GABA-Synthesizing Enzymes in Calbindin and Calretinin Neurons in Monkey Prefrontal Cortex.

Non-overlapping groups of cortical γ-aminobutyric acid-releasing (GABAergic) neurons are identifiable by the presence of calbindin (CB), calretinin (CR), or parvalbumin (PV). Boutons from PV neuron subtypes are also distinguishable by differences in protein levels of the GABA-synthesizing enzymes GAD65 and GAD67. Multilabel fluorescence microscopy was used to determine if this diversity extends to boutons of CB and CR neurons in monkey prefrontal cortex. CB and CR neurons gave rise to 3 subpopulations of GAD-containing boutons: GAD65+, GAD67+, and GAD65/GAD67+. Somatostatin and vasoactive intestinal peptide-expressing neurons, subtypes of CB and CR neurons, respectively, also gave rise to these distinct bouton subpopulations. At the transcript level, CB and CR neurons contained mRNA encoding GAD67-only or both GADs. Thus, the distinct subpopulations of CB/GAD+ and CR/GAD+ boutons arise from 2 unique subtypes of CB and CR neurons. The different CB and CR GAD-expressing neurons targeted the same projection neurons and neuronal structures immunoreactive for PV, CR, or CB. These findings suggest that GABA synthesis from CB/GAD67+ and CR/GAD67+ neurons would presumably be more vulnerable to disease-associated deficits in GAD67 expression, such as in schizophrenia, than neurons that also contain GAD65.

Laminar-Specific Alterations in Calbindin-Positive Boutons in the Prefrontal Cortex of Subjects With Schizophrenia.

BACKGROUND: Cognitive deficits in schizophrenia are associated with altered GABA (gamma-aminobutyric acid) neurotransmission in the prefrontal cortex (PFC). GABA neurotransmission requires GABA synthesis by 2 isoforms of glutamic acid decarboxylase (GAD65 and GAD67) and packaging by the vesicular GABA transporter (vGAT). Current postmortem findings suggest that GAD67 messenger RNA is lower in a subset of the calbindin-expressing (CB+) class of GABA neurons in schizophrenia. Hence, we assessed if CB+ GABA neuron boutons are affected in schizophrenia. METHODS: For 20 matched pairs of subjects with schizophrenia and unaffected comparison subjects, PFC tissue sections were immunolabeled for vGAT, CB, GAD67, and GAD65. The density of CB+ GABA boutons and levels of the 4 proteins per bouton were quantified. RESULTS: Some CB+ GABA boutons contained both GAD65 and GAD67 (GAD65+/GAD67+), whereas others contained only GAD65 (GAD65+) or GAD67 (GAD67+). In schizophrenia, vGAT+/CB+/GAD65+/GAD67+ bouton density was not altered, vGAT+/CB+/GAD65+ bouton density was 86% higher in layers 2/superficial 3 (L2/3s), and vGAT+/CB+/GAD67+ bouton density was 36% lower in L5-6. Bouton GAD levels were differentially altered across bouton types and layers. In schizophrenia, the sum of GAD65 and GAD67 levels in vGAT+/CB+/GAD65+/GAD67+ boutons was 36% lower in L6, GAD65 levels were 51% higher in vGAT+/CB+/GAD65+ boutons in L2, and GAD67 levels in vGAT+/CB+/GAD67+ boutons were 30% to 46% lower in L2/3s-6. CONCLUSIONS: These findings indicate that schizophrenia-associated alterations in the strength of inhibition from CB+ GABA neurons in the PFC differ across cortical layers and bouton classes, suggesting complex contributions to PFC dysfunction and cognitive impairments in schizophrenia.

Altered Parvalbumin Basket Cell Terminals in the Cortical Visuospatial Working Memory Network in Schizophrenia.

