Introduction
Several neurodegenerative, neuromuscular, and inherited diseases are strongly related to both mitochondrial dysfunction and optic nerve atrophy [
1]. Concerning the latter, optic neuropathies are frequently associated with chronic visual impairment that may lead to blindness in both pediatric and adult populations [
2]. In fact, the high-energy requirements of the neural visual pathway increase the likelihood of excessive oxidative stress and neural damage when mitochondrial dysfunction occurs.
Autosomic dominant optic neuropathy (ADOA, Kjer’s disease, MIM no. 165500) is one of the most common hereditary mitochondrial diseases with an estimated prevalence set between 1:12,000 (Denmark, founder effect) and 1:50,000 [
3]. ADOA has a progressive degeneration profile that “commences as a degeneration of retinal ganglion cells, with secondary ascending optic atrophy and changes of the corresponding tract and areas in the lateral geniculate body” [
4].
Phenotypically, this disorder is characterized by moderate to severe central vision deterioration and/or blindness due to optic nerve atrophy and retinal nerve fiber layer (RNFL) degeneration [
5]. Despite the well-documented expression of ADOA as an isolated optic nerve pathology [
6], less is known regarding the impact on the brain. In fact, several patients develop extraocular complications such as neurosensory hearing loss, myopathy, peripheral neuropathy, chronic progressive external ophtalmoplegia being commonly named “plus” phenotypes [
7]. These syndromic forms highlight the high vulnerability of the peripheral and possibly also the central nervous system (CNS) to degeneration and dysfunction in ADOA.
Genetic studies have associated this disorder to pathogenic mutations mainly within the nuclear gene OPA1 in chromosome 3q28-q29 that encodes for inner mitochondrial membrane proteins [
8]. These mutations have an impact on the subunit I of the mitochondrial transport chain leading to the disruption of mitochondrial dynamics and influencing the normal mitochondrial fusion and fission balance [
6,
9].
A previous study showed that the OPA1 gene is ubiquitously expressed in both retina, optic tract, and brain [
10] rendering quite intriguing the assumption of the predominant selectivity of impairment of retinal ganglion cells (RGCs) in genetic mitochondrial disorders. Interestingly, the smaller-caliber RGCs within the inner retina constituting the papillomacular bundle are particularly vulnerable while melanopsin-containing RGCs are relatively spared [
2].
The retina and the brain are closely related and share embryological, anatomical, and physiological features [
11]. It is therefore important to understand whether changes in the retina are correlated with alterations in cortical and subcortical structures and whether they are independent. In our previous study of preclinical Leber Hereditary Optic Neuropathy (LHON) carriers, we found higher cortical thickness values in the peripheral visual extrastriate representations and these changes were predicted by the swelling of macular RGC axons [
12,
13]. Other authors have also established retinocortical-associated changes in healthy aging, cognitive impairment and Alzheimer’s disease [
14], and in other pathologies such as multiple sclerosis [
15], glaucoma [
16], and in patients with long-lasting retinal visual defects [
17] in both anatomical and functional domains [
18].
Also, disturbances in the excitation/inhibition balance in cortical neural networks have been implied not only in several disorders such as autism spectrum disorders and schizophrenia [
19] but also during cortical development [
20] and plasticity processes [
21]. The maintenance of this balance, mainly through the regulation of glutamate (excitatory) and GABA (inhibitory) levels, is pivotal for normal brain physiology.
This is the first study investigating both structural and excitation/inhibition cortical phenotypes of ADOA, a condition that leads to the loss of central vision with optic nerve atrophy and retinal ganglion cell degeneration. More specifically, motivated by the hypothesis that visual cortex neurochemistry and structure might be altered due to the fact that the OPA1 gene is expressed in the brain, we performed cortical thickness and volumetric analysis, and also in vivo neurochemical analysis using proton MR spectroscopy (
1H-MRS) in the occipital cortex of a group of OPA1-ADOA patients from a previously studied cohort in a vision research study [
22] to (indirectly) assess neurotransmission.
Discussion
In this work, we investigated the hypothesis that cortical neurochemistry and in particular inhibitory balance is impaired in patients with genetically determined mitochondrial dysfunction. We report the first cortical morphometry analysis and in vivo measurements of glutamate and GABA in patients with OPA1-ADOA. We found a significant decrease of GABA+/tCr levels in the occipital lobe of ADOA patients, in contrast with preserved glutamate levels, suggesting a functional decrease in cortical inhibition. Structural measures of volumetric VBM and cortical thickness did not reveal changes in the occipital cortex of these patients.
