Alqadah A et al. (2017) Dev Dyn "Asymmetric development of the nervous system."

Alqadah A, Chuang CF, Hsieh YW, Morrissey ZD

[Dev Dyn, 2017]

The human nervous system consists of seemingly symmetric left and right halves. However, closer observation of the brain reveals anatomical and functional lateralization. Defects in brain asymmetry correlate with several neurological disorders, yet our understanding of the mechanisms used to establish lateralization in the human central nervous system is extremely limited. Here, we review left-right asymmetries within the nervous system of humans and several model organisms, including rodents, zebrafish, chickens, Xenopus, Drosophila, and the nematode Caenorhabditis elegans. Comparing and contrasting mechanisms used to develop left-right asymmetry in the nervous system can provide insight into how the human brain is lateralized. This article is protected by copyright. All rights reserved.

Alqadah A et al. (2014) Cell Cycle "A molecular link between distinct neuronal asymmetries."

Alqadah A, Chuang CF, Hsieh YW

[Cell Cycle, 2014]

Alqadah A et al. (2016) Cell Cycle "Sox2 goes beyond stem cell biology."

Alqadah A, Chuang CF, Hsieh YW

[Cell Cycle, 2016]

Alqadah A et al. (2013) Front Cell Neurosci "microRNA function in left-right neuronal asymmetry: perspectives from C. elegans."

Alqadah A, Hsieh YW, Chuang CF

[Front Cell Neurosci, 2013]

Left-right asymmetry in anatomical structures and functions of the nervous system is present throughout the animal kingdom. For example, language centers are localized in the left side of the human brain, while spatial recognition functions are found in the right hemisphere in the majority of the population. Disruption of asymmetry in the nervous system is correlated with neurological disorders. Although anatomical and functional asymmetries are observed in mammalian nervous systems, it has been a challenge to identify the molecular basis of these asymmetries. C. elegans has emerged as a prime model organism to investigate molecular asymmetries in the nervous system, as it has been shown to display functional asymmetries clearly correlated to asymmetric distribution and regulation of biologically relevant molecules. Small non-coding RNAs have been recently implicated in various aspects of neural development. Here, we review cases in which microRNAs are crucial for establishing left-right asymmetries in the C. elegans nervous system. These studies may provide insight into how molecular and functional asymmetries are established in the human brain.

Alqadah A et al. (2015) EMBO J "Postmitotic diversification of olfactory neuron types is mediated by differential activities of the HMG-box transcription factor SOX-2."

Hsieh YW, Vidal B, Chang C, Chuang CF, Alqadah A, Hobert O

[EMBO J, 2015]

Diversification of neuron classes is essential for functions of the olfactory system, but the underlying mechanisms that generate individual olfactory neuron types are only beginning to be understood. Here we describe a role of the highly conserved HMG-box transcription factor SOX-2 in postmitotic specification and alternative differentiation of the Caenorhabditis elegans AWC and AWB olfactory neurons. We show that SOX-2 partners with different transcription factors to diversify postmitotic olfactory cell types. SOX-2 functions cooperatively with the OTX/OTD transcription factor CEH-36 to specify an AWC "ground state," and functions with the LIM homeodomain factor LIM-4 to suppress this ground state and drive an AWB identity instead. Our findings provide novel insights into combinatorial codes that drive terminal differentiation programs in the nervous system and reveal a biological function of the deeply conserved Sox2 protein that goes beyond its well-known role in stem cell biology.

Alqadah A et al. (2016) Philos Trans R Soc Lond B Biol Sci "Stochastic left-right neuronal asymmetry in Caenorhabditis elegans."

Chuang CF, Alqadah A, Hsieh YW, Xiong R

[Philos Trans R Soc Lond B Biol Sci, 2016]

Left-right asymmetry in the nervous system is observed across species. Defects in left-right cerebral asymmetry are linked to several neurological diseases, but the molecular mechanisms underlying brain asymmetry in vertebrates are still not very well understood. The Caenorhabditis elegans left and right amphid wing 'C' (AWC) olfactory neurons communicate through intercellular calcium signalling in a transient embryonic gap junction neural network to specify two asymmetric subtypes, AWC(OFF) (default) and AWC(ON) (induced), in a stochastic manner. Here, we highlight the molecular mechanisms that establish and maintain stochastic AWC asymmetry. As the components of the AWC asymmetry pathway are highly conserved, insights from the model organism C. elegans may provide a window onto how brain asymmetry develops in humans.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

Alqadah, A. et al. (2015) International Worm Meeting "Postmitotic diversification of olfactory neuron types is mediated by differential activities of the HMG-box transcription factor SOX-2."

