Rats transgenic for Huntington’s disease (tgHD51 CAG rats), surviving up to two years, represent an animal model of HD similar to the late-onset form of human disease. This enables us to follow histopathological changes in course of neurodegenerative process (NDP) within the striatum and compare them with postmortem samples of human HD brains. A basic difference between HD pathology in human and tgHD51 rats is in the rate of NDP progression that originates primarily from slow neuronal degeneration consequently resulting in lesser extent of concomitant reactive gliosis in the brain of tgHD51 rats. Although larger amount of striatal neurons displays only gradual decrease in their size, their number is significantly reduced in the oldest tgHD51 rats. Our quantitative analysis proved that the end of the first year represents the turn in the development of morphological changes related to the progression of NDP in tgHD51 rats. Our data also support the view that all types of CNS glial cells play an important, irreplaceable role in NDP. To the best of our knowledge, our findings are the first to document that tgHD51 CAG rats can be used as a valid animal model for detailed histopathological studies related to HD in human.
Huntington’s disease (HD) is an autosomal dominant inherited disorder belonging to the group of systemic brain atrophies. General clinical symptoms are defined by early changes in personality and cognitive functions, followed by typical involuntary choreatic movements; advanced stages of the disease include bradykinesia, rigidity, and dementia. The first symptoms appear frequently between 35 and 50 years of age. The disease is always fatal with an average survival of 10–15 years after the onset of the first symptoms. The disease may develop at any time, even during childhood and adolescence. Of note, juvenile form of HD represents 6% of HD patients [
Histopathologically, HD is characterised by premature death particularly of medium-sized (mainly GABA-ergic) striatal neurons but large interneurons are mostly spared (e.g., [
Severity of the degeneration is evaluated using grading system proposed by Vonsattel and coauthors [
Genetic mutation on short arm of chromosome 4, which causes HD, was discovered in 1983 [
Mutant form of huntingtin (mhtt) comprises up to 40 repeats and individuals with 36–39 CAG repeats are in risk of developing adult (late-onset) form of HD. Juvenile form of HD develops in patients with 55 and more repeats (e.g., [
Generation of transgenic mice as well as transgenic rats advanced significantly the understanding of HD pathology. Indeed, over 20 different rodent models of this disease have been generated to date (for the review see [
In 2003, von Hörsten and coworkers generated the first transgenic rat model of HD (tgHD51 rats; Sprague-Dawley background), which carries a truncated htt cDNA fragment with 51 CAG repeats under the control of the native rat htt promoter [
Astrocytes are the most numerous type of glial cells in mammalian CNS. Currently, they are considered as highly active cellular component of the CNS parenchyma with functional pleiotropy essential for neuronal survival and function (e.g., [
The communication between neurons and local blood flow mediated by astrocytes is elementary for the maintenance of functional microenvironment in the grey matter of the entire CNS parenchyma; therefore the term neuronal-glial-vascular unit is used [
Glial fibrillary acidic protein (GFAP [
Beta-subunit of Ca2+ binding protein (S100
NG2 glia (polydendrocytes or synantocytes) represent a fourth type of glia in the CNS (e.g., [
Microglia, the immunocompetent highly motile cells of the CNS, are extremely plastic and undergo a variety of structural changes based on their location and current role [
It is commonly known that the neurodegenerative process of HD phenotype is a chronic process, morphologically characterized by the progressive degeneration of neurons, principally in the striatum, but gradually affecting almost all parts of the brain. This results in a reduction of grey matter and brain atrophy with compensatory enlargement of the lateral brain ventricles. Nevertheless, also as the second component of the brain parenchyma, the glial cells play an irreplaceable role in this process. The reaction of astrocytes to any damage of the CNS parenchyma in a sense of their conversion into the reactive intensely GFAP-positive subset is well known already for long time. Although the participation of other types of glial cells, particularly of microglia and NG2 glia, in neurodegenerative process has been studied in last two decades, the histopathological interrelations among all above-mentioned cell types have not been well described yet. Moreover, the validation of existing transgenic rat model of HD51 from this point of view is still lacking.
