Pyroptotic cell death in the R6/2 mouse model of Huntington’s disease: new insight on the inflammasome

Mechanisms of tissue damage in Huntington’s disease involve excitotoxicity, mitochondrial damage, and neuroinflammation, including microglia activation. In the present study, we investigate the role of pyroptosis process in the striatal neurons of the R6/2 mouse model of Huntington’s disease. Transgenic mice were sacrificed at 4 and 13 weeks of age. After sacrifice, histological and immunohistochemical studies were performed. We found that NLRP3 and Caspase-1 were intensely expressed in 13-week-old R6/2 mice. Moreover, NLRP3 expression levels were higher in striatal spiny projection neurons and in parvalbumin interneurons, which are prone to degenerate in HD.


Introduction
Pyroptosis is a Caspase-1 (Casp-1) dependent programmed cell death that leads to a rapid lysis of the cell 1 . Similar to apoptosis, pyroptosis is a programmed cell death but is dependent on a different caspase. Pyroptosis indeed shares several features with apoptosis, but at a closer look the two processes are found to be fairly distinct. In pyroptosis, DNA damage occurs and TUNEL assay is positive 2,3 . However, while the apoptotic nucleus becomes pyknotic and subsequently breaks up in karyorrhexis, the nucleus in pyroptosis undergoes a chromatin condensation, but remains intact and karyorrhexis does not occur 4 .
Inflammasomes are complexes of proteins that are assembled in response to various cellular stresses, and their activation leads in turn to the recruitment and activation of procaspase 1 2 . The major components of the inflammasome complex are pattern recognition receptors (PRRs) 5 . Several inflammasomes have been described, which include NLR family pyrin domain containing 3 (NLRP3), NLRP1, AIM2, and others. Among them, NLRP3 inflammasome was described to display a major role in inflammatory and immune system-related disorders 6 , and also in the pathogenesis of several neurodegenerative diseases 7,8 .
Thus, the interest in this topic is based on its association with various diseases which is leading to the development of therapeutics that target inflammasome activity 9 . With this in mind, the aim of our work was to investigate the relevance of pyroptosis in the neuropathology of Huntington's disease. In fact, such devastating neurodegenerative disease is still lacking a valid therapy, and we speculate that pyroptosis could represent a new target for future therapies. Thus, in this study, we carried out a research to investigate the presence of pyroptosis in the striatal cells of R6/2 Huntington's disease mouse model by comparing it to the distribution of apoptosis.

Results
In order to evaluate the development and the (activation of the different inflammasome components that suggest the involvement of pyroptosis) presence of inflammasome and the involvement of pyroptosis in the neuropathology of HD, we chose to study two different time points, i.e. the early presymptomatic stage of 4 weeks and the fully symptomatic 13 weeks.

Caspase-8 and NLRP3 expression in the R6/2 mice striatum
We studied the distribution of the activated apoptosis marker Caspase-8 ( Fig. 1) compared to the inflammasome marker NLRP3 (Fig. 2) in the mice striatum. The number of striatal neurons positive for Caspase-8 appeared higher in the young R6/2 mice than in 13 weeks old R6/2 mice P < 0.001 (Fig. 1m), as well as a significant expression of Caspase-8 was also present in 4-week-old Wt (Fig. 1a-c). Cleaved Caspase-8 product at the different time points was also confirmed by Western Blotting analysis. These data suggested that Caspase-8 activation at this stage was linked to the cell homeostasis. In fact, the expression of cleaved Caspase-8 was significantly reduced in the 13week-old Wt and R6/2 mice P < 0.001 (Fig. 1n). Immunofluorescence analysis revealed that 13-week-old R6/2 mice had a significantly higher expression of NLRP3 in the striatal neurons compared to the apoptosis marker Caspase-8 (Fig. 2); P < 0.001.
NLRP3 contributes to Caspase-1 activation in the R6/2 mice striatum Caspase-1 immunohistochemistry was performed for the detection of pyroptosis (Bergsbacken et al. 10 ) in striatal neurons of all mice from each experimental groups. Immunostaining for Caspase-1 in 13-week-old R6/2 mice revealed an intense immunohistochemical reaction, where neurons positive for Caspase-1 were significantly more numerous compared to their wild-type littermates F1,36 = 18,030; P < 0.001 (Fig. 3a-e).
Protein analysis, performed by western blot, revealed the expression of procaspase-1 in all the experimental groups, however, in the R6/2 at 13 weeks of age the cleavage product of procaspase-1 was significantly higher compared to the other groups (Fig. 3f).

