Abstract
Secondary degeneration in areas beyond ischemic foci can inhibit poststroke recovery. The cysteine protease Cathepsin B (CathB) regulates cell death and intracellular protein catabolism. To investigate the roles of CathB in the development of secondary degeneration in the ventroposterior nucleus (VPN) of the ipsilateral thalamus after focal cerebral infarction, infarct volumes, immunohistochemistry and immunofluorescence, and Western blotting analyses were conducted in a distal middle cerebral artery occlusion (dMCAO) stroke model in adult rats. We observed marked neuron loss and gliosis in the ipsilateral thalamus after dMCAO, and the expression of CathB and cleaved caspase-3 in the VPN was significantly upregulated; glial cells were the major source of CathB. Although it had no effect on infarct volume, delayed intracerebroventricular treatment with the membrane-permeable CathB inhibitor CA-074Me suppressed the expression of CathB and cleaved caspase-3 in ipsilateral VPN and accordingly alleviated the secondary degeneration. These data indicate that CathB mediates a novel mechanism of secondary degeneration in the VPN of the ipsilateral thalamus after focal cortical infarction and suggest that CathB might be a therapeutic target for the prevention of secondary degeneration in patients after stroke.
INTRODUCTION
Injuries caused by focal cerebral artery occlusion are not restricted to the territory of the artery; accumulating data have demonstrated there is damage that sequentially spreads to areas that are remote from but functionally connected with the ischemic territory. This phenomenon is now widely recognized as poststroke secondary degeneration. For example, in a cohort of 173 patients with hemispheric stroke, Schaapsmeerders et al ( 1 ) found that brain structural changes, that is, ipsilateral hippocampal atrophy, were associated with long-term memory deficits within a follow-up time period up to 10 years. In a magnetic resonance imaging study of patients with cerebral infarction within the MCA distribution, Ogawa et al ( 2 ) reported that hyperintense lesions appear in ipsilateral thalamus 1–12 months after symptom onset, also indicating that secondary degeneration is remote from ischemic foci. Similar changes have been identified in the contralateral anterior horn of the spinal cord ( 3 ) and even in the contralateral hemisphere ( 4 ).
The advent of brain network techniques has demonstrated that secondary damage following focal cerebral infarcts injures the functional connectivity network ( 5 , 6 ) and thus leads to additional impairment of functional recovery of stroke patients ( 7 ). Because of its delayed occurrence, poststroke secondary degeneration occurs in a much extended time window and thus may be a promising target for neuroprotective therapy ( 8 ).
Different mechanisms have been proposed for secondary degeneration in remote areas after stroke. These include retrograde degeneration, anterograde degeneration and transneuronal degeneration, neurotoxic and neuroinhibitory factors, such as β-amyloid and Nogo-A accumulation, inflammation, oxidative stress, autophagy, and others ( 9–16 ).
Cathepsin B (CathB) is a major cysteine protease in brain tissue. It is distributed almost exclusively within neurons in which it is scattered throughout the cytosol, dendrites, and synapses ( 17 , 18 ). As a result, CathB is extensively involved in intracellular protein catabolism, autophagosome formation ( 19 ), and axon outgrowth ( 20 ). Under pathological conditions, CathB activation can lead to cellular autolysis, apoptosis, excessive autophagy, and damage to neighboring cells ( 18 , 21 , 22 ). There is also evidence that CathB participates in axon degeneration ( 23 ). Recently, Xing et al ( 15 ) reported that the level of activated CathB and the formation of autophagosomes in the ipsilateral thalamus in an experimental ischemic stroke model were significantly increased after cortical ischemic stroke, suggesting that it may be a mediator of poststroke secondary degeneration.
In this study, using a focal cortical infarction rodent model, we aimed to test the roles of CathB in secondary degeneration in ipsilateral thalamus after stroke. We hypothesized that the cell membrane-permeable CathB inhibitor L-3-trans-(Propyl-carbamoyloxirane-2-carbonyl]-L-isoleucyl-L-proline methyl ester (CA-074Me) could attenuate poststroke secondary degeneration in the ipsilateral thalamus.
