategory of lysosomal storage diseases (LSDs) caused by mutations in enzymes of the sphingolipid catabolic pathway. Like many LSDs, neurological involvement in sphingolipidoses leads to early mortality with limited treatment options. Given the role of myelin loss as a major contributor toward LSD-associated neurodegeneration, we investigated the pathways contributing to demyelination in a CRISPR-Cas9-generated zebrafish model of combined saposin (psap) deficiency. psap knockout (KO) zebrafish recapitulated major LSD pathologies, including reduced lifespan, reduced lipid storage, impaired locomotion and severe myelin loss; loss of myelin basic protein a (mbpa) mRNA was progressive, with no changes in additional markers of oligodendrocyte differentiation. Brain transcriptomics revealed dysregulated mTORC1 signaling and elevated neuroinflammation, where increased proinflammatory cytokine expression preceded and mTORC1 signaling changes followed mbpa loss. We examined pharmacological and genetic rescue strategies via water tank administration of the multiple sclerosis drug monomethylfumarate (MMF), and crossing the psap KO line into an acid sphingomyelinase (smpd1) deficiency model. smpd1 mutagenesis, but not MMF treatment, prolonged lifespan in psap KO zebrafish, highlighting the modulation of acid sphingomyelinase activity as a potential path toward sphingolipidosis treatment.

Prosaposin PS18 reduces dopaminergic neurodegeneration in a 6-hydroxydopamine rat model of Parkinson's disease.

Saposin and its precursor prosaposin are endogenous proteins with neurotrophic and anti-apoptotic properties. Prosaposin or its analog prosaposin-derived 18-mer peptide (PS18) reduced neuronal damage in hippocampus and apoptosis in stroke brain. Its role in Parkinson's disease (PD) has not been well characterized. This study aimed to examine the physiological role of PS18 in 6-hydroxydopamine (6-OHDA) cellular and animal models of PD. We found that PS18 significantly antagonized 6-OHDA -mediated dopaminergic neuronal loss and TUNEL in rat primary dopaminergic neuronal culture. In SH-SY5Y cells overexpressing the secreted ER calcium-monitoring proteins, we found that PS18 significantly reduced thapsigargin and 6-OHDA-mediated ER stress. The expression of prosaposin and the protective effect of PS18 were next examined in hemiparkinsonian rats. 6-OHDA was unilaterally administered to striatum. The expression of prosaposin was transiently upregulated in striatum on D3 (day 3) after lesioning and returned below the basal level on D29. The 6-OHDA-lesioned rats developed bradykinesia and an increase in methamphetamine-mediated rotation, which was antagonized by PS18. Brain tissues were collected for Western blot, immunohistochemistry, and qRTPCR analysis. Tyrosine hydroxylase immunoreactivity was significantly reduced while the expressions of PERK, ATF6, CHOP, and BiP were upregulated in the lesioned nigra; these responses were significantly antagonized by PS18. Taken together, our data support that PS18 is neuroprotective in cellular and animal models of PD. The mechanisms of protection may involve anti-ER stress.

Expression patterns of prosaposin and its receptors, G protein-coupled receptor (GPR) 37 and GPR37L1, in the mouse olfactory organ.

Prosaposin is a glycoprotein conserved widely in vertebrates, because it is a precursor for saposins that are required for normal lysosomal function and thus for autophagy, and acts as a neurotrophic factor. Most tetrapods possess two kinds of olfactory neuroepithelia, namely, the olfactory epithelium (OE) and the vomeronasal epithelium (VNE). This study examined the expression patterns of prosaposin and its candidate receptors, G protein-coupled receptor (GPR) 37 and GPR37L1, in mouse OE and VNE by immunofluorescence and in situ hybridization. Prosaposin immunoreactivity was observed in the olfactory receptor neurons, vomeronasal receptor neurons, Bowman's gland (BG), and Jacobson's gland (JG). Prosaposin expression was mainly observed in mature neurons. Prosaposin mRNA expression was observed not only in these cells but also in the apical region of the VNE. GPR37 and GPR37L1 immunoreactivities were found only in the BG and/or the JG. Prosaposin was suggested to secrete and facilitate the autophagic activities of the neurons and modulate the mucus secretion in mouse olfactory organ.

Expression patterns of prosaposin and its receptors, G protein-coupled receptor (GPR) 37 and GPR37L1 mRNAs, in the chick inner ear.

