Identification of CDC42 Effectors Operating in FGD1-Dependent Trafficking at the Golgi.

Mikhail Egorov,Roman Polishchuk

doi:10.3389/fcell.2019.00007

Abstract

Loss of function mutations in the FGD1 gene cause a rare X-linked disease, faciogenital dysplasia (FGDY, also known as Aarskog-Skott syndrome), which is associated with bone and urogenital abnormalities. The FGD1 gene encodes à CDC42-specific guanine nucleotide exchange factor. The mutations are frequently located in the DH module of FGD1 preventing its transformation to the active form. We previously reported that Golgi-associated FGD1 regulates post-Golgi transport of some conventional and bone-specific proteins in a CDC42-dependent manner. However, the downstream targets of FGD1/CDC42 signaling that operate to support transport from the Golgi remain elusive. Here, we demonstrate that Golgi-localized CDC42 effectors might be involved in FGD1-mediated post-Golgi transport, probably through coordination of Golgi membrane and cytoskeleton dynamics. Overexpression of effector-specific CDC42 mutants (exhibiting preferential affinities for PAK1, IQGAP1, N-WASP, or PAR6) only partially rescue membrane trafficking in FGD1-deficient cells, indicating that the orchestrated activities of several downstream targets of CDC42 are required to support FGD1-mediated export from the Golgi. Our findings provide new insights into understanding the molecular mechanisms behind FGD1/CDC42-dependent transport events and uncover new targets whose potential might be explored for correction of membrane trafficking in FGDY.

Highlights

Faciogenital dysplasia (FGDY) is a rare X-linked disorder that manifests in defects of bone development such as disproportional acromelic short stature, abnormal face shape, as well as cardiac, ocular, urogenital abnormalities and mental retardation (Aarskog, 1970; Orrico et al, 2015)
To determine whether Golgi-associated CDC42 targets such as IQGAP1, PAK1 and N-WASP operate in FGD1/CDC42dependent trafficking events, we assessed the ability of these proteins to influence post-Golgi transport by monitoring constitutive export
We found that VSVG strongly co-localized with the trans-Golgi marker TGN46 in both control and IQGAP1, PAK1, or N-WASP-depleted cells at the end of the 20◦C temperature block (Figure 1A, upper row; see Figure 1C for silencing efficiency), indicating that suppression of these CDC42 effectors did not impact on the delivery of cargo proteins to the transGolgi network (TGN)

Summary

Introduction

Faciogenital dysplasia (FGDY) is a rare X-linked disorder that manifests in defects of bone development such as disproportional acromelic short stature, abnormal face shape, as well as cardiac, ocular, urogenital abnormalities and mental retardation (Aarskog, 1970; Orrico et al, 2015). FGDY is caused by loss-of-function mutations in the FGD1 gene that encodes a 961 amino acid FGD1 protein that acts as a specific GEF for the small Rho GTPase CDC42 (Pasteris et al, 1994; Zheng et al, 1996). The predicted frequency of clinical manifestation of FGDY is about 1:25,000 (Orrico et al, 2015). The FGD1 protein is expressed in regions of active osteogenesis in developing long bones (Gorski et al, 2000). The highest levels of FGD1 expression have been observed in bone tissue, kidney, liver, lung, heart and brain (Pasteris et al, 1994; Olson, 1996). The pattern of postnatal FGD1 expression strongly correlates with clinical manifestations of FGDY.

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