ADNP Regulates Cognition: A Multitasking Protein
- 1Tel Aviv University, Israel
With the advantage of rapid progress of DNA/RNA sequencing techniques, it has become feasible to identify the cause of developmental disorders encompassing intellectual disabilities to single de novo mutated genes (e.g. [1; 2; 3]). It is my opinion that we should study in depth, the leading identified genes, to acquire better understanding of the molecular basis for human cognitive functions. Furthermore, from a translational science point of view, understanding genes regulating cognition will facilitate drug development to currently untreatable devastating disease, which hamper cognition. Here, I focus on Activity-dependent neuroprotective protein (ADNP)  showing a tight association with cognition, and in my opinion, a key gene regulating cognitive functions.
Activity-dependent neuroprotective protein (ADNP)
Our original studies identified vasoactive intestinal peptide (VIP)  as a gene/protein highly expressed at the time of synapse formation , which was translated to VIP-associated neuroprotection  and VIP-related synaptogenesis, through astrocyte activation . Astrocyte activation entailed secretion of protein growth factors, leading to the cloning/discovery of ADNP and its active neuroprotective site, NAP (NAPVSIPQ) [9; 10]. To elucidate ADNP's in vivo activity we knocked out the ADNP gene and showed that this gene is essential for neural tube closure and brain formation . At the single cell level, ADNP is found in the nucleus and upon neuronal maturation, the protein is found also in the cytoplasm with specific RNA silencing resulting in loss of microtubules/loss of neurites . While complete knockout of ADNP is lethal, haploinsufficient (heterozygous) mice survive, showing cognitive impairment . Further results indicate microtubule insufficiency, reduced axonal transport  and reduced dendritic spines  in the Adnp+/- mice. These findings are in line with patient results showing intellectual disabilities in case of ADNP gene heterozygous microdeletion or truncating mutation [16; 17; 18]. Given the fact that ADNP is a large protein it includes many identified signature motifs for macromolecular interactions and here I will concentrate on the ADNP motifs, protein interactors and the strong link to cognition.
ADNP binding motifs
ADNP contains a nuclear localization signal (NLS) and a homeobox domain profile [9; 10]. ADNP has heterochromatin protein 1 (HP1) binding domains [19; 20] and interacts with DNA in a sequence-specific manner, as well as with HP1 [19; 20] and chromodomain-helicase-DNA-binding protein 4 (CHD4) . ADNP was discovered to bind and affect the SWItch/Sucrose Non-Fermentable (SWI/SNF) chromatin remodeling complex  also associated with alternative RNA splicing . The DNA/chromatin binding characteristics have been further implicated in promoter/control gene specific regions for ADNP binding and direct regulation of RNA expression [19; 24]. Complete gene array analysis, RNA-seq and high-throughput platform BioMark™ HD System (Fluidigm) identified hundreds of ADNP regulated transcripts [14; 15; 19] suggesting a master gene regulator function.
In the cytoplasm, ADNP was found to bind eukaryotic initiation factor 4E (Eif4e), implicating an involvement in the protein translation machinery  and the autophagy complex, by direct binding to microtubule associated protein 1 light chain 3B (LC3B) [26; 27]. ADNP provides potent neurotrophic/neuroprotective activity that can be attributed, at least in part, to NAP (davunetide, AL-108 or CP201 [4; 9]. In short, the SIP domain in NAP interacts with microtubule end binding proteins (EB1 and EB3) enhancing ADNP  and tau  interaction with microtubules. This SxIP (SKIP) domain in NAP further protects against deficits in axonal transport occurring because of ADNP deficiency  and NAP enhances ADNP interaction with the autophagosome membrane protein LC3B . In vivo NAP restores multiple anomalies caused by ADNP haploinsufficiency [13; 15]. Lastly, our original studies have shown that ADNP has a glutaredoxin active site .
