Background
Haploinsufficiency of SHANK3 is widely recognized as the major cause of Phelan–McDermid syndrome (PMS), a complex neurodevelopmental disorder [
1]. Major clinical features of the syndrome include global developmental delay, moderate-to-severe intellectual impairment, absent or delayed speech and neonatal hypotonia. Over 80% of patients with PMS exhibit autistic-like behavior including impaired communication and social interaction, poor eye contact, anxiety and self-stimulatory conduct, and decreased perception of pain [
2,
3]. In addition, more than 25% of the PMS cases present epileptic symptoms, such as grand mal, focal and absence seizures [
2,
4,
5]. Of note numerous studies have linked the SHANK3 gene with ASD [
6‐
8], and it is estimated that mutations or deletions in the SHANK3 gene account for approximately 1% of all ASD cases [
8].
Despite extensive research on the synaptic functions of Shank3, the lack of effective therapies for PMS and ASD underscores the critical need to identify the underlying pathogenic mechanisms of these disorders. Brain activity relies on the interplay between excitation (
E) and inhibition (
I) via synaptic communication between glutamatergic and GABAergic neuron. Although GABAergic neurons comprise only about 20% of cortical neurons, they have an important role in modulating cortical functions and plasticity; indeed, GABAergic neurons are essential for synchronizing network activity and maintaining the excitatory and inhibitory dynamics [
9‐
12]. In addition to their role in modulating cortical functions and plasticity, GABAergic neurons are also critical for the maturation of neural circuits [
13] and are implicated in developmental processes, such as regulation of neuronal proliferation and migration [
14,
15]. Therefore, loss or dysfunction of these neurons has been implicated in several neurological disorders including epilepsy, schizophrenia, and autism [
12,
16‐
18]. Interestingly, preferential deletion of
Shank3 in GABAergic cortical neurons has been associated with pyramidal neuron hyperexcitability and sensory hypersensitivity [
19]. Among GABAergic neurons, parvalbumin-positive (PV) neurons have been implicated in the etiology of neuropsychiatric disorders based on a growing body of evidence [
16,
20].
The PV neurons, which constitute 50% of GABAergic cells, include fast-spiking basket cells and chandelier cells. Fast-spiking PV neurons are capable of generating action potentials at high-frequency mediating fast and precise inhibition of target neurons. These cells are involved in important functions, including the regulation of plasticity, the control of gain control in the cortex and the establishment and maintenance of cortical rhythms [
9,
19,
21,
22]. The activation of PV neurons is particularly critical for the generations of gamma frequency oscillations (30–80 Hz), which are a key feature of pyramidal cell synchronization and are implicated in higher brain functions, such as sensory perception, motor behavior and memory formation [
13,
22‐
24]. Shank3 is expressed in PV GABAergic neurons, and its absence results in reduced PV expression in the striatum of Shank3 KO mice [
25]. Additionally, null mutant
Shank3 exhibits delayed circuit maturation during development particularly in perisomatic PV neuronal circuit function [
26].
In this study, we recorded local field potentials and visual evoked responses in the visual cortex of Shank3∆11
−/− (Shank3 KO) mice [
27] and observed that responses to visual stimuli had a limited dynamic range likely resulting from impaired gain control in
Shank3 KO compared with Wt mice. Since gain adaptation is due to the feedback from PV interneurons [
28], we hypothesized that this was due to reduced inhibition. Our hypothesis was supported by the fact that gain adaptation was restored following the potentiation of GABA
A receptor activity with Midazolam, suggesting a role for Shank3 in the modulation of GABAergic neurons.
In order to investigate whether deletion of
Shank3 in PV-expressing neurons leads to behavioral deficits in mice, we generated a new mouse model by crossing
Shank3 floxed mice [
29] with Pv-Cre mice. Our findings indicate that the heterozygous deletion of
Shank3 specifically in PV neurons is sufficient to induce specific behavioral alterations, including repetitive behaviors, motor problems and memory impairments. Additionally, LFP recordings of Pv-Cre +/−
Shank3Wt/Fl mice revealed that selective deletion of
Shank3 in PV neurons caused a severe phenotype that led to cortical hyperexcitability. Finally, we demonstrated that the potentiation of GABA
A receptor activity may represent a possible therapeutic strategy to ameliorate some of the behavioral symptoms caused by Shank3 deletion.
