Parvalbumin interneuron deficits in schizophrenia

Parvalbumin-expressing (PV + ) interneurons represent one of the most abundant subclasses of cortical interneurons. Owing to their specific electrophysiological and synaptic properties, PV + interneurons are essential for gating and pacing the activity of excitatory neurons. In particular, PV + interneurons are critically involved in generating and maintaining cortical rhythms in the gamma frequency, which are essential for complex cognitive functions. Deficits in PV + interneurons have been frequently reported in postmortem studies of schizophrenia patients, and alterations in gamma oscillations are a prominent electrophysiological feature of the disease. Here, I summarise the main features of PV + interneurons and review clinical and preclinical studies linking the developmental dysfunction of cortical PV + interneurons with the pathophysiology of schizophrenia.


Introduction
Schizophrenia is a complex and heterogeneous disorder generally characterised by recurrent psychosis, deterioration of social life, and cognitive dysfunction (Carpenter and Buchanan, 1994;Howes and Murray, 2014;Kahn et al., 2015).Schizophrenia symptoms are often classified into positive, negative, and cognitive categories, although they are by no means exclusive to the disorder.Positive symptoms are linked to recurrent episodes of psychosis driven by hallucinations and delusions and characterised by loss of contact with reality, disorganised speech, and agitated behaviour.Negative symptoms are often pervasive and include social withdrawal, affective flattening, anhedonia, and lack of initiative.Schizophrenia is also often associated with broad cognitive deficits impacting executive functions, including working memory, cognitive flexibility, planning, and metacognition.
Schizophrenia is diagnosed following the recurrence of psychotic episodes, generally during late adolescence, but the onset of the illness may precede the first psychotic episode by more than ten years.Indeed, cognitive development is compromised early in life in patients later diagnosed with schizophrenia (Kahn and Keefe, 2013), and differences have also been reported for critical neurodevelopmental milestones and associated behaviours (Hyde et al., 2008;Jääskeläinen et al., 2008;Sørensen et al., 2010).These observations are consistent with the prevailing hypothesis that schizophrenia is caused by changes in the typical trajectory of brain development (Murray and Lewis, 1987;Weinberger, 1987), which progressively lead to alterations in brain function and behaviour.Indeed, many of the genes that have been consistently associated with schizophrenia are expressed early in life and control critical aspects of brain development (Pocklington et al., 2015;Ripke et al., 2014;Singh et al., 2022;Trubetskoy et al., 2022), and many epidemiological risk factors are linked to early life events, such as obstetric complications and childhood adversity (Cannon et al., 2002;Stilo and Murray, 2010).
Ultimately, schizophrenia is the result of altered development trajectories that affect how the brain processes information and adapts to life experiences.However, identifying consistent pathophysiological changes in the brains of schizophrenia patients has proven difficult.Gross brain pathology is not a characteristic of schizophrenia.Instead, postmortem studies suggest alterations in synaptic connectivity and the functional state of specific neuronal and glial cell populations.Although postmortem studies are limited by a myriad of epiphenomena (e.g., treatment, chronicity, comorbidities), several observations have been replicated across multiple studies.For instance, multiple studies have found structural alterations in the dendrites of glutamatergic neurons across multiple regions of the cerebral cortex in schizophrenia patients, including diminished dendrite length and complexity and reduced spine density (Hu et al., 2015).Another highly replicated finding in the brain of schizophrenia patients is a reduction in the expression of one of the biosynthetic enzymes of GABA, glutamate acid decarboxylase 67 (GAD67), along with other GABAergic markers (Dienel and Lewis, 2019).Postmortem evidence of alterations in the dopaminergic system in patients with schizophrenia has been more inconsistent.However, data from positron emission tomography (PET) neuroimaging studies have repeatedly found increased striatal dopamine uptake in schizophrenia patients (Kesby et al., 2018;McCutcheon et al., 2019).
Positive symptoms in schizophrenia are generally associated with the presynaptic dysregulation of the dopaminergic modulation of striatal function, likely caused by abnormal activation of midbrain dopaminergic neurons (Howes et al., 2012;Lyon et al., 2011;Murray et al., 2008).Striatal dopaminergic dysfunction has been linked with psychosis-driven agitation and aberrant salience processing, but it is unclear how it may relate to the emergence of hallucinations.Abnormalities linked to negative symptoms may also be related to reward processing and emotional regulation.For example, ventral striatal responses to reward are consistently reduced in schizophrenia (Juckel et al., 2006), and deficits in the function of the amygdala have also been reported (Aleman and Kahn, 2005).Finally, schizophrenia is associated with broad cognitive impairment, including working and episodic memory, cognitive flexibility, and executive functions such as planning (Kahn and Keefe, 2013;McCutcheon et al., 2023).Defects in cognitive function have been primarily associated with disruption of cortical circuits in certain areas, most notably the dorsolateral prefrontal cortex, cingulate cortex, and hippocampal formation (Achim and Lepage, 2005;Fusar-Poli et al., 2007;Meyer-Lindenberg et al., 2005;Weinberger et al., 1986).Because working memory and other high-order cognitive functions require specific activity patterns across many of these regions (Goldman-Rakic, 1995), the mechanisms mediating the emergence of such patterns are likely key to understanding the neurobiological basis of schizophrenia.