OPA1-ADOA is a rare disease associated with point mutations in the OPA1 gene ubiquitously expressed that is crucial for mitochondrial function regulation. In this disease, the central papillomacular bundle is especially affected due to the retinal ganglion cell (RGC) degeneration leading to optic nerve atrophy and ensuing central visual field loss related to mitochondrial dysfunction [
30].
In our recent work with asymptomatic Leber Hereditary Optic Neuropathy (LHON) individuals, also an inherited mitochondrial optic neuropathy, we found evidence for enhanced developmental mechanisms of cortical plasticity in visual extrastriate cortex [
12]. We also found that this regionally specific cortical reorganization is triggered by changes in macular retinal ganglion cell axonal layer thickness [
13], suggesting a retinal cause. We did not find structural cortical changes in the occipital lobe of ADOA patients, evaluated with both cortical thickness and volumetric measures, as based on the anatomical definition of our ROIs. Our results are consistent with the findings of Rocca and colleagues [
31] who used VBM and Tract-Based Spatial Statistics (TBSS) to assess regional GM and WM changes in ADOA patients. They found significant WM atrophy of the chiasm and optic tract but no atrophy of GM regions.
In spite of the absence of volumetric changes, we found independent changes in cortical biochemistry. A previous study with OPA1 patients revealed several brain imaging abnormalities but normal creatine,
N-acetylaspartate, and choline levels [
32], suggesting preserved metabolism. However, neurotransmitter levels were not accessed. Indeed, our hypothesis was based on the tenet that impaired mitochondrial functioning can cause alterations in neurotransmission [
33] in this disorder. Also, it is possible that intracortical reorganization mechanisms may be counteracting in the visual cortex against retinal degeneration processes. A shift in excitatory/inhibitory balance is consistent with this idea. A developmental origin for our findings is corroborated by the absence of a relation with structural (OCT) and functional markers of visual loss (psychophysical, as indexed by visual acuity and color testing along cone contrast axes; and electrophysiological, as indexed by pattern ERG).
In our study, we focused the changes in excitatory/inhibitory balance in OPA1 patients, as assessed by the quantification of glutamate and GABA. First and foremost, glutamate is the main excitatory neurotransmitter of the CNS. Glutamate effects are short-lived and its concentration needs to be strictly controlled due to excitotoxic effects. Excessive excitatory neurotransmission can be triggered by mitochondrial dysfunction, oxidative stress, calcium overload, and energy deficiency, ultimately leading to neurodegeneration [
34,
35]. In our study, glutamate levels were not different between patients and controls.
On the other hand, GABA is a surrogate marker of inhibitory neurotransmission, and it is commonly associated with brain function as assessed from neurophysiology and the behavioral points of view [
36‐
38]. GABA level changes have been implicated in the pathophysiology of several neurologic disorders [
39], brain maturation [
40], neuroplasticity events [
41,
42], and recovery post-stroke [
43].
As a cautionary note, in vivo spectroscopy does not allow the discrimination between intracellular or extracellular GABA, i.e., the metabolic (cytosolic) GABA or the neurotransmitter (vesicular) (GABA). In any case, the decrease of GABA in this mitochondrial disorder may be linked with disrupted Ca
2+ homeostasis that occurs in ADOA due to OPA1 loss [
44,
45] leading to changes in neurotransmitter release [
46].
Also, the physiological impact of this specific change in the excitatory/inhibitory balance may possibly be related to plasticity-related developmental phenomena [
47]. As stated above, the fact that there is no correlation of GABA/tCr with visual dysfunction (VA, OCT, CCT, and pattern ERG) renders less likely a mechanism based on lack of sensory input.
The pathophysiological implications of the observed lower levels of GABA in the visual cortex, in the absence of volumetric changes, should be further explored from the mechanistic point of view. In this line, lowering of GABA levels is possibly linked with modulation by factors such as insulin-like growth factor 1 (IGF1) [
48], which has been suggested to be a potentially effective therapy for mitochondrial protection and recovery [
49].
Future studies, with larger cohorts and complementary neuroimaging techniques might be helpful in elucidating the nature of putative homeostatic mechanisms, for example by measuring GABA-A receptor binding potential which could be addressed by positron-emission tomography (PET) imaging.
We provide evidence for impaired cortical physiology (reduced inhibition vs. excitation as assessed by reduced GABA levels with preserved glutamate levels) with structural sparing in ADOA, given the identified GABAergic changes in the visual cortex. These results suggest a novel cortical physiological alteration that may be relevant for the exploration of hitherto unexpected brain dysfunction of this retinal ganglion cell disorder. Future studies should address the impact of this novel phenotype on visual and cognitive function, as identified in other neurodegenerative disorders.