Edited by: Shin-ichi Hisanaga, Tokyo Metropolitan University, Japan
Reviewed by: Yi Zhou, State College of Florida, Manatee-Sarasota, United States; Jaewon Ko, Daegu Gyeongbuk Institute of Science and Technology (DGIST), South Korea
*Correspondence: Kazuhito Toyo-oka
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
The 14-3-3 proteins are a family of highly conserved, multifunctional proteins that are highly expressed in the brain during development. Cumulatively, the seven 14-3-3 isoforms make up approximately 1% of total soluble brain protein. Over the last decade, evidence has accumulated implicating the importance of the 14-3-3 protein family in the development of the nervous system, in particular cortical development, and have more recently been recognized as key regulators in a number of neurodevelopmental processes. In this review we will discuss the known roles of each 14-3-3 isoform in the development of the cortex, their relation to human neurodevelopmental disorders, as well as the challenges and questions that are left to be answered. In particular, we focus on the 14-3-3 isoforms and their involvement in the three key stages of cortical development; neurogenesis and differentiation, neuronal migration and neuromorphogenesis and synaptogenesis.
The 14-3-3 protein family consists of seven isoforms in mammals, encoded by seven separate genes, each denoted by a Greek letter (β, γ, ε, ζ, η, τ and σ). This family was given their name when these proteins were originally discovered in 1967 as abundant proteins in the mammalian brain. This name was chosen due to the particular elution and migration pattern of these proteins in DEAE-cellulose chromatography and gel electrophoresis, with the 14-3-3 proteins eluting in the 14th fraction of bovine brain homogenate on DEAE-cellulose and in position 3.3 in the gel (Moore and Perez,
14-3-3 proteins are acidic proteins that form and function as both homodimers and heterodimers, with the exception of the 14-3-3σ isoform, which preferentially forms homodimers (Benzinger et al.,
Schematic illustration of some of the known functions of 14-3-3 proteins.
In addition to the diversity and number of 14-3-3 binding targets, 14-3-3 proteins can facilitate a number of different functions once they have bound their targets (Pozuelo Rubio et al.,
The first function of 14-3-3 proteins was described in 1987 when it was found that 14-3-3 proteins can activate tyrosine and tryptophan hydroxylases, which are the rate limiting enzymes in the synthesis of dopamine and other neurotransmitters (Ichimura et al.,
14-3-3 proteins are highly conserved and share a large amount of structural similarity between isoforms, suggesting functional redundancy. However, while there is evidence showing some functional overlap, the 14-3-3 proteins in general show a surprising amount of binding target specificity and functional specificity between the isoforms. It has been found that 14-3-3 isoforms often have specific binding partners that are not able to be bound by other 14-3-3 dimer combinations (Comparot et al.,
With 14-3-3 proteins having so many diverse cellular and molecular roles and functions it comes with no surprise that this family is also associated with a number of human disorders. 14-3-3 proteins have been implicated in everything from cardiomyopathy and cancer to even hair pigmentation (Morrison,
Interestingly over the last decade, evidence has now accumulated implicating the importance of the 14-3-3 family in the development of the nervous system, in particular cortical development, and have more recently been recognized as key regulators in a number of neurodevelopmental processes. In this review we will discuss the known roles of each 14-3-3 isoform in the development of the cortex, as well as the challenges and questions that are left to be answered. In particular, we will focus on the 14-3-3 isoforms and their involvement in the three key stages of cortical development; neurogenesis and differentiation, neuronal migration and neuromorphogenesis and synaptogenesis.
During brain development, the majority of projections neurons that will eventually form the cortical layers of the brain are generated from neural progenitor cells localized in the ventricular zone (VZ; Kriegstein and Alvarez-Buylla,
Illustration demonstrating the role of radial glial cells and intermediate progenitor cells (IPCs) in neurogenesis during cortical development. Three isoforms, 14-3-3ε, ζ and γ, are known to be expressed in these cells during cortical development. Further analysis is required for the remaining isoforms. VZ, Ventricular Zone; SVZ, Subventricular Zone.
