Cerebellar hyper-plasticity in the IB2 KO mouse model of autism
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1
University of Pavia, Dept. Brain and Behavioral Sciences, Italy
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2
Museo storico della fisica e Centro studi e Ricerche "Enrico Fermi", Italy
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3
University of Pavia, Dept. of Biology and Biotechnology "L. Spallanzani", Italy
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4
Dept. of Biological sciences, Hunter college, United States
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5
Brain Connectivity Center, C. Mondino National Neurological Institute, Italy
Introduction
Autism spectrum disorders (ASDs) are pervasive neurodevelopmental disorders that include syndromes with familial conditions. Among these, the Phelan-McDermid syndrome is associated with the co-deletion of SHANK3 and IB2 genes at the chromosome 22q terminus. Although much attention has been devoted to characterize SHANK3 mutations, very little is known about the role of IB2 in ASDs. The IB2 protein is expressed at synapses and takes part to the NMDA receptor (NMDAR) interactome in the postsynaptic densities. Experimental disruption of the IB2 gene in transgenic mice determined an enhanced NMDAR-mediated transmission at the mossy fiber-granule cell (MF-GrCs) synapse in the cerebellum. Moreover, IB2 knocked-out (IB2 KO) mice showed motor and cognitive deficits, making them a reliable ASD model [1]. Herein, we further investigated the synaptic and circuit modifications in IB2 KO mice. In particular, we addressed the potential alterations in the excitatory/inhibitory (E/I) balance and long-term potentiation (LTP) induction in the cerebellar granular layer of IB2 KO mice.
Methods
We exploited a combination of whole-cell patch-clamp electrophysiology with voltage sensitive dye imaging (VSDi) in acute parasagittal cerebellar slices (220 µm thick) from 18- to 25 day-old wild-type WT and KO (129Svev) mice. GrCs responses were elicited by MF stimulation with a bipolar tungsten electrode. LTP was induced by high-frequency MF stimulation (100 x 100 pulses Hz). Data are reported as mean ± S.E.M. Electrophysiology: whole-cell patch-clamp recordings were performed using K+-gluconate based intra-pipette solution and extracellular Krebs solution. Voltage Sensitive Dye imaging (VSDi): slices for optical recordings were loaded with Di-4-ANEPPS; fluorescence signals (DF/F0) were collected through a CCD camera, at a 0.5 kHz rate.
Results
In addition to the enhanced NMDAR mediated currents reported in a previous work from our group [1],electrophysiological recordings showed highly increased GrCs excitability in IB2 KO compared to WT mice. Furthermore, we investigated the spatial distribution of excitation (E) and inhibition (I) in the granular layer by using VSDi, thereby discovering an unbalanced E/I ratio in IB2 KO respect to WT mice. In particular, the E in the center was expanded while the I in the surround was reduced in IB2 KO mice as compared to their WT littermates, that might reflect the enhanced NMDA component of E in the former. Consistent with this observation, whole-cell patch-clamp recordings revealed that high frequency MF stimulation induced a larger LTP in IB2 KO mice compared to WT (WT 47.0±3.0%; n=7, p<0.03; KO 150±47.1%; n=8, p<0,05) (figure 1). We then examined whether long-term changes in intrinsic excitability of GrCs were affected by deletion of the IB2 gene. Before the induction protocol, discharge frequency was already significantly higher in IB2 KO mice as compared to WT. Accordingly, the plasticity of intrinsic excitability underwent a minor increase in IB2 KO mice, as respect to WT. These data were supported by VSDi, which revealed an increase in LTP/LTD area in IB2 KO with respect to WT mice (9.3 ± 4.3% vs. 3.3 ± 1.5%, n=4 for both; p<0.05) (figure 2). VSDi also confirmed that LTP magnitude was enhanced in IB2 KO mice as compared to control animals. The center-surround organization of LTP and LTD in the IB2 KO cerebellar granular layer showed several abnormalities as compared to WT: 1) the LTP magnitude in the center was higher 2) the LTD was less deep 3) the area of the center/LTP was larger. Interestingly, the spatial distribution in center (LTP)-surround (LTD) structures showed alterations in IB2 KO mice (with larger centers and less deep surrounds), thereby suggesting a shift form a classic “Mexican hat” to a “stove-pipe” shape profile hypothesized by Casanova (2006)[6], which is characterized by a large core of excitation with little inhibitory surround. Moreover, the spatial distribution of LTP showed larger potentiated cores and thinner depressed surrounds in IB2 KO than WT mice. Our results extend to the cerebellum the main hypothesis of ASD pathogenic mechanisms, suggesting that hyper-excitability and hyper-plasticity altered the E/I balance and the spatial configuration of plasticity and signal processing.