Methods
Mice
Pv-Cre
+/− Shank3Wt/Fl mice were generated by breeding
Shank3 floxed mice (kindly provided by Tobias Böeckers, from the Institute for Anatomy and Cell Biology of Ulm University, Germany) with Pv-Cre
+/− mice (
PValbtm1(cre)Arbr, The Jackson laboratory). To visualize PV
+ interneurons, we generated Pv-Cre
+/− TdTomato
Fl/− Shank3Fl/Wt mice using TdTomato
+/+ knock-in mice (
Gt(ROSA)26Sortm14(CAG−tdTomato)Hze, The Jackson laboratory). The
Shank3Δ11−/− mice were generated as previously described by Schemeisser et al., 2012 [
27] and re-derived in a C57BL/6 background (Charles River Laboratories, Calco, Italy). These mice have already been characterized by [
30]. All experiments conducted for this project involved mice that were housed in an animal facility maintained at a constant temperature (22 +/− 1 °C) and humidity (50%), with a 12-h light/dark cycle, and provided with ad libitum access to food and water. The experiments were performed in accordance with the guidelines approved by the European Communities Council and the Italian Ministry of Health (Rome, Italy) for the ethical use of laboratory animals in research. All possible efforts were taken to minimize the number of mice used and reduce their suffering. Animals from both sexes were included in all experiments, and subsequently, the data were combined, as no statistical differences were observed between the two groups.
Mice genotyping
All primers were provided by Thermo Fisher; the REDExtract-N-Amp PCR Reaction Mix™
(Sigma-Aldrich) and the MyFi™ DNA Polymerase (Meridian Bioscience) were used for the polymerase reaction. PCR genotyping was performed using the following sets of oligonucleotide primers: parvalbumin for Wt allele forward 5′-CAGAGCAGGCATGGTGACTA-3′, for Wt allele reverse 5′-AGT ACCAAGCAGGCAGGAGA-3′, for mutant allele forward 5′-GCGGTCTGGCAGTAAAAACTATC-3′, for mutant allele reverse 5′-GTGAAACAGCATTGCTGTCACTT-3′; Shank3 floxed forward 5′-GTCTCTGTGGTTGGGGTGTC-3′, reverse 5′-CAGTGAAGAAGCCCCAGAAG-3′ for both Wt and mutant allele; TdTomato for Wt allele forward 5′- AAGGGAGCTGCAGTGGAGTA-3′, for Wt allele reverse 5′-CCGAAAATCTGTGGGAAGTC-3′, for mutant allele forward 5′- CTGTTCCTGTACGGCATGG -3′, for mutant allele reverse 5′- GGCATTAAAGCAGCGTATCC -3′; Shank3Δ11−/− for Wt allele forward 5′-CAAGTTCATCGCTGTGAAGG-3′, for mutant allele forward 5′-CCTCTAGGCCTGCTAGCTGTT-3′, reverse 5′-AAGAAGCCCCAGAAGTGACA-3′ for both Wt and mutant allele.
Surgical procedures
Mice were anesthetized with a solution of 20% urethane in physiological solution (0.9% NaCl) with a final dose of 0.8 ml/hg ([
23,
24,
31,
32]; 1.6 g/kg). The depth of anesthesia was evaluated by monitoring the pinch withdrawal reflex and other physical signs (respiratory and heart rate). Additional doses (10% of initial dose) were intraperitoneally administered to maintain the level of anesthesia if necessary. The brain surface was routinely moistened with the addition of artificial cerebrospinal fluid (ACSF) at body temperature. The head of the mouse was fixed in a stereotaxic apparatus, and a portion of the skull overlying the visual cortex (0.0 mm anteroposterior and 2.7 mm lateral to the lambda suture) was drilled on the right hemisphere. A chamber was created around the craniotomy applying a thin layer of dental cement (Vertex Dental). The mouse was placed with its left eye in front of the monitor at 30 cm, oriented 45
° with respect to the medial sagittal plane of the animal. Local field potentials (LFP) were recorded with glass micropipettes (impedance ~ 2 M
Ω), filled with ACSF solution and connected to the amplifier head stage with an Ag/AgCl electrode. The microelectrode was positioned into the visual cortex at a depth of 250–300 μm (II/III layer) with a motorized micromanipulator. A common reference Ag/AgCl electrode was placed on the cortical surface in the ACSF cortical bath. Electrophysiological signal was amplified 1000-fold with a multichannel differential amplifier (EXT-02F, NPI), band-pass-filtered (0.1–1000 Hz) and sampled at 2 kHz with 16-bit precision by a National Instruments (NI-usb6251) ADC board controlled by custom-made LabView software. Line frequency 50 Hz noise was removed by means of a linear noise eliminator (Humbug, Quest Scientific). LFP recording lasted around 45 min, after which some animals have been superfused over the visual cortex with Midazolam (5 mg/ml in ACSF; 200 µl volume; BI- Istituto Biochimico Italiano Giovanni Lorenzini S.p.A.) or its vehicle (ACSF). After administration, the mouse was left resting, in the dark, for 1 h. Then, a second round of LFP recording was performed. Visual stimuli were provided via an LCD monitor (120 Hz frame rate, Asus). The stimuli were generated with a custom-made MATLAB program based on Psychophysics Toolbox 3 [
