Perineuronal Nets: Subtle Structures with Large Implications

Perineuronal nets (PNNs) are specialized structures of the extracellular matrix that surround the soma and proximal dendrites of certain neurons in the central nervous system, particularly parvalbumin-expressing interneurons. Their appearance overlaps the maturation of neuronal circuits and the closure of critical periods in different regions of the brain, setting their connectivity and abruptly reducing their plasticity. As a consequence, the digestion of PNNs, as well as the removal or manipulation of their components, leads to a boost in this plasticity and can play a key role in the functional recovery from different insults and in the etiopathology of certain neurologic and psychiatric disorders. Here we review the structure, composition, and distribution of PNNs and their variation throughout the evolutive scale. We also discuss methodological approaches to study these structures. The function of PNNs during neurodevelopment and adulthood is discussed, as well as the influence of intrinsic and extrinsic factors on these specialized regions of the extracellular matrix. Finally, we review current data on alterations in PNNs described in diseases of the central nervous system (CNS), focusing on psychiatric disorders. Together, all the data available point to the PNNs as a promising target to understand the physiology and pathologic conditions of the CNS.

Parvalbumin Interneurons and Perineuronal Nets in the Hippocampus and Retrosplenial Cortex of Adult Male Mice After Early Social Isolation Stress and Perinatal NMDA Receptor Antagonist Treatment

Both early life aversive experiences and intrinsic alterations in early postnatal neurodevelopment are considered predisposing factors for psychiatric disorders, such as schizophrenia. The prefrontal cortex and the hippocampus have protracted postnatal development and are affected in schizophrenic patients. Interestingly, similar alterations have been observed in the retrosplenial cortex (RSC). Studies in patients and animal models of schizophrenia have found alterations in cortical parvalbumin (PV) expressing interneurons, making them good candidates to study the etiopathology of this disorder. Some of the alterations observed in PV+ interneurons may be mediated by perineuronal nets (PNNs), specialized regions of the extracellular matrix, which frequently surround these inhibitory neurons. In this study, we have used a double hit model (DHM) combining a single perinatal injection of an NMDAR antagonist (MK801) to disturb early postnatal development and post-weaning social isolation as an early life aversive experience. We have investigated PV expressing interneurons and PNNs in the hippocampus and the RSC of adult male mice, using unbiased stereology. In the CA1, but not in the CA3 region, of the hippocampus, the number of PNNs and PV + PNN+ cells was affected by the drug treatment, and a significant decrease of these parameters was observed in the groups of animals that received MK801. The percentage of PNNs surrounding PV+ cells was significantly decreased after treatment in both hippocampal regions; however, the impact of isolation was observed only in CA1, where isolated animals presented lower percentages. In the RSC, we observed significant effects of isolation, MK801 and the interaction of both interventions on the studied parameters; in the DHM, we observed a significantly lower number of PV+, PNNs, and PV+PNN+cells when compared to control mice. Similar significant decreases were observed for the groups of animals that were just isolated or treated with MK801. To our knowledge, this is the first report on such alterations in the RSC in an animal model combining neurodevelopmental alterations and aversive experiences during infancy/adolescence. These results show the impact of early-life events on different cortical regions, especially on the structure and plasticity of PV+ neurons and their involvement in the emergence of certain psychiatric disorders.

PSA Depletion Induces the Differentiation of Immature Neurons in the Piriform Cortex of Adult Mice

Immature neurons are maintained in cortical regions of the adult mammalian brain. In rodents, many of these immature neurons can be identified in the piriform cortex based on their high expression of early neuronal markers, such as doublecortin (DCX) and the polysialylated form of the neural cell adhesion molecule (PSA-NCAM). This molecule plays critical roles in different neurodevelopmental events. Taking advantage of a DCX-CreERT2/Flox-EGFP reporter mice, we investigated the impact of targeted PSA enzymatic depletion in the piriform cortex on the fate of immature neurons. We report here that the removal of PSA accelerated the final development of immature neurons. This was revealed by a higher frequency of NeuN expression, an increase in the number of cells carrying an axon initial segment (AIS), and an increase in the number of dendrites and dendritic spines on the immature neurons. Taken together, our results demonstrated the crucial role of the PSA moiety in the protracted development of immature neurons residing outside of the neurogenic niches. More studies will be required to understand the intrinsic and extrinsic factors affecting PSA-NCAM expression to understand how the brain regulates the incorporation of these immature neurons to the established neuronal circuits of the adult brain.

