The Journal of Neuroscience, January 25, 2017 • 37(4):739–741 • 739

Journal Club

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The Role of in the Cerebellum Highlights the Diversity of Synapse-Specifying Molecules

X Jessica Messier Department of Neuroscience and Medical Scientist Training Program, Baylor College of Medicine, Houston, Texas 77030, and The Cain Foundation Laboratories, Jan and Dan Duncan Neurological Research Institute, Texas Children’s Hospital, Houston, Texas 77030 Review of Zhang and Su¨dhof

The brain integrates and processes sensory 2012). Many families of cell adhesion mole- mediated excitation in the cortex, hippocam- input to generate outputs that determine cules, including postsynaptic neuroligins, pus, and amygdala; Nlgn2 regulates inhibition our actions, emotions, and thoughts. This cerebellins, calsyntenins, and leucine-rich inthehippocampus,dentategyrus,brainstem, impressive task is accomplished by trans- repeat transmembrane neuronal and cortex, and, at least in the somatos- mitting signals through synapses between (LRRTMs), which can all bind to presynap- ensory cortex, is specific to parvalbumin (Pv)- , which are interconnected in a tic to specify synapses, are in- expressing interneuron-mediated inhibition; highly specific pattern. During the process volved in synapse formation. Many of these and Nlgn3 regulates excitation in the hip- of synapse formation, there first is estab- molecules can undergo , pocampus and cerebellum, but not in the so- lishment of synaptic contacts, followed by leading to altered binding affinities to other matosensory cortex or dorsal striatum (Hu et the assembly of specific presynaptic and adhesion molecules, furthering adding to al., 2015). In addition, Nlgn3 enhances inhibi- postsynaptic molecular machinery and their diversity (de Wit and Ghosh, 2016). tion onto D1, but not D2 dopamine receptor- functional specification to confer specific The large diversity of synaptic cell adhesion expressing medium spiny neurons (MSNs) of properties to the synapse. Functional speci- molecules suggests that they may provide a the dorsal striatum (Rothwell et al., 2014), fication determines the extent to which the trans-synaptic code that can differentially whereas it acts to attenuate inhibition medi- postsynaptic receptors will respond to the participate in synapse specification depend- ated by cholecystokinin-expressing basket neurotransmitters released from the pre- ing on the brain region and/or cell types, cells, but not Pv-expressing cells, in hip- synaptic cell. Synaptic cell adhesion mole- which could explain the diversity of syn- pocampal pyramidal cells (Fo¨ldy et al., cules are a family of proteins that contribute apses found in the brain. However, the ex- 2013). These results suggest that neuroli- to synapse specification. Different synaptic tent to which synapse-specifying molecules gins might regulate synaptic specification adhesion molecules located on the presyn- act across different brain regions and and function in a circuit- and synapse- aptic and postsynaptic cells can interact in a synapses to establish connectivity and regul- specific manner. heterophilic manner through their extracel- ate network properties is incompletely To further unravel the extent to which lular domains and then can transmit trans- understood. synapse-specifying molecules act across dif- synaptic signals intracellularly through their Of the synaptic cell adhesion molecules, ferent brain regions and synapses, Zhang cytoplasmic tails to recruit different presyn- neuroligins are probably the most studied be- and Su¨dhof (2016) investigated the role of aptic and postsynaptic molecular machiner- cause they are associated with various neuro- neuroligins specifically in cerebellar stellate ies, thereby contributing to the functional psychiatric disorders, including and cells. These cells are GABAergic interneu- properties of that synapse (Missler et al., schizophrenia (Su¨dhof, 2008). Neuroligins are rons that receive direct excitation from par- a class of four postsynaptic cell adhesion mol- allel fibers, but not climbing fibers, and Received Oct. 26, 2016; revised Dec. 10, 2016; accepted Dec. 12, 2016. ecules (Nlgn1–Nlgn4) that are localized to the inhibition from stellate cells and other in- J.M. is a McNair Student Scholar at Baylor College of Medicine. I thank postsynaptic membrane of glutamatergic terneurons. The authors used mouse lines in Dr. Mingshan Xue for his continued support, comments, and discussions. (Nlgn1 and Nlgn3), GABAergic (Nlgn2 and which the expression of any of the three ma- Correspondence should be addressed to Jessica Messier, 1250 Mour- sund Street, Suite 1250, Houston, TX 77030. E-mail: [email protected]. Nlgn3), cholinergic (Nlgn2), or glycinergic jor neuroligins present in the cerebellum DOI:10.1523/JNEUROSCI.3317-16.2017 (Nlgn4) synapses (Hu et al., 2015). Recent (Nlgn1, Nlgn2, and Nlgn3) can be elimi- Copyright © 2017 the authors 0270-6474/17/370739-03$15.00/0 work suggests that Nlgn1 regulates NMDAR- nated in cells expressing Cre recombinase. 740 • J. Neurosci., January 25, 2017 • 37(4):739–741 Messier • Journal Club

