Paolo Spaiardi

Developmentally coordinated extrinsic signals drive human pluripotent stem cell differentiation toward authentic DARPP-32(+) medium-sized spiny neurons

Medium-sized spiny neurons (MSNs) are the only neostriatum projection neurons, and their degeneration underlies some of the clinical features of Huntington’s disease. Using knowledge of human developmental biology and exposure to key neurodevelopmental molecules, human pluripotent stem (hPS) cells were induced to differentiate into MSNs. In a feeder-free adherent culture, ventral telencephalic specification is induced by BMP/TGFβ inhibition and subsequent SHH/DKK1 treatment. The emerging FOXG1+/GSX2+ telencephalic progenitors are then terminally differentiated, resulting in the systematic line-independent generation of FOXP1+/FOXP2+/CTIP2+/calbindin+/DARPP-32+ MSNs. Similar to mature MSNs, these neurons carry dopamine and A2a receptors, elicit a typical firing pattern and show inhibitory postsynaptic currents, as well as dopamine neuromodulation and synaptic integration ability in vivo. When transplanted into the striatum of quinolinic acid-lesioned rats, hPS-derived neurons survive and differentiate into DARPP-32+ neurons, leading to a restoration of apomorphine-induced rotation behavior. In summary, hPS cells can be efficiently driven to acquire a functional striatal fate using an ontogeny-recapitulating stepwise method that represents a platform for in vitro human developmental neurobiology studies and drug screening approaches.

Dual modulation of inward rectifier potassium currents in olfactory neuronal cells by promiscuous G protein coupling of the oxytocin receptor

J. Neurochem. (2010) 114, 1424–1435.

Rac1 and Rac3 GTPases Control Synergistically the Development of Cortical and Hippocampal GABAergic Interneurons

The intracellular mechanisms driving postmitotic development of cortical γ-aminobutyric acid (GABA)ergic interneurons are poorly understood. We have addressed the function of Rac GTPases in cortical and hippocampal interneuron development. Developing neurons express both Rac1 and Rac3. Previous work has shown that Rac1 ablation does not affect the development of migrating cortical interneurons. Analysis of mice with double deletion of Rac1 and Rac3 shows that these GTPases are required during postmitotic interneuron development. The number of parvalbumin-positive cells was affected in the hippocampus and cortex of double knockout mice. Rac depletion also influences the maturation of interneurons that reach their destination, with reduction of inhibitory synapses in both hippocampal CA1 and cortical pyramidal cells. The decreased number of cortical migrating interneurons and their altered morphology indicate a role of Rac1 and Rac3 in regulating the motility of cortical interneurons, thus interfering with their final localization. While electrophysiological passive and active properties of pyramidal neurons including membrane capacity, resting potential, and spike amplitude and duration were normal, these cells showed reduced spontaneous inhibitory currents and increased excitability. Our results show that Rac1 and Rac3 contribute synergistically to postmitotic development of specific populations of GABAergic cells, suggesting that these proteins regulate their migration and differentiation.

Functional Interactions Within the Parahippocampal Region Revealed by Voltage-Sensitive Dye Imaging in the Isolated Guinea Pig Brain

The massive transfer of information from the neocortex to the entorhinal cortex (and vice versa) is hindered by a powerful inhibitory control generated in the perirhinal cortex. In vivo and in vitro experiments performed in rodents and cats support this conclusion, further extended in the present study to the analysis of the interaction between the entorhinal cortex and other parahippocampal areas, such as the postrhinal and the retrosplenial cortices. The experiments were performed in the in vitro isolated guinea pig brain by a combined approach based on electrophysiological recordings and fast imaging of optical signals generated by voltage-sensitive dyes applied to the entire brain by arterial perfusion. Local stimuli delivered in different portions of the perirhinal, postrhinal, and retrosplenial cortex evoked local responses that did not propagate to the entorhinal cortex. Neither high- and low-frequency-patterned stimulation nor paired associative stimuli facilitated the propagation of activity to the entorhinal region. Similar stimulations performed during cholinergic neuromodulation with carbachol were also ineffective in overcoming the inhibitory network that controls propagation to the entorhinal cortex. The pharmacological inactivation of GABAergic transmission by local application of bicuculline (1 mM) in area 36 of the perirhinal cortex facilitated the longitudinal (rostrocaudal) propagation of activity into the perirhinal/postrhinal cortices but did not cause propagation into the entorhinal cortex. Bicuculline injection in both area 35 and medial entorhinal cortex released the inhibitory control and allowed the propagation of the neural activity to the entorhinal cortex. These results demonstrate that, as for the perirhinal-entorhinal reciprocal interactions, also the connections between the postrhinal/retrosplenial cortices and the entorhinal region are subject to a powerful inhibitory control.

