Autism Spectrum Disorders (ASDs) are neurodevelopmental syndromes, characterized by behavioral deficits and a strong genetic background.
In the last decades, several autism-linked mutations have been found in the Neuroligins (NLGNs) family of proteins. Between these mutations, the R451C substitution in NLGN3 has been highly characterized. It is known from in vitro studies, that the R451C mutation affects the folding of the extracellular domain of the protein, causing its retention in the Endoplasmic Reticulum (ER). It is estimated that only 10% of the mutant protein can reach the synapse and it has been shown, both in vitro and in vivo, that the loss of NLGN3 on the cell surface causes alterations in synaptic functions in several brain regions.
We have generated and characterized a new cell-based model system that allow us to study protein trafficking and its modulation by chemical compounds with chaperone-like activity.
The aim of this project is to screen a library of compounds approved by the American Food and Drug Administration with the aim to restore normal trafficking of the autism-linked R451C NLGN3 protein, to the cell surface.
In the last years, the research field regarding diseases correlated to the accumulation of misfolded protein has been growing quickly. Endoplasmic Reticulum (ER) stress and Unfolded Protein Response (UPR) have been considered as common features of several neurological and non-neurological diseases, such as Alzheimer¿s disease or Diabetes (Koss et al., 2017; Iwawaki and Oikawa, 2013).
Our interest is focused on the biological effects caused by the Arg451 to Cys substitution (R451C), in the extracellular domain of the post-synaptic protein Neuroligin3 (NLGN3). This mutation has been found in patients with Autism Spectrum Disorders (ASDs) and causes loss of the protein at the synapse where it normally plays its role as an adhesion molecule.
Our recent work established the first evidence between the retention of the R451C NLGN3 in the ER and the activation of UPR (Ulbrich et al., 2016).
Two research groups have independently generated a transgenic mouse carrying the R451C mutation in the endogenous NLGN3 gene (Tabuchi, et al. 2007; Chadman at al., 2008), allowing to study this human mutation in a Knock-In (KI) mouse model. This mouse strain shows autistic-like behaviours and electrophysiological alterations, such as the increase of inhibitory transmission in the somatosensory cortex and an enhancement of excitatory transmission in the hippocampus. These alterations have not been detected neither in the WT nor in the NLGN3 Knock-Out (KO) mice, indicating that the R451C mutation might cause a gain of function phenotype.
Data from our lab have also shown alterations in the excitatory current in the cerebellum of R451C NLGN3 KI mice, the only brain region in which we observed UPR activation (unpublished data). Since UPR has been found to regulate some neuronal functions, such as synaptic plasticity, it might represent one of the causes of the electrophysiological alterations observed in this KI mouse.
The development of drugs to treat multifactorial diseases represent a challenge. Moreover, the huge number of mutations associated to ASDs and the influence of environmental factors makes impossible to develop a cure for these disorders. Indeed, so far only therapies against the main symptoms are available.
A possible strategy to treat the monogenic form of autism caused by the R451C mutation in NLGN3, would be the reintroduction of the mutant protein on the cell surface. It might be possible by using chemical or pharmacological chaperones, which help the folding of the mutant protein, allowing it to escape the ER and reach the cell membrane.
In literature there are several examples of diseases in which this approach has been used in vitro with promising and encouraging results (Van Goor et al., 2006; Cortez and Sim, 2014).
It is noteworthy that helping the mutant NLGN3 to exit the ER would have a double effect. In fact, beside restoring its normal localization, it would alleviate the overload of the ER and it could reduce UPR activation.
This project can be considered as part of a new field of research, which aims to find a possible treatment for diseases correlated with protein misfolding. This field is new and exciting and it opens a high number of possibilities for all those pathologies untreatable so far.
Moreover, alleviation of the UPR is emerging as a possible treatment for those diseases in which it is activated. In 2016, Valenzuela and colleagues published a review with a sum up of the available treatments against UPR developed so far.
The correlation between ASDs, ER stress and UPR establishes a new and stimulating point of view on these disorders.