MATERIALS AND METHODS
Adult male Sprague-Dawley rats weighing 280–320 g (10–12 weeks old) were obtained from Southern Medical University (Guangzhou, China). Rats were housed under standard temperature (22 ± 1 °C), a 12-hour light/dark controlled environment with free access to food and water. Weight gain and health conditions of the rats were comparable among the different groups. All animal procedures were performed in accordance with Animal Research: Reporting In Vivo Experiments (ARRIVE) guidelines and were approved and monitored by the Animal Care and Use Committee of Guangzhou Medical University. All efforts had been made to minimize the suffering of animals and the number of animals used.
Animal Model
Permanent occlusion of distal branch of middle cerebral artery (dMCAO) was performed using an electrocoagulation methodology, as previously described ( 13 ). In brief, rats were placed in the anesthesia induction box supplied with 3%–4% isoflurane in 100% oxygen. Anesthesia was maintained with 1.5%–2.5% isoflurane in 100% oxygen, delivered through a nose mask (SurgiVet, Waukesha, WI) during the surgical procedure. The distal striatal branch of the MCA was exposed and occluded by unipolar electrocoagulation under an operating microscope. Rectal temperature of the animals was monitored and maintained at approximately 37 °C throughout the procedure. Sham-operated animals were performed with the same surgical procedures except for electrocoagulation of the dMCAO. After surgery, the rats were allowed to wake up, and the neurological status was evaluated as previously described ( 14 ). Rats without neurologic deficit or cortical infarcts were excluded from the study.
Histology
Animals were intracardially perfused with normal saline, followed by 4% paraformaldehyde in phosphate buffer saline (PBS) (0.01M, pH 7.4) under anesthesia with 10% chloral hydrated administered intraperitoneally. The brains were postfixed for 12 hours in 4% paraformaldehyde and then cryoprotected with 10%, 20%, 30% sucrose in the same fixative overnight. Coronal tissue blocks (bregma 1.7 mm to −5.8 mm) were cut on a freezing microtome (Leica CM1950, Heidelberg, Germany) into 30-μm-thick sections. Nissl staining was performed according to standard procedures with 0.1% Cresyl violet (Sigma, St. Louis, MO), and then sections were dehydrated with 90% and 100% ethanol and immersed into dimethylbenzene. Fluoro-Jade B (FJ-B) staining was used to label degenerating neurons. Briefly, sections were mounted on gelatin-coated glass slides, air-dried, and were then immersed in 100% ethanol for 3 minutes followed 70% ethanol and water for 1 minute each. Sections were then transferred to 0.06% potassium permanganate for 15 minutes. After 2 more rinses, they were placed in 0.001% FJ-B (Millipore, Bedford, MA) in 0.1% acetic acid for 60 minutes at room temperature in the dark and then water-washed and mounted with distrene plasticizer xylene (Sigma). The FJ-B-stained sections were examined under a fluorescence microscope (Leica Microsystems, Wetzlar, Hessen, Germany).
The sections from Nissl staining were examined under a light microscope (×400). Surviving cells showed well-stained Nissl bodies, whereas damaged cells were either swollen with loss of stainable Nissl material or were necrotic with deeply staining fragmented dendrites. FJ-B-stained neurons were quantified in 3 sections of each animal. For each section, the number of FJ-B-positive cells was estimated by counting the cells from 3 nonoverlapping fields under ×200 magnification and presented as the average cell number per field on each section.
To confirm the presence of infarction, rats were killed at 24 hours after dMCAO under anesthesia with 10% chloral hydrate. The brains were quickly removed and were frozen at −20 °C for 10 minutes, dissected into 5 coronal slices and immediately incubated in 2% 5-triphenyltetrazolium chloride (TTC) at 37 °C for 10 minutes. Viable brain tissues were stained dark red, whereas infarcted tissue was unstained.
For infarct volume evaluation, a glial fibrillary acidic protein (GFAP)/diaminobenzidine (DAB) immunostaining experiment was performed. A series of sections from both sides of all brain tissue blocks of each animal were selected for this purpose. Each stained section was photographed. The territory of infarction and the total area of brain were outlined and quantitated digitally using Image J software (NIH, Bethesda, MD). The infarction volume was then determined by multiplying by the block thickness, and the percentage volume of infarction was normalized by the volume of the contralateral nonischemic hemisphere.
