Stress-induced recruitment of epiplakin to keratin networks increases their resistance to hyperphosphorylation-induced disruption

Abstract

Epiplakin is a large (>725 kDa) cytoskeletal protein exclusively expressed in epithelial tissues. It has a unique structure, consisting entirely of plakin repeat domains (PRDs), one of the hallmarks of spectraplakin protein family members. Previous studies, including the phenotypic analyses of knockout mice, failed to reveal the biological function of epiplakin. Using in vitro binding assays, we show here that all but one of the 16 PRDs of mouse epiplakin bind to keratins of basal keratinocytes. Nevertheless, in primary keratinocyte cell cultures, epiplakin only partially colocalized with keratin intermediate filament networks. However, upon application of cellular stress in the form of keratin hyperphosphorylation, osmotic shock or UV irradiation, the entire cytoplasmic epiplakin pool became associated with keratin. In response to such types of stress, epiplakin initially translocated to the stillintact keratin filament network and remained associated with keratin after its disruption and transformation into granular aggregates. Time-course experiments revealed that serine/threonine (okadaic acid) and tyrosine (orthovanadate) phosphatase inhibitor-induced filament disruption in differentiated keratinocytes proceeded faster in epiplakindeficient cells compared with wild-type cells. Our data suggest that epiplakin plays a role in keratin filament reorganization in response to stress, probably by protecting keratin filaments against disruption in a chaperone-like fashion.

Epiplakin accelerates the lateral organization of keratin filaments during wound healing

“…The binding of EPPK to K5/K14 has been reported previously [3,5]. The association of EPPK with other keratins, as suggested by our observations, remains to be confirmed by analyses in vitro.…”

Epiplakin Deficiency Aggravates Murine Caerulein-Induced Acute Pancreatitis and Favors the Formation of Acinar Keratin Granules

“…This finding and epiplakin’s unique structure comprising 16 similar PRDs, most of which bind to keratins [12] suggest that keratins are epiplakin’s most important and probably sole binding partners in acinar cells. ii) During acute pancreatitis epiplakin and keratins are upregulated in a concerted manner in both mice and humans, indicating a close functional relationship.…”

“…We and others generated epiplakin knock-out (EPPK −/− ) mice, which, surprisingly, showed no obvious spontaneous or stress-induced in vivo phenotype [10], but accelerated migration of keratinocytes ex vivo [11]. Subsequent analyses revealed that epiplakin protects from hyperphosphorylation-induced keratin disruption in keratinocytes, thereby indicating a function in stress response [12]. Epiplakin interacts with keratin 8 and 18 (K8 and K18), the main keratins expressed in simple epithelia [8].…”

Plakin Proteins, Hemidesmosomes and Human Disease

“…It is unusual in that it consists solely of 13 plakin repeat domains ( Figure 1). Its function is unknown although one report has suggested that it may play a role in protecting keratin intermediate filaments from damage when cells are exposed to UV radiation and other forms of stress (Spazierer et al, 2008). To date no diseases have been linked to mutations in either the MACF1 or epiplakin genes.…”