Part:BBa_K2807014
eGFP-GLB1 WT
Usage and Biology
GLB1 encodes a human lysosomal acid β-galactosidase (GLB), an enzyme that is responsible for the cleavage of terminal β-linked galactose residues from glycoproteins, sphingolipids, keratan sulfate, and other glycoconjugates. The loss of GLB in physiological systems will result in autosomal recessive lysosomal storage diseases such as GM1 gangliosidosis and Morquio B disease (Suzuki et al., 2001).
This eGFP-GLB1 part is a GFP fusion protein linked to the human GLB1 gene. GLB1 encodes a human lysosomal acid beta galactosidase, an enzyme that is responsible for the cleavage of terminal β-linked galactose residues from glycoproteins, sphingolipids, keratan sulfate, and other glycoconjugates. The eGFP-GLB1 fusion protein was designed as an alternative base editing reporter for potential application in a disease model in the future. In our project, it served the purpose of detecting whether our base editing system, rAPOBEC1-XTEN-dPspCas13b along with a guide RNA, is able to make the specific base changes.
Over hundreds of single nucleotide mutations have been implicated in this disease. One of the challenges in building a disease model lies in manipulating GLB1 without affecting normal cellular function since it is an endogenous housekeeping gene. The GLB1-deficient fibroblasts are not easily available for experimental work. Moreover, while the E. coli β-galactosidase gene LacZ is widely used as a reporter in mammalian systems, this bacterial ortholog cannot be reliably used to create accurate representations of human GLB1 related diseases caused by mutations in the gene (Naderian et al., 2011). Therefore, we want to investigate if overexpression of exogenous GLB1 is possible in mammalian cell lines. Ideally, overexpressed GLB1 activity would be much higher than endogenous activity, leading to a high signal to noise ratio, without affecting the normal function of the cells. This would provide tools for researchers to study the effect of GLB1 mutation without knocking out endogenous GLB1 expression. We also generated T --> C mutants that are reported to be found in GM1 galactosidase patients (Hofer et al., 2010) to investigate if the loss of enzymatic function is due to enzyme expression and degradation, or enzyme inactivation. We hope this overexpression system can be used to test the ability of Cas13b-APOBEC which is designed to perform C-> U base editing to restore the function of GLB1. This would shed light on the potential of RNA editing to treat diseases such as those involving GLB1 mutations.
1.Construction of EGFP-GLB1 reporter plasmid
We amplified the flag epitope GLB1 from Genscript GLB1 cDNA ORF clone (NM_000404.3) using PCR, then cloned it into pSB1C3 (BBa_K2807014) and a mammalian expression vector, pEGFP-C1. Subsequently, we carried out point mutations to the wild-type GLB gene to generate GLB1 TCT mutant (BBa_K2807016) and GLB1 CCG mutant (BBa_K2807015) which are point mutations that are reported to reduce GLB1 enzymatic activity by more than 95% in patients suffered from GM1 gangliosidosis (Hofer et al., 2010). To allow normalization of transfection and expression efficiency of GLB1 in cells, we retained EGFP in C1 vector. To eliminate the possibility of the fusion protein to affect enzymatic activity, we separated EGFP and GLB1 by a stop codon and frameshift linker sequence. We also included a Kozak sequence in front of GLB1 to allow effective ribosome binding to translate GLB1.
2. Full-length precursor GLB1 and its mutant can be overexpressed in mammalian cells
To verify the expression of GLB1 WT and mutants, we expressed both plasmids in HEK293T cells and then tested for the protein expression levels by Western blot. Cell lysates were separated on an SDS-PAGE gel, and transferred onto a nitrocellulose membrane for blotting. The plasmids with GLB1 had a Flag epitope tag as well as an EGFP tag preceding the GLB1 coding sequence. We probed the membrane with anti-EGFP, anti-Flag and anti-Actin antibodies. From the Western blot in Figure 1, we can see that GLB1 protein is expressed as a visible band in both GLB WT and the GLB mutants at 84kDa, the expected size of GLB1 precursor, with the non-transfected lane as the negative control. In addition, a 27kDa band corresponding to EGFP was seen for both GLB WT and the GLB mutants, but not the non-transfected control, confirming our transfection efficiency. Therefore, we can conclude that our WT and mutant EGFP-GLB1 construct is able to express both EGFP and precursor GLB1 at the correct length. Moreover, we confirmed that the reported single nucleotide mutation in GLB1 does not change gene translation level or degradation rate. As such, any change in apparent enzymatic activity of the mutant GLB1 is likely to due to a change in the catalytic efficiency of the enzyme but not the amount of enzyme.
