Irradiation exacerbates cortical cytopathology in the Eker rat model of tuberous sclerosis complex, but does not induce hyperexcitability

mTOR inhibitor improves autistic-like behaviors related to Tsc2 haploinsufficiency but not following developmental status epilepticus

Background

Tuberous sclerosis complex (TSC), a multi-system genetic disorder often associated with autism spectrum disorder (ASD), is caused by mutations of TSC1 or TSC2, which lead to constitutive overactivation of mammalian target of rapamycin (mTOR). In several Tsc1+/- and Tsc2+/- animal models, cognitive and social behavior deficits were reversed by mTOR inhibitors. However, phase II studies have not shown amelioration of ASD and cognitive deficits in individuals with TSC during mTOR inhibitor therapy. We asked here if developmental epilepsy, common in the majority of individuals with TSC but absent in most animal models, could explain the discrepancy.

Methods

At postnatal day P12, developmental status epilepticus (DSE) was induced in male Tsc2+/- (Eker) and wild-type rats, establishing four experimental groups including controls. In adult animals (n = 36), the behavior was assessed in the paradigms of social interaction test, elevated plus-maze, light-dark test, Y-maze, and novel object recognition. The testing was carried out before medication (T1), during a 2-week treatment with the mTOR inhibitor everolimus (T2) and after an 8-week washing-out (T3). Electroencephalographic (EEG) activity was recorded in a separate set of animals (n = 18).

Results

Both Tsc2+/- mutation and DSE caused social behavior deficits and epileptiform EEG abnormalities (T1). Everolimus led to a persistent improvement of the social deficit induced by Tsc2+/-, while deficits related to DSE did not respond to everolimus (T2, T3).

Conclusions

These findings may contribute to an explanation why ASD symptoms in individuals with TSC, where comorbid early-onset epilepsy is common, were not reliably ameliorated by mTOR inhibitors in clinical studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s11689-021-09357-2.

Revisiting Brain Tuberous Sclerosis Complex in Rat and Human: Shared Molecular and Cellular Pathology Leads to Distinct Neurophysiological and Behavioral Phenotypes

Tuberous sclerosis complex (TSC) is a dominant autosomal genetic disorder caused by loss-of-function mutations in TSC1 and TSC2, which lead to constitutive activation of the mammalian target of rapamycin C1 (mTORC1) with its decoupling from regulatory inputs. Because mTORC1 integrates an array of molecular signals controlling protein synthesis and energy metabolism, its unrestrained activation inflates cell growth and division, resulting in the development of benign tumors in the brain and other organs. In humans, brain malformations typically manifest through a range of neuropsychiatric symptoms, among which mental retardation, intellectual disabilities with signs of autism, and refractory seizures, which are the most prominent. TSC in the rat brain presents the first-rate approximation of cellular and molecular pathology of the human brain, showing many instructive characteristics. Nevertheless, the developmental profile and distribution of lesions in the rat brain, with neurophysiological and behavioral manifestation, deviate considerably from humans, raising numerous research and translational questions. In this study, we revisit brain TSC in human and Eker rats to relate their histopathological, electrophysiological, and neurobehavioral characteristics. We discuss shared and distinct aspects of the pathology and consider factors contributing to phenotypic discrepancies. Given the shared genetic cause and molecular pathology, phenotypic deviations suggest an incomplete understanding of the disease. Narrowing the knowledge gap in the future should not only improve the characterization of the TSC rat model but also explain considerable variability in the clinical manifestation of the disease in humans.

Tuberous Sclerosis (tsc2+/-) Model Eker Rats Reveals Extensive Neuronal Loss with Microglial Invasion and Vascular Remodeling Related to Brain Neoplasia

Tuberous sclerosis complex (TSC) is a genetic disorder characterized by frequent noncancerous neoplasia in the brain, which can induce a range of severe neuropsychiatric symptoms in humans, resulting from out of control tissue growth. The causative spontaneous loss-of-function mutations have been also identified in rats. Herein, we studied histopathological and molecular changes in brain lesions of the Eker rat model carrying germline mutation of the tsc2 gene, predisposed to multiple neoplasias. Predominant subcortical tumors were analyzed, along with a rare form occurring within the pyriform lobe. The uniform composition of lesions supports the histochemical parity of malformations, with immunofluorescence data supporting their neuro-glial origin. Massive depletion of mature neurons and axonal loss were evident within lesions, with occasional necrotic foci implying advanced stage of pathology. Enrichment of mesenchymal-derived cell markers with hallmarks of neurogenesis and active microglia imply enhanced cell proliferation, with local immune response. The depletion of capillaries within the core was complemented by the formation of dense mesh of nascent vessels at the interface of neoplasia with healthy tissue, implying large-scale vascular remodeling. Taken as a whole, these findings present several novel features of brain tumors in Eker rat model, rendering it suitable for studies of the pathobiology and progression of primary brain tumors, with therapeutic interventions.