Immunohistochemistry
A sham-operated group (n = 6), dMCAO group (n = 24), vehicle group (n = 6), and CA-074Me group (n = 8) were used for immunohistochemistry studies. The animals from dMCAO group were killed at 1, 2, 3, and 4 weeks, and animals from the CA-074Me group were killed at 1 week after operation. Single-label immunohistochemistry was conducted using the avidin-biotin-peroxidase complex method ( 24 ). Briefly, sections from bregma −3.14 to −4.16 mm were rinsed with 0.01M PBS, treated with 3% H 2 O 2 for 30 minutes, followed by 5% normal serum for 1 hour at room temperature, and then incubated overnight at 4 °C with the following primary antibodies: rabbit polyclonal anti-CathB (1:1000, Millipore), mouse anti-neuron-specific nuclear-binding protein ([NeuN], 1:2000; Millipore), rabbit polyclonal anti-GFAP (1:2000, Millipore), and mouse anti-ionized calcium-binding adapter molecule-1 ([Iba-1], 1:1000; Millipore). After 3 washes with 0.01M PBS, the sections were incubated with biotinylated secondary immunoglobulin G antibody for 2 hours at room temperature. After washing with PBS, the sections were incubated with the avidin-biotin-peroxidase complex for 30 minutes at room temperature. The peroxidase reaction was visualized with DAB. Some anti-NeuN- and GFAP-stained sections were counterstained with Cresyl violet. Immunopositive cells in the ventroposterior nucleus (VPN) of ipsilateral thalamus were quantified in 3 sections of each animal. For each section, numbers of immunoreactive cells for NeuN, GFAP, and Iba-1 were counted. Only cells with reaction products that were within a clear and regular-shaped cytoplasmic border were quantified from 3 nonoverlapping fields under ×200 magnification; data are presented as the average cell number per field on each section. For densitometry analysis, the average intensity of CathB-positive staining in the VPN of thalamus was determined using ImageJ software (NIH, Bethesda, MD).
Triple-fluorescence immunohistochemistry was performed as previously described ( 25 ). Sections were preincubated with 5% normal goat serum (containing 2% Triton X-100) for 1 hour at room temperature and then incubated overnight at 4 °C with mixtures of the following rabbit and mouse primary antibodies: rabbit anti-CathB (1:100), mouse anti-NeuN (1:1000), mouse anti-GFAP (1:1000), and mouse anti Iba-1 (1:1000). After rinsing in 0.01M PBS, sections were incubated for 1 hour at room temperature with the following secondary antibodies: Cy3-conjugated goat anti-mouse IgG antibody (1:100; Invitrogen, Carlsbad, CA) and FITC-conjugated goat anti-rabbit antibody (1:50; Invitrogen). After that, they were PBS washed and mounted with mounting medium containing 4',6-diamidino-2-phenylindole (DAPI). Slides were analyzed with a confocal laser microscope (Leica Microsystems).
Western Blotting
The rats were killed at 1, 2, and 3 weeks after dMCAO (n = 3 in each group) or at 1 week after vehicle or CA-074Me treatment (n = 3). According to the “rat brain in stereotaxic coordinates” ( 26 ), the brain tissue was cut into 2-mm coronal slices using a brain matrix (Shuolinyuan Technology Co., Ltd., Beijing, China) and the VPN of thalamus (−2.3 mm to −4.3 mm, bregma) was quickly dissected under a stereomicroscope. The proteins extracted from VPN were separated with 15% SDS–PAGE gel, transferred to PVDF membranes, and incubated with antibodies against CathB (1:1000), NeuN (1:6000), GFAP (1:6000), cleaved caspase-3 (1:500; Cell Signaling Technology, Danvers, MA), and glyceraldehyde-3-phosphate dehydrogenase ([GAPDH], 1:10000, Invitrogen) in Tris-buffered saline containing 0.2% Tween-20 (TBST) and 5% nonfat dry milk at 4 °C overnight. Membranes were washed and incubated with second antibody in TBST for 2 hours. Densitometry analysis for the quantification of the bands was performed using image analysis software (Quantity One, Bio-Rad Laboratories, Inc., Hercules, CA). Relative optical densities of protein bands were calibrated with GAPDH and normalized to those in sham-operated rats.