33]. The stimulus consisted in a black and white contrast-reversing checkerboard (0.5 Hz; 0.04 cycles/degree). The checkerboard contrast was changed between different traces. The contrast values were calculated using the formula:
$$K = \frac{{\frac{L}{l} - \frac{{L_{\min } }}{{l_{\min } }}}}{{\frac{{L_{\max } }}{{l_{\max } }} - \frac{{L_{\min } }}{{l_{\min } }}}}*100\% ,$$
where
K indicates contrast;
L and
l indicate, respectively, the maximum and minimum luminance of the stimulus;
Lmax and
lmax indicate, respectively, the maximum and minimum luminance of the highest luminance stimulus and
Lmin and
lmin the maximum and minimum luminance of the lowest luminance stimulus. Screen luminance was measured using a photometer (Konica Minolta). Six contrast values were used from maximum to minimum and randomly alternated (100%, 28%, 5.7%, 2.3%, 1.8% and 0%).
Up states (US) and down states (DS) detection
Analysis of slow-wave oscillations was performed computing the short-time root mean square (RMS) power of the data band passed in the gamma band (25–80 Hz). The time course of the RMS power was computed on a moving window of 250 ms width and 150 ms overlap. The distribution of the logarithm of the time-resolved gamma power was bimodal, reflecting high gamma activity during USs and low gamma activity during down states. This distribution was then fitted with a double Gaussian function and the threshold for the discrimination of USs was chosen, minimizing false positives and false negatives, by computing the ROC curve. A cutoff in the minimum up (down) states duration was set to 100 ms, and up (down) state shorter than the cut-off was assigned to the ongoing down (up) state (see [
32] for details).
Spectral analysis
Spectrograms have been computed using the
mtspecgramc function of the Chronux toolbox, with an overlapping Hammer window of 150 ms width and 50 ms overlap. Multitaper power spectra have been calculated on 90-s traces recorded from anesthetized mice with no visual stimulation, using the
mtspectrumc function of the Chronux toolbox [
25].
Analysis of visual evoked responses
The visual evoked potential (VEP) of each mouse was calculated as the average of 120–180 trials. The peak amplitude was measured in a window from 90 to 500 ms after the stimulus, with respect to a baseline level computed in the 30–80 ms window. Contrast transfer functions obtained for every mouse have been fitted with a Michaelis Menten function:\(A(k) =\frac{{A}_{\mathrm{max}}*k}{{k}_{H}+k}\), where A is the response amplitude, k is contrast, Amax is the response at "saturating" contrast levels and kH is the contrast at which the amplitude is equal to half M. The VEP slope was calculated as the angular coefficient of a line fitting the VEP in a window of 80 ms centered on the half-peak value. Gamma band power (25–80 Hz) during the response was calculated for every trial as the RMS power of the filtered trace in two different time windows. The windows were 30–80 ms and 90–500 ms after the stimulus, called, respectively, "baseline" and "response"; then the base-10 logarithm of the ratio between response and baseline gamma power has been calculated and averaged among trials.
Statistical analysis for electrophysiology
In the figure legends are reported the number of replicates (n) and the statistical analyses used for every experiment. Based on the number of condition and the number of groups in the comparison, appropriate statistical tests were used to analyze the data. First was analyzed the data distribution by the Shapiro–Wilk normality test. For the comparisons, parametric tests including Student t-tests and one-way and two-way analysis of variance (ANOVA) with appropriate post hoc test were used if the data were normally distributed; nonparametric test including the Mann–Whitney test and Kruskal–Wallis test with appropriate post hoc test were used for data with non-normal distribution. Results are presented as box plots presenting the 1st and 3rd quartile, the median and the mean. Since each data point represents an experimental mouse, the total number of data points is limited and the box plots do not include whiskers, but all data points are presented, thus allowing a proper understanding of scattering and range. Statistics were computed as described in the figure legends.