Heterozygous Tsc2 (Tsc2+/–) mouse model to study induced renal cancer in response to ionizing radiation at low doses

Kidneys are one of the main dose-limiting organs in radiotherapeutic procedures of lower abdomen. Likewise, the threat of exposure of radiosensitive organs such as kidneys in warfare or radiation accidents among military personal or due to terrorist activities in general public is of increasing concern. These events warrant the need for appropriate animal models to study the acute and chronic effects of low- and high-dose rate radiation exposures. In this study, for the first time, we validated Tsc2+/– mouse model to study whether radiation accelerates carcinogenesis in kidneys. Tsc2+/– mice at increasing age groups at 8 and 10 months were exposed to repeated doses of gamma radiation (0.4 Gy × 5) and assessed for aggravated kidney tumor formation at 2 months post-irradiation. Animals from irradiated group showed a significant increase in numbers of bilateral, multifocal tumors compared with mock-irradiated animals. Intra-glomerular reactive oxygen species (ROS) levels measured by dihydroethidium florescence showed significant increases in ROS production in irradiated Tsc2+/– mice compared with non-irradiated animals. Similarly, selective hematological parameters and glomerular filtration rate were further reduced significantly in irradiated Tsc2+/– mice. Tsc2 protein, tuberin in irradiated mice, however, remains at the same reduced levels as that of the mock-irradiated heterozygous Tsc2 mice. The results indicate that radiation alters kidney homeostatic function and influences high spontaneous incidence of renal cell carcinoma in this rodent model. Repurposing of Tsc2+/– mice model will, therefore, provide a unique opportunity to study acute and delayed effects of radiation in the development of kidney cancers.

Dampened hippocampal oscillations and enhanced spindle activity in an asymptomatic model of developmental cortical malformations

Developmental cortical malformations comprise a large spectrum of histopathological brain abnormalities and syndromes. Their genetic, developmental and clinical complexity suggests they should be better understood in terms of the complementary action of independently timed perturbations (i.e., the multiple-hit hypothesis). However, understanding the underlying biological processes remains puzzling. Here we induced developmental cortical malformations in offspring, after intraventricular injection of methylazoxymethanol (MAM) in utero in mice. We combined extensive histological and electrophysiological studies to characterize the model. We found that MAM injections at E14 and E15 induced a range of cortical and hippocampal malformations resembling histological alterations of specific genetic mutations and transplacental mitotoxic agent injections. However, in contrast to most of these models, intraventricularly MAM-injected mice remained asymptomatic and showed no clear epilepsy-related phenotype as tested in long-term chronic recordings and with pharmacological manipulations. Instead, they exhibited a non-specific reduction of hippocampal-related brain oscillations (mostly in CA1); including theta, gamma and HFOs; and enhanced thalamocortical spindle activity during non-REM sleep. These data suggest that developmental cortical malformations do not necessarily correlate with epileptiform activity. We propose that the intraventricular in utero MAM approach exhibiting a range of rhythmopathies is a suitable model for multiple-hit studies of associated neurological disorders.

Are developmental dysplastic lesions epileptogenic?

Cortical dysplasia of various types, reflecting abnormalities of brain development, have been closely associated with epileptic activities. Yet, there remains considerable discussion about if/how these structural lesions give rise to seizure phenomenology. Animal models have been used to investigate the cause-effect relationships between aberrant cortical structure and epilepsy. In this article, we discuss three such models: (1) the Eker rat model of tuberous sclerosis, in which a gene mutation gives rise to cortical disorganization and cytologically abnormal cellular elements; (2) the p35 knockout mouse, in which the genetic dysfunction gives rise to compromised cortical organization and lamination, but in which the cellular elements appear normal; and (3) the methylazoxymethanol-exposed rat, in which time-specific chemical DNA disruption leads to abnormal patterns of cell formation and migration, resulting in heterotopic neuronal clusters. Integrating data from studies of these animal models with related clinical observations, we propose that the neuropathologic features of these cortical dysplastic lesions are insufficient to determine the seizure-initiating process. Rather, it is their interaction with a more subtly disrupted cortical "surround" that constitutes the circuitry underlying epileptiform activities as well as seizure propensity and ictogenesis.

Zebrafish model of tuberous sclerosis complex reveals cell-autonomous and non-cell-autonomous functions of mutant tuberin

Tuberous sclerosis complex (TSC) is an autosomal dominant disease caused by mutations in either the TSC1 (encodes hamartin) or TSC2 (encodes tuberin) genes. Patients with TSC have hamartomas in various organs throughout the whole body, most notably in the brain, skin, eye, heart, kidney and lung. To study the development of hamartomas, we generated a zebrafish model of TSC featuring a nonsense mutation (vu242) in the tsc2 gene. This tsc2^vu242 allele encodes a truncated Tuberin protein lacking the GAP domain, which is required for inhibition of Rheb and of the TOR kinase within TORC1. We show that tsc2^vu242 is a recessive larval-lethal mutation that causes increased cell size in the brain and liver. Greatly elevated TORC1 signaling is observed in tsc2^vu242/vu242 homozygous zebrafish, and is moderately increased in tsc2^vu242/+ heterozygotes. Forebrain neurons are poorly organized in tsc2^vu242/vu242 homozygous mutants, which have extensive gray and white matter disorganization and ectopically positioned cells. Genetic mosaic analyses demonstrate that tsc2 limits TORC1 signaling in a cell-autonomous manner. However, in chimeric animals, tsc2^vu242/vu242 mutant cells also mislocalize wild-type host cells in the forebrain in a non-cell-autonomous manner. These results demonstrate a highly conserved role of tsc2 in zebrafish and establish a new animal model for studies of TSC. The finding of a non-cell-autonomous function of mutant cells might help explain the formation of brain hamartomas and cortical malformations in human TSC.

