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Trabajo presentado en el 19th International Congress of Developmental Biology, celebrado en Guia (Portugal) del 16 al 20 de octubre de 2022., Regeneration in mammals is limited to the digit tips. The amputation of the distal tip of the terminal phalanx triggers the formation of an undifferentiated pool of progenitor cells called a blastema that will regenerate the multiple tissues required to restore the missing part. Digit tip regeneration is linked to the presence of the dorsal nail organ, as only amputations that conserve the nail matrix regenerate. However, the regenerative potential of digits without nails has not yet been tested. To evaluate nailless digits, we used dLARM1/2 mutants that develop digits with no nails due to the limb-specific loss of the dorsal limb determinant Lmx1b. We report that dLARM1/2 mutant digits are unable to regenerate, although they form a blastema. To test whether the lack of regeneration was due to the absence of dorso-ventral (DV) polarity, a concept suggested in amphibia, we evaluated Del(27) mutants that also lack DV polarity with nails on both dorsal and ventral aspects of distal phalanges due to ventral overexpression of Lmx1b. Unlike dLARM1/2 mutants, Del(27) mutant digits regenerated indicating that DV polarity is dispensable for regeneration. Interestingly, Lmx1b expression (which continues in the nail dermis postnatally) is detected uniformly in the blastema of WT, but not dLARM1/2 mutant mice. In addition, comparison of dLARM1/2 vs WT blastemal transcriptomes revealed that the mutants had defective pro regenerative processes such as vasculogenesis and extracellular matrix remodeling, while inflammatory pathways were exacerbated. Gene expressions typical of the nail dermis were also lost in dLARM1/2 digit tips, suggesting regulation by Lmx1b. Collectively, our data suggest a direct role for Lmx1b in digit tip regeneration maintaining the nail dermis and promoting a competent blastema.

Trabajo presentado en el 16th International Conference on Limb Development, Regeneration, and Evolution, celebrado en Cambridge (Estados Unidos) del 08 al 11 de agosto de 2022., Mammalian limb regeneration is restricted to the distal third of terminal phalanges. Amputation of distal digit tips results in blastema formation and restoration of the missing part. This regenerative response has been linked to the presence of the nail, but digits without nails have not been evaluated. Here we interrogate the regenerative ability of the double-ventral digits of dLARM1/2 mutants, which lack dorsal nails due to the limb-specific absence of Lmx1b. We report that these nail-less digits fail to regenerate, supporting the nail requirement. Because the absence of dorso-ventral (DV) polarity could also impair regeneration, as reported in amphibians, we tested the double-dorsal double-nail digits of Del(27) mutants, which lack DV polarity due to the limb-restricted loss of En1. Since Del(27) digits did regenerate, we concluded that DV polarity is not required for digit tip regeneration. We documented continued dorsal expression of Lmx1b postnatally in the wild-type (WT) nail dermis. Moreover, there is uniform Lmx1b expression in WT regeneration-competent blastemas. Transcriptomic comparison between WT and dLARM1/2 mutant blastemas revealed differential up-regulation of gene signatures functionally associated with vascularization and connective tissue remodeling in WT mice. In contrast, the dLARM1/2 non-regenerative blastemas that lack Lmx1b expression show differential up-regulation of genes associated with inflammation and inflammatory processes. Conjointly, our data implicate a direct role for Lmx1b in digit tip regeneration by maintaining the nail dermis and promoting a competent blastema.

Trabajo presentado en The Company of Biologists Workshop Molecular mechanisms of developmental and regenerative biology (EMBO), celebrado en modalidad virtual del 26 al 29 de abril de 2022., The regeneration of the digit tip is an exceptional model of multi-tissue regeneration in mammals, with remarkable potential to advance regenerative medicine. Digit tip regeneration positively correlates with the presence of the nail organ, most likely because of the environment of the nail stem cells, but the underlying mechanisms are still unclear. The fact that the nail is a dorsal structure links regeneration with dorso-ventral (DV) polarity. Here, to evaluate the nail function more directly, and to disentangle it from DV positional information, we tested the regenerative ability of double-ventral digits and double-dorsal digits resulting from the limb-specific loss of Lmx1b and of En1 expression, respectively. Our results indicate that DV polarity is not required for digit tip regeneration but that an excessive inflammatory response prevents regeneration in the absence of Lmx1b. The possible causes of the exacerbated inflammation in the absence of Lmx1b are discussed.