Immunohistochemistry
Mice were anesthetized and perfused transcardially with sucrose 5% and paraformaldehyde 4%. Brains were removed and post-fixed overnight in paraformaldehyde 4%. The day after three washes with PBS were performed and brains were incubated for at least eight hours in 30% sucrose. Finally, brains were included in cryomolds with Tissue-Tek OCT compound (Bio-Optica) and put at − 80 °C until cryostat sectioning. Brains were cut with a cryostat and 20 μm-thick coronal slices were collected on polysine microscope adhesion slides (ThermoFisher). Slices were incubated first in blocking solution (3%BSA, 10% goat serum, 0.4% Triton-X-100, diluted in PBS) for at least 30 min and then they were incubated with primary antibodies overnight at 4 °C. Subsequently, three wash (10 min each) with PBS were performed and brain slices were incubated with fluorophore-conjugated secondary antibodies (Jackson ImmunoResearch Laboratories) for 1 h at room temperature. After the antibody incubation, brain slices were washed and 4',6diamidino-2-phenylindole (DAPI) staining (ThermoFisher) was carried out (DAPI diluted in PBS to a final concentration of 0.5 μg/ml). Finally, another washing step is performed before mounting the coverslips with Mounting Medium (Vecta Shield). Primary antibodies used were: anti-Parvalbumin (Swant, GP72), Shank3 (homemade SHANK3rb [
35]).
Microscopic analysis
Brain areas were selected according to the mouse brain atlas of Paxinos and Franklin (Paxinos & Franklin, 2008) from each of the following brain regions at the following bregma coordinates: prefrontal cortex (from 2.4 to 1.7 mm), visual cortex (from − 2.06 to − 2.70 mm), hippocampus (from − 1.06 to − 2.2 mm). Confocal images were obtained using LSM800 confocal microscope (Carl Zeiss) with Zeiss 10 × objective at a resolution of 1024 × 1024 pixels, under the same condition across different mice and brain slices. PV-positive cells were counted in the bilateral areas of each section using ImageJ software.
For Shank3 staining, imaging was performed on a laser-scanning microscope (Leica DMi8) with 63 × oil DIC immersion objective (ACS Apo, NA 1.3) with xy resolution 159 nm and z resolution 345 nm with a stack size of 1.73 µm. Deconvolution of the images was performed using Huygens Essentials 22.04 software (Scientific Volume Imaging B.V.). Subsequently, an analysis of the images was performed with surface and spot tool with Imaris 9.9 Software (Oxford Instruments). Parvalbumin-positive neurons within a specific region of interest (ROI) were selected with the surface tool by tdTomato signal. With the spot tool Shank3-positive spots close to the surfaces (If < or equal to 0.5 µm distance) were determined. Number of spots close to surface within one ROI was normalized on the surface volume within the ROI. Two sections (3 ROIs per section) were analyzed for each hemisphere per animals.
Biochemistry
Cortex and hippocampi were dissected from mouse brain of both sexes. Tissues were homogenized in buffer containing 10 mM Hepes pH 7.4, 2 mM EDTA, protease inhibitors (Roche), and phosphatase inhibitors (Sigma, P8340). Samples were centrifuged at 500 × g for 5 min at 4 °C. Resulting supernatants were centrifuged at 10,000 × g for 15 min at 4 °C. After the centrifugation, pellets were resuspended in buffer composed by 50 mM Hepes pH 7.4, 2 mM EDTA, 2 mM EGTA, 1% triton-X-100 (Sigma), protease inhibitors, and phosphatase inhibitors and centrifuged at 20,000 × g for 80 min at 4 °C. Finally, pellets were resuspended in buffer containing 50 mM Tris pH 9, 1% sodium deoxicholate (Sigma, D6750). Samples were quantified by BCA protein assay (EuroClone) to assess protein concentration. Equal amounts of each sample were separated using SDS-PAGE and subsequently blotted on nitrocellulose membranes using the Trans- Blot Turbo System (BioRad). Membranes were washed in Tris-buffered saline-Tween (TBS-T) (200 mM Tris pH 7.4, 1.5 M NaCl (both Sigma-Aldrich) and 0.1% Tween 20% (BioRad). After 1 h blocking at room temperature with 5% bovine serum albumin or milk in TBS-T, membranes were incubated overnight at 4 °C with primary antibody (Shank3, Santa Cruz Biotechnology, cat. H-160; GABA-A-R-alpha1, Neuromab, cat. 75136; Parvalbumin, Swant, cat. GP72; β3-tubulin, Sigma, cat. T8578). Membranes were washed in TBS-T and then incubated with HRP-conjugated secondary antibodies (Jackson ImmunoResearch) for 1 h at room temperature. After three washes, chemiluminescence was induced using an ECL Western Blotting Substrate kit and further detected using a ChemiDoc XRS + machine. All signals were quantified using ImageLab software and normalized against the values of the respective signal for βIII-tubulin.
