Abstract:
Heparin-binding regions of several proteins, such as neural cell adhesion molecule, fibronectin, laminin, midkine, and anti-thrombin III have been shown to promote neurite extension on two-dimensional surfaces. The effect of heparin-binding peptides on neurite extension through three-dimensional matrices was investigated by culturing embryonic chick dorsal root ganglia (DRG) within fibrin gels containing chemically attached heparin-binding peptide (HBP). The length of neurites within fibrin gels containing cross-linked HBP was increased by more than 70% over extension through fibrin gels containing no peptide. The HBP sequence of antithrombin III was incorporated into the fibrin gel as the C-terminal domain of a bidomain, chimeric peptide; the N-terminal second domain of this peptide contained the α2-plasmin inhibitor substrate for Factor XIIIa. Factor XIIIa, a transglutaminase, was used to chemically attach the HBP-containing chimeric peptide to the fibrin gels during polymerization. The amount of HBP cross-linked into the fibrin gels was determined, after degradation by plasmin using gel permeation chromatography, to be approximately 8 moles of peptide per mole fibrinogen. A peptide (HBP), where the cross-linking glutamine was replaced with glycine, showed no increase in extension in comparison with fibrin gels. The addition of heparin to the gel precursors resulted in no increase in neurite extension in comparison with fibrin gels. HBPs promote neurite extension by binding to cell surface proteoglycans on the DRG.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to the fields of materials useful in the support of tissue and cell incorporation and growth. More particularly, it concerns particular substrates of protein, such as fibrin, to which bioactive peptides and combinations of peptides are attached.  
         BACKGROUND OF THE INVENTION  
         [0002]    The active domain of many proteins may in some cases be mimicked at least in part through the use of short peptide sequences derived from the active site of the protein (Massia and Hubbell, 1991, Yamada, 1991). Through this method, the activity of a specific protein can be conferred to an otherwise nonactive surface or matrix. This method allows a much higher concentration of active sequences to be immobilized onto a surface than is found naturally. While many peptides have been shown to have a monotonic correlation between density and cellular activity, other peptides are known to reach a maximum activity at a moderate level of peptide density. The best example is migration of cells on a surface coated with RGD (SEQ. I.D. NO. 2). If the concentration of RGD (SEQ. I.D. NO. 2) is too high, the surface binds too strongly to the cells, inhibiting cellular migration. However, if the RGD (SEQ. I.D. NO. 2) density is too low, then there is not enough traction for these cells to effectively migrate across the surface, leading to a maximal migration rate at a moderate surface concentration of peptide. (DiMilla, et al., 1991) Unfortunately, there is not very much research of the concentration dependent effect of these peptides in a three dimensional matrix. It is not possible to predict which peptides will show saturating behavior and it is not possible to predict at what peptide concentration maximal benefits will be observed.  
           [0003]    While individual peptides can partially mimic the effect of the whole protein, the magnitude of this effect is typically lower. This is due to several reasons, including possible changes in conformation, peptide accessibility and changes in solubility between the peptide and the protein. One additional difference is that the interaction between cells and individual proteins or entire extracellular matrix involve simultaneously binding to multiple peptide sequences. (Martin, 1987, Kleinman, et al., 1993) Sometimes these sequences are on the same protein, but often they are on different proteins. In general it is not possible to predict which combinations might interact negatively, which might interact additively and which might interact synergistically.  
           [0004]    Reports in the literature relate to findings that heparin-binding domains of proteins as well as receptor-mediated binding domains promote neuron adhesion and neurite extension. Many heparin binding domains have been identified (Table 1) and furthermore, heparin binding regions of several proteins such as neural cell adhesion molecule, fibronectin, laminin, midkine, and anti-thrombin III have been reported to promote neurite extension on two-dimensional surfaces. (Edgar, et al., 1984, Borrajo, et al., 1997, Kallapur and Akeson, 1992, Kaneda, et al., 1996, Rogers, et al., 1985) These heparin-binding domains have been reported by indirect evidence to interact with cell-surface proteoglycans by a number of methods including inhibition by soluble heparin, enzymatic removal of cell surface proteoglycans, and biochemical inhibition of proteoglycan synthesis (Kallapur and Akeson, 1992). These peptides have only been studied in 2-dimensional systems.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention discloses a variety of particularly biologically active cell matrices to which a number of cell types may attach and grow. By way of example, the invention provides for protein matrices that have been modified to include chemically cross-linked peptides. These peptides may be defined as having a sequence that mimics heparin-binding regions of several proteins, such as neural cell adhesion molecule, fibronectin, laminen, midkine, and antithrombin III. These peptides are chemically cross-linked to protein and/or fibrin gels to provide particularly efficacious platforms for the attachment and extension of a variety of different cell types.  
