[Retracted] Therapeutical Significance of Serpina3n Subsequent Cerebral Ischemia via Cytotoxic Granzyme B Inactivation

Aslam, Mehwish Saba; Aslam, Mobeena Saba; Aslam, Komal Saba; Iqbal, Asia; Yuan, Liudi

doi:https://doi.org/10.1155/2022/1557010

BioMed Research International

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Abstract Introduction Materials and Methods Results Discussion Data Availability Conflicts of Interest Authors’ Contributions Acknowledgments References Copyright Related Articles

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Volume 2022 | Article ID 1557010 | https://doi.org/10.1155/2022/1557010

[Retracted] Therapeutical Significance of Serpina3n Subsequent Cerebral Ischemia via Cytotoxic Granzyme B Inactivation

Mehwish Saba Aslam,¹Mobeena Saba Aslam,²Komal Saba Aslam,³Asia Iqbal,⁴and Liudi Yuan^1,5

Academic Editor: Cai Mei Zheng

Received25 Feb 2022

Accepted16 Apr 2022

Published29 May 2022

Abstract

Ischemic stroke is a devastating CNS insult with few clinical cures. Poor understanding of underlying mechanistic network is the primary limitation to develop novel curative therapies. Extracellular accumulation of granzyme B subsequent ischemia promotes neurodegeneration. Inhibition of granzyme B can be one of the potent strategies to mitigate neuronal damage. In present study, we investigated the effect of murine Serpina3n and human (homolog) SERPINA3 against cerebral ischemia through granzyme B inactivation. Recombinant Serpina3n/SERPINA3 were expressed by transfected 293 T cells, and eluted proteins were examined for postischemic influence both in vitro and in vivo. During in vitro test, Serpina3n was found effective enough to inhibit granzyme B, while SERPINA3 was ineffectual to counter cytotoxic protease. Treatment of hypoxic culture with recombinant Serpina3n/SERPINA3 significantly increased cell viability in dosage-dependent manner, recorded maximum at the highest concentration (4 mM). Infarct volume analysis confirmed that 50 mg/kg dosage of exogenous Serpina3n was adequate to reduce disease severity, while SERPINA3 lacked behind in analeptic effect. Immunohistochemical test, western blot analysis, and protease activity assay’s results illustrated successful diffusion of applied protein to the ischemic lesion and reactivity with the target protease. Taken together, our findings demonstrate therapeutic potential of Serpina3n by interfering granzyme B-mediated neuronal death subsequent cerebral ischemia.

1. Introduction

Ischemic stroke is a pathological state accounting 80,0000 victims/year across the globe [1] with limited effective therapeutical approaches for functional restrain [2]. Regardless of constant efforts to develop alternative drugs, tissue plasminogen activator (tPA) is the only available FDA-approved clinical treatment against ischemia [3]. Major obstacles in finding novel effective therapeutics have been intricate responses of surveillance mechanisms and their complexed interactivities. This multifaceted pathophysiology involves stimulation of numerous immune responses that leads to neuroinflammation with heterogenous forms of neuronal death [4]. One of the factors with detrimental effect is secretion of proteases subsequent ischemia [5, 6]. Among proteases, granzyme B is a prime proapoptotic acid hydrolase released by cytotoxic T-lymphocytes at the lesion site that initiates neuronal demyelination by activating caspases and other apoptotic progenitor protein molecules [7–11]. Inhibition of granzyme B can be one of the possible approaches to reduce clinical severity and therefore serves research requisite.