Alqadah, A., Hobert, O., Vidal, B., Chang, C., Hsieh, Y.-W., Chuang, C.-F.

[International Worm Meeting, 2015]

Neuronal diversification is essential for the function of the olfactory system. One way to achieve neuronal diversification is to specify distinct neuronal subclasses through combinatorial control of gene expression mediated by cooperative binding of transcription factors. However, the molecular mechanisms of how combinatorial control of gene expression functions to diversify neuronal subclasses are only partly understood. Here we describe a novel role of the highly conserved HMG-box transcription factor SOX-2 in postmitotic specification and alternative differentiation of the C. elegans AWC and AWB olfactory neurons. We show that SOX-2 partners with different transcription factors to diversify postmitotic olfactory cell types. SOX-2 functions cooperatively with the OTX/OTD transcription factor CEH-36 to specify an AWC "ground state", and functions with the LIM homeodomain factor LIM-4 to suppress this ground state and drive an alternate AWB fate. Our findings provide novel insights into combinatorial codes that drive terminal differentiation programs in the nervous system and reveal a novel biological function of the deeply conserved Sox2 protein that goes beyond its well-known role in stem cell biology.

Alqadah, A. et al. (2019) International Worm Meeting "A universal transportin protein controls asymmetric olfactory neuron differentiation via specific nuclear transport of a sox-2-activating factor."

Lesch, B., Chuang, C.-F., Alqadah, A., Hsieh, Y.-W., Xiong, R., Chang, C.

[International Worm Meeting, 2019]

Defects in brain laterality are associated with several neurological diseases. Left-right asymmetry in the nervous system is observed in many species; however, the mechanisms used to establish brain lateralization are not well understood. The Caenorhabditis elegans AWC olfactory neuron pair asymmetrically differentiates into the default AWCOFF and induced AWCON subtypes in a stochastic manner. Stochastic choice of the AWCON subtype is established using gap junctions and SLO BK potassium channels to repress a calcium-activated protein kinase pathway. However, it is unknown how the potassium channel-repressed calcium signaling is translated into the induction of the AWCON subtype. To address this question, we performed a forward genetic screen to discover suppressors of slo-1(gf) mutants that act as modifiers of K+ channels (mok genes). Out of 6,000 genomes screened, we identified 16 mok mutants that suppressed the slo-1(gf) 2AWCON phenotype. The mok-5(vy10) mutant by itself displays a 2AWCOFF phenotype, suggesting an essential role of mok-5 in promoting the AWCON subtype. We found that mok-5(vy10) is a missense mutation in a highly conserved importin protein using whole genome sequencing (kindly performed by Oliver Hobert's lab). mok-5 loss-of-function mutants are not viable; here, we identify a viable mok-5 allele from an unbiased forward genetic screen that reveals a specific role of mok-5 in AWC olfactory neuron asymmetry. Double mutant analysis places mok-5 downstream of the calcium-triggered MAP kinase cascade to promote the AWCON subtype. Consistent with the genetic data, mok-5 importin is asymmetrically expressed in the AWCON neuron and acts cell autonomously to specify the AWCON subtype. Furthermore, we show that mok-5 importin functions to mediate transport of a homeodomain transcription factor required for AWCON subtype specification into the nucleus of AWC neurons. Lastly, we show that the homeodomain transcription factor activates the expression of the HMG transcription factor sox-2 by direct binding to the sox-2 promoter to promote the AWCON identity. Together, our results show that mok-5 specifically controls stochastic choice of olfactory neurons by mediating nuclear transport of the sox-2-activating factor. This study sets a precedent by mechanistically implicating a widely expressed transportin in neuronal cell fate specification. Our findings also provide structure-function insight into a conserved amino acid residue of transportins in brain development and suggest its mutation may lead to human neurological disorders.

Alqadah, A. et al. (2017) International Worm Meeting "An importin protein controls asymmetric olfactory neuron differentiation by mediating nuclear transport of a homoeodomain transcription factor for sox-2 expression."

Chuang, C.-F., Hsieh, Y.-W., Alqadah, A., Lesch, B.J.