Cortical PVþ interneurons
Information processing in the cerebral cortex critically depends on the precise spatial and temporal coordination of the activity of glutamatergic principal cells (also known as pyramidal cells) and GABAergic interneurons.Pyramidal cell activity is controlled by a rich diversity of interneurons, which control different temporal dynamics, the formation of neuronal ensembles, network oscillations, and brain states (Kepecs and Fishell, 2014;Klausberger and Somogyi, 2008).
Fast-spiking expressing the calcium-binding protein parvalbumin (PV+) cells are among the most abundant cortical interneurons.They have unique electrophysiological characteristics associated with high energy expenditure, including fast action potential kinetics and ion conductances (Table 1).In the context of this review, fast-spiking PV+ interneurons will primarily refer to PV+ basket cells.However, other interneurons, like chandelier cells and translaminar interneurons, also have fast-spiking characteristics and often express PV (Lim et al., 2018).
PV+ interneurons can generate action potentials at high frequencies (>100 Hz) for prolonged periods with weak or no accommodation (Hu et al., 2014).Several key features support the fast-spiking behaviour of these cells.For example, their intrinsic membrane properties show resonance in the gamma-frequency band, and their 'resting' membrane potential is about 10-15 mV closer to the spike threshold than in pyramidal cells.In addition, their action potential has very high conduction velocity due to the high density of voltage-gated Na + channels in the axon.The soma and dendrites of PV+ basket cells are densely covered with excitatory synapses (Gulyás et al., 1999), which facilitates the generation of fast excitatory postsynaptic potentials.
There are additional features that are characteristic of cortical PV+ interneurons.Most PV+ basket cells are enwrapped by a condensed form of extracellular matrix known as perineuronal nets (Fawcett et al., 2019).These structures regulate the contribution of PV+ interneurons to developmental critical periods and synaptic plasticity (Hensch, 2005;Reichelt et al., 2019).In addition, PV+ basket cells contain considerably higher numbers of mitochondria than other interneurons and pyramidal cells.Mitochondria in PV+ basket cells are also larger and enriched with proteins crucial for the electron transport chain, such as cytochrome c oxidase (complex IV) and cytochrome c (Fitzgerald et al., 2012;Gulyás et al., 2006;Takács et al., 2015).
The axons of PV+ basket cells are extensively arborised and make abundant perisomatic synapses on pyramidal cells (Freund and Buzsáki, 1996;Klausberger and Somogyi, 2008).This characteristic allows PV+ interneurons to tightly control the output of a local network by suppressing the generation of action potentials in principal cells.PV+ basket cells also exhibit mutual inhibitory innervation (Avermann et al., 2012;Bezaire and Soltesz, 2013;Galarreta and Hestrin, 1999;Gibson et al., 1999;Somogyi et al., 1998) and are frequently self-connected by autapsessynapses that a neuron makes onto itselfwhich allow them to adjust their temporal firing interval with extreme accuracy (Deleuze et al., 2019;Szegedi et al., 2020).Notably, the axons of PV+ basket cells are intermittently myelinated, a rather exceptional feature among cortical GABAergic interneurons in rodents and humans (Micheva et al., 2016;Stedehouder et al., 2017).Myelin function in PV+ cells remains unclear, but it has been suggested that myelin may provide trophic support to mitochondria in the axons of these cells to sustain high-frequency firing (Kole et al., 2022;Micheva et al., 2016).
The ability of PV+ interneurons to generate action potentials at high frequencies and their dense interconnectivity with pyramidal cells allows them to modulate network oscillations in the gamma frequency (~30-80 Hz).Synchronous oscillations in neuronal activity in various frequency bands are essential for information processing and enable the coordinated function of different cortical areas.Gamma oscillations specifically increase during tasks requiring complex processing of sensory information, attention, and working and episodic memory (Fries, 2009;Griffiths and Jensen, 2023;Ni et al., 2016;Uhlhaas and Singer, 2012), and fast-spiking interneurons are uniquely required for their generation and maintenance (Buzsáki and Wang, 2012;Cardin et al., 2009;Sohal et al., 2009).

