Biology subjects

Asakawa, K.

Publications and source records attributed to Asakawa, K..

7 recordsLinked to original sources

bioRxiv2025

An orthogonal Cro/OR3 binary system for targeted gene expression

Precise spatiotemporal control of gene expression is essential for dissecting complex biological systems. Here, we introduce a phage-derived Cro/OR3 binary system for transcriptional activation that functions robustly in zebrafish and human cells. A Cro repressor fused to the VP16 activation domain efficiently drives expression of OR3 operator-linked genes, and its activity can be targeted to specific cell types using enhancer-trap and promoter-controlled zebrafish lines. As a proof of concept, we manipulate glutamate transporter Glast/Slc1a3b-expressing cells in the central nervous system by Cro/OR3-mediated expression of a human ataxia-associated, uptake-deficient Glast/Slc1a3b variant, causing spastic, uncoordinated movement and axial deformation. Concurrent Gal4/UAS-mediated visualization reveals substantially disrupted positioning of motor neurons, highlighting their selective sensitivity to extracellular glutamate homeostasis during spinal circuit assembly. Together, these results establish the Cro/OR3 system as an orthogonal platform for targeted gene expression, enabling precise mechanistic dissection of how specific cell types and their interactions underlie diverse biological processes.

bioengineering↗

bioRxiv2025

Intrinsic and TDP-43 dysfunction-induced catabolic stress elicit neuroprotective cellular degradation in ALS-vulnerable motor neurons

Selective neuronal vulnerability is a defining feature of neurodegenerative disorders, exemplified by motor neuron degeneration in amyotrophic lateral sclerosis (ALS). The nature of motor neurons underlying this selectivity remains unresolved. Here, by monitoring autophagy at single-cell resolution across the translucent zebrafish spinal cord, we identify motor neurons as the cell population with the highest autophagic flux. Large spinal motor neurons (SMNs), most susceptible to ALS, exhibit higher flux compared to smaller SMNs and ALS-resistant ocular motor neurons. Notably, large SMNs accelerates both autophagy and proteasome-mediated degradation, which are further augmented by TDP-43 loss. Additionally, acceleration of multiple unfolded protein response pathways indicates their innate tendency to accumulate misfolded proteins. Enhanced cellular degradation in large SMNs is neuroprotective as its inhibition halts axon outgrowth. These findings propose that cell size-associated degradation load underlies selective neuronal vulnerability in ALS, highlighting the alleviation of catabolic stress as a target of therapy and prevention.

neuroscience↗

bioRxiv2024

Gene identification for ocular congenital cranial motor neuron disorders using human sequencing, zebrafish screening, and protein binding microarrays

PurposeTo functionally evaluate novel human sequence-derived candidate genes and variants for unsolved ocular congenital cranial dysinnervation disorders (oCCDDs). MethodsThrough exome and genome sequencing of a genetically unsolved human oCCDD cohort, we previously identified variants in 80 strong candidate genes. Here, we further prioritized a subset of these (43 human genes, 57 zebrafish genes) using a G0 CRISPR/Cas9-based knockout assay in zebrafish and generated F2 germline mutants for seventeen. We tested the functionality of variants of uncertain significance in known and novel candidate transcription factor-encoding genes through protein binding microarrays. ResultsWe first demonstrated the feasibility of the G0 screen by targeting known oCCDD genes phox2a and mafba. 70-90% of gene-targeted G0 zebrafish embryos recapitulated germline homozygous null-equivalent phenotypes. Using this approach, we then identified three novel candidate oCCDD genes (SEMA3F, OLIG2, and FRMD4B) with putative contributions to human and zebrafish cranial motor development. In addition, protein binding microarrays demonstrated reduced or abolished DNA binding of human variants of uncertain significance in known and novel sequence-derived transcription factors PHOX2A (p.(Trp137Cys)), MAFB (p.(Glu223Lys)), and OLIG2 (p.(Arg156Leu)). ConclusionsThis study nominates three strong novel candidate oCCDD genes (SEMA3F, OLIG2, and FRMD4B) and supports the functionality and putative pathogenicity of transcription factor candidate variants PHOX2A p.(Trp137Cys), MAFB p.(Glu223Lys), and OLIG2 p.(Arg156Leu). Our findings support that G0 loss-of-function screening in zebrafish can be coupled with human sequence analysis and protein binding microarrays to aid in prioritizing oCCDD candidate genes/variants.

neuroscience↗

bioRxiv2023

Dysregulated TDP-43 proteostasis perturbs excitability of spinal motor neurons during brainstem-mediated fictive locomotion in zebrafish