Serpina3n is a ~44-55 kDa murine serine protease inhibitor which shares 61% homology with human ortholog SERPINA3 [12] and exhibits wide range of target proteases [13, 14]. Specie-specific Serpina3n/SERPINA3 performs important physiological roles during both quiescent and pathological states by inhibiting attributed proteases [15]. Reported functions of Serpina3n/SERPINA3 during various pathologies especially during CNS calamities have made them emerging aspirations for research studies [16, 17]. Upregulated expression of Serpina3n (by reactive astrocytes [18] or neurons [19]) during CNS (central nervous system) insults reduces neuronal damage by protease inhibition [17] and has been proposed as a biomarker of astrogliosis [18], while SERPINA3 still needs validation for such scenario. Serpina3n has a special affinity for granzyme B and is believed to be the only known extracellular inhibitor of granzyme B till date, while its analog SERPINA3 is unable to counter granzyme B [20]. Owing to the inhibitory and anti-inflammatory properties of Serpina3n during CNS pathologies and taking SERPINA3 as a counterpart, in the current study, we attempted to investigate the following:(i)Potential therapeutical function of Serpina3n subsequent ischemic stroke via granzyme B inactivation(ii)Postischemic effect of SERPINA3(iii)Brief comparison between therapeutical efficacy of Serpina3n and SERPINA3 against ischemia(iv)Delivery of the exogenous protein to the lesion site(v)Immunoreactivity of the applied proteins

2. Materials and Methods

2.1. Materials and Reagents

Lentivirus plasmids, empty backbone of pLenti CMV Blast DEST (plasmid #17451) addgene, pLenti CMV Puro DEST (plasmid #17452) addgene, psPAX2, pMD2.G, 293 T cells, N2A cells, XL 1 E. Coli, DMEM (Gibco), FBS (BioExcel), Blastidine (Invivogen company), puromycin (Invivogen company), prestained marker (Yeaseen), streptomycin/penicillin solution (Gibco), anti-rabbit granzyme B (Abcam, cat. No. ab53097), anti-goat GFAP (Abcam, cat. No. ab53554), anti-hamster Serpina3n (Merck cat. No. MABC1182), anti-chicken MAP-2 (Abcam cat. No. ab5392), Hoechst 33258 (Sigma-Aldrich cat. No. 14530), anti-mouse 6his-tag (Proteintech, cat. No. 66005-1-Ig), anti-rabbit C-Myc (Proteintech, cat. No. 10828-1-AP), glucose oxidase (Sigma, cat. No. G7141), and catalase (Sangon, cat. No. A001847). Cell culture plates (Nest Biotechnology Co., Ltd.). Human granzyme B gene sequences were stored in our laboratory (EC:3.4.21.79).

2.2. Gene Amplification and Construction of Vector

Transfer vectors were constructed by inserting gene of interest using pLenti CMV Blast plasmid backbone and packaged as previously described [21, 22]. Briefly, Serpina3n (Accession no: NM_009252.2) and SERPINA3 (Accession no: NM_001085.5) genes were amplified with listed primers under following conditions (Table 1).

Lentiviral transfer vectors carrying genes of interest, i.e., SERPINA3 and Serpina3n, with reporter peptide sequences (c-Myc, 6-histidine, and mluc2) driven by CMV promotor were constructed and multiplied by ampicillin resistant XL1 E. Coli (Table 2). Successful plasmid construction was analyzed and confirmed by double enzyme digestion and direct DNA sequencing.

2.3. Transfection of 293 T Cells

Lentiviral preparations were used to get stable expression of recombinant proteins by human embryonic kidney derived 293 T cells [23]. Briefly, 6-well plate was seeded with 293 T cells in glucose-rich DMEM+10% FBS supplemented with penicillin and streptomycin followed by incubation at 37°C with 5% CO₂ for 18 hrs or till 60-70% confluency. Lentiviral transfection mix was prepared using PEI transfection reagent (1 mg/ml), transfection buffer (15 mM NaCl), transformed vector, and packaging system psPAX2 and pMD2.G in 1 : 2 : 1, respectively, and incubated at room temperature for 30 mins. The suspension was mixed by pipetting and gently added dropwise to 293 T cells and plate was returned to incubator. 48 hrs posttransfection, 70% confluence was confirmed through green-polarized microscopy, and cell media were replaced with fresh complete growth media supplemented with Blastidine (selection marker 1 : 1000) followed by incubation at 37°C and 5% CO₂.

2.4. Selection and Propagation of Recombinant Clone

To get consistent expression of recombinant proteins and to rule out any ambiguity in future experiments, single stable clone for each recombinant protein, i.e., Serpina3n/SERPINA3, was selected. After 4 days, transfected cells were trypsinized (0.05% trypsin-EDTA) and diluted in Blastidine-supplemented growth media to inoculate ~50cells/well in 96-well plate. Plates were incubated at 37°C with 5% CO₂ till the survived cells colonized large enough. Transformants were stored in liquid nitrogen as master stocks.