They crossed these mice with Pv-Cre mice to possible that cerebellin-1 regulates AMPAR- normally use, or are able to compensate remove neuroligins from Purkinje cells, mediated excitation from parallel fiber syn- with, trans-synaptic molecules that are not stellate cells, and other interneurons in the apses onto stellate cells through Glu␦1ina Nlgn 1, Nlgn2, or Nlgn3, whereas the stel- cerebellum (Pv-Nlgn123 cKO). While re- manner that is similar to how it regulates late–Purkinje cell synapse and the inhibitory cording from stellate cells in an acute parallel fiber–Purkinje cell synapses synapse onto D1-MSNs do not have this slice preparation, the authors stimulated the through Glu␦2, explaining the lack of an ability. Since LRRTMs, calsyntenins, and nearby molecular layer of the cerebellum to AMPAR synaptic phenotype observed in cerebellins are also able to bind to presynap- activate parallel fibers and stellate cells. They -deficient stellate cells. However, tic neurexins to specify synapses (de Wit and pharmacologically blocked inhibitory or ex- it is unknown whether of Nlgn1, Ghosh, 2016), it is potentially one of these citatory synaptic transmission to measure Nlgn2, and Nlgn3 from cerebellar stellate postsynaptic cell adhesion molecules that is the EPSCs from parallel fibers or IPSCs cells, during early development, as per- partially responsible for regulating the stel- from stellate cells, respectively. formed in the study by Zhang and Su¨dhof late–stellate inhibitory synapse. Neuroligin-deficient cerebellar stellate (2016), produces no parallel fiber AMPAR- Another potential explanation for the cells had a large reduction in glutamatergic mediated synaptic phenotype because an- modest reduction in inhibition onto neur- EPSCs mediated by NMDARs postsynaptic other , potentially oligin-deficient stellate cells is that the dele- to parallel fiber inputs, but there was no cerebellin-1, normally has this role or be- tion of Nlgn2 or Nlgn3 alone would have change in AMPAR-mediated EPSCs at these cause it is assuming a compensatory role. A opposite effects on IPSCs. As previously synapses (Zhang and Su¨dhof, 2016). This possible compensatory role of cerebellin-1 is mentioned, Nlgn2 deletion alone will most same phenotype was seen in stellate cells supported by recent work (Zhang et al., likely decrease inhibition in stellate cells. In lacking only Nlgn1. This result is interesting 2016) showing that the deletion of Nlgn3 contrast, Nlgn3 deletion might result in in- because NMDARs in stellate interneurons from the calyx of Held during early develop- creased inhibition onto stellate cells by re- are expressed perisynaptically rather than ment results in no detectable phenotype due ducing tonic endocannabinoid signaling, within the synaptic density (Clark and Cull- to compensation from cerebellin-1, whereas thus increasing the probability of GABA re- Candy, 2002), and thus this result indicates the deletion of Nlgn3 during late develop- lease, because such an effect was reported that, in addition to regulating synaptic ment produces a strong synaptic phenotype. for hippocampal pyramidal neurons lacking NMDAR-mediated EPSCs in the cortex, To test whether compensation is occurring, Nlgn3 (Fo¨ldy et al., 2013). Cerebellar stellate hippocampus, and amygdala (Hu et al., Nlgn1, Nlgn2, and Nlgn3 should be deleted cells express type 1 endocannabinoid recep- 2015), Nlgn1 can also regulate extrasynaptic from cerebellar stellate cells at a later time tors (CB1Rs) on their presynaptic terminals NMDAR-mediated EPSCs in cerebellar point in development. (Ashton et al., 2004), and stellate cells can stellate cells. Whether Nlgn1 interacts di- Zhang and Su¨dhof (2016) also showed release endocannabinoids to activate pre- rectly or indirectly with extrasynaptic that neuroligin-deficient cerebellar stellate synaptic CB1Rs on parallel fibers to attenu- NMDARs is unknown, however. A next cells had a modest (ϳ20%) reduction in ate synaptic transmission (Beierlein and step might be to investigate the association miniature IPSC (mIPSC) amplitude. Al- Regehr, 