Discussion and conclusions
Hyperconnected, hypereactive and hyperplastic microcircuits have been observed in different brain areas, neocortex [2] somatosensory cortex [3], prefrontal cortex [4], amygdale [5], which were featured by a massive and selective hyperfunctioning of NMDARs. Interestingly, NMDAR hyperfunctioning has been reported in IB2 KO mice in the cerebellum, supporting the generality of the NMDAR hypothesis for ASDs. This is likely to play a role in the altered E/I balance, modifying the temporal window for integrating sensory signals. These changes may also be related to the hypothesized modification of center-surround responses, which shift from the normal mexican-hat to the stove-pipe shape, altering receptive and cognitive fields in a way that might explain the cognitive dysfunction in ASDs as well as in other brain diseases [6].Therefore, these results support the hypothesis of the enhancement of NMDAR-mediated glutamatergic transmission in the cerebellar cortex of the IB2 KO mouse model of ASDs, further characterizing the enhancement in cerebellar plasticity. In particular, these data show impressive cerebellar microcircuit alterations in the IB2 KO model, according to the growing number of evidences accounting for a major role of the cerebellum in ASDs.
Acknowledgements
Acknowledgments. This work was supported by: European Union grant Human Brain Project (HBP-29 604102) to ED and Fermi grant [13(14)] to ED and TS. The authors declare no competing financial interests.
References
[1] Giza J, MJ Urbanski, F Prestori, B Bandyopadhyay, A Yam, V Friedrich, K Kelley, E D'Angelo and M Goldfarb. (2010).Behavioral and cerebellar transmission deficits in mice lacking the autism-linked gene islet brain-2.J Neurosci 30:14805-16.
[2] Rinaldi Kulangara K, Antoniello K, Markram H. (2007)Elevated NMDA receptor levels and enhanced postsynaptic long-term potentiation induced by prenatal exposure to valproic acid.Proc Natl Acad Sci U S A. 2007 Aug 14;104(33):13501-6.
[3] Rinaldi T, G Silberberg and H Markram. (2008). Hyperconnectivity of local neocortical microcircuitry induced by prenatal exposure to valproic acid. Cereb Cortex 18:763-70. 74.
[4] Rinaldi T, C Perrodin and H Markram. (2008). Hyper-connectivity and hyper-plasticity in the medial prefrontal cortex in the valproic Acid animal model of autism.Front Neural Circuits 2:4
[5] Markram K, T Rinaldi, D La Mendola, C Sandi and H Markram. (2008). Abnormal fear conditioning and amygdala processing in ananimal model of autism. Neuropsychopharmacology 33:901-12.
[6] Casanova MF, IA van Kooten, AE Switala, H van Engeland, H Heinsen, HW Steinbusch, PR Hof, J Trippe, J Stone and C Schmitz. (2006). Minicolumnar abnormalities in autism. Acta Neuropathol 112:287-303.
Keywords:
autism,
Cerebellum,
NMDA receptor,
Long Term Plasticity,
IB2
Conference:
The Cerebellum inside out: cells, circuits and functions
, ERICE (Trapani), Italy, 1 Dec - 5 Dec, 2016.
Presentation Type:
poster
Topic:
Cellular & Molecular Neuroscience
Citation:
Soda
T,
Mapelli
L,
Locatelli
F,
Botta
L,
Goldfarb
M,
Prestori
F and
D‘Angelo
E
(2019). Cerebellar hyper-plasticity in the IB2 KO mouse model of autism.
Conference Abstract:
The Cerebellum inside out: cells, circuits and functions
.
doi: 10.3389/conf.fncel.2017.37.000028
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Received:
30 Nov 2016;
Published Online:
25 Jan 2019.
*
Correspondence:
Dr. Teresa Soda, University of Pavia, Dept. Brain and Behavioral Sciences, Pavia, Italy, teresa.soda@unicz.it
Dr. Lisa Mapelli, University of Pavia, Dept. Brain and Behavioral Sciences, Pavia, Italy, lisa.mapelli@unipv.it
Dr. Francesca Prestori, University of Pavia, Dept. Brain and Behavioral Sciences, Pavia, Italy, francesca.prestori@unipv.it
Prof. Egidio D‘Angelo, University of Pavia, Dept. Brain and Behavioral Sciences, Pavia, Italy, dangelo@unipv.it