Proteins interacting with ADNP
Ten ADNP-interacting proteins were identified when analyzing (string) for human genes and 9 proteins when searching for mouse associations, with 6 overlapping proteins (Figure 1). Some of these proteins are described in the section above. The common mouse and human proteins, not described above, include ZFP161 - Zinc finger protein 161 homolog (mouse), which is a transcriptional activator of the dopamine transporter (DAT). ZFP161 also acts as a repressor of the FMR1 gene (fragile X syndrome). We have originally shown that ZF5 is linked to regulation by ADNP in the developing mouse embryos . Another shared mouse and human protein, EBNA1BP2 is linked to early onset Alzheimer’s disease (https://www.malacards.org/card/early_onset_familial_alzheimer_disease). A third one, SAP18 enhances the ability of SIN3-HDAC1-mediated transcriptional repression. When tethered to the promoter, it can direct the formation of a repressive complex to core histone proteins. SAP18 is an auxiliary component of the splicing-dependent multiprotein exon junction complex (EJC) deposited at splice junction on mRNAs, and our laboratory has shown interaction of ADNP with the RNA splicing machinery . ADNP-interacting proteins described for either human or mouse, include actin-interacting proteins (EMD – nuclear), SEPT2 –cytoplasmic and Spna2 – associated with the cytoskeleton. Other interacting proteins are NFIA, linked to viral infection, PHGDH, linked to cytoplasmic energy metabolism and SAP18b (Gm10094, (http://www.informatics.jax.org/marker/MGI:1277978).
ADNP and cognition
Our studies showed that VIP and VIP derivatives protected against Alzheimer-like pathology [30; 31]. Furthermore, the VIP receptor, VPAC2, controlling ADNP expression , has been linked to schizophrenia and autism spectrum disorders [33; 34] and VIP regulates ADNP expression in vivo . Our discovery of the requirement of ADNP for brain formation  coupled with the finding that a major phenotypic outcome of ADNP haploinsufficency in mice leads to cognitive impairments, placed ADNP as a key regulatory gene for brain function . The direct involvement of ADNP in cognitive function was reported in our 2007 Adnp haploinsufficient mouse model  coupled with a paper showing that deletion in the chromosomal area including ADNP (20q12–13.2 ) specifically, 20q13.13-q13.2  resulted in developmental delays and intellectual disabilities in humans. Both animal studies [14; 15; 25] as well as the human studies were repeated and extended showing axonal/synaptic/behavioral dysfunctions at the mouse level [14; 15] mirroring the human situation when the ADNP gene is partially deleted  or pathologically mutated [4; 17; 36; 37; 38; 39; 40; 41]. Over the last 4 years it became apparent that the mutated ADNP gene is consistently reported as one of the most frequent causes of syndromic autism and intellectual disability [1; 2; 3; 39].
Notably, the involvement of ADNP in cognitive performance is not limited to the ADNP syndrome but is extended to schizophrenia  and Alzheimer’s disease  with ADNP transcripts dysregulated in lymphocytes in both diseases and with ADNP blood levels correlating with intelligence . Thus, the current opinion combines mechanisms to cognitive protection.
Furthermore, NAP activity is not limited to the mouse model, but has shown efficacy in amnestic mild cognitive impairment patients, prodromal to Alzheimer’s disease (protecting short term memory) and in schizophrenia patients (protecting functional activities of daily living as reviewed [43; 44]). Currently, Coronis Neurosciences (www.coronisns.com) is developing NAP (CP201) for the ADNP syndrome.
This opinion article connects ADNP to a network of proteins linked with cognitive abilities. As many cases within the autism spectrum disorders and developmental disorders are caused by single gene mutations, it is of great interest to understand the protein interactions to get a comprehensive understanding of the molecular basis of cognition. Specifically, in the case of ADNP, which has been correlated with intelligence in the developing child and in the elderly, in autism spectrum disorders, the ADNP syndrome, in Alzheimer’s disease and cognitive impairments associated with schizophrenia. The case of ADNP is unique with the identification of its active neuroprotective site, NAP. Outlined above are protein interacting with the multitasking ADNP, which are linked in part to neurodevelopment and cognition. For example, mutations in CHD4 (OMIM # 617159) cause neurodevelopmental delays, chromatin remodelers have been associated with cognition , Eif4e has been tightly linked with autism  and autophagy with autism, brain degeneration and schizophrenia . Finally, ADNP’s interaction with cytoskeletal proteins shapes the synapse and contributes to brain plasticity [4; 15]. Understanding ADNP multitasks and interacting proteins, will allow the development of NAP and pipeline for other related diseases, syndromes affected by single gene mutations and allow cross-over drug repositioning clinical developments for the benefit of the cognitively impaired patient, families and society at large.
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Keywords: ADNP (activity dependent neuroprotective protein), ADNP gene, Cognition, Protein interaction, Neurodegenaration
Received: 04 Oct 2018;
Accepted: 08 Nov 2018.
Edited by:Mark P. Burns, Georgetown University, United States
Reviewed by:Corrado Romano, Associazione Oasi Maria SS. Onlus (IRCCS), Italy
Copyright: © 2018 Gozes. 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) and the copyright owner(s) 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.
* Correspondence: Prof. Illana Gozes, Tel Aviv University, Tel Aviv, Israel, email@example.com