Ongoing Projects

Based on a previous cell-based high-throughput screening (HTS) assay developed by our lab (Geng et al. 2011), a quantitative cell-based HTS assay for globoid-cell leukodystrophy (GLD), known mostly as Krabbe disease, we developed two different quantitative cell-based HTS assay have been developed b-galactocerebrosidase (GALC). GLD is rapidly progressive, invariably fatal LSDs manifested mostly in infancy (Wenger D et al. 2001). GLD is a classical neurological lysosomal storage disease (LSD) caused by the deficiency of galactocerebrosidase (GALC), a lysosomal hydrolase catalyzing the removal of galactose from two sphingolipids galactosylceramide and galactosylsphingosine, mostly known as psychosine. In affected patients with GLD, psychosine is found at high levels in both the central (CNS) and peripheral nervous system (PNS). Psychosine is present at low levels in myelin-forming cells in a physiological state. However, in GLD, psychosine becomes extremely cytotoxic at high concentrations, resulting in the loss of oligodendrocytes (CNS), Schwann cells (PNS), and subsequent diffuse demyelination and leukodystrophy.

Using the brain-derived cell line established in our lab from the cortices of the GLD murine model, the Twitcher (145M-Twi), we developed and implemented two throughput screening assays and identified two classes of potential therapeutic molecules for GLD (Fig.1a). First, we developed and implemented a live cell-based throughput LC-MS/MS assay for psychosine (Fig.1b). Through this high-throughput screening (HTS) assay, we identified molecules that reduce their levels to a nearly physiological range. Second, expressing a common human GALC misfolding mutation in the 145M-Twi cell line, we developed and implemented a cell-based HTS assay and identified molecules that assist in folding and prevent mutant GALC degradation, enhancing its residual activity (Fig.1c). Through validation assays, the small molecule ‘hits’ showed in vitro and in vivo effects on psychosine and GALC residual activity.

Here, we are examining whether psychosine-reducing and GALC-folding assistant molecules are therapeutic agents able to reduce psychosine levels resulting in the arrest and even prevention of the rapid demyelination in GLD. We developed two cell-based assays for characterizing compounds using differentiated neural cells from induced-neural stem cells (iNSC) established from four early-onset and four late-onset GLD patients' fibroblasts. The iNSC cells will be differentiated into oligodendrocytes to further study several synthesized analogs from two classes of small molecules. In collaboration with NIH/NCATS. We are currently performing SAR activities and analog synthesis, followed by in vitro screening of analogs in cell-based assays to test their efficacy in reducing cytotoxic GALC-substrates, such as psychosine (Fig.1d). In vitro ADME and in vivo PK/PD will also be assessed for top small molecule ‘hits.’ Subsequently, we will evaluate and prioritize top analogs based on neurobehavioral outcomes, morphometric myelin assessments, and BBB penetration using established GLD mouse models (Fig.1e).

Fig.1. Discovery and characterization of GALC folding-assistant and psychosine-reducing small molecule agents for globoid-cell leukodystrophy (GLD). (a) From newborn, brain cortices, brain-derived cell line (145M-Twi) from GLD mouse model (Twitcher) were established and shown to accumulate high-levels of psychosine (Ribbens et al 2014). (b) Using LC-MS/MS throughput screening, psychosine-reducing small molecules were identified. (c) In another high-throughput screening (HTS), using the 145M-Twi cell line expressing the human GALC, we identified several GALC-folding assisting molecules. Both HTS were quantitative and performed in multiple concentrations of the screened libraries (e.g. seven concentration or the psychosine-reducing library).

Funding: NIH/NINDS R61NS118407-02

2. Investigation of Pathogenic Cascades in LSDs as potential therapeutic targets

From a clinical perspective, patients suffering from LSDs present involvement of multiple organs and systems, predominantly affecting the central nervous system. From a cellular and molecular standpoint, the general lysosomal dysfunction may disturb several molecular pathways ultimately resulting in cell death. In LSDs, specific molecular mechanisms resulting in apoptosis have not been understood.