Behavioral analysis
Mice of P60-P90 were used for behavioral experiments. Animals were housed in groups of four or five individuals. All of the tests were conducted during the light portion of the cycle. The sample size of animals required for behavioral analysis was estimated using the G*Power 3.1 statistical analysis program, setting a 95% confidence limit (α err. prob. = 0.05) and a power of 80%. The effect size was calculated using the formula Δ/SD (Δ = difference between the groups; SD = standard deviation). Means and standard deviations were estimated based on previous experiments in the laboratory. Based on these parameters, we determined that a sample size of at least 5 animals was necessary to detect any differences between groups.
Repetitive self-grooming test
The spontaneous self-grooming behavior was assayed as described in [
36]. Single mouse was placed into a standard cylinder (46 × 23.5 × 20 cm). Cylinders were empty to eliminate digging in the bedding, which is a potentially competing behavior. The room was illuminated at about 40 lx. A front-mounted CC TV camera (Security Cameras Direct) was placed at circa 1 m from the cages to record the sessions. Sessions were video-taped for 20 min. The first 10 min of habituation was not scored. Cumulative time spent grooming all the body regions during the second 10 min of the test session was measured.
Novel object recognition test
The novel object recognition test was performed in an open plastic arena (60 × 50 × 30 cm). The test had three phases: the habituation on the first day, when mice were accustomed to the test arena for 10 min; the familiarization; and the novel object recognition the day after. In the familiarization phase, two identical objects were placed in the middle of the arena equidistant from the walls and from each other. Mice were placed between the two objects until it had completed 30 s of cumulative object exploration (20 min cut-off). Experimenter measured the time mice was within approximately 1 cm of an object with its nose toward the object. Climbing the object or pointing the nose toward ceiling near the object were not considered exploring behaviors. After familiarization, mice returned to the home cage until they were tested for novel recognition after 5 min, 120 min and 24 h. In the novel recognition phase, a novel object (never seen before) took the place of one of the two familiars. Scoring of object recognition was performed as during the familiarization phase. For each mouse, the role (familiar or new object) as well as the relative position of the two objects were randomly permuted. The objects used for the test were white plastic cylinders and colored plastic Lego stacks of different shapes. The arena was cleaned with 70% ethanol after each trial. Performance was analyzed by calculating the discrimination index (N − F/N + F), where N = the time spent exploring the novel object, and F = the time spent exploring the familiar object.
Spatial object recognition test
Spatial object recognition test was performed in an arena according to [
30], with minor modifications that consisted in an opaque white Plexiglass cage (58 × 50 × 43 cm) that was dimly lit from above (27 lx) and two visual cues that were placed above two adjacent walls. The visual cues consisted of a black and white stripped pattern (21 × 19.5 cm) that was affixed to the center of the northern wall and a black and gray checkered pattern (26.5 × 20 cm) that was placed in the center of the western wall. Objects were placed across the visual cues. Mice were habituated to the arena for 10 min the day before the test. Twenty-four hours later, it is initially performed a training session where mice were allowed to familiarize with two different objects. The experimenter measured the time spent in sniffing both objects until the mouse completed 30 s in exploring objects (cut-off 20 min). Exploring behavior was defined as mouse having its nose directed toward the object and within approximately 1 cm of the object [
37]; climbing or sitting were not considered exploration behaviors. After 5 min, 120 min and 24 h, mice were allowed to re-explore the cage where one object was moved in a new position. Between two sessions, mice returned to their home cage. Cage and object were carefully cleaned with 70% ethanol before and after all behavioral procedures. Performance was analyzed by calculating a discrimination index (
N −
F/
N +
F), where
N = the time spent exploring the moved object during the test, and
F = the time spent exploring the unmoved object during the test.
Balance beam walking test
The beam apparatus consisted of 1 m beams with a flat surface of 12 mm or 6 mm width resting 50 cm above the table top on two poles. A black box was placed at the end of the beam as the finish point. Nesting material from home cages was placed in the black box to attract the mouse to the finish point. A lamp (with 60-W light bulb) was used to shine light above the start point and served as an aversive stimulus. A video camera was set on a tripod to record the performance. On training days, each mouse crossed the 12 mm beam 3 times and then the 6 mm beam 3 times. The time required to cross to the escape box at the other end (80 cm away) was measured with a stopwatch. The stopwatch started when the nose of the mouse began to cross the beam, and stopped when the animal reaches the escape box. Once the mice are in the safe box, they are allowed some time (~ 15 secs) to rest there. Before the next trial the mice rest for 10 min in their home cages between training sessions on the two beams. On the test day, times to cross each beam were recorded. Two successful trials in which the mouse did not stall on the beam are averaged. The beams and box were cleaned with towels soaked with 70% ethanol and then water before the next beam was placed on the apparatus.
