3. Diffusion barriers and the control of aging.

Compartmentalization of the plasma membrane into distinct domains underlies many cellular processes, such as cell migration, differentiation and asymmetric cell division. In many instances, such as in epithelial cells, neurons and budding yeast, compartmentalization involves the establishment of diffusion barriers that limit the lateral diffusion of plasma membrane and cortical proteins across domain boundaries.

Over the last few years, we discovered the existence of a number of septin-dependent, lateral diffusion barriers in the plasma and endoplasmic membranes (Barral et al., Mol. Cell 2000; Dobbelaere and Barral, Science 2004, Luedeke et al., J. Cell Biol., 2005; Shcheprova et al., Nature, 2008). These barriers are located at the bud neck and limit the exchange of membrane-inserted proteins between mother and bud. These barriers play interesting roles in cell division, in the maintenance of cell polarity and in the segregation of fate determinants. Particularly, we recently showed that they play a key role in the retention of aging factors in the mother cell and hence in the ability of already aged yeast mother cells to produce rejuvenated buds (Shcheprova et al., Nature, 2008).

Our current research activities around diffusion barriers follow four directions: 1- What is the molecular composition and mechanism of these diffusion barriers? 2- What are the relative contributions of these different barriers in rejuvenation, cell division, cell polarity and phenotypic diversity? 3- Is barrier formation regulated? 4- Are similar barriers found in other organisms?

A. Diffusion from the mother cortex to the bud cortex is slow in metaphase cells. FLIP experiments were performed on WT (left) cells expressing Sec61-GFP (ER marker). The bleaching region is depicted by a red circle. The diffusion barrier is Bud6 dependent. FLIP experiments were performed on bud6∆ cells, diffusion between mother and bud is fast. (Adapted from: Luedeke et al., J. Cell Biol., 2005)
B. Three Diffusion Barriers at the Yeast Bud Neck
Sectional view of a yeast cell depicting the plasma (black line), the ER (green), and the nuclear envelope (light red) membranes. These membranes are continuous through the bud neck (middle) yet are compartmentalized into mother (bottom) and bud (top) domains. The septin ring (red) governs the formation of the diffusion barriers responsible for this compartmentalization (dashed lines). Ribosomes (black dots) associate with the ER membranes throughout the cell except at the bud neck, where the ER is smooth. Old nuclear pore complexes (brown) and associated extra-chromosomal rDNA circles (blue) are retained in the mother, and new nuclear pore complexes (orange) are inserted in the nuclear envelope of the bud. Spindle pole bodies (gray) together with spindle and cytoplasmic microtubules (gray lines) are also depicted.
(Adapted from: Caudron and Barral, Dev Cell, 2009)