Neuregulin 1 in autism spectrum disorder

Introduction

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social interactions and communication, restricted and repetitive behaviors, interests or activities. There is a strong genetic component, with approximately 800 genes and dozens of genetic syndromes which have been found to be implicated in the susceptibility to ASD (Butler et al. 2005). About 50% of ASD cases involve chromosome deletions/duplications (particularly 15q11.2, 16p11.2) (Ho et al. 2016). Microdeletions or chromosomal duplications have been reported for a list of chromosomal regions in ASD (for a full list, see Miles 2011). A GWAS of ASD and broad autism phenotypes in extended pedigrees was conducted in Canada and the United States and reported other chromosome regions such as 1p36.22, 2p13.1, 6q27, 8q24.22, 9p21.3, 9q31.2, 12p13.31, 16p13.2 and 18q21.1 (Woodbury-Smith et al. 2018).
There are 41 genes that were found to be more associated with ASD compared to other neurodevelopmental disorders (Leblond et al. 2021). A shared gene that was associated with both ASD and attention-deficit hyperactivity disorder was mannosidase beta (Mattheisen et al. 2022), and in both ASD and schizophrenia shared genes were exostosin 1, astrotactin 2, mono-ADP ribosylhydrolase 2, and histone deacetylase 4 (Autism Spectrum Disorders Working Group of The Psychiatric Genomics Consortium, 2017). Known genetic syndromes or single gene disorders that are linked to ASD include X-linked Rett (MECP2 gene) and fragile X (FMR1 gene) (Canitano and Bozzi 2024). These findings provide some explanation of the heterogeneity of the presentation of ASD and also provides insight into key biological pathways that may be affected in many other neurological disorders such as schizophrenia, which shares genetic factors.
Social deficits are a shared clinical characteristic between schizophrenia and ASD (American Psychiatric Association 2013). Impairments in neurocognitive measures of social cognition have also been found to occur in people with schizophrenia and ASD (Jutla et al. 2022), and both these disorders co-occur at a higher rate than in the general population, with the pooled prevalence of schizophrenia occurring in ASD close to 6% (Lugo Marín et al. 2018). Several of the same genes are found to be altered in both schizophrenia and autism, including neuregulin 1 (NRG1) (Prats et al. 2022). NRG1 has been found to be altered in schizophrenia and is associated with an increased risk of developing schizophrenia (Stefansson et al. 2002, 2004). NRG1 deficiency is also associated with deregulation of neurotransmitter receptors such as N-methyl-D-aspartate (NMDA), g-aminobutyric acid (GABA) and acetylcholine receptors and disturbed migration of cortical neuronal precursor changes in the brain, which may increase the risk of developing schizophrenia (Corfas et al. 2004).
NRG1 is also a potential candidate gene in the etiology of ASD. NRG1 mRNA has been implicated in executive functioning in individuals with ASD (Abbasy et al. 2018). NRG1 signaling can result in deficiencies in neurite outgrowth, myelination, axon projection, GABAergic and pyramidal neuron migration, and synapse formation and plasticity (Iwakura and Nawa, 2013; Ledonne and Mercuri, 2020; Mei and Xiong 2008). NRG1 interacts with several other genes which are implicated in ASD, such as: ErbB4, which is involved in neurodevelopment and synaptic plasticity; disrupted in schizophrenia 1 (DISC1), which involved in neurodevelopment and associated with various psychiatric disorders (Unda et al. 2016); and catechol-O- methyltransferase (COMT), which is involved in dopamine metabolism and cognitive function (Quednow et al. 2009).
The human NRG1 gene is located on chromosome 8p12. NRG1 belongs to a family of trophic factors that are encoded by four genes (NRG 1-4) and its receptors are ErbB receptor tyrosine kinases (Mei and Xiong 2008). Alternative promoters and splicing processes produce over 30 different isoforms, which are categorized into six different types of NRG1 protein (types I-VI) with distinct amino-terminal sequences (Mei and Xiong 2008). NRG1 types I, II, IV and V contain an immunoglobin (Ig) like domain which is linked to the epidermal growth factor like domain with or without the spacer region. In NRG1 type III, the Ig like domain is not present and instead is located further down the terminal region. NRG1 type III is the only type where the N-terminal region is unique and contains a cysteine-rich domain and an N-terminal transmembrane domain (Mei and Nave 2014; Mei and Xiong 2008).
The receptors of NRG1 are ErbB receptors or HER (human epidermal growth factor receptor) proteins, which are a family of receptor tyrosine kinases that play crucial roles in cell signaling, growth, and differentiation (Fig. 1). They are also involved in neuroplasticity and neuroprotection by protecting neurons from various forms of stress and damage (Longart et al. 2022; Pankratova et al. 2018). ErbB receptors bind to growth factors such as epidermal growth factor and neuregulin. Structurally, ErbB receptors have an extracellular ligand-binding domain, a transmembrane domain, and an intracellular tyrosine kinase domain. There are four types of ErbB receptors. ErbB4 is the only NRG1-specific ErbB that can bind with the ligand and become activated by it. ErbB4 and NRG1 interaction is crucial for neurodevelopment. ErbB4 and NRG1 interaction is essential for heart development and repair of heart injury (Bersell et al. 2009) and mammary gland development (Hayes and Gullick 2008).