All animal procedures were performed in accordance with the directive of the EEC (86/609/EEC) and the use of animals in our experiments was reviewed and approved by the Animal Ethical Committee of Charles University in Prague, Faculty of Medicine in Hradec Králové.
Male homozygous tgHD51 CAG rats (+/+;
The brains of three patients with approximately 2-, 8-, and 20-year clinical manifestation of HD (sex/age: ♀/52, ♂/38, and ♀/52) were studied. Three control brains of patients (sex/age/weight: ♂/33/1440 g, ♀/43/1510 g, and ♂/56/1400 g) with no history of neurologic disorder or brain lesion were also taken for the study. The clinical features of HD (described in autopsy records) were characteristic for the given stages in all three patients. However, detailed neurological records or results of genetic testing were not available because old archival material was used. Surprisingly, the total brain weight was markedly reduced in all HD cases independent of the sex and duration of the illness (duration/sex/weight: 2 y/♀/1160 g, 8 y/♂/1150 g, and 20 y/♀/1120 g) in comparison with control human brains. The severity of striatal histopathological changes was graded (grades 0–4) according to Vonsattel and coauthors [
Paraffin blocks of brain tissue from autopsies were taken from the neostriatum (the caudate nucleus and putamen) at the level of the globus pallidus and at the level of the nucleus accumbens. The blocks were donated by Fingerland’s Department of Pathology, Faculty Hospital in Hradec Králové.
Animal brains were processed either with formalin fixation (4% neutral formaldehyde) and embedded in paraffin or with 4% paraformaldehyde (in 0.1 M phosphate buffer) fixation and frozen sections preparation. The transcardial perfusion with fixative solution under deep anaesthesia followed by postfixation (for 3 days or 2 hours, resp.) was made in all animals. Brains were transversely cut using the Brain Blocker (Better Hospital Equipment Corp., USA) to obtain the identical blocks of brain tissue. After postfixation, the brain hemispheres were separated and processed separately.
Histological processing was the same for both the experimental material and autopsies. Serial coronal sections (7
For immunohistochemical detection, deparaffinized and rehydrated sections were used. Pretreatment in microwave 3 × 5 min at 800 W in the sodium citrate buffer (pH 6.0) and washing in 0.01 M PBS was mostly performed. For detection of polyglutamine deposits the pretreatment with 98% formic acid (5 min at room temperature) was required. Incubation in blocking solution (water solution of H2O2) for 20 min was followed by incubation with primary antibodies (Table
Antibodies used.
Antibody | Host | Dilution | Source | Report |
---|---|---|---|---|
Nestin | Rat monoclonal anti-mouse | 1 : 4 | DSHB | Progenitor cells marker |
Nestin | Mouse monoclonal |
1 : 200 | Millipore | Progenitor cells marker |
|
Mouse monoclonal | 1 : 20 | Exbio | Neuronal marker |
MAP2 | Mouse monoclonal | 1 : 500 | Sigma-Aldrich | Neuronal marker |
MAP2 | Rabbit polyclonal | 1 : 700 | Millipore | Neuronal marker |
NeuN | Mouse monoclonal | 1 : 100 | Millipore | Marker of mature neurons |
Synaptophysin | Mouse monoclonal | 1 : 20 | Dako | Marker of neuronal synapses |
Vimentin (Cy3 conjugated) | Mouse monoclonal | 1 : 100 | Sigma-Aldrich | Astrocyte and radial glia marker |
NG2 | Rabbit polyclonal | 1 : 400 | Millipore | Oligodendrocyte precursor and pericyte marker |
APC | Mouse monoclonal | 1 : 200 | Calbiochem | Oligodendrocyte and astrocyte marker |
GFAP | Mouse monoclonal | 1 : 400 | Sigma-Aldrich | Astrocyte marker |
GFAP | Rabbit polyclonal | 1 : 400 | Dako | Astrocyte marker |
S100 |
Mouse monoclonal | 1 : 1000 | Sigma-Aldrich | Astrocyte marker |
S100 |
Rabbit polyclonal | 1 : 300 | Dako | Astrocyte marker |
Iba1 | Mouse monoclonal | 1 : 300 | Millipore | Microglia and macrophage marker |
PolyQ-huntingtin | Mouse monoclonal | 1 : 20000 | Millipore | Polyglutamine inclusions marker |
Postfixed blocks of rat brains were placed stepwise in solutions with gradually increasing sucrose concentrations (10%, 20%, and 30%) for cryoprotection at 4°C. Serial coronal cryostat free-floating sections (30
The sequential technique for immunofluorescent double-labelling of antibodies (Ab) was same for both types of sections. Avidin or appropriate secondary Ab was labelled with Cy-3 or Alexa Fluor 660 (red) and with Alexa Fluor 488 or 594 (green) and nuclei were counterstained with DAPI (blue). The negative control, omitting the primary antibody, was made in each labelling.