           [0006]    The protein matrices of the present invention may include a single or multiple peptide chemically cross-linked thereto. By way of example, one such peptide is HAV (SEQ. I.D. NO. 6). By way of example, the HAV (SEQ. I.D. NO. 6) peptide may be cross-linked into said fibrin gels of the present invention at a concentration of about 2 mol peptide/mol fibrin gel. Another peptide that may be employed with the matrices of the present invention is RGD (SEQ. I.D. NO. 2). By way of example, said peptide may be included in cross-linked fashion to the matrix at a concentration of about 1.5 mol RGD/peptide mol fibrin gel. Yet another peptide that may be included with the matrices of the invention is IKVAV (SEQ. I.D. NO. 1). An exemplary concentration of this peptide found efficacious by the present inventors was a concentration of about 8 mol peptide/mol fibrin gel. Other examples of peptides that may be used in the practice of the present invention are the peptide YIGSR (SEQ. I.D. NO. 3) and the peptide RNIAEIIKDI (SEQ. I.D. NO. 5). These particular peptides have been found to be particularly efficacious in the practice of the present invention when employed at a concentration of about 6 mol YIGSR (SEQ. I.D. NO. 3) peptide/mol fibrin gel and at a concentration of about 8 mol RNIAEIIKDI (SEQ. I.D. NO. 5) peptide/mol fibrin gel.  
           [0007]    In another aspect, the present invention defines a protein matrix that include a mixture of at least two peptides. By way of example, said peptides may be further defined as peptide HAV (SEQ. I.D. NO. 6), IKVAV (SEQ. I.D. NO. 1), RNIAEIIKDI (SEQ. I.D. NO. 5), YIGSR (SEQ. I.D. NO. 3), DGEA (SEQ. I.D. NO. 4), a combination thereof, a combination of peptide fragments comprising HAV (SEQ. I.D. NO. 6), IKVAV (SEQ. I.D. NO. 1), RNIAEIIKDI (SEQ. I.D. NO. 5), YIGSR (SEQ. I.D. NO. 3), and DGEA (SEQ. I.D. NO. 4), or a fusion peptide comprising these peptides. By way of example, an embodiment of the invention may comprise a fibrin gel matrix that includes a mixture of peptides cross-linked thereto, this particular mixture of peptides being defined as comprising IKVAV (SEQ. I.D. NO. 1), RGD (SEQ. I.D. NO. 2), YIGSR (SEQ. I.D. NO. 3), and RNIAEIIKDI (SEQ. I.D. NO. 5), wherein the particular fibrin gel matrix is essentially free of HAV (SEQ. I.D. NO. 6) peptide. As used in the description of the present invention, the term “essentially free” is defined as essentially absent any concentration of the HAV (SEQ. I.D. NO. 6) peptide that would have a neurite extension inhibiting effect on neurite cells cross-linked to a fibrin gel.  
           [0008]    In some embodiments, the invention may be defined as a protein gel that comprises a neurite extension promoting amount of a defined ratio of peptide HAV (SEQ. I.D. NO. 6) and peptide YIGSR (SEQ. I.D. NO. 3). A ratio of about 1 HAV (SEQ. I.D. NO. 6):3 YIGSR (SEQ. I.D. NO. 3) cross-linked into a protein gel matrix has been found by the present inventors to be particularly useful in the practice of the present invention. In yet another aspect, the protein gel may include a neurite extension promoting amount of a ratio of the peptide HAV (SEQ. I.D. NO. 6) and the peptide RNIAEIIKDI (SEQ. I.D. NO. 5). Where these are the two peptides of choice, they may be included at a ratio of about 1:3 as cross-linked to the protein gel. In yet other embodiments of the invention, the protein gel may include a first peptide RGD (SEQ. I.D. NO. 2) and a second peptide YIGSR (SEQ. I.D. NO. 3). Alternatively, the protein gel may comprise a ratio of the peptide RGD (SEQ. I.D. NO. 2) together with the peptide DGEA (SEQ. I.D. NO. 4). The ratio of these two peptides relative to one another that may be employed in the present invention is about 1:3.  
