Enhanced ATPase activities as a primary defect of mutant valosin‐containing proteins that cause inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia

Abstract

Valosin‐containing protein (VCP) has been shown to colocalize with abnormal protein aggregates, such as nuclear inclusions of Huntington disease and Machado‐Joseph disease, Lewy bodies in Parkinson disease. Several mis‐sense mutations in the human VCP gene have been identified in patients suffering inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia (IBMPFD). Recently, we have shown that VCP possesses both aggregate‐forming and aggregate‐clearing activities. Here, we showed that in cells treated with proteasome inhibitors VCP first appeared as several small aggregates throughout the cells; and then, these small aggregates gathered together into a single big aggregate. Subcellular localization and ATPase activity of VCP clearly influenced the localization of the aggregates. Furthermore, all tested IBMPFD‐causing mutant VCPs, possessed elevated ATPase activities and enhanced aggregate‐forming activities in cultured cells. In Drosophila, these mutants and VCP(T761E), a super active VCP, did not appear to spontaneously induce eye degeneration, but worsened the phenotype when co‐expressed with polyglutamines. Unexpectedly, these VCPs did not apparently change sizes and the amounts of polyglutamine aggregates in Drosophila eyes. Elevated ATPase activities, thus, may be a hidden primary defect causing IBMPFD pathological phenotypes, which would be revealed when abnormal proteins are accumulated, as typically observed in aging.

Altered Intersubunit Communication Is the Molecular Basis for Functional Defects of Pathogenic p97 Mutants

“…4B), corroborating previous observations (19,20,22). We noticed that the enhancement in ATPase activity is not uniform across all IBMPFD mutants.…”

“…Although some reports have shown that pathogenic mutants have ATPase activities similar to that of the wild type (6,21), others have demonstrated significantly enhanced ATPase activity (19,20,22). No mechanistic interpretation has been offered in the latter reports as to why the ATPase activity is elevated.…”

“…It is conceivable that such deviations in the nucleotide-binding properties of mutant p97 may also be reflected in their ATPase activities. However, conflicting observations have been reported showing that the pathogenic p97 mutants have either similar (6,21) or higher (19,20,22) ATPase activities compared with the wild type.…”

Control of p97 function by cofactor binding

“…Due to an altered inter-subunit communication in IBMPFD mutants the D1 domains fail to regulate their respective nucleotide-binding states as evidenced by a lower amount of pre-bound ADP and weaker affinity for ADP, resulting in a better accessibility for ATP in comparison to the wild-type protein [29,30]. The altered conformational equilibrium of the N domain in p97 disease mutants is accompanied by elevated ATPase activities in vitro [43,115,116] and altered interactions with some but not all cofactors in cells, which has been proposed to be key to the pathogenesis of IBMPFD [10,[116][117][118] (for a recent review see [119]). Specifically, increased amounts of p47, UFD1-NPL4, and ataxin-3, but decreased amounts of UBXD1 and UBE4B have been found.…”

VCP, a Major ATPase in the Cells, as a Novel Drug Target for Currently Incurable Disorders

“…In 2004 and 2010, VCP mutations were identifi ed that are causative for IBMPFD (inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia) [ 5 ], a human hereditary disease with dementia, and for rare cases of familial amyotrophic lateral sclerosis (ALS) [ 6 ], respectively. In our evaluation, all tested pathogenic VCPs showed signifi cant elevations of the ATPase activities, as compared with wild-type VCP [ 7 ], and thus we proposed the possibility that the constitutive elevation of its ATPase activity is a pathogenic mechanism.…”