Photomicrographs were made with Lucia G/F software version 4.82 (Laboratory Imaging, Prague, Czech Republic) or Quick Photo Camera 2.3 software (Promicra, Prague, Czech Republic).
In order to characterize the progression of NDP in the striatum of tgHD51 rats, we used the quantitative analysis of the median diameter of neuronal nuclei as a marker of proposed significant process in a course of neurodegeneration in tgHD rats; it means the shrinkage of striatal neurons. We would like also to determine the onset of significant neuronal degeneration in the striatum of tgHD51 rats and the possible participation of age-related changes.
The region of rat brain, used for analysis, was determined according to the brain atlas [
The groups of tgHD and wt rats were divided into two basic subgroups of “young” 3- and 6-month-old (0–6 months) and “old” 12-, 18-, and 24-month-old (>6 months) rats. The number of analysed sections was the following: 6 sections in the “young_wt” group, 6 sections in the “young_tgHD” group, 18 sections in the “old_wt” group, and 24 sections in the “old_tgHD” group. Neuronal nucleus median diameter was obtained from 50 independent measurements in the central area of the striatum on each analysed section. Due to possible distortions of the shape of neuronal nucleus in the section, the largest size of the nucleus was considered the nucleus diameter. Each group of rats of the same age was represented by the set of all medians in given group.
Statistical analyses of the differences between groups were performed using MS Excel 2007 (Microsoft Corp., Redmond, WA, USA) and NCSS 2007 (NCSS LLC, Kaysville, Utah, USA). The median diameters were compared using Kruskal-Wallis Multiple-Comparison
In tgHD51 rats, the NDP in the striatum starts to develop only after 6 months of age. Surprisingly, the most distinct changes in striatal grey matter develop by the end of the first year of age (probably between 9 and 12 months). The end of the first year represents the turn in the development of morphological changes related to the progression of NDP within the striatum of tgHD51 rats. These findings correspond to the course of HD in human brain, where the motor and behavioural changes precede the loss of striatal neurons [
It is almost impossible to dissociate the alterations referring to neurons and glia in a course of NDP because of very close relationship and mutual influence of both main components of striatal parenchyma. However, we would like to stress some features specific for each of them in a course of the development of NDP within the striatum of both rat and human brains. For that reason, we described their involvement in progression of NDP separately.
When we compare the brains of 2- and 3-month-old, (young adult) wild-type and tgHD rats, there is no difference in morphology of the striatum. Also lateral brain ventricles are narrow, of the same shape in both mentioned groups (Figure
(a) Lateral brain ventricles are quite narrow in young (3-month-old) control wt rats, unlike (b) notably enlarged ventricles in 18-month-old tgHD51 rats, owing to the progression of striatal atrophy which confirms the development of NDP.
