“…The specific importance of SapA for mediating GALC activity in vivo is highlighted by two key observations: clinical data has identified that a mutation in SapA causes Krabbe disease despite normal GALC activity 17 , and a transgenic mouse with defective SapA develops a phenotype resembling late-onset Krabbe disease 18 . In support of this specificity, mutations that result in defective SapC cause Gaucher disease, not Krabbe disease 19 – 21 . However, in vitro studies of saposin-mediated degradation of galactosphingolipids have shown that both SapA and SapC can stimulate GALC activity 32 , 33 .…”
Section: Discussionmentioning
confidence: 95%
“…Importantly, mutation of the GALC-associated saposin SapA can also cause Krabbe disease 17 , 18 . Similarly, Gaucher disease is caused by loss of glucocerebrosidase activity and also by loss of SapC, highlighting the critical role of saposins in sphingolipid processing 19 – 21 . Fig.…”
Sphingolipids are essential components of cellular membranes and defects in their synthesis or degradation cause severe human diseases. The efficient degradation of sphingolipids in the lysosome requires lipid-binding saposin proteins and hydrolytic enzymes. The glycosphingolipid galactocerebroside is the primary lipid component of the myelin sheath and is degraded by the hydrolase β-galactocerebrosidase (GALC). This enzyme requires the saposin SapA for lipid processing and defects in either of these proteins causes a severe neurodegenerative disorder, Krabbe disease. Here we present the structure of a glycosphingolipid-processing complex, revealing how SapA and GALC form a heterotetramer with an open channel connecting the enzyme active site to the SapA hydrophobic cavity. This structure defines how a soluble hydrolase can cleave the polar glycosyl headgroups of these essential lipids from their hydrophobic ceramide tails. Furthermore, the molecular details of this interaction provide an illustration for how specificity of saposin binding to hydrolases is encoded.
“…The specific importance of SapA for mediating GALC activity in vivo is highlighted by two key observations: clinical data has identified that a mutation in SapA causes Krabbe disease despite normal GALC activity 17 , and a transgenic mouse with defective SapA develops a phenotype resembling late-onset Krabbe disease 18 . In support of this specificity, mutations that result in defective SapC cause Gaucher disease, not Krabbe disease 19 – 21 . However, in vitro studies of saposin-mediated degradation of galactosphingolipids have shown that both SapA and SapC can stimulate GALC activity 32 , 33 .…”
Section: Discussionmentioning
confidence: 95%
“…Importantly, mutation of the GALC-associated saposin SapA can also cause Krabbe disease 17 , 18 . Similarly, Gaucher disease is caused by loss of glucocerebrosidase activity and also by loss of SapC, highlighting the critical role of saposins in sphingolipid processing 19 – 21 . Fig.…”
Sphingolipids are essential components of cellular membranes and defects in their synthesis or degradation cause severe human diseases. The efficient degradation of sphingolipids in the lysosome requires lipid-binding saposin proteins and hydrolytic enzymes. The glycosphingolipid galactocerebroside is the primary lipid component of the myelin sheath and is degraded by the hydrolase β-galactocerebrosidase (GALC). This enzyme requires the saposin SapA for lipid processing and defects in either of these proteins causes a severe neurodegenerative disorder, Krabbe disease. Here we present the structure of a glycosphingolipid-processing complex, revealing how SapA and GALC form a heterotetramer with an open channel connecting the enzyme active site to the SapA hydrophobic cavity. This structure defines how a soluble hydrolase can cleave the polar glycosyl headgroups of these essential lipids from their hydrophobic ceramide tails. Furthermore, the molecular details of this interaction provide an illustration for how specificity of saposin binding to hydrolases is encoded.