Figure 1: Western Blot analysis of cell lysates from transfected HEK293T cells expressing GLB1, GLB1 CCG mutant and GLB1 TCT mutant. Cell lysates were run on SDS-PAGE and probed using antibodies against EGFP, Flag epitope tag and Actin. Anti-actin antibodies were used to probe for the housekeeping gene actin in order to ensure equal protein loading across samples.
3. Precursor GLB1 is not enzymatically active
Next, we wanted to investigate if the precursor form of GLB1 is enzymatically active by itself. We transfected EGFP-GLB1 plasmid into HEK293T cells and carried out β-galactosidase functional assay using cell lysate. To optimise the method of cell lysis for obtaining the β-galactosidase protein, we tried three methods, namely Promega Passive Lysis Buffer, 1% Triton and freeze-Thaw method using dry ice. Upon running a Western blot with the lysates from the varying methods, we could see that the Promega passive lysis buffer produced the least number of non-specific bands as well as the correct band size for GLB1 at about 84kDa as seen in Figure 2. As such, this lysis buffer was used for subsequent enzymatic assays.
Figure 2: Western Blot analysis of cell lysates from transfected HEK293T cells expressing GLB1, GLB1 CCG mutant and GLB1 TCT mutant using Promega Lysis Buffer, 1% Triton and freeze-Thaw method. The cell lysates were run on SDS-PAGE and probed using antibodies against Flag epitope tag to verify the expression levels of GLB1 protein (~84kDa).
We carried out two β-galactosidase assays, using galactoside analog o-nitrophenylβ- D-galactopyranoside (ONPG) and 5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside (X-gal) as the substrate respectively. Functional β-galactosidase converts the colourless galactoside analog o-nitrophenylβ-D-galactopyranoside (ONPG) into galactose and chromophore o-nitrophenol, producing a bright yellow product. The final product can then be quantified using a spectrophotometer at 420 nm to determine the amount of substrate converted into the yellow chromophore. On the other hand, GLB1 cleaves X-gal to produce bright blue 5-bromo-4 -chloro-3-hydroxyindole and galactose as products. The appearance of the blue colour indicates that the sample contains a functional β-galactosidase enzyme, while a lack of blue colouration will be expected for the GLB1 mutants.
As shown in Figure 3, we incubated ONPG with cell lysates from pGEMT expressing DH5α with 6hr or 24hr IPTG induction (Tubes A and B) as positive control, HEK293T transfected with empty vector (Tube C), HEK293 cells transfected with GLB1 WT (Tubes D) and GLB1 TCT and GLB1 CCG mutants (Tubes E and F). The absorbance reading at 420nm is shown in Table 1. Tubes A and B (Figure 3), our positive controls, had significant β-galactosidase activity. However, for Tubes C to F (Figure 3), no colour change was observed. Taking into consideration that GLB1 is a lysosomal enzyme with optimal pH at 4, we repeated the experiment at pH=4.5 and there was no observable colour change. Similar results were observed in the dot activity assay using X-gal, where both non-transfected cells, wild-type and mutant GLB1 produced very faint blue colouration (Figure 4C-F) as compared to the positive control (Figure 4A-B).
The above results confirmed that precursor β-galactosidase is not functional, as post-translational modification via proteolytic cleavage at the C-terminal of the GLB1 precursor protein in the acidic environment of the lysosome is required for maturation of beta-galactosidase (Kreutzer et al., 2008). Therefore, in the future, we will engineer EGFP-GLB1 plasmid to include the cleavage signal to produce 64kDa and 22kDa proteolytic fragment that makes up the mature enzyme.
