Cruz-Martín, AlbertoPhadke, Rhushikesh A.2025-09-122025-09-122024https://hdl.handle.net/2144/512222024The immune complement pathway is essential in regulating synaptic wiring and influencing neuroimmune adaptations. Members of the pathway have also been shown to play a pivotal role in synaptic plasticity. Disruptions in the complement pathway have been linked to devastating brain diseases exhibiting pathological synaptic loss, such as schizophrenia (SCZ) and Alzheimer’s disease. Here the effect of multiple components of the complement pathway on synaptic connectivity and plasticity is investigated.Our group previously demonstrated that increasing the levels of complement component 4 (C4A or just C4) – a member of the complement cascade and a SCZ risk gene – leads to hypoconnectivity of developing cortical neurons. C4-mediated decrease in synaptic connectivity is thought to be mediated by downstream activation of the canonical pathway that involves complement receptor 3 (CR3) and recruitment of microglia. Here, I report that decreased connectivity caused by overexpression of C4 (C4-OE) is CR3 independent. Instead, C4-OE triggers GluR1 degradation through an intracellular mechanism involving endosomal trafficking protein SNX27, resulting in pathological synaptic loss. Moreover, the connectivity deficits associated with C4-OE were rescued by increasing levels of SNX27, linking excessive complement activity to an intracellular endolysosomal recycling pathway affecting synapses. Alongside C4, complement component 1q (C1q) has also been shown to be involved in synaptic wiring. Hence, I aimed to determine the role of microglia-specific C1q on the connectivity and network properties of cells of the hippocampus. We found that knocking out microglial C1q in mice led to an increase in the density of dendritic spines in the stratum radiatum (SR) and the molecular layer (ML) of the dentate gyrus. We observed a reduction in the engulfment of PSD95, an excitatory post-synapse marker. Lack of C1q also led to changes in the morphology of microglia, which suggests a change in microglial physiology. In support of the lack of C1q in microglia contributing to increased connectivity of hippocampal neurons on a network scale, the knockout of C1q led to increased CA1 pyramidal cell activity. My studies have led me to conclude that the complement pathway regulates synaptic function in the brain through both canonical and non-canonical mechanisms involving C1q and C4. Deciphering brain function through disease mouse models requires understanding complex interactions between genetics, neural circuitry, and behavior. To aid this process, I developed REVEALS (Rodent BEhaVior Multi-camErA Laboratory AcquiSition), a user-friendly graphical user interface (GUI) for efficient recording and analysis of rodent behavior using USB3 FLIR cameras. I designed REVEALS to allow user-friendly control of simultaneous recording from multiple cameras while streamlining the data acquisition process, enabling researchers to collect and analyze large datasets of rodent behavior efficiently. I used REVEALS to study the behavioral effects of C4-OE in fast-spiking parvalbumin interneurons (PV-INs) using DeepLabCut (DLC) pose-estimation pipelines. Overall, these studies provide insight into the complex world of neuroimmune interactions and elucidate how components of the complement system can regulate and disrupt synaptic wiring. We also provide one of the first proofs of an intracellular non-canonical complement mechanism that affects spine stability and opens the doors to the exploration of potential novel therapeutic targets.en-USNeurosciencesMolecular biologyCellular biologyAMPAR degradationComplement 4Endosomal recyclingNeuroimmuneNon-canonicalSchizophreniaThesis/Dissertation2025-09-110009-0008-7097-7593