2006). Thus, it is possible that stel- of Nlgn1 with G-␣-interacting - though they did not investigate which neu- late cells can release endocannabinoids to interacting protein, C terminus, a PDZ pro- roligin subtype was responsible for the activate CB1Rs on presynaptic stellate cell tein that is preferentially associated with modest reduction in inhibition, it is most extrasynaptic NMDARs in cultured hip- likely attributable to Nlgn2, which is synaptic terminals, resulting in a reduction pocampal neurons (Yi et al., 2007). required for proper stellate and basket in GABA release probability at the stellate– The role of neuroligins in cerebellar stel- cell-mediated inhibition of Purkinje cells stellate synapse. The loss of Nlgn3 could de- late cells differs from their role in Purkinje (Zhang et al., 2015), for proper inhibition of crease tonic endocannabinoid release from cells. In a previous study, Zhang et al. (2015) CA1 hippocampal pyramidal cells (Poulo- neuroligin-deficient stellate cells, resulting showed that Purkinje cells lacking Nlgn1, poulos et al., 2009), and for Pv cell-mediated in enhanced GABA release probability, and Nlgn2, and Nlgn3; both Nlgn1 and Nlgn3; inhibition of cortical pyramidal cells (Gib- thus larger IPSCs, from stellate cells onto Nlgn1 alone; or Nlgn3 alone all had a signif- son et al., 2009). The question that remains neighboring stellate cells. Consequently, icant reduction in EPSCs mediated by AM- is why do stellate cells lacking Nlgn1, Nlgn2, when both Nlgn2 and Nlgn3 are deleted si- PARs at climbing fiber synapses, with no and Nlgn3 have only a modest reduction in multaneously, the decreased and increased change in EPSCs at parallel fiber synapses. stellate cell-mediated inhibition when, in inhibition, respectively, result in a net mod- Since neuroligins do not regulate AMPAR- the same neural circuit, Purkinje cells lack- est reduction in the inhibition seen in these mediated excitation from parallel fibers ing Nlgn1, Nlgn2, and Nlgn3 have a large cells. This hypothesis can be tested in onto Purkinje cells or stellate cells, then 45% and 80% reduction in mIPSC ampli- Nlgn3-null stellate cells. Nevertheless, the what molecules do? It is possible that tude and frequency, respectively, from the result of a modest reduction in inhibition cerebellin-1 is involved in this role. It is same presynaptic stellate cells (Zhang et al., seen in neuroligin-deficient stellate cells known that cerebellin-1 is secreted by paral- 2015). A similar phenomenon has been ob- would further support the notion that dif- lel fibers and is essential for the formation of served with Nlgn3 deletion in MSNs of the ferent neuroligins play distinct roles in indi- parallel fiber–Purkinje cell synapses by me- dorsal striatum, where the deletion of Nlgn3 vidual synapses and circuits. diating the interaction between presynaptic from MSNs expressing D1 dopamine recep- In conclusion, the study by Zhang and neurexins on parallel fibers and postsynap- tors (D1-MSNs) reduces inhibition in these Su¨dhof (2016) not only revealed that Nlgn1 tic Glu␦2 on Purkinje cells (Uemura et al., cells, whereas the deletion of Nlgn3 in regulates extrasynaptic NMDAR-mediated ␦ 2010). In addition, Glu 1 is required for the MSNs expressing D2 dopamine receptors excitation at the parallel fiber synapse onto formation of parallel fiber–stellate cell syn- (D2-MSN) does not affect inhibition (Roth- cerebellar stellate cells, but also further high- apses (Konno et al., 2014). Although the role well et al., 2014). One possibility is that the lighted the diversity of synapse-specifying of cerebellin-1 in regulating parallel fiber– stellate–stellate synapse in the cerebellum molecules across different brain regions and stellate cell synapses is unknown, it is and the inhibitory synapse onto D2-MSNs synapse types. Future studies should aim to Messier • Journal Club J. Neurosci., January 25, 2017 • 37(4):739–741 • 741 uncover the role of neuroligins in other cell Gibson JR, Huber KM, Su¨dhof TC (2009) boost repetitive behaviors. 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