During development, multipotent neural precursors give rise to oligodendrocyte progenitor cells (OPCs), which migrate and divide to produce additional OPCs. Near the end of embryogenesis and during postnatal stages, many OPCs stop dividing and differentiate as myelinating oligodendrocytes, whereas others persist as nonmyelinating cells. Investigations of oligodendrocyte development in mice indicated that the Nkx2.2 transcription factor both limits the number of OPCs that are formed and subsequently promotes their differentiation, raising the possibility that Nkx2.2 plays a key role in determining myelinating versus nonmyelinating fate. We used in vivo time-lapse imaging and loss-of-function experiments in zebrafish to further explore formation and differentiation of oligodendrocyte lineage cells. Our data show that newly specified OPCs are heterogeneous with respect to gene expression and fate. Whereas some OPCs express the nkx2.2a gene and differentiate as oligodendrocytes, others that do not express nkx2.2a mostly remain as nonmyelinating OPCs. Similarly to mouse, loss of nkx2.2a function results in excess OPCs and delayed oligodendrocyte differentiation. Notably, excess OPCs are formed as a consequence of prolonged OPC production from neural precursor cells. We conclude that Nkx2.2 promotes timely specification and differentiation of myelinating oligodendrocyte lineage cells from species representing different vertebrate taxa.

Acid-sensing ion channels (ASICs) evolved to sense changes in extracellular acidity with the divalent cation calcium (Ca2+) as an allosteric modulator and channel blocker. The channel-blocking activity is most apparent in ASIC3, as removing Ca2+ results in channel opening, with the site’s location remaining unresolved. Here we show that a ring of rat ASIC3 (rASIC3) glutamates (Glu435), located above the channel gate, modulates proton sensitivity and contributes to the formation of the elusive Ca2+ block site. Mutation of this residue to glycine, the equivalent residue in chicken ASIC1, diminished the rASIC3 Ca2+ block effect. Atomistic molecular dynamic simulations corroborate the involvement of this acidic residue in forming a high-affinity Ca2+ site atop the channel pore. Furthermore, the reported observations provide clarity for past controversies regarding ASIC channel gating. Our findings enhance understanding of ASIC gating mechanisms and provide structural and energetic insights into this unique calcium-binding site.

Stress is the most common trigger among episodic neurologic disorders. In episodic ataxia type 2 (EA2), physical or emotional stress causes episodes of severe motor dysfunction that manifest as ataxia and dystonia. We used the tottering (tg/tg) mouse, a faithful animal model of EA2, to dissect the mechanisms underlying stress-induced motor attacks. We find that in response to acute stress, activation of α 1 -adrenergic receptors (α1-Rs) on Purkinje cells by norepinephrine leads to their erratic firing and consequently motor attacks. We show that norepinephrine induces erratic firing of Purkinje cells by disrupting their spontaneous intrinsic pacemaking via a casein kinase 2 (CK2)–dependent signaling pathway, which likely reduces the activity of calcium-dependent potassium channels. Moreover, we report that disruption of this signaling cascade at a number of nodes prevents stress-induced attacks in the tottering mouse. Together, our results suggest that norepinephrine and CK2 are required for the initiation of stress-induced attacks in EA2 and provide previously unidentified targets for therapeutic intervention.

Background Cytoplasmic dynein provides the main motor force for minus-end-directed transport of cargo on microtubules. Within the vertebrate central nervous system (CNS), proliferation, neuronal migration and retrograde axon transport are among the cellular functions known to require dynein. Accordingly, mutations of DYNC1H1, which encodes the heavy chain subunit of cytoplasmic dynein, have been linked to developmental brain malformations and axonal pathologies. Oligodendrocytes, the myelinating glial cell type of the CNS, migrate from their origins to their target axons and subsequently extend multiple long processes that ensheath axons with specialized insulating membrane. These processes are filled with microtubules, which facilitate molecular transport of myelin components. However, whether oligodendrocytes require cytoplasmic dynein to ensheath axons with myelin is not known. Results We identified a mutation of zebrafish dync1h1 in a forward genetic screen that caused a deficit of oligodendrocytes. Using in vivo imaging and gene expression analyses, we additionally found evidence that dync1h1 promotes axon ensheathment and myelin gene expression. Conclusions In addition to its well known roles in axon transport and neuronal migration, cytoplasmic dynein contributes to neural development by promoting myelination.

Vertebrate craniofacial development requires coordinated morphogenetic interactions between the extracellular matrix (ECM) and the differentiating chondrocytes essential for cartilage formation. Recent studies reveal a critical role for specific lysyl oxidases in ECM integrity required for embryonic development. We now demonstrate that loxl3b is abundantly expressed within the head mesenchyme of the zebrafish and is critically important for maturation of neural crest derived cartilage elements. Histological and ultrastructural analysis of cartilage elements in loxl3b morphant embryos reveals abnormal maturation of cartilage and altered chondrocyte morphology. Spatiotemporal analysis of craniofacial markers in loxl3b morphant embryos shows that cranial neural crest cells migrate normally into the developing pharyngeal arches but that differentiation and condensation markers are aberrantly expressed. We further show that the loxl3b morphant phenotype is not due to P53 mediated cell death but likely to be due to reduced chondrogenic progenitor cell proliferation within the pharyngeal arches. Taken together, these data demonstrate a novel role for loxl3b in the maturation of craniofacial cartilage and can provide new insight into the specific genetic factors important in the pathogenesis of craniofacial birth defects.