BACKGROUND: Visuospatial working memory (vsWM), which is commonly impaired in schizophrenia, involves information processing across the primary visual cortex, association visual cortex, posterior parietal cortex, and dorsolateral prefrontal cortex (DLPFC). Within these regions, vsWM requires inhibition from parvalbumin-expressing basket cells (PVBCs). Here, we analyzed indices of PVBC axon terminals across regions of the vsWM network in schizophrenia. METHODS: For 20 matched pairs of subjects with schizophrenia and unaffected comparison subjects, tissue sections from the primary visual cortex, association visual cortex, posterior parietal cortex, and DLPFC were immunolabeled for PV, the 65- and 67-kDa isoforms of glutamic acid decarboxylase (GAD65 and GAD67) that synthesize GABA (gamma-aminobutyric acid), and the vesicular GABA transporter. The density of PVBC terminals and of protein levels per terminal was quantified in layer 3 of each cortical region using fluorescence confocal microscopy. RESULTS: In comparison subjects, all measures, except for GAD65 levels, exhibited a caudal-to-rostral decline across the vsWM network. In subjects with schizophrenia, the density of detectable PVBC terminals was significantly lower in all regions except the DLPFC, whereas PVBC terminal levels of PV, GAD67, and GAD65 proteins were lower in all regions. A composite measure of inhibitory strength was lower in subjects with schizophrenia, although the magnitude of the diagnosis effect was greater in the primary visual, association visual, and posterior parietal cortices than in the DLPFC. CONCLUSIONS: In schizophrenia, alterations in PVBC terminals across the vsWM network suggest the presence of a shared substrate for cortical dysfunction during vsWM tasks. However, regional differences in the magnitude of the disease effect on an index of PVBC inhibitory strength suggest region-specific alterations in information processing during vsWM tasks.

Cell Type-Specific Gene Expression of Alpha 5 Subunit-Containing Gamma-Aminobutyric Acid Subtype A Receptors in Human and Mouse Frontal Cortex.

Converging evidence suggests that deficits in somatostatin (SST)-expressing neuron signaling contributes to major depressive disorder. Preclinical studies show that enhancing this signaling, specifically at α5 subunit-containing γ-ami-nobutyric acid subtype A receptors (α5-GABAARs), provides a potential means to overcome low SST neuron function. The cortical microcircuit comprises multiple subtypes of inhibitory γ-aminobutyric acid (GABA) neurons and excitatory pyramidal cells (PYCs). In this study, multilabel fluorescence in situ hybridization was used to characterize α5-GABAAR gene expression in PYCs and three GABAergic neuron subgroups - vasoactive intestinal peptide (VIP)-, SST-, and parvalbumin (PV)-expressing cells - in the human and mouse frontal cortex. Across species, we found the majority of gene expression in PYCs (human: 39.7%; mouse: 54.14%), less abundant expression in PV neurons (human: 20%; mouse: 16.33%), and no expression in VIP neurons (0%). Only human SST cells expressed GABRA5, albeit at low levels (human: 8.3%; mouse: 0%). Together, this localization suggests potential roles for α5-GABAARs within the cortical microcircuit: (1) regulators of PYCs, (2) regulators of PV cell activity across species, and (3) sparse regulators of SST cell inhibition in humans. These results will advance our ability to predict the effects of pharmacological agents targeting α5-GABAARs, which have shown therapeutic potential in preclinical animal models.

The Role of Dendritic Brain-Derived Neurotrophic Factor Transcripts on Altered Inhibitory Circuitry in Depression.

BACKGROUND: A parallel downregulation of brain-derived neurotrophic factor (BDNF) and somatostatin (SST), a marker of inhibitory gamma-aminobutyric acid interneurons that target pyramidal cell dendrites, has been reported in several brain areas of subjects with major depressive disorder (MDD). Rodent genetic studies suggest that they are linked and that both contribute to the illness. However, the mechanism by which they contribute to the pathophysiology of the illness has remained elusive. METHODS: With quantitative polymerase chain reaction, we determined the expression level of BDNF transcript variants and synaptic markers in the prefrontal cortex of patients with MDD and matched control subjects (n = 19/group) and of C57BL/6J mice exposed to chronic stress or control conditions (n = 12/group). We next suppressed Bdnf transcripts with long 3' untranslated region (L-3'-UTR) using short hairpin RNA and investigated changes in cell morphology, gene expression, and behavior. RESULTS: L-3'-UTRs containing BDNF messenger RNAs, which migrate to distal dendrites of pyramidal neurons, are selectively reduced, and their expression was highly correlated with SST expression in the prefrontal cortex of subjects with MDD. A similar downregulation occurs in mice submitted to chronic stress. We next show that Bdnf L-3'-UTR knockdown is sufficient to induce 1) dendritic shrinkage in cortical neurons, 2) cell-specific MDD-like gene changes (including Sst downregulation), and 3) depressive- and anxiety-like behaviors. The translational validity of the Bdnf L-3'-UTR short hairpin RNA-treated mice was confirmed by significant cross-species correlation of changes in MDD-associated gene expression. CONCLUSIONS: These findings provide evidence for a novel MDD-related pathological mechanism linking local neurotrophic support, pyramidal cell structure, dendritic inhibition, and mood regulation.

