Sandell Lab

Linda Sandell Linda Sandell, Ph.D

Professor Emeritus

Debabrata Patra, Ph.D

Associate Professor


Skeletal diseases and abnormalities such as osteopenia and osteoarthritis (OA) affects humans worldwide and are crucial components of the spectrum of diseases that puts a significant financial burden on the human society. These diseases are often characterized by aberrant gene expressions and the difficulty to treat and/or repair. One approach to understanding skeletal diseases that would allow the design of an effective therapeutic program is to understand cartilage and skeletal development. Mammalian skeletal development is one of the most complex biological process, requiring the participation of a large number of matrix-encoding genes, transcription factors, and signaling pathways that come together to execute a well-planned developmental outcome. Over the years our laboratory has shown that the proprotein convertase, Site-1 protease (S1P) plays several critical roles in skeletal development, both cartilage and bone. Mutations in S1P in a human with significant loss of S1P functions have recently been identified (Journal of Clinical Investigation Insight 3:e121596 This human suffers from moderately severe skeletal dysplasia including growth retardation and reduced bone density, phenotypes similar to that we see in our mouse models. Using a number of powerful techniques combining experimental and computational approaches, our laboratory aims to mechanistically delineate the various roles that S1P performs during mammalian skeletal development. We hope to elucidate the various regulatory mechanisms critical for chondrogenesis and osteogenesis, which could allow designing strategies to control cartilage and/or bone development during development, tissue engineering and repair, to ultimately design creative strategies to control skeletal abnormalities.

1. Site-1 Protease and Cartilage Development

We are interested in the dynamics of protein synthesis by chondrocytes, the developmental program that allows for the synthesis of the specialized extracellular matrix production that is cartilage. Chondrocytes are secretory cells that like other secretory cells in the body such as the differentiating plasma B cells and the cells of exocrine tissues such as the pancreas, produce copious amounts of protein which mandates the application of special pathways and cellular apparatus for normal matrix production. Very little is however known about these pathways in the chondrocytes. Our recent study of the properties of the cartilage matrix in the S1Pcko mutant mice underscores the importance of S1P in chondrocyte during cartilage development (Journal of Cell Biology, vol. 179, pages 687 -700, 2008). Site-1 Protease (S1P) is a Golgi-resident, proprotein convertase that processes endoplasmic reticulum (ER) membrane-bound, latent transcription factors such as SREBPs and ATF6 to their free and active form. While SREBPs play a role in fatty acid and cholesterol homeostasis, ATF6 is involved in ER stress signaling to alleviate ER stress. When we created a cartilage-specific S1P knockout mouse (S1Pcko), we found that the mutant matrix shows a drastic reduction in processed type IIB collagen protein, which is a major component of cartilage. Ultrastructural analysis shows that chondrocytes in S1Pcko suffer from the entrapment of type II collagen in the ER. Currently we are exploring the nature of this entrapment and S1P’s requirement for type IIB collagen secretion through the use of S1Pcko mice.

2. Site-1 Protease and Bone and Spine Development

Ablation of S1P in bone progenitors (osterix-expressing cells) results in mice with drastically reduced bone. These mice are small, exhibit osteochondrodysplasia, with fragile bones that fracture easily, and are often characterized with varying degrees of scoliosis (twisting of the spine) (Biology Open, 7, bio032094. doi:10.1242bio.032094). Remarkably, our studies show that removal of S1P in the bone progenitors results in a decrease of bone marrow-resident skeletal stem cells along with a defect in osteogenic induction, though no defect is seen in adipogenesis. Thus, this is not a generic defect on all progenitors, but a specific effect on osteoprogenitors which is seen in the absence of S1P in the osterix -lineage. Through the use of these mice, we are attempting to understand how S1P regulates bone development and how this is related to spinal aberrations such as scoliosis.