2.5. Protein Purification

For large-scale protein expression, selected clones of Serpina3n and SERPINA3 were propagated by inoculating about cells in complete growth media plus Blastidine (1 : 1000) per 100 mm plate at 37°C and 5% CO₂. Collected supernatants were dialyzed through cellulose dialysis tube at 4°C with two exchanges of 1XPBS after 8-10 hrs. Later, expressed proteins were purified in two steps.(i)Purification: 400 ml of dialyzed supernatants was centrifuged at 12000 rpm, 4°C for 15 mins to remove traces of cellular debris, and loaded onto preequilibrated Ni-NTA column followed by washing with 250 ml of 10 mM imidazole and eluted with 25 ml of 250 mM imidazole(ii)Enrichment: eluted part was ultrafiltered by centrifugation at 14000 rpm, 4°C for 30mins with exchange of imidazole to 1XPBS. Obtained protein samples were analyzed through western blots probed with 6-histidine, Serpina3n, and c-Myc primary antibodies

3. In Vitro Studies

3.1. Cell Culture and Maintenance of Cell Line

Mouse neuroblastoma cells (N2A) were defrosted, cultured, and maintained using DMEM supplemented with 10% FBS, 1% penicillin/streptomycin solution at 37°C with 5% CO₂, and 95% air. Upon confluency, the cells were trypsinized and re-plated.

3.2. In Vitro Ischemic Model

Enzymatic ischemia/reperfusion was introduced to the cultured cells as described earlier [24]. Briefly, N2A cells were seeded in 96-well plate at about 10⁴ cells/100 μl/well in normal growth media and incubated till 70-80% confluency. Hypoxic media was prepared by diluting stock solutions of glucose oxidase (50 mM sodium acetate buffer, ) and catalase (50 mM Tris-HCL buffer, ) in glucose free DMEM at a constant ratio of 10 : 1 supplemented with (5 mM) 2-deoxyglucose to get uniform 2% oxygen concentration at cell surface. Upon confluency, hypoxia was introduced to test groups by incubating cells with hypoxic media for 4 hrs at 37°C and 5% CO₂, while in negative control, media were replaced with fresh media (4 replicates/group). Reperfusion was introduced for 3 hrs by replacing hypoxic media with normal growth media supplemented with different concentrations of recombinant Serpina3n and SERPINA3 proteins, i.e., 1 mM, 2 mM, 3 mM, and 4 mM, while positive control was supplied with complete growth media only. Cell viability was measured by adding CCK8 10 μl/well, and absorbance was measured at 450 nm by microplate reader.

3.3. Generation of Granzyme B Transformants

Lentiviral transfection method was used to get stable expression of human granzyme B by 293 T cells [22, 23]. Briefly, 6-well plate was inoculated with cells in glucose-rich media and placed in incubator for 18 hrs. Lentiviral transfection mix containing PEI (1 mg/ml), transfection buffer (15 mM NaCl), transformed vector (pLenti CMV puro DEST), and packaging system psPAX2 and pMD2.G were mixed in 1 : 2 : 1 proportion and left at room temperature for half an hour. The suspension was thoroughly mixed and used to transfect 293 T cells. After 48 hrs, transfection efficiency was confirmed by RFP (red fluorescent protein), and media were replaced with fresh complete media supplemented with puromycin (selection marker 1 : 5000) to screen transformed cells. The expression of the screened clone was confirmed by immunoblot probed with reporter 6-histidine and granzyme B primary antibodies. Transformants were stored in liquid nitrogen as master stocks.

3.4. Fluorometric Granzyme B Activity Assay

For the protease activity assay, 96-well plate was seeded with granzyme B transformants ~10⁴ cells/well [25]. Upon 80-90% confluency, the media were decanted and cells were incubated with reaction mix containing two different concentrations, either 1 mM or 2 mM of Serpina3n and SERPINA3 in a reaction buffer (50 mM HEPES, 10% sucrose, 0.1% CHAPS, and 5 mM DTT) to the total volume of 50 μl/well for 1 hr at 37°C. To assess inhibition of granzyme B, colorimetric substrate acetyl (Ac)–IEPD–p-nitroaniline (pNA) (Sigma) 200 μM final concentration in a reaction buffer 20 μl/well was incubated with test groups for 30 mins. The substrate cleavage was measured at 405 nm.

