Motor neuron output in the crustacean cardiac ganglion is organized and maintained by homeostatic conductance relationships

Generating appropriate and functional neuronal output is a matter of fundamental importance to nervous systems across species and phyla. Potential perturbations to a neurons intrinsic excitability alone include Hebbian style plasticity, ion channel turnover and neuromodulation. With these dynamic processes in place, it is remarkable that neurons are able to maintain stable and functional output throughout the life of an organism. Yet little is known about the cellular mechanisms in neurons and networks which act to stabilize functional output. Here we utilize large cell motor neurons of the Cancer borealis cardiac ganglion to investigate how these cell's important burst output is generated, maintained and synchronized across cells in a rhythmic motor network. We find that large cell motor neurons utilize multiple intrinsic currents to mediate burst potentials, and these currents are in part localized to the LC somata, distal from action potential generating currents. Next we examine the functional organization of these intrinsic currents and show that LC motor neurons, which have synchronous burst activity during normal network behavior, have differential intrinsic tuning even across LCs in a single network. This differential tuning results in conserved voltage output across LCS in part due to compensatory conductance relationships.Includes bibliographical references (pages 139-148)

Correlated Levels of mRNA and Soma Size in Single Identified Neurons: Evidence for Compartment-specific Regulation of Gene Expression

In addition to the overall complexity of transcriptional regulation, cells also must take into account the subcellular distribution of these gene products. This is particularly challenging for morphologically complex cells such as neurons. Yet the interaction between cellular morphology and gene expression is poorly understood. Here we provide some of the first evidence for a relationship between neuronal compartment size and maintenance of mRNA levels in neurons. We find that single-cell transcript levels of 18S rRNA, GAPDH, and EF1-alpha, all gene products with primary functions in the cell soma, are strongly correlated to soma size in multiple distinct neuronal types. Levels of mRNA for the K+ channel shal, which is localized exclusively to the soma, are negatively correlated with soma size, suggesting that gene expression does not simply track positively with compartment size. Conversely, levels of beta-actin and beta-tubulin mRNA, which are major cytoskeletal proteins of neuronal processes, do not correlate with soma size, but are strongly correlated with one another. Additionally, actin/tubulin expression levels correlate with voltage-gated ion channels that are uniquely localized to axons. These results suggest that steady-state transcript levels are differentially regulated based on the subcellular compartment within which a given gene product primarily acts

Intrinsic mechanisms in the gating of resurgent Na+ currents

The resurgent component of the voltage-gated sodium current (

Intracellular FGF14 (iFGF14) is required for spontaneous and evoked firing in cerebellar Purkinje neurons and for motor coordination and balance

Mutations in FGF14, which encodes intracellular fibroblast growth factor 14 (iFGF14), have been linked to spinocerebellar ataxia (SCA27). In addition, mice lacking Fgf14 (Fgf14(−/−)) exhibit an ataxia phenotype resembling SCA27, accompanied by marked changes in the excitability of cerebellar granule and Purkinje neurons. It is not known, however, whether these phenotypes result from defects in neuronal development or if they reflect a physiological requirement for iFGF14 in the adult cerebellum. Here, we demonstrate that the acute and selective Fgf14-targeted short hairpin RNA (shRNA)-mediated in vivo “knock-down” of iFGF14 in adult Purkinje neurons attenuates spontaneous and evoked action potential firing without measurably affecting the expression or localization of voltage-gated Na(+) (Nav) channels at Purkinje neuron axon initial segments. The selective shRNA-mediated in vivo “knock-down” of iFGF14 in adult Purkinje neurons also impairs motor coordination and balance. Repetitive firing can be restored in Fgf14-targeted shRNA-expressing Purkinje neurons, as well as in Fgf14(−/−) Purkinje neurons, by prior membrane hyperpolarization, suggesting that the iFGF14-mediated regulation of the excitability of mature Purkinje neurons depends on membrane potential. Further experiments revealed that the loss of iFGF14 results in a marked hyperpolarizing shift in the voltage dependence of steady-state inactivation of the Nav currents in adult Purkinje neurons. We also show here that expressing iFGF14 selectively in adult Fgf14(−/−) Purkinje neurons rescues spontaneous firing and improves motor performance. Together, these results demonstrate that iFGF14 is required for spontaneous and evoked action potential firing in adult Purkinje neurons, thereby controlling the output of these cells and the regulation of motor coordination and balance

Generation of Human Striatal Neurons by MicroRNA-Dependent Direct Conversion of Fibroblasts

SummaryThe promise of using reprogrammed human neurons for disease modeling and regenerative medicine relies on the ability to induce patient-derived neurons with high efficiency and subtype specificity. We have previously shown that ectopic expression of brain-enriched microRNAs (miRNAs), miR-9/9∗ and miR-124 (miR-9/9∗-124), promoted direct conversion of human fibroblasts into neurons. Here we show that coexpression of miR-9/9∗-124 with transcription factors enriched in the developing striatum, BCL11B (also known as CTIP2), DLX1, DLX2, and MYT1L, can guide the conversion of human postnatal and adult fibroblasts into an enriched population of neurons analogous to striatal medium spiny neurons (MSNs). When transplanted in the mouse brain, the reprogrammed human cells persisted in situ for over 6 months, exhibited membrane properties equivalent to native MSNs, and extended projections to the anatomical targets of MSNs. These findings highlight the potential of exploiting the synergism between miR-9/9∗-124 and transcription factors to generate specific neuronal subtypes

Loss of Navβ4-Mediated Regulation of Sodium Currents in Adult Purkinje Neurons Disrupts Firing and Impairs Motor Coordination and Balance

Summary: The resurgent component of voltage-gated Na+ (Nav) currents, INaR, has been suggested to provide the depolarizing drive for high-frequency firing and to be generated by voltage-dependent Nav channel block (at depolarized potentials) and unblock (at hyperpolarized potentials) by the accessory Navβ4 subunit. To test these hypotheses, we examined the effects of the targeted deletion of Scn4b (Navβ4) on INaR and on repetitive firing in cerebellar Purkinje neurons. We show here that Scn4b−/− animals have deficits in motor coordination and balance and that firing rates in Scn4b−/− Purkinje neurons are markedly attenuated. Acute, in vivo short hairpin RNA (shRNA)-mediated “knockdown” of Navβ4 in adult Purkinje neurons also reduced spontaneous and evoked firing rates. Dynamic clamp-mediated addition of INaR partially rescued firing in Scn4b−/− Purkinje neurons. Voltage-clamp experiments revealed that INaR was reduced (by ∼50%), but not eliminated, in Scn4b−/− Purkinje neurons, revealing that additional mechanisms contribute to generation of INaR. : Loss of Navβ4 attenuates, but does not eliminate, the resurgent sodium current (INaR) in cerebellar Purkinje neurons, revealing that additional mechanism(s) contribute to the generation of INaR. Ransdell et al. also find that INaR magnitude tunes the firing rate of Purkinje neurons and that Navβ4−/− animals display balance and motor deficits. Keywords: cerebellum, resurgent sodium current, Scn4b−/−, Scn4b-targeted shRNA, dynamic clam