Striatal neurons (
Significant decrease in the size (diameter) of the striatal neurons in a course of the development of NDP is marked by accompanied changes in the diameter of their nuclei, due to the maintenance of the nucleo-cytoplasmic rate. Detection of the
Significant difference in the number and size of neurons/neuronal nuclei (NeuN+) is evident if we compare (a) 2-month-old wt rats and (b) 18-month-old tgHD rats; (b) concomitant reactive astrogliosis is already developed in 18-month-old tgHD rats; it is also apparent that the degeneration of neurons in tgHD rats is typically selective (alike in human HD brain).
In postmortem samples of human HD brain, (b, c) gradual progression of chronic striatal NDP marked by massive neuronal degeneration and severe concomitant astrogliosis is evident in comparison with (a) intact control brain. (a) Control (♂/56), (b) HD duration 8 years, grade 3 (♂/38), and (c) HD duration 20 years, grade 4 (♀/52).
Nuclei of striatal neurons are very characteristic, especially due to their large size and fine loosely arranged chromatin in comparison with significantly smaller nuclei with more densely arranged chromatin of glial cell. Despite the fact that striatal neurons become gradually smaller in course of HD progression (compare Figures
In the human HD brain, grades 1-2 (with approximately 2-year clinical manifestation), the degeneration and loss of neurons were only random; therefore, the loosening of the neuropil has not been apparent yet. On the other hand, in grade 3 (approximately 8-year clinical history), neuronal degeneration was already obvious (Figure
Additionally, we confirmed that, alike in human HD brain, neuronal degeneration is selective, that is, affecting primarily certain groups of neurons in the striatum of investigated senescent tgHD rats and moreover that age-related changes contribute to final extent of NDP.
We supposed that neuronal degeneration in the striatum of tgHD rats manifests primarily by the decrease in a volume/size of neuronal bodies including their nuclei. In order to precisely characterize the progression of NDP within the striatum of tgHD51 rats, our morphological findings were supplemented by quantitative analysis of the diameter of neuronal nuclei labelled with NeuN. Also the proportion of age-related changes in this process was assessed.
The progression in decrease of the median diameter of neuronal nuclei with age of rats in both wt and tgHD groups of rats is documented by Progress Chart (Figure
(a) Progress Chart: the progression in decrease of the median diameter of neuronal nuclei with age of rats in wt groups (black line) and tgHD51 rats (grey line). (b) Box Plot: statistical characteristic of the groups of rats. The multiple comparison of the median diameter of neuronal nuclei of the groups of rats: “young_wt” and “young_tgHD” are groups of rats 3 and 6 months (0–6 months) old; “old_wt” and “old_tgHD” are groups of rats 12, 18, and 24 months old (>6 months).
Statistical characteristic of the groups of rats using Box Plot (Figure
Results of Kruskal-Wallis Multiple-Comparison
Group | young_tgHD | young_wt | old_tgHD | old_wt |
---|---|---|---|---|
young_tgHD | ||||
young_wt | 0.2844 | |||
old_tgHD | 4.9557* | 4.5959* | ||
old_wt | 1.813 | 1.4646 | 4.5135* |
We can conclude that the rate of neuronal degeneration reaches maximum at the end of the first year of animal’s age and then, in the following 12 months, it proceeds rather slowly. Moreover, it is potentiated with age-related changes particularly in the oldest animals. Unexpectedly, the transitional amelioration of the process up to slight improvement appeared in both groups (wt and tgHD) of 18-month survivors. Neuronal degeneration in wt rats can be attributed only to the debit of the aging process; the decrease in size of nuclei was slow and the difference between 2-3-month-old rats and 24-month-old ones was only 8.3% (Figure
Striatal atrophy, in the case of HD, is primarily caused by the degeneration of striatal neurons. Of course, the most prominent feature, seen on histological preparations, is a gradual reduction of neuronal bodies marked by the nuclei. Indeed, the reduction in a volume of neuropil is at least of the same importance. Large amounts of dendrites (with dendritic spines) extending from neuronal bodies (labelled with anti-MAP2—Figure
(a) In young control animals (2 months old), synapses within the neuropil are very numerous, fine and of uniform size; (b) the most conspicuous alteration in old (here 18 months) tgHD rats is variable size and enlargement (coarsening) of most of synapses; however, also their decreased number participating in loosening of neuropil is evident.