4. In Vivo Studies

4.1. Animals

All animal experiments were performed according to the guidelines issued by the Institutional Ethics Committee of Southeast University, Nanjing, China, and National Institutes of Health Guide for the Care and Use of Laboratory Animals (NIH Publications No. 8023, revised 1978). Animals were housed in cages at constant room temperature of 25°C-26°C with free access to food and water.

4.2. Photothrombotic Stroke Model

Photothrombotic stroke model was produced by the photoillumination of preinjected photosensitive dye Rose Bengal [26]. Briefly, C57BL/6 mice weighing 20-25 g were anesthetized with 1% pentobarbital intraperitoneal injection (75 mg/kg) and placed on stereotaxic frame supplied with DC temperature control module to maintain body’s temperature at 37°C throughout the experiment. Toes were gently pinched to ensure deep anesthesia. The skull hair was removed with hair removal cream and area was sterilized with 70% ethanol. A small cranial window of about 2-3 mm was made by incising skin along midline of the skull, and skin retractors were used to keep the scalp exposed. Eye gel was applied and 1% Rose Bengal was injected subcutaneously at the scruff of the neck followed by right parietal cortex exposure to 532 nm laser beam (50 mW) for 20-30 mins to induce focal ischemia. Ischemia induction procedure remained the same for all animal experiments except in vivo treatment test.

4.3. Triple Immunofluorescence

In order to investigate postischemic endogenous upregulated expression of Serpina3n specifically at focal region and its probable secretory source, photothrombotic stroke models were randomly assigned to 3 groups (/group) tagged with postischemic euthanasia time points, i.e., 24 hrs, 48 hrs, and 72 hrs [27]. Ischemic mice were sacrificed at 3 different time points 24 hrs, 48 hrs, and 72 hrs. 25 μm thick brain coronal sections derived from each cohort were stained with cocktail of primary antibodies, i.e., anti-chicken MAP-2 (1 : 250), anti-goat GFAP (1 : 1000), anti-hamster Serpina3n (1 : 500), and Hoechst (1 : 1000). Images were photographed through confocal microscope and processed by ImageJ software.

4.4. In Vivo Serpinas Treatments

Reversible MCA occlusion surgery was performed as previously described [28]. Briefly, mice were anesthetized with 4% isoflurane and maintained at 2%. Body temperature was maintained at by employing heating pad throughout the surgery. Through midline incision on the neck, right common carotid artery (CCA), right external carotid artery (ECA), and right internal carotid artery (ICA) were exposed. A 3-0 silicon coated suture was advanced into the ICA till it hit the opening of MCA. After 60 mins of occlusion, monofilament was withdrawn. Incisions were closed and mice were maintained at 37°C. Ischemic mice were randomly assigned to 3 treatment groups, designated as ischemic-untreated group, Serpina3n-treated group, and SERPINA3-treated group (/group). Healthy mice were included as control group. After 2 hrs of stroke induction, testing cohorts were administered with either 50 mg/kg of Serpina3n or 50 mg/kg of SERPINA3 via tail vein. Ischemic-untreated group was given purified supernatants of nontransfected 293 T cells. After treatment, animals were caged with sufficient food and water for recovery.

4.5. Neurobehavioral Deficits

24 hrs posttreatment, neurological scores were assigned on a scale of 0 to 5 [29]: normal (0), difficulty in extending forelimb (1), unable to extend forelimb (2), mild circling (3), severe circling (4), and falling on contralateral side (5).

For motor function examination, rotarod test was performed after 2-5 mins of neurodeficit scoring. Before surgery, mice were trained for 3 consecutive days. Treatment cohorts (/group) were placed on the rotarod with gradual increase of speed from 10 to 30 revolutions per minute (rpm) over 120 sec and latency to fall was recorded. The mean duration (in seconds) was recorded from 3 rotarod measurements and analyzed by comparison.