“…For example, in vitro assays have identified that SapC and SapA are both able to enhance the activity of β-galactosylceramidase (GALC) to promote degradation of the galactosphingolipids 60,61 . However, this does not reflect the specificity of saposins in whole organisms as loss of SapA causes Krabbe disease, similar to that of loss of GALC, while mutations in SapC do not 20,21,[23][24][25] . Why these in vitro assays do not recapitulate the specificity of saposins in a whole organism remains unclear but may suggest a non-specific detergentlike effect of saposins when present at relatively high concentrations.…”
Section: Discussionmentioning
confidence: 97%
“…The saposins can exist in a "closed" monomeric, globular conformation or, at low pH, adopt a more "open" conformation, forming higher-order oligomers enclosing a hydrophobic cavity into which lipid acyl chains can be buried [14][15][16][17][18][19] . Each saposin functions in conjunction with specific hydrolases to facilitate the degradation of different glycosphingolipids and the loss of saposin function phenotypically resembles the loss of the associated hydrolases [20][21][22][23][24][25] . There are two proposed mechanisms for how saposins assist in lipid presentation to hydrolases: the "solubiliser" model, whereby saposins encapsulate lipids within a hydrophobic oligomeric complex for presentation to soluble enzymes, and the "liftase" model, where saposins bind directly to membranes destabilising them allowing membrane-associated hydrolases access to lipid substrates.…”
Lipid antigens are presented on the surface of cells by the CD1 family of glycoproteins, which have structural and functional similarity to MHC class I molecules. The hydrophobic lipid antigens are embedded in membranes and inaccessible to the lumenal lipid-binding domain of CD1 molecules. Therefore, CD1 molecules require lipid transfer proteins for lipid loading and editing. CD1d is loaded with lipids in late endocytic compartments, and lipid transfer proteins of the saposin family have been shown to play a crucial role in this process. However, the mechanism by which saposins facilitate lipid binding to CD1 molecules is not known and is thought to involve transient interactions between protein components to ensure CD1-lipid complexes can be efficiently trafficked to the plasma membrane for antigen presentation. Of the four saposin proteins, the importance of Saposin B (SapB) for loading of CD1d is the most well-characterised. However, a direct interaction between CD1d andSapB has yet to be described. In order to determine how SapB might load lipids onto CD1d, we used purified, recombinant CD1d and SapB and carried out a series of highly sensitive binding assays to monitor direct interactions. Using equilibrium binding analysis, chemical cross-linking and cocrystallisation experiments, under a range of different conditions, we could not demonstrate a direct interaction. This work establishes comprehensively that the role of SapB in lipid loading does not involve direct binding to CD1d. We discuss the implication of this for our understanding of lipid loading of CD1d and propose several factors that may influence this process.
“…12 A patient heterozygous for PSAP c.1133C > G (p. Pro378Arg) and deletion of exon 2 to 7 presented with hepatosplenomegaly, thrombocytopenia, and anemia. 13 Herein, we present the first reported case from India of an acute neuronopathic Gaucher-like disorder caused by homozygosity for a novel mutation in the PSAP gene.…”
This is the first reported case of prosaposin (PSAP) mutation from India manifesting as an acute neuronal Gaucher disease-like condition. A 2-month-old male baby presented with encephalopathy, resistant tonic–clonic seizures, moderate hepatosplenomegaly, hypotonia, and cherry red spot in the retinae. The child had anemia, thrombocytopenia, elevated chitotriosidase, and normal activity of acid sphingomyelinase and low normal activity of β-glucosidase 1 (β-glucocerebrosidase 1, GBA). The child succumbed in the fourth month of life due to persistent respiratory distress and refractory seizures. The clinical phenotype, cherry red spots, elevated chitotriosidase, and lysosomal assays led to the suspicion of Gaucher disease. Exome sequencing revealed a homozygous stop codon mutation in the PSAP gene (c.G1228T, p.Glu410ter). Prenatal diagnosis in the next pregnancy revealed a carrier fetus, who was unaffected postnatally. The diagnosis of specific activator deficiency such as saposin C and saposin D deficiency (in the current study) should be considered and tested for when Gaucher disease is suspected in an infant with partially deficient or near normal GBA activity.
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