Spinal motor neurons (SMNs) are the primary target of degeneration in amyotrophic lateral sclerosis (ALS). Degenerating motor neurons accumulate cytoplasmic TAR DNA-binding protein 43 (TDP-43) aggregates in most ALS cases. This SMN pathology can occur without mutation in the coding sequence of the TDP-43-encoding gene, TARDBP. Whether and how wild-type TDP-43 drives pathological changes in SMNs in vivo remain largely unexplored. In this study, we develop a two-photon calcium imaging setup in which tactile-evoked neural responses of motor neurons in the brainstem and spinal cord can be monitored using the calcium indicator GCaMP. We devise a piezo-assisted tactile stimulator that reproducibly evokes a brainstem descending neuron upon tactile stimulation of the head. A direct comparison between caudal primary motor neurons (CaPs) with or without TDP-43 overexpression in contiguous spinal segments demonstrates that CaPs overexpressing TDP-43 display attenuated Ca2+ transients during fictive escape locomotion evoked by the tactile stimulation. These results show that excessive amounts of TDP-43 protein reduce the neuronal excitability of SMNs and potentially contribute to asymptomatic pathological lesions of SMNs and movement disorders in patients with ALS.

neuroscience↗

bioRxiv2020

Gsx2 is involved in specification of neurons in the inferior olivary nuclei from Ptf1a-expressing neuronal progenitors in zebrafish

Neurons in the inferior olivary nuclei (IO neurons) send climbing fibers to Purkinje cells to elicit functions of the cerebellum. IO neurons and Purkinje cells are derived from neural progenitors expressing the proneural gene ptf1a. In this study, we found that the homeobox gene gsx2 was co-expressed with ptf1a in IO progenitors in zebrafish. Both gsx2 and ptf1a zebrafish mutants showed a strong reduction or loss of IO neurons. The expression of ptf1a was not affected in gsx2 mutants and vice versa. In IO progenitors, the ptf1a mutation increased apoptosis whereas the gsx2 mutation did not, suggesting that ptf1a and gsx2 are independently regulated and have distinct roles. The fibroblast growth factors (Fgf) 3/8a and retinoic acid signals negatively and positively, respectively, regulated gsx2 expression and thereby the development of IO neurons. mafba and hox genes are at least partly involved in the Fgf- and retinoic acid-dependent regulation of IO neuronal development. Our results indicate that gsx2 mediates the rostro-caudal positional signals to specify the identity of IO neurons from ptf1a-expressing neural progenitors. SummaryThe homeobox gene gsx2 mediates rostro-caudal positional signaling to specify the identify of neurons in the inferior olivary nuclei from neural progenitors expressing the proneural gene ptf1a.

developmental biology↗

bioRxiv2019

Optogenetic modulation of TDP-43 oligomerization fast-forwards ALS-related pathologies in the spinal motor neurons

Cytoplasmic aggregation of TDP-43 characterizes degenerating neurons in most cases of amyotrophic lateral sclerosis (ALS), yet the mechanisms and cellular outcomes of TDP-43 pathology remain largely elusive. Here, we develop an optogenetic TDP-43 variant (opTDP-43), whose multimerization status can be modulated in vivo through external light illumination. Using the translucent zebrafish neuromuscular system, we demonstrate that short-term light stimulation reversibly induces cytoplasmic opTDP-43 mislocalization, but not aggregation, in the spinal motor neuron, leading to an axon outgrowth defect associated with myofiber denervation. In contrast, opTDP-43 forms pathological aggregates in the cytoplasm after longer-term illumination and seeds non-optogenetic TDP-43 aggregation. Furthermore, we find that an ALS-linked mutation in the intrinsically disordered region (IDR) exacerbates the light-dependent opTDP-43 toxicity on locomotor behavior. Together, our results propose that IDR-mediated TDP-43 oligomerization triggers both acute and long-term pathologies of motor neurons, which may be relevant to the pathogenesis and progression of ALS.

neuroscience↗

bioRxiv2018

Protocadherin-mediated cell repulsion controls the central topography and efferent projections of the abducens nucleus

Cranial motor nuclei in the brainstem innervate diverse types of head and neck muscles. Failure in establishing these neuromuscular connections causes congenital cranial dysinnervation disorders (CCDDs) characterized by abnormal craniofacial movements. However, mechanisms that link cranial motor nuclei to target muscles are poorly understood at the molecular level. Here, we report that protocadherin-mediated repulsion mediates neuromuscular connection in the ocular motor system in zebrafish. We identify pools of abducens motor neurons that are topographically arranged according to soma size and convergently innervate a single muscle. Disruptions of Duane retraction syndrome-associated transcription factors reveal that these neurons require Mafba/MAFB, but not Sall4/SALL4, for differentiation. Furthermore, genetic perturbations of Pcdh17/Protocadherin-17 result in defective axon growth and soma clumping, thereby abolishing neuromuscular connectivity. Our results suggest that protocadherin-mediated repulsion forms the central topography and efferent projection pattern of the abducens nucleus following Mafba-dependent specification, and imply potential involvement of protocadherins in CCDD etiology.

neuroscience↗