Title: Preventing cognitive impairment in mouse genetic models of Down syndrome by early postnatal suppression of Kir3.2 channel signaling Abstract Neural mechanisms responsible for cognitive impairment in DS are still unclear, and there is no treatment to prevent this most common genetic disability. Our long-term goal is to develop treatments ameliorating cognitive impairment in DS. Recently we discovered that synaptic plasticity and cognition can be restored in DS models by genetic removal of an extra copy of Kcnj6, the gene encoding Kir3.2 (Girk2) subunits of potassium channels. These channels, known for their critical role in neuronal hyperpolarization, are highly expressed during early postnatal development suggesting their involvement in the formation of neural circuits. Our central hypothesis is that, in DS models, increased signaling through Kir3.2 channels reduces the spontaneous activity of neonatal neurons, thus affecting synaptogenesis and the formation of neural circuits, which ultimately leads to cognitive impairment. Accordingly, suppression of Kir3.2 channel signaling during the early postnatal period can normalize the neural circuits, thereby reducing neuronal, synaptic, and cognitive abnormalities in DS.
Two Specific Aims will used to test this hypothesis.
Aim1. To examine the effects of the Kcnj6 gene dose on the development and maturation of neural circuits in mouse genetic models of DS. Formation and maturation of GABAergic and glutamatergic synaptic connections will be examined in the hippocampus of DS mice containing either 2 or 3 copies of Kcnj6 (Ts65Dn/Kcnj6++- vs. Ts65Dn/Kcnj6+++ mice). Electrophysiological, biochemical, and histochemical techniques will be used to assess the effects of Kcnj6 gene dose on: (i) Neuronal properties affecting the activity of neonatal neurons; (ii) The efficiency of synaptic depolarizing GABA action before the ?excitatory-to-inhibitory GABA switch? (P5 - P12); (iii) The efficiency of GABAergic inhibition after the ?excitatory-to-inhibitory GABA switch? (P14 ? P22); (iv) Development of synchronized neonatal neuronal activity indicative of the formation of neural circuits; (v) Developmental changes in synaptic plasticity. These studies will show the role of the Kcnj6 gene in the formation of neural circuits and give a basis for monitoring post-treatment changes in the Aim 2.
Aim 2. To examine the effects of early postnatal treatments reducing signaling through the Kir3.2 subunit-containing potassium channels on the development and maturation of neural circuits in DS models. Kir3.2 channel signaling will be reduced in a time- and region- specific manner via: (i) Neonatal intraventricular AAV delivery of Kcnj6 siRNA; (ii) Intraperitoneal injections of selective GABAB receptor antagonists. Consequences of these treatments will be examined in neonatal and adult mice. Following properties will be assessed: changes in Kir3.2 channel signaling, properties of neonatal neural circuits, post-treatment changes in GABAergic synaptic transmission and synaptic plasticity. This data will provide information on the role of Kcnj6 gene in synaptic, structural, and functional abnormalities in DS, and provide a ?proof of principle? that time-limited postnatal suppression of Kir3.2 channel signaling can be used to improve brain functions and cognition in DS.
(3 sentence limit) In mouse genetic models of Down syndrome (DS), enhanced inhibitory efficiency limits hippocampal synaptic plasticity thus negatively affecting cognition. Here we hypothesized that these changes occur during early postnatal periods as a compensatory reaction to excessive neuronal hyperpolarization, caused by increased signaling through Kir3.2 channels, preventing neonatal neurons from ?seeing? each other during the formation of nascent neural circuits. While testing this hypothesis, we will examine the therapeutic potential of antagonists of the GABAB receptors as cognitive enhancers in DS.
