Relationship of cell growth to the regulation of tissue‐specific gene expression during osteoblast differentiation

Abstract

The relationship of cell proliferation to the temporal expression of genes characterizing a developmental sequence associated with bone cell differentiation can be examined in primary diploid cultures of fetal calvarial-derived osteoblasts by the combination of molecular, biochemical, histochemical, and ultrastructural approaches. Modifications in gene expression define a developmental sequence that has 1) three principal periods: proliferation, extracellular matrix maturation, and mineralization; and 2) two restriction points to which the cells can progress but cannot pass without further signals. The first restriction point is when proliferation is down-regulated and gene expression associated with extracellular matrix maturation is induced, and the second when mineralization occurs. Initially, actively proliferating cells, expressing cell cycle and cell growth regulated genes, produce a fibronectin/type I collagen extracellular matrix. A reciprocal and functionally coupled relationship between the decline in proliferative activity and the subsequent induction of genes associated with matrix maturation and mineralization is supported by 1) a temporal sequence of events in which an enhanced expression of alkaline phosphatase occurs immediately after the proliferative period, and later an increased expression of osteocalcin and osteopontin at the onset of mineralization; 2) increased expression of a specific subset of osteoblast phenotype markers, alkaline phosphatase and osteopontin, when proliferation is inhibited; and 3) enhanced levels of expression of the osteoblast markers when collagen deposition is promoted, suggesting that the extracellular matrix contributes to both the shutdown of proliferation and development of the osteoblast phenotype. The loss of stringent growth control in transformed osteoblasts and in osteosarcoma cells is accompanied by a deregulation of the tightly coupled relationship between proliferation and progressive expression of genes associated with bone cell differentiation.

Pericytes resident in postnatal skeletal muscle differentiate into muscle fibres and generate satellite cells

“…AP is broadly expressed in many pre-and postnatal tissues, and in different cell types such as osteoblasts 37 , embryonic stem cells 38 , bone marrow stroma cells 39 , liver and kidney cells 40 , granulocytes 41 and B cells 42 ; however, in postnatal mouse striated muscles, AP is expressed only by vessel-associated cells, in both endothelial cells and pericytes, as previously reported 9,[14][15][16] , and exhaustively confirmed in this work through a variety of histochemical and immunochemical methods. Most importantly, by using virtually all possible experimental approaches, we demonstrated that both AP and Cre expression are invariably absent in quiescent satellite cells, as well as in proliferating and differentiating myogenic cells in vitro or in vivo, even after cardiotoxin-induced damage.…”

A comparison of osteogenesis-related gene expression of mesenchymal stem cells during the osteoblastic differentiation induced by Type-I collagen and/or fibronectin

“…ALP gene expression has a two phase expression, in other words, two peaks of expression profiles, and some suggest that the first peak is strongly related to differentiation and that the second one may be related to mineral deposition (14,26). In our study, ALP gene expression of MSCs with type I collagen or type I collagen and fibronectin showed a peak level at day 12 and then decreased.…”

“…In our study, ALP gene expression of MSCs with type I collagen or type I collagen and fibronectin showed a peak level at day 12 and then decreased. It was also repotted that the extracellularmatrix (ECM) supported the expression of tissue-specific genes during osteoblast differentiation and suppressed the proliferation of cells (14). Therefore, MSCs with type I collagen and type I collagen and fibronectin The expression of ALP mRNA and enzyme activity before the initiation of osteoblast mineralization suggests that ALP may be involved in preparing the ECM for an ordered deposition of minerals.…”

“…Some of the noncollagenous proteins are also expressed at high levels in osteoblasts at the front of the mineralization (13). The deposition of collagen is required for subsequent events that are necessary to render the ECM competent for mineralization (14). Mineralization induces cell shape and metabolic changes in osteoblasts, which develop into osteocytes surrounded by mineralized bone matrices or flattened bone lining cells apposed to the mineralized surface.…”

“…Initially, actively proliferating cells, expressing cell cycle and cell growth regulated genes, produce a fibronectin/type I collagen extracellular matrix. A reciprocal and functionally coupled relationship between the decline in proliferative activity and the subsequent induction of genes associated with matrix maturation and mineralization is supported by (1) a temporal sequence of events in which an enhanced expression of alkaline phosphatase occurs immediately after the proliferative period and later an increased expression of osteocalcin and osteopontin occurs at the onset of mineralization; (2) increased expression of a specific subset of osteoblast phenotype markers, alkaline phosphatase, and osteopontin when proliferation is inhibited; and (3) enhanced levels of expression ofthe osteoblast markers when collagen deposition is promoted, suggesting that the extracellular matrix contributes to both the shutdown of proliferation and development of the osteoblast phenotype (14).…”