Rotarod test
The rotarod apparatus (Ugo Basile, Biological Research Apparatus, Varese, Italy) was used to measure fore and hindlimb motor coordination and balance. During the training period, each mouse was placed on the rotarod at a constant speed (12 and 32 rpm) for a maximum of 120 s, and the latency to fall off the rotarod within this time period was recorded. Mice received four trials per day for 4 straight days. The fourth trial of each day was evaluated for statistical analysis.
Pole test
In the pole test, the mouse was placed on a vertical wire-mesh pole (90 cm length, 1 cm diameter) with its head facing upwards. Mice were habituated to descend the pole in 2 trials per day for 2 days. On test day (third day) mice were subjected to 5 trials: the total time taken to turn the body and to descend the pole was recorded according to [
38]. A cutoff of 60 s was given. Data were shown as mean of 5 trials evaluated during the test day.
Wire hanging test
The limb force was tested by positioning a mouse on the top of a wire cage lid (19 × 29 cm) that was turned upside down at approximately 25 cm above a surface with the bedding material. The grip of the mouse was ensured by gently waving three times before rotating the lid as described in [
39]. The latency to fall onto the bedding was recorded over a maximum period of 300 s.
Sociability and social novelty test
The sociability tests were performing in an apparatus called three-chambered box. It is formed by a rectangular transparent polycarbonate apparatus with three-chamber (width = 42.5 cm, height = 22.2 cm, central chamber length = 17.8 cm, and side chamber lengths = 19.1 cm) as previously described in [
36]. The three-chamber box was illuminated by diffuse incandescent lighting (15 lx), and a video camera was located directly over the center of the open field.
First, the test includes a habituation phase during which the mouse was placed in the middle chamber free to explore all the compartments for 10 min. After this period, novel conspecific mouse, that had no previous interaction with the test mouse, was placed in one of the side chambers. The doors were unblocked and the subject mouse was given the possibility to explore for 10 min either an empty chamber or a chamber containing the stranger mouse. In the 10-min trial, the duration of contact with the counterpart was recorded. Immediately after sociability test, without cleaning the apparatus, it was performed the social novelty test putting an unfamiliar mouse in the empty wire cage. Time spent in each chamber was recorded. The data were expressed in sociability index (SI) and social novelty preference index (SNI) as follows: SI = (time exploring novel mouse 1 – time exploring empty cage) / (time exploring novel mouse 1 + time exploring empty cage) and SNI = (time exploring novel mouse 2 – time exploring familiar mouse) / (time exploring novel mouse 2 + time exploring familiar mouse).
Pharmacological treatment
Pv-Cre+/− Shank3Wt/Wt, Pv-Cre+/− Shank3Fl/Wt, Shank3 Wt and Shank3 KO mice were treated with a selective positive allosteric modulator of GABAA receptors: Ganaxolone. Ganaxolone (Tocris Cat. No. 2531) was resuspended in ethanol and betaciclodextrin (Sigma-Aldrich 332607). Mice received an intraperitoneal injection of ganaxolone (5 mg/Kg) or vehicle 30 min before each behavioral test.
Discussion
This study provides significant evidence demonstrating that Shank3 plays a crucial role in modulating the activity of inhibitory circuit in vivo. Our data indicate that
Shank3 deletion leads to a severe impairment in the recruitment of PV interneurons and that the deletion of Shank3 specifically in PV-positive neurons is sufficient to recapitulate some of the behavioral deficits observed in individuals with PMS, such as repetitive behavior, memory impairments and motor problems. Additionally, local field potential recordings of Pv-Cre
+/− Shank3Wt/Fl demonstrated that these behavioral deficits are associated to cortical hyperexcitability. Finally, we showed that enhancing the activity of GABA
A receptors with ganaxolone may be a viable strategy for ameliorating some of the behavioral symptoms resulting from
Shank3 deletion. Previous research has shown that alterations in the balance between excitation and inhibition (E-I) are present in various models of neurodevelopmental disorders and have been strongly linked to epilepsy [
17,
49‐
52]. Notably, at least a third of individuals with PMS develop epilepsy [
3] which is consistent with the impairment of the PV inhibitory feedback observed in our study.
Although GABAergic neurons represent a minority cell type, they control the activity level of principal neurons in the brain. The loss or dysfunction of interneurons has been implicated in numerous neuropsychiatric disorders [
49,
53‐
55] and multiple studies support the hypothesis that specific symptoms of ASDs may be caused by an increase in the ratio of excitatory to inhibitory synaptic transmission [
56,
57].