Effects of Aging on the Structure and Expression of NMDA Receptors of Somatostatin Expressing Neurons in the Mouse Hippocampus

Changes in the physiology, neurochemistry and structure of neurons, particularly of their dendritic spines, are thought to be crucial players in age-related cognitive decline. One of the most studied brain structures affected by aging is the hippocampus, known to be involved in different essential cognitive processes. While the aging-associated quantitative changes in dendritic spines of hippocampal pyramidal cells have already been studied, the relationship between aging and the structural dynamics of hippocampal interneurons remains relatively unknown. Spines are not a frequent feature in cortical inhibitory neurons, but these postsynaptic structures are abundant in a subpopulation of somatostatin expressing interneurons, particularly in oriens-lacunosum moleculare (O-LM) cells in the hippocampal CA1. Previous studies from our laboratory have shown that the spines of these interneurons are highly plastic and influenced by NMDA receptor manipulation. Thus, in the present study, we have investigated the impact of aging on this interneuronal subpopulation. The analyses were performed in 3−, 9−, and 16-month-old GIN mice, a strain in which somatostatin positive interneurons express GFP. We studied the changes in the density of dendritic spines, en passant boutons, and the expression of NMDA receptors (GluN1 and GluN2B) using confocal microscopy and image analysis. We observed a significant decrease in dendritic spine density in 9-month-old animals when compared with 3-month-old animals. We also observed a decrease in the expression of the GluN2B subunit in O-LM cells, but not of that of GluN1, during aging. These results will constitute the basis for more advanced studies of the structure and connectivity of interneurons during aging and their contribution to cognitive decline.

Effects of Dopamine on the Immature Neurons of the Adult Rat Piriform Cortex

The layer II of the adult piriform cortex (PCX) contains a numerous population of immature neurons. Interestingly, in both mice and rats, most, if not all, these cells have an embryonic origin. Moreover, recent studies from our laboratory have shown that they progressively mature into typical excitatory neurons of the PCX layer II. Therefore, the adult PCX is considered a “non-canonical” neurogenic niche. These immature neurons express the polysialylated form of the neural cell adhesion molecule (PSA-NCAM), a molecule critical for different neurodevelopmental processes. Dopamine (DA) is a relevant neurotransmitter in the adult CNS, which also plays important roles in neural development and adult plasticity, including the regulation of PSA-NCAM expression. In order to evaluate the hypothetical effects of pharmacological modulation of dopaminergic neurotransmission on the differentiation of immature neurons of the adult PCX, we studied dopamine D2 receptor (D2r) expression in this region and the relationship between dopaminergic fibers and immature neurons (defined by PSA-NCAM expression). In addition, we analyzed the density of immature neurons after chronic treatments with an antagonist and an agonist of D2r: haloperidol and PPHT, respectively. Many dopaminergic fibers were observed in close apposition to PSA-NCAM-expressing neurons, which also coexpressed D2r. Chronic treatment with haloperidol significantly increased the number of PSA-NCAM immunoreactive cells, while PPHT treatment decreased it. These results indicate a prominent role of dopamine, through D2r and PSA-NCAM, on the regulation of the final steps of development of immature neurons in the adult PCX.

Phenotype and Distribution of Immature Neurons in the Human Cerebral Cortex Layer II

This work provides evidence of the presence of immature neurons in the human brain, specifically in the layer II of the cerebral cortex. Using surgical samples from epileptic patients and post-mortem tissue, we have found cells with different levels of dendritic complexity (type I and type II cells) expressing DCX and PSA-NCAM and lacking expression of the mature neuronal marker NeuN. These immature cells belonged to the excitatory lineage, as demonstrated both by the expression of CUX1, CTIP2, and TBR1 transcription factors and by the lack of the inhibitory marker GAD67. The type II cells had some puncta expressing inhibitory and excitatory synaptic markers apposed to their perisomatic and peridendritic regions and ultrastructural analysis suggest the presence of synaptic contacts. These cells did not present glial cell markers, although astroglial and microglial processes were found in close apposition to their somata and dendrites, particularly on type I cells. Our findings confirm the presence of immature neurons in several regions of the cerebral cortex of humans of different ages and define their lineage. The presence of some mature features in some of these cells suggests the possibility of a progressively integration as excitatory neurons, as described in the olfactory cortex of rodents.