           [0009]    In yet another aspect, the protein gel matrix of the present invention may include a mixture of 3 or more peptides. By way of example, a mixture of peptide RGD (SEQ. I.D. NO. 2), peptide YIGSR (SEQ. I.D. NO. 3), and peptide RNIAEIIKDI (SEQ. I.D. NO. 5) may be included in chemically cross-linked fashion to the protein gel, these three peptides to be included at a ratio of about 1:1:1, respectively.  
           [0010]    All of the above protein and gel matrices may be further defined as three-dimensional matrices. Surprisingly, these three-dimensional matrices modified to include the peptides described herein have been found to advantageously provide enhanced and, in some cases synergistic enhancement of neurite cell growth and extension.  
           [0011]    In some embodiments, the invention provides for fibrin gel matrices. These particular embodiments again comprising a neurite extension promoting ratio of a mixture of peptides. These peptides are again cross-linked to the fibrin gel. The particular matrices of the present invention may also comprise a protein gel other than fibrin. Some embodiments of the protein gel include cross-linked thereto a mixture of peptides as described above at a neurite extension promoting ratios.  
           [0012]    The invention in yet another aspect provides for a bi-domain peptide. The bi-domain peptide in some embodiments may be defined as comprising a first domain of a heparin-binding domain and a second domain consisting essentially of a Factor XIIIa substrate or a bioactive peptide. The bi-domain in some particular embodiments may be further defined as comprising a first domain of a peptide K(βA)FAKLAARLYRKA (SEQ. I.D. NO. 8), YKKIIKKL (SEQ. I.D. NO. 9), KHKGRDVILKKDVR (SEQ. I.D. NO. 10), or a mixture thereof. Protein gels that include chemically cross-linked thereto the afore-described bi-domain peptides may thus also be provided according to the present invention.  
           [0013]    Yet another aspect of the invention provides for fibrin that has been modified to include the bi-domain peptides as described herein. By way of example, the bi-domain peptide employed to modify fibrin may be further defined as comprising a heparin binding domain from ATIII (SEQ. I.D. NO.18). The particular heparin-binding domain peptide that may be employed according to the present invention to modify a fibrin matrix or a cell matrix that includes at least in part fibrin may be defined as a heparin binding domain peptide that elutes from a heparin-affinity column at [NaCl]&gt;0.34 mol.  
           [0014]    The various peptide modified fibrin matrices described herein may be further defined as a protein gel comprising cross-linked peptides. A protein gel that comprises a chemically bound protease inhibitor provides yet another embodiment of the present invention. By way of example, the protease inhibitor may be defined as a2-PI. The protease inhibitor may be even further defined as a modified aprotinin with a Factor XIIIa substrate site.  
           [0015]    The protease inhibitor may be further defined as comprising a modified aprotinin, a Factor XIIIa substrate, and a protease degradation site. In some embodiments, the protease inhibitor may be further defined as comprising a modified aprotinin and a heparin-binding domain sequence. In yet other embodiments, the protease inhibitor may be defined as comprising a modified aprotinin having an hbd and a protease degradation site. The protein gel of the invention maybe further defined as comprising a peptide having a protease binding site covalently incorporated to said gel. The protease binding site in some embodiments of the invention may be further defined as an enzymatic cleavage site, this enzymatic cleavage site having a low kcat and a high km. In yet other embodiments, the protease binding site may be further defined as an enzymatic binding domain.  
           [0016]    In yet another aspect, the present invention provides for three-dimensional matrices capable of supporting neurite cell extension. In some embodiments, the matrix may be defined as comprising a neurite cell extension promoting amount of a mixture of peptides, wherein each of said peptides comprise a sequence that defines a heparin-binding sequence. These three-dimensional matrices may be further defined as fibrin gels or protein gels. Where the three-dimensional matrix includes a mixture of peptides, these peptides may be further defined as having a sequence comprising a heparin-binding peptide of antithrombin II. The heparin-binding peptide will be chemically attached to said three-dimensional matrix, and may be particularly described as cross-linked to the matrix. Particular embodiments of the three-dimensional matrix of the invention may be further defined as comprising a fibrin gel. Where the peptide included within the three-dimensional matrix is a heparin-binding peptide, the peptide may be further defined as a sequence of antithrombin III.  