Pharmacologic Interventions
Implantation of an intracerebroventricular injection cannula into the right lateral ventricle was performed stereotaxically under anesthesia with 3%–4% isoflurane. The cannula was placed through a burr hole opened on the right parietal skull at 1.5 mm lateral, 1.0 mm posterior, and 3.6 mm dorsal with respect to the bregma and affixed to the skull with stainless steel screws and cranioplastic cement. Rats were randomly divided into 3 groups: sham, vehicle, and CA-074Me group. All rats were allowed to recover from surgery for 1 week before treatment. CA-074Me (16 nmol/d, Millipore) or the vehicle (2% dimethyl sulfoxide [DMSO] in 0.01M PBS) were injected into lateral cerebral ventricle for 6 days; sham-operated rats received no interventional injection.
Data Analyses
All compared data are expressed as mean ± SD and analyzed using SPSS 13.0 (SPSS, Inc., Chicago, IL). Statistical significance was determined by one-way ANOVA, followed by LSD post hoc test or 2-tailed Student t -test; p < 0.05 was considered statistically significant.
RESULTS
Mortality
One hundred and nineteen rats were used for the experiments. Three rats died during anesthetization. Four rats in the dMCAO groups died during the surgical procedure and rats in the dMCAO groups died after surgery. Two rats in the CA-074Me group died after intracerebroventricular injection. Fourteen rats were excluded because neither neurologic deficit nor cortical infarction after dMCAO was observed.
Animal Model
TTC staining indicated that the infarction was strictly limited within the cerebral cortex and did not involve the ipsilateral thalamus at 24 hours after dMCAO; the corpus callosum separated the infarcted cortical lesion (pale) and the nonischemic intact subcortical regions, such as the striatum, thalamus, and hippocampus (red), indicating that the focal cortical infarction model was successfully developed ( Fig. 1C ). Simultaneously, GFAP/DAB-staining assay in Figure 1B also clearly showed that ischemic foci were limited within the cortex, whereas there was secondary degeneration in the VPN of the ipsilateral thalamus remote from the ischemic foci at 2 weeks after dMCAO.
Focal cortical infarction is induced by dMCAO in rats. (A) Schematic illustration (blue area indicates the cortical infarct and red area indicates the VPN of the ipsilateral thalamus). (B) GFAP/DAB staining of brain coronal sections at 2 weeks after dMCAO. Ischemic foci are confined to the left somatosensory cortex; secondary degeneration occurs in the VPN of the ipsilateral thalamus (square), remote from the ischemic foci. (C) TTC-stained coronal brain sections illustrate the extent of focal cortical infarction at 24 hours after dMCAO. The infarcted tissue appeared white, while the intact tissue is colored.
Secondary Degeneration in the VPN of Ipsilateral Thalamus After Focal Cortical Infarction
Progressive neuronal damage characterized by reduced numbers of NeuN-positive cells was observed in the VPN of ipsilateral thalami. The numbers of NeuN-positive cells at 1, 2, 3 and 4 weeks, respectively, were significantly decreased in the VPN of ipsilateral thalamus after dMCAO when compared with the sham-operated group ( Fig. 2C ). FJ-B staining indicated that neuron degeneration was most significant in the first 1 week after surgery. In contrast to the reduced number of neurons, numbers of GFAP-positive cells at 1–4 weeks and Iba-1-positive cells at 2–4 weeks post-dMCAO at the same sites were increased markedly when compared with the sham-operated group. These glial cells were characterized by their typical hypertrophic shape and thickened processes ( Fig. 2A ). Up to 2 weeks after dMCAO, GFAP- and Iba-1-positive cells were seen over the entire VPN of ipsilateral thalami. Thus, secondary degeneration in the ipsilateral thalami presented not only as profound neuron loss but also as extensive gliosis. No significant change in numbers of NeuN-positive, GFAP-positive, or Iba-1-positive cells was observed in the VPN of ipsilateral thalami of sham-operated rats ( Fig. 2A ).
