Energy deficit in parvalbumin neurons leads to circuit dysfunction, impaired sensory gating and social disability
Introduction
GABAergic parvalbumin interneurons (PV INs) display fast-spiking action potentials and innervate the somata and axonal initial segments of their postsynaptic targets (Ascoli et al., 2008). These properties make them potent regulators of neuronal circuits (Woodruff and Yuste, 2008), and PV cortical INs (cINs) are vital for the balance of excitation and inhibition in the cortex (Inan et al., 2013, Rubenstein and Merzenich, 2003). Aberrant excitation/inhibition (E/I) balance in cortical microcircuits has been linked to neuropsychiatric disorders such as autism spectrum disorders (ASDs) and schizophrenia (Lisman, 2012, Uhlhaas and Singer, 2015, Zikopoulos and Barbas, 2013). Disruption of the E/I balance via solely enhancing excitation in the medial prefrontal cortex (mPFC) by optogenetic stimulation in freely behaving mice impaired their sociability (Yizhar et al., 2011), a phenotypic hallmark of both ASD and schizophrenia (Couture et al., 2010, Kaidanovich-Beilin et al., 2011, Sugranyes et al., 2011). This impairment in sociability was rescued (at least partially) by the concurrent optogenetic activation of PV cINs (Yizhar et al., 2011). Additional studies have demonstrated that deficits in PV IN function are associated with behavioral defects of both ASD and schizophrenia, including abnormal sociability and sensory gating (Barnes et al., 2015, Cho et al., 2015, Gogolla et al., 2009, Lewis et al., 2012). These results emphasize the importance of E/I balance in establishing social behavior, and shed light onto the pathophysiology underlying neuropsychiatric disorders.
Behavioral impairments in neuropsychiatric disorders and in animal models of these disorders have also been associated with altered network oscillations in the gamma range (30–80 Hz) (Kikuchi et al., 2011, Kogan et al., 2015, Lodge et al., 2009, Orekhova et al., 2007, Rubenstein, 2010, Spencer, 2008, Spencer, 2011, Spencer et al., 2003, Spencer et al., 2004, Wilson et al., 2007). GABAergic transmission is required for the coordination of network oscillations through synchronization of cortical circuits by generating a narrow window for effective excitation (Szabadics et al., 2001, Támas et al., 2000, Uhlhaas and Singer, 2010). In particular, hippocampal and neocortical PV INs are required for synchronization of the high frequency gamma oscillations (Lewis et al., 2012, Sohal et al., 2009, Uhlhaas and Singer, 2010), and loss of PV cINs in the mPFC correlates with reduced gamma-band response in a mouse model of schizophrenia (Lodge et al., 2009).
Both gamma oscillations and the fast-spiking firing properties of PV INs have high energy demands (Carter and Bean, 2009, Carter and Bean, 2011). The fast-spiking property of PV INs, but not the firing of excitatory pyramidal neurons, was compromised in the presence of mitochondrial respiratory chain inhibitors (Whittaker et al., 2011). Additionally, hippocampal PV INs were reported to have higher mitochondrial content compared to other neurons (Gulyas et al., 2006). Together, these observations suggest that PV INs rely heavily on mitochondrial energy metabolism, and it has been suggested that mitochondrial dysfunction in PV INs could result in abnormalities of gamma oscillations and consequently in impairment of complex information processing (Kann et al., 2014). However, the in vivo cellular and behavioral effects of mitochondrial dysfunction specifically in PV cINs are still unknown.