           [0017]    In yet a further aspect, the invention provides for chimeric peptides. In some embodiments, the chimeric peptide comprises a first C-terminal domain comprising a heparin-binding peptide sequence of antithrombin III; and a second N-terminal domain comprising an a2-plasmin inhibitor substrate for Factor XIIIa. Three-dimensional matrices of protein and/or fibrin in particular, having chemically cross-linked thereto the afore-described chimeric peptides are also provided with the present invention. By way of example, a fibrin gel according to this embodiment may be further defined as comprising a neurite extension promoting amount of about 8 mols of the chimeric peptide/mol of fibrin.  
                                                   SEQUENCE I.D. TABLE            SEQ. I.D. NO.   SEQUENCE                    1   IKVAV               2   RGD               3   YIGSR               4   DGEA               5   RNIAIEIIKDI               6   HAV               7   NCAM               8   K(βA)FAKLAARLYRKA               9   YKKIIKKL               10   KHKGRDVILKKDVR               11   YEKPGSPPREVVPRPRPCV               12   KNNQKSEPLIGRKKT               13   KDPKRL               14   YRSRKY               15   YKKPKL               16   AKRSSKM               17   CRKRCN               18     LNQEQVSP  K(βA)FAKLAARLYRKA               19     LNQEQVSP  YKKIIKKL               20     LNQEQVSP  KHKGRDVILKKDVR                  
 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein.  
         [0019]    [0019]FIG. 1 Enhancement of neurite outgrowth with HAV (SEQ. I.D. NO. 6) modified fibrin gels. A concentration series of HAV (SEQ. I.D. NO. 6) cross-linked into fibrin was tested in triplicate with day 8 chick dorsal root ganglia. The level of growth at 24 and 48 hours was calculated and normalized to growth in unmodified fibrin. Mean and standard error of the mean are shown. Legend to FIG. 1: -□- 24 Hours, -⋄- 48 Hours.  
         [0020]    [0020]FIG. 2 Enhancement of neurite outgrowth with RGD (SEQ. I.D. NO. 2)-modified fibrin gels. A concentration series of RGD (SEQ. I.D. NO. 2) cross-linked into fibrin was tested in triplicate with day 8 chick dorsal root ganglia. (because fibrin naturally contains two active RGD (SEQ. I.D. NO. 2) sites, the level of incorporation of RGD (SEQ. I.D. NO. 2) becomes 2-10 mol RGD (SEQ. I.D. NO. 2)/mol fg instead of 0-8) The level of growth at 24 and 48 hours was calculated and normalized to growth in unmodified fibrin. Mean and standard error of the mean are shown. Legend to FIG. 2: -□- 24 Hour Data, -⋄- 48 Hour Data.  
         [0021]    [0021]FIG. 3 Enhancement of neurite outgrowth with IKVAV (SEQ. I.D. NO. 1)-modified fibrin gels. A concentration series of IKVAV (SEQ. I.D. NO. 1) cross-linked into fibrin was tested in triplicate with day 8 chick dorsal root ganglia. The level of growth at 24 and 48 hours was calculated and normalized to growth in unmodified fibrin. Mean and standard error of the mean are shown. Legend to FIG. 3: -□- 24 Hours, -⋄- 48 Hours  
         [0022]    [0022]FIG. 4 Enhancement of neurite outgrowth with RNIAEIIKDI (SEQ. I.D. NO. 5) modified fibrin gels. A concentration series of RNIAEIIKDI (SEQ. I.D. NO. 5) cross-linked into fibrin was tested in triplicate with day 8 chick dorsal root ganglia. The level of growth at 24 and 48 hours was calculated and normalized to growth in unmodified fibrin. Mean and standard error of the mean are shown. Legend to FIG. 4: -□- 24 Hours, -⋄- 48 Hours  
         [0023]    [0023]FIG. 5 Enhancement of neurite outgrowth with YIGSR (SEQ. I.D. NO. 3) modified fibrin gels. A concentration series of YIGSR (SEQ. I.D. NO. 3) cross-linked into fibrin was tested in triplicate with day 8 chick dorsal root ganglia. The level of growth at 24 and 48 hours was calculated and normalized to growth in unmodified fibrin. Mean and standard error of the mean are shown. Legend to FIG. 5: -□- 24 Hours, -⋄- 48 Hours  
         [0024]    FIG. 6 Peptides were grafted in combination at a total concentration of 8 mol peptide/mol fg. HAV (SEQ. I.D. NO. 6) demonstrated a maximum efficacy at 2 mol/mol fg. Therefore, HAV (SEQ. I.D. NO. 6) was incorporated at this concentration and the second peptide shown was incorporated at 6 mol/mol fg. The growth of neurites relative to growth in unmodified fibrin is shown in the first bars of this graph. The normalized growth of the second peptide grafted alone is shown as well as the theoretical additive growth derived from the two peptides grafted alone. Mean and standard error of the mean are shown. Legend to FIG. 6: □ Combination Growth,          Growth Alone,          Additive Results from Growth Alone.  