Secondary degeneration in the VPN of ipsilateral thalamus after dMCAO. (A) Representative microphotographs of immunostaining for NeuN, GFAP and Iba-1, and FJ-B staining in the VPN of ipsilateral thalamus at 1–4 weeks after dMCAO or sham operation. (a–d) Sham-operated group; ( e–h ) 1 week after dMCAO; ( i–l) 2 weeks after dMCAO; ( m–p) 3 weeks after dMCAO; ( q–t) 4 weeks after dMCAO. Scale bar, 75 μm. The inset panels at the bottom right corners are magnified from the outlined territory in the VPN of the thalami. (B, a) Representative microphotographs of immunostaining of NeuN counterstained with Nissl in the VPN of ipsilateral thalamus at sham operation; ( b) GFAP counterstained with Nissl in the VPN of ipsilateral thalamus at sham operation; ( c–e) representative microphotographs of fluorescent double-staining of Iba-1 (red, c ) and DAPI (blue, d ), and merged (e) in the VPN of ipsilateral thalamus at 2 weeks after dMCAO. (C) Quantitative analysis of NeuN, FJ-B, GFAP, and Iba-1-positive cells in the VPN of ipsilateral thalami. Each bar represents the mean ± SD (n = 6 in each group). *p < 0.05 versus sham group. NeuN, neuron-specific nuclear-binding protein; FJ-B, Fluoro-Jade B staining; GFAP, glial fibrillary acidic protein; Iba-1, ionized calcium-binding adapter molecule-1; Sham; sham operated; w, weeks.
CathB Is Upregulated in the VPN of Ipsilateral Thalami After Focal Cortical Infarction
In sham-operated rats, immunoreactivity for CathB was found mainly in cells with round nuclei, spindle cell-shaped bodies, and elongated axons, that is, typical neuron-like morphology ( Fig. 3A, a ). Double labeling with immunofluorescence assay further confirmed that these cells were NeuN-positive ( Fig. 4A, a–e ), indicating a predominantly neuronal localization of CathB in sham-operated brains. However, CathB granules in the VPN of ipsilateral thalami became progressively larger and irregular, forming aggregates or diffuse cytoplasmic staining from 1 to 3 weeks after dMCAO ( Fig. 3A, B ). Quantitative analysis of CathB-positive cell densities showed that the expression of CathB in the VPN of thalamus was increased after dMCAO ( Fig. 3D ). Western blotting confirmed that the expression of CathB in the VPN of ipsilateral thalamus increased and peaked at 2 weeks after dMCAO ( Fig. 3C, E ). Alternatively, immunoreactivity of CathB at 3 weeks after dMCAO appeared mostly in cells with elongated, irregular nuclei ( Fig. 3A, d ), and double labeling revealed that these cells were either GFAP-positive or Iba-1-positive ( Fig. 4B, f–o ), indicating that CathB in the VPN of ipsilateral thalami was localized predominantly in astrocytes and microglia at 3 weeks after dMCAO. Enlarged images of the fluorescent double staining showed that CathB colocalized in neuronal granular vesicles in sham-operated rats ( Fig. 4A, e ), whereas in the degenerating VPN of ipsilateral thalamus at 3 weeks after dMCAO, CathB immunoreactivity was more evenly distributed in the cytoplasm of NeuN-positive neurons, and the displacement of CathB led to the loss of normal shape and membrane collapse of the cell, that is, degeneration of the neurons ( Fig. 4B, e ).
CathB is upregulated in the VPN of ipsilateral thalamus after dMCAO. (A, B) Representative microphotographs of immunohistochemistry and immunofluorescence staining of CathB in the VPN of thalami at 1–3 weeks after dMCAO or sham operation. The insets at the bottom right corner are magnified from the square in the VPN of thalamus. (a–d) The VPN of ipsilateral thalamus after dMCAO; ( e–h ) VPN of the contralateral thalamus after dMCAO. Scale bar, 75 μm. (C) Western blot analysis of CathB protein in the VPN of thalami after dMCAO. (D) Quantitative analysis of CathB-positive density in the VPN of thalamus after dMCAO. Data are expressed as percentage of value of sham-operated animals. Each bar represents the mean ± SD. *p < 0.05 versus sham-operated animals (n = 6 in each group). (E) Quantitative analysis of CathB protein in the VPN of thalami after normalization by GAPDH protein of the same sample. Data are expressed as percentage of value of sham-operated animals. Each bar represents the mean ± SD. *p < 0.05 versus the sham-operated animals (n =3 in each group). GAPDH, glyceraldehyde 3-phosphate dehydrogenase; Ip, ipsilateral; Con, contralateral.