The 14-3-3ε isoform is encoded by the gene
In recent work with the use of
Further experiments also suggest that this increase in proliferation and the number of IPCs in 14-3-3ε and 14-3-3ζ dKO mice results in an increase of neurons (Toyo-Oka et al.,
Next, the molecular mechanism responsible for this change in neurogenesis and neurodifferentiation in 14-3-3ε and 14-3-3ζ deficient mice was examined. Mackie and Aitken (
Catenin proteins are known to regulate the activity of RhoA, Rac1 and cdc42. Furthermore, the activation of Rho family GTPases results in the activation of Limk1, which once phosphorylated will then phosphorylate cofilin, inactivating it. Active cofilin (non-phosphorylated form) typically severs F-actin, therefore when cofilin is inactivated F-actin formation is accelerated (Luo,
Schematic model of the functions of 14-3-3ε and 14-3-3ζ in neurogenesis and neuronal differentiation during cortical development, reproduced with permission from the Society for Neuroscience (Toyo-Oka et al.,
Interestingly, it was shown that the 14-3-3ε and 14-3-3ζ dKO mice often displayed seizures when moved to a new cage, with the seizers typically lasting around 15 s and showing similar behaviors as a Racine Class II seizure described by Racine et al. (
The roles of the remaining 14-3-3 isoforms in neurogenesis and neuronal differentiation have yet to be studied and are for the most part unknown. In general, it has been well established that 14-3-3 proteins play key roles in cell cycle control, apoptosis and cancer progression in non-neuronal cells (van Hemert et al.,
The 14-3-3γ isoform is encoded by the gene
The 14-3-3σ isoform has long been known to play a regulatory role in cell cycle control. Furthermore, 14-3-3σ has been classified as a tumor suppressor and is known to be down-regulated in breast cancer (Hermeking et al.,
Neuronal migration is responsible for the proper distribution of neurons throughout the entire nervous system and is essential for establishing the basic organization of the brain. In this section we will focus on radial migration in the cortex. Following the generation of neurons in the VZ and SVZ, these neurons must then migrate toward the cortical plate in order to form proper cortical layers. Disruptions in this process can lead to a number of disorders including; epilepsy, lissencephaly, mental retardation and brain malformations.
The importance of 14-3-3ε in neuronal migration has been fairly well established in the literature, with the first findings appearing over a decade ago in 2003 when it was found that the 14-3-3ε isoform is an essential protein for radial neuronal migration during cortical development (Toyo-Oka et al.,
Further experiments have verified the essential role of 14-3-3ε and perhaps 14-3-3ζ as well in neuronal migration. Using 14-3-3ε and 14-3-3ζ dKO mice, it was elucidated that these mice showed a severe disruption in cortical layering and decreased neuron travel distance from the VZ (Toyo-Oka et al.,
The 14-3-3γ gene is highly expressed in the brain during embryonic mouse development, however its expression rapidly decreases at birth, suggesting its importance in brain development (Wachi et al.,
Recent work by Cornell et al. (
Interestingly, to expand upon these observations, when 14-3-3γ is knocked down; a nearly identical neuronal migration phenotype was identified when 14-3-3γ was overexpressed
To our knowledge the remaining 14-3-3 isoforms (β, η, τ and σ) have not been studied in regard to their involvement in neuronal migration. However, some studies have been performed using non-neuronal models indicating that 14-3-3ε, ζ and γ may not be the only isoforms involved in regulating neuronal migration during cortical development.
The 14-3-3β isoform is encoded by the gene
The 14-3-3τ isoform is encoded by the gene
Once neurons have completed their migration from the VZ to the cortical plate to form cortical layers, they then must grow a complex dendritic arbor and extend long axons to their targets in a process collectively known as neuromorphogenesis. Following the extension of their dendrites, these neurons then must grow dendritic spines and form synaptic connections. Together, these processes are essential for the formation of a functional and immensely complex brain.