NLRP3 immunoreactivity in Calbindin positive neurons
We investigated the distribution of the NLRP3 in the striatal projection neurons, labeled by Calbindin immunoreactivity. Calbindin labeled striatal projection cells were present in the 13-week-old Wt mice where they constituted the majority of neurons, whereas they were dramatically reduced in the R6/2 mice. Very low levels of NLRP3 immunoreactivity were present in 4-week-old Wt, and no immunoreactivity at all was observed at 13 weeks ( Fig. 4a-f). Conversely, the double immunostaining revealed an early suffering of calbindin positive striatal projection neurons in the 4-week-old R6/2 mice as is showed in the graph (Fig. 4m), where we observed a statistically significant decrease in number and in immunofluorescence intensity in 13-week-old R6/2 mice compared to 4-and 13-week-old Wt (Fig. 4j-l). Such specific decrease of calbindin labeled projection neurons and immunofluorescence intensity was associated with the overexpression of NLRP3 in the 13-week-old R6/2 as shown in Fig. 4n. Coloc 2 analysis revealed a statistically significant colocalization of NLRP3 in calbindin neurons in the R6/2 mice compared with both 4-and 13-week-old Wt mice (Fig. 4o).

Analysis of NLRP3 expression levels in the interneurons subtypes
Along with the distribution of NLRP3 in projection neurons, we studied the pattern of its localization in the different interneurons subtypes, each of which express a peculiar susceptibility to HD degeneration. The intensity of NLRP3 immunoreaction product in the interneurons subtypes was investigated in all experimental groups ( Fig.  5a-c). We confirmed a significant reduction of CALR interneurons in the 13-week-old R6/2 mice, where NLRP3 expression, in terms of intensity of immunoreactivity, was significantly increased (Fig. 5b). In the cholinergic interneurons, which are resistant to the specific neurodegenerative process in HD 11,12 , we observed no colocalization of NLRP3 (Fig. 5a, f). Our immunolabeling study showed, in fact, an immunofluorescence intensity of Chat interneurons that was comparable between R6/2 and WT at both time points (Fig. 5d). No NLRP3 immunoreactivity was observed in those interneurons (Fig. 5f). Our study revealed an initial expression of NLRP3 in neurons containing parvalbumin in the 4-week-old R6/2 mice. NLRP3 expression levels culminated in the 13-week-old R6/2 mice in which, also, the number and the immunofluorescence intensity of parvalbumin containing neurons resulted statistically significant decreased (Fig. 5e). The mean calculated Mander's colocalization coefficient was near 1 (Fig. 5f). In the 4-and 13-week-old Wt mice, in which there was no statistically significant expression of NLRP3, parvalbumin neurons did not decrease in number.

Microglia activation in R6/2 mice
Iba-1 immunofluorescence was performed to study the activation of microglia at different time points. Both at 4 and at 13 weeks, wild-type mice displayed a ramified or primed microglia morphology, with a bigger cell body, but with processes similar to that of ramified microglia. The presence of dystrophic microglia in the 13-week-old-wildtype mice, on the other hand, can be attributed to the aging/senescence process. The immunostaining for Iba-1 in 4 and 13 weeks R6/2 group revealed an intense microglial reaction, where microglial cells appeared numerous and displayed an ameboid cell body in which are still present few ramified or unramified processes (Fig. 6b, c). Microglial reaction increased over time, concurrently with the significantly higher expression levels of NLRP3 in other cells of the striatum. However, unlike previously described 13 , we did not observe NLRP3 immunoreaction in the Iba-1 labeled microglia 14 .

