A zyxin-nectin interaction facilitates zyxin localization to cell-cell adhesions.

Cell-cell junction remodeling is associated with dramatic actin reorganizations. Several actin regulatory systems have been implicated in actin remodeling events as cell-cell contacts are assembled and disassembled, including zyxin/LPP-VASP complexes. These complexes facilitate strong cell-cell adhesion by maintaining actin-membrane connections. It has been proposed that zyxin and LPP localize to cell-cell junctions via a well-defined interaction with alpha-actinin. This was recently confirmed for LPP, but zyxin localization at cell-cell contacts occurs independently of alpha-actinin binding. Here we seek to map the zyxin sequence responsible for localization to cell-cell contacts and identify the protein that docks zyxin at this cellular location. Previous results have shown that a zyxin fragment excluding the alpha-actin binding site and the LIM domains (amino acids 51-392) can independently localize to cell-cell contacts. Here, expression of smaller zyxin fragments show that zyxin localization requires amino acids 230-280. A yeast-two-hybrid screen, using the central region of zyxin as bait, resulted in the identification of the cell-cell adhesion receptor nectin-4 as a zyxin binding partner. Further demonstrating zyxin-nectin interactions, zyxin binds the intracellular domain of nectin-2 in vitro. Depletion of nectin-2 from L cells expressing E-cadherin results in a loss of zyxin localization to cell-cell contacts, demonstrating that the zyxin-nectin interaction plays a critical role in zyxin targeting to these sites.

Retinaldehyde dehydrogenase 1 deficiency inhibits PPARγ-mediated bone loss and marrow adiposity.

PPARγ, a ligand-activated nuclear receptor, regulates fundamental aspects of bone homeostasis and skeletal remodeling. PPARγ-activating anti-diabetic thiazolidinediones in clinical use promote marrow adiposity, bone loss, and skeletal fractures. As such, delineating novel regulatory pathways that modulate the action of PPARγ, and its obligate heterodimeric partner RXR, may have important implications for our understanding and treatment of disorders of low bone mineral density. We present data here establishing retinaldehyde dehydrogenase 1 (Aldh1a1) and its substrate retinaldehyde (Rald) as novel determinants of PPARγ-RXR actions in the skeleton. When compared to wild type (WT) controls, retinaldehyde dehydrogenase-deficient (Aldh1a1(-/-)) mice were protected against bone loss and marrow adiposity induced by either the thiazolidinedione rosiglitazone or a high fat diet, both of which potently activate the PPARγ-RXR complex. Consistent with these results, Rald, which accumulates in vivo in Aldh1a1(-/-) mice, protects against rosiglitazone-mediated inhibition of osteoblastogenesis in vitro. In addition, Rald potently inhibits in vitro adipogenesis and osteoclastogenesis in WT mesenchymal stem cells (MSCs) and hematopoietic stem cells (HSCs) respectively. Primary Aldh1a1(-/-) HSCs also demonstrate impaired osteoclastogenesis in vitro compared to WT controls. Collectively, these findings identify Rald and retinoid metabolism through Aldh1a1 as important novel modulators of PPARγ-RXR transactivation in the marrow niche.