Prosaposin is a glycoprotein that is widely conserved in vertebrates. It serves as a precursor for saposins A, B, C, and D, which are necessary activators of lysosomal sphingolipid hydrolases. It can also act as a neurotrophic factor. Prosaposin plays a crucial role in the mammalian vestibuloauditory system because it prevents progressive deafness and severe vestibular dysfunction. Prosaposin can exhibit a neurotrophic effect through the G protein-coupled receptor (GPR), and GPR37 and GPR37L1 are its candidate receptors. In this study, we examined the expression patterns of prosaposin, GPR37, and GPR37L1 mRNAs in postnatal day 0 chick vestibuloauditory organs by in situ hybridization. Prosaposin mRNA expression was observed in all vestibular end organs, the vestibular and spiral ganglions, whereas no hybridization signal was observed in the auditory organ, namely basilar papilla. While GPR37L1 mRNA expression was observed in the oligodendrocytes/Schwann cells in the vestibular ganglion, GPR37 mRNA expression was observed in the crista ampullaris base region. These findings suggest that prosaposin expression in the auditory hair cells is acquired uniquely in mammals partly due to the loss of regeneration upon maturation and improved autophagic activity in mammalian auditory hair cells. In addition, as GPR37L1 expression in the chick glial cells differed from GPR37 expression in mammalian glial cells, the roles of GPR37 and GPR37L1 for prosaposin may differ between birds and mammals.

A saposin domain-containing protein of tongue sole Cynoglossus semilaevis: Antimicrobial activity and mechanism.

Prosaposin is a precursor that can be processed into four different saposins, designated as A, B, C, and D, which have multiple functions in mammals, including neuroprotection and immune modulation. The immune function of saposin in teleost remains largely unknown. In the present study, a saposin (SAP) domain-containing protein was identified in half-smooth tongue sole Cynoglossus semilaevis and named CsSDP. CsSDP harbors one SAP A domain and two SAP B domains. When expressed in HEK293T cells, CsSDP was specifically localized in the lysosome. When overexpressed in Escherichia coli, CsSDP markedly inhibited bacterial growth, and the inhibitory effect depended on two specific regions in the SAP A and SAP B domains. Two polypeptides (P32 and P30) derived from the above SAP A and B domains could bind to and inhibit the growth of both Gram-negative and Gram-positive bacteria. The ultrastructural analysis revealed that P32 and P30 killed target bacteria by disrupting the bacterial cell wall and inducing substantial release of cytoplasmic contents. These results shed new lights on the immune function of saposin domain-containing protein in teleost.

Cab45 deficiency leads to the mistargeting of progranulin and prosaposin and aberrant lysosomal positioning.

The trans-Golgi Network (TGN) sorts molecular "addresses" and sends newly synthesized proteins to their destination via vesicular transport carriers. Despite the functional significance of packaging processes at the TGN, the sorting of soluble proteins remains poorly understood. Recent research has shown that the Golgi resident protein Cab45 is a significant regulator of secretory cargo sorting at the TGN. Cab45 oligomerizes upon transient Ca2+ influx, recruits soluble cargo molecules (clients), and packs them in sphingomyelin-rich transport carriers. However, the identity of client molecules packed into Cab45 vesicles is scarce. Therefore, we used a precise and highly efficient secretome analysis technology called hiSPECs. Intriguingly, we observed that Cab45 deficient cells manifest hypersecretion of lysosomal hydrolases. Specifically, Cab45 deficient cells secrete the unprocessed precursors of prosaposin (PSAP) and progranulin (PGRN). In addition, lysosomes in these cells show an aberrant perinuclear accumulation suggesting a new role of Cab45 in lysosomal positioning. This work uncovers a yet unknown function of Cab45 in regulating lysosomal function.

Saposin C, Key Regulator in the Alpha-Synuclein Degradation Mediated by Lysosome.