Discussion
Anomalous apoptosis has been reported to play a role in Huntington's disease pathology for many years [15][16][17] . Apoptotic cells and DNA degradation products were observed both in HD patients brains 15,16,18 and in experimental HD models 16,19,20 . Moreover, our group previously described the neuroprotective effects exherted by Poly (ADP-ribose) polymerase 1 (PARP-1) inhibition in the mouse R6/2 model 21 , that are postulated to be mediated by an inhibition of apoptosis 22 . An emerging role for non-apoptotic programmed cell death has been described in neurodegenerative diseases in most recent years 23 . Pyroptosis is an inflammatory form of regulated necrosis which is alternative to apoptosis 23 . Recent studies have shown that pathways of cell death are associated in different ways according to the activation of signaling cascades inducing apoptosis, necroptosis, and pyroptosis 24 . An increasing body of evidence suggests that receptor interacting protein kinase 1 and caspase-8 are the mediators of the cross-talk between apoptosis, necroptosis, and pyroptosis and determine the type of cell death induced in response to activation of cell death signaling 25 .
In this paper, we investigated the presence and behavior of pyroptosis in the R6/2 mouse model of Huntington's disease 26 .
The comparative study of the distribution of selected pyroptosis and apoptosis markers in the striatum showed that young mice, both wild type and R6/2, express high levels of the cleavage product of Caspase-8. We associate this phenomenon to the physiological process of cell homeostasis 25,27 . This was also confirmed by the lack of immunoreactivity for cleaved Caspase-8 in the 13-weekold mice, both wild type and R6/2.
The expression of NLRP3 was significantly high in the 13-week-old R6/2 mouse striatum, where it was particularly distributed into the spiny projection neurons labeled by calbindin immunofluorescence. These are the neurons that degenerate massively in Huntington's disease 11,[28][29][30] .
The novelty of our data is represented by the finding that the degeneration of these neurons is sustained by pyroptosis, which can account, at least in part, for their degeneration. Our data were corroborated by our immunohistochemical and molecular study of the pyroptosis marker, namely, cleaved Caspase-1 31 , which confirmed that those cells were undergoing a pyroptotic cell death 32 .
Furthermore, we observed that striatal interneurons that are typically resistant to neurodegeneration in HD such as cholinergic neurons, expressed low levels of NLRP3, thus confirming that they were excluded from the ongoing inflammasome process and related pyroptosisassociated striatal neuronal degeneration. Conversely, evidence of the inflammasome complex, which is part of the pyroptosis pathway, was observed in the parvalbuminergic and calretininergic striatal neurons, which are prone to degenerate in HD 31,33 . Thus, we confirmed that healthy neurons did not show signs of pyroptosis, while degenerating ones did.
Clearly, further studies are necessary to investigate the role of caspase 1 in promoting cleavage of IL1β and IL-18.
We studied the distribution of inflammasome NLRP3 in microglia. An activation of microglia associated with neuroinflammation is observed in Huntington's disease, and compounds that reduce microglia activation proved beneficial 12,13,22 . We observed here that reactive microglial cells were intensely stained for Iba1 in the 13-weekold R6/2 mice and devoid of NLRP3 immunoreactivity. NLRP3 immunoreactive product was observed in brain areas where microglia was activated, although we did not observe a colocalization. In a recent study 13 , microglial cells were found immunoreactive for NLRP3 in mice injected lentiviruses harboring shLgals3: Slight differences in the animal model used could explain this discrepancy. Moreover, it is conceivable that the inflammasome is contained in the degenerating cells rather than in those cells that facilitate this process.
However, our data support the evidence that reactive microglia is part of the inflammasome and contributes functionally to the pyroptosis process 34,35 .
Our interest in pyroptosis is driven by the concept that the inhibition of inflammatory response and pyroptosis was recently shown to counteract neurodegeneration in several disease models. Indeed, the inhibition of pyroptosismediated neurotoxicity is beneficial in cellular and animal AD models 36 . Moreover, cerebral ischemia-induced brain damage is alleviated by downregulating inflammatory response and pyroptosis mediated by Bromodomaincontaining protein 4(BRD4) 10 .
With this in mind, we have demonstrated that pyroptosis plays a role in the degeneration of striatal neurons in HD, and we infer that counteracting the process and the inflammasome with appropriate compounds could be beneficial in fighting Huntington's disease 37 .

