The mammalian brain is the most complex system in all of biology. The human brain has ~100 billion neurons and perhaps 100 trillion synapses, contact sites between neurons through which information flows. The elaboration of specific neural circuits during brain development underlies all mature function, and disruption of this process underlies a wide variety of genetic and syndromic disorders, including forms of autism spectrum disorders, mental retardation, and schizophrenia. My laboratory is focused on identifying the molecular cues that mediate specific cell-cell interactions in the developing nervous system. We have identified a gene cluster that encodes a diverse array of neural cell adhesion molecules, the gamma-protocadherins, as candidates. We utilize a wide range of genetic, biochemical, pharmacological, molecular and cell biological techniques to pursue the functions of these and other molecules in neural circuit formation in the mouse brain. As several synaptic adhesion molecules, including protocadherins, have been identified as candidate genes in human neurodevelopmental disorders, our work has direct implications for human neural circuit formation.
Three large clusters of cadherin-related genes (Protocadherin-α, -β, and -γ) lie in a tandem array on a single chromosome in mammals. The γ cluster, on which we focus, consists of 22 "variable" exons, each of which encodes the extracellular, transmembrane, and partial cytoplasmic domains of a single protocadherin isoform. Each variable exon is spliced to a set of three "constant" exons which encode a shared C-terminal domain. Thus, a variety of adhesive specificities can link into a common signaling pathway. (Figure 1) Our work has shown that γ-protocadherins are expressed in the nervous system during development, and are found at a subset of synapses as well as perisynaptically, where they regulate neuron-astrocyte interactions that are critical for synaptogenesis. Mice in which the entire γ-protocadherin locus is deleted die at birth due to massive disruption of spinal cord synaptogenesis, which results in exacerbated patterns of developmental neuronal cell death (Prasad et al., Development, 2008). When we utilize conditional mouse alleles and Cre-Lox technology to restrict γ-protocadherin disruption to particular neuronal or glial subsets, we can uncover a wide range of phenotypes, all related to neural circuit formation. Our recent discoveries include:
- The γ-protocadherins act as homophilic adhesion molecules. They form cis-tetramers with no isoform specificity, but then these tetramers interact in trans with strict homophilic specificity. Because of the combinatorial nature of γ-protocadherin interactions, the 22 proteins could specify over 10,000 distinct adhesive interfaces (Schreiner and Weiner, PNAS, 2010)
- The γ-protocadherin are critical for proper dendrite arborization in cortical neurons, which they promote by inhibiting a signaling pathway involving FAK, PKC, and the PKC target MARCKS (Garrett et al., Neuron, 2012; Figure 2).
- Astrocytic γ-protocadherins interact with those on developing neurons, and this interaction stabilizes nascent synapses, allowing for the proper timecourse of synaptogenesis in the spinal cord. Individual astrocytes express subsets of the γ-protocadherins, just as neurons do (Garrett and Weiner, J. Neurosci., 2009; Figure 3).
- The γ-protocadherins are also expressed in the choroid plexus, a brain epithelial tissue responsible for secreting cerebrospinal fluid, as well as for regulating immune cell entry into the brain. Choroid plexus-specific knockout of the γ-protocadherins leads to reduced ventriclular volume, suggesting that CSF regulation is disrupted (Lobas et al., J. Neurochem., 2012; Figure 4). We are currently testing the hypothesis that the γ-protocadherins regulate the entry of T cells into the brain via the choroid plexus in a mouse model of multiple sclerosis.
Multiple projects are currently available for ambitious graduate students, postdocs, and undergraduate Honors students. Our work is funded by grants from the NIH, the March of Dimes, and the National Multiple Sclerosis Society.
Weiner, J.A. and Jontes, J. (2013). Protocadherins, not prototypical: a complex tale of their interactions, expression, and functions. Frontiers in Molecular Neuroscience, 6:4.
Garrett, A.M..*, Schreiner, D..*, Lobas, M.A., and Weiner, J.A. (2012). γ-Protocadherins control cortical dendrite arborization by regulating the activity of a FAK/PKC/MARCKS signaling pathway. Neuron, 74: 269-276. *co-first authors
Lobas, M.A., Helsper, L., Vernon, C.G., Schreiner, D., Zhang, Y., Holtzman, M.J., Thedens, D.R., and Weiner, J.A. (2012). Molecular heterogeneity in the choroid plexus epithelium: the 22-member γ-protocadherin family is differentially expressed, apically localized, and implicated in CSF regulation. Journal of Neurochemistry, 120: 913-927.
Jannie, K.M., Stipp, C.S., and Weiner, J.A. (2012). ALCAM regulates motility, invasiveness, and adherens junction formation in uveal melanoma cells. PLoS ONE, in press.
Prasad, T. and Weiner, J.A. (2011). Direct and indirect regulation of spinal cord Ia afferent terminal formation by the γ-Protocadherins. Frontiers in Molecular Neuroscience, 4: 54.
Schreiner, D., and Weiner, J.A. (2010). Combinatorial homophilic interactions between multimerized gamma-protocadherin family proteins greatly expand the molecular diversity of cell adhesion. Proceedings of the National Academy of Sciences, 107: 14893-14898. Featured as a \"Must Read\" paper on Faculty of 1000 Biology
Garrett, A.M., and Weiner, J.A. (2009). Control of CNS synapse development by γ-protocadherin-mediated astrocyte-neuron contact. Journal of Neuroscience, 29:11723–11731. Featured on cover and \"This Week In the Journal\".
Buhusi, M., Demyanenko, G.P., Jannie, K.M., Dalal, J., Weiner, J.A.*, and Maness, P.F.* (2009). ALCAM regulates mediolateral retinotopic mapping in the superior colliculus. Journal of Neuroscience, 29:15630 –15641. *Co-corresponding authors. Featured on cover.
Garrett, A.M., Schreiner, D., and Weiner, J.A. (2009). The Cadherin Superfamily in Synapse Formation and Function. In The Sticky Synapse (Hortsch, M. and Umemori, H., eds.). Springer: New York.
Prasad, T., Wang, X., Gray, P.A., Weiner, J.A. (2008). A differential, developmental pattern of spinal interneuron apoptosis during synaptogenesis: Insights from the genetic analysis of the protocadherin-gamma gene cluster. Development, 135: 4153-4164. Featured in \"In This Issue\"
Weiner, J.A., Wang, X., Tapia, J.C., and Sanes, J.R. (2005). Gamma protocadherins are required for synaptic development in the spinal cord. Proceedings of the National Academy of Sciences, 102: 8-14
Yamagata, M., Sanes, J.R., and Weiner, J.A. (2003). Synaptic adhesion molecules. Current Opinion in Cell Biology, 15: 621-632.
Wang, X.*, Weiner, J.A.*, Levi, S., Craig, A.M., Bradley, A., and Sanes, J.R. (2002). Gamma protocadherins are required for survival of spinal interneurons. Neuron, 36: 843-854. *co-first authors
Yamagata, M., Weiner, J.A., and Sanes, J.R. (2002). Sidekicks: Synaptic adhesion molecules that promote lamina-specific connectivity in the retina. Cell, 110: 649-660.