Cellular localization of CathB in the VPN of ipsilateral thalami after dMCAO. (A) Representative images of fluorescent triple staining of CathB (green), DAPI (blue), and the neuron marker (NeuN, red); CathB (green), DAPI (blue), and the astrocyte marker (GFAP, red); CathB (green) DAPI (blue) and the marker of microglia (Iba-1, red) in the VPN of ipsilateral thalamus at sham-operated group. The overlapped images show that NeuN is surrounded by CathB (a–e) . No colocalization of CathB and GFAP (f–j) , Iba-1 (k–o) was observed in the VPN of the sham-operated group. (B) Representative images of fluorescent triple staining of CathB (green), DAPI (blue), and NeuN (red); CathB (green), DAPI (blue), and GFAP (red); CathB (green), DAPI (blue), and the microglial marker (Iba-1, red) in the VPN of ipsilateral thalamus at 3 weeks after dMCAO. The overlapped images show that NeuN is surrounded by CathB (a–e) , and CathB is colocalized with GFAP (f–j) and Iba-1 (k–o) in thalamic VPN at 3 weeks after dMCAO. Scale bar, 75 μm. NeuN, neuron-specific nuclear-binding protein; GFAP, glial fibrillary acidic protein; Iba-1, ionized calcium-binding adapter molecule-1.
Inhibition of CathB Alleviates Secondary Degeneration in the VPN of Ipsilateral Thalami After dMCAO
At 1 week after dMCAO, relative infarct volume computed from GFAP/DAB-stained sections in vehicle treated rats was 11.39% ± 2.92%, and 13.55% ± 2.84% in CA-074Me rats. There was no significant difference in the infarct volume between the 2 groups. Immunofluorescence assay of CathB demonstrated that intracerebroventricular administration of 16 nmol CA-074Me starting at 24 hours after dMCAO significantly decreased the expression of CathB in the VPN of ipsilateral thalami ( Fig. 5A ). Western blot evaluation of CathB protein further confirmed that CathB protein in the CA-074Me group was reduced compared with the vehicle group at 1 week after dMCAO ( Fig. 5A, C ). Meanwhile, the expression of cleaved caspase-3 was significantly increased in VPN of the vehicle groups but was reversed by CA-074Me ( Fig. 5B, D ).
Inhibition of CathB and cleaved caspase-3 in the thalamic VPN after dMCAO. (A, B) Western blot analysis of CathB and cleaved caspase-3 in the VPN of bilateral thalami at 1 week after dMCAO. (C, D) Quantitative analysis of CathB and cleaved caspase-3 protein levels after normalization with GAPDH of the same sample. Data are expressed as percentage of value of sham-operated animals. Each bar represents the mean ± SD. *p < 0.05 versus vehicle group; #p < 0.05 versus vehicle group (n = 3 in each group). (E) Representative microphotographs show fluorescent staining of CathB (green) after dMCAO with or without CA-074Me treatment. The panels at the bottom right corner are magnified in the VPN of thalamus. ( a, b ) VPN of bilateral thalamus of the sham group; ( c, d ) VPN of bilateral thalamus of the vehicle group; ( e, f ) VPN of bilateral thalamus of the CA-074Me-treated group. Scale bar, 75 μm. Ip, ipsilateral; Con, contralateral.
Accordingly, CA-074Me treatment significantly decreased the degeneration of neurons and reduced gliosis in the VPN of ipsilateral thalamus, for example, the number of surviving cells with pale stained nuclei and intact Nissl substance, and NeuN-positive cells were significantly higher and GFAP-positive and FJ-B positive cells were evidently lower in the CA-074Me group at 1 week after dMCAO when compared with the vehicle group ( Fig. 6A–H ). This was also the case for the comparison of NeuN and GFAP proteins ( Fig. 6I–L ). These results indicate that CA-074Me alleviated post-dMCAO secondary degeneration in the VPN of ipsilateral thalami.
Delayed CA-074Me treatment alleviates secondary degeneration in the VPN of the ipsilateral thalamus after dMCAO. (A–D) Representative microphotographs of immunostaining of Cresyl violet, NeuN, GFAP, and FJ-B staining in the VPN of bilateral thalami. Scale bar, 100 μm. (a, b ) Sham group; ( c, d) Vehicle group at 1 week after dMCAO; ( e, f ) CA-074Me-treated group at 1 week after dMCAO. (E–H) Quantitative analyses of survival of NeuN-positive cells, GFAP-positive cells, and FJ-B-labeled cells in the VPN of thalami at 1 week after dMCAO. Each bar represents the mean ± SD. *p < 0.05 versus sham group and #p < 0.05 versus vehicle group (n = 6 in each group). (I, J) Western blot analysis of protein for NeuN and GFAP in the VPN of ipsilateral thalamus at 1 week after dMCAO. (K, L) Quantitative analysis of NeuN and GFAP protein after normalization with GAPDH of the same sample. Data are expressed as percentage of value of sham-operated animals. Each bar represents mean ± SD. *p < 0.05 versus sham group; #p < 0.05 versus vehicle group (n = 3 in each group). Ip, ipsilateral; Con, contralateral; NeuN, neuron-specific nuclear-binding protein; GFAP, glial fibrillary acidic protein.