The accumulation of glycosphingolipids and glycosaminoglycans in LSDs can potentially induce LMP with the subsequent release of lysosomal proteolytic enzymes into the cytosol, demonstrated to activate several signaling pathways leading to apoptosis. Additionally, substantial evidence supports LMP as the primary event rather than occurring at later stages of apoptosis (Boya P et al. 2008). LMP involves the selective release of lysosomal hydrolases and proteases, particularly cathepsins, that translocate from the lysosomal lumen into the cytosol (Li W et al. 2004); (Fig.2a). These cathepsins remain partially active at neutral pH and function as apoptotic trigger elements by directly activating caspases and calpains. Cathepsins B (CB) and (CD) were shown to cleave BH-3-only proteins (Bid), which are B-cell lymphoma-2 (Bcl-2)-family proteins that are activated by proteolytic cleavage (Cirman T et al. 2004; fig.2a). The activated “truncated” Bid (tBid) promotes the assembly of Bak (Bcl-2 antagonist/killer-1) and Bax (Bcl-2 –associated X-protein) oligomers in the mitochondrial outer membrane, initiating its permeabilization. The mitochondrial outer membrane permeabilization (MOMP) results in cytochrome c release into the cytosol, which then activates caspases-3 and -7, effector caspases that orchestrate the dismantling of diverse cell structures (Boya et al. 2008). Although not currently recognized as a pathogenic cascade in LSD, LMP will be examined separately. It may potentially consist of a common cascade resulting in the activation of apoptotic events in LSDs. The examination of skin fibroblast lines from patients with various LSDs may offer a unique opportunity to evaluate any variation of LMP among these diseases. At low concentrations (0.5-1.5 mg/mL), AO, a lysosomotropic metachromatic fluorochrome, accumulated in acidic organelles, including lysosomes emitting red signal visualized by live-cell confocal fluorescence microscopy or measured by flow cytometry. AO was also shown to emit green fluorescence at low concentrations in the cytosol and nucleus. Thus, AO-treated cells manifest decreased red fluorescence and increased green fluorescence after LMP. I will also evaluate LMP using specific antibodies against CB and CD to demonstrate the relocalization of these cathepsins from lysosomes to the cytosol as described in preliminary results (Fig.2b).

Lysosomes are the endpoint for the autophagy pathway. Macroautophagy is a catabolic pathway responsible for the turnover of long-lived cytosolic proteins and organelles. Basically, autophagic cargo is sequestered by double-membrane vesicles (autophagosomes) and is ultimately degraded after fusion with lysosomes (Settembre C et al. 2008). In addition to its crucial role during cellular stress, autophagy may also function as a pathway that results in apoptosis caused by the impairment of autophagosome-lysosome fusion or by exacerbating an uncontrolled autophagy response. To evaluate autophagy, we measure the expression levels of two autophagic vesicular markers: LC3, both in its soluble (LC3-I) and phosphatidylethanolamine (PE)-conjugated form (LC3-II), and Beclin-1 (34). Confocal immunofluorescence assays with specific antibodies to LC3-II and Beclin, using the lysosomal-associate-membrane (LAMP1 and 2), as lysosomal labeling antibodies, are tools to assess the level of autophagy flux (Fig.2c). As mentioned above, we also perform immunoblots of lysosomal fractions from cultured skin fibroblast from the LSDs and controls. The LC3-II/LC3-I and Beclin are considered reliable makers to evaluate autophagy.

Fig.2. Pathogenic cascades were observed in cultured patients’ cells with neurological lysosomal diseases. (a) Sketch depicts the increased lysosomal membrane permeability (LMP) in lysosomal storage diseases (LSDs) and subsequent pathogenic cascades to activating the caspase-dependent apoptotic signaling pathway. (b) Lysosomal membrane stability assay. Using real-time confocal immunofluorescence, live skin fibroblasts from three patients with MPS II, IVA, and VI and control were treated with low concentrations of acridine orange, AO (0.5 ug/mL for 15 min). AO localizes in lysosomes at low concentrations, emitting red fluorescence at acidic pH. Under blue light (488 nm), AO is photo-oxidized, and loss of lysosomal integrity is visualized by the loss of red signal (lysosomes) and increase in the green signal (cytoplasm and nuclei). MPS II, IVA, and VI fibroblasts showed decreased lysosomal membrane stability (demonstrated by the rapid loss of red signal loss) when compared to the control cell line (upper panel) over 80 seconds of blue-light exposure time. An assay based on lysosomal membrane stability assay described in Brunk UT et al. 1997. (b) Using specific antibodies for LC3-II (red), a key protein in autophagy (text), a control (A, B), and an MPS-II patient cell line (C, D) are shown. A specific antibody against LAMP-1 is shown as a lysosomal marker (green). In comparison to control (A, B), the MPS-II fibroblasts showed increased expression of LC3-II, which is adjacent to the LAMP1 location (C). LC3-II location is more appreciable at higher magnification panels (control B; MPS-II D).