Long term effects of peripubertal stress on the thalamic reticular nucleus of female and male mice

Adverse experiences during infancy and adolescence have an important and enduring effect on the brain and are predisposing factors for mental disorders, particularly major depression. This impact is particularly notable in regions with protracted development, such as the prefrontal cortex. The inhibitory neurons of this cortical region are altered by peripubertal stress (PPS), particularly in female mice. In this study we have explored whether the inhibitory circuits of the thalamus are impacted by PPS in male and female mice. This diencephalic structure, as the prefrontal cortex, also completes its development during postnatal life and is affected by adverse experiences. The long-term changes induced by PPS were exclusively found in adult female mice. We have found that PPS increases depressive-like behavior and induces changes in parvalbumin-expressing (PV+) cells of the thalamic reticular nucleus (TRN). We observed reductions in the volume of the TRN, together with those of parameters related to structures/molecules that regulate the plasticity and connectivity of PV+ cells: perineuronal nets, matricellular structures surrounding PV+ neurons, and the polysialylated form of the neural cell adhesion molecule (PSA-NCAM). The expression of the GluN1, but not of GluN2C, NMDA receptor subunit was augmented in the TRN after PPS. An increase in the fluorescence intensity of PV+ puncta was also observed in the synaptic output of TRN neurons in the lateral posterior thalamic nucleus. These results demonstrate that the inhibitory circuits of the thalamus, as those of the prefrontal cortex, are vulnerable to the effects of aversive experiences during early life, particularly in females. This vulnerability is probably related to the protracted development of the TRN and might contribute to the development of psychiatric disorders.

Long-term effects of a double hit murine model for schizophrenia on parvalbumin expressing cells and plasticity-related molecules in the thalamic reticular nucleus and the habenula

The exposure to aversive experiences during early-life affects brain maturation and induces changes in behavior. Additionally, when these experiences coincide with subtle neurodevelopmental alterations, they may contribute to the emergence of psychiatric disorders, such as schizophrenia. Studies in patients and animal models have identified changes in parvalbumin (PV) expressing inhibitory neurons, highlighting their significance in the etiology of this disorder. Most studies have been focused on the cortex, but PV+ neurons also provide inhibitory input to diencephalic regions, particularly to the thalamus (through cells in the thalamic reticular nucleus, TRN) and the habenula. Remarkably, alterations in both nuclei have been described in schizophrenia. Some of these changes in PV+ cells may be mediated by perineuronal nets (PNN), specialized regions of the extracellular matrix that often surround them and regulate their synaptic input and activity. Interestingly, the physiological maturation and integration of PV+ neurons, which involves the assembly of PNN, occurs during early postnatal life. Plasticity molecules associated to inhibitory neurons, such as PSA-NCAM, or NMDA receptors (NMDAR) can also influence the structure and function of these cells. Growing evidence also indicates that glial cells regulate the physiology of PV+ neurons by influencing their maturation and modulating their synaptic connectivity. To explore the impact of early-life aversive experiences and concomitant subtle neurodevelopmental alterations on diencephalic PV+ cells, we analyzed adult male mice subjected to a double-hit model (DHM) of schizophrenia, combining a single injection of an NMDAR antagonist at P7 and post-weaning social isolation. We observed that exploratory behavior, PV+ neurons and their associated PNN, as well as PSA-NCAM and NMDAR expression and glial cells, in the TRN and the habenula were affected by the DHM or one of its factors. To our knowledge, this is the first report on such alterations in these diencephalic structures in an animal model combining neurodevelopmental alterations and early-life stress during adolescence. Our findings complement previous work on PV+ neurons in cortical regions and underscore the importance of studying diencephalic inhibitory networks and their intricate interactions with aversive experiences and neurodevelopmental alterations during early life in the context of schizophrenia.