Expression pattern of membrane-associated guanylate kinases in interneurons of the visual cortex

In the cortex, a variety of biochemically, morphologically, and functionally distinct subtypes of GABAergic interneurons regulate the activity of local microcircuits (Markram et al., 2004; Silberberg et al., 2005). The excitatory glutamatergic input onto cortical interneurons is a key determinant of their function and can be altered with activity (Kullmann and Lamsa, 2007) and development (Hennou et al., 2002). In addition, abnormal inhibitory (GABAergic) circuits are implicated in certain psychiatric and neurodevelopmental diseases, such as schizophrenia and autism (Di Cristo, 2007; Lisman et al., 2008). Schizophrenia, for example, may reflect in part a decreased glutamatergic innervation and NMDAR expression on GABAergic interneurons in the prefrontal cortex (Lisman et al., 2008). Still, despite their important role in regulating GABAergic activity, the molecular structure, development, and dynamics of glutamatergic synapses on inter-neurons are poorly defined. Although glutamatergic synapses on interneurons may share key features and mechanisms with pyramidal neurons, GABAergic inter-neuron subtypes often express distinct glutamate receptor subunits (e.g., only Ca2+-permeable AMPAR subunits and the NMDAR GluN2C subunit; Geiger et al., 1995; Angulo et al., 1997; Moga et al., 2002; Martina et al., 2003), can lack spines (Kawaguchi and Kubota, 1993; Goldberg et al., 2003; Goldberg and Yuste, 2005), and can show distinct features of synaptic plasticity (Kullmann and Lamsa, 2007; Nissen et al., 2010). Membrane-associated guanylate kinases (MAGUKs) are scaffolding proteins and central organizers of glutamatergic synapses. The MAGUK family has four members that can be expressed in the postsynaptic density (PSD) of excitatory synapses: PSD-95, PSD-93, SAP97, and SAP102 (Funke et al., 2005; Sheng and Hoogenraad, 2007). These proteins share high sequence similarity as well as similar domain structure: three PDZ domains followed by an SH3 and a GK domain (Funke et al., 2005; Sheng and Hoogenraad, 2007). In general, these PSD proteins have been implicated in trafficking, clustering, and regulating GluRs (Elias and Nicoll, 2007; Newpher and Ehlers, 2008; Robertson et al., 2009), as intracellular adaptor proteins (Funke et al., 2005), and as elements in brain dysfunction (Gardoni, 2008). They interact with different GluR subunits (Leonard et al., 1998; Kim and Sheng, 2004), GluR-interacting proteins (see, e.g., Chen et al., 2000; Bats et al., 2007), and cytoplasmic proteins (Sheng and Hoogenraad, 2007). Accordingly, they have different functions at specific synapses (Regalado et al., 2006; Elias et al., 2008). In addition, EM studies suggest preferential localization of MAGUK members in different excitatory synapses and subsynaptic compartments (synaptic vs. extrasynaptic; Aoki et al., 2001). All members of the MAGUK family are expressed in the mouse and rat brain, including the cortex (Kim et al., 1996; Allen Brain Atlas; http://mouse.brain-map.org/). However, they show strong developmental and brain-region- and cell-type-specific expression patterns (Fukaya et al., 1999; Fukaya and Watanabe, 2000; Aoki et al., 2001). In addition, although their cell-type-specific expression can be defined in certain brain regions with well-defined structure (e.g., cerebellum; http://mouse.brain-map.org/; Muller et al., 1995; Brenman et al., 1996; Chetkovich et al., 2002), this is not possible in the cortex, where pyramidal neurons and a variety of interneuron subtypes are mixed together. In the cortex, the two most prominent interneuron subtypes are parvalbumin (PV)- and somatostatin (SOM)-positive. These nonoverlapping interneuron subtypes of the cortex differ not only in terms of biochemical markers but also in morphology, membrane properties, synaptic targeting, and subcellular architecture (Markram et al., 2004; Burkhalter, 2008). PV+ interneurons are mainly multipolar basket cells that are fast-spiking and that target exclusively perisomatic domains of pyramidal neurons, having a strong influence on action potential generation (Freund, 2003; Freund and Katona, 2007). On the other hand, SOM+ interneurons are bitufted Martinotti cells that are regular-spiking and that target mainly more distal dendrites, influencing primarily synaptic integration (Thomson and Bannister, 2003; Markram et al., 2004). Notably, glutamatergic synapses on PV+ interneurons are axodendritic (aspiny), whereas on SOM+ interneurons they are axospinous (spiny; Kawaguchi and Kubota, 1993; Goldberg and Yuste, 2005). In addition, although the identity of GluR subtypes is not well-defined on these cells, they both express Ca2+-permeable AMPARs (Geiger et al., 1995; Moga et al., 2002). Given the importance of MAGUKs in organizing and regulating pyramidal-to-pyramidal synapses (Montgomery et al., 2004; Sheng and Hoogenraad, 2007), we defined their cell-type-specific expression profiles in PV+ and SOM+ interneurons as a function of developmental age. To do so, we used a combination of in situ hybridization and immunohistochemistry to characterize the mRNA expression pattern of MAGUKs in these interneurons in the visual cortex. Surprisingly, we find that the four MAGUKs show a comparable expression pattern in the two interneuron subtypes. However, SAP97 was developmentally regulated, with a subset of both PV+ and SOM+ interneurons showing greatly decreased expression in adult. The identity of these interneuron subsets and the functional consequences of SAP97 absence in adults remain unknown.