Postnatal signalling with homeoprotein transcription factors

Prochiantz, Alain,Fuchs, Julia,Di Nardo, Ariel A. Royal Society 2014 Philosophical transactions. Biological sciences Vol.369 No.1652

<P>Homeoprotein (HP) transcription factors were originally identified for their embryonic cell-autonomous developmental functions. In this review, we discuss their postnatal and adult physiological functions based on the study of Otx2, Engrailed-1 and Engrailed-2 (collectively Engrailed). For Engrailed, we discuss its function in the cell-autonomous regulation of ventral midbrain dopaminergic neuron survival and physiology and in the non-cell-autonomous maintenance of axons. For Otx2, we describe how the protein is expressed in the choroid plexus and transported into cortical parvalbumin cells where it regulates plasticity in the visual cortex. These two examples illustrate how the understanding of HP postnatal and adult functions, including signalling functions, may lead to the identification of disease-associated genetic pathways and to the development of original therapeutic strategies.</P>

Reports : Perspective ; Donating Otx2 to support neighboring neuron survival

( Hyoung-tai Kim ),( Alain Prochiantz ),( Jin Woo Kim ) 생화학분자생물학회 2016 BMB Reports Vol.49 No.2

Mutations of orthodentricle homeobox 2 (OTX2) in human and mice often cause retinal dystrophy and nyctalopia, suggesting a role of OTX2 in mature retina, in addition to its functions in the development of the eye and retina. In support of this, the number of bipolar cells in Otx2+/. post-natal mouse retina was found to be significantly lower than normal. Degeneration of the cells becomes greater as the mice age, leading to the loss of vision. Especially, the type-2 OFF-cone bipolar cells, which do not express Otx2 mRNA but carry Otx2 protein, are most sensitive to Otx2 haplodeficiency. Interestingly, this bipolar cell subpopulation imports Otx2 protein from photoreceptors to protect itself from glutamate excitotoxicity in the dark. Moreover, in the bipolar cells, the exogenous Otx2 relocates to the mitochondria to support mitochondrial ATP synthesis. This novel mitochondrial activity of exogenous Otx2 highlights the therapeutic potential of Otx2 protein transduction in retinal dystrophy. [BMB Reports 2016; 49(2): 69-70]

Local homeoprotein diffusion can stabilize boundaries generated by graded positional cues

Quió,ñ,inao, Cristó,bal,Prochiantz, Alain,Touboul, Jonathan The Company of Biologists Limited 2015 Development (Cambridge) Vol.142 No.10

<P>Boundary formation in the developing neuroepithelium decides on the position and size of compartments in the adult nervous system. In this study, we start from the French Flag model proposed by Lewis Wolpert, in which boundaries are formed through the combination of morphogen diffusion and of thresholds in cell responses. In contemporary terms, a response is characterized by the expression of cell-autonomous transcription factors, very often of the homeoprotein family. Theoretical studies suggest that this sole mechanism results in the formation of boundaries of imprecise shapes and positions. Alan Turing, on the other hand, proposed a model whereby two morphogens that exhibit self-activation and reciprocal inhibition, and are uniformly distributed and diffuse at different rates lead to the formation of territories of unpredictable shapes and positions but with sharp boundaries (the ‘leopard spots’). Here, we have combined the two models and compared the stability of boundaries when the hypothesis of local homeoprotein intercellular diffusion is, or is not, introduced in the equations. We find that the addition of homeoprotein local diffusion leads to a dramatic stabilization of the positioning of the boundary, even when other parameters are significantly modified. This novel Turing/Wolpert combined model has thus important theoretical consequences for our understanding of the role of the intercellular diffusion of homeoproteins in the developmental robustness of and the changes that take place in the course of evolution.</P><P><B>SUMMARY:</B> Mathematical modeling and simulations suggest that the local diffusion of homeoproteins during neuronal differentiation is enough to create smooth and reliable boundaries between cortical areas.</P>

A Mouse Model for Conditional Secretion of Specific Single-Chain Antibodies Provides Genetic Evidence for Regulation of Cortical Plasticity by a Non-cell Autonomous Homeoprotein Transcription Factor

Bernard, Clé,mence,Vincent, Clé,mentine,Testa, Damien,Bertini, Eva,Ribot, Jé,rô,me,Di Nardo, Ariel A.,Volovitch, Michel,Prochiantz, Alain Public Library of Science 2016 PLoS genetics Vol.12 No.5

