Neuronal postdevelopmentally acting SAX-7S/L1CAM can function as cleaved fragments to maintain neuronal architecture inCaenorhabditis elegans

Abstract

Whereas remarkable advances have uncovered mechanisms that drive nervous system assembly, the processes responsible for the lifelong maintenance of nervous system architecture remain poorly understood. Subsequent to its establishment during embryogenesis, neuronal architecture is maintained throughout life in the face of the animal’s growth, maturation processes, the addition of new neurons, body movements, and aging. The C. elegans protein SAX-7, homologous to the vertebrate L1 protein family of neural adhesion molecules, is required for maintaining the organization of neuronal ganglia and fascicles after their successful initial embryonic development. To dissect the function of sax-7 in neuronal maintenance, we generated a null allele and sax-7S-isoform-specific alleles. We find that the null sax-7(qv30) is, in some contexts, more severe than previously described mutant alleles, and that the loss of sax-7S largely phenocopies the null, consistent with sax-7S being the key isoform in neuronal maintenance. Using a sfGFP::SAX-7S knock-in, we observe sax-7S to be predominantly expressed across the nervous system, from embryogenesis to adulthood. Yet, its role in maintaining neuronal organization is ensured by post-developmentally acting SAX-7S, as larval transgenic sax-7S(+) expression alone is sufficient to profoundly rescue the null mutants' neuronal maintenance defects. Moreover, the majority of the protein SAX-7 appears to be cleaved, and we show that these cleaved SAX-7S fragments together, not individually, can fully support neuronal maintenance. These findings contribute to our understanding of the role of the conserved protein SAX-7/L1CAM in long-term neuronal maintenance, and may help decipher processes that go awry in some neurodegenerative conditions.

SAX-7/L1CAM acts with the adherens junction proteins MAGI-1, HMR-1/Cadherin, and AFD-1/Afadin to promote glial-mediated dendrite extension

“…Interestingly, SAX-7 was recently shown to undergo proteolytic cleavage in vivo and to function, at least in some contexts, in its cleaved form (Desse et al, 2021). All of our mutant alleles are predicted to produce truncated SAX-7 proteins reminiscent of the cleaved SAX-7 products (Fig.…”

“…We therefore examined a molecular null allele of sax-7 generated by CRISPR/Cas9-mediated deletion of the entire sax-7 locus (sax-7(qv30), Fig. 1D) (Desse et al, 2021).…”

Gene bookmarking by the heat shock transcription factor programs the insulin-like signaling pathway

“…We wanted to find out whether offspring of heat-shocked mothers expressed higher basal levels of the molecular chaperone proteins HSP70 (HSP-1 in C. elegans) and HSP90 (DAF-21 in C. elegans), which inhibit HSF-1 activation but can increase thermotolerance, because this might explain the apparently contradictory results of increased stress resilience and decreased HSF-1 stress inducibility. However, this was also not the case; western blot analysis using antibodies to endogenous hemagglutinin (HA)-tagged HSP-1 (Figure S3E) and DAF-21 (Figure S3F; see also Blanchette et al, 2015;Desse et al, 2021) showed no differences in these protein levels between offspring of stressed mothers and non-stressed mothers. Inducible HSP-70 protein levels were also not higher in animals that had experienced early-life stress exposure (Figure S3G).…”

Loss of the Extracellular Matrix Protein DIG-1 Causes Glial Fragmentation, Dendrite Breakage, and Dendrite Extension Defects

“…Briefly, the morphology of every neuron and glial cell is highly stereotyped and has been carefully catalogued; individual cells can be readily visualized in live intact animals throughout life; and it is straightforward to perform forward genetic screens by visually identifying mutants that show aberrant morphology of single neurons or glial cells. Recent screens have revealed roles for extracellular factors (i.e., ECM components and cell surface adhesion molecules) in diverse aspects of neurodevelopment, including neuroblast migration [2][3][4], axon and dendrite extension [5][6][7][8][9], fasciculation of neurites into nerve bundles [10][11][12][13][14][15], and synapse development [16][17][18][19][20][21].…”