The current advances in LSD treatment are exemplified by enzyme replacement therapy (ERT), which consists of weekly or biweekly intravenous administrations of the recombinant enzymes deficient in the patients. Although generally well-tolerated, the intrathecally delivery of ERT showed limited CNS biodistribution in neuronopathic forms of mucopolysaccharidosis (MPS-III). Therefore, ERT agents are large molecules and are incapable of crossing the blood-brain barrier (BBB). The lack of reliable and efficacious delivery across the BBB severely limits the ERT and other therapies to treat neurological symptoms. Hence, the development of delivery strategies for therapeutic agents for LSDs is an unmet and urgent need and will have a substantial impact on affected patients with these and other neurodegenerative disorders.

The BBB is a homeostatic defense mechanism of the brain against pathogens and toxins (Anton N et al. 2007). The ideal drug candidates permeable through BBB are traditionally small, lipophilic, hydrophobic, and compact molecules. Whereas 98% of small-molecule drugs and every biological agent, including peptides, recombinant proteins, monoclonal antibodies, genes, and short interfering RNAs, cannot cross the BBB. Numerous CNS delivery strategies have been developed to overcome the often stated ‘problem-behind the problem’ of many neurological conditions.

Recent studies have shown that extracellular vehicles (EVs), or exosomes, multi-cargo nanosized vesicles, provide a physiological means to deliver proteins or miRNA across BBB and, even remotely regulating the integrity of BBB (Zhao Z et al. 2017; Yuan D et al. 2017). Exosomes are released by cells into the circulation and bodily fluids, displaying distinct cargo profiles dependable upon their cellular origin. Exosomes are extracellular nanovesicles of ~ 30-130 nm in diameter and originate from inward budding of the limiting membrane of multivesicular bodies, late endosomal compartments present in the cell (Revenfeld AL et al. 2014). To take advantage of these new developments, we hypothesize that EVs are therapeutic nanovesicles that allow penetration through the BBB to treat neuropathic forms of LSDs. Recently, we have established a murine neural cell line that secretes neural-derived exosomes containing specific lysosomal enzymes.

First, we are characterizing and further improving the exosomal production from an established murine neural cell line derived from a Twitcher (Twi) mouse model of a severe neurological LSD. We are optimizing the production and purification process of EVs from neural immortalized Twi murine cell line stably expressing human GALC (galactocerebrosidase) (Fig.3). Second, the therapeutic potential of neural-derived exosomes as a CNS-delivery system in the Twitcher, mouse model. We are examining the biodistribution and efficacy of neural-derived exosomes in the neurological course using Twi mice (Fig.4). Significant CNS penetration and neuoronal uptake was observed in the Twi mice receiving intravenously (IV) solutions containing 0.5-2 x 10^6 particles (Fig.6). As mediators of intercellular communication and surrogates of intracellular changes in pathological states, exosomes become a reliable source to deliver therapies to LSDs. This proof-of-principle study will reveal exosomes as a powerful and robust CNS- ‘nanovesicle-delivery’ strategy of therapeutic applications to treat neurodegenerative disorders.

Fig.4. Extracellular vehicles (EVs) purification and characterization. (a) A representation of EVs isolation methods examined: ultracentrifugation (UDC) and size-exclusion chromatography (SEC). In both methods, conditional media of brain-derived 145M-Twi-hGALC was harvested. Chromatography UV profile and showing resolution of EVs fractions and individual NTA analysis. (A) Entire sample chromatography UV absorbance profile. (B) The EVs chromatography peak dissected by NTA fraction-by-fraction. (b) Using trans-electron microscopy (TEM), the EVs purified by UDC and SEC methods were evaluated for ultrastructural morphology. (c) Comparable EVs sizes and (d) NTA profile were noted from UDC and SEC isolation methods. Increased EVs yield was noted from SEC method. (e) Immunoblot of 4 different fractions revealed the presence of GALC, FLOT1, and CD63 with their associated kDa ladder. (f) GALC enzymatic activity measurements of different fractions showed that native human GALC is active in the lysates form different fractions of the EVs SEC isolation. (g) Timeline of the live cell-uptake of cultured brain-derived exosomes. Each individual signal on 10X magnification was capture and counted and divided by 1,000 cells counted in a 96-well plate surface (0.32 cm2).

Fig.5. Biodistribution Assessments of the GALC-containing extracellular vesicles. (a) A series of mice underwent IV injection of Dir-labeled EVs and underwent in vivo fluorescence studies (IVIS) showing increased CNS penetration in the Twitcher murine model of Krabbe disease. (b) Post-mortem 24hs organ immunofluorescence showed analysis confirmed the organ uptake pattern.

Fig.6. Brain sections from mouse treated with Dir-labeled exosomes from 145M-Twi-hGALC cell line. (a) Magnification of 10X with DAPI showing significant uptake of Dir-labeled exosomes. Increased magnifications of 20X (b) and 60X (c).

Funding: R21NS113649-01