Wednesday, October 23, 2013

Tom Maniatis, Ph.D.
Columbia University

The Clustered Protocadherins Provide a Barcode for Single Cell Diversity in the Mammalian Brain

The mammalian brain consists of over 100 million neurons, each of which can engage in hundreds of synaptic interactions. Proper neural circuit assembly during development and in response to sensory and cognitive input is a complex process that requires individual neurons to distinguish self from non-self. Adhesive cell surface proteins have emerged as essential components of a neuronal self-recognition “barcode” for individual neurons.

The best-studied example of a cell surface barcode is provided by the Drosophila Dscam proteins in which thousands of distinct cell surface isoforms are generated in each neuron through a unique genomic organization and stochastic alternative pre-mRNA splicing. Each neuron generates a unique set of protein isoforms on its cell surface, which interact with Dscam proteins on the surface of other neurons in a strictly homophilic manner. That is, individual protein isoforms at the cell surface are capable of interacting only with the same isoform on the apposing cell surface. Thus, because each neuron displays a unique combinatorial set of Dscam proteins on its surface, dendrites of the same neuron engage in homophilic interactions, but do not engage dendrites from nearby neurons that display a different set of isoforms. Studies have shown that homophilic interactions between identical Dscam isoforms lead to “self-avoidance” a phenomenon in which dendrites from the same neuron do not cross over or interact. Neurons that fail to express Dscam functional proteins display a self-avoidance phenotype in which dendrites from an individual neuron cross over and clump.

Remarkably, the genomic organization of mammalian Dscam genes does not lead to the generation of extensive Dscam diversity through alternative splicing. Rather, it appears that cell surface diversity in the mammalian brain is provided by cell surface proteins called the clustered “protocadherins”. These proteins are encoded in three closely linked gene clusters (α,β and γ). Millions of distinct combinatorial sets of protocadherin isoforms can be expressed in individual neurons through a mechanism that involves stochastic promoter choice and alternative pre-mRNA splicing. Similar to the situation with Dscam proteins in Drosophila, protocadherin mutants in mice lead to a self- avoidance phenotype.

In this lecture I will review our current understanding of the mechanisms by which protocadherin diversity is generated in the mamma- lian brain, and the function of protocadherins in neural circuit assembly.

Wednesday, November 13, 2013

Denise Montell, Ph.D.
University of California-Santa Barbara

Life, Death, and Resurrection at the Cellular Level

My laboratory studies the formation and maintenance of normal adult tissues. We investigate mechanisms of stem cell maintenance, cell fate specification, cell differentiation, morphogenesis, survival, and migration. Cell migrations are essential for normal development, wound healing, angiogenesis, and tumor metastasis. Cell migration research has focused primarily on individual cells mi- grating on extracellular matrix. However in vivo, cell migrations are diverse. Many cells migrate in interconnected groups and they can move on, or in between, other cells. E-cadherin is a major homophilic cell-cell adhesion molecule that inhibits motility of individual cells on matrix. However its contribution to migration of cells through cell-rich tissues is unclear. We developed an in vivo optical sensor of mechanical tension across E-cadherin molecules and a method for statistical classification of migration phenotypes called morphodynamic profiling. We used these approaches in conjunction with cell type specific RNAi and photo-activatable Rac to investigate the in vivo function of E-cadherin during border cell migration. We discovered that E-cadherin plays distinct roles in different subcellular locations. Surprisingly, adhesion between border cells and their substrate, the nurse cells, was required in a positive feedback loop with Rac to stabilize forward directed protrusion and directionally persistent movement. Adhesion between individual border cells was essential for communication of direction and collective guidance. E-cadherin-mediated adhesion between the motile border cells and the polar cells, a pair of non-migratory cells, holds the cluster together and provides each individual cell with polarity. Together, these results establish E-cadherin as a multi-functional core component of the cell migration machinery in vivo.

Another crucial feature of tissue homeostasis is maintaining the proper balance of cell survival and death. In order to eliminate abnormal or danger- ous cells, organisms have evolved cell suicide mechanisms, most famously the form of programmed cell death known as apoptosis. However excess cell death can cause degenerative diseases, so it is crucial to achieve the proper balance between survival and death. We have discovered that cells that have progressed far along the apoptotic pathway, past previously identified points of no return, can actively reverse the process and survive. This process, which we have named anastasis (Greek for “rising to life”) has implications for cancer, degenerative disease, and regenerative medicine.