Bacurd1/Kctd13 and Bacurd2/Tnfaip1 are interacting partners to Rnd proteins which influence the long-term positioning and dendritic maturation of cerebral cortical neurons
© Gladwyn-Ng et al. 2016
Received: 10 November 2015
Accepted: 28 February 2016
Published: 11 March 2016
The development of neural circuits within the embryonic cerebral cortex relies on the timely production of neurons, their positioning within the embryonic cerebral cortex as well as their terminal differentiation and dendritic spine connectivity. The RhoA GTPases Rnd2 and Rnd3 are important for neurogenesis and cell migration within the embryonic cortex (Nat Commun 4:1635, 2013), and we recently identified the BTB/POZ domain-containing Adaptor for Cul3-mediated RhoA Degradation family member Bacurd2 (also known as Tnfaip1) as an interacting partner to Rnd2 for the migration of embryonic mouse cortical neurons (Neural Dev 10:9, 2015).
We have extended this work and report that Bacurd1/Kctd13 and Bacurd2/Tnfaip1 are interacting partners to Rnd2 and Rnd3 in vitro. Given that these genes are expressed during cortical development, we performed a series of in utero electroporation studies in mice and found that disruptions to Bacurd1/Kctd13 or Bacurd2/Tnfaip1 expression impair the long-term positioning of E14.5-born cortical neurons within the postnatal (P17) mouse cerebral cortex. We also find that forced expression of Bacurd1/Kctd13 and Bacurd2/Tnfaip1 alters the branching and dendritic spine properties of layer II/III projection neurons.
We identify Bacurd1/Kctd13 and Bacurd2/Tnfaip1 as interacting partners to Rnd proteins which influence the development of cortical neurons. Their neurodevelopmental functions are likely to be relevant to human brain development and disease.
KeywordsNeuronal migration Dendritic branching Dendritic spines Bacurd
During development, newborn neurons undergo directional cell migration to position themselves appropriately within the embryonic cerebral cortex before establishing their branching characteristics and finally establishing dendritic spine connections (reviewed in [1, 2]). Members of the Rnd family of RhoA GTPases such as Rnd2 and Rnd3 play critical roles in the production of neurons [3, 4] as well as their migration within the embryonic cortex [4–7]. On the other hand, Bacurd proteins comprise Bacurd1/Kctd13, Bacurd2/Tnfaip1 and Bacurd3/Kctd10, and the functions of these proteins are increasingly recognised to be relevant to the development of neurons as well as in synaptic signalling [8–10]. Indeed, elevations in the dosage of BACURD1 and BACURD2 are associated with human neurodevelopmental disorders [11–13], but the pathological consequences of BACURD overexpression in neurons remain poorly understood. We recently identified Bacurd2/Tnfaip1 as an interacting partner to Rnd2 which influences neuronal migration in a concentration-sensitive manner, and our studies provided the first insight into the potential roles for Bacurd proteins in neuronal development . However, we were interested to expand on our findings by asking whether other Bacurd family members might also interact with Rnd proteins and play a role in neuronal development. Here, we report that Bacurd1/Kctd13 and Bacurd2/Tnfaip1 are interacting partners to Rnd2 and Rnd3, and our functional studies demonstrate that alterations to Bacurd expression disrupt the development of neurons within the mouse cerebral cortex.