4.6. Brain Infarct Analysis

After neurobehavioral testing, brains were harvested and sliced into 2 mm thick sections. The sections were stained with 2% TTC at 37°C for 30 mins. Viable brain tissue appeared brick red, whereas infarct portion remained unstained and appeared as white. ImageJ software was used by blind observer to calculate the infarction volume by processing the images of stained sections.

4.7. Immunoreactivity and Histochemical Detection of Recombinant Protein

After in vivo treatment, it was necessary to confirm delivery and immunoreactivity of applied protein specifically at focal area [30]. To assess in vivo reactivity 24 hrs posttreatment, brains from Serpina3n-treated and ischemic-untreated groups (/group) were harvested on ice and washed with chilled 1XPBS followed by dissection into six equal parts. Each tissue chunk was aliquoted in 500 μl of lysis buffer (0.1 M EDTA, 0.5% () SDS, and 10 mM Tris-Cl (pH 8.0)) containing protease inhibitors and lysed mechanically with tissue homogenizer on ice. The lysates were centrifuged at 12000 rpm, 4°C for 20 mins. The supernatants were collected and analyzed by western blot probed with reporter c-Myc and granzyme B primary antibodies. In order to investigate inhibition of granzyme B in lysates, 50 μl aliquots of lysed supernatants from Serpina3n-treated and ischemic-untreated group were incubated with 200 μM final concentration of granzyme B fluorometric substrate acetyl (Ac)–IEPD–p-nitroaniline for 0.5 hr at 37°C. The substrate cleavage was measured at 405 nm. Moreover, for the detection of exogenous protein at lesion, 20 μm thick cryosections (~10 sections/animal) derived from Serpina3n-treated, ischemic-untreated, and healthy cohorts were stained with mix of anti-rabbit c-Myc (1 : 800) and anti-hamster Serpina3n (1 : 500) at 4°C. Fluorescent images were obtained using confocal microscope and processed by ImageJ.

5. Statistical Analysis

Statistical data is presented as . Two-way ANOVA analysis followed by Bonferroni post hoc test or Tukey’s multiple comparisons test was performed for statistical differences among groups. was set as significant difference in all cases.

6. Results

6.1. Recombinant Serpina3n and SERPINA3 Protein Expression and Purification

Cell supernatants containing Serpina3n and SERPINA3 were collected and purified. The protein’s expression was confirmed by immunoblot probed with 6-histidine, Serpina3n, and c-Myc primary antibodies, all displayed ~48 kDa band (Figure 1). Purified proteins were further used in in vitro and in vivo experiments to assess proposed therapeutical efficacy during cerebral infarction.

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6.2. Reduction in Neuronal Death In Vitro

Ischemia is associated with oxygen and glucose deprivation that destroys physiological balance between crucial cellular mechanics triggering apoptotic phenotypes, so we asked whether Serpina3n/SERPINA3 can improve cell survival against it. Posthypoxia significant difference was observed in cell viability between the treated and untreated groups. The survived cells were significantly higher in wells containing higher concentrations of Serpina3n or SERPINA3 as compared to the positive control indicating therapeutical effect of recombinant proteins in pseudo stroke model. However, different concentrations of SERPINA3 did not show as much increase in cell viability as observed in the groups treated with Serpina3n (Figure 2).

6.3. Inhibition of Granzyme B by Recombinant Proteins

Expression of granzyme B by selected clone was confirmed by RFP and western blot probed with granzyme B and 6-histidine primary antibodies that displayed ~37 kDa band (Figure 3). In order to confirm Serpina3n-granzyme B reactivity, monolayer culture was used to analyze inhibition of protease in extracellular microenvironment. Preincubation of transformed cells with exogenous Serpina3n effectively inhibited cleavage of colorimetric substrate, while SERPINA3 was not observed with any inhibitory effect against the protease (Figure 3).

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Figure 3

Expression of granzyme B was confirmed through (a) RFP by 293 T cells. (b) Immunoblot of cell lysates probed with granzyme B ~37 kDa. (c) Reprobed with 6-histidine reporter peptide representing same band weight ~37 kDa. (d) Fluorometric assay of granzyme B indicating significant inhibitory effect of Serpina3n in vitro at different concentrations. The data is expressed as and analyzed for statistical significance using two-way ANOVA with Bonferroni post hoc test taking as significant value (, , , and vs. positive control).