(a) Synapses in intact (control, ♂/56) human brain are (like in rats) uniform and densely accumulated within the neuropil; (b) they also become coarser and of variable size with the progression of NDP in HD patients (here ♀/52, grade 4, duration 20 years); continuous decrease in their number significantly participates in rarefaction of the neuropil, most prominent in terminal stage of NDP.
(a) Only fine polyQ deposits are spread in the nuclei of striatal neurons in control 6-month-old rats unlike (b) a higher density of polyQ deposits in age-matched tgHD rats; (c) significantly increased density of polyQ inclusions, in both neurons and some glial cells
Detection of polyglutamine deposits using polyQ-huntingtin provides interesting findings, which give a complete histopathological picture of HD progression. In wt rats, polyQ detects a normal polyglutamine domain huntingtin encoded by lower number (about 35 or less) of consecutive glutamine repeats; therefore, only fine polyQ deposits are spread in the nuclei of striatal neurons (Figure
Moreover, the highest density of deposits in shrunk/hyperchromic terminally degenerated neurons (less prominent in glial cells) corresponds to the hypothesis that accumulation of mhtt results in the cell death.
It is evident that the developments of changes in glial cell morphology, and certainly also in their function, are conditioned by the intensity and rate of neuronal degeneration in the context of the neuron-glia relationship.
Protoplasmic astrocytes are the most numerous component of the striatal parenchyma. Despite their standard visualisation by detection of GFAP, most of them are GFAP-negative.
The shape of astrocytes changes during the progression of NDP; however specific prominent alterations occur only in human HD brains, where the astrogliosis gradually develops (Figures
(a) In young (2-month-old) wt rats S100
In HD human brain, significant “reactive” astrogliosis develops with the progression of NDP (b, c) in comparison with control brain (a); also the fine GFAP+ processes become numerous and of characteristic arborization; specific terminal swellings are in most of the densely GFAP+ processes; although the amount of S100
In HD human brains, besides the gradually increased number of reactive astrocytes, their fine GFAP+ processes become numerous although shorter in comparison with control brains and of characteristic arborization, forming a fine loosely arranged network, whose density increases with the progression of the disease. Moreover, we identified the specific terminal swellings in most of densely GFAP+ processes, whose number and also size slowly but gradually increase with the progression of HD (Figures
Despite the fact that the astrocytes, engaged in NDP, are described as “reactive,” it is necessary to point out that they are of quite different structure in comparison with typical reactive astrocytes appearing after the acute brain injury. First of all, in both HD patients and tgHD rats, generation of reactive astrocytes proceeds gradually and slowly, unlike the almost immediate appearance of reactive astrocytes after the acute brain damage. Indeed, their bodies are not significantly enlarged (hypertrophic); contrariwise, a part of them also undergoes the degeneration and they are scavenged by microglia (Figures
S100
They are closely related to the development of myelinating oligodendrocytes, whose precursors are considered. We were interested in their possible alterations related to the progression of NDP in tgHD rats and also in senescence. Due to technical reasons (particularly the length of formalin fixation and the use of paraffin sections only), we were not able to detect NG2 glia cells in postmortem samples of human brains. In the rat brain, we did not prove any significant changes either in their number or in morphology in a course of the development of NDP, even in ageing process. In all examined samples, they were numerous (e.g., they outnumbered the GFAP+ astrocytes) and large number of their fine branched processes forms a very dense three-dimensional network throughout the entire striatal parenchyma, particularly in the grey matter (Figures
NG2 glia forms a dense 3D network, particularly prominent on thick (30
(a) Microglia
(a) In control human brain (♂/33), microglia
They represent a special type of professional phagocytes occurring only in CNS, which are spread out primarily within grey matter (Figures
In human brain, in relation to advancing NDP, the growing number of microglia is also observed (Figure