Numerous studies have provided evidence that Shank family proteins are extensively expressed in GABAergic neurons where they seem to be crucial for their function and development [
10,
58]. Preferential deletion of
Shank3 from interneurons has been linked to hyperexcitability of pyramidal neurons and increased sensitivity to sensory stimuli [
19]. Recently, the visual cortex has emerged as a valuable model for evaluating cortical processing in mouse models of neurodevelopmental disorders [
40‐
43,
59,
60]. Altered visual cortical function has been observed in Rett syndrome patients and in mouse models carrying mutations in MeCP2 or FOXG1, thereby supporting the notion that VEPs can serve as a dependable biomarker for assessing the pathological state of the brain [
61,
62]. Although we did not observe spontaneous epileptic seizures in
Shank3 KO mice, we found a higher degree of excitability in anesthetized mice both in resting state and in response to visual stimulation. In resting state, the increased excitability of the
Shank3 KO mice manifested as up states that were longer and had higher power in the 25–80 Hz range. During up states, cortical cell membrane potential is depolarized to subthreshold level and cells are more excitable [
63] as clearly demonstrated by the transients observed in the spectrograms (Fig.
1A). The increased duration of up states is reflected by an increased power in the 10–100 Hz range (Fig.
1B). Our findings align with those of a mouse model of Fragile X Syndrome, which also displayed longer up states and hyperexcitability [
64]. In addition to studying slow-wave activity, we have also investigated gain control, a fundamental property of neurons that allows to scale the response to excitatory inputs by means of feed-forward inhibition, thus reducing the incremental change of the response as the input strength increases [
28]. Our results reveal that contrast gain control is impaired in the
Shank3 KO mice, and we hypothesized that this is due to a reduction in the recruitment of inhibitory interneurons by excitatory neurons. Indeed, the administration of midazolam, a positive modulator of GABA currents, reversed the gain control deficit observed in Shank3 KO mice. Previous research has demonstrated that contrast sensitivity gain control is regulated by the feedback of fast-spiking PV interneurons that are pivotal regulators of contrast transfer function in V1 [
45]. Interestingly, the changes in spectral power observed in the
Shank3 KO and in the Pv-Cre
+/− Shank3Wt/Fl are complementary (Fig.
4A). In the Pv-Cre
+/− Shank3Wt/Fl the spectral power in the gamma band is similar to that of the control group, while there is a clear increase in power at low (< 5 Hz) and high frequencies (> 100 Hz). The increased power at low frequencies is likely due to the larger amplitude of up states that determine a larger component of the fundamental harmonic of slow-wave activity. Furthermore, EEG monitoring in both animal models and humans has shown that some epileptic conditions are associated with enhanced slow oscillations at frequencies < 0.5 Hz [
65,
66]. On the opposite end of the Fourier spectra, increased power at frequencies > 100 Hz may indicate the presence of an epileptic focus. Indeed, intracranial recordings from the epileptic hippocampus (both in animal and human models) have reported fast ripples associated with increased epileptogenicity [
67].
Different studies have shown that Shank proteins play a significant role in regulating synaptic transmission in parvalbumin (PV) neurons. Specifically, Shank1 has been shown to be highly expressed in PV neurons and is involved in the regulation of excitatory synaptic transmission in PV basket cells [
68]. Additionally, deletion of Shank2 in PV neurons has been found to cause hyperactivity, increased self-grooming, and suppressed brain excitation [
69]. Similarly, Shank3 has been found to be expressed in PV neurons and its knockout has been associated with reduced numbers of PV neurons in the striatum of mice [
25].
To directly address the roles of Shank3 in regulating E/I balance and better understand the mechanisms responsible for the hyperexcitable state observed in the
Shank3 KO mouse, we used a PV-restricted conditional knockout approach. Our findings revealed that cell type-specific heterozygous deletion of
Shank3 in PV neurons was sufficient to induce behavioral alterations in contrast to what we found in
Shank3 full KO mice that did not exhibit cognitive defects in heterozygosis [
30], strengthening the importance of the modulation of E-I balance during brain maturation [
70]. Our study revealed that conditional deletion of
Shank3 in PV neurons caused an increase in repetitive behavior, reduced memory performance and alterations in motor coordination.