Elementary properties of Ca2+ channels and their influence on multivesicular release and phase-locking at auditory hair cell ribbon synapses

Voltage-gated calcium (Cav1.3) channels in mammalian inner hair cells (IHCs) open in response to sound and the resulting Ca2+ entry triggers the release of the neurotransmitter glutamate onto afferent terminals. At low to mid sound frequencies cell depolarization follows the sound sinusoid and pulses of transmitter release from the hair cell generate excitatory postsynaptic currents (EPSCs) in the afferent fiber that translate into a phase-locked pattern of action potential activity. The present article summarizes our current understanding on the elementary properties of single IHC Ca2+ channels, and how these could have functional implications for certain, poorly understood, features of synaptic transmission at auditory hair cell ribbon synapses.

Distinct roles of Eps8 in the maturation of cochlear and vestibular hair cells.

Several genetic mutations affecting the development and function of mammalian hair cells have been shown to cause deafness but not vestibular defects, most likely because vestibular deficits are sometimes centrally compensated. The study of hair cell physiology is thus a powerful direct approach to ascertain the functional status of the vestibular end organs. Deletion of Epidermal growth factor receptor pathway substrate 8 (Eps8), a gene involved in actin remodeling, has been shown to cause deafness in mice. While both inner and outer hair cells from Eps8 knockout (KO) mice showed abnormally short stereocilia, inner hair cells (IHCs) also failed to acquire mature-type ion channels. Despite the fact that Eps8 is also expressed in vestibular hair cells, Eps8 KO mice show no vestibular deficits. In the present study we have investigated the properties of vestibular Type I and Type II hair cells in Eps8-KO mice and compared them to those of cochlear IHCs. In the absence of Eps8, vestibular hair cells show normally long kinocilia, significantly shorter stereocilia and a normal pattern of basolateral voltage-dependent ion channels. We have also found that while vestibular hair cells from Eps8 KO mice show normal voltage responses to injected sinusoidal currents, which were used to mimic the mechanoelectrical transducer current, IHCs lose their ability to synchronize their responses to the stimulus. We conclude that the absence of Eps8 produces a weaker phenotype in vestibular hair cells compared to cochlear IHCs, since it affects the hair bundle morphology but not the basolateral membrane currents. This difference is likely to explain the absence of obvious vestibular dysfunction in Eps8 KO mice.

An allosteric gating model recapitulates the biophysical properties of IK,L expressed in mouse vestibular type I hair cells

Vestibular type I and type II hair cells and their afferent fibres send information to the brain regarding the position and movement of the head. The characteristic feature of type I hair cells is the expression of a low‐voltage‐activated outward rectifying K+ current, IK,L, whose biophysical properties and molecular identity are still largely unknown. In vitro, the afferent nerve calyx surrounding type I hair cells causes unstable intercellular K+ concentrations, altering the biophysical properties of IK,L. We found that in the absence of the calyx, IK,L in type I hair cells exhibited unique biophysical activation properties, which were faithfully reproduced by an allosteric channel gating scheme. These results form the basis for a molecular and pharmacological identification of IK,L.

Glutamic acid decarboxylase 67 expression by a distinct population of mouse vestibular supporting cells.

The function of the enzyme glutamate decarboxylase (GAD) is to convert glutamate in γ-aminobutyric acid (GABA). Glutamate decarboxylase exists as two major isoforms, termed GAD65 and GAD67, that are usually expressed in GABA-containing neurons in the central nervous system. GAD65 has been proposed to be associated with GABA exocytosis whereas GAD67 with GABA metabolism. In the present immunofluorescence study, we have investigated the presence of the two GAD isoforms in the semicircular canal cristae of wild type and GAD67-GFP knock-in mice. While no evidence for GAD65 expression was found, GAD67 was detected in a distinct population of peripherally-located supporting cells, but not in hair cells or in centrally-located supporting cells. GABA, on the other hand, was found in all supporting cells. The present result indicate that only a discrete population of supporting cells use GAD67 to synthesize GABA. This is the first report of a marker that allows to distinguish two populations of supporting cells in the vestibular epithelium. On the other hand, the lack of GABA and GAD enzymes in hair cells excludes its involvement in afferent transmission.

Analysis of the noise associated to the muscarinic modulation of the mouse perirhinal cortex

The perirhinal cortex (PRC) is a polymodal associative area which plays a key role in several memory processes. It has been shown that cholinergic muscarinic modulation plays a crucial role in regulating PRC functions. In this work, changes in the signal noise during muscarinic modulation of the PRC have been observed and analyzed. The relationship between these changes and the information processing performed by neurons of the PRC are inevstigated. In more details, the effects of muscarinic modulation on the membrane potential and on the voltage membrane noise of pyramidal neurons and GABAergic interneurons are inevstigated, both from deep and superficial layers of the PRC.