Failed remyelination of the nonhuman primate optic nerve leads to axon degeneration,retinal damages,and visual dysfunction

White matter disorders of the central nervous system (CNS), such as multiple sclerosis (MS), lead to failure of nerve conduction and long-lasting neurological disabilities affecting a variety of sensory and motor systems, including vision. While most disease-modifying therapies target the immune and inflammatory response, the promotion of remyelination has become a new therapeutic avenue to prevent neuronal degeneration and promote recovery. Most of these strategies have been developed in short-lived rodent models of demyelination, which spontaneously repair and do not reflect the size, organization, and biology of the human CNS. Thus, well-defined nonhuman primate models are required to efficiently advance therapeutic approaches for patients. Here, we followed the consequence of long-term toxin-induced demyelination of the macaque optic nerve on remyelination and axon preservation, as well as its impact on visual functions. Findings from oculomotor behavior, ophthalmic examination, electrophysiology, and retinal imaging indicate visual impairment involving the optic nerve and retina. These visual dysfunctions fully correlated at the anatomical level, with sustained optic nerve demyelination, axonal degeneration, and alterations of the inner retinal layers. This nonhuman primate model of chronic optic nerve demyelination associated with axonal degeneration and visual dysfunction, recapitulates several key features of MS lesions and should be instrumental in providing the missing link to translate emerging repair promyelinating/neuroprotective therapies to the clinic for myelin disorders, such as MS.

White matter disorders are a large group of neurological diseases of various origins. Those affecting the central nervous system (CNS), such as multiple sclerosis (MS), lead to failure of nerve conduction, axon degeneration, and result in long-lasting neurological disabilities and tissue atrophy (1). The loss of myelin and healthy axons are believed to be responsible for irreversible damages, which affect a variety of sensory and motor systems, including vision. In MS, 70% of patients are affected with optic neuritis. It can manifest in an acute episode with decreased vision that can recover over several weeks in the majority of patients, while permanent visual symptoms persist in 40 to 60% of patients (2, 3). Chronic optic neuritis can lead to significant optic nerve atrophy and retinal alterations, affecting mainly the retinal inner layers, including the retinal nerve fiber and ganglion cell layers (4). Several visual assays, including visual fields (VF) (5), pupillary responses to luminance and color (pupillary light reflex, PLR) (6), electroretinograms (ERG) (7), optical coherence tomography (OCT) (4, 8), and visual evoked potential (VEP) (9–11) are routinely performed to assess noninvasively the anatomical and electrophysiological perturbations of visual functions in MS. While functional recovery was reported in some patients (9), the lack of anatomical–electrophysiological correlation has prevented to attribute directly these improvements to remyelination or other regenerative processes.Animal models of demyelination induced by toxins, such as lyso-phosphatidylcholine (LPC), are suitable for studying the mechanisms of demyelination/remyelination and developing approaches aimed at promoting CNS remyelination, as they show little inflammation and, therefore, provide means to assay directly the effect of a therapy on remyelination. However, most of these models are developed in short-lived rodents and spontaneously repair, thus lacking the long-lasting progressive degenerative disease context of MS. Besides, these models do not reflect the size or complex organization of the human primate CNS (12). They do not inform on the biology of primate cells, which differs from rodents (13, 14), nor on the security, toxicity, and long-term efficacy of cell- or compound-based promyelinating/neuroprotective therapies. Thus, experiments in long-lived nonhuman primates appear an essential step toward clinical trials.While promoting remyelination may prevent axon degeneration, only a few promyelination strategies have been translated to the clinic (15,16). One of the roadblocks is the absence of studies addressing the clinical benefit of promyelination approaches that could be applied to the clinic (17). A positive correlation between changes in VEP parameters and the degree of demyelination/remyelination was established in rodents (18–21), cats (22), and dogs (23), and exploited successfully to follow promyelination therapies in rodents (24, 25). OCT has been used to identify loss of optic nerve and retinal damages in animal models of myelin disorders as well (23, 26). While used seldomly in nonhuman primates (27), none of these clinical assays were exploited to monitor the impact of optic nerve demyelination in nonhuman primates.We previously demonstrated that LPC injection in the macaque optic nerve induced demyelination with fair axon preservation but little remyelination up to 2 mo post demyelination (28). Taking advantage of the fact that nonhuman primates are long-lived and are able to perform several tasks awake, as do humans, we questioned whether this model could be used to follow the consequence of long-term demyelination on axon preservation, and whether multimodal noninvasive assays, such as VF, VEP, OCT, and PLR could be instrumental to follow/predict the functional and anatomical outcome of optic nerve demyelination. Using multidisciplinary approaches, we provide compelling evidence that LPC-induced demyelination of the macaque optic nerve leads to modified VF, VEP, PLR, and altered inner retinal layers, but preserved photoreceptors based on OCT and ERG. These clinical and functional anomalies were correlated at the histological level with failed remyelination and progressive optic nerve axon loss, followed by neuronal and fiber loss of the inner retinal layers. The postmortem validation of OCT, VEP, and PLR as pertinent markers of optic nerve demyelination/degeneration could further help the translation of therapeutic strategies toward the clinic for myelin diseases associated with long-term demyelination of the optic nerve.     