[International Worm Meeting, 2017]

Defects in brain laterality are associated with several neurological diseases. Left-right asymmetry in the nervous system is observed in many species; however, the mechanisms used to establish brain lateralization are not well understood. The pair of AWC olfactory neurons displays molecular and functional left-right asymmetry in the nematode C. elegans. The neurons are differentiated into the AWCOFF and AWCON subtypes that express different sets of genes and have unique functions. The AWC neurons display stochastic asymmetry, such that either subtype has equal probability of residing on the left or right side of the animal. This asymmetry is established using a calcium-triggered signaling pathway consisting of voltage-gated calcium channels, CaMKII, TIR-1 (Sarm1) adaptor protein, and a MAP kinase cascade to promote the default AWCOFF identity. The contralateral neuron represses the calcium signaling pathway through NSY-5 gap junctions and downstream redundant SLO-1/SLO-2 BK potassium channels to induce the AWCON cell identity. It is unknown what molecules function downstream of the SLO-1 and SLO-2 potassium channels to induce AWCON. To address this question, we performed a forward genetic screen to discover suppressors of slo-1(gf) mutants that act as modifiers of K+ channels (mok genes). Out of 6,000 genomes screened, we identified 16 mok mutants. Using whole genome sequencing (kindly performed by Oliver Hobert's lab), we found that the mok-5 gene encodes a highly conserved importin protein. A vy10 missense mutation in the mok-5 importin gene results in a 2AWCOFF phenotype, suggesting an essential role of mok-5 in promoting the AWCON subtype. Double mutant analysis places mok-5 downstream of the calcium-triggered MAP kinase cascade to promote the AWCON subtype. Consistent with the genetic data, mok-5 importin is asymmetrically expressed in the AWCON neuron and acts cell autonomously to specify the AWCON subtype. Furthermore, we show that mok-5 importin functions to mediate transport of a homeodomain transcription factor required for AWCON subtype specification into the nucleus of AWC neurons. Lastly, we show that the homeodomain transcription factor regulates the expression of HMG transcription factor sox-2 by direct binding to the sox-2 promoter to promote the AWCON identity. To the best of our knowledge, this is the first study to implicate an importin protein in the development of left-right asymmetry in the nervous system.

Alqadah A et al. (2016) PLoS Genet "SLO BK Potassium Channels Couple Gap Junctions to Inhibition of Calcium Signaling in Olfactory Neuron Diversification."

Jorgensen EM, Alqadah A, Schumacher JA, Bayne B, Merrill SA, Wang X, Millington G, Hsieh YW, Chuang CF

[PLoS Genet, 2016]

The C. elegans AWC olfactory neuron pair communicates to specify asymmetric subtypes AWCOFF and AWCON in a stochastic manner. Intercellular communication between AWC and other neurons in a transient NSY-5 gap junction network antagonizes voltage-activated calcium channels, UNC-2 (CaV2) and EGL-19 (CaV1), in the AWCON cell, but how calcium signaling is downregulated by NSY-5 is only partly understood. Here, we show that voltage- and calcium-activated SLO BK potassium channels mediate gap junction signaling to inhibit calcium pathways for asymmetric AWC differentiation. Activation of vertebrate SLO-1 channels causes transient membrane hyperpolarization, which makes it an important negative feedback system for calcium entry through voltage-activated calcium channels. Consistent with the physiological roles of SLO-1, our genetic results suggest that slo-1 BK channels act downstream of NSY-5 gap junctions to inhibit calcium channel-mediated signaling in the specification of AWCON. We also show for the first time that slo-2 BK channels are important for AWC asymmetry and act redundantly with slo-1 to inhibit calcium signaling. In addition, nsy-5-dependent asymmetric expression of slo-1 and slo-2 in the AWCON neuron is necessary and sufficient for AWC asymmetry. SLO-1 and SLO-2 localize close to UNC-2 and EGL-19 in AWC, suggesting a role of possible functional coupling between SLO BK channels and voltage-activated calcium channels in AWC asymmetry. Furthermore, slo-1 and slo-2 regulate the localization of synaptic markers, UNC-2 and RAB-3, in AWC neurons to control AWC asymmetry. We also identify the requirement of bkip-1, which encodes a previously identified auxiliary subunit of SLO-1, for slo-1 and slo-2 function in AWC asymmetry. Together, these results provide an unprecedented molecular link between gap junctions and calcium pathways for terminal differentiation of olfactory neurons.