BACURD1/KCTD13 and BACURD2/TNFAIP1 are expressed during mammalian cerebral cortex development in mice and humans (Additional file 1: Figure S1C-D; as well as [8, 15–17]) and we previously reported that forced expression of Bacurd2/Tnfaip1 impaired cell migration within the murine embryonic cortex but not their early neuronal differentiation, as determined by staining for βIII-tubulin . However, it remained unclear as to whether perturbations to Bacurd expression levels caused a delay or a defect in the long-term positioning of cortical neurons. Given that elevations in the dosages of BACURD1/KCTD13 and BACURD2/TNFAIP1 genes are associated with structural brain disorders in humans [11, 13], we wanted to study the effects of Bacurd1/Kctd13 and Bacurd2/Tnfaip1 overexpression on the development of cortical neurons. For this, we cloned FLAG epitope-tagged expression constructs for Bacurd1/Kctd13 and Bacurd2/Tnfaip1 into a mammalian expression construct which also comprises a GFP cassette (pCIG2) and investigated the consequences of their forced expression within cells of the embryonic E14.5 dorsal telencephalon by in utero electroporation. We collected the brains of successfully electroporated postnatal day 17 (P17) mice, a timepoint in which these E14.5-born cortical neurons have completed their migration [1, 18]. As shown in Fig. 1c, forced expression of Bacurd1/Kctd13 or Bacurd2/Tnfaip1 led to a significant impairment in the positioning of GFP-labelled cortical neurons compared with control (GFP only) treatment, observed as a significant decrease in the proportion of GFP+ cells within layers II/III and a concomitant increase in the proportion of layer V cells (Fig. 1d). The relative expression of exogenously derived FLAG-tagged Kctd13 and Tnfaip1 proteins was not significantly different between both treatments (Additional file 2: Figure S2A-B). Also, we found that treatment with Bacurd1/Kctd13 or Bacurd2/Tnfaip1 did not significantly alter the neuronal identity of GFP-labelled cells, as determined by co-localisation of the projection neuron marker Cux1 (Fig. 1e and Additional file 2: Figure S2C). Consistent with the notion that appropriate levels of Bacurd1/Kctd13 and Bacurd2/Tnfaip1 underpin cortical neuron positioning, we find that knockdown with targeting shRNAs also leads to a disruption in cell positioning (Additional file 2: Figure S2D-H).
This study identifies Bacurd1/Kctd13 and Bacurd2/Tnfaip1 as interacting partners to Rnd2 and Rnd3. We have explored the functions for Bacurd1/Kctd13 and Bacurd2/Tnfaip1 in the development of E14.5-born cortical projection neurons within the postnatal cerebral cortex. Our study suggests that appropriate levels of Bacurds are necessary for the long-term positioning of cortical projection neurons, and we extend from our previous work  to show that forced expression of either Bacurd1/Kctd13 or Bacurd2/Tnfaip1 leads to a defect in the radial positioning of cortical projection neurons, rather than a delay in their migration. In addition, we find that treatment with Bacurd1/Kctd13 or Bacurd2/Tnfaip1 leads to distinct effects on the dendritic arborisation of layer II/III cortical projection neurons. Notably, forced expression of Bacurd2/Tnfaip1 results in a more severe impairment in cell positioning as well as neurite branching compared with Bacurd1/Kctd13, and this suggests that Bacurd family members might have distinct functions in neurons. While our in vivo evaluation of the expression constructs for Bacurd1/Kctd13 and Bacurd2/Tnfaip1 indicated that the steady-state levels of exogenously derived protein were not significantly different within cells of the P17 cortex (Additional file 2: Figure S2A-B), we cannot rule out the possibility that different phenotypes in our gain-of-function studies might reflect subtly different levels of overexpression rather than different intrinsic activities in cortical neurons. Nevertheless, our results are consistent with previous findings by Chen and coworkers which demonstrated that Xenopus xBACURDα and xBACURDβ have non-identical functions during gastrulation .
During nervous system development, intracellular RhoA signalling in immature neurons is critical for multiple aspects of their maturation, including directional cell migration, neurite and axon outgrowth, dendritic branching and synaptogenesis [22–24]. It is noteworthy that both the Bacurds and the Rnds are important mediators of RhoA signalling, albeit in different ways. The Bacurds recruit Cul3 to ubiquitinate RhoA so as to suppress protein levels, leading to F-actin cytosketal remodelling and inhibition of cell migration [21, 25]. Significantly, a recent study implicated a KCTD13-Cul3-RhoA signalling pathway to be important for human nervous system development and psychiatric disease . In the case of Rnd proteins, both Rnd2 and −3 negatively regulate RhoA signalling to induce cytoskeletal changes in neurons, thereby affecting neurite outgrowth and cell migration in a mechanism involving several protein partners including p190RhoGAP and Plexin B2 [4–6]. Since Rnd2 and Rnd3 regulate multiple steps of radial migration by embryonic cortical neurons as they reach the cortical plate [4, 5], we draw the link here to suggest that such roles for Rnd proteins might also involve functional interactions with Bacurd proteins . In particular, we postulate that Bacurds and Rnds cooperate to influence RhoA signalling in immature neurons for their proper development within the developing cerebral cortex, including their bipolar transition within the IZ and locomotion within the CP as they complete their migration before undergoing dendritic maturation. It remains to be determined if disruptions to Bacurd levels alter the binding of Rnd2 and Rnd3 to their other partners in neurons (such as p190RhoGAP and Plexin B2), or influence RhoA signalling, or both.