Developmental and metabolic vulnerability of PVþ interneurons
PV+ interneurons follow a very protracted period of maturation that enables the fast-spiking properties of these cells (Doischer et al., 2008;Goldberg et al., 2011).This process occurs concurrently with an increase in the density of glutamate synapses received by these cells (Miyamae et al., 2017), suggesting that excitatory inputs shape the maturation of PV+ basket cells.Indeed, PV levels correlate with the degree of  (Chung et al., 2017).Perineuronal nets are also strongly regulated during development, increasing with the neurochemical and physiological maturation of PV+ fast-spiking interneurons (Condé et al., 1996;Nowicka et al., 2009;Reynolds and Beasley, 2001;Rio et al., 1994;Rogers et al., 2018;Ye and Miao, 2013).Recent experiments suggest that early network dynamics in the mouse postnatal cortex prevent the premature maturation of PV+ interneurons (Mòdol et al., 2023).In the adult cortex, the density of glutamatergic synapses targeting PV+ interneurons predicts cellular PV levels in both mice and humans (Chung et al., 2016;Donato et al., 2013).Because PV+ basket cells might not fully mature in rodents and primates until puberty (Fung et al., 2010;Miyamae et al., 2017), their contribution to cognitive function during adolescence and early adult life might be significantly different than in adulthood.
The protracted development of PV+ basket cells and the sensitivity of this process to neuronal activity make these cells particularly susceptible to schizophrenia risk factors, such as early life adversity and social isolation.For example, early life stress affects the development of PV+ interneurons in the mouse prefrontal cortex (Goodwill et al., 2018).The function of PV+ interneurons in the adult cortex is also particularly susceptible to changes in neuronal activity during adolescence (Canetta et al., 2022).Consistently, social interactions during this period strongly influence the function of cortical PV+ interneurons in mice (Jeon et al., 2023).
PV+ interneurons have considerable metabolic demands due to their electrophysiological characteristics.Energy is required for the dissipation of the strong ion gradients caused by the abundant excitatory and inhibitory inputs PV+ basket cells receive, their ability to sustain highfrequency firing and contribute to gamma oscillations, and the sizeable quantal size and high probability of GABA release of these cells (Kann, 2016).PV+ interneurons can meet these high energy demands due to their elevated mitochondrial content, but this also renders these cells more susceptible to oxidative stress and alterations of mitochondrial function.

