Role of alpha(v) integrins in adenovirus cell entry and gene delivery

Abstract

Adenoviruses (Ad) are a significant cause of acute infections in humans; however, replication-defective forms of this virus are currently under investigation for human gene therapy. Approximately 20 to 25% of all the gene therapy trials (phases I to III) conducted over the past 10 years involve the use of Ad gene delivery for treatment inherited or acquired diseases. At present, the most promising applications involve the use of Ad vectors to irradicate certain nonmetastatic tumors and to promote angiogenesis in order to alleviate cardiovascular disease. While specific problems of using Ad vectors remain to be overcome (as is true for almost all viral and nonviral delivery methods), a distinct advantage of Ad is the extensive knowledge of its macromolecular structure, genome organization, sequence, and mode of replication. Moreover, significant information has also been acquired on the interaction of Ad particles with distinct host cell receptors, events which strongly affect virus tropism. This review provides an overview of the structure and function of Ad attachment (coxsackievirus and Ad receptor [CAR]) and internalization (alpha(v) integrins) receptors and discusses their precise role in virus infection and gene delivery. Recent structure studies of integrin-Ad complexes by cryoelectron microscopy are also highlighted. Finally, unanswered questions arising from the current state of knowledge of Ad-receptor interactions are presented in the context of improving Ad vectors for future human gene therapy applications.

FIG. 1

Schematic illustration of the interaction of Ad2 with different cellular receptors involved in infection. High-affinity virus attachment is mediated by the interaction of the fiber capsid protein (white) with a 46-kDa receptor known as CAR. A second interaction of the penton base capsid protein (red) with α_v integrins promotes virus internalization.

FIG. 2

An alignment of penton base sequences from different Ad serotypes. Identical amino acid residues, located primarily at the N and C termini of the proteins, are indicated by vertical lines. Gaps indicated by dotted lines were used to maximize the alignment. Note the conserved RGD sequence indicated in boldface type with asterisks. The Ad12 sequence was obtained from reference .

FIG. 3

Schematic diagram of the signaling events involved in Ad internalization and cell migration. The signaling molecules not selectively involved in α_v integrin-mediated cell migration are shown in the boxed region on the left.

FIG. 4

Cryo-EM reconstruction of the vertex region of the Ad2/DAV-1 Fab fragment. The viral capsid proteins are displayed in the same color scheme as in Fig. 5 and with the hexons in blue. (A) The reconstructed Fab density (magenta) is weak and diffuse. (B) A model showing the cryo-EM Ad2 vertex density together with five crystallographic Fab fragments filtered to 19-Å resolution (magenta). Each Fab fragment is shown in a different orientation to suggest mobility. The wire mesh corresponds to the total model density obtained from Fab fragments in eight distinct orientations. The scale bar is 100 Å. Reprinted from reference with permission of the publisher.

FIG. 5

Ad2 penton base (yellow and red) and fiber (green) from a cryo-EM reconstruction of the intact virus particle. Each of the five penton base protrusions has a region of weak density (red) at the top corresponding to the mobile RGD loop. Note that during the Ad reconstruction process, icosahedral (60-fold) symmetry was imposed. Regions of the virus that do not follow perfect icosahedral symmetry, such as the flexible fiber, are not fully reconstructed. The actual length of the fiber protein is approximately six times longer than is shown here. (A) Top view. (B) Side view. Bar, 25 Å. Reprinted from reference with permission of the publisher.

FIG. 6

Cryo-EM reconstruction of the Ad12-α_vβ₅ complex. (A) Full virus-receptor complex, viewed along an icosahedral threefold axis. The penton base is shown in yellow, the fiber is shown in green, and the rest of the viral capsid is shown in blue. The integrin density is shown in red. (B) Enlarged view of the vertex region. The integrin appears to have two domains, a globular domain bound to the penton base and an extended tail domain farther from the viral surface. Bars, 100 Å. Reprinted from Chiu et al. (14a) with permission of the publisher.

FIG. 7

The ring formed by five RGD-binding globular domains of integrin from the Ad12-α_vβ₅ integrin cryo-EM reconstruction, shown color coded by height from the viral surface. (A) Top view. Note that the bound integrin heterodimers form a continuous ring with close associations between adjacent globular domains. (B) Side view. Five columns of density (red) connect the integrin globular domains to the more flexible tail domains (not shown). (C) Bottom view. The arrows mark five clefts where the RGD-containing protrusions of the penton base bind. Bar, 100 Å. Reprinted from Chiu et al. (14a) with permission of the publisher.