The transmission of signals between cells and tissues is fundamental for normal cell growth and development. Only recently have we begun to appreciate the significance of information transfer between intracellular organelles. Eukaryotic cells are divided into distinct membrane-bound organelles with unique identities and specialized metabolic functions. Communication between organelles must take place to regulate the size, shape, and composition of individual organelles, as well as to coordinate transport between organelles. Research in our laboratory focuses on the regulatory networks that facilitate inter-organelle signaling and membrane trafficking between organelles.
Regulation of ER-PM Crosstalk
Our research group is currently investigating the structural requirements and functional roles for crosstalk between the endoplasmic reticulum (ER) and the plasma membrane (PM) in the regulation of cell signaling, membrane trafficking, ER architecture and function, and PM domain organization. In numerous cell types, the cortical ER network forms close associations with the PM (spanning 10-30 nm, see example below). ER-PM connections are thought to serve as sites for communication between these two membrane compartments.
The ER is the foundation of the secretory pathway and it has essential roles in protein secretion and quality control, lipid biosynthesis, and calcium signaling. Numerous proteins and lipids synthesized in the ER are ultimately destined for transport to the plasma membrane PM. As such, biosynthesis in the ER must be modulated, corresponding to changes in PM composition. To balance ER metabolism with changes in PM composition, the ER and PM engage in crosstalk at membrane junctions– where these two organelles become closely apposed without undergoing membrane fusion (see example above). The rapid transfer of information between these membranes provides the cell a way to regulate essential ER functions (calcium dynamics, protein and lipid synthesis) and may allow the ER to launch responses to ensure the integrity and function of the PM even under stress conditions.
Regulation of ER-PM Junction Formation and Architecture
Proteins responsible for forming ER-PM contacts have remained unclear until recently (Manford, Stefan, et al., 2012). In yeast, three conserved protein families serve as ER-PM tethers: the VAP proteins Scs2/22, Ist2 (related to the TMEM16 channel family), and the tricalbin proteins Tcb1/2/3 (orthologs of the extended synaptotagmin-like proteins E-Syt1/2/3). The ER-PM tethers are anchored in the ER and interact with the PM via cytoplasmic lipid- and protein-binding domains (see model below). Loss of the ER-PM tethers results in a massive reduction in ER-PM contacts and accumulation of internal ER structures. Interestingly, cells lacking ER-PM contacts undergo ER stress, suggesting a role for ER-PM contacts in ER homeostasis. However, our understanding of the structural components that assemble ER-PM contacts is limited, and there is much to learn about the regulation and function of these proteins in the transmission of signals between the ER and PM. Our research aims to identify novel regulatory factors for the ER-PM tether proteins and to characterize their roles in ER-PM crosstalk.
Control of ER-PM Crosstalk by PI Kinase Signaling Networks
ER-PM contacts have fascinated cell biologists for decades, and ER-PM junctions have well-established functions in the movement of small molecules, such as calcium ions, between the ER and PM. More recent findings have revealed critical roles for phoshoinositide lipids in the transmission of signals between the ER and PM (Stefan et al., 2011; Stefan et al., 2013). Phosphoinositide (PIP) lipids are essential signaling molecules that impact cell growth and development, cell polarity, and membrane trafficking pathways (see below). Mis-regulation of PIP metabolism is implicated in numerous human diseases, including cancer, diabetes, and neurodegenerative disorders. Thus, it is critical to understand how cells maintain the proper balance of PIP levels, as well as to identify effector proteins and signaling networks regulated by PIP lipids. Our research group is utilizing cell biology, biochemistry, and system-wide (genomic, proteomic, lipidomic) strategies to discover new roles for PIP signaling in membrane trafficking and information transfer between organelles to further understand how cells respond to cues in their environment, such as growth factors, nutrients, and stress. Currently, we are investigating novel roles for PIP signaling networks and ER-PM crosstalk in the control of PM integrity and ER homeostasis.
Crosstalk between the ER and Additional Organelles
The ER consists of a continuous system of membrane sheets and tubules that that contacts and participates in crosstalk with several organelles in the cell (e.g. the PM, Golgi compartments, endosomes, lysosomes, and mitochondria). In this way, the ER coordinates with multiple membrane compartments along the secretory and endocytic systems. Our research is aimed at developing a better understanding of the roles for phosphoinositide (PIP) signaling networks in inter-organelle crosstalk and membrane trafficking pathways. PIP lipids form part of a complex spatial code for defining organelle identity and function in eukaryotic cells: with PI3P on endosomes, PI(3,5)P2 on lysosomes, PI4P on the Golgi complex, and both PI4P and PI(4,5)P2 on the plasma membrane (see below). A long-term goal is to understand how PIP regulatory networks, cell signaling, and membrane trafficking pathways are regulated at additional ER-organelle junctions.