DISCUSSION
CathB is a lysosomal cysteine peptidase that plays pivotal roles in intracellular protein catabolism, autophagosome formation, and axon outgrowth. However, under pathological stimulation, CathB activation can lead to cellular autolysis, apoptosis, excessive autophagy, and even damage to neighboring cells ( 18 , 21 , 22 ). Moreover, there is also evidence that CathB takes part in the process of axon degeneration ( 23 ). Selective inhibitors of CathB would be powerful tools for clarifying the functions of CathB. CA-074 has been used widely to inactivate CathB in vivo and in vitro , because it exhibits 10,000–30,000 times greater inhibitory effect on purified rat CathB than on cathepsin H and L ( 27 ). Because CA-074 is impermeable to cell membranes ( 28 ), its intracellular activity is too weak for cathepsin L, but it is still sufficiently potent to abolish intracellular CathB activity when it is used in a relatively high concentration in vitro ; this also indicates that CA-074 has much higher selectivity in inhibiting CathB than cathepsin L. The cell membrane impermeable nature of CA-074 restricts its use in vivo . This led to the development of CA-074Me, which has the highly CathB inhibitory effect from CA-074, and is cell membrane-permeable. In this study, using CA-074Me, we investigated the roles of CathB in the formation of secondary degeneration in the VPN of ipsilateral thalamus after focal cortical infarction.
We first confirmed substantial progressive neuronal loss and gliosis in the VPN of ipsilateral thalami, which are remote from the primary ischemic infarction in the somatosensory cortex. Moreover, we found that the expression of CathB was significantly increased and that caspase-3 was activated; CA-074Me reversed the release of CathB from the lysosomes into the cytoplasm and the activation of caspase-3 in the VPN of ipsilateral thalamus after dMCAO. In addition, inhibition of CathB by CA-074Me treatment led to significant attenuation of the delayed neuronal damage in the ipsilateral thalamus. To the best of our knowledge, this is the first study to show that abnormal accumulation of CathB-mediated secondary degeneration in the VPN of ipsilateral thalamus after dMCAO in adult rats.
The ipsilateral thalamus is the most common site used for poststroke secondary degeneration studies, and secondary degeneration in the ipsilateral thalamus after focal cortical stroke has been widely documented in rodents, nonhuman primates, and humans ( 29–32 ). In this study, we electrocoagulated the MCA distal to striate branch similar to Xing et al ( 13 ) did in their rodent model. The blood supply of the rat thalamus is from the thalamo-geniculate arteries of posterior cerebral artery, anterolateral thalamostriate artery of middle cerebral artery, and recurrent branches of the anterior cerebral artery, which parallel the olfactory tract ( 33 , 34 ). Thus, occlusion of the distal MCA to the striate branches does not affect the blood supply of the thalamus ( 35–37 ). Our results of TTC staining confirmed that the area of infarction did not extended into thalamic structures, that is, there were no direct injuries caused by the artery occlusion. In addition, significant neuron loss and gliosis in the VPN of ipsilateral thalamus were evident 1 week after focal cortical infarction sequentially and progressed over time, suggesting these changes in ipsilateral thalamus developed secondary to the focal cortical infarction.