Cortical PV interneuron deficits in schizophrenia
Although GABAergic deficits in schizophrenia may implicate several types of interneurons, the reduction in specific markers suggests a prominent involvement of fast-spiking basket cells in the pathophysiology of the disorder (Table 2).For instance, the levels of GAD67, PV, and individual components of perineuronal nets are specifically reduced in PV+ basket cells in schizophrenia patients across multiple cortical areas (Fung et al., 2010;Glausier et al., 2014;Hashimoto et al., 2008Hashimoto et al., , 2003;;Mauney et al., 2013;Zhang and Reynolds, 2002).Importantly, this reduction is not due to a decrease in the density of PV+ interneurons but rather in the levels of these critical proteins (Enwright et al., 2016;Hashimoto et al., 2003).Because the expression of PV and GAD67 is activity-dependent, it has been suggested that a reduction in the excitatory drive of PV+ basket cells may account for these observations.In agreement with this idea, the density of excitatory synapses contacting PV+ interneurons in the prefrontal cortex is lower in schizophrenia patients than in controls (Chung et al., 2016).
The reduction in the levels of PV and GAD67 in schizophrenia has been interpreted as a reduction in the activity of PV+ interneurons because the expression of these markers is strongly linked to the activity of PV+ basket cells.Since the inhibition of pyramidal cells by PV+ basket cells is required for the generation of gamma oscillations (Metzner et al., 2016;Vierling-Claassen et al., 2008), a reduction in the activity of these interneurons should lead to deficits in neural synchrony in the cerebral cortex.Consistently, abnormalities in gamma frequency oscillations have been described across multiple cortical areas in schizophrenic patients (Cho et al., 2006;Gallinat et al., 2004;Gonzalez-Burgos et al., 2015;Kikuchi et al., 2011;Kwon et al., 1999;Light et al., 2006;Uhlhaas and Singer, 2010).For example, the power of gamma oscillations in the prefrontal cortex increases during working memory tasks in healthy volunteers but not in schizophrenia patients (Cho et al., 2006).Oscillatory activity can also be probed in humans independently of a task, for instance, by recording steady-state responses to trains of sensory stimuli at different frequencies.Schizophrenia patients consistently exhibit reduced power and phase delay to 40-Hz auditory stimulation (Kwon et al., 1999;Thuné et al., 2016), suggesting that deficits in neural synchrony in the gamma range are pervasive in schizophrenia.Significantly, drug-naïve patients with first-episode psychosis may have increased resting state gamma power across multiple cortical regions (Reilly et al., 2018;Tikka et al., 2013;Yadav et al., 2021).
Mitochondrial function has long been suggested to influence the development and course of schizophrenia (Clay et al., 2011;Cuenod et al., 2022;Hjelm et al., 2015).Mitochondria play critical roles in cellular bioenergetics, calcium homeostasis, redox signalling, and apoptosis (Friedman and Nunnari, 2014).Postmortem studies have identified a reduction in the expression of genes encoding mitochondrial complexes in schizophrenia (Arion et al., 2015;Glausier et al., 2020;Karry et al., 2004;Prabakaran et al., 2004), and similar abnormalities have been detected in iPSC-derived cortical neurons from schizophrenia patients (Brennand et al., 2015;Kathuria et al., 2020;Ni et al., 2020).Moreover, excitatory neurons derived from individuals with 22q11.2deletion syndrome and schizophrenia, but not those without schizophrenia, have mitochondrial dysfunction (Li et al., 2021(Li et al., , 2019)).Neurons derived from 22q11.2 deletion syndrome individuals without schizophrenia have increased mitochondrial function compared to control neurons (Li et al., 2021), suggesting that the development of schizophrenia in 22q11.2deletion syndrome patientsand perhaps other individuals at high clinical risk for psychosismight be influenced by compensatory mitochondrial function.This observation is consistent with genome-wide association studies suggesting that genetic variants related to bioenergetics may increase the risk of schizophrenia (Ripke et al., 2014;Trubetskoy et al., 2022).However, alterations in gene expression related to mitochondrial function seem more prominent in pyramidal cells than in PV+ basket cells (Enwright et al., 2017).

Preclinical studies linking PV interneuron dysfunction and schizophrenia
Experiments in mice have demonstrated that disrupting the development of PV+ interneurons can reproduce many pathophysiological

Table 2
Alterations of cortical PV+ interneurons in schizophrenia.