Lysosomal dysfunction has been proposed as one of the most important pathogenic molecular mechanisms in Parkinson disease (PD). The most significant evidence lies in the GBA gene, which encodes for the lysosomal enzyme ß-glucocerebrosidase (ß-GCase), considered the main genetic risk factor for sporadic PD. The loss of ß-GCase activity results in the formation of α-synuclein deposits. The present study was aimed to determine the activity of the main lysosomal enzymes and the cofactors Prosaposin (PSAP) and Saposin C in PD and healthy controls, and their contribution to α-synuclein (α-Syn) aggregation. 42 PD patients and 37 age-matched healthy controls were included in the study. We first analyzed the ß-GCase, ß-galactosidase (ß-gal), ß-hexosaminidase (Hex B) and Cathepsin D (CatD) activities in white blood cells. We also measured the GBA, ß-GAL, ß-HEX, CTSD, PSAP, Saposin C and α-Syn protein levels by Western-blot. We found a 20% reduced ß-GCase and ß-gal activities in PD patients compared to controls. PSAP and Saposin C protein levels were significantly lower in PD patients and correlated with increased levels of α-synuclein. CatD, in contrast, showed significantly increased activity and protein levels in PD patients compared to controls. Increased CTSD protein levels in PD patients correlated, intriguingly, with a higher concentration of α-Syn. Our findings suggest that lysosomal dysfunction in sporadic PD is due, at least in part, to an alteration in Saposin C derived from reduced PSAP levels. That would lead to a significant decrease in the ß-GCase activity, resulting in the accumulation of α-syn. The accumulation of monohexosylceramides might act in favor of CTSD activation and, therefore, increase its enzymatic activity. The evaluation of lysosomal activity in the peripheral blood of patients is expected to be a promising approach to investigate pathological mechanisms and novel therapies aimed to restore the lysosomal function in sporadic PD.

Decreased Prosaposin and Progranulin in the Cingulate Cortex Are Associated with Schizophrenia Pathophysiology.

Prosaposin (PSAP) and progranulin (PGRN) are two lysosomal proteins that interact and modulate the metabolism of lipids, particularly sphingolipids. Alterations in sphingolipid metabolism have been found in schizophrenia. Genetic associations of PSAP and PGRN with schizophrenia have been reported. To further clarify the role of PSAP and PGRN in schizophrenia, we examined PSAP and PGRN levels in postmortem cingulate cortex tissue from healthy controls along with patients who had suffered from schizophrenia, bipolar disorder, or major depressive disorder. We found that PSAP and PGRN levels are reduced specifically in schizophrenia patients. To understand the role of PSAP in the cingulate cortex, we used an AAV strategy to knock down PSAP in neurons located in this region. Neuronal PSAP knockdown led to the downregulation of neuronal PGRN levels and behavioral abnormalities. Cingulate-PSAP-deficient mice exhibited increased anxiety-like behavior and impaired prepulse inhibition, as well as intact locomotion, working memory, and a depression-like state. The behavioral changes were accompanied by increased early growth response protein 1 (EGR-1) and activity-dependent cytoskeleton-associated protein (ARC) levels in the sensorimotor cortex and hippocampus, regions implicated in circuitry dysfunction in schizophrenia. In conclusion, PSAP and PGRN downregulation in the cingulate cortex is associated with schizophrenia pathophysiology.

Prosaposin Reduces α-Synuclein in Cells and Saposin C Dislodges it from Glucosylceramide-enriched Lipid Membranes.

Parkinson's disease (PD) is the second most common progressive neurodegenerative disorder affecting over 1% of the 65 + age population. Saposin C, a lysosomal protein required for the normal activity of glucocerebrosidase (GCase), may serve as a disease modifier in PD. Saposin C is cleaved from its precursor, Prosaposin (PSAP), which is secreted as an uncleaved protein and exerts neuroprotective effects. In this study, we aim to elucidate the neuroprotective roles of PSAP and saposin C in PD by evaluating their effects on α-synuclein accumulation in human neuroblastoma cells. Stable overexpression of PSAP reduced monomeric α-synuclein levels in SH-SY5Y cells, while PSAP knockdown by small interfering RNA led to the opposite effect, and those effects were independent of GCase activity. Autophagy flux was decreased by stable PSAP overexpression. Furthermore, a flow-through assay revealed that recombinant saposin C was able to detach α-synuclein from artificial glucosylceramide-enriched lipid membranes at the lysosomal pH. Taken together, our findings provide further evidence that PSAP and saposin C as key proteins involved in α-synuclein clearance by dislodging it from lipid membranes.

Prosaposin, tumor-secreted protein, promotes pancreatic cancer progression by decreasing tumor-infiltrating lymphocytes.