Mitochondrial dysfunction has also been linked to ASD and schizophrenia (Hjelm et al., 2015, Legido et al., 2013, Rajasekaran et al., 2015, Rossignol and Frye, 2014, Toker and Agam, 2015). Therefore, we hypothesized that a mitochondrial defect that is restricted to PV INs in mice may result in phenotypes that resemble abnormalities seen in individuals with ASD and schizophrenia. To investigate the consequences of mitochondrial dysfunction in PV cINs in vivo, we generated conditional knockout (CKO) mice by eliminating cox10 expression in PV cells (PV-Cox10 CKO). Cox10 is a heme farnesyl transferase required for the assembly of cytochrome oxidase (COX), the terminal enzyme of the mitochondrial electron transfer chain. In earlier studies, conditional deletion of cox10 in forebrain excitatory neurons resulted in a slowly progressive neurodegenerative phenotype (Diaz et al., 2012), suggesting that this is a viable approach to induce a progressive decline of oxidative phosphorylation (OXPHOS) in neurons. Using PV-Cox10 CKO mice, we examined how mitochondrial impairment in PV cINs affects their intrinsic physiology, connectivity at single-cell as well as network levels, and mouse behavior. Since impaired sociability is a hallmark of ASDs and schizophrenia, this behavioral aspect was explicitly tested using the three-chamber sociability task. Furthermore, pre-pulse inhibition (PPI) of startle reflex was used as a model to study sensorimotor gating and its deficits in neuropsychiatric disorders, such as schizophrenia.
Section snippets
Mice
Knock-in mice carrying floxed alleles of cox10 (cox10f/f) were a generous gift from Dr. Carlos Moraes at the University of Miami. These mice were crossed with the pvalbCre/+ knock-in line (Jackson Laboratories Stock no: 008069). The progeny of this mating were then inter-crossed to generate pvalbCre/+::cox10f/f (PV-Cox10 conditional KO, CKO) mice. Wild-type (pvalb+/+::cox10f/f) littermates were used as controls in order to avoid any confounding effect due to the loss of one copy of pvalb or
Progressive loss of COX in PV-Cox10 CKO mice
We first examined the COX content in different cIN subgroups of wild type mice at P60 using Cox1 immunolabeling. Cox1 is the catalytic subunit of COX, encoded by the mitochondrial DNA (Diaz et al., 2005, Mogi et al., 1994, Soto et al., 2012). Cox1 is a viable marker of COX content because the COX holoenzyme cannot be formed in the absence of Cox1 (Dennerlein and Rehling, 2015, Khalimonchuk and Rödel, 2005, Lemaire et al., 1998, Nijtmans et al., 1998). In PV-Cox10 CKO mice, the heme of Cox1
Discussion
In this study, we investigated the effect of mitochondrial dysfunction in PV cINs by generating PV-Cox10 CKO mice. As predicted, elimination of cox10 led to progressive reduction of Cox1 expression, as Cox1 cannot be assembled into COX holoenzyme in the absence of Cox10 and is degraded. Since Cox1 is the catalytic subunit of COX and is required for the assembly of this holoenzyme, it is very likely that PV cells in the PV-Cox10 CKO mice had a progressive OXPHOS deficit (Diaz et al., 2012).
Funding
This work was supported by Autism Speaks Translational Postdoctoral Fellowship (to M.I.), CTSC UL1 RR 024996 Pilot Grant (to M.Z.) and Cornell University Ithaca-WCMC seed grant (to M.Z.).
Acknowledgments
We thank Dr. Gareth Tibbs and Dr. Tim Petros for their valuable comments on the manuscript and the Departments of Anesthesiology and BMRI, WCMC for support.
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2022, Biological PsychiatryCitation Excerpt :Our cell type–specific studies in SZ revealed larger alterations in mitochondrial-related gene expression in layer 3 PNs than in PVBCs (121), whereas a primary problem in mitochondrial energy production might have produced the opposite findings, given that PVBCs are more active and require greater adenosine triphosphate production than PNs (122–126). Genetic manipulations that impaired mitochondrial function in PV neurons were associated with greater excitatory inputs to PV neurons and greater gamma band power (127), the opposite of findings in SZ (96,128). Because PVBCs in cortical layers 3 and 4 are a major recipient of excitatory inputs from layer 3 PNs (129), hypoactivity of layer 3 PNs in SZ would be expected to produce activity-dependent alterations in PVBCs, such as lower levels of PV and GAD67.