BTBD3

BTBD3 is an activity-regulated protein that couples neuronal activity to dendritic morphogenesis and circuit formation in the developing cerebral cortex. In response to neuronal activity, BTBD3 translocates to the nucleus and orients primary dendrites toward active axon inputs; this process is conserved across species and cortical areas, and is enabled by Lhx2-dependent transcriptional induction of Btbd3 in postmitotic layer 4 neurons (PMID:24179155, PMID:28122236). Loss of Btbd3 in the ventral hippocampus produces compulsive-like and exploratory behavioral phenotypes that are partially rescued by serotonergic pharmacology, linking BTBD3-dependent circuit formation to behavioral regulation (PMID:31501410). BTBD3 also functions as an E3 ubiquitin ligase that ubiquitinates β-catenin for degradation, thereby suppressing Wnt/β-catenin signaling (PMID:39227913), and undergoes NAA10-mediated N-α-acetylation that promotes its interaction with the actin-capping protein CapZb to drive neurite outgrowth [bio_10.1101_2024.05.09.583166].

1. 2011 Medium

   Identifying upstream regulators of BTBD3 expression, this work showed that BTBD3 mRNA is a direct target of miR-9 and miR-181c, placing it in a post-transcriptional regulatory network relevant to the amyloid-β response in hippocampal neurons.

   Evidence 3′ UTR luciferase reporter assay with miRNA overexpression in hippocampal cultures

   PMID:21720722

   Open questions at the time

   • Endogenous BTBD3 protein changes upon miRNA modulation not demonstrated
   • Functional consequence of miRNA-mediated BTBD3 repression on neuronal phenotype not assessed
2. 2013 High

   The central discovery that BTBD3 is an activity-dependent nuclear translocating protein that orients dendrites toward active axons resolved how sensory-driven activity instructs dendritic architecture in developing barrel and visual cortex.

   Evidence In vivo gain- and loss-of-function in mouse and ferret cortex; immunostaining and biochemical fractionation for nuclear translocation; dendritic morphology readouts across species and cortical areas

   PMID:24179155

   Open questions at the time

   • Nuclear binding partners and transcriptional targets of BTBD3 upon translocation remain unknown
   • Signal transduction pathway linking neuronal activity to BTBD3 nuclear import not identified
3. 2017 High

   The upstream transcriptional control of BTBD3 was established by showing that Lhx2 directly activates Btbd3 transcription in postmitotic layer 4 neurons, explaining how neurons acquire competency for activity-dependent dendritic orientation.

   Evidence Conditional Lhx2 knockout in postmitotic cortical neurons with ChIP and reporter assays confirming Btbd3 as a direct target

   PMID:28122236

   Open questions at the time

   • Whether Lhx2 regulation of Btbd3 operates in non-cortical brain regions is untested
   • Other Lhx2 targets that may cooperate with Btbd3 in dendritic orientation are uncharacterized
4. 2019 High

   Extending BTBD3 function from cortical dendritic orientation to behavioral output, this study demonstrated that hippocampal Btbd3 is required for normal exploratory and goal-directed behavior, and that its loss produces compulsive-like phenotypes reversible by serotonergic drugs.

   Evidence Btbd3 KO and heterozygous mice; neonatal viral knockdown targeting whole vs. dorsal hippocampus; multiple behavioral assays; pharmacological rescue with fluoxetine

   PMID:31501410

   Open questions at the time

   • Molecular mechanism through which Btbd3 shapes hippocampal circuitry to regulate behavior is unknown
   • Whether dendritic orientation defects underlie the behavioral phenotype has not been tested
5. 2024 Medium

   A molecular enzymatic function was assigned to BTBD3 by demonstrating it acts as an E3 ubiquitin ligase that ubiquitinates β-catenin for proteasomal degradation, establishing a direct mechanism by which BTBD3 suppresses Wnt signaling in colorectal cancer cells.

   Evidence Co-IP; ubiquitination assays; overexpression and knockdown in CRC cell lines; in vivo xenograft tumor growth assay

   PMID:39227913

   Open questions at the time

   • The Cullin partner or E2 enzyme cooperating with BTBD3 for β-catenin ubiquitination is not identified
   • Whether BTBD3-mediated β-catenin degradation operates in neurons or only in cancer contexts is unknown
   • Independent replication of E3 ligase activity in a second lab is lacking
6. 2024 Medium

   A post-translational regulatory mechanism was uncovered whereby NAA10-mediated N-α-acetylation of BTBD3 enables its interaction with the actin-capping protein CapZb, thereby promoting CapZb–F-actin binding and neurite outgrowth, linking BTBD3 directly to cytoskeletal regulation.

   Evidence (preprint) Hippocampal CA1-specific Naa10 KO mice; acetylation-defective Btbd3 mutant; co-IP of Btbd3/CapZb; F-actin binding assays; cytochalasin D rescue; neurite morphometry in primary neurons

   PMID:bio_10.1101_2024.05.09.583166

   Open questions at the time

   • Preprint not yet peer-reviewed
   • Whether N-α-acetylation is required for BTBD3 nuclear translocation or E3 ligase activity is untested
   • Structural basis for acetylation-dependent BTBD3–CapZb interaction is unknown

• Key unresolved questions include the nuclear targets/effectors through which BTBD3 translocation directs gene expression to remodel dendrites, whether its E3 ubiquitin ligase activity and cytoskeletal roles operate through shared or independent pathways, and the identity of the Cullin scaffold with which BTBD3 assembles.
• No transcriptional targets of nuclear BTBD3 have been identified
• Relationship between E3 ligase activity toward β-catenin and dendritic orientation function is unexplored
• The Cullin adaptor complex incorporating BTBD3 has not been defined