Methodology

PubMed and Scopus were the databases used for this review. Original research papers and reviews were eligible for inclusion, whilst letters to editors and short communication reports were excluded. All papers available in English from 2000 were considered. Additional relevant papers were identified through references found. The keywords used include “neuregulin”, “autism”, “post-mortem brain”, “neuroimaging”, “ErbB” and “GABA”.

NRG1 in human brains

NRG1 mRNA in the post mortem human brain is expressed in multiple regions such as the prefrontal cortex, hippocampus, substantia nigra pars compacta and cerebellum (Law et al. 2004). All six types of NRG1 were detected in the cerebral cortex of the healthy human brain as measured by quantitative real-time polymerase chain reaction (RT-qPCR). NRG1 protein, measured by western blot and immunohistochemistry, were detected in cell populations in pyramidal neurons, Purkinje cells, white matter and brainstem nuclei of the post mortem human brain, which was consistent with mRNA data. This study, however, did not include double staining of NRG1 with glial or neuronal markers to quantify or localize the extent of NRG1 protein in the post mortem human brain. NRG1 isoform composition in the cerebral cortex as measured by RT-qPCR was found to change with age, with the major NRG1 isoform detected in the human cortex being type III, followed by types II and I (Liu et al. 2011). The same major NRG1 isoforms were also detected in the rat brain, with the most abundant NRG1 isoforms in the rat embryonic brains being types I, II, and III, indicating that these isoforms play an important role in neural development (Liu et al. 2011). NRG1 isoforms (types I-IV) showed distinct expression patterns during fetal and postnatal brain development. NRG1-I mRNA expression in human post mortem prefrontal cortex tissue samples using RT-qPCR was the highest in the early second trimester and decreased with fetal age, then remained stable postnatally (Paterson et al. 2014). Additionally, NRG1-II mRNA expression was stable during fetal development but decreased after birth, whilst NRG1-III mRNA expression increased dramatically during the second trimester, reaching a peak at birth, then declined in adolescence. A novel NRG1 isoform, NRG1-IVNV, was identified and found to be expressed only from 16 weeks gestation until 3 years of age (Paterson et al. 2014).

