Zmpste24 deficiency in mice causes spontaneous bone fractures, muscle weakness, and a prelamin A processing defect

Abstract

Zmpste24 is an integral membrane metalloproteinase of the endoplasmic reticulum. Biochemical studies of tissues from Zmpste24-deficient mice (Zmpste24(-/-)) have indicated a role for Zmpste24 in the processing of CAAX-type prenylated proteins. Here, we report the pathologic consequences of Zmpste24 deficiency in mice. Zmpste24(-/-) mice gain weight slowly, appear malnourished, and exhibit progressive hair loss. The most striking pathologic phenotype is multiple spontaneous bone fractures-akin to those occurring in mouse models of osteogenesis imperfecta. Cortical and trabecular bone volumes are significantly reduced in Zmpste24(-/-) mice. Zmpste24(-/-) mice also manifested muscle weakness in the lower and upper extremities, resembling mice lacking the farnesylated CAAX protein prelamin A. Prelamin A processing was defective both in fibroblasts lacking Zmpste24 and in fibroblasts lacking the CAAX carboxyl methyltransferase Icmt but was normal in fibroblasts lacking the CAAX endoprotease Rce1. Muscle weakness in Zmpste24(-/-) mice can be reasonably ascribed to defective processing of prelamin A, but the brittle bone phenotype suggests a broader role for Zmpste24 in mammalian biology.

Figure 1

(a) Retarded growth in male Zmpste24^−/− mice (n = 4) vs. littermate male Zmpste24^+/+ mice (n = 13). Similar results were obtained with female mice (not shown). (b) Photograph of a 3-month-old Zmpste24^−/− mouse and littermate Zmpste24^+/+ mouse.

Figure 2

(a) Thorax of a 24-week-old Zmpste24^−/− mouse. Fibrotic lesions surrounding broken ribs are indicated by arrows. (b) Radiograph of a Zmpste24^−/− mouse, revealing callus formation surrounding rib fractures (arrows). (c) Surface rendering of a μCT scan of the lower thoracic spine of a Zmpste24^−/− mouse. Callus is visible at the tips of several ribs near the costovertebral joints (arrowheads). (d) Maximal intensity projection from a μCT scan of the lower thoracic spine of a Zmpste24^−/− mouse, revealing multiple rib fractures (arrowheads). (e and f) Hematoxylin- and eosin-stained sections of rib fractures (arrows) and surrounding fibrosis (arrowheads). (g) Avulsion of a fragment of bone (arrowhead) from the cortex of the humerus. (h) Fracture of the mandible with fibrosis between bone fragments (arrowhead).

Figure 3

μCT scans illustrating bony lesions in Zmpste24^−/− mice. (a–c) Surface renderings of top, lateral, and bottom surfaces of the skulls of 8-week-old Zmpste24^−/− and Zmpste24^+/+ mice. Arrowheads indicate the destroyed zygomatic arch in a and c; an arrow indicates the small mandible in b. (d) Maximal intensity projection of the lateral view of the skulls of 8-week-old mice, revealing an increase in the length of the incisors and the size of other teeth. (e) Surface rendering of the thoracic spines of 8-week-old Zmpste24^−/− and Zmpste24^+/+ mice, revealing increased numbers of pock marks in the Zmpste24^−/− mice. (f) Surface rendering of the femur, knee, tibia/fibula of 2-month-old Zmpste24^−/− and Zmpste24^+/+ mice, revealing increased pock marks in the proximal fibula of the Zmpste24^−/− mouse as well as effacement of the tibial tuberosity.

Figure 4

Photograph of a Zmpste24^−/− mouse, illustrating a characteristic dragging of the hind limb.

Figure 5

(a) Paravertebral muscles from 19-week-old male Zmpste24^−/− and Zmpste24^+/+ mice. (b) Sections through the hearts of 19-week-old male Zmpste24^−/− and Zmpste24^+/+ mice. (c) Sections through the skin of 15-week-old female Zmpste24^−/− and Zmpste24^+/+ mice, showing reduced adipose tissue in Zmpste24^−/− mice.

Figure 6

Western blots of Zmpste24^−/−, Icmt^−/−, and Rce1^−/− fibroblast extracts with a C-terminal prelamin A antibody and an N-terminal lamin A/C antibody. Also shown are Western blots of extracts of wild-type fibroblasts and fibroblasts treated with 5 μM SCH66336, a farnesyltransferase inhibitor.