Spine dynamics of PSD-95-deficient neurons in the visual cortex link silent synapses to structural cortical plasticity

Critical periods (CPs) are time windows of heightened brain plasticity during which experience refines synaptic connections to achieve mature functionality. At glutamatergic synapses on dendritic spines of principal cortical neurons, the maturation is largely governed by postsynaptic density protein-95 (PSD-95)-dependent synaptic incorporation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors into nascent AMPA-receptor silent synapses. Consequently, in mouse primary visual cortex (V1), impaired silent synapse maturation in PSD-95-deficient neurons prevents the closure of the CP for juvenile ocular dominance plasticity (jODP). A structural hallmark of jODP is increased spine elimination, induced by brief monocular deprivation (MD). However, it is unknown whether impaired silent synapse maturation facilitates spine elimination and also preserves juvenile structural plasticity. Using two-photon microscopy, we assessed spine dynamics in apical dendrites of layer 2/3 pyramidal neurons (PNs) in binocular V1 during ODP in awake adult mice. Under basal conditions, spine formation and elimination ratios were similar between PSD-95 knockout (KO) and wild-type (WT) mice. However, a brief MD affected spine dynamics only in KO mice, where MD doubled spine elimination, primarily affecting newly formed spines, and caused a net reduction in spine density similar to what has been observed during jODP in WT mice. A similar increase in spine elimination after MD occurred if PSD-95 was knocked down in single PNs of layer 2/3. Thus, structural plasticity is dictated cell autonomously by PSD-95 in vivo in awake mice. Loss of PSD-95 preserves hallmark features of spine dynamics in jODP into adulthood, revealing a functional link of PSD-95 for experience-dependent synapse maturation and stabilization during CPs.

Early life of an animal is characterized by time windows of functionally and structurally enhanced brain plasticity known as critical periods (CPs), which have been described initially in the primary visual cortex (V1) of kittens (1). During CPs, experience refines the connectivity of principal excitatory neurons to establish the mature functionality of neural networks. This refinement is governed by the constant generation and elimination of nascent synapses on dendritic spines that sample favorable connections to be consolidated and unfavorable ones to be eliminated (2–5). A fraction of nascent synapses is or becomes α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-receptor silent, expressing N-methyl-D-aspartate (NMDA) receptors only (6–8). At eye opening, silent synapses are abundant in the primary visual cortex (V1) (9, 10) and mature during CPs by stable AMPA receptor incorporation (11–14). The pace of silent synapse maturation is governed by the opposing yet cooperative function of postsynaptic density protein of 95 kDa (PSD-95) and its paralog PSD-93, two signaling scaffolds of the postsynaptic density of excitatory synapses (12, 13). However, whether silent synapses are preferential substrates for spine elimination during CPs remains to be investigated.In juvenile mice (postnatal days [P] 20 to 35), a brief monocular deprivation (MD) of the dominant contralateral eye results in a shift of the ocular dominance (OD) of binocular neurons in V1 toward the open eye, mediated by a reduction of responses to visual stimulation of the deprived eye (15–17). Structurally, MD induces an increase in spine elimination in apical dendrites of layer (L) 2/3 and L5 pyramidal neurons (PNs) which is only observed during the CP and constitutes a hallmark of juvenile OD plasticity (jODP) (18–20). After CP closure, cortical plasticity declines progressively, and in standard cage-raised mice beyond P40, a 4-d MD no longer induces the functional nor anatomical changes associated with jODP (21–24).At least three different mechanisms involved in experience-dependent maturation of cortical neural networks have been described, but the molecular and cellular mechanisms that cause CP closure remain highly debated (18, 25, 26). First, plasticity of local inhibitory neurons, such as increased inhibitory tone or a reduction of release probability by experience-dependent endocannabinoid receptor 1 (CB1R) activation was reported to close the critical period in rodent V1 (27–29). Second, the expression of so-called “plasticity brakes,” such as extracellular matrix (ECM), Nogo receptor 1 (NgR1), paired immunoglobulin-like receptor B (PirB), and Lynx1 were correlated with the end of critical periods (30–33). Experimentally decreasing the inhibitory tone or absence of plasticity brakes enhanced ODP expression in various knockout (KO) mouse models (32, 34, 35), among which only Lynx1 KO mice were shown to exhibit functional hallmarks of jODP, such as selective deprived eye depression after a short MD (36). Structurally, Lynx1 KO mice exhibited elevated spine dynamics at baseline; however, MD induced a reduction in spine elimination in apical dendrites of L5 PNs, whereas in L2/3 PNs there was no change (37). Thus, the effects of removing plasticity brakes on structural plasticity are variable, and it remains unclear to what extend manipulating the plasticity brakes can reinstate cellular signatures of CP plasticity in the adult wild-type (WT) brain (38). Third, the progressive maturation of AMPAR-silent synapses was correlated with the closure of the CP for jODP (12, 13). Consequently, in PSD-95 KO mice, the maturation of silent synapses is impaired; their fraction remains at the eye opening level, and jODP is preserved lifelong (13). Furthermore, visual cortex-specific knockdown (KD) of PSD-95 in the adult brain reinstated jODP. In contrast, in PSD-93 KO mice, silent synapses mature precociously and the CP for jODP closes precociously (12), correlating the presence of silent synapses with functional plasticity during CPs.While these three mechanisms of CP closure are not mutually exclusive in regulating cortical plasticity (26), it remains elusive whether CP-like structural plasticity can be expressed in the adult brain and whether silent synapses might be substrates for it. Here, we performed chronic two-photon imaging of dendrites of L2/3 pyramidal neurons in binocular V1 of PSD-95 KO (and KD) and WT mice, tracking the same dendritic spines longitudinally before, during, and after a 4-d period of MD. As previous studies have reported anesthesia effects on spine dynamics (39–41), we performed our experiments in awake mice, thoroughly trained for head fixation under the two-photon microscope. Our chronic spine imaging experiments revealed that in adult PSD-95 KO and KD mice, a brief MD indeed increased spine elimination about twofold, while adult WT mice did not display experience-dependent changes in spine elimination or spine formation. Thus, the loss of PSD-95 led to a high number of AMPAR-silent synapses which were correlated with jODP after MD, and with juvenile-like structural plasticity even in the adult brain, underscoring the importance of silent synapses for CP-timing and network maturation and stabilization.