We find that overexpression of Bacurd1/Kctd13 or Bacurd2/Tnfaip1 leads to aberrant dendrite arborization, as well as impairments in basal but not apical dendritic spines. It will be important in future studies to determine if down-regulation of endogenous levels of Bacurds influence dendritic spines. Notably, our results suggest that Bacurds influence the dendritic maturation of cortical projection neurons in different ways. While this could be explained by differences in developmental timing for apical versus basal dendrites, the mechanistic effects of Bacurd overexpression on dendritic spine properties could also be explained by disruptions to RhoA signalling, a critical mechanism for the formation and stabilization of spines . Altogether, we find that disruptions to endogenous levels of Bacurd1/Kctd13 and Bacurd2/Tnfaip1 are likely to impact on the development of neurons, and thus we predict that perturbations to Bacurd expression lead to disruptions in neuronal signalling, with ongoing consequences for the balance between neuronal excitation and inhibition within the cerebral cortex. Given the association between elevations in BACURD1/KCTD13 gene dosage and psychosis , and BACURD2/TNFAIP1 with PHACE syndrome (a congenital condition in which infants present with structural brain abnormalities and seizures) , it is plausible that excessive BACURD1/KCTD13 and BACURD2/TNFAIP1 gene expression may have direct, detrimental effects on the formation of cerebral cortical circuitry, and could lead to neurological dysfunction in humans.
DNA constructs, cell culture, co-immunoprecipitation and western blotting
Complementary DNAs (cDNAs) encoding FLAG-tagged Rnd2, Rnd3, Bacurd1/Kctd13 and Bacurd2/Tnfaip1 and GFP-tagged Bacurd1/Kctd13 and Bacurd2/Tnfaip1 were cloned by PCR and ligated to the mammalian expression construct vector pCMV using compatible ends. Cloned cDNAs encoding Bacurd1/Kctd13 and Bacurd2/Tnfaip1 harbouring MfeI restriction sites were cloned into the EcoRI site of the mammalian expression vector pCIG2. All constructs were sequence verified and plasmids prepared using a Qiagen midipre PLUS kit (Qiagen, Australia?). HEK293T cells were cultured according to conventional protocols, while co-immunoprecipitation studies and Western blotting were performed as previously described . Antibodies used in this study include mouse anti-IgG (millipore) anti-β-actin (Sigma-Aldrich), anti-FLAG (Sigma-Aldrich), anti-GFAP (millipore), anti-FLAG (Cell Signaling), anti-CDP (Cux1, Santa Cruz); and chicken anti-GFP (Abcam). Preimmunised goat serum (Sigma) was used for immunoprecipitation experiments as a control. Cell nuclei were visualized with DAPI (4′6-Diamidino-2-Phenylindole).
In utero electroporation
All procedures were performed in accordance to guidelines the Animal Ethics Committee at Monash University (MARP-2012-069), and in accordance with Institutional guidelines. In utero electroporation was performed on E14.5 embryos of time-mated C57/Black6J mice as previously indicated . Injections were carried out with 1 μg/μl of each constituent plasmid. Successfully electroporated animals were verified by GFP epifluorescence inspection through the skull of P0 pups. At postnatal day P17, transcardial perfusion was performed to preserve the brain tissue with fixative (4 % paraformaldehyde/phosphate-buffered saline), and the tissue processed as previously indicated . Further methods related to electroporation experiments performed with small hairpin RNA (shRNA) plasmid constructs are provided in Additional file 4.
GFP-labelled P17 neurons were captured as 3-dimensional confocal images of 40 μm-thick mouse brain sections. For cell morphology analysis, GFP-labelled cortical projection neurons residing within layers II/III of the somatosensory cortex of successfully electroporated P17 brains were captured at 40× magnification. For dendritic spine studies, neurites were imaged at 60× magnification, with 1 μm z-stack step size. In both cases, rendering of neuronal cell morphology as well as the identification of dendritic spines was performed manually, while the classification of spine types was automatically calculated based on pre-defined parameters, as described recently . Raw images were analysed and digitally reconstructed using Filament Tracer (Imaris 7.6.2, Bitplane) for the detection of neurons, dendritic trees, axons and spines in 3D.
BTB/POZ-domain containing Adaptor for Cul3-mediated RhoA Degradation
Green Fluorescent Protein
This work was supported by a grant from the Australian Research Council (DP130103328) as well as funding from the Victorian Government through the Operational Infrastructure Scheme. The Australian Regenerative Medicine Institute is supported by grants from the State Government of Victoria and the Australian Government.
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