Alteration Type of study Reference
Reduced PV levels Human postmortem Enwright et al., 2016;Fung et al., 2010;Glausier et al., 2014;Hashimoto et al., 2003;Hashimoto et al., 2008 Cho et al., 2006;Gallinat et al., 2004;Kikuchi et al., 2011;Kwon et al., 1999;Light et al., 2006;Reilly et al., 2018;Thuné et al., 2016;Tikka et al., 2013;Yadav et al., 2021Mouse models Carlén et al., 2011;Korotkova et al., 2010;Pino et al., 2013 O. Marín observations made in schizophrenia patients.For instance, work from several laboratories has revealed that signalling downstream of the Erb-B2 receptor tyrosine kinase 4 (ErbB4) controls the formation of excitatory synapses on PV+ interneurons (Exposito-Alonso et al., 2020;Fazzari et al., 2010;Müller et al., 2018;Pino et al., 2013;Ting et al., 2011).ErbB4 activation leads to the expression and local translation of a molecular programme required to form glutamatergic synapses onto PV+ interneurons (Bernard et al., 2022).Consistently, conditional deletion of ErbB4 from PV+ interneurons reduces the density of glutamatergic synapses contacting PV+ basket cells (Pino et al., 2013).This alteration causes a prominent decrease in the expression of PV and GAD67, impairs the recruitment of PV+ interneurons, disrupts gamma oscillations in the hippocampus and prefrontal cortex, and perturbs cognitive function in mice (Pino et al., 2013).Loss of ErbB4 in PV+ interneurons also causes a non-cell autonomous reduction in the density of dendritic spines in pyramidal cells (Pino et al., 2013;Yin et al., 2013), suggesting that this characteristic pathological feature of schizophrenia might beat least theoreticallya secondary adaptation to defective inhibition in the cerebral cortex.Importantly, ErbB4 expression in PV+ interneurons is conserved in rodents and primates, including humans (Neddens et al., 2011).Rare mutations and tandem repeat expansion in ERBB4 have been linked to schizophrenia and intellectual disability (Kasnauskiene et al., 2013;Mojarad et al., 2022;Walsh et al., 2008), and common genetic variation in this locus also seems to be associated with the disorder (Marenco et al., 2011;Pardiñas et al., 2018;Trubetskoy et al., 2022).Dysregulated alternative splicing of ERBB4 has also been reported in the cerebral cortex of schizophrenia patients (Chung et al., 2015).Irrespectively, these studies suggest that the defective development of excitatory synapses contacting PV+ interneurons is a plausible pathophysiological mechanism in schizophrenia.
Mouse genetic studies investigating the function of N-methyl-Daspartate (NMDA) receptors in PV+ interneurons further support the idea that disrupting the excitatory drive these cells receive from pyramidal cells impacts their function in mature circuits.For example, deletion of the mandatory GRIN1 subunit of NMDA receptors from interneurons during early postnatal development results in reduced expression in PV and GAD67, disinhibition of pyramidal cells, and abnormal neural synchrony in the mouse cerebral cortex (Belforte et al., 2010).Although these genetic experiments may involve interneurons other than PV+ basket cells, conditional mutants deleting GRIN1 exclusively from PV+ interneurons also exhibit enhanced baseline gamma rhythms and impaired cognitive function (Carlén et al., 2011;Korotkova et al., 2010).Genetic variation in GRIN1 has not been associated with schizophrenia, but rare mutations in GRIN2A, which encodes another subunit of NMDA receptors, increase the risk of developing schizophrenia by almost 20-fold (Singh et al., 2022).
One important consideration regarding the findings linking defects in the glutamatergic recruitment of PV+ interneurons and the contribution of these cells to the pathophysiology of schizophrenia is timing.Early deletion of ErbB4 and GRIN1 from PV+ interneurons produces molecular and behavioural abnormalities that are not observed if the loss of these proteins occurs after adolescence (Batista-Brito et al., 2023;Belforte et al., 2010;Mallien et al., 2021).Consistently, adult re-expression of ErbB4 only partially reverses the phenotypes observed following the developmental deletion of this protein (Wang et al., 2018).Moreover, the contribution of NMDA currents in immature PV+ interneurons is very significant but subsequently declines during postnatal development until reaching low levels in adulthood (Wang andGao, 2009, 2010).These observations emphasise the unique role of ErbB4 and NMDA receptors in the maturation and wiring of PV+ interneurons and suggest that developmental disruption of cortical inhibitory circuits mediated by PV+ interneurons is central to the pathophysiology of schizophrenia.
Beyond alterations in the afferent connectivity of PV+ interneurons, other mechanisms may account for the dysfunction of these cells in schizophrenia.For instance, genetic and pharmacological manipulations of the redox system significantly impact PV+ interneurons (Cuenod et al., 2022).This observation is consistent with the notion that PV+ interneurons have high energy demands satisfied by oxidative phosphorylation at mitochondria, making these cells particularly vulnerable to redox imbalance.Interestingly, PV+ interneurons are more susceptible to redox dysregulation during early postnatal development rather than later in life (Cabungcal et al., 2013a(Cabungcal et al., , 2014)).This might be due to the immaturity of perineuronal nets, which seem to protect PV+ interneurons from oxidative stress (Cabungcal et al., 2013b).Oxidative stress has been reported in many animal models of neurodevelopmental disorders, leading to the suggestion that redox dysregulation might be upstream of PV+ interneuron deficits in schizophrenia (Steullet et al., 2017).This hypothesis requires additional support from clinical studies.
PV+ interneurons may also be particularly susceptible to myelin pathology due to the small diameter of their axons.In agreement with this notion, experimental demyelination disrupts inhibitory transmission by PV+ interneurons, compromising gamma oscillations (Dubey et al., 2022).Myelin also seems to regulate the spatial distribution of mitochondria in the axons of PV+ basket cells, which tend to accumulate in the myelinated segments (Kole et al., 2022).Intriguingly, disrupting mitochondria trafficking in PV+ interneurons reduces their density in axons and severely impairs gamma oscillations (Kontou et al., 2021).Similarly, experimental demyelination of PV+ interneurons disrupts gamma oscillations in vivo (Dubey et al., 2022).Although myelination deficits have been extensively implicated in schizophrenia (Fitzsimmons et al., 2013), and effect sizes are more significant in the grey matter than in the white matter (Miyakawa et al., 1972;Uranova et al., 2011), specific defects in PV+ interneurons in the cortex of schizophrenia patients have not yet been reported.

