Radiosynthesis and Early Evaluation of a Positron Emission Tomography Imaging Probe [18F]AGAL Targeting Alpha-Galactosidase A Enzyme for Fabry Disease.

Success of gene therapy relies on the durable expression and activity of transgene in target tissues. In vivo molecular imaging approaches using positron emission tomography (PET) can non-invasively measure magnitude, location, and durability of transgene expression via direct transgene or indirect reporter gene imaging in target tissues, providing the most proximal PK/PD biomarker for gene therapy trials. Herein, we report the radiosynthesis of a novel PET tracer [18F]AGAL, targeting alpha galactosidase A (α-GAL), a lysosomal enzyme deficient in Fabry disease, and evaluation of its selectivity, specificity, and pharmacokinetic properties in vitro. [18F]AGAL was synthesized via a Cu-catalyzed click reaction between fluorinated pentyne and an aziridine-based galactopyranose precursor with a high yield of 110 mCi, high radiochemical purity of >97% and molar activity of 6 Ci/µmol. The fluorinated AGAL probe showed high α-GAL affinity with IC50 of 30 nM, high pharmacological selectivity (≥50% inhibition on >160 proteins), and suitable pharmacokinetic properties (moderate to low clearance and stability in plasma across species). In vivo [18F]AGAL PET imaging in mice showed high uptake in peripheral organs with rapid renal clearance. These promising results encourage further development of this PET tracer for in vivo imaging of α-GAL expression in target tissues affected by Fabry disease.

Development of a selective activity-based probe for glycosylated LIPA.

Loss of LIPA activity leads to diseases such as Wolman's Disease and Cholesterol Ester Storage Disease. While it is possible to measure defects in LIPA protein levels, it is difficult to directly measure LIPA activity in cells. In order to measure LIPA activity directly we developed a LIPA specific activity based probe. LIPA is heavily glycosylated although it is unclear how glycosylation affects LIPA activity or function. Our probe is specific for a glycosylated form of LIPA in cells, although it labels purified LIPA regardless of glycosylation.

Expression and functional characterization of human lysosomal acid lipase gene (LIPA) mutation responsible for cholesteryl ester storage disease (CESD) phenotype.

Lysosomal acid lipase (LAL) is a serine hydrolase which hydrolyzes cholesteryl ester and triglycerides delivered to the lysosomes into free cholesterol and free fatty acids. Mutations in the LAL gene (LIPA) result in accumulation of triglycerides and cholesterol esters in various tissues of the body, leading to pathological conditions such as Wolman's disease (WD) and cholesteryl ester storage disease (CESD). CESD patients homozygous for His295Tyr (H295Y) mutation have less than 5% of normal LAL activity. To shed light on the molecular basis for this loss-of-function phenotype, we have generated the recombinant H295Y enzyme and studied its biophysical and biochemical properties. No significant differences were observed in the expression levels or glycosylation patterns between the mutant and the wild type LAL. However, the H295Y mutant displayed only residual enzymatic activity (<5%) compared to the wild type. While wild type LAL is mostly a monomer at pH 5.0, the vast majority H295Y exists as a high molecular soluble aggregate. Besides, the H295Y mutant has a 20°C lower melting temperature compared to the wild type. Transient expression studies in WD fibroblasts showed that mutation of His295 to other amino acids resulted in a significant loss of enzymatic activity. A homology model of LAL revealed that His295 is located on an α-helix of the cap domain and could be important for tethering it to its core domain. The observed loss-of-function phenotype in CESD patients might arise from a combination of protein destabilization and the shift to a non-functional soluble aggregate.

Cardiac manifestations of Fabry disease in G3Stg/GlaKO and GlaKO mouse models-Translation to Fabry disease patients.