Animals and tissue processing
All animal experiments, which satisfied ARRIVE guidelines, were performed in accordance with European Communities Council Directive (2010/63 EU) as adopted by the Santa Lucia Foundation Animal Care and Use and approved by Italian Ministry of Health. Transgenic R6/2 mice were obtained by ovarian transplant of hemi zygote females × B6CBAF1/J males, all obtained from Jackson Laboratories (Bar Harbor, ME). F1 mice were used to perform all experiments. Ten mice per group were used. No randomization was performed. The study groups included: R6/2 mice at 4 weeks of age, R6/2 mice at 13 weeks of age, Wild-type mice at 4 and 13 weeks. Mice were handled under the same conditions by one investigator at the same day and time. Genotyping was performed at 21 days of age, and all mice were weaned and housed four in each cage under standard conditions (room temperature: 20 ± 2°C; humidity: 60%) and a 12/ 12-h light/dark cycle (7:00 am-7:00 pm) with ad libitum access to food and water. Forty animals (10 R6/2 4 weeks, 10 R6/2 13 weeks, 10 wild type 4 weeks and 10 wild type 13 weeks) were transcardially perfused under deep anesthesia with saline solution containing 0.01 ml heparin, brains were removed and cut in half. One half brain was post-fixed in 14 ml of 4% paraformaldehyde, 10% sucrose and 20% glycerol in 0.1 M phosphate buffer (PB) at +4°C for 48 h. Sectioning was performed on a sliding frozen microtome at 40 μm thickness. Observers who were in blind, collected primary data.

Immunohistochemical studies Evaluation of activated Caspase-8 and NLRP3
Coronal brain sections were incubated with the apoptotic marker Caspase-8 (polyclonal anti-Caspase-8, Abcam, Novus Biologicals, Italy). The antibody specifically recognized the cleaved and activated Caspase-8 protein. Antigen retrieval was performed in Citrate Buffer (pH 6) for 20 min at 80°C. After that, sections were retained in this buffer solution while allowing it to cool at room temperature. Sections were rinsed three times for 5 min at room temperature and subsequently incubated with the primary antibody. The immunohistochemical staining was completed with the streptavidin-biotin amplification (Jackson Immunoresearch, West Grove, PA, USA). A set of sections was processed for the inflammasome marker NLRP3 (polyclonal anti-NLRP3, Abcam, Novus Biologicals, Italy) at 1:200 dilution in a 0.1 M PB solution containing 0.3% Triton X-100 for 72 h at 4°C. All sections were rinsed three times for 5 min at room temperature and subsequently counterstained with Neurotrace TM (Nissl-like fluorescent marker, Jackson, USA) to calculate the number and the immunofluorescence intensity of neurons containing Caspase-8 and NLRP3. Sections were mounted on specimen slides, cover slipped with GEL-MOUNT (Sigma-Aldrich, Italy) and a confocal laser scanning microscope (Zeiss LSM 800) was used to acquire all the images. A sample of about 100 neurons for each of three sections obtained by the 10 mice per group were analyzed, in order to determine the distribution of the cleavage product of Caspase-8 and the immunoreaction product of NLRP3 in the striatum of R6/ 2 and wild-type mice at the different time points 4 and 13 weeks.

Immunohistochemistry for Caspase-1
To evaluate the involvement of inflammasome complex in our system, we performed an immunohistochemistry for Caspase-1. Peroxidase-antiperoxidase diaminobenzidine tetrahydrochloride single-label immunohistochemistry for Caspase-1 was performed to identification and quantification of neurons undergoing pyroptosis in the striatum. diaminobenzidine tetrahydrochloride-labeled sections were then washed in distillated water, placed in 0.1 M PB, mounted on gelatin-coated slides, dried, dehydrated and coverslipped. Caspase-1 positive cells count was performed on collected images obtained by Neurolucida TM Stereo Investigator software (Zeiss, Rochester, NY, USA).

