Characterisation of anatomical and functional deficits in a mouse model of Rett Syndrome

Weng, Shih-Ming (2012) Characterisation of anatomical and functional deficits in a mouse model of Rett Syndrome. PhD thesis, University of Glasgow.

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Rett syndrome (RTT), a disorder caused almost exclusively by mutations in the X-linked gene, MECP2, has a clinical phenotype thought to be primarily of neurological origin. Disruption of Mecp2 in mice results in a prominent RTT-like phenotype and Mecp2 knock-out animal models provide an excellent platform for investigating the role of MeCP2 in the brain development. In this thesis, I used the Mecp2-stop mouse model to address the effects of MeCP2 deficiency in the central nervous system. First, I assessed the genotype-phenotype relationship at the level of the whole organism by detailed temporal mapping of the RTT-like neurological signs. I also addressed the genotype-phenotype relationship at the level of neuronal networks by assessing alterations in neuronal (cortical) density including potential alterations across morphologically discrete cell subtypes. Finally, I also investigated the genotype-phenotype relationship at the level of the synapse by assessing RTT-related changed in synaptic plasticity.
Whole organism phenotyping using observational scoring revealed the male hemizygous Mecp2-mutant mice (Mecp2stop/y) to show an early (from ~5 weeks) and aggressive onset of signs including locomotor and other general features. Correlated studies at the level of the synapse revealed the severity of gross organismal pathology to mirror a progressive decline in both short- and long-term forms of synaptic plasticity as measured in the CA1 region of the hippocampus. Specifically, extracellular field recordings in acute hippocampal slices from strongly symptomatic Mecp2-stop mice showed long-term plasticity (LTP) at the Schaffer-collateral-to-CA1 pyramidal synapse to be 40.2 ± 1.6 % of age-matched wild-type littermate controls. In addition, putative presynaptic short-term forms of plasticity (post-tetanic potentiation (PTP) and paired-pulse facilitation (PPF)) were also decreased in the Mecp2-stop hippocampus (45 ± 18.8 % and 78 ± 0.1 % of wild type for PTP and PPF respectively; all p<0.05). Moreover, the impairment in LTP was associated with symptom severity score whereby mice with a more ‘severe’ symptom score showed a more profound deficit in LTP.
Refined axon stimulation protocols revealed evidence of pronounced LTP saturation in symptomatic Mecp2stop/y mice, suggesting an LTP ‘ceiling’ effect. I therefore assessed the action of the weak NMDA receptor blocker memantine, shown previously to reverse LTP saturation-related LTP deficits, in the hippocampus of control and Mecp2-stop mice. Application of memantine (1μM), resulted in substantial reversal of short-term plasticity deficits in the Mecp2-stop samples, without affecting plasticity in wild-type mice. However, systemic administration of memantine (30mg/Kg) in vivo did not have any observable effect on RTT-like phenotype at the organismal level including symptom onset, progression and survival.
Utilising immunohistochemistry (NeuN) and histological staining (DAPI) to quantify cell density in layer five of the somatosensory cortex, I demonstrated that the symptomatic Mecp2-stop mouse had a higher cortical cell density compared to wild-type controls (1.28 times of whole cortical cell density and 1.41 times of neuronal cell density). Detailed analyses of distinct neuronal subpopulations (parvalbumin-, somatostatin-, calretinin- and calbindin-immunopositive cells) showed that calretinin (CR)- and somatostatin (SOM)- immunopositive cells had a lower cell density in Mecp2-stop mouse somatosensory cortex. However, the distribution patterns of different neuronal subtypes (using the same markers) in Mecp2-stop hippocampus were preserved and were grossly similar to those of WT brains.
In summary, in this thesis, I demonstrated that the cell densities of CR- and SOM- positive neurons were altered in the somatosensory cortex in symptomatic Mecp2-stop mice. At the synaptic functional level, I showed both short-term and long-term plasticity deficits in the hippocampus of Mecp2-stop brains. Memantine, a clinically widely-used Alzheimer drug, partially restored the synaptic plasticity deficits in vitro. These data together supported that deficits in specific neuronal populations and progressive functional synaptic impairment may be key features in the RTT brain and also demonstrated the potential for the pharmacological restoration of synaptic plasticity function.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Subjects: Q Science > QH Natural history > QH426 Genetics
Colleges/Schools: College of Medical Veterinary and Life Sciences > School of Molecular Biosciences > Molecular Biosciences
College of Medical Veterinary and Life Sciences > School of Psychology & Neuroscience
Supervisor's Name: Bailey, Dr. Mark E.S. and Cobb, Dr. Stuart R.
Date of Award: 2012
Depositing User: MR SHIH-MING WENG
Unique ID: glathesis:2012-3099
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 12 Jan 2012
Last Modified: 10 Dec 2012 14:03

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