Conclusions
The dysfunction of PV+ interneurons is a consistent finding in schizophrenia, but the underlying pathophysiological mechanisms remain ambiguous.Considering the close relationship between pyramidal cells and PV+ interneurons in cortical circuits, it is conceivable that multiple primary alterations can lead to the same changes in GABAergic markers observed in schizophrenia patients.As summarised above, many of the alterations observed in the cortex of schizophrenia patients, including non-cell autonomous defects in pyramidal cells, can be caused by primary alterations in PV+ interneurons.However, it is equally conceivable that all these changes arise secondary to a primary problem in pyramidal cells.For example, genetic variation may primarily reduce glutamatergic transmission among pyramidal cells, and this could, in turn, lead to a secondary reduction in the activity of PV+ basket cells (Dienel et al., 2022).Therefore, there might not be a unique, common pathway leading to the characteristic alterations of the cerebral cortex in schizophrenia patients.Nonetheless, the dysfunction of this critical inhibitory motif seems unequivocally linked with the disruption of neuronal synchrony in the gamma range and the accompanying defects in cognitive function, independently of the causal mechanism.
It is worth emphasising that the mechanisms mediating the onset of psychosis are likely different across patient populations.NMDA receptor antagonists and autoimmune encephalitis caused by antibodies against NMDA receptor subunits can trigger psychosis in adults without a history of developmental problems (Fukata et al., 2018;Moghaddam and Krystal, 2012).In contrast, experiments in mice indicate that alterations in the developmental trajectory of PV+ interneurons likely drive the deficits observed in the adult cortex.It is still unclear why such early developmental problems give rise to cognitive and social problems during childhood and early adolescence and eventually translate into a psychotic disorder in early adulthood.One possibility is that alterations in cortical circuits cause pathological maladaptation in other brain circuits, including the dopaminergic system, which would explain why cognitive deficits predate the onset of psychosis in schizophrenia.Although some experimental evidence supports this idea (Howes and Shatalina, 2022), future experiments should aim to identify a causal link between the alteration of cortical PV+ interneurons and striatal hyperdopaminergia.

Contributions
O.M. wrote this review.

Table 1
Characteristics of cortical PV+ basket cells.