6.4. Endogenous Overexpression of Murine Serpina3n and Triple Immunostaining

Postischemic upregulated expression of endogenous Serpina3n was confirmed by harvesting brains from photothrombotic mice at 3 different time points, i.e., 24 hrs, 48 hrs, and 72 hrs. Coronal sections were immunostained with cocktail of antibodies, GFAP, MAP-2, Serpina3n, and Hoechst. Serpina3n fluorescence was distinctly prominent at focal region of ischemic brain as compared to healthy brain (Figure 4). Our data showed colocalized fluorescence of Serpina3n and MAP-2 indicating that neurons secrete Serpina3n and contribute to Serpina3n concentration during traumatic injury under our experimental conditions.

6.5. Neurological Functions of Treated Mice

After 24 hrs of treatment, neurological scores and motor deficit readings were recorded and analyzed. Mice from ischemic-untreated batch demonstrated higher scores by exhibiting serious neurological impairments as compared to healthy and other treatment groups. Significant improvement in neurological functions was observed in the group administrated with Serpina3n. However, SERPINA3-treated mice did not show impressive improvement in comparison to ischemic-untreated animals.

During rotarod test, healthy and Serpina3n groups showed significantly longer retention time on rod as compared to ischemic-untreated and SERPINA3-treated mice. No significant difference in neurodeficit scores and retention time on rod was observed between Serpina3n- and SERPINA3-treated groups (Figure 5).

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Figure 5

(a) Graph representing neurodeficit score between treatment groups. The data is expressed as and analyzed for statistical significance using two-way ANOVA with Bonferroni post hoc test (, , , and vs. ischemic-untreated group). (b) Representative graph for time on rotarod by different treatment cohorts. Values expressed as analyzed by two-way ANOVA with Tukey’s test for significant difference (, , , and vs. ischemic-untreated group; ^#, ^##, ^###, and ^#### vs. SERPINA3-treated group). (c) TTC-stained coronal brain sections from different treatment groups showing infarction volume. (d) Graph representing comparative infarction area between different treatment cohorts. The data is expressed as and analyzed for statistical significance using one-way ANOVA with Tukey’s multiple comparisons test (, , , and vs. ischemic-untreated group; ^#, ^##, ^###, and ^#### vs. SERPINA3-treated group).

6.6. Brain Infarct Volume Analysis

After witnessing neuroprotective effect of recombinant proteins in vitro, we attempted to assess their therapeutical efficacy in vivo. TTC-stained brain sections derived from Serpina3n group showed significantly reduced infarct as compared to ischemic-untreated group. No significant difference was observed in infarct volume between Serpina3n- and SERPINA3-treated groups. (Figure 5).

6.7. Blood-Brain Barrier and Serpina3n-Granzyme B Reactivity

The immunoblot probed with c-Myc antibody contained bands of ~48 kDa (reflecting exogenous Serpina3n) and ~80 kDa (reflecting Serpina3n-granzyme B complex as calculated), while no such bands were observed for ischemic-untreated and healthy control. Second blot probed with granzyme B antibody exhibited bands at ~32 kDa (representing endogenous granzyme B) and ~80 kDa (representing Serpina3n-granzyme B complex) for both ischemic-untreated and Serpina3n-treated mice but not in healthy control as expected, demonstrating successful infiltration and reactivity of Serpina3n at lesion. Immunohistochemical analyses revealed colocalized staining of reporter c-Myc and Serpina3n among treated mice indicating successful diffusion of exogenous Serpina3n into focal region. Dense fluorescence of Serpina3n in treated group reflected the presence of both applied and endogenous Serpina3n in treated mice. Fluorescence exhibited by ischemic untreated reflected postischemic innate response. Serpina3n and granzyme B reactivity was further confirmed by significantly reduced cleavage of colorimetric granzyme B substrate in lysates derived from Serpina3n-treated cohort as compared to ischemic-untreated group (Figure 6). This data suggested successful in vivo delivery and immunoreactivity of applied Serpina3n subsequent ischemia.

