Versatility and biocompatibility of amphiphilic diblock copolypeptide hydrogels in the central nervous system

Amphiphilic diblock copolypeptide hydrogels (DCHs) are synthetic materials whose properties can be varied readily and predictably by altering copolymer chain length or composition and which are of potential interest for biomaterial applications. We tested the biocompatibility in the central nervous system (CNS) of DCH composed of lysine, homoarginine or glutamate in combination with leucine. A range of DCH formulations with rheological properties similar to brain tissue were injected into mouse forebrain and examined after 1-8 weeks using light microscopy, immunohistochemistry, and electron microscopy. DCH deposits elicited no more gliosis, inflammation, or toxicity to neurons, myelin or axons than did injections of physiological saline. The size, rigidity, and density of DCH deposits could be varied subtly by altering DCH composition and concentration. For any given DCH formulation, increased concentration correlated with increased gel strength in vitro and increased deposit size in vivo. DCHs of lysine and leucine (KmLn) were selected for detailed analyses because these formed deposits with desirable physical properties and since lysine is routinely used as a substrate for neural cell cultures. Deposits of unmodified K180L20 exhibited time-dependent in-growth of blood vessels and of certain glial cells, and limited in-growth of nerve fibers. These findings show that DCHs are injectable, re-assemble in vivo to form 3-dimensional deposits, exhibit little or no detectable toxicity in the CNS, integrate well with brain tissue and represent a new class of synthetic biomaterials with potential for applications as depots or scaffolds in the CNS. To test such feasibilities, chondroitinase ABC (chABC) and laminin-1 were individually incorporated into K180L 20 hydrogels. Several recent investigations carried out in different laboratories and experimental models have shown that chABC digestion on condroitin sulfate proteoglycans (CSPGs) from the CNS extracellular matrix enhances axonal plasticity, and improves the functional outcome after an injury. We found that bioactive chABC could be released from the DCHs, and its overall in vivo digestive capability on CSPGs was prolonged by DCHs. Laminin-1 is an extracellular matrix protein that promotes cell adhesion, neurite outgrowth, and angiogenesis. Deposits of laminin-1 conjugated K180L20 promoted rapid angiogenesis and neurogenesis after 1 week in vivo.