C3 glomerulopathy (C3G) encompasses a group of severe renal diseases characterized by “dominant C3” deposition in the renal glomerulus. Patients typically present as nephritic nephrotics, with hematuria, hypertension, heavy proteinuria and edema. Within ten years of diagnosis, 50% of affected patients progress to end-stage renal disease and require dialysis or renal transplantation. No treatment is available to halt disease progression and thus both disease recurrence and allograft loss are common after transplantation. Genetic studies of C3G have firmly implicated dysregulation of the alternative pathway (AP) of complement in disease pathogenesis. In addition to genetic factors, acquired factors like autoantibodies can also exaggerate AP activity in the circulation to cause C3G. Although AP dysregulation in the circulation (i.e. fluid-phase dysregulation) has been well studied in these patients, AP activity in the glomerular microenvironment is not well understood. In this body of work, we used MaxGel, an ex-vivo surrogate for the glomerular extracellular matrix, to study AP activity and regulation. We showed that C3 convertase can be assembled on MaxGel and elucidated the dynamics of its formation and decay in the presence of complement regulators. We confirm that on MaxGel factor H (fH) inhibits C3 convertase formation and accelerates its decay, while properdin has a stabilizing effect. We also show that the complement factor H-related proteins (FHRs) are vital to the regulation of AP activity. Consistent with our MaxGel data, CFHR gene-fusion events have been reported as genetic drivers of disease in a few familial cases of C3G. One such familial case in which we identified and characterized the rearrangement event results from a novel CFHR5-CFHR2 fusion gene. The fusion gene is translated into a circulating FHR-5/-2 protein that consists of the first two SCRs of FHR-5 followed by all four SCRs of FHR-2. The structural repetition of SCR1-2 followed by another SCR1-2 motif facilitates the formation of complex FHR-1, FHR-2 and FHR-5 multimers, which have enhanced affinity for C3b and by out-competing fH, lead to impaired C3 convertase regulation in the glomerular microenvironment. Finally, we tested gene therapy as a tool to rescue the disease phenotype and restore fluid-phase AP complement control in a mouse model of C3G (Cfh-/-/huCR1-Tg mice). Using the piggyBac transposon system, we introduced a construct derived from complement regulator 1 (CR1) into Cfh-/-/huCR1-Tg mice. Delivery of sCR1-AC via hydrodynamic tail vein injection provided constitutive circulatory expression of sCR1-AC, and in animals followed for 6 months, we found that long-term expression of this complement regulator rescued the renal phenotype. These results suggest that sCR1 may be a potential therapy for patients with this disease.