In summary, the hallmark of NDP in tgHD51 rats is a slow degeneration of striatal neurons, manifested primarily by gradual decrease in size of neuronal bodies/nuclei (with maintenance of nucleo-cytoplasmic rate) accompanied with the rarefaction of neuropil. Using the quantitative analysis, we clearly demonstrated for the first time that the turn point in the progression of neurodegenerative process in tgHD51 rats is before the end of the first year of animal age. Then, between 12 and 24 months of age, the further progression is gradual but at a slower rate, resulting in death of many neurons. Moreover, we confirmed that the development of NDP within the striatum is accompanied with gradual degeneration of cortical, particularly pyramidal neurons. We also documented significant participation of the glia, of which function in the development of NDP is irreplaceable. Most prominent is the involvement of GFAP+ astrocytes, particularly their transformation into the specific type of reactive astrocytes. This transformation is responsible for alterations in a structure (and therefore also in a function) of the perivascular glial limiting membrane, loosening of neuropil, and other changes. Surprisingly, we cannot confirm noticeable changes in morphology or number of NG2 glia in tgHD rats, unlike significant participation of microglia and, although less prominent, involvement of S100
The aim of our study was primarily motivated by the absence of histopathological characteristics of chronic neurodegenerative process of HD phenotype in transgenic HD51 CAG rats, which, unlike the other transgenic animal models of HD, survive up to 2 years. Moreover, this transgenic model comprises relatively smaller number (51) of CAG repeats. Both mentioned hallmarks create conditions for similarity to the late-onset form of HD. For this reason, we tried to define to which extent is possible to make a parallel between rat tgHD model and real NDP in human HD brain from histopathological point of view. On the other hand, behavioural symptoms were already widely studied on these animals
Despite the fact that HD takes place exclusively within human brain and each type of existing animal models is not able to replicate completely mechanisms participating in NDP of the HD phenotype, transgenic models represent a crucial part in the field of the research on HD pathogenesis.
It is generally known that HD is a neurodegenerative movement disorder caused by genetic mutation and morphologically characterized by progressive but selective loss of neurons, primarily within the striatum, followed by the development of reactive gliosis (e.g., [
Reactive astrogliosis is a gradual continuous process of progressive alterations in gene expression and cellular changes. The intensity and extent of the reactive gliosis are determined primarily by signals from damaged cells (e.g., [
Owing to the fact that only very low (if any) response of neural progenitors to the CNS damage is found in all nonneurogenic regions, the source for generation of GFAP+ reactive astrocytes remains under discussion—two plausible possibilities are proposed: (1) transformation or dedifferentiation from resident astrocytes which has been confirmed after different neurotoxic lesions in rodent brain (e.g., [
It is evident that the development of reactive gliosis and the alterations in astrocyte morphology are conditioned by the intensity and rate of neuronal degeneration in the context of the neuron-glia relationship [
Except for others, very important is also the relationship of astrocytes to the vessels. Our previous findings documented the thickening of end-feet (and this way of the perivascular limiting membrane), which markedly outlines vessel walls forming typical “rings” with the progression of NDP in excitotoxic rat model of HD [
It is also well known that astrocytes are involved in degradation of different metabolites and toxic products under the normal conditions. Their ability to function as phagocytes is then enhanced under the pathological conditions (e.g., [
To our surprise, we did not find any significant alterations in number and morphology of NG2 polydendrocytes in the presence of pathology. They formed a typical dense network in all tested groups of tgHD and wt rats. Due to technical reasons, we were unfortunately not able to follow them in postmortem specimens of human brains, either with HD or intact. It is probably the main reason why the involvement of NG2 glia in NDP has not been reported yet in brains of HD patients. The elegant study employing postmortem specimens of human brain tissue with multiple sclerosis (MS) lesions or glioma (using both frozen and paraffin sections) was published by Staugaitis and Trapp [
It is obvious that glia, particularly GFAP-positive astrocytes, play a critical role in the progression of NDP. Nevertheless, their involvement is always in a context of changes affecting simultaneously all components—cellular and noncellular—of the nervous tissue microenvironment. Also microglia is suggested to be an important player in complex response of nervous tissue to the chronic damage, principally by the activation of cascade of the immune response.