Figure 3: Representative image of reaction tubes containing ONPG with respective cell lysates. (A) IPTG induced DH5α cell lysate (6 hours incubation), (B) IPTG induced DH5α cell lysate (24 hours incubation), (C) GLB1 CCG mutant transfected HEK293T cell lysate, (D) GLB1 TCT mutant transfected HEK293T cell lysate, (E) GLB1 wild type transfected HEK293T cell lysate (6 hours incubation), (F) GLB1 wild type transfected HEK293T cell lysate (24 hours incubation).
| Sample Name | Abs1 | Abs2 |
|---|---|---|
| Non-transfected | 0.291 | 0.202 |
| EGFP-GLB1 WT | 0.200 | 0.202 |
| EGFP-GLB1 CCG | 0.130 | 0.128 |
| EGFP-GLB1-TCT | 0.135 | 0.132 |
| Arith. Mean | 0.103 | 0.086 |
| 6hr IPTG induction | 1.307 | 1.304 |
| 24hr IPTG induction | 1.259 | 1.252 |
This lack of significant enzymatic activity as seen in both experiments could be attributed to the lack of post-translational processing Hence, the GLB1 enzyme produced was enzymatically inactive.
Figure 4: Enzymatic activity on nitrocellulose membranes using dot-blot method upon incubation with X-Gal. (A) IPTG induced DH5α cell lysate (6 hours incubation), (B) IPTG induced DH5α cell lysate (24 hours incubation), (C) empty vector transfected HEK293T cell lysate (6 hours incubation), (D) GLB1 wild-type transfected HEK293T cell lysate (24 hours incubation), (E) GLB CCG mutant transfected HEK293T cell lysate, (F) GLB1 TCT mutant transfected HEK293T cell lysate.
References
Kreutzer, R., Kreutzer, M., Pröpsting, M. J., Sewell, A. C., Leeb, T., Naim, H. Y., & Baumgärtner, W. (2008). Insights into the post-translational processing of β-galactosidase in an animal model resembling late infantile human GM1-gangliosidosis. Journal of Cellular and Molecular Medicine, 12(5a), 1661–1671. http://doi.org/10.1111/j.1582-4934.2007.00204.x
Hofer, D., Paul, K., Fantur, K., Beck, M., Roubergue, A., Vellodi, A., ... & Paschke, E. (2010). Phenotype determining alleles in GM1 gangliosidosis patients bearing novel GLB1 mutations. Clinical genetics, 78(3), 236-246.
Naderian, H., Rezvani, Z., Atlasi, M. A., Nikzad, H., & Antoine, A. de V. (2011). Expression Cloning of Recombinant Escherichia coli lacZ Genes Encoding Cytoplasmic and Nuclear β-galactosidase Variants. Iranian Journal of Basic Medical Sciences, 14(4), 369–375.
Suzuki Y, Oshima A, Nanba E. 2001. ß-galactosidase deficiency (ß-galactosidosis) GM1-gangliosidosis and Morquio Bdisease. In: Scriver CR, Beaudet AL, Sly WS, Valle D, editors. The Metabolic and Molecular Bases of Inherited Disease. New York: McGraw-Hill Publishing Co. p 3775-3809.
Sequence and Features
- 10COMPATIBLE WITH RFC[10]
- 12INCOMPATIBLE WITH RFC[12]Illegal NheI site found at 16
- 21INCOMPATIBLE WITH RFC[21]Illegal BglII site found at 764
Illegal BglII site found at 797
Illegal BamHI site found at 828
Illegal BamHI site found at 2185
Illegal BamHI site found at 2521
Illegal XhoI site found at 768
Illegal XhoI site found at 801 - 23COMPATIBLE WITH RFC[23]
- 25INCOMPATIBLE WITH RFC[25]Illegal AgeI site found at 25
- 1000COMPATIBLE WITH RFC[1000]
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