SAP97 Binding Partner CRIPT Promotes Dendrite Growth in Vitro and in Vivo

The dendritic tree is a key determinant of neuronal information processing. In the motor system, the dendritic tree of spinal cord neurons undergoes dramatic remodeling in an activity-dependent manner during early postnatal life. This leads to the proper segmental spinal cord connectivity that subserves normal locomotor behavior. One molecular system driving the establishment of dendrite architecture of mammalian motor neurons relies on AMPA receptors assembled with the GluA1 subunit and this occurs in an NMDA-R- independent manner. The dendrite growth promoting activity of GluA1-containing AMPA receptors depends on its intracellular binding partner, SAP97, and SAP97’s PDZ3 domain. We show here that CRIPT is a bona fide SAP97 PDZ3-domain binding partner, localizes to synapses with GluA1 and SAP97 along the dendritic tree and is a determinant of the dendritic growth of mammalian spinal cord neurons. We further show that CRIPT has a well-conserved ortholog in the nematode, Caenorhabditis elegans, and animals lacking CRIPT display decreased dendrite branching of the well-studied PVD neuron in vivo. The lack of CRIPT leads to a selective defect in touch perception and this is rescued by expression of wild type human CRIPT in the nervous system. This work brings new light into the molecular machinery that drives dendritic growth during development and may prove relevant to the promotion of nervous system plasticity following insult.

Significance Statement Proper dendritic growth is a critical step in the development of neuronal connectivity that underlies proper neuronal communication. Much is known about how NMDA receptors drive neuronal development and plasticity, but less is known about how AMPA receptors contribute in an independent manner. While SAP97 plays a critical role in this process, the molecular mechanisms and binding partners that subserve these effects are under active exploration. Here we show that the cysteine-rich interactor of PDZ3 (CRIPT) is a bona fide binding partner of SAP97 in biochemical assays and resides in dendrites in the vicinity of putative AMPAergic synapses. In knockdown experiments, we find that CRIPT is essential for SAP97-dependent dendrite growth in vitro. We extend these studies to an in vivo model and show that CRIPT is also essential for dendrite growth and mechanosensory function in C.elegans. This work links AMPA receptors, MAGUKs and CRIPT to essential neuronal cell biology and C.elegans behavior.