         [0025]    [0025]FIG. 7 Peptides were grafted in combination at a total concentration of 8 mol peptide/mol fg. RGD (SEQ. I.D. NO. 2) demonstrated a maximum efficacy at 2 mol/mol fg. Therefore, RGD (SEQ. I.D. NO. 2) was incorporated at this concentration and the second peptide shown was incorporated at 6 mol.mol fg. The growth of neurites relative to growth in unmodified fibrin is shown in the first bars of this graph. The normalized growth of the second peptide grafted alone is shown as well as the theoretical additive growth derived from the two peptides grafted alone. The final series of data labeled laminin represents the four relevant laminin derived peptides, RGD (SEQ. I.D. NO. 2), IKVAV (SEQ. I.D. NO. 1), RNIAEIIKDI (SEQ. I.D. NO. 5) and YIGSR (SEQ. I.D. NO. 3), these peptides were grafted in equimolar quantities at 2 mol peptide/mol fg. Mean and standard error of the mean are shown. Legend to FIG. 7: □ Grafted with RGD,          Sample Grafted Alone,          Cumulative Value for Sample and RGD.  
         [0026]    [0026]FIG. 8 Heparin binding peptides derived from three separate proteins, NCAM (SEQ. I.D. NO. 7), platelet factor 4 and antithrombin III were cross-linked into fibrin at 8 mol/mol fg. The level of neurite outgrowth at 48 hours was normalized against growth in unmodified fibrin and is shown below. Additionally, controls of crosslinked peptide with heparin or just soluble peptide were tested. The growth is shown below with levels that are significantly different from unmodified fibrin (p&lt;0.05) noted with a star. Mean and standard error of the mean are shown. Legend for FIG. 8:          Peptide Grafted at 8 mol/mol Fg, □ Peptide Grafted at 8 mol/mol Fg with Soluble Heparin,          Peptide Added in Soluble Phase.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    Following long-standing patent law convention, the terms ‘a” and “an” mean “one or more˜ when used in this application, including the claims.  
         [0028]    The following examples are included to demonstrate preferred embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples which follow represent techniques discovered by the inventor to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.  