The Relative Contributions of Cell-Dependent Cortical Microcircuit Aging to Cognition and Anxiety.

BACKGROUND: Aging is accompanied by altered thinking (cognition) and feeling (mood), functions that depend on information processing by brain cortical cell microcircuits. We hypothesized that age-associated long-term functional and biological changes are mediated by gene transcriptomic changes within neuronal cell types forming cortical microcircuits, namely excitatory pyramidal cells (PYCs) and inhibitory gamma-aminobutyric acidergic neurons expressing vasoactive intestinal peptide (Vip), somatostatin (Sst), and parvalbumin (Pvalb). METHODS: To test this hypothesis, we assessed locomotor, anxiety-like, and cognitive behavioral changes between young (2 months of age, n = 9) and old (22 months of age, n = 12) male C57BL/6 mice, and performed frontal cortex cell type-specific molecular profiling, using laser capture microscopy and RNA sequencing. Results were analyzed by neuroinformatics and validated by fluorescent in situ hybridization. RESULTS: Old mice displayed increased anxiety and reduced working memory. The four cell types displayed distinct age-related transcriptomes and biological pathway profiles, affecting metabolic and cell signaling pathways, and selective markers of neuronal vulnerability (Ryr3), resilience (Oxr1), and mitochondrial dynamics (Opa1), suggesting high age-related vulnerability of PYCs, and variable degree of adaptation in gamma-aminobutyric acidergic neurons. Correlations between gene expression and behaviors suggest that changes in cognition and anxiety associated with age are partly mediated by normal age-related cell changes, and that additional age-independent decreases in synaptic and signaling pathways, notably in PYCs and somatostatin neurons, further contribute to behavioral changes. CONCLUSIONS: Our study demonstrates cell-dependent differential vulnerability and coordinated cell-specific cortical microcircuit molecular changes with age. Collectively, the results suggest intrinsic molecular links among aging, cognition, and mood-related behaviors, with somatostatin neurons contributing evenly to both behavioral conditions.

Laminar Distribution of Subsets of GABAergic Axon Terminals in Human Prefrontal Cortex.

In human prefrontal cortex (PFC), ~85% of γ-aminobutyric acid (GABA)-expressing neurons can be subdivided into non-overlapping groups by the presence of calbindin (CB), calretinin (CR) or parvalbumin (PV). Substantial research has focused on the differences in the laminar locations of the cells bodies of these neurons, with limited attention to the distribution of their axon terminals, their sites of action. We previously reported that in non-human primates subtypes of these cells are distinguishable by differences in terminal protein levels of the GABA synthesizing enzymes glutamic acid decarboxylase 65 (GAD65) and GAD67. Here we used multi-label fluorescence microscopy in human PFC to assess: (1) the laminar distributions of axon terminals containing CB, CR, or PV; and (2) the relative protein levels of GAD65, GAD67 and vesicular GABA transporter (vGAT) in CB, CR and PV terminals. The densities of the different CB, CR and PV terminal subpopulations differed across layers of the PFC. PV terminals comprised two subsets based on the presence of only GAD67 (GAD67+) or both GADs (GAD65/GAD67+), whereas CB and CR terminals comprised three subsets (GAD65+, GAD67+, or GAD65/GAD67+). The densities of the different CB, CR and PV GAD terminal subpopulations also differed across layers. Finally, within each of the three calcium-binding protein subpopulations intra-terminal protein levels of GAD and vGAT differed by GAD subpopulation. These findings are discussed in the context of the laminar distributions of CB, CR and PV cell bodies and the synaptic targets of their axons.

Shifting priorities: highly conserved behavioral and brain network adaptations to chronic stress across species.

Parallel clinical and preclinical research have begun to illuminate the biological basis of stress-related disorders, including major depression, but translational bridges informing discrete mechanistic targets for intervention are missing. To address this critical need, we used structural MRI in a mouse model and in a large human sample to examine stress effects on brain structure that may be conserved across species. Specifically, we focused on a previously unexplored approach, whole-brain structural covariance, as it reflects synchronized changes in neuroanatomy, potentially due to mutual trophic influences or shared plasticity across regions. Using the unpredictable chronic mild stress (UCMS) paradigm in mouse we first demonstrate that UCMS-induced elevated behavioral emotionality correlates with increased size of the amygdala and other corticolimbic regions. We further identify focal increases in the amygdala's 'hubness' (degree and strength) set against the background of a global stress-related loss of network clustering and modularity. These macroscopic changes are supported on the molecular level by increased postsynaptic density-95 protein in the amygdala, consistent with stress-induced plastic changes and synaptic strengthening. Finally, we provide clinical evidence that strikingly similar structural network reorganization patterns exist in young adults reporting high childhood trauma and increased mood symptoms. Collectively, we provide initial translational evidence for a conserved stress-related increase in amygdala-centered structural synchrony, as measured by enhanced structural covariance, which is paralleled by a decrease in global structural synchrony. This putative trade-off reflected in increased amygdala-centered plastic changes at the expense of global structural dedifferentiation may represent a mechanistic pathway for depression and related psychopathology.