NRG1 in autism spectrum disorder

A cross-sectional magnetic resonance imaging (MRI) study in autistic children aged between 12 and 24 months observed enlarged cerebral and white matter by the age of 2.5 (Schumann et al. 2010). In males, the frontal and temporal lobe gray matter volumes were significantly enlarged whilst abnormal brain growth was more widespread in females, which included cerebral white, cerebral gray, frontal and temporal lobes. Females have been found to have a more pronounced pathology in early childhood (Bloss and Courchesne 2007; Schumann et al. 2009).
The amygdala was enlarged in children with autism but not in teenagers as measured by MRI (Schumann et al. 2004). The hippocampus was enlarged in children and teenagers with autism (Schumann et al. 2004). These changes in early childhood support the hypothesis that the autistic brain undergoes abnormal development. A recent study looking at retrospective MRIs from individuals who were later diagnosed with autism found that ASD biomarkers could be detected as early as the fetal period (Ortug et al. 2024), suggesting that abnormal development could be occurring as early as in utero. Future research could benefit from including a younger population as many of the early childhood studies involved participants from 12 months of age. Additionally, longitudinal studies that involve neuroimaging and biomarkers could be advantageous in tracking ASD throughout key development timepoints.
Post mortem studies have previously found neuroanatomical changes in the ASD brain. In individuals of all ages with ASD, smaller neuronal cell size and increased densities were found in the hippocampus, limbic system, entorhinal cortex, and amygdala (Kemper and Bauman 1998). A reduction in Purkinje cell numbers has also been found in the cerebellum (Kemper and Bauman 1998). Another study in male children with ASD found a 67% increase in neuronal numbers in the prefrontal cortex compared to controls (Courchesne et al. 2011). Additionally, age-related changes in microglial and synaptic gene expression trajectories in ASD were observed over the first two decades of life (Parikshak et al. 2016). These imaging studies and pathological findings suggest a dysregulation in brain development and neuronal proliferation which are long lasting in the autism brain.
NRG1 serum levels in ASD patients measured by ELISA were found to be significantly higher than in controls (Esnafoglu 2018). In this study, the median age of ASD patients was 3 years old and the sample size of each group was 32. Further studies could include measurements across different ages, a larger cohort size and additional neuroimaging tests for a better understanding of the extent of NRG1 changes. Strong downregulation of NRG1 and in particular types I, II and III was detected in the blood of ASD patients (Abbasy et al. 2018). ASD patients showed significant deficiencies in executive functions compared to controls, and downregulation of NRG1 types I and III was associated with poorer performance in executive function tests. Downregulation of NRG1 types I and III was associated with poorer performance in executive function tests. Boys with ASD showed significantly lower expression of NRG1 type III compared to girls. Type I and III downregulation correlated with poorer performance in all integrated visual and auditory (IVA) continuous performance tests and working memory tests. These findings suggest that NRG1 expression levels correlate with executive function deficits, potentially explaining some cognitive challenges in ASD. Type III downregulation correlated with poorer performance in all IVA tests and higher spatial working memory strategy scores (Abbasy et al. 2018). Neuroimaging tests alongside these executive function tests would be able to provide a map of areas of the brain and intensity of activation when completing these tasks.
Protein, mRNA and immunostaining of NRG1 in the postmortem autism brain have yet to be investigated and could provide a more detailed understanding of what is occurring in the ASD brain. Findings from these studies could inform appropriate in vivo animal models to find a suitable animal model where altered NRG1 levels can be studied.
Disruptions in NRG1 signaling may lead to imbalances in excitatory and inhibitory neurotransmission, a common feature observed in autism. Potential mechanisms that lead to this imbalance include alterations in glutamate and GABA signaling, change in the neurodevelopmental processes, disruption in synaptic formation, and function and abnormalities in gene expression regulators (Rubenstein and Merzenich 2003).
A disproportionately high level of excitation or weak inhibition in specific neural circuits is hypothesized to cause some forms of autism (Rubenstein and Merzenich, 2003). The excitation/inhibition imbalance could result from genetic factors, environmental influences, or a combination of both. This excitation/inhibition imbalance is hypothesized to lead to noisy and unstable cortical networks, which may explain various symptoms of autism, including sensory hypersensitivity and epilepsy. There are two contrasting hypotheses about the excitation/inhibition imbalance in ASD. The first is that there is increased excitation relative to inhibition, and the second is that there is decreased excitation relative to inhibition (Nelson and Valakh 2015). Due to the heterogeneity of ASD and different brain regions with different brain circuitry, homeostatic plasticity mechanisms play a crucial role in regulating excitatory and inhibitory balance, but these mechanisms may be inadequate or maladaptive in ASD (Nelson and Valakh 2015).

ErbB in human brains

ErbB1 is expressed in neural stem cells and progenitors and regulates neurogenesis and gliogenesis, as observed in mice (Aguirre et al. 2005, 2010). ErbB2 acts primarily as a co-receptor (Tzahar et al. 1996), whilst ErbB3 is found in oligodendrocytes and is involved in their propagation and differentiation in mice (Makinodan et al. 2012).
ErbB4 is expressed in interneurons tangentially migrating from the ventral to the dorsal cortex, suggesting that neuregulin signaling may play an important role in regulating migration and differentiation of interneurons in the developing rat brain (Yau et al. 2003). NRG1 is a key axonal signal that regulates Schwann cell development and myelination through binding to ErbB2/3 receptors, with different NRG1 isoforms (particularly types I and III) having distinct functions in Schwann cell and muscle development (Newbern and Birchmeier 2010).