Does ketogenic diet alter seizure sensitivity and cell loss following fluid percussion injury?

Traumatic brain injury (TBI) frequently leads to epilepsy. The process of epileptogenesis - the development of that seizure state - is still poorly understood, and effective antiepileptogenic treatments have yet to be identified. The ketogenic diet (KD) has been shown to be effective as an antiepileptic therapy, but has not been extensively tested for its efficacy in preventing the development of the seizure state, and certainly not within the context of TBI-induced epileptogenesis. We have used a rat model of TBI - fluid percussion injury (FPI) - to test the hypothesis that KD treatment is antiepileptogenic and protects the brain from neuronal cell loss following TBI. Rats fed a KD had a higher seizure threshold (longer latency to flurothyl-induced seizure activity) than rats fed a standard diet (SD); this effect was seen when KD was in place at the time of seizure testing (3 and 6 weeks following FPI), but was absent when KD had been replaced by SD at time of testing. FPI caused significant hippocampal cell loss in both KD-fed and SD-fed rats; the degree of cell loss appeared to be reduced by KD treatment before FPI but not after FPI. These results are consistent with prior demonstrations that KD raises seizure threshold, but do not provide support for the hypothesis that KD administered for a limited time directly before or after FPI alters later seizure sensitivity; that is, within the limits of this model and protocol, there is no evidence for KD-induced antiepileptogenesis.

Mechanisms of epileptogenesis in tuberous sclerosis complex and related malformations of cortical development with abnormal glioneuronal proliferation

Malformations of cortical development (MCDs) are increasingly recognized as causes of medically intractable epilepsy. In order to develop more effective, rational therapies for refractory epilepsy related to MCDs, it is important to achieve a better understanding of the underlying mechanisms of epileptogenesis, but this is complicated by the wide variety of different radiographic, histopathological, and molecular features of these disorders. A subset of MCDs share a number of characteristic cellular and molecular abnormalities due to early defects in neuronal and glial proliferation and differentiation and have a particularly high incidence of epilepsy, suggesting that this category of MCDs with abnormal glioneuronal proliferation may also share a common set of primary mechanisms of epileptogenesis. This review critically analyzes both clinical and basic science evidence for overlapping mechanisms of epileptogenesis in this group of disorders, focusing on tuberous sclerosis complex, focal cortical dysplasia with balloon cells, and gangliogliomas. Specifically, the role of lesional versus perilesional regions, circuit versus cellular/molecular defects, and nonneuronal factors, such as astrocytes, in contributing to epileptogenesis in these MCDs is examined. An improved understanding of these various factors involved in epileptogenesis has direct clinical implications for optimizing current treatments or developing novel therapeutic approaches for epilepsy in these disorders.

The natural history and treatment of epilepsy in a murine model of tuberous sclerosis

PURPOSE

Patients with tuberous sclerosis complex (TSC) often have severe epilepsy that is intractable to available therapies. The development of novel treatments for epilepsy in TSC would benefit greatly from a suitable animal model, but most animal models of TSC to date have few reported neurological abnormalities, such as epilepsy. We previously described a novel model of TSC, due to conditional inactivation of the Tsc1 gene in glia (Tsc1(GFAP)CKO mice), in which mice develop epilepsy and premature death. Here, we characterize the natural history of the epilepsy in Tsc1(GFAP)CKO mice in more detail and report acute effects of treatment with standard antiepileptic drugs on seizures in these mice.

METHODS

Video-EEG recordings were obtained from Tsc1(GFAP)CKO mice on a weekly basis, starting at 4 weeks of age until death, to monitor progression of interictal EEG abnormalities and seizures. In separate experiments, Tsc1(GFAP)CKO mice were monitored for interictal EEG abnormalities and seizures before and during treatment with phenobarbital, phenytoin, or saline.

RESULTS

Tsc1(GFAP)CKO mice developed seizures around 4-6 weeks of age and subsequently had progressive worsening of the interictal EEG background and seizure frequency over a month, culminating in death. Treatment with phenobarbital or phenytoin caused a reduction in seizure frequency, but did not improve EEG background or prevent death.

CONCLUSIONS

Tsc1(GFAP)CKO mice develop progressive epilepsy. Acute treatment with standard antiepileptic drugs suppresses seizures in these mice, but does not affect long-term prognosis. Tsc1(GFAP)CKO mice represent a good model to test other drugs that may have true antiepileptogenic actions in TSC.