Trabajo presentado en el EMBO Workshop: Enhanceropathies: understanding enhancer function to understand human disease, celebrado en Santander (España) del 06 al 09 de octubre de 2021., Lmx1b is the key regulator of dorso-ventral (DV) pattern in the limb, as it is both necessary and sufficient to specify dorsal fates. Accordingly, targeted disruption of Lmx1b results in double ventral limbs. In humans, mutations in the LMX1B gene are responsible for the autosomal dominant Nail-Patella Syndrome (OMIM #161200), characterized by incomplete limb dorsalization. Here, we report on two enhancers located upstream of the gene, LARM1 and LARM2, that regulate Lmx1b specifically in the limb. These sequences contain several canonical Lmx1b binding sites that mediate a positive loop of Lmx1b autoregulation. Deletion of these enhancers together results in mice with double ventral limbs, like these of Lmx1b-nul mice. Remarkably, deletion of each individual enhancer produced a double ventral phenotype clearly restricted to the posterior half of the limb in LARM1 mutants and to the anterior in LARM2 mutants, indicating differential activity of these enhancers along the anterior-posterior axis. These results reflect an unexpected complex, modular and autoregulatory mode of Lmx1b transcriptional regulation during limb development. We also report on several NPS patient families with normal LMX1B coding sequence but variations in the LARM region that exhibit limb restricted phenotypes, stressing the role of regulatory modules in disease pathogenesis.

Resumen del trabajo presentado en el 17th Spanish Society for Developmental Biology Meeting Virtual Meeting, celebrado en modalidad virtual del 18 al 20 de noviembre de 2020., Lmx1b is a LIM homeodomain transcription factor that plays essential roles in a wide range of developmental processes including the development of the kidney, the eye, the dopaminergic and serotonergic neurons and the limb. In the limb, Lmx1b expression is restricted to the dorsal mesenchyme and is responsible for limb dorsalization. Mice lacking functional Lmx1b die at birth because of multisystemic malformations. Homozygous Lmx1b mutants develop a double-ventral limb phenotype with loss of dorsal structures such as the patella, whereas the ventral ones such as the sesamoid bones are duplicated and dorsal tendons and muscles are transformed into mirror images of ventral ones. At superficial level, hair and nails are absent while typical ventral features such as pads are also duplicated. Herein, we report on two conserved Lmx1bassociated cis-regulatory modules (LARM1 and LARM2) located upstream of Lmx1b that are bound by Lmx1b and mediate an autoregulatory loop required to provide adequate levels of Lmx1b expression necessary for limb dorsalization. Combined removal of both enhancer lead to a limb restricted phenotype identical to that of the Lmx1b Knockout mice with no other Lmx1b-related systemic defects, indicating that they are limb specific enhancers. Unexpectedly, the CRISPR/Cas9 deletion of each individual enhancer produced a double ventral phenotype clearly restricted to the posterior or anterior half of the autopod in LARM1 and LARM2 respectively, unveiling spatial modularity in the transcriptional control of Lmx1b in the limb. In humans, LMX1B haploinsufficiency causes Nail-patella syndrome (NPS; MIM 161200), characterized by nail dysplasia, absent/hypoplastic patellae, chronic kidney disease, and glaucoma. Of most interest, we report on two NPS patient families with normal LMX1B coding sequence and limb-restricted phenotype, but with loss-of-function variations in the LARM1/2 region, stressing the role of regulatory modules in disease pathogenesis.

Resumen del trabajo presentado en el 17th Spanish Society for Developmental Biology Meeting Virtual Meeting (2020), celebrado en modalidad virtual del 18 al 20 de noviembre de 2020., In recent years, the development of CRISPR technologies has provided an excellent tool for genomic editing. We have taken advantage of this technology to set up a CRISPR service at our Institute that based on the electroporation of mouse zygotes (day 0.5) generates desired mouse genomic modifications in a highly efficient, rapid and unexpensive manner. We have applied this methodology to the generation of several mouse models of human congenital hand malformations, especially focussing on the Split Hand Foot Malformation (SHFM), a rare and highly variable malformation characterized by the loss or deformity of the central digit rays. We have previously shown the involvement of Sp6 and Sp8 transcription factors in the generation of SHFM phenotypes in mouse and the interaction of these two factors with Dlx family members. Dlx genes are the genes involved in human SHFM types 1 (Dlx5/Dlx6) and 5 (Dlx1/Dlx2). With the aim of fully investigating the implication of the Dlx-Sp interactions in the pathogenesis of SHFM, we have generated a couple of mouse models: the Dlx5/Dlx6 double deletion and the Sp6- V5 tagged knock-in. Additionally, and based on studies in Drosophila indicating that Sp factors control appendage development through the Notch pathway, we have also generated a Jag2 KO mutant. We are currently using the generated animal models to advance our understanding of the mechanisms subjacent to the SHFM.