Glycoproteins produced by tumor cells are involved in cancer progression, metastasis, and the immune response, and serve as possible therapeutic targets. Considering the dismal outcomes of pancreatic ductal adenocarcinoma (PDAC) due to its unique tumor microenvironment, which is characterized by low antitumor T-cell infiltration, we hypothesized that tumor-derived glycoproteins may serve as regulating the tumor microenvironment. We used glycoproteomics with tandem mass tag labeling to investigate the culture media of three human PDAC cell lines, and attempted to identify the key secreted proteins from PDAC cells. Among the identified glycoproteins, prosaposin (PSAP) was investigated for its functional contribution to PDAC progression. PSAP is highly expressed in various PDAC cell lines; however, knockdown of intrinsic PSAP expression did not affect the proliferation and migration capacities. Based on the immunohistochemistry of resected human PDAC tissues, high PSAP expression was associated with poor prognosis in patients with PDAC. Notably, tumors with high PSAP expression showed significantly lower CD8+ T-cell infiltration than those with low PSAP expression. Furthermore, PSAP stimulation decreased the proportion of CD8+ T cells in peripheral blood monocytes. Finally, in an orthotopic transplantation model, the number of CD8+ T cells in the PSAP shRNA groups was significantly increased, resulting in a decreased tumor volume compared with that in the control shRNA group. PSAP suppresses CD8+ T-cell infiltration, leading to the promotion of PDAC progression. However, further studies are warranted to determine whether this study contributes to the development of a novel immunomodulating therapy for PDAC.

Phenotype Expansion for Atypical Gaucher Disease Due to Homozygous Missense PSAP Variant in a Large Consanguineous Pakistani Family.

Atypical Gaucher disease is caused by variants in the PSAP gene. Saposin C is one of four homologous proteins derived from sequential cleavage of the saposin precursor protein, prosaposin. It is an essential activator for glucocerebrosidase, which is deficient in Gaucher disease. Although atypical Gaucher disease due to deficiency of saposin C is rare, it exhibits vast phenotypic heterogeneity. Here, we report on a Pakistani family that exhibits features of Gaucher disease, i.e., prelingual profound sensorineural hearing impairment, vestibular dysfunction, hepatosplenomegaly, kyphosis, and thrombocytopenia. The family was investigated using exome and Sanger sequencing. A homozygous missense variant c.1076A>C: p.(Glu359Ala) in exon 10 of the PSAP gene was observed in all affected family members. In conclusion, we identified a new likely pathogenic missense variant in PSAP in a large consanguineous Pakistani family with atypical Gaucher disease. Gaucher disease due to a deficiency of saposin C has not been previously reported within the Pakistani population. Genetic screening of patients with the aforementioned phenotypes could ensure adequate follow-up and the prevention of further complications. Our finding expands the genetic and phenotypic spectrum of atypical Gaucher disease due to a saposin C deficiency.

A Type 3 Gaucher-Like Disease Due To Saposin C Deficiency in Two Emirati Families Caused by a Novel Splice Site Variant in the PSAP Gene.

Gaucher disease is caused by glucocerebroside accumulation in different tissues due to beta-glucocerebrosidase enzyme deficiency. Genetic defects in proteins involved in beta-glucocerebrosidase processing and activation may indirectly lead to Gaucher-like phenotypes in affected individuals. Saposin C, derived from the prosaposin precursor, is a crucial activator for beta-glucocerebrosidase, and its deficiency has been linked to Gaucher-like phenotypes in several clinical reports. Here, we report two Emirati families with Gaucher-like disorder due to Saposin C deficiency. Affected patients from both families carry the homozygous state of the novel c.1005 + 1G > A splice site (first to be reported) variant in the PSAP gene. Molecular analysis showed that the underlying variant is predicted to result in the retention of intron 9-10 and the formation of a premature stop codon leading to the complete loss of Saposin C. Clinical examination of the affected patients showed a wide heterogeneity in the patients' age of onset and symptoms ranging from Gaucher-like type 3 phenotype with severe refractory myoclonic epilepsy to Gaucher-like type 1 phenotype with growth retardation and hepatosplenomegaly. Collectively, the available clinical and molecular data confirms the pathogenicity of the reported PSAP splice site variant. The reported clinical cases expand the genetic and clinical spectrum of Saposin C deficiency.

Lysosomal ceramides regulate cathepsin B-mediated processing of saposin C and glucocerebrosidase activity.