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Figure 6

(a) Immunoblot of brain lysates probed with granzyme B marked bands at ~32 kDa and ~80 kDa representing endogenous granzyme B and Serpina3n-granzyme B complex, respectively. (b) Immunoblot probed with c-Myc exhibited bands at ~48 kDa and ~80 kDa representing recombinant Serpina3n and Serpina3n-granzyme B complex, respectively. (c) Delivery of exogenous proteins to the focal region was confirmed by extensive overlaid fluorescence of Serpina3n and reporter c-Myc among treatment group, while no such overlaid staining appeared in ischemic-untreated group. (d) Fluorometric assay of granzyme B with brain lysates derived from different treatment groups demonstrating significant inhibitory effect of Serpina3n in vivo. The data is expressed as and analyzed for statistical significance using two-way ANOVA with Bonferroni post hoc test taking as significant value (, , , and vs. ischemic untreated).

7. Discussion

In current times, keen efforts are devoted for developing new therapeutical strategies against ischemic stroke through mechanistic scrutiny of perplexed contributors [31]. Posttrauma protease-mediated neuronal apoptosis is one of the intrinsic contributing mechanisms [5, 11] which needs effective intervention to prevent disease advancement. Serpina3n and SERPINA3 are specie-specific serine protease inhibitors with a range of potent and practicing medical implications [32]. Serpina3n is an emerging protease inhibitor with an extensive literature reporting its potentialities during different pathologies [32] as compared to SERPINA3 that exhibits slightly limited functional exploration [33]. In present study, we attempted to investigate potential therapeutical efficacy of recombinant murine Serpina3n and its human isomer SERPINA3 against CNS stroke by specifically targeting a proapoptotic protease granzyme B, which is distinguishing target between the two [20] and also involved in neuronal demyelination during CNS insults [17].

Histochemical test was performed to confirm the endogenous upregulated expression of Serpina3n and to investigate the probable source of expression after traumatic injury. Immunofluorescence findings confirmed postischemic-inflated expression of endogenous Serpina3n as reported earlier by Zamanian et al. [18]. Our data illustrated overlaid fluorescence of Serpina3n with MAP-2 instead of GFAP which is in coherence with the recent study reporting 79% neurons positive for Serpina3n after hippocampal injury [19]. Presumably in addition to neurons, astrocytes and oligodendrocytes might also contribute to the density of Serpina3n at lesion site. However, it needs further investigation and experimental proofs to validate the conflicting astrocytic or neuronal upregulated expression of Serpina3n.

In vitro enzymatic ischemia/reperfusion model was generated to study the effect of Serpina3n/SERPINA3 in monolayer culture. Findings demonstrated neuroprotective effect of Serpina3n and SERPINA3 on hypoxic cells with significantly pronounced cell viability at the highest concentration (4 mM). The key mechanism behind the neurorestorative behavior of Serpina3n in hypoxic culture might be due to an effective obstruction of apoptotic cascades followed by MMP-2 [19] and MMP-9 [34]. Serpina3n effectively impedes MMP-2 and MMP-9 to restore neuronal integrity. Neuroprotection induced by SERPINA3 was limited in comparison to Serpina3n, yet there was promising cell survival against positive control which opens a new door to investigate the probable mechanism.

Granzyme B retains 70% of its activities in plasma [35]. For that reason, upregulated protease levels subsequent ischemic injury become more pernicious [36]. Serpina3n is the only known secretory protease inhibitor that inactivates granzyme B by forming irreversible complex [37] and prevents proteolytic disruption. In order to investigate, whether Serpina3n/SERPINA3 inhibit cytotoxic protease granzyme B, purified proteins were incubated with granzyme B transformants. Results revealed that Serpina3n successfully inhibited granzyme B in vitro as indicated by the reduced cleavage of (Ac)–IEPD–p-nitroaniline, while SERPINA3 was unable to bind to granzyme B due to lack of substrate specificity. Resulting data was consistent with the previous reports signifying the critical role of RCL (reactive central loop) for proteolytic inactivation [20]. The inability of SERPINA3 to inhibit granzyme B might be due to amino acids present at P1, P3´, and P4´ positions of RCL, which have been identified as the most crucial loci to specify the target protease [15]. Additionally, difference in innate meta stable stressed state, cofactors, and glycosylation pattern might also be the contributing factors.

