Fig. 1: Design principle (schematic) for our synthetic osteoyeast platform.

a Hydroxyapatite (HAp) synthesis catalyzed by osteoblasts. Dense calcium phosphate particles are formed in mitochondria, and they interact with lysosomes (Step 1). Osteoblasts start to transport, accumulate, and store phosphate and calcium within their lysosomes (Step 2). As the lysosomes are filled with amorphous calcium phosphate (ACP), they are secreted into the extracellular milieu as matrix vesicles (MVs) (Step 3). These MVs interact with proteins such as collagen and are gradually deformed, releasing ACP (Step 4). Lastly, platelet-like HAp is formed within the gaps of collagen fibrils, which act as templates (Step 5). b The design principle used for engineering the osteoyeast platform. The ureolytic enzyme is overexpressed, which triggers the activity of an antiporter (Vcx1) to exchange cytosolic Ca²⁺ with intra-vacuolar H⁺. Ca²⁺ is accumulated and stored in the form of ACP (Step 1). The ACP in the vacuoles is translocated to extracellular vesicles (EVs) and secreted into the media (Step 2). The EVs merge, and ACP inside the EVs transforms into HAp (Step 3). VTC vacuolar transporter chaperone.