Surprisingly, the selective knockdown of
Shank3 in PV neurons did not significantly affect social behavior, despite the crucial role played by PV-positive neurons in regulating social behavior [
68,
71,
72] and the strong social deficits observed in
Shank3 KO mice [
30]. This suggests that the expression of
Shank3 in PV cells may not be critical for regulating social behavior in mice, or that compensatory mechanisms may be at play in the conditional knockout (cKO) mice, which could mask the effects of
Shank3 deletion in PV cells on social behavior. It is possible that other GABAergic or excitatory neurons could compensate for the loss of
Shank3 in PV cells, thereby maintaining social behavior in the cKO mice. Interestingly, a similar observation has been made regarding Shank2, where the deletion of
Shank2 specifically in PV-positive neurons has minimal impact on social interaction and communication, despite global
Shank2 knockout mice exhibiting pronounced impairments in social behavior [
69]. This highlights the complexity of the role of Shank proteins in regulating behavior and the need for further investigation to fully understand their functions in different neuronal populations.
The electrophysiological phenotype of the Pv-Cre
+/− Shank3Wt/Fl mice is complex. Up states have a larger amplitude with a high spectral power only at frequencies > 100 Hz compared to
Shank3 KO mice. However, Pv-Cre
+/− Shank3Wt/Fl mice don’t exhibit increased power in the 25–80 Hz gamma band range compared to Wt (Fig.
4). We frequently observed large amplitude events (> 1 mV) suggestive of hypersynchronous, interictal events. Thus, the partial deletion of
Shank3 in a subset of cortical neurons causes a more severe hyperexcitable phenotype than the complete KO. On the other hand, the two models have subtle differences in phenotype: the functional and behavioral phenotype in the Pv-Cre
+/− Shank3Wt/Fl mouse is associated with hyperexcitability, while the
Shank3 KO mouse presents a more complex ASD-like picture but no clear signs of hyperexcitability except for the deficit in gain control. A possible interpretation of this difference is that the loss of
Shank3 may lead to impairments in excitatory synapses, which in the complete KO mouse, could result in a reduction of excitatory drive on both pyramidal neurons and interneurons, thus partially compensating for the E-I balance. In contrast, the PV model with
Shank3 deletion only in PV interneurons has reduced excitatory drive on these specific interneurons, leading to a stronger imbalance toward excitation. However, further studies are needed to fully understand the mechanisms underlying the observed differences in phenotype between these two models.
The reduction in PV neurons observed in the hippocampus and mPFC of PV-Cre
+/− Shank3
Wt/Fl mice highlights the importance of GABAergic signaling in proper brain development and function. This finding also suggests that compounds targeting deficits in the GABAergic system, such as ganaxolone, may have therapeutic potential for treating brain disorders characterized by E/I imbalances, such as ASD [
57,
73]. Indeed, the administration of midazolam showed that pharmacological reinforcement of inhibition was sufficient to restore gain control in
Shank3 KO mice, possibly normalizing the E/I imbalance. However, although being effective GABA
A receptor agonists, benzodiazepines often cause side effects including sedation. Ganaxolone is a synthetic neurosteroid that acts as a positive allosteric modulator of GABAA receptors, enhancing the activity of the inhibitory neurotransmitter GABA. Importantly, ganaxolone has been shown to be effective in reducing seizures and improving behavior in animal models of ASD [
74,
75]. Additionally, ganaxolone has a more favorable side effect profile than benzodiazepines [
76,
77] making it a promising candidate for clinical use. We demonstrated that a single dose of ganaxolone was effective in restoring normal behavior in Pv-Cre
+/− Shank3
Wt/Fl mice. Additionally, while we did not observe any changes in the number of PV neurons in
Shank3 KO mice, our results showed that
Shank3 knockdown led to a significant reduction in the expression of GABA-A-R-alpha1 in the hippocampus. Treatment with ganaxolone improved memory deficits and repetitive behaviors in
Shank3 KO mice, indicating that modulation of the GABAergic system may be a viable therapeutic strategy for individuals with Shank3 mutations.
Although a more comprehensive molecular characterization remains to be determined, our results suggest that that targeting the GABAergic system, specifically by enhancing GABAergic transmission through allosteric modulation of GABAA receptors, may represent a potential therapeutic strategy for treating some symptoms associated with Shank3-related disorders. Our study demonstrated that Shank3 is involved in the modulation of PV neurons activity and emphasize the challenges and importance of defining the specific neuronal population and circuits causative of behavioral deficits due to Shank3 deletion in order to develop effective therapies for PMS and ASD.
Future studies should focus on identifying the molecular mechanisms underlying the effects of Shank3 deletion on PV neurons and its impact on other GABAergic neurons. Finally, chronic treatment with ganaxolone should be further evaluated to assess its long-term effects on behavior in Shank3 KO mice.
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