                             TABLE 1                           List of heparin binding peptides and the proteins       they are derived from.                Heparin-binding           Protein   domain   Reference               Anti-thrombin   K(βA)FAKLAARLYRKA   Tyler-Cross       III   (SEQ. I.D. NO. 8)   et al., 1994       Platelet   YKKIIKKL   Zeuker and       Factor 4   (SEQ. I.D. NO.9)   Katz, 1991       Neural Cell   KHKGRDVILKKDVR   Kallapur,       Adhesion   (SEQ. I.D. NO. 10)   1992       Molecule       Fibronectin   YEKPGSPPREVVPRPRPCV   Haugen, et           (SEQ. I.D. NO. 11)   al, 1992           KNNQKSEPLIGRKKT           (SEQ. I.D. NO. 12)       bFGF   KDPKRL   SwissProt:           (SEQ. I.D. NO. 13)   P09038           YRSRKY           (SEQ. I.D. NO. 14)       aFGF   YKKPKL   SwissPROT:           (SEQ. I.D. NO. 15)   P05230       LPL   AKRSSKM   Hata, et           (SEQ. I.D. NO. 16)   al., 1993           CRKRCN           (SEQ. I.D. NO. 17)                  
 
       EXAMPLE 1  
       [0029]    The biological effect of incorporated peptides in a three dimensional matrices demonstrated in the present example. Multiple peptide sequences from extracellular matrix proteins have been chosen. These proteins were chosen in part because of their reported ability to enhance neurite outgrowth (Yamada, 1991, Tashiro, et al., 1989). These proteins have been tested at various concentrations. To test these peptides, specific selected peptide sequences were cross-linked into a three dimensional fibrin matrix with a day 8 dorsal root ganglia embedded into the gels. The neurites were grown for 48 hr, and the migration rate of the neurites extending from the ganglia was quantified for each condition at both 24 and 48 hr. This growth was then normalized to the growth in unmodified fibrin. The ability for some of these peptides to enhance neurite outgrowth was found to increase with peptide concentration, while other peptides reach a maximal enhancement at a moderate peptide concentration. Two peptides that were tested, one from N-Cadherin, namely the tripeptide HAV (SEQ. I.D. NO. 6) (FIG. 1), and one which is present in many extracellular matrix proteins, namely RGD (SEQ. I.D. NO. 2) (FIG. 2), were shown to reach a maximal effect at a moderate concentration of incorporated peptide. HAV (SEQ. I.D. NO. 6) achieved a maximal effect at 2 mol/mol fg while RGD (SEQ. I.D. NO. 2) achieved a maximal effect at 1.5 mol/mol fg of incorporated peptide. In contrast, the peptide sequences IKVAV (SEQ. I.D. NO. 1) (FIG. 3), RNIAEIIKDI (SEQ. I.D. NO. 5) (FIG. 4) and YIGSR (SEQ. I.D. NO. 3) (FIG. 5) were shown to have a linear correlation between peptide concentration and the level of enhancement. YIGSR (SEQ. I.D. NO. 3), IKVAV (SEQ. I.D. NO. 1) and RNIAEIIDI (SEQ. I.D. NO. 5) showed maximal enhancement at 6, 8 and 8 mol peptide/mol fibrinogen respectively.  
       EXAMPLE 2  
     Fibrin Gels with Multiple Peptides  
       [0030]    Because two of the peptides that were tested were found to have maximal effect on the neuronal cell model employed at low concentrations, it is possible to incorporate these peptides at a low concentration and still observe a large neuronal effect, leaving many cross-linking sites open. The remaining sites can then be occupied with a different peptide which has it&#39;s maximal effect at a high concentration.  
         [0031]    The present inventors employed the above approach using with several peptides. In one example, HAV (SEQ. I.D. NO. 6) was cross-linked at 2 mol/mol fibrinogen in combination with the following peptides at 6 mol/mol fibrinogen: IKVAV (SEQ. I.D. NO. 1), RNIAEIIKDI (SEQ. I.D. NO. 5), YIGSR (SEQ. I.D. NO. 3) and DGEA (SEQ. I.D. NO. 4). The growth obtained with the peptides grafted together, with the peptide grafted alone and the theoretical sum derived from the results of the two peptides grafted separately is shown in FIG. 6. The cross-linking of HAV (SEQ. I.D. NO. 6) with IKVAV (SEQ. I.D. NO. 1) results in a negative interaction, where the effect on neuronal outgrowth is lower than when IKVAV (SEQ. I.D. NO. 1) is grafted alone. The cross-linking of HAV (SEQ. I.D. NO. 6) with DGEA (SEQ. I.D. NO. 4) had relatively little to no additional effect on neurite extension, resulting in growth similar to when DGEA (SEQ. I.D. NO. 4) is cross-linked into the fibrin alone. The cross-linking of HAV (SEQ. I.D. NO. 6) with YIGSR (SEQ. I.D. NO. 3) had an additive effect on neurite extension.  
         [0032]    In another example, RGD (SEQ. I.D. NO. 2) was cross-linked at 2 mol/mol fibrinogen with the same series of peptides at 6 mol/mol fibrinogen. In several examples, the effect was similar. The incorporation of RGD (SEQ. I.D. NO. 2) with IKVAV (SEQ. I.D. NO. 1) seemed to have a negative effect as the level of neurite outgrowth was similar to that in fibrin. When RGD (SEQ. I.D. NO. 2) was cross-linked with RNIAEIIKDI (SEQ. I.D. NO. 5), it had a neutral effect in that neurite growth was similar to that seen in gels modified with RNIAEIIKDI (SEQ. I.D. NO. 5) alone. Once again, grafting with YIGSR (SEQ. I.D. NO. 3) appeared to lead to an additive effect on neurite outgrowth.  