Fluorescence-based cell-specific detection for laser-capture microdissection in human brain.

Cell-specific molecular investigations of the human brain are essential for understanding the neurobiology of diseases, but are hindered by postmortem conditions and technical challenges. To address these issues we developed a multi-label fluorescence in situ hybridization protocol and a novel optical filter device to identify cell types and control for tissue autofluorescence. We show that these methods can be used with laser-capture microdissection for human brain tissue cell-specific molecular analysis.

Alterations in a Unique Class of Cortical Chandelier Cell Axon Cartridges in Schizophrenia.

BACKGROUND: The axons of chandelier cells (ChCs) target the axon initial segment of pyramidal neurons, forming an array of boutons termed a cartridge. In schizophrenia, the density of cartridges detectable by gamma-aminobutyric acid (GABA) membrane transporter 1 immunoreactivity is lower, whereas the density of axon initial segments detectable by immunoreactivity for the α2 subunit of the GABAA receptor is higher in layers 2/superficial 3 of the prefrontal cortex. These findings were interpreted as compensatory responses to lower GABA levels in ChCs. However, we recently found that in schizophrenia, ChC cartridge boutons contain normal levels of the 67 kDa isoform of glutamic acid decarboxylase (GAD67) protein, the enzyme responsible for GABA synthesis in these boutons. To understand these findings we quantified the densities of ChC cartridges immunoreactive for vesicular GABA transporter (vGAT+), which is present in all cartridge boutons, and the subset of cartridges that contain calbindin (CB+). METHODS: Prefrontal cortex tissue sections from 20 matched pairs of schizophrenia and unaffected comparison subjects were immunolabeled for vGAT, GAD67, and CB. RESULTS: The mean density of vGAT+/CB+ cartridges was 2.7-fold higher, exclusively in layer 2 of schizophrenia subjects, whereas the density of vGAT+/CB- cartridges did not differ between subject groups. Neither vGAT, CB, or GAD67 protein levels per ChC bouton nor the number of boutons per cartridge differed between subject groups. CONCLUSIONS: Our findings of a greater density of CB+ ChC cartridges in prefrontal cortex layer 2 from schizophrenia subjects suggests that the normal developmental pruning of these cartridges is blunted in the illness.

Markedly Lower Glutamic Acid Decarboxylase 67 Protein Levels in a Subset of Boutons in Schizophrenia.

BACKGROUND: Convergent findings indicate that cortical gamma-aminobutyric acid (GABA)ergic circuitry is altered in schizophrenia. Postmortem studies have consistently found lower levels of glutamic acid decarboxylase 67 (GAD67) messenger RNA (mRNA) in the prefrontal cortex (PFC) of subjects with schizophrenia. At the cellular level, the density of GABA neurons with detectable levels of GAD67 mRNA is ~30% lower across cortical layers. Knowing how this transcript deficit translates to GAD67 protein levels in axonal boutons is important for understanding the impact it might have on GABA synthesis. In addition, because reductions in GAD67 expression before, but not after, the maturation of GABAergic boutons results in a lower density of GABAergic boutons in mouse cortical cultures, knowing if GABAergic bouton density is altered in schizophrenia would provide insight into the timing of the GAD67 deficit. METHODS: PFC tissue sections from 20 matched pairs of schizophrenia and comparison subjects were immunolabeled for the vesicular GABA transporter (vGAT) and GAD67. RESULTS: vGAT+ bouton density did not differ between subject groups, consistent with findings that vGAT mRNA levels are unaltered in the illness and confirming that the number of cortical GABAergic boutons is not lower in schizophrenia. In contrast, in schizophrenia subjects, the proportion of vGAT+ boutons with detectable GAD67 levels (vGAT+/GAD67+ boutons) was 16% lower and mean GAD67 levels were 14% lower in the remaining vGAT+/GAD67+ boutons. CONCLUSIONS: Our findings suggest that GABA production is markedly reduced in a subset of boutons in the PFC of schizophrenia subjects and that this reduction likely occurs after the maturation of GABAergic boutons.