The evaluation of the contribution of GFAP+ astrocytes to neurodegenerative process remains still unclear, albeit their dual role—neuroprotective and neurodegenerative—was already mentioned by some authors (e.g., [
Regardless of the interesting and challenging findings related to the different types of glial cells, the degeneration and loss of neurons, primarily within the striatum, accompanied with loosening of the neuropil still remain the hallmark of HD. However, the disintegration of the neuropil not only results from gradual reduction of neuronal processes, but also includes the alterations of the glial cells. Indeed, regression of the neuropil represents the progressive disruption of the striatal microenvironment, including the vascular niche, essential for the viability and functioning of all the components of striatal tissue, which even more contributes to worsening of the entire process. The reduction of the grey matter results in a severe brain (primarily striatal) atrophy in advanced NDP.
Transgenic HD51 CAG rats were used also by other research groups, mainly to validate changes in their behaviour, which indicates impaired striatal function (e.g., [
Concerning morphological evaluation, it was shown that polyQ aggregates appear within the striatum of tgHD rats at 12 months of age [
In this study, histopathological features, significant for alterations in human brains suffered from HD and brains of tgHD51 CAG rats, were compared. The significant difference is primarily in the intensity of neuronal degeneration which, even in the oldest rats, does not reach the severity of the damage characteristic for advanced stages of HD in humans. In tgHD51 rats, quite large amounts of striatal neurons display only gradual decrease in their size and do not fully degenerate and disappear until the death of animal (at about 2 years of age). Therefore, also concomitant reactive astrogliosis is not extensive in old tgHD rats, unlike the advanced stages of HD in humans. On the other hand, striatal atrophy (the reduction of grey matter) develops gradually, and it is distinguishable from 12 months of age in tgHD rats, manifested primarily by compensatory enlargement of lateral brain ventricles. The reason for striatal shrinkage is the same in rats and humans; it originates from the gradual degeneration of neurons resulting in rarefaction of the neuropil including the alterations in the density and character of synapses. Our data also support the view that all types of CNS glial cells play an important, irreplaceable role in neurodegenerative process. There are different indications that changes in their character and function try to balance the development of serious irreversible damage of the striatum as long as possible. However, when this battle exceeds all their capacities, the system collapses and their involvement becomes detrimental, finally resulting in death of an individual. All the available results in this field directly or indirectly confirm that the response of nervous tissue to any damage is always complex (moreover in relation to the entire organism) and therefore has to be considered in this context.
To the best of our knowledge, our findings are the first to document that tgHD51 CAG rats can be used as a valid animal model for detailed histopathological studies related to HD in human. On the other hand, since HD occurs only in humans and we are not able to reproduce fully this process in animals till now, the interpretation of all findings has to be always carefully weighted and sufficiently critical
The authors declare that there is no conflict of interests regarding the publication of this paper.
All authors have read and approved the final paper.
The authors would like to thank M. Hetešová, S. Vrchotová, Z. Komárková, J. Hošková, laboratory technicians at the Department of Histology and Embryology, and H. Pavlíková at the Department of Cellular Neurophysiology, for excellent technical assistance, and also D. Ježková from Radioisotope Laboratories and Vivarium for perfect care of rats. This study was supported by the Charles University Research Program PRVOUK P3706140/06 and PRVOUK P3706140/09 and by the Grant Agency of the Czech Republic (GAČR) P304/12/G069.