<▼1><P>During postnatal life the cerebral cortex passes through critical periods of plasticity allowing its physiological adaptation to the environment. In the visual cortex, critical period onset and closure are influenced by the non-cell autonomous activity of the Otx2 homeoprotein transcription factor, which regulates the maturation of parvalbumin-expressing inhibitory interneurons (PV cells). In adult mice, the maintenance of a non-plastic adult state requires continuous Otx2 import by PV cells. An important source of extra-cortical Otx2 is the choroid plexus, which secretes Otx2 into the cerebrospinal fluid. Otx2 secretion and internalization requires two small peptidic domains that are part of the DNA-binding domain. Thus, mutating these “transfer” sequences also modifies cell autonomous transcription, precluding this approach to obtain a cell autonomous-only mouse. Here, we develop a mouse model with inducible secretion of an anti-Otx2 single-chain antibody to trap Otx2 in the extracellular milieu. Postnatal secretion of this single-chain antibody by PV cells delays PV maturation and reduces plasticity gene expression. Induced adult expression of this single-chain antibody in cerebrospinal fluid decreases Otx2 internalization by PV cells, strongly induces plasticity gene expression and reopens physiological plasticity. We provide the first mammalian genetic evidence for a signaling mechanism involving intercellular transfer of a homeoprotein transcription factor. Our single-chain antibody mouse model is a valid strategy for extracellular neutralization that could be applied to other homeoproteins and signaling molecules within and beyond the nervous system.</P></▼1><▼2><P><B>Author Summary</B></P><P>Classically, cell signaling is based on the secretion of molecules that bind cell surface receptors. Lipophilic agents can do without cell-surface receptors due to their ability to diffuse through the plasma membrane, but this is normally not the case for proteins, which cannot pass the membrane barrier. However, homeoprotein transcription factors represent an exception as they are secreted and internalized by live cells owing to two peptidic domains. An important illustration of this novel signaling mechanism is provided by Otx2, a homeoprotein that travels from the choroid plexus to specific inhibitory neurons in the cerebral cortex, where it regulates physiological plasticity throughout life. Because the two transfer peptides are in the DNA-binding domain of Otx2, it is impossible to mutate them without altering both cell signaling and cell-autonomous functions. We have therefore developed a mouse in which a secreted anti-Otx2 single-chain antibody can be induced to trap extracellular Otx2 while leaving its cell autonomous function untouched. We show that neutralizing extracellular Otx2 modifies the expression of plasticity genes in the visual cortex, thus providing the first genetic demonstration for homeoprotein signaling in a mammal.</P></▼2>

Graded Otx2 activities demonstrate dose-sensitive eye and retina phenotypes

Bernard, Clé,mence,Kim, Hyoung-Tai,Torero Ibad, Raoul,Lee, Eun Jung,Simonutti, Manuel,Picaud, Serge,Acampora, Dario,Simeone, Antonio,Di Nardo, Ariel A.,Prochiantz, Alain,Moya, Kenneth L.,Kim, Ji Oxford University Press 2014 Human Molecular Genetics Vol.23 No.7

<P>In the human, mutations of <I>OTX2</I> (<I>Orthodenticle homeobox 2</I> transcription factor) translate into eye malformations of variable expressivity (even between the two eyes of the same individual) and incomplete penetrance, suggesting the existence of subtle thresholds in OTX2 activity. We have addressed this issue by analyzing retinal structure and function in six mutant mice with graded Otx2 activity: <I>Otx2<SUP>+/+</SUP></I>, <I>Otx2<SUP>+/AA</SUP></I>, <I>Otx2<SUP>+/GFP</SUP></I>, <I>Otx2<SUP>AA/AA</SUP></I>, <I>Otx2<SUP>AA/GFP</SUP></I> and <I>Otx2<SUP>GFP/GFP</SUP></I>. Null mice (<I>Otx2<SUP>GFP/GFP</SUP></I>) fail to develop the head and are embryonic lethal, and compound heterozygous <I>Otx2<SUP>AA/GFP</SUP></I> mice show a truncated head and die at birth. All other genotypes develop until adulthood. We analyzed eye structure and visual physiology in the genotypes that develop until adulthood and report that phenotype severity parallels Otx2 activity. <I>Otx2<SUP>+/AA</SUP></I> are only mildly affected whereas <I>Otx2<SUP>+/GFP</SUP></I> are more affected than <I>Otx2<SUP>+/AA</SUP></I> but less than <I>Otx2<SUP>AA/AA</SUP></I> mice. <I>Otx2<SUP>AA/AA</SUP></I> mice later manifest the most severe defects, with variable expressivity. Electrophysiological and histological analyses of the mouse retina revealed progressive death of bipolar cells and cone photoreceptors that is both Otx2 activity- and age-dependent with the same ranking of phenotypic severity. This study demonstrates the importance of gene dosage in the development of age-dependent pathologies and underscores the fact that small gene dosage differences can cause significant pathological states.</P>

Engrailed homeoprotein blocks degeneration in adult dopaminergic neurons through LINE‐1 repression

Blaudin de Thé,, Franç,ois‐,Xavier,Rekaik, Hocine,Peze‐,Heidsieck, Eugenie,Massiani‐,Beaudoin, Olivia,Joshi, Rajiv L,Fuchs, Julia,Prochiantz, Alain EMBO 2018 The EMBO journal Vol.37 No.15