The mechanisms of development of secondary degeneration are far from clear. Because secondary degeneration happens only in areas that have connections with infarct loci, it is possible that retrograde degeneration, anterograde degeneration, and transneuronal degeneration might be the main underlying mechanisms for the development of secondary degeneration. It is possible that injuries spread through projections between areas that are connected such as the cerebral infarct loci and ipsilateral thalamus. In our previous study ( 17 ) and the study of Seyfried et al ( 38 ), CathB has been documented to be a critical mediator for ischemic injuries in infarct loci and peri-infarct tissue. In this study, chronological observations indicated that CathB was significantly upregulated in the VPN of ipsilateral thalamus after focal cortical infarction, and it then appeared to redistribute from lysosomal lumens and vesicles to cytoplasm in neurons. Meanwhile, we observed a mild to moderate upregulation of CathB immunoreactivity in the contralateral thalamus, which was also confirmed by Western blot experiments. Unlike CathB in the ipsilateral thalamus, however, upregulated CathB protein in contralateral thalamus was mostly restricted within lysosomal granules and the immunoreactivity of CathB was much less strong. As a result, there was likely little secondary degeneration in contralateral thalamus. However, we did observe mild neuron loss in the contralateral thalamus 4 weeks after MCAO and these observations are consistent with those of Patience et al ( 39 ), who noticed that astrogliosis appeared not just within ipsilateral hemisphere after stroke, but also significantly, and widespread in contralateral hemisphere.
Our data suggest that activation of CathB in the VPN may be involved in apoptotic regulation via cleaved caspase-3 and that inhibition of CathB confers neuroprotection against secondary degeneration. On one hand, the redistribution of CathB might act either by necrotic or apoptotic mechanisms and cause neuronal death ( 21 ). On the other hand, significantly upregulated CathB would also initiate excessive autophagy-lysosome processes ( 40 ) and thus add to the secondary degeneration ( 15 ). In this study, CathB inhibition by CA-074Me significantly alleviated the neuron loss and gliosis thus confirming that CathB is involved in the secondary degeneration in the VPN of ipsilateral thalamus. However, further investigations are needed to determine whether CathB transmits injuries from the ischemic foci to the remote areas after dMCAO.
CathB might work differently in neurons versus glial cells. In general, the activation of CathB would lead to the rupture of cellular and lysosomal membrane and then cause autolysis and programed cell death to host cells ( 21 ). However, glial cells seem do not appear to synthesize CathB until ischemia begins, whereas neurons are constitutively rich in CathB. In the meantime, glial CathB is more likely to be secreted by its host into the extracellular space for the regulation of neuronal death or for the production of other injury mediators, rather than functioning intracellularly as does neuronal CathB ( 41–45 ). Thus, neurons are prone to cell death upon the activation of CathB ( 21 ). This hypothesis also needs to be confirmed by further experiments.
Since the activation of CathB happens quickly after ischemic onset ( 46 ) and leaves a very limited time window for CathB inhibition treatment in acute stroke ( 21 ), prestroke administration of CathB inhibitor is usually needed ( 17 , 47 ). However, administration of CathB inhibitor before stroke is difficult to implement in clinical practice. In this study, CA-074Me was administrated 24 hours after stroke onset; no significant difference in infarct volume was found between the CA-074Me group and vehicle groups, but the protective effect of CathB inhibition against secondary degeneration in decreasing neuron loss and gliosis in the VPN of ipsilateral thalami was sustained from 1 week to 4 weeks after dMCAO or even longer. Furthermore, in regard to glial CathB constitutes the major resource of CathB in the ipsilateral thalamus at the later stage of stroke. Because there is evidence that CathB plays a critical role in angiogenesis, which might enhance repair, further investigation of its behavior is needed ( 48–50 ). Nevertheless, in this study, 2 weeks after dMCAO and beyond, though CathB is majorly coming from glial cells, CA-074Me was still capable of attenuating delayed neuron loss. This leads us to the speculation that, except for the direct inhibition of the enzyme activity of CathB within neurons ( 51 ), CA-074Me might decrease the secretion of CathB from glial cells and help with the stabilization of the cellular and lysosomal membrane as well. Our results indicate that targeting the prevention of secondary degeneration might offer a much-extended therapeutic window for stroke treatment and that CathB could be a potent candidate for this therapeutic strategy ( 52 ). Furthermore, on the basis of our findings from FJ-B staining, the first 2 weeks after stroke onset might be the best time period for the prevention of poststroke secondary degeneration in the ipsilateral thalamus.
Taken together, we suggest that secondary degeneration in the VPN of ipsilateral thalamus after focal cortical infarction is mediated and progressed by CathB and that CathB inhibition may be a potent therapy target for the prevention of poststroke secondary damage.
REFERENCES
Author notes
Xialin Zuo and Qinghua Hou contributed equally to this work.
This study was supported by the National Natural Science Funds of China (Grant No. 81100880 and Grant No. 81501952) and Project Funded by China Postdoctoral Science Foundation (Grant No. 2014M552184).