Variants in multiple lysosomal enzymes increase Parkinson's disease (PD) risk, including the genes encoding glucocerebrosidase (GCase), acid sphingomyelinase (ASMase) and galactosylceramidase. Each of these enzymes generates ceramide by hydrolysis of sphingolipids in lysosomes, but the role of this common pathway in PD pathogenesis has not yet been explored. Variations in GBA1, the gene encoding GCase, are the most common genetic risk factor for PD. The lysosomal enzyme cathepsin B has recently been implicated as an important genetic modifier of disease penetrance in individuals harboring GBA1 variants, suggesting a mechanistic link between these enzymes. Here, we found that ceramide activates cathepsin B, and identified a novel role for cathepsin B in mediating prosaposin cleavage to form saposin C, the lysosomal coactivator of GCase. Interestingly, this pathway was disrupted in Parkin-linked PD models, and upon treatment with inhibitor of ASMase which resulted in decreased ceramide production. Conversely, increasing ceramide production by inhibiting acid ceramidase activity was sufficient to upregulate cathepsin B- and saposin C-mediated activation of GCase. These results highlight a mechanistic link between ceramide and cathepsin B in regulating GCase activity and suggest that targeting lysosomal ceramide or cathepsin B represents an important therapeutic strategy for activating GCase in PD and related disorders.

Rare PSAP Variants and Possible Interaction with GBA in REM Sleep Behavior Disorder.

BACKGROUND: PSAP encodes saposin C, the co-activator of glucocerebrosidase, encoded by GBA. GBA mutations are associated with idiopathic/isolated REM sleep behavior disorder (iRBD), a prodromal stage of synucleinopathy. OBJECTIVE: To examine the role of PSAP mutations in iRBD. METHODS: We fully sequenced PSAP and performed Optimized Sequence Kernel Association Test in 1,113 iRBD patients and 2,324 controls. We identified loss-of-function (LoF) mutations, which are very rare in PSAP, in three iRBD patients and none in controls (uncorrected p = 0.018). RESULTS: Two variants were stop mutations, p.Gln260Ter and p.Glu166Ter, and one was an in-frame deletion, p.332_333del. All three mutations have a deleterious effect on saposin C, based on in silico analysis. In addition, the two carriers of p.Glu166Ter and p.332_333del mutations also carried a GBA variant, p.Arg349Ter and p.Glu326Lys, respectively. The co-occurrence of these extremely rare PSAP LoF mutations in two (0.2%) GBA variant carriers in the iRBD cohort, is unlikely to occur by chance (estimated co-occurrence in the general population based on gnomAD data is 0.00035%). Although none of the three iRBD patients with PSAP LoF mutations have phenoconverted to an overt synucleinopathy at their last follow-up, all manifested initial signs suggestive of motor dysfunction, two were diagnosed with mild cognitive impairment and all showed prodromal clinical markers other than RBD. Their probability of prodromal PD, according to the Movement Disorder Society research criteria, was 98% or more. CONCLUSION: These results suggest a possible role of PSAP variants in iRBD and potential genetic interaction with GBA, which requires additional studies.

Loss of Lysosomal Proteins Progranulin and Prosaposin Associated with Increased Neurofibrillary Tangle Development in Alzheimer Disease.

Alzheimer disease (AD) is a progressive neurodegenerative disease causing cognitive decline in the aging population. To develop disease-modifying treatments, understanding the mechanisms behind the pathology is important, which should include observations using human brain samples. We reported previously on the association of lysosomal proteins progranulin (PGRN) and prosaposin (PSAP) with amyloid plaques in non-demented aged control and AD brains. In this study, we investigated the possible involvement of PGRN and PSAP in tangle formation using human brain tissue sections of non-demented aged control subjects and AD cases and compared with cases of frontotemporal dementia with granulin (GRN) mutations. The study revealed that decreased amounts of PGRN and PSAP proteins were detected even in immature neurofibrillary tangles, while colocalization was still evident in adjacent neurons in all cases. Results suggest that neuronal loss of PGRN preceded loss of PSAP as tangles developed and matured. The GRN mutation cases exhibited almost complete absence of PGRN in most neurons, while PSAP signal was preserved. Although based on correlative data, we suggest that reduced levels of PGRN and PSAP and their interaction in neurons might predispose to accumulation of p-Tau protein.