Resumen del trabajo presentado en 17th Spanish Society for Developmental Biology Meeting Virtual Meeting, celebrado en modalidad virtual del 18 al 20 de noviembre de 2020., Epimorphic regeneration is a type of multi-tissue regeneration defined by the formation of a blastema. In contrast to amphibians, which can regenerate their entire limbs, mammals can only regenerate the distal tip of their digits, hence investigating the mechanisms involved is of maximum interest for regenerative medicine. Interestingly, in mice and humans this regeneration associates with the nail organ, particularly with the Wnt/ß-catenin active nail matrix. Nails are ectodermal appendages of the dorsal tip of the digits. Their development reflects the dorso-ventral polarity of the limb, established in the early limb bud ectoderm by the interaction of three central molecules. En1, expressed in the ventral ectoderm, restricts Wnt7a to the dorsal ectoderm. Wnt7a induces Lmx1b, the dorsal determinant, in the subjacent mesoderm. Lmx1b-null mice display bi-ventral distal limbs and die perinatally due to multisystemic defects. Recent studies have identified two Lmx1b limb-specific enhancers named LARM1 and LARM2. CRISPR/Cas9-mediated deletion of these two enhancers (LARM1/2-/-) yielded mice with a limb-restricted Lmx1b-null phenotype, but no other systemic defects. LARM1/2 null mutants show absence of nails in their digit tips, providing an opportunity to directly test the involvement of the nail in digit tip regeneration. As expected, LARM1/2 mutants fail to regenerate their digit tips, as histological and ¿CT analyses demonstrate. Importantly, disregarding the lack of regeneration, a blastema does form at the tip of the LARM1/2 stump. Our preliminary results point to a reduction of proliferation in the LARM1/2 blastema compared to wild-type, and to an absence of Wnt/ß-catenin active epidermis in mutants, as an explanation of the regenerative failure. Our results confirm that bi-ventral digits do not regenerate their digit tips and set the LARM1/2 mutant as a useful model to investigate the mechanisms of regenerative blastema, with the aim of enhancing regeneration.

Trabajo presentado en el 41 Congreso de la Sociedad Española de Bioquímica y Biología Molecular SEBBM, celebrado en Santander (España) del 10 al 13 de septiembre de 2018.

Resumen del trabajo presentado a los Congresos: 17th International Congress of Developmental Biology, 72nd Annual Meeting of the Society for Developmental Biology, VII Latin American Society of Developmental Biology Meeting y XI Congreso de la Sociedad Mexicana de Biologia del Desarrollo, celebrados en Cancún (México) del 16 al 20 de junio de 2013., The apical ectodermal ridge (AER) is a specialized epithelium located at the distal dorso-ventral (DV) rim of the developing limb that is crucial for limb bud development. The induction of the AER is a complex process that relies on intricate interactions among the FGF, WNT, and BMP signaling pathways operating within the ectoderm and between the ectoderm and mesoderm of the early limb bud. Furthermore, induction of the AER is linked to the establishment of DV patterning. Sp6 and Sp8 are two members of the Specificity Protein family of transcription factors that are expressed in the limb bud ectoderm and function downstream of WNT/ β Catenin signaling and upstream of Fgf8. Their individual genetic inactivations result in a mild syndactyly phenotype for Sp6 and limb truncation, due to the premature regression of the AER, for Sp8. To investigate a possible functional redundancy between Sp6 and Sp8, we generated double Sp6;Sp8 null mutants. We also generated Sp6-null; Sp8-conditional mutants using a Sp8 floxed allele with the Ap2-Cre and with the Msx2-Cre deleter lines. Our results show that double Sp6;Sp8 mutants are tetra-amelic. Initial budding occurs, but Fgf8 and Bmp4 are not activated in the limb ectoderm and the dorsal marker Wnt7a persists throughout the limb bud ectoderm. The phenotype of mutants bearing a single functional copy of Sp6 (Sp6 +/-;Sp8-/-) is indistinguishable from that of the double mutants, whereas the presence of a single functional allele of Sp8 (Sp6-/-;Sp8+/-) results in a Split Hand Foot Malformation phenotype. We conclude that Sp6 and Sp8 work in a redundant manner as indispensable mediators of WNT/ β Catenin signaling in the limb ectoderm and that their function links the Proximo-distal and DV axes.