Amiloride, a diuretic used in the treatment of hypertension and congestive heart failure, and 2-guanidine-4-methylquinazoline (GMQ) are guanidine compounds that modulate acid-sensing ion channels. Both compounds have demonstrated affinity for a variety of membrane proteins, including members of the Cysloop family of ligand-gated ion channels, such as the heteromeric GABA-A abg receptors. The actions of these guanidine compounds on the homomeric GABA-A r1 receptor remains unclear, especially in light of how many GABA-A abg receptor modulators have different effects in the GABA-A r1 receptors. We sought to characterize the influence of amiloride and GMQ on the human GABA-A r1 receptors using whole-cell patch-clamp electrophysiology. The diuretic amiloride potentiated the human GABA-A r1 GABA-mediated current, whereas GMQ antagonized the receptor. Furthermore, a GABA-A second transmembrane domain site, the intersubunit site, responsible for allosteric modulation in the heteromeric GABA-A receptors mediated amiloride's positive allosteric actions. In contrast, the mutation did not remove GMQ antagonism but only changed the guanidine compound's potency within the human GABA-A r1 receptor. Through modeling and introduction of point mutations, we propose that the GABA-A r1 intersubunit site plays a role in mediating the allosteric effects of amiloride and GMQ.

Guanidine compounds act as ion channel modulators. In the case of Cys-loop receptors, the guanidine compound amiloride antagonized the heteromeric GABA-A, glycine, and nicotinic acetylcholine receptors. However, amiloride exhibits characteristics consistent with a positive allosteric modulator for the human GABA-A (hGABA-A) ρ1 receptor. Site-directed mutagenesis revealed that the positive allosteric modulation was influenced by the GABA-A ρ1 second transmembrane domain 15' position, a site implicated in ligand allosteric modulation of Cys-loop receptors. There are a variety of amiloride derivatives that provide opportunities to assess the significance of amiloride functional groups (e.g., the guanidine group, the pyrazine ring, etc.) in the modulation of the GABA-A ρ1 receptor activity. We utilized 3 amiloride derivatives (benzamil, phenamil, and 5-(N, N-Hexamethylene) amiloride) to assess the contribution of these groups toward the potentiation of the GABA-A ρ1 receptor. Benzamil and phenamil failed to potentiate on the wild type GABA-A ρ1 GABA-mediated current while HMA demonstrated efficacy only at the highest concentration studied. The hGABA-A ρ1 (I15'N) mutant receptor activity was potentiated by lower HMA concentrations compared to the wild type receptor. Our findings suggest that an exposed guanidine group on amiloride and amiloride derivatives is critical for modulating the GABA-A ρ1 receptor. The present study provides a conceptual framework for predicting which amiloride derivatives will demonstrate positive allosteric modulation of the GABA-A ρ1 receptor.

γ‐ amino butyric acid (GABA) is the major inhibitory neurotransmitter in the vertebrate brain, and targets the ionotropic GABAA receptors. GABAC, or GABAA‐rho, is a subclass of GABAA receptors composed entirely of rho (ρ) subunits and are located on the axonal terminal of retinal bipolar cells, where it not only exhibits a tonic inhibitory current, but also regulates the GABA‐A and other GABAA‐rho synaptic currents (Jones et al 2011). GABAA‐rho exhibits unique properties, such as insensitivity to select antagonists of the heteromeric GABAA receptors (Korpi et al., 2002). A group of ligands, which possess a guanidine group, have been shown to influence GABAA receptors. These compounds, such as (S)‐2‐Guanidinopropionic acid and guanidine acetic acid were competitive antagonists for the GABAA‐rho1 receptor. Other guanidine compounds that are acid sensing ion channel (ASIC) ligands, might also exhibit unique effects on the GABAA‐rho1 receptor. We hypothesize that these ASIC ligands will exhibit unique intrinsic activities on the GABAA‐rho1 receptor, which is different from that of the heteromeric GABAA receptor. The human GABAA‐rho1 receptors were expressed in HEK‐293T cells, and activity was analyzed using whole cell patch‐clamp electrophysiology. When co‐applied with GABA and compared to the GABA concentration profile, one ligand was found to decrease the maximal response, with no change in the GABA EC50. This indicates characteristics of non‐competitive inhibition, while a different ligand shifted the GABA EC50 to lower GABA concentrations, suggesting this guanidine compound is a positive allosteric modulator. When applied alone, it failed to directly activate GABA rho1 receptors. These contrasting effects suggest that these ligands act at two binding sites within the GABAA‐rho architecture. Future experiments will focus on additional characterization of these novel effects on GABA rho receptors and offer a novel chemical structure to design novel GABAA‐rho therapeutics. Grant Funding Source: Supported by: Welch Foundation Grant No: BK‐1736, NIH training grant: NBA T32 AG020494