         [0033]    One study was done where the four peptides derived from laminin were cross-linked into the fibrin at equimolar concentrations. Since 8 mol peptide/mol fibrinogen can be obtained, this material then had 2 mol/mol fibrinogen of IKVAV (SEQ. I.D. NO. 1), RGD (SEQ. I.D. NO. 2), YIGSR (SEQ. I.D. NO. 3), and RNIAEIIKDI (SEQ. I.D. NO. 5). When neurites were grown in this material, the effect led to 75% improvement, which was higher than the combined effects from the peptides grafting alone. These peptides were thus demonstrated in the present studies to act synergistically when co-grafted into fibrin. (FIG. 7)  
         [0034]    By utilizing a peptide-modified fibrin matrix, a novel method has been developed that employs several active peptides in 3 dimensions. It is shown that the addition of protease inhibitors has no effect on 2-dimensional neurite outgrowth, but that addition of these same inhibitors decreases neurite outgrowth 3-dimensionally (Herbert, et al., 1996). Therefore, it becomes impossible to predict the activity of these peptides in 3 dimensions from the 2-dimensional model, because the effect of proteolysis often becomes the limiting factor for 3-dimensional migration.  
         [0035]    Bi-domain peptides containing both a factor XIIIa substrate and a heparin-binding domain (Table 2) were synthesized and cross-linked into fibrin gels as previously demonstrated with one variation.  
         [0036]    Table 2 Exact sequences of bi-domain peptides containing the factor XIIIa substrate sequence and the heparin binding domain sequence. The source for each heparin binding domain is shown as well.  
                       TABLE 2                       Peptide               Name   Sequence   Source                   ATIII     LNQEQVSP  K(βA)FAKLAARLYRKA   Antithrombin           (SEQ. I.D. No. 18)   III       NCAM     LNQEQVSP  YKKIIKKL   neural cell           (SEQ. I.D. No. 19)   adhesion               molecule       PF4     LNQEQVSP  KHKGRDVILKKDVR   platelet           (SEQ. I.D. No. 20)   factor 4                          
 
         [0037]    In one example, the peptide was cross-linked into the gel alone and in the second example, it was incorporated in the presence of heparin. These two methods led to gels where an identical concentration of bi-domain peptide was covalently bound to the fibrin, but the first example resulted in free heparin binding domains being present in the gel, while the addition of heparin in the example condition resulted in this domain being occupied with a heparin fragment. Therefore, the effects of interaction between the growing neurites and either a heparin binding domain or heparin itself could be determined. When the peptide was incorporated without heparin present, it was able to enhance the extension of neurites from day 8 chick dorsal root ganglia. Three bi-domain peptides, each with a different heparin binding domain, were tested and gave statistically better growth than unmodified fibrin (FIG. 8). The level of improvement ranged from 75% to 25%. This level of enhancement could be correlated to the binding affinity of the heparin binding domain that was incorporated (Table 3). When heparin was added at the beginning of the study, it was discovered that this abolished the effect, resulting in growth similar to that seen in unmodified fibrin.  
         [0038]    Table 3 Provides results of the relative heparin binding affinity and enhancement of three dimensional neurite outgrowth for several heparin binding peptides. There is a correlation between the binding strength for each peptide and the percent enhancement of neurite outgrowth.  
                                     TABLE 3                       Peptide   Elution from Heparin Affinity   Enhancement of Neurite       Name   Column (Mol NaCl)   Outgrowth (%)                                ATIII   0.67   73.1       NCAM   0.35   24.5       PF4   0.34   20.2                  
 
       EXAMPLE 3  
     Custom-Designed Gel Matrix for Neurite Growth  
       [0039]    There are four components necessary for creating a cross-linked fibrin gel; fibrinogen, calcium, thrombin and factor XIIIa, and the structural characteristics of the material. These four components can be modified by changing the concentration of any one of them. There are two main characteristics that determine the structure of the fibrin; the density of the fibrin bundles and the thickness of each individual bundle. These two properties will then control the ability of cells to infiltrate the matrix.  
