Confocal and two-photon fluorescence microscopy have advanced the exploration of complex, three-dimensional biological structures at submicron resolution. We have developed a voxel-based three-dimensional (3-D) imaging program (Voxx) capable of near real-time rendering that runs on inexpensive personal computers. This low-cost interactive 3-D imaging system provides a powerful tool for analyzing complex structures in cells and tissues and encourages a more thorough exploration of complex biological image data.

Transforming growth factor ␤ (TGF-␤)1 is a member of a family of growth factors that regulate cellular proliferation, cellular differentiation, embryonic development, wound healing, and angiogenesis in a cell-specific manner (1). TGF-␤ regulates this diverse array of cellular processes through binding three high-affinity cell surface receptors, the TGF-␤ type I, type II, and type III receptors. The type I and II receptors contain serine/threonine protein kinases in their intracellular domain. TGF-␤ initiates cellular signaling by either binding to type III receptors, which then presents TGF-␤ to type II receptors, or binding to type II receptors directly. Once activated by TGF-␤, the type II receptor recruits, binds, and transphosphorylates the type I receptor, thereby stimulating its protein kinase activity. The activated type I receptor phosphorylates Smad2 or Smad3 that then bind to Smad4. The resulting Smad complex then translocates into the nucleus where it interacts in a cellspecific manner with numerous transcription factors to regulate the transcription of TGF-␤-responsive genes.How this simplistic pathway regulates the diverse array of biology attributed to TGF-␤ remains to be elucidated. Numerous proteins that interact with the type I or type II receptors and the Smad proteins to modulate TGF-␤ signaling have been described (2). Another method by which diversity may be generated is through the formation of distinct receptor complexes that could then utilize distinct TGF-␤ pathways. Indeed, Smadindependent signaling and signaling through mitogen-activated protein kinase and other cellular signaling pathways have been reported recently (3-7).In the process of retroviral expression cloning screens to identify additional members of the downstream signaling pathway for TGF-␤, we cloned GIPC, a PDZ domain-containing protein. This protein had been cloned previously by several groups using the yeast two-hybrid system as a protein that interacted with Class I PDZ binding motifs in Tax (8), RGS-GAIP (9), Glut-1 (10), SemaF (11), neuropilin (12), syndecan (13), tyrosinase-related protein-1 (14) and integrins ␣ 5 , ␣ 6A , ␣ 6B (15). Inspection of the TGF-␤ receptors revealed that the type III receptor contained a Class I PDZ binding motif in the cytoplasmic domain. Indeed, GIPC bound to the type III receptor in vivo and in vitro. In Mv1Lu cells, binding of the type III receptor to GIPC resulted in enhanced expression of the type III receptor at the cell surface. In L6 myoblasts, which normally do not express the type III receptor, GIPC decreased the expression of transiently expressed type III receptor but increased the expression of stably expressed type III receptor. Increased expression of the type III receptor was due to stabilization at the cell surface and was sufficient to enhance cellular responsiveness to TGF-␤ both in terms of inhibition of proliferation and induction of PAI-based promoter-driven gene expression. The type III receptor lacking the Class I PDZ binding motif did not bind GIPC and was not re...

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