In order to address the postischemic role, in vivo study was performed using MCAO mice that underwent different treatments. As ischemic insult is associated with dementia, behavioral, and sensory-motor impairments, our neurological recordings revealed that a single intravenous dose (50 mg/kg) of Serpina3n was adequate to reduce clinical severity in ischemic mice. Analysis of infarct volume illustrated that SERPINA3 was lacking behind in curative effect. The prime reason behind pronounced therapeutical efficacy of Serpina3n can be its aptness to inhibit a wide range of serine proteases, i.e., MMP-2, MMP-9, leukocyte elastase, and granzyme B which contribute to the neuronal damage [11, 16, 19, 38, 39]. Postischemic-elevated granzyme B levels promote neuronal death not only through activation of cellular caspases, i.e., caspase-3, PARP, lamins, HSP-70, and Bid [10, 11], but also through a noikis [11, 40, 41]. In such case, Serpina3n treatment may prevent granzyme B-mediated neuronal degeneration in more than one way to reduce the ultimate damage of ischemia. On that account, it becomes essential to appraise the neuroprotective function of Serpina3n against neuronal degeneration during cerebral stroke.

In order to protect the brain from any relapse, the blood-brain barrier performs a unique service by becoming a major check point for all circulating entities and tightly regulating the exchange of diffusible materials [35, 42]. Most of the circulating molecules, cells, and potential risky substances do not succeed to infiltrate brain across highly selective semipermeable border [36]. After witnessing the therapeutical efficacy of exogeneous Serpina3n against ischemia, we aimed to confirm the successful delivery and immunoreactivity of applied protein at the injured region. For this purpose, we have performed western blot, immunohistochemical, and granzyme B activity tests with ischemic-treated and ischemic-untreated mice. Two immunoblots probed with different antibodies displayed a similar band of ~80 kDa. This band weight signified effective inhibition of granzyme B as well as successful diffusion of applied Serpina3n into the brain. Colocalized dense fluorescence of Serpina3n and c-Myc among treated group further validated the presence of exogenous Serpina3n at lesion site. Serpina3n fluorescence seen in ischemic-untreated group reflected postischemic overexpression of endogenous Serpina3n [18]. Granzyme B activity assay has shown reduced cleavage of calorimetric substrate in Serpin3n-treated group as compared to ischemic-untreated cohort indicating effective inhibition of granzyme B in vivo.

8. Concluding Remarks

In conclusion, inhibition of granzyme B by Serpina3n can be considered as one of the possible approaches to mitigate postischemic damage. This hypothesis was verified and supported by our experimental data. In present study, we have analyzed and confirmed postischemic neuroprotective role of recombinant Serpina3n both in vivo and in vitro by assuming granzyme B as target protease responsible for neuronal apoptosis [17]. However, the promising restorative effect of SERPINA3 in hypoxic culture was taken as a pleasant surprise which needs further studies to investigate complex underlying mechanisms and identification of involved elements intervening postischemic inflammation and neurodegeneration.

Data Availability

All experimental and analyzed data is included in the manuscript.

Conflicts of Interest

Authors declared no conflict of interest.

Authors’ Contributions

Experiments were designed, conducted, and analyzed by Mehwish Saba Aslam accompanied by apt and fruitful discussions with Miss Liudi Yuan. Cerebral ischemia was induced and analyzed by Dr. Mobeena Saba Aslam along with the provision of necessary support and expert view for the experiments. Dr. Komal Saba Aslam participated in animal experiments and performed immunohistochemical tests. Asia Iqbal helped in data analysis. The manuscript was drafted by Mehwish Saba Aslam and reviewed by Miss Liudi Yuan.

Acknowledgments

We would like to thank Mr. Sheng Zhao (Southeast University, Nanjing) for providing necessary support at the initiation of the hypothesis. This work was supported by the Open Funds of the Key Laboratory for Developmental Genes and Human Disease, Ministry of Education, China (201801).

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Copyright © 2022 Mehwish Saba Aslam et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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