**At the Intersection of the Pathways for Exocytosis and Autophagy**

D.A. Brooks, C. Bader, Y.S. Ng, R.D. Brooks, G.N. Borlace and T. Shandala *Mechanisms in Cell Biology and Diseases Research Group, School of Pharmacy and Medical Sciences, Sansom Institute for Health Research University of South Australia Australia* 

#### **1. Introduction**

108 Crosstalk and Integration of Membrane Trafficking Pathways

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Exocytosis and autophagy may be competitively, cooperatively or independently regulated, depending upon the nature of the intracellular and/or extracellular environment. In response to conditions of low or high energy demand, there would be an advantage to the cell in reducing the energy consuming process of secretion, where the membrane from exocytic vesicles could be utilized to enable rapid expansion of the autophagic compartment (i.e. competitive regulation). During an immune response there may be concomitant stimulation of autophagy to degrade an intracellular bacterial pathogen, and exocytosis to release second messengers and antimicrobial effectors. Alternatively, it may be necessary to only upregulate an individual process, which is the case for increased autophagy during organelle and cytoplasm turnover under restricted nutrient supply (e.g. in the bone growth plate) (He & Klionsky, 2009), or increased exocytosis during proteoglycan delivery to the

At the Intersection of the Pathways for Exocytosis and Autophagy 111

Fig. 1. Proposed model for crosstalk between autophagy and exocytosis.

dual function.

membrane.

**2.1 The molecular machinery for exocytosis** 

route, cargo can be trafficked via a number of compartments including recycling endosomes, early endosomes, multivesicular bodies, secretory granules and secretory lysosomes. This pathway is utilized for the immune related secretion of cytokines and antimicrobial peptides, following exposure to pathogens or inflammatory stimuli. Similarly, for neurotransmitter release, exocytosis is stimulated by an increase in the intracellular calcium ion concentration in neurons; which allows the propagation of neuronal function. These different pathways are in dynamic balance with the endocytic pathway, which, apart from facilitating uptake into the cell, allows the recovery of membrane from the plasma membrane following exocytosis, enabling the cell to control its surface area (Khandelwal *et al*., 2010). The molecular machinery that drives exocytosis therefore operates in conjunction with the endocytic machinery, and in some cases may involve common elements that have

The molecular machinery that facilitates the process of exocytosis can vary with respect to the specific cell type and specialist cargo being transported, although two key molecular complexes are conserved for most membrane associated exocytic events; the exocyst and the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) complex (Liu & Guo, 2011; Nair *et al*., 2011). Through interaction with a number of effector molecules, these complexes mediate the tethering, docking and fusion of vesicles with the plasma

The exocyst is an octomeric complex that is required for the efficient delivery of exocytic vesicles to the plasma membrane (TerBush *et al*., 1996). The components of the exocyst complex were first identified for yeast in the early 1990's, with mammalian orthologues

cell surface (Franken *et al*., 2003). Finally, during the removal of dysfunctional exocytic vesicles or the rapid cessation of secretion (e.g. during neurotransmission), the cell requires organelle-specific molecular machinery, for the nucleation of autophagy (Geng *et al*., 2010).

Despite defects in multiple human syndromes that demonstrate changes in both exocytosis and autophagy, the mutual dependence of these processes on common molecular machinery has only recently been investigated. Evidence indicates that the exocyst complex and its regulator Ral (Ras like GTPase), both of which are known to have a critical function in exocytosis, also appear to be essential for the initiation of autophagy (Bodemann *et al*., 2011). Similarly, the small GTPase Rab11 has a critical role in exocytosis at the recycling endosome and in exocytic vesicle function (van Ijzendoorn, 2006), although in times of starvation where autophagy is induced, Rab11 containing vesicular compartments have been shown to asscociate with autophagosomes (Rab11 positive amphisomes; (Fader *et al*., 2008)). Finally, disruption of the exocytic Rab GTPase Sec4 and its guanine nucleotide exchange factor Sec2, can have significant effects on the anterograde movement of the integral autophagosome membrane protein Atg9 (Autophagy related protein 9), thereby influencing the recruitment of Atg8 to the phagophore assembly site (PAS) (Bodemann *et al*., 2011). The movements of Atg9 and Atg8 are of particular interest as they are both important during the initiation of autophagy (Geng *et al*., 2010; Wang *et al*., 2009). Atg9 has, in turn, been reported to reside on exocytic vesicles that can be converted into a phagophore assembly site (Mari *et al*., 2010; Mari & Reggiori, 2010). The aim of this chapter is to provide an overview of the exocytic and autophagic processes with a focus on the common molecular machinery acting at critical control points. It is this machinery that may facilitate communication between these functionally distinct vesicular compartments and may act as potential sites for regulation.