17p13.3 microduplication syndrome is a newly identified genetic syndrome that is characterized by duplications of various size in the 17p13.3 chromosome locus. Patients with this syndrome exhibit severe neural developmental disorders, including autism spectrum disorder (ASD), epilepsy and mental retardation (Roos et al.,
Recently Cornell et al. (
Schematic illustration of neurite initiation. Illustration of the typical stages of neurite initiation. In stage 1, actin based lamellipodia type structures rapidly form and retract. In stage 2, microtubules begin to invade and stabilize lamellipodia structures preventing their collapse. In stage 3, neurites that have been invaded by microtubules become stable structures and begin to extend in stage 4. The overexpression of 14-3-3ε prevents the invasion of microtubules into forming neurites as seen in stage 2, thus disrupting neurite formation.
Schematic illustration of the regulation of neurite initiation by 14-3-3ε during cortical development.
Interestingly, Pramparo et al. (
A number of studies have shown an association between the 14-3-3ζ isoform and the development of schizophrenia, with 14-3-3ζ knockout mice becoming a model for schizophrenia (Jia et al.,
Very little is known regarding the involvement of the remaining 14-3-3 isoforms in neurite and synapse formation. Prior to the studies described above regarding 14-3-3ε in neuromorphogenesis, Yoon et al. (
The members of the 14-3-3 protein family perform a vast array of functions by binding hundreds of target proteins throughout the body. This family has more recently been found to play essential roles in the developing cerebral cortex and human neurodevelopmental disorders. However, due to the multitude of functions and targets, there is still much that is unknown about this protein family. In particular, a number of the 14-3-3 isoforms have yet to be analyzed in regard to their involvement in any of the three stages of cortical development; neurogenesis, neuronal migration and neuromorphogenesis (Table
Summary of the involvement of 14-3-3 isoforms in three stages of cortical development and each isoform potential involvement in related neurodevelopmental disorders.
Neurogenesis | Neuronal migration | Neuromorphogenesis | Potential involvement in neuro-developmental disorders | References | |
---|---|---|---|---|---|
14-3-3ε | + | + | + | Miller-Dieker Syndrome |
Toyo-Oka et al. ( |
14-3-3ζ | + | + | N/A | Epilepsy |
Cheah et al. ( |
14-3-3γ | N/A | + | N/A | Atypical Williams Syndrome |
Fountoulakis et al. ( |
14-3-3σ | N/A | N/A | N/A | N/A | |
14-3-3β | N/A | N/A | N/A | N/A | |
14-3-3η | N/A | N/A | N/A | N/A | |
14-3-3τ | N/A | N/A | N/A | N/A |
One challenge that is presented when studying 14-3-3 proteins is the fact that 14-3-3 proteins function as homodimers and heterodimers. In this regard, 14-3-3 isoforms may have functional redundancy. However, 14-3-3 isoforms also show target specificity depending on the particular heterodimer or homodimer. Furthermore, alterations in the levels of a particular isoform may produce indirect effects by changing the balance of the 14-3-3 population resulting in an altered distribution of heterodimers and homodimers as non-preferential dimers are formed when the preferred dimer is no longer possible. To further confound the situation, 14-3-3 proteins also undergo post-translational modifications that can alter their function and binding specificity. In addition, 14-3-3 isoforms are known to have multiple targets and thus can act upon multiple signaling pathways simultaneously. Together, the vast functionality of these simple dimers provides a number of challenges when studying their roles in cortical development and developmental disorders. The importance of the 14-3-3 family in cortical development and the challenges involved in studying this family leaves much to be elucidated.
BC wrote the manuscript, and KT finalized it.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We would like to acknowledge the NINDS for their support.