         [0040]    Increasing fibrin concentration from 5-15 mg/mL in the precursor mixture was found to result in fibrin gels with smaller fibrin bundles that are much denser. This resulting material has been shown to be more difficult for neurites to migrate through. When the calcium concentration was increased from 2-10 mM, the fibrin bundles got thicker, but the spacing between these bundles became greater. Changing the fibrin density clearly can have a direct effect on cellular migration while changes in the fibril morphology does not. Degradation of the fibrin matrix is dependent on the morphology of these fibers. Therefore, the ability for cells to infiltrate the fibrin and the overall degradation of the gel can be controlled independently. The kinetics of fibrin formation are dependent on the amount of the two enzymes, thrombin and factor XIII, that are present. Increasing the concentration of thrombin decreases time for gelatin while increasing the factor XIII concentration increases the rate of cross-link formation. By varying the concentrations of these four precursor components, the fibrin morphology and kinetics are manipulated to provide a matrix with optimal properties.  
         [0041]    The present example demonstrates methods to control the cell-mediated rate of fibrin degradation. One method to control the degradation of the material is to modify the structural characteristics through the initial concentration of the precursor components. By making a fibrin gel that is denser, the rate of cellular infiltration, and related cellular degradation will be decreased. This can be accomplished by increasing the density of the gel through either an increase in the fibrinogen concentration or an increase in the cross-linking through the amount of factor XIIIa.  
         [0042]    Another example involves controlling the rate of degradation by crosslinking exogenous protease inhibitors into the fibrin gel. In order to maximize the efficacy of these inhibitors, they must be covalently bound to the gel to provide a high local concentration. Therefore, proteases will be either selected or engineered to have a factor XIIIa substrate site. In the example of 2-plasmin inhibitor, a substrate sequence is already present. For other proteins, like aprotinin, this site in some embodiments of the invention will be built into the protein. These proteins will then be added to the coagulation mixture and allowed to cross-link into the fibrin. The inhibitors will remain in the gel through this covalent cross-link until enzymatic degradation of the fibrin begins. This initial degradation will then release the bound inhibitors, preventing widespread degradation of the fibrin matrix. This method can also be further modified to incorporate a enzymatic degradation site in the protease inhibitor between the cross-linking domain and the active protein domain. This method will then allow the protease inhibitor to be released free of fibrin degradation products, potentially increasing the efficacy of the released inhibitor.  
         [0043]    Another example involves cross-linking bi-domain peptides into the fibrin that include a factor XIIIa substrate sequence and a protease binding domain. The binding domain in the peptide could be a degradation site that was selected to have a low Km and a low Kcat leading to high level of enzyme binding at the site but a low level of enzymatic activity. This would then serve to allow the incorporated peptides to act as a competitive inhibitor for fibrin degradation sites. However, since the incorporated peptide would be cleaved at a very slow rate, it would effectively immobilize the protease and prevent it from degrading the fibrin matrix. Additionally, the binding domain could interact with the protease through a site unrelated to the enzymatic activity. (i.e. heparin binding) This would then allow the protease to be sequestered in the fibrin before it could degrade the gel as well.  
         [0044]    All of the compositions and methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the composition, methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. More specifically, it will be apparent that certain agents which are both chemically and physiologically related may be substituted for the agents described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims.  
         [0045]    References  
         [0046]    The following references, to the extent that they provide exemplary procedural or other details supplementary to those set forth herein, are specifically incorporated herein by reference.  
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                                                   SEQUENCE LISTING            SEQ. I.D. NO.   SEQUENCE                    1   IKVAV               2   RGD               3   YIGSR               4   DGEA               5   RNIAEIIKDI               6   HAV               7   NCAM               8   K(βA)FAKLAARLYRKA               9   YKKIIKKL               10   KHKGRDVILKKDVR               11   YEKPGSPPREVVPRPRPCV               12   KNNQKSEPLIGRKKT               13   KDPKIRL               14   YRSRKY               15   YKKPKL               16   AKRSSKM               17   CRKRCN               18     LNQEQVSP  K(βA)FAKLAARLYRKA               19     LNQEQVSP  YKYIIKKL               20     LNQEQVSP  KHKGRDVILKKDVR