Fabry disease (FD) is an X-linked disorder of glycosphingolipid metabolism caused by mutations in the GLA gene encoding alpha-galactosidase A (α-Gal). Loss of α-Gal activity leads to progressive lysosomal accumulation of α-Gal substrate, predominately globotriaosylceramide (Gb3) and its deacylated derivative globotriaosylsphingosine (lyso-Gb3). FD manifestations include early onset neuropathic pain, gastrointestinal symptoms, and later onset life-threatening renal, cardiovascular and cerebrovascular disorders. Current treatments can preserve kidney function but are not very effective in preventing progression of cardiovascular pathology which remains the most common cause of premature death in FD patients. There is a significant need for a translational model that could be used for testing cardiac efficacy of new drugs. Two mouse models of FD have been developed. The α-Gal A-knockout (GlaKO) model is characterized by progressive tissue accumulation of Gb3 and lyso-Gb3 but does not develop any Fabry pathology besides mild peripheral neuropathy. Reports of minor cardiac function abnormalities in GlaKO model are inconsistent between different studies. Recently, G3Stg/GlaKO was generated by crossbreeding GlaKO with transgenic mice expressing human Gb3 synthase. G3Stg/GlaKO demonstrate higher tissue substrate accumulation and develop cellular and tissue pathologies. Functional renal pathology analogous to that found in early stages of FD has also been described in this model. The objective of this study is to characterize cardiac phenotype in GlaKO and G3Stg/GlaKO mice using echocardiography. Longitudinal assessments of cardiac wall thickness, mass and function were performed in GlaKO and wild-type (WT) littermate controls from 5-13 months of age. G3Stg/GlaKO and WT mice were assessed between 27-28 weeks of age due to their shortened lifespan. Several cardiomyopathy characteristics of early Fabry pathology were found in GlaKO mice, including mild cardiomegaly [up-to-25% increase in left ventricular (LV mass)] with no significant LV wall thickening. The LV internal diameter was significantly wider (up-to-24% increase at 9-months), when compared to the age-matched WT. In addition, there were significant increases in the end-systolic, end-diastolic volumes and stroke volume, suggesting volume overload. Significant reduction in Global longitudinal strain (GLS) measuring local myofiber contractility of the LV was also detected at 13-months. Similar GLS reduction was also reported in FD patients. Parameters such as ejection fraction, fractional shortening and cardiac output were either only slightly affected or were not different from controls. On the other hand, some of the cardiac findings in G3Stg/GlaKO mice were inconsistent with Fabry cardiomyopathy seen in FD patients. This could be potentially an artifact of the Gb3 synthase overexpression under a strong ubiquitous promoter. In conclusion, GlaKO mouse model presents mild cardiomegaly, mild cardiac dysfunction, but significant cardiac volume overload and functional changes in GLS that can be used as translational biomarkers to determine cardiac efficacy of novel treatment modalities. The level of tissue Gb3 accumulation in G3Stg/GlaKO mouse more closely recapitulates the level of substrate accumulation in FD patients and may provide better translatability of the efficacy of new therapeutics in clearing pathological substrates from cardiac tissues. But interpretation of the effect of treatment on cardiac structure and function in this model should be approached with caution.

Silencing of enzymes involved in ceramide biosynthesis causes distinct global alterations of lipid homeostasis and gene expression.

Dysregulation of ceramide synthesis has been associated with metabolic disorders such as atherosclerosis and diabetes. We examined the changes in lipid homeostasis and gene expression in Huh7 hepatocytes when the synthesis of ceramide is perturbed by knocking down serine pal mitoyltransferase subunits 1, 2, and 3 (SPTLC123) or dihydroceramide desaturase 1 (DEGS1). Although knocking down all SPTLC subunits is necessary to reduce total ceramides significantly, depleting DEGS1 is sufficient to produce a similar outcome. Lipidomic analysis of distribution and speciation of multiple lipid classes indicates an increase in phospholipids in SPTLC123-silenced cells, whereas DEGS1 depletion leads to the accumulation of sphingolipid intermediates, free fatty acids, and diacylglycerol. When cer amide synthesis is disrupted, the transcriptional profiles indicate inhibition in biosynthetic processes, downregulation of genes involved in general endomembrane trafficking, and upregulation of endocytosis and endosomal recycling. SPTLC123 silencing strongly affects the expression of genes involved with lipid metabolism. Changes in amino acid, sugar, and nucleotide metabolism, as well as vesicle trafficking between organelles, are more prominent in DEGS1-silenced cells. These studies are the first to provide a direct and comprehensive understanding at the lipidomic and transcriptomic levels of how Huh7 hepatocytes respond to changes in the inhibition of ceramide synthesis.

A Small-Molecule Anti-secretagogue of PCSK9 Targets the 80S Ribosome to Inhibit PCSK9 Protein Translation.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a secreted protein that downregulates low-density lipoprotein (LDL) receptor (LDL-R) levels on the surface of hepatocytes, resulting in decreased clearance of LDL-cholesterol (LDL-C). Phenotypic screening of a small-molecule compound collection was used to identify an inhibitor of PCSK9 secretion, (R)-N-(isoquinolin-1-yl)-3-(4-methoxyphenyl)-N-(piperidin-3-yl)propanamide (R-IMPP), which was shown to stimulate uptake of LDL-C in hepatoma cells by increasing LDL-R levels, without altering levels of secreted transferrin. Systematic investigation of the mode of action revealed that R-IMPP did not decrease PCSK9 transcription or increase PCSK9 degradation, but instead caused transcript-dependent inhibition of PCSK9 translation. In support of this surprising mechanism of action, we found that R-IMPP was able to selectively bind to human, but not E. coli, ribosomes. This study opens a new avenue for the development of drugs that modulate the activity of target proteins by mechanisms involving inhibition of eukaryotic translation.