ErbB in autism spectrum disorder

ErbB receptors have been implicated in ASD. ErbB4 knockout mice exhibit behaviors reminiscent of ASD, including reduced social interaction and communication deficits (Shamir et al. 2012). Higher levels of ERbB3 were detected in the blood of autistic participants compared to controls (Russo et al. 2021). Fragile X syndrome (FXS) is a common known cause of ASD and exhibits overlap of similar symptoms. The substantia nigra pars compacta dopamine neurons in an FXS model mice showed hyperactivity compared to wild-type mice, driven by overexpression and hyperfunction of mGluR1 receptors (D’Addario et al. 2025). There was upregulation of ErbB4 and ErbB2 receptors in substantia nigra pars compacta dopamine neurons of FXS mice, and inhibition of ErbB signaling normalized dopamine neuron activity and reduced repetitive behaviors in FXS mice (D’Addario et al. 2025). These findings suggest that ErbB receptors could provide a novel target for therapies in FXS and ASD.

NRG1 and presynaptic function/GABA

NRG1 also interacts with ErbB receptors to regulate neurotransmitter release, receptor function and synaptic strength. NRG1 plays a crucial role in the development and function of GABAergic neurons. They are involved in the formation of GABAergic synapses, expression of GABA receptors, and regulation of GABA release (Fazzari et al. 2010; Kato et al. 2010; Woo et al. 2007).
Downregulation of NRG1 could therefore potentially lead to reduced development of GABAergic neurons, decreased formation of GABAergic synapses (Ting et al. 2011), altered GABA receptor expression, and/or impaired GABA release (Abbasy et al. 2018).
In adults with ASD, there were significantly higher GABA concentrations in the left dorsal lateral prefrontal cortex compared to controls as measured by magnetic resonance spectroscopy (Maier et al. 2022). Higher levels of plasma GABA were found in male ASD children and a significantly lower glutamate/GABA ratio (Al-Otaish et al. 2018).
This is in contrast to the decreased levels of GABA levels found in children with ASD measured in vivo using GABA-edited magnetic resonance spectroscopy (MRS) (Puts et al. 2017) and the general decrease in GABA found in previous research in the frontal lobe using MRS (Kubas et al. 2012), motor and auditory areas of interest (Gaetz et al. 2014), and anterior cingulate cortex (Ito et al. 2017). No change in GABA levels has also been observed between adults with ASD and age-matched neurotypical adults (Kolodny et al. 2020). This unexpected finding suggests that the increased GABA concentrations in the adult ASD brain may reflect age-related changes and/or be a compensatory mechanism. N
RG1 and ErbB4 play important roles in regulating inhibitory neural circuits crucial for cognitive functions such as attention and working memory. These signaling molecules are also involved in critical periods of cortical development and plasticity. NRG1 suppresses long-term potentiation (LTP) in the hippocampus, which is mediated by increased GABA transmission. ErbB4, the receptor for NRG1, is critical for these effects and is primarily expressed in parvalbumin-positive (PV+) interneurons (Chen et al. 2010). Selective deletion of ErbB4 in PV+ interneurons prevents NRG1 from enhancing GABAergic transmission and suppressing long-term potentiation. Mice that lack ErbB4 in PV+ interneurons show impaired contextual fear conditioning, suggesting a role in hippocampus-dependent learning and memory (Chen et al. 2010). Similarly, NRG1 reversed LTP at CA1 hippocampal synapses as measured in mice hippocampal slices; however, it was unclear whether NRG1 blocks LTP induction or expression (Kwon et al. 2005). PV+ interneurons are significantly reduced in the ASD prefrontal cortex, although it could not be confirmed whether this was due to a decrease in the number of interneurons or loss of protein expression (Hashemi et al. 2017). Mice lacking PV+ interneurons (PV knockout and PV heterozygous) displayed some ASD-like behaviors, including social deficits, communication impairments, and impaired reversal learning (Wöhr et al. 2015).
These findings on how NRG1/ErbB4 signaling affects neural circuitry, synaptic function, and behavior could potentially inform our understanding of similar processes in autism. NRG1, its receptor ErbB4, or specific interneurons are therefore potential therapeutic targets for neurodevelopmental disorders such as schizophrenia and ASD. Therapeutic strategies might involve reducing NRG1 availability at synapses on PV+ interneurons or using viral vectors to manipulate NRG1 or ErbB4 in specific cell types and brain regions. However, challenges would still remain in translating these approaches to an effective clinical treatment, such as potential side effects and specific targeting.

Acknowledgments

I would like to thank Sophie Richards for critical comments on the manuscript.

Disclosures

This research received no external funding.
Institutional review board statement: Not applicable.
The author declares no conflict of interest.

References