Analysis of NLRP3 in striatal projection neurons
Double-label immunofluorescences were carried out to evaluate the distribution of NLRP3 in calbindin labeled striatal neurons and its expression levels in the mice striatum. Coronal brain sections of mice were incubated with a cocktail of anti-NLRP3 antibody and a mouse antibody against Calbindin protein (mouse anti-Calbindin, Abcam, Novus Biologicals, Italy) at 1:200 dilution in a 0.1 M PB solution containing 0.3% Triton X-100 for 72 h at 4°C. After that, sections were rinsed three times for 5 min at room temperature and subsequently incubated with the Alexa Fluor 555 and 488 secondary antibodies (Immunological Science, Italy) for 2 h at room temperature at 1:200 dilution in a 0.1 M PB solution containing 0.3% Triton X-100. Sections were then mounted on slides, cover slipped with GEL-MOUNT (Sigma-Aldrich, Italy). A confocal laser scanning microscope (Zeiss LSM 800) was used to acquire all of the images. Three separate fields (dorsolateral, central and medial each 1 mm in diameter) in each of three rostro caudally spaced sections of ten mice per group were examined. NLRP3 and Calbindin immunofluorescence intensity was measured and quantified by using the Java image processing and analysis program available in, Fiji ImageJ. All confocal images were acquired under no saturation conditions, with a ×20 objective raising a ×1 zoom with value 0 of Offset and producing images in the format 1024 × 1024, Airy Units 1.0. The same set configuration was performed for all samples. Colocalization of NLRP3 in striatal projection neurons was calculated performing Coloc2 analysis, the plugin provided by Fiji ImageJ.
The mean Mander's coefficients, obtained by the plugin Coloc2 performing Costes Autothresholds on samples regions of interest (ROIs) and running the analysis on 100 randomized images, varies from 0 to 1, corresponding to non-overlapping images and 100% or quite colocalization between the two images, respectively.

Striatal interneurons characterization
A double immunohistological staining for striatal interneurons markers and the pyroptosis marker NLRP3 was performed. Brain sections were incubated with goat anti-choline acetyl transferase (ChAT; Nova biological, CA, USA); mouse anti-calretinin (CALR; Chemicon International, Inc., Temecula, CA, USA); mouse antiparvalbumin (PARV, Chemicon International, Inc., Temecula, CA, USA) and polyclonal anti-NLRP3. All primary antibodies were used at a 1:200 dilution, in 0.1 M PB containing 0.3% Triton X-100 for 72 h at 4°C. Sections were rinsed three times for 5 min at room temperature and subsequently incubated with secondary antibodies Alexa Fluor 488 and 555 for 2 h at room temperature at 1:200 dilution in a 0.1 M PB solution containing 0.3% Triton X-100. Subsequently, sections were mounted on slides, cover slipped with GEL-MOUNT and examined under an epi-illumination fluorescence microscope (Zeiss Axioskop 2). The confocal laser scanner microscopy (Zeiss LSM800) was used to acquire images. Immunofluorescence intensity and colocalization analysis were performed by using the Java image processing and plugin analysis program included in Fiji ImageJ.

Microglial morphology
Microglial morphology was studied by a double immunostaining with an antibody for microglia (goat anti-Iba-1 from Novus Biologicals, Italy) and NLRP3. Striatal brain sections were incubated with the primary antibodies for 72 h at 4°C, followed by incubation with the appropriate secondary antibodies for 2 h at room temperature. Images were acquired by confocal laser scanner microscopy (Zeiss LSM 800) in order to perform soma size analysis. Microglia cells in the area of interest were captured using a ×20 objective raising a ×1 zoom and images in the format 1024×1024, Airy Units 1.0 were produced. This configuration was used for all samples. Collected images were exported in TIFF format, brightness and contrast were adjusted. The area of soma of Iba-1 positive cells was characterized by using ×63 Z-stack images, performing the school analysis available in the Java image processing and analysis program Fiji Image J. The Iba-